MEMBRANE FILTRATION GUIDANCE MANUAL

United States

Environmental 

Protection Agency

Office of Water

(4601)

EPA 815-D-03-008

June 2003 

Proposal Draft
Note on the Membrane Filtration Guidance Manual

June 2003 Draft

Purpose

The purpose of this guidance manual, when finalized, is solely to provide technical information 

on the use of membrane filtration and application of the technology for compliance with the 

Long Term 2 Enhanced Surface Water Treatment Rule, which would require certain systems to 

provide additional treatment for Cryptosporidium. This rule, along with the companion Stage 2 

Disinfectants and Disinfection Byproducts Rule, are summarized in Chapter 1 of this manual.

This guidance is not a substitute for applicable legal requirements, nor is it a regulation itself. 

Thus, it does not impose legally binding requirements on any party, including USEPA, States, or 

the regulated community. Interested parties are free to raise questions and objections to the 

guidance and the appropriateness of using it in a particular situation. Although this manual 

covers many aspects of implementing membrane filtration, it is not intended to be a 

comprehensive resource on the subject. The mention of trade names or commercial products 

does not constitute endorsement or recommendation for use.

Authorship

This manual was developed under the direction of USEPA’s Office of Water, and was prepared 

by Malcolm Pirnie, Inc., Separation Processes, Inc., and The Cadmus Group, Inc. Questions 

concerning this document should be addressed to:

Steve Allgeier

United States Environmental Protection Agency

Office of Ground Water and Drinking Water

Technical Support Center (MS 140)

26 West Martin Luther King Dr.

Cincinnati, OH 45268

Voice: (513) 569-7131

Fax: (513) 569-7191

Email: allgeier.steve@epa.gov

Request for Comments

USEPA is releasing this manual in draft form in order to solicit public review and comment. The 

Agency would appreciate comments on the content and organization of technical information 

presented in this manual. Please submit any comments no later than 90 days after publication of 

the Long Term 2 Enhanced Surface Water Treatment Rule proposal in the Federal Register. 
Comments on this draft guidance manual should be submitted to USEPA’s Water Docket. 

Comments may be submitted electronically, by mail, or through hand delivery/courier, as 

follows:

Submitting comments via USEPA’s electronic public docket

Go directly to USEPA Dockets at http://www.epa.gov/edocket and follow the online instructions 

for submitting comments. Once in the system, select “search,” and then key in Docket ID No. 

OW-2002-0039. 

Submitting comments via e-mail

Send comments to OW_Docket@epa.gov, Attention Docket ID No. OW-2002-0039. EPA’s email system automatically captures the sender’s e-mail address, which is then included as part of 

the comment that is placed in the official public docket.

Submitting comments on a disk or CD ROM

Mail any comments on disk or CD to address given below. These electronic submissions will be 

accepted in WordPerfect or ASCII file format. Avoid the use of special characters and any form 

of encryption. 

Water Docket

United States Environmental Protection Agency

Mail Code 4101T

1200 Pennsylvania Ave., NW

Washington, DC, 20460

Attention Docket ID No. OW-2002-0039

Submitting comments by mail

Send three copies of comments and any associated enclosures to: 

Water Docket

United States Environmental Protection Agency

Mail Code 4101T

1200 Pennsylvania Ave., NW

Washington, DC 20460

Attention Docket ID No. OW-2002-0039

Submitting comments by hand delivery or courier

Deliver comments to: 

Water Docket

USEPA Docket Center

United States Environmental Protection Agency

Room B102

1301 Constitution Ave., NW

Washington, DC 20460

Attention Docket ID No. OW-2002-0039
Please identify the appropriate docket identification number in the subject line on the first page 

of your comment. If you submit an electronic comment, please include a name, mailing address, 

and an e-mail address or other contact information in the body of the comment. This contact 

information should also be included on the outside of any disk or CD ROM submitted, as well as 

in any cover letter accompanying the disk or CD ROM. 

USEPA policy is that public comments, whether submitted electronically or in paper, will be 

made available for public viewing in USEPA’s electronic public docket as USEPA receives them 

and without change, unless the comment contains copyrighted material, CBI, or other 

information whose disclosure is restricted by statute.
Acknowledgements

This document was prepared by the United States Environmental Protection Agency, 

Office of Ground Water and Drinking Water, Standards and Risk Management Division, 

Technical Support Center. The Work Assignment Manager was Steven Allgeier, and the 

Contract Project Officer was Jane Holtorf.

Technical consultants played a significant role in the development of this document. The 

work was conducted jointly by Malcolm Pirnie, Inc and Separation Processes, Inc. under contract 

with The Cadmus Group, as administered by Maureen Donnelly. The primary contributors to the 

document included:

• Steven Allgeier (USEPA)

• Brent Alspach (Malcolm Pirnie)

• James Vickers (Separation Processes). 

Other contributors included Steve Alt (Separation Processes), Maureen Donnelly (The Cadmus 

Group), Christopher Hill (Malcolm Pirnie), and Sheryl Patrick (Separation Processes).

Addition support for this work was provided by a panel of peer reviewers, including: 

Pierre Côté (ZENON Environmental), Scott Freeman (Black & Veatch), Joseph Jacangelo 

(MWH), Larry Landsness (Wisconsin Department of Natural Resources), James Lozier 

(CH2M Hill), Charles Liu (Pall Corporation), David Paulson (GE Osmonics), Richard Sakaji 

(California Department of Health Services), and James Schaefer (Pall Corporation).
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Contents

1.0 INTRODUCTION................................................................................................................ 1-1

1.1 Regulating Membranes for the Drinking Water Industry....................................1-2

1.2 Summary of Applicable Regulations ...................................................................1-3

 1.2.1 Long Term 2 Enhanced Surface Water Treatment Rule..........................1-3

 1.2.2 Stage 2 Disinfectants and Disinfection Byproducts Rule ........................1-4

1.3 Requirements for Membrane Filtration Under the LT2ESWTR.........................1-5

 1.3.1 Definition of a Membrane Filtration Process...........................................1-6

 1.3.2 Challenge Testing ....................................................................................1-7

 1.3.3 Direct Integrity Testing............................................................................1-7

 1.3.4 Continuous Indirect Integrity Monitoring................................................1-8

1.4 Considering Existing Membrane Filtration Facilities Under the LT2ESWTR ...1-8

1.5 Membrane Terminology Used in the Guidance Manual....................................1-10

1.6 Guidance Manual Objectives.............................................................................1-12

1.7 Guidance Manual Organization.........................................................................1-13

2.0 OVERVIEW OF MEMBRANE FILTRATION........................................................................ 2-1

2.1 Introduction..........................................................................................................2-1

2.2 Basic Principles of Membrane Filtration.............................................................2-2

 2.2.1 Microfiltration and Ultrafiltration............................................................2-3

 2.2.2 Nanofiltration and Reverse Osmosis .......................................................2-4

 2.2.3 Membrane Cartridge Filtration ................................................................2-5

 2.2.4 Membrane Pore Size and Filtration Removal Efficiency........................2-6

 2.2.5 Electrodialysis and Electrodialysis Reversal...........................................2-6

2.3 Membrane Materials, Modules, and Systems ......................................................2-7

 2.3.1 Membrane Materia ls................................................................................2-7

 2.3.2 Membrane Modules .................................................................................2-9

2.3.2.1 Hollow-Fiber Modules............................................................2-10

2.3.2.2 Spiral-Wound Modules ...........................................................2-11

2.3.2.3 Membrane Cartridges.............................................................2-13

2.3.2.4 Other Module Configurations.................................................2-14

 2.3.3 Types of Membrane Filtration Systems.................................................2-15

2.3.3.1 Hollow-Fiber (MF/UF) Systems.............................................2-15

2.3.3.2 Spiral-Wound (NF/RO) Systems.............................................2-17

2.4 Basic Principles of Membrane Filtration System Design and Operation..........2-20

 2.4.1 General Concepts...................................................................................2-20

 2.4.2 MF, UF, and MCF Processes.................................................................2-22

 2.4.3 NF and RO Processes.............................................................................2-26

2.5 Hydraulic Configurations...................................................................................2-27

 2.5.1 Deposition Mode....................................................................................2-29

 2.5.2 Suspension Mode ...................................................................................2-31

2.5.2.1 Plug Flow Reactor Model.......................................................2-32

2.5.2.2 Crossflow Model .....................................................................2-36

2.5.2.3 Continuous Stirred Tank Reactor Model ................................2-42
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 2.5.3 Summary................................................................................................2-48

3.0 CHALLENGE TESTING..................................................................................................... 3-1

3.1 Introduction..........................................................................................................3-1

3.2 Summary of Challenge Testing Requirements ....................................................3-3

3.3 Test Organization Qualification...........................................................................3-5

3.4 General Procedure for Designing a Challenge Test Protocol..............................3-6

3.5 Module Specifications..........................................................................................3-8

3.6 Non-Destructive Performance Testing.................................................................3-9

3.7 Selection of Modules for Challenge Testing .....................................................3-11

3.8 Small-Scale Module Testing..............................................................................3-12

3.9 Target Organisms and Challenge Particulates...................................................3-13

 3.9.1 Selecting a Target Organism..................................................................3-13

 3.9.2 Surrogate Characteristics .......................................................................3-14

3.9.3 Surrogates for Cryptosporidium ............................................................3-16

3.9.3.1 Alternate Microorganisms......................................................3-17

3.9.3.2 Inert Particles .........................................................................3-18

3.9.3.3 Molecular Markers .................................................................3-19

3.10 Challenge Test Solutions ...................................................................................3-19

 3.10.1 Test Solution Water Quality ..................................................................3-20

 3.10.2 Test Solution Volume ............................................................................3-21

3.10.3 Test Solution Concentration ..................................................................3-23

3.10.4 Challenge Particulate Seeding Method..................................................3-25

3.10.5 Example: Challenge Test Solution Design ............................................3-27

3.11 Challenge Test Systems.....................................................................................3-31

 3.11.1 Test Apparatus .......................................................................................3-31

 3.11.2 Test Operating Conditions .....................................................................3-36

3.12 Sampling ............................................................................................................3-37

3.12.1 Sampling Methods .................................................................................3-38

3.12.2 Sample Port Design and Location..........................................................3-38

3.12.3 Process Monitoring ................................................................................3-39

3.12.4 Sample Plan Development.....................................................................3-40

3.13 Analysis and Reporting of Challenge Test Results............................................3-41

3.13.1 Calculation of Removal Efficiency........................................................3-41

3.13.2 Statistical Analysis.................................................................................3-42

3.13.3 Reporting................................................................................................3-43

3.14 Re-Testing of Modified Membrane Modules ....................................................3-44

3.15 Grandfathering Challenge Test Data From Previous Studies............................3-45

4.0 DIRECT INTEGRITY TESTING.......................................................................................... 4-1

4.1 Introduction..........................................................................................................4-1

4.2 Test Resolution ....................................................................................................4-3

4.2.1 Pressure-Based Tests................................................................................4-3

4.2.2 Marker-Based Tests .................................................................................4-5

4.3 Test Sensitivity.....................................................................................................4-6

4.3.1 Pressure-Based Tests................................................................................4-6
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4.3.1.1 Basic Concepts..........................................................................4-6

4.3.1.2 Calculating Sensitivity ..............................................................4-8

4.3.1.3 Diffusive Losses / Baseline Decay ..........................................4-13

4.3.2 Marker-Based Tests ...............................................................................4-18

4.4 Test Frequency...................................................................................................4-19

4.5 Establishing Control Limits...............................................................................4-19

4.6 Example: Established Direct Integrity Test Parameters ....................................4-22

4.7 Test Methods......................................................................................................4-29

4.7.1 Pressure Decay Test...............................................................................4-29

4.7.2 Vacuum Decay Test...............................................................................4-32

4.7.3 Diffusive Airflow Test...........................................................................4-33

4.7.4 Water Displacement Test.......................................................................4-35

4.7.5 Marker-Based Integrity Tests.................................................................4-38

4.8 Diagnostic Testing .............................................................................................4-41

4.8.1 Visual Inspection....................................................................................4-42

4.8.2 Bubble Testing.......................................................................................4-42

4.8.3 Sonic Testing..........................................................................................4-43

4.8.4 Conductivity Profiling ...........................................................................4-44

4.8.5 Single Module Testing...........................................................................4-45

4.9 Data Collection and Reporting...........................................................................4-46

5.0 CONTINUOUS INDIRECT INTEGRITY MONITORING........................................................ 5-1

5.1 Introduction..........................................................................................................5-1

5.2 Turbidity Monitoring ...........................................................................................5-4

5.2.1 Methods....................................................................................................5-5

5.2.2 Control Limits..........................................................................................5-6

5.2.3 Advantages and Limitations.....................................................................5-7

5.3 Particle Counting and Particle Monitoring ..........................................................5-8

5.3.1 Methods....................................................................................................5-9

5.3.2 Control Limits........................................................................................5-10

5.3.3 Advantages and Limitations...................................................................5-12

5.4 Other Indirect Monitoring Methods...................................................................5-13

5.5 Data Collection and Reporting...........................................................................5-15

6.0 PILOT TESTING................................................................................................................ 6-1

6.1 Introduction..........................................................................................................6-1

6.2 Planning ...............................................................................................................6-2

6.2.1 Process Considerations ............................................................................6-3

6.2.2 Screening and System Selection..............................................................6-3

6.2.3 Scheduling................................................................................................6-5

6.3 Testing Objectives................................................................................................6-6

6.3.1 Membrane Flux Optimization..................................................................6-7

6.3.2 Backwash Optimization...........................................................................6-8

6.3.3 Chemical Cleaning Optimization.............................................................6-9

6.4 Testing and Monitoring......................................................................................6-13

6.4.1 Operational Parameter Monitoring ........................................................6-13
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6.4.2 Water Quality Monitoring......................................................................6-13

6.4.3 Microbial Monitoring.............................................................................6-15

6.4.4 Integrity Testing.....................................................................................6-15

6.5 Report Development ..........................................................................................6-16

7.0 IMPLEMENTATION CONSIDERATIONS............................................................................. 7-1

7.1 Introduction..........................................................................................................7-1

7.2 Operational Unit Processes..................................................................................7-1

7.2.1 Pretreatment .............................................................................................7-2

7.2.1.1 Prefiltration...............................................................................7-2

7.2.1.2 Chemical Conditioning .............................................................7-3

7.2.2 Backwashing............................................................................................7-5

7.2.3 Chemical Cleaning...................................................................................7-6

7.2.4 Integrity Testing.......................................................................................7-8

7.2.5 Post-Treatment.......................................................................................7-10

7.3 System Design Considerations ..........................................................................7-11

7.3.1 Membrane Flux......................................................................................7-11

7.3.2 Water Quality.........................................................................................7-11

7.3.3 Temperature Compensation...................................................................7-15

7.3.4 Cross-Connection Control .....................................................................7-19

7.3.5 System Reliability .................................................................................7-21

7.4 Residuals Treatment and Disposal.....................................................................7-22

7.4.1 Backwash Residuals...............................................................................7-24

7.4.2 Chemical Cleaning Residuals ................................................................7-25

7.4.3 Concentrate ............................................................................................7-26

8.0 INITIAL START-UP ........................................................................................................... 8-1

8.1 Introduction..........................................................................................................8-1

8.2 Temporary System Interconnections ...................................................................8-2

8.3 Flushing and Testing Without Membranes..........................................................8-2

8.4 Membrane Installation.........................................................................................8-3

8.5 System Disinfection.............................................................................................8-4

8.5.1 Chlorine-Tolerant Membrane ..................................................................8-4

8.5.2 Chlorine-Intolerant Membranes...............................................................8-5

8.6 Initial Direct Integrity Testing .............................................................................8-5

8.7 Acceptance Testing..............................................................................................8-5

8.8 Operator Training.................................................................................................8-6
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Appendices

A DEVELOPMENT OF A COMPREHENSIVE INTEGRITY VERIFICATION PROGRAM............A-1

A.1 Introduction......................................................................................................... A-1

A.2 Direct Integrity Testing....................................................................................... A-3

A.3 Continuous Indirect Integrity Monitoring......................................................... A-10

A.4 Diagnostic Testing ............................................................................................ A-15

A.5 Membrane Repair and Replacement................................................................. A-17

A.6 Data Collection and Analysis............................................................................ A-20

A.7 Reporting........................................................................................................... A-23

A.8 Summary........................................................................................................... A-24

B OVERVIEW OF BUBBLE POINT THEORY.........................................................................B-1

B.1 Introduction..........................................................................................................B-1

B.2 The Bubble Point Equation..................................................................................B-2

B.3 Bubble Point Equation Parameters ......................................................................B-4

C CALCULATING THE AIR-LIQUID CONVERSION RATIO ..................................................C-1

C.1 Introduction......................................................................................................... C-1

C.2 Darcy Pipe Flow Model...................................................................................... C-3

C.3 Orifice Model...................................................................................................... C-6

C.4 Hagen-Poiseuille Model...................................................................................... C-8

D EMPIRICAL METHOD FOR DETERMINING THE AIR-LIQUID CONVERSION RATIO 

FOR A HOLLOW-FIBER MEMBRANE FILTRATION SYSTEM .............................. D-1

D.1 Introduction......................................................................................................... D-1

D.2 Methodology....................................................................................................... D-4

D.3 Example: Using the CAM Method to Determine the ALCR.............................. D-5

E APPLICATION OF MEMBRANE FILTRATION FOR VIRUS REMOVAL...............................E-1

E.1 Introduction..........................................................................................................E-1

E.2 Overview of Current Regulatory Policy ..............................................................E-2

E.3 Direct Integrity Testing........................................................................................E-5

E.4 Challenge Testing ................................................................................................E-7

E.5 Continuous Indirect Integrity Monitoring............................................................E-8
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List of Tables

No. Title Page

1.1 LT2ESWTR Additional Treatment Requirements for Unfiltered Systems.....................1-4

2.1 Summary of VCF Values for a NF/RO System Modeled as a PFR ..............................2-36

2.2 Increase in VCF as a Function of the Number of Detention Times...............................2-45

2.3 Typical Range of VCF Values for Various Hydraulic Configurations..........................2-48

2.4 Summary of VCF Equations for Various Hydraulic Configurations.............................2-49

3.1 Potential Target Organisms for Challenge Testing........................................................3-14

3.2 Comparative Summary of Cryptosporidium and Potential Surrogates..........................3-16

3.3 Potential Microbiological Surrogates for Cryptosporidium ..........................................3-17

3.4 Example Challenge Test Solution Volume for Various Types of Modules ..................3-23

3.5 Typical Parameters for Various Types of Modules.......................................................3-37

3.6 Typical Range of VCF Values for Various Hydraulic Configurations..........................3-45

4.1 Approaches for Calculating the ALCR..........................................................................4-12

6.1 Water Quality Parameters to Measure Prior to Piloting ..................................................6-8

6.2 Suggested Water Quality Sampling Schedule for Membrane Piloting..........................6-14

7.1 Typical Membrane System Prefiltration Requirements...................................................7-3

7.2 Chemical Cleaning Agents...............................................................................................7-7

7.3 Estimated NF/RO Membrane Fluxes as a Function of SDI...........................................7-12

8.1 Schedule of Training Events............................................................................................8-7

B.1 Surface Tension of Water at Various Temperatures........................................................B-5

C.1 Approaches for Calculating the ALCR........................................................................... C-2

E.1 Virus Removal Studies Using MF and UF ......................................................................E-3

E.2 Summary of State Virus Removal Credit for MF and UF ...............................................E-3

E.3 Summary of State Giardia and Cryptosporidium Removal Credit for MF/UF ...............E-4

E.4 Required Direct Integrity Test Pressure as a Function of Target Pathogen Size 

(i.e., Resolution)...................................................................................................E-6
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List of Figures

No. Title Page

2.1 Filtration Application Guide for Pathogen Removal.......................................................2-3

2.2 Conceptual Diagram of Osmotic Pressure .......................................................................2-5

2.3 Membrane Construction and Symmetry..........................................................................2-9

2.4 Hollow Fiber Cross-Section Photomicrograph..............................................................2-11

2.5 Inside-Out and Outside-In Modes of Operation ............................................................2-12

2.6 Spiral-Wound Membrane Module .................................................................................2-12

2.7 Membrane Cartridge Filter.............................................................................................2-13

2.8 Schematic of a Typical Pressure-Driven Hollow-Fiber (MF/UF) System....................2-16

2.9 Schematic of a Typical Vacuum-Driven Hollow-Fiber (MF/UF) System....................2-17

2.10 Typical Spiral-Wound (NF/RO) Module Pressure Vessel.............................................2-18

2.11 Typical 2:1 (Relative) Array of Pressure Vessels..........................................................2-19

2.12 Schematic of a Typical Spiral-Wound (NF/RO) System...............................................2-20

2.13 Schematic of a System Operating in Deposition Mode .................................................2-29

2.14 Conceptual Illustration of Deposition Mode Operation.................................................2-30

2.15 Conceptual Illustration of Suspension Mode Operation................................................2-31

2.16 Flow Diagram for a Plug Flow Reactor ........................................................................2-32

2.17 Schematic of a Typical Small Volume Crossflow System............................................2-37

2.18 Concentration Profile in a Small Volume Crossflow System........................................2-39

2.19 Schematic of a Typical Large Volume Crossflow System............................................2-40

2.20 Concentration Profile in a Large Volume Crossflow System........................................2-41

2.21 Schematic of a Typical Pressure-Driven CSTR Without Backwashing........................2-44

2.22 Schematic of a Typical Vacuum-Driven CSTR Without Backwashing........................2-44

2.23 Concentration Profile in a CSTR Without Backwashing ..............................................2-45

2.24 Schematic of a Typical Pressure-Driven CSTR With Backwashing.............................2-46

2.25 Concentration Profile in a CSTR With Backwashing....................................................2-47

3.1 Schematic of Acceptable and Unacceptable Sample Injection Ports.............................3-26

3.2 Schematic of a Typical Pressure-Driven System in Deposition Mode with Batch 

Seeding and Composite Sampling .....................................................................3-32

3.3 Schematic of a Typical Pressure-Driven System in Deposition Mode with 

Continuous Seeding and Grab Sampling ...........................................................3-33

3.4 Schematic of a Typical Pressure-Driven System in Suspension Mode with 

Continuous Seeding and Grab Sampling ...........................................................3-34

3.5 Schematic of a Typical Vacuum-Driven System in Deposition Mode with 

Continuous Seeding and Grab Sampling ...........................................................3-35

3.6 Schematic of a Typical Vacuum-Driven System in Suspension Mode with 

Continuous Seeding and Grab Sampling ...........................................................3-35

3.7 Schematic of Acceptable and Unacceptable Sampling Ports.........................................3-39

4.1 Schematic Illustrating a Pressure Decay Test................................................................4-30

4.2 Schematic Illustrating a Vacuum Decay Test................................................................4-32

4.3 Schematic Illustrating a Diffusive Airflow Test............................................................4-34

4.4 Schematic Illustrating a Water Displacement Test........................................................4-36
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4.5 Schematic Illustrating a Particulate Marker Test...........................................................4-38

4.6 Schematic Illustrating a Molecular Marker Test............................................................4-39

4.7 Sample Summary Report Form for Pressure Decay Testing.........................................4-47

6.1 Sample Chemical Cleaning Test Profile ........................................................................6-11

6.2 Sample Pilot Study Sequence Overview........................................................................6-12

7.1 Double Block and Bleed Valving Arrangement ............................................................7-20

B.1 Diagram of a Membrane Pore Modeled as a Capillary Tube ..........................................B-1

D.1 CAM Technique Airflow Measurement Apparatus........................................................ D-3

D.2 Example Airflow and Water Flow Correlation Curves .................................................. D-3

D.3 Example ALCR Correlation............................................................................................ D-6
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Acronyms

AFT Alternate Filtration Technology

ALCR Air-Liquid Conversion Ratio

ANSI American National Standards Institute

APHA American Public Health Administration

ASTM American Society for Testing and Materials

AWWA American Water Works Association

AWWARF American Water Works Association Research Foundation

BOD Biochemical Oxygen Demand

CA Cellulose Acetate

CAM Correlated Airflow Measurement

CIP Clean-In-Place

CL Control Limit

CSTR Continuous Stirred Tank Reactor

CT (Disinfectant Residual) Concentration (mg/L) × (Contact) Time (minutes)

CWS Community Water System

DBP Disinfection Byproduct

DBPR Disinfectants and Disinfection Byproducts Rule

DOC Dissolved Organic Carbon

ED Electrodialysis

EDR Electrodialysis Reversal

ETV Environmental Technology Verification 

FR Federal Register

GWR Ground Water Rule
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GWUDI Ground Water Under the Direct Influence (Of Surface Water)

HAA Haloacetic Acid

HAA5 (sum of five) Haloacetic Acids

HFF Hollow Fine Fiber

HPC Heterotrophic Plate Count

IDSE Initial Distribution System Evaluation

IESWTR Interim Enhanced Surface Water Treatment Rule

IMS Integrated Membrane System

ISO International Organization for Standardization

IVP Integrity Verification Program

LCL Lower Control Limit

LED Light Emitting Diode

LRAA Locational Running Annual Average

LRC Log Removal Credit

LRV Log Removal Value

LT1ESWTR Long Term 1 Enhanced Surface Water Treatment Rule

LT2ESWTR Long Term 2 Enhanced Surface Water Treatment Rule

MCF Membrane Cartridge Filtration

MCL Maximum Contaminant Level

MF Microfiltration

MWCO Molecular Weight Cut-Off

NA Not Applicable

NDP Net Driving Pressure

NDPT Non-Destructive Performance Test

NF Nanofiltration
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NSF National Sanitation Foundation

NTNCWS Non-Transient Non-Community Water System

OEM Original Equipment Manufacturer

PA Polyamide

PAC Powdered Activated Carbon

PAN Polyacrylonitrile 

PES Polyethersulfone

PFR Plug Flow Reactor

PP Polypropylene

PS Polysulfone

PVDF Polyvinylidene Fluoride

QA Quality Assurance

QC Quality Control

QCRV Quality Control Release Value

RAA Running Annual Average

RO Reverse Osmosis

SDI Silt Density Index

SEM Scanning Electron Microscopy

SMP Standard Monitoring Program

SSDR Stock Solution Delivery Rate

SSS System-Specific Study

SUVA Specific Ultraviolet Absorbance

SWTR Surface Water Treatment Rule

TCEQ Texas Commission on Environmental Quality

TCF Temperature Correction Factor
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TCPP Total Challenge Particulate Population

TDS Total Dissolved Solids

TMP Transmembrane Pressure

TOC Total Organic Carbon

TSS Total Suspended Solids

TTHM Total Trihalomethanes

UCL Upper Control Limit

UF Ultrafiltration

USEPA United States Environmental Protection Agency

UV Ultraviolet (Light)

VCF Volumetric Concentration Factor
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Symbols

Units

Symbol Description

English Metric

A1

Membrane area based on the inside of a hollow 

fiber ft2 m

2

A2

Membrane area based on the outside of a hollow 

fiber ft2 m

2

A20

Membrane area (required for design at a reference 

temperature of 20 oC) ft2 m

2

Ad Design membrane area (corrected for temperature) ft2 m

2

Am Membrane area ft2 m

2

ALCR Air-liquid conversion ratio - -

BP Backpressure psi

kPa

dynes/cm2

BPmax Maximum backpressure psi

kPa

dynes/cm2

BPmin Minimum backpressure psi

kPa

dynes/cm2

C Coefficient of discharge - -

Cc Concentrate concentration -

mg/L

number/L

CfInfluent feed water concentration -

mg/L

number/L

Cf-max Maximum feed concentration (in a challenge test) -

mg/L

number/L

Cf-min Minimum feed concentration (in a challenge test) -

mg/L

number/L

Cm

Concentration maintained on the feed side of the 

membrane -

mg/L

number/L

Cm-max

Maximum concentration maintained on the feed 

side of the membrane -

mg/L

number/L
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Units

Symbol Description

English Metric

Cm(t-1)

Concentration on the feed side of the membrane 

immediately after the previous backwash 

operation

-

mg/L

number/L

Cm(x)

Concentration maintained on the feed side of the 

membrane at a function of position with the 

membrane unit

-

mg/L

number/L

Cp Filtrate concentration -

mg/L

number/L

Ctest

Feed concentration of the challenge particulate in a 

challenge test -

mg/L

number/L

Css

Concentration of challenge particulate in stock 

solution -

mg/L

number/L

Dam

Diffusion coefficient for air through a saturated 

semi-permeable membrane - cm2

/s

Daw Diffusion coefficient for air in a water matrix - cm2/s

dcap Capillary diameter in

cm

mm

μm

dfect Defect diameter in

ft

cm

mm

μm

dfiber Fiber diameter in

ft

cm

mm

μm

dres Integrity test resolution requirement in

cm

mm

μm

DL Detection limit - -

DOC Dissolved organic carbon - mg/L

e Specific roughness in cm

mm

f Moody friction factor - -
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Units

Symbol Description

English Metric

g Gravitational constant 2

lbf s

lbm ft

−

−

2

s

kg − m

H Henry’s constant for air-water system 3

psi m

mol

−

J Flux gfd Lmh

J20

Normalized flux at 20 °C

(common for MF, UF, and MCF systems) gfd Lmh

J25

Normalized flux at 25 °C

(common for NF and RO systems) gfd Lmh

JT Flux at temperature T gfd Lmh

K Flow resistance coefficient - -

Kair Resistance coefficient of air - -

Kwater Resistance coefficient of water - -

L Defect length in

ft

cm

mm

LRC Log removal credit - -

LRV Log removal value - -

LRVt Target log removal value in a challenge test - -

LRVDIT

Log removal value that can be verified by the direct 

integrity test (i.e., method sensitivity) - -

LRVC-Test

Log removal value demonstrated during challenge 

testing - -

M Permeability or specific flux gfd/psi Lmh/kPa

NDP Net driving pressure psi kPa

P Pressure psi kPa

Patm Atmospheric pressure psi

kPa

dynes/cm2

atm

Pbp Bubble point pressure psi kPa

Pc Concentrate pressure psi kPa
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Units

Symbol Description

English Metric

Pf Feed pressure psi kPa

Pp Filtrate pressure psi kPa

Ptest

Pressure at which a pressure-based direct integrity 

test is conducted psi

kPa

dynes/cm2

? P Differential pressure across the membrane during 

the direct integrity test psi

kPa

dynes/cm2

? Peff Effective integrity test pressure psi

kPa

dynes/cm2

? Ptest Rate of pressure decay / loss psi/min kPa/min

Qair Flow of air 

ft3/s

ft3/min

cm3/s

cm3/min

L/min

Qb Backwash flow gpm L/min

Qbreach

Flow from an integrity breach associated with the 

smallest integrity test response that can be 

reliably measured

ft3/s

ft3/min

cm3/s

cm3/min

L/min

Qc Concentrate (i.e., bleed or reject) flow gpm L/min

Qdiff

Diffusion of air through the water matrix in the 

pores of a fully-wetted membrane ft3

/min L/min

cm3/min

Qf Feed flow gpm L/min

Qp Filtrate flow

gpm

gpd

MGD

L/hr

L/min

Qp(x)

Filtrate flow as a function of position within a 

membrane unit

gpm

gpd

MGD

L/hr

L/min

R Recovery % %

Re Reynolds number - -

R(x)

Recovery as a function of position within a 

membrane unit % %

Rf Foulant layer resistance 

gfd cp

psi

− L cp

bar hr m

−

− −

2
Membrane Filtration Guidance Manual xvii June 2003

Proposal Draft

Units

Symbol Description

English Metric

Rg Universal gas constant 

mol K

L psia

−

−

Rm Intrinsic membrane resistance 

gfd cp

psi

− L cp

bar hr m

−

− −

2

Rt Total membrane resistance 

gfd cp

psi

− L cp

bar hr m

−

− −

2

r1 Inner radius of a hollow fiber in cm

r2 Outer radius of a hollow fiber in cm

SF Safety factor - -

SSDR Stock solution delivery rate gpm L/min

SUVA Specific ultraviolet absorbance - L/mg-m

T Temperature oF

oC

t Filtration cycle time min min

tb Backwash duration min min

tf Filtration cycle duration min min

Tmin Challenge test duration min min

TCF Temperature correction factor - -

TCPP Total challenge particulate population number

lbs

number

g

TDSc

Total dissolved solids concentration in the 

concentrate stream - mg/L

TDSf

Total dissolved solids concentration in the feed 

stream - mg/L

TDSp

Total dissolved solids concentration in the filtrate 

stream - mg/L

TMP Transmembrane pressure psi

kPa

dynes/cm2

TMP25 Transmembrane pressure at 25 °C psi kPa

TMPT Transmembrane pressure at temperature T psi kPa
Membrane Filtration Guidance Manual xviii June 2003

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Units

Symbol Description

English Metric

U

Membrane-specific temperature correction factor 

constant (manufacturer-supplied) - 1/K

UCL Upper control limit (top = airflow)

(bottom = pressure decay rate)

-

psi/min

L/min

kPa/min

UV254 Ultraviolet absorbance at 254 nm -

1/m

1/cm

VCF Volumetric concentration factor - -

VCF(x)

Volumetric concentration factor as a function of 

position within a membrane unit - -

VCF(t) Volumetric concentration factor as a function of 

time - -

VCFavg

Average volumetric concentration factor in a 

membrane unit - -

VCFmax

Maximum volumetric concentration factor in a 

membrane unit - -

Veq

System volume required to attain equilibrium feed 

concentration during a challenge test gal L

Vhold

Unfiltered test solution volume remaining in the 

system at the end of a challenge test (i.e. the 

hold-up volume)

gal L

Vr Total recirculation loop volume gal L

Vss Challenge particulate stock solution volume gal L

Vsys

Volume of pressurized air in a membrane system 

during a pressure- or vacuum-decay test ft3 L

Vtest Minimum challenge test solution volume gal L

x Position in a membrane unit in the direction of flow ft m

xmax

End position in a membrane unit in the direction of 

flow ft m

Y

Net expansion factor for compressible flow through 

a pipe to a larger area - -

z Membrane thickness in cm
Membrane Filtration Guidance Manual xix June 2003

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Units

Symbol Description

English Metric

f(Qair) airflow correlation function psi-min/mL psi-min/mL

f(Qbreach) water flow correlation function psi-min/mL psi-min/mL

ε Membrane porosity - -

? Liquid-membrane contact (i.e., “wetting”) angle degrees degrees

κ Pore shape correction factor - -

σ Surface tension - dynes/cm

ρair Density of air lb/ft3g/m3

ρw Density of water lb/ft3g/cm3

μ20 Viscosity of water at 20 °C lbs/ft-s cp

μair Viscosity of air lbs/ft-s cp

μT Viscosity of water at temperature T lbs/ft-s cp

μw Viscosity of water lbs/ft-s cp

J Solids retention time min min
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Glossary

Anion – a negatively charged ion resulting from the disassociation of salts, acids, or bases in 

aqueous solution

Anti-Scalant – a chemical agent added to water to inhibit the precipitation or crystallization of 

salt compounds; also referred to as a “scale-inhibitor”

Anti-Telescoping Device – a rigid structure firmly attached to each end of a spiral-wound 

nanofiltration (NF) or reserve osmosis (RO) membrane module that prevents telescoping, 

unwinding, or other undesirable movement of the membrane module

Array – a description of a nanofiltration (NF) or reverse osmosis (RO) membrane system based 

upon the ratio of the number of pressure vessels in each stage that operate in parallel (e.g., a 

24:12 (absolute) two-stage array or a 2:1 (relative) two-stage array 

Asymmetric – having a varying consistency throughout (e.g., a membrane that varies in density 

or porosity across its structure)

Backwash– the intermittent waste stream from a microfiltration (MF) or ultrafiltration (UF) 

membrane system; also, a term for a cleaning operation that typically involves periodic reverse 

flow to remove foulants accumulated at the membrane surface

Biofouling – membrane fouling (and associated decreases in flux) that is attributable to the 

deposition and growth of microorganisms on the membrane surface and/or the adsorptive fouling 

of secretions from microorganisms

Bleed – the continuous waste stream from a microfiltration (MF) or ultrafiltration (UF) system 

operated in a crossflow hydraulic configuration 

Boundary Layer – thin layer of water at the surface of a semi-permeable membrane containing 

the rejected contaminants from the filtrate (i.e., permeate) flow in higher concentrations than the 

bulk feed/brine stream (called concentration polarization), affecting the osmotic pressure and salt 

passage

Brackish Water – saline water in which the dissolved solids content generally falls between that 

of drinking water and seawater

Breach – see Integrity Breach

Brine – a saline solution with a concentration of dissolved solids exceeding that of seawater (i.e., 

approximately 35,000 mg/L)
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Brine Seal – a rubber seal around the circumference of a spiral-wound module between the 

module and the interior pressure vessel wall that separates the feed water from the concentrate 

stream, preventing the bypass of feed between the module and the inside of the pressure vessel 

wall

Bubble Point – the amount of applied air pressure required to evacuate the largest pores of a 

fully-wetted porous membrane 

Cation– a positively charged ion resulting from the dissociation of salts, acids, or bases in 

aqueous solution

Cartridge – a term commonly used to describe a disposable backwashable or non-backwashable 

filter element; included under the term “module” for the purposes of the LT2ESWTR 

Challenge Particulate – the target organism or acceptable surrogate used to determine the log 

removal value (LRV) during a challenge test

Challenge Test – (as defined under the LT2ESWTR) a study conducted to determine the 

removal efficiency (i.e., log removal value (LRV)) of a membrane material for a particular 

organism, particulate, or surrogate

Clean-In Place (CIP) – the periodic application of a chemical solution or (series of solutions) to 

a membrane unit for the intended purpose of removing accumulated foulants and thus restoring 

permeability and resistance to baseline levels; commonly used term for in-situ chemical cleaning

Colloid – type of particulate matter ranging in size from approximately 2 - 1,000 nm in diameter 

that does not settle out rapidly 

Compaction– the compression or densification of a membrane as a result of exposure to applied 

pressure over a period of time, which typically results in decreased productivity

Composite – made from different materials (e.g., a membrane manufactured from two or more 

different materials in distinct layers)

Concentrate – the continuous waste stream (typically consisting of concentrated dissolved 

solids) from a membrane process, usually in association with nanofiltration (NF) and reverse 

osmosis (RO) processes; in some cases also used to describe a continuous bleed stream of 

concentrated suspended solids wasted from microfiltration (MF) and ultrafiltration (UF) systems 

operated in a crossflow (or feed-and-bleed), hydraulic configuration

Concentrate Staging – a configuration of spiral-wound nanofiltration (NF) and reverse osmosis 

(RO) membrane systems in which the concentrate from each stage of a multi-stage system 

becomes the feed for the subsequent stage

Concentration Polarization– a phenomenon that occurs when dissolved and/or colloidal 

materials concentrate on or near the membrane surface in the boundary layer
Membrane Filtration Guidance Manual xxii June 2003

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Conductivity – a measure of the ability of an aqueous solution to conduct an electric charge; 

related to the amount of total dissolved solids (TDS)

Continuous Indirect Integrity Monitoring – (as defined under the LT2ESWTR) monitoring 

some aspect of filtrate water quality that is indicative of the removal of particulate matter at a 

frequency of at least once every 15 minutes

Control Limit (CL) – (as defined under the LT2ESWTR) a response from an integrity test, 

which, if exceeded, indicates a potential problem with the [membrane filtration] system and 

triggers a response; synonymous with “upper control limit” (UCL) as used in the Membrane 

Filtration Guidance Manual to distinguish from additional voluntary or state-mandated “lower 

control limits” (LCLs)

Crossflow– the application of water at high velocity tangential to the surface of a membrane to 

maintain contaminants in suspension; suspension mode hydraulic configuration typically 

associated with spiral-wound nanofiltration (NF) and reverse osmosis (RO) systems and a few 

hollow-fiber microfiltration (MF) and ultrafiltration (UF) systems

Dalton – a unit of mass equal to 1/12th the mass of a carbon-12 atom (i.e., one atomic mass unit 

(amu)); typically used as a unit of measure for the molecular weight cutoff (MWCO) of a 

ultrafiltration (UF), nanofiltration (NF), or reverse osmosis (RO) membrane

Dead End Filtration– term commonly used to describe the deposition mode hydraulic 

configuration of membrane filtration systems; also synonymous with “direct filtration”

Deposition Mode – a hydraulic configuration of membrane filtration systems in which 

contaminants removed from the feed water accumulate at the membrane surface (and in 

microfiltration (MF)/ultrafiltration (UF) systems subsequently removed via backwashing)

Desalination– the process of removing dissolved salts from water

Differential Pressure – pressure drop across a membrane module or unit from the feed inlet to 

concentrate outlet (as distinguished from transmembrane pressure (TMP), which represents the 

pressure drop across the membrane barrier)

Direct Filtration – (as used with respect to membrane filtration) term commonly used describe 

the deposition mode hydraulic configuration of membrane filtration systems; also synonymous 

with “dead end filtration”

Direct Integrity Testing – (as defined under the LT2ESWTR) a physical test applied to a 

membrane unit in order to identify and/or isolate integrity breaches

Electrodialysis (ED) – a process in which ions are transferred through ion-selective membranes 

by means of an electromotive force from a less concentrated solution to a more concentrated 

solution
Membrane Filtration Guidance Manual xxiii June 2003

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Electrodialysis Reversal (EDR) – a variation of the electrodialysis process in which the polarity 

of the electrodes is periodically reversed on a prescribed time cycle, thus changing the direction 

of ion movement, in order to reduce scaling

Element – a term commonly used to describe an encased spiral-wound membrane module; 

included under the term “module” for the purposes of the LT2ESWTR

Feed-and-Bleed Mode – a term used to describe a variation of the suspension mode hydraulic 

configuration of membrane filtration systems in which a portion of the crossflow stream is 

wasted (i.e., bled) rather than recirculated

Feed Channel Spacer – a plastic mesh spacer that separates the various leaves in a spiral-wound 

module, providing a uniform channel for feed water to reach the membrane surface and 

promoting turbulence in order to minimize the formation of a boundary layer at the membrane 

surface

Feed Water – the influent stream to a water treatment process 

Filtrate – the water produced from a filtration process; typically used to describe the water 

produced by porous membranes such those used in membrane cartridge filtration (MCF), 

microfiltration (MF), and ultrafiltration (UF) process, although used in the context of the 

LT2ESWTR to describe the water produced from all membrane filtration processes, including 

nanofiltration (NF) and reverse osmosis (RO)

Flux – (as defined under the LT2ESWTR) the throughput of a pressure-driven membrane 

filtration system expressed as flow per unit of membrane area (e.g., gallons per square foot per 

day (gfd) or liters per hour per square meter (Lmh))

Foulant – any substance that causes fouling

Fouling – the gradual accumulation of contaminants on a membrane surface or within a porous 

membrane structure that inhibits the passage of water, thus decreasing productivity

Heterogeneous – composed of a combination of different materials (e.g., composite and some 

asymmetric membranes)

Hollow-Fiber Module – a configuration in which hollow-fiber membranes are bundled 

longitudinally and either encased in a pressure vessel or submerged in a basin; typically 

associated with microfiltration (MF) and ultrafiltration (UF) membrane processes

Hollow-Fine-Fiber (HFF) Module – a relatively uncommon configuration in which very small 

diameter (i.e., approximately 50 μm (inside diameter)) semi-permeable hollow-fiber membranes 

are bundled in “U” shape and potted into a pressure vessel; typically associated with reverse 

osmosis (RO) membrane processes
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Homogenous – composed of the same material throughout (e.g., symmetric and some 

asymmetric membranes)

Hydraulic Configuration– the pattern of flow through a membrane process by which the feed 

contaminants are removed or concentrated (e.g., crossflow, dead-end, etc.) 

Hydrophilic – the water attracting property of membrane material

Hydrophobic – the water repelling property of membrane material

Integrity Breach – (as defined under the LT2ESWTR) one or more leaks that could result in the 

contamination of the filtrate

Irreversible Fouling – any membrane fouling that is permanent and cannot be removed by 

either backwashing (if applicable) or chemical cleaning

Leaf – a sandwich arrangement of flat sheet, semi-permeable membranes placed back-to-back 

and separated by a fabric spacer (i.e., permeate carrier) in a spiral-wound module

Log Removal Value (LRV) – (as defined under the LT2ESWTR) filtration removal efficiency 

for a target organism, particulate, or surrogate expressed as log10 (i.e., log10(feed concentration) –

log10(filtrate concentration))

Lower Control Limit (LCL) – an additional control limit (CL) that is not mandated by the 

LT2ESWTR but which is instead voluntarily implemented or which may be required by the state 

at its discretion

Lumen– the center or bore of a hollow-fiber membrane

Membrane Filtration– (as defined under the LT2ESWTR) a pressure or vacuum-driven 

separation process in which particulate matter larger than 1 μm is rejected by an engineered 

barrier, primarily through a size-exclusion mechanism and which has a measurable removal 

efficiency of a target organism that can be verified through the application of a direct integrity 

test (includes common membrane classifications microfiltration (MF), ultrafiltration (UF), 

nanofiltration (NF), and reverse osmosis (RO), as well as any “membrane cartridge filtration” 

(MCF) device that satisfies this definition)

Membrane Cartridge Filtration (MCF) – (as defined under the LT2ESWTR) any cartridge 

filtration devices that meet the definition of membrane filtration as specified under the 

LT2ESWTR

Membrane Softening – semi-permeable membrane treatment process designed to selectively 

remove hardness (i.e., calcium, magnesium, and certain other multi-valent cations) but allow 

significant passage of monovalent ions; typically used to describe the application of 

nanofiltration (NF) for hardness removal
Membrane Filtration Guidance Manual xxv June 2003

Proposal Draft

Membrane Unit – (as defined under the LT2ESWTR) a group of membrane modules that share 

common valving which allows the unit to be isolated from the rest of the system for the purpose 

of integrity testing or other maintenance

Molecular Weight Cutoff (MWCO) – a measure of the removal characteristic of a membrane in 

terms of atomic weight (or mass), as opposed to pore size; typically measured in terms of 

Daltons

Microfiltration (MF) – a pressure-driven membrane filtration process that typically employs 

hollow-fiber membranes with a pore size range of approximately 0.1 – 0.2 μm (nominally 

0.1 μm)

Module – (as defined under the LT2ESWTR) the smallest component of a membrane unit in 

which a specific membrane surface area is housed in a device with a filtrate outlet structure; used 

in the Membrane Filtration Guidance Manual to refer to all types of membrane configurations, 

including terms such as “element” or “cartridge” that are commonly used in the membrane 

treatment industry

Nanofiltration (NF) – a pressure-driven membrane separation process that employs the 

principles of reverse osmosis to remove dissolved contaminants from water; typically applied for 

membrane softening or the removal of dissolved organic contaminants

Net Driving Pressure (NDP) – the pressure available to force water through a semi-permeable 

nanofiltration (NF) or reverse osmosis (RO) membrane, defined as the average feed side pressure 

(i.e., the average of the feed and concentrate pressures) less the filtrate side backpressure and the 

osmotic pressure of the system 

Non-Destructive Performance Test (NDPT) – a physical quality control test typically 

conducted by a manufacturer to characterize some aspect of process performance without 

damaging or altering the membrane or membrane module 

Normalization– the process of evaluating membrane system performance at a given set of 

reference conditions (e.g., at standard temperature, per unit pressure, etc.), allowing the direct 

comparison and trending of day-to-day performance independent of changes to the actual system 

operating conditions

Osmosis – the passage of a solvent (e.g., water) through a semi-permeable membrane from a 

solution of lower concentration to a solution of higher concentration so as to equalize the 

concentrations on either side of the membrane

Osmotic Pressure – the amount of pressure that must be applied to stop the natural process of 

osmosis

Permeability – the ability of a membrane barrier to allow the passage or diffusion of a substance 

(i.e., a gas, a liquid, or solute)
Membrane Filtration Guidance Manual xxvi June 2003

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Permeate – the water that passes through a nanofiltration (NF) or reverse osmosis (RO) 

membrane; synonymous with the term filtrate, which is used in the context of the LT2ESWTR

Permeate Staging – a configuration of spiral-wound nanofiltration (NF) and reverse osmosis 

(RO) membrane systems in which the permeate (or filtrate) from each stage of a multi-stage 

system becomes the feed for the subsequent stage

Permeate Tube – the perforated tube in the center of a spiral-wound module that collects 

permeate (or filtrate) and transports it out of the membrane module

Permeate (Filtrate) Carrier– the fabric spacer in between two sheets of membrane material in 

one leaf of a spiral-wound module, serving to transfer the water that permeates through the 

membrane(s) (i.e., the filtrate) to a perforated central collector tube (i.e., the permeate tube)

Plate-and-Frame Module – a relatively uncommon configuration consisting of a series of flat 

sheet membranes separated by alternating filtrate spacers and feed/concentrate spacers; used with 

electrodialysis reversal (EDR) membrane systems

Plugging – the physical blockage of the feed side flow passages of a membrane or membrane 

module (e.g., a blockage in the lumen of an hollow-fiber module operated in inside-out mode or 

in the spacer of a spiral-wound module)

Pore Size – the size of the openings in a porous membrane expressed either as nominal (average) 

or the absolute (maximum), typically in terms of microns

Porosity – for a membrane material, the ratio of the volume of voids to the total volume

Post-Treatment – any treatment applied to the filtrate of a membrane process in order to meet 

given water quality objectives

Pretreatment – any treatment applied to the feed water to a membrane process to achieve 

desired water quality objectives and/or protect the membranes from damage or fouling

Productivity – the amount of filtered water that can be produced from a membrane filtration unit 

or system over a period of time, accounting for the use of filtrate in backwash and chemical 

cleaning operations, as well as otherwise productive time that a unit or system is off-line for 

routine maintenance processes such as backwashing, chemical cleaning, integrity testing, or 

repair 

Quality Control Release Value – a minimum quality standard of a non-destructive performance 

test (NDPT) established by the manufacturer for membrane module production that ensures that 

the module will attain the targeted log removal value (LRV) demonstrated during challenge 

testing in compliance with the LT2ESTWR 
Membrane Filtration Guidance Manual xxvii June 2003

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Rack – in a nanofiltration (NF) or reverse osmosis (RO) spir al-wound membrane filtration 

system, a group of pressure vessels that share common valving and which can be isolated as a 

group for testing, cleaning, or repair; synonymous with the terms train and skid; included under 

the term “unit” for the purposes of the LT2ESWTR

Recovery – (as defined under the LT2ESWTR) the volumetric percent of feed water that is 

converted to filtrate in the treatment process over the course of an uninterrupted (i.e., by 

chemical cleaning or a solids removal process such as backwashing) operating cycle (i.e., 

excluding losses that occur due to the use of filtrate in backwashing or cleaning operations)

Reject – a continuous waste stream from a membrane system; used synonymously with the term 

concentrate for nanofiltration (NF) and reverse osmosis (RO) membrane systems

Rejection – the prevention of feed water constituents from passing through a semi-permeable 

membrane; typically used in association with dissolved solids rather than particulate matter

Resistance – the measurement of degree to which the flow of water is impeded by a membrane 

material or fouling

Resolution– (as defined under the LT2ESWTR) the smallest leak that contributes to a response 

from a direct integrity test; also applicable to some continuous indirect integrity monitoring 

methods

Reverse Osmosis (RO) – the reverse of the natural osmosis process – i.e., the passage of a 

solvent (e.g., water) through a semi-permeable membrane from a solution of higher 

concentration to a solution of lower concentration against the concentration gradient, achieved 

by applying pressure greater than the osmotic pressure to the more concentrated solution; also, 

the pressure-driven membrane separation process that employs the principles of reverse osmosis 

to remove dissolved contaminants from water

Salinity – amount of salt in a solution; usually used in association with salt solutions in excess of 

1,000 mg/L and synonymously with the term total dissolved solids (TDS)

Salt Passage – the transport of a salt through a semi-permeable membrane; typically expressed 

either as a percentage or as mass of salt per unit of membrane area per unit time

Salt Rejection (Solids Rejection) – the amount of salt in the feed water that is rejected by a 

semi-permeable membrane, expressed as a percentage

Scale Inhibitor – a chemical agent added to water to inhibit the precipitation or crystallization of 

salt compounds; also referred to as “anti-scalant”

Scaling – the precipitation or crystallization of salts on a surface (e.g., on the feed side of a 

membrane)
Membrane Filtration Guidance Manual xxviii June 2003

Proposal Draft

Semi-Permeable – the property of a membrane barrier that allows it to selectively pass certain 

molecules in a solution while restricting the passage of others

Sensitivity – (as defined under the LT2ESWTR) the maximum log removal value (LRV) that 

can be reliably verified by a direct integrity test; also applicable to some continuous indirect 

integrity monitoring methods

Skid – in a nanofiltration (NF) or reverse osmosis (RO) spiral-wound membrane filtration 

system, a group of pressure vessels that share common valving and which can be isolated as a 

group for testing, cleaning, or repair; synonymous with the terms train and rack; included under 

the term “unit” for the purposes of the LT2ESWTR

Softening – the removal of hardness (i.e., divalent metal ions, primarily calcium and 

magnesium) from water

Spacer (Feed Water Spacer or Brine Spacer) – the material that separates the semi-permeable 

membrane layers and creates flow passages in a spiral-wound module; also called feed water 

spacer or brine spacer

Spiral-Wound Module – a configuration in which sheets of a semi-permeable membrane, a 

porous support matrix, and a spacer are wrapped around a central filtrate collector tube; typically 

associated with nanofiltration (NF) and reverse osmosis (RO) membrane processes

Stage – a group of membrane units operating in parallel

Suspension Mode – a hydraulic configuration of membrane filtration systems in which 

contaminants are maintained in suspension through the application of an external force, typically 

either air or water tangential to the membrane surface

Surrogate – a challenge particulate that is a substitute for the target microorganism of interest 

and which is removed to an equivalent or lesser extent by a membrane filtration device

Symmetric – having the same consistency throughout

Telescoping – the physical deformation of a spiral-wound membrane module due to high 

differential pressure in which the membrane, support, and spacer layers are displaced axially 

(i.e., in the direction of the feed flow) from the center, causing membrane fracture and element 

failure

Train – in a nanofiltration (NF) or reverse osmosis (RO) spiral-wound membrane filtration 

system, a group of pressure vessels that share common valving and which can be isolated as a 

group for testing, cleaning, or repair; synonymous with the terms rack and skid; included under 

the term “unit” for the purposes of the LT2ESWTR

Transmembrane Pressure (TMP) – the difference in pressure from the feed (or feedconcentrate average, if applicable) to the filtrate across a membrane barrier
Membrane Filtration Guidance Manual xxix June 2003

Proposal Draft

Tubular Module – a relatively uncommon configuration (in drinking water applications) similar 

to that of a hollow-fiber module but utilizing membranes of much larger diameter; may be 

associated with either microfiltration (MF)/ultrafiltration (UF) or nanofiltration (NF)/reverse 

osmosis (RO) membrane processes

Turbidimeter – an instrument used to measure turbidity via the scattering of a light beam 

through a solution that contains suspended particulate matter

Volumetric Concentration Factor (VCF) – the degree to which the feed water becomes 

concentrated in a membrane filtration system operating in a suspension mode hydraulic 

configuration; defined as the suspended solids concentration on the high pressure side of the 

membrane relative to the that of the ambient feed water

Ultrafiltration (UF) – a pressure-driven membrane filtration process that typically employs 

hollow-fiber membranes with a pore size range of approximately 0.01 – 0.05 μm (nominally 

0.01 μm)

Upper Control Limit (UCL) – a control limit (CL) for a membrane filtration system that is 

required by the LT2ESWTR; used in the Membrane Filtration Guidance Manual to distinguish 

CLs mandated by the rule from additional “lower control limits” (LCLs) that are either

voluntarily implemented or which may be required by the state at its discretion
Membrane Filtration Guidance Manual 1-1 June 2003

Proposal Draft

1.0 Introduction

Currently, the most common form of drinking water treatment for surface water sources 

involves the chemical/physical removal of particulate matter by coagulation, flocculation, 

sedimentation, and filtration processes, along with disinfection to inactivate any remaining 

pathogenic microorganisms. Filtration remains the cornerstone of drinking water treatment, 

conventionally in the form of granular media depths filters. Although granular media filters can 

produce high quality water, they represent a probabilistic rather than an absolute barrier; 

consequently, pathogens can still pass through the filters and pose a health risk. The disinfection 

process provides an additional measure of public health protection by inactivating these 

microorganisms. However, some microorganisms, such as Cryptosporidium, are resistant to 

common primary disinfection practices such as chlorination and chloramination. Furthermore, 

drinking water regulations have established maximum contaminant levels (MCLs) for 

disinfection byproducts (DBPs) that may create incentive for drinking water utilities to minimize 

the application of some disinfectants. As a result of the concern over chlorine-resistant 

microorganisms and DBP formation, the drinking water industry is increasingly utilizing 

alternative treatment technologies in effort to balance the often-competing objectives of 

disinfection and DBP control. One such alternative technology that has gained broad acceptance 

is membrane filtration.

Although the use of membrane processes has increased rapidly in recent years, the 

application of membranes for water treatment extends back several decades. Reverse osmosis 

(RO) membranes have been used for the desalination of water since the 1960’s, with more 

widespread use of nanofiltration (NF) for softening and the removal of total organic carbon 

(TOC) dating to the late 1980s. However, the commercialization of backwashable hollow-fiber 

microfiltration (MF) and ultrafiltration (UF) membrane processes for the removal of particulate 

matter (i.e., turbidity and microorganisms) in the early 1990’s has had the most profound impact 

on the use, acceptance, and regulation of all types of membrane processes for drinking water 

treatment.

USEPA developed this guidance manual in support of the Long Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESTWR), which has identified membrane filtration (i.e., 

MF, UF, NF, RO and certain types of cartridge filters) as one treatment technology in a 

“toolbox” of options that may be used to achieve the required level of Cryptosporidium

treatment. Although the LT2ESWTR only regulates membrane filtration in terms of its 

application for compliance with the requirements of the rule, the concepts and guidance provided 

in this manual may also be relevant to the Interim Enhanced Surface Water Treatment Rule 

(IESWTR), the Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR), the 

Stage 2 Disinfectants and Disinfection Byproducts Rule (DBPR), the Ground Water Rule 

(GWR), and other regulations, at the discretion of the state1.

 

1 For the purposes of this manual, the term “state” refers to the state or primacy agency that is responsible for 

enforcement of drinking water standards.
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1.1 Regulating Membranes for the Drinking Water Industry

Driven by the need to protect public health from waterborne pathogens, USEPA has 

progressively developed regulations that require higher standards for filtered water quality to 

prevent the passage of infectious pathogens through the treatment process and into the finished 

drinking water supply. In 1986, the Surface Water Treatment Rule (SWTR) required surface 

water systems to provide treatment equivalent to 3 log Giardia and 4 log virus reduction via a 

combination of removal and inactivation. USEPA estimated conventional filtration plants (i.e., 

including coagulation and sedimentation) meeting the filter effluent turbidity requirements 

provided a minimum of 2.5 log Giardia and 2 log virus reduction, while direct filtration plants 

(i.e., without sedimentation) provided 2 log Giardia and 1 log virus removal. The IESWTR (FR 

63(241):69518) focused on filter effluent turbidity control, setting a combined filter effluent 

turbidity limit of 0.3 NTU (40 CFR 41.173) and requiring individual filter turbidity monitoring 

(40 CFR141.174) for systems serving at least 10,000 people. The rule also introduced a 

requirement for 2 log Cryptosporidium removal. The IESWTR also required that turbidity 

monitoring be conducted “continuously” (i.e., every 15 minutes) on individual filters. Based on 

available data, USEPA believed conventional and direct filtration plants meeting the IESWTR 

filter effluent turbidity requirements provided a minimum 2 log removal of Cryptosporidium. 

The LT1ESWTR (FR 67(9):1812) promulgated the requirements of the IESWTR, with some 

modifications, for all water systems serving less than 10,000 people. 

Under the existing surface water treatment rules, log removal credits for alternative 

filtration technologies (AFTs), such as membrane processes and bag and cartridge filters, were 

not explicitly addressed, but instead covered under a special state primacy requirement. For 

compliance with the SWTR, IESWTR, and LT1ESWTR, many states grant removal credits to 

membrane processes based on the guidelines for AFTs in the Guidance Manual for Compliance 

With the Filtration and Disinfection Requirements for Public Water Systems Using Surface 

Water Sources (commonly called the SWTR Guidance Manual) (USEPA 1991). However, there 

is significant variability in the manner in which states regulate membrane processes, as 

summarized for MF/UF systems in the USEPA report Low-Pressure Membrane Filtration for 

Pathogen Removal: Application, Implementation, and Regulatory Issues (2001).

The LT2ESWTR builds on the previous surface water treatment rules by requiring 

additional treatment for those systems with elevated influent Cryptosporidium levels. The rule 

identifies a number of “toolbox” technologies that may be employed to achieve additional 

Cryptosporidium treatment requirements. The range of removal or inactivation credits allocated 

to each of the various toolbox options under the rule varies based on the capabilities of the 

particular treatment technology. Because the various types of membrane filtration processes 

represent one of these toolbox alternatives, utilities have the option of using membrane filtration 

for compliance with the rule requirements as a distinct technology rather than simply as a general 

AFT. Consequently, USEPA has developed specific regulatory requirements and associated 

guidance, as contained in this manual, for membrane filtration processes used for compliance 

with the LT2ESWTR. 
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1.2 Summary of Applicable Regulations

Although the regulatory framework presented in this guidance manual is directly 

applicable only to the use of membrane filtration for compliance with the LT2ESWTR, the 

concepts may be extended to other applications of the technology at the discretion of the state. 

Some regulations for which this regulatory framework may be useful for utilities and states 

include the Stage 2 DBPR and the various existing surface water treatment regulations. Because 

the Stage 2 DBPR will make DBP limits more stringent, some utilities may consider shifting 

treatment strategies to obtain a greater portion of their respective total pathogen control 

requirements using removal processes such as membrane filtration rather and less from 

disinfection, if such flexibility is permitted by the state. Since membrane filtration will not be 

specifically addressed under the Stage 2 DBPR, the Membrane Filtration Guidance Manual may

serve as a useful resource for states and utilities under the regulatory scenario noted above. In 

addition, this regulatory framework and associated guidance manual may also be a useful 

resource for states to develop distinc t regulatory policies for membrane filtration relative to the 

existing surface water treatment rules (i.e., those promulgated prior to the LT2ESWTR) to 

separate the technology from the broader AFT classification.

The basic requirements of both the LT2ESWTR and the Stage 2 DBPR are described in 

the following subsections. The information provided is not specific to membrane treatment, but 

simply represents a general overview of the rule requirements.

1.2.1 Long Term 2 Enhanced Surface Water Treatment Rule

Under the LT2ESWTR, systems that use either a surface water or ground water under the 

direct influence of surface water (GWUDI) source (collectively referred to as surface water 

systems) are required to conduct source water monitoring to determine average Cryptosporidium

concentrations. Based on the average Cryptosporidium concentration, a system is classified in 

one of four possible bins, as shown in Table 1.1 (40 CFR 141.720). The bin assignment dictates 

the supplemental level of Cryptosporidium treatment required in addition to the existing 

requirements of the SWTR, IESWTR, and LT1ESWTR. Utilities may comply with additional 

treatment requirements by implementing one or more management or treatment techniques from 

a toolbox of options that includes membrane filtration. Guidance for the use of membrane 

filtration for compliance with the LT2ESWTR is provided in this manual. A separate guidance 

manual has also been developed for the use of ultraviolet (UV) disinfection for LT2ESWTR 

compliance (USEPA 2003). Guidance for the use of all other toolbox options is given in the 

Toolbox Guidance Manual for the Long Term 2 Enhanced Surface Water Treatment Rule

(USEPA 2003).
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Table 1.1 LT2ESWTR Additional Treatment Requirements for Filtered Systems

LT2ESWTR Category Type of Existing Filtration

Bin

Cryptosporidium

Concentration

(oocysts/L)

Conventional 

Filtration

Direct 

Filtration

Slow Sand 

or

Diatomaceous 

Earth Filtration

Alternative 

Filtration 

Technologies6

1 < 0.075 No additional 

treatment

No additional 

treatment

No additional 

treatment

No additional 

treatment

2 > 0.075 and < 1.0 1 log11.5 log11 log1 As determined 

by the state1, 3

3 > 1.0 and < 3.0 2 log22.5 log22 log2 As determined 

by the state2, 4

4 > 3.0 2.5 log23 log22.5 log2 As determined 

by the state2, 5

1 Systems may use any technology or combination of technologies from the toolbox

2 Systems must achieve at least 1 log of the required treatment using ozone, chlorine dioxide, UV disinfection, membranes, 

bag/cartridge filters, or bank filtration

3 Total Cryptosporidium treatment must be at least 4 log

4 Total Cryptosporidium treatment must be at least 5 log

5 Total Cryptosporidium treatment must be at least 5.5 log

6 Includes membrane filtration

1.2.2 Stage 2 Disinfectants and Disinfection Byproducts Rule

The Stage 2 DBPR is designed to reduce DBP occurrence peaks in the distribution 

system by changing compliance monitoring requirements. The requirements of the 

Stage 2 DBPR apply to all community water systems (CWSs) and non-transient non-community 

water systems (NTNCWSs) – both ground and surface water systems – that either add a 

chemical disinfectant (i.e., a disinfectant other than UV light) or deliver water that has been 

treated with a chemical disinfectant. The primary components of the rule are summarized as 

follows.

Initial Distribution System Evaluations

Under the Stage 2 DBPR, systems are required to conduct an initial distribution system 

evaluation (IDSE) to identify compliance monitoring locations with high total trihalomethane 

(TTHM) and haloacetic acid (HAA5) levels. The IDSE consists of either a standard monitoring 

program (SMP) or a system-specific study (SSS). NTNCWSs serving fewer than 10,000 people 

are not required to conduct an IDSE, and other systems may be eligible to receive waivers from 

the IDSE requirement.
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Compliance Determination and Schedule

The Stage 2 DBPR changes the way DBP sampling results are averaged to determine 

compliance, implementing a methodology based on a locational running annual average (LRAA) 

rather than the system-wide running annual average (RAA) used under the Stage 1 DBPR. This 

new methodology introduced under the Stage 2 DBPR is introduced in two phases: Stage 2A 

and Stage 2B. Under Stage 2A, all systems must comply with TTHM and HAA5 MCLs of 

120 μg/L and 100 μg/L, respectively, measured as LRAAs at each Stage 1 DBPR monitoring 

site, while continuing to comply with the respective TTHM and HAA5 Stage 1 DBPR MCLs of 

80 μg/L and 60 μg/L, measured as RAAs. Subsequently, under Stage 2B, systems must comply 

with respective TTHM and HAA5 MCLs of 80 μg/L and 60 μg/L at the sampling locations 

identified under the IDSE.

Compliance Monitoring

Stage 2B compliance monitoring requirements (in terms of number of sites and frequency 

of sampling) are expected to be similar to the Stage 1 DBPR requirements for most, but not all, 

systems. Some small systems will have to add an additional monitoring location if the highest 

TTHM and highest HAA5 site do not occur at the same location.

Significant Excursion Evaluations

Because Stage 2 DBPR MCL compliance is based on an annual average of DBP 

measurements, a system could from time to time have DBP levels significantly higher than the 

MCL (referred to as a significant excursion) while still maintaining compliance. If a significant 

excursion occurs, a system must conduct a significant excursion evaluation and discuss the 

evaluation with the state prior to the next sanitary survey.

1.3 Requirements for Membrane Filtration Under the LT2ESWTR

In order to receive removal credit for Cryptosporidium removal under the LT2ESWTR, a 

membrane filtration system must meet the following three criteria (40 CFR 141.728):

1. The process must comply with the definition of membrane filtration as stipulated by 

the rule.

2. The remova l efficiency of a membrane filtration process must be established through 

a product-specific challenge test and direct integrity testing.

3. The membrane filtration system must undergo periodic direct integrity testing and 

continuous indirect integrity monitoring during operation.
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The rule does not prescribe a specific removal credit for membrane filtration processes. 

Instead, removal credit is based on system performance as determined by challenge testing and 

verified by the direct integrity testing. Thus, the maximum removal credit that a membrane 

filtration process may receive is the lower value of either (40 CFR 141.728):

• The removal efficiency demonstrated during challenge testing; OR

• The maximum log removal value that can be verified by the direct integrity test used 

to monitor the membrane filtration process 

Based on this framework, a membrane filtration process could potentially meet the Bin 4 

Cryptosporidium treatment requirements, as shown in Table 1.1. Additionally, if a membrane 

filtration system has been previously approved for 5.5 log Cryptosporidium removal by the state, 

the utility would not be required to conduct source monitoring under the LT2ESWTR 

(40 CFR 141.728).

These primary elements of the regulatory requirements for membrane filtration under the 

LT2ESWTR, including the definition of membrane filtration, as well as challenge testing, direct 

integrity testing, and continuous indirect integrity monitoring, are summarized in the following 

sections.

1.3.1 Definition of a Membrane Filtration Process

For the purposes of compliance with the LT2ESWTR, membrane filtration is defined as a 

pressure- or vacuum-driven separation process in which particulate matter larger than 1 μm is 

rejected by a non-fibrous engineered barrier primarily through a size exclusion mechanism and 

which has a measurable removal efficiency of a target organism that can be verified through the 

application of a direct integrity test (40 CFR 141.2). This definition is intended to include the 

common membrane technology classifications: MF, UF, NF, and RO. In addition, any cartridge 

filtration device that meets the definition of membrane filtration and which can be subject to 

direct integrity testing in accordance with rule requirements would also be eligible for 

Cryptosporidium removal credit as a membrane filtration process under the LT2ESWTR 

(40 CFR 141.728). In this guidance manual, such processes are called membrane cartridge 

filtration (MCF). Filtration processes that are reliant on mechanisms such as adhesion to filter 

media or accumulation of a fouling layer to remove particulate matter are excluded from the 

definition of membrane filtration. Examples of processes that would not be considered 

membrane filtration devices for the purposes of LT2ESWTR compliance include bag filters and 

cartridge filters using a fibrous filtration media.
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1.3.2 Challenge Testing

Since there are no uniform design criteria that can be used to assure the removal 

efficiency of a membrane process, challenge testing is required to demonstrate the ability of a 

treatment process to remove a specific target organism. The removal efficiency demonstrated 

during challenge testing establishes the maximum removal credit that a membrane process would 

be eligible to receive, provided that this value is less than or equal to the maximum log removal 

value that can be verified by the direct integrity test (40 CFR 141.728), as described in the 

following section. The LT2ESWTR only requires product-specific challenge testing; once the 

removal efficiency has been demonstrated, additional testing is not required unless the product is 

significantly modified, as described in Chapter 3. Data from challenge studies conducted prior to 

promulgation of this regulation can be considered in lieu of additional testing at the discretion of 

the state (40 CFR 141, Subpart W, Appendix C). However, the prior testing must have been 

conducted in a manner that demonstrates removal efficiency for Cryptosporidium equivalent to 

or greater than the treatment credit awarded to the process.

1.3.3 Direct Integrity Testing

While challenge testing can demonstrate the ability of an integral membrane process to 

remove the target organism, integrity breaches can develop in the membrane during routine 

operation that could allow the passage of microorganisms. In order to verify the removal 

efficiency of a membrane process during operation, direct integrity testing is required for all 

membrane filtration processes used to comply with LT2ESWTR (40 CFR 141.728). A direct 

integrity test is defined as a physical test applied to a membrane unit in order to identify and 

isolate integrity breaches. The rule does not mandate the use of a specific type of direct integrity 

test, but rather performance criteria that any direct integrity test must meet. These criteria 

include requirements for resolution, sensitivity, and frequency (40 CFR 141, Subpart W, 

Appendix C):

• Resolution: The direct integrity test must be applied in a manner such that a 3 μm 

hole contributes to the response from the test.

• Sensitivity: The direct integrity test must be capable of verifying the log removal 

value awarded to the membrane process.

• Frequency: The direct integrity test must be applied at a frequency of at least once 

per day.

A control limit must also be established for a direct integrity test, representing a threshold 

response which, if exceeded, indicates a potential integrity problem and triggers subsequent 

corrective action. For the purposes of LT2ESWTR compliance, this threshold response must be

indicative of an integral membrane unit capable of achieving the Cryptosporidium removal credit 

awarded by the state.
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The criteria for direct integrity testing are discussed in greater detail in Chapter 4, along 

with guidance describing how these criteria apply to commonly used direct integrity tests.

1.3.4 Continuous Indirect Integrity Monitoring

Because currently available direct integrity test methods require the membrane unit to be 

temporarily taken out of service, or are either too costly or infeasible to apply continuously, 

direct testing is only conducted periodically. Thus, in the absence of continuous direct integrity 

test that meets the resolution and sensitivity requirements of the LT2ESWTR, continuous 

indirect integrity monitoring is required (40 CFR 141.728). Although the indirect monitoring 

methods are typically not as sensitive as direct tests for detecting losses of membrane integrity, 

the indirect methods do provide some measure of performance assessment between applications 

of direct testing. For the purposes of the LT2ESWTR, continuous indirect integrity monitoring 

is defined as monitoring some filtrate water parameter that is indicative of the removal of 

particulate matter at a frequency of no less than once every 15 minutes (40 CFR 141.728). 

Although turbidity monitoring is specified as the default method of continuous indirect integrity 

monitoring under the rule, other methods, such as particle counting or particle monitoring, may 

be used in lieu of turbidity monitoring at the discretion of the state (40 CFR 141.728). For any 

indirect method used, a control limit must be established that is indicative of acceptable 

performance. Monitoring results exceeding the control limit for a period of more than 15 

minutes must trigger direct integrity testing (40 CFR 141.728). The requirements and associated 

guidance for continuous indirect integrity monitoring are detailed in Chapter 5.

1.4 Considering Existing Membrane Facilities Under the LT2ESWTR

As shown in Table 1.1, the LT2ESWTR only requires additional treatment measure for 

those drinking water systems with source water Cryptosporidium levels greater than or equal to 

0.075 oocysts/L – bins 2, 3, or 4. Thus, existing systems utilizing membrane filtration that fall 

into bin 1 may continue to operate under the previous surface water treatment rules (i.e., the 

SWTR and either the IESWTR or the LT1ESWTR) as administered by the state. 

Utilities with existing membrane filtration facilities will be affected by the LT2ESWTR 

in one of five ways. These cases are summarized as follows:

• Case 1: The utility has previously been awarded 5.5 log Cryptosporidium treatment 

credit via a combination of physical removal (which may include membrane 

filtration) and chemical inactivation. In this case the utility is not required to conduct 

source water monitoring (40 CFR 141.701(f)).

• Case 2: The utility conducts source water monitoring for Cryptosporidium, and 

determines that it is in bin 1 (i.e., concentrations less than 0.075 oocysts/L). In this 

case, the system may continue to operate under the previous surface water treatment 

rules (i.e., the SWTR and either the IESWTR or the LT1ESWTR) as administered by 

the state. No additional action is required under the LT2ESWTR. 
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• Case 3: The utility conducts source water monitoring for Cryptosporidium, and 

determines that it is in either bins 2, 3, or 4 (i.e., concentrations greater than or equal 

to 0.075 oocysts/L). The utility then successfully demonstrates to the state that its 

membrane filtration system can achieve the total required Cryptosporidium treatment 

credit, as listed in the right hand column of Table 1.1, up to a maximum of 5.5 log 

credit.

• Case 4: The utility conducts source water monitoring for Cryptosporidium, and 

determines that it is in either bins 2, 3, or 4 (i.e., concentrations greater than or equal 

to 0.075 oocysts/L). The utility then successfully demonstrates to the state that its 

membrane filtration system can achieve part of the required Cryptosporidium

treatment credit, as listed in the right hand column (i.e., under “Type of Existing 

Filtration – Alternative Filtration Technologies”) of Table 1.1. In this case the utility 

would be required to use other toolbox options to obtain the balance of 

Cryptosporidium treatment credit required under the rule.

• Case 5: The utility conducts source water monitoring for Cryptosporidium, and 

determines that it is in either bins 2, 3 or 4 (i.e., concentrations greater than or equal 

to 0.075 oocysts/L). The utility then successfully demonstrates to the state that its 

membrane filtration system can achieve part of the required Cryptosporidium

treatment credit, as listed in the right hand column (i.e., under “Type of Existing 

Filtration – Alternative Filtration Technologies”) of Table 1.1. However, the utility 

opts not to use its membrane filtration system for the purposes of LT2ESWTR 

compliance. In this case the utility would be required use other toolbox options to 

obtain all of the Cryptosporidium treatment credit required under the rule.

Under the LT2ESTWR, the regulatory basis for a membrane filtration process to receive 

treatment credit for Cryptosporidium is the demonstration of removal efficiency through 

challenge testing and the demonstration of membrane system integrity through routine direct 

integrity testing and continuous indirect integrity monitoring. These criteria form the basis for 

the potential for membrane filtration systems to be award up to a maximum of 5.5 log 

Cryptosporidium removal credit for complying with the requirements of the previously 

promulgated surface water treatment rules in combination with any additional Cryptosporidium

treatment credit that may be required under the LT2ESWTR.

With respect to the challenge testing requirements of the rule, the two most likely options 

available to utilities with existing membrane filtration systems are to grandfather data generated 

during the pilot testing conducted as part of the permitting process (if applicable) or to use data 

from challenge testing conducted by the membrane manufacturer in its effort to qualify its 

product(s) for Cryptosporidium removal credit under LT2ESWTR (since challenge testing is 

required on a product-specific and not a site-specific basis). Challenge testing, including 

recommendations for grandfathering data, is described in detail in Chapter 3.
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Existing membrane facilities will also have to meet the direct integrity testing and 

continuous indirect integrity monitoring requirements of the LT2ESWTR to qualify for treatment 

credit under the rule. This may necessitate that some facilities implement new integrity 

verification practices, since state requirements vary widely, and some may not require direct 

integrity testing at all (USEPA 2001). However, this is not anticipated to be problematic for 

many existing facilities, since most membrane filtration systems applied to surface water are 

equipped with the ability to conduct some form of direct integrity testing. In addition, although 

states may not have explicit indirect integrity monitoring requirements, turbidity monitoring (the 

default method of continuous indirect integrity monitoring under the LT2ESWTR) is nonetheless 

required for compliance with the various existing surface water treatment rules. In some cases, 

utilities may need to purchase additional equipment to comply with the integrity verification 

requirements of the rule. Detailed guidance on the use of direct integrity testing and continuous 

indirect integrity monitoring for LT2ESWTR compliance is provided in Chapters 4 and 5, 

respectively.

Another consideration for existing membrane facilities required to meet the LT2ESWTR 

criteria is replacement membrane modules. When replacement modules are installed, it is 

necessary to verify that the specific modules used meet the quality control release value of the 

nondestructive performance test as a means of indirectly verifying removal efficiency. 

Additional guidance regarding the nondestructive performance test is provided in section 3.6.

The regulatory framework developed for membrane filtration under the LT2ESWTR 

addresses many of the specific capabilities and requirements of the technology, and thus may 

introduce new concepts that might not be included in a given state’s current regulatory approach 

for membrane processes, particularly if the state currently considers membrane filtration as an 

AFT, as described the Guidance Manual for Compliance With the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources (USEPA 1991). Although 

states may choose to adopt aspects of the LT2ESWTR framework for broader regulation of 

membrane filtration systems, USEPA only requires that this regulatory framework be applied to 

systems that utilize membrane filtration to meet the additional Cryptosporidium treatment 

requirements of the LT2ESWTR.

1.5 Membrane Terminology Used in the Guidance Manual

In the development of the regulatory language and associated guidance for LT2ESWTR, 

it was necessary to select the most appropriate terminology for various aspects of membrane 

treatment, with the understanding that use of such terminology can vary widely throughout the 

industry. The purpose of this section is to clarify the use of membrane treatment terminology 

associated with the LT2ESWTR and note generally synonymous terms that are also in common 

use, where applicable. This section also presents some new terms defined under the rule that are 

critical to the regulatory framework.

There are twelve terms formally defined under the language for the LT2ESWTR, as 

follows:
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• Membrane filtration– a pressure- or vacuum-driven separation process in which 

particulate matter larger than 1 μm is rejected by a nonfibrous engineered barrier, 

primarily through a size exclusion mechanism and which has a measurable removal 

efficiency of a target organism that can be verified through the application of a direct 

integrity test (40 CFR 141.2). The definition of a membrane filtration process and 

what it represents is further discussed in section 1.3.1.

• Module – the smallest component of a membrane unit in which a specific membrane 

surface area is housed in a device with a filtrate outlet structure (40 CFR 141, 

Subpart W, Appendix C). For the purposes of the LT2ESWTR, this term 

encompasses hollow-fiber modules and cassettes, spiral-wound elements, cartridge 

filter elements, plate-and-frame modules, and tubular modules, among other 

membrane devices of similar scope and purpose.

• Membrane unit – a group of membrane modules that share common valving which 

allows the unit to be isolated from the rest of the system for the purpose of integrity 

testing or other maintenance (40 CFR 141, Subpart W, Appendix C). For the 

purposes of the LT2ESWTR, “membrane unit” is intended to include the commonlyused synonymous terms rack, train, and skid.

• Challenge test – a study conducted to determine the removal efficiency (i.e., log 

removal value (LRV)) of the membrane filtration media (40 CFR 141, Subpart W, 

Appendix C). Challenge testing is discussed in detail in Chapter 3, and the 

requirements for challenge testing under the LT2ESWTR are summarized briefly in 

section 1.3.2.

• Flux – flow per unit of membrane area (40 CFR 141, Subpart W, Appendix C).

• Recovery – the ratio of filtrate volume produced by a membrane to feed water 

volume applied to a membrane over the course of an uninterrupted operating cycle 

(40 CFR 141, Subpart W, Appendix C). In the context of the LT2ESWTR, the term 

recovery does not consider losses that occur due to the use of filtrate in backwashing 

or cleaning operations.

• Direct integrity test – a physical test applied to a membrane unit in order to identify 

and isolate integrity breaches (40 CFR 141, Subpart W, Appendix C). Direct 

integrity testing is discussed in detail in Chapter 4, and the requirements for direct 

integrity testing under the LT2ESWTR are summarized briefly in section 1.3.3.

• Integrity breach– one or more leaks [in a membrane filtration system] that could 

result in the contamination of the filtrate (40 CFR 141, Subpart W, Appendix C).

• Resolution– the smallest leak that [i.e., integrity breach] that contributes to a 

response from a direct integrity test (40 CFR 141, Subpart W, Appendix C).
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• Sensitivity – the maximum log removal value that can be reliably verified by the 

direct integrity test [associated with a membrane filtration system] (40 CFR 141, 

Subpart W, Appendix C).

• Control limit – an integrity test response that, if exceeded, indicates a potential 

problem with the system and triggers a response (40 CFR 141, Subpart W, Appendix 

C). In the context of this guidance manual the terms upper control limit (UCL) and 

lower control limit (LCL) are also used. The term upper control limit is always used 

in reference to the control limit that is mandated under the LT2ESWTR, or the last 

control limit that could be exceeded before the unit must be taken off-line for 

diagnostic testing and repair. The term lower control limit was established to 

distinguish any more conservative voluntary or additional state-mandated control 

limits that may trigger increased monitoring or other action short of taking the unit 

off-line.

• Indirect integrity monitoring – monitoring some aspect of filtrate water quality that 

is indicative of the removal of particulate matter (40 CFR 141.728). In the context of 

indirect integrity monitoring, continuous is defined as a frequency of no less than 

once every 15 minutes (40 CFR 141.728). Continuous indirect integrity monitoring is 

discussed in detail in Chapter 5, and the requirements for continuous indirect integrity 

monitoring under the LT2ESWTR are summarized briefly in section 1.3.4.

In addition to these terms, it is important to note that the term filtrate, as used in both the 

rule language and this guidance manual, includes the synonymous term permeate, which is 

commonly used in the industry in association with the treated water from NF and RO semipermeable membrane processes. All of the terms clarified in this section, as well as numerous 

others used in the context of this manual, are defined in the glossary.

1.6 Guidance Manual Objectives

The purpose of this manual is to establish a clear and consistent framework for the 

application and regulation of membrane filtration for compliance with the requirements of the 

LT2ESWTR. Specifically, the objective of the manual is to provide utilities, state regulators, 

membrane module and system manufacturers, and consulting engineers with guidance in the 

following respective areas:

• Utilities and consulting engineers: specific guidance on meeting the criteria 

specified in LT2ESWTR in order to receive removal credit for Cryptosporidium

through the application of membrane filtration, as well as additional guidance 

regarding industry practices for pilot testing, implementing, and starting up a 

membrane filtration facility.
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• State regulators: guidance on evaluating membrane filtration systems and 

determining appropriate Cryptosporidium removal credits for these processes under 

the LT2ESWTR

• Manufacturers: guidance on qualifying membrane filtration systems for 

Cryptosporidium removal credits under the LT2ESWTR

Note that the guidance provided in this manual applies to systems that meet the definition 

of membrane filtration under the LT2ESWTR, including MF, UF, NF, and RO processes, as well 

as qualifying cartridge filtration systems. Electrodialysis (ED) and electrodialysis reversal 

(EDR) processes are not considered in this document, since they do not provide a physical 

barrier to pathogens and consequently are not considered membrane filtration under the rule.

1.7 Guidance Manual Organization

The Membrane Filtration Guidance Manual expands on the requirements for utilizing 

membrane filtration for compliance with the LT2ESWTR and provides guidance to facilitate 

compliance with these requirements. In addition, this manual describes some recommended 

industry practices for the application of membrane filtration systems for the removal of 

pathogens in the drinking water treatment process. These recommended practices are not

required under LT2ESTWR and are provided as general guidance only. 

Chapters 1, 3, 4, and 5 elaborates on LT2ESTWR requirements and associated guidance, 

while Chapters 6, 7, and 8 discuss recommended industry practices for membrane filtration that 

are not specifically related to the rule. Chapter 2 presents an overview of membrane filtration for 

readers unfamiliar with the technology. It is recommended that even readers with significant 

membrane process experience review Chapter 2 to better understand how the concepts and 

terminology are used in the context of the guidance manual. 

The guidance manual is organized as follows:

Chapter 1: Introduction

Chapter 1 presents the objectives of the guidance manual and provides an 

overview of the regulatory requirements for membrane filtration under the 

LT2ESWTR.

Chapter 2: Overview of Membrane Filtration

Chapter 2 provides an overview of the basic theory and concepts of 

membrane filtration, including: types of membrane processes; types of 

membrane materials, modules, and systems; fundamental principles; and 

hydraulic models describing various configurations.
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Chapter 3: Challenge Testing

Chapter 3 provides guidance on designing a challenge study to 

demonstrate the removal efficiency of a membrane module with respect to 

Cryptosporidium, as required under the LT2ESWTR.

Chapter 4: Direct Integrity Testing

Chapter 4 provides guidance on meeting the LT2ESWTR performancebased requirements for direct integrity testing of membrane filtration 

systems and describes the commonly used direct integrity test methods in 

the context of these requirements.

Chapter 5: Continuous Indirect Integrity Monitoring

Chapter 5 provides guidance on meeting the LT2ESWTR requirements for 

continuous indirect integrity monitoring, including both the default 

approach using turbidity as well as a discussion of potential alternative 

methods.

Chapter 6: Pilot Testing

Chapter 6 discusses aspects of pilot testing membrane filtration systems, 

including objectives, planning, operation, and data collection and analysis.

Chapter 7: Implementation Considerations

Chapter 7 discusses a variety of design and operational considerations for 

implementing membrane filtration processes.

Chapter 8: Initial Startup

Chapter 8 discusses issues associated with the start-up and shakedown of a 

new membrane filtration system, as well as some recommended practices 

to facilitate this process.

Appendix A: Development of a Comprehensive Integrity Verification Program

Appendix A presents a framework for developing an integrity testing and 

monitoring program that utilizes a variety of tools to ensure system 

performance.

Appendix B: Overview of Bubble Point Theory

Appendix B presents an overview of bubble point theory, which serves as 

the basis for pressure-based direct integrity tests.

Appendix C: Calculating the Air-Liquid Conversion Ratio

Appendix C provides supplemental guidance for calculating the air-liquid 

conversion ratio (ALCR) (as described in Chapter 4), a method for 

converting the results of pressure-based direct integrity tests to the flow of 

water through an integrity breach during normal system operation for the 

purpose of determining both test sensitivity and appropriate control limits.
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Appendix D: Empirical Method for Determining the Air-Liquid Conversion Ratio for a 

Hollow-Fiber Membrane Filtration System

Appendix D describes an empirical method called the correlated airflow 

measurement (CAM) procedure for determining the ALCR, as described 

in Chapter 4.

Appendix E: Application of Membrane Filtration for Virus Removal

Appendix E provides a general overview of the issue of applying 

membrane filtration, and in particular UF, for virus removal, including the 

applicability of the LT2ESWTR regulatory framework for this objective.
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2.0 Overview of Membrane Filtration

2.1 Introduction

There are several classes of treatment processes that constitute membrane filtration for 

the purposes of Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) 

compliance. These processes include: microfiltration (MF), ultrafiltration (UF), nanofiltration 

(NF), and reverse osmosis (RO). In addition, cartridge filtration devices that meet the criteria for 

a membrane filtration process as defined under the rule (see sections 1.3.1 and 2.2) would also be 

eligible for Cryptosporidium removal credit as membrane filtration (40 CFR 141.728). For the 

purposes of this guidance manual, these devices are termed membrane cartridge filtration (MCF).

Each of these technologies utilizes a membrane barrier that allows the passage of water 

but removes contaminants. The membrane media is generally manufactured as flat sheets or as 

hollow fibers and then configured into membrane modules. The most common membrane 

module configurations are hollow-fiber (consisting of hollow-fiber membrane material), spiralwound (consisting of flat sheet membrane material wrapped around a central collection tube), 

and cartridge (consisting of flat sheet membrane material that is often pleated to increase the 

surface area). Although the spiral-wound and cartridge configurations are also termed as 

“elements” and “cartridges,” respectively, under the LT2ESWTR the term “module” – defined as 

the smallest component of a membrane unit in which a specific membrane surface area is housed 

in a device with a filtrate outlet structure (see section 1.5) – is used to refer to all of the various 

membrane module configurations for simplicity of nomenclature (40 CFR 141.728).

In addition to the various module configurations, there are a number of different types of 

membrane materials, hydraulic modes of operation, and operational driving forces (i.e., pressure 

or vacuum) that can vary among the different classes of membrane filtration (i.e., MF, UF, NF, 

RO, and MCF). Each of these characteristics of membrane filtration systems may be considered 

tools that a manufacturer may utilize to meet the particular treatment objectives for a given 

application.

The purpose of Chapter 2 of the Membrane Filtration Guidance Manual is to provide an 

overview of the various membrane filtration processes, including descriptions of the various 

classes, membrane material, geometry, module construction, driving forces, basic principles of 

design and operation, and hydraulic configuration(s). Emphasis is given to the manner in which 

each of these characteristics relates to membrane filtration applied for pathogen removal, as 

would be the case for compliance with the LT2ESWTR.

This chapter is divided into the following sections: 

Section 2.2: Basic Principles of Membrane Filtration

This section reviews the basic treatment mechanisms of the various classes 

of membrane treatment processes, as well as the common principle of 

operation that distinguishes membrane filtration systems, particularly in 

the context of the LT2ESWTR.
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Section 2.3: Membrane Materials, Modules, and Systems

This section describes the types of membrane materials, modules, and 

types of systems associated with the various classes of membrane 

filtration.

Section 2.4: Basic Principles of Membrane Filtration System Design and Operation

The section presents some the basic concepts and equations governing 

membrane filtration system design and operation in order to facilitate a 

more complete general understanding of membrane processes.

Section 2.5: Hydraulic Configurations

This section describes the various hydraulic modes of operation for 

membrane filtration systems and provides equations for calculating the 

volumetric concentration factor.

2.2 Basic Principles of Membrane Filtration

For the purposes of the LT2ESWTR, a membrane filtration process is defined by two 

basic criteria (40 CFR 141.2):

1. The filtration system must be a pressure- or vacuum-driven process and remove 

particulate matter larger than 1 micron (μm) using a non-fibrous, engineered barrier, 

primarily via a size exclusion mechanism.

2. The process must have a measurable removal efficiency of a target organism that can 

be verified through the application of a direct integrity test.

The ability of each of type of membrane filtration system to remove various drinking 

water pathogens of interest on the basis of size is illustrated in Figure 2.1. The figure shows the 

approximate size range of viruses, bacteria, and Cryptosporidium and Giardia cysts, as well as 

the ability of MF, UF, NF, RO, and MCF, respectively, to remove each of these pathogens on the 

basis of size exclusion. Overlap between the range covered by a membrane filtration processes 

in the figure with a given pathogen size range indicates the ability of that process to remove the 

pathogen. Note that the molecular weights listed do not correspond precisely to the indicated 

pathogen size range, but are rough generalizations depicted as a result of the fact that NF, RO, 

and some UF processes are rated according to a “molecular weight cutoff” on the basis of their 

ability to remove dissolved solids and larger macromolecules. 

Although each of the classes of membrane filtration functions as a filter for various sizes 

of particulate matter, the basic principles of operation vary between MF/UF, NF/RO, and MCF 

systems. Each of these types of systems is described in the following sections.
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Figure 2.1 Filtration Application Guide for Pathogen Removal

2.2.1 Microfiltration and Ultrafiltration

MF and UF are the two processes that are most often associated with the term 

“membrane filtration.” MF and UF are characterized by their ability to remove suspended or 

colloidal particles via a sieving mechanism based on the size of the membrane pores relative to 

that of the particulate matter. However, all membranes have a distribution of pore sizes, and this 

distribution will vary according to the membrane material and manufacturing process. When a 

pore size is stated, it can be presented as either nominal (i.e., the average pore size), or absolute 

(i.e., the maximum pore size) in terms of microns (μm). MF membranes are generally 

considered to have a pore size range of 0.1 – 0.2 μm (nominally 0.1 μm), although there are 

exceptions, as MF membranes with pores sizes of up to 10 μm are available. For UF, pore sizes 

generally range from 0.01 – 0.05 μm (nominally 0.01μm) or less, decreasing to an extent at 

which the concept of a discernable “pore” becomes inappropriate, a point at which some discrete 

macromolecules can be retained by the membrane material. In terms of a pore size, the lower 

cutoff for a UF membrane is approximately 0.005 μm.

Because some UF membranes have the ability to retain larger organic macromolecules, 

they have been historically characterized by a molecular weight cutoff (MWCO) rather than by a 

particular pore size. The concept of the MWCO (expressed in Daltons – a unit of mass) is a 

measure of the removal characteristic of a membrane in terms of atomic weight (or mass) rather 

than size. Thus, UF membranes with a specified MWCO are presumed to act as a barrier to 

compounds or molecules with a molecular weight exceeding the MWCO. Because such organic 

macromolecules are morphologically difficult to define and are typically found in solution rather 

Cryptosporidium

RO

Membrane Filtration 

Process

MCF

MF

UF

NF

Size (mm) 0.0001 0.001 0.01 0.1 1.0 10 100

Molecular Weight

(Daltons)

Drinking Water 

Pathogens

200 20,000 200,000

Bacteria

Giardia

Viruses
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than as suspended solids, it may be convenient in conceptual terms to use a MWCO rather than a 

particular pore size to define UF membranes when discussed in reference to these types of 

compounds. Typical MWCO levels for UF membranes range from 10,000 to 500,000 Daltons, 

with most membranes used for water treatment at approximately 100,000 MWCO. However, UF 

membranes remove particulate contaminants via a size exclusion mechanism and not on the basis 

of weight or mass; thus, UF membranes used for drinking water treatment are also characterized 

according to pore size with respect to microbial and particulate removal capabilities.

2.2.2 Nanofiltration and Reverse Osmosis

NF and RO constitute the class of membrane processes that is most often used in 

applications that require the removal of dissolved contaminants, as in the case of softening or 

desalination. The typical range of MWCO levels is less than 100 Daltons for RO membranes, 

and between 200 and 1,000 Daltons for NF membranes. While NF and RO are sometimes 

referred to as “filters” of dissolved solids, NF and RO utilize semi-permeable membranes that do 

not have definable pores. NF and RO processes achieve removal of dissolved contaminants 

through the process of reverse osmosis, as described below. However, these membrane 

processes also represent a barrier to particulate matter and thus are considered membrane 

filtration under the LT2ESWTR (40 CFR 141.2).

NF/RO membranes are designed to remove dissolved solids through the process of 

reverse osmosis. Osmosis is the natural flow of a solvent, such as water, through a semipermeable membrane (acting as a barrier to dissolved solids) from a less concentrated solution to 

a more concentrated solution. This flow will continue until the chemical potentials (or 

concentrations, for practical purposes) on both sides of the membrane are equal. The amount of 

pressure that must be applied to the more concentrated solution to stop this flow of water is 

called the osmotic pressure. An approximate rule of thumb for the osmotic pressure of fresh or 

brackish water is approximately 1 psi for every 100 mg/L difference in total dissolved solids 

(TDS) concentration on opposite sides of the membrane.

Reverse osmosis, as illustrated in Figure 2.2, is the reversal of the natural osmotic 

process, accomplished by applying pressure in excess of the osmotic pressure to the more 

concentrated solution. This pressure forces the water through the membrane against the natural 

osmotic gradient, thereby increasingly concentrating the water on one side (i.e., the feed) of the 

membrane and increasing the volume of water with a lower concentration of dissolved solids on 

the opposite side (i.e., the filtrate or permeate). The required operating pressure varies 

depending on the TDS of the feed water (i.e., osmotic potential), as well as on membrane 

properties and temperatur e, and can range from less than 100 psi for some NF applications to 

more than 1,000 psi for RO seawater desalting.
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Figure 2.2 Conceptual Diagram of Osmotic Pressure

Both the NF and RO are pressure driven separation processes that utilize semi-permeable 

membrane barriers. NF differs from RO only in terms of its lower removal efficiencies for 

dissolved substances, particularly for monovalent ions. This results in unique applications of 

NF, such as the removal of hardness ions at lower pressures than would be possible using RO. 

Consequently, NF is often called “membrane softening.” The differences between NF and RO 

are irrelevant with respect to the removal of particulate matter, and as a result, these two 

membrane processes are functionally equivalent for the purposes of the LT2ESWTR.

Because semi-permeable NF and RO membranes are not porous, they have the ability to 

screen microorganisms and particulate matter in the feed water; however, they are not 

necessarily absolute barriers. NF and RO membranes are specifically designed for the removal 

of TDS and not particulate matter, and thus the elimination of all small seal leaks that have only 

a nominal impact on the salt rejection characteristics is not the primary focus of the 

manufacturing process. Consequently, spiral-wound elements are not intended to be sterilizing 

filters and some passage of particulate matter may occur despite the absence of pores in the 

membrane, which can be attributed to slight manufacturing imperfections (Meltzer 1997). 

Nonetheless, NF and RO are eligible for Cryptosporidium removal credit under the LT2ESWTR 

based on the demonstrated ability of these technologies to remove pathogens, as well as on the 

high probability that these processes can meet the requirements for membrane filtration specified 

in the rule.

2.2.3 Membrane Cartridge Filtration

The principles of MCF system operation are similar those for MF/UF systems in that 

MCF filters particles via a sieving mechanism on the basis of size exclusion. While cartridge 

filtration has not traditionally been considered a membrane treatment process, a cartridge 

filtration device that utilizes a membrane filtration media capable of removing particles 1.0 μm 

and larger and which can be subjected to direct integrity testing would satisfy the requirements 

for membrane filtration (40 CFR 141.728). However, many cartridge filters have pore sizes that 

are larger than 1.0 μm, and the utilization of a direct integrity test with these filters is relatively 

Dilute 

Solution

Concentrated 

Solution

Osmosis

Semi-permeable 

membrane

Osmotic 

Pressure

P

Reverse 

Osmosis

P
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new in drinking water treatment applications. Thus, it is important to note that only cartridge 

filters that meet both criteria of the definition are considered membrane filtration under the 

LT2ESWTR.

2.2.4 Membrane Pore Size and Filtration Removal Efficiency

Although the concept of using the nominal or absolute pore size is sometimes used in 

reference to the filtration capabilities of membrane material, this concept is overly simplistic and 

does not fully cha racterize the removal efficiency of a membrane. For example, the mechanisms 

of filtering particles close in size to the pore distribution of a membrane becomes more complex 

than sieving, as particles smaller than most pores may be removed through probabilistic 

interception through the depth of the filter media. In addition, for some membrane materials, 

particles may be rejected through electrostatic repulsion and adsorption to the membrane 

material. The filtration properties of the membrane may also depend on the formation of a cake 

layer during operation, as the deposition of particles can obscure the pores over the course of a 

filter run, thus increasing the removal efficiency.

Because there are currently no standard methods for characterizing and reporting the pore 

sizes for the various membrane filtration processes, the meaning of this information can vary 

between different membrane manufactures, thereby limiting its value. In addition, the concept of 

pore size has no significance for NF and RO, which utilize semi-permeable membranes that do 

not have pores. The concept of pore size also does not address the integrity of the manufactured 

membrane module assembly, which could potentially pass particles larger than the indicated pore 

size.

Consequently, for the purpose of LT2ESWTR compliance, the rule requires that 

membrane filtration performance be determined by challenge testing in which the ability of the 

membrane module to reject Cryptosporidium (or an approved surrogate) is demonstrated. These 

studies provide a means to empirically determine the actual exclusion characteristic of the 

membrane module for Cryptosporidium (or other contaminant of interest) and thus account for 

all the factors that contribute to removal efficiency. Thus, unlike a measure of pore size, the use 

of an empirically determined exclusion characteristic facilitates the direct comparison of 

different membrane filtration systems for the removal of a target contaminant and provides a 

direct measure of membrane performance. Challenge testing is described in further detail in 

Chapter 3.

2.2.5 Electrodialysis and Electrodialysis Reversal

Although both electrodialysis (ED) and electrodialysis reversal (EDR) utilize membranes 

and are classified as membrane processes, these treatment technologies do not constitute 

membrane filtration as defined by the LT2ESWTR (40 CFR 141.2). Unlike NF and RO, which 

use pressure to force water through the membranes while rejecting dissolved solids, the driving 

force for separation in ED and EDR processes is electric potential, and an applied current is 

utilized to transport ionic species across selectively permeable membranes. Because the water 
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does not physically pass through the membrane in either the ED or EDR process, particulate 

matter is not removed. Thus, ED and EDR membranes are specifically applied for the removal 

of dissolved ionic constituents and are not considered filters. Consequently, ED/EDR processes 

are not addressed further in this guidance manual.

2.3 Membrane Materials, Modules, and Systems

There are a number of different types of membrane materials, modules, and associated 

systems that are utilized by the various classes of membrane filtration. While several different 

types of membrane modules may be employed for any single membrane filtration technology, 

each class of membrane technology is typically associated with only one type of membrane 

module in water treatment applications. In general, MF and UF use hollow-fiber membranes, 

and NF and RO use spiral-wound membranes. MCF systems use flat sheet material configured 

into a cartridge filtration device. The terms hollow-fiber, spiral-wound, and cartridge refer to the 

module in which the membrane media is manufactured. Section 2.3 describes each of these 

types of membrane modules, as well as the materials from which the membranes are made and 

the systems into which they are configured.

2.3.1 Membrane Materials

The membrane material refers to the substance from which the membrane itself is made. 

Normally, the membrane material is manufactured from a synthetic polymer, although other 

forms, including ceramic and metallic membranes, may be available. Currently, almost all 

membranes manufactured for drinking water production are made of polymeric material, since 

they are significantly less expensive than membranes constructed of other materials.

The material properties of the membrane may significantly impact the design and 

operation of the filtration system. For example, membranes constructed of polymers that react 

with oxidants commonly used in drinking water treatment should not be used with chlorinated 

feed water. Mechanical strength is another consideration, since a membrane with greater 

strength can withstand larger transmembrane pressure (TMP) allowing for greater operational 

flexibility and the use of higher pressures with pressure-based direct integrity testing (see 

section 4.7). Similarly, a membrane with bi-directional strength may allow cleaning operations 

or integrity testing to be performed from either the feed or the filtrate side of the membrane. 

Material properties influence the exclusion characteristic of a membrane as well. A membrane 

with a particular surface charge may achieve enhanced removal of particulate or microbial 

contaminant of the opposite surface charge due to electrostatic attraction. In addition, a 

membrane can be characterized as being hydrophilic (i.e., water attracting) or hydrophobic (i.e., 

water repelling). These terms describe the ease with which membranes can be wetted, as well as 

the propensity of the material to resist fouling to some degree.

MF and UF membranes may be constructed from a wide variety of materials, including 

cellulose acetate (CA), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polypropylene 

(PP), polysulfone (PS), polyethersulfone (PES), or other polymers. Each of these materials has 
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different properties with respect to surface charge, degree of hydrophobicity, pH and oxidant 

tolerance, strength, and flexibility. NF and RO membranes are generally manufactured from 

cellulose acetate or polyamide materials (and their respective derivatives), and there are various 

advantages and disadvantages associated with each. While cellulose membranes are susceptible 

to biodegradation and must be operated within a relatively narrow pH range of about 4 to 8, they 

do have some resistance to continuous low-level oxidant exposure. In general, for example, 

chlorine doses of 0.5 mg/L or less may control biodegradation as well as biological fouling 

without damaging the membrane. Polyamide (PA) membranes, by contrast, are able to be used 

under a wide range of pH conditions and are not subject to biodegradation. Although PA 

membranes have very limited tolerance for the presence of strong oxidants, they are compatible 

with weaker oxidants such as chloramines. PA membranes require significantly less pressure to 

operate and have become the predominant material used for NF and RO applications.

A characteristic that influences the performance of all membranes is the trans-wall 

symmetry, a quality that describes the level of uniformity throughout the cross-section of the 

membrane. There are three types of construction that are commonly used in the production of 

membranes: symmetric, asymmetric (including both skinned and graded density variations), and 

composite. Cross-sectional diagrams of membranes with different trans-wall symmetry are 

shown in Figure 2.3. Symmetric membranes are constructed of a single (i.e., homogenous) 

material, while composite membranes use different (i.e., heterogeneous) materials. Asymmetric 

membranes may be either homogeneous or heterogeneous. 

In a symmetric membrane, the membrane is uniform in density or pore structure 

throughout the cross-section, while in an asymmetric membrane there is a change in the density 

of the membrane material across the cross sectional area. Some asymmetric membranes have a 

graded construction, in which the porous structure gradually decreases in density from the feed 

to the filtrate side of the membrane. In other asymmetric membranes, there may be a distinct 

transition between the dense filtration layer (i.e., the skin) and support structure. The denser 

skinned layer is exposed to the feed water and acts as the primary filtration barrier, while the 

thicker and more porous understructure serves primarily as mechanical support. Some hollowfibers may be manufactured as single- or double- skinned membranes, with the double skin 

providing filtration at both the outer and inner walls of the fibers. Like the asymmetric skinned 

membranes, composite membranes also have a thin, dense layer that serves as the filtration 

barrier. However, in composite membranes the skin is a different material than the porous 

substructure onto which it is cast. This surface layer is designed to be thin so as to limit the 

resistance of the membrane to the flow of water, which passes more freely through the porous 

substructure. NF and RO membrane construction is typically either asymmetric or composite, 

while most MF, UF, and MCF membranes are either symmetric or asymmetric.
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Figure 2.3 Membrane Construction and Symmetry

2.3.2 Membrane Modules

Membrane filtration media is usually manufactured as flat sheet stock or as hollow fibers 

and then configured into one of several different types of membrane modules. As defined for the 

purposes of the LT2ESWTR, a membrane module represents the smallest discrete filtration unit 

in a membrane system (40 CFR 141.728). (Terminology associated with the LT2ESWTR and 

used in the Membrane Filtration Guidance Manual is described in section 1.5.) Module 

construction typically involves potting or sealing the membrane material into a corresponding 

assembly, which may incorporate an integral containment structure, such as with hollow-fiber 

modules. These types of modules are designed for long-term use over the course of a number of 

years. Spiral-wound modules are also manufactured for long-term use, although the design of 

membrane filtration systems that utilize spiral-wound modules requires that the modules be

encased in a separate pressure vessel that is independent of the module itself. Alternatively, a 

module may be configured as a disposable cartridge with a useful life that is typically measured 

in weeks or months rather than years. Membrane cartridges may either be inserted into pressure 
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vessels that are separate from the module (as with spiral-wound modules) or manufactured 

within a casing that serves as an integral pressure vessel. Each of these three types of modules, 

along with some other less common module designs, is discussed in the following subsections. 

(The membrane filtration systems that utilize these types of modules are subsequently described 

under section 2.3.3.)

2.3.2.1 Hollow-Fiber Modules

Most hollow-fiber modules used in drinking water treatment applications are 

manufactured to accommodate porous MF or UF membranes and designed to filter particulate 

matter. As the name suggests, these modules are comprised of hollow-fiber membranes, which 

are long and very narrow tubes that may be constructed of any of the various membrane 

materials described in section 2.3.1. The fibers may be bundled in one of several different 

arrangements. In one common configuration used by many manufacturers, the fibers are 

bundled together longitudinally, potted in a resin on both ends, and encased in a pressure vessel 

that is included as a part of the hollow-fiber module. These modules are typically mounted 

vertically, although horizontal mounting may also be utilized. One alternate configuration is 

similar to spiral-wound modules in that both are inserted into pressure vessels that are 

independent of the module itself. These modules (and the associated pressure vessels) are 

mounted horizontally. Another configuration in which the bundled hollow fibers are mounted 

vertically and submerged in a basin does not utilize a pressure vessel. A typical commercially 

available hollow-fiber module may consist of several hundred to over 10,000 fibers. Although 

specific dimensions vary by manufacturer, approximate ranges for hollow-fiber construction are 

as follows:

• Outside diameter: 0.5 – 2.0 mm

• Inside diameter: 0.3 – 1.0 mm

• Fiber wall thickness: 0.1 – 0.6 mm

• Fiber length: 1 – 2 meters

A cross section of a symmetric hollow-fiber is shown in Figure 2.4.

Hollow-fiber membrane modules may operate in either an “inside-out” or “outside-in” 

mode. In inside-out mode, the feed water enters the fiber lumen (i.e., center or bore of the fiber) 

and is filtered radially through the fiber wall. The filtrate is then collected from outside of the 

fiber. During outside-in operation, the feed water passes from outside the fiber through the fiber 

wall to the inside, where the filtrate is collected in the lumen. Although inside-out mode utilizes 

a well-defined feed flow path that is advantageous when operating under a crossflow hydraulic 

configuration (see section 2.5), the membrane is somewhat more subject to plugging as a result 

of the potential for the lumen to become clogged. The outside-in mode, utilizes a less well-
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defined flow feed flow path, but increases the available membrane surface area for filtration per 

fiber and avoids potential problems with clogging of the lumen bore. 

Figure 2.4 Hollow Fiber Cross-Section Photomicrograph

Both the inside-out and outside-in operating modes for hollow-fiber modules are 

illustrated in Figure 2.5. When a hollow-fiber module is operated in an inside-out mode, the 

pressurized feed water may enter the fiber lumen at one or both ends of the module, with the 

filtrate exiting in the center or end(s). In outside-in mode, the feed water typically enters the 

module in the center at the ends and is filtered into the fiber lumen, where the filtrate collects 

before exiting at the end(s). Most hollow-fiber systems operate in “dead-end” or direct filtration 

mode (see section 2.5) and are periodically backwashed to remove the accumulated solids.

2.3.2.2 Spiral-Wound Modules

Spiral-wound modules were developed as an efficient configuration for the use of semipermeable membranes to remove dissolved solids, and thus are most often associated with 

NF/RO processes. The basic unit of a spiral-wound module is a sandwich arrangement of flat 

membrane sheets called a leaf wound around a central, perforated tube. One leaf consists of two 

membrane sheets placed back to back and separated by a fabric spacer called a permeate carrier. 

The layers of the leaf are glued along three edges, while the unglued edge is sealed around the 

perforated central tube. A single spiral-wound module 8 inches in diameter may contain up to 

approximately 20 leaves, each separated by a layer of plastic mesh called a spacer that serves as 

the feed water channel.

Feed water enters the spacer channels at the end of the spiral-wound element in a path 

parallel to the central tube. As the feed water flows across the membrane surface through the 

spacers, a portion permeates through either of the two surrounding membrane layers and into the 

permeate carrier, leaving behind any dissolved and particulate contaminants that are rejected by 

the semi-permeable membrane. The filtered water in the permeate carrier travels spirally inward 

around the element toward the central collector tube, while the water in the feed spacer that does 

not permeate through the membrane layer continues to flow across the membrane surface, 

becoming increasingly concentrated in rejected contaminants. This concentrate stream exits the 
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element parallel to the central tube through the opposite end from which the feed water entered. 

A diagram of a spiral-wound element is shown in Figure 2.6.

Figure 2.5 Inside-Out and Outside-In Modes of Operation

Inside-Out Hollow-Fiber Configuration Outside-In Hollow-Fiber Configuration

Figure 2.6 Spiral-Wound Membrane Module

Permeate

Feed

Sealed Around 

Outside Edge

Membrane(s)

Permeate Carrier 

Glue Line(s)

Feed/Reject Spacer

Concentrate
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Spiral-wound membranes for drinking water treatment are commercially available in a 

variety of sizes. Modules that are either 4 or 8 inches in diameter and either 40 or 60 inches long 

are most common, although other sizes may be used. Some bench- and pilot-scale applications 

utilize modules that are 2.5 inches in diameter, while modules up to 16 inches in diameter or 

more may be used in full-scale facilities.

2.3.2.3 Membrane Cartridges

Under the LT2ESWTR, cartridge filters that meet the criteria specified in section 2.2 

would be eligible to receive Cryptosporidium removal credit as a membrane filtration process, 

even though cartridge filtration has not traditionally been considered a membrane process. In 

this case, the cartridge filter element would constitute a membrane module for the purposes of 

the rule. The ability of these modules to be subjected to direct integrity testing in the field during 

the course of normal operation, a feature that has not been widely utilized in association with 

cartridge filters in municipal water treatment applications, is a critical aspect of these systems 

that distinguishes what is considered to be MCF under the LT2ESWTR.

Membrane cartridge filters are manufactured by placing flat sheet membrane media 

between a feed and filtrate support layer and pleating the assembly to increase the membrane 

surface area within the cartridge. The pleat pack assembly is then placed around a center core 

with a corresponding outer cage and subsequently sealed, via adhesive or thermal means, into its 

cartridge configuration. End adapters, typically designed with a double o-ring sealing 

mechanism, are attached to the filter to provide a positive seal with the filter housing. A diagram 

of membrane cartridge filter is shown in Figure 2.7.

Figure 2.7 Membrane Cartridge Filter
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Proposal Draft

Most membrane cartridge filters are manufactured as disposable components that are 

inserted into a housing. Once the filter fouls to the point at which the maximum transmembrane 

pressure (TMP) is reached, the cartridge is replaced. Because the cartridges are designed to be 

disposable, and thus relatively inexpensive to replace, cartridge filtration systems have not 

historically utilized backwashing or chemical cleaning. However, some systems that feature 

these processes have recently been introduced. Cartridge filters are available in various sizes and 

pore sizes, although the device would have to be constructed using a non-fibrous membrane 

barrier and be capable of filtering particulate matter larger than 1 μm to be considered a 

membrane filter under the LT2ESWTR.

2.3.2.4 Other Module Configurations

In addition to hollow-fiber and spiral-wound modules, there are several other types of 

less common configurations that may be used in membrane filtration systems. These 

configurations include hollow-fine-fiber (HFF), tubular, and plate-and-frame type modules. 

While these configurations are seldom employed in membrane filtration systems applied for 

drinking water treatment, each of these modules could be applied for LT2ESWTR compliance, 

and thus are briefly described in this section.

Semi-permeable HFF membranes were the original hollow-fiber type membranes and 

were developed for desalting (i.e., RO) applications. With the development of widely used 

porous hollow-fiber MF and UF membranes for particulate filtration with much larger fiber 

diameters, the semi-permeable variety gradually became known as hollow-fine-fiber membranes. 

HFF membranes are bundled length-wise and shaped into a “U” arrangement (called a “Utube”), which is potted in a cylindrical pressure vessel. Feed water enters a HFF module via a 

perforated tube in the middle of the vessel and flows radially outward through the membrane 

bundle. The water that permeates the membrane is collected in the fiber lumen and exits the 

element at the open end of the U-tube. The remaining water that does not permeate into the fiber 

lumen carries the concentrated salts and suspended solids out of the pressure vessel through the 

concentrate port. Typical hollow-fine fibers are only about 40 μm in diameter (inside), allowing 

a very large number of fibers to be contained in a single pressure vessel and maximizing the 

available membrane surface area per unit volume in the pressure vessel. However, the high 

packing density also significantly inc reases the potential for fouling and reduces the flux to 

levels well below those possible using spiral-wound membranes, more than offsetting the 

advantage in increased surface area. HFF membranes are most commonly used today in 

seawater desalting applications, particularly in the Middle East.

Tubular membranes are essentially a larger, more rigid version of hollow-fiber 

membranes. With diameters as large as 1-2 inches, the tubes are not prone to clogging and the 

membrane material (i.e., the tube wall) is comparatively easy to clean. However, the large tubes 

also result in a very inefficient amount of membrane surface area per unit volume in the pressure 

vessel. Both porous (for MF/UF) and semi-permeable (for NF/RO) membranes have been 

manufactured in tubular configurations. Ceramics have been considered as non-traditional 

material for tubular MF/UF membranes, although there are currently no commercially promoted 

ceramic MF/UF systems for drinking water applications.
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A plate-and-frame configuration, one of the earliest membrane modules developed, is 

simply a series of flat sheet membranes separated by alternating filtrate spacers and 

feed/concentrate spacers. Because of the very low surface area to volume ratio, the plate-andframe configuration is considered inefficient and is therefore seldom used in drinking water 

applications. One notable exception (although not a membrane filtration process under the 

LT2ESWTR) is in the case of EDR systems, which utilize a design that lends itself well to the 

use of plate-and-frame membranes.

2.3.3 Types of Membrane Filtration Systems

In drinking water treatment applications, each of the four traditional types of pressuredriven membrane processes (i.e., MF, UF, NF, and RO) is generally associated with a single type 

of membrane filtration system that is designed around a specific type of module. MF and UF 

systems typically utilize hollow-fiber modules, while NF and RO system typically utilize spiralwound modules. An overview of each of the two types of systems that utilize these respective 

modules is provided in the following subsections. Because the concept of a MCF system as 

defined under the LT2ESWTR is a new concept introduced with the rule, a standard type of 

MCF system has not yet been developed. 

2.3.3.1 Hollow-Fiber (MF/UF) Systems

With few exceptions, most MF/UF processes utilize systems designed around hollowfiber modules. Hollow-fiber membrane filtration systems are designed and constructed in one or 

more discrete water production units, also called racks, trains, or skids. A unit consists of a 

number of membrane modules that share feed and filtrate valving, and the individual units can 

usually be isolated from the rest of the system for testing, cleaning, or repair. A typical hollowfiber system is composed of a number of identical units that combine to produce the total filtrate 

flow.

Most of the currently available hollow-fiber membrane systems are proprietary, such that 

a single supplier will manufacture the entire filtration system, including the membranes, piping, 

appurtenances, control system, and other features. The manufacturer also determines the 

hydraulic configuration and designs the associated operational sub-processes – such as 

backwashing, chemical cleaning, and integrity testing – that are specific to its particular system. 

As a result, there are significant differences in the proprietary hollow-fiber membrane systems 

produced by the various manufacturers, and the membranes and other components are not 

interchangeable.

Although each manufacturer’s system is distinct, all of the hollow-fiber membrane 

systems fall into one of two categories – pressure-driven or vacuum-driven – according to the 

driving force for operation. In a pressure-driven system, pressurized feed water is piped directly 

to the membrane unit, where it enters the module and is filtered through the membrane. Typical 

operating pressures for range from 3 to 40 psi. Most applications require designated feed pumps 
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Proposal Draft

to generate the required operating pressure, although there are some water treatment plants that 

take advantage of favorable hydraulic conditions to operate a MF or UF via gravity flow.

A schematic of a typical pressure-driven hollow-fiber membrane filtration system is 

shown in Figure 2.8. In the example shown, the system is operated in a “dead-end” hydraulic 

configuration (see section 2.5) and uses a liquid backwash. 

Figure 2.8 Schematic of a Typical Pressure-Driven Hollow-Fiber (MF/UF) System

While all hollow-fiber systems employ pressure as a fundamental driving force, a 

vacuum-driven system is distinguished by its utilization of negative pressure and, consequently, 

its significantly different design and configuration. Unlike the pressure-driven systems, in which 

each membrane module incorporates a pressure vessel, vacuum-driven systems utilize hollowfiber modules that are “submerged” or “immersed” in an open tank or basin. While the ends are 

fixed, the lengths of the hollow-fibers are exposed to the feed water in the basin.

Because the feed water is contained in an open basin, the outside of the fibers cannot be 

pressurized above the static head in the tank. Therefore, a vacuum of approximately -3 to -12 psi 

is induced at the inside of the fibers via pump suction. The water in the tank is drawn through 

the fiber walls, where it is filtered into the lumen. By design, vacuum-driven membrane 

filtration systems cannot be operated via gravity nor in an inside-out mode. However, a 

favorable hydraulic gradient might enable the use of a gravity-based siphon to generate the 

suction required to drive the filtration process in a vacuum-driven system. In some cases with a 

substantial hydraulic gradient, the large amount of available head could be used to generate the 

power for suction pumps via on-site turbines.

A representative schematic of a vacuum-driven system is shown in Figure 2.9. In the 

example shown, the membrane process may be designed with either continuous (Option A) or 

intermittent discharge (Option B) of concentrated waste.
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Figure 2.9 Schematic of a Typical Vacuum-Driven Hollow-Fiber (MF/UF) System

2.3.3.2 Spiral-Wound (NF/RO) Systems

Virtually all NF and RO membrane processes applied for potable water treatment in the 

United States utilize systems designed for spiral-wound membrane modules. Although some MF 

and UF membranes may also be manufactured as spiral-wound modules, these are seldom used 

in municipal drinking water applications. Consequently, the discussion in this section is focused 

on NF/RO spiral-wound systems.

In a spiral-wound membrane filtration system, the spiral-wound modules are contained in 

a pressure vessel that is independent of the module itself. Typically, a single pressure vessel 

houses six or seven modules, although vessels that accommodate other numbers of modules can 

be custom manufactured. The modules are arranged in series in the pressure vessel such that the 

concentrate from each preceding element represents the feed water for the next. A brine seal 

around the outside of the feed end of each element separates the feed water from the concentrate 

and prevents the feed water from bypassing the element. Although the recovery for a single 

NF/RO module is typically less than 15 percent, the cumulative recovery associated with a sixmodule pressure vessel may be 50 percent or more. A diagram of a typical pressure vessel 

containing spiral-wound modules is shown in Figure 2.10.
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Figure 2.10 Typical Spiral-Wound (NF/RO) Module Pressure Vessel

A group of pressure vessels operating in parallel collectively represent a single stage of 

treatment in a NF/RO spiral-wound system. The total system recovery is increased by 

incorporating multiple stages of treatment in series, such that the combined concentrate (or 

reject) from the first stage becomes the feed for the second stage. In some cases in which higher 

recovery is an objective, a third stage may also be used. This configuration is sometimes 

referred to as “concentrate staging.” Because some fraction of the feed to the first stage has been 

collected as filtrate (or permeate), the feed flow to the second stage will be reduced by that 

fraction. As a result, the number of total pressure vessels (and hence the number of modules) in 

the second stage is also typically reduced by approximately that same fraction. Similar flow, 

module, and pressure vessel reductions are propagated through all successive stages, as well. 

Although the potential system recovery is a function of the feed water quality, as a rough 

approximation, a two-stage design may allow recoveries up to 75 percent, while the addition of a 

third stage can potentially achieve recoveries up to 90 percent.

Although concentrate staging is most often used in drinking water applications, another 

arrangement called “permeate staging” may also be employed. In this configuration the filtrate 

(or permeate) from a stage (rather than the concentrate) becomes the feed water for the 

subsequent stage. This arrangement is more commonly employed in ultra-pure water 

applications (typically in industry), it may also be used for drinking water treatment when the 

source water salinity is very high, such as with seawater desalination. In these cases, the product 

water must pass through multiple stages to remove a sufficient amount of salinity to make the 

water potable quality.

The combination of two or more stages in series is called an array, which is identified by 

the ratio of pressure vessels in the sequential stages. An array may be defined by the ratio of 

either the actual number or relative number of pressure vessels in each stage. For example, a 

32:16:8 array expressed as the actual number of pressure vessels) may be alternatively called a 

4:2:1 array in relative terms. Two-stage arrays, such as 2:1 and 3:2 (relative), are most common 

Feed

Concentrate

Permeate

Head

End Adapter Spiral-Wound Module

O-rings Interconnector Brine Seal

Pressure Vessel

Retaining Ring

Head Seal

Thrust Cone
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Proposal Draft

in drinking water treatment, although the specific array required for a particular application is 

dictated in part by the feed water quality and targeted overall system recovery. Figure 2.11 

illustrates the configuration of a typical 2:1 (relative) array, showing both plan and endperspective views.

Figure 2.11 Typical 2:1 (Relative) Array of Pressure Vessels

As with hollow-fiber systems, spiral-wound membrane systems are designed and 

constructed in discrete units that share common valving and which can be isolated as a group for 

testing, cleaning, or repair. For spiral-wound systems these uniform units are typically called 

trains, or alternatively racks or skids. NF and RO treatment processes consist of one or more 

trains that are typically sized to accommodate a feed flow of up to about 5 MGD per train. A 

schematic of a typical NF/RO system is shown in Figure 2.12.

Unlike hollow-fiber systems, spiral-wound membrane filtration systems are not 

manufactured as proprietary equipment. With the exception of the membrane modules, spiralwound systems are generally custom-designed by an engineer or an original equipment 

manufacturer (OEM) to suit a particular application. Although the membrane modules are 

proprietary, standard-sized spiral-wound NF/RO modules share the same basic construction, and 

thus membranes from one manufacturer are typically interchangeable with those from others.

Feed

Permeate

Concentrate

Concentrate Feed

Plan View End-Perspective View

Stage 1:

24 Vessels

144 Modules

Stage 2:

12 Vessels

72 Modules
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Figure 2.12 Schematic of a Typical Spiral-Wound (NF/RO) System

2.4 Basic Principles of Membrane Filtration System Design and Operation

Familiarity with the basic principles underlying membrane filtration system design and 

operation is important to the comprehension and interpretation of the material presented in this 

guidance manual. The material presented in this subsection is intended to provide an overview 

of these basic principles. Although all of the types of membrane filtration addressed in this 

manual (i.e., MF, UF, NF, RO, and MCF) utilize pressure (or vacuum) as a driving force, there 

are fundamental differences in the models used to describe systems using porous membranes 

(MF, UF, and MCF) and semi-permeable membranes (NF and RO). The basic principles of 

these respective models, along with some general concepts that are applicable to all membrane 

filtration systems, are discussed in the following subsections. Note that these discussions are 

intended to be informative for the purposes of fostering an understanding the technology and not 

a specific guide for system design and operation.

2.4.1 General Concepts

There are a number of general concepts that are applicable to all types of pressure-driven 

membrane filtration systems and which serve as the underlying basic principles for system 

design and operation. These concepts include flux, recovery, and flow balance, each of which is 

discussed in general terms below. Additional concepts that are specific to either MF, UF, and 

MCF systems or NF and RO systems are discussed in sections 2.4.2 and 2.4.3, respectively.
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Membrane filtration system throughput or productivity is typically characterized by the 

system flux, which is defined as the filtrate flow per unit of membrane filtration area, as shown 

in Equation2.1:

m

p

A

Q

J = Equation 2.1

Where: J = flux (gfd)

Qp = filtrate flow (gpd)

Am = membrane surface area (ft2)

The recovery of a membrane unit, is defined under the LT2ESWTR as the amount of feed 

flow that is converted to filtrate flow, expressed as a decimal percent, as shown in Equation 2.2:

f

p

Q

Q

R = Equation 2.2 

Where: R = recovery of the membrane unit (decimal percent)

Qp = filtrate flow produced by the membrane unit (gpd)

Qf = feed flow to the membrane unit (gpd)

The recovery as defined under the rule does not account for the use of filtrate for routine 

maintenance purposes (such as chemical cleaning or backwashing) or lost production during 

these maintenance operations. Because the definition of recovery is not necessarily consistent 

throughout the water treatment industry, it is important to identify how recovery is defined in any 

particular discussion. However, the use of the term recovery as defined in Equation 2.2 is 

consistent throughout the LT2ESWTR rule language and the Membrane Filtration Guidance 

Manual. Note that for some types of membrane systems, particularly those that operate in 

suspension mode that can be modeled as plug flow reactors (see section 2.5), recovery can also 

be defined as a function of position within a membrane unit. This is simply a variation of 

Equation 2.2 for systems in which the filtrate flow increases in the direction of feed flow through 

the membrane unit, thus increasing the recovery in direct proportion. The limit of this recovery 

in the direction of flow (i.e., the recovery at the furthest position in the unit) is equivalent to the 

overall membrane unit recovery, as defined in Equation 2.2.
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A general flow balance that can be applied to all membrane filtration systems is shown in 

Equation 2.3:

Qf = Qc + Qp Equation 2.3

Where: Qf = feed flow to the membrane unit (gpd)

Qc = concentrate flow from the membrane unit (gpd)

Qp = filtrate flow from the membrane unit (gpd)

Note that the concentrate (i.e., bleed or reject) flow, Qc, is zero for systems operating in 

deposition (i.e., dead-end) mode, as discussed in section 2.5. For the purpose of sizing a 

membrane filtration system, it may be desirable to account for the additional filtered water used 

for both backwashing and chemical cleaning in the determination of the filtrate flow, Qp. 

Similarly, an estimate of the total required feed flow Qf to the system should incorporate any raw 

water that may be used in these routine maintenance processes.

2.4.2 MF, UF, and MCF Processes

The driving force for the transport of water across a microporous membrane – that 

utilized by MF, UF, and MCF processes – is a pressure gradient across the membrane, or the 

TMP. The TMP is defined by the pressure on the feed side of the membrane minus the filtrate 

pressure, commonly called the backpressure, as shown in Equation 2.4:

TMP = Pf − Pp Equation 2.4

Where: TMP = transmembrane pressure (psi)

Pf = feed pressure (psi)

Pp = filtrate pressure (i.e., backpressure) (psi)

For systems that operate in suspension mode and thus utilize a concentrate stream that is 

wasted or recirculated (as described in section 2.5), the pressure on the feed side of the 

membrane is not constant, but can instead be approximated by a linear pressure gradient from the 

feed inlet to the concentrate outlet. In this case, the pressure on the feed side of the membrane 

may be represented by the average of the feed and concentrate pressures, as shown in 

Equation 2.5:
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p

f c

P

P P

TMP −

+

=

2

Equation 2.5

Where: TMP = transmembrane pressure (psi)

Pf = feed pressure (psi)

Pc = concentrate pressure (psi)

Pp = filtrate pressure (i.e., backpressure) (psi)

The resistance to flow acting in opposition to the driving force and inhibiting the 

transport of water across the membrane can also be quantified. This resistance has two 

components: the intrinsic resistance of the membrane and the resistance attributable to the 

accumulated foulant layer at the membrane surface at any point during operation. The total 

resistance is represented by the sum of these two components, as shown in Equation 2.6:

Rt = Rm + Rf Equation 2.6

Where: Rt = total membrane resistance (psi/gfd-cp)

 Rm = intrinsic membrane resistance (psi/gfd-cp)

 Rf = resistance of the foulant layer (psi/gfd-cp)

While the intrinsic resistance of the membrane should remain constant for all practical purposes 

and can generally be obtain from the membrane manufacturer (if necessary), the increase in 

fouling during normal operation and the decrease in fouling as a result of backwashing and 

chemical cleaning causes the fouling resistance to fluctuate.

If the total membrane resistance is known, the flux can be calculated as a function of the 

TMP and water viscosity, as shown in Equation 2.7:

t T

T

R

TMP J

• m

= Equation 2.7

Where: JT = flux at temperature T (gfd) 

TMP = transmembrane pressure (psi)

Rt = total membrane resistance (psi/gfd-cp)

μT = viscosity of water at temperature T (cp) 

Because the viscosity of water increases with decreasing temperature, larger TMPs (by 

application of increased pressure or vacuum) are required to maintain constant flux. Values for 

water viscosity can be found in the literature or approximated using the empirical relationship 

expressed in Equation 2.8:
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( ) ( ) ( )

2 5 3

T =1.784 − 0.0575 •T + 0.0011• T − 10 • T

−

m Equation 2.8 

Where: μT = viscosity of water at temperature T (cp)

T = water temperature (oC)

Since water temperature can have a significant impact on flux, it is common practice to 

“normalize” the flux to a reference temperature during operation for the purposes of monitoring 

system productivity independent of changes in water temperature. For MF/UF and MCF 

processes, a reference temperature of 20 oC is typically used for convenience, since the viscosity 

of water is approximately 1 cp at 20 oC. For constant TMP and total membrane resistance, a 

form of Equation 2.7 can be used to illustrate the relationship between the normalized flux and 

viscosity at 20 oC and the actual flux and viscosity at a given temperature of interest T, as shown 

in Equation 2.9:

T T

J 20 • m20 = J • m Equation 2.9

Where: J20 = normalized flux at 20 oC (gfd)

μ20 = viscosity of water at 20 oC (cp)

JT = actual flux at temperature T (gfd)

μT = viscosity of water at temperature T (cp)

Substituting the value of 1 cp for the viscosity at 20 oC (μ20) and Equation 2.5 for the 

viscosity of water at temperature T (μT) yields and expression for normalized flux at 20 oC as a 

function of the actual flux and the temperature, as shown in Equation 2.10:

[ ( ) ( ) ( )]

2 5 3

20 J = J T

• 1.784 − 0.0575 • T + 0.0011 •T − 10 • T

− Equation 2.10

Where: J20 = normalized flux at 20 oC (gfd)

JT = actual flux at temperature T (gfd)

T = water temperature (oC)

It is important to note that the normalized flux (J20) does not represent an actual operating 

condition. This term simply represents what the flux would be at 20 oC for a constant TMP and 

total membrane resistance. Thus, changes in the value of J20 during the course of normal 

operation are indicative of changes in pressure and/or membrane resistance due to fouling. If 

values for viscosity are known, the polynomial expression for viscosity as a function of 

temperature in Equation 2.10 may be simplified to a temperature correction factor (TCF). For a 
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MF, UF, or MCF process, the TCF is defined as the ratio of the viscosity at temperature T to the 

viscosity at 20 oC, as shown in Equation 2.11:

m20

mT

TCF = Equation 2.11

Where: TCF = temperature correction factor (dimensionless)

μT = viscosity of water at temperature T (cp)

μ20 = viscosity of water at 20 oC (cp)

Note that the term TCF is often used generically to refer to any type correction factor used to 

adjust a parameter for temperature. Thus, the specific equation for the TCF may vary depending 

on the parameter to which it is applied. For example, in the context of membrane filtration, the 

TCF applied to reference MF, UF, and MCF flux to a standard temperature, as defined in 

Equation 2.11, is different than that applied to NF and RO flux to a standard temperature, as 

shown in Equation 2.15. Thus, it is important to always consider the context in which the term 

TCF is used.

Because the TCF is a dimensionless ratio, the values for viscosity can be expressed in any 

convenient and consistent units. Thus, the temperature-normalized flux can be expressed in 

simplified as terms, as shown in Equation 2.12:

J 20 = J T•TCF Equation 2.12

Where: J20 = normalized flux at 20 oC (gfd)

JT = actual flux at temperature T (gfd)

TCF = temperature correction factor (dimensionless)

Generally, in order to identify changes in productivity (as measured by flux) that are 

specifically attributable to membrane fouling, it is desirable to normalize the flux for pressure as 

well as temperature, as shown in Equation 2.13. Note that the temperature- and pressurenormalized flux is often referred to as either the specific flux or permeability.

TMP

J

M

20

= Equation 2.13

Where: M = temperature- and pressure-normalized flux (gfd/psi)

J20 = normalized flux at 20 oC (gfd)

TMP = transmembrane pressure (psi)
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2.4.3 NF and RO Processes

As with the microporous MF, UF, and MCF membranes, the driving force for the 

transport of water across a semi-permeable membrane – such as that utilized by NF and RO 

processes – is a pressure gradient across the membrane. However, because NF and RO 

processes reject dissolved salts, the resulting osmotic pressure gradient, which acts against the 

transport of water from the feed to the filtrate side of the membrane, must also be taken into 

account. Typically, the osmotic pressure gradient is approximated from the concentration of 

total dissolved solids (TDS) on the feed and filtrate sides of the membrane. The corrected 

driving force across semi-permeable membrane is termed the net driving pressure (NDP) and can 

be calculated using Equation 2.14 (AWWA 1999):

( ) p p

f c f c

P

mg L

psi TDS

P P TDS TDS

NDP −

















•

















−















 +

−















 +

=

/

0.01

2 2

Equation 2.14

Where: NDP = net driving pressure (psi)

Pf = feed pressure (psi)

Pc = concentrate pressure (psi)

TDSf = feed TDS concentration (mg/L) 

TDSc = concentrate TDS concentration (mg/L)

TDSp = filtrate TDS concentration (mg/L)

Pp = filtrate pressure (i.e., backpressure) (psi)

Equation 2.14 can be considered as three distinct components, each shown above in 

parentheses. The first term represents the average pressure on the feed side of the membrane; the 

second term represents the average osmotic backpressure; and the third term represents the 

filtrate backpressure. The conversion factor of 0.01 in the osmotic pressure term comes from a 

widely used rule of thumb for fresh and brackish waters indicating that there is approximately 

1 psi of osmotic pressure for every 100 mg/L of TDS, as discussed in section 2.2.2. In many 

cases the filtrate TDS concentration (TDSp) is small and can be neglected.

While the flux associated with MF, UF, and MCF systems is typically referenced to a 

temperature of 20 oC for the purposes of assessing operational performance, it is common to 

reference the flux associated with NF and RO systems to 25 oC (298 K). Accordingly, the 

appropriate TCF for NF and RO systems is shown in Equation 2.15:
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Proposal Draft

























−

+

= •

298

1

273

1

exp

T

TCF U Equation 2.15

Where: TCF = temperature correction factor (dimensionless)

T = water temperature (oC)

U = membrane-specific manufacturer-supplied constant (1/K)

Once the TCF has been determined, the flux normalized to 25 oC can be calculated 

according to Equation 2.16:

J J (TCF) T = • 25 Equation 2.16

Where: J25 = normalized flux at 25 oC (gfd)

JT = actual flux at temperature T (gfd)

TCF = temperature correction factor (dimensionless)

As with MF, UF, and MCF systems, it is important to note that the normalized flux (J25) 

for NF and RO systems does not represent an actual operating condition. This term simply 

represents what the flux would be at 25 oC for the purposes of comparing membrane 

performance independent of temperature-related affects. Similarly, it is also common to 

normalize the flux for pressure in order to identify changes in productivity that are attributable to 

fouling, as shown in Equation 2.17:

NDP

J

M

25

= Equation 2.17

Where: M = temperature- and pressure-normalized flux (gfd/psi)

J25 = normalized flux at 25 oC (gfd)

NDP = net driving pressure (psi)

2.5 Hydraulic Configurations 

The term hydraulic configuration is used to describe the manner in which the feed water 

and associated suspended solids are is processed by a membrane filtration system. Although 

there are a number of different hydraulic configurations in which the various membrane filtration 

systems can operate, each of these configurations can be categorized into one of two basic modes 

of operation: deposition mode and suspension mode. The hydraulic configuration of a system is 

determined from operational conditions such backwash, concentrate flow, and recycle flow, 

where applicable.
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For the purposes of the LT2ESWTR, one of the most important implications of a 

system’s hydraulic configuration is its impact on the degree to which suspended solids are 

concentrated on the feed side of the membrane. This concentration effect is characterized by the 

volumetric concentration factor (VCF), a dimensionless parameter representing the ratio of the 

concentration of suspended solids on the feed side of the membrane relative to that of the 

influent feed to the membrane filtration process, as shown in Equation 2.18:

f

m

C

C

VCF = Equation 2.18

Where: VCF = volumetric concentration factor (dimensionless)

Cm = concentration of suspended solids maintained on the feed 

side of the membrane (number or mass / volume)

Cf = concentration of suspended solids in the influent feed water 

to the membrane system (number or mass / volume)

By definition, the VCF is equal to 1 for a system that does not concentrate suspended 

solids on the feed side of the membrane (i.e., Cm = Cf); these are defined as deposition mode 

systems. However, some hydraulic configurations concentrate suspended solids on the feed side 

of the membrane to degrees much greater than the influent feed concentration, with a 

corresponding VCF greater than 1; these are the suspension mode systems. Consequently, the 

VCF can significantly affect the quantity of particulate matter that can flow through an integrity 

breach, and thus must be considered in the determination of direct integrity test method 

sensitivity, as discussed in section 4.3 (40 CFR 141, Subpart W, Appendix C). For example, if a 

system concentrates particula te matter by a factor of 10 (i.e., VCF = 10), then the direct integrity 

test used to monitor the system must be ten times more sensitive than a test applied to a system 

that does not concentrate particulate matter (i.e., VCF = 1), in order to demonstrate the same log 

removal value (LRV). 

The primary purposes of this section are to describe the various hydraulic configurations 

in which membrane filtration systems can operate and present the associated equations used to 

determine the respective VCFs. The discussion of the various hydraulic configurations in this 

section is predicated on three basic assumptions:

1. In the absence of a hydraulic force tangential to the membrane surface, particulate 

matter in the feed stream is deposited on the membrane and held in place by the TMP.

2 In the presence of a hydraulic force tangential to the membrane surface, all of the 

particulate matter remains in suspension, resulting in elevated concentrations of 

suspended particulate matter on the feed side of the membrane. This increase in 

concentration is characterized by the VCF, which can vary as a function of position 

and/or time for various hydraulic configurations.

3. The membrane is a complete barrier to the passage of the particulate contaminants.
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-29 June 2003

Proposal Draft

Deposition modes systems and the various types of suspension mode systems are 

discussed in sections 2.5.1 and 2.5.2, respectively. Although the majority of systems utilize one 

of the hydraulic configurations described in this section, there may be unusual cases in which a 

system-specific analysis is necessary to characterize the VCF. For example, some systems that 

are designed to operate in deposition mode may still exhibit some degree of particle suspension, 

and conversely, some degree of particle deposition may occur in a system operating primarily in 

suspension mode. In such cases in which characteristics of both types of hydraulic 

configurations may be observed, the VCF should be calculated using the most conservative 

applicable assumptions that result in the highest anticipated VCF values. (Note that the most 

conservative condition for a particular system is that which results in the highest estimated 

concentration of suspended particulate matter.) As an alternative to the theoretical calculations, 

the VCF may be measured experimentally under realistic operating conditions. This approach 

may be advantageous for systems that may not necessarily be well described by mathematical 

modeling. In addition to both the modeling and experimental approaches, there may be some 

cases in which a manufacturer of a proprietary membrane filtration system has developed a 

system-specific method for determining the VCF.

2.5.1 Deposition Mode

Membrane filtration systems operating in deposition mode utilize no concentrate stream 

such that there is only one influent (i.e., the feed) and one effluent (i.e., the filtrate) stream, as 

shown in the schematic in Figure 2.13. These systems are also commonly call “dead-end” or 

“direct” filtration systems and are analogous to conventional gr anular media filters in terms of 

hydraulic configuration. In the deposition mode of operation, contaminants suspended in the 

feed stream accumulate on the membrane surface and are held in place by hydraulic forces acting 

perpendicular to the membrane, forming a cake layer, as illustrated in Figure 2.14.

Figure 2.13: Schematic of a System Operating in Deposition Mode
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-30 June 2003

Proposal Draft

Figure 2.14 Conceptual Illustration of Deposition Mode Operation

In a deposition mode hydraulic configuration, the concentration of suspended material on 

the feed side of the membrane (Cm) is assumed to be equivalent to the concentration of influent 

feed stream (Cf), independent of time or position in the membrane system, as the suspended 

contaminants are removed from the process stream and deposited in the accumulated cake layer. 

Therefore, all systems operating in deposition mode have a VCF equal to one. MCF and most 

hollow-fiber MF and UF systems operate in deposition mode. Typically, the accumulated solids 

are removed from MF/UF systems by backwashing, while most MCF systems simply operate 

until the accumulated solids reduce the flow and/or TMP to an unacceptable level, at which point 

the membrane cartridge is replaced.

Some MF/UF systems utilize a periodic “backpulse” – a short interval of reverse flow 

(which may include air and/or the addition of small doses of oxidants) designed to dislodge 

particles from the membrane surface without removing these solids from the system. This 

process re-suspends particles, effectively concentrating the suspended solids in the feed near the 

membrane surface and increasing the potential for pathogens or other particulate to pass through 

an integrity breach and contaminate the filtrate. Consequently, systems that do not utilize a 

concentrate stream but still practice backpulsing may be more appropriately and conservatively 

modeled as operating in suspension mode.
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-31 June 2003

Proposal Draft

2.5.2 Suspension Mode

In membrane filtration systems that operate in suspension mode, a scouring force using 

water and/or air is applied parallel (i.e., tangential) to the membrane surface during the 

production of filtrate in a continuous or intermittent manner, as illustrated in Figure 2.15. The 

objective of operating in this mode is to minimize the accumulation of contaminants at the 

membrane surface or boundary layer, thus reducing fouling. As shown in Equation 2.18, the 

VCF quantifies the increase in the feed side concentration of suspended solids relative to that of 

the influent feed stream that occurs in a suspension mode of operation. 

Figure 2.15 Conceptual Illustration of Suspension Mode Operation

The three most common suspension mode hydraulic configurations are the plug flow 

reactor (PFR) model, the crossflow model, and the continuous stirred tank reactor (CSTR) 

model. Systems operating under a crossflow hydraulic configuration may be further categorized 

as either small volume or large volume systems, since the volume affects the manner in which 

suspended solids are concentrated in such a system. Depending on the particular hydraulic 

configuration, the VCF may vary temporally or spatially. In a PFR, the VCF increases in the 

direction of feed flow as a function of position in the system; however, in a CSTR or crossflow 

reactor, the VCF increases with time over the course of a filtration cycle. Methods for 

calculating both the average and maximum VCF are presented for PFR, crossflow, and CSTR 

systems are discussed in the following subsections.
Chapter 2 – Overview of Membrane Filtration

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Proposal Draft

2.5.2.1 Plug Flow Reactor Model

Membrane filtration systems that operate as PFRs have a VCF that varies as a function of 

position in the system. Examples of such systems include spiral-wound NF/RO systems and 

vacuum-driven MF/UF systems submerged in tanks with large length-to-width ratios. The 

concentration profile of these two types of PFR systems is illustrated in Figure 2.16. 

Figure 2.16 Flow Diagram for a Plug Flow Reactor
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Proposal Draft

The concentration of suspended solids on the feed side of the membrane in a PFR can be 

expressed as a function of the recovery at any position within the system, as shown in 

Equation 2.19:

( )

R(x)

C

C x

f

m

−

=

1

Equation 2.19

Where: Cm(x) = concentration on the feed side of the membrane at position x

in the membrane unit (number or mass / volume)

x = position in the membrane unit in the direction of tangential 

flow (i.e., x = 0 at the entrance to the first module)

Cf = feed concentration at the inlet to the membrane unit

(number or mass / volume)

R(x) = recovery as a function of position within the membrane unit 

(decimal percent)

Equations 2.18 and 2.19 can be combined to create an expression for the VCF as a 

function of position in a PFR:

( )

R(x)

VCF x

−

=

1

1

Equation 2.20

Where: VCF(x) = VCF as a function of position the membrane unit 

(dimensionless)

x = position in the membrane unit in the direction of tangential 

flow (i.e., x = 0 at the entrance to the first module)

R(x) = recovery as a function of position within the membrane unit 

(dimensionless decimal)

The recovery in a PFR varies from zero at the inlet to membrane unit to the value of the 

overall unit recovery at the outlet of the tangential flow stream in the membrane unit. 

Accordingly, between these inlet and outlet boundary conditions, the VCF increases from 1.0 at 

the system inlet (where the recovery is equal to zero), to a maximum value (VCFmax) at the 

concentrate outlet of the membrane unit, as shown in Equation 2.21:
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-34 June 2003

Proposal Draft

R

VCF

−

=

1

1

max Equation 2.21

Where VCFmax = maximum VCF (dimensionless)

R = membrane unit recovery (decimal percent)

The value of the VCF yielded by Equation 2.21 represents the largest (and thus most 

conservative) such value in a PFR. Suspended particulate matter is concentrated to this degree 

only at the far end of the unit (in the direction of flow) at the point of the tangential flow stream 

outlet; the rest of the membrane unit may be characterized by a lower VCF. Thus, it may be 

appropriate to consider an average value of the VCF that is more representative of concentrations 

over the membrane unit as a whole.

A simplistic method for estimating the average VCF (VCFavg) is to simply divide the 

maximum VCF by two. However, a more accurate method involves calculating a positionaveraged VCF(x) from the system inlet to the concentrate stream outlet. If a mathematical 

expression for R(x) is known, it can be inserted into Equation 2.20 to yield a known expression 

for the VCF as a function of position (VCF(x)) in a PFR. Subsequently, if this resulting 

expression can be integrated, then the average value function can be applied to determine an 

accurate value for VCFavg, as shown in Equation 2.22:

( )

( )

1

1 max 1

0

max

dx

x R x

VCF

x

avg ∫

−

= • Equation 2.22

Where: VCFavg = average VCF (dimensionless)

xmax = end position of the membrane unit in the direction of 

tangential flow (i.e., at the outlet of the tangential flow 

stream)

R(x) = recovery as a function of position within the membrane unit 

(dimensionless decimal)

x = position in the membrane unit in the direction of tangential 

flow (i.e., x = 0 at the entrance to the first module)

The parameter R(x) in Equation 2.20 is typically a function of pressure, membrane area, 

and flow, all of which may vary as a function of position within the membrane unit. For 

example, NF and RO systems experience a decrease in NDP due to pressure losses through the 

system and increasing osmotic pressure gradients. For MF and UF membrane systems operating 

as PFRs, there may also be a flow imbalance across the membrane surface in the direction of 

flow resulting from the increasing concentration of suspended solids, which may increase 

resistance to the flow of water through the membrane (i.e., the filtrate flow) toward the outlet the 

tangential flow stream. In addition, the back-mixing of suspended solids can also create non-
Chapter 2 – Overview of Membrane Filtration

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Proposal Draft

ideal flow and concentration conditions (Cote et al. 2003). However, in general, the VCF 

increases exponentially in the direction of the tangential flow stream in a PFR.

If an expression for the VCF as a function of position (VCF(x)) cannot be developed or if 

the resulting equation cannot be integrated, a flow-weighted average can be computed using 

numerical techniques, as shown in Equation 2.23:

p

p

avg Q

VCF x Q x

VCF ∑ •=

[ ( ) ( )]

Equation 2.23

Where: VCFavg = (flow-weighted) average VCF (dimensionless)

VCF(x) = VCF at discrete position “x” within the membrane unit 

(dimensionless)

x = position in the membrane unit in the direction of tangential 

flow (i.e., x = 0 at the entrance to the first module)

Qp(x) = filtrate flow at position “x” within the membrane unit (gpd)

Qp = filtrate flow (gpd)

The use of Equation 2.23 first requires determination of the filtrate flow over a differential 

membrane area in the unit at a specific location. The resulting filtrate flow at position “x ” 

(Qp(x)) can be used to calculate the recovery at position “x” (R(x)) within the membrane unit 

using Equation 2.2 (see section 2.4). The VCF at position “x” can then be calculated using 

Equation 2.20. These calculation steps are repeated for numerous positions throughout the 

membrane unit, and a flow-weighted average is computed from this data using Equation 2.23. 

Typically, values for the filtrate flow at discrete intervals within a membrane unit may be 

obtained from modeling software available from the membrane module manufacturer, and the 

various calculations can be facilitated by the use of a spreadsheet.

Both the maximum and flow-weighted average VCFs (as calculated using Equations 2.21 

and 2.23, respectively) for a typical NF/RO system over a range of overall systems recoveries 

from 70 to 99 percent are summarized in Table 2.1.

Table 2.1 shows that maximum VCF is substantially higher than the average VCF, 

particularly for higher recoveries. For example, a NF/RO system operating at an overall 

recovery of 85 percent has a maximum VCF approximately three times larger than its average 

VCF. Thus, the concentration of suspended particulate matter at the outlet of the tangential flow 

stream in the membrane unit is similarly three times higher than the average concentration. As a

result, a membrane breach located close to the outlet of the tangential flow stream in membrane 

unit operating as a PFR has the potential to allow a significantly larger concentration of 

contaminants to pass into the filtrate than a breach occurring at a location further upstream.
Chapter 2 – Overview of Membrane Filtration

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Proposal Draft

Table 2.1 Summary of VCF Values for a NF/RO System Modeled as a PFR

VCF VCF VCF Recovery

(percent) Avg.1 Max.

Recovery

(percent) Avg.1 Max.

Recovery

(percent) Avg.1 Max.

70 1.73 3.33 80 2.02 5.0 90 2.56 10.0

71 1.75 3.45 81 2.06 5.26 91 2.65 11.1

72 1.78 3.57 82 2.11 5.56 92 2.75 12.5

73 1.80 3.70 83 2.15 5.88 93 2.87 14.3

74 1.83 3.84 84 2.20 6.25 94 3.00 16.7

75 1.86 4.0 85 2.25 6.67 95 3.16 20.0

76 1.89 4.17 86 2.31 7.14 96 3.37 25.0

77 1.92 4.35 87 2.37 7.69 97 3.63 33.3

78 1.95 4.55 88 2.43 8.33 98 4.01 50

79 1.99 4.76 89 2.49 9.09 99 4.68 100

1 Ideal, theoretical average

If information regarding the filtrate flow as a function of membrane position cannot be 

obtained or approximated, the VCF data presented in Table 2.1 may be used as an estimate for 

NF/RO systems. However, it is strongly recommended that Equation 2.20, 2.22, or 2.23 be used 

to obtain a more accurate, system-specific estimate for the average VCF. Alternatively, the 

maximum VCF (i.e., the most conservative value) can be calculated from the overall recovery 

using Equation 2.21.

2.5.2.2 Crossflow Model

The objective of crossflow filtration is to maintain a high scour velocity across the 

membrane surface to minimize particle deposition and membrane fouling. Crossflow membrane 

processes operate in an unsteady-state manner in which suspended solids accumulate on the feed 

side of the membrane over the course of a filtration cycle. Thus, in crossflow systems, the VCF 

varies as a function of time. At the end of each filtration cycle the membrane unit is backwashed 

to remove the accumulated solids. Crossflow filtration has traditionally been used in conjunction 

with inside-out hollow-fiber membrane processes to increase the scouring velocity in the fiber 

lumen in order to minimize fouling.

For crossflow systems, a portion of the concentrate stream (i.e., the tangential flow) is 

recirculated or recycled to the system inlet and mixed with the incoming feed stream. Because 

the concentrate stream has a higher concentration of suspended solids than the influent feed 

stream to the membrane process, the VCF for crossflow systems is greater than one. Although 

the recycled concentrate stream in a crossflow system may be as small as 10 percent of the 

combined feed flow or less, it is typically 5 to 20 times higher than the influent feed flow. The 
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-37 June 2003

Proposal Draft

manner in which crossflow systems concentrate suspended particulate matter depends on the 

volume of the feed in the membrane process. For simplicity, two variations of the crossflow 

configuration are considered in this discussion: small volume and large volume systems.

Small Volume Crossflow Systems

In a small volume crossflow system, the unfiltered concentrate stream is returned to the 

inlet of the membrane system and blended with the incoming feed after any prefiltration that may 

be incorporated into the treatment process. The line that connects the concentrate outlet to the 

feed inlet is termed the recirculation loop. A schematic of a typical small volume crossflow 

system configuration is shown in Figure 2.17. The small volume crossflow configuration, in 

which the contaminants accumulated during a filtration cycle are removed via backwashing, is 

relatively common among hollow-fiber MF/UF membrane systems.

Figure 2.17 Schematic of a Typical Small Volume Crossflow System

In crossflow systems, the concentration of suspended solids on the feed side of the 

membrane (Cm in Figure 2.17) increases linearly over the filtration cycle. The rate at which the 

concentration increases is a function of the feed flow (Qf in Figure 2.17) and volume of the 

recirculation loop (Vr). As shown in Figure 2.17 this volume would include the volume on the 

feed side of the membrane modules back to the recirculation blending point, as well as the 

recirculation loop piping.

In analyzing such systems, it is useful to define the solids retention time (J), which 

represents the time required for a one-fold increase in the concentration of suspended particulate 

matter on the feed side of the membrane. The solids retention time is calculated as the ratio of 

the total volume of the recirculation loop to the feed flow, as shown in Equation 2.24:
Chapter 2 – Overview of Membrane Filtration

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Proposal Draft

f

r

Q

V

t = Equation 2.24

Where: J = solids retention time (min)

Vr = total volume of the recirculation loop (gallons)

Qf = feed flow (gpm)

The VCF is a function of the solids retention time and the operational time within a 

filtration cycle, as shown in Equation 2.25:

( )

t

t

VCF t = Equation 2.25

Where: VCF(t) = VCF as a function of filtration cycle time (dimensionless)

t = filtration cycle time (min)

J = solids retention time (min)

At the end of the filtration cycle, backwashing is used to flush the accumulated solids 

from the membrane system. Since most crossflow systems are backwashed with filtrate water, 

and the concentration of solids in the filtrate is negligible relative to that of the feed and 

recirculation streams, it can be assumed that the backwash removes all of the solids from the 

entire volume of the recirculation loop. Consequently, the concentration of suspended solids at 

the membrane surface is equal to zero immediately after backwash (i.e., at t = 0, both Cm = 0 and 

VCF = 0). Figure 2.18 illustrates the periodic variation in the concentration of suspended solids 

on the feed side of the membrane for a typical small volume crossflow system with a 20 minute 

filtration cycle.

As shown in Figure 2.18, for a small volume crossflow system, the maximum VCF 

occurs at the end of the filtration cycle (i.e., t = tf) and is calculated according to Equation2.35.

t

f

t

VCFmax = Equation 2.26

Where: VCFmax = maximum VCF (dimensionless)

tf = filtration cycle duration (min)

J = hydraulic detention time (min)
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-39 June 2003

Proposal Draft

Figure 2.18 Concentration Profile in a Small Volume Crossflow System 

Since the VCF increases linearly from zero to VCFmax over the course of a filtration cycle 

in a crossflow system, the average VCF is simply one half of the maximum value, as shown in 

Equation 2.27:

















= •

t

f

avg

t

VCF 0.5 Equation 2.27

Where: VCFavg = average VCF (dimensionless)

tf = filtration cycle duration (min)

J = hydraulic detention time (min)

Note that some small volume crossflow systems may be operated at lower crossflow 

velocities, resulting in incomplete particle suspension. In these cases the mathematical model 

may overestimate the VCF. While this calculated VCF would represent conservative estimate, 

the VCF could also be measured experimentally under realistic operating conditions in order to 

determine a more accurate value.

0

20

40

60

80

100

0 20 40 60 80 100

Filtration Time

(minutes)

C

m/Cm-max

(percent)

Backwash
Chapter 2 – Overview of Membrane Filtration

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Proposal Draft

Large Volume Crossflow Systems

In a large volume crossflow system the concentrate recycle stream is returned to a large 

feed tank, as shown in Figure 2.19, which greatly increases the total volume of the recirculation 

loop (Vr) and changes the manner in which solids accumulate in the membrane system.

Figure 2.19 Schematic of a Typical Large Volume Crossflow System

Like small volume systems, large volume crossflow systems experience a linear increase 

in the solids concentration on the feed side of the membrane over the course of a filtration cycle. 

However, in a large volume system, backwashing does not remove all of the contaminants that 

have accumulated in the recirculation loop and on the feed side of the membrane; thus 

concentration on the feed side of the membrane (Cm) and the VCF do not return to zero after a 

backwash operation. As a result, the solids concentration gradually increases over multiple 

filtration cycles, a phenomenon which must be considered when calculating the VCF. The 

periodic decrease in concentration occurs during a backwash operation when a portion of the 

suspended solids is removed. The suspended particulate matter continues to accumulate over 

successive filtration cycles until a stable process condition is reached, in which the amount of 

solids delivered to the membrane system during a filtration cycle is equal to the amount of solids 

that are discharged during the backwash process. The “saw-tooth pattern” concentration profile 

and the gradual establishment of a stable process condition for a large volume crossflow system 

are illustrated in Figure 2.20.
Chapter 2 – Overview of Membrane Filtration

Membrane Filtration Guidance Manual 2-41 June 2003

Proposal Draft

Figure 2.20 Concentration Profile in a Large Volume Crossflow System

Equation 2.28 describes VCF as a function of time for a large volume crossflow system, 

taking into account both the buildup of solids over a filtration cycle and the partial removal of 

solids during backwash:















 •

• −

−

= +

r

b b

f

m

V

Q t

C

t C t

VCF t 1

( 1)

( )

t

Equation 2.28

Where: VCF(t) = VCF as a function of filtration cycle time (dimensionless)

t = filtration cycle time (min)

J = solids retention time (min)

Cm(t-1) = concentration on the feed side of the membrane 

immediately after the previous backwash operation 

(number or mass / volume)

Cf = feed concentration at the inlet to the membrane unit

(number or mass / volume)

Qb = backwash flow (gpm)

tb = backwash duration (min)

Vr = total volume of the recirculation loop (gallons)

The maximum VCF for a large volume crossflow system can be calculated as the ratio of 

feed volume processed over a filtration cycle to backwash water volume, as shown in 

Equation 2.29:

0

20

40

60

80

100

0 1 2 3 4 5

Hydraulic Detention Time

(minutes)

C

m/Cm-max

(percent)
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Proposal Draft

b b

f f

t Q

t Q

VCF

•

•

max = Equation 2.29

Where: VCFmax = maximum volumetric concentration factor (dimensionless)

tf = filtration cycle duration (min)

Qf = feed flow (gpm)

tb = backwash duration (min)

Qb = backwash flow (gpm)

The average VCF for a large volume crossflow system can be calculated using 

Equation 2.30:

























•

•

• •

• −

•

•

=

b b

f f

f

b b

f f

avg

t Q

t Q

t

t Q

t Q

VCF

2 t

1 Equation 2.30

Where: VCFavg = average VCF (dimensionless)

tf = filtration cycle duration (min)

Qf = feed flow rate (gpm)

tb = backwash duration (min)

Qb = backwash flow (gpm)

J = solids retention time (min) 

2.5.2.3 Continuous Stirred Tank Reactor Model

The CSTR (also known as feed-and-bleed in some applications) hydraulic configuration 

is similar to that of a crossflow system in that the particulate matter is held in suspension and 

increases in concentration on the feed side of the membrane as a function of time. However, the 

CSTR incorporates a continuous concentrate waste stream (also referred to as the reject or bleed 

stream) that removes suspended solids from the system. Since the solids are continuously 

removed from the system, steady-state operation is achieved when the rate at which solids are 

removed with the concentrate stream is equal to the rate at which solids enter the system with the 

feed water. Although a PFR also utilizes a continuous concentrate stream and operates at steadstate, the CSTR model assumes complete mixing and thus a uniform concentration of suspended 

solids on the feed side of the membrane. A PFR, by contrast, has a feed side concentration 

profile that increases in the direction of feed flow through the membrane unit. Thus, the CSTR 

model is more appropriate than the PFR model for systems that are expected to have a relatively 

uniform suspended solids concentration on the feed side of the membrane. 
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Proposal Draft

CSTR theory generally describes systems that operate continuously without 

backwashing. For example, some MF and UF systems utilize a periodic short duration reverse 

flow operation (i.e., a back-pulse) to remove solids from the membrane surface but collect the 

dislodged solids in the feed tank. Since this operation does not constitute backwashing (i.e., 

removal of solids from the system), these MF and UF systems can be modeled as CSTRs without 

backwashing. In addition, a small-scale NF or RO membrane unit that is too small to be 

considered a PFR (e.g., a single stage system using a high recirculation flow or a single module 

system) can also be modeled as CSTR that does not utilize backwashing. However, there may be 

types of membrane filtration systems that can be effectively modeled as a CSTR with 

backwashing, such as a small, immersed vacuum-driven membrane system in which the 

backwash water is removed from the tank after the backwash operation. Thus, CSTR models 

both with and without backwashing, respectively, are described as follows.

CSTR Without Backwashing

Figures 2.21 and 2.22 provide representative schematics illustrating the types of pressureand vacuum-driven membrane filtration systems, respectively, that can be modeled as CSTRs 

without backwashing. Note that a submerged, vacuum-driven system with mechanical agitation 

(e.g., due to aeration) can be modeled as either a CSTR without backwashing or a PFR, 

depending on the number of modules. Systems with a very small number of modules may be 

modeled as a CSTR without backwashing, as shown in Figure 2.22. However, vacuum-driven 

systems with a larger number of modules may be more accurately modeled as a PFR, as shown 

in Figure 2.16. Alternatively, the VCF may be measured experimentally if neither model is 

sufficient to describe the system within an acceptable degree of accuracy.

The VCF for a CSTR without backwashing increases as a function of filtration cycle time 

and can be defined in terms of the hydraulic detention time and the recovery, as shown in 

Equation 2.31:























 −

• − 











−

=

t

t

R

VCF t 1 exp

1

1

( ) Equation 2.31

Where: VCF(t) = VCF as a function of filtration cycle time (dimensionless)

R = recovery (dimensionless decimal)

t = filtration cycle time (min)

J = solids retention time (min)
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Proposal Draft

Figure 2.21 Schematic of a Typical Pressure-Driven CSTR Without Backwashing

Figure 2.22 Schematic of a Typical Vacuum-Driven CSTR Without Backwashing

The concentration profile for a CSTR without backwash exhibits an exponential rise in 

the VCF over time to an equilibrium value of VCFmax, as illustrated in Figure 2.23. Additional 

detail is provided in Table 2.2, which shows the percent of equilibrium value (i.e., VCFmax) that 

is achieved in a CSTR without backwashing as a function of the number of hydraulic detention 

times. In general, a CSTR without backwashing approaches equilibrium rapidly – Table 2.2 

shows that the concentration of suspended solids within a CSTR without backwashing reaches 

95 percent of the equilibrium value after only three hydraulic detention times have elapsed.
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Figure 2.23 Concentration Profile in a CSTR Without Backwashing

Table 2.2 Increase in VCF as a Function of Number of Detention Times

No. of Detention 

Times

(t / J)

Fraction of 

VCFmax

0 0

1 .632

2 .865

3 .950

4 .982

5 .993

Since most systems are operated significantly longer than three hydraulic detention times 

before being purged and cleaned, a CSTR without backwashing generally operates under steadystate conditions at the maximum VCF. Consequently, the average and maximum VCF are 

equivalent, and both can be calculated as shown in Equation 2.41.

0

20

40

60

80

100

0 1 2 3 4 5

Hydraulic Detention Time

(minutes)

C

m/Cm-max

(percent)
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R

VCF VCFavg

−

= =

1

1

max Equation 2.32

Where: VCFmax = maximum VCF (dimensionless)

VCFavg = average VCF (dimensionless)

R = recovery (dimensionless decimal)

CSTR With Backwashing

Figure 2.24 shows a schematic of a representative pressure-driven membrane system that 

can be modeled as a CSTR with periodic backwashing. Note that the concentration profile for a 

CSTR with backwashing is similar to that for a small volume crossflow system, although the 

latter the maximum VCF is approached linearly rather than exponentially. Another important 

difference between these two types of hydraulic configurations is that the CSTR with 

backwashing utilizes a concentrate waste (i.e., bleed) stream, whereas a crossflow system does 

not use a concentrate bleed.

Figure 2.24 Schematic of a Typical Pressure-Driven CSTR With Backwashing

As with a CSTR without backwashing, the VCF for a CSTR with backwashing increases 

over time according to the first order exponential function given in Equation 2.31. However, in a 

CSTR with backwashing, the concentration of suspended solids on the feed side of the 

membrane is periodically reduced. Because filtrate is typically used in the backwash process, the 

backwash water introduces a negligible amount of particulate matter into the system; thus, it 
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reasonable to assume that the backwash process removes all suspended solids from the feed side 

of the membrane. As a result, the VCF is reduced to zero at beginning of each successive 

filtration cycle (i.e., at t = 0). Figure 2.25 illustrates the concentration profile of a CSTR with 

backwashing over time for a system that is backwash once every four hydraulic detention times.

Figure 2.25 Concentration Profile in a CSTR With Backwashing

The maximum VCF for a CSTR with backwashing is determined by evaluating 

Equation 2.31 at the end of the filtration cycle (i.e., t = tf), just prior to backwashing, as shown in 

Equation 2.33:































 −

• − 











−

=

t

f

t

R

VCF 1 exp

1

1

max Equation 2.33

Where: VCFmax = maximum VCF (dimensionless)

R = recovery (dimensionless decimal)

tf = filtration cycle duration (min)

J = solids retention time (min)

The average VCF for a CSTR with backwashing is determined by applying the average 

value function to Equation 2.33 over a filtration cycle. The resulting expression for VCFavg is 

shown in Equation 2.34:

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8

Hydraulic Detention Time

(minutes)

C

m/Cm-max

(percent)

Backwash
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





























































 −

• − • −

−

=

t

t f

f

avg

t

R t

VCF 1 1 exp

1

1

Equation 2.43

Where: VCFavg = average VCF (dimensionless)

R = water recovery (dimensionless decimal)

tf = duration of the filtration cycle (min)

J = solids detention time (min)

2.5.3 Summary

The hydraulic configuration of a membrane filtration system governs the concentration of 

suspended solids on the feed side of membrane and whether it increases as a function of time or 

position in a membrane unit. Accordingly, the VCF has a significant impact on the amount of 

particulate matter that could pass through an integrity breach and thus on the sensitivity of a 

direct integrity test (as discussed in section 4.3). Systems that operate in deposition mode have a 

VCF equal to one, while the various suspension mode hydraulic configurations have VCFs 

greater than one, as summarized in Table 2.3. 

Table 2.3 Typical Range of VCF Values for Various Hydraulic Configurations

Hydraulic Configuration VCF

Deposition 

mode Dead-end 1

PFR 3 - 20

Crossflow 4 - 20 Suspension 

mode

CSTR 4 - 20

A summary of the equations used to calculate the average and maximum VCFs for each 

of the various hydraulic configurations is presented in Table 2.4. The table also indicates the 

types of membrane filtration systems that are generally associated with the respective hydraulic 

configurations. Note that it is also possible to determine the VCF experimentally for a particular 

system (either full-scale or representative pilot-scale) by measuring the influent feed 

concentration (Cm) and the concentration on the feed side of the membrane (Cf).
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Table 2.4 Summary of VCF Equations for Various Hydraulic Configurations

VCF Equation

Hydraulic 

Configuration

Typical 

Membrane 

Process(es) Average1 Maximum

Deposition 

Mode

• MCF

• MF/UF

1 1

Small Volume 

Crossflow

[(tb ?Qb) < Vf]

• MF/UF

(w/o feed tank) 2t

f

t

t

f

t

Large Volume 

Crossflow

[(tb ?Qb) > Vf]

• MF/UF

(w/ feed tank)

























•

•

• •

• −

•

•

b b

f f

f

b b

f f

t Q

t Q

t

t Q

t Q

2 t

1

b b

f f

t Q

t Q

•

•

CSTR

w/o 

backwashing

• NF/RO 

(small-scale)

• MF/UF

(w/ bleed)

1− R

1

1− R

1

CSTR

w/ backwashing

• MF/UF

(w/ bleed) 

























































 −

• − • −

− t

t f

f

t

R t

1 1 exp

1

1



























 −

• − 











− t

f

t

R

1 exp

1

1

PFR • NF/RO

p

p

Q

∑VCF(x) • Q (x)

1− R

1

1 Ideal, theoretical average
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3.0 Challenge Testing

3.1 Introduction

The Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires that 

any membrane filtration system used to comply with the Cryptosporidium treatment 

requirements of the rule undergo challenge testing (40 CFR 141.728). The primary purpose of 

this challenge testing is to establish the log removal value (LRV) that an integral membrane can 

achieve. Under the LT2ESWTR, the maximum removal credit that a membrane filtration system 

is eligible to receive is the lower of the two values established as follows (40 CFR 141.728):

• The removal efficiency demonstrated during challenge testing; or

• The maximum LRV that can be verified by the particular direct integrity test used 

during the course of normal operation

The requirement for challenge testing under the LT2ESWTR is intended to be productspecific such that site-specific demonstration of Cryptosporidium removal efficiency is not 

necessary. Once the LRV of a membrane has been established through a challenge test that 

meets the requirements of LT2ESWTR, additional challenge testing is not required unless 

significant modifications are made to the membrane process (as discussed in section 3.14). The 

rule specifies criteria for the following aspects of challenge testing:

• Full-scale vs. small-scale module testing

• Appropriate challenge particulates

• Challenge particulate concentrations

• Test operating conditions

• Calculation of removal efficiency

• Verifying characteristic removal efficiency for untested modules

• Module modifications

Specific requirements of the rule are summarized in section 3.2 and further discussed in the 

appropriate context in subsequent sections of Chapter 3. The discussion of challenge testing in 

this chapter applies similarly to microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), 

reverse osmosis (RO), and membrane cartridge filtration (MCF), except as otherwise noted. 

Although the primary focus of challenge testing as required under the LT2ESTWR is 

demonstration of Cryptosporidium removal, the general framework for challenge testing 

developed in this guidance manual may be adapted for use in establishing removal efficiencies 
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for other microbial pathogens of concern, including bacteria, viruses, and other protozoa such as 

Giardia.

Chapter 3 is organized into sections that describe the various issues to be considered in 

the design and implementation of a challenge test. Furthermore, it presents the various aspects 

of challenge testing in order from the planning and design phase, through test implementation, 

and finally to the analysis and interpretation of results.

This chapter is divided into the following sections:

Section 3.2: Summary of Challenge Testing Requirements

This section summarizes the requirements for challenge testing under the 

LT2ESWTR.

Section 3.3: Test Organization Qualification

This section provides an overview of factors to consider when selecting an 

organization for conducting a challenge test.

Section 3.4: General Procedure for Developing a Challenge Test Protocol

This section describes the general procedures for developing a challenge 

testing protocol that meets the requirements of the LT2ESWTR.

Section 3.5: Module Specifications

This section outlines the particular module specifications that are 

important considerations in the development of a challenge test.

Section 3.6: Non-Destructive Performance Testing

This section describes the important role of non-destructive performance 

testing – a common quality control procedure used in the production of 

membrane modules – in the challenge testing process.

Section 3.7: Selection of Modules for Challenge Testing

This section discusses some considerations and two potential approaches 

for both selecting particular modules for challenge testing and identifying 

an appropriate number of modules to test

Section 3.8: Small-Scale Module Testing

This section describes some the issues associated with testing small-scale 

rather than full-scale modules.

Section 3.9: Target Organisms and Challenge Particulates

This section discusses factors to consider in selecting a challenge 

particulate for evaluating the removal efficiency of a membrane filtration 

process, including attributes of potential surrogates for Cryptosporidium.
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Section 3.10: Challenge Test Solutions

This section describes methods and procedures for preparing a challenge 

test solution and for seeding challenge particulates into the solution.

Section 3.11: Challenge Test Systems

This section provides some examples of typical challenge test apparatuses 

used under different conditions, as well as the appropriate operational 

parameters to use during the testing.

Section 3.12: Sampling

This section describes aspects of challenge testing associated with 

sampling, including sampling methods, sample port design and location, 

process monitoring, and the development of a thorough sampling plan.

Section 3.13: Analysis and Reporting of Challenge Test Results

This section discusses the calculation of removal efficiency based on the 

results of challenge testing, as well as suggestions for statistical analyses 

and the preparation of a summary report for state review.

Section 3.14: Re-Testing of Modified Membrane Modules

This section provides guidance with respect to product modifications that 

would warrant re-testing of a membrane filtration device.

Section 3.15: Grandfathering Challenge Test Data From Previous Studies

This section discusses factors that should be considered when evaluating 

data from previously conducted removal studies, that may not meet all of 

the specific requirements for challenge testing under the LT2ESWTR, for 

potential acceptance for satisfying the rule requirements.

3.2 Summary of Challenge Testing Requirements 

The LT2ESWTR specifies the core requirements that a challenge test must meet in order 

to demonstrate the removal efficiency of a membrane filtration system with respect to 

Cryptosporidium. These requirements are summarized as follows:

• Full-Scale vs. Small-Scale Module Testing: 

Challenge testing must be conducted on a full-scale membrane module identical in 

material and construction to the membrane modules proposed for use in full-scale 

treatment facilities. Alternatively, challenge testing may be conducted on a smaller 

scale module that is identical in material and similar in construction to the full-scale 

modules. (40 CFR 141, Subpart W, Appendix C).
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• Appropriate Challenge Particulates: 

Challenge testing must be conducted using Cryptosporidium oocysts or a surrogate 

that has been determined to be removed no more efficiently than Cryptosporidium

oocysts. The organism or surrogate used during challenge testing is referred to as the 

challenge particulate. The concentration of the challenge particulate must be 

determined using a method capable of discretely quantifying the specific challenge 

particulate used in the test; indirect water quality measurements such as turbidity, 

particle counting, or conductivity cannot be used for this purpose (40 CFR 141, 

Subpart W, Appendix C).

• Challenge Particulate Concentrations:

The maximum allowable feed water concentration used during a challenge test is 

based on the detection limit of the challenge particulate in the filtrate and is 

determined according to the following equation:

Maximum Feed Concentration = (3.16 x 106) x (Filtrate Detection Limit)

This expression allows for the demonstration of up to 6.5 log removal during 

challenge testing if the challenge particulate is removed to the detection limit 

(40 CFR 141, Subpart W, Appendix C).

• Test Operating Conditions:

Challenge testing must be conducted under representative hydraulic conditions at the 

maximum design flux and maximum design system recovery specified by the 

manufacturer (40 CFR 141, Subpart W, Appendix C).

• Calculation of Removal Efficiency:

The removal efficiency of a membrane filtration process as determined from the 

results of the challenge test is expressed in terms of a LRV according to the following 

equation:

log( ) log( ) LRV = C f − Cp

Where: LRV = log removal value demonstrated during challenge testing

Cf = feed concentration used during challenge testing 

(number or mass/ volume)

Cp = filtrate concentration observed during challenge testing 

(number or mass/ volume)

In order for this equation to be valid, equivalent units must be used for both the feed 

and filtrate concentrations. If the challenge particulate is not detected in the filtrate, 

then the term Cp is set equal to the detection limit. A single LRV is calculated for 

each module tested. The overall removal efficiency demonstrated during challenge 

testing is referred to as LRVC-Test . If fewer than 20 modules are tested, then LRVC-Test

is assigned a value equal to the lowest of the representative LRVs among the various 

modules tested. If 20 or more modules are tested, then LRVC-Test is assigned a value 
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equal to the 10th percentile of the representative LRVs among the various modules, as 

defined by [i/(n+1)], where “i” is the rank of “n” individual data points ordered from 

lowest to highest. It may be necessary to calculate the 10th percentile using linear 

interpolation (40 CFR 141, Subpart W, Appendix C).

• Verifying Characteristic Removal Efficiency for Untested Modules:

Because the LT2ESWTR does not require that every membrane module be subject to 

challenge testing, a non-destructive performance test (NDPT) (e.g., bubble point test, 

diffusive airflow test, pressure or vacuum decay test, etc.) must be applied to each 

production membrane module that did not undergo challenge testing in order to verify 

removal efficiency. A quality control release value (QCRV) must be established for 

the NDPT that is directly related to the removal efficiency of the membrane filtration 

process as demonstrated during challenge testing. Membrane modules that do not 

meet the established QCRV are not eligible for the removal credit demonstrated 

during challenge testing (40 CFR 141, Subpart W, Appendix C).

• Module Modifications:

Any significant modification to the membrane media (e.g., a change in the polymer 

chemistry), hydraulic configuration (e.g., changing from suspension to deposition 

mode), or any other modification that could potentially affect removal efficiency or 

NDPT parameters would require additional challenge testing to both demonstrate the 

removal efficiency of the modified module and define a new QCRV for the NDPT 

(40 CFR 141, Subpart W, Appendix C).

Beyond these core requirements, the rule provides substantial flexibility in the design of a 

challenge test. Guidance for planning, designing, and implementing a challenge test, including 

elaboration on the core requirements in the appropriate context, is provided in the subsequent 

sections of this chapter. 

3.3 Test Organization Qualification 

The LT2ESWTR does not specify any requirements with respect to the qualifications of 

an organization conducting a challenge test, as long as the test is performed according to the 

criteria mandated under the rule (40 CFR 141, Subpart W, Appendix C). Each state has 

discretion in approving the results from a challenge test conducted by any organization. 

However, since challenge testing is intended to be product-specific, it is important that there be 

some consensus regarding what constitutes an acceptable test. Thus guidance provided in this 

section is intended to outline the skills and capabilities that a test organization should posses in 

order to produce quality data.
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In general, conducting a successful challenge test necessitates that the testing 

organization demonstrate effective knowledge of the following: 

• The various membrane processes commonly used in drinking water treatment

• Operation of membrane filtration equipment and related system processes, including 

pretreatment, post-treatment, backwashing, chemical cleaning, and integrity testing 

• Proper challenge particulate seeding and sampling techniques

• Analytical techniques for the enumeration of the challenge particulate used in the 

challenge test, including analyses of microorganisms, inert particulate markers, or 

molecular markers (as applicable)

• Adequate quality assurance (QA)/quality control (QC) procedures to ensure that data 

quality objectives are achieved

• Basic statistical procedures that may be used in data analysis 

• Preparation of reports for regulatory agencies

Historically, many utilities and states have used independent, third party organizations to conduct 

verification testing (i.e., challenge testing) in order to ensure an unbiased evaluation of the 

process. While there are advantages to this approach, a membrane manufacturer may have the 

ability to conduct an acceptable challenge test if it can demonstrate that appropriate QA/QC 

procedures are used. 

3.4 General Procedure for Designing a Challenge Test Protocol

The core challenge test requirements of the LT2ESWTR should be incorporated into a 

detailed protocol for implementing the test that documents the necessary equipment, procedures, 

and analyses. Due to the variety of membrane systems available from numerous suppliers, it is 

not possible to develop a single comprehensive protocol. However, the following general list of 

procedures describes the basic steps in the development of such a protocol. 

1. Document basic membrane module specifications (as indicated in section 3.5), 

including:

• Maximum design flux

• Mode of operation

(i.e., inside-out or outside-in)

• Hydraulic configuration

(i.e., deposition or suspension)
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• Module dimensions and filtration area

• Operating constraints

(e.g., maximum feed pressure, temperature, pH range, oxidant tolerance, etc.)

• Backwash and chemical cleaning procedures

2. Document the manufacturer’s procedure for conducting non-destructive performance 

testing and ensure that the associated quality control release value (i.e., the minimal 

result from the NDPT that constitutes an acceptable product) is indicative of a NDPT 

resolution of 3 μm in order to demonstrate Cryptosporidium removal capability, as 

discussed in section 3.6. If available, the statistical distribution of the NDPT results 

for the product line may also be useful. 

3. Determine the number of modules that will be evaluated during the challenge test and 

the method or criteria that will be used to select specific modules for testing (see 

section 3.7).

4. Determine whether or not small-scale module testing is an option (see section 3.8).

5. Identify the target organism or contaminant for the test. For the purposes of 

compliance with the LT2ESWTR, the target organism is Cryptosporidium (see 

section 3.9).

6. Establish the target LRV for the challenge test. Because challenge testing is intended 

to be product-specific under the LT2ESWTR, it is generally advantageous for a 

manufacturer to set this target at the maximum LRV for which it is anticipated that 

the system will qualify.

7. Select the challenge particulate to be used for testing. If it is not feasible or desirable 

to use the target organism as the challenge particulate, it is necessary to identify an 

acceptable surrogate that is removed on an equivalent or more conservative basis (see 

section 3.9).

8. Select an analytical method that will be used to discretely quantify (i.e., enumerate) 

the challenge particulate and collect information relevant to the methodology for use 

in developing a sampling plan (see section 3.9). Determine the detection limit for the 

challenge particulate in the filtrate based on the method capabilities and filtrate 

sample volume.

9. Design the challenge test solution and establish the method for seeding the challenge 

particulates into the solution (e.g., continuous or batch seeding) (see section 3.10).

10. Design and construct the testing apparatus, and select appropriate operational 

parameters (see section 3.11).
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11. Develop a sampling and monitoring plan that specifies (as described in section 3.12):

• The number of feed and filtrate samples to be collected and analyzed 

• The frequency of feed and filtrate sample collection

• The feed and filtrate sample volumes

• Procedures for sample collection

• Additional operating and water quality parameters to be monitored and associated 

monitoring frequency

After completing the steps outlined above, the specific protocol for conducting challenge 

testing should be documented and submitted for state approval, if required. Note that the 

LT2ESWTR does not require that the challenge test protocol be reviewed or approved by the 

USEPA; however, each state may exercise its discretion regarding whether approval of the 

protocol is required before results of the challenge testing are accepted in that state.

3.5 Module Specifications

Because there are significant differences in the numerous types of membrane modules 

that are commercially available from various suppliers, it is important to document the particular 

specifications of the module of interest prior to developing a product-specific challenge test. 

Membrane equipment suppliers typically provide product specification sheets that contain 

general information about a particular product line. These sheets are generally applicable to all 

the modules in a particular product line and typically contain the following information:

• Membrane module (i.e., product) designation

• Type of membrane material

• Membrane pore size or molecular weight cut-off (MWCO) – both mean and 

maximum values

• Feed side membrane filtration area within a module 

• Module configuration

(e.g., hollow-fiber, spiral-wound, etc.)

• Module dimensions
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• Membrane media dimensions

? Hollow-fiber modules: inside and outside fiber diameters, membrane 

thickness, fiber length, etc.

? Spiral-wound modules: sheet dimensions, sheet thickness, etc.

• Membrane media symmetry

(e.g., symmetric, asymmetric, composite, etc.)

• Maximum design flux

• Maximum feed and transmembrane pressure (TMP)

• Mode of operation

(i.e., inside-out or outside-in)

• Hydraulic configuration

(i.e., deposition or suspension)

• Operating constraints

(e.g., temperature limit, pH range, oxidant tolerance, etc.)

Although not all of the information listed above or provided with any particular product 

specification sheet may be necessary for developing a challenge test, it is nevertheless prudent to 

compile as much available information about the product as possible not only for its potential use 

in challenge testing, but for long-term use during operation of the full-scale facility.

It is also common for manufacturers to supply data for each specific module produced. 

Membrane module data sheets are applicable only to the particular module with the listed serial 

number and typically contain the results from a manufacturer’s QC process. If available, it is 

important that these data sheets be obtained for each module that is to undergo challenge testing, 

since it is critical to document as much information about these particular modules, as possible. 

For reference, it may also be useful to document the QC procedures associated with the 

production of modules to be used for challenge testing.

3.6 Non-Destructive Performance Testing

While challenge testing is used to establish the LRV of an integral module of a particular 

product type, it does not necessarily guarantee that all such modules produced will achieve the 

same level of performance due to variability in the manufacturing process. In order to address 

this issue, a NDPT is applied to all subsequently manufactured modules that are not subject to 

challenge testing to ensure that these modules comply with the minimum standards for 

Cryptosporidium removal under the LT2ESWTR. A NDPT is a physical test applied to the 
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membrane module with the objective of characterizing some aspect of process performance and 

which does not alter or damage the membrane.

In order to be utilized in a membrane filtration system that is applied for the purpose of 

receiving Cryptosporidium removal credit under the LT2ESWTR, each module must pass a 

NDPT that is consistent with the 3-μm resolution requirement of the rule. For example, one 

commonly used type of NDPT is the bubble point test, which characterizes the largest pore (or 

defect) in a membrane module. (Note that the “bubble point test” – in this context a type of 

NDPT – is not the same as the “bubble test” – a diagnostic direct integrity test – described in 

Chapter 4, although these two procedures are both based on the same principles of bubble point 

theory, as described in Appendix B.) In a bubble point test, a pressure is applied to a fullywetted membrane module and gradually increased. The pressure at which water is first 

evacuated from the pores represents the bubble point of the membrane associated with a 

particular module. If the established bubble point of the membrane is sufficient to demonstrate 

that there are no pores (or defects) larger than 3 μm (as described in section 4.2.1.), then the 

NDPT is consistent with resolution specified in the rule for the removal of Cryptosporidium. 

The minimum passing test result for a NDPT is known as the quality control release 

value (QCRV). In the context of the LT2ESWTR, a test result that surpasses the QCRV 

indicates both that a module can adequately remove Cryptosporidium and is sufficiently similar 

in quality to the modules subjected to challenge testing to demonstrate the ability of the module 

of interest (which would not have been subject to challenge testing) to achieve the same LRV. 

After a group of modules has been subject to challenge testing, the NDPT is applied to those 

modules to determine an appropriate QCRV associated with the removal efficiency observed 

during the test. Subsequently, all modules that are not subjected to challenge testing must pass 

the same NDPT by exceeding the established QCRV applicable to Cryptosporidium removal 

under the LT2ESWTR. Modules that do not pass the NDPT at the QCRV would not be eligible 

for Cryptosporidium removal credit under the rule and could not be used in any membrane 

filtration systems applied for this purpose.

The LT2ESWTR does not specify a particular procedure for determining the QCRV from 

the various modules that are subjected to challenge testing. Thus, the manufacturer or 

independent testing organization may exercise its discretion in selecting an appropriate 

methodology. The QCRV may be selected as the average result among the various modules 

tested, the most conservative result (to establish the most stringent QA/QC standards), or the 

least conservative result (to maximize the number of modules eligible for removal credit under 

the LT2ESWTR). Alternatively, a methodology similar to that required by the LT2ESWTR for 

determining the overall removal efficiency based on the number of LRV observations could be 

applied to the various NDPT results to yield an appropriate QCRV, as described in section 3.13.1 

(40 CFR 141, Subpart W, Appendix C). The method of module selection for challenge testing, 

as discussed in section3.7, should also be considered when determining the QCRV from the test 

data.

Note that the rule does not specify the manner in which the QCRV is determined from the 

challenge test data; however, the methodology must be acceptable to each state in which the 

product line is applied for the purpose of receiving Cryptosporidium removal credit under the 
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LT2ESWTR. It is recommended that each module subjected to challenge testing also undergo 

subsequent non-destructive performance testing for the purpose of establishing a QCRV. The 

manner in which the NDPT results are used to determine the QCRV should reflect the manner in 

which modules are selected, as well as the range of LRVs observed during the test. If modules 

are selected in a conservative manner (see section 3.7), and if the range of LRVs observed during 

challenge testing is small, the average of the respective NDPT results from the modules 

subjected to challenge testing might be selected to represent the QCRV. However, if a statistical 

distribution of modules is selected for challenge testing, or if the range of LRVs observed during 

challenge testing is significant, a more conservative value is recommended for the QCRV, such 

as the minimum of the NDPT results observed among the tested modules. 

Because it is common for manufacturers to conduct some type of NDPT on every module 

as a routine component of its QA/QC program, the NDPT requirements of the LT2ESWTR 

simply ensure that the QCRV used by the manufacturer is sufficient to justify the LRV for 

Cryptosporidium demonstrated via challenge testing. Note that because different NDPTs may be 

used by the various membrane module manufacturers, the rule does not specify a particular type 

of NDPT. However, the NDPT used must be consistent with the resolution requirements of the 

LT2ESWTR in order for a module to be eligible for Cryptosporidium removal credit.

3.7 Selection of Modules for Challenge Testing

The intent of challenge testing under the LT2ESWTR is to characterize the removal 

efficiency of a specific membrane product without requiring challenge testing for all production 

modules. In addition, the rule does not specify a particular number of modules that are required 

to undergo challenge testing in order to demonstrate Cryptosporidium removal efficiency. 

However, because it is important that manufacturing variability in the product line be considered 

in the development of an appropriate challenge test, the number of modules subject to challenge 

testing, as well as the particular modules chosen, should be carefully selected on a rational and 

scientific basis. Although manufacturers or independent testing organizations may develop any 

number of different procedures for module selection, two common approaches are discussed in 

this guidance as illustrative examples:

1. Selection of modules based on previously collected QC data for the product line

2. Random sampling of membrane modules from several manufactured lots according to 

a statistical sample design

Use of the first approach listed above is predicated on the existence of significant QC 

data for the product line accumulated over time by the manufacturer. Since manufacturers 

typically conduct some kind of NDPT on all modules produced to ensure quality and 

characterize the variability of a product line independent of the link established between nondestructive performance testing and challenge testing under the LT2ESWTR, such data should 

generally be available. Because the modules subject to challenge testing will be subsequently recharacterized with the NDPT to establish an acceptable QCRV required for all modules to be 
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eligible for Cryptosporidium removal credit under the LT2ESWTR, it may be most 

advantageous for a manufacturer to select modules for challenge testing that are near the lower 

end of the statistical distribution of acceptable (i.e., under the manufacturer’s in-house QC 

procedures) NDPT results based on historical data. If these tested modules yield a QCRV that is 

consistent with the resolution requirement of the rule, then it is likely that the majority of 

production modules will also meet the established QCRV and thus be eligible for 

Cryptosporidium removal credit. Using this approach, the number of modules selected for 

challenge testing is generally at the discretion of the manufacturer or independent testing 

organization.

If historical QC data for the product line is not available from the manufacturer, the 

second approach listed above may represent an appropriate option. This method involves the 

evaluation of a statistically significant random sample of modules from a number of production 

lots. Because the modules at the lower end of the QC data are not artificially selected (as with 

the first approach described above), it is likely that this method will result in a higher QCRV, 

resulting in a somewhat higher rejection rate of modules eligible for Cryptosporidium removal 

credit under LT2ESWTR. This number of modules selected for challenge testing using this 

approach will likely be dictated by the particular statistical sampling technique used.

Either of these two approaches or other rationale approach developed by the 

manufacturer or independent testing organization could be utilized to select modules for 

challenge testing. Regardless of the method used, it is suggested that at least five (5) membrane 

modules from different manufactured lots be evaluated during a challenge test. 

3.8 Small-Scale Module Testing

The evaluation of small-scale (as opposed to full-scale) modules during a challenge test is 

permitted under the LT2ESWTR to allow for cases in which it may not be feasible or practical to 

test a full-scale module (40 CFR 141, Subpart W, Appendix C). For example, if it is desirable to 

conduct challenge testing using the target organism (i.e., Cryptosporidium for the purposes of the 

LT2ESWTR) rather than a surrogate, the use of a small-scale module may be the only 

economically viable alternative.

All challenge testing requirements under the LT2ESWTR (as well as the associated 

guidance) are equally applicable to both full-scale and small-scale modules. However, any 

small-scale module tested must be similar in design to the full-scale modules of the product of 

interest such that it can be operated (and thus tested) under a hydraulic configuration and at a 

maximum design flux and recovery that are representative of the full-scale modules. Simulating 

the full-scale recovery and hydraulic configuration are important considerations for small-scale 

challenge testing, since both of these parameters affect the concentration of suspended solids on 

the feed side of the membrane.

Although the decision to allow the use of small-scale module testing is left to the 

discretion of the state, the option is permitted under the LT2ESWTR since it is considered a 

valid approach for characterizing removal efficiencies. For the purposes of consistency, it is 
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recommended that manufacturers or independent testing agencies that opt to subject a product 

line to challenge testing using small-scale modules utilize a protocol that has been accepted by a 

wide range of stakeholders. Such a protocol has been proposed for use under the National 

Sanitation Foundation (NSF) Environmental Technology Verification (ETV) program and is 

available online at http://www.nsf.org/etv/dws/pdf/Bench_Memo_Protocol_Stakeholders.pdf. If 

the link is inactive, contact the NSF at (800) 673-6275 for further information.

3.9 Target Organisms and Challenge Particulates

The purpose of a challenge test is to determine the removal efficiency of a membrane 

module for one or more target organisms or pathogens. Challenge testing can be conducted 

using either the target organism itself or an appropriate surrogate; the organism or surrogate used 

in the test is referred to as the challenge particulate. The selection of a suitable challenge 

particulate is critical to the design of a challenge test.

This section provides guidance for selecting an appropriate challenge particulate, 

including the selection of a target organism for the test and characteristics of suitable surrogates 

for the target organism. A more detailed discussion of particular surrogates for Cryptosporidium

is also provided.

3.9.1 Selecting a Target Organism

The target organism or pathogen of interest for the purposes of challenge testing is 

selected based on the treatment objectives for the membrane filtration system. For example, 

Cryptosporidium would be the target organism in a challenge test conducted to demonstrate the 

ability of a membrane filtration system to comply with the treatment requirements of the 

LT2ESWTR. However, in some cases it may be desirable to determine the removal efficiency of 

a system for multiple target organisms. In such cases, the most conservative target organism 

should be selected for the purpose of designing a challenge test. For example, if the challenge 

test is designed to evaluate the removal efficiency of a system for both Cryptosporidium and 

Giardia, then the smaller of these two pathogens should be used as the target organism. 

Although the approximate size ranges for these two organisms overlap to some degree, as shown 

in Table 3.1, Cryptosporidium has the smaller lower bound. Since membrane filtration is a 

barrier technology based primarily on the principle of size exclusion, the removal efficiency for 

the smallest organism of interest should be conservative for larger pathogens.
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Table 3.1 Potential Target Organisms for Challenge Testing

Target Organism Size Range

(:m)

Enteric viruses 0.03 – 0.1

Fecal coliform 1 - 4

Cryptosporidium 3 - 7

Giardia 7 - 15

3.9.2 Surrogate Characteristics

Although use of the target organism as the challenge particulate offers the advantages of 

directly measuring removal efficiency for the pathogen of interest and eliminates issues 

regarding the appropriateness of a surrogate, it may not be practical or feasible as a result of 

economic considerations or concerns about working directly with the pathogen. Thus, the use of 

surrogates may be the most viable option for challenge testing. An ideal surrogate should have 

characteristics that are likely to affect removal efficiency which are similar to those of the target 

organism, while a conservative surrogate would have characteristics that may result in a lower 

removal efficiency relative to the target organism. In general, it is necessary to use a 

conservative surrogate unless there are data to support the use of an ideal surrogate.

As a result of the cost and potential health concerns associated with conducting a 

challenge test using Cryptosporidium oocysts, the LT2ESWTR allows challenge testing to be 

administered with a surrogate that has been verified to be removed no more efficiently than 

Cryptosporidium oocysts (i.e., an ideal or conservative surrogate) (40 CFR 141, Subpart W, 

Appendix C). The most direct means of demonstrating that a surrogate is ideal or conservative is 

through a comparative test in which removal of the surrogate and the target organisms are 

evaluated side-by-side. However, if the characteristics of a surrogate are sufficiently 

conservative, direct verification may not be necessary. Key physical characteristics to consider 

when evaluating the suitability of a surrogate for Cryptosporidium removal using membrane 

filtration include size, shape, and surface charge. Other important considerations include ease of 

use and measurement, as well as cost. Each of these factors is discussed as follows.

Particle size and shape 

The effective size of an appropriate surrogate should be equivalent to or smaller than the 

lower bound of the size range of the target organism. Furthermore, the effective size of the 

surrogate should be characterized using an upper bound of its size distribution such the 99th or 

99.9th percentile rather than the median. Ideally, a surrogate would have a relatively narrow size 

distribution and a high uniformity coefficient. For example, the lower size range of 

Cryptosporidium is approximately 3 μm, and thus a conservative surrogate might be one in 

which 99 percent of particles have a diameter of 1 μm or less.
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Generally, it is desirable to use a surrogate tha t is the same shape as the target organism. 

In the case of Cryptosporidium, an appropriate surrogate would have a spherical shape, although 

in some cases a non-spherical surrogate might be considered. If a non-spherical surrogate is 

used, it is recommended that the smallest dimension be considered as the effective size since 

particles can interact with a membrane barrier at any orientation.

Another consideration is the surface structure of the proposed surrogate. A particle that 

has a highly irregular surface structure may be removed more efficiently than a similarly sized 

particle that has a smooth surface. While it may be difficult to completely characterize the 

surface of a potential surrogate, those with rough surfaces that are known to exhibit a high 

degree of adherence may be removed through mechanisms other than size exclusion, and thus 

may not provide a conservative estimate of removal efficiency.

The manner in which the surrogate disperses in the challenge test solution has a 

significant impact on the effective size and shape of the challenge particulate. Some may 

agglomerate or become attached to other particles while in solution, which would yield larger 

effective particle sizes. For example, organisms such as Staphylococci exist as clumps and 

Streptococci exist as chains. In its aggregate form each of these organisms is too large to be 

considered conservative surrogates for Cryptosporidium. Surface structure also impacts the 

tendency of particles to agglomerate, and in general particles with a smooth surface are more 

likely to be mono-dispersed in solution. 

Particle surface charge

A conservative challenge particulate should have a neutral surface charge, since charged 

particles may interact with other particles and surfaces, thus enhancing removal. The solution 

pH can also affect the charge of some surrogates and thus should be considered in the 

preparation of a test solution. If there is a concern regarding the charge of the surrogate such that 

mechanisms of particle retention other than size exclusion may be responsible for surrogate 

removal in a MF or UF system, a nonionic surfactant could be used in the challenge test solution 

to significantly reduce the impact of charge-related removal mechanisms.

Ease of Use and Measurement 

Although factors such as ease of handling and measurement are not critical to 

determining the appropriateness of a surrogate, these nevertheless may be important factors to 

consider. Handling the surrogate could expose personnel to the challenge particulate, and thus 

should the surrogate should be selected to minimize unacceptable risk to the technicians 

conducting the test. The material should also be easy to work with and dose accurately since 

repeated tests may be conducted in which reproducibility is desirable. Surrogates that could 

degrade during the test, resulting in an inconsistent challenge concentration, should be avoided. 

It is also desirable to use a surrogate that is easy to enumerate through established analytical 

techniques. Furthermore, the LT2ESWTR requires that the concentration of challenge particles 
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determined using a discrete measuring technique such that gross measurements such as turbidity 

are not acceptable. 

Cost 

The cost of seeding and analysis may preclude the use of some surrogates. Both the cost 

of the surrogate itself and cost of the required analytical techniques should be considered, as well 

as any other miscellaneous costs associated with the surrogate.

3.9.3 Surrogates for Cryptosporidium

In the absence of an acceptable surrogate, formalin- or heat-fixed Cryptosporidium 

parvum could be used as the challenge particulate for compliance with the requirements of the 

LT2ESWTR. However, the rule does permit surrogates for the purpose of challenge testing, and 

several different surrogates have been successfully used in studies evaluating physical removal 

of Cryptosporidium. There are three general classifications of surrogates: alternate 

microorganisms, inert particles, and molecular markers. It important to note that not all of these 

classes of surrogates are appropriate for each type of membrane filtration system, and it is critical 

that these be compatible for the purposes of challenge testing. Generally, particulate surrogates 

such as alternate microorganisms and inert particles are appropriate for MF, UF, and MCF 

systems, while molecular markers would not be removed by these types of membranes. It may 

be necessary to use molecular makers with NF and RO membrane systems that can remove 

dissolved substances and which are not designed to accommodate large particulate 

concentrations. Each of these surrogate classes is discussed in further detail in the following 

subsections. Some of the potential advantages and disadvantages associated with each class are 

summarized in Table 3.2.

Table 3.2 Comparative Summary of Cryptosporidium and Potential Surrogates

Challenge 

Particulate Size Range Advantages Disadvantages

Cryptosporidium 

parvum

3 - 5 μm

• No verification of 

surrogate required

• High cost

• Difficult to measure

Alternate

microorganisms 0.01 - 1 μm

• Low cost

• Easy to measure

• Accepted use

• Difficult to handle

• Potential clumping

Inert particles < 1 μm

• Moderate cost

• High uniformity

• Easy to use

• Difficult to measure 

accurately

Molecular markers < 100,000 Daltons • Low cost

• Easy to measure

• Inappropriate for 

some applications
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3.9.3.1 Alternate Microorganisms

Microorganisms other than the target can be used as surrogates for the purposes of 

challenge testing for MF, UF, and MCF systems. Numerous organisms that have a history of use 

in filter evaluation studies are smaller than 1 μm (when mono-dispersed in solution), and these 

could be considered conservative surrogates for Cryptosporidium. A number of these organisms 

and an appropriate enumeration method are listed in Table 3.3, including both bacterial and 

viruses. Table 3.3 also includes common surrogates for Giardia and enteric viruses. S. 

marcessans and P. dimunita have been widely used as surrogates within the membrane filtration 

industry, and the use of MS2 bacteriophage has generally been accepted as a surrogate for enteric 

viruses, since it is similar in size and shape to the poliovirus and hepatitis virus.

Table 3.3 Potential Microbiological Surrogates for Cryptosporidium

Microorganism Size Range

(:m) Target Organism Enumeration Method

Micrococcus l. 7 - 12 Giardia Standard Methods 

9222

Bacillus subtilis ~ 1 Cryptosporidium Barbeau et al. 

(1997)

E. coli 1 - 4 Cryptosporidium Standard Methods 

9222

P. dimunita 0.3 Cryptosporidium Standard Methods 

9222

S. marcessans 0.5 Cryptosporidium Standard Methods 

9222

MS2 bacteriophage 0.01 Enteric virus Adams (1959)

Although Bacillus subtilis has been used as a surrogate for Cryptosporidium for testing 

the removal efficiencies of conventional treatment processes, it is not necessarily a suitable 

Cryptosporidium surrogate for challenge testing membrane filtration devices. Because there is 

limited data currently available regarding the use of Bacillus subtilis in membrane challenge 

studies, a rigorous characterization of this organism would be necessary in order to determine 

whether it could be used as a Cryptosporidium surrogate for the purposes of challenge testing 

under the LT2ESWTR. Based on the size range cited in Table 3.3, Bacillus subtilis could 

potentially be considered an ideal surrogate (see section 3.9.2) for Cryptosporidium, pending a 

rigorous comparison of other characteristics (e.g., shape, surface charge, etc.) between these two 

organisms. However, because of this same size range overlap, Bacillus subtilis could not be 

considered a conservative surrogate for Cryptosporidium.

The primary advantage of many microbial surrogates is that enumeration is fairly simple 

and inexpensive, typically involving culturing the test organisms present in the feed and filtrate 
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samples. The ease with which these organisms can be cultured allows many to be grown in a 

laboratory to produce a stock for use in challenge testing. Bacteria can be cultured to yield stock 

concentrations in the range of 105 to 109 organisms per 100 mL, while MS2 bacteriophage can be 

grown at concentrations in the range of 107 to 1012 organisms per 100 mL. Any microbial stock 

used for the purpose of seeding during a challenge test should be enumerated prior to conducting 

the challenge test in order to facilitate seeding at the target level.

3.9.3.2 Inert Particles

Inert particles may also be used as a surrogate for Cryptosporidium under the 

LT2ESWTR. For example, polystyrene latex microspheres (i.e., latex beads) have been used as 

a surrogate for Cryptosporidium in a number of studies. Historically, microspheres have been 

used in the calibration of particle counters and similar optical equipment in which a challenge 

particle of a known size and geometry is required by the investigator. Microspheres can be 

manufactured with very high particle uniformity and a smooth surface, both of which are 

important considerations when selecting a conservative surrogate. Microspheres are chemically 

inert, easy to handle, and relatively inexpensive. Furthermore, microspheres without a 

significant surface charge can be produced to minimize the potential for adsorption and 

interaction with either other particles or the membrane surface. Microspheres are also readily 

available with particle concentrations ranging from 107 to 109 particles per mL. 

The primary difficulty associated with the use of microspheres is particulate 

enumeration. Although particle counting is a simple means of enumeration, this technique may 

not meet the rule requirement that the challenge particulate be discretely quantified as a result of 

the potential for background particles other than the microspheres to affect the results. 

Furthermore, other problems such as coincidence error and the dynamic range of most particle 

counting instruments may also skew the results. Any clumping of microspheres may also 

complicate particulate enumeration. A more reliable, albeit more expensive, means of 

enumerating microspheres is through capture (normally on a laboratory grade membrane filter) 

and direct examination. The use of fluorescent microspheres is recommended to facilitate 

particulate identification. Methods for microscopic analysis of fluorescent microspheres are 

reported in the literature (Abbaszadegan et al. 1997; Li et al. 1997).

The appropriateness of microspheres as a surrogate for Cryptosporidium could be directly 

verified through a comparative study; however, microspheres that meet certain criteria might be 

deemed conservative surrogates that would not require direct verification. For example, neutral, 

spherical shaped microspheres with a maximum diameter of 1 μm and which are completely 

mono-dispersed in solution might constitute a conservative surrogate for Cryptosporidium that 

would not require direct verification.
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3.9.3.3 Molecular Markers

The suitability of molecular markers as surrogates for Cryptosporidium should be 

considered on a case-by-case basis. While the justification for using microorganisms and inert 

particles as surrogates for Cryptosporidium is more straightforward given that all are 

particulates, molecular markers are dissolved substances that are fundamentally different from 

particulate contaminants. As such, the removal mechanisms for molecular markers may be 

different than for those associated with discrete particles in many cases. However, semipermeable membranes that are capable of achieving very high removal efficiencies for dissolved 

substances may be capable of achieving similar removal of particulates such as Cryptosporidium. 

In addition, porous membranes with very fine pore sizes may be able to remove large 

macromolecules via mechanisms similar to those that filter discrete particles. Thus, the use of 

molecular challenge particulates is permitted for the purposes of challenge testing under the 

LT2ESWTR if the molecular marker used is determined to be conservative for Cryptosporidium

and is discretely quantifiable.

A variety of molecular markers have been historically used to characterize the pore size 

or removal capabilities of membrane processes. For example, macromolecular protein 

compounds are used to determine the MWCO for many UF membranes. In addition, fluorescent 

dyes such as Rhodamine WT and FDC Red #40 are used to characterize NF and RO membranes. 

These substances have high spectrophotometric absorbance characteristics that allow 

measurement and detection at the ng/L level (Lozier et al. 2003). However, these low molecular 

weight (~500 Daltons) solutions could only be used with RO and less permeable NF membranes. 

If molecular markers are considered for challenge testing, it is desirable to use compounds that 

are more similar to discrete particles such as macromolecular proteins. It is also recommended 

that a mass balance be conducted on the feed, filtrate, and concentrate streams prior to challenge 

testing to assess the efficacy of using a particular molecular marker.

With some molecular markers, it may difficult to demonstrate removal in excess of 3 log 

unless sufficiently sensitive instrumentation is used. For challenge tests conducted with 

molecular markers, the feed and filtrate concentrations are typically quantified in terms of mass 

per unit volume. If the analytical method is specific for the molecular marker used in the test, 

use of a mass based concentration is acceptable since the mass of a known substance can be 

related to moles, which is a discrete quantification. As is the case with any challenge particulate, 

gross measurements cannot be used for the purpose of quantification. This requirement would 

preclude the use of analytical techniques such as total organic carbon (TOC) monitoring and 

conductivity monitoring in most cases.

3.10 Challenge Test Solutions

Generation of an appropriate challenge test solution is an essential component of an 

effective test program. The purpose of the challenge test solution is to deliver the challenge 

particulate to the module of interest under the established test conditions. The design of the 

challenge test solution includes establishing acceptable water quality of the solution, determining 
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volume requirements and the challenge particulate concentration, and selecting a seeding 

method. These considerations are discussed in the following subsections.

3.10.1 Test Solution Water Quality

While the LT2ESWTR does not stipulate any constraints on the design of the challenge 

test solution, it is desirable to conduct the test in a manner that would be considered valid under 

any anticipated conditions to which the product undergoing testing might be applied in the field. 

Thus, the water qua lity of the test solution should be taken into consideration. In designing the 

test solution, it is important to consider that the primary objective of challenge testing is to 

evaluate the removal efficiency of the challenge particulate during filtration. While water quality 

may not have a significant impact on the removal efficiency, particulate matter in the feed water 

can enhance removal of smaller contaminants. Thus, it is generally accepted that a test solution 

matrix comprised of high-quality water provides the most conservative estimate of removal 

efficiency. Note that challenge testing is not intended to yield meaningful information regarding 

membrane productivity and fouling potential, and thus the use of a high quality water matrix 

should not be considered inhibitory to testing these measures of membrane performance that are 

unrelated to the primary objective of challenge testing – to evaluate the removal efficiency of the 

challenge particulate. Productivity and fouling are addressed using site-specific pilot testing (see 

Chapter 6) and not on a product-specific basis.

Some particular considerations regarding water quality characteristics and their 

implications for challenge testing are as follows:

• High quality water or “particle–free water” with a low concentration of suspended 

solids (e.g., membrane filtrate) should be used as the matrix for the challenge 

solution, minimizing the potential for formation of a fouling layer during the 

challenge test that would enhance removal of the challenge particulate.

• No oxidants, disinfectants, or other pretreatment chemicals should be added to the test 

solution unless necessitated by process requirements (e.g., acid addition and/or scale 

inhibitor which may be necessary with NF/RO processes). 

• If the challenge particulate is a molecular marker, the water quality of the matrix for 

the test solution should not interfere with the introduction, dispersion, or 

measurement of the marker. Thus, the impact of water quality parameters, such as 

pH and ionic strength, on the chemical characteristics and speciation of molecular 

markers should be considered in the design of the test solution. This is particularly 

critical for NF or RO membrane modules, which can concentrate organic and 

inorganic solutes, potentially interfering with some molecular markers.

• If a microbial challenge particulate is used, it may be necessary to add buffers or 

other materials to maintain the viability of the organisms. Any additives used must 

not interfere with any aspect of the test or result in a change in the concentration of 

the challenge particulate over the duration of the test. In addition, because water 
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quality parameters such as pH and ionic strength can affect microbial aggregation, 

these solution characteristics should also be considered.

• It is recommended that the challenge test solution be characterized with respect to 

basic water quality parameters, such as pH, turbidity, temperature, total dissolved 

solids (TDS), TOC, and any other water quality parameters that are critical to the test 

or interpretation of the results.

3.10.2 Test Solution Volume 

The solution volume necessary to conduct a challenge test depends on several factors 

determined during the test design, including:

• Filtrate flow 

• Recovery

• Test duration

• Hold-up volume of the test system

• Equilibration time for the test solution 

Taking these factors into account, the volume of solution required for challenge testing 

may be calculated using Equation3.1:

V V SF

R

Q T

V hold eq

p

test •

















+ +

•

=

min Equation 3.1

Where: Vtest = minimum challenge test solution volume (gallons)

Qp = filtrate flow (gpm)

Tmin = challenge test duration (min)

R = system recovery during test (decimal percent)

Vhold = hold-up volume of the test system (gallons)

Veq = system volume required to attain equilibrium feed 

concentration (gallons)

SF = safety factor (dimensionless)

The safety factor in Equation 3.1 accounts for unanticipated circumstances that might 

require additional solution volume. The value of the safety factor should be at least 1.0 and may 

be as high as 2.0 under conservative conditions. 
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The parameters Qp and R are dictated by the system operating conditions (see 

section 3.11.2). The LT2ESWTR requires that the flux and recovery used during the challenge 

test be set at the maximum design values for each parameter, as per manufacturer specifications.

The hold-up volume of the test system, Vhold, is the unfiltered test solution volume that 

would remain in the system on the feed side of the membrane barrier at the end of the test (i.e., 

after the system is shut down). At a minimum, this volume would include the feed side volume 

of the membrane module and associated piping. In general, it is desirable to design the system 

with a small hold-up volume, which could potentially allow Vhold to be ignored in Equation 3.1. 

However, the hold-up volume can be significant in some systems, and in such cases Vhold should 

be measured and taken into account. Another approach for dealing with the system hold-up 

volume is to finish the test with a “chaser” of clean water (i.e., without the challenge particulate) 

through the system allowing the entire test solution to be filtered. However, because this 

approach can dilute the feed concentration of the challenge particulate, the use of a clean water 

chaser is only recommended when the entire filtrate stream is sampled as a single composite (see 

Figure 3.1).

The equilibrium volume (Veq) is the quantity of the test solution that must pass through 

the membrane module(s) at the beginning of the test before the system stabilizes (i.e., the feed 

side concentration reaches an equilibrium value). In general, filtrate sampling cannot begin until 

at least this equilibrium volume has passed through the system. For most test apparatuses, a 

reasonable assumption is that a system achieves 90 percent of its equilibrium condition after 

three hold-up volumes (i.e., 3? Vhold) have passed through the system.

The duration of the challenge test, Tmin, as given in Equation 3.1, does not include the 

time required for the test solution to come to equilibrium, as this is taken into account by the 

parameter Veq. Thus, Tmin represents the time necessary to implement the sampling program 

associated with the challenge test (see section 3.12), which typically requires less than one hour.

Table 3.4 provides examples of the challenge test solution volumes (i.e., Vtest) required as 

calculated using Equation 3.1 for various membrane configurations under the listed conditions, 

in these cases assuming the system hold-up volume (Vhold) and volume required to achieve 

equilibrium (Veq) are negligible and a safety factor of 1.1. The filtrate flow (Qp) is not shown 

specifically in the table, but is calculated simply by multiplying the membrane area and 

maximum flux. Table 3.4 is intended to be illustrative only. Thus, it is recommended that the 

solution volume requirements for a specific challenge test be determined according to the 

procedure described above. Also, note that the values listed in Table 3.4 are examples only, and 

that particular product specifications will vary by module manufacturer.
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Table 3.4 Example Challenge Test Solution Volume for Various Types of Modules

Module

Example 

Membrane 

Area

{ft2 (m2)} 

Example 

Maximum 

Flux

{gfd (Lmh)}

Test 

Duration

(min)

Recovery

(percent)

Volume (Vtest)

{gal (L)}

Cartridge filter15 (0.46) 1,364 (2,316) 30 100 155 (586)

4” diameter 75 (7.0) 17.8 (30.2) 30 85 36.1 (137)

Spiral-wound

8” diameter 350 (32.5) 17.8 (30.2) 30 85 168 (635)

Outside-in 350 (32.5) 53.5 (90.8) 30 100 429 (1623)

Hollow-fiber

Inside-out 1,400 (130) 107 (182) 30 100 3,438 (13,013)

1 Note that becaus e MCF is a new concept introduced with the LT2ESWTR, the example specifications cited are for cartridge 

filters, in general, not necessarily for MCF devices

3.10.3 Test Solution Concentration

The concentration of the test solution is based on the target LRV to be demonstrated 

during the challenge test (LRVt) and the detection limit for the challenge particulate in the filtrate 

samples. Since challenge testing is intended to be a one-time, product-specific requirement, it is 

generally advantageous to select a LRVt at or near the maximum of 6.5 log removal that can be 

demonstrated under the LT2ESWTR. The detection limit is a function of the analytical technique 

used to enumerate the challenge particulate and the filtrate sample volume. For example, if the

method can detect 1 particle in a sample, and the filtrate sample volume is one liter, the detection 

limit is 1 particle/L. The detection limit and maximum 6.5 LRV are used to calculate the 

maximum feed concentration that can be used during a challenge test, as shown in Equation 3.2 

(40 CFR 141, Subpart W, Appendix C):

Cf − = (3.16 •10 ) • DL 6

max Equation 3.2

Where: Cf-max = maximum feed concentration (number or mass / volume)

DL = detection limit in the filtrate (number or mass / volume)

The coefficient in Equation 3.2 represents the antilog 6.5, thus capping the maximum feed 

concentration in the test solution to allow a maximum of 6.5 log removal to be demonstrated if 

the challenge particulate is removed to the detection limit in the filtrate. The 6.5 log limit under 

the rule is intended to prevent excessive over-seeding that can result in artificially high LRVs. 

The minimum required feed concentration can be calculated from the LRVt and the 

detection limit using Equation 3.3:
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C DL LRVt

f = • −min 10 Equation 3.3

Where: Cf-min = minimum feed concentration (number or mass / volume)

LRVt = the target log removal value for the challenge test 

(dimensionless)

DL = detection limit in the filtrate (number or mass / volume)

Equation 3.3 implicitly assumes complete removal of the challenge particulate by an integral 

membrane as a conservative means of estimating the minimum feed concentration. Note that 

Equations 3.2 and 3.3 are identical for a LRVt of 6.5. Equations 3.2 and 3.3 also demonstrate 

that the requisite feed concentration is a function of the detection limit associated with the 

analytical technique used to enumerate the challenge particulate in the filtrate. The detection 

limit is typically expressed in terms of the number of challenge particulates per unit volume, or 

in the case of a molecular marker, mass per unit volume. For a given analytical method with a 

known sensitivity, the detection limit can be reduced by increasing the sample volume analyzed. 

For example, if a microbiological method is capable of detecting one organism in a sample, the 

detection limit can be improved by an order of magnitude by increasing the volume analyzed 

from 100 mL to 1,000 mL; however, many methods have limitations with respect to the 

maximum sample volume that can be analyzed. Also note that the feed and filtrate 

concentrations must be expressed in terms of equivalent volumes for the purposes of calculating 

log removal, even if different sample volumes are collected and analyzed during the test.

If the LRVt selected is less than the maximum of 6.5 permitted under the LT2ESWTR, 

the maximum and minimum feed concentrations will be different. Note that it is desirable to use 

a concentration greater than the minimum since use of the minimum feed concentration would 

only demonstrate the LRVt if the challenge particulate were removed to the detection limit in the 

filtrate. After the maximum and minimum concentrations are established using Equations 3.2 

and 3.3, respectively, a convenient value between these boundaries can be selected for the 

challenge particulate concentration (Ctest). 

Once the required test solution volume and concentration have been determined, the total 

number of challenge particulates required for the test can be calculated using Equation 3.4. If a 

molecular marker is used, Equation 3.4 would be used to determine the total mass of the marker 

required. 

TCPP Ctest Vtest = • Equation 3.4

Where: TCPP = total challenge particulate population 

(number or mass of particles)

Ctest = feed concentration of challenge particulate 

(number or mass/ volume)

Vtest = challenge test solution volume (gallons) 
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3.10.4 Challenge Particulate Seeding Method

There are two approaches commonly used to introduce the challenge particulate into the 

test solution: batch seeding and in-line injection. Batch seeding involves the introduction of the 

total challenge particle population (TCPP) into the entire volume of test solution followed by 

complete mixing to a uniform concentration. In-line injection allows for continuous or 

intermittent introduction of challenge particulates into the feed stream entering the membrane 

filtration system. The specific method used may depend on the circumstances of the particular 

challenge test, although either is permitted under the LT2ESWTR.

Batch seeding is simpler and requires less equipment than in-line injection, but it is only 

feasible when the entire test solution volume is contained in a reservoir. Furthermore, mixing 

the challenge particulates into large volumes of water to create a uniform concentration can be 

logistically problematic. For these reasons, batch seeding is typically only used in challenge 

studies for small-scale modules with a relatively small membrane area, for which test solution 

volumes are easier to handle and mix.

In-line injection is the most common seeding approach used in challenge studies, 

particularly for those involving full-scale modules with greater membrane area. In-line injection 

allows the challenge particulate to be introduced into the feed on either a continuous or 

intermittent basis. In general, continuous seeding is advantageous for challenge testing, although 

intermittent seeding may be appropriate for long-term studies in which it is only necessary to 

seed organisms at key times during an operational cycle. If intermittent seeding is used, it is 

necessary to ensure that equilibrium is achieved during each seeding event prior to collection of 

any feed or filtrate samples.

In-line injection requires additional equipment, such as chemical feed pumps, injection 

ports, and in-line mixers. These components must be properly designed and integrated into the 

test apparatus to ensure a consistent challenge particulate concentration in the feed. A chemical 

metering pump that delivers an accurate and steady flow of challenge material is recommended, 

while pumps that create a pulsing action should be avoided. The injection port should introduce 

the challenge material directly into the bulk feed stream to aid in dispersion. Examples of 

acceptable and unacceptable injection ports are shown in Figure 3.1. An in-line static mixer 

should be placed downstream of the injection port, and a feed sample tap should be located 

approximately ten pipe diameters downstream of the mixer.
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Figure 3.1 Schematic of Acceptable and Unacceptable Sample Injection Ports

The in-line injection method of seeding delivers challenge particulates from a stock 

solution with a known concentration as calculated using Equation 3.5:

SS

SS V

TCPP C = Equation 3.5

Where: Css = challenge particulate concentration in the stock solution 

(number or mass / volume)

TCPP = total challenge particulate population 

(number or mass of particles)

Vss = challenge particulate stock solution volume (gallons)

The TCPP is calculated according to Equation 3.4, while the stock solution volume, Vss, can be 

selected for convenience. However, the Vss should be between 0.5 and 2 percent of the total test 

solution volume, as determined using Equation 3.1.

Once the concentration of the stock solution has been determined, the stock solution 

delivery rate (SSDR) for the in-line injection method of seeding can be calculated using 

Equation 3.6:
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C R

C Q

SSDR

ss

test p

•

•

= Equation 3.6

Where: SSDR = stock solution delivery rate (gpm)

Ctest = feed concentration of challenge particulate 

(number or mass/ volume)

Qp = filtrate flow (gpm)

Css = challenge particulate concentration in the stock solution

(number or mass / volume)

R = system recovery during test (decimal percent)

The SSDR represents the setting for the chemical feed pump used to inject the challenge 

particulate into the feed stream.

3.10.5 Example: Challenge Test Solution Design

Scenario:

Using the methodology described in sections 3.10.1 through 3.10.4, design a challenge 

test solution using the following assumptions and parameters: 

• The target LRV for the challenge test is 4 log.

• The membrane module has an area of 100 m2.

• The maximum flux for the module 85 Lmh. 

• The module operates in deposition mode.

• A test duration of 30 minutes is required to conduct the required sampling

• A system hold-up volume of 200 L.

• The filtrate sample volume used during the challenge test is 500 milliliters.

• The detection limit for the filtrate sampling technique is 1 particle per 500 mL.

• Challenge particulate seeding is conducted via continuous in-line injection.

Solution:

Step 1: Determine the required test solution volume.

V V SF

R

Q T

V hold eq

p

test •

















+ +

•

=

min Equation 3.1

Tmin = 30 minutes from given information

R = 100 percent standard for deposition mode 

hydraulic configuration 
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Vhold = 200 L from given information

Veq = ? to be determined

SF = ? to be determined

Qp = ? to be determined

Assume that assume that the equilibrium volume is equal to three times the holdup volume, as discussed in section 3.10.2: 

Veq = 3•Vhold

Veq = 3•(200 L)

Veq = 600 L

Also, as discussed in section 3.10.2, a suitable safety factor is approximately in 

the range of 1.1 to 1.5. Since no other information is given in this example, a 

value for the safety factor is arbitrarily assumed.

SF = 1.2

The filtrate flow, Qp, can be calculated simply by multiplying the given maximum 

flux and the membrane area (and converting to convenient units), as shown 

below:













•

=

hour

Lmh m

Qp min 60

85( ) 100( )

2

Qp = 142 L/min

Therefore, the required test solution volume can be calculated as follows:

200 600 1.2

1

30(min)

min

142

•

























+ +

 •











= L L

L

Vtest

Vtest = 6,072 L 
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Step 2: Determine feed challenge particulate concentration within the bounds established by 

Equations 3.2 and 3.3. 

Minimum feed concentration:

C DL LRVt

f = • −min 10 Equation 3.3

LRVt = 4 log from given information

DL = 1 particles per 500 mL from given information

= 2 particles per L

Cf 10 2 particles/ L

4

min = • −

Cf 2 10 particles/ L

4

min = • −

Maximum feed concentration:

Cf − = (3.16 •10 ) • DL 6

max

DL = 1 particles per 500 mL from given information

= 2 particles per L

Cf(3.16 10 ) 2 particles / L

6

max = • • −

Cf 6.32 10 particles/ L

6

max = • −

Select a feed challenge particle concentration between the minimum 

(2 • 104particles/L) and maximum (6.32 • 106 particles/L). Arbitrarily:

Ctest = 5 • 104 particles/L
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Step 3: Determine the total challenge particulate population required for the test solution. 

TCPP Ctest Vtest = • Equation 3.4

Ctest = 5 ? 104 particles/L as calculated in step 2 above

Vtest = 6,072 L as calculated in step 1 above

(5 10 / ) (6,072 )

4

TCPP = • particles L • L

TCPP particles 8= 3.036 •10

Step 4: Determine the challenge particulate stock solution concentration using Equation 3.5.

SS

SS V

TCPP C = Equation 3.5

TCPP = 3.036 ? 108 particles as calculated in step 3 above

Vss = ? to be determined

As discussed in section 3.10.4, select a stock solution volume that is between 

0.5 and 2 percent of the total test solution volume of 6,072 L (as determined in 

step 1 above). Thus:

(0.005 ? 6,072 L) < Vss < (0.02 ? 6,072 L)

(30.4 L) < Vss < (121.4 L)

 Vss = 100 L arbitrary selection

Substituting values:

L

particles CSS 1003.036 108•

=

CSS 3.036 10 particles / L

6

= •
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Step 5: Determine the challenge particulate stock solution delivery rate for continuous, in-line 

injection. 

C R

C Q

SSDR

ss

test p

•

•

= Equation 3.6

Ctest = 5 ? 104 particles/L as calculated in step 2 above

Qp = 142 L/min as calculated in step 2 above

Css = 3.036 ? 106 particles/L as calculated in step 4 above

R = 100 percent standard for deposition mode 

hydraulic configuration

Substituting values:

(3.036 10 / ) 1

(5 10 / ) (142 / min)

6

4

• •

• •

=

particles L

particles L L

SSDR

SSDR = 2.53 L/min

3.11 Challenge Test Systems

A system used for challenge testing must be carefully designed to both meet the 

objectives of the test and simulate full-scale operation to the greatest practical extent. Guidance 

for both designing an appropriate test apparatus and determining operational parameters for 

challenge testing is provided in the following subsections.

3.11.1 Test Apparatus

The equipment used to conduct challenge testing is product-specific to some extent, 

although there are some basic components that are common to all systems. In many cases, a 

manufacturer may maintain a special test apparatus to check individual modules as a component 

of its QA/QC program. Such an apparatus may be suitable for conducting challenge testing and 

typically includes equipment such as pumps, valves, instrumentation, and controls necessary to 

evaluate full-scale modules. This same type of equipment would be used in the design of 

systems for testing small-scale modules.

Both the seeding and sampling methods selected for challenge testing, as well as the 

hydraulic configuration of the system, affect the design of the test apparatus. Batch seeding 

requires a feed tank and mixing equipment, while continuous seeding requires a stock solution 

reservoir, chemical metering pump, and in-line mixers. Sampling requirements may dictate the 
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location and design of sample taps in the system. In addition, the test apparatus should be 

designed to mimic the hydraulic configuration of the full-scale system as much as practical; 

however, the test apparatus may alternatively utilize a more conservative recovery than the fullscale system. If 100 percent recovery (i.e., the most conservative scenario) is used, a crossflow 

system must operate without a bleed stream such that all of the concentrate is recirculated (see 

section 2.5.2.2), and a deposition mode system (see section 2.5.1) must filter the entire test 

solution volume. Note that a full-scale crossflow system could not operate at 100 percent 

recovery on a sustained basis, since the feed would become increasingly concentrated. However, 

operation at 100 percent recovery is feasible for a short-term challenge test in the interest of 

generating conservative results. The test apparatus should allow the membrane module to 

undergo direct integrity testing both before and after the challenge test. Figures 3.2 through 

Figure 3.6 are schematic representations of typical apparatuses for challenge testing under 

various conditions. Note that ancillary equipment and operational processes (e.g., backwash, 

chemical cleaning, and integrity testing) are not shown. 

Figure 3.2 illustrates a pressure-driven apparatus operating in deposition mode with batch 

seeding and composite sampling. This type of system may be well suited for a MCF or other 

membrane module with limited surface area. With this apparatus the test solution is prepared as 

a batch and a composite filtrate sample would be generated, yielding a single data pair (i.e., a 

feed sample and a composite filtrate sample) for the purposes of calculating the log removal 

efficiency for the challenge particulate. 

Figure 3.2 Schematic of a Typical Pressure-Driven System in Deposition Mode 

with Batch Seeding and Composite Sampling

Figure 3.3 illustrates an apparatus similar to that shown in Figure 3.2, but designed for 

continuous in-line seeding and grab sampling rather than batch seeding and composite sampling. 

With this apparatus the challenge particulate is introduced from a stock solution reservoir and 

mixed prior to the feed sampling point. The use of grab sampling allows the collection of 

multiple feed and filtrate samples from a single test run.
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Figure 3.3 Schematic of a Typical Pressure-Driven System in Deposition Mode 

with Continuous Seeding and Grab Sampling

Figure 3.4 shows a schematic of a pressure-driven apparatus operated in suspension mode 

with continuous seeding and grab sampling. In systems that operate in suspension mode, the 

concentration of suspended solids increases on the feed side of the membrane, as discussed in 

section 2.5. While it may not be practical to accurately replicate the solids concentration profile 

for a membrane system in which the feed side concentration of suspended solids varies as a 

function of filtration time, concentrate recirculation can produce conservative feed side 

conditions for the purpose of challenge testing, assuming appropriate operating conditions for 

recovery and recycle ratio are selected. Selection of an appropriate recovery for challenge 

testing can be complicated by the fact that system recoveries can vary significantly in some cases 

(particularly for NF/RO systems). Guidance regarding the selection of an appropriate recovery 

for a challenge test of a membrane system operated in suspension mode is provided in section 

3.11.2. The recycle ratio should be selected such that velocities across the membrane surface are 

high enough to keep particles in suspension. The manufacturer can typically recommend a 

minimum scour velocity for a crossflow system.

For systems that utilize concentrate recycle there are some additional considerations that 

are important to take into account regarding the feed side system volume and the location of the 

feed sample point. In general, larger feed side system volumes require longer system 

equilibration times. For example, Figure 3.4 shows the concentrate return location at the feed 

tank rather than directly into the module feed line, thus increasing the effective feed side system 

volume significantly. If such an arrangement is necessary (e.g., to provide an air break in the 

recirculation system), then the feed tank volume should be minimized. In an apparatus utilizing 

concentrate recycling, the feed sample point must be located upstream of the return point, as 

shown in Figure 3.4.
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Figure 3.4 Schematic of a Typical Pressure-Driven System in Suspension 

Mode with Continuous Seeding and Grab Sampling

Figure 3.5 shows a typical vacuum-driven test apparatus operated in deposition mode 

with continuous seeding and grab sampling. Although the module is immersed in a tank, the 

feed water is not agitated, thus allowing particles to deposit on the membrane surface. With this 

apparatus, the filtrate sampling point must be located downstream of the vacuum pump. 

Figure 3.6 illustrates an apparatus for a vacuum-driven system operated in suspension 

mode with continuous seeding and grab sampling. With this apparatus the feed tank is 

mechanically agitated to keep particles in suspension and can be modeled as a continuous stirred 

tank reactor (CSTR) (see section 2.5). As with the vacuum-driven apparatus shown in 

Figure 3.5, the filtrate sampling point must be located downstream of the vacuum pump.
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Figure 3.5 Schematic of a Typical Vacuum-Driven System in Deposition Mode 

with Continuous Seeding and Grab Sampling

Figure 3.6 Schematic of a Typical Vacuum-Driven System in Suspension Mode 

with Continuous Seeding and Grab Sampling
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3.11.2 Test Operating Conditions

The design of a challenge test includes specifications for the following operating 

conditions: flux, recovery, and hydraulic configuration. The LT2ESWTR requires the challenge 

test to be conducted at the maximum design flux and recovery (40 CFR 141, Subpart W, 

Appendix C), and that the test apparatus be operated under representative or conservative 

hydraulic conditions (40 CFR 141, Subpart W, Appendix C). These requirements dictate the 

operating conditions for the test apparatus during challenge testing. Note that under the 

LT2ESWTR, recovery is defined as the volumetric percent of feed water that is converted to 

filtrate in the treatment process over the course of an uninterrupted operating cycle (i.e., 

excluding losses that occur due to the use of filtrate in backwashing or cleaning operations).

Testing at the maximum recovery is important to ensure that the volumetric concentration 

factor (VCF) simulated during challenge testing is representative of (or conservative for) fullscale system operation. For systems that operate in deposition (i.e., direct flow or “dead-end”) 

mode, such as most MF/UF and MCF systems, the value of the VCF is one, and thus the 

recovery does not have a significant impact on the suspended solids concentration. However, it 

is still recommended that systems operating in deposition mode process at least 90 percent of the 

challenge test solution, thus resulting in an effective recovery of at least 90 percent. Similarly, 

for MF/UF systems that operate in suspension mode but without a concentrate waste stream (i.e., 

a bleed stream), it is also recommended that at least 90 percent of the challenge test solution be 

processed in order to generate an effective recovery of at least 90 percent. For MF/UF systems 

that operate with a concentrate waste stream, it is recommended that a recovery of at least 

75 percent be utilized for challenge testing unless a more representative system recovery can be 

demonstrated by the manufacturer; a recovery of 100 percent would represent the most 

conservative case. NF/RO systems, for which the utilization of a concentrate waste stream is 

standard, should be operated at a recovery of at least 45 percent for the purposes of challenge

testing, which is representative of a flow-weighted average recovery for a module in such a 

system operating at an overall recovery of greater than 90 percent for a single stage. If challenge 

testing is conducted on a small-scale NF/RO system (such as one using a single module), then 

concentrate recycle should be used to increase the recovery to at least 45 percent.

The membrane area is also typically given in the specifications, and the maximum filtrate 

flow can be calculated by multiplying the membrane area and the maximum flux. Although the 

LT2ESWTR stipulates specific requirements for challenge testing flux, the filtrate flow is also 

necessary for designing the challenge test solution, as described in section 3.10. Table 3.5 

summarizes some typical specifications for membrane area and maximum flux associated with 

various types of membrane modules, as well as the corresponding filtrate flow. For the purposes 

of challenge testing, the membrane area exposed to the feed (i.e., as opposed to that to the 

filtrate) should be used in all calculations. Note that the values listed in Table 3.5 are examples 

only, and that particular product specifications will vary by module manufacturer.
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Table 3.5 Typical Parameters for Various Types of Modules 

Module

Example 

Membrane Area

{ft2 (m2)} 

Example 

Maximum Flux

{gfd (Lmh)}

Filtrate Flow

{gpm (L/min)}

Cartridge filter 5 (0.46) 1,364 (2,316) 4.8 (18.2)

4” diameter 75 (7.0) 17.8 (30.2) 0.9 (3.4)

Spiral-wound

8” diameter 350 (32.5) 17.8 (30.2) 4.3 (16.3)

Outside-in 350 (32.5) 53.5 (90.8) 13.1 (49.6)

Hollow-fiber

Inside-out 1,400 (130) 107 (182) 103 (390)

1 Note that because MCF is a new concept introduced with the LT2ESWTR, the example specifications cited 

are for cartridge filters, in general, not necessarily for MCF devices

A challenge test should be designed to simulate the hydraulic configuration of full-scale 

system operation, since it affects the concentration of suspended solids on the feed side of the 

membrane and thus the removal efficiency of the process. A membrane filtration system can be 

operated in either suspension of deposition mode. (The various hydraulic configurations for 

membrane filtration systems are discussed in further detail in section 2.5.) While it is relatively 

straightforward to simulate a system operating in deposition mode during a challenge test, it may 

not be possible to simulate all variations of suspension mode operation using a single module 

challenge test apparatus. For example, it is not practical to simulate a plug flow reactor (PFR) 

configuration typical of full-scale RO systems. In such cases, the challenge test apparatus should 

be designed and operated as a CSTR, since a CSTR configuration generally results in the highest 

concentration of suspended solids on the feed side of the membrane. If the challenge test can 

successfully demonstrate the target LRV with a higher concentration of suspended solids on the 

feed side of the membrane than expected with the full-scale system, then the use of a CSTR in 

place of a PFR configuration would represent more conservative, and thus acceptable, challenge 

test conditions. A single RO module can be operated as a CSTR by including a concentrate 

recycle loop, as shown in Figure 3.4. 

3.12 Sampling 

Although the LT2ESWTR does not stipulate any particular requirements for sampling, it 

is an important component of a challenge test. An effective sampling program is dependent on 

the development of detailed and thoroughly documented sampling plan, as well as the selection 

of appropriate sampling methods and locations. These critical aspects of sampling, along with 

the monitoring of operational parameters during the execution of the challenge test, are discussed 

in the following subsections.
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3.12.1 Sampling Methods 

The two most common approaches for sampling are the grab and composite methods. 

Grab sampling involves the collection of one or more aliquots from the feed or filtrate stream, 

while composite sampling involves collection of the entire process stream for processing and 

subsequent analysis. The concentration of challenge particulates in the feed solution is typically 

characterized through grab sampling, while the filtrate stream may be sampled using either grab 

or composite sampling. If grab sampling is used for both the feed and filtrate streams, the 

number of feed and filtrate samples does not need to be equivalent, and samples can be collected 

on different schedules during the challenge test. In many cases, it may be advantageous to 

collect more filtrate samples than feed samples, since filtrate concentrations are expected to be 

very low, and an error of just a few particles in a filtrate sample can have a significant impact on 

the demonstrated removal efficiency. Moreover, if batch seeding is used, the feed concentration 

should not vary significantly over the course of challenge testing, assuming appropriate feed 

stock mixing. However, if continuous seeding is used, paired sampling may be preferred for 

simplicity of data reduction.

Grab sampling typically involves the collection of a predetermined volume of water in an 

appropriate collection vessel at predetermined times, as documented in the sampling plan. In 

some cases, a composite sample is collected directly into a sampling vessel; however, it is more 

common to capture the challenge particulate during composite sampling. A composite sample is 

usually compiled by passing the entire filtrate stream through an absolute filter capable of 100 

percent capture of the challenge particulate. The challenge particulate would then be enumerated 

either directly from the filter media or removed from the filter for subsequent analysis. Grab 

samples typically do not require this type of processing in the field, and any extraction or 

concentration steps for grab samples are typically conduct in the lab.

It is important that good sampling practices be employed during challenge testing, such 

as flushing taps prior to sample collection (if applicable) and isolating filtrate sampling locations 

from feed sampling locations to prevent cross-contamination. Furthermore, appropriate QA/QC 

measures should be implemented during sampling, such as collection of duplicate samples and 

blanks.

3.12.2 Sample Port Design and Location

As with the challenge particulate injection port, the design of the feed and filtrate sample 

withdraw ports should yield a uniform sample. Figure 3.7 illustrates examples of both 

acceptable and unacceptable sampling ports. The unacceptable apparatus requires the sample to 

be pulled from a large section of pipe and has an area where stagnation of sample flow may 

occur. By contrast, the acceptable apparatus has a quill that extends into the center of the pipe to 

obtain a more representative sample.
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Figure 3.7 Schematic of Acceptable and Unacceptable Sampling Ports

The feed sample tap should be located at least ten pipe diameters downstream of 

challenge particulate injection points and in-line mixers to ensure uniform concentration. For 

apparatuses that utilize concentrate recycle, as illustrated in Figure 3.4, the feed sample tap 

should be located upstream of the T-connection where the concentrate is blended with the 

incoming feed water. A check valve may be used to prevent backflow of concentrate into the 

feed line. As a guideline, the feed sample rate should be no more than 1 percent of the flow to 

the membrane. Filtrate samples should be collected at a point after the filtrate passes through 

any filtrate side instrumentation such that any important measurements are not affected by the 

sampling event. In vacuum-driven apparatuses, the filtrate sample tap must be located 

downstream of the filtrate pump, as shown in Figures 3.5 and 3.6. Note that the filtrate 

sampling valve should be positioned as close as possible to the filtrate port in order to minimize 

error due to any potential for adhesion of the challenge particulate to the piping prior to the 

sampling point. In addition, if a microorganism is used as the challenge particulate, it is prudent 

to use a metal or heat-resistant sampling valve to allow the tips of the valve to be flamesterilized. 

3.12.3 Process Monitoring 

During the challenge test it is important that the operational parameters be monitored to 

ensure that the test conditions remain constant. Continuous monitoring for flow (or flux) and 

pressure is should be conducted if the apparatus is equipped with the appropriate 

instrumentation. If periodic monitoring must be utilized, it is important that operational 

parameters be checked at least both before beginning and after completing sampling for the 

challenge particulate. Operational data collected prior to initiating sampling should be used 

verify that the flux and recovery are at the required levels. It is also important that the membrane 
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module(s) undergo direct integrity testing both before and after challenge testing to verify that 

the modules were integral during the test. 

If the water quality of the test solution matrix is a particular concern, other water quality 

parameters should be sampled accordingly. Some examples of other parameters that may be 

important in some cases include pH, temperature, turbidity, TDS, TOC, and ionic strength. The 

measurement of such water quality parameters may be most relevant if a molecule marker is 

used as the challenge particulate, as discussed in section 3.10.1.

All aspects of process monitoring, including what parameters to monitor, how often to 

monitor, and the range of acceptable results should be included the sampling plan for the 

challenge test (see section3.12.4). If any results are outside acceptable tolerances, the challenge 

test should be restarted.

3.12.4 Sample Plan Development

The primary purpose of a sampling plan is to define the samples to be collected and 

provide an accompanying sampling schedule for the challenge test. A sampling plan should 

include:

• Type of sample(s) (i.e., composite vs. grab)

• Number of feed and filtrate samples to be collected

• Sample locations

• Sampling interval

• Estimate of time required to collect each sample

• Sampling equipment required

• Sample volume(s)

• Process monitoring requirements (see section 3.12.3)

The sampling plan should also specify any particular requirements associated with the analytical 

technique to be employed, and well as procedures for shipping the sample(s) if they are to be 

analyzed at an off-site laboratory. Samples should be collected, preserved, stored, prepared, and 

analyzed using methods and techniques appropriate for the challenge particulate.

Sampling should not begin until the system has stabilized (i.e., reached equilibrium 

concentration). Most test apparatuses achieve greater than 90 percent of equilibrium 

concentrations after three hold-up volumes have passed through the system. Thus, the system 

hold-up volume and feed flow can be used to estimate the point at which the system is near 
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equilibrium and thus the time at which sampling can begin. Both the hold-up volume and time 

required for system stabilization (as discussed in section 3.10.2) should be included in the 

sampling plan.

3.13 Analysis and Reporting of Challenge Test Results 

After challenge testing is completed for a particular product, the results must be analyzed 

to determine the established removal efficiency of the module (i.e., LRVC-Test) for the purposes of 

LT2ESWTR compliance. The following subsections provide guidance regarding the calculation 

of removal efficiency under the rule, the statistical analysis of the challenge test results, and 

summarizing challenge testing in a report for state review.

3.13.1 Calculation of Removal Efficiency

The removal efficiency established during challenge testing – LRVC-Test – is determined 

from the various LRVs generated during the testing process. The LT2ESWTR requires that a 

single LRV be generated for each module tested for the product line under evaluation. The 

LRVs for each respective module tested are then combined to yield a single value of LRVC-Test

that is representative of the product line. 

Under the LT2ESWTR, the LRV is calculated according to Equation 3.7 (40 CFR 141, 

Subpart W, Appendix C):

log( ) log( ) LRV = C f − Cp Equation 3.7

Where: LRV = log removal value demonstrated during a challenge test

Cf = feed concentration of the challenge particulate 

(number or mass/ volume)

Cp = filtrate concentration of the challenge particulate 

(number or mass/ volume)

Note that the feed and filtrate concentrations must be expressed in identical units (i.e., based on 

equivalent volumes) in order for Equation 3.7 to yield a valid LRV. If the challenge particulate 

is not detected in the filtrate, then the term Cp is set equal to the detection limit.

The overall value of LRVC-Test (i.e., the removal efficiency of the product) is based on the 

entire set of LRVs obtained during challenge testing, with one representative LRV established 

per module tested. The manner in which LRVC-Test is determined from these individual LRVs 

depends on the number of modules tested. Under the LT2ESWTR, if fewer than 20 modules are 

tested, then the lowest representative LRV among the various modules tested is the LRVC-Test . 

If 20 or more modules are tested, then the 10th percentile of the representative LRVs is the 

LRVC-Test . The percentile is defined by [i/(n+1)] where “i" is the rank of “n” individual data 
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points ordered from lowest to highest. It may be necessary to calculate the 10th percentile using 

linear interpolation (40 CFR 141, Subpart W, Appendix C).

Although the LT2ESWTR requires that one representative LRV be established per 

module tested, the rule does not restrict the manner in which the representative LRV for each 

module is calculated. Consequently, there are numerous methods that could be used to calculate 

the representative LRV for a module. If multiple feed/filtrate sample pairs are collected, a LRV 

can be calculated for each set of paired data, and the LRV for the tested module could be 

selected as the lowest LRV (more conservative) or the average of the LRVs (less conservative). 

Another approach is to average all the respective feed and filtrate concentrations from among the 

various samples collected and calculate a single LRV for a tested module using Equation 3.7. A 

more conservative approach would be to use the average feed concentration but the maximum 

filtrate concentration sampled, which would result in a lower representative LRV for a tested 

module. Likewise, a still more conservative approach would be to use the minimum feed and 

maximum filtrate concentrations. Note that these methods simply represent potential options; 

other approaches may be used for calculating a representative LRV for each module tested. 

Regardless of the particular method used, the range of data collected for a single module can 

provide some indication about the experimental error associated with the study (i.e., errors due to 

seeding, sampling, analysis, etc.). If a statistically valid sampling method was used to select the 

modules for challenge testing (as described in section 3.7), a comparison of the LRVs across the 

different modules tested would provide an indication of variability within the product line.

3.13.2 Statistical Analysis

If a sufficient number of modules are evaluated in the course of a challenge test to be 

considered a statistically significant sample of the product line, it may be useful to conduct a 

formal statistical analysis of the data in order to make inferences about the entire population 

from the sample set. Note that such a statistical analysis is not considered a substitute for the 

methodology for determining LRVC-Test required under the rule, but could provide the 

manufacturer with useful information. For example, if challenge testing were conducted using a 

random sampling of membranes from a production lot, a statistical analysis of the challenge test 

data (i.e., the LRV observed for each of the modules tested) would provide an estimate of the 

range of removal efficiency for the entire product line. This information could be used to infer 

the number of membrane modules in the product line that would be expected to achieve the 

LRVC-Test . If it is desirable to apply a statistical analysis to the LRV data generated during 

challenge testing, the particular method of analysis should be considered in the design of the 

challenge test protocol, since the method to be employed may dictated the number of modules 

selected for testing.
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3.13.3 Reporting

The LT2ESWTR does not mandate any reporting requirements for challenge testing. 

However, the results of the challenge test must be acceptable to any state that permits a particular 

membrane module product line to be used for obtaining Cryptosporidium removal credit under 

the rule. In order to facilitate this acceptance by the various states, a sample outline of a 

challenge test report is provided as follows. Note that this outline should be customized to meet 

any particular state requirements.

1) Introduction

a) Description of testing organization

b) Test site

c) Description of membrane filtration product

d) Testing objectives (including target LRV)

2) Membrane Modules and Test Apparatus

a) Membrane module specifications for each module evaluated

b) Considerations for small-scale module testing (if applicable)

c) Non-destructive performance testing

d) Description of test apparatus 

3) Challenge Test Protocol

a) Challenge particulate (including rationale for selection)

b) System operating conditions

c) Challenge test solution design 

d) Seeding method

e) Process monitoring

f) Detailed sampling plan

g) QA/QC procedures

h) Data management

4) Results and Discussion

a) Summary of measured system operating conditions

b) Summary of LRV results for each module tested

c) Summary of system integrity evaluation

d) Determination of removal efficiency

e) Summary of NDPT results for each module tested

f) QCRV determination based on the results of non-destructive performance testing

g) Statistical evaluation of results (if applicable)

5) Summary and Conclusions

a) Summary description of membrane filtration product

b) Summary of challenge test protocol

c) LRV demonstrated during challenge testing

d) Quality control release value for non-destructive performance testing
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3.14 Re-Testing of Modified Membrane Modules

As a component of ongoing innovation and product development, manufacturers may 

make changes to a particular product line or its associated manufacturing process. If such a 

change affects the fundamental characteristics of the module, the removal efficiency, and/or the 

NDPT results and associated QCRV, the LT2ESWTR requires the modified product to be resubjected to challenge testing (40 CFR 141, Subpart W, Appendix C).

Because it is not possible to develop a comprehensive listing of the potential 

modifications that would require re-testing, the need to re-test a modified product must be 

evaluated on a case-by-case basis and at the discretion of the state. Some examples of membrane 

properties, which if modified, might alter the fundamental removal characteristics of a module, 

include:

• Membrane material

(e.g., a change in the polymer or backing material)

• Pore size (nominal and absolute)

• Porosity

• Permeability 

• Membrane symmetry

(i.e., symmetric, asymmetric, or composite)

As shown in the examples given above, most of the changes that may necessitate retesting are modifications to the membrane material itself, such as changes to the polymer 

chemistry and/or pore size distribution. Even if the change to the membrane media is not 

intended to affect removal efficiency or NDPT parameters, it may be necessary to re-test the 

module, since a modification to an intrinsic property of the membrane media could have an 

impact on one or both of these criteria.

Minor changes or changes that are not related to the fundamental removal characteristics 

of the membrane media or NDPT parameters, would not require re-testing. For example, a 

change in the membrane area within a module would not be expected to affect either the removal 

efficiency or non-destructive performance testing.

Although not directly related to the product itself, any modifications to the 

manufacturer’s NDPT may also require that the product line be re-subjected to challenge testing, 

since the results of the specific NDPT used ensure that the modules produced meet the minimum 

requirements for achieving the demonstrate LRV. However, if the modified NDPT can be 

correlated to the specific test associated with product line’s challenge test, additional challenge 

testing may not need to be conducted, at the discretion of the state.
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Modifications to the hydraulic configuration of a membrane filtration system might 

warrant re-testing, since the concentration of suspended solids on the feed side of the membrane 

may be affected, in turn potentially affecting removal efficiency. In determining whether or not 

a modification to the hydraulic configuration would require re-testing, one significant factor is 

whether the hydraulic conditions of the original challenge test are cons ervative for the new 

hydraulic configuration of the system. In general, testing performed under a hydraulic 

configuration with a higher VCF than that associated with the modification would be considered 

conservative. A summary of the various hydraulic configurations and associated VCFs is 

provided in Table 2.3, and duplicated for reference as Table 3.6, below. 

Table 3.6 Typical Range of VCF Values for Various Hydraulic Configurations

Hydraulic Configuration VCF

Deposition 

mode Dead-end 1

PFR 3 - 20

Crossflow 4 - 20 Suspension 

mode

CSTR 4 - 20

In general, most modifications to the module itself (as opposed to the membrane media) 

are unlikely to affect either removal efficiency or non-destructive performance testing, although 

re-testing might be warranted in some cases. 

The product modifications addressed in this section are provided as guidance to help 

assess whether or not a particular change might require a product line to undergo a new 

challenge test under the LT2ESWTR. This discussion is not intended to be comprehensive. In 

addition, the inclusion of a particular modification in this section does not necessarily imply that 

re-testing is required for all such modifications. Although the manufacturer can exercise its 

discretion in deciding the circumstances under which additional testing is necessary, the state has 

authority regarding whether or not to accept a particular change such that the product line would 

still be eligible to receive Cryptosporidium removal credit under the rule without re-testing.

3.15 Grandfathering Challenge Test Data From Previous Studies

As a result of the commercialization of membrane treatment processes for drinking water 

production, a number of states, consulting engineers, and independent testing organizations have 

developed programs to demonstrate the removal efficiency of membrane systems. Under these 

existing programs, numerous challenge studies have been conducted in which complete removal 

of Cryptosporidium was demonstrated, independent of the challenge level (USEPA 2001). The 
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state has the discretion to accept data from a previous study provided that the testing was 

conducted in a manner that demonstrates removal efficiency for Cryptosporidium commensurate 

with the treatment credit awarded to the process (40 CFR 141, Subpart W, Appendix C). 

While a previously conducted challenge test may adequately demonstrate the required 

Cryptosporidium removal efficiency, it may be more difficult to correlate the results of a prior 

test to the NDPT currently used by the manufacturer. If the state determines that the 

grandfathered test does not meet the intent of the LT2ESWTR, a new challenge test that is 

consistent with the rule requirements must be conducted.

As a general guide, the following challenge test conditions have been identified as 

potentially yielding results that do not satisfy the intent of the rule:

• Challenge testing conducted on obsolete products: Refer to section 3.14 for guidance 

on the re-testing of modified membrane modules.

• Challenge testing conducted on small-scale modules: Small-scale module testing is 

permitted under the LT2ESWTR if certain criteria are met. Refer to section 3.8 for 

guidance regarding the testing of small-scale modules.

• Challenge testing using unacceptable surrogates for Cryptosporidium: The challenge 

particulate used in a grandfathered test must provide equivalent or sufficiently 

conservative removal efficiency relative to Cryptosporidium oocysts. Refer to 

section 3.9 regarding the selection of surrogates for use in challenge testing. 

• Challenge particulate enumeration using unacceptable methodology: The challenge 

particulate must have been quantified using an acceptable method. Specifically, gross 

measurements are generally considered unacceptable. Refer to section 3.9 regarding 

methods for enumerating various challenge particulates.

• Unavailable QCRV: If non-destructive performance testing was not used to establish 

a suitable QCRV in a previous study, it may be difficult or impossible to relate the 

demonstrated removal efficiency to the NDPT results for untested modules that are 

produced. 

There may also be cases in which deviations from challenge testing requirements under 

the LT2ESWTR may not be significant, such that additional testing would not be required. 

Some potential cases in which challenge test data that do not specifically comply with the rule 

might receive favorable consideration for grandfathering, at the discretion of the state, are listed 

below.

• Removal efficiency determine using different method: It may be necessary to 

recalculate the removal efficiency (i.e., LRVC-Test) from a previous challenge test 

according to the requirements of the LT2ESWTR. If some moderate deficiencies 

exist in the data such that the LRVC-Test cannot be calculated according to the 
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procedure described in this guidance manual, the state may exercise discretion as to 

whether or not to approve the prior test. The state may also evaluate the impact of 

such deficiencies and consequently consider reducing the LRVC-Test on the basis of 

this evaluation.

• Elevated feed challenge particulate concentration: The LT2ESTR places a limit on 

the feed concentration of the challenge particulate that essentially establishes a 

maximum LRV of 6.5 in order to prevent excessive over-seeding during challenge 

studies. However, exceeding this maximum feed concentration does not necessarily 

invalidate a prior test. In reviewing grandfathered data, it is most important to 

consider whether or not high LRVs were achieved through excessive over-seeding. 

For example, if a removal study was conducted at a challenge level of 108 particles/L 

and all filtrate concentrations were less than 10 particles/L, the process could 

potentially qualify for the maximum removal credit of 6.5 log. By contrast, if the 

same challenge level was applied to a membrane that allowed 103 particles/L to pass 

into the filtrate, states may opt not to qualify the test for a LRV of 5, since the high 

LRV is likely a result of over-seeding an otherwise inadequate or non-integral 

membrane.

• Challenge testing at a flux or recovery other than the maximum design value(s): 

Deviations from testing at the maximum flux and/or recovery might not exclude a 

prior test from consideration for grandfathering. Most available data suggests that 

flux and recovery will not significantly impact the removal efficiency of a membrane 

filtration process for Cryptosporidium-sized particles. If complete removal of the 

challenge particulate was achieved during the challenge test, then deviations from 

testing at maximum design flux and recovery values can probably be ignored.

• Challenge testing using a poor quality feed water matrix: Although it is 

recommended that challenge testing be conducted using clean make-up water for the 

test solution and unfouled membranes to provide a conservative estimate of removal 

efficiency, prior studies conducted with waters with a greater fouling potential (e.g., 

untreated surface waters) can potentially be considered to meet the LT2ESWTR 

challenge test requirement, since the removal of Cryptosporidium-sized particles is 

generally unaffected by the presence of foulants for membrane filtration processes as 

defined under the LT2ESWTR. However, recent research suggests that integrity 

defects on the order of 200 μm can be obscured by foulants in some cases, improving 

pathogen rejection (Lozier et al. 2003). Thus, prior studies conducted with poorer 

quality water should be considered for grandfathering on a case-by-case basis. 
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4.0 Direct Integrity Testing

4.1 Introduction

In order for a membrane process to be an effective barrier against pathogens and other 

particulate matter, the filtration system must be integral, or free of any leaks or defects resulting 

in an integrity breach. Thus, it is critical that operators are able to demonstrate the integrity of 

this barrier on an ongoing basis during system operation. Direct integrity testing represents the 

most accurate means of assessing the integrity of a membrane filtration system that is currently 

available.

Under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), a direct 

integrity test is defined as a physical test applied to a membrane unit in order to identify and 

isolate integrity breaches (40 CFR 141, Subpart W, Appendix C). In order to receive 

Cryptosporidium removal credit for compliance with the rule, the removal efficiency of a 

membrane filtration process must be routinely verified during operation using direct integrity 

testing. The direct integrity test must be applied to the physical elements of the entire membrane 

unit, including membranes, seals, potting material, associated valves and piping, and all other 

components which could result in contamination of the filtrate under compromised conditions 

(40 CFR 141, Subpart W, Appendix C).

There are two general classes of direct integrity tests that are commonly used in 

membrane filtration facilities: pressure-based tests and marker-based tests. The pressure-based 

tests are based on bubble point theory (as described in Appendix B) and involve applying a 

pressure or vacuum (i.e., negative pressure) to one side of a membrane barrier and monitoring for 

parameters such as pressure loss or the displacement of air or water in order to establish whether 

an integrity breach is present. The various pressure-based tests include the pressure- and vacuum 

decay tests, the diffusive airflow test, and the water displacement test. Marker-based tests utilize 

either a spiked particulate or molecular marker to verify membrane integrity by directly 

assessing removal of the marker, similar to a challenge test.

The LT2ESWTR does not require the use of a particular direct integrity test for rule 

compliance, but rather that any test used meet the specified performance criteria for resolution, 

sensitivity, and frequency. Thus, a particular system may utilize an appropriate pressure- or 

marker-based test or any other method that both meets the performance criteria and is approved 

by the state. The performance criteria for direct integrity tests are summarized as follows:

• Resolution: The direct integrity test must be responsive to an integrity breach on the 

order of 3 μm or less (40 CFR 141, Subpart W, Appendix C).

• Sensitivity: The direct integrity test must be able to verify a log removal value 

(LRV) equal to or greater than the removal credit awarded to the membrane filtration 

process (40 CFR 141, Subpart W, Appendix C).
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• Frequency: A direct integrity test must be conducted on each membrane unit at a 

frequency of no less than once every 24 hours of operation (40 CFR 141.728). (The 

definition of a membrane unit under the LT2ESWTR is provided in section 1.5.)

In addition to the performance criteria, the rule also requires the establishment of a control limit

for the direct integrity test that is indicative of an integral membrane unit capable of achieving 

the Cryptosporidium removal credit awarded by the state. If the results of the direct integrity test 

exceed this limit, the rule requires that the affected membrane unit be taken off-line for 

diagnostic testing and repair (40 CFR 141.728). The performance criteria for direct integrity 

tests, as well as the establishment of control limits, are described in further detail in section 4.2. 

The objective of Chapter 4 is to describe the various pressure- and marker-based direct 

integrity tests currently in use and how these tests can be applied to meet the performance criteria 

specified under the LT2ESWTR. Diagnostic tests, data collection, and reporting are also 

addressed.

This chapter is divided into the following sections:

Section 4.2: Test Resolution

This section discusses the determination of pressure- and marker-based 

direct integrity test resolution for meeting the performance criteria 

required by the rule.

Section 4.3: Test Sensitivity

This section discusses the determination of pressure- and marker-based 

direct integrity test sensitivity for meeting the performance criteria 

required by the rule, including general concepts and methods.

Section 4.4: Test Frequency

This section reviews the direct integrity testing frequency requirement of 

the rule.

Section 4.5: Establishing Control Limits

This section describes the mathematical and experimental determination of 

control limits for direct integrity testing.

Section 4.6: Example: Establishing Direct Integrity Test Parameters

This section illustrates the calculation of some of the critical direct 

integrity test performance criteria, including test resolution, sensitivity, 

and control limits for an example membrane filtration system.

Section 4.7: Test Methods

This section provides an overview of the various types of pressure- and 

marker-based tests, including generic test protocols as well as some 

advantages and disadvantages of each.
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Section 4.8: Diagnostic Testing

This section describes some of the diagnostic tests that are used to identify 

and isolate integrity breaches following a failed direct integrity test.

Section 4.9: Data Collection and Reporting

This section provides guidance on direct integrity test data collection and 

reviews the associated reporting requirements of the rule.

4.2 Test Resolution

Resolution is defined as the smallest leak that contributes to the response from a direct 

integrity test. Any direct integrity test applied to meet the requirements of the LT2ESTWR is 

required to have a resolution of 3 μm or less (40 CFR 141, Subpart W, Appendix C). This 

resolution criterion is based on the lower size range of Cryptosporidium oocysts and is intended 

to ensure that any integrity breach large enough to pass oocysts contributes to a response from 

the direct integrity test used. The manner in which the resolution criterion is met depends on 

whether the direct integrity test is pressure-based or marker-based, as described in the following 

subsections.

4.2.1 Pressure-Based Tests

In order to achieve a resolution of 3 μm with pressure-based direct integrity tests, the net 

pressure applied during the test must be great enough to overcome the capillary forces in a 3 μm 

hole, thus ensuring that any breach large enough to pass Cryptosporidium oocysts would also 

pass air during the test. The minimum applied test pressure necessary to achieve a resolution of 

3 μm is calculated using Equation 4.1:

( ) max Ptest = 0.193 •k •s • cosq + BP Equation 4.1

Where: Ptest = minimum test pressure (psi)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

BPmax = maximum backpressure on the system during the test (psi)

0.193 = constant that includes the defect diameter (i.e., 3 μm 

resolution requirement) and unit conversion factors 

Equation 4.1 is based on bubble point theory and is derived from the balancing of 

capillary static forces. Note that the constant of 0.193 accounts for the LT2ESWTR resolution 

requirement of responding to a defect of 3 μm in diameter, as well as the appropriate unit 

conversion factors, in order to simplify the equation for the purposes of rule compliance. The 

general form of Equation4.1 includes the capillary diameter as a variable and represents an 
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expression relating this diameter to the bubble point pressure. A discussion of bubble point 

theory and a derivation of Equation 4.1 is provided in Appendix B.

Values for several parameters in Equation 4.1 must be determined in order to calculate 

the minimum test pressure necessary to achieve a resolution of 3 μm when using a pressurebased integrity test. The parameters κ and θ are intrinsic properties of the membrane. In the 

absence of data supplied by the membrane manufacturer, conservative values of κ = 1 and θ = 0 

should be used. Appendix B provides an additional discussion of these parameters. The surface 

tension, s, is inversely related to temperature; consequently, the surface tension at the coldest 

anticipated water temperature should be used to calculate a conservative value for the minimum 

required test pressure. As a point of reference, the surface tension of water at 5 oC is 

74.9 dynes/cm. Substituting these three values (i.e., κ = 1, θ = 0, and s = 74.9 dynes/cm) into 

Equation 4.1 yields the following simplified equation:

5 max Ptest =14. + BP Equation 4.2

Where: Ptest = minimum test pressure (psi)

BPmax = maximum backpressure on the system during the test (psi)

Equation 4.2 indicates that the minimum test pressure necessary to achieve a 3-μm 

resolution is 14.5 psi plus the maximum backpressure on the system during application of a 

pressure-based direct integrity test. Ideally, there should be no hydrostatic backpressure on the 

system during the test. However, it is not always practical to perform the test without any 

hydrostatic backpressure, and in these cases the additional backpressure must be considered in 

establishing the minimum test pressure necessary to meet the resolution criterion. For example, 

there might be hydrostatic pressure on the undrained side of the membrane if a pressure-driven 

membrane module remains filled with water or if a vacuum-driven (i.e., immersed) membrane 

remains under water in a basin. Thus, if the bottom of the membrane is under 7 feet of water, 

BPmax would be approximately 3 psi, yielding a Ptest value of 17.5 psi to achieve a resolution of 

3 μm.

Both Equations 4.1 and 4.2 assume that the applied pressure remains constant during the 

direct integrity test. However, in many cases there may be some baseline decay (e.g., that 

attributable to diffusion) that is measurable over the duration of the test. In this case, it is 

important to account for this baseline decay in the resolution calculation. Thus, in order to 

ensure that the resolution requirement is satisfied throughout the duration of the test, the 

anticipated pressure at the end of the direct integrity test should be used to calculate the 

resolution. This value can be estimated using the initial applied pressure, the typical rate of 

baseline pressure decay for a fully integral system, and the duration of the test. If the baseline 

decay is small enough such that the final test pressure is within approximately 5 percent of the 

initial applied pressure, the baseline decay can be assumed to be negligible, and the initial 

applied pressure may be used to calculate the test resolution.
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The LT2ESTWR does not establish the minimum test pressure to be used during a 

pressure-based direct integrity test, but rather only requires that the test achieve a 3-μm 

resolution. If a membrane manufacturer has information to support the use of values other than 

κ = 1 and θ = 0, and these less conservative values are approved by the state, then Equation 4.1 

can be used to calculate the minimum required test pressure. It is essential that the use of values 

other than κ = 1 and θ = 0 be scientifically defensible, since the use of inappropriate values could 

result in the use of a test pressure that does not meet the resolution criterion established by the 

rule. One approach for determining membrane specific values for κ and θ is through direct 

experimental evaluation. It is also possible that data from the non-destructive performance test 

(NDPT) (as described in section 3.6) may be used to support the use of a (κ ? cosθ) product less 

than 1.0 (i.e. κ < 1 and/or θ > 0). The latter approach would require an empirical correlation to 

be developed between test pressure and minimum pore size. 

Although the rule does not include a frequency requirement for the recalculation of 

resolution, the resolution should be recalculated if the system backpressure during direct 

integrity testing is adjusted. Alternatively, if desired, direct integrity test instrumentation and 

data recording system could be configured to calculate the resolution after each application of the 

direct integrity test using the applied test pressure, system backpressure, and surface tension 

corresponding to the temperature at which the test is conducted. Note that the liquid-membrane 

contact angle can also change over the life of a membrane module (e.g., as a result of the 

adsorption of organic matter by the membrane material), and these changes may not necessarily 

be uniform among the various modules in a unit (Childress et al. 1996; Jucker et al. 1994). Thus, 

if a value other θ = 0 (i.e., the most conservative value) is used, then it may be appropriate to 

periodically recalculate the resolution based on a revised estimate of the actual value of θ, an 

exercise that may necessitate destructive testing of a representative sample of membrane 

modules in the system.

4.2.2 Marker-Based Tests

A marker-based direct integrity test can be viewed as a “mini-challenge study,” in which 

a surrogate is periodically applied to the feed water in order to verify the integrity of a membrane 

filtration system. In order to meet the resolution criterion of the rule, the surrogate used in a 

marker-based test must have an effective size of 3 μm or smaller, as described in section 3.9.2. 

A marker-based direct integrity test can use either particulate or molecular surrogates, but in 

either case, it must be established that the surrogate meets the resolution criterion. Section 3.9 

presents guidelines for the selection of a conservative surrogate for Cryptosporidium during 

challenge testing, and these same guidelines are applicable to selection of an appropriate 

surrogate for a marker-based direct integrity test. The effective size of the marker can be 

established through any accepted methodology such as size distribution analysis of particulate 

markers or estimation techniques based on the molecular weight and geometry of molecular 

markers.
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4.3 Test Sensitivity

Sensitivity is defined as the maximum log removal value that can be verified by the direct 

integrity test (i.e., LRVDIT) (40 CFR 141, Subpart W, Appendix C). The sensitivity of the direct 

integrity test establishes the maximum log removal credit that a membrane filtration process is 

eligible to receive if it is less than or equal to that demonstrated during challenge testing (i.e., 

LRVC-Test). For example, if the challenge test demonstrated a LRVC-Test of 5.5 log, and the direct 

integrity test is capable of verifying a LRVDIT of 4.5 log, the membrane filtration process would 

be eligible for removal credit up to 4.5 log. Although the sensitivity of the direct integrity test 

should not be expected to vary significantly over time, the determination of sensitivity as 

described in this section is design to produce a conservative result that would remain applicable 

over the life of the membrane filtration system. However, if significant changes occur in terms 

of operational parameters, direct integrity test conditions, or any basic assumptions that might 

affect the value of the direct integrity test sensitivity, it is suggested that the sensitivity be 

reestablished to verify that it is at least equal to the removal credit awarded to the process.

The sensitivity of a direct integrity test is logarithmic in nature. For example, a test with 

a LRVDIT of 5 log is 100 times more sensitive than a test with a LRVDIT of 3 log. Thus, when a 

higher sensitivity is required, the test must be capable of measuring very small changes in the 

direct integrity test response and distinguishing these results from background or baseline data. 

Data suggest that many direct integrity tests, as currently applied, have sensitivities in excess of 

4 log; however, sensitivity must be determined on a case-by-case basis using the information 

provided by the membrane manufacturer and the guidance in this document. While 

determination of integrity test sensitivity is complex, it provides a rational basis for awarding 

high removal credits to membrane processes that are commensurate with their abilities. As was 

the case with resolution, the manner in which sensitivity is determined depends on whether the 

type of direct integrity test used is pressure- or marker-based.

4.3.1 Pressure-Based Tests

The discussion in this section regarding the calculation of sensitivity for pressure-based 

direct integrity tests is divided into three parts, as follow. First, the basic concepts that are 

applicable to all pressure-based tests are introduced. The subsequent section describes 

calculation of the sensitivity for pressure-based direct integrity tests based on this general 

conceptual framework. A third section discusses the determination of diffusive (or baseline) 

losses in a fully integral system during the application of a pressure-based direct integrity test.

4.3.1.1 Basic Concepts

The determination of sensitivity for pressure- based direct integrity tests is more complex 

than for marker-based tests. The equation used to determine the sensitivity of a pressure-based 

integrity test is specified in the LT2ESWTR (40 CFR 141, Subpart W, Appendix C) and given 

below as Equation 4.3:
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Membrane Filtration Guidance Manual 4-7 June 2003

Proposal Draft 

















•

=

breach

p

DIT VCF Q

Q

LRV log Equation 4.3

Where: LRVDIT = direct integrity test sensitivity in terms of LRV 

(dimensionless)

Qp = membrane unit design capacity filtrate flow (L/min) 

Qbreach = flow from the breach associated with the smallest integrity 

test response that can be reliably measured, referred to as 

the critical breach size (L/min) 

VCF = volumetric concentration factor (dimensionless)

Equation 4.3 represents a dilution model that assumes water passing through the intact 

membrane barrier is free of the particulate contaminant of interest and that water flowing through 

an integrity breach has a particulate contaminant concentration equal to that on the high pressure 

side of the membrane. Under these assumptions, LRVDIT is a function of the ratio of total filtrate 

flow to flow through the critical breach (i.e., Qp/Qbreach), which quantifies the dilution of the 

contaminated stream passing through the breach as it mixes with treated filtrate. For a 

membrane unit of a given capacity (i.e., constant Qp), LRVDIT will increase as Qbreach decreases. 

This implies that a more sensitive direct integrity test capable of detecting a smaller breach can 

verify a higher log removal value and thus potentially increase the removal credit that a 

membrane filtration system is eligible to receive.

The term VCF is a dimensionless term that accounts for the increase in the suspended 

solids concentration that occurs on the feed side of the membrane for some hydraulic 

configurations. In the denominator of Equation 4.3, the term Qbreach is multiplied by the VCF to 

incorporate the impact of this concentration effect on the sensitivity of the direct integrity test. 

The VCF is calculated as the ratio of the concentration of suspended solids maintained on the 

feed side of the membrane to that in the influent feed water, as shown in Equation 4.4:

f

m

C

C

VCF = Equation 4.4 (2.18)

Where: VCF = volumetric concentration factor (dimensionless)

Cm = concentration of suspended solids maintained on the feed 

side of the membrane (number or mass / volume)

Cf = concentration of suspended solids in the influent feed water 

to the membrane system (number or mass / volume)

The VCF generally ranges between 1 and 20, and the value depends on the hydraulic 

configuration of the system. Membrane systems that operate in deposition mode do not increase 
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the concentration of suspended solids on the feed side of the membrane and thus have a VCF 

equal to 1. In contrast, membrane systems that operate in suspension mode, such as a crossflow 

hydraulic configuration, typically have a VCF in the range of 4 to 20, representing a 4 to 20-fold 

increase in the suspended particle concentration on the feed side of the membrane. The methods 

and equations used to calculate the VCF for various hydraulic configurations are provided in 

section 2.5, and Table 2.4 presents equations for calculating both the average and maximum 

VCF for various hydraulic configurations. Alternatively, the VCF could be determined 

experimentally. 

Note that the LT2ESWTR does not specify use of the maximum or average VCF value in 

calculating sensitivity, but the rule does require that the increase in suspended solids 

concentration on the high-pressure side of the membrane, as occurs with some hydraulic 

configurations, be considered in the calculation. The maximum VCF typically ranges from 1 to 

20 and provides the most conservative value for LRVDIT, while the average VCF typically ranges 

from 1 to 7. In selecting between the maximum, average, or any other value for the VCF, 

consideration should be given to the concentration profile along the membrane surface in the 

direction of water flow and the implication of integrity breaches at various locations within in the 

membrane unit. For example, although the maximum VCF does provide the most conservative 

value for LRVDIT, this value represents only a very small portion of the concentration profile and 

thus is only representative of breaches that occur at the extreme end of the membrane unit. 

Similarly, some systems exhibit a concentration profile as a function of time within a filtration 

cycle, and the maximum VCF only occurs at the end of the filtration cycle immediately before a 

backwash event; prior to this time, the VCF is significantly lower.

4.3.1.2 Calculating Sensitivity

The sensitivity of a pressure-based direct integrity test can be calculated by converting 

the response from a pressure-based test that measures the flow of air (e.g., the diffusive airflow 

test) or rate of pressure loss (e.g., the pressure decay test) to an equivalent flow of water through 

an integrity breach during normal operation, as described in the following subsection. The

second subsection outlines a general procedure for determining the threshold response of a 

pressure-based direct integrity test experimentally, if this information is not available from the 

membrane filtration system manufacturer.

Calculating Sensitivity Using the Air-Liquid Conversion Ratio

In order to calculate the LRVDIT, the flow through the critical breach for a direct integrity 

test (i.e., Qbreach) must be determined (as shown in Equation 4.3). Since most direct integrity 

tests do not directly measure Qbreach, it is necessary to establish a correlation between the direct 

integrity test response and the flow of water through the critical breach during system operation. 

In some of the most commonly used pressure-based direct integrity tests, including the pressureor vacuum-decay test and the diffusive airflow test, air is applied to the drained side of a 

membrane and subsequently flows through any integrity breach that exceeds the test resolution. 

The response from such a test is typically measured as pressure decay or airflow. In order to 
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Proposal Draft 

relate the response from a pressure-based integrity test to Qbreach, it is necessary to establish a 

correlation between airflow and liquid flow through the critical integrity breach.

This correlation can be characterized through the air-liquid conversion ratio (ALCR), 

which is defined as the ratio of air that would flow through a breach during a direct integrity test 

to the amount of water that would flow through the breach during filtration, as defined in 

Equation 4.5: 

















=

breach

air

Q

Q

ALCR Equation 4.5

Where: ALCR = air-liquid conversion ratio (dimensionless)

Qair = flow of air through the critical breach during a pressurebased direct integrity test (L/min)

Qbreach = flow of water through the critical breach during filtration 

(L/min)

The ALCR can be used to express the liquid flow through a breach in terms of an 

equivalent flow of air, as shown in Equation 4.6:













=

ALCR

Q

Q

air

breach Equation 4.6

Where: Qbreach = flow of water through the critical breach during filtration 

(L/min)

Qair = flow of air through the critical breach during a pressurebased direct integrity test (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

Substituting Equation 4.6 into the general expression for sensitivity (Equation 4.3) yields 

the following expression:

















•

•

=

Q VCF

Q ALCR

LRV

air

p

DIT log Equation 4.7

Where: LRVDIT = direct integrity test sensitivity in terms of LRV 

(dimensionless)

Qp = membrane unit design capacity filtrate flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)
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Qair = flow of air through the critical breach during a pressurebased direct integrity test (L/min)

VCF = volumetric concentration factor (dimensionless)

Equation 4.7 can be used to directly calculate the sensitivity for any pressure-based direct 

integrity test that is based on bubble point theory and measures the flow of air (Qair) through an 

integrity breach. The four parameters that need to be determined to calculate sensitivity are: Qp, 

VCF, ALCR, and Qair. The VCF can be established as previously described in section 4.3.1.1. 

Qp is the design capacity filtrate flow approved by the state, expressed in liters per minute, from 

the membrane unit to which the direct integrity test is applied. For a constant Qbreach, higher 

filtrate flows yield greater direct integrity test sensitivity. Thus, if different sizes of membrane 

units with varying capacity are used in the same treatment system, the sensitivity of the direct 

integrity test should be independently determined for each unit size. The flow of air, Qair, is 

related to the response from the direct integrity test. For tests that measure the airflow through 

an integrity breach directly, such as the diffusive airflow test, Qair is simply the results of the test. 

On the other hand, methods such as the pressure- and vacuum-decay tests yield results in terms 

of pressure loss per unit time, which must be converted to an equivalent flow of air using 

Equation 4.8:

+ 14.7

Δ •

=

BP

P V

Q

test sys

air Equation 4.8

Where: Qair = flow of air (L/min)

ΔPtest = rate of pressure decay during the integrity test (psi/min)

Vsys = volume of pressurized air in the system during the test (L)

BP = backpressure on the system during the test (psi)

Note that Equation 4.8 assumes that the temperature of the water and air are the same 

since the air temperature should rapidly equilibrate with the water temperature. In addition, Vsys

and the ALCR should be measured at the same reference temperature and pressure. Vsys

encompasses the entire pressurized volume, including all fibers (typically the inside of the fibers 

is pressurized), piping, and other void space on the pressurized side.

Substituting Equation 4.8 into Equation 4.7 yields Equation 4.9, which can be used to 

calculate the sensitivity of a direct integrity test that measures the rate of pressure or vacuum 

decay.
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Proposal Draft 

















Δ • •

• • +

=

P V VCF

Q ALCR BP

LRV

test sys

p

DIT

( 14.7)

log Equation 4.9

Where: LRVDIT = direct integrity test sensitivity in terms of LRV 

(dimensionless)

Qp = membrane unit design capacity filtrate flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

BP = backpressure on the system during the test (psi)

ΔPtest = rate of pressure decay during the integrity test (psi/min)

Vsys = volume of pressurized air in the system during the test (L)

VCF = volumetric concentration factor (dimensionless)

Regardless of whether the flow of air (Qair) or pressure decay rate (ΔPtest) is measured 

during the direct integrity test, the smallest response from the test that can be reliably measured 

and associated with an integrity breach should be used in the sensitivity calculation. This should 

not be confused with the baseline integrity test response from an integral membrane unit, since 

there may be a small flow of air or pressure decay due to diffusion of air through water in the 

wetted pores, even if there are no breaches in the system. In many cases, this smallest 

measurable response associated with an integrity breach may be provided by the membrane 

filtration system manufacturer. If this information is not available from the system 

manufacturer, it may be determined experimentally by progressively creating small integrity 

breaches of a known size in an otherwise integral membrane unit in order to determine the 

smallest measurable response from the direct integrity test that is distinguishable from the 

baseline response for an integral membrane unit. A general procedure for this experimental 

method is described in the following subsection (“Measuring The Threshold Direct Integrity Test 

Response Experimentally”). 

The basic procedure for calculating the ALCR involves first making a reasonable 

assumption regarding whether the flow through the critical breach is laminar or turbulent for the 

particular membrane filtration system of interest. Then the ALCR can be calculated using the 

appropriate equation, as summarized in Table 4.1. Note that the equations in Table 4.1 assume 

that the flow regimes for the passage of air and water through an integrity breach are the same 

(i.e., both either laminar or turbulent). If this assumption is not considered appropriate for a 

specific system and/or application such that inaccurate estimates for direct integrity test 

sensitivity may result, a hybrid approach may be considered, as described in Appendix C.
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Table 4.1 Approaches for Calculating the ALCR

Module Type Defect Flow 

Regime Model ALCR Equation Appendix C Equation

Turbulent2 Darcy pipe 

flow

( )

( T ) TMP

P BP P

Y

test test

+ •

− • +

• •

460

( 14.7)

173 C.4

Hollow-fiber1

Laminar HagenPoiseuille3

(460 )

527 (175 2.71 0.0137 )

2

TMP T

Peff T T

• +

• Δ • − • + •

C.15

Turbulent Orifice 

( )

( T ) TMP

P BP P

Y

test test

+ •

− • +

• •

460

( 14.7)

173 C.9

Flat sheet4

Laminar HagenPoiseuille3

(460 )

527 (175 2.71 0.0137 )

2

TMP T

Peff T T

• +

• Δ • − • + •

C.15

1 Or hollow -fine-fiber

2 Typically characteristic of larger diameter fibers and higher differential pressures

3 The binomial in the Hagen_Poiseuille equation (C.15) approximates the ratio of water viscosity to air viscosity and is valid for 

temperatures ranging from approximately 32 to 86 oF. Additional details are provided in Appendix C.

4 Includes spiral-wound and cartridge configurations

The various parameters given in the ALCR equations listed in Table 4.1 include the 

following:

Y = net expansion factor for compressible flow through a pipe to a larger 

flow area (dimensionless)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the direct integrity test (psi)

ρw = density of water (lbs/ft3)

T = water temperature (oF)

TMP = transmembrane pressure (psi)

ΔPeff = effective integrity test pressure (psi)

Additional guidance for both calculating the ALCR and determining appropriate values for the 

component parameters of the respective ALCR equations (as listed above) is provided in 

Appendix C, along with derivations of the ALCR equations. Further information regarding the 

net expansion factor (Y) may be found in various hydraulics references, including Crane (1988). 

Note that although the Darcy and orifice equations for the ALCR are identical, the method for 

determining the net expansion factor (Y) is different for these two models, as described in 

Appendix C.
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The ALCR can also be determined via empirical means, which would be applicable to 

any flow regime or configuration of membrane material and are independent of a particular 

hydraulic model. Some manufacturers may have developed empirical models that could be used 

to determine the ALCR. If an empirical approach is preferred for determining the ALCR, and a 

valid empirical model is not available for the system, it may be necessary to develop one. One 

conceptual procedure for empirically deriving the ALCR for hollow-fiber membrane filtration 

systems is the correlated airflow measurement (CAM) technique; the details of this procedure are 

presented in Appendix D.

Measuring the Threshold Direct Integrity Test Response Experimentally

The smallest measurable response of a pressure-based direct integrity test that is 

associated with a known breach can be evaluated experimentally if this information is not 

available from the membrane filtration system manufacturer. For pressure-based tests, this 

response corresponds to the value of ΔPtest that should be used in the calculation of sensitivity. 

This experimental evaluation involves intentionally compromising system integrity in small, 

discrete, and quantifiable steps and monitoring the corresponding integrity test responses. In the 

case of MF/UF systems, several fiber-cutting studies conducted to evaluate the threshold 

response of various direct integrity tests have been documented in the literature (Adham et al. 

1995; Landsness 2001). In general, the procedure for measuring the threshold response 

experimentally involves the following steps:

1. The membrane system is determined to be integral through the application of a direct 

integrity test.

2. The investigator intentionally compromises a membrane to generate a known defect. 

Examples of such compromises include generating a hole in the membrane using a 

pin of a known diameter or cutting a hollow fiber at a predetermined location. In 

order to identify the threshold response, it is desirable to utilize a small integrity 

breach, such as a single cut fiber in a membrane unit. 

3. After compromising the membrane, the integrity of the membrane unit is measured 

using the designated direct integrity test. 

4. The process is repeated with additional defects of progressively increasing size or 

quantity until a measurable response from the direct integrity test is detected. This 

minimum measurable response represents ΔPtest for the purposes of calculating the 

sensitivity of a direct integrity test.

4.3.1.3 Diffusive Losses / Baseline Decay

If it is determined to be appropriate for the membrane under consideration, the diffusive 

losses that constitute the baseline integrity test response may be subtracted from the smallest 
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measurable response associated with an integrity breach for the purpose of determining 

sensitivity. For example, if a pressure decay rate of 0.05 psi/min is typical for an integral 

membrane unit and the limitations of the test are such that the smallest pressure decay rate that 

can be reliably associated with an integrity breach is 0.12 psi/min, the incremental response 

associated with an integrity breach is 0.07 psi/min, and this value may be used in the sensitivity 

calculation. In general, diffusive losses are most likely to be observed over the duration of the 

direct integrity test for thin-skinned, asymmetric membranes. The manufacturer should be able 

to provide information regarding whether or not diffusive losses are expected to be significant. 

If a high level of sensitivity is required for a membrane filtration system, baseline diffusive 

losses may be an important consideration. 

For porous MF, UF, and MCF membranes, diffusive losses occur during direct integrity 

testing because a certain amount of compressed air used during the test dissolves into the water 

in the fully-wetted pores and is transported across the membrane surface. In order to calculate 

the diffusive losses, it can be assumed that the water fills the pores of the membrane and forms a 

film of thickness z, and that diffusion directly through the membrane material itself is 

insignificant in comparison to the diffusion across the film of water. Using these assumptions, 

Equation 4.10 illustrates the relationship between diffusive losses and the various parameters that 

influence these losses.

( )

















+

•

 •









 • • − • •∈

= •

14.7

6

BP

R T

z

D A P BP H

Q

gas aw m test

diff Equation 4.10

Where: Qdiff = diffusion of air through the water held in the membrane 

pores (L/min)

Daw = diffusion coefficient for air in water (cm2/s) 

Am = total membrane surface area to which the direct integrity 

test is applied (m2)

Ptest = membrane test pressure (psi)

BP = backpressure on the system during the test (psi)

H = Henry’s constant for air-water system (mol/psi-m

3

)

ε = membrane porosity (dimensionless decimal)

z = membrane thickness (mm)

Rgas = universal gas constant (L-psia/mol-K)

T = water temperature during direct integrity test (K)

6 = unit conversion factor

Note that the dimensionless porosity (ε) is defined as the ratio of area of pores to the total 

membrane area in the unit. This term should not be confused with the pore size of porous MF, 

UF, and MCF membranes, which is given in terms of the limiting dimension of the openings in 

the membrane. The porosity of the membrane material can typically be provided by the 

manufacturer, if necessary. In addition, the diffusion flow path, which is affected by porosity, 

tortuosity, and the differential pressure across the membrane, is approximated by the membrane 

thickness (z) in Equation 4.10. Because membrane porosity and tortuosity may be difficult to 
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Proposal Draft 

measure, thus making it problematic to accurately quantify the actual length of the diffusion flow 

path, a more precise empirical method accounting for these two factors as a combined term has 

been developed by Farahbakhsh (2003). Both the diffusion coefficient (Daw) and Henry’s 

constant (H) vary with temperature, and Henry’s constant also varies somewhat with the 

concentration of dissolved solids in the water. However, these affects may partially offset and 

may not be significant. Values for Daw and H as a function of these variables (as applicable) may 

be found in standard tables in the literature.

The parameters given above for Equation 4.10 are applicable to flat sheet porous 

membranes, such as those used in membrane cartridge configurations. For porous membranes in 

a hollow-fiber configuration, such as most MF and UF systems, the following modifications are 

required:

Am = log mean total membrane area to which the direct integrity 

test is applied (m2)

= (A2-A1)/ln(A2/A1) 

A1 = total membrane surface area to which the direct 

integrity test is applied based on the inside fiber 

diameter

A2 = total membrane surface area to which the direct 

integrity test is applied based on the outside fiber 

diameter

z = differential fiber radius (mm)

= r2- r1 

r1 = inside radius of the hollow fiber

r2 = outside radius of the hollow fiber

It is generally accepted that diffusion of air across the membrane can reduce the 

measured LRVDIT. Under most circumstances, the amount of diffusion is small in comparison to 

the flow of air through a breach. However, if the membrane has a propensity to diffuse a 

significant amount of air (e.g., if the porosity is unusually high) or if a high level of sensitivity is 

required, it may be necessary to account for diffusive losses. Typically, there is only limited 

information available regarding the amount of diffusion that occurs for membrane processes used 

in water treatment under production conditions; however, MF/UF membrane manufacturers can 

typically provide a value for the baseline decay during a pressure-based direct integrity test for 

their specific proprietary systems, so it is generally not necessary to explicitly calcula te diffusive 

losses using Equation 4.10. 
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In the absence of information provided by the manufacturer, it may be advantageous to 

directly measure the baseline pressure decay on an integral membrane unit, a process that should 

be conducted using clean membranes to avoid the potential for fouling to artificially hinder 

diffusion. Because the diffusive loss is directly proportional to temperature (as shown in 

Equation 4.10), and the diffusion coefficient (which is also directly proportional to diffusive 

loss) also increases with temperature, it is also important to characterize the baseline decay for 

membrane filtration system at the highest anticipated water temperature in order to generate a 

conservative value for diffusive loss. 

With semi-permeable NF and RO membranes, diffusive losses occur via the diffusion of 

air through the saturated membrane material itself. However, because NF and RO modules are 

manufactured separately from the accompanying filtration systems and inserted manually into 

pressure vessels, small seal leaks may occur that can be difficult to distinguish from baseline 

decay. Thus, it is recommended that baseline response for a pressure-based direct integrity test 

be evaluated for each unit in a NF/RO system on a site-specific basis.

If the membrane module manufacturer has information for typical diffusion airflow rates 

per unit of membrane area, the expected diffusive airflow for the entire membrane unit should be 

calculated and compared against the baseline observed during the unit-specific evaluation. If the 

observed baseline is significantly higher than the predicted diffusive losses, the result could be 

indicative of an integrity problem, and diagnostic testing (see section 4.8) may be necessary to 

identify the source of additional airflow or pressure loss. If the membrane module manufacturer 

is only able to provide a diffusion coefficient for air through the membrane material, 

Equation 4.11 may be used to estimate the diffusive airflow for a membrane unit with a known 

membrane area, if necessary:

( )

















+

•

 •









 • • − •

= •

14.7

6

BP

R T

z

D A P BP H

Q

gas am m test

diff Equation 4.11

Where: Qdiff = diffusion of air through a semi-permeable membrane

 (L/min)

Dam = diffusion coefficient for air through a saturated semipermeable membrane material (cm2

/s)

Am = total membrane surface area to which the direct integrity 

test is applied (m2)

Ptest = membrane test pressure (psi)

BP = backpressure on the system during the test (psi)

z = membrane thickness (mm)

Rgas = universal gas constant (L-psia/mol-K)

T = water temperature during direct integrity test (K)

6 = unit conversion factor

Note that the equations for the diffusion of air through porous and semi-permeable 

membranes – Equation 4.10 and 4.11, respectively – are very similar. These equations differ 
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Proposal Draft 

only in that Equation 4.11 for semi-permeable membranes does not require the membrane 

porosity and utilizes a diffusion coefficient for a composite membrane layer consisting of both 

the membrane and the water of saturation bound in the microscopic interstices of the membrane 

material.

Because spiral-wound NF/RO membranes are typically composite structures consisting 

of two or more layers (as described in section 2.3.1), it is important that the membrane thickness 

(z) used corresponds to the layer to which the diffusion coefficient (Dam) provided by the 

membrane manufacturer applies. For example, the diffusion coefficient may apply to the thin, 

active, semi-permeable layer, in which case the membrane thickness would correspond to this 

layer only. Alternatively, if the diffusion coefficient is a composite representing the diffusion of 

air through all the layers of the membrane taken as whole, then the thickness used should be that 

of the entire membrane, including all layers.

Both Equations 4.10 and 4.11 show that diffusive airflow is directly proportional to 

membrane area, the applied direct integrity test pressure, and the system backpressure. As a 

result, the decay should be quantified for each membrane unit of different size in the system and 

also recalculated if either the applied test pressure or system backpressure are modified. 

If the sensitivity is calculated using the ALCR approach described in section 4.3.1.2 and 

if diffusion is significant, Equation 4.6 can be modified to compensate for diffusive airflow as 

shown in Equation 4.12:

ALCR

Q Q

Q

air diff

breach

−

= Equation 4.12

Where: Qbreach = flow of water through the critical breach during filtration 

(L/min) 

Qair = flow of air through the critical breach during a pressurebased direct integrity test (L/min)

Qdiff = diffusive air flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

Combining Equation 4.12 with Equation 4.7 yields Equation 4.13, which enables the 

calculation of sensitivity using the ALCR approach, taking into account diffusive losses. 
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Membrane Filtration Guidance Manual 4-18 June 2003

Proposal Draft 

( )















• −

•

=

air diff

p

DIT VCF Q Q

Q ALCR

LRV log Equation 4.13

Where: LRVDIT = direct integrity test sensitivity in terms of LRV 

(dimensionless)

Qp = filtrate flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

VCF = volumetric concentration factor (dimensionless)

Qair = flow of air (L/min)

Qdiff = diffusive air flow (L/min)

Note that Equations 4.12 and 4.13 are applicable to MCF, MF/UF, and NF/RO membrane 

filtration systems.

4.3.2 Marker-Based Tests

Sensitivity for marker-based direct integrity tests is determined via a straightforward 

calculation of the log removal value, similar to the determination of log removal values during a 

challenge study, as shown in Equation 4.14 (reference to similar equation in Chapter 3 is given in 

parentheses):

log( ) log( ) LRV DIT = Cf − Cp Equation 4.14 (3.7)

Where: LRVDIT = direct integrity test sensitivity in terms of LRV 

(dimensionless)

Cf = feed concentration (number or mass / volume)

Cp = filtrate concentration (number or mass / volume)

In the use Equation 4.14 to calculate sensitivity of a marker-based test, the LT2ESWTR 

specifies that the feed concentration, Cf, is the typical feed concentration of the marker used in 

the test, and that the filtrate concentration, Cp, is the baseline filtrate concentration of the marker 

from an integral membrane unit. Due to variability in dosing the marker, the day-to-day LRV is 

likely to vary during system operation. Thus, in order to establish the sensitivity of a markerbased direct integrity test using Equation 4.14, it is necessary to assume an appropriately 

conservative feed concentration such that the LRVs determined on a day-to-day basis meet or 

exceed the LRVDIT unless there is an integrity breach. An example of such a conservative 

approach would be the use of the lower bound of the anticipated concentration range of seeded 

marker as the feed concentration for the purposes of calculating the sensitivity of the test. 

In order to optimize sensitivity and reliability of a marker-based direct integrity test, it is 

important to use an accurate method for quantifying the feed and filtrate concentrations. Since 
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Proposal Draft 

the feed and filtrate concentrations will differ by orders of magnitude, an analytical method with 

a wide dynamic range is desired. If such a range is not available using a single device, two 

different instruments may need to be used to measure these respective concentrations. 

Regardless of the dynamic range of the instrument(s), it is likely that different analytical volumes 

will need to be used to deal with the different concentration ranges, but the concentrations will 

have to be expressed in terms of equivalent volumes for the purpose of calculating an LRV. 

Some specific considerations regarding the use of particulate and molecular marker-based direct 

integrity tests are discussed in section 4.7.5.

4.4 Test Frequency

Most currently available direct integrity tests require the membrane unit to be taken offline for testing and thus are conducted in a periodic manner, requiring a balance between the 

need to verify system integrity with desire to minimize system downtime and lost productivity. 

In addition, although some marker-based tests may be conducted while the membrane unit is online and in production, it is general neither practical nor cost effective to implement these tests 

on a continual basis. Thus, the frequency at which direct integrity testing is conducted for 

membrane filtration systems represents a compromise between these competing objectives.

The LT2ESWTR requires that direct integrity testing be conducted on each membrane 

unit at least once every 24 hours of unit operation for rule compliance. This minimum test 

frequency is intended to balance the need to verify system integrity as often as possible with cost 

and production considerations and is based in part on a USEPA report that indicated daily direct 

integrity testing was relatively common practice at membrane filtration facilities (USEPA 2001).

The state may require more frequent integrity testing for compliance with the rule at its 

discretion. An alternate test frequency may be based on a variety of considerations, such as the 

frequency at which integrity breaches occur and the integration of process safety practices, such 

as maintaining filtrate storage with a detention time equivalent to or longer than the time 

between direct integrity test events. Although unrelated to rule compliance, more frequent 

testing may also be appropriate under certain specific circumstances, such during initial facility 

start-up, as described in Chapter 8.

If the state utilizes the LT2ESWTR regulatory framework for membrane filtration for 

applications other than rule compliance, it may also permit less frequent direct integrity testing. 

However, an underlying factor that should be considered before implementing less frequent 

testing is the implication of letting an integrity breach remain undetected for an extended period 

of time while water is being supplied to the public.

4.5 Establishing Control Limits

A control limit (CL) is defined as a response that, if exceeded, indicates a potential 

problem with the system and triggers a response. Multiple CLs can be set at different levels to 

indicate the severity of the potential problem. In the context of direct integrity testing, CLs are 
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-20 June 2003

Proposal Draft 

set at levels associated with various degrees of integrity loss. Under the provisions of the 

LT2ESWTR, a control limit (CL) for direct integrity testing must be established at the threshold 

test response that is indicative of an integral membrane unit capable of achieving the 

Cryptosporidium removal credit awarded by the state for rule compliance (40 CFR 141.728). 

Because utilities or states would have the option to implement a series of tiered CLs that may 

represent progressively greater levels of integrity loss leading up to the specific CL required 

under the rule, in this guidance manual the LT2ESWTR-mandated CL is referred to as an upper 

control limit (UCL). If the integrity test response is below the UCL, the membrane unit should 

be achieving a LRV equal to or greater than the removal credit awarded to the process. 

Alternatively, if the UCL is exceeded, the membrane unit is required to be taken off-line for 

diagnostic testing (as describe in section 4.8) and repair.

The same principles used to establish direct integrity test sensitivity are also used to 

establish the UCL. For pressure-based tests, the UCL may be calculated using the ALCR 

methodology. A modified version of Equation 4.7 yields an expression for the UCL for, as 

shown in Equation 4.15:

VCF

Q ALCR

UCL LRC

p

•

•

=

10

Equation 4.15

Where: UCL = upper control limit in terms of airflow (L/min)

Qp = membrane unit design capacity filtrate flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

LRC = log removal credit awarded (dimensionless)

VCF = volumetric concentration factor (dimensionless)

Similarly, Equation 4.9 can be rearranged to establish an expression for calculating the 

UCL in terms of a pressure decay rate, as shown in Equation 4.16:

V VCF

Q ALCR BP

UCL

sys

LRC

p

• •

• • +

=

10

( 14.7)

Equation 4.16

Where: UCL = upper control limit in terms of pressure decay rate (psi/min)

Qp = membrane unit design capacity filtrate flow (L/min)

ALCR = air-liquid conversion ratio (dimensionless)

BP = backpressure on the system during the test (psi)

LRC = log removal credit (dimensionless)

Vsys = volume of pressurized air in the system during the test (L)

VCF = volumetric concentration factor (dimensionless)
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-21 June 2003

Proposal Draft 

Values for the other parameters in Equations 4.15 and 4.16 should be the same as those used to 

calculate sensitivity using Equations 4.7 and 4.9, respectively. Note that to the extent possible, 

these values should be selected to yield a conservative result for the UCL. 

Equations 4.15 and 4.16 establish the maximum direct integrity test response that can be 

used as a UCL for the log removal credit (LRC) awarded by the state. The LRC, in turn, must be 

less than or equal to the lower value of either the log removal value determined during challenge 

testing (LRVC-Test) or the sensitivity of the direct integrity test used (LRVDIT). 

In the context of Equations 4.15 and 4.16, CLs are expressed in terms of the actual 

response from the direct integrity test (i.e., flow or air or pressure decay rate, respectively). In 

this form the CLs may be most useful for operators, since these could be directly compared to 

integrity test results. However, it may also be useful to calculate the corresponding LRVs for 

both the CL(s) and the individual direct integrity test responses using the generic forms of 

Equations 4.7 (for tests yielding results in terms of the flow of air) or 4.9 (for tests yielding 

results in terms of pressure decay). In this case these equations will simply yield a LRV 

corresponding to a particular direct integrity test result (i.e., a general LRV) rather than the 

sensitivity of the test (i.e., LRVDIT). Many membrane systems have automated data acquisition 

equipment that could be programmed to calculate the LRV based upon the results of the most 

recent integrity test results and current operating conditions. These parameters may be displayed 

and trended to track system performance. Additional guidance on data analysis is provided in 

section 4.9.

For marker-based tests, which yield results in terms of log remova l, the UCL is simply 

equal to the log removal credit awarded by the state.

Any CLs other than that mandated by the LT2ESWTR that are either voluntarily 

implemented by the utility or required by the state are referred to as lower control limits (LCLs). 

For example, a LCL may be established to provide operators with an indication that there may be 

an integrity breach before the breach becomes a compliance concern. In this scenario the LCL 

could be used in the context of preventative maintenance, and excursions above the LCL could 

prompt investigation and repair during scheduled downtime for the unit rather than require an 

immediate shutdown. The use of CLs and integrity testing in the context of a comprehensive 

integrity verification program is discussed further in Appendix A.

Unlike the UCL that is established by the log removal credit awarded to the process, a 

LCL can be established based on the needs and objectives of the utility. However, any LCLs 

should be above the baseline for an integral membrane and below the UCL in order to be useful. 

The baseline integrity test value for a membrane unit can be established during the 

commissioning of the facility, after the membrane system has been fully wetted and determined 

to be integral. The baseline level is described as the normal range of direct integrity test results 

that would occur for an integral membrane unit. A practical lower bound for any LCLs is the 

sensitivity of the direct integrity test (i.e., the lowest response that can be reliably measured that 

is indicative of an integrity breach).
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-22 June 2003

Proposal Draft 

4.6 Example: Establishing Direct Integrity Test Parameters

Scenario:

A submerged, vacuum-driven hollow-fiber membrane system that operates in a deposition (i.e., 

dead-end) mode hydraulic configuration is required to provide a total of at least 3 log removal 

for Cryptosporidium by the state under the LT2ESWTR. A pressure decay test is applied to one 

of the membrane units in the system. Applicable parameters are as follows:

Operational parameters

• The permitted design capacity of the membrane unit is 1,200 gpm.

• The maximum anticipated water temperature is 75 oF (24 oC).

• The maximum anticipated backpressure that might be exerted on the units during 

direct integrity testing is 75 inches of water column.

• The minimum anticipated backpressure that might be exerted on the units during 

direct integrity testing is 60 inches of water column.

• The backpressure measured prior to the most recent pressure decay test was 65 inches 

of water column.

• The filtrate flow measured prior to the most recent pressure decay test was 

1,000 gpm.

• The TMP measured prior to the most recent pressure decay test was 10 psi.

Direct integrity test parameters

• The volume of pressurized piping during the test is 285 L.

• The initial applied test pressure is 15 psi.

• The duration of the pressure decay test is 10 minutes.

• Baseline (i.e., diffusive) decay is negligible over the duration of the test.

• The smallest verifiable rate of pressure decay under known compromised conditions 

is 0.10 psi/min.

• The most recent pressure decay test yielded a result of 0.13 psi/min.

• The temperature of both the water and the applied air were 68 oF (20 oC) during the 

most recent pressure decay test.

Unit and membrane characteristics

• The maximum rated TMP is 30 psi.

• The pore shape correction factor (?) for the membrane material was not determined 

experimentally, and thus a conservative value of 1 is assumed.

• The membrane material is relatively hydrophilic and has a liquid-membrane contact 

angle (i.e., “wetting angle”) of 30o.

• The membrane surface variation (i.e., roughness) is 0.3 μm, as provided by the 

manufacturer.

• The hollow-fiber lumen diameter is 500 μm.

• The depth of the membrane into the potting material is 50 mm.

• All flow through any integrity breach that may be present is assumed to be turbulent.
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Membrane Filtration Guidance Manual 4-23 June 2003

Proposal Draft 

Calculate:

1. The minimum direct integrity test pressure commensurate with the required resolution of 

3 μm for the removal of Cryptosporidium

2. The sensitivity of the direct integrity test

3. The UCL for this system

4. The LRV verified by the most recent direct integrity test

Solution:

1. Calculate the minimum direct integrity test pressure commensurate with the required 

resolution of 3 μm for the removal of Cryptosporidium.

( ) max Ptest = 0.193 •k •s • cosq + BP Equation 4.1

? = 1 from given information

s = 72 dynes/cm the surface tension of water at 20 oC 

θ = 30ofrom given information

BPmax = 75 inches (of water column) from given information

psi

inches H O

inches H O

cm

dynes

dyne

psi cm P

o

test

2

2

27.7

75 0.193 1.0 72 cos(30 )

⋅

⋅

+













• • •

⋅

=

P psi psi test = 12.0 + 2.7

P psi test = 14 .7

Because the problem scenario states that baseline diffusive losses are negligible, the pressure 

calculated above represents the lowest permissible initial applied test pressure. If diffusive 

losses could not be neglected, Ptest would represent the lower bound above which the pressure 

must be maintained to ensure that the resolution is maintained throughout the duration of the 

test. If this were the case, the applied test would have to be increased over Ptest by total 

anticipated pressure decay over the duration of the test. In this particular example above, 

since the applied test pressure is given as 15 psi, the resolution requirement of the 

LT2ESWTR is satisfied.
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-24 June 2003

Proposal Draft 

2. Calculate the sensitivity of the direct integrity test.

















Δ • •

• • +

=

P V VCF

Q ALCR BP

LRV

test sys

p

DIT

( 14.7)

log Equation 4.9

Qp = 1,200 gpm design capacity filtrate flow

(from given information)

BP = 60 inches (of water column) minimum backpressure 

(from given information)

Note that the minimum backpressure that might be exerted during direct integrity 

testing is used to establish a conservative value for sensitivity.

? Ptest = 0.10 psi/min smallest verifiable decay rate

(from given information)

Vsys = 285 L from given information

VCF = 1 standard for deposition mode 

hydraulic configuration

ALCR = ? to be determined

Consult Table 4.1 and Appendix C – use Darcy pipe flow model for a hollowfiber membrane filtration system under conditions of turbulent flow (as specified):

( )

( T) TMP

P BP P

ALCR Y

test test

+ •

− • +

= • •

460

( 14.7)

173 Equation C.4

Ptest = 15 psi initial applied test pressure

(from given information)

Note that if diffusion through an integral membrane unit (i.e., baseline 

pressure decay was significant, the cumulative decay over the duration of 

the test would be subtracted from the initial test pressure before applying 

this parameter to Equation C.4 to yield a conservative result for the 

ALCR.

BP = 60 inches (of water column) minimum backpressure 

(from given information)

T = 75 oF maximum anticipated 

temperature

(from given information)
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-25 June 2003

Proposal Draft 

TMP = 30 psi maximum allowable TMP

(from given information)

Note that the values for system backpressure, temperature, and TMP were 

selected establish a conservative (i.e., lower) value for the ALCR, which 

in turn yields a conservative value for sensitivity.

Y = ? net expansion factor

(to be determined)

As indicated in Equation C.5 in Appendix C, the gas 

compressibility factor (Y) is a function of the applied test pressure 

(Ptest), the system backpressure during the test (BP), and a flow 

resistance coefficient (K), as follows:













































+

−

∝ K

P

P BP

Y

test

test

,

14.7

1

Equation C.5

Note that smaller values for Y are generated with larger values of 

the first parameter in Equation C.5 and smaller values for K. Thus, 

values for these variables should be selected to produce smaller 

values of Y, which in turn yield smaller values for the ALCR and a 

more conservative value for the test sensitivity.

Quantifying the first parameter in Equation C.5:

Ptest = 15 psi

BP = 75 inches (of water column)

0.41

15 14.7

27.7

75

15

14.7

2

2

=





































+

⋅

⋅

−

=

















+

−

psi psi

psi

inches H O

inches H O

psi

P

P BP

test

test

The flow resistance coefficient is defined by Equation C.6, as 

described in Appendix C:
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Membrane Filtration Guidance Manual 4-26 June 2003

Proposal Draft 

d fiber

L

K = f • Equation C.6

L = 50 mm potting depth

(from given information)

dfiber = 0.5 mm fiber diameter

(from given information)

f = ? Darcy friction factor

(to be determined)

The Darcy friction factor can be obtained the iterative 

method described in Appendix C.

f = 0.037 Darcy friction factor

(from iterative method)

3.7

0.5

50 (0.037) *  =











= • =

mm

mm

d

L

K f

fiber

Using the appropriate chart on page A-22 from Crane (1988) with 

the values calculated above:

0.41

14.7

=















+

−

test

test

P

P BP

, K = 3.7

...yields a value for Y as shown below.

Y = 0.79

Having determined a value for Y, the ALCR can be calculated follows:

( )

( T) TMP

P BP P

ALCR Y

test test

+ •

− • +

= • •

460

( 14.7)

173
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-27 June 2003

Proposal Draft 

( )

0.5

2

2

460 75 30

(15 14.7 )

27.7

75

15

173 (0.79)





















































+ •

• +

























⋅

⋅

−

= • •

F F psi

psi psi

psi

inches H O

inches H O

psi

ALCR o o

ALCR = 7.0

Substituting values into Equation 4.9 for sensitivity:

















































• •

























+

⋅

⋅

• •

















•

=

285 1

min

0.10

14.7

27.7

60

1,200 3.785 7.0

log

2

2

L

psi

psi

psi

inches H O

inches H O

gal

L

gpm

LRVDIT

LRVDIT = 4.1

Therefore, the maximum removal value that this membrane filtration system is capable of 

verifying is 4.1 log.

3. Calculate the UCL for this system.

V VCF

Q ALCR BP

UCL

sys

LRC

p

• •

• • +

=

10

( 14.7)

Equation 4.16

Qp = 1,200 gpm design capacity filtrate flow

(from given information)

ALCR = 7.0 as determined in Part 2 of this 

example, above

BP = 60 inches (of water column) minimum backpressure 
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-28 June 2003

Proposal Draft 

(from given information)

Note that the minimum backpressure that might be exerted during direct integrity 

testing is used to establish a conservative value for the UCL.

LRC = 3 from given information

Vsys = 285 L from given information

VCF = 1 standard for deposition mode

hydraulic configuration

10 285 1

14.7

27.7

60 1,200 3.785 7.0

3

2

2

• •

























+

⋅

⋅

• •

















•

=

L

psi

psi

inches H O

inches H O

gal

L

gpm

UCL

UCL = 1.9 psi/min

4. Calculate the LRV verified by the most recent integrity test.

In addition to calculating the sensitivity, Equation 4.9 can also be used to determine the LRV 

verified by the most recent direct integrity test via applying values for the variables specific to 

this test event.

















Δ • •

• • +

=

P V VCF

Q ALCR BP

LRV

test sys

p

( 14.7)

log Equation 4.9

Qp = 1,000 gpm flow measured prior to testing

(from given information)

BP = 65 inches (of water column) backpressure measured prior to 

testing

(from given information)

? Ptest = 0.13 psi/min measure test decay rate

(from given information)

Vsys = 285 L from given information

VCF = 1 standard for deposition mode 

hydraulic configuration
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-29 June 2003

Proposal Draft 

ALCR = 7.0 as determined in part 2, above

Note that because the ALCR is designed to be a conservative value, it is not 

necessary to recalculate the ALCR for a specific pressure decay test in order to 

use Equation 4.9 for the purpose of determining the LRV verified by that test.

Substituting values into Equation 4.9 for sensitivity:

















































• •

























+

⋅

⋅

• •

















•

=

285 1

min

0.13

14.7

27.7

65

1,000 3.785 7.0

log

2

2

L

psi

psi

psi

inches H O

inches H O

gal

L

gpm

LRV

LRV = 4.1

4.7 Test Methods

The LT2ESWTR does not specify a particular type of direct integrity test for rule 

compliance, but instead allows the use any test that meets the requirements of the rule 

(40 CFR 141.728). The two general classes of tests currently employed in municipal water 

treatment applications are pressure- and marker-based tests. Within these two categories, the 

particular types of tests most commonly used are described in the following subsections, 

including the pressure- and vacuum-decay tests, the diffusive airflow test, the water displacement 

test, and particulate- and molecular-marker tests. General procedures for conducting each of 

these tests are provided, along with a listing of some of the advantages and disadvantages of each 

method. The particular manner in which each of these tests is applied may vary according to 

manufacturer or site- or system-specific conditions.

The specific tests addressed in this guidance manual are not intended to represent a 

comprehensive list of all types of direct integrity tests that could be used to comply with the 

requirements of the LT2ESWTR. Any method that is both consistent with the definition of a 

direct integrity test under the rule and capable of meeting the specified resolution, sensitivity, 

and frequency requirements could be used for rule compliance at the discretion of the state.

4.7.1 Pressure Decay Test

The pressure-based pressure decay test is the most common direct integrity test currently 

in use and is generally associated with microfiltration (MF), ultrafiltration (UF), membrane 

cartridge filtration (MCF) systems, which utilize porous membranes. In a pressure decay test, a 
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-30 June 2003

Proposal Draft 

pressure below the bubble point value of the membrane is applied, and the subsequent loss in 

pressure is monitored over several minutes. An integral membrane unit will maintain the initial 

test pressure or exhibit a very slow rate of decay. Note that the pressure decay test is applicable 

to currently available pressure-driven and vacuum-driven systems, since this test is conducted 

under positive pressure for both types of systems. A schematic illustrating a pressure decay test 

is shown in Figure 4.1.

Figure 4.1 Schematic Illustrating a Pressure Decay Test

An outline of a generic protocol for a pressure decay test as is as follows:

1. Drain the water from one side of the membrane.

For hollow-fiber systems, typically the inside of the fiber lumen is drained, which 

may represent the feed or the filtrate, depending on whether the system is operated in 

an “inside-out” or “outside-in” mode, respectively.

2. Pressurize the drained side of the fully wetted membrane . 

The applied pressure must be lower than the bubble point pressure of the membrane 

(i.e., the pressure required to overcome the capillary forces that hold water in the 

membrane pores). Pressures ranging from 4 to 30 psi are typically applied during the 

pressure decay test, depending on the particular system. Membrane construction may 

limit the pressure at which a membrane can be tested. For compliance with 

LT2ESWTR requirements, the applied pressure must be sufficient to meet the 

resolution criterion of 3 μm based on Equation 4.1. For systems that utilize 

membranes submerged in an open basin, the test is typically applied to the filtrate 

side of the membranes without draining of the basins. As a result, the hydrostatic 

pressure at the deepest part of the membrane unit at which feed water passes through 

the membrane must be considered in determining the resolution actually achieved by 

the test, as discussed in section 4.2.
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Proposal Draft 

3. Isolate the pressure source and monitor the decay for a designated period of time. 

If there are no leaks in the membrane, process plumbing, or other pressurized system 

components, then air can only escape by diffusing through the water contained in the 

pores of the fully wetted membrane. Typically, this test is monitored over a period of 

5 to 10 minutes such that a stable rate of decay can be determined. The rate of 

pressure decay should be compared to the UCL (or any LCLs that may also be 

established) for the test to determine what, if any, subsequent action is triggered. 

Advantages of the pressure decay test include:

• Ability to meet the resolution criterion of 3 μm under most conditions

• Able to detect integrity breaches on the order of single fiber breaks and small holes in 

the lumen wall of a hollow fiber, depending on test parameters and system-specific 

conditions

• Standard feature of most MF and UF systems

• High degree of automation

• Widespread use by utilities and acceptance by states

• Simultaneous use as a diagnostic test to isolate a compromised module in a 

membrane unit in some cases (e.g., see section 4.8.1)

Limitations of the pressure decay test include:

• Inability to continuously monitor integrity

• Calculation of test method sensitivity requires measurement of the volume of 

pressurized air in the system (see Equation 4.8)

• Potential to yield false positive results if the membrane is not fully wetted (which 

may occur with newly installed and hydrophobic membranes that are difficult to wet, 

or when the test is applied immediately after a backwash process that includes air)

• More difficult to apply to membranes that are oriented horizontally as a result of 

potential draining and air venting problems 

In addition to the disadvantages cited above, another important concern associated with 

the pressure decay test is the potential for larger decay rates even within the UCL to affect the 

accuracy of the test. For example, if the total pressure decay over the duration of the test reduces 

the applied pressure on the membrane to a level below that sufficient to meet the resolution 
Chapter 4 – Direct Integrity Testing

Membrane Filtration Guidance Manual 4-32 June 2003

Proposal Draft 

criterion, the test would not comply with the requirements of the rule. Consequently, parameters 

should be established so as to ensure that the pressure decay test meets the 3-μm resolution 

criterion throughout the duration of the test, as discussed in section 4.2.

4.7.2 Vacuum Decay Test

The vacuum decay test is analogous to the pressure decay test with the exception that the 

test pressure is applied by drawing a vacuum on the membrane and monitoring the rate of 

vacuum (as opposed to pressure) decay over a period of time. An integral membrane unit will 

maintain the initial test vacuum or exhibit a very slow rate of decay. This test is generally 

associated with spiral-wound nanofiltration (NF) and reverse osmosis (RO) membranes. A 

schematic illustrating a vacuum decay test is shown in Figure 4.2.

Figure 4.2 Schematic Illustrating a Vacuum Decay Test

An outline of a generic protocol for a vacuum decay test as is as follows:

1. Drain the water from one side of the membrane. 

Typically, the filtrate side of a spiral-wound NF or RO membrane is drained.

2. Apply a vacuum to the drained side of the fully wetted membrane.

A vacuum of 20 to 26 inches Hg is typically applied during the test. Membrane 

construction may limit the vacuum at which a membrane can be tested. For 

compliance with LT2ESWTR requirements, the applied vacuum must be sufficient to

meet the resolution criterion of 3 μm. 
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3. Isolate the vacuum source and monitor the decay for a designated period of time. 

If there are no leaks in the membrane, process plumbing, or other system components 

under vacuum, then the vacuum should not decay over the duration of the test. 

Typically, this test is monitored over a period of 5 to 10 minutes to allow a stable rate 

of decay to be determined. The rate of pressure decay should be compared to the 

UCL (or any LCLs that may also be established) for the test to determine what, if any, 

subsequent action is triggered. 

Advantages of the vacuum decay test include:

• Ability to test spiral-wound membranes or other systems that cannot be pressurized 

on the filtrate side of the membrane

• Ability to meet the resolution criterion of 3 μm under most conditions

Limitations of the vacuum decay test include:

• Inability to continuously monitor integrity

• Not widely used for full-scale systems in current practice

• Difficulty in removing entrained air after the test has been completed

• Calculation of test method sensitivity requires measurement of the volume of air 

under vacuum in the system (see Equation 4.8)

As with the pressure decay test, another important concern associated with the vacuum 

decay test is the potential for larger decay rates even within the UCL to affect the accuracy of the 

test. Thus, if the total vacuum decay over the duration of the test reduces the applied vacuum on 

the membrane to a level below that sufficient to meet the resolution criterion, then the test would 

not comply with the requirements of the rule. Consequently, test parameters should be 

established so as to ensure that the vacuum decay test meets the 3-μm resolution criterion 

throughout the duration of the test.

4.7.3 Diffusive Airflow Test

The diffusive airflow test provides a direct measurement of the airflow through an 

integrity breach (i.e., Qair). Like the pressure decay test, the diffusive airflow test is also based 

bubble point theory and generally associated with MF, UF, and MCF membranes. However, 

instead of measuring the rate of pressure decay, the test pressure is kept constant and the airflow 

through a breach is measured. If there are no breaches in the system, there will typically be a 

small flow of air resulting from the diffusion of air through water held in the fully wetted 
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membrane pores. This diffusive airflow represents the baseline test response that is indicative of 

an integral membrane.

The diffusive airflow test has not been commonly used in municipal water treatment 

plants, at least partially as a result of the incorporation of pressure decay test as a standard 

feature with most MF and UF systems. However, the diffusive airflow test has long been used 

for measuring the integrity of cartridge filters used in sterilization applications and has been 

described in the literature by a number of authors, including Meltzer (1987), Vickers (1993), and 

Johnson (1997), among others. Note that this test is not applicable to submerged membrane 

filtration systems. A schematic illustrating a diffusive airflow test is shown in Figure 4.3.

Figure 4.3 Schematic Illustrating a Diffusive Airflow Test

The outline of a generic protocol for a diffusive airflow test is similar to that for the pressure 

decay test, and is as follows:

1. Drain the water from one side of the membrane. 

For hollow-fiber systems, typically the inside of the fiber lumen is drained, which may 

represent the feed or the filtrate, depending on whether the system is operated in an 

“inside-out” or “outside-in” mode, respectively.

2. Pressurize the drained side of the fully wetted membrane .

The applied pressure must be lower than the bubble point pressure of the membrane (i.e., 

the pressure required to overcome the capillary forces that hold water in the membrane 

pores). As with the pressure decay test, pressures ranging from 4 to 30 psi are typically 

applied during the diffusive airflow test, depending on the particular system. Membrane 

construction may limit the pressure at which a membrane can be tested. For compliance 

with LT2ESWTR requirements, the applied pressure must be sufficient to meet the 

resolution criterion of 3 μm.
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3. Maintain constant pressure and monitor the airflow for a designated period of time.

If there are no leaks in the membrane, process plumbing, or other pressurized system 

components, then air can only escape is by diffusing through the water contained in the 

pores of the fully wetted membrane. Typically, this test is monitored over a period of 5 

to 10 minutes to allow a stable airflow to be established. The flow of air should be 

compared to the UCL (or any LCLs that may also be established) for the test to determine 

what, if any, subsequent action is triggered. 

Advantages of the diffusive airflow test include:

• Ability to directly measure the flow of air through an integrity breach

• Ability to meet the resolution criterion of 3 μm under most conditions

Limitations of the diffusive airflow test include:

• Inability to continuously monitor integrity

• Requires measurement equipment to measure flow of air that is generally not a 

standard inclusion with a membrane filtration system

• Not widely used for full-scale systems in current practice

• Potential to yield false positive results if the membrane is not fully wetted (which 

may occur with newly installed and hydrophobic membranes that are difficult to wet, 

or when the test is applied immediately after a backwash process that includes air)

4.7.4 Water Displacement Test

The water displacement test is similar to the diffusive airflow test with the exception that 

the volume of displaced water as a result of an integrity breach is measured rather than the flow 

of air through a breach. Specifically, the air that passes across the membrane displaces an 

equivalent volume of water on the opposite side. This test can only be applied to membrane 

systems operated under positive pressure (i.e., as opposed to immersed systems that operate 

under vacuum). The volume of displaced water collected over a known period of time is

measured and converted to equivalent volume of air to determine the airflow (Qair). If there are 

no breaches in the system, there will typically be a small flow of displaced water resulting from 

the diffusion of air. This flow represents the baseline test response that is indicative of an 

integral membrane. A larger volume (or flow) of displaced water would indicate an integrity 

breach. It is important to note that although this test measures the flow of water through an 

integrity breach, it does not constitute the flow of water through the critical breach (Qbreach) that 

is necessary for determining the direct integrity test sensitivity.
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The water displacement test is described in an American Water Works Association 

Research Foundation (AWWARF) report prepared by Jacangelo et al. (1997). While this test is 

not commonly used in municipal drinking water treatment plants, it is relatively easy to use, 

requires minimal equipment, and can detect very small integrity breaches on the order of a single 

broken fiber. Since publication of this AWWARF report, a membrane filtration facility in 

Tauranga, New Zealand has adopted the use of this integrity test. A schematic illustrating a 

water displacement test is shown in Figure 4.4.

Figure 4.4 Schematic Illustrating a Water Displacement Test

The outline of a generic protocol for a water displacement test is similar to that for the 

diffusive airflow test, and is as follows:

1. Drain the water from one side of the membrane.

For hollow-fiber systems, typically the inside of the fiber lumen is drained, which 

may represent the feed or the filtrate, depending on whether the system is operated in 

an “inside-out” or “outside-in” mode, respectively.

2. Pressurize the drained side of the fully wetted membrane .

The applied pressure must be lower than the bubble point pressure of the membrane 

(i.e., the pressure required to overcome the capillary forces that hold water in the 

membrane pores). Pressures ranging from 4 to 30 psi are typically applied during the 

pressure decay test, depending on the particular system. Membrane construction may 

limit the pressure at which a membrane can be tested. For compliance with 

LT2ESWTR requirements, the applied pressure must be sufficient to meet the 

resolution criterion of 3 μm. Since one side of the membrane must remain flooded 

with water during the test, there is a potential for some hydrostatic backpressure. If 

this pressure is determined to be significant, it must be considered in the resolution 

calculations as discussed in section 4.2. 
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3. Maintain constant pressure and monitor the flow for a designated period of time. 

If there are no leaks in the membrane, process plumbing, or other pressurized system 

components, then air can only escape is by diffusing through the water contained in 

the pores of the fully wetted membrane. Typically, this test is monitored over a 

period of 5 to 10 minutes to allow a stable flow to be established. The air displaces 

water that in turn is measured via either a flow meter or a graduated cylinder and 

timing device. Since the flow of water is equivalent to the flow of air, the resulting 

flow should be compared to the UCL (or any LCLs that may also be established) for 

the test to determine what, if any, subsequent action is triggered. Note that the flow 

meter and associated sample line used in this test must be configured to prevent the 

gravity flow of water.

Advantages of the water displacement test include:

• Ability to directly measure the flow of air via an equivalent flow of water provided 

the backpressure on the system does not result in the compression of diffused air

• Ability to meet the resolution criterion of 3 μm under most conditions

• Able to detect integrity breaches on the order of single fiber breaks and small holes in 

the fiber lumen wall of a hollow 

• Ease of measuring flow of water relative to that of air

Limitations of the water displacement test include:

• Inability to continuously monitor integrity

• Not widely used for full-scale systems

• Potential to yield false positive results if the membrane is not fully wetted (which 

may occur with newly installed and hydrophobic membranes that are difficult to wet, 

or when the test is applied immediately after a backwash process that includes air)
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4.7.5 Marker-Based Integrity Tests

Marker-based tests are advantageous in that they provide a direct assessment of the LRV 

achieved by the process. Both particulate- and molecular marker-based direct integrity tests may 

be used to meet the direct integrity test requirements of the LT2ESWTR provided the specified 

performance criteria for resolution, sensitivity, and frequency are satisfied (40 CFR 141, Subpart 

W, Appendix C). Since a marker-based direct integrity test is essentially a form of challenge 

testing, much of the guidance provided in Chapter 3 may be useful in designing a marker-based 

test. Because these tests are applied to water treatment equipment in active use, the particulate or 

molecular marker used must be inert and suitable for use in a water treatment facility (e.g.,

approved by Food and Drug Administration (FDA) or certified by the National Sanitation 

Foundation (NSF) as an NSF-60 approved material). Because MF, UF, and MCF systems utilize 

porous membranes that would not remove a molecular marker, only particulate marker tests 

should be used with these systems. (Note that one possible exception is a system utilizing UF 

membranes with a low MWCO, which could potentially remove larger molecular markers.) 

Conversely, because the semi-permeable NF and RO membranes are not specifically designed to 

accommodate large particle loads and typically cannot be backwashed to removed particulate 

matter from the membrane surface, only molecular markers should be used with NF and RO 

systems. Schematics illustrating the particulate and molecular marker tests are shown in 

Figures 4.5 and 4.6, respectively. 

Figure 4.5 Schematic Illustrating a Particulate Marker Test
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Figure 4.6 Schematic Illustrating a Molecular Marker Test

The generic protocols for both particulate and molecular marker tests are similar, and are as 

follows:

1. Select an appropriate marker and verify that it meets the resolution criterion. 

(Additional guidance is given in section 3.9.)

2. Ensure that the expected feed and filtrate sampling concentrations fall within the 

limits for the measuring instrumentation used. 

3. Calculate the feed dosage rate and filtrate sample volume(s). 

(Additional guidance is given in section 3.10.)

4. Make sure that any operational processes such as chemical cleaning or backwashing 

are not initiated during the marker-based test.

5. Determine the amount of time the marker has to be applied to attain a steady state or 

equilibrium condition.

(Additional guidance is given in section 3.10.)

6. Feed the marker into the system.

7. Commence filtrate sampling when the equilibrium is reached. 

8. Continue simultaneous feed and filtrate sampling until results are attained. 

9. Discontinue dosing.
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Advantages of marker-based integrity tests include: 

• Provides direct evalua tion of the LRV of a membrane unit

• Potential for application while unit is on-line and producing water (i.e., normal 

operation)

• Instruments used for filtrate monitoring may also potentially be used for continuous 

indirect integrity monitoring between application for the direct integrity test at the 

discretion of the state

Limitations of marker-based integrity tests include:

• Potential for an improperly selected marker to cause fouling and interfere with normal 

system operation 

• Cost and calibration of instrumentation

• Cost of marker

• May require disposal of filtered water produced during the test

• May require special considerations for disposal of the filtered marker waste 

• Measurement of marker concentrations is typically not continuous

Additional Considerations for Particulate Marker Tests

Typically particulate marker tests require the use of particle counters to measure the feed 

and filtrate particle concentrations during the test, thus enabling the LRV to be directly 

calculated. Consequently, there are some important considerations regarding particle counters to 

consider in implementing a particulate marker test. First, it is important that the feed and filtrate 

particle counters continually monitor baseline concentrations between test events so that 

background particle levels can be taken into account in the determination of the LRV of 

measured by the test during routine operation. The background levels recorded in both the feed 

and the filtrate should be subtracted from the respective particle count measurements during the 

test event in order to yield an accurate LRV. In addition, particle counters are subject to 

coincidence error (i.e., two or more particles passing through the sensor simultaneously that are 

consequently measured as a single larger particle), particularly at high particle concentrations. 

This potential error may limit the maximum particle concentration that can be counted, which 

may in some cases be problematic for measuring the feed concentration during a particulate 

marker test. In order to compensate for possible coincidence error, some particle counters can 

allow for the sample to be diluted. However, because any particles in the dilution water would 
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introduce error into the measurement, the quality of any dilution water used should be also be 

taken into account in measuring the particle concentration of interest during a particulate marker 

test. It is also important that the particulate marker selected does not clump, which could 

exacerbate coincidence error or clog the instrument sensor.

All particle counters (or other instruments used to measure particulate concentrations 

during a particulate marker-based test) should be routinely calibrated at a frequency 

recommended by the manufacturer or required by the state. The calibration should be targeted 

toward counting particles at the specified size and concentration used in the particulate marker 

test. The particles used for the particulate marker test should be inert and compatible with the 

membrane in order to avoid irreversible fouling. 

Additional Considerations for Molecular Marker Tests

Although the ambient levels of a molecular marker in the feed and filtrate may be 

negligible in many cases, these background concentrations should nevertheless still be measured

and accounted for in the LRV calculation. The background concentrations of the molecular 

marker in the feed and filtrate should be subtracted from the respective measurements collected 

during the molecular marker-based direct integrity test in order to accurately quantify the LRV of 

the membrane unit. In addition, as with particle counters, all instruments that may be used to 

measure molecular marker concentrations should be calibrated regularly at a frequency 

recommended by the manufacturer or required by the state.

4.8 Diagnostic Testing

Diagnostic tests are types of direct integrity tests that are specifically used to identify and 

isolate any integrity breaches that are detected, thus supplementing primary methods such as the 

pressure- or marker-based test described in section 4.7, which simply determine whether or a not 

a leak is present in a membrane unit. Note that there are no sensitivity or resolution requirements 

for diagnostic tests under the LT2ESWTR, since the objective of these test is simply to isolate 

particular compromised module and/or broken fiber. However, it is important that a diagnostic 

test have sufficient resolution to enable the detection of the integrity breach that caused the UCL 

to be exceeded.

Many diagnostic tests are most useful for identifying a particular compromised module 

within a unit; however, for hollow-fiber MF/UF systems some diagnostic tests may be used 

locate a specific broken fiber in a module. The LT2ESWTR requires that if the results of a direct 

integrity test exceed the UCL, the affected unit be immediately taken off-line for diagnostic 

testing and subsequent repair (40 CFR 141.728). Although many different types of diagnostic 

tests may be available, four of the tests most commonly used are described in the following 

subsections, including visual inspection, bubble testing, sonic testing, conductivity profiling, and 

single module testing. Note that not all of these tests are applicable to every type of membrane 

filtration. Visual inspection, bubble testing, and sonic testing are generally applicable to MF, 
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UF, and MCF systems, while conductivity probing is used with NF and RO systems. Single 

module testing may be applicable to all types of membrane filtration systems.

4.8.1 Visual Inspection

Visually inspecting a membrane unit for leaks is the simplest form of diagnostic testing 

and is generally applicable to MF, UF, and MCF systems. Since many pressure-based direct 

integrity tests apply air to one side of the membrane while maintaining water on the other side, it 

may be possible to see air bubbles form in a compromised module. If a pressure-based direct 

integrity is used, visual inspection may be able to be conducted simultaneously with the direct 

integrity test to identify a compromised module. In order to perform a visual inspection, some 

component of the module housing must be transparent; accordingly, some pressure-driven 

membrane systems have inspection ports or clear tubing at the top of the membrane module to 

allow an operator to identify the particular compromised module. For vacuum-driven membrane 

systems submerged in basins, this test simply involves observing the water surface to identify the 

module that is the source of the bubbles rising to the surface.

Advantages of visual inspection include:

• Ability to identify specific compromised fibers and leaking seals

• Easy to use and interpret results

• No additional equipment required

• Equally applicable to systems using enclosed pressure vessels or immersed modules

• Does not require membrane modules to be removed from the unit

Limitations of visual inspection include:

• Manual application

• Requires system design considerations for implementation (e.g., sight tubes, 

removable plates or carriages, etc.) 

4.8.2 Bubble Testing

The bubble test is based on bubble point theory (as described in Appendix B) and is 

applicable to MF, UF, and MCF membranes. In conducting a bubble test, the module of interest 

is first removed from the membrane unit. The external shell of the module is then drained and 

pressurized to a level below the bubble point of the membrane but higher than the pressure 
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required to achieve the required 3-μm resolution for compliance with the LT2ESWTR. The 

pressures applied in a bubble test are generally similar to those applied for the pressure-based 

direct integrity tests. The end cap is removed and a dilute surfactant solution is applied to the 

open ends of the membrane fibers at the end of the module. The formation of bubbles in the 

surfactant solution can be used pinpoint specific leaking fibers. The defective fiber may be 

repaired by plugging the lumen with a stainless steel pin or epoxy adhesive dispensed from a 

syringe. The membrane module manufacturer should be consulted for specific recommendations 

regarding module repair. Note that the bubble test is distinct from the bubble point test described 

in section 3.7.3. The bubble test described in this section is applied to identify leaks, and thus 

the pressure must be kept below the bubble point of the porous membrane. By contrast, the 

bubble point test is applied for the specific purpose of determining the bubble point of the 

membrane, and thus the pressure must be gradually increased until the bubble point is achieved.

Advantages of the bubble test include:

• Ability to identify specific compromised fibers and leaking seals

• Easy to use and interpret results

• Equally applicable to systems using enclosed pressure vessels or immersed modules

Limitations of the bubble test include:

• Manual application

• May require removal of a membrane module from the rack

4.8.3 Sonic Testing

The principle of sonic testing is that water passing through broken fibers or other 

damaged system components will make a unique sound that can be detected using specialized 

equipment. The analysis is typically conducted by manually applying an accelerometer (an 

instrument used to detect vibrations) to one or more locations on each membrane module. Using 

headphones, the operator listens for vibrations generated by leaking air. Since the test is applied 

to modules on an individual basis, it is useful for identifying the module that has a potential 

integrity breach when the direct integrity test indicates that there may be a problem in a 

membrane unit. Sonic testing is generally applicable to MF, UF, and MCF systems.

A sonic test is most effectively administered by a skilled and experienced operator, 

particularly since the results are more subjective than the other forms of integrity testing, either 

direct or indirect. Nonetheless, it is a useful diagnostic tool that can help isolate a compromised 

membrane module. Adham et al. (1995) reported that sonic testing was able to detect a breach as 

small as a 0.6 mm needle puncture in the wall of one out of over 22,000 hollow fibers.
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An automated sonic testing system could have the potential to eliminate the subjectivity 

of this test and serve as an on-line, continuous, and direct means of integrity testing. The earlystages of development and testing of such an automated system was described in a paper by 

Glucina et al. (1999). This new acoustic monitoring system utilizes a sensor on each membrane 

module to detect changes in noise caused by pressure fluctuations in any compromised fibers. 

Test results indicated that the system was capable of detecting a single compromised fiber, 

although performance was affected by the level of background mechanical noise associated with 

the membrane filtration system. The system described by Glucina et al. is currently still under 

development.

Advantages of sonic testing include:

• Ability to identify a specific compromised module within a membrane unit

• Ease of use (assuming the test is conducted by a trained operator)

Limitations of sonic testing include:

• Manual application

• Potential for subjective interpretations of results

• Requires the purchase of additional equipment

• Not feasible for immersed membrane systems

4.8.4 Conductivity Profiling

Conductivity profiling is a common practice associated with NF and RO systems to 

identify leaks in modules, o-rings, and seals. A long sample tube is inserted into the permeate 

port and used to withdraw a sample. The conductivity of the sample is monitored at various 

points within the module and then indexed along the length of the permeate tube. Any integrity 

breaches are identified by significant changes in conductivity. The most common location of 

leaks in a NF or RO system is at the module filtrate tube collector and end adaptor o-ring seals.
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Advantages of conductivity profiling include:

• Ability to identify a specific compromised module within pressure vessel

Limitations of conductivity profiling include:

• Manual application

• Requires operator skill for indexing probe

• Potential for subjective interpretations of results

• Requires the purchase of additional equipment

4.8.5 Single Module Testing

Single module testing may be applicable to all types of membrane filtration systems and 

involves removing potentially compromised modules from a membrane unit that is known to 

contain an integrity breach and testing each module on an individual basis. Although a number 

of different methods of integrity testing could potentially be utilized screen individual modules 

with this diagnostic technique, single module testing generally refers to the use of a small 

apparatus to conduct a pressure or vacuum decay test on one module at a time. One advantage of 

this diagnostic test is that the pressure or vacuum decay associated with an integrity breach may 

be much more pronounced in a single compromised module at a given test pressure than in one 

compromised module in an entire membrane unit that is otherwise integral, enabling a breach to 

be more readily detected. Thus, while the process of single module testing is labor-intensive, it 

may be especially useful for isolating a compromised module in cases in which other methods of 

diagnostic testing have not been successful. 

Advantages of the single module testing include:

• Increased ability to detect compromised modules compared to in-situ testing applied 

to an entire membrane unit

• May be able to isolate compromised modules in cases in which other diagnostic tests 

have been unsuccessful 

• Generally applicable to MF, UF, NF, RO, and MCF systems
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Limitations of single module testing include:

• Manual application

• Labor-intensive testing process

• May require modules to be removed from the membrane unit

• May require separate testing apparatus 

4.9 Data Collection and Reporting

The LT2ESWTR requires that direct integrity testing be conducted on each membrane 

unit at a frequency of at least once each 24 hours of operation (40 CFR 141.728). At a 

minimum, the direct integrity test results that exceed the UCL and result in a membrane unit 

being taken off line must be reported (40 CFR 141.728). The corrective action taken as a result 

of the UCL exceedance should also be reported. Routine direct integrity test results that do not 

exceed the UCL are not required to be reported under the LT2ESWTR; however, the state may 

require that additional results be reported at its discretion. The state may require that direct 

integrity test results be reported as measured (e.g., the rate of pressure decay rate for the pressure 

decay test or the flow of air for a diffusive airflow test) and/or as converted to equivalent LRVs, 

as calculating using the methodology described under section 4.5. Note that it is often most 

advantageous for a utility to record the actual process value(s) measured (e.g., the rate of 

pressure decay) to track the results of the direct integrity test over time. Additional guidance 

regarding data collection and the use of this data for system optimization is provided in 

Appendix A in the context of developing a comprehensive integrity verification program.

A sample summary report form for a hollow-fiber system using the pressure decay test 

(the most common type of direct integrity test) is provided in Figure 4.8. The form includes the 

following components:

• Facility information, membrane unit no., and date (i.e., month and year)

• System parameters and test constants

• Test conditions and results

• UCL violations

The sample summary report form also contains a column for recording the LRV that is verified 

for each particular daily application of the direct integrity test, as well as for the flow, TMP, and 

ALCR parameters that are required to calculate the LRV. Note that these data are not required 

for reporting purposes under the LT2ESWTR, but are included to underscore the utility of these 

values for tracking overall unit performance over time.
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 Figure 4.7 Sample Summary Report Form for Pressure Decay Testing

Month Utility

Year Facility Name

Membrane Unit No. Test Duration min

Volume of System (Vsys) L VCF

UCL Total No. of UCL Violations

Signed Dated

Pressure

Day (psi)

Initial Final

)P

(psi/min)

Within

UCL?

Corrective Action Taken

(if required)

Filtrate 

Flow

(gpm)

TMP

(psi) ALCR LRV

Verified

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

Min

Max

Ave
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5.0 Continuous Indirect Integrity Monitoring

5.1 Introduction

As the name suggests, the various indirect integrity monitoring methods are not physical 

tests applied specifically to a membrane module or membrane unit, but instead involve 

monitoring some aspect of filtrate water quality as a surrogate measure of membrane integrity. 

Because the quality of membrane filtrate is very consistent and independent of fluctuations in 

feed water turbidity or particle levels, a marked decline in filtrate quality may indicate an 

integrity problem. Although indirect integrity monitoring is not as sensitive as direct testing for 

detecting integrity breaches, the indirect methods offer some significant benefits that make them 

an important and useful tool in an overall integrity verification strategy. These benefits include 

both the ability to be operated in a continuous, on-line mode and the non-proprietary nature of 

the indirect techniques, such that the same indirect method is similarly applicable to any 

membrane filtration system independent of manufacturer, configuration, or other system 

parameters.

Chapter 4 discussed the use of direct integrity testing as a means of verifying the removal 

efficiency of a membrane filtration process at a level commensurate with the removal credit 

awarded by the state. However, while direct integrity tests can be extremely sensitive and thus 

can potentially verify high log removal values, most direct test methods require that the 

membranes be taken off-line (i.e., out of production mode) to undergo testing. Thus, these direct 

tests are limited to periodic application in order to minimize system down time. A failed 

periodic direct integrity test indicates that an integrity breach occurred at some time between the 

most recent direct test in which integrity has been verified and the failed test, but indicates 

nothing about integrity over the period between direct test applications. Continuous monitoring 

using indirect methods does provide a real-time indication of membrane integrity, albeit with less 

sensitivity in most cases. Consequently, the advantages of the direct and indirect integrity 

monitoring approaches are complementary, and both are critical elements of a comprehensive 

integrity verification program (IVP). (The development of a comprehensive IVP, including the 

complementary value of direct and indirect methods of assessing system integrity, is discussed in 

Appendix A.) 

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires

continuous indirect integrity monitoring in the absence of a continuous direct integrity test 

method (40 CFR 141.728). Currently, continuous direct integrity monitoring techniques that 

meet the performance criteria of the rule are not available, and thus it is anticipated that utilities 

employing membrane filtration to comply with the LT2ESWTR requirements will have to 

implement both periodic direct integrity testing and continuous indirect integrity monitoring at 

the time of rule promulgation. However, the development of a sufficiently sensitive and reliable 

continuous direct integrity test is potentially feasible, and implementation of such a test would 

preclude the requirement to conduct indirect integrity monitoring, additional state integrity 

monitoring requirements notwithstanding.
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“Continuous” indirect integrity monitoring is defined as monitoring some aspect or 

component of filtrate water quality that is indicative of the removal of particulate matter at a 

frequency of no less than once every fifteen minutes (40 CFR 141.728). The LT2ESWTR 

specifies turbidity monitoring on the filtrate of each membrane unit as the default methodology 

for continuous indirect integrity monitoring, and that two consecutive filtrate turbidity readings 

above 0.15 NTU triggers direct integrity testing (40 CFR 141.728). Turbidity was selected as the 

default indirect integrity monitoring technique because it is an accepted monitoring technology 

within the water treatment industry, it is used as both a relative and an absolute indicator of water 

quality, and it is required for plants employing both conventional and alternative technologies 

under both the Surface Water Treatment Rule (SWTR) and the Interim Enhanced Surface Water 

Treatment Rule (IESWTR) (54 FR 27486, 63 FR 69477, and 65 FR 19046). However, because 

several other techniques are generally accepted for indirect integrity monitoring in the water 

treatment industry, a number of which are more sensitive than turbidity monitoring, the 

LT2ESWTR contains a provision to allow states to approve alternative methods for continuous 

indirect integrity monitoring.

Independent of the method used (i.e., turbidity monitoring or an alternative method 

approved by the state), the LT2ESWTR specifies the following requirements for indirect 

integrity monitoring:

1. The filtrate from each membrane unit of the filtration system must be monitored 

independently. (The definition of a membrane unit under the LT2ESWTR is 

provided in section 1.5.) 

2. The filtrate must be monitored continuously from each membrane unit. For the 

purposes of indirect integrity monitoring under the LT2ESWTR, continuous 

monitoring is defined as taking measurements at least once every 15 minutes.

3. A performance-based control limit must be established such that readings exceeding 

the control limit for a period of greater than 15 minutes (or two consecutive 15-

minute readings exceeding the upper control limit) would trigger direct integrity 

testing. (The establishment of control limits is further discussed in the context of 

each indirect integrity monitoring method addressed in Chapter 5. Additional 

information regarding control limits and their role in a comprehensive IVP is 

provided in Appendix A.)

4. Utilities must report a summary of all excursions above the established control limit 

that trigger direct integrity testing to the state on a monthly basis.

Aside from turbidity monitoring, particle counting may be the most commonly employed 

means of indirect integrity monitoring for compliance with the LT2ESWTR. Both of these 

techniques are widely used throughout the water treatment industry. Particle monitoring is 

similar to particle counting, although the collected data are presented in a relative manner rather 

than as absolute values. The tracking of particles and turbidity in the filtrate is based on the 

passage of particulate matter through a membrane and is thus appropriate for all types of 

membrane filtration systems. However, other surrogate measures of integrity that monitor 
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dissolved solids or other miscible water quality parameters such as conductivity may also satisfy 

the indirect integrity monitoring requirements of the LT2ESWTR for nanofiltration (NF) and 

reverse osmosis (RO) membranes capable of removing these water quality constituents. Still, it 

is important to note that any indirect integrity monitoring method used for the purposes of 

LT2ESWTR compliance other than turbidity monitoring must be approved by the state.

Under the LT2ESWTR, resolution and sensitivity are both defined with respect to their 

applicability to direct integrity testing, as described in Chapter 4. However, unlike the direct 

methods, there are no requirements relating to test sensitivity or resolution with respect to the 

indirect integrity monitoring methods under the LT2ESWTR. Nevertheless, these two concepts, 

where applicable, may be useful tools for optimizing the ability of an indirect test to yield 

meaningful information about potential integrity breaches. Within the context of continuous 

indirect integrity monitoring, resolution and sensitivity may be described as specified in 

Chapter 4, although without the references to direct integrity testing, as specified in the rule 

language. Thus, resolution may be described as the smallest leak that contributes to a response 

to an indirect integrity monitoring method, and sensitivity may be expressed as the maximum log 

removal value that can be reliably verified by an indirect integrity monitoring method. 

Resolution and sensitivity are discussed in association with each respective indirect integrity 

monitoring method addressed in Chapter 5, noting the extent to which each concept applies to 

each method.

The purpose of Chapter 5 is to describe the most commonly used techniques for indirect 

integrity monitoring and discuss their use for compliance with the requirements of the 

LT2ESWTR. Considerations for selecting other indirect integrity monitoring methods are also 

addressed. 

This chapter is divided into the following sections:

Section 5.2: Turbidity Monitoring

This section discusses the use of turbidity monitoring as the default 

indirect integrity monitoring technique under the LT2ESWR, including 

methods of application, control limits, and advantages and limitations.

Section 5.3: Particle Counting and Particle Monitoring

This section describes the use of particle counting or particle monitoring 

as a potential alternative to turbidity monitoring for indirect integrity 

monitoring, including methods of application, control limits, and 

advantages and limitations.

Section 5.4: Other Indirect Monitoring Surrogates

This section discusses some general considerations for selecting an 

alternative indirect integrity monitoring method not specifically addressed 

in this guidance.
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Section 5.5: Data Analysis and Reporting

This section summarizes the reporting requirements for continuous 

indirect integrity monitoring results and provides guidance on the analysis 

and interpretation of the results.

Note that a non-continuous indirect method (e.g., a silt density index (SDI) test) has 

limited value for integrity monitoring, offering neither the ability to directly test the membranes 

nor the advantage of continuous, on-line monitoring. As a result, non-continuous indirect 

methods do not satisfy the indirect integrity monitoring requirements of the LT2ESWTR and are 

not addressed in this chapter

5.2 Turbidity Monitoring

Conventional turbidimeters (also referred to as “turbidity meters”) detect the intensity of 

light scattered at one or more angles to an incident beam of light. The angular distribution of 

scattered light depends on a number of conditions, including the wavelength of the incident light, 

as well as particle size, shape, and composition. Consequently, it is difficult to correlate the 

turbidity with the number or mass concentration of particles in suspension. However, 

turbidimeters are in widespread use throughout the water industry, and the turbidity data 

generated by these instruments is broadly recognized as a meaningful gauge of water quality. As 

such, turbidity measurements have been used as an indicator of finished water quality for 

previous surface water regulations.

Under the LT2ESWTR, turbidity monitoring is specified as the default method for 

continuous indirect integrity monitoring unless the state approves an alternate approach. Using 

the default approach specified in the rule, filtrate turbidity must be monitored on each discrete 

membrane unit at a minimum frequency of once every 15 minutes. Two consecutive readings 

that exceed a control limit of 0.15 NTU for any one unit would require that unit to undergo direct 

integrity testing and subsequent diagnostic testing, as necessary. Any such events that trigger 

direct integrity testing must be reported to the state on a monthly basis. Further information 

regarding data collection, analysis, and reporting is provided in section 5.5. 

Because turbidimeters do not yield any information about particulate size, these 

instruments cannot provide any specific indication about the size of an integrity breach through 

which turbidity may have passed. Similar turbidity readings may result from either significant 

particulate material passing through a small breach, or a smaller amount of larger particulate 

matter passing through a larger breach. As a result, the concept of test method resolution as 

defined under the LT2ESWTR is not applicable to turbidity monitoring. However, the concept 

of method sensitivity can apply to turbidity monitoring, even though there is no formal 

sensitivity requirement for continuous indirect integrity monitoring methods, as fiber cutting 

studies may be conducted to correlate a particular reduction in log removal capability of the 

membrane system to approximate increases in filtrate turbidity. The sensitivity of turbidity 

monitoring and the use of this technique to detect integrity breaches are further discussed in the 

context of establishing control limits in section 5.2.2.
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The following sections describe turbidity monitoring methods, approaches for 

establishing control limits, and advantages and limitations, respectively.

5.2.1 Methods

There are two basic types of on-line turbidimeters that may have the capability to provide 

continuous indirect integrity monitoring as required under the LT2ESWTR: conventional 

turbidimeters and laser turbidimeters. The primary difference between these two types of 

instruments lies in the type of light source used. Conventional turbidimeters typically utilize a 

tungsten lamp or other light-emitting diode (LED) as a light source, whereas laser turbidimeters 

employ a laser light source.

Because laser turbidimetry is a relatively new technique, its effectiveness as a monitoring 

tool is still being evaluated within the water industry. However, recent research generally 

indicates that laser turbidimeters are more sensitive than conventional turbidimeters and may 

perform comparably to particle counters for detecting membrane integrity breaches (Banerjee et 

al. 2000; Colvin et al. 2001). Available manufacturer specifications indicate that laser 

turbidimeters may have increased sensitivity in excess of two orders of magnitude over 

conventional turbidimeters (in terms of measuring turbidity, not necessarily with respect to the 

ability of the technique to measure reductions in log removal efficiency resulting from 

membrane integrity breaches). Additional research indicates laser turbidimeters can be 

optimized to measure very low turbidities, in the range of 0 to 1 NTU (Banerjee et al. 1999a; 

1999b; and 2000). Since most microfiltration (MF) and ultrafiltration (UF) systems produce 

filtrate water consistently in the range of 0.03 to 0.07 NTU as measured by conventional 

turbidimeters, laser turbidimeters may be better suited to monitor membrane filtrate.

Currently, there are four USEPA-approved analytical methods for the measurement of 

turbidity (USEPA 1999). These are as follows:

• USEPA Method 180.1, Determination of Turbidity by Nephelometry (USEPA 1993)

• Standard Method 2130B, Turbidity – Nephelometric Method (American Public 

Health Association (APHA) 1998)

• Great Lakes Instrument Method 2 (GLI 2) (USEPA 1999)

• Hach FilterTrak Method 10133

Systems must utilize turbidimeters that conform to one of these four methods for 

compliance monitoring purposes. Additional guidance on the installation, calibration, operation, 

and maintenance of on-line turbidimeters is provided in Guidance Manual for Compliance with 

the Interim Enhanced Surface Water Treatment Rule: Turbidity Provisions (USEPA-815-R-99-

010, April 1999).
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5.2.2 Control Limits

A control limit (CL) is defined as a threshold response from an integrity test that triggers 

a specific action. The LT2ESWTR requires CLs to be established for both direct and indirect 

integrity testing. In the context of this guidance manual the CLs required under the rule are 

referenced as upper control limits (UCLs), signifying that these limits serve as indicators of 

minimally acceptable system performance. For direct integrity tests, the UCL corresponds to the 

smallest integrity breach that would compromise the log removal credit awarded by the state. 

However, for indirect integrity monitoring methods the UCL is simply designed to serve as a 

general indication that a system integrity breach may have occurred.

Unless the state approves an alternate approach (e.g., particle counting, particle 

monitoring, or other method), the LT2ESWTR requires continuous filtrate turbidity monitoring 

on each membrane unit with a control limit of 0.15 NTU. Because most membrane filtration 

systems consistently produce filtrate well-below 0.15 NTU, a sustained high-turbidity event with 

filtrate readings above 0.15 NTU may suggest a potentially serious integrity problem. 

Consequently, the LT2ESWTR requires that two consecutive filtrate turbidity readings above 

0.15 NTU on any membrane unit trigger direct integrity testing on that unit, as described in 

Chapter 4. (Note that the state may require a more stringent UCL at its discretion.) If the unit in 

question passes the triggered direct integrity test, the unit may continue in production. However, 

if the unit fails the direct integrity test, further diagnostic testing and repair of any integrity 

breach(es) would be required. The unit may only be returned to service upon passing a direct 

integrity test.

The LT2ESWTR defines continuous monitoring as one reading at least every 15 minutes. 

The rule is designed to allow states latitude for interpreting how the continuous indirect 

monitoring data collected by a utility would translate into a measurement for the purposes of 

reporting and rule compliance. Some potential strategies that states may adopt for data 

collection and reporting in interpretation of this requirement are further discussed in both 

section 5.5 and Appendix A. In the absence of any particular state requirements, a utility may 

adopt a data interpretation strategy that is most appropriate for its system.

While only the UCL is required under the LT2ESWTR, some utilities may choose to 

establish a tiered approach to control limits with one or more lower control limits (LCLs), in 

which the level of corrective action increases with each successive CL that is further removed 

from normal system performance. For example, a utility may opt to implement a LCL in which 

a filtrate turbidity reading exceeding 0.10 NTU triggers increased monitoring frequency. 

Alternatively, with hollow-fiber membrane filtration systems, a utility may choose to conduct 

fiber-cutting studies to correlate a quantifiable loss of log removal capability with a certain level 

of increased filtrate turbidity. Research has indicated that turbidity monitoring may be sensitive 

to some smaller integrity breaches, although this sensitivity is dependent on the type membrane 

system, the number of modules linked to a single instrument, the number of fibers per module, 

the hydraulic configuration, and other system-specific parameters (Adham et al. 1995). One 

study conducted by the Wisconsin Department of Natural Resources demonstrated that for one 

system in Kenosha, WI turbidity monitoring was unable to detect any loss of system integrity 

even with a substantial number of cut fibers (Landsness 2001). Consequently, as this study 
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indicates, for some systems turbidity monitoring may not be sufficiently sensitive to allow a 

meaningful correlation with integrity loss for the purposes of establishing LCLs. In general, it is 

suggested that a utility work with the state and its membrane manufacturer to establish any 

additional CLs and associated corrective actions that may be appropriate for a particular 

membrane filtration system.

5.2.3 Advantages and Limitations

One of the most significant advantages of tracking turbidity as a means of indirect 

integrity testing is that system operators throughout the water treatment industry are familiar 

with the use of conventional turbidimeters. Because filtrate turbidity monitoring has been 

required for media filtration under previous federal regulations for surface water treatment, 

conventional turbidimeters are widely used in water treatment plants across the country.

Many of the other advantages of both conventional and laser turbidimeters are relative to 

other continuous indirect integrity monitoring methods. For example, neither conventional nor 

laser turbidimeters are subject to the same accuracy and precision inconsistencies that can be 

problematic for particle counters. While two well-calibrated turbidimeters are likely to yield 

similar data for the same water, achieving such consistency from two separate particle counters 

may be more difficult. In addition, turbidimeters have the advantages of measuring a parameter 

that is absolute (as opposed to a relative measure, as with particle monitors), broad acceptance as 

a meaningful gauge of water quality, and application as a water quality benchmark in federal 

surface water treatment regulations.

Conventional turbidimeters are also significantly less expensive than particle counters. 

Although laser turbidimeters are typically more expensive than conventional turbidimeters, the 

ability to multiplex these instruments may help to minimize the associated cost of utilizing laser 

turbidimetry (Banerjee et al. 2001). In multiplexing, multiple sensors are connected to a single 

laser light source, detector, and control system via fiber optics. Sensors can be attached to 

monitor the filtrate from each membrane unit or individual membrane module, if desired. At the 

time of publication, however, multiplexing laser turbidimetry systems are still under 

development and not yet commercially available.

The most significant limitation of on-line turbidimeters is insensitivity to small changes 

in water quality. Since membrane systems produce a high quality filtrate, a turbidimeter may be 

unable to detect minor changes in filtrate water quality as a result of a small integrity breach. 

Laser turbidimeters are more sensitive than conventional turbidimeters, but this sensitivity is still 

subject to variation based on the number of modules per sensor, the system hydraulic 

configuration, and other system-specific parameters. 

Turbidimeters are also subject to air entrainment error. Any air bubbles introduced into 

the system either during production, backwashing, chemical cleaning, or integrity testing may be 

falsely detected as particulate matter, artificially increasing the turbidity reading. Consequently, 

after a backwash cycle or chemical cleaning (particularly if air is utilized in the process), 

turbidity measurements may not be representative of filtrate quality until any entrained air is 
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purged from the system. This purge time will vary between different membrane filtration 

systems and their respective backwash or chemical cleaning practices. Typically, bubble traps 

are employed with turbidimeters (both conventional and laser) to minimize or eliminate this 

error. 

Advantages of turbidity monitoring include:

• Widely used in surface water treatment plants throughout the country

• Less expensive instrumentation (conventional turbidimeters)

• Consistent, precise measurements

• Absolute (as opposed to relative) measure of water quality

Limitations of turbidity monitoring include:

• Less sensitive to smaller integrity breaches than particle counters or particle monitors 

(conventional turbidimeters)

• Susceptible to air entrainment error

5.3 Particle Counting and Particle Monitoring

Particle counters use a laser-based light scattering technique to count particles and group 

them according to size. Particle monitors also operate on the principle of light obstruction; 

however, rather than counting particles and grouping them by size, particle monitors measure 

particulate water quality on a dimensionless scale relative to an established baseline.

Although neither particle counting nor particle monitoring are specified under the 

LT2ESWTR, either may potentially be approved by a state as an alternative technique for 

satisfying the continuous indirect integrity monitoring requirement of the rule. However, any 

approved technique must still be utilized in such a way as to satisfy the four basic criteria for 

indirect integrity monitoring, as specified in section 5.1. In summary, these criteria are that the 

filtrate of each membrane unit must be monitored independently; indirect integrity monitoring on 

the filtrate of each membrane unit must be continuous (i.e., at a frequency no less than once 

every 15 minutes); two consecutive excursions above a pre-established, performance-based 

upper control limit must trigger direct integrity testing; and all excursions above the control limit 

that trigger direct integrity testing must be reported to the state on a monthly basis.

The ability of particle counters to convey information about particle size renders the 

concept of resolution (as defined under the LT2ESWTR) applicable to this method of integrity 

monitoring. Although there are no requirements for continuous indirect integrity monitoring 
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method resolution (or sensitivity) under the LT2ESWTR, the ability of particle counters to yield 

resolution information may help to optimize the usefulness of this technique for detecting 

potential integrity breaches. Any significant increase in the number of particles exceeding 3 μm 

in size may indicate that a breach allowing the passage of Cryptosporidium oocysts may have 

occurred. Any particle counters that are used for the purpose of membrane filtrate monitoring to 

satisfy the continuous indirect integrity monitoring requirement of the LT2ESWTR should be 

well calibrated to detect particles in the size range of Cryptosporidium oocysts (i.e., 3 to 8 μm). 

Conversely, because particle monitoring instruments do not measure particulate size, there is no 

specific resolution associated with the particle monitoring technique. 

As with turbidity monitoring, the concept of test method sensitivity (as defined under the 

LT2ESWTR) applies to both particle counting and particle monitoring in that fiber cutting 

studies may be conducted to correlate a particular reduction in log removal capability to 

increases in the quantity of particles in the filtrate. Although Adham et al. (1995) determined 

that particle counting was the most sensitive of the three common methods of continuous indirect 

integrity monitoring (i.e., conventional turbidity monitoring, particle counting, and particle 

monitoring), particle counting instruments have a number of well-established operational 

problems that can potentially distort both the accuracy and precision of their measurements. 

Particle monitoring devices, which are more sensitive then conventional turbidimeters but less 

sensitive than particle counters, share many of the same limitations of particle counters (Adham 

et al. 1995). The advantages and limitations of both particle counters and particle monitors are 

discussed in section5.3.3. The sensitivity of particle counting and particle monitoring methods 

for the detection of integrity breaches is further discussed in section 5.3.2 in the context of 

establishing control limits for these indirect integrity monitoring techniques.

5.3.1 Methods

USEPA does not currently approve particle counting and particle monitoring as 

compliance monitoring methods. However, either of these methods may be used for compliance 

with the continuous indirect integrity monitoring requirements of the LT2ESWTR if approval is 

granted by the state. Both the International Organization for Standardization (ISO) and the 

American Society for Testing and Materials (ASTM) have published standards relating to 

particle counting techniques, as follows:

• ISO 11943 – Hydraulic fluid power – On-line automatic particle counting systems for 

liquids – Methods of calibration and validation (1999)

• ASTM F658-00a – Standard Practice for Calibration of a Liquid-Borne Particle 

Counter Using an Optical System Based Upon Light Extinction (2000)

In addition, there are some relevant references on the use of particle counters in water 

treatment applications that may serve as a useful source of additional information:
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• Particle Count Method Development for Concentration Standards and Sample 

Stabilization. American Water Works Association Research Foundation 

(AWWARF) 2000.

• Fundamentals of Drinking Water Particle Counting. AWWARF 2000.

Currently there are no standards for particle monitoring, and little information has been 

published regarding the use of particle monitors in potable water treatment applications.

5.3.2 Control Limits

As discussed in section 5.2.2 (and throughout Chapter 4), a CL is a response to an 

integrity test that results in some measure of corrective action. UCLs refer to the required limits 

mandated under the LT2ESWTR, while LCLs are voluntary or additional state-mandated limits. 

In the context of continuous indirect integrity monitoring, the UCL is a response to the results of 

an indirect integrity method that triggers direct integrity testing. The UCL for the default method 

of turbidity monitoring is defined in the LT2ESWTR; however, the rule does not establish 

control limits for other continuous indirect integrity monitoring methods (e.g., particle counting 

or particle monitoring). 

Because particle count and particle monitoring data can vary significantly between two 

different instruments, even between adjacent membrane units applied to the same source water 

and within the same filtration system, site-specific CLs must be established when particle 

counting or particle monitoring is used as an alternative method of continuous indirect integrity 

monitoring. Moreover, it is recommended for these two techniques that CLs be developed on a 

membrane-unit-specific basis. One such approach that has been used by the Texas Commission 

on Environmental Quality (TCEQ) is establishing a CL for particle counting at the 95-percent 

confidence interval for all the data collected by a particular instrument monitoring the filtrate of 

a particular membrane unit over the previous month. However, because the particle counts could 

slowly increase over time without exceeding the 95-percent confidence interval (either as a result 

of slow membrane degradation or the instrument slipping out of calibration), this approach also 

incorporates an additional, higher control limit that is absolute (i.e., at a fixed number of particles 

per unit volume) (Schulze 2001). The magnitude of this UCL would be sufficient to indicate a 

probable membrane integrity breach independent of variations in particle count readings between 

different instruments. Note that if the 95-percent confidence interval of the data collected over 

the previous month is used to establish a new CL on an ongoing basis, it is important to exclude 

any data corresponding to a known integrity breach. Otherwise, a small amount of abnormally 

elevated data that are not indicative of typical membrane unit performance could skew the CL 

for the following month to an unacceptably high level, thereby diminishing the ability to detect 

small integrity breaches.

Absolute CLs may also be used without a lower, relative CL (e.g., the 95-percent 

confidence interval of the previous month’s data) provided the absolute CLs are sufficiently 

conservative and established using a scientific methodology that is approved by the state. For 
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example, studies may be conducted prior to placing the membrane filtration system into service 

in order to establish baseline particle count or particle monitoring data levels for each membrane 

unit. Building on the example of the framework developed by the TCEQ, the 95-percent 

confidence interval of this fixed data set could then represent an absolute CL. Although this 

method (and other membrane-unit-specific methods) may result in different CLs for each 

membrane unit, if studies indicate that baseline filtrate particle count or particle monitoring data 

are similar among all the units (such that the control limits would also be similar), the most 

conservative (i.e., lowest) CL may be applied to all the units. One advantage of using a 

statistical method such as the 95-percent confidence interval for setting CLs is that it minimizes 

the possibility that normal fluctuations and noise in the data that are unrelated to an integrity 

problem will exceed the control limit, triggering additional testing and resulting in lost unit 

productivity.

For hollow-fiber membrane filtration systems, fiber cutting studies may also be used to 

establish control limits for particle counting or particle monitoring by correlating quantifiable 

losses of log removal capability to specific increases in the number of particles in the 3 to 8 μm 

size range. However, while Adham et al. (1995) demonstrated that particle counting and particle 

monitoring were both more sensitive than turbidity monitoring (and that particle counting was 

more sensitive than particle monitoring) for detecting integrity breaches, a study conducted by 

Landsness (2001) indicated that both particle counting and turbidity monitoring may be unable to 

reliably detect an integrity breach of fewer than the equivalent of several hundred fibers in a 

membrane unit. As with turbidity monitoring, however, the sensitivity of both particle counters 

and particle monitors is dependent on the type of membrane system, the number of modules 

linked to a single instrument, the number of fibers per module, the hydraulic configuration, and 

other system-specific parameters.

For particle counting, particle monitoring, or any approved method of continuous indirect 

integrity monitoring, one reading must be taken on the filtrate of each membrane unit at least 

every 15 minutes. If the readings exceed the UCL for a period of fifteen minutes (or, in the case 

that readings are taken only at 15 minute intervals, if two consecutive 15 minute readings exceed 

the UCL), then direct integrity testing is required (40 CFR 141.728). Any membrane unit taken 

off-line for further integrity testing (i.e., direct and potentially diagnostic testing) may be 

returned to service upon passing a direct integrity test. All excursions beyond the UCL that 

trigger direct integrity testing must be reported to the state on a monthly basis (40 CFR 141.728). 

Also, although not required under the LT2ESWTR, utilities may also voluntarily choose to 

establish one or more LCLs that would trigger increased data collection frequency or require an 

operator to monitor the membrane performance for a specified period of time. Data analysis and 

reporting for continuous indirect integrity monitoring are further discussed in section 5.5, and 

guidance for the development of a comprehensive integrity verification program is provided in 

Appendix A.
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5.3.3 Advantages and Limitations

The primary advantage associated with particle counters and particle monitors is the 

sensitivity of the instruments relative to conventional turbidimeters. Both have been shown to be 

more sensitive to breaches in membrane integrity than conventional turbidimeters, with particle 

counters being the most sensitive of the three instruments (Adham et al. 1995; Jacangelo et al. 

1997). 

However, particle counters and particle monitors have a number of well-known potential 

operational problems that may distort the accuracy and precision of their measurements. First, 

two separate but otherwise identical particle counters or particle monitors applied to the filtrate 

of the same membrane unit may yield different responses, the magnitude of which may be 

particularly significant at the very low particle concentrations that are typical of membrane 

filtrate quality. This inconsistency could potentially make it necessary to establish CLs on a 

membrane-unit-specific basis, since the magnitude of a particle count or particle monitor reading 

that may begin to suggest a small integrity breach may not be the same for two different 

instruments applied to the filtrate of different membrane units. In addition, once baseline filtrate 

data are established for each membrane unit, this inconsistency could complicate changing 

instruments, if necessary, since new baseline data may need to be generated. The impact of this 

potential problem may not be significant, however, if the relative differences between 

instruments are negligible in comparison to the differences between the baseline filtrate quality 

and that associated with an integrity breach of sufficient magnitude to exceed the CL.

The variation between the readings of different instruments at low particle concentrations 

may be particularly problematic for using particle counters to verify log removal capabilities in 

association with challenge testing or a particulate-based direct integrity test. For example, the 

difference between 1.0 particles/mL and 0.1 particles/mL, which is within the range of observed 

variance for different particle counters at very low particulate concentrations, translates into a 

1 log discrepancy in terms of removal capability. This variation must be considered when using 

particle counters to verify log removals. Considerations and guidance for challenge testing are 

addressed in Chapter 3, and particulate-based direct integrity tests are discussed in section4.7.5.

Another disadvantage associated with particle counters and particle monitors is the 

potential for air entrainment to cause the instruments to overestimate filtrate particle 

concentrations. For example, in hollow-fiber membrane filtration systems, a significant amount 

of air may be introduced into the piping during the backwash process, particularly if the process 

utilizes air for scouring or pulsing the membranes. Any air bubbles remaining in the system after 

the backwash cycle is complete and the unit is returned to production may be falsely detected as 

particles. Thus, anomalous particle concentration spikes that may indicate an integrity problem 

could be masked until the air is purged from the system. As a result, the effective use of particle 

counters and particle monitors as an integrity monitoring tool is somewhat diminished after 

backwashing until the air is expelled and the particle concentrations return to baseline levels. 

The time required for this air purge varies with the particular membrane system and the 

backwash operating parameters. As with turbidimeters, bubble traps can be used in some cases 

to minimize or eliminate this potential source of error.
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Particle counters and particle monitors are also susceptible to coincidence error and 

sensor clogging, both of which can cause particle concentrations to be underestimated. 

However, this may not represent a significant problem for these instruments when applied to 

monitor membrane filtrate. Because membrane filtrate consistently has a very low concentration 

of particulate matter, even under compromised conditions, coincidence or clogging errors are 

much less likely with membrane filtration systems than for most other potable water treatment 

applications.

Advantages of particle counting and particle monitoring are generally similar, and include the 

following: 

• More sensitive to smaller integrity breaches than conventional turbidimeters

• Widely used in surface water treatment plants (particle counters)

• Absolute (as opposed to relative) measure of water quality (particle counters)

• Ability to yield information regarding test resolution (particle counters)

Limitations of particle counting and particle monitoring include:

• Imprecision between instruments at low particulate concentrations 

• Susceptible to air entrainment error

• Susceptible to coincidence and clogging error at higher particle concentrations

• More expensive instrumentation (particle counters)

• Relative measure of water quality (particle monitors)

• More operation and maintenance support

5.4 Other Indirect Monitoring Methods

In addition to the three methods of continuous indirect integrity monitoring described in 

this chapter – turbidity monitoring, particle counting, and particle monitoring – other methods 

may also be used. Note that turbidity monitoring is the only method specified under the 

LT2ESWTR for compliance with the rule, although other methods (including both particle 

counting and particle monitoring) may be used with state approval. All three of the methods 

described in this chapter relate to monitoring particulate matter in the filtrate, as these methods 

are applicable to all types of membrane filtration systems – MF/UF, NF/RO, and membrane 

cartridge filtration (MCF). However, because NF/RO processes also remove dissolved 
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constituents, methods that monitor these constituents in the filtrate have the potential to be used 

for continuous indirect integrity monitoring with NF/RO systems.

An example of one potential method that could be used for continuous indirect integrity 

monitoring with NF/RO systems is conductivity monitoring. Because monitoring filtrate (i.e., 

permeate) conductivity is routinely used as a means of assessing semi-permeable membrane 

performance, it may be advantageous for utilities using NF/RO systems for compliance with the 

LT2ESWTR to employ this technique for continuous indirect integrity monitoring, if approved 

by the state. However, there are some limitations with conductivity monitoring that are also 

applicable to other indirect methods that monitor dissolved constituents. First, increased salt 

passage over time may be a function of either an uncontrolled increase in membrane 

permeability or a planned increase in system recovery or flow, parameters unrelated to the 

physical integrity of the membranes and their ability to serve as a barrier to particulate matter 

such as Cryptosporidium. Although increases in membrane permeability that do not represent 

integrity breaches can manifest over longer periods of time and may not occur over the 15-

minute intervals at which continuous indirect integrity monitoring must be conducted, this is a 

consideration that should be taken into account by utilities. In addition, filtrate conductivity (and 

other dissolved constituents) may vary with water quality parameters such as pH and 

temperature, factors that are likewise unrelated to membrane integrity. While fluctuations in 

conductivity with these factors may potentially be minimal relative to those that might occur as a 

result of an integrity breach, it is nevertheless important that a NF/RO system be operated under 

consistent water quality and operational conditions for conductivity to yield reliable and 

meaningful results relative to membrane integrity. 

Whether using a technique that measures particulate matter or dissolved constituents, 

there are several general criteria that should be considered when selecting an alternative method 

for continuous indirect integrity monitoring:

• The monitoring technique should be approved for use by the state.

• For systems that apply membrane filtration specifically for compliance with the 

LT2ESWTR, the technique must be approved by the state specifically for compliance 

with the rule.

• The monitored parameter must be able to be measure on-line. 

(i.e., while the membrane unit is in production)

• The parameter must be able to be monitored continuously.

(i.e., at a frequency of at least one measurement every 15 minutes for the purposes of 

compliance with the LT2ESWTR)

• The parameter must be monitored on the filtrate of each membrane unit.

• A significant change in the levels of the parameter should generally be indicative of 

an integrity breach.
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• The instrumentation used to measure the parameter should be sufficiently sensitive 

such that relatively small integrity breaches can be detected, thus enabling the 

establishment of a meaningful UCL. (Note that there are no specific sensitivity 

requirements for continuous indirect integrity monitoring under the LT2ESWTR.)

5.5 Data Collection And Reporting

The LT2ESWTR requires utilities both to record a continuous indirect integrity 

monitoring reading for each membrane unit at least once every 15 minutes and to report any 

sequence of two consecutive excursions above the upper control limit (which would trigger 

direct integrity testing) to the state on a monthly basis (40 CFR 141.728). Within this regulatory 

framework, utilities and/or states have some latitude with respect to the specifics of how 

continuous indirect integrity monitoring data are collected and reported. For example, provided 

the basic requirements specified in the rule are satisfied, compliance strategies may vary in 

regard to the following:

• Frequency of data collection

• Method of data reduction

• Monitoring location

Some utilities may desire to collect data more frequently than once every 15 minutes, 

particularly since most of the instruments associated with the various methods of continuous 

indirect integrity monitoring are capable of recording data at much shorter intervals. Even if a 

utility is not required by the state to report data any more frequently than at 15-minute intervals, 

the additional data may be useful for monitoring system performance with greater precision, 

identifying trends and patterns, and ultimately optimizing system operation. If a utility chooses 

to measure and record indirect integrity monitoring data at a frequency greater than every 

15 minutes (or is required to do so by the state), the state may require that data collected at 

intervals shorter than 15 minutes be compared to the UCL for purposes of potentially triggering 

direct integrity testing and compliance reporting.

If a utility does opt to collect continuous indirect integrity monitoring data more 

frequently than every 15 minutes, the LT2ESWTR allows some flexibility regarding how the 

collected data are utilized for compliance purposes. For example, if a utility records one 

turbidity reading each minute, there are several methods in which the data collected over a 15-

minute period may be reduced. One option would be to average the data to yield a single 

statistic that is representative of the performance of each membrane unit for a given 15-minute 

interval. Two consecutive 15-minute averages exceeding the UCL would trigger direct integrity 

testing and subsequent notification of the state in the next monthly report. Another option for 

this example would be to make the maximum value of the 15 measurements the representative 

statistic. Likewise, any two consecutive 15-minute intervals with maximum values exceeding 

the UCL would trigger direct integrity testing and require the excursion to be reported. 
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Alternatively, independent of how often data are collected, a utility may opt to use only the 

reading recorded at the 15-minute mark for comparison with the UCL. Note that while the rule 

allows for some flexibility for data collection and reduction, the state may require a specific 

methodology at its discretion. 

The critical interval of 15 minutes cited in the LT2ESWTR may also be shortened at the 

discretion of the utility if a more conservative approach to monitoring membrane integrity is 

desired. For example, a utility that records a particle count measurement once every five 

minutes may configure its system to trigger direct integrity testing if any two consecutive 5 

minute readings exceed the UCL. A more conservative approach to the location of indirect 

integrity monitoring may be adopted, as well, if desired. Although monitoring is required for the 

filtrate of every membrane unit, additional monitoring may be conducted on one or more 

individual membrane modules, pressure vessels, or stages within a membrane unit. Monitoring 

smaller incremental membrane process units, where applicable and cost-effective, may help to 

isolate any integrity breaches that are detected and provide additional data for process 

optimization and control.

The LT2ESWTR does not require utilities to report all excursions above the UCL, but 

rather only those sequences of two consecutive UCL excursions that would trigger direct 

integrity testing. While single excursions may be anomalous measurements and not be indicative 

of an integrity breach, two consecutive measurements exceeding the UCL represent more 

sustained evidence of an integrity problem that warrants further testing and subsequent reporting. 

In conjunction with these exceedances, the rule also requires a corresponding summary of any 

corrective action taken in each case, including direct integrity testing and subsequent measures 

(40 CFR 141.728). The ongoing record of these responses over time should also help the utility 

identify and streamline any appropriate future corrective action that may be necessary. Note that 

the state may require more detailed reporting at its discretion. 

In general, the LT2ESWTR allows for some flexibility in compliance strategy with 

respect to continuous indirect integrity monitoring data collection and reporting to accommodate 

site-specific considerations, provided the basic requirements of both the rule and those of the 

state are satisfied. Additional guidance for data collection and analysis strategies, as well as for 

using this data for system optimization, is provided in Appendix A in the context of developing a 

comprehensive integrity verification program.
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6.0 Pilot Testing

6.1 Introduction

Pilot testing a membrane filtration process is often conducted as part of the design effort 

for a membrane treatment facility. The purpose of this chapter is to describe the considerations 

that are associated with pilot testing. It should be noted that the Long Term 2 Enhanced Surface 

Water Treatment Rule (LT2ESWTR) does not contain any requirements for pilot testing

membrane filtration systems; thus, this chapter is simply intended to provide general guidance in 

terms of widely recognized industry practices. However, any particular state requirements would 

supersede the guidance provided in this chapter. This chapter is not intended to serve as a 

comprehensive guide, but rather to highlight some of the important benefits and considerations 

associated with pilot testing.

Pilot testing should not be confused with challenge testing. Challenge testing, as 

described in Chapter 3, is typically conducted on a product-specific basis and characterizes a 

membrane filtration module in terms of removal efficiency. Pilot testing, by contrast, is 

conducted on a site- and system-specific basis and is used to collect information for full-scale 

facility design. Pilot testing need not include challenge testing unless specifically required by 

the state. In cases in which a particular membrane product that has not undergone challenge 

testing is proposed for compliance with LT2ESWTR requirements, it may be convenient to 

include a challenge test in the scope of pilot testing. 

The primary goal of a membrane pilot study is to obtain information such as treated water 

quality (e.g., turbidity) and operating parameters (e.g., flux) that are necessary for the design of a 

membrane filtration facility. The treated water quality data provides assurance that the treatment 

objectives can be achieved, while the pre-determination of operating parameters allows for 

proper sizing of the membrane facility and minimizes the uncertainties regarding footprint and 

utility requirements. In general, the use of pilot data helps account for unforeseen conditions that 

may otherwise have gone undetected. Pilot testing also helps to familiarize operators with the 

membrane treatment equipment. 

This chapter is divided into the following sections:

Section 6.2: Planning

This section discusses important considerations that should be taken into 

account before conducting a pilot test. 

Section 6.3: Testing Objectives

This section provides an overview of testing objectives such as the 

optimization of membrane flux, backwashing, and chemical cleaning, as 

well as balancing the flux with backwashing and chemical cleaning 

frequencies to maximize operational efficiency.
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Section 6.4: Testing and Monitoring

This section discusses important data that are typically collected during 

pilot testing, including operational, water quality, microbial, and integrity 

testing parameters. 

Section 6.5: Report Development

This section outlines considerations for preparing a summary report after 

the pilot testing is completed.

Most of the information contained in this chapter is generally applicable to both 

microfiltration (MF)/ultrafiltration (UF) and nanofiltration (NF)/reverse osmosis (RO) systems. 

Distinctions between the two systems are noted where differences occur. Pilot testing for 

membrane cartridge filtration (MCF) systems is typically simpler relative to testing other types 

of membrane filtration and is often performed simply to verify the replacement frequency and 

productivity for the filters. Consequently, pilot testing MCF systems is not specifically 

addressed in this chapter. However, MCF piloting guidelines are generally similar to those for 

MF/UF with the exception of references to cleaning intervals, since membrane cartridges are not 

typically cleaned and instead disposed of when fouled.

6.2 Planning

The planning phase prior to implementing pilot testing is an important component of an 

overall pilot test program. Careful planning helps ensure that all the pilot test objectives are 

achieved both efficiently and economically without unexpected delays. The most important 

element of the planning process is the development of a comprehensive pilot test protocol 

specifying how the testing should be conducted. This protocol should include not only 

instructions for carrying out the testing, but also specific testing objectives and strategies for 

optimizing performance in terms of flux, backwashing, and chemical cleaning (see section 6.3). 

A plan for collecting water quality, integrity test, and microbial (where applicable) data should 

also be incorporated in the test protocol (see section 6.4). In addition, it is recommended that the 

pilot protocol be developed in conjunction with the state to ensure that any particular state 

requirements are satisfied.

In addition to the test protocol, a number of other planning considerations should be 

taken into account. For example, it should be determined whether the pilot test units can be 

provided by the membrane manufacturers at no cost, rented or custom-fabricated, as budget 

constraints allow. Also, once the particular membrane filtration systems to be tested are 

selected, the utility requirements (i.e., water, electricity, and/or air) for the pilot units must be 

accommodated at the test site. Appropriately sized plumbing connections should be provided for 

the feed, filtrate, and concentrate streams, and provisions must be made for the disposal of 

backwash (for MF/UF) and chemical cleaning residuals. Labor requirements for operating the 

pilot units should be estimated, and it is also important to ensure that the designated testing site 

has sufficient area to accommodate all the units to be tested. One consideration that is 
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sometimes overlooked is shipping the pilot units; the proper equipment for loading and 

unloading the pilot units must be available at the testing site or be provided with the unit(s). 

Special attention in the planning phase should be given to process considerations such as 

scalability, screening appropriate membrane filtration systems to test, and test scheduling. These 

three aspects of pilot test planning are discussed in sections 6.2.1, 6.2.2, and 6.2.3, respectively.

6.2.1 Process Considerations

It is important that the pilot process be representative of the full-scale system. For 

example, if pretreatment is part of the planned full-scale process, then the pilot process should 

incorporate similar pretreatment. Likewise, piloting should also accommodate production (e.g., 

filtration) and intermittent (e.g., backwashing) design considerations to the extent that these 

parameters have been defined prior to the pilot testing phase. Thus, if it has been determined 

that the membrane filtration system(s) that will be used at full-scale (e.g., if an MF/UF system 

has been pre-selected) requires air at a specified flow rate and duration during a backwash 

process, then the pilot system should be designed and operated in a similar manner. The design 

of the membrane system should also mimic the hydraulic configuration of the full-scale system, 

as described in section 2.4. However, it should be recognized that it might not be possible to 

design a pilot system using the same hydraulic configuration used at full scale for all membrane 

processes. For example, it may not be economically feasible to design a NF/RO pilot unit with 

hydraulic characteristics identical to the full-scale system. In this case, the methodology of 

conversion from pilot-scale data to full-scale design should be included in the development of 

the pilot test protocol. 

Membrane modules should also be scaleable with respect to appropriate operational 

characteristics. Section 3.8 includes a discussion pertaining to the use of a small-scale 

membrane module, which may be appropriate under some circumstances. Although the 

discussion in section 3.8 is presented in the context of challenge testing, the concepts are also 

applicable to piloting, and small-scale module testing may be viable for pilot testing depending 

on the objectives of the study

6.2.2 Screening and System Selection

Spiral-wound NF and RO membrane modules are standardized such that the membranes 

from different manufacturers are interchangeable and system design is somewhat uniform. Thus, 

in terms of screening for appropriates membranes, the primary consideration involves the 

selection of a membrane material that provides desirable productivity, resistance to fouling, and 

removal characteristics.

 

Screening is somewhat more complex for MF/UF systems, which are largely proprietary 

in design. The various commercially available systems may use either pressure or vacuum as the 

driving force and can be designed to filter from the inside-out or outside-in direction relative to 

the fiber lumen. In addition, the various membrane materials have differences that may be 
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important, including removal efficiency and pH and oxidant tolerance. Membranes of different 

materials also have varying degrees of compatibility with water treatment chemicals such as 

coagulants and powdered activated carbon (PAC) that may affect performance and cost. It is 

important to consider such differences in MF/UF membranes and membrane systems and how 

these may impact system selection. For example, if the source water were periodically 

prechlorinated, a membrane that is not compatible with chlorine would be undesirable. 

A list of some questions to consider when screening membranes for a pilot study is 

provided below:

• What are the treatment objectives of the application?

• What operational constraints/goals are to be considered in membrane selection?

• Has the membrane been used on similar waters at other sites?

• What are the pH and oxidant tolerances of the membrane? Are these compatible with 

the application?

• Is the membrane compatible with any pretreatment chemicals aside from oxidants 

that may be in use, such as alum, ferric chloride, PAC, and polymers?

• Is this membrane compatible with solids and total organic carbon (TOC) levels in the 

raw water?

• Does the membrane have prior applicable state regulatory approval and any required 

certifications (e.g., NSF 61)? 

• Does the supplier have experience with full-scale operation for a facility of this size 

and for treating similar water with the same configuration to be used in the pilot?

• Does the system require proprietary items such as spare parts or cleaning chemicals? 

• Are there any unusual operational considerations associated with the membrane 

filtration system, such as significant power requirements, frequent membrane 

replacement, or substantial or undesirable chemical use?

These types of questions should be considered as a part of the screening process before selecting 

any membrane filtration systems to pilot. An additional consideration for MF/UF systems is that 

because these systems are proprietary, the membrane modules vary in design and are not 

presently interchangeable. Thus, a utility is limited to obtaining replacement membranes from 

only one supplier after a system has been installed.

In some cases, screening tools for MF/UF membranes may be limited to experience at 

other sites or chemical incompatibility. Therefore, pilot testing is an important consideration. 

For NF/RO, proprietary software programs that are available from the various membrane 
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manufacturers can predict membrane system removal performance with a fairly good degree of 

accuracy for a particular membrane product. Small-scale module testing (see section 3.8) may 

be an option for screening some MF/UF membranes. Similarly, NF/RO membranes could be 

screened for some parameters using flat-sheet studies or single element tests.

6.2.3 Scheduling

Scheduling is another important aspect of pilot test planning. One factor to consider 

when scheduling a pilot test for surface waters is seasonal variations in water quality, since 

turbidity, temperature, algae content, taste and odor, and other parameters can potentially vary 

significantly throughout the year. A description of typical seasonal variation in surface water 

quality is summarized below. Note that not all of the seasonal variations described will be 

applicable for every site, or necessarily pertinent for all types of membrane filtration systems.

Summer– Because user demand is typically highest in the summer, the filtration system 

may also have to produce more water during this season. However, greater production is 

facilitated by warmer water temperature. The degree to which a membrane process is 

affected by a change in water temperature is related to the viscosity of the water. Since 

the viscosity of the water is lower at higher temperatures, the membrane flux will likely 

be at its peak in this season. (The relationship between water temperature and viscosity 

as it relates to flux is discussed in further detail in Chapter 7.) Warmer water also 

facilitates enhanced algae growth that may be problematic for membrane system 

operation. Taste and odor events may also manifest during the summer months.

Autumn – In areas with hardwood cover in the watershed, autumn months typically 

bring an increase in the organic content of surface water resulting from the decay of 

fallen leaves. Turbidity may sharply increase, as well. Furthermore, cooler air 

temperatures and wind cause surface waters in reservoirs to “turnover,” bringing deeper, 

more anaerobic water to the surface and thus creating the potential for both taste and odor 

and iron and manganese problems.

Winter – Although winter months yield the coldest water temperatures and thus typically 

the lowest membrane fluxes, the demand for water is typically at its lowest, as well. Both 

the cold temperature and decrease in demand are likely to minimize required membrane 

flux in this season. (As previously noted, the relationship between water temperature and 

viscosity as it relates to flux is discussed in Chapter 7.) Most membrane manufacturers 

have membrane-specific temperature compensation factors for cold-water operations.

Spring – Spring months usually yield increases in water temperature, as well as in the 

potential for turbidity spikes related to run-off caused from snow melt. There is also 

potential for springtime turnover of a reservoir, resulting in subsequent degradation in 

feed water quality.
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Ideally, the pilot study should be conducted during the time of year yielding the most 

difficult water quality to treat, so that design parameters resulting from the study, such as flux 

and chemical cleaning frequency, would be conservative for year round operation. Some water 

quality effects can be accurately modeled, such as that of temperature on productivity. Multiseason piloting is advantageous if scheduling and cost constraints will allow. In fact, some states 

require multi-season piloting for membrane treatment facilities.

The duration of the pilot study is also an important scheduling consideration. In general, 

piloting through at least three cleaning cycles is recommended practice. The target cleaning 

frequency for hollow-fiber MF/UF systems is typically at least 30 days of continuous operation. 

Through the first cleaning cycle the membrane flux and backwash frequency are usually targeted 

to provide for 30 days of operation before cleaning is required. The second cycle provides an 

opportunity for optimization and operational improvement. The third cycle establishes 

repeatability if the operating conditions remain the same. Note that because new membranes 

typically perform better than membranes that have been previously fouled and subsequently 

cleaned, it may be beneficial to add an extra cycle; to study the effects of repeated fouling and 

cleaning on membrane performance. Because this strategy represents three operational cycles of 

at least 30 days, a pilot test duration of 90 days (or 3 months) is recommended, if possible. For 

spiral-wound NF/RO membrane systems, longer cleaning intervals are desirable, which results in 

fewer operational cycles than for a MF/UF pilot of similar duration.

For more thorough MF/UF pilot studies, approximately 4 to 7 months (3,000 to 5,000 

hours) of cumulative operational time is usually recommended. A longer pilot study may be 

appropriate for newer, less proven membrane filtration systems or for applications in which the 

water quality is extremely variable. NF/RO pilot studies generally range from about 2 to 7 

months (1,500 to 5,000 hours) of cumulative operational time, with longer studies used for 

waters of variable quality. The state may also have minimum requirements for the duration of 

pilot studies.

6.3 Testing Objectives

Membrane flux and system productivity are typically the most important design 

parameters to optimize, as these dictate the number of membranes (and hence a large portion of 

the capital cost) required for the full-scale plant. Because these two parameters are inversely 

related to a certain extent, the pilot testing process may help to establish the optimum balance. 

For example, typically operating at higher fluxes increases the rate of fouling, in turn requiring 

more frequent backwashing and chemical cleaning. However, the system productivity is limited 

by the backwashing (where applicable) and chemical cleaning frequency. Backwashing and 

chemical cleaning not only use filtrate as process water (thus affecting the productivity), but also 

represents time during which filtrate cannot be produced (thus affecting overall system 

productivity). The effect of more frequent backwashes and chemical cleanings on the system 

productivity may be an important consideration if the source water is limited, or alternatively if 

waste disposal is problematic. In addition, chemical cleaning may be a labor-intensive operation 

that requires the handling of harsh chemicals and produces a waste stream that may be difficult 

to dispose. However, chemical cleaning is a necessary process associated with membrane 
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filtration, and establishing operating practices that extend the time between cleanings is one 

objective of a membrane pilot study. 

This section provides guidelines for balancing flux, productivity, backwash frequency 

(where applicable), and chemical cleaning intervals during pilot testing. It is important to note 

that in order for cause and effect to be analyzed properly, only one process variable should be 

changed at a time during the pilot study. Economic and time constraints often dictate the 

duration of the pilot study and may not allow complete optimization of each of these parameters. 

Therefore, it is important to understand which of these parameters is most important for a 

particular application of membrane filtration and to structure the pilot test protocol accordingly. 

Thus, the result of a pilot study is not necessarily the “optimum” design, but rather a set of 

operational conditions that will result in feasible and economic water treatment over the 

anticipated range of operating and source water conditions. 

6.3.1 Membrane Flux Optimization

Membrane manufacturers can recommend fluxes for particular applications and a given 

water quality. Table 6.1 lists some of the important water quality data that should be provided to 

membrane suppliers to facilitate a fairly accurate initial estimation of anticipated membrane 

productivity. 

If scheduling permits, it is typically advantageous to begin piloting MF/UF systems at a 

conservative flux and then increase it based upon the rate of fouling observed. The flux may be 

increased either after a chemical cleaning or during a filter run if the pilot unit has undergone 

sustained operation with only a nominal increase in fouling. In subsequent filter runs (i.e., 

between chemical cleanings) the pilot unit flux may be either increased if the fouling rate of the 

previous run was still within acceptable tolerances or decreased if the fouling occurred at an 

unacceptably high rate. Note that the backwash frequency may also be adjusted either during a 

filter run or between filter runs to minimize fouling at a particular flux (see section 6.3.2). This 

fine-tuning process of adjusting the flux and/or backwashing interval (if possible, only one 

parameter should be adjusted at a time) between filter runs may be repeated to the extent that 

budget and scheduling constraints allow, or to the point at which such adjustments reach the 

point of diminishing returns. NF/RO performance can typically be accurately gauged by the 

manufacturers based on water quality models, although it is often beneficial to adjust the flux 

over a series of filter runs to optimize productivity based on pilot test results. (Note, however, 

that manufacturers’ models do not predict fouling.) The pilot test protocol should include the 

expected range of membrane fluxes and guidelines on how these fluxes should be adjusted based 

upon the results of early testing. Models may also indicate chemical pretreatment requirements 

necessary to control scaling on NF/RO membranes and thus to help maintain acceptable fluxes.
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Table 6.1 Water Quality Parameters to Measure Prior to Piloting

Parameter MF/UF NF/RO

Cations

Aluminum X

Ammonia X

Barium X

Calcium X X

Iron X X

Magnesium X X

Manganese X X

Potassium X

Sodium X

Strontium X

Anions

Chloride X

Fluoride X

Nitrate X

Silica X X

Sulfate X

Other Chemical / Physical Parameters

Algae X

Alkalinity X

pH X X

SDI X

TDS X

TOC X X

TSS X

Turbidity X X

UV-254 X X

6.3.2 Backwash Optimization

Backwashing is a periodic reverse flow process used with many hollow-fiber MF/UF 

systems to remove accumulated contaminants from the membrane surface, thus maintaining 

sufficient flux and minimizing the rate of long-term, irreversible fouling. A MF/UF membrane 

manufacturer can provide information on the backwash protocol required for its system, as well 

as a recommended backwash frequency. It is important to understand the following issues 

regarding the backwash process, which are applicable to both piloting and full-scale operation:

• Large water and/or air flows over short periods of time may be included as a part of 

the backwash process. The flows of air and water vary depending on the system 

manufacturer.
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• Backwash effluent water contains approximately 10 to 20 times the concentration of 

the feed contaminants. 

• Many manufacturers add free chlorine to the backwash process water to reduce 

membrane fouling via disinfection (a process often referred to as a “chemically 

enhanced backwash”). Other manufacturers may include acid or caustic solution to 

remove inorganic or organic foulants. As a result, the backwash effluent may contain 

both solids and/or chemical residuals.

• In locations where the disposal of backwash water may be problematic, consideration 

should be given to the volume of backwash water generated. Backwash water 

treatment should also be considered in order to maximize system productivity and 

minimize residuals.

• Increased backwash efficiency can significantly enhance system performance, 

allowing higher fluxes and recoveries.

Variations in the backwash frequency will influence the waste flow, as well as the 

productivity of the system. If higher residual flows can be easily accommodated and 

productivity is not a critical design consideration, increasing the backwashing frequency may be 

a viable strategy for achieving longer filter run times or higher membrane fluxes.

Based upon the water quality data provided, MF/UF manufacturers can estimate the 

initial membrane flux and backwash frequency for the pilot test. The backwash frequency can be 

optimized during pilot testing to provide the most economical process. The potential for 

increased flux and useful membrane life that may be commensurate with increased backwash 

frequency should be balanced against decreased productivity, increased waste volume, and 

higher chemical usage (if applicable).

6.3.3 Chemical Cleaning Optimization

The chemical cleaning of a membrane filtration system is a necessary process that results 

in lost production time, produces chemical waste, and requires operator attention. Consequently, 

pilot testing should be used to minimize chemical cleaning frequency for both MF/UF and 

NF/RO systems, while maintaining acceptable productivity and controlling long-term fouling. 

The typical membrane cleaning cycle consists of recirculating a heated cleaning solution for a 

period of several hours. Some manufacturers require that the cleaning solution be prepared with 

softened or demineralized water, which should be accounted for during the pilot test. The 

specific cleaning regime will depend upon the feed water quality and the particular membrane 

system. Some membranes may be degraded by excessive cleaning frequencies, shortening 

membrane life. For MF/UF systems the typical chemical cleaning frequency assumed for design 

purposes is approximately once every 30 to 60 days. NF/RO systems are generally designed 

with significantly longer cleaning intervals, typically in the range of 3 months to 1 year. It is 

important for a utility to understand the impacts of chemical cleaning on the system and to 
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determine whether to increase the flux at the expense of the consequent increase in chemical 

cleaning frequency. One of the objectives of a pilot test should be to determine a chemical 

cleaning strategy that restores membrane permeability without damaging the membrane 

integrity. Thus, the pilot test should be designed to include at least one chemical cleaning, even 

if the pilot schedule is very restrictive. If necessary, the flux can be significantly increased to 

induce membrane fouling for the purposes of conducting chemical cleaning.

It is important to demonstrate the effectiveness of the chemical cleaning during the pilot 

study. Because membrane cleaning is a time-, labor-, and chemical-intensive process, it is 

important to avoid experimentation on the full-scale plant. The best way to evaluate the success 

of a particular chemical cleaning scheme is to conduct the cleaning in a methodical manner and 

evaluate the performance of each step in the process. Before and after each step, a plot of the 

transmembrane pressure (TMP) as a function of flux (or filtrate flow) should be developed. This 

method of evaluation is normally described as a “clean water flux test.” This test requires that 

the membrane unit be placed into filtration mode in between each of the various steps in the 

chemical cleaning process in order to observe the effect on the flux (or filtrate flow). (Note that 

filtrate produced during this process should be considered chemical waste.) For example, if a 

chemical cleaning process consists of recirculating citric acid and caustic in succession, three 

plots of TMP versus flux (or filtrate flow) should be generated. Figure 6.1 illustrates the clean 

water flux test technique for this example. The three applicable plots shown in Figure 6.1 are as 

follows:

1. TMP vs. flux (or filtrate flow) for the fouled membrane prior to cleaning

2. TMP vs. flux (or filtrate flow) after citric acid recirculation / cleaning

3. TMP vs. flux (or filtrate flow) after caustic recirculation / cleaning 

(i.e., after all cleaning steps are completed)
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Figure 6.1 Sample Chemical Cleaning Test Profile

In an ideal cleaning, the final plot (i.e., after caustic) of TMP versus flux (or filtrate flow) 

would be similar to that after the previous clean such that the plots generated after each

successive chemical cleaning (i.e., the final plot of each cleaning) would overlap. Thus, the plots 

established during a clean water flux test facilitate this comparison and provide an indication as 

to the effectiveness of the cleaning regimen. This is illustrated in Figure 6.1 with plots showing 

the increase in TMP per incremental change in flux (or filtrate flow). Smaller slopes indicate 

lower pressur e requirements for operation at a given flux (or filtrate flow), and thus a more 

effective step in the chemical cleaning process.

After cleaning, a direct integrity test should be conducted before the pilot unit is returned 

to service. If the membrane is integral and an acceptable flux has been restored, then the 

cleaning is deemed successful. If not, alternate cleaning strategies should be considered. A flow 

chart outlining a general pilot study sequence of events is shown in Figure 6.2, illustrating a

typical series of test runs and subsequent chemical cleaning events.
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Figure 6.2 Sample Pilot Study Sequence Overview

Set Up

Equipment

Base Run

Establish Flux / 

BW Scheme

Integrity test

Yes

Run #1

New BW

Protocol

Integrity test No

Run #2

Change Flux or

BW Frequency

Yes No

Integrity test Yes No

Pilot Testing 

Complete

Final Clean

& Integrity Test

 Successful

BW?

 Successful

Clean?

 Successful

Clean?

New Cleaning

Protocol

New Cleaning

Protocol

Run #3

Same as Run #2

(Redundancy)
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6.4 Testing and Monitoring

A thorough and carefully developed testing and monitoring plan is a critical component 

of a pilot test program, as this is the means by which system performance is assessed. There are 

several categories of testing and monitoring that should be addressed in a pilot test protocol, 

including operational parameters, water quality, and microbial monitoring (whe re applicable), 

and integrity testing. Each of these four categories is discussed in further detail in the following 

subsections.

6.4.1 Operational Parameter Monitoring

Monitoring pilot unit operational parameters is an important means of assessing system 

performance and tracking the rate of membrane fouling. The following operational parameters 

should be monitored continuously, if possible:

1. Elapsed run time

2. Pressure (feed, filtrate, concentrate)

3. Flow (feed, filtrate, concentrate)

4. Temperature (feed or filtrate)

Another important consideration in the evaluation of a membrane process is operational 

data collection for the intermittent process sequences, such as backwashing. For these 

intermittent processes, the design parameters (e.g., flows, times, volumes) associated with air, 

water, or chemical usage during the sequence should be established and verified during pilot 

testing. It is not uncommon for the pilot unit to operate under different parameters than those 

that would be considered appropriate for a full-scale unit. For example, a pilot system generally 

has faster pneumatic valve actuation times and a shorter overall backwash sequence than a fullscale unit. Some of these discrepancies are unavoidable; however, these scale-up issues should 

be noted for consideration prior to initiating pilot testing. 

6.4.2 Water Quality Monitoring

The particular water quality data collected during a pilot study will depend upon the type 

of membrane filtration system, site-specific treatment objectives, and foulants of concern. In 

general, the following table can be used as a guideline:
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Table 6.2 Suggested Water Quality Sampling Schedule for Membrane Piloting 1

MF/UF NF/RO Parameter

Feed Concentrate2Filtrate Feed Concentrate Filtrate

General Water Quality Parameters

Algae W W W

Color W M W W M W

HPC M M M M

Particle counts C C

pH D W W C W W

SDI W

Taste & odor M M M M M M

Temperature C C

TOC W M W W M W

Total coliform W W W

TSS M M M

Turbidity C M C C W

UV-254 W M W W W

Dissolved Solids

Alkalinity W M W W W W

Barium W W W

Hardness M W W W

Iron W W W W

Manganese W W W W

Silica W W W

Sulfate W W W W

TDS M C W C

Simulated Distribution System Disinfection Byproducts

HAAs M M

TTHMs M M

1 Generic recommendations only; the specific applicable parameters will vary with each application

2 Where applicable, based on particular hydraulic configuration used

Key:

C = Continuous

D = Daily

W = Weekly

M = Monthly

In addition to those flows listed in Table 6.2, it is recommended that the MF/UF backwash flow 

be checked weekly for total suspended solids (TSS) and turbidity. Note that the recommended 

parameters and sampling frequencies should be modified to meet the requirements and objectives 

of each particular site-specific pilot study. The sampling frequencies may also be modified 

during the course of a pilot study if conditions warrant.
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6.4.3 Microbial Monitoring

One advantage of membrane filtration is that it provides a physical barrier through which 

water must pass, virtually eliminating pathogens larger than the membrane exclusion 

characteristic. As with the full-scale facility, it is important that the pilot test demonstrate that 

this barrier is intact and is rejecting pathogens. A site-specific microbial challenge test may not 

be required by the state during the pilot study for a membrane filtration system that has received 

prior regulatory approval. For the purposes of LT2ESWTR compliance, product-specific

demonstration of Cryptosporidium removal efficiency is accomplished during challenge testing, 

although microbial monitoring may be conducted during a pilot study for additional verification 

of membrane performance at the discretion of the utility or if required by the state. Although 

additional microbial testing can be conduct for any pathogen of concern during piloting, 

typically, coliform bacteria, and sometimes Heterotrophic plate count (HPC) are used as an 

indicator for microbial removal efficiency. 

6.4.4 Integrity Testing 

The importance of integrity testing under the LT2ESWTR necessitates that greater 

emphasis be placed upon integrity testing during piloting for applications of membrane filtration 

intended for LT2ESWTR compliance. For direct integrity testing, the membrane manufacturer 

should use a method approved or mandated by the state. The standard direct integrity test 

sequences intended for the full-scale system should be incorporated into the pilot unit. Direct 

integrity testing should be conducted at least as frequently as required by the state for the fullscale facility. In the absence of a particular state requirement, it is generally recommended that 

direct integrity testing be conducted on a daily basis, a frequency consistent with the 

LT2ESWTR requirements. If an integrity breach occurs during the piloting testing, then 

diagnostic testing and membrane repair techniques may also be practiced. Continuous indirect 

integrity monitoring can usually be accomplished using data that are collected during a typical 

pilot test program (e.g., turbidity or particle count data). However, any state requirements 

regarding data that must be collected for the purposes of continuous indirect integrity monitoring 

for the full-scale facility should also be implemented during the pilot test, and may be required 

by the state in any case.

Note that because the number of membrane modules to which an integrity test is applied 

affects the sensitivity of the test (for both direct integrity testing and continuous indirect integrity 

monitoring), any control limits established for a full-scale system will generally not be applicable 

to a pilot unit, which typically utilizes only a small fraction of the number of modules in a fullscale membrane unit. Guidance on establishing full-scale system control limits for both direct 

integrity testing and continuous indirect integrity monitoring for the purposes of compliance with 

the LT2ESWTR are provided in Chapters 4 and 5, respectively, and also in Appendix A –

Development of a Comprehensive Integrity Monitoring Program.
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6.5 Report Development

After the pilot test is complete, a report should be prepared to summarize the procedures 

and the test results. The pilot study report should contain sufficient detail to establish design 

parameters for the full-scale plant to the extent for which the testing was intended. Flux, 

chemical cleaning, backwashing, and integrity testing, should be addressed, where applicable. 

Collected water quality data should also be included with emphasis given to unanticipated test 

results. Details on the operational parameters for each filter run (i.e., between chemical 

cleanings) should be summarized. The state may also have specific requirements for the report if 

the piloting is required.
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7.0 Implementation Considerations

7.1 Introduction

The successful implementation of a membrane filtration system requires a general 

understanding of the major considerations that influence system design and operation. Although 

there are many such considerations, this chapter focuses on the most significant of these issues, 

categorized as operational unit processes, system design considerations, and residuals treatment 

and disposal. The purpose of this chapter is to present a general overview of some of these 

critical considerations, highlighting their respective roles in the implementation of the 

technology. Note that this chapter is intended to provide information and recommendations 

regarding industry practices, and is not directly related to compliance with the Long Term 2 

Enhanced Surface Water Treatment Rule (LT2ESWTR).

This chapter is divided into the following sections:

Section 7.2: Operational Unit Processes

This section reviews the purpose(s) and practices associated with the 

major operational unit processes of membrane filtration systems, including 

pretreatment, backwashing, chemical cleaning, integrity testing, and posttreatment.

Section 7.3: System Design Considerations

This section discusses some of the most significant conceptual design 

issues associated with membrane filtration systems, including flux, water 

quality, temperature compensation, cross-connection control, and system 

reliability. 

Section 7.4: Residuals Treatment and Disposal

This section outlines management practices for the various waste streams 

produced by membrane filtration systems, such as backwash and chemical 

cleaning residuals, as well as concentrate.

7.2 Operational Unit Processes

All membrane filtration systems have associated operational unit processes that are 

essential for maintaining and optimizing system performance and therefore critical to the 

successful implementation of the technology. These operational processes include backwashing, 

chemical cleaning, and integrity testing. For the purposes of this discussion, pretreatment and 

post-treatment are also considered operational unit processes associated with membrane 

filtration. Each of these processes and its role in the operation of a membrane filtration system is 

described in the following sections. Although not every membrane filtration system utilizes all 

of these processes, many utilize each process to some degree. The optimization of these 

operational unit processes can be address during pilot testing, as discussed in Chapter 6.
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7.2.1 Pretreatment

 Pretreatment is typically applied to the feed water prior to entering the membrane system 

in order to minimize membrane fouling, but in some cases may be used to address other water 

quality concerns or treatment objectives. Pretreatment is most often utilized to remove foulants, 

optimize recovery and system productivity, and extend membrane life. Pretreatment may also be 

used to prevent physical damage to the membranes. Different types of pretreatment can be used 

in conjunction with any given membrane filtration system, as determined by site-specific 

conditions and treatment objectives. Pilot testing can be used to compare various pretreatment 

options, optimize pretreatment, and/or demonstrate pretreatment performance. Several different 

types of commonly used pretreatment for membrane filtration systems are discussed in the 

following subsections.

7.2.1.1 Prefiltration

Prefiltration, including screening or coarse filtration, is a common means of pretreatment 

for membrane filtration systems that is designed to remove large particles and debris. 

Prefiltration can either be applied to the membrane filtration system as a whole or to each 

membrane unit separately. The particular pore size associated with the prefiltration process 

varies depending on the type of membrane filtration system and the feed water quality. For 

example, although hollow-fiber microfiltration (MF) and ultrafiltration (UF) systems are 

designed specifically to remove suspended solids, large particulate matter can damage or plug 

the membranes fibers. For these types of systems the pore size of the selected prefiltration 

process may range from as small 100 μm to as large as 3,000 μm or higher, depending on the 

influent water quality and manufacturer specifications. Generally, hollow-fiber MF/UF systems 

that are operated in an inside-out mode are more susceptible to fiber plugging and thus may 

require finer prefiltration. 

Because nanofiltration (NF) and reverse osmosis (RO) utilize non-porous semipermeable membranes that cannot be backwashed and are almost exclusively designed in a 

spiral-wound configuration for municipal water treatment, these systems must utilize much finer 

prefiltration in order to minimize exposure of the membranes to particulate matter of any size. 

Spiral-wound modules are highly susceptible to particulate fouling, which can reduce system 

productivity, create operational problems, reduce membrane life, or in some cases damage or 

destroy the membranes. If the feed water has a turbidity less than approximately 1 NTU or a silt 

density index (SDI) less than approximately 5, cartridge filters with a pore size range from about 

5 to 20 μm are commonly used NF/RO prefiltration. However, if the feed water turbidity or SDI 

exceeds these values, a more rigorous method of particulate removal, such as conventional 

treatment (including media filtration) or MF/UF membranes, is recommended as pretreatment for 

NF/RO.

In conventional applications of cartridge filtration technology, high quality source waters 

are treated, and thus additional prefiltration is typically not required. However, membrane 

cartridge filtration (MCF) systems may warrant some level of prefiltration, possibly using more 

conventional cartridge or bag filters, to protect and extend the life of the more effective and more 
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expensive MCF, particularly if the MCF system is applied to surface waters specifically for the 

purposes of LT2ESWTR compliance. Since MCF is a new class of technology defined under the 

LT2ESTWR, pretreatment practices for MCF will likely evolve as this technology is applied. 

Prefiltration requirements will likely be similar to those for MF/UF.

A summary of the typical prefiltration requirements associated with the various types of 

membrane filtration is presented in Table 7.1.

Table 7.1 Typical Membrane System Prefiltration Requirements

Membrane System Prefiltration Requirements

Classification Configuration Size 

(μm) Type(s)

Membrane Cartridge 

Filtration (MCF)1 Cartridge 300 - 3,000 Strainers;Bag Filters

Hollow-Fiber,

Inside-Out 100 - 300 Strainers;Bag Filters Microfiltration (MF) /

Ultrafiltration (UF)

Hollow-Fiber,

Outside-In 300 - 3,000 Strainers;Bag Filters

Nanofiltration (NF) / 

Reverse Osmosis (RO) Spiral-Wound 5 - 20 Cartridge Filters

1 Prefiltration is not necessarily required for MCF systems

In some cases, one type of membrane filtration may be used as prefiltration for another. 

This type of treatment scheme is commonly known as an integrated membrane system (IMS). 

Typically, this involves the use of MF/UF as pretreatment for NF/RO in applications that require 

the removal of particulate matter and microorganisms as well as some dissolved contaminants 

such as hardness, iron and manganese, or disinfection byproduct (DBP) precursors. One of the 

most significant advantages of an IMS treatment scheme is that the MF/UF filtrate is of 

consistently high quality and allows the NF/RO system to maintain stable operation by reducing 

the rate of membrane fouling.

7.2.1.2 Chemical Conditioning

Chemical conditioning may be used for a number of pretreatment purposes, including pH 

adjustment, disinfection, biofouling control, scale inhibition, or coagulation. Some type of 

chemical conditioning is almost always used with NF/RO systems, most often the addition of an 

acid (to reduce the pH) or a proprietary scale inhibitor recommended by the membrane 

manufacturer to prevent the precipitation of sparingly soluble salts such as calcium carbonate 

(CaCO3), barium sulfate (BaSO4), strontium sulfate (SrSO4), or silica species (e.g., SiO2). 
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Software programs that simulate NF/RO scaling potential based on feed water quality are 

available from the various membrane manufactures. In some cases, such as for those NF and RO 

membranes manufactured from cellulose acetate, the feed water pH must also be adjusted to 

maintain the pH within an acceptable operating range to minimize the hydrolysis (i.e., chemical 

deterioration) of the membrane. The addition of chlorine or other disinfectants may also be used 

as pretreatment for primary disinfection or to control biofouling. However, because some 

NF/RO membrane materials are readily damaged by oxidants, it is important that any 

disinfectants added upstream be quenched prior to contact with such membranes.

A number of different chemicals may be added as pretreatment for MF or UF, depending 

on the treatment objectives for the system. For example, lime may be added for softening 

applications; coagulants may be added to enhance removal of total organic carbon (TOC) with 

the intent of minimizing the formation of DBPs or increasing particulate removal; disinfectants 

may be applied for either primary disinfection or biofouling control; and various oxidants can be 

used to precipitate metals such iron and manganese for subsequent filtration. However, as with 

NF and RO membranes, it is important to ensure that the applied pretreatment chemicals are 

compatible with the particular membrane material used. As with conventional media filters, presettling may be used in conjunction with pretreatment processes such as coagulation and lime 

softening. While a MF/UF system may be able to operate efficiently with the in-line addition of 

lime or coagulants, pre-settling in association with these pretreatment processes can enhance 

membrane flux and increase system productivity by minimizing backwashing and chemical 

cleaning frequency.

Although chemical pretreatment is generally not associated with cartridge filters, if the 

filters are compatible, disinfectants may be added upstream of MCF systems to maximize the 

time for primary disinfection (i.e., CT). Because cartridge filters are designed to be disposable 

and are generally not backwashed, pretreatment chemicals such as lime or coagulants could 

rapidly foul the cartridges and thus are not applied upstream these systems.

With any form of chemical pretreatment, it is very important to understand whether any 

chemical under consideration for use is compatible with the membrane material. In addition to 

irreversible fouling and/or physical damage to the membranes, the use of an incompatible 

chemical may void the manufacturer’s warranty. Some chemicals such as oxidants can be 

quenched upstream, while others such as coagulants and lime cannot be counteracted prior to 

membrane exposure. In general, most NF/RO membranes and some MF/UF membranes are not

compatible with disinfectants and other oxidants. However, some MF/UF membranes that are 

incompatible with stronger oxidants such as chlorine may have a greater tolerance for weaker 

disinfectants such as chloramine, which may allow for a measure of biofouling control without 

damaging the membranes. Certain types of both MF/UF and NF/RO membranes require 

operation within a certain pH range. Coagulants and lime are incompatible with many NF/RO 

membranes but are typically compatible with most types of MF/UF membranes. Polymers are 

incompatible with NF/RO membranes, and generally not compatible with MF/UF membranes, as 

well, although this depends to some degree on the charge of the polymer relative to the charge 

associated with the membrane. A polymer with a charge opposite to that of the membrane is 

likely to cause rapid and potentially irreversible fouling. Chemical compatibility with various 
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types of membrane materials is briefly discussed in section 2.3.1; however, it is critical to consult 

with the membrane manufacturer prior to implementing any form of chemical pretreatment.

7.2.2 Backwashing

 The backwash process for membrane filtration systems is similar in principle to that for 

conventional media filters and is designed to remove contaminants accumulated on the 

membrane surface. Each membrane unit is backwashed separately and in a staggered pattern so 

as to minimize the number of units in simultaneous backwash at any given time. During a 

backwash cycle, the direction of flow is reversed for a period ranging from about 30 seconds to 3 

minutes. The force and direction of the flow dislodge the contaminants at the membrane surface 

and wash accumulated solids out through the discharge line. Membrane filtration systems are 

generally backwashed more frequently than conventional media filters, with intervals of 

approximately 15 to 60 minutes between backwash events. Typically the membrane backwash 

process reduces system productivity in the range of 5 to 10 percent due to the volume of filtrate 

used during the backwash operation.

Of the various types of membrane filtration systems, backwashing is almost exclusively 

associated with hollow-fiber MF and UF processes. Some MCF systems may also be 

backwashed, depending on how the system is designed and configured; however it is more 

common to replace the cartridge filters, which are manufactured to be disposable, when they 

become fouled. 

Backwashing is conducted periodically according to manufacturer specifications and sitespecific considerations. Although more frequent backwashing maintains higher fluxes, this 

benefit is counterbalanced by the decrease in system productivity. In general, a backwash cycle 

is triggered when a performance-based benchmark is exceeded, such as a threshold for operating 

time, volumetric throughput, increase in transmembrane pressure (TMP), and/or flux decline. 

Ideally, the backwash process restores the TMP to its baseline (i.e., clean) level; however, most 

membranes exhibit a gradual increase in the TMP that is observed after each backwash, 

indicating that the accumulation of foulants that cannot be removed by the backwash process 

alone. These foulants are addressed through chemical cleaning (see section 7.2.3).

Some systems also utilize pressurized air and/or chlorine (if a compatible membrane 

material is used) in combination with filtered water to remove solids, provide a measure of 

pathogen inactivation and biofouling control, and improve overall backwash effectiveness. A 

disinfectant such as chlorine may be added at a frequency ranging from every backwash to about 

once per day. If chloramines are added upstream of the membrane process, the benefits of 

adding chlorine during the backwash process should weighed against the cost of breakpoint 

chlorination to achieve a free chlorine residual and possible subsequent ammonia addition to 

regenerate a chloramine residual for secondary disinfection in the distribution system.

Some manufacturers have implemented backwash strategies for their proprietary MF/UF 

systems that involve the use of chemicals other than chlorine (such as acids, bases, surfactants, or 

other proprietary chemicals). These strategies are used as a method to enhance membrane flux 
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and extend intervals between chemical cleanings, thus lowering the cost of operation. In these 

cases state regulators may require enhanced cross-connection control measures for the backwash 

piping similar to those used for chemical cleaning systems (see section 7.3.4), depending on the 

backwash frequency and the type(s) and concentration(s) of chemical(s) used. Special 

provisions for rinsing the membranes at the end of the backwash process may also be required. 

Because the design of spiral-wound membranes generally does not permit reverse flow, 

NF and RO membrane systems are not backwashed. For these systems, membrane fouling is 

controlled primarily with chemical cleaning, as well as through flux control and crossflow 

velocity. The inability of spiral-wound membranes to be backwashed is one reason that NF and 

RO membranes are seldom applied to directly treat water with high turbidity and/or suspended 

solids. 

7.2.3 Chemical Cleaning

Chemical cleaning is another means of controlling membrane fouling, particularly those 

foulants such as inorganic scaling and some forms of organic and biofouling that are not 

removed via the backwash process. As with backwashing, chemical cleaning is conducted for 

each membrane unit separately and is typically staggered to minimize the number of units 

undergoing cleaning at any time. While chemical cleaning is conducted on both MF/UF and 

NF/RO systems, because non-porous, semi-permeable membranes cannot be backwashed, 

chemical cleaning represents the primary means of removing foulants in NF/RO systems. 

Although cleaning intervals may vary widely on a system-by-system basis, the gradual 

accumulation of foulants makes eventual chemical cleaning virtually inevitable. Membrane 

cartridge filters are an exception, however, in that cartridge filters are usually designed to be 

disposable and thus are typically not subject to chemical cleaning.

As with backwashing, the goal of chemical cleaning is to restore the TMP of the system 

to its baseline (i.e., clean) level. Any foulant that is removed by either the backwash or chemical 

cleaning process is known as reversible fouling. Over time, membrane processes will also 

typically experience some degree of irreversible fouling which cannot be removed through either 

chemical cleaning or backwashing. Irreversible fouling occurs in all membrane systems, albeit 

over a wide range of rates, and eventually necessitates membrane replacement.

There are a variety of different chemicals that may be used for membrane cleaning, and 

each is generally targeted to remove a specific form of fouling. For example, citric acid is 

commonly used to dissolve inorganic scaling, and other acids may be used for this purpose as 

well. Strong bases such as caustic are typically employed to dissolve organic material. 

Detergents and surfactants may also be used to remove organic and particulate foulants, 

particularly those that are difficult to dissolve. Chemical cleaning may also utilize concentrated 

disinfectants such as a strong chlorine solution to control biofouling. Due to the variety of 

foulants that are present in many source waters, it is often necessary to use a combination of 

different chemicals in series to address multiple types of fouling. The various types of chemical 

cleaning agents used are summarized in Table 7.2.
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Table 7.2 Chemical Cleaning Agents

Category Chemicals Commonly Used Typical Target 

Contaminant(s)

Acid • Citric Acid

• Hydrochloric Acid (HCl) Inorganic scale

Base • Caustic (NaOH) Organics

Oxidants / 

Disinfectants

• Sodium Hypochlorite (NaOCl)

• Chlorine (Cl2) Gas

• Hydrogen Peroxide

Organics;

Biofilms

Surfactants • Various Organics;

Inert particles

Numerous proprietary cleaning chemicals are also available, and these specialty cleaning 

agents may be useful in cases in which more conventional chemicals are ineffective. For 

example, under some circumstances enzymatic cleaners have been found to be effective at 

dissolving organic contaminants. Chemical cleaning options are more limited for membranes 

that cannot tolerate oxidants and/or extreme pH levels. A chemical cleaning regimen may be 

specified by the manufacturer or identified based on site-specific pilot testing and source water 

quality analyses to determine the prevalent form(s) of fouling experienced at a particular facility. 

The term clean-in-place (CIP) is often used to describe the chemical cleaning process, 

since it is typically conducted while the membrane modules remain in the membrane unit (i.e., 

in-situ). The cleaning process generally involves recirculating a cleaning solution through the 

membrane system at high velocities (to generate scouring action) and elevated temperature (to 

enhance the solubility of the foulants). A soak cycle follows the recirculation phase. After the 

soak cycle is completed, the membrane system is flushed to remove residual traces of the 

cleaning solution(s). The process may be repeated using a different cleaning solution to target 

different types of foulants until the membranes have been successfully cleaned. Under some 

circumstances the use of softened or demineralized water may be required for the cleaning 

solution or as rinse water. 

While backwashing may be conducted at more regular intervals or on a routine basis, 

chemical cleaning is typically conducted only when necessary. A chemical cleaning is generally 

necessary for MF and UF systems when the ability of periodic backwashing to restore system 

productivity (i.e., increase flux and reduce TMP) reaches a point of diminishing returns. For NF 

and RO systems, a 10 to 15 percent decline in temperature- and pressure-normalized flux or 

about a 50 percent increase in differential pressure may indicate the need for chemical cleaning. 

Delaying necessary chemical cleaning can accelerate irreversible fouling, reduce production 

capacity, and shorten membrane life. 
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Isolating the cleaning chemicals from the treated (i.e., filtered) water is an important 

consideration for membrane filtration systems. Guidelines for cross-connection control are 

discussed in section 7.3.4. In addition, it is important to properly flush the membrane unit after 

the cleaning process and before restarting the filtration cycle. The flushed water should be 

diverted to waste until filtrate water quality parameters (such as turbidity for MF/UF systems and 

pH for NF/RO systems) return to normal production mode levels. Note that the volume of 

flushed water can be significant in cases in which surfactants are used.

For MF/UF systems, it is common to recycle as much as 90 percent of the cleaning 

chemicals for reuse, thus reducing the volume of chemical waste as well as the cost associated 

with cleaning. Recycling cleaning solutions is less common with NF/RO systems, since the used 

cleaning solutions accumulate dissolved constituents with repeated use, diminishing the 

effectiveness of the cleaning agents. The treatment and disposal of spent chemical cleaning 

waste is discussed in section 7.4.2.

7.2.4 Integrity Testing

Integrity testing is a means of determining whether or not a membrane system is 

“integral,” or free of any breaches, leaks, or defects that might allow unfiltered water to bypass 

the membrane barrier, passing contaminants that are normally removed. As with both 

backwashing and chemical cleaning, integrity testing is conducted on each membrane unit (or 

smaller division in some cases) separately and is typically staggered to minimize the number of 

units simultaneously undergoing testing. The use of periodic or continuous integrity testing and 

monitoring methods allows ongoing operational verification that the membranes are performing 

as expected based on their established exclusion characteristics. This verification is an essential 

component of any membrane filtration system, particularly when the constituents of concern are 

pathogenic microorganisms. Integrity testing and is described in detail in Chapters 4 and 5 in the 

context of applying membrane filtration for compliance with the LT2ESWTR. The discussion in 

this section is intended to be a general overview of integrity testing as an important operational 

unit process for membrane filtration systems.

There are a number of potential modes of failure associated with membrane filtration 

systems that would result in an integrity breach. For example, membranes may become damaged 

via exposure to oxidants, pH levels outside the recommended range, or other chemicals or 

operating conditions to which the membranes are sensitive. In addition, membranes may break 

or puncture as a result of extreme pressure, scratches or abrasions, or operational fatigue over 

time. Spiral-wound membranes can be damaged at glue lines if the pressure on the filtrate side 

of the membrane exceeds that on the feed side. Factory imperfections such as glue line gaps or 

potting defects may cause integrity breaches, as well. Improper installation of membrane 

modules can also create integrity problems at o-rings or interconnections. 

The various types of integrity tests are generally divided into two categories: direct and 

indirect test methods. Direct methods are physical tests that are applied specifically to the 

membrane unit to detect integrity breaches and/or determine their sources. Indirect methods are 
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surrogate measures of integrity that involve filtrate water quality monitoring such that a 

significant decline in filtrate quality may indicate an integrity problem. Both direct integrity 

testing and continuous indirect integrity monitoring are required for compliance with 

LT2ESWTR. 

Direct test methods are non-destructive techniques that can be used to identify and/or 

isolate leaks. While these methods yield direct information about membrane integrity, they 

cannot be conducted continuously while the membrane filtration system is in operation. Thus, 

the longer and more often that a direct test is conducted, the greater the impact on the overall 

system productivity. The minimum frequency requirements for direct testing vary among 

different states, although daily testing is commonly recommended as good practice and is 

required for those utilities using membrane filtration for LT2ESWTR compliance. 

Although there are a number of types of direct integrity tests, the most common method 

is the pressure decay test, which measures the rate of pressure loss across the membrane relative 

to a maximum acceptable threshold. Almost all currently available proprietary MF and UF 

systems are designed with the capability to conduct this test automatically at regular intervals. 

MCF systems may also be equipped to conduc t automatic pressure decay tests. A similar test 

using a vacuum can also be conducted on NF and RO membranes, although currently spiralwound systems are generally not designed to conduct this testing automatically during the course 

of normal operation. Other on-line direct test methods measure the flow of air or water from 

integrity breaches, or the concentration of either spiked inert particles or a molecular marker. 

Additional information regarding direct integrity testing, including common methods and their 

application for facilitating compliance with the LT2ESWTR, is provided in Chapter 4.

The various indirect methods consist primarily of water quality monitoring practices that 

are common throughout the water treatment industry, such as turbidity monitoring and particle 

counting. Particulate-based indirect monitoring techniques are applicable to all membrane 

classes used for filtration. For NF/RO membranes capable of removing dissolved contaminants, 

other parameters such as conductivity or sulfide may also potentially be used as surrogate 

measures of membrane integrity. The effectiveness of indirect methods is a function of the 

ability of membrane filtration to produce very consistent, high quality filtrate, such that a marked 

decline in filtrate quality is likely to indicate an integrity problem. Although indirect methods 

are not as sensitive as the direct methods for detecting integrity breaches, the primary advantage 

of indirect methods is that they allow continuous monitoring, a capability not possible with 

currently available direct testing methods. Typically, if an indirect method indicates a potential 

integrity problem, a direct test will be conducted to determine more conclusively whether or not 

a breach has occurred. Indirect monitoring methods are further addressed in Chapter 5, including 

advantages and disadvantages of the various techniques, as well as requirements for utilizing 

these tests in the course of applying membrane filtration for LT2ESWTR compliance.

Guidance for the development of comprehensive integrity verification program, including 

the use of both direct and indirect methods, is included in Appendix A.
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7.2.5 Post-Treatment

Post-treatment for membrane filtration systems typically consists of chemical 

conditioning and/or disinfection and is typically applied to combined filtrate. Most chemical 

conditioning is associated with NF and RO systems, because the removal of dissolved 

constituents that is achieved by these processes has a more significant impact on water chemistry

than the filtering of suspended solids alone. For example, because NF and RO pretreatment 

often includes acid addition to lower the pH and, consequently, increase the solubility of 

potential inorganic foulants, a portion of the carbonate and bicarbonate alkalinity in the water is 

converted to aqueous carbon dioxide, which is not rejected by the membranes. The resulting 

filtrate can thus be corrosive given the combination of a low pH, elevated carbon dioxide levels, 

and minimal buffering capacity of the filtrate. Other dissolved gases such as hydrogen sulfide, 

will also readily pass through the semi-permeable membranes, further augmenting the corrosivity 

of the filtrate and potentially causing turbidity and taste and odor problems. As a result, the 

primary goal of chemical conditioning is the stabilization of NF/RO filtrate with respect to pH, 

buffering capacity, and dissolved gases.

Degasification is commonly achieved via packed tower aeration (i.e., air stripping). Air 

stripping also increases dissolved oxygen levels, which may be very low in the case of an 

anaerobic groundwater source. The pH of the water may subsequently be readjusted to typical 

finished water levels (i.e., approximately 6.5 - 8.5) by adding a base such as lime or caustic. 

Alkalinity (e.g., in the form of sodium bicarbonate) may also be added, if necessary, to increase 

the buffering capacity of the water. Alternatively, if the pH is raised prior to degasification (thus 

converting the dissolved carbon dioxide to bicarbonate), much of the alkalinity may be 

recovered. However, this post-treatment strategy also converts any dissolved hydrogen sulfide 

gas into dissociated sulfide, which may readily react with other dissolved species to produce 

sparingly soluble sulfide compounds that may precipitate. Because MF, UF, and MCF systems 

do not directly affect the pH or remove alkalinity, these processes do not generally require 

chemical conditioning to stabilize the filtrate. 

While the use of membrane filtration does not specifically necessitate disinfection posttreatment as a result of process considerations, the need for post-disinfection is generally 

required by regulation for primary and/or secondary disinfection. However, in some states the 

use of membrane filtration may reduce primary disinfection (i.e., CT) requirements, thus helping 

to control DBP formation. Because membrane filtration is often the last major process in the 

treatment scheme, it is common to apply a disinfectant to the filtrate prior to entry into a 

clearwell and/or the distribution system. This application is particularly important if 

disinfectants were either neutralized or not added at all prior to the membrane filtration process 

to avoid damaging oxidant-intolerant membranes. 

For NF and RO membrane processes, if the disinfectant is applied prior to filtrate pH 

adjustment, post-disinfection may have the additional benefit of oxidizing sulfide to sulfate, thus 

reducing the potential for both sulfide precipitation and taste and odor concerns. Corrosion 

inhibitors may also be added prior to distribution, particularly for NF and RO systems that 

produce more corrosive water.
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7.3 System Design Considerations

In the process of planning and implementing a membrane filtration system, there are 

several important issues that can have a particularly significant impact on system design and 

operation, thus warranting special consideration. These issues include membrane flux, water 

quality, temperature compensation, cross-connection control, and system reliability. Each of 

these subjects is discussed briefly in the following subsections. 

7.3.1 Membrane Flux

The flux – the flow per unit of membrane area, as defined in section 2.4 – is one of the 

most fundamental considerations in the design of a membrane filtration system, since this 

parameter dictates the amount of membrane area necessary to achieve the desired system 

capacity and thus the number of membrane modules required. Because the membrane modules 

represent a substantial component of the capital cost of a membrane filtration system, 

considerable attention is given to maximizing the membrane flux without inducing excessive 

reversible fouling, thereby minimizing the number of modules required.

Typically, the maximum flux associated with a particular membrane filtration system is 

determined during pilot testing, mandated by the state, or established via a combination of these 

two in cases in which the state specifies a maximum operating flux based on the pilot results. 

Independent of maximum flux, pilot testing is also commonly used to determine a reasonable 

operating range that balances flux with backwash and chemical cleaning frequencies. Because 

higher fluxes accelerate fouling, backwashing and chemical cleaning must usually be conducted 

more frequently at higher fluxes. The process of using pilot testing to optimize the flux relative 

to the backwash and chemical cleaning frequencies is described in section 6.3. The upper bound 

of the range of acceptable operating fluxes (provided this bound does not exceed the statemandate maximum) is sometimes called the “critical” flux, or the point at which a small increase 

in flux results in a significant decrease in the run time between chemical cleanings. A membrane 

filtration system should operate below this critical flux to avoid excessive downtime for cleaning 

and the consequent wear on the membranes over time due to increased chemical exposure.

The flux through a membrane is influenced by a number of factors, including pore size 

(for MF, UF, and MCF membranes), module type (i.e., cartridge, hollow-fiber, spiral-wound, 

etc.), membrane material, and water quality. However, it is important to note that higher fluxes 

do not necessarily indicate that one membrane is better than another for a particular application. 

Factors such as estimated membrane life, fouling potential, frequency and effectiveness of 

chemical cleaning, chemical use, and energy requirements to maintain a given flux should also 

be considered.

7.3.2 Water Quality

Because water quality can have a significant impact on membrane flux, feed water 

quality is also a primary design consideration for membrane filtration systems. Poorer water 
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quality will require lower fluxes, which in turn increase the necessary membrane area and 

required number of modules, augmenting both the cost and the size of the system. However, 

pretreatment (as described in section 7.2.1) can often improve feed water quality at a lower cost 

than additional membrane area. Conversely, better water quality will allow higher fluxes, 

reducing the required membrane area, the size of the system, and the capital cost. Typically, 

membrane flux is determined through pilot testing, as described in Chapter 6. In the absence of 

pilot test data, it is important to have some understanding of how critical water quality 

parameters such as SDI, turbidity, organic carbon, and dissolved solids affect the flux. The 

influence of each of these parameters on flux is briefly described in this section. Temperature 

also has a significant impact on membrane flux, and this relationship is discussed separately in 

section 7.3.3.

Silt Density Index

The silt density index, or SDI, is an empirical dimensionless measure of particulate 

matter in water and is generally useful as a rough gauge of the suitability of a source water for 

efficient treatment using NF/RO processes. The American Society for Testing and Materials 

(ASTM) Standard D4189-95: Standard Test Method for Silt Density Index (SDI) of Water

details the procedure for determining SDI. In general, SDI measurements are taken by filtering a 

water sample through a 0.45-μm flat sheet filter with a 47-mm diameter at a pressure of 30 psi. 

The time required to collect two separate 500 mL volumes of filtrate is measured, and the 

resulting data become the inputs to a formula used to calculate SDI. Water samples that contain 

greater quantities of particulate matter require longer to filter and thus have higher SDI values. 

As a general rule of thumb, spiral-wound NF and RO modules are not effective for 

treating water with a SDI of 5 or greater, as this quality of water contains too much particulate 

matter for the non-porous, semi-permeable membranes, which would foul at an unacceptably 

high rate. Thus, some form of pretreatment to remove particulate matter is generally required for 

SDI values exceeding five in NF/RO applications. NF/RO membrane module manufacturers can 

usually provide a rough estimate of the range of anticipated operating fluxes based on the type of 

source water, which is roughly associated with a corresponding range of SDI values. A 

summary of these estimates is included in Table 7.3.

Table 7.3 Estimated NF/RO Membrane Fluxes as a Function of SDI

Source SDI

(dimensionless)

Estimated NF/RO Flux

(gfd)

Surface Water 2 - 4 8 - 14

Ground Water < 2 14 - 18
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However, SDI is only one measure of water quality, and there are a number of site- and 

system-specific water quality and operational factors that combine to dictate the flux for a given 

system. Thus, the ranges cited in Table 7.3 should only be used as a rough guideline. Caution 

should also be exercised when interpreting SDI results, as measurements can vary from test to 

test and with the analyst, as well as with both temperature and the specific type of membrane 

used. Consequently, it is important that the results are given for comparable conditions when 

evaluating SDI data. Note that because SDI is a batch process, it is not conducted continuously 

on-line and thus is not typically utilized as gauge of water quality or system performance during 

daily operation in the way turbidity or conductivity monitoring are often employed.

SDI is typically not used as a tool for estimating flux for MF, UF, and MCF systems. 

Since these systems are designed to filter particulate matter, parameters such turbidity that more 

commonly associated with conventional drinking water filtration are used to assess system 

performance.

Turbidity

Turbidity is a measure of the scatter of incident light caused by particulate matter in 

water. Because turbidity is widely used as a performance gauge for conventional media filters, 

among the various types of membrane filtration systems turbidity is most often used as an 

assessment tool for MF, UF, and MCF, since these systems are specifically designed to remove 

particulate matter. Higher turbidity measurements are indicative of greater quantities of 

suspended solids, and thus the potential to cause more rapid membrane fouling. Therefore, water 

with higher turbidity is usually filtered at lower fluxes to minimize fouling and the consequent 

backwash and chemical cleaning frequency. In some cases when turbidity levels are extremely 

elevated, it may be more economical to provide pretreatment for a MF/UF system to reduce the 

solids loading to the membranes. In general, if the turbidity of the water normally exceeds 

10 NTU on a sustained basis, some type of pretreatment (e.g., prefiltration or pre-settling) should 

be considered. MCF systems may be applied to untreated source waters; however, as turbidity 

levels increase, disposable MCF cartridges will foul more rapidly, requiring more frequent 

replacement. In some applications with higher turbidity source water, coarse bag or cartridge 

filters may be used as pretreatment for MCF systems. 

Because spiral-wound NF and RO membrane modules are not designed to handle 

significant solids loading, these systems are typically not applied to treat water with turbidity 

levels exceeding approximately 1 NTU. For water turbidity levels greater than 1 NTU, 

pretreatment would be necessary to reduce solids loading upstream of NF or RO.

Organic Carbon

Another water quality constituent that influences membrane flux is the organic carbon 

content, which is typically expressed in terms of either total or dissolved organic carbon (DOC). 

Organic carbon in the feed water can contribute to membrane fouling, either by adsorption of the 

dissolved fraction onto the membrane material or obstruction by the particulate fraction. Thus, 
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lower fluxes may be necessary if membrane filtration is applied to treat a water with significant 

organic carbon content. The tendency for a membrane to be affected by TOC is partially 

influenced by the nature of the organic matter in the water. TOC can be characterized as either 

hydrophilic or hydrophobic in composition, and studies suggest that the hydrophobic fraction 

contributes more significantly to membrane fouling. The character of the organic carbon content 

can be roughly quantified by measuring the specific ultraviolet absorbance (SUVA) of the water, 

as calculated using the following equation:

DOC

UV

SUVA 254= Equation 7.1

Where: SUVA = Specific UV absorbance (L/mg-m)

UV254 = UV absorbance at 254 nm (1/m)

DOC = Dissolved organic carbon (mg/L)

Because TOC is more commonly measured than DOC in drinking water treatment, SUVA is 

sometimes estimating using values for TOC in place of those for DOC.

Higher SUVA values tend to indicate a greater fraction of hydrophobic organic material, 

thus suggesting a greater potential for membrane fouling. Generally SUVA values exceeding 

4 L/mg-m are considered somewhat more difficult to treat. However, organic carbon (as well as 

turbidity) can often be removed effectively via coagulation and pre-settling, particularly if more 

hydrophobic in character, thus minimizing the potential for membrane fouling and facilitating 

operation at higher fluxes. Coagulation can also be conducted in-line (i.e., without pre-settling) 

with MF/UF systems. Pretreatment using the injection of powdered activated carbon (PAC) may 

also reduce DOC in the membrane feed; however, because spiral-wound membrane modules 

cannot be backwashed, PAC should not be used in conjunction with NF/RO systems unless 

provisions are made to remove the particles upstream.

Dissolved Solids

The total dissolved solids (TDS) and the particular species of dissolved solids present in

the membrane feed are both critical considerations for NF/RO systems. Species such as silica, 

calcium, barium, and strontium, which can precipitate as sparingly soluble salts, can cause 

scaling and a consequent rapid decline in flux under certain conditions. Scaling is typically 

controlled using pretreatment chemicals such as an acid to lower the pH and/or a proprietary 

scale inhibitor, as discussed in section 7.2.1.2. However, the total quantity of dissolved solids of 

any species also influences system operation, as the net driving pressure required to achieve a 

target flux is related to the osmotic pressure of the system, which is directly proportional to the 

TDS (as discussed in section 2.2.2). Thus, as the TDS increases, so does the required feed 

pressure. 
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TDS is generally not a significant consideration for MF, UF, and MCF systems, since 

these processes do not remove dissolved solids. In some cases, however, the use of upstream 

oxidants may cause the precipitation of iron or manganese salts (either unintentionally or by 

design as a pretreatment process), which could accelerate membrane fouling.

7.3.3 Temperature Compensation

Like other water quality parameters such as turbidity and TDS (for NF/RO systems), the 

temperature of the feed water also affects the flux of a membrane filtration system. At lower 

temperatures water becomes increasingly viscous; thus, lower temperatures reduce the flux 

across the membrane at constant TMP or require an increase in pressure to maintain constant 

flux. The means of compensating for this phenomenon varies with the type of membrane 

filtration system used. General viscosity-based means of compensating for temperature 

fluctuations for both MF/UF and NF/RO systems are described below, although membrane 

manufacturers may have a preferred product-specific approach.

MF/UF membrane systems usually operate within a relatively narrow range of TMPs, 

which may limit increasing the TMP in order to maintain constant flux as the water temperature 

decreases. Because the membrane modules can be damaged if the TMP exceeds an upper limit, 

as specified by the manufacturer, it may not be possible to operate the system at a TMP that is 

sufficient to meet the required treated water production during colder months if demand remains 

high. As a result, additional treatment capacity (i.e., membrane area or number of membrane 

modules) is incorporated into the design of the system such that the water treatment production 

requirements can be satisfied throughout the year. 

For the microporous MF/UF membranes, the relationship between flux, TMP, and water 

viscosity is given by the following equation, with a cross-reference to the same equation as listed 

in Chapter 2 given in parentheses:

Rt w

TMP J

• m

= Equation 7.2 (2.7)

Where: J = flux (gfd) 

TMP = transmembrane pressure (psi)

Rt = total membrane resistance (psi/gfd-cp)

μw = viscosity of water (cp)

If the system is operated at constant flux, then increases in viscosity require proportional 

increases in operating TMP (assuming constant membrane resistance). However, once the TMP 

approaches the rated maximum for the membranes, further increases in viscosity necessitate a 

reduction in flux. Thus, in order to maintain the required filtered water production flow (so as to 

satisfy customer demand), the membrane area must increase in proportion to the flux decrease, as 

shown in Equation 7.3:
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m

p

A

Q

J = Equation 7.3 (2.1)

Where: J = flux (gfd)

Qp = filtrate flow (gpd)

Am = membrane surface area (ft2)

Combining Equations 7.2 and 7.3 demonstrates that the additional membrane area 

required is directly proportional to the increase in water viscosity (for constant flow, TMP, and 

membrane resistance), as shown in Equation 7.4:

m t w

p

R

TMP

A

Q

• m

= Equation 7.4 

Where: Qp = filtrate flow (gpd)

Am = membrane surface area (ft2)

TMP = transmembrane pressure (psi)

Rt = total membrane resistance (psi/gfd-cp)

μw = viscosity of water (cp)

Membrane filtration systems are commonly designed to operate at a particular flux (e.g., 

as determined via pilot testing or mandated by the state) to produce a specific flow (i.e., rated 

system capacity) at a reference temperature of 20 oC. Thus, the required membrane area at 20 oC 

can be calculated using Equation 7.3. The increased membrane area required to compensate for 

cold weather flow can be determined by multiplying this area by the ratio of the viscosity at the 

coldest anticipated temperature (e.g., the coldest average monthly temperature) to that at the 

reference temperature of 20 oC. Values for water viscosity can be found in the literature or 

approximated using Equation 7.5:

( ) ( ) ( )

2 5 3

T =1.784 − 0.0575 •T + 0.0011• T − 10 • T

−

m Equation 7.5 (2.8)

Where: μT = viscosity of water at temperature T (cp)

T = water temperature (oC)

After the appropriate values for water viscosity have been determined for both the 

reference temperature (commonly 20 oC for MF/UF systems) and the coldest anticipated 

temperature, then the design membrane area, as compensated for seasonal temperature variation, 

can be calculated as shown in Equation 7.6:
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















= •

20

20

m

mT Ad A Equation 7.6

Where: Ad = design membrane area (as adjusted for temperature) (ft2)

A20 = membrane area required at 20 oC reference temperature (ft2)

μT = viscosity of water at temperature T (cp)

μ20 = viscosity of water at temperature T (cp)

Equation 7.4 can also be utilized to calculate the required membrane area using a less 

conservative approach that accounts for seasonal fluctuations in demand. First, if the 

information is available, the average daily flow over and temperature over each calendar month 

can be tabulated, and the temperature data converted into associated values for water viscosity 

using Equation 7.5. Then, the 12 sets of paired flow (Qp) and viscosity (μ) data can be applied to 

Equation 7.4 to generate 12 values of membrane area. The largest of these values for the 

required membrane area over each of the 12 calendar months, along with its corresponding flow, 

is applied to Equation 7.3 to generate an associated flux. If this flux is less than the maximum 

permitted value, then this largest of the 12 calculated values for membrane area represents the 

design value for the system. However, the resulting flux exceeds this threshold, the design area 

must be increased to lower the flux to the maximum permitted value. 

Note that that use of Equation 7.4 in this approach requires values for both the TMP and 

the total membrane resistance, Rt, both of which should be considered constants for the purposes 

of calculate the various values for membrane area. While a reasonable TMP can be easily 

identified, appropriate values of Rt are more difficult to determine. The total membrane 

resistance represents the sum of the intrinsic resistance of the membrane (which may be 

considered a constant and can generally be obtained from the manufacturer) and the resistance 

attributable to fouling at any given point during operation, as shown in Equation 7.7:

Rt = Rm + Rf Equation 7.7 (2.6)

Where: Rt = total membrane resistance (psi/gfd-cp)

 Rm = intrinsic membrane resistance (psi/gfd-cp)

 Rf = resistance of the foulant layer (psi/gfd-cp)

 Because it difficult to both identify and justify a single, specific value for the fouling 

resistance for use with this approach, the contribution attributable to fouling may be ignored for 

practical purposes. Thus, for the purposes of calculating membrane area, the total resistance 

used in Equation 7.4 may be approximated by the membrane’s intrinsic resistance. This 

approximation may be reasonable for the membrane system at the start of a filtration cycle when 

the fouling resistance is minimal. Since the minimum value for membrane resistance expected 

over a filtration cycle is used, the minimum TMP anticipated over a filtration cycle should also 

be used. This minimum TMP occurs at the beginning of a filtration cycle before gradually 
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increasing until the system must be backwashed. Use of the intrinsic membrane resistance and 

minimum TMP should result in a reasonable and not excessively conservative estimate of the 

membrane area requirements. In rare cases in which the membrane is experiencing significant 

fouling under conditions of peak demand such that the flux and/or TMP are approaching their 

maximum values, the backwash and/or chemical cleaning frequencies can be increased 

temporarily to compensate and keep fouling to a minimum. Alternatively, standby membrane 

units may be used when necessary, as described in section7.3.5. 

The MF/UF membrane manufacturer may have an alternate preferred method of 

determining the design membrane area for a particular application based on temperature, 

membrane material, or other site- or system-specific factors. It is recommended that the utility 

collaborate with the state, the membrane manufacturer, and its engineer (if applicable) to select 

the most appropriate method for determining the required area. Note that the addition of 

membrane area to compensate for low-temperature flow will also help the system to meet higher 

flow demands during warm weather without operating at an exceedingly high membrane flux. 

Temperature compensation for MCF systems, if necessary, can be determined using the 

methodology for MF/UF systems.

Because spiral-wound NF/RO membrane modules are designed to operate over a larger 

range of TMPs than MF/UF modules, in NF/RO systems the TMP is simply increased to 

maintain constant flux as the temperature of the feed water decreases. The required increase in 

TMP at the temperature of interest relative to that at a given reference temperature (typically 

25 oC for NF/RO systems) is dependent on the specific proprietary membrane used and can be 

calculated by means of a temperature correction factor (TCF), as shown in Equation 7.8:

























−

+

= •

298

1

273

1

exp

T

TCF U Equation 7.8 (2.15)

Where: TCF = temperature correction factor (dimensionless)

T = water temperature (oC)

U = membrane-specific manufacturer-supplied constant

Alternatively, many NF/RO membrane manufacturers may supply tables specifying TCF 

values over a range of temperatures for a given membrane. Once the appropriate TCF is known, 

the required TMP at temperature T of interest can be calculated by dividing the TMP at the 

reference (i.e., design) temperature (commonly 25 oC for NF/RO systems) by the TCF value, as 

shown in Equation 7.9:
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TCF

TMP

TMPT

25

= Equation 7.9

Where: TMPT = transmembrane pressure (temperature-corrected) (psi)

TMP25 = transmembrane pressure at 25 oC reference temperature (psi)

TCF = temperature correction factor (dimensionless)

Note that Equations 7.7 and 7.8 also allow the calculation of the TMP reduction resulting 

from decreased water viscosity at higher feed water temperatures during warmer months of the 

year.

7.3.4 Cross-Connection Control

In the context of membrane filtration systems, cross-connection control measures are 

implemented to prevent chemicals from the cleaning process from contaminating the feed or 

filtrate streams. States may have particular requirements for cross-connection control, although 

in general there are two strategies that are commonly used: a double block and bleed valving 

arrangement or a removable spool. These strategies are applicable to both MF/UF and NF/RO 

systems; however, because MCF systems typically utilize modules that are disposable cartridges, 

these systems are not usually subject to chemical cleaning and thus cross-connection control 

measure are generally unnecessary for these systems.

The double block and bleed valve arrangement is the most common method of crossconnection control for large membrane filtration systems. A schematic illustrating this method is 

shown in Figure 7.1. In summary, two isolation valves (V-1A and V-2A in Figure 7.1) are 

placed in the feed line to isolate it from the cleaning chemicals. These are the block and bleed 

valves, respectively. During the cleaning process, valve V-4A and V-6A are opened to bring 

cleaning solution(s) into Membrane Rack (i.e., unit) A. Although valve V-3A is kept closed 

during this operation, if it were to leak it would allow chemicals to pass into the common feed 

manifold, causing contamination. In order to prevent this potential contamination, valve V-1A is 

also closed and valve V-2A is opened. With this configuration, if valve V-3A leaks, the cleaning 

solution(s) simply flows through valve V-2A to waste. A similar block and bleed valve 

arrangement is utilized to prevent the cleaning chemicals from contaminating the filtrate 

manifold as they are recirculated, which gives rise to the term “double” block and bleed. If the 

cleaning process starts automatically, valves V-1A and V-2A (i.e., the block and bleed valves) 

must be actuated automatically. 

As an alternative to the double block and bleed valving arrangement, a removable spool 

(i.e., a short section of pipe) can be placed between valves V-1A and V-3A in lieu of utilizing 

valve V-2A. The spool is then removed during the chemical cleaning. A removal spool should 

not be used for cross-connection control if chemical cleaning is automated, since the process 

could potentially be initiated with the spool still in place.
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Figure 7.1 Double Block and Bleed Valving Arrangement
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7.3.5 System Reliability

System reliability is an important consideration in the design of a membrane filtration 

system. Design capacities are typically maximum values and may not necessarily account for 

the possibility of one or more units being taken out of service for repair or routine maintenance. 

Even standard operational unit processes such as backwashing, chemical cleaning, and integrity 

testing that are normally accounted for in accurately sizing the facility and determining system 

capacity may be problematic if it becomes necessary to conduct these processes more frequently 

than was planned. For example, most membrane filtration systems are designed with sufficient 

storage and equalization capacity to meet average demand even when a unit is taken out of 

service for chemical cleaning. However, this storage would generally not allow the system to 

operate at capacity for extended periods (e.g., longer than about 24 hours) with a unit of out 

service for repair. Reliability issues such as this may be particularly pronounced for smaller 

systems with fewer membrane units, since one out-of-service unit may significantly impact 

overall system capacity. Note that any state sizing and redundancy requirements should be 

considered in the design of the facility. 

In order to ensure system reliability, it is common for filtration systems to incorporate 

some measure of redundancy. One method of providing this redundancy is by oversizing the 

membrane units at the design flux. Oversizing the membrane units generally allows for 

operation at average flow as a minimum with one unit out of service and may allow for operation 

at or near system capacity in some cases. For example, a water treatment plant with rated 

capacity of 7 MGD may be designed with four membrane units rated at 2 MGD each rather than 

1.75 MGD. Thus, with one unit out of service and the remaining three operating at capacity, the 

system can produce as much as 6 MGD (at the design flux) consistently. Although the design in 

this example would not allow the system to operate at maximum capacity, it is likely that the 

average flow could be met or exceeded. 

A system could also be designed to operate at the maximum rated capacity with one unit 

out of service (in this example, using four units at 2.33 MGD each), although this ability may be 

cost prohibitive, particularly for smaller systems, since the percentage that each unit must be 

oversized increases as the total number of units is reduced. Note that because this method of 

providing system redundancy is based on the assumption that the rated capacities are all 

specified with respect to the same design flux, the rate of fouling would not be increased by 

operating three of the four oversized units at capacity. In addition, continuing this example of a 

treatment plant with 7 MGD of permitted capacity consisting of four 2.33-MGD membrane 

filtration units, with all four units in operation the system could operate at reduced flux and still 

produce 7 MGD in accordance with its permit. This operation at reduced flux would decrease 

the rate of fouling and lower operating costs to partially offset the increase in capital cost 

associated with the extra unit capacity.

Another method of adding system redundancy is by providing an additional membrane 

unit. Thus, using the same example of a water treatment plant permitted for a maximum rated 

capacity of 7 MGD, the membrane filtration system would consist of five 1.75-MGD units, 

rather than four. This design would always allow for operation at capacity with one unit either 

out of service or in standby mode. In this case the standby unit can be rotated such that any time 
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a unit is taken offline for chemical cleaning the standby unit is activated. The newly cleaned unit 

then reverts to standby mode until the next unit is due for chemical cleaning. This method of 

adding redundancy is more cost effective for larger facilities with a greater number of membrane 

units, such that the addition of an extra unit does not represent a large percent increase in capital 

cost. This approach is recommended when the treatment facility utilizing membrane filtration is 

the primary source of drinking water for the public water system, such that operating the 

membrane system at capacity may be critical to the ability of the utility to satisfy customer 

demand. Alternatively, under this scenario all the units may be operated at lower flux to extend 

the interval between chemical cleanings.

Independent of the particular strategy for providing some redundant capacity for the 

membrane filtration system, utilities should also provide some redundancy for any major 

ancillary mechanical equipment that may be utilized, such as pumps, compressors, and blowers. 

Utilities must comply with any state-mandated redundancy requirements for either membrane 

capacity or ancillary equipment.

Membrane filtration systems may also have some inherent redundancy if the system is 

normally operated significantly below the maximum flux permitted by the state. In this case, if 

one membrane unit is taken out of the service, the utility can increase the temperature- and 

pressure-normalized flux through the remaining units to partially or fully compensate for the loss 

of production attributable to the out-of-service unit. However, if a unit is out of service for a 

prolonged period, the increase in flux may accelerate fouling in the remaining in-service units to 

an unacceptable rate.

Timely membrane replacement is another consideration for maintaining system 

performance and reliability. Membrane replacement is usually conducted on an as-needed basis, 

typically either in cases in which the membranes have been damaged or the flux has declined to 

an unacceptable level as a result of irreversible fouling. Although the useful life of membranes 

is commonly cited in the range of 5 to 10 years (a period generally consistent with manufacturer 

warranties), the use of membrane technology – particularly MF and UF – has increased almost 

exponentially in the decade preceding this guidance manual, and thus most membrane filtration 

systems have not been in continuous operation for more than 5 to 10 years. Consequently, there 

is limited field data available to document the typical useful life of membrane filtration modules. 

Nevertheless, it is recommended that utilities keep a small number of surplus membrane modules 

on site in the event that emergency replacement becomes necessary. 

7.4 Residuals Treatment and Disposal

As with many water treatment processes, membrane filtration systems may gene rate 

several different types of residuals that must be treated and/or disposed of, including backwash 

and chemical cleaning residuals and concentrate. The types of residuals that are generated vary 

with both the type of membrane filtration system and the hydraulic configuration in which 

system in operated. For example, NF/RO systems produce a continuous concentrate stream and 

periodic chemical cleaning waste, but because these systems are not backwashed, no backwash 

residuals are generated. MF/UF systems are regularly backwashed and undergo periodic 
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chemical cleanings and thus produce residuals from both these operational unit processes. 

However, only MF/UF systems that are operated in a crossflow hydraulic configuration and 

waste (rather than recirculate or recycle to the plant influent) the unfiltered flow – the so-called 

“feed and bleed” mode – will generate a concentrate stream. Because cartridge filters operate in 

a deposition mode hydraulic configuration (see section 2.5) and are designed to be disposable, 

there are typically no residuals streams associated with MCF systems. Nevertheless, the spent 

filter cartridges (as well as all membrane filtration residuals) must be properly disposed of in 

accordance with any applicable state and local regulations, particularly if the cartridges have 

been used to filter any potentially hazardous materials.

The various potential residuals streams from membrane filtration systems – backwash 

residuals, chemical cleaning residuals, and concentrate – are discussed in the following 

subsections. Note that these discussions are not meant to represent a comprehensive review of 

residuals treatment and disposal, but rather a general overview of some of the primary 

considerations that should be taken into account when planning and designing a membrane 

filtration system. For additional information, the reader is referred to the following references:

• Water Treatment Plant Waste Management (1987)

(Prepared by Cornwell et al. of Environmental Engineering & Technology for the

American Water Works Association Research Foundation)

• Treatment of MF Residuals for Contaminant Removal Prior to Recycle (2002)

(Prepared by MacPhee et al. of Environmental Engineering & Technology for the 

American Water Works Association Research Foundation)

• Current Management of Membrane Plant Concentrate (2000)

(Prepared by Kenna et al. of Clarkson University for the American Water Works 

Association Research Foundation)

• Major Ion Toxicity in Membrane Concentrate (2000)

(Prepared by Mickley of Mickley & Associates for the American Water Works 

Association Research Foundation)

• Membrane Concentrate Disposal: Practices and Regulation (2001)

(Prepared by Mickley of Mickley & Associates for the United States Bureau of 

Reclemation)

• Membrane Concentrate Disposal (1993)

(Prepared by Mickley et al. for the American Water Works Association Research 

Foundation)

• The Desalting and Water Treatment Membrane Manual: A Guide to Membranes 

for Municipal Water Treatment (2nd Ed.) (1998)

(Prepared by the United States Bureau of Reclamation)
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• Reverse Osmosis and Nanofiltration (M46) (1999)

(Prepared the American Water Works Association)

7.4.1 Backwash Residuals

 Among the various types of membrane filtration, only MF/UF systems employ 

backwashing, thus generating backwash residuals. Although the frequency varies on a site- and 

system-specific basis, backwashing is typically conducted every 15 to 60 minutes. Under normal 

operating conditions the backwash frequency should remain relatively consistent, allowing for 

the quantity of residuals generated to be estimated fairly accurately.

As a general rule of thumb, the residual stream produced from backwashing MF/UF 

membranes has a concentration of suspended solids that is approximately 10 to 20 times greater 

than that of the feed water. Although MF/UF systems remove approximately the same types of 

feed water constituents as conventional media filters, the volume and characteristics of the 

residuals may be significantly different. In many current applications of MF/UF for municipal 

water treatment, filter aids such as coagulants and polymers are not necessary. In these cases, 

the amount of solids removed in the backwash process may be significantly less than that for a 

comparable conventional filtration plant. In addition, disposal of these coagulant- and polymerfree MF/UF backwash residuals may be less problematic. However, in some applications of 

MF/UF, coagulants may be added in-line (i.e., without pre-settling) to help facilitate the removal 

of TOC, which is not normally removed to a significant degree by MF or UF. In such 

applications the MF/UF backwash residuals characteristics will generally be similar to those for 

conventional media filtration.

Disposal options for MF/UF backwash residuals are similar to those for conventional 

water treatment plants, and typically include the following:

• Discharge to a suitable surface water body

• Discharge to the sanitary sewer 

• Treatment with supernatant recycle and solids disposal

The discharge of backwash residuals to surface water bodies or the sanitary sewer is 

likely to be subject to state and/or local regulations and, in the case of surface water discharge, to 

require a permit. Moreover, the potential to utilize one of these options may be complicated if 

the residua ls include chemical wastes. In addition to the use of coagulants added to the feed, 

some backwash procedures utilize chlorine or other chemicals, as described in section 7.2.2. 

Small amounts of chlorine may be quenched and acids or bases can be neutralized prior to 

discharge, although larger amounts or other types of chemicals added to the backwashing 

processes may require additional treatment or preclude discharge to a surface water body or 

sanitary sewer.
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On-site treatment options for MF/UF backwash residuals are also similar to those that 

might be used with conventional media filtration, and include clarification, sedimentation 

lagoons, gravity thickening, centrifuging, belt filter presses, or a combination of these processes. 

A second stage of MF or UF may also be utilized to further concentrate residuals and increase 

process recovery. If a sedimentation process is used to treat MF/UF backwash residuals, the 

addition of a coagulant may be necessary to improve the settling characteristics of the solids if 

coagulant is not already applied in the MF/UF pretreatment process. With on-site treatment, the 

supernatant is generally recycled to the treatment plant influent while the concentrated solids are 

transported off site for landfilling or other means of disposal. As with discharging, the addition 

of chlorine or other chemicals to the backwash process may complicate residuals treatment.

It is important to note that backwash residuals concentrate any pathogenic organisms that 

are present in the feed water, as well as other suspended solids. Although the control of such 

concentrated pathogens in filter backwash residuals is largely unregulated, the potential 

treatment of this stream should be taken into consideration if these residuals are to be discharged 

into a surface water receiving body.

7.4.2 Chemical Cleaning Residuals

Both MF/UF and NF/RO membranes undergo periodic chemical cleaning, and thus both 

types of systems generate spent chemical waste as a byproduct of these processes. As with the 

backwashing process for MF/UF, the frequency of chemical cleaning varies on both a site- and 

system-specific basis. Although chemical cleaning is conducted much less frequently than 

backwashing, the frequency is also more difficult to predict. Generally, MF/UF systems are 

cleaned no more frequently than once every month for efficient operation and the minimization 

of system downtime, although it is not uncommon for these systems to operate for much longer 

without requiring chemical cleaning. The cleaning frequency for NF/RO may vary anywhere 

from 3 months to 1 year, depending on the feed water quality and the effectiveness of feed water 

pretreatment for minimizing fouling. However, because chemical cleaning is a relatively 

infrequent batch process, estimating the quantity of residuals generated is not as critical for dayto-day operation, as is the case with backwashing.

Chemical cleaning residuals are generally treated on-site and discharged to either a 

suitable surface water body or sanitary sewer, subject to state and/or local regulations. Oxidants 

such as chlorine used in the chemical cleaning process can be quenched prior to discharge, and 

acids and bases can be neutralized. The use of other chemicals, such as surfactants or proprietary 

cleaning agents, may complicate the process of obtaining regulatory approval for discharge and 

could require additional treatment. 

Note that the rinse water applied to the membranes after the cleaning process also 

represents a chemical waste and must be treated prior to discharge. Although the rinse water 

increases the volume the chemical cleaning residuals, this increase can be balanced somewhat by 

the recovery and reuse of a significant portion of the cleaning solutions. In some cases, as much 

as 90 percent of the applied cleaning solutions can be reused, reducing residuals treatment and 

disposal costs, as well as chemical usage.
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7.4.3 Concentrate

The term “concentrate” is usually associated with the continuous waste stream of 

concentrated dissolved solids produced by NF and RO processes. This waste stream is typically 

4 to 10 times more concentrated than the feed with respect to suspended and dissolved 

constituents and represents about 15 to 25 percent of the total feed flow, although it can exceed 

50 percent or more in some cases. As a result, concentrate disposal is a significant logistical and 

regulatory concern for utilities and is often a critical factor in the planning and design of an NF 

or RO facility.

There are a number of methods for concentrate treatment and disposal, including the 

following options:

• Surface water discharge

• Sanitary sewer discharge

• Land application / irrigation

• Deep well injection

• Evaporation 

Because there are complicating factors associated with each of these options, no single option is 

ideal or most appropriate for every application. In many cases surface water discharge is the 

least expensive option, although the permitting process may be difficult, since there are potential 

environmental impacts if the salinity of the concentrate is significantly higher than that of the 

receiving body. Discharge to the sanitary sewer may have similar issues, since the wastewater 

treatment process does not typically affect dissolved solids concentrations, and the treatment 

plant effluent may ultimately be discharged to surface water receiving body. High salinity or 

major ion toxicity may also preclude land application of the concentrate if levels exceed 

threshold levels tolerated by the irrigated crops. Bioaccumulation of metals has also been cited 

as a potential concern for land application of NF/RO concentrate. Deep well injection is an 

effective and commonly used technique for concentrate disposal, although this method risks the 

escape of brackish water into less saline or freshwater aquifers and may have unknown long-term 

environmental affects. The use of evaporation ponds is generally limited to areas with low 

precipitation and high evaporation rates, as well as an abundance of inexpensive and available 

land. 

Another option for dealing with NF/RO concentrate is the concept of “zero liquid 

discharge,” which involves sufficiently concentrating the residual stream through the use of such 

technologies as crystalizers and evaporators to allow remaining solids to be landfilled. While 

zero liquid discharge is typically an expensive option, is does offer a number of advantages, 

including avoiding the discharge permitting process and the ability to be utilized at any location 

independent of factors such as the proximity to a suitable surface water body or available land 

for evaporation ponds. In addition, zero liquid discharge maximizes facility recovery and has 
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minimal environmental impact. Although still fairly uncommon, this option has become 

increasingly feasible relative to other methods as environmental and discharge regulations have 

become more stringent.

The considerations noted above represent only a cursory discussion of the issues 

associated with the various options for concentrate treatment and disposal. More detailed 

information is available in several of the references cited in section 7.4. Note that all of these 

options are subject to applicable federal, state, and local regulations.

Some MF/UF systems that are operated in a suspension mode and waste the unfiltered 

flow generate a continuous concentrate stream. This stream differs from that associated with 

NF/RO systems in two significant ways: MF/UF systems concentrate suspended rather than 

dissolved solids; and the concentrate stream represents only a small fraction of total feed flow. 

(Note that NF/RO membranes represent a barrier to particulate matter, and thus these systems 

will also concentrate suspended solids; however, because suspended solids rapidly foul the semipermeable membranes (which cannot be backwashed), most particulate matter is typically 

removed with pre-filters.) An MF/UF concentrate stream has characteristics similar to those of 

backwash residuals, and therefore can be considered comparable for the purposes of treatment 

and disposal. Alternatively, these two residuals streams may be blended together.
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8.0 Initial Start-Up

8.1 Introduction

The initial start-up phase is a critical step in the successful installation of a full-scale 

membrane filtration system and thus is an essential consideration in the facility planning and 

design process. This period includes such tasks as initial system flushing and disinfection, 

system diagnostic checks, membrane module installation, integrity testing new equipment, and 

operator training, all of which must be completed prior to placing the system into service. Note 

that the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) does not contain 

any requirements for the initial start-up of a membrane filtration system. Consequently, this 

chapter is included in the Membrane Filtration Guidance Manual as a useful reference for 

utilities that summarizes widely recognized industry best practices and important considerations 

for the start-up process. However, state requirements that overlap with any of the subjects 

covered under this reference on initial start-up would supersede the guidance provided in this 

chapter. 

In general, the primary objectives of the initial phase are to ensure that the installation is 

successfully completed and that the equipment is in proper working order and ready to produce 

potable water that achieves all target quality standards. A well-planned initial start-up phase can 

thus help facilitate the proper execution of the membrane filtration system start-up and create a 

smooth transition from testing to drinking water production. This chapter discusses important 

general start-up considerations, pointing out any significant differences between nanofiltration 

(NF)/reverse osmosis (RO) and microfiltration (MF)/ultrafiltration (UF) systems. For the 

purposes of this discussion, membrane cartridge filtration (MCF) systems are considered to be 

similar to MF/UF systems, except as otherwise noted.

This chapter is divided into the following sections. 

Section 8.2: Temporary System Interconnections 

This section describes the provisional requirements that may be necessary 

during start-up of a membrane filtration system.

Section 8.3: Flushing and Testing Without Membranes

This section describes the general procedures that are associated with 

flushing a membrane filtration system and confirming that each unit is 

operating properly without the membrane modules installed.

Section 8.4: Membrane Installation

This section reviews considerations associated with the installation of 

membrane modules. 

Section 8.5: System Disinfection 

This section discusses the initial disinfection process for systems using 

both chlorine-tolerant and chlorine-intolerant membranes. 
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Section 8.6: Initial Direct Integrity Testing 

This section outlines some considerations for conducting initial direct 

integrity testing on the newly installed membrane modules.

Section 8.7: Acceptance Testing 

This section describes some of the typical practices associated with 

acceptance testing.

Section 8.8: Operator Training 

This section describes the operator training that the equipment supplier 

should provide during the start-up phase. 

8.2 Temporary System Interconnections

During the start-up process, the filtered water produced from the membrane units may not 

be acceptable for distribution. Therefore, facility design should include provisions for the 

recycle and/or temporary disposal of feed and filtrate water. Provisions for disposal usually 

consist of a removable pipe spool or a tee with a “dump” valve placed in the common feed or 

filtrate line. This water can usually be diverted to the sanitary sewer. If the feed and filtrate 

piping are interconnected, the connection is removed and replaced with blind flanges after startup has been completed. Some facilities simply divert and recycle the treated water to the inlet 

structure until the system is fully commissioned. Any state requirements governing the disposal 

and potential recycle of the water produced during the start-up phase should be incorporated into 

the planning process.

 

8.3 Flushing and Testing Without Membranes

Prior to installing the membrane modules, any debris introduced during construction 

should be flushed from the system. This flushing is usually conducted by running the 

appropriate pump(s) at high velocity and low pressure through the piping and discharging the 

water to a suitable drain. Typically, the flush water can be discharged to the sanitary sewer, 

although the utility should comply with any state requirements for disposal. Because the intent 

of the flushing process is to remove debris from the system, this water should not be recycled 

unless pretreatment processes would be expected to remove contaminants flushed from the 

system.

After the piping system has been flushed, the operational sequences and chemical 

addition systems should be tested to ensure that they are operating properly before installing the 

membrane modules. Because of the complexity of the equipment involved, it is suggested that a 

plan for conducting these diagnostic checks be developed in advance. In general, the 

recommended testing can be divided into the following categories: 
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• Mechanical Equipment – Inspect automated mechanical equipment to ensure that it 

is properly installed and that there are no leaks in the piping system. Operate 

mechanical equipment in manual mode and then in automatic mode to verify that it is 

working as designed. 

• Instrumentation– Verify that the instrumentation is properly installed and 

calibrated. Confirm that the instruments are operating correctly and are responsive to 

the control system.

• Control System– Verify control inputs and outputs (both digital and analog), 

instrumentation alarm limits, programming logic, instrumentation loops, and 

operational sequences. 

• Membrane Units and Related Systems – Verify that the membrane filtration system 

and each respective membrane unit can be both started and shutdown smoothly. 

Operate the chemical feed systems (if any) to verify that they are each in proper 

working condition and tha t all chemicals are delivered at the proper dosages. 

For NF/RO systems, the procedure for testing the operating sequences is commonly 

known as the “48-hour test,” as it may take two or more days to complete. Unlike the initial 

flushing, the 48-hour test should be conducted at actual operating parameters, including both 

typical pressures and flows. Therefore, it is common to insert flow restriction devices in the 

system in place of the membrane modules during this operation to simulate the anticipated 

system backpressure. These devices typically consist of orifice plugs placed in the permeate 

(i.e., filtrate) port inside the pressure vessel. The orifice plugs are sized to simulate the flow and 

pressure parameters of the operating membrane filtration system. Alternatively, an isolation 

valve on the permeate piping could be throttled to the appropriate backpressure. Although the 

term “48-hour test” is less commonly used in association with MF and UF, these systems are 

also generally tested either with “dummy” modules that have similarly designed orifices or by 

throttling filtrate effluent valves to create sufficient backpressure during the testing sequences.

8.4 Membrane Installation

Membrane module installation should be conducted according to the instructions 

obtained from the manufacturer. Care should be taken not to damage the membranes during the 

installation process. It is recommended that the location of each individual membrane in the 

system be recorded according to its serial number. Factory test data are often shipped with the 

membranes, and this information should be collected and filed. 

Membrane modules are typically shipped “wet” with a liquid preservative solution. The 

particular preservative depends upon the type of membrane. Most membranes are preserved 

with a 1 percent solution of sodium bisulfite, a reducing agent that acts as a biocide to control 

microbial growth. Prior to installation, the membrane modules should be stored in an 

appropriate manner such that they are not subjected to freezing conditions that could damage the 
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membranes. Regardless of the membrane type, the preservative should be flushed to waste after 

the membranes are installed. Note that the disposal of any preservative solutions should be 

conducted in accordance with any applicable regulations.

Some membrane modules are shipped with more problematic storage solutions. For 

example, glycerin solutions may pose a waste treatment issue because of the biochemical oxygen 

demand (BOD) that may result from the solution being flushed down the drain. The use of 

formaldehyde, once a common membrane preservative is generally no longer acceptable, and it 

may pose a significant disposal problem. Both state and local regulations regarding chemical 

disposal may apply to membrane preservative solutions, and these may dictate whether it is 

permissible to discharge these preservatives to the sanitary sewer or if collection and alternate 

disposal and/or treatment is required.

 

8.5 System Disinfection

Initial system disinfection typically involves the application of a disinfectant, such as a 

chlorine solution, throughout the entire system, including both the feed and filtrate piping. 

Although both soaking and recirculating procedures are used, recirculating the solution through 

the system will generally provide more effective disinfection. This disinfection step may be 

required by the state, and in any case is recommended to inactivate bacteria or other pathogenic 

organisms that may contaminate the membrane filtration system and associated piping. Because 

some membranes have limited compatibility with disinfecting chemicals, the manufacturer may 

have specific requirements for the disinfection process, and approval will be necessary if 

manufacturer recommendations differ from state requirements. General procedures for the 

disinfection of both chlorine-tolerant and chlorine-intolerant membranes are described in the 

following sections. In either case, once the system disinfection is complete, the entire system 

(including the membranes) should be flushed prior placing the membrane unit(s) into continuous 

service. This final flush can be conducted according same guidelines outlined in section 8.3 and 

should continue until the desired filtered water quality is attained as me asured by site- or 

membrane system-specific water quality parameters.

8.5.1 Chlorine-Tolerant Membranes

The American Water Works Association (AWWA) and the American National Standards 

Institute (ANSI) have developed standards pertaining to the disinfection of water treatment 

facilities. Specifically, ANSI/AWWA Standards C651 and C653 provide guidance for the 

disinfection of water treatment plants and associated piping systems. The procedure involves 

surface contact with a high strength chlorine solution for a specific time period. The disinfection 

is complete when bacteriological sampling and testing indicate the absence of coliform 

organisms. For systems that use chlorine-tolerant membranes, the membrane modules should be 

installed prior to initiating the disinfection procedure. If the disinfection process is conducted 

while the membranes are in place, sufficient pressure should be applied to ensure adequate flow 

across the membranes such the filtrate side piping is thoroughly disinfected. Some MF/UF 

membranes and most MCF membrane are chlorine-tolerant.
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8.5.2 Chlorine-Intolerant Membranes

Most NF and RO membranes, as well as some MF and UF membranes, are not chlorinetolerant. For systems that use these types of membranes, there are two options that are generally 

available for system disinfection. One option consists of conducting the system disinfection with 

chlorine as described in section 8.5.1 but prior to membrane installation. Subsequently, the 

membranes are installed and flushed, and then another disinfection specific to the chlorineintolerant membrane is conducted. This second disinfection typically utilizes a high pH (i.e., 

~10 or greater) solution of caustic or other alkaline chemical that is very effective for 

bacteriological inactivation. The membrane manufacturer should be consulted to ensure that this 

second disinfection is conducted within the pH tolerance limits of the membrane. A second 

option, if permissible under state regulatory requirements, involves the elimination of chlorine or 

other oxidants and using caustic (or other alkaline chemical) to disinfect the entire system, 

including the membranes. The state should be consulted prior to performing this type of 

alternative disinfection procedure. If an alternate disinfection procedure is used, additional 

microbiological monitoring may be warranted to ensure the efficacy of the disinfection process.

8.6 Initial Direct Integrity Testing

Once the system is thoroughly flushed and disinfected, a direct integrity test should be 

conducted on each membrane unit. (Both general guidance for direct integrity testing and 

specific requirements for compliance with the LT2ESWTR are provided in Chapter 4.) If the 

direct integrity test utilized requires that the membrane be fully wetted, the system may need to 

be operated for a period of time prior to conducting the test. After the direct integrity test has 

been completed, any defective membrane modules or leaks in o-rings, pipe connections, or valve 

seals should be repaired or replaced, as necessary. Until the direct integrity test has been 

successfully completed, the filtrate should be discharged to waste or recycled, as appropriate.

For most membrane systems it is advisable to conduct relatively frequent direct integrity 

testing during the initial start-up phase, as there may be a higher incidence of integrity failure 

observed during the initial stages of facility operation. As a general rule, most membrane 

manufacturing related defects will manifest within 72 hours of continuous operation. However, 

the propensity to exhibit such defects varies significantly among different membranes. In order 

to ensure that all initial manufacturing defects are detected, it is recommended that direct 

integrity testing be conducted 2 to 6 times per day during start-up until the test results are stable.

8.7 Acceptance Testing 

The purpose of acceptance testing is to demonstrate equipment performance as a 

condition for transferring responsibility over the membrane filtration equipment from the 

manufacturer or contractor to the utility. Acceptance testing is conducted after other phases of 

the start-up and commissioning process (i.e., flushing disinfection, and integrity testing) have 

been completed. Typically, acceptance testing is the final phase of the commissioning process 

and consists of the following two criteria:
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• Operation– The entire system, including all membrane units, is continuously 

operated for a pre-determined period of time, usually from 3 to 30 days. Any 

interruptions in continuous operation as a result of faulty equipment or controls 

during this period of time may result in restarting or temporarily suspending the test. 

Design parameters (e.g., flux and backwash frequency (if applicable)) are also 

typically verified as a component of the operational criterion.

• Water Quality Testing – Periodic sampling and analysis is conducted to ensure that 

the treated water quality objectives are continuously satisfied. The required sampling 

can vary widely in terms of the number of constituents of interest and is very sitespecific. The sampling frequency for each parameter is typically once per day. If the 

system is unable to produce the required water quality, the manufacturer or contractor 

should be obligated to correct the problem before the acceptance testing is determined 

to be successfully completed. 

Utilities that apply membrane filtration for compliance with the LT2ESWTR may use the 

acceptance testing period to establish the required control limits for direct integrity testing and

continuous indirect integrity monitoring, as discussed in Chapters 4 and 5, respectively, subject 

to any particular state requirements.

8.8 Operator Training

Because operators may not be familiar with the various types of membrane filtration 

systems, operator training is an important program element to include in the initial start-up 

phase. Even if some experience was gained during pilot testing, this experience should serve as 

a supplement to training on the completed full-scale facility, rather than a substitute. This 

training can also help facilitate a smooth transition of responsibility from the equipment supplier 

to the utility. A sample operator training schedule is shown in Table 8.1.
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Table 8.1 Schedule of Training Events

Time Period Topic Duration1

(hours)

System Overview / Review of Unit Operations 1 - 4

Facility Walk Through 1 - 2 

Principles of Membrane Operation 1 - 4

Control System 4 - 24

Pretreatment Unit Operations 1 - 4

Operation / Pumps / Instrumentation 4 - 16

Post-Treatment 1 - 2

Chemical Cleaning (Including Demonstration) 4 - 16

Integrity Testing / Module Isolation and Repair 4 - 8

Monitoring / Troubleshooting / Data Normalization 2 - 4

Details of Control System / Remote Monitoring 1 - 2

During Construction 

and Commissioning

Total 24 - 86

After Commissioning Open Discussion with Membrane Manufacturer 1 - 3 days

After Three Months Open Discussion with Membrane Manufacturer 1 - 3 days

1 Higher ends of the ranges cited are associated with more complex systems and less operator familiarity
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Farahbakhsh, K., and D. Smith. 2003. Estimating air diffusion contribution to pressure decay 

during membrane integrity tests. AWWA Membrane Technology Conference 

Proceedings, March 2-5, 2003. Atlanta, GA.
References

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Glucina, K., J-M Laine, C. Anselme, M. Chamant, and P. Simonie. 1999. Acoustic sensor: a 

novel technique for low pressure membrane integrity monitoring. AWWA Membrane 

Technology Conference Proceedings, February 28 – March 3, 1999. Long Beach, CA.

International Organization for Standardization (1999) 11943 – Hydraulic fluid power – on-line 

automatic particle counting systems for liquids – methods of calibration and validation. 

Geneva, Switzerland.

Jacangelo, J., S. Adham, and J-M Laine. 1997. Membrane filtration for microbial removal. 

Report No. 90715. Denver, CO: American Water Works Association Research 

Foundation.

Johnson, W.T. 1997. Predicting log removal performance of membrane systems using in-situ 

integrity testing. AWWA Annual Conference Proceedings, June 15 – 19, 1997. 

Atlanta, GA.

Jucker, C. and M. Clark. 1994. Adsorption of aquatic humic substances on hydrophobic 

ultrafiltration membranes. J. Membrane Science. 97:253-268.

Kenna, Eric N., and Amy K. Zander. 2000. Current management of membrane plant 

concentrate. Denver, CO: American Water Works Association Research Foundation.

Landsness, L.B. 2001. Accepting MF/UF technology – making the final cut. AWWA Membrane 

Technology Conference Proceedings, March 4 – 7, 2001. San Antonio, TX.

Li, S.Y., J.A. Goodrich, J.H. Owens, G.E. Willeke, F.W. Schaefer III, and R.M. Clark. 1997. 

Reliability of non-hazardous surrogates for determining Cryptosporidium removal in bag 

filters. Journal AWWA. 89(5):90-99.

Lozier, Jim, Christian Colvin, Jae-Hong Kim, Mehmet Kitis, Benito Mariñas, and Baoxia Mi. 

2003. Microbial removal and integrity monitoring of high-pressure membranes. Denver, 

CO: American Water Works Association Research Foundation.

MacPhee, Michael J., Yann LeGouellec, and David Cornwell. 2002. Treatment of MF residuals 

for contaminant removal prior to recycle. Denver, CO: American Water Works 

Association Research Foundation.

Meltzer, Theodore H. 1997. High-purity water preparation for the semiconductor, 

pharmaceutical, and power industries. Littleton, CO: Tall Oaks Publishing, Inc.

Meltzer, Theodore H. 1987. Filtration in the pharmaceutical industry. New York, NY: Marcel

Dekker, Inc.

Mickley, Michael M. 2001. Membrane concentrate disposal: practices and regulation. Denver, 

CO: United States Bureau of Reclamation Technical Service Center, Water Treatment 

Engineering and Research.
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Mickley, Michael M. 2000. Major ion toxicity in membrane concentrate. Denver, CO: 

American Water Works Association Research Foundation.

Mickley, Michael M., Robert Hamilton, Lana Gallegos, and Jeffrey Truesdall. 1993. Membrane 

concentrate disposal. Denver, CO: American Water Works Associa tion Research 

Foundation.

National Sanitation Foundation International. 2002. ETV protocol for equipment verification 

testing for physical removal of microbiological and particulate contaminants. Ann Arbor, 

MI.

Schulze, Jack C. 2001. The Texas approach to regulating MF/UF drinking water applications. 

AWWA Membrane Technology Conference Proceedings, March 4-7, 2001. San Antonio, 

TX.

United States Bureau of Reclamation. 1998. The desalting and water treatment membrane 

manual: a guide to membranes for municipal water treatment (2nd Ed.). Technical 

Service Center, Water Treatment Engineering and Research, Denver, CO.

United States Department of Health and Human Services, Food and Drug Administration. 2002. 

Code of Federal Regulations Title 21 (Food and Drugs), Part 820 – Quality System 

Regulation (21 CFR 820).

USEPA. 2003. Toolbox guidance manual for the Long Term 2 Enhanced Surface Water 

Treatment Rule, Proposal Draft.

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implementation, and regulatory issues. EPA 815-C-01-001, April 2001.

USEPA. 1999. Guidance manual for compliance with the Interim Enhanced Surface Water 

Treatment Rule: turbidity provisions. EPA 815-R-99-010, April 1999.

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samples. EPA 600-R-93-100, August 1993.

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March 1991.

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membrane filters. Technical Paper – Memtec America Corporation.
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Appendix A

Development of a Comprehensive Integrity Verification 

Program
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Appendix A:

Development of a Comprehensive Integrity Verification Program

A.1 Introduction

The ability to maintain system integrity is one of the most important operational concerns 

associated with any membrane filtration facility, whether applied for compliance with the Long 

Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) or for any other treatment 

objective. Because a membrane represents a physical barrier to pathogens and other drinking 

water contaminants, the means to ensure that this barrier remains uncompromised is critical for 

the ongoing protection of public health. Moreover, the number and variety of integrity-related 

compliance requirements for membrane filtration under the LT2ESWTR, ranging from various 

forms of testing to repair to data collection and reporting, as specified in Chapters 4 and 5, 

illustrate the potential complexity of the process of maintaining system integrity. As a result, 

this appendix has been prepared to serve as a tool to guide utilities in the development of a 

comprehensive, effective, and efficient integrity verification program (IVP). The various 

sections of this appendix are organized into a series of introductory questions that would be 

posed in the preparation of an IVP and corresponding discussions to elaborate on how these 

questions would be addressed in the context of an IVP. 

What is a comprehensive IVP? 

A comprehensive IVP is a customized site- and system-specific program that details all 

operating procedures associated with maintaining membrane filtration system integrity, including 

both federal and state requirements and any additional practices that are voluntarily implemented 

at the discretion of the utility. In the broadest terms, an IVP should serve as a master plan for 

preserving system integrity.

What is the purpose of an IVP?

The primary purpose of an IVP is to provide a utility with rational and systematic 

blueprint for applying appropriate tools and techniques to efficiently conduct the following 

procedures:

1. Verifying integrity on an ongoing basis

2. Identifying and correcting any integrity problems

3. Recording and analyzing integrity test data

4. Preparing any required compliance reporting
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The successful execution of these procedures, in turn, allows a utility to track system 

performance and determine whether or not it is consistent with either that established by 

challenge testing or other requirements that may be applicable.

Why is an IVP important?

Because the process of maintaining system integrity has many aspects, an IVP is critical 

for organizing all of the various operating procedures relating to system integrity into a 

comprehensive plan. As an organizational tool, an IVP is also important for its ability to provide 

a framework to assist operators with conducting the proper procedures in the correct sequence 

under both normal operating conditions and when an integrity breach is either suspected or 

confirmed. As a whole, these IVP functions help to both ensure the production of safe drinking 

water and facilitate regulatory compliance.

What are the regulatory requirements associated with an IVP?

Although an IVP is not required for membrane filtration systems under the LT2ESWTR, 

the development of IVP can facilitate regulatory compliance by organizing the rule requirements 

into a comprehensive operational program. However, whether membrane filtration is applied for 

LT2ESWTR compliance or to meet any other treatment objectives, any IVP should be consistent 

with all USEPA and state requirements governing the operation of membrane treatment 

facilities. In addition, any requirements relating to maintaining system integrity should be 

incorporated into an IVP. Thus, while the development of an IVP is not required under the 

LT2ESWTR, it is strongly recommended for all utilities that utilize membrane filtration, 

particularly for disinfection applications.

What are the components of an IVP?

An IVP should incorporate all of the operational procedures associated with maintaining 

system integrity, including as a minimum the following major program elements:

• Direct integrity testing

• Continuous indirect integrity monitoring

• Diagnostic testing

• Membrane repair and replacement

• Data collection and analysis

• Reporting
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Other site-specific procedures or compliance requirements applicable to system integrity, but not 

specifically addressed under any of the above program elements, should also be included as 

components of an IVP.

How is IVP guidance presented in this appendix?

This appendix is organized into sections according to the major program elements 

described above. Each of these sections provides an overview of what the IVP should address 

with respect to that particular element and the various associated considerations that should be 

taken into account in the process of developing and implementing an IVP. Because compliance 

with the LT2ESWTR is critically related to maintaining system integrity, requirements from the 

rule are used as examples to illustrate program development, as well as how the various IVP 

components fit together into an integrated program.

The order of the sections in this appendix also illustrates the tiered approached to an IVP, 

ranging from the fundamental direct integrity test to successive levels of monitoring, testing, and 

repair that may be necessary. Note that this appendix is applicable to all types of membrane 

filtration addressed under the LT2ESWTR and consequently covered in this Membrane 

Filtration Guidance Manual, including microfiltration (MF), ultrafiltration (UF), nanofiltration 

(NF), reverse osmosis (RO), and membrane cartridge filtration (MCF). Significant technologyspecific nuances associated with one or more particular types of membrane filtration are 

addressed in context whenever practical.

A.2 Direct Integrity Testing

Direct integrity testing is the primary means of verifying system integrity and thus 

represents a fundamental component of an IVP. The series of questions into which this section is 

organized represents important aspects direct integrity testing that should be addressed in an IVP. 

In addition, the questions are presented in a logical progression designed to parallel the step-bystep process of formulating a direct integrity testing strategy.

What is the purpose of direct integrity testing?

Under the LT2ESWTR, direct integrity testing is defined as a physical test applied to a 

membrane unit in order to identify and isolate integrity breaches. Because direct integrity testing 

is the most accurate and precise means of determining whether or not a breach has occurred, it 

has historically been the primary means used to assess membrane integrity in potable water 

treatment applications in which pathogen removal is a principal concern. In addition, the test 

parameters and results can be correlated to the desired treatment objectives (e.g., log removal 

values) to yield a quantifiable assessment of system performance. In terms of LT2ESWTR 

compliance, as discussed in Chapter 4, requirements for direct integrity test resolution and 

sensitivity are specified for ensuring the necessary level of Cryptosporidium removal at a 

particular facility.
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For example, the LT2ESWTR resolution requirement specifies that the direct integrity 

test parameters must be fixed such that a breach on the order of the smallest Cryptosporidium

oocyst (i.e., 3 μm) is physically capable of contributing to a test response. Thus, for pressurebased direct integrity tests, the applied pressure (or vacuum) must be great enough to overcome 

the capillary static forces that hold water in a breach (i.e., the bubble point) of 3 μm in diameter 

in a fully-wetted membrane, thereby allowing air to escape through a Cryptosporidium–sized 

hole and consequently enabling this loss of air to be potentially detected. If the applied pressure 

(or vacuum) were insufficient to overcome the bubble point, then the direct integrity test would 

be physically incapable of detecting any number of Cryptosporidium–sized breaches that would 

allow the passage of pathogens to the filtrate. Similarly, with marker-based tests, the marker 

used must be smaller than 3 μm to ensure that it would pass through a Cryptosporidium–sized 

hole, thus enabling a breach of this size to be potentially detected by the instrumentation that 

measures the concentration of the marker in the filtrate. Note that the 3 μm (or less) resolution 

requirement applies to all facilities utilizing membrane filtration for compliance with the 

LT2ESWTR.

However, unlike the resolution, the required sensitivity – the maximum log removal value 

(LRV) that can be reliably verified by the direct integrity test – may vary among different 

facilities under the LT2ESWTR, as the rule stipulates only that the test used have a sensitivity 

that exceeds the Cryptosporidium log removal credit awarded by the state. In some cases, the 

sensitivity of the test may be determined from the threshold test result that signifies the smallest 

detectable integrity breach, information that can be provided by the membrane filtration system 

supplier. This result, which represents the test sensitivity, may then be converted into a LRV 

using the methodology described in Chapter 4 or an alternative methodology approved by the 

state.

Although the compliance framework developed for direct integrity testing under the 

LT2ESWTR applies only to those utilities that are subject to the Cryptosporidium removal 

requirements of the rule, the methodology could be applied for other pathogen either by mandate 

of the state or at the discretion of the utility. If this methodology is applied to more than one 

different pathogen simultaneously at the same treatment facility, then the direct integrity test 

used would be required to have a resolution corresponding to the smallest of the pathogens; in 

addition, the test would have to have a sensitivity greater than the removal credit awarded by the 

state for each pathogen to which the methodology was applied. Even if a compliance framework 

similar to that for the LT2ESWTR is not applicable to a particular utility’s membrane filtration 

system, a direct integrity test is still the most reliable means of determining whether or not a 

breach has occurred. 

For any system to which a compliance framework similar to that for the LT2ESWTR is 

applied, the resolution and sensitivity requirements for direct integrity testing should be 

established prior to placing the facility into service and specified in the IVP. If a different 

method is used to determine threshold integrity test results for applications other than for 

LT2ESWTR compliance (e.g., fiber-cutting studies), these critical values should also be 

incorporated into the IVP. As a minimum, the IVP should include the direct integrity test result 

threshold specified by the manufacturer as indicative of a potential integrity breach. 
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What type of direct integrity test should be used?

Currently, there are two general types of direct integrity tests that are commercially 

available for use with membrane filtration facilities: 1) pressure-based tests; and 2) marker-based 

tests. The various specific tests that fall under each of these two general categories are described 

in Chapter 4. The particular test utilized in conjunction with a given system may depend on 

regulatory requirements, the type of membrane filtration system, target organism (i.e., in terms of 

the required test resolution), test sensitivity, or utility preference.

The LT2ESWTR does not mandate the use of a particular direct integrity test for 

regulatory compliance; any test utilized must simply meet the criteria specified under the rule for 

resolution, sensitivity, and frequency (40 CFR 141, subpart W, Appendix C). However, some 

states do stipulate the use of a specific test. In some cases, even if no particular test is specified, 

the required sensitivity relative to that of each potential direct integrity test method may govern 

which test(s) may be used. If no specific test is required by regulatory mandate, then any type of 

direct integrity test approved for use by the state may be used at the utility’s discretion.

The direct integrity test used also depends to some extent on the type of membrane 

filtration system utilized, as some tests are not compatible with certain types of systems. For 

example, while a particulate marker test may be used with a MCF, MF, or UF system, a 

molecular marker test would not be a feasible means of assessing MCF, MF, or UF integrity, 

since the molecular marker may not be sufficiently removed by these membrane systems to 

demonstrate a reasonable LRV (e.g., 3 log). Conversely, a particulate marker test would not be 

used with NF or RO systems, since the particles could not easily be flushed from a spiral-wound 

membrane module and would be likely to irreversibly foul or otherwise damage the membranes. 

Thus, for NF/RO systems, a molecular marker would be a more appropriate marker-based direct 

integrity test. 

Pressure (or vacuum) decay tests are compatible with all the various types of membrane 

filtration as defined under the LT2ESWTR (i.e., MF, UF, NF, RO, and MCF), and the equipment 

necessary to conduct this type of test is typically included with most of the currently available 

proprietary MF/UF systems. Similarly, some types of direct integrity tests may not be available 

from a proprietary membrane filtration system supplier. If several types of tests are available 

and the utility is not otherwise constrained by regulatory requirements, the selection of a direct 

integrity test should take into account any site- or system-specific considerations that would

either favor or preclude certain tests.

The type of direct integrity test used for a particular system and the justifying rationale 

should both be included in the facility IVP, as well as specific procedures for conducting the test. 

If the test is automated (as is common), the procedures specified in the IVP should include how 

the test works in terms of the automatic sequencing that the system undergoes. In addition, the 

procedure for manually conducting the test whenever necessary should also be specified, as well 

as any other responsibilities that system operators may have with respect to direct integrity 

testing. One particular consideration for systems with automated direct integrity testing is 

whether or not an operator must be present during the test. Although the presence of an operator 

may be beneficial, particularly if an integrity breach is detected, the ability of an operator to 
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respond quickly to an automated alarm and notification system may render this unnecessary. If 

the state does not require direct operator supervision during direct integrity testing, the utility 

should exercise its own discretion in determining whether the presence of an operator is critical 

based on site- and system-specific considerations and degree of comfort with unsupervised 

testing.

How frequently should direct integrity testing be conducted?

Because none of the various types of direct integrity tests currently available can feasibly 

assess integrity on a continuous basis while the system is on-line and producing filtered water, a 

membrane unit to be tested must be taken off-line and out of production during the testing 

process. Thus, more frequent direct integrity testing results in increased downtime for each 

membrane unit and consequently decreased system productivity. As a result, the frequency of 

direct integrity testing is an important parameter that hinges on striking an acceptable balance 

between the competing desires to maximize both confirmation of system integrity and treated 

water production.

A number of states have established minimum test frequency requirements for membrane 

filtration systems; currently, these requirements range from as often as every four hours to as 

relatively infrequently as once per week, depending on the state. If a utility applies membrane 

filtration for compliance with the LT2ESWTR, the rule requires that direct integrity testing be 

conducted on each membrane unit once per day at a minimum (40 CFR 141.728). However, the 

state may require testing on a more frequent basis at its discretion. 

Even if subject to minimum test frequency requirements by federal or state regulations, a 

utility could opt to conduct direct integrity testing more frequently. In addition, a utility that is 

not otherwise constrained by any regulatory requirements should use its discretion to determine 

an appropriate direct integrity test frequency. In establishing a test frequency, careful 

consideration should be given to the factors that may influence this decision. For example, 

testing less frequently may increase overall facility production and minimize the mechanical 

stress on the membrane module(s) that repeating testing might induce. However, increased 

testing provides more frequent assurance of system integrity, or in the case of an integrity

breach, less operating time under compromised conditions that may allow the passage of 

pathogens or other undesirable pre-filtered water constituents. The most important concern 

should be the maximum length of time that a utility feels comfortable potentially operating with 

an integrity breach of any magnitude, which would dictate the minimum acceptable integrity test 

frequency. Although the use of continuous indirect integrity monitoring (see Chapter 5 and 

section A.3) provides some measure of integrity confirmation between direct test applications, 

currently available indirect integrity monitoring techniques may not be able to detect a breach of 

potentially significant magnitude. It is important to note that the frequency of direct integrity 

testing should be based on public health considerations rather than the observed or anticipated 

frequency of the occurrence of integrity breaches. 
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When should direct integrity testing be conducted?

In the process of developing an IVP, some consideration should be given to the timing of 

conducting direct integrity testing relative to the normal operating cycle(s) of the system. For 

example, with systems that utilize some type of regular reverse flow process to remove foulants 

from the membrane surface (e.g., backwashing), the membrane will be the least obstructed 

immediately after this reverse flow process is complete. Implementing a direct integrity test at 

this point in the operational cycle would provide the most conservative estimate of system 

integrity, since the accumulated foulants that could plug any breaches, and thus effectively mask 

potential integrity problems, would be minimized. However, using this same example, for 

systems that employ a pressure-based direct integrity test, it is important that the test not be 

conducted too soon after the reverse flow processes (particularly if air is used), since the 

membrane must be fully-wetted for the integrity test to be effective. 

It is also recommended that a direct integrity test be conducted after chemical cleaning or 

any other routine or emergency maintenance in order to ensure that the system integrity has not 

been compromised by the procedure(s). The affected membrane module(s) or unit(s) should be 

returned to service only after system integrity has been confirmed by a direct integrity test. In 

cases in which a membrane unit has been taken out of service for diagnostic testing and/or repair, 

systems applying membrane filtration for LT2ESWTR compliance are required to conduct direct 

integrity testing on the affected unit to demonstrate system integrity prior to returning the unit to 

service.

Any special circumstances that might call for direct integrity testing (i.e., in addition to 

the regularly scheduled periodic testing), such as subsequent to chemical cleaning or membrane 

repair, whether in accordance with federal or state requirements or at the discretion of the utility, 

should be specified in the IVP.

How should the direct integrity test results be interpreted?

For a given resolution, a marker- or pressure-based direct integrity test indicates whether 

or not an integrity breach has occurred by comparing the results of the test to the threshold result 

known to represent a breach. This threshold represents the test sensitivity and can be determined 

either by information provided by the manufacturer or through an on-site, system-specific 

assessment such as a fiber-cutting study. 

Under the compliance framework of the LT2ESWTR, the ongoing test results obtained 

during facility operation for each membrane unit (as well as the threshold result representing the 

test sensitivity) can be converted into LRVs using the methodology described in Chapter 4 or 

other method approved by the state. Each successive test result (or LRV, as converted) is then 

compared to the log removal credit allocated to the system by the state (either as per the 

requirements of the LT2ESWTR or for other treatment objectives) to determine compliance on 

an ongoing basis. If the LRV yielded by the direct integrity test is greater than the regulatory 

allocation, the system remains in compliance. However, if the LRV is below the required log 
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removal, the membrane unit must be taken off-line for diagnostic testing and repair (see sections 

A.4 and A.5, respectively).

Thus, under the LT2ESWTR compliance framework, the direct integrity test results yield 

two important pieces of information: 1) whether or not an integrity breach has occurred; and 2) 

the maximum LRV that can be verified at the time of the test (when converted using the 

technique(s) described in Chapter 4 or alternate methodology approved by the state). Because 

the test sensitivity may often be greater (i.e., higher LRV) than the required log removal for any 

particular pathogen, it is possible for the direct integrity test to indicate that an integrity breach of 

some magnitude has occurred without the system being out of compliance. Thus, under the 

LT2ESWTR framework, a system could knowingly continue to operate with some level of 

integrity breach and still meet regulatory requirements. However, USEPA recommends (and the 

state may require) that a membrane unit with a detectable integrity breach of any magnitude be 

immediately taken out of service for diagnostic testing and repair. At a minimum, should 

conduct diagnostic testing and potential repair when the unit is taken off-line for chemical 

cleaning, previously scheduled maintenance, or other routine purpose.

The LT2ESWTR defines a control limit as an integrity test response, which, if exceeded, 

indicates a potential integrity problem and triggers subsequent action. For the purposes of 

LT2ESWTR compliance, a direct integrity test control limit would be established as the test 

result that translates into an LRV equal to the Cryptosporidium removal credit awarded by the 

state. As noted previously, any membrane unit for which the test results exceed the control limit 

(i.e., the LRV drops below the awarded removal credit) must be immediately taken off-line for 

diagnostic testing and repair. Other control limits could also be established if the LT2ESWTR 

compliance framework is simultaneously applied for the removal of other pathogens with 

membrane filtration. However, because the system would be out of compliance if either of the 

two (or more) control limits were exceeded, the most stringent control limit (i.e., the highest log 

required log removal value) would always represent the governing limit.

In addition to the critical or “upper” control limit (UCL) that governs regulatory 

compliance for a membrane filtration facility under the LT2ESWTR framework, “lower” control 

limits (LCLs) may be established as performance benchmarks either at the discretion of the 

utility or by mandate of the state. One or more LCLs may be identified between the integrity test 

result that indicates the smallest detectable breach and a breach that reduces the performance of 

the system such that it is just capable of meeting the require log removal (i.e., the UCL). LCLs 

may be useful for alerting system operators to the presence of an integrity breach, even if the 

detected breach is not sufficient to bring the system out of compliance. Rather than triggering 

unit shutdown and subsequent diagnostic testing, as with an UCL, exceeding a LCL might 

trigger increased operator attention or an investigation that can be conducted by operators while 

the system is still on-line in an attempt to determine the cause of the integrity problem and 

prevent the breach from expanding, if possible. Alternatively, in cases in which there is a 

particularly large gap between the test sensitivity and the UCL a utility may choose to 

voluntarily take a membrane unit off-line for diagnostic testing if a preferred LCL is exceeded in 

order to minimize the risk to public health via the potential for pathogens to pass through a 

barrier known to be compromised, even if only to a small degree that is within regulatory 

tolerances. 
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As an example, even with the smallest detectable breach the direct integrity test might 

still be able to verify 6 log removal (i.e., the test sensitivity), although the state only awards 

2.5 log Cryptosporidium removal credit (i.e., the UCL) for the membrane filtration process. 

Under this scenario, a direct integrity test result yielding a LRV of 4 would indicate the presence 

of a significant integrity breach, even though the membrane filtration system would still be 

capable of achieving the required log removal. Thus, a utility might opt to establish a LCL at a 

LRV of 4 if an integrity breach of this magnitude represents an unacceptable public health risk 

independent of the ability of the system to maintain regulatory compliance.

Fiber-cutting studies that allow integrity test results to be quantified and correlated to 

certain number of fiber breaks or particular reductions in log removal capacity may be used to 

determine a single appropriate LCL or a series of tiered LCLs. Setting a LCL equal to the test 

sensitivity (i.e., the maximum log removal value that can be reliably verified by the direct 

integrity test) would represent the most conservative scenario, as in this case any detectable 

integrity breach would, at a minimum, alert an operator and potentially trigger some responsive 

action. All control limits and the action triggered by exceeding each limit should be clearly 

specified in the IVP.

Another issue that should be addressed in the IVP is the possibility of false positive and 

false negative direct integrity test results. For example, if a periodic direct integrity test indicates 

the presence of a significant integrity breach, but the instrumentation used for continuous indirect 

integrity monitoring (e.g., turbidimeters – see section A.3) has consistently yielded baseline 

results that are well within normal operating tolerances, the direct integrity test may have 

generated a false positive result (i.e., a result incorrectly indicating a breach in a fully integral 

system). Alternatively, if the continuous indirect integrity monitoring data are also unusually 

high after the direct integrity test, an integrity breach may have occurred during the direct test. 

Nevertheless, because the direct integrity tests are more sensitive to integrity breaches, 

LT2ESWTR compliance requires that any membrane unit for which a direct integrity test result 

exceeds an UCL be taken off-line for diagnostic testing and repair, independent of indirect 

monitoring data. However, evidence such as indirect monitoring results that may suggest a false 

positive result may used to help guide an operator investigation and identify the source of the 

problem, which may be a function of the direct integrity test rather than an integrity breach. If a 

false positive result is suspected, the operator should check isolation valves and fittings on the 

system that are associated with the direct integrity test. In addition, a follow-up direct integrity 

test should be conducted both to confirm the results of the first test and to closely monitor the 

test for any system malfunctions.

False negative results (i.e., results that indicate either a fully integral system in the 

presence of an integrity breach or which significantly underestimate a substantial integrity 

breach) of direct integrity tests may be more difficult to identify; since the methods of 

continuous indirect integrity monitoring are less sensitive to integrity breaches, indirect 

monitoring data may not be capable of detecting a breach that is masked by a false negative 

direct test result. However, if the continuous indirect integrity monitoring data does suggest an 

integrity breach in contradiction of the direct test results, the membrane unit should be taken out 

of service to investigate the source of the discrepancy. One potential scenario that might result 
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in a false negative result is an integrity breach that occurs in a membrane that is partially fouled. 

The accumulated foulants may obscure the breach, thus masking the integrity problem until after 

the next backwash or chemical cleaning. Nevertheless, in this case the false negative result may 

not represent a significant concern; if the direct integrity test is functioning properly, the false 

negative result would suggest that the system is functionally integral even with an integrity 

problem as a result of the breach being plugged. In this case, the breach would likely be detected 

after the next backwash or chemical cleaning that successfully removes the foulants from 

blocking the breach. A more problematic scenario involving false negative results might occur if 

the integrity test equipment is malfunctioning. Thus, is it important to incorporate a routine 

maintenance program for the integrity test system as part of the IVP.

Any particular strategies for minimizing the potential for false positive and false negative 

results should be specified in IVP documentation. A maintenance schedule for direct integrity 

monitoring equipment should also be specified. It is recommended that the direct integrity 

monitoring system receive a thorough check-up on at least an annual basis.

A.3 Continuous Indirect Integrity Monitoring

Continuous indirect integrity monitoring is a secondary means of verifying membrane 

filtration system integrity intended to detect significant breaches between direct test applications. 

Thus, in the absence of a continuously applied direct test, continuous indirect integrity 

monitoring is critical to an IVP. As with section A.2, this section is organized into a series of 

questions that parallel the step-by-step process of formulating strategy for continuous indirect 

integrity monitoring. Each of these questions represents an important aspect of that strategy that 

should be included to some extent in an IVP.

What is the purpose of indirect integrity monitoring?

For the purposes of LT2ESWTR compliance, indirect integrity monitoring is defined as 

monitoring some aspect of filtrate water quality that is indicative of the removal of particulate 

matter. Although the various indirect monitoring methods are less sensitive techniques for 

assessing membrane integrity than the direct integrity tests, the value of utilizing the indirect 

methods is that they can be applied to assess integrity continuously while the system is on-line 

and producing water. In fact, since by definition indirect monitoring is applied to the filtrate, 

these techniques require that the membrane unit be in continuous production to assess membrane 

integrity. 

Because currently available methods of direct integrity testing cannot be applied 

continuously, a successful direct test only indicates that no breach has occurred since the 

previous application of the test. Consequently, if the system were to become compromised 

immediately after a successful direct integrity test had been conducted, the breach might not be 

detected until the next regularly scheduled direct test, which may be as long as 24 hours for the 

purposes of LT2ESWTR compliance. During this interval, a potentially significant breach could 

have occurred, allowing pathogens or other particulate matter to contaminate the filtrate for a 
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period as long as an entire day. For applications other than LT2ESWTR compliance, this 

interval may be as short as four hours or as long as one week or more, depending on the state of 

jurisdiction. Thus, although continuous indirect integrity monitoring may not be able to detect 

small compromises in integrity, these techniques do provide the ability to identify larger 

breaches on an ongoing basis during production. As a result, periodic direct integrity testing and 

continuous indirect integrity monitoring are complementary tools for assessing system integrity, 

and both are critical for a comprehensive IVP.

Under the LT2ESWTR, continuous indirect integrity monitoring is required in the 

absence of a direct integrity test that can be applied continuously and which meets the resolution 

and sensitivity requirements of the rule. States may also have regulations requiring some form of 

continuous indirect integrity monitoring. In the absence of any applicable requirements, a utility 

may opt to employ some form of continuous indirect integrity monitoring at its discretion; 

however, note that turbidity monitoring, which is required under the various federal surface 

water treatment regulations as a measure of overall system performance, may also serve the dual 

purpose of a means of continuous indirect integrity monitoring.

Unlike direct testing, there are no specific resolution or sensitivity requirements for 

continuous indirect integrity monitoring under the LT2ESWTR. However, these concepts, 

where applicable, may be useful tools for optimizing the ability of the various continuous 

indirect integrity monitoring methods to yield meaningful information about potential integrity 

breaches, as described in Chapter 5.

What type of indirect integrity monitoring method should be used?

There are a number of different methods and associated devices that may be used for 

continuous indirect integrity monitoring, including particle counting, particle monitoring, 

turbidimetry, laser turbidimetry, and conductivity monitoring. In general, any method that 

measures particulate matter in the filtrate as an indirect means of assessing integrity (such as 

particle counting, turbidimetry, etc.) is applicable to any of the various types of membrane 

filtration systems. Other methods that may measure dissolved constituents in the filtrate, such as 

conductivity monitoring, would only be applicable to NF or RO systems. The particular method 

of continuous indirect integrity monitoring employed by a utility for its system may be a function 

of regulatory requirements, test resolution or sensitivity, cost, confidence in the technology, or 

simply preference based on prior experience or other subjective criteria.

The LT2ESWTR requires the use of turbidity monitoring on each membrane unit as the 

default method of continuous indirect integrity monitoring for compliance, unless an alternate 

method is approved by the state. Because the various federal surface water treatment regulations 

(i.e., the Surface Water Treatment Rule (SWTR), the Long-Term 1 Enhanced Surface Water 

Treatment Rule (LT1ESWTR), and the Interim Enhanced Surface Water Treatment Rule 

(IESWTR)) require turbidity monitoring as a means of assessing filtration performance, surface 

water facilities implementing membrane filtration for LT2ESWTR compliance may use turbidity 

monitoring to satisfy both requirements. States may have other specific requirements for 

continuous indirect integrity monitoring independent of the LT2ESWTR or may approve other 
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methods for LT2ESWTR compliance. All federal and state requirements must be satisfied in a 

utility’s IVP. 

If not otherwise constrained by regulatory requirements, a utility’s decision to use a 

specific type of indirect integrity monitoring technique may be influenced by the ability of a 

particular method to provide sufficient resolution or sensitivity. For example, because particle 

counters have been demonstrated to be more sensitive to breaches than particle monitors or 

conventional turbidimeters, a utility may choose to use particle counting to maximize the ability 

to detect compromises in system integrity between periodic direct integrity test events 

(Jacangelo, et al., 1997). Other utilities may select laser turbidimetry, which has been shown in 

some studies to perform comparably to particle counting in terms of sensitivity to integrity 

breaches (Banerjee, et al., 2000; Colvin, et al., 2001). In this case, if laser turbidimetry is 

approved by the state for both purposes, a utility may use laser turbidimeters for compliance with 

the applicable surface water treatment rules with the additional benefit of improved sensitivity 

for detecting integrity breaches over conventional turbidimeters. 

Sensitivity may also be improved for any given continuous indirect integrity monitoring 

method by applying instrumentation to smaller groupings of membrane modules, such that any 

integrity breach would have a greater impact on filtrate quality. In this case, the benefits of the 

increased sensitivity should be weighed against the cost of additional instruments. This approach 

may be advantageous if the gain in sensitivity by using a greater number of less expensive 

instruments is justified by the comparable cost of fewer more expensive instruments. Note that 

for LT2ESWTR compliance, an instrument for continuous indirect integrity monitoring must be 

applied to each membrane unit. A utility may apply instruments to smaller groupings of 

membrane modules at its discretion. For other applications, state requirements for monitoring 

various groupings of membrane modules in an overall system must be satisfied in a utility’s IVP.

If test resolution is an important criterion, than a utility might strongly consider particle 

counting. Because particle counting is the only method that assesses the size of particulate 

matter, it is the only method of indirect integrity monitoring to which the concept of resolution 

applies. For example, if membranes are applied specifically to remove Cryptosporidium, the 

particle counters should be well-calibrated to accurately detect particles approximately 3 μm in 

size or larger. The target resolution may vary depending on the particular contaminant of 

concern.

What constitutes “continuous” indirect integrity monitoring?

Under the LT2ESWTR, “continuous” monitoring is defined as monitoring conducted at a 

frequency of no less than once every 15 minutes. However, because the instrumentation used for 

the various methods of indirect integrity monitoring may allow data collection at much more 

frequent intervals, the state may have more stringent data collection requirements. In the 

absence of specific regulatory requirements, a utility may collect data at a frequency it 

determines to be appropriate. Nevertheless, since data acquisition can be automated, it is 

recommended that data be collected at interval no less than every 15 minutes, even if no other 

regulatory requirements apply.
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While more frequent data collection both provides increased integrity monitoring and 

additional data to track system performance, there may be some potential complications from 

collecting data too frequently. For example, data could be collected frequently enough such that 

a backwash (for applicable systems) cannot be completed between readings. In this case, a 

utility must program the system to cease data collection during the backwash cycle and for any 

length of time afterwards that the data remain artificially high, such that a direct integrity test is 

not triggered. Data collection and analysis are further discussed in section A.6. The implications 

of collecting indirect integrity monitoring data at different intervals should be considered in 

developing an IVP, and the frequency and any other associated qualifying guidelines or 

restrictions should be specified in the facility IVP documentation. 

How should indirect integrity monitoring results be interpreted? 

Continuous indirect integrity monitoring is primarily intended to provide some indication 

of system integrity between direct integrity test applications. The indirect monitoring results are 

continuously compared to an established performance threshold that is known to represent a 

potential integrity breach. If this threshold level is exceeded, some type of response is triggered 

to further investigate the problem.

Under the LT2ESWTR, this performance threshold represents the UCL for continuous 

indirect integrity monitoring. If the UCL is exceeded, direct integrity testing is automatically 

triggered as a means to assess system integrity using a more sensitive technique. Unlike that for 

direct testing, the UCL for continuous indirect integrity monitoring does not necessarily 

correspond to a specific and quantifiable integrity loss. For example, the UCL established by the 

LT2ESWTR for turbidity monitoring (i.e., the default method in the absence of another 

technique specified by the state) is 0.15 NTU independent of site- or system-specific 

considerations; filtrate turbidity readings exceeding 0.15 NTU for a period of 15 minutes (or two 

consecutive 15-minute readings exceeding 0.15 NTU) would trigger direct testing. The 0.15 

NTU threshold was selected because it is significantly below the 0.3 NTU threshold for filter 

performance required by the IESWTR for 95 percent of all turbidity samples, yet because 

membrane filtrations systems are well-documented to consistently produce filtered water below 

0.05 NTU, a sustained filtrate turbidity exceeding 0.15 NTU strongly suggests a potential 

integrity problem. Note that if turbidity monitoring with the default UCL of 0.15 NTU is used as 

a means of continuous indirect integrity monitoring, a utility could simultaneously be in 

compliance with the IESWTR but not with the LT2ESWTR.

Although the LT2ESWTR specifies a UCL of 0.15 NTU with the default method of 

turbidity monitoring, the state may establish a more stringent standard at its discretion. In 

addition, for any approved method of continuous indirect integrity monitoring, the state may 

require a site- or system-specific performance-based UCL that is linked to a certain level of 

integrity loss (in terms of a specific number of broken fibers or LRV) as determined by a fiber 

cutting study. These studies may also serve as the basis for establishing LCLs that trigger a 

particular response at a threshold prior to the point at which direct integrity testing would be 

required, either mandated by the state or voluntarily implemented by the utility. Voluntary LCLs 
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could also be implemented after the membrane filtration system has been in operation for a 

certain period of time; this staggered implementation would allow sufficient baseline data to be 

collected such that operators could identify threshold levels that represent elevated or otherwise 

unusual results that are still below the UCL and which do not necessarily indicate an integrity 

breach, but nevertheless warrant observation. Some examples of actions associated with LCLs 

might be increased operator attention or diagnostic checks of the continuous indirect integrity 

monitoring instrumentation. All CLs both mandated by the state and voluntarily implemented by 

the utility should be documented in an IVP, along with the rationale supporting the establishment 

of the CLs as well as the respective subsequent action associated with the exceedance of each.

As with direct integrity tests, false negative and false positive results are also possible 

with indirect integrity monitoring. For example, some indirect integrity monitoring instruments 

may indicate elevated levels of a parameter (e.g., turbidity, particle counts, etc.) after a routine 

maintenance event such as a backwash, particularly if air is employed in the process to scour or 

pulse the membrane surface. If significant, this air entrainment error could cause a CL to be 

exceeded, generating a false positive result (i.e., a result incorrectly suggesting a breach in a fully 

integral system); in the case of the UCL, this exceedance would inappropriately trigger direct 

integrity testing (and, under the LT2ESWTR, consequent reporting). This type of false positive 

result can be minimized by first characterizing typical system performance under a variety of 

operating conditions (such as after a backwash) and subsequently programming the data 

acquisition system to account for regularly occurring data aberrations of previously quantified 

magnitude and duration which are known not to represent an integrity problem, even if CLs are 

exceeded. (The importance collecting and analyzing baseline data for IVP optimization is 

further discussed in section A.6.) In some cases, devices such as bubble traps (e.g., in the case of 

air entrainment) may be utilized to minimize modes of error that might generate false positive 

results.

False negative results (i.e., results that indicate either a fully integral system in the 

presence of an integrity breach or which significantly underestimate a substantial integrity 

breach) may be more common with indirect integrity monitoring methods. Because currently 

available indirect integrity monitoring techniques are less sensitive to breaches than direct 

integrity tests, it is possible that small breaches may occur that could be detectable via direct but 

not indirect methods. This potential may be minimized by utilizing a more sensitive method of 

continuous indirect integrity monitoring (such as the use of laser vs. conventional turbidimeters), 

if a utility is permitted such flexibility under state regulations. Alternatively, a utility could 

increase indirect method sensitivity by utilizing a greater number of instruments (i.e., decreasing 

the number of membrane modules monitored per instrument). However, a utility considering 

these options should evaluate whether the cost of increasing indirect method sensitivity (i.e., via 

purchasing a greater number of instruments or more sensitive instruments or both) is justified by 

the consequent level of heightened integrity monitoring ability between direct test events. 

Any particular strategies for minimizing the potential for false positive and false negative 

results should be specified in IVP documentation. A calibration schedule for continuous indirect 

integrity monitoring instrumentation should also be specified. It is recommended that this 

instrumentation be calibrated on at least an annual basis.
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A.4 Diagnostic Testing

Diagnostic testing is a process of identifying and isolating integrity breaches that have 

already been detected and confirmed using other methods, and thus is a critical component of an 

IVP. As with previous sections in this appendix, this section is organized into a series of 

questions that parallel the step-by-step process of formulating a diagnostic testing strategy for an 

IVP. Each of the questions presented in this section represents some aspect of diagnostic testing 

that should be clearly addressed and documented in an IVP.

What is the purpose of diagnostic testing?

The purpose of diagnostic testing is to identify and isolate integrity breaches in a 

membrane module that are detected via other methods. Because direct integrity testing and 

continuous indirect integrity monitoring techniques can only detect the existence of a breach, 

diagnostic testing complements these methods, serving as a tool to pinpoint the exact location of 

a breach. In this way, diagnostic testing also serves as a critical link between identifying an 

integrity problem during the course of operation and repairing the breach. (Membrane repair and 

replacement is further discussed in section A.5). Thus, if an integrity breach is known or 

suspected in a membrane unit, that unit should be taken out of service for diagnostic testing in 

order to facilitate repairs. IVP documentation should clearly note the purpose of diagnostic 

testing so as to distinguish it from other forms of testing.

Under what circumstances should diagnostic testing be applied?

The LT2ESWTR requires that a membrane unit be taken out of service for diagnostic 

testing and repair if the established upper control limit (i.e., that associated with the log removal 

credit awarded to the membrane process) is exceeded during a direct integrity test. The use of 

diagnostic testing under these circumstances is also suggested for those utilities that do not use 

their respective membrane filtration systems for LT2ESWTR compliance. Thus, in essence 

diagnostic testing is recommended any time an integrity breach is detected from the results of a 

direct integrity test.

Note that the LT2ESWTR does not link diagnostic testing directly to continuous indirect 

integrity monitoring. Even if indirect monitoring results clearly indicate an integrity problem, it 

is advisable to confirm the existence of a breach using more sensitive direct integrity testing 

methods. In any case, under the compliance framework for the LT2ESWTR, any continuous 

indirect integrity monitoring results that would clearly indicate an integrity breach would almost 

certainly exceed the upper control limit and thus trigger direct integrity testing, as required. 

However, a utility that is not otherwise constrained by regulatory requirements could voluntarily 

take a membrane unit out of service for diagnostic testing based on indirect monitoring results 

alone.

An IVP should clearly identify the specific circumstances under which diagnostic testing 

should be applied, including both those conditions that require diagnostic testing under a 
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regulatory framework and those that might trigger the use of diagnostic testing on a voluntary 

basis by the utility. 

What type(s) of diagnostic testing should be used?

By definition, most diagnostic tests are categorized as types of direct integrity tests; 

however, diagnostic tests are distinguished from other types of direct tests by their ability to not 

just detect an integrity breach, but to help locate the specific module or fiber containing the 

breach, as well. In addition, most methods considered to be diagnostic tests are designed to be 

applied to specific membrane units on an as-needed basis, and thus are impractical for 

implementation on a scale that would satisfy the direct integrity testing requirements of the 

LT2ESWTR. For example, sonic testing – one type of diagnostic test – requires an operator to 

manually apply an accelerometer to various locations on the membrane module to listen for 

vibrations caused by leaking air. While this technique fits the definition of direct integrity test, it 

would be infeasible to use such a test to check every module in a membrane filtration system for 

integrity breaches every 24 hours.

In addition to the sonic test described above, other methods of diagnostic testing include 

bubble testing, conductivity probing, and simple visual inspection (where applicable). Each of 

these methods is described in further detail in section 4.8. A pressure (or vacuum) decay test 

applied to a smaller number of modules (i.e., a subset of a full membrane unit) in order to 

identify a particular breached module may also be used as a form of diagnostic test. Single 

module testing represents the smallest form of incremental membrane unit testing. This type of 

diagnostic test generally involves removing the individual modules from the membrane unit and 

testing each on a specially designed single module apparatus. Other types of direct integrity tests 

may also have the potential to serve as diagnostic tests in a scaled-down form.

In some cases, a battery of diagnostic tests may be used to identify an integrity breach. 

For example, if a MF membrane unit fails a direct integrity test (i.e., the results exceed the upper 

control limit), after the unit is taken out of service a sonic test might be applied to each module in 

the unit in turn to identify the affected module(s). (Note that although the membrane unit is 

taken out of service, it must still remain in operation in order to facilitate some types of 

diagnostic tests, such as the sonic test referenced in this example and described in section 4.8.3. 

Therefore, in such cases, the unit must be operating in filter-to-waste mode.) The module(s) may 

then be removed from the unit to conduct a bubble test (see section 4.8.2) in order to isolate 

particular fibers that may be subsequently removed from service permanently by pinning or 

sealing (as described in section A.5). Thus, just as diagnostic testing complements direct 

integrity testing or continuous indirect integrity monitoring, different diagnostic tests can also 

complement each other.

A utility should develop its own system-specific protocol for diagnostic testing and 

document the procedures in its IVP. The IVP should specify which diagnostic tests are 

prescribed, the particular purpose of each test (e.g., to isolate a module or identify a particular 

fiber), the circumstances under which each test should be conducted, a list of necessary testing 

equipment, and detailed instructions for conducting the test. Some membrane filtration system 
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manufacturers may provide guidance in developing an appropriate diagnostic testing plan. 

Although diagnostic testing may not be commonly employed, a utility should still keep all the 

equipment necessary to conduct each diagnostic test specified in its system IVP both on site and 

in good working order. Note that some diagnostic tests (such as sonic testing) require more 

some level of skill in both conducting the test and interpreting the results; operators that will 

have responsibility for conducting these tests should be designated and trained in advance in 

order to minimize membrane unit downtime.

A.5 Membrane Repair and Replacement

In the context of this guidance, “membrane repair and replacement” does not necessarily 

apply to just the membranes themselves, but to any component of a membrane filtration system 

that might allow an integrity breach if it were to fail. This section is organized into a series of 

questions presented in a logical order intended to parallel the step-by-step process of considering 

membrane repair and replacement in the context of an IVP. Each of the questions presented in 

this section should be addressed in a utility’s IVP to an appropriate degree.

What is the purpose of membrane repair / replacement?

The purpose of conducting repairs on a membrane filtration system or replacing 

irreparably damaged components is to correct any integrity breaches that have been detected, 

thus restoring and maintaining a fully-integral system. In cases in which membrane filtration is 

applied for the removal of one or more specific pathogens of interest (e.g., for compliance with 

the LT2ESWTR), a more specific objective is to enable the system to maintain the full removal 

credit allocated by the state. As previously noted, although other system components may be 

essential for overall system operation, in this context repair and replacement are discussed in 

regard to only those system components that are critical to system integrity. 

When should membrane repair / replacement be conducted?

In the simplest terms, repair or replacement should be conducted whenever an integrity 

breach is detected (e.g., using direct integrity testing or continuous indirect integrity monitoring). 

After the source of the breach has been isolated (e.g., via diagnostic testing), the problem should 

be corrected via component repair or replacement, as appropriate.

Under the LT2ESWTR, if the results of a direct integrity test exceed the upper (i.e., 

mandated) control limit, the affected membrane unit(s) must be taken out of service for 

diagnostic testing and repair. Because the membrane unit(s) cannot be returned to service until a 

direct integrity test confirms that the UCL is no longer exceeded, some type of repair must be 

conducted to correct the integrity problem. If a utility has voluntarily implemented one or more 

tiered LCLs, it may in some cases be able to detect an integrity problem without exceeding the 

UCL. In this case the utility could opt to take corrective action, conducting diagnostic testing 

and subsequent repair immediately, or instead keep the affected membrane unit(s) in continued 
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operation under increased observation until the next scheduled maintenance event. Although 

repair of any breach is recommended as soon as possible, a utility may exercise discretion in 

determining whether or not to implement any repairs based on severity of the breach (assuming 

the UCL has not been exceeded). For example, if a utility is required to achieve only 2 log 

Cryptosporidium removal credit for LT2ESWTR compliance, it is possible that a significant 

integrity breach could occur without jeopardizing the ability of the membrane filtration system to 

obtain this credit. However, continued operation with a known integrity breach of any 

magnitude may not be permitted by a state. Even if operation under some compromised 

conditions is not explicitly prohibited, a utility should carefully consider the risk associated with 

the potential for pathogen passage before engaging in continued operation.

If a utility does not apply its membrane filtration system for compliance with the 

LT2ESWTR, it may have more flexibility with respect to the timing and necessity of conducting 

immediate repairs if an integrity breach is detected, unless otherwise constrained by state 

requirements. In the absence of any regulations prescribing system repair requirements, it is 

recommended that utilities adopt a conservative approach in order to help ensure the integrity of 

the membrane barrier against pathogens. The system IVP should clearly specify any regulatory 

requirements relating to membrane system repair, as well as other circumstances under which the 

utility would conduct repairs.

Membrane module repair – as opposed to replacement – is often advisable, if possible, 

since new membranes are typically expensive. However, if a membrane module is subject to 

repeated integrity breaches, a utility should consider replacing the module. Chronic repairs 

adversely affect treated water production and prevent operators from carrying out regular plant 

responsibilities. Also, if it is critical that an integrity breach be repaired as quickly as possible, 

for some types of membrane filtration systems it may be more efficient to replace the module 

with a new one from the utility’s supply of spare components maintained on-site. Thus, in some 

cases the most expedient and cost-effective system “repair” could actually be membrane 

replacement.

Although the LT2ESWTR requires the use of direct integrity testing to confirm the 

success of any system repairs concerning integrity problems before returning the affect 

membrane unit(s) to service, this practice is recommended for all utilities, even if membrane 

filtration is not conducted for LT2ESWTR compliance. It is also recommended that this testing 

be conducted after the module has been inserted into the system, as in this case the direct 

integrity test would not only be able to confirm the success of the module repair, but also 

whether it was properly re-installed. After any repair or replacement measures have been 

completed, both direct integrity test and continuous indirect integrity monitoring results for the 

repair unit(s) should be closely tracked for an extended period to gauge the long-term success of 

the repair and perhaps whether the problem would be likely to recur. 
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What are some common modes of integrity breaches?

The types of integrity breaches to which a particular membrane filtration system is most 

susceptible can depend on both the type of system (i.e., MCF, MF/UF, or NF/RO) and the 

manufacturer. For example, while many NF/RO membranes are subject to chemical degradation 

by oxidants such as chlorine, only some MF/UF membranes are vulnerable to oxidation, 

depending on the membrane material used. This same example of chemical oxidation illustrates 

potential causes of integrity breaches that are specific to a particular treatment application of 

membrane filtration, as well. A treatment process that utilizes chlorine as a disinfectant 

upstream of RO membranes has an inherent potential source of chemical degradation, even 

though dechlorination may be implemented prior to the membranes. If the dechlorination system 

fails or is miscalibrated, the membranes could be subject to chlorine exposure. By contrast, an 

identical RO system used for a different application in which upstream disinfection is not 

required to meet treatment objectives would not have this additional element of risk. It is 

important to note that this example should not be interpreted as a recommendation against 

utilizing oxidants upstream of oxidant intolerant membranes. It is not uncommon to use 

upstream oxidants effectively, particularly for disinfection and biofouling control. However, a 

utility should be aware of the potential for chemical degradation of membrane material and take 

appropriate measures to prevent potential integrity breaches and protect treatment equipment.

In addition to chemical degradation, the most common causes of integrity breaches with 

NF and RO membranes are associated with o-rings and seals, which can be cracked, rolled, 

and/or improperly sized. Each of these defects can result in an integrity breach, as can foreign 

matter such as hairs or other fibrous material underneath o-rings. Other mechanisms for integrity 

breaches may be related to membrane defects, such as failures that may occur along glue lines or 

at weak spots in the membrane (i.e., creases or thin areas). Fiber breaks and potting problems 

are the most common types of integrity breaches associated with MF/UF membranes, although 

chemical degradation may also result in membrane failure. MCF systems are most likely to be 

compromised by improperly seated/sealed membrane cartridges or tears or punctures in the 

membrane material. MCF membranes may also exhibit integrity breaches along folds and 

creases, depending on the construction of the filter.

Although it is possible for breaches to occur at any time during operation, it is most 

common for integrity problems to occur during system start-up. These breaches typically result 

from either manufacturing defects or improper installation. As a result, it is important that an 

initial shakedown period be included as a part of the start-up process to enable these initial 

problems to be identified and corrected prior to putting the system into service.

An IVP should identify and document the most probable types of integrity breaches for a 

utility’s specific membrane filtration system. These modes may be identified initially by 

consultation with the membrane manufacturer and consideration of site-specific circumstances 

(e.g., the use of upstream oxidants). Subsequently, the shakedown period and ongoing 

operational experience may yield important information about the most frequently occurring 

types of integrity problems. Also, the experience of other utilities using the same or similar 

filtration equipment may yield valuable information about the types of integrity failures common 

to a particular system.
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How should membrane repair / replacement be conducted?

The types of repairs that may be conducted to correct integrity breaches vary significantly 

with the type of membrane filtration system. For example, for MF/UF systems, fiber breaks (the 

most common mode of failure) are not technically repaired, but rather isolated by inserting small 

pins or epoxy in the end(s) of the broken fiber, effectively removing them from service 

permanently and thus eliminating the system integrity breach. By contrast, it is generally not 

possible to repair comprised NF/RO membranes, although problems with o-rings and other seals 

may often be corrected by replacing the seals and making sure the membrane is properly seated 

in its housing. Likewise, because MCF systems utilize cartridges that are designed to be 

disposable, unless an integrity breach is the result of a seal problem, a damaged membrane 

cartridge would generally not be repaired. Thus, any integrity problems associated directly with 

either NF/RO or MCF membranes themselves generally necessitate membrane replacement.

Prior to placing the system into operation, it is important to consult with the membrane 

manufacturer to determine what types of repairs can be made and which types of integrity 

breaches require membrane replacement. The manufacturer should also be able to provide both 

instructions and training for all applicable repair procedures. Because it is somewhat common 

for some types of integrity problems to occur during the system shakedown phase, it is 

recommended that this period be use for practicing repair procedures under the directions of a 

qualified manufacturer’s representative. 

When integrity problems occur during operation, it is important to identify the cause of 

the integrity problem, as well as its source. While some fiber breaks in MF/UF systems may be 

expected due to wear or mechanical stress over time, other breaches may have a specific cause 

that can be isolated and corrected to avoid further integrity problems. If an integrity breach 

occurs, it is important to check both the membrane filtration system as well as any upstream 

treatment process to ensure that these are operating properly. For example, if a chemical 

incompatible with the membrane material is added upstream but is not being properly removed 

or quenched, some membrane damage and loss of integrity may occur. 

Any necessary repair equipment or spare parts, including replacement modules that may 

be required in the event of an integrity breach should be kept on site. The system IVP should 

specify a list of these components, along with any applicable instructions. The IVP should also 

include suggestions for troubleshooting integrity problems.

A.6 Data Collection and Analysis

Diligent and rigorous collection and analysis of integrity testing and monitoring data are 

an important component of any IVP. Careful data collection and analysis can serve as useful 

tools for preventing integrity problems, as well as for optimizing system performance and 

troubleshooting problems. Like other sections in this appendix, the following discussion is 

organized by addressing a series of frequently asked questions in regard to data collection and 

analysis, each of which should be encompassed in a comprehensive IVP. 
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What is the purpose of data collection and analysis?

Although the primary purpose of data collection and analysis may be to demonstrate 

regulatory compliance, a thorough and well-planned program can result in a number of other 

important benefits for preventing integrity problems and optimizing system performance. For 

example, a consistently maintained record of membrane unit performance during both direct 

integrity testing and continuous indirect integrity monitoring may help determine when some 

membranes are approaching the end of their useful lives. In addition, because the resistance or 

permeability of some membranes changes after an initial “setting” period, a careful record of 

integrity test results may indicate that either the UCL (subject to regulatory approval) or any 

voluntarily implemented LCLs should be adjusted. 

Continuous indirect integrity monitoring data also have a number of advantages in 

addition to helping gauge membrane integrity. This data may be used to identify performance 

trends, such as those that may occur between backwashing or chemical cleaning events, or over 

the entire life of the membranes. A well-documented data record can also identify any 

systematic or periodic trends and potentially help isolate the cause(s). In addition, data records 

facilitate the comparison of performance trends among different membrane units. (Note that a 

normal amount of instrument variation should be taken into account when conducting a 

statistical analysis comparing data among different membrane units.) A substantial amount of 

data collected over time may also enable operators to identify aberrations that do not necessarily 

indicate integrity problems. For example, if the turbidity is consistently higher than normal after 

a backwash, operators may be able to attribute this consistent aberration to air-entrainment error 

associated with the instrumentation. It is important that these types of aberrations are identified 

such that the system can be programmed not to trigger direct integrity testing during these 

events. Direct integrity testing data can also be a valuable tool for identifying trends; however, 

because continuous indirect integrity monitoring data are typically collected much more 

frequently and while the unit is on-line, it may often be the most practical means of tracking 

performance trends.

What data should be collected?

In addition to any regulatory requirements regarding data collection during operation, a 

utility should collect baseline data for each membrane unit (both with respect to direct integrity 

testing and continuous indirect integrity monitoring) before putting the plant into service. This 

data will serve as a reference baseline against which to evaluate membrane unit performance and 

also help refine a utility’s strategy for collecting continuous indirect integrity monitoring data 

during regular operation. For example, for MF/UF systems the baseline data should demonstrate 

how long after a backwash event turbidity or particle count data might remain elevated. Based 

on this information, a utility can account for such data spikes that are known to not represent 

integrity problems, such that direct integrity testing and consequent loss of production are not 

unnecessarily triggered. Throughout operation, as well as during integrity testing conducted 

during routine maintenance or repair, it is recommended that data be collected in a spreadsheet or 
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with software that establishes a database and allows data to be readily plotted in order to identify 

trends occurring over time.

What are some methods for reducing continuous indirect integrity monitoring data?

As noted in section A.3, the LT2ESWTR defines “continuous” monitoring as a frequency 

of no less than once every 15 minutes. However, instruments used to collect integrity 

monitoring data – particle counters, partic le monitors, turbidimeters, etc. – may allow data 

collection at much more frequency intervals. Thus, in the absence of more specific requirements 

from the state, a utility may collect data as often as possible, if desired, provided the minimum 

frequency is met or exceeded. Furthermore, in the absence of specific guidelines for how a large 

amount of data should be reduced for compliance and reporting purposes, a utility has the 

latitude to select a statistical method that it determines to be appropriate for its system. Some 

potential methods include:

• Maximum value

• 95th percentile

• Average value

• Singular timed measurement

Using the example of “continuous” monitoring as defined under the LT2ESWTR (i.e., minimum 

frequency of once every 15 minutes), each of the above methods are described in context as 

follows.

Maximum Value

Using this method, the maximum value that occurs over every 15-minute period represents the 

entire data set. Thus, direct integrity testing is triggered if even one measurement exceeds the 

UCL. This method is very conservative and could potentially result in excessive direct integrity 

testing and subsequent loss of filtered water production from any anomalous data spikes, which 

may be attributable to any number of factors aside from an integrity problem.

95th Percentile

With this method, 95th percentile datum represents the entire 15-minute data set, effectively 

screening out the largest five percent of data spikes. Direct integrity testing would be triggered if 

this datum exceeded the UCL. This method is less conservative than the maximum value 

approach and is more likely to screen anomalous data spikes that are not indicative of an 

integrity problem. The rationale behind this method is that if an integrity breach occurs, it may 

be likely that more than five percent of the data collected exceed the UCL. The premise for this 

method may be used with any percentile, and it may be advantageous for a utility using this 

technique to conduct a statistical analysis to determine an appropriate percentile to eliminate 

anomalous spikes without screening data that might indicate an integrity breach.
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Average Value

Using this method, the average value represents the entire 15-minute data set. This technique is 

roughly equivalent to a 50th percentile approach, and thus is less conservative than the 95th

percentile method. However, average value method lessens the need to artificially exclude 

anticipated integrity spikes that are known not to indicate an integrity problem (e.g., data 

collected immediate after a backwash with MF/UF systems), as the effect of these spikes may be 

sufficiently dampened such that the average value is below the UCL. 

Singular Timed Measurement

The singular timed measurement approach uses one reading collected exactly every 15 minutes 

for comparison to the UCL (i.e., for regulatory compliance), independent of how frequently data 

are collected between these compliance readings. Because many other non-compliance 

measurements collected during these 15-minute intervals could exceed the UCL without 

triggering direct integrity testing, this method is one of the least conservative approaches. This 

technique represents the minimum requirement for compliance with the LT2ESWTR.

Note that because the LT2ESWTR does not specify a statistical reduction technique for 

data collected more frequently that at 15-minute intervals, the methods described in this section 

are not specific compliance options under the rule. These methods are not meant to represent an 

exclusive or exhaustive list and are simply examples of approaches that utilities could employ if 

not otherwise constrained by state requirements. A utility’s IVP should clearly specify its data 

collection and analysis practices for both continuous indirect integrity monitoring and direct 

integrity testing.

A.7 Reporting

Utilities that use membrane filtration for compliance with the LT2ESWTR are required to 

submit a monthly operating report to the state. Because these monthly reports are directly linked 

to integrity testing results, it is important that reporting be incorporated into a comprehensive 

IVP. As with other sections in this appendix, the following discussion on reporting is organized 

into a series of critical questions that should be addressed in an IVP.

What is the purpose of reporting?

As it relates to integrity verification, the primary purpose of reporting is to document the 

ability of a membrane filtration system to meet its required log removal or other performancebased objectives on an ongoing basis. Under the minimum requirements of the LT2ESWTR, this 

documentation generally consists of all direct integrity test and continuous indirect integrity 

monitoring results that exceed the respective UCL and the subsequent corrective action that was 

taken in each case. Note that collecting, recording, and storing an abundance of integrity test 

data may be very beneficial for optimizing membrane filtration system performance, as 

discussed in section A.6, even if the majority of the accumulated data are not necessary for 

complying with reporting requirements.
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If membrane filtration is not applied specifically for LT2ESWTR compliance, reporting 

may not necessarily be directly related to integrity verification, depending on state requirements. 

For example, membrane filtration might be considered an alternative filtration technology (as 

provided for under the federal SWTR), in which case reporting requirements might simply 

include turbidity, similar to those for conventional media filters. In this case, including reporting 

requirements in an IVP may not be critical for a utility. Nevertheless, because verifying and 

preserving membrane integrity are critical to successful membrane filtration system operation, it 

is recommended that even turbidity, particle counts, or other required filtrate quality data always 

be considered continuous indirect integrity monitoring results, thus linking reporting 

requirements to membrane integrity and, subsequently, an IVP.

What should an IVP include with respect to reporting?

An IVP should specify any state requirements for reporting, including both the content 

and the reporting frequency. (Reporting requirements for utilities that employ membrane 

filtration for compliance with the LT2ESWTR are addressed in Chapters 4 and 5 for direct 

integrity testing and continuous indirect integrity monitoring, respectively.) Any utility-specific 

procedures for preparing the compliance report should also be included in an IVP, along with a 

sample report form.

A.8 Summary

Although not required under the LT2ESWTR, the development of a comprehensive IVP 

can be a valuable organizational tool to help a utility verify and maintain membrane system 

integrity. An IVP should essentially serve as a utility’s system-specific “how-to” guide for all 

aspects operation and maintenance that are related to system integrity, including (but not 

necessarily limited to) the following:

• Regulatory requirements

• Voluntarily implemented system-specific practices

• Clear objectives for all IVP procedures

• Instructions for all IVP procedures

• Equipment listing, description, and purpose

• System troubleshooting tips

• Guidelines for interpreting test results

• Sample calculations (where applicable)
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• Membrane manufacturer contact information

A well-developed IVP containing these elements can make the process of integrity verification 

more effective and efficient, thus helping a utility maximize the benefit of its membrane 

filtration system for serving as a barrier to pathogens and other particulate matter.
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Appendix B

Overview of Bubble Point Theory
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Appendix B:

Overview of Bubble Point Theory

B.1 Introduction

The various methods of pressure-based direct integrity testing are all predicated on 

capillary theory as described by the bubble point equation, which is derived from a balance of 

static forces on the meniscus in a capillary tube. The bubble point itself is defined as the 

threshold gas pressure required to displace liquid from the pores or capillary-like breaches of a 

fully-wetted membrane. In the context of porous membranes, bubble point theory was originally 

used as the basis for developing a test to characterize the pore sizes. Because the bubble point 

equation describes an inverse relationship between the applied pressure and capillary (or pore) 

diameter, the pressure at which bubbles are first detected in a fully-wetted membrane can be used 

to calculate the diameter of the largest pore (see Equation B.1). Accordingly, larger threshold 

pressures are indicative of membranes with smaller pores. A diagram illustrating a membrane 

pore as a capillary tube is shown in Figure B.1.

Figure B.1 Diagram of a Membrane Pore Modeled as a Capillary Tube

Bubble point theory has also been applied to the detection of integrity breaches in the 

form of the various pressure-based direct integrity tests, such as the pressure or vacuum decay 

tests (sections 4.7.1 and 4.7.2, respectively), the diffusive airflow test (section 4.7.3), and the 

water displacement test (section 4.7.4). Integrity breaches such as broken hollow fibers or holes 

in the surface of the membrane are analogous to pores, and larger test pressures enable the 

detection of smaller breaches. If the applied pressure is below the bubble point of the rated 

membrane pore size and does not decay (pressure and vacuum decay tests), generate airflow 

(diffusive airflow test), or displace water (water displacement test) to a degree that exceeds 
Appendix B – Overview of Bubble Point Theory

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normal tolerances over the duration of the direct integrity test, the membrane is determined to be 

integral at the level of the threshold pore or breach size corresponding to that applied pressure. 

In the context of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), this 

threshold pore or breach size is called the test resolution, as described in section 4.2.

The purpose of this appendix is to provide a general overview of bubble point theory as it 

relates to direct integrity testing under the LT2ESWTR. In addition to the background provided 

in this introduction, subsequent sections of this appendix describe the bubble point equation and 

its parameters.

B.2 The Bubble Point Equation

The bubble point equation is derived from a balance of static forces on the meniscus in a 

capillary tube and is given as Equation B.1 (without specific units). A derivation of this equation 

is given in the literature by Meltzer (1987).

cap

bp d

P

4 • s • cosq

= Equation B.1

Where: Pbp = bubble point pressure

s = surface tension at the air-liquid interface

θ = liquid-membrane contact angle

dcap = capillary diameter

Converting Equation B.1 to a form that utilizes convenient units for the various 

parameters yields Equation B.2:

cap

bp d

P

0.58 • s • cosq

= Equation B.2

Where: Pbp = bubble point pressure (psi)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

dcap = capillary diameter (μm)

Because the structure of most membranes cannot be accurately represented by a perfectly 

cylindrical capillary, a shape correction factor, κ, is typically applied to account for non-ideal 

conditions, as shown in Equation B.3:
Appendix B – Overview of Bubble Point Theory

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cap

bp d

P

0.58 • k •s • cosq

= Equation B.3

Where: Pbp = bubble point pressure (psi)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

dcap = capillary diameter (μm)

Equation B.3 serves as the general form of an expression for the resolution of a pressurebased direct integrity test, yielding the minimum required test pressure as a function of the 

resolution. Equation B.4 represents this same expression, but incorporates variables more 

specific to the application of determining pressure-based direct integrity test resolution.

res

test d

P

0.58 •k •s • cosq

= Equation B.4

Where: Ptest = minimum test pressure (psi)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

dres = direct integrity test resolution requirement (μm)

Substituting the direct integrity test resolution requirement (dres) of 3 μm for membrane 

filtration under the LT2ESWTR into Equation B.4 yields an expression specific to the rule, as 

shown in Equation B.5. 

Ptest = 0.193• k •s • cosq Equation B.5

Where: Ptest = minimum test pressure (psi)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

If there is any backpressure in the membrane filtration system during direct integrity 

testing, this pressure must also be accounted for in calculating the minimum test pressure as a 

function of the required resolution. Because the backpressure acts in opposition to the applied 

test pressure, the test pressure must be increased by this same amount, as shown in Equation B.6. 

Note that this equation is also cited as Equation 4.1 in section 4.2.1.
Appendix B – Overview of Bubble Point Theory

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( ) max Ptest = 0.193 •k •s • cosq + BP Equation B.6 (4.1)

Where: Ptest = minimum test pressure (psi)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

BPmax = maximum backpressure on the system during the test (psi)

For the purposes of pressure-based direct integrity testing, Equation B.3 can be 

rearranged to yield an expression for the diameter of smallest integrity breach that can be 

detected for a given applied pressure, as shown in Equation B.7:

test

fect P

d

0.58 • k • s • cosq

= Equation B.7

Where: dfect = defect diameter (μm)

κ = pore shape correction factor (dimensionless)

s = surface tension at the air-liquid interface (dynes/cm)

θ = liquid-membrane contact angle (degrees)

Ptest = integrity test pressure (psi)

B.3 Bubble Point Equation Parameters

As shown in Equations B.4-B.6, the pore shape correction factor (κ), liquid-membrane 

contact angle (θ), and the surface tension (s) all affect the minimum direct integrity test pressure 

necessary to meet a given resolution requirement. Each of these parameters is discussed in the 

following subsections, including considerations for selecting appropriate values.

Liquid-Membrane Contact Angle (q)

The liquid-membrane contact angle ranges from 0-90o and is primarily a function of the 

membrane hydrophilicity, which can be characterized in general terms as the affinity of the 

membrane material for water or ability of the membrane to become wetted with water. For an 

ideally hydrophilic membrane, the liquid-membrane contact angle is 0 degrees. Although many 

membranes used for drinking water applications are manufactured using hydrophilic materials, 

an ideally hydrophilic membrane is purely theoretical. However, a value of 0 degrees yields the 

largest minimum integrity test pressure (as shown in Equations B.4-B.6), and thus represents the 

most conservative contact angle. In the absence of specific information from the membrane 

manufacturer regarding a more accurate value of θ for a particular material, a contact angle of 

0 degrees should be assumed. The determination of values for θ is discussed in the literature 
Appendix B – Overview of Bubble Point Theory

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(Meltzer 1987). Because a less conservative contact angle can significantly reduce the minimum 

required integrity test pressure, any value for θ other than 0 degrees should be well-documented 

and approved by the state if used for the purposes of regulatory compliance, such as under the 

LT2ESWTR.

Pore Shape Correction Factor (k)

The dimensionless pore shape correction factor ranges from 0-1 and is a function of the 

pore structure, accounting for deviations from perfectly cylindrical pores, as well as for the 

torturous flow path across the membrane. A correction factor of 1 represents a perfectly 

cylindrical pore, and as shown in Equations B.4-B.6, maximizes the value of the minimum 

required test pressure. However, it is generally recognized that almost all microporous 

membranes used for drinking water treatment do not have perfectly or approximately cylindrical 

pores, thus have associated correction factors less than 1. Nevertheless, because pore shape 

correction factor can significantly reduce the minimum required test pressure, it is important that 

any value less than 1 be well-documented and approved by the state if used for the purposes of 

regulatory compliance, such as under the LT2ESWTR. In the absence of such data supporting 

the use of a non-ideal pore shape correction factor, a conservative value of 1 should be used.

Surface Tension

Although the surface tension of water does not vary significantly over the typical range of 

ambient water temperatures, it still can affect the minimum direct integrity test pressure 

necessary to meet a given resolution requirement, as shown in Equations B.4-B.6. Thus, because 

the surface tension varies inversely with temperature, the surface tension at the lowest 

anticipated water temperature should be used to calculate a conservative value for the minimum 

required test pressure. For reference, Table B.1 includes values of surface tension over a range 

of water temperatures.

Table B.1 Surface Tension of Water at Various Temperatures1

Temperature

(

oC)

Surface Tension

(dynes/cm)

5 74.9

10 74.2

15 73.5

20 72.8

25 72.0

30 71.2

40 69.6

1 CRC Handbook, 66th ed., 1985-1986
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Appendix C

Calculating the Air-Liquid Conversion Ratio
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Appendix C:

Calculating the Air-Liquid Conversion Ratio

C.1 Introduction

As described in Chapter 4, the regulatory framework for the Long-Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR) requires that the flow through the smallest 

integrity breach that generates a measurable response from the direct integrity test (i.e., the 

critical breach size - Qbreach) be determined in order to establish the sensitivity of the test method 

(40 CFR 141, Subpart W, Appendix C). (Under the LT2ESWTR, sensitivity is defined as the 

maximum log removal value (LRV) that can be reliably verified by the direct integrity test (i.e., 

LRVDIT) (40 CFR 141, Subpart W, Appendix C).) However, because most pressure-based direct 

integrity tests yield results in terms of airflow or pressure decay, it may be necessary to convert 

these results to an equivalent value for the flow of water through the critical breach under typical 

filtration conditions. This conversion is necessary for calculating both the sensitivity and the 

upper control limit (UCL) for a direct integrity test.

Although there are a number of methods for converting a direct integrity test response to 

a corresponding flow of water, each can be generally categorized as one of two types of 

approaches: mathematical modeling or experimental determination. This appendix describes a 

mathematical approach based on a parameter called the air-liquid conversion ratio (ALCR), 

which is defined as the ratio of air that would flow through a breach during a direct integrity test 

to the amount of water that would flow through the breach during filtration, as shown in 

Equation C.1 (also Equation 4.5):

















=

breach

air

Q

Q

ALCR Equation C.1 (4.5)

Where: ALCR = air-liquid conversion ratio (dimensionless)

Qair = flow of air through the critical breach during a pressurebased direct integrity test (volume / time)

Qbreach = flow of water through the critical breach during filtration 

(volume / time)

Because of the many variations in membrane configurations, breaches in the membrane 

may exhibit either turbulent or laminar flow characteristics depending upon the location and size 

of the defect, as well as the applied pressure differential. In addition, there are fundamental 

differences between hollow-fiber and flat sheet membrane breaches, since the most common 

breaches associated with hollow-fiber modules exhibit pipe flow characteristics, while flat sheet 

breaches are best represented by an orifice model. Consequently, three different hydraulic 

models have been developed for determining the ALCR for a particular membrane system, 

depending on the configuration of the membrane material (i.e., hollow-fiber vs. flat sheet) and 
Appendix C – Calculating the Air-Liquid Conversion Ratio

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the type of flow (i.e., laminar vs. turbulent) that is expected through the critical breach. These 

three models include the Darcy pipe flow model (for breaches in a hollow-fiber (or hollow-finefiber) module under conditions of turbulent flow), the orifice model (for modules utilizing flat 

sheet membranes such as spiral-wound and membrane cartridge configurations under conditions 

of turbulent flow), and the Hagen-Poiseuille model (for any configuration under conditions of 

laminar flow). Table C.1 summarizes the various approaches for calculating the ALCR based on 

these three models and the conditions under which the use of each model is appropriate. 

Table C.1 Approaches for Calculating the ALCR

Module Type Defect Flow 

Regime Model

Turbulent2 Darcy Pipe Flow

Hollow-fiber1

Laminar HagenPoiseuille

Turbulent Orifice

Flat sheet3

Laminar HagenPoiseuille

1 Or hollow -fine-fiber

2 Typically characteristic of larger diameter fibers and higher 

differential pressures

3 Includes spiral-wound and cartridge configurations

Note that the various methods presented in this appendix for determining the ALCR 

implicitly assume that the flow regime for airflow through a breach during direct integrity testing 

is the same as that for liquid flow though a breach during filtration (i.e., either both laminar or 

both turbulent). If this assumption is determined to be inappropriate for a given membrane 

filtration system such that inaccurate and non-conservative estimates for sensitivity may result, 

then a hybrid approach may be considered. An example of such a hybrid approach is to assume 

laminar water flow and turbulent airflow, which could be modeled through the application of the 

Hagen-Poiseuille equation for water and the Darcy equation for air.

Procedures for calculating the ALCR and subsequently the sensitivity and upper control 

limit (UCL) for applicable pressure-based direct integrity tests are given in Chapter 4, but the 

derivations of the various hydraulic models that form the basis for the respective ALCR 

equations are provided in the following sections of this appendix as additional information. Note 

that while the derivation of the ALCR equations relies on various hydraulic models that could be 

used to directly calculate the flow of air (Qair) and water (Qbreach) through an integrity breach, 

direct application of these equations requires knowledge of the critical breach size, which is 

difficult and impractical to accurately quantify. The advantage of the ALCR is that the terms 

relating to the size of the breach cancel out, yielding equations for the ALCR that are a function 

of either known and/or more easily determined parameters and independent of the critical breach 

size or geometry.
Appendix C – Calculating the Air-Liquid Conversion Ratio

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For the derivation of the various models described in this appendix, airflow equations 

have been developed using a standard temperature and pressure of 68 oF (528 R, 293 K, or 

20 oC) and 0 psi (14.7 psia) as. These standard conditions were selected to be consistent with the 

convention for airflow measurement devices. The equations can be modified to different set of 

reference conditions through application of the ideal gas law expressed in terms of absolute 

temperature and pressure, if necessary. In addition, the temperature of the air used in a pressurebased direct integrity test is assumed to be the same as that for the water in the membrane 

filtration system, since these temperatures are expected to rapidly equilibrate.

Additional background on the hydraulic modeling developed in this appendix may be 

found in Crane’s Flow of Fluids Through Valves, Fittings, and Pipes (1988). All of the basic 

hydraulic equations used in the derivation of the ALCR equations are included in this text.

C.2 Darcy Pipe Flow Model

The Darcy pipe flow model is used to describe turbulent flow through an integrity breach 

with characteristics similar to broken hollow-fiber. Generally, turbulent flow may be expected 

through larger diameter broken fibers and at higher differential pressures. The Darcy equations 

for the flow of air and water through a pipe are given in Equations C.2 and C.3, respectively: 

( )

( ) air

test test

air fiber T K

P BP P

Q Y d

+ •

− • +

= • • •

460

( 14.7)

11.5

2

Equation C.2

Where: Qair = flow of air at standard conditions (ft3/sec)

Y = net expansion factor for compressible flow through a pipe 

to a larger flow area (dimensionless)

dfiber = fiber diameter (in)

Ptest = integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

T = water temperature (oF)

Kair = resistance coefficient of air (dimensionless)

water w

breach fiber K

TMP

Q d

• r

= • •

2

0.525 Equation C.3

Where: Qbreach = flow of water through the critical breach during filtration

(ft3/s)

dfiber = fiber diameter (in)

TMP = transmembrane pressure (psi)

Kwater = resistance coefficient of water (dimensionless)

ρw = density of water (lbs/ft3
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Assuming that the resistance coefficients for air and water are similar (i.e., Kair ≈ Kwater) 

and applying a value of 62.4 lbs/ft3 for the density of water, the ratio of Equation C.2 to 

EquationC.3 yields an expression for the ALCR, as given by Equation C.4:

( )

( T) TMP

P BP P

ALCR Y

test test

+ •

− • +

= • •

460

( 14.7)

173 Equation C.4

Where: ALCR = air-liquid conversion ratio (dimensionless)

Y = net expansion factor for compressible flow through a pipe 

to a larger flow area (dimensionless)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

T = water temperature (oF)

TMP = transmembrane pressure (psi)

The ALCR is used in the equations for determining both the sensitivity and the upper 

control limit (UCL) for direct integrity testing, as described in Chapter 4. Consequently, the 

values of the parameters in Equation C.4 should be selected to yield a conservative value for the 

ALCR. 

The net expansion factor for compressible flow (Y) may be obtained from charts in 

various hydraulics references, such as Crane (1988) (page A-22). Using the appropriate chart for 

airflow, values for Y are given as a function of pressure and the flow resistance coefficient, as 

shown in Equation C.5 (a non-specific expression illustrating the relationship between Y and its 

variables):













































+

−

∝ K

P

P BP

Y

test

test

,

14.7

1

Equation C.5

Where: Y = net expansion factor for compressible flow through a pipe 

to a larger area (dimensionless)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

K = flow resistance coefficient (dimensionless)

The flow resistance coefficient (K) is a common fluid flow parameter described by most 

hydraulics texts and is defined as shown in Equation C.6:
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d fiber

L

K = f • Equation C.6

Where: K = flow resistance coefficient (dimensionless)

f = Darcy friction factor (dimensionless)

L = length of the defect (in)

dfiber = fiber diameter (in)

Using the conservative scenario of a fiber break at the point where the fiber enters the pot, the 

length of the defect (L) is represented by the length of the lumen encasement into the membrane 

pot. The friction factor (f) may be estimated from a Moody diagram or the corresponding 

tabulated values, both of which are readily in available in most hydraulics references. The 

relative roughness (e/dfiber) that is required to estimate the value for the Darcy fiction faction may 

be calculated by either obtaining a product-specific value for the specific roughness (e) from the 

manufacturer or by using the membrane pore size as an estimate of the specific roughness. 

Note that the net expansion factor (Y) should remain constant over time for practical 

purposes if appropriately conservative values are used. Thus, the determination of Y should 

represent a one-time, site-specific calculation. Also, because the ALCR is directly proportional 

to Y (as shown in Equation C.4), lower values for Y result in lower, more conservative values for 

the ALCR.

An iterative solution may be required to determine a value for the net expansion 

factor (Y). A general outline for one such iterative process is given as follows. The use of a 

spreadsheet may help facilitate the various calculations required.

1. Select a reasonable value for the friction factor (f).

2. Calculate the flow resistance coefficient (K) using Equation C.6.

3. Obtain a value for the Reynolds number (Re) from tabulated values for the friction 

factor (f) as a function of the Reynolds number (Re) and the relative roughness 

(e/dfiber).

4. Calculate airflow (Qair) from the equation for the Reynolds number (Re) as a function 

of equivalent diameter, air velocity, and dynamic viscosity (as referenced in fluid 

mechanics and dynamics texts). For the purposes of determining the equivalent 

diameter and velocity (i.e., the flow (Qair) divided by the cross-sectional area), assume 

that the applicable integrity breach may be represented by a pipe (e.g., a hollow fiber) 

flowing full with air. Use the maximum anticipated temperature and the minimum 

pressure applied over the duration of the direct integrity test (i.e., accounting for 

baseline decay) to generate a conservative (i.e., low) value for the dynamic viscosity 

and, thus, in turn conservative (i.e., low) values for airflow (Qair) and the ALCR.
Appendix C – Calculating the Air-Liquid Conversion Ratio

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5. Using tables available in hydraulics texts (e.g., page A-22 of Crane (1988)), apply the 

flow resistance coefficient (K) and the pressure ratio to determine a value for the net 

expansion factor (Y), as shown in Equation C.5.

6. Calculate airflow (Qair) using Equation C.2. Assume that K ≈ Kair.

7. If the airflow (Qair) calculated in steps 4 and 6 (above) is approximately the same, then 

the net expansion factor (Y) determined in step 5 is correct. Otherwise, select a revise 

value for the friction factor (f) and repeat steps 1-7 in an iterative process until the two 

calculated values for airflow (Qair) converge.

C.3 Orifice Model

The orifice flow model may be used to approximate turbulent flow through an integrity 

breach with characteristics similar to a hole in a flat sheet membrane that may be configured as a 

disposable cartridge or a spiral-wound module. The representative equations for airflow and 

water flow through an orifice are given as Equations C.7 and C.8, respective ly:

( )

T

P BP P

Q Y d C

test test

air fiber +

− • +

= • • • •

460

( 14.7)

11.5

2

Equation C.7

Where: Qair = flow of air at standard conditions (ft3/s)

Y = net expansion factor for compressible flow through a pipe 

to a larger area (dimensionless)

dfiber = fiber diameter (in)

C = coefficient of discharge (dimensionless)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

T = water temperature (oF)

w

breach fiber

TMP

Q d C

r

= • • •

2

0.525 Equation C.8

Where: Qbreach = flow of water through the critical breach during filtration 

(ft3/s)

dfiber = fiber diameter (in)

C = coefficient of discharge (dimensionless)

TMP = transmembrane pressure (psi)

ρw = density of water (lbs/ft3) 
Appendix C – Calculating the Air-Liquid Conversion Ratio

Membrane Filtration Guidance Manual C-7 June 2003

Proposal Draft

The ratio of Equation C.7 to Equation C.8 yields an expression for the ALCR, as given in 

Equation C.9. Note that this equation incorporates a value of 62.4 lbs/ft3 for density of water.

( )

( T) TMP

P BP P

ALCR Y

test test

+ •

− • +

= • •

460

( 14.7)

173 Equation C.9

Where: ALCR = air-liquid conversion ratio (dimensionless)

Y = net expansion factor for compressible flow through a pipe 

to a larger area (dimensionless)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

ρw = density of water (lbs/ft3) 

T = water temperature (oF)

TMP = transmembrane pressure (psi)

Note that although derivations are slightly different, the resulting ALCR equations for the Darcy 

and orifice models (Equations C.4 and C.9, respectively) are identical. However, the two models 

utilize different methodologies for determining the net expansion factor for compressible flow 

(Y). As described in section C.2 for the Darcy model, the values of the parameters in 

EquationC.10 should be selected to yield a conservative value for the ALCR.

Also, as with the Darcy model, the net expansion factor for compressible flow (Y) may 

be obtained from charts in various hydraulics references, such as Crane (1988) (page A-21). 

However, for the orifice model, EquationC.10 may also be used to calculate the net expansion 

factor, as follows:

































+

+

= − • −

14.7

14.7

1 0.293 1

Ptest

BP

Y Equation C.10

Where: Y = net expansion factor for compressible flow through a pipe 

to a larger area (dimensionless)

BP = backpressure on the system during the integrity test (psi)

Ptest = direct integrity test pressure (psi)

As with the Darcy model, because the ALCR is directly proportional to Y (as shown in 

EquationC.9), lower values for Y result in lower, more conservative values for the ALCR. Also 

like Darcy model, note that the net expansion factor (Y) should remain constant over time for 

practical purposes if appropriately conservative values are used. Thus, the determination of Y 

should represent a one-time, site-specific calculation.
Appendix C – Calculating the Air-Liquid Conversion Ratio

Membrane Filtration Guidance Manual C-8 June 2003

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C.4 Hagen-Poiseuille Model

The Hagen-Poiseuille model is appropriate for sma ll integrity breaches (such as a pin 

hole or flow through a broken, small-diameter hollow-fiber under low differential pressure) that 

would result in laminar flow. Using this model, the equation for airflow through a small defect 

under laminar flow conditions is given by Equation C.11:

(460 )

49.5

4

L T

d P g

Q

air

eff

air

fect

• • +

• • • Δ •

=

m

p

Equation C.11

Where: Qair = flow of air at standard conditions (ft3/s)

dfect = defect diameter (in)

ΔPeff = effective integrity test pressure (psi)

g = gravitational constant (32.2 lbm-ft/lbf-s

2

)

L = length of the defect (in)

μair = viscosity of air (lbs/ft-s)

T = water temperature (oF)

Because air is a compressible fluid, the airflow is determined using the effective integrity 

test pressure, ΔPeff, calculated according to Equation C.12. 

[ ]











 +

•













•

+ + +

Δ = − •

14.7

( 14.7)

2 14.7

( 14.7) ( 14.7)

( )

P BP BP P P BP test

eff test Equation C.12

Where: ΔPeff = effective air integrity test pressure (psi)

Ptest = direct integrity test pressure (psi)

BP = backpressure on the system during the integrity test (psi)

The elements of the effective integrity test pressure include three primary terms, as 

individually bracketed in EquationC.12. These three terms are, respectively: 

• the differential pressure across the membrane during the integrity test

• a term that accounts for the average velocity gradient of the compressed air as it 

passes across the membrane

• a multiplier that is necessary to convert the backpressure as it leaves the membrane to 

standard atmospheric conditions 
Appendix C – Calculating the Air-Liquid Conversion Ratio

Membrane Filtration Guidance Manual C-9 June 2003

Proposal Draft

The Hagan-Poiseuille equation for liquid flow through a breach under conditions of 

laminar flow is shown as Equation C.13:

w

fect

breach L

d g TMP

Q

m

p

•

• • • •

=

4

0.094

Equation C.13

Where: Qbreach = flow of water through the critical breach during filtration 

(ft3/s)

dfect = defect diameter (in)

g = gravitational constant (32.2 lbm-ft/lbf-s

2

) 

TMP = transmembrane pressure (psi)

L = length of the defect (in)

μw = viscosity of water (lbs/ft-s) 

The ratio of Equation C.11 to Equation C.13 yields an expression for the ALCR, as given 

by Equation C.14:

(460 )

527

TMP T

P

ALCR

air

eff w

• • +

• Δ •

=

m

m

Equation C.14

Where: ALCR = air-liquid conversion ratio (dimensionless)

ΔPeff = effective integrity test pressure (psi)

μw = viscosity of water (lbs/ft-s)

TMP = transmembrane pressure (psi)

μair = viscosity of air (lbs/ft-sec)

T = water temperature (oF)

The ratio of the viscosity of water to the viscosity of air (μw /μair) may be combined and 

expressed as a single function of the water temperature that is derived by fitting a curve to 

discrete data points for the viscosity ratio. Equation C.15 simplifies the calculation of the ALCR 

to an expression that is function of only measured pressures and the water temperature. Note 

that this form of the equation is only valid in the temperature range from 32 to 86 oF, in 

accordance with the limitations of the binomial fit for the viscosity ratio. If the temperature is 

outside of this range, the more general expression in Equation C.14 should be used.
Appendix C – Calculating the Air-Liquid Conversion Ratio

Membrane Filtration Guidance Manual C-10 June 2003

Proposal Draft

(460 )

527 (175 2.71 0.0137 )

2

TMP T

P T T

ALCR eff

• +

• Δ • − • + •

= Equation C.15

Where: ALCR = air-liquid conversion ratio (dimensionless)

ΔPeff = effective integrity test pressure (psi)

T = water temperature (oF)

TMP = transmembrane pressure (psi)

As with the Darcy and orifice models, the values for the parameters ΔPeff and TMP used 

in Equation C.14 or C.15 should be selected to yield a conservative value for the ALCR.
Membrane Filtration Guidance Manual June 2003

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Appendix D

Empirical Method for Determining the Air-Liquid Conversion 

Ratio for a Hollow-Fiber Membrane Filtration System
Membrane Filtration Guidance Manual D-1 June 2003

Proposal Draft

Appendix D:

Empirical Method for Determining the Air-Liquid Conversion Ratio for 

a Hollow-Fiber Membrane Filtration System

D.1 Introduction

As described in Chapter 4, the regulatory framework for the Long-Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR) requires that the flow through the smallest 

integrity breach that generates a measurable response from the direct integrity test (i.e., the 

critical breach size) be determined in order to establish the sensitivity of the test method. (Under 

the LT2ESWTR, sensitivity is defined as the maximum log removal value (LRV) that can be 

reliably verified by the direct integrity test (i.e., LRVDIT).) However, because most pressurebased direct integrity tests yield results in terms of airflow or pressure decay, it may be necessary 

to convert these results to an equivalent value for the flow of water through the critical breach 

under typical filtration conditions. Although there are a number of methods for converting a 

direct integrity test response to a corresponding flow of water, each can be generally categorized 

as one of two types of approaches: mathematical modeling or experimental determination. This 

appendix describes an empirical approach based on a parameter called the air-liquid conversion 

ratio (ALCR), which is defined as the ratio of air that would flow through a breach during a 

direct integrity test to the amount of water that would flow through the breach during filtration, 

as shown in Equation D.1 (also Equation 4.5):

















=

breach

air

Q

Q

ALCR Equation D.1 (4.5)

Where: ALCR = air-liquid conversion ratio (dimensionless)

Qair = flow of air through the critical breach during a pressurebased direct integrity test (L/min)

Qbreach = flow of water through the critical breach during filtration 

(L/min)

While Appendix C describes the hydraulic models that could be used to calculate the 

ALCR for various types of membrane filtration systems and under different flow regimes, this 

appendix provides an example of an empirical method that can be used to determine the ALCR 

for system using microporous hollow-fiber membranes based on bubble point theory – the 

correlated airflow measurement (CAM) technique. The CAM technique measures the flow of air 

and water through a worst-case fiber break scenario to empirically determine the ALCR of a 

membrane filtration unit. This method is specific to hollow-fiber membrane processes in which 

the geometry of the fiber and associated module is known. 

The test involves applying air pressure that is below the bubble point of the membrane to 

determine the airflow that corresponds to the worst-case broken fiber scenario. For most hollow-
Appendix D – Empirical Method for Determining the Air-Liquid Conversion Ratio for a Hollow-Fiber Membrane 

Filtration System

Membrane Filtration Guidance Manual D-2 June 2003

Proposal Draft

fiber systems, this condition occurs when the fiber is broken at the membrane pot, since it 

represents the shortest flow path for feed water to pass unfiltered to the filtrate side of the 

membrane. Thus, for any fiber of a given length and diameter, the flow of water through a 

breach during normal operation can be correlated to the flow of air via the integrity test pressure, 

as shown in Equation D.2: 

f (Qbreach ) = f (Qair ) = P Equation D.2

Where: f(Qbreach) = water flow correlation function (psi-min/mL)

Qbreach = flow of water through a broken fiber (mL/min)

f(Qair) = airflow correlation function (psi-min/mL)

Qair = flow of air through a broken fiber (mL/min)

P = pressure (psi)

The relationship between the air and water flow is determined through empirical 

determination of the water flow and airflow correlation functions – f(Qbreach) and f(Qair) –

respectively – which require the water flow and airflow through a broken fiber to be measured at 

various differential pressures. (A schematic illustrating an apparatus for measuring airflow using 

the CAM technique is shown in Figure D.1.) These relationships are mathematical functions that 

may be linear or non-linear in nature and are dependent upon the type of defect. The data 

obtained from these measurements may be fitted with respective equations to establish empirical 

relationships, as illustrated in Figure D.2. Since both functions are measurements of flow (either 

air or water) and pressure, this commonality can be used to establish a direct relationship 

between the flow of air to water, which can in turn be used to determine the ALCR. As 

illustrated in Figure D.2, one such relationship exists at the direct integrity test pressure (Ptest). 

However, it is also possible to determine the relationship between air and water flow at other 

conditions, as well, such as the integrity test pressure and the transmembrane pressure (TMP). 

The ALCR varies as a function of the TMP during operation, since a highe r TMP results in a 

greater flow of water through the defect and thus a lower ALCR. Note that the relationship 

between the airflow and water flow correlation functions assumes constant temperature, since 

changes in air or water temperature may change the functional relationships.
Appendix D – Empirical Method for Determining the Air-Liquid Conversion Ratio for a Hollow-Fiber Membrane 

Filtration System

Membrane Filtration Guidance Manual D-3 June 2003

Proposal Draft

Figure D.1 CAM Technique Airflow Measurement Apparatus

Figure D.2 Example Airflow and Water Flow Correlation Curves

Although the CAM technique for determining the ALCR empirically is more laborintensive than calculating the ALCR using a hydraulic model, the procedure does have several 

advantages. First, because the measurement is empirical, it is more accurate than calculations 

based on general hydraulic models. In addition, the CAM procedure does not rely on 

assumptions that are necessary to estimate the ALCR from hydraulic models, but instead 

facilitates direct determination of the ALCR based on measured air and water flows through a 

known defect. 
Appendix D – Empirical Method for Determining the Air-Liquid Conversion Ratio for a Hollow-Fiber Membrane 

Filtration System

Membrane Filtration Guidance Manual D-4 June 2003

Proposal Draft

D.2 Methodology

The following general procedure is provided as a guide for conducting the CAM 

technique for experimentally determining the ALCR: 

1. Determine the worst-case integrity breach scenario. For example, for a single-exit 

hollow-fiber module, the worst-case integrity breach is at the interface of the membrane 

fiber and the potting at the filtrate outlet end of the module. Modify a bench or fullscale module to artificially create this worst-case integrity breach.

2. Measure and characterize the water flow through the integrity breach (i.e., Qbreach) over 

the range of operating TMPs.

3. Develop an equation for a fitted curve that represents the water flow through the worstcase integrity breach as a function of TMP. This relationship is characterized by the 

parameter f(Qbreach) in Equation D.2.

4. Determine the minimum bubble point of the porous membrane material. (This 

information should generally be available from the manufacturer.)

5. Establish the direct integrity test pressure. As a general rule, the test pressure should be 

less than 80 percent of the bubble point pressure for the membrane and below the 

maximum TMP. However, the test pressure must be sufficient to meet the resolution 

requirement of the LT2ESWTR for the removal of Cryptosporidium. 

6. Determine the baseline integrity response for an integral membrane (see section 4.3.1.3 

for a discussion of diffusive flow through the wetted pores of an integral membrane).

7. If the diffusive flow is significant (i.e., approximately 10 percent of the total airflow), 

then lower test pressures should be evaluated (or the diffusive flow will have to be 

accounted for in determining the ALCR, as described in section 4.3.1.3).

8. Measure the airflow from the worst-case integrity breach at the intended test pressure 

(Ptest) during the pressure-based direct integrity test.

9. If Ptest is not known or is potentially variable, it may be necessary to measure airflow 

over a range of integrity test pressures and develop an empirical relationship to 

characterize the f(Qair) function. For example, if the hydrostatic backpressure is may 

vary between different direct integrity test applications, it may be advantageous to 

measure airflow at a variety of potential integrity test pressures.
Appendix D – Empirical Method for Determining the Air-Liquid Conversion Ratio for a Hollow-Fiber Membrane 

Filtration System

Membrane Filtration Guidance Manual D-5 June 2003

Proposal Draft

10. Determine the ALCR using Equation D.1. 

breach

air

Q

Q

ALCR =

Where: ALCR = air-liquid conversion ratio (dimensionless)

Qair = flow of air through a broken fiber at the direct 

integrity test pressure (mL/min)

Qbreach = flow of water through a broken fiber at the 

reference TMP (mL/min)

Note that the reference TMP described in association with the variable Qbreach above 

refers to the TMP that is used in the determination of the ALCR for the purpose of 

establishing the direct integrity test sensitivity for regulatory compliance, as described 

in section 4.3.1.2 and Appendix C. 

11. Calculate the direct integrity test method sensitivity (i.e., LRVDIT) using the test result 

(Qair) (either as directly measured with the diffusive airflow test or as converted from 

the pressure decay rate (ΔPtest) using Equation4.8 as measured with the pressure decay 

test) and the ALCR determined in the previous step, as described in Chapter 4.

D.3 Example: Using the CAM Method to Determine the ALCR

Scenario:

Airflow and water flow correlation curves have been established for a hollow fiber, as 

shown in Figure D.3. Determine the ALCR for a direct integrity test pressure of 15 psi and a 

reference TMP of 10 psi.

Solution:

Step 1: Determine the airflow (Qair) at the direct integrity test pressure from the f(Qair) curve in 

Figure D.3 (denoted as Point A).

Qair = 180 mL/min

Step 2: Determine the water flow (Qbreach) at the reference TMP from the f(Qbreach) curve in 

Figure D.3 (denoted as Point C).

Qbreach = 15 mL/min
Appendix D – Empirical Method for Determining the Air-Liquid Conversion Ratio for a Hollow-Fiber Membrane 

Filtration System

Membrane Filtration Guidance Manual D-6 June 2003

Proposal Draft

Step 3: Calculate the ALCR.

12

15 / min

180 / min

= = =

mL

mL

Q

Q

ALCR

breach

air

Figure D.3 Example ALCR Correlation
Membrane Filtration Guidance Manual June 2003

Proposal Draft

Appendix E

Application of Membrane Filtration for Virus Removal
Membrane Filtration Guidance Manual E-1 June 2003

Proposal Draft

Appendix E:

Application of Membrane Filtration for Virus Removal

E.1 Introduction

Although the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) only 

regulates the use of membrane filtration for the removal of Cryptosporidium for the purposes of 

compliance with the rule requirements, states could opt to apply the regulatory framework in a 

broader context to other applications of membrane filtration, at their discretion. For the removal 

of Giardia or other relatively large pathogens that are approximately the same size as 

Cryptosporidium in order-of-magnitude terms, the LT2ESWTR regulatory framework can be 

applied almost directly with only minor modifications, such as adjusting the direct integrity test 

resolution to coincide with the lower bound of the size range characterizing the pathogen of 

interest. However, for pathogens that are significantly smaller than Cryptosporidium, there are a 

number of important considerations that must be taken into account and which may limit the 

applicability of the LT2ESWTR regulatory framework in some cases. The purpose of this 

appendix is to summarize some of the critical issues associated with the application of membrane 

filtration for the removal of very small pathogens using the LT2ESWTR regulatory framework. 

Because viruses represent the smallest class of pathogen and are directly regulated under the 

Surface Water Treatment Rule (SWTR), this appendix is primarily focused on the application of 

membrane filtration for virus removal; however, it is important to note that these same issues 

may generally apply to other small pathogens of comparable size, as well. 

Among the five classes of membrane filtration discussed in the Membrane Filtration

Guidance Manual – MF, UF, nanofiltration (NF), reverse osmosis (RO), and membrane cartridge 

filtration (MCF) – only MF, UF and MCF are typically applied specifically for the purpose of 

pathogen (or other particulate) removal. However, because the nominal pore sizes of the types 

of membranes associated with these processes vary widely (UF = 0.01 μm; MF = 0.1 μm; 

MCF = 1 μm or smaller), their removal characteristics are different. While all three processes 

can remove Cryptosporidium, with a size range lower bound of about 3 μm, only UF membranes 

have pores small enough to filter viruses, which generally range in size from about 0.01 to 

0.1 μm. Thus, UF is the primary membrane filtration technology utilized for the objective of 

virus reduction. Accordingly, the discussion in this appendix is primarily presented in the 

context of virus removal using UF. Nevertheless, note that while MF cannot generally be used 

as an effective means of virus treatment, it may be applied to remove organisms such as some 

species of bacteria that are larger than 0.1 μm but still as much as an order of magnitude smaller

than Giardia or Cryptosporidium. In these cases, some of the issues addressed in the appendix 

may be applicable to MF. It is important to note that although some virus removal by MF has 

been reported in the literature, it is generally attributed to formation of a cake layer on the 

membrane surface. Since this cake layer is dynamic and removed by the backwash process, the 

removal of particulate matter via this mechanism varies during the course of an operational cycle 

and thus is not consistent with the LT2ESWTR framework, which considers only the removal 

efficiency of the membrane barrier itself.
Appendix E – Application of Membrane Filtration for Virus Removal

Membrane Filtration Guidance Manual E-2 June 2003

Proposal Draft

While NF and RO utilize non-porous semi-permeable membranes that represent a barrier 

to viruses, the associated modules are generally not manufactured to be aseptic. In addition, 

because these membranes are not able to be backwashed, particulate matter can cause rapid 

irreversible fouling. As a result, NF and RO are not typically applied to directly treat raw water 

supplies with significant concentrations of suspended solids. However, NF or RO may be 

directly applied (i.e., without significant pretreatment for particulate removal) to remove 

dissolved solids from a groundwater source that is subject to virus contamination but very low in 

suspended solids. In such cases, NF and RO may be used to obtain virus reduction credits under 

the Ground Water Rule (GWR). Under these circumstances, the issues addressed in this 

appendix may also be applicable to NF or RO. 

In general, application of the LT2ESWTR regulatory framework for the removal of 

viruses necessitates that the membrane filtration process comply with appropriate pathogenspecific criteria for the three primary regulatory elements: challenge testing, direct integrity 

testing, and continuous indirect integrity monitoring. The subsequent sections of this appendix 

address some of the practical and regulatory issues with respect to virus removal for each of 

these three program elements. An additional preceding section summarizes current state 

regulatory policy for the use of UF for virus removal and highlights some associated 

considerations for the application of the LT2ESWTR regulatory framework.

E.2 Overview of Current Regulatory Policy

Because there is no general federal framework for the broad regula tion of membrane 

filtration as a water treatment technology, states have developed policies that are widely varying 

in some cases. However, most states are consistent in allowing very little virus removal credit 

for membrane filtration. While numerous challenge studies have demonstrated a clear difference 

in the ability of MF and UF to remove viruses (as shown in Table E.1), a 2001 survey of state 

primacy agencies conducted by USEPA indicated that only 6 of 29 states with regulatory policies 

specific to membrane filtration technology differentiated between these two processes in terms of 

the virus removal credit awarded, as summarized in Table E.2 (USEPA 2001).

The most significant factor limiting the virus removal credit awarded to UF is the 

infeasibility of using current direct integrity test methods to detect a virus-sized breach (as 

discussed in section E.3). Thus, it is possible that a number of very small integrity breaches 

could allow the passage of viruses through the membrane barrier undetected, contaminating the 

filtrate. While this mode of failure may not be as common as a broken fiber, such a very small 

integrity breach may occur as the membranes age or as a result of exposure to incompatible 

treatment chemicals.
Appendix E – Application of Membrane Filtration for Virus Removal

Membrane Filtration Guidance Manual E-3 June 2003

Proposal Draft

Table E.1 Virus Removal Studies Using MF and UF

Researcher(s) Year Process Log Removal

Jacangelo et al. 1997 MF 0 – 2.4

Wilinghan et al. 1992 MF 0.3 – 4

Schneider et al. 1999 MF 1.1

Trussel et al. 1998 MF 0.4 – 3.2

Jacangelo et al. 1997 UF 6.0 – 7.9

Dwyer et al. 1994 UF 6.2 – 6.81

Trussel et al. 1998 UF > 6.91

Kruithof et al. 1999 UF > 5.4

1 Removal to levels below detection limit

Table E.2 Summary of State Virus Removal Credit for MF and UF

Virus Log Number of States

Removal Credit MF UF

No standard credit18 8

0 18 14

0.5 3 1

1.0 - 1

1.5 - -

2.0 - -

2.5 - -

3.0 - 1

3.5 - -

4.0 - 4

1 Credit typically awarded on a case-by-case basis

Although this rigorous degree of testing and performance verification is generally not 

required of conventional treatment (e.g., daily testing to ensure the absence of short-circuiting in 

media filters), the combined processes of flocculation/sedimentation and media filtration 

represent a multiple contaminant barrier that provides some degree of insurance against difficultto-detect treatment deficiencies (such as small integrity breaches or short-circuiting). The 

removal credits awarded to conventional treatment under the SWTR account for this multiple 
Appendix E – Application of Membrane Filtration for Virus Removal

Membrane Filtration Guidance Manual E-4 June 2003

Proposal Draft

barrier treatment concept, while the log removal credits referenced for UF in Tables E.1 and E.2 

represent membrane filtration as a stand-alone process. Consequently, is possible that a multibarrier treatment process scheme including flocculation, sedimentation, and UF (or other 

membrane filtration process) could be awarded equivalent or greater virus removal credit than 

conventional treatment by some states, at their discretion.

In addition, because viruses are easily inactivated by low chlorine doses and contact 

times that are unlikely to yield significant disinfection byproducts (DBPs), there is less of a 

necessity for states to award high virus removal credits to any alternative treatment process if 

some level of primary disinfection is required. By contrast, states generally award removal 

credit on par with that for conventional treatment for both MF and UF for larger pathogens such 

as Giardia and Cryptosporidium, which have more robust chlorine inactivation requirements 

relative to viruses and which are large enough such that similarly sized integrity breaches can be 

detected using common direct integrity test techniques (as discussed in section E.3). Stateawarded log removal credits for both Giardia and Cryptosporidium using MF/UF (which are 

considered comparable technologies for Giardia and Cryptosporidium removal) are summarized 

in Table E.3 for the same 29 states with specific membrane filtration policies (USEPA 2001). 

Table E.3 Summary of State Giardia and Cryptosporidium Removal Credit for 

MF/UF

Virus Log Number of States

Removal Credit Giardia Cryptosporidium

No standard credit18 22

0 - -

0.5 - -

1.0 1 -

1.5 1 -

2.0 2 4

2.5 7 -

3.0 9 2

3.5 - -

4.0 1 1

1 Credit typically awarded on a case-by-case basis

In awarding virus removal credit to UF, states must balance the studies that consistently 

demonstrate the ability of UF to achieve 5 to 7 log virus removal with the practical problem of 

verifying virus-sized integrity breaches during operation (as discussed in section E.3) and the 

potential impact that such small integrity breaches might have on the ability of UF to achieve the 

awarded credit. Another consideration is the emphasis that many states place on multiple barrier 

protection. In cases in which chemical disinfection is utilized as a component of multiple barrier 
Appendix E – Application of Membrane Filtration for Virus Removal

Membrane Filtration Guidance Manual E-5 June 2003

Proposal Draft

treatment, the level of virus inactivation generally negates the need to award virus removal 

credits to UF. As shown in Table E.2, most states currently consider the issues of integrity 

verification and/or multiple barrier protection more significant factors, since few states award UF 

any virus removal credit as a stand-alone process. However, even without significant stateawarded removal credit, utilities may still specifically opt to install UF as a barrier against 

viruses on the basis of the many studies indicating near complete virus removal and that even 

with some very small and undetectable integrity breaches UF may remove viruses more 

efficiently than either MF or conventional treatment.

E.3 Direct Integrity Testing

In terms of direct integrity testing, a membrane process applied to obtain virus removal 

under the LT2ESWTR regulatory framework would need to meet applicable criteria for 

sensitivity, resolution, and frequency. While applying the LT2ESWTR framework for virus 

removal is similar to that for Cryptosporidium for sensitivity and frequency of direct integrity 

testing (since both of these concepts are independent of the target pathogen), the resolution that 

would be necessary for very small pathogens such as viruses is more problematic. Because 

viruses range in size from approximately 0.01 – 0.10 μm, the resolution must be a very small 

0.01 μm – two orders of magnitude smaller than that for Cryptosporidium – in order to provide 

an appropriate level of conservatism. A resolution requirement this small may be very difficult 

to achieve with currently available direct integrity tests.

A virus-sized resolution requirement is particularly problematic for pressure-based direct 

integrity tests, which are based on bubble point theory (as described in Appendix B). A form of 

the bubble point equation is shown as Equation E.1, which relates the required direct integrity 

test pressure to the size of an integrity breach. 

fect

test d

P

0.58 •k •s • cosq

= Equation E.1

Where: Ptest = required test pressure (psi)

0.58 = constant including applicable unit conversion factors 

κ = pore shape correction factor (dimensionless) 

s = surface tension at the air-liquid interface (dynes/cm) 

θ = liquid-membrane contact angle (degrees)

dfect = defect diameter (μm) 

In the context of determining a minimum necessary test pressure, the defect diameter is 

equal to the required resolution (i.e., the lower bound on the size range of the pathogen of 

interest). Table E.4 summarizes the required direct integrity test pressure as a function of the 

target pathogen size (i.e., the test resolution) for two sets of conditions. In the most conservative 

case, the pore shape correction factor (κ) is equal to 1, and the liquid-membrane contact angle (θ) 

is equal to zero (indicating a perfectly hydrophilic membrane). These respective values should 
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be used as a default in the absence of more specific information about the true values of these 

two parameters. The second case utilizes less conservative values of κ = 0.25 and θ = 45o to 

illustrate the affect that these two parameters can have on the required test pressure. Both cases 

assume a conservative surface tension (s ) of 74.9 dynes/cm (i.e., at a temperature of 5 oC). Note 

that Table E.4 is logarithmic, citing pathogen sizes from 0.01 to 0.1 μm in 0.01 μm increments, 

and then continuing to 1.0 μm in 0.1 μm increments.

Table E.4 Required Direct Integrity Test Pressure as a Function of Target 

Pathogen Size (i.e., Resolution)

Required Test Pressure

Target (psi)

Pathogen 

Size

(mm)

Most Conservative

k = 1

q = 0o

Less Conservative

k = 0.25

q = 45o

0.01 4,344 768

0.02 2,172 384

0.03 1,448 256

0.04 1,086 192

0.05 869 154

0.06 724 128

0.07 621 110

0.08 543 96

0.09 483 85

0.1 434 77

0.2 217 38

0.3 145 26

0.4 109 19

0.5 87 15

0.6 72 13

0.7 62 11

0.8 54 10

0.9 48 9

1.0 43 8

As shown in Table E.4, for the most conservative case, the required test pressure for a 

virus-sized resolution 0.01 μm is over 4,000 psi, a value far in excessive of what any current, 

commercially available water treatment membrane could withstand without rupturing. Even the 

strongest hollow-fiber membrane can withstand a transmembrane pressure (TMP) of no more 

than 100 psi at most, limiting the smallest target pathogen (and thus the resolution) to about 

0.4 μm in the most conservative case and perhaps just below 0.1 μm if more specific, less 

conservative values of κ and θ are known. Other small pathogens that are larger than viruses 
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(e.g., bacteria in the 0.5 – 1.0 μm range) require lower test pressures and may allow more 

feasible application of LT2ESWTR regulatory framework. In addition, some manufacturers 

have experimented with stronger ceramic “membrane” processes with pore sizes similar to MF 

or UF, which may allow the application of direct integrity tests at higher pressures, thus 

facilitating the use of the LT2ESTWR regulatory framework for smaller pathogens than are 

possible with current polymeric membrane materials. Note that Table E.4 does not account for 

the system backpressure during the application of a pressure-based direct integrity test, which 

can increase the required test pressure to achieve a target resolution, as described in Equation 4.1 

in Chapter 4.

Resolution requirements on the order of virus also present some problems for the use of 

marker-based direct integrity tests. Suitable virus-sized challenge particulates may be 

prohibitively expensive, making daily testing infeasible. In addition, it may be difficult to 

manufacture these particulates to meet acceptable tolerances for size range variation. Thus, even 

if the practical problems associated with virus-sized direct integrity test resolution could be 

overcome and significant virus removal credit would be awarded by the state under the 

LT2ESWTR regulatory framework, it may be substantia lly less expensive for a utility using UF 

to apply a small amount of chlorine to inactivate viruses. In this case, because the achievement 

of virus removal credit using UF is an issue of operational verification and not removal 

efficiency as demonstrated in challenge studies, the utility would have the benefit of a multiple 

barrier treatment process even if no removal credit is awarded to UF by the state.

E.4 Challenge Testing

As shown in Table E.1, a number of studies conducted in recent years have demonstrated 

the ability of UF to achieve high log removals of viruses. While it is not necessary to use the 

pathogen of interest itself for the purposes of conducting a challenge test in accordance with the 

LT2ESWTR regulatory framework, it is important to ensure that any challenge particulate 

selected is conservatively (i.e., equivalently or less efficiently) removed (as discussed in 

section 3.9). 

One significant consideration in regard to challenge testing under the LT2ESWTR 

regulatory framework as applied to virus removal is the issue of representative performance 

testing to verify the removal efficiency of all membrane modules in a product line that are not 

subject to challenge testing. In order to demonstrate Cryptosporidium removal efficiency, all 

modules must be subjected to non-destructive performance testing that is consistent with the 

appropriate resolution requirement, as discussed in section 3.6. The results of this testing are 

compared to the results of similar testing conducted on the representative modules in the same 

product line that were subject to challenge modules as an indicator of performance. Common 

non-destructive performance tests (NDPTs) are types of direct integrity tests, such as the bubble 

test (see section4.8.2). However, as discussed in section E.3, because pressure-based direct 

integrity tests may not be able to achieve the required 0.01 μm resolution requirement for 

viruses, non-destructive performance testing may not be possible. Thus, it may be necessary to 

develop different criteria and methods for validating the virus removal capability of modules in a 

product line that are not subjected to challenge testing, such as representative destructive 
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performance testing on a statistically significant number of modules in each production lot. One 

example of such a destructive performance test might be a scanning electron microscopy (SEM) 

analysis of the membrane media to confirm the pore size distribution.

E.5 Continuous Indirect Integrity Monitoring

Although the LT2ESWTR regulatory framework does not specify any particular 

resolution or sensitivity criteria for continuous indirect integrity monitoring, related issues may 

impact the effectiveness of indirect integrity monitoring if the framework is applied to virus 

removal. For example, very small integrity breaches that permit the passage of viruses may not 

allow enough particulate matter across the membrane to be detected by indirect methods such as 

turbidity monitoring or particle counting. Thus, it may be difficult to establish a meaningful 

control limit for continuous indirect integrity monitoring that is specific to the application of 

membrane filtration for virus removal. As a result, continuous indirect integrity monitoring 

should be used as a general gauge of gross membrane integrity that is independent of the 

particular pathogen of concern.