IWC 11-2

72nd ANNUAL INTERNATIONAL WATER CONFERENCE (IWC)

RECOVERY AND RECYCLING OF 

INDUSTRIAL SIDE-STREAM 

WASTEWATER

Michael Chan, P.E. 

Duraflow, LLC

Tewksbury, MA

KEYWORDS: Industrial wastewater, side-stream wastewater, difficult-to-treat, recycle, reuse, 

chemical softening, pre-treatment, membrane, microfiltration, reverse osmosis 

ABSTRACT

Most industrial operations generate a substantial quantity of “side-stream” wastes. These streams 

are characterized by their high concentrations and difficult-to-treat property. Recent studies have 

identified side-stream wastes as a major cause of performance problem to treatment operations. 

As the national water scarcity crisis escalates, industries have started looking into recycling of 

the less desirable water sources as a long-term sustainable solution to the problem. This 

presentation shall explore the technical challenges associated with side-streams management.

INTRODUCTION

It is well documented that water is a 

scarce resource in many parts of the world 

including in the U.S. In some areas water 

availability is the key issue while in others, 

water quality is the major problem. Some 

locations, unfortunately, have to cope with 

both quality and quantity issues. In response 

to these growing concerns, industry has been 

challenged to find ways to utilize poorer 

quality water, such as the side-stream

wastes, which in general is not being 

considered a viable source in the past.

For most industrial water and 

wastewater treatment operations, a 

substantial quantity of side-stream wastes is 

generated from the manufacturing and 

water/wastewater treatment processes as byproducts of the operations. These streams 

are characterized by high concentrations in

total dissolved solids (TDS), suspended 

particles, difficult-to-treat chemicals and 

organic substances.

SIDESTREAM SOURCES

As shown in Figure 1, side-stream 

wastes are generated from three primary 

sources: (1) Manufacturing operations, (2) 

Process water supply treatment operations, 

and (3) Wastewater treatment or recycling 

operations. The types of side-streams from 

these sources vary according to the industry, 

production processes, technology employed 

and method of operation. As a general 

characteristic, side-streams have high 

instantaneously flow rate but small average 

volume relative to the main stream, and they

contain high concentrations of various 

contaminants as indicated in Table 1.
IWC 11-2

Table 1 – Side-stream Contaminants

Side-stream Contaminants

Reverse Osmosis Brine High TDS, hardness, 

silica, and metals

Filter/Screen Backwash High TSS

Cooling Tower 

Blowdown

High TDS, TSS, 

hardness, silica, and 

organic

Equipment Wash-down Complex cleaning 

chemicals, and trace 

heavy metals 

Scrubber Blow-down High chloride, sulfate, 

heavy metals and 

organic

Ion Exchange 

Regeneration Wastes

High TDS, TSS, 

hardness

Boiler Blowdown High TDS, metals and 

complex chemicals

IMPACT ON WWTP

Most of the traditional wastewater 

treatment plant (WWTP) combines the sidestream wastes with the main wastewater

stream for treatment. It was reported that the 

side-stream wastes cause significant 

interferences, malfunctions and damages, in 

many cases, to the treatment operation. The 

degree of impact on the WWTP will depend 

largely on factors such as the flow volume,

surge frequency, chemical characteristic, 

TSS concentration, and mixing ratio and 

blending method for the side-streams. The 

following examples were documented for 

some of the major impacts observed and 

reported.

 Foul the RO membrane pre-maturely 

and cleaning the membrane is difficult.

 Foul or bind the activated carbon and 

sand filter medium, and increase the 

backwash frequency and volume.

 Interfere with the coagulation reaction 

and cause discharge compliance issues.

 Cause settling difficulties in clarifier 

resulting in carry-over of pin flocs in the 

overflow.

 Induce watery sludge cake generated 

from the de-watering process.

SIDE-STREAM RECYCLING 

TECHNOLOGY

Reverse Osmosis (RO) - RO 

membranes are designed for removal of 

dissolved solids with high efficiency, but 

adversely affected or fouled by suspended 

solids, colloidal material or scale. Common 

examples of such foulants are calcium 

precipitates, metal oxides, colloidal silica 

and various organics. Industrial side-stream

wastewater typically contains most, if not 

all, of these fouling substances. Once fouled, 

limited by its membrane material properties, 

only mild cleaning chemicals such as citric 

acid and detergent can be used to restore the 

flux. Stronger or more effective cleaning 

Process Water 

Treatment

Production Operation

WW Treatment or 

Recycle

Figure 1 – Industrial Side-stream Sources

Industrial Side-streams

 RO Brine (Reject)

 Filter/Screen Backwash

 IX Regenerant

 Boiler Blowdown

 De-watering Filtrate / Centrate

 Cooling Tower Blowdown

 Plant & Equip. Washdown

 Scrubber Blowdown

Water 

Recycle

Water Supply
IWC 11-2

chemicals, such as sulfuric acid, 

hydrochloric acid, bleach, and peroxide 

cannot be applied as they will cause 

irreversible damages to the RO membranes. 

In wastewater recycling applications, RO 

can rarely function independently without 

any protection from the fouling materials. 

Appropriate pretreatment must be provided 

to achieve stable performance of RO 

membranes.

Microfiltration (MF) – Certain MF products 

have demonstrated the ability to handle 

concentrated wastewater and significantly 

reduce membrane fouling to provide stable, 

predictable RO performance. The success of 

MF for this application can be attributed to 

the following key reasons:

 MF membrane is designed to remove 

suspended solids and colloidal particles

(RO foulants). Some of the MF products, 

such as the one with tubular 

configuration, can handle very high TSS 

of >5,000 mg/L in the influent. 

 MF membrane can be made of a variety 

of polymeric materials, including PVDF, 

which exhibit strong resistance to 

concentrated chemicals. As a result, the 

membrane can be cleaned with mineral 

acids, oxidizers (bleach, peroxide), 

caustic and selected organic solvents for 

removal of different persistent fouling 

elements, both inorganic and organic, 

which are difficult to be removed with 

weak cleaning chemicals.

 MF filtration produces a quality product 

water stream with NTU (<1.0) and SDI 

(<3.0) values in full compliance with the 

feed water criteria specified by all RO 

manufacturers. 

 The chemical or physical characteristic 

of the foulants in the wastewater can be 

converted into particles suitable for 

removal by MF filtration.

 The MF can be designed to remove 

regulated contaminants in the sidestream wastes and make the RO reject 

more manageable for recovery or 

discharge.

Chemical pre-treatment plays a key role in 

membrane-based treatment or recycle. 

Properly designed and controlled chemistry 

mitigates the effects of the fouling 

components on membrane because it 

induced a change in the characteristics of the 

foulants. The principle pre-treatment 

chemistries for different contaminants are 

presented in Table 2.

Table 2 – Pre-treatment Chemistry 

Examples

Contaminants MF Pre-treatment 

Chemistry

Hardness

(Ca & Mg)

 Chemical softening - Lime 

& soda ash

Heavy Metals  Hydroxide precipitation –

Caustic/lime

 Sulfide precipitation –

Sodium sulfide

 Organo-sulfur 

precipitation – Sodium 

Dithiocarbamate (DTC)

Fluoride  De-fluoridation –

Calcium/aluminum salts

Sulfate  Sulfate precipitation - lime 

and aluminum salt.

Silica  Chemical adsorption –

Magnesium salt

Oil & Grease  Adsorption – Lime / 

activated carbon

 Acid Cracking – Sulfuric 

acid

Organic  Adsorption – Lime / 

activated carbon

 Coagulation –

Aluminum/iron salts

 Oxidation destruction –

Bleach / Peroxide
IWC 11-2

CASE STUDY

SIDE-STREAMS RECYCLING

Project Data

Plant Type/Location

An automotive components manufacturers

located in Mexico.

Background

(1) There was severe shortage of quality 

water around the plant site. 

(2) Performance of the main WWPT plant 

was unstable and subject to frequent 

upsets due to high variation of the 

influent characteristic. 

Project Objective

Segregate the difficult-to-treat side-streams, 

specifically the cooling tower blowdown 

and RO brine, for water recycling and 

improvement of the central WWTP 

operation.

Years of Operation

Over 8 years.

Side-stream Flow

100 to 150 GPM.

Recycle Concept

Selected side-streams, RO brine, cooling 

tower blowdown and others from the 

production processes and main wastewater 

treatment plant are directed to an 

equalization tank. The process starts with 

chemical softening / coagulation and 

followed by microfiltration and RO. The RO 

permeate is returned to the cooling tower for 

reuse, and the reject stream is discharge to 

an on-site evaporation pond for de-watering.

Design Considerations

The side-streams contain a variety of 

membrane foulants including hardness 

(Calcium and Magnesium), silica, organic 

(Anti-scalant & Dispersants) and total 

suspended solids (TSS). Fouling control 

plays a critical role in the success of 

operating a membrane-based recycling

system. The Fouling Mitigation Technique 

(FMT) employed consists of Chemical 

Softening for converting all foulants into 

compatible forms followed by MF for 

separation of the resulting suspended solids.

The MF membranes employed in this 

installation are manufactured in a tubular 

configuration designed to handle high solid 

concentration. The membranes, made of 

PVDF, are cast on the surface of porous 

polymeric tubes to produce a nominal pore 

size of 0.1 micron. Figure 2 illustrates the 

MF membrane modules installed in the 

system. Bleach and hydrochloric acid are 

typically used for membrane cleaning.

 

The chemically pre-treated wastewater is 

processed through the MF membrane 

modules designed for separation of the

precipitates from water. The wastewater is

pumped at a velocity of 12 – 15 ft/sec 

through the membrane modules connected 

in series as shown in Figure 3. The 

turbulent flow, parallel to the membrane 

surface, produces a high-shear scrubbing 

action which minimizes deposition of solids 

on the membrane surface. During operation, 

filtrate permeates through the membrane, 

while the suspended solids retained in the 

re-circulation loop are periodically purged 

for further de-watering. An automatic backpulse mechanism provides surface cleaning 

by periodically reversing the filtrate flow 

direction. The MF has been operated with an 

average flux of 300 GFD. The entire 

treatment process is depicted in Figure 4.

Figure 2

MF Module End View

Figure 3

MF System Configuration
IWC 11-2

Membrane Process Performance Data

The removal efficiency for selected 

elements in the MF filtrate and RO permeate 

is presented in Table 3.

Table 3 – Selected Membrane Process Performance Data

Selected

Elements

Influent

(mg/L as ion)

MF Filtrate

(mg/L as ion)

RO Permeate 

(mg/L as ion)

Ca 150 – 300 <35.0 <1.0

Mg 100 – 250 <5.0 <0.5

SiO2 100 - 200 <10.0 <4.0

Na 900 – 1,200 <1200 <60

SO4 1,000 – 1,500 <1500 <40

Cl 500 - 900 <900 <45

TSS 250 – 1,500 <1.0 ND

TDS 3,000 – 4,000 <4,000 <200

SDI ---- <2.0 ----

Figure 4 – Simplified Side-stream Wastewater Recycling Process Flow Diagram

Reverse Osmosis Tubular Microfultration

Manufacturing Plant

 Side-stream EQ

 Process Water RO Brine

 Cooling Tower Blowdown

 Filter/Screen Backwash

 De-watering Filtrate

 Plant & Equip. Washdown

Two-Stage 

Chemical Softening & 

Coagulation

Central Wastewater 

Treatment & 

Recycling Plant

RO Permeate To 

Cooling Tower

Softening Coagulants

Chemicals Selected Sidestreams

Effluent

Discharge

RO Reject to 

Evaporation Pond

Sludge Slurry to

Central De-watering

Side-stream 

Recycle System

Main WW

Recycle Stream

Water
IWC 11-2

CONCLUSION

Side-stream wastewater is known to cause 

adverse impact to the operation and 

performance of a conventional wastewater

treatment system. Most of the side-streams 

can be clearly defined, but for large 

industrial plant operations, some of them 

will require thorough site survey to track 

and identify. The removal of side-stream 

wastes from the main wastewater stream has 

a two-fold advantage: (1) Improve and 

optimize the performance of the main 

treatment system, and (2) maximize the inhouse resource available for recycle and 

reuse. Membrane based technology

(MF/RO) is proven as a technically viable 

approach. Chemical pre-treatment that 

transforms wastewater into membranecompatible form provides the needed 

flexibility to cope with the complicated and 

fast-changing side-streams chemistry. To 

assure a successful side-stream control 

project from initial design to final operation, 

careful evaluation of potential sources of 

reuse water is needed and detailed treatment 

processes must be tailored to the site 

specific needs of a particular plant.

 

REFERENCE

1. Cheryan, Munir, 1998, Ultrafiltration

and Microfiltration Handbook, 2nd

edition, CRC Press, Boca Raton, 

Florida, pp. 237 – 254.

2. Duraflow, 2005 – 2011, Internal

Technical Files.

3. EPA, December 1985, Sidestreams in

Advanced Wastewater Treatment 

Plants: Problems and Remedies, pp. 

3 - 4.

4. Hydranautics, January 2001, 

Pre-treatment, Technical Service 

Bulletin.