Municipal Nutrient Removal Technologies 

Reference Document 

EPA 832-R-08-006, September 2008

This document was prepared by the following staff from the Ann Arbor, Michigan, and 

Fairfax, Virginia, offices of Tetra Tech, Inc., under U.S. Environmental Protection Agency 

(EPA) Contract EP-C-05-046. 

Shin Joh Kang, Ph.D., P.E., Principal Author 

Kevin Olmstead, Ph.D., P.E., Author 

Krista Takacs, P.E., Author 

James Collins, Author and Project Manager 

Acknowledgements 

EPA Office of Wastewater Management staff and the authors would like to acknowledge the 

efforts of the following EPA-state project workgroup members: 

Dan Murray ─ EPA, ORD 

David Pincumbe ─ EPA Region 1 

Dave Ragsdale ─ EPA Region 10 

Denny Rowland ─ CT, DEP 

Gary Johnson ─ CT, DEP (formerly) 

Dr. Ta-Shan Yu ─ MD, MDE

The authors appreciate the review and feedback on an early draft of the document that was 

conducted by the EPA Regions and also by several state permitting and water quality 

agencies, as coordinated by the Association of State and Interstate Water Pollution Control 

Agencies. 

In addition, a formal technical review of the draft document was conducted by professionals 

with experience in wastewater treatment in accordance with EPA Peer Review Guidelines. 

While every effort was made to accommodate all of the Peer Review comments, the results 

and conclusions do not indicate consensus and may not represent the views of all the 

reviewers. The technical reviewers of this document included the following: 
Ms. Jeanette Brown, Executive Director, Water Pollution Control Authority, Stamford, CT 

Sudhir Murthy, Ph.D., P.E., Manager for Process Development, D.C. Water and Sewer 

Authority (DCWASA), Washington, DC 

J.B. Neethling, Ph.D., PE, BECC, Senior Vice president, HDR, Inc., Folsom, CA 

Amit Pramanik, Ph.D., BCEEM, Senior Program Director, Water Environment Research 

Foundation, Alexandria, VA 

Clifford W. Randall, Ph.D., Professor, Virginia Polytechnic Institute and State University, 

Blacksburg, VA 

H. David Stensel, Ph.D., PE, Professor, University of Washington, Seattle, WA 

Printed copies of this document are printed with vegetable-based ink on paper that contains a 

minimum of 50 percent post-consumer fiber content and is chlorine free. 

Electronic copies of this document can be downloaded from EPA’s Office of Wastewater 

Management web site at: www.epa.gov/owm

Cover photo credits: 

Photo (lower left) courtesy of Enviroquip, Inc., Austin. Texas. 

Photo (upper right) courtesy of Natural Resources Conservation Service 

Photo inserts credit: Tetra Tech, Inc. 
Preface 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting 

the nation’s land, air, and water resources. Under a mandate of environmental laws, the 

Agency strives to formulate and implement actions leading to a balance between human 

activities and the ability of ecosystems to support and sustain life. To meet this mandate, the 

Office of Wastewater Management (OWM) provides information and technical support to 

solve environmental problems today and to build a knowledge base necessary to protect 

public health and the environment well into the future. This publication was prepared under 

contract to EPA, by Tetra Tech, Inc. The document provides current state of development as 

of the publication date; however, it is expected that this document will be revised 

periodically to reflect advances in this rapidly evolving area. Except as noted, information, 

interviews, and data development were conducted by the contractor. While there are many 

proven, cost-effective nutrient removal technologies and numerous new technologies or 

modifications of existing technologies available to detailed study, the case studies in this 

document were selected on the basis of specific criteria. The criteria included the ability to 

provide as least one year of full-scale operating and performance data, capability of 

providing detailed capital and operation and maintenance cost breakdowns, and the ability to 

provide the data within the time frame established for completing the document. It is 

anticipated that as the document is updated, additional case studies on new technologies 

could be included. Some of the information, especially related to emerging technologies, was 

provided by the manufacturer or vendor of the equipment or technology and could not be 

verified or supported by a full-scale case study. In some cases, cost data were based on 

estimated savings without actual field data. When evaluating technologies, estimated costs, 

and stated performance, efforts should be made to collect current and more up-to-date 

information. 

The mention of trade names, specific vendors, or products does not represent an actual or 

presumed endorsement, preference, or acceptance by EPA or the federal government. Stated 

results, conclusions, usage, or practices do not necessarily represent the views or policies of 

EPA. 
How to use this document 

EPA is providing this reference document to make information available on recent advances 

in nutrient removal technology and practices. The goal of this document is straightforward – 

to provide information that will assist local decision makers and regional and state regulators 

plan cost-effective nutrient removal projects for municipal wastewater treatment facilities. 

Volume 1 provides key information on technologies, case studies, and capital and O&M 

costs for retrofitting or expanding existing facilities. 

Chapter 1 provides a brief history of nutrient removal in the United States which sets the 

stage for the detailed presentation of the current status of nutrient removal practices and 

costs. 

Chapter 2 includes detailed information on commonly used biological and physiochemical 

nutrient removal technologies and preliminary information on emerging phosphorus removal 

and side stream nitrogen removal processes. It presents detailed technical and cost 

information about both biological and physiochemical treatment technologies. The technical 

information includes detailed process descriptions and operating factors for more than 40 

different treatment alternatives for removing nitrogen, phosphorus, or both from municipal 

wastewater streams. The information also includes data on process performance and 

reliability that were developed from full-scale operating data obtained voluntarily from 30 

wastewater treatment plants throughout the United States and in Canada. This extensive 

analysis allows decision makers to evaluate full-scale performance data obtained from 

specific facilities. 

Chapter 3 provides a synthesis of the information generated from the nine fully scale case 

studies. The case studies facilities represented a variety of technologies in both cold- and 

warm-weather locations were the subject of in-depth discussion of the factors involved in 

successful process design and operation, as well as a detailed process cost analysis. The full 

case study reports are provided in Volume 2 of this document. 

Chapter 4 contains information on general cost estimates for many of the available nutrient 

removal technologies for both retrofits and expansion of existing facilities. The accuracy of 

the cost estimate will vary depending on the level of detail provided in the evaluation. If the 

cost estimate is based on cost curves or costs from similar facilities or technologies with very 

little consideration of local conditions, the cost estimate might be accurate to within only 

approximately 50 percent. If more detailed studies such as soil borings, preliminary 

engineering design drawings, and draft specifications are prepared, the estimate will be more 

accurate. 

Chapter 5 presents a set of considerations and an approach for planning for process upgrades 

that includes projecting future loads, assessing existing capabilities, preparing a mass balance 

that includes all return and recycle flows and loads, developing the needed expansion and 

upgrade that should incorporate flexibility into the operation of the plant to account for future 
uncertainties, evaluating feasible alternatives, and selecting the recommended plan. Chapter 

5 also presents a list of technologies capable of meeting the selected target effluent range for 

nitrogen, phosphorus, or both and technology selection factors to be considered in identifying 

and evaluating feasible technologies on the basis of design and operational and cost factors. 

Volume 2, Appendix A contains detailed case study reports for each of the nine facilities 

evaluated as part of this project. The objective of the case studies was to present the data 

from selected technologies for a one-year period; to identify the factors that contribute to the 

reliability of nitrogen and phosphorus removal; to identify the factors that contribute to the 

costs of various removal technologies; and to evaluate the reliability of nutrient removal 

through a simple, yet sound statistical method by which performance data could be presented 

and compared. 

Volume 2, Appendix B provides the technical and statistical basis of using coefficient of 

variation to describe performance reliability. 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

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Contents 

Volume 1 – Technical Report 

Executive Summary 

Chapter 1. Introduction and History 

1.1 Overview ......................................................................................................1-1 

1.2 Background of Nitrogen and Phosphorus Removal in the United States.....1-2 

1.2.1 Lake Tahoe .....................................................................................1-3 

1.2.2 Great Lakes.....................................................................................1-4 

1.2.3 The Occoquan Reservoir and the Chesapeake Bay.......................1-6 

1.3 NPDES Permitting........................................................................................1-8 

1.3.1 Watershed-based Permitting and Water Quality Trading................1-8 

1.4 References .................................................................................................1-10 

Chapter 2. Treatment Technologies and Technology Matrix 

2.1 Overview ......................................................................................................2-1 

2.2 Nitrogen Removal Processes.......................................................................2-1 

2.2.1 Nitrogen Species in Wastewater .....................................................2-1 

2.2.2 Nitrogen Removal Factors ..............................................................2-3 

2.2.3 Nitrogen Removal Technologies .....................................................2-9 

2.3 Phosphorus Removal Processes ...............................................................2-27 

2.3.1 Biological Phosphorus Removal ...................................................2-27 

2.3.2 Chemical Phosphorus Removal....................................................2-31 

2.3.3 Phosphorus Removal Technologies .............................................2-35 

2.4 Nitrogen and Phosphorus Removal Processes..........................................2-48 

2.4.1 Nitrogen and Phosphorus Removal Factors .................................2-48 

2.4.2 Nitrogen and Phosphorus Removal Technologies........................2-50 

2.5 Full-Scale Nutrient Removal Process Cases..............................................2-57 

2.5.1 Nitrogen Removal Matrix and Variability Data...............................2-57 

2.5.2 Phosphorus Removal Matrix and Variability Data.........................2-66 

2.5.3 Combined Nitrogen and Phosphorus Removal Matrix and 

Variability Data ..............................................................................2-74 

2.6 Summary ....................................................................................................2-77 

2.6.1 Performance and Variability ..........................................................2-77 

2.6.2 Nitrogen Removal Technologies ...................................................2-80 

2.6.3 Phosphorus Removal Technologies .............................................2-80 

2.6.4 Combined Nitrogen and Phosphorus Removal Technologies.......2-80 

2.7 References .................................................................................................2-81 

Attachment 1: Locations Providing Data .............................................................2-91 

Chapter 3. Case Studies and Reliability Factors 

3.1 Introduction and Overview............................................................................3-1 

3.1.1 Permit Limits for the Case Study Facilities......................................3-1 

3.2 Summary of Case Studies............................................................................3-4 

3.2.1 Total Nitrogen and Phosphorus Removal at Low Concentration 

Limits (3 mg/L or less in TN) ...........................................................3-4 
Municipal Nutrient Removal Technologies Reference Document September 2008 

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3.2.2 Total Nitrogen and Phosphorus Removal at Mid-Level 

Concentration Limits (3 to 6 mg/L)................................................ 3-15 

3.2.3 Phosphorus Removal Low Level (less than 0.1 mg/L).................. 3-24 

3.3 Reliability Factors....................................................................................... 3-29 

3.3.1 Wastewater Characteristics .......................................................... 3-29 

3.3.2 Fermenter and VFA Generation.................................................... 3-30 

3.3.3 Bioreactor Design and Process Parameters................................. 3-31 

3.3.4 Secondary Sludge Thickening ...................................................... 3-32 

3.3.5 Sludge Digestion........................................................................... 3-32 

3.3.6 Recycle Flows and Loads ............................................................. 3-32 

3.3.7 Wet-Weather Flow Management .................................................. 3-33 

3.3.8 Tertiary Filters ............................................................................... 3-34 

3.3.9 Tertiary Clarifier............................................................................. 3-34 

3.4 Cost Factors............................................................................................... 3-37 

3.4.1 Capital Costs................................................................................. 3-37 

3.4.2 Operation and Maintenance Costs................................................ 3-40 

3.4.3 Unit Costs...................................................................................... 3-41 

3.5 Summary.................................................................................................... 3-43 

3.5.1 Discharge Permits......................................................................... 3-43 

3.5.2 Phosphorus Removal.................................................................... 3-44 

3.5.3 Nitrogen Removal ......................................................................... 3-46 

3.5.4 Costs for Capital and O&M ........................................................... 3-48 

3.6 References................................................................................................. 3-49 

Chapter 4. Cost Factors 

4.1 Modifying Existing Facilities ......................................................................... 4-1 

4.1.1 Literature Review ............................................................................ 4-1 

4.1.2 Case Studies................................................................................... 4-3 

4.2 Retrofit Process Cost Models....................................................................... 4-4 

4.2.1 CAPDETWorks ............................................................................... 4-5 

4.2.2 Retrofit Phosphorus Removal Technologies................................... 4-8 

4.2.3 Retrofit Nitrogen Removal Technologies ...................................... 4-12 

4.2.4 Combined Nitrogen and Phosphorus Scenarios........................... 4-16 

4.3 Expansion Process Cost Models................................................................ 4-19 

4.3.1 Phosphorus Removal Technologies ............................................. 4-19 

4.3.2 Nitrogen Removal Technologies................................................... 4-24 

4.3.3 Combined Nitrogen and Phosphorus Scenarios........................... 4-29 

4.4 Discussion of Cost Factors......................................................................... 4-33 

4.5 Summary.................................................................................................... 4-34 

4.6 References................................................................................................. 4-35 

Chapter 5. Upgrading Existing Facilites

5.1 General Approach to Upgrading................................................................... 5-1 

5.1.1 Success Criteria.............................................................................. 5-1 

5.1.2 Facility Planning.............................................................................. 5-2 

5.2 Available Technologies ................................................................................ 5-2 

5.3 Technology Selection Criteria ...................................................................... 5-6 

5.3.1 Nitrogen Removal ......................................................................... 5-11 

5.3.2 Phosphorus Removal Plant Factors.............................................. 5-14 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

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5.3.3 Nitrogen and Phosphorus Removal Plant Factors ........................5-18 

5.4 Design and Operational Factors in Nutrient Removal ................................5-18 

5.4.1 Influent Wastewater Characteristics..............................................5-21 

5.4.2 Sources of Biodegradable Carbon ................................................5-23 

5.4.3 Impact of Wet-Weather Flows.......................................................5-24 

5.4.4 Managing Sludge-Handling Processes .........................................5-25 

5.4.5 Recycle Flows ...............................................................................5-26 

5.4.6 SCADA Requirements and Sensors .............................................5-29 

5.4.7 Staffing Requirements...................................................................5-29 

5.4.8 Training Needs..............................................................................5-29 

5.4.9 Pilot Testing ..................................................................................5-30 

5.5 Finalizing Process Selection ......................................................................5-30 

5.6 Summary ....................................................................................................5-32 

5.7 References .................................................................................................5-34 

Volume 2 - Appendices 

Appendix A – Case Studies 

Denitrification 

Central Johnston County, NC 

Lee County, FL 

Phosphorus removal 

Kalispell, MT 

Clark County, NV 

Nitrogen and phosphorus removal 

Kelowna, BC 

Marshall Street in Clearwater, FL 

Noman Cole in Fairfax County, VA 

North Cary, NC 

Western Branch in Upper Marlboro, MD 

Appendix B – Reliability and Coefficient of Variation 

Figures 

Figure 2-1. Denitrifying filter process. ................................................................................2-10 

Figure 2-2. Biological upflow filter. .....................................................................................2-11 

Figure 2-3. Modified Ludzack-Ettinger process..................................................................2-12 

Figure 2-4. Cyclically aerated activated-sludge process....................................................2-12 

Figure 2-5. Four-stage Bardenpho process. ......................................................................2-13 

Figure 2-6. Oxidation ditch process. ..................................................................................2-14 

Figure 2-7. Integrated fixed-film activated sludge process.................................................2-16 

Figure 2-8. Moving-bed biofilm reactor process.................................................................2-17 

Figure 2-9. MBR process. (a) External filter in lieu of clarifier;...........................................2-18 

Figure 2-9. MBR process. (b) In-tank filter.........................................................................2-19 

Figure 2-10. Step-feed activated sludge process...............................................................2-20 

Figure 2-11. Biodenitro process.........................................................................................2-21 

Figure 2-12. Schreiber countercurrent aeration process....................................................2-22 
Municipal Nutrient Removal Technologies Reference Document September 2008 

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Figure 2-13. Sequencing batch reactor..............................................................................2-23 

Figure 2-13A. InNitri process. ............................................................................................2-24 

Figure 2-13B. BABE process. ............................................................................................2-25 

Figure 2-13C. SHARON process .......................................................................................2-26 

Figure 2-13D. ANAMMOX process....................................................................................2-27 

Figure 2-14. Fermentation process....................................................................................2-35 

Figure 2-15. A/O (Phoredox) process. ...............................................................................2-36 

Figure 2-16. Oxidation ditch with anaerobic zone..............................................................2-37 

Figure 2-17. Phosphorus chemical/filter process...............................................................2-38 

Figure 2-18. Parkson Dynasand D2 advanced filter system..............................................2-44 

Figure 2-19. CoMag process. ............................................................................................2-46 

Figure 2-20. Blue-PRO process.........................................................................................2-47 

Figure 2-21. U.S. Filter Trident process.............................................................................2-47 

Figure 2-22. A2O process..................................................................................................2-50 

Figure 2-23. Five-stage modified Bardenpho process. ......................................................2-51 

Figure 2-24. University of Cape Town process..................................................................2-52 

Figure 2-25. Modified University of Cape Town process. ..................................................2-53 

Figure 2-26. Virginia Initiative process...............................................................................2-54 

Figure 2-27. Johannesburg process. .................................................................................2-54 

Figure 2-28. Biodenipho (phased isolation ditch) process. ................................................2-55 

Figure 2-29. Blue Plains process. ......................................................................................2-56 

Figure 2-30. Westbank process (Kelowna, British Columbia)............................................2-57 

Figure 2-31. Monthly average frequency curves for TN: low-range removal. ....................2-60 

Figure 2-32. Concentric oxidation ditch. ............................................................................2-63 

Figure 2-33. Monthly average frequency curves for TN—mid-range removal. ..................2-65 

Figure 2-34. Monthly average frequency curves for ammonia nitrogen.............................2-66 

Figure 2-35. Monthly average frequency curves for TP—low-end removal.......................2-70 

Figure 2-36. Monthly frequency curves for TP removal—mid-range removal....................2-73 

Figure 3-1. Western Branch WWTP: Monthly frequency curves for TP...............................3-5 

Figure 3-2. Western Branch WWTP: Monthly frequency curves for ammonia nitrogen.......3-6 

Figure 3-3. Western Branch WWTP: Monthly frequency curves for TN...............................3-7 

Figure 3-4. Lee County, Florida: Monthly frequency curves for TP. ....................................3-8 

Figure 3-5. Lee County, Florida: Monthly frequency curves for TN. ....................................3-8 

Figure 3-6. Johnston County, North Carolina: Monthly average frequency curves for 

TP.................................................................................................................................3-10 

Figure 3-7. Johnston County, North Carolina: Monthly average frequency curves for 

ammonia nitrogen. .......................................................................................................3-10 

Figure 3-8. Johnston County, North Carolina: Monthly average frequency curves for 

TN. ...............................................................................................................................3-11 

Figure 3-9. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average 

frequency curves for TP...............................................................................................3-13 

Figure 3-10. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average 

frequency curves for ammonia nitrogen.......................................................................3-13 

Figure 3-11. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average 

frequency curves for TN...............................................................................................3-14 

Figure 3-12. North Cary, North Carolina: Monthly average frequency curves for TP. .......3-15 

Figure 3-13. North Cary, North Carolina: Monthly average frequency curves for 

ammonia nitrogen. .......................................................................................................3-16 

Figure 3-14. North Cary, North Carolina: Monthly average frequency curves for TN. .......3-16 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

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Figure 3-15. Kelowna, British Columbia: Monthly average frequency curves for TP. ........3-18 

Figure 3-16. Kelowna, British Columbia: Monthly average frequency curves for 

ammonia nitrogen. .......................................................................................................3-19 

Figure 3-17. Kelowna, British Columbia: Monthly average frequency curves for TN.........3-19 

Figure 3-18. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly 

average frequency curve for TP...................................................................................3-21 

Figure 3-19. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly 

average frequency curve for ammonia nitrogen...........................................................3-22 

Figure 3-20. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly 

average frequency curve for TN...................................................................................3-22 

Figure 3-21. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly 

average frequency curve for TP...................................................................................3-25 

Figure 3-22. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly 

average frequency curve for TN...................................................................................3-26 

Figure 3-23. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly 

average frequency curve for ammonia nitrogen...........................................................3-26 

Figure 3-24. Kalispell, Montana: Monthly average frequency curve for TP. ......................3-28 

Figure 3-25. Kalispell, Montana: Monthly average frequency curve for ammonia 

nitrogen. .......................................................................................................................3-28 

Figure 3-26. Kalispell, Montana: Monthly average frequency curve for TN. ......................3-29 

Figure 4-1. O&M costs for retrofit phosphorus removal technologies ($/MG treated)........4-11 

Figure 4-2. Capital costs for retrofit phosphorus technologies ($ per gpd capacity). .........4-11 

Figure 4-3. Life-cycle costs for retrofit phosphorus removal technologies ($/MG 

treated).........................................................................................................................4-12 

Figure 4-4. Capital costs for retrofit nitrogen removal scenarios........................................4-14 

Figure 4-5. O&M costs for retrofit nitrogen removal scenarios...........................................4-15 

Figure 4-6. Life-cycle costs for retrofit nitrogen removal scenarios....................................4-15 

Figure 4-7. Capital costs for retrofit nitrogen plus phosphorus removal technologies. ......4-17 

Figure 4-8. O&M costs for retrofit nitrogen plus phosphorus removal technologies. .........4-18 

Figure 4-9. Life-cycle costs for retrofit nitrogen plus phosphorus removal technologies. ..4-18 

Figure 4-10. O&M costs for expansion phosphorus removal technologies ($/MG 

treated).........................................................................................................................4-22 

Figure 4-11. Capital costs for expansion phosphorus removal technologies.....................4-22 

Figure 4-12. Life-cycle costs for expansion phosphorus removal technologies.................4-23 

Figure 4-13. Component percentages of total O&M costs for 1 MGD expansion 

phosphorus removal technologies. ..............................................................................4-23 

Figure 4-14. Component percentages of total O&M costs for 10 MGD expansion 

phosphorus removal technologies. ..............................................................................4-24 

Figure 4-15. Capital costs for expansion nitrogen removal scenarios. ..............................4-26 

Figure 4-16. O&M costs for expansion nitrogen removal scenarios. .................................4-27 

Figure 4-17. Life-cycle costs for expansion nitrogen removal scenarios. ..........................4-27 

Figure 4-18. Component percentages of total O&M costs for 1 MGD expansion 

nitrogen removal technologies. ....................................................................................4-28 

Figure 4-19. Life-cycle costs for expansion nitrogen plus phosphorus removal 

technologies. ................................................................................................................4-28 

Figure 4-20. Component percentages of total O&M costs for 10 MGD expansion 

nitrogen removal technologies. ....................................................................................4-29 

Figure 4-21. Capital costs for expansion nitrogen plus phosphorus removal 

technologies. ................................................................................................................4-31 
Municipal Nutrient Removal Technologies Reference Document September 2008 

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Figure 4-22. O&M costs for expansion nitrogen plus phosphorus removal technologies. .4-31 

Figure 4-23. Life-cycle costs for expansion nitrogen plus phosphorus removal 

technologies.................................................................................................................4-32 

Figure 4-24. Component percentages of total O&M costs for 1 MGD expansion 

nitrogen plus phosphorus removal technologies..........................................................4-32 

Figure 4-25. Component percentages of total O&M costs for 10 MGD expansion 

nitrogen plus phosphorus removal technologies..........................................................4-33 

Tables 

Table ES-1. Cost estimates for retrofit technologies.............................................................14 

Table ES-2. Cost estimates for expansion technologies for 10 MGD...................................20 

Table 2-1. Process Performance Data: Nitrogen Removal—Plant Effluent.......................2-58 

Table 2-2. Detailed curve information for Case Study Facilities in Figure 2-31 .................2-61 

Table 2-3. Detailed curve information for Case Study Facilities in Figure 2-33 .................2-65 

Table 2-4. Detailed curve information for Case Study Facilities in Figure 2-34 .................2-66 

Table 2-5. Process Performance Data: Phosphorus Removal—Plant Effluent .................2-67 

Table 2-6. Detailed curve information for Case Study Facility in Figure 2-35....................2-70 

Table 2-7. Detailed curve information for Case Study Facilities in Figure 2-36 .................2-73 

Table 2-8. Process Performance Data: Nitrogen and Phosphorus Removal— 

Plant Effluent................................................................................................................2-78 

Table 3-1. Discharge permit limits and performance data summary ...................................3-2 

Table 3-2. Statistical comparison of Canadian and U.S. permit limits .................................3-4 

Table 3-3. Monthly variation of wastewater characteristics at Fairfax County, Virginia .....3-30 

Table 3-4. Case study facilities’ treatment processes........................................................3-35 

Table 3-5. Case study facilities’ capital costs .......................................................................38 

Table 4-1. Upgrade costs for Maryland, Connecticut, and others........................................4-2 

Table 4-2. Upgrade costs for case studies ..........................................................................4-3 

Table 4-3. Cost factors used in CAPDETWorks estimates..................................................4-6 

Table 4-4. Comparison of CAPDETWorks and actual costs for Clearwater, Florida, and 

Kalispell, Montana..........................................................................................................4-7

Table 4-5. Cost model influent wastewater parameters.......................................................4-8 

Table 4-6. Process parameters for phosphorus removal scenarios ..................................4-10 

Table 4-7. Process parameters for nitrogen removal scenarios ........................................4-13 

Table 4-8. Process parameters for nitrogen removal scenarios ........................................4-16 

Table 4-9. Process parameters for phosphorus removal scenarios ..................................4-20 

Table 4-10. Process parameters for nitrogen removal scenarios ......................................4-25 

Table 4-11. Process parameters for combined nitrogen and phosphorus removal 

scenarios......................................................................................................................4-30 

Table 5-1. Process list: TP...................................................................................................5-3 

Table 5-2. Process list: TN...................................................................................................5-4 

Table 5-3. Process list: ammonia nitrogen...........................................................................5-4 

Table 5-4. Process list: TN plus TP .....................................................................................5-5 

Table 5-5. Technology selection matrix: nitrogen removal ..................................................5-7 

Table 5-6. Technology selection matrix: phosphorus removal.............................................5-8 

Table 5-7. Technology selection matrix: nitrogen and phosphorus removal........................5-9 

Table 5-8. Decision matrix example 1................................................................................5-31 

Table 5-9. Decision matrix example 2................................................................................5-32 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

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Acronyms/Abbreviations 

Al3 Aluminum sulfate (or Alum) 

ANAMMOX anaerobic ammonia oxidation 

ASCE American Society of Civil Engineers 

A/O anoxic/oxic 

A2/O anaerobic/anaerobic/oxic 

AS activated sludge 

AT 3 Aeration Tank 3 process 

BABE bio-augmentation batch enhanced 

BAF biological aerated filter 

BAR bioaugmentation reaeration 

BASIN biofilm activated sludge innovative nitrifiation 

BNR biological nutrient removal 

BOD biochemical oxygen demand 

BOD-to-TKN biochemical oxygen demand-to-total Kjeldahl nitrogen ratio

BOD-to-TP biochemical oxygen demand-to-total phosphorus ratio 

BPR biological phosphorus removal 

CANON completely autotrophic nitrogen removal over nitrite 

CAS cyclic activated sludge 

CBOD carbonaceous biochemical oxygen demand 

COD chemical oxygen demand 

COV coefficient of variation 

CP central plant 

CWSRF Clean Water State Revolving Fund 

DAF dissolved-air flotation unit 

DO dissolved oxygen 

DON dissolved organic nitrogen 

EBPR enhanced biological phosphorus removal 

ENR Engineering News-Record
Municipal Nutrient Removal Technologies Reference Document September 2008 

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EPA Environmental Protection Agency 

FeCl3 ferric chloride 

FFS fixed-film systems 

GAO glycogen accumulating organism 

GPD gallons per day 

HDWK headworks 

HCO3

-

 bicarbonate 

H2CO3 carbonic acid 

HRT hydraulic retention time 

IFAS integrated fixed-film activated sludge 

I&C instrumentation and control 

kWh/year kilowatt-hours per year 

MAUREEN mainstream autotrophic recycled enhanced N-removal 

MBBR moving-bed biofilm reactor 

MBR membrane bioreactor 

MG million gallons 

MGD million gallons per day 

mg/L milligrams per liter (equivalent to parts per million) 

LE modified Ludzack-Ettinger 

MLSS mixed liquor suspended solids 

MW molecular weight 

N nitrogen 

NH4 ammonium 

NH4-N ammonia nitrogen 

NL no limit 

NPDES National Pollutant Discharge Elimination System 

OLAND oxygen limited aerobic nitrification-denitrification 

ORP oxidation-reduction potential 

O&M operation and maintenance 

PAO phosphate accumulating organisms 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

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PID phased isolation ditch 

PLC programmable logic controller 

PO4

3- phosphate 

POTWs publicly owned treatment works 

RAS return-activated sludge 

rbCOD readily biodegradable carbonaceous oxygen demand 

rDON refractory dissolved organic nitrogen 

SCADA supervisory control and data acquisition 

SBR sequencing batch reactor 

SE secondary effluent 

SND simultaneous nitrification and denitrification 

SF subsurface flow 

SHARON single reactor high-activity ammonia removal over nitrate 

SRT solids retention time 

TMDL total maximum daily load 

TKN total Kjeldahl nitrogen 

TN total nitrogen 

TP total phosphorus 

TRPA Tahoe Regional Planning Agency 

TSS total suspended solids 

UCT University of Cape Town 

UV ultraviolet 

VFA volatile fatty acids 

VIP Virginia Initiative process 

VSS volatile suspended solids 

WAS waste activated sludge 

WEF Water Environment Federation 

WERF Water Environment Research Foundation 

WQBEL water quality-based effluent limit 

WWTP wastewater treatment plant 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-1

Executive Summary 

One of the major concerns regarding constituents in municipal wastewater treatment plant 

discharges is the concentration of nutrient compounds, in particular nitrogen and phosphorus. 

Nutrients stimulate the growth of microorganisms (including algae) and other aquatic 

vegetation in receiving waters, leading to decreased oxygen levels. Excess nutrients are a 

significant water quality concern in many of the nation’s waters and a leading cause of 

impairment of designated uses. Wastewater treatment plants that employ conventional 

biological treatment processes designed to meet secondary treatment effluent standards 

typically do not remove total nitrogen (TN) or total phosphorus (TP) to an extent sufficient to 

protect certain receiving waters. Wastewater treatment facilities are increasingly being 

required to address this issue by implementing treatment processes that reduce effluent 

nutrient concentrations to levels that regulators deem sufficiently protective of the 

environment. Such implementation usually involves retrofitting the plant to enhance the 

biological treatment processes or to include chemical treatment to effect phosphate 

precipitation. The challenge for those facilities, however, is to determine which treatment 

alternatives will best meet their needs, both technically and financially, and to make the 

decision that is most sustainable. 

Purpose of this Document 

This reference document includes technical information developed to assist municipal 

decisionmakers and regional and state regulators in planning for nutrient removal from 

municipal wastewater streams. Consequently, it is not intended to be a design manual for use 

by engineers in generating design parameter values or drawings. It presents detailed technical 

and cost information about both biological and physiochemical treatment technologies. The 

technical information includes detailed process descriptions and operating factors for more 

than 40 different treatment alternatives for removing nitrogen, phosphorus, or both from 

municipal wastewater streams. The information also includes data on process performance 

and reliability that were developed from full-scale operating data obtained voluntarily from 

30 wastewater treatment plants throughout the United States and in Canada. This extensive 

analysis allows decisionmakers to evaluate full-scale performance data obtained from 

specific facilities. Nine of the facilities were the subject of in-depth case studies that further 

examined the factors involved in successful process design and operation, as well as process 

cost analysis. Case study summaries are provided in Volume II, Appendix A. 

Cost information for various technologies was also developed from literature sources, as well 

as from the facilities contacted for the case studies for this document. Capital and operation 

and maintenance (O&M) cost estimates in 2007 dollars were determined for the nine case 

study facilities. In addition, capital, O&M, and life-cycle costs were estimated for 12 retrofit 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-2 Executive Summary 

and 20 expansion alternatives using CAPDETWorks software (Hydromantis Corporation, 

Ontario, Canada). 

Content of Document 

The document has five chapters. Chapter 1 provides a brief introduction and history of 

nutrient removal at wastewater treatment plants. Chapter 2 provides an overview of more 

than 40 alternative technologies that are available for providing nitrogen and phosphorus 

removal in municipal wastewater treatment. Nitrogen removal technologies are based on 

biological nitrification-denitrification because that is the generally preferred method for 

removing nitrogen. Both chemical and biological methods for phosphorus removal are 

described. The technologies discussed range in complexity from one-point chemical addition 

for phosphorus precipitation to a 5-stage Bardenpho system for combined biological 

phosphorus and nitrogen removal. The descriptions include process configurations, factors 

important in design and operation, and observed ranges of effluent concentrations. Chapter 3 

of the document summarizes important findings of the case studies and associated 

technologies. Information on capital operations, maintenance, and life-cycle costs is provided 

in Chapter 4. Finally, Chapter 5 presents information about upgrading existing facilities for 

those who are evaluating the use of nutrient removal technologies. 

Technologies, Performance, and Reliability 

Chapter 2 provides operating results from 30 full-scale treatment facilities. Full-scale 

operating data were obtained from these facilities on a voluntary basis and analyzed for the 

reliability of nitrogen and phosphorus removal, as applicable under the facility’s National 

Pollutant Discharge Elimination System (NPDES) permit or, in some cases, where facilities 

voluntarily achieved removal results above and beyond the NPDES requirements. Most of 

the facilities are throughout the United States; one is in Canada. It should be emphasized that 

the performance data for these facilities reflect differences in operating philosophy, permit 

limitations, temperature, influent conditions, flow conditions, and the relative plant load 

compared to design. Thus, the documented performance does not necessarily represent 

optimum operation of the technologies presented. 

Common Statistical Base 

Performance data from the facilities are presented in Tables 2-1, 2-5, and 2-8 for nitrogen 

removal, phosphorus removal, and combined nitrogen plus phosphorus removal, 

respectively. In general, performance was affected by both the selected technology and the 

permit limit for each substance. The data presented include the range of monthly average 

effluent concentrations observed at the facility, as well as concentrations corresponding to 

the statistically derived values for the annual average, maximum month, maximum week, and 

maximum day, as data were available. These concentrations were derived by plotting the 

monthly average effluent concentrations for a year in ascending order on probability paper. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-3

The annual average corresponded to the average, or the 50 percent probability, while the 

monthly maximum corresponded to the 92 percent probability. The slopes of these reliability 

curves corresponded to the reliability, or variability as defined statistically: lower slopes 

meant that the process as implemented at that facility would produce more consistent effluent 

parameter values compared to processes implemented elsewhere with higher slopes. These 

slopes are related to the coefficient of variation (COV) for the data set, which is defined as 

one standard deviation divided by the mean. To attain the same target effluent concentrations 

(e.g., meet the same NPDES limits), treatment processes with a low (e.g., 20 percent) COV 

are considered more reliable, whereas those with a high (e.g., 60 percent) COV are 

considered less reliable, because they are more variable. Appendix B, in Volume II of this 

document, presents detailed information about this technique. COV alone might not be 

sufficient to evaluate the performance of treatment processes. 

Performance of Technologies 

Technologies are available to reliably attain an annual average of 0.1 milligram per liter 

(mg/L) or less for TP and 3 mg/L or less for TN. Reliability curves were developed for the 9 

case studies and for 21 other facilities for which suitable data were acquired from plants that 

removed TN, TP, and both TN and TP. The reliability of the plants that were required to 

remove ammonia nitrogen is also included. 

Nitrogen Removal 

The nitrogen removal processes evaluated for this document all employ biological 

nitrification of ammonia nitrogen and organic nitrogen under aerobic conditions. Most of the 

systems also employ biological denitrification under anoxic conditions. Table 2-1 presents 

effluent TN concentrations reported by 19 facilities. The performances were grouped into 

three categories: (1) high effluent nitrogen (annual average concentrations above 5 mg/L 

TN); (2) medium effluent nitrogen (annual average concentrations between 3 and 5 mg/L); 

and (3) low effluent nitrogen (annual average concentrations below 3 mg/L TN). Reliability 

curves are presented in Chapter 2, Figures 2-31 and 2-33, for low and medium effluent TN, 

respectively. 

The following seven systems reported annual average concentrations above 5 mg/L: 

• Johannesburg process, Hagerstown, Maryland, full-year data, 7.86 mg/L, COV 

21 percent 

• Virginia initiative process (VIP), literature report, 6.12 mg/L, no COV available 

• Step-feed activated-sludge (AS) process, Cumberland, Maryland, full-year data, 6.7 

mg/L, COV 27 percent 

• Anerobic/anox/oxic (A2

O) process, literature report, 7.3 to 9.0 mg/L, no location or 

COV available 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-4 Executive Summary 

• Schreiber process, literature report, 8 mg/L, no location or COV available 

• Blue Plains process, as done at the Blue Plains facility in Washington, D.C., 7.5 

mg/L, no COV available 

• Step-feed AS, Fairfax County, Virginia, case study, 5.25 mg/L, 12 percent COV 

Six processes were found to generate annual average effluent TN between 3 and 5 mg/L: 

• Cyclic on-off (operational air adjustment), Ridgefield, Connecticut, full-year data, 

4.59 mg/L, COV 25 percent 

• Sequencing batch reactor, Thomaston, Connecticut, full-year data, 4.59 mg/L, 

50 percent COV 

• Modified Ludzack-Ettinger (MLE), Westminster, Maryland, full-year data, 4.35 

mg/L, 23 percent COV 

• Westbank process, Kelowna, British Columbia (Canada), case study, 4.38 mg/L, 

12 percent COV 

• Phased isolation ditches (PIDs), Jewett City, Connecticut, full-year data, 4.2 mg/L, 42 

percent COV 

• Biological anoxic filters, Cheshire, Connecticut, two-thirds-year data, 3.6 mg/L, 

62 percent COV 

Seven processes were found to produce annual average effluent TN levels below 3 mg/L: 

• Integrated fixed-film activated sludge (IFAS), literature, 2.8 mg/L (low observed 

value), no location or COV available 

• Concentric ring oxidation ditch, Hammonton, New Jersey, full-year data, 3 mg/L, 

32 percent COV 

• Step-feed AS, Piscataway, Maryland, full-year data, 2.58 mg/L, 57 percent COV 

• 5-stage Bardenpho, Clearwater, Florida–Marshall Street facility, case study, 

2.32 mg/L, 16 percent COV 

• 5-stage Bardenpho, Clearwater, Florida–Northeast facility, full-year data, 2.04 mg/L, 

42 percent COV 

• Denitrification filter, Central Johnston County, North Carolina, case study, 2.14 

mg/L, 16 percent COV 

• Denitrification filter, Lee County, Florida, case study, 1.71 mg/L, 28 percent COV 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-5

• 3-sludge process including denitrifying AS, Western Branch, Maryland, case study, 

1.6 mg/L, 36 percent COV 

Phosphorus Removal 

Table 2-5 in Chapter 2 presents effluent TP concentration reported by 17 facilities. Most 

systems employed chemical treatment to effect part or all of the TP removal, except as noted. 

The performances were grouped into two categories––low effluent phosphorus (annual 

average concentrations between 0.1 and 0.5 mg/L TP) and very low effluent phosphorus 

(annual average concentrations below 0.1 mg/L TP). The corresponding groups of reliability 

curves are presented in Chapter 2, Figures 2-35 and 2-36, for very low and low effluent TP, 

respectively. 

The following six systems reported producing an annual average effluent TP between 0.1 and 

0.5 mg/L: 

• VIP, literature report, 0.4 mg/L, no COV available 

• University of Cape Town (UCT) process with filter, Penticton, British Columbia, 

literature report, 0.3 mg/L, no COV available 

• Anoxic/oxic (A/O) process (no chemical and no filter), Genesee County, Michigan, 

full-year data, 0.24 mg/L, 50 percent COV 

• The Westbank process with fermenter and filter, Kelowna, British Columbia, case 

study, 0.139 mg/L, 12 percent COV 

• A2O with volatile fatty acid (VFA), chemical, tertiary clarifier, and filter, Durham, 

Oregon, full-year data, 0.132 mg/L, 33 percent COV 

• Modified UCT with fermenter and filter, Kalispell, Montana, case study, 0.12 mg/L, 

19 percent COV 

Eleven systems reported producing an annual average effluent TP at or below 0.1 mg/L: 

• A/O with filter, Clark County, Nevada, case study, 0.1 mg/L, 30 percent COV 

• PhoStrip with filter, Truckee Meadows, Nevada, literature report, < 0.1 mg/L, no 

COV available 

• Oxidation ditch with denitrification filter with alum, Lee County, Florida, case study, 

0.098 mg/L, 47 percent COV 

• Chemical addition with flocculating clarifier, Chelsea, Michigan, full-year data, 

0.09 mg/L, 14 percent COV 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-6 Executive Summary 

• Step-feed AS with fermenter and filter, Fairfax County, Virginia, case study, 

0.09 mg/L, 21 percent COV 

• Membrane bioreactor, Hyrum, Utah, full-year data, 0.07 mg/L, 107 percent COV 

• Chemical addition with tertiary clarifier and filter, McMinnville, Oregon, seasonal 

(6 months) data, 0.058 mg/L, 63 percent COV 

• 5-Stage Bardenpho with chemical and filter, Pinery, Colorado, 0.031 mg/L, 

34 percent COV 

• Membrane bioreactor, Lone Tree Creek, Colorado, 0.027 mg/L, 27 percent COV 

• Enhanced biological phosphorus removal with chemical addition and filter, 

Breckenridge, Colorado, literature report, 0.01 mg/L, no COV available 

• Chemical addition with tertiary filter and infiltration basin, Brighton, Michigan, full 

year data, 0.01 mg/L, 0 percent COV 

Combined Nitrogen and Phosphorus Removal 

Table 2-8 in the document presents effluent TN and TP concentrations reported by 12 

facilities. Most systems employed chemical treatment to effect part or all of the TP removal, 

except as noted. The facilities were divided into three groups: those with TN greater than 5 

mg/L; those with TN less than 5 mg/L and TP between 0.1 mg/L and 0.5 mg/L; and those 

with TN less that 5 mg/L and TP less than 0.1 mg/L. The reliability curves for nitrogen 

removal for these facilities are included in Chapter 2, Figures 2-31 and 2-33; the reliability 

curves for phosphorus removal for these facilities are included in Figures 2-35 and 2-36. 

The following five systems reported an annual average TN over 5 mg/L and variable TP: 

• UCT, literature report, 8.9 to 10 mg/L TN, TP and location unspecified, COV not 

available 

• IFAS, literature report, Broomfield, Colorado, 5.6 to 11.3 mg/L TN, 0.2 to 1.7 mg/L 

TP, COV not available 

• VIP, literature report, 3 to 10 mg/L TN, 0.19 to 5.75 mg/L TP, location unspecified, 

COV not available 

• VIP with VFA addition, literature report, 5 to 10 mg/L TN, 0.6 to 0.8 mg/L TP, COV 

not available 

• Modified UCT with VFA addition, McDowell Creek, North Carolina, literature 

report, 5 to 6 mg/L TN, 0.1 to 2.7 mg/L TP, COV not available 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-7

Five systems reported annual average TN levels less than 5 mg/L and annual average TP 

levels between 0.1 and 0.5 mg/L: 

• Biodenipho/PID, North Cary, North Carolina, case study, 3.55 mg/L TN, 0.31 mg/L 

TP, 26 percent COV (TN), 87 percent COV (TP) 

• Three-stage AS system with denitrifying sludge and filter, Western Branch, 

Maryland, case study, 1.63 mg/L TN, 0.43 mg/L TP, 36 percent COV (TN), 

62 percent COV (TP) 

• 5-Stage Bardenpho with chemical addition and filter, Clearwater, Florida, Marshall 

Street facility, case study, 2.32 mg/L TN, 0.11 mg/L TP, 16 percent COV (TN), 

64 percent COV (TP) 

• 5-Stage Bardenpho with chemical addition and filter, Clearwater, Florida, Northeast 

facility, full-year data, 2.04 mg/L TN, 0.20 mg/L TP, 42 percent COV (TN), 

82 percent COV (TP) 

• Denitrification filter with chemical addition, Central Johnston County, North 

Carolina, case study, 2.14 mg/L TN, 0.26 mg/L TP, 16 percent COV (TN), 60 percent 

COV (TP) 

Two systems reported an annual average TN less than 5 mg/L and an annual average TP less 

than 0.1 mg/L: 

• Step-feed AS with fermenter and filter, Piscataway, Maryland, full-year data, 

2.59 mg/L TN, 0.09 mg/L TP, 57 percent COV (TN), 89 percent COV (TP) 

• Denitrification filter with chemical addition, Lee County, Florida, Fiesta Village 

facility, case study, 1.38 mg/L TN, 0.098 mg/L TP, 40 percent COV (TN), 47 percent 

COV (TP) 

Performance Factors for Design and Operation 

Key factors found to affect the performance of nutrient removal processes are discussed in 

Chapters 2 and 3. 

Nitrogen Removal 

For nitrogen removal, key factors are presented in Section 2.2.2. They include an adequate 

supply of carbon from internal or external sources, the number of anoxic zones, favorable 

temperature, sufficient alkalinity, the sludge age and maintenance of a deep sludge blanket in 

the secondary clarifier, and proper management of the recycle flows. 

An adequate supply of carbon is needed to meet one of the following: chemical oxygen 

demand (COD)-to-total Kjeldahl nitrogen (TKN) ratio, readily biodegradable COD-to-TKN 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-8 Executive Summary 

ratio, biochemical oxygen demand (BOD)-to-TKN ratio, or VFAs. A fermenter is a good 

source of internal carbon at a facility. If an insufficient amount of carbon is present inside the 

facility, however, external carbon sources are needed. Typically, methanol or another locally 

available material is selected. 

The number of anoxic zone is a function of the target concentration and the existing facility. 

For facilities with a target concentration of 3 mg/L or less, typically two anoxic zones are 

required for a single-sludge system. Having a number of swing zones provides significant 

protection against changing wastewater characteristics and other conditions. In addition, the 

size of the anoxic zone depends on the carbon source: smaller zones are designed with a 

readily biodegradable carbon like VFAs, whereas larger zones are designed with a lessbiodegradable carbon source in the wastewater. A separate denitrifying filter, however, is not 

affected by these factors. 

Alkalinity is an essential requirement for nitrification, and its stoichiometry is well 

established. Denitrification in the same sludge system enables recovery of approximately 

30 percent of the alkalinity in accordance with the stoichiometry. Supplementary alkalinity, 

in the form of caustic or lime, can be used in soft-water regions. 

The sludge age is an operating parameter that varies from region to region, reflecting the 

temperature and changing characteristics of wastewater during the year. It varied from 10 to 

50 days in the case studies. 

Operationally, many facilities reported a strategy by which a significant amount of 

denitrification was accomplished by maintaining 3 to 4 feet of sludge blanket in the 

secondary clarifier. 

Recycle flows contributed to a significant amount of nutrient and affected the performance of 

the technologies. Ammonia nitrogen from anaerobic digestion and dewatering are significant, 

and flow equalization reduces shock effects on the biological processes. 

Phosphorus Removal 

For phosphorus removal, the key factors are presented in Section 2.3. They are based on a 

system that consists of multiple processes conducted in series to reach an extremely low 

target concentration. This proven practice consists of a biological process, followed by a 

chemical process, and eventually by a physical process in which solids are separated from the 

effluent water. The key factors included, for biological removal, an adequate supply of VFAs 

in the wastewater (and the use of a fermenter to generate additional VFAs where needed), the 

size of the anaerobic and aerobic zones, the number of swing zones, the sludge age, the 

control of secondary release, and the depth of the sludge blanket in the secondary clarifier. 

For chemical removal, the key factors included the number of chemical application points, 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-9

the dosage, the need for a tertiary clarifier, and the type of filters for final polishing. 

Management of recycle flows is another key factor for reliable operations. 

The recommended carbon supply is expressed in many ways, which include a VFA-to-TP 

ratio of 4 or higher, a BOD-to-TP ratio of greater than 20, and a readily biodegradable 

COD-to-TP ratio of 15 or greater. If insufficient VFAs are present in the wastewater yearround, a fermenter is a necessity for reliable performance. The typical design includes a 

sludge age of 4 days or longer and a hydraulic retention time of longer than 6 hours, 

depending on the temperature and location. The sizes of the anaerobic and aerobic zones, 

along with the swing zone, are important factors for ensuring reliable performance. Because 

of varying wastewater characteristics during the year, the swing zone provides added 

assurance of reliable performance. 

Maintaining 2 to 4 feet of sludge blanket in the secondary clarifier was reported to be 

beneficial to many facilities, including those described in the Kelowna, Lee County, and 

Central Johnston County case studies. Clark County, on the other hand, reported use of a 

no-blanket policy to prevent the secondary release of phosphorus. 

Tertiary clarifiers were effective in reducing the load to the tertiary filter, where the target 

concentration was low. Fairfax County; Clark County; and Brighton, Michigan, have shown 

added flexibility and reliability. 

Special filters were effective in reaching a phosphorus concentration below 0.1 mg/L. They 

included a membrane filter at Hyrum, Utah, and at Lone Tree Creek, Colorado, a Dynasand 

filter at Breckenbridge, Colorado, and a Trident filter at Pinery, Colorado. Land application 

of tertiary clarifier effluent through 6 feet of soil layer in Brighton produced an effluent 

concentration of 0.01 mg/L at all times, making it the most reliable of all the technologies. 

The management of recycle flows is necessary to avoid high phosphorus concentrations from 

building up in the wastewater remaining in the facility (phosphorus that is not disposed of 

with the biosolids). Therefore, opportunities for sludge to be anaerobic for long periods 

should be minimized. This is done by avoiding the use of long-term sludge storage as well as 

avoiding the use of an anaerobic digester or thickener. The use of lime or ferric chloride to 

chemically remove phosphorus from the recycle stream might be needed in such instances. 

Combined Nitrogen and Phosphorus Removal 

For combined removal of nitrogen and phosphorus, the key factors presented above apply. In 

addition, a careful approach is necessary to balance the needs for each process in a singlesludge system (see Section 2.2 for details). These needs include optimal allocation of carbon 

sources, the size of the anoxic zone, control of dissolved oxygen and nitrate nitrogen, and the 

number and placement of chemical application points. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-10 Executive Summary 

The VFA from the same sources can be allocated between the anaerobic zone and the anoxic 

zone, depending on the needs and the control philosophy. Both the step-feed AS and the 

Westbank processes use this principle with a varying split formula to produce reliable 

performance. Under this approach, the size of anoxic zone can be reduced in comparison 

with the normal-mode AS. 

Good control of dissolved oxygen and nitrate nitrogen is achieved with online sensors and 

automation of control functions, as is the case in Clark County and Clearwater. 

For two-sludge and three-sludge systems, the controls are separated, and thus less effort is 

required to balance the needs for nitrogen removal versus phosphorus removal. These 

systems include the denitrification filters at Lee County and Central Johnston County and the 

three-sludge system in Western Branch. A simpler control and good performance are 

advantages at these facilities; the disadvantages include the new facilities––with their larger 

footprint and high cost for carbon––required. 

Wet-Weather Flow 

The reliability of technologies also depends in part on how the facility manages wet-weather 

flows. The key factors are the size and location of the equalization basin, selection of 

treatment processes, and operational flexibility available. 

The size of the equalization basin depends on the sewer system in place. The peaking factors 

and the ability to bypass upstream treatment process are site-specific constraints. The 

reliability of the overall technologies is ensured with a large-enough basin, as is the case in 

North Cary, which has 58 percent of the average daily flow rate; Fairfax County, which has 

11.5 percent of the average daily flow rate in the raw wastewater and 20 percent in the 

secondary effluent; and Kelowna, with 7.5 percent of the daily average flow rate. 

Step-feed AS offers a distinct advantage in maintaining a higher sludge inventory in the 

aeration basin while maintaining a low solids loading rate on the secondary clarifiers, as 

shown in Fairfax County and Kelowna. 

To address extreme storm conditions, many facilities have developed storm modes of 

operation, under which aeration is suspended in a section of the aeration basin during the 

high-flow period. The size and duration depend strictly on the size of the storm and the 

layout of the facility. 

The PID at North Cary has a storm mode of operation, which allows quick adjustment of the 

cycle time and thus affords protection of the sludge inventory from washouts. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-11

Case Study Locations 

Kelowna, British Columbia (Canada) 

Clearwater, Florida (Marshall Street Plant) 

Lee County, Florida 

Upper Marlboro, Maryland (Western Branch 

Plant) 

Kalispell, Montana 

Clark County, Nevada 

Central Johnston County, North Carolina 

North Cary, North Carolina 

Fairfax County, Virginia (Noman Cole Plant) 

Emerging Technologies 

Chapter 2 also includes presents emerging technologies that show promise for effective 

nutrient removal but have not yet had extensive full-scale operation so that their performance 

and reliability could be assessed. These include the Co-Mag process, which entails ballasted 

flocculation with magnetic separation of the ballast; the Blue PRO process, which involves 

ferric chloride-aided adsorption of phosphorus on ferric oxide; and the Trident HS process, 

which employs metal precipitation of phosphorus, two-stage clarification, and filtration. 

Case Studies Involving Nine Municipalities 

Chapter 3 provides a synthesis of the information generated from the nine case studies that 

were developed for this document. Most of the case study facilities are throughout the United 

States; one plant is in Canada. Facilities in both cold and warm weather locations were 

included in the study. The facilities varied with respect to whether phosphorus removal, 

nitrogen removal, or both were required under the NPDES permit. All the facilities exhibited 

outstanding performance. The objective of the case studies was four-fold: 

• To present the data from selected technologies for a one-year period 

• To identify the factors that contribute to the reliability of nitrogen and phosphorus 

removal 

• To identify the factors that contribute to the costs of various removal technologies 

• To evaluate the reliability of nutrient removal through a simple, yet sound statistical 

method by which performance data could be presented and compared 

The performance of the technologies in use at the case study facilities is summarized below. 

For biological phosphorus removal, efficient and reliable performance was shown at four 

facilities, two of which had dedicated fermenters. The annual average effluent concentration 

was 0.12 mg/L in Kalispell (modified UCT 

process with fermenter) and 0.14 mg/L in 

Kelowna (3-stage Westbank process with 

fermenter), with corresponding COVs of only 

19 percent and 21 percent, respectively. North 

Cary (PID process) had an average 

concentration of 0.38 mg/L with a COV of 64 

percent, and Central Johnston County (MLE 

with long anoxic detention time and high 

internal recirculation) had an average 

concentration of 0.26 mg/L with a COV of 62 

percent. Key factors included favorable 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-12 Executive Summary 

wastewater characteristics, an on-site fermenter, and a good design that included flexible 

features like swing zones, multiple points of carbon feed, safeguards against secondary 

release and minimal return of recycle flows from sludge handling, and good controls 

developed and practiced based on key process parameters. Operational factors included 

sludge blankets (also referred to as denitrification blankets) in the secondary clarifiers, 

maintained at Central Johnston County and Kelowna; optimization of fermenter parameters 

throughout the year; and control strategies based on nitrate concentration, blanket 

monitoring, dissolved oxygen, and oxidation-reduction potential measurements at some 

facilities. 

Three facilities practiced a combination of biological phosphorus removal and chemical 

addition. The average annual effluent concentrations were lowered down to 0.09 mg/L with 

COVs of 30 percent in Clark County (A/O process) and 21 percent in Fairfax County (stepfeed AS process with fermenter). Clearwater (5-stage Bardenpho process) achieved an 

average concentration of 0.132 mg/L with a COV of 40 percent. Key factors for increased 

removal included the added flexibility of having a tertiary clarifier, multiple feed points for 

chemical addition, a good filter system, and prevention of secondary release. 

Two facilities––Lee County and Western Branch––relied on adding chemicals to their AS 

processes for phosphorus removal. The average annual concentrations for the two facilities 

were 0.1 mg/L and 0.47 mg/L, with COVs of 56 and 62 percent, respectively. 

For ammonia nitrogen removal, two facilities were studied. The Kalispell and Clark County 

facilities removed ammonia with the highest efficiency and reliability––0.07 mg/L and 

0.12 mg/L as an annual average with COVs of 0 percent in Kalispell and 14 percent in Clark 

County. Contributing factors included adequate sludge age and a sufficient supply of oxygen. 

Six facilities were required to remove TN, and they all met their respective permit limits 

efficiently and reliably. The average annual effluent concentrations were 2.32 mg/L with a 

COV of 16 percent in Clearwater (5-Stage Bardenpho), 3.7 mg/L with a COV of 14 percent 

in North Cary (PID), and 4.38 mg/L with a COV of only 12 percent at Kelowna (Westbank 

process). Denitrification filters were installed to remove nitrogen at Central Johnston County 

and Lee County; their performance was good and reliable. The average concentrations were 

2.14 mg/L and 1.57 mg/L with COVs of 16 and 34 percent, respectively. 

To achieve low effluent nitrogen concentrations, Western Branch added a denitrifying AS 

process with a methanol feed system to its existing two-stage AS-nitrifying sludge system. 

This created a three-sludge system, the performance of which was good and reliable. The 

average concentration was 1.7 mg/L with a COV of 36 percent. The key factors for this high 

reliability included favorable wastewater characteristics, an adequate supply of carbon, a 

flexible design with swing zones, separate control of mixing and aeration, multiple carbon 

feed points, minimization of recycle loads, and proper design of the sludge-handling 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-13

processes. Operating factors included a denitrification blanket at some locations; proper 

sludge age; online monitoring; and automation of process control based on nitrate, dissolved 

oxygen, and oxidation-reduction potential at some locations. 

Costs 

Chapter 4 presents comprehensive cost information for nutrient removal technologies at 

municipal facilities. The information is based on the literature reviewed, the data collected 

specifically for the case studies prepared for this document, and the use of the 

CAPDETWorks software. 

The literature from which cost information was obtained includes studies surveying facilities 

in Maryland, Connecticut and Colorado. The Maryland studies include the costs for capital 

and O&M (except labor). The Connecticut study had capital and O&M costs (including labor 

costs) for one facility and capital and O&M costs (excluding labor costs) for the other 

facility. The Colorado study includes only capital costs. These literature data were assumed 

to be accurate. The U.S. Environmental Protection Agency (EPA) does not assume 

responsibility for the reported data. 

Additional cost information was obtained in conjunction with developing the case studies 

prepared for this report. The costs reported from these sources included capital costs updated 

to 2007 dollars using the Engineering News-Record Index, and O&M costs for energy and 

chemicals but not for labor or maintenance. These cost data were provided and verified by 

the study facilities and were assumed to be accurate. EPA does not assume responsibility for 

the reported data. 

In addition, cost estimates were developed for all cost items using CAPDETWorks software 

for 12 retrofit and 20 expansion alternatives. The cost estimates are based on unit costs 

updated to 2007 using published cost index values. CAPDETWorks was run for three flow 

rates: 1 million gallons per day (MGD), 5 MGD, and 10 MGD. These rates were selected to 

represent the majority of facilities in the country. On the basis of information from EPA’s 

Clean Watersheds Needs Survey database, nearly 80 percent of existing facilities are less 

than 1 MGD, and about 97 percent of those facilities are 10 MGD or less. The 

CAPDETWorks cost database was assumed to be accurate. Capital costs were automatically 

updated to 2007 dollars. EPA does not assume responsibility for the data. 

Capital, O&M, and Life-Cycle Costs for Retrofits 

Table ES-1 presents the incremental costs of retrofit technologies obtained from the 

literature, case studies, and projections using CapdetWorks arranged by the amount of 

treatment achieved by the various systems. The sources of the data are shown on the table, 

and the flow rate in each system is noted. CAPDETWorks results are presented below for 

1- and 10-MGD flow; more complete cost curves are presented in Chapter 4. For all 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-14 Executive Summary 

technologies, costs vary substantially, which reflects the selected technologies, chemical use, 

power use, and for CAPDETWorks estimates, projected labor requirements. The size of the 

system is a significant cost factor: Higher unit costs are associated with smaller facilities 

compared to larger facilities because of economies of scale. 

Table ES-1. Cost estimates for retrofit technologies 

Target 

concentration 

(annual 

average) 

Initial 

concentration 

(annual 

average) 

Technologies

(conversion or 

add-on 

indicated by 

footnote) Location 

Flow 

rate

(MGD)

Capital 

$/gpd 

capacity 

O&M 

$/MG 

treated 

Life-cycle 

$/MG 

treated 

Total N target only 

TN, 

5.1 mg/L 

9.6 mg/L TN Cyclic on/off 

aerationa

Ridgefield, 

CT 

1 $0.20 $111 --- 

TN, 

5.0 mg/L 

7 mg/L 

TN 

Denitrification 

filterb

Cheshire, 

CT 

3.5 $1.65 $136 --- 

8 mg/L TN MLEÆ4-stage 

Bardenphoa

Seneca, 

MD 

20 $0.21 $63 --- 

8 mg/L TN MLEÆ4-stage 

Bardenphoa

Freedom, 

MD 

3.5 $0.99 --- --- 

8 mg/L TN MLEÆ4-stage 

Bardenphoa

Cumberland, MD 15 $1.10 $122 --- 

15 mg/L TN LagoonÆ4-

stage 

Bardenphoa

Hurlock, 

MD 

1.5 $4.12 --- --- 

8 mg/L TN Denitrification 

filterb

Baltimore, 

MD 

180 $1.39 --- --- 

8 mg/L TN Denitrification 

filterb

Cox 

Creek, MD

15 $1.74 $104 --- 

6.5 mg/L TN 5-stage 

Bardenpho + 

denitrification 

filterb

Frederick, 

MD 

7 $1.41 --- --- 

40 mg/L TNd Phased 

oxidation ditcha

CW 10 $0.47 $44 $157 

40 mg/L TNd MLE retrofita CW 10 $0.71 $82 $164 

40 mg/L TNd Step-feed 

retrofita

CW 10 $0.65 $91 $245 

TN, 

3 mg/L or less 

40 mg/L TNd

 Denitrification 

filterb

CW 10 $0.71 $156 $324 

TN, 1 mg/L 42 mg/L TNe 5-stage 

Bardenpho with 

MBR and 

reverse 

osmosisb

Las 

Virgenes, 

Calabasas, 

CA 

16 $5.20 --- --- 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-15

Table ES-1. Cost estimates for retrofit technologies (continued)

Target 

concentration 

(annual 

average) 

Initial 

concentration 

(annual 

average) 

Technologies

(conversion or 

add-on 

indicated by 

footnote) Location 

Flow 

rate

(MGD)

Capital 

$/gpd 

capacity 

O&M 

$/MG 

treated 

Life-cycle 

$/MG 

treated 

Total P Target Only 

5 mg/L TP Fermenter 

retrofit, no filterb

CW 10 $0.18 $7 $50 

5 mg/L TP 1-point chemical 

addition, 

no filterb

CW 10 $0.03 $91 $98 

TP, 

0.5 mg/L 

5 mg/L TP Fermenter + 

sand filter 

retrofitb

CW 10 $0.44 $25 $130 

5 mg/L TP Fermenter + 

sand filter + 

1-point chemical 

additionb

CW 10 $0.47 $106 $218 

TP, 

0.1 mg/L 

5 mg/L TP 2-point chemical 

addition + filterb

CW 10 $0.29 $215 $283 

Ammonia-N + TP limits 

AmmoniaN+TP, 1.5 

mg/L & 1 mg/L 

37 mg/L 

Ammonia-N, 

10 mg/L TP 

Cyclic on/off 

aerationa

 for 

ammonia-N 

Broomfield

, CO 

8 $1.00 --- --- 

AmmoniaN+TP, 1.5 

mg/L & 1 mg/L 

37 mg/L 

Ammonia-N, 

10 mg/L TP 

IFASa

 for 

ammonia-N 

Broomfield

, CO 

8 $0.85 --- --- 

AmmoniaN+TP, 1.5 

mg/L & 1 mg/L 

37 mg/L 

Ammonia-N, 

10 mg/L TP 

MBBRb

 for 

nitrification/ 

denitrification 

Broomfield

, CO 

8 $1.70 --- --- 

AmmoniaN+TP, 1.4 

mg/L & 1 mg/L 

24 mg/L 

Ammonia-N, 4 

mg/L TP 

Modified UCT 

with fermenter 

and sand filterc

Kalispell, 

MT, case 

study 

3 $3.03 $108 --- 

AmmoniaN+TP, 

1 mg/L & 0.18 

mg/L 

18.9 mg/L 

Ammonia-N, 

6.4 mg/L TP 

Step-feed AS 

with dual-media 

and deep-bed 

filter + 1-point 

chemical 

additionc

Fairfax, 

VA, case 

study 

67 $1.07 $106 --- 

AmmoniaN+TP, 0.6 

mg/L & 0.2 

mg/L 

27 mg/L 

Ammonia-N, 

5.8 mg/L TP 

A/O with VFA 

and dual media 

filters and chemical additiona

Clark Co., 

NV, case 

study 

100 $2.01 $183 --- 

TN+TP limits 

TN+TP, 6 mg/L 

& 0.25 mg/L 

28.8 mg/L TN, 

6 mg/L TP 

3-stage 

Westbank with 

fermentersa

Kelowna, 

BC, case 

study 

10.5 $3.25 $77 --- 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-16 Executive Summary 

Table ES-1. Cost estimates for retrofit technologies (continued)

Target 

concentration 

(annual 

average) 

Initial 

concentration 

(annual 

average) 

Technologies

(conversion or 

add-on 

indicated by 

footnote) Location 

Flow 

rate

(MGD)

Capital 

$/gpd 

capacity 

O&M 

$/MG 

treated 

Life-cycle 

$/MG 

treated 

TN+TP, 3.9 

mg/L & 2 mg/L 

56 mg/L TN,

7.7 mg/L TP 

Oxidation ditch 

with sand filtera

North 

Cary, NC, 

case study

12 $2.84 $60 --- 

TN+TP, 3.7 

mg/L & 1 mg/L 

31.2 mg/L TN, 

5.8 mg/L TP 

Plug flow AS 

with 

denitrification 

filterc

Central 

Johnston 

Co., NC 

case study

7 $0.58 $221 --- 

TN+TP, 3 mg/L 

& 1 mg/L 

24 mg/L TN, 

3.7 mg/L TP 

3-stage activated 

sludge with 

chemical 

additionb

Western 

Branch, 

MD, case 

study 

30 $1.73 $165 --- 

TN+TP, 3 mg/L 

& 1 mg/L 

28 mg/L TN, 5 

mg/L TP 

5-stage 

Bardenpho with 

sand filter + 

chemical 

additiona

Clearwater

, FL 

(Marshall 

Street) 

case study

10 $2.95 $242 --- 

TN+TP, 3 mg/L 

& 0.5 mg/L 

33.2 mg/L TN, 

3.8 mg/L TP 

Denitrification 

filter + chemical 

additionb

Lee Co., 

FL (Fiesta 

Village) 

case study 

5 $2.79 $265 --- 

40 mg/L TN, 5 

mg/L TP 

PID retrofit with 

1-point chemical 

addition, clarifier, 

and filtera

CW 10 $0.89 $199 $411 

40 mg/L TN, 5 

mg/L TP 

5-stage 

Bardenpho 

retrofit with 

chemical 

additiona

CW 10 $1.30 $256 $566 

TN+TP, 

3 mg/L & 

 0.1 mg/L 

40 mg/L TN, 5 

mg/L TP 

Nitrification/ 

chemical 

addition/ 

denitrification 

filter retrofitb

CW 10 $0.75 $448 $626 

Notes: 

Case = case study as described in Chapter 3; O&M does not include labor. 

CW = cost from CAPDETWorks 

A/O with VFA = anoxic/oxic enriched with volatile fatty acids 

AS = activated sludge 

CT = Connecticut Study. CT-1 plant included labor in the O&M cost; CT-2 did not. 

IFAS = integrated fixed-film activated sludge

MD = Maryland Study. Incremental cost for retrofitting from 8 mg/L TN to 3 mg/L TN does not include labor in O&M. 

MLE = modified Ludzack-Ettinger process 

MBBR = moving-bed biofilm reactor 

MBR = membrane bioreactor 

PID = phased isolation ditch 

Other = other literature sources 

TN = total nitrogen as N 

TP = total phosphorus as P 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-17

UCT = University of Cape Town process 

For combined TN and TP technologies (TN+TP), first target number is for TN, second for TP a

 Conversion 

b

 Add-on 

c

 Combination conversion and add-on 

d

 Secondary effluent (as influent for add on processes) 

e

 5-stage Bardenpho with MBR and reverse osmosis removes both TN and TP but the study reported only on TN 

Life-cycle costs were calculated for the CAPDETWorks results by first annualizing the 

capital cost at 20 years at 6 percent interest. The annualized capital cost was then added to 

the annual O&M cost to obtain a total annual cost. This cost was then divided by the annual 

flow to get the life-cycle cost per million gallons (MG) treated. 

For nitrogen removal technologies, the capital costs ranged between $0.99 and $1.74 per 

gallon per day (gpd) capacity. The low cost represents converting AS facilities from meeting 

8 mg/L to meeting 3 mg/L, while the high cost represents adding new denitrification filters. 

For nitrogen removal, the O&M cost reported in the Maryland study ranged from $63 to 

$122 and did not include labor in the O&M cost. The Connecticut plant at $111 per MG 

treated included labor in the O&M cost, but the Connecticut plant at $136 per MG treated did 

not include labor in the O&M cost. 

For chemical phosphorus removal, the capital costs ranged between $0.03 and $0.29 per gpd 

capacity. The low cost was for chemical feed equipment and storage tanks for one chemical 

addition point, while the higher cost was for two chemical addition points. For biological 

phosphorus removal, the costs ranged between $0.44 and $0.47 per gpd capacity. The costs 

were from conversion of the AS process and the addition of new fermenters. 

For chemical phosphorus removal, the O&M cost range was $91 to $215 per MG treated. 

This cost included the cost of chemicals, power, labor, and the handling of the additional 

sludge caused by chemical addition. The low cost represents single-point chemical addition 

without filtration. The high cost represents two-point chemical feed with tertiary filtration to 

achieve a lower target limit, 0.1 mg/L. 

The rest of the costs presented in Table ES-1 are for combined processes that remove 

phosphorus and a nitrogen species. The following rules were applied to rationally allocate 

costs between nitrogen and phosphorus removal during preparation of the case studies 

described in Chapter 3: 

• Electrical power was proportional to oxygen demand (e.g., nitrification versus BOD 

removal). 

• The external carbon source (e.g., methanol) was entirely for nitrogen removal. 

• Fermenters were assumed to be entirely for phosphorus removal. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-18 Executive Summary 

• The tertiary filter was assumed to be entirely for phosphorus removal. 

• The denitrification filter (capital and O&M) was assumed to be 95 percent for 

nitrogen removal and 5 percent for phosphorus removal via filtration. 

• The coagulation chemicals were assumed to be entirely for phosphorus removal. 

• Other costs were allocated in proportion to the hydraulic retention times for nitrogen 

and phosphorus removal. 

Although the focus of this document is the removal of both nitrogen and phosphorus, some of 

the study facilities have limits for only phosphorus and ammonia nitrogen. The capital cost 

estimates for these facilities ranged from $0.85 per gpd capacity (Broomfield, Colorado, for 

IFAS installation) to $3.03 per gpd capacity (Kalispell). O&M costs were available for only 

three case study facilities––Kalispell, Clark County, and Fairfax County. They ranged from 

$106 to $183 per MG treated; the lower numbers resulted from use of a fermenter, rather 

than chemical addition, to obtain TP removal. 

For combined nitrogen and phosphorus removal technologies, capital costs ranged between 

$0.58 and $3.25 per gpd capacity. The high-cost facilities typically included a fermenter, 

additional piping for the anoxic zone, swing zones, and filters. The factors that most affected 

O&M costs were the use of an external carbon source, power for the equipment installed, the 

use of chemical coagulants and polymers, and increased sludge production. 

The combined nitrogen and phosphorus removal O&M costs ranged from $60 to $265 per 

MG treated for the case study facilities. Those costs did not include labor estimates. The 

lower costs were associated with systems that use fermenters to aid in biological phosphorus 

removal (with no chemical addition), while the higher costs were associated with chemical 

addition for phosphorus or nitrogen removal. 

The O&M cost estimates generated using CAPDETWorks ranged from $199 to $448 per MG 

treated. The CAPDETWorks estimates included labor costs. The low costs reflect one-point 

chemical feed, while the high cost represents methanol feed, chemicals for phosphorus 

removal, and filtration. The life-cycle cost range was from $411 to more than $1,000 per MG 

treated. The wide range of life-cycle costs reflects the selected unit operations, chemical use, 

power use, and projected labor requirements. Details of the costs are provided in Chapter 4. 

For nitrogen removal, nitrification/denitrification attained 5–8 mg/L without extra chemicals, 

using a single anoxic zone; the capital cost ranged from $0.63 to $2.17 per gpd capacity. The 

low cost was for a PID, and the high cost was for a 4-stage Bardenpho. The range of O&M 

costs was from $122 to $453 per MG treated, representing all cost items, including 

chemicals, labor, sludge handling, and power. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-19

Capital, O&M, and Life-Cycle Costs for Expansion 

Table ES-2 presents the CAPDETWorks-generated cost estimates for 20 expansion 

technologies for a 10-MGD average flow for the indicated target nutrient concentrations. The 

term expansion, as used here, is defined as a parallel train and no increase in design flow. 

Chapter 4 presents a full set of cost curves, including values for 1 and 5 MGD. For all 

technologies, costs vary; higher unit costs are associated with smaller facilities compared to 

larger facilities because of economies of scale. In general, costs are higher for the expansion 

processes compared to corresponding retrofit processes because there is no opportunity to use 

the existing facilities. Chapter 4 also includes bar graphs that show the breakdown of O&M 

for the expansion technologies into labor, energy, material, and chemical/sludge costs. The 

graphs show that the percentage of the O&M cost devoted to labor was higher for 1-MGD 

plants (labor at 40 to 50 percent of total O&M) than for 10-MGD facilities (labor at 15 to 25 

percent of total O&M). The percentages devoted to energy and chemical costs remained 

proportional to the flow. 

For chemical phosphorus removal, the cost ranged from $0.03 to $0.29 for chemical feed 

equipment and storage tanks. The O&M cost range was $91 to $215 per MG treated, based 

on one or two chemical feed points. 

For biological phosphorus removal to reach 1 mg/L without extra chemicals, the cost was 

estimated at $1.21 per gpd capacity. With a fermenter and tertiary filter, the cost went up to 

$1.52 per gpd capacity, and the O&M cost went up from $280 to $308 per MG treated. 

For combined nitrogen and phosphorus removal, the capital cost range was $1.36 to $2.48 

per gpd capacity. The lower costs were for target TN concentrations of 5 mg/L and target TP 

concentrations of 1 mg/L, while the high costs were for target concentrations of 3 mg/L TN 

and 0.1 mg/L TP. The higher costs also reflect the use of two chemical addition points and 

methanol feed. The O&M costs ranged from $259 to $477. The low cost represents high 

target TN and TP, while the high cost represents low target TN and TP limits, as shown in 

Table ES-2. 

To achieve a TP level of 0.1 mg/L or below, two-point chemical addition followed by a 

clarifier and high-performance filters might be needed. High-performance filters that can 

remove phosphorus to this level include the Dynasand D2, DensaDeg, membrane filters, and 

Trident. Emerging technologies include CoMag and Blue PRO. Cost information for these 

processes should be obtained from the manufacturers of the technologies. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-20 Executive Summary 

Table ES-2. Cost estimates for expansion technologies for 10 MGD 

Nutrient/ 

target 

concentration 

Initial 

concentration Technology 

Capital 

$/gpd 

capacity 

O&M $/MG 

treated 

Life-cycle 

$/MG 

treated 

40 mg/L TN PID $0.63 $122 $273 

40 mg/L TN MLE $1.61 $309 $695 

40 mg/L TN 4-stage Bardenpho $2.17 $453 $971 TN, 5 mg/L 

40 mg/L TN Denitrification filter $0.71 $156 $324 

TP, 1 mg/L 5 mg/L TP A/O, no additional 

equipment 

$1.21 $280 $568 

5 mg/L TP 1-point chemical addition $0.03 $91 $98 

5 mg/L TP A/O with fermenter $1.26 $290 $590 TP, 0.5 mg/L 

5 mg/L TP A/O with fermenter and 

sand filter 

$1.52 $308 $670 

5 mg/L TP 2-point chemical addition 

with filter 

$0.29 $215 $283 

TP, 0.1 mg/L 5 mg/L TP A/O with fermenter, filter, 

and chemical addition 

$1.55 $389 $758 

40 mg/L TN, 5 

mg/L TP 

Step-feed $1.36 $299 $625 

40 mg/L TN, 5 

mg/L TP 

TN+TP, 5 mg/L SBR $1.94 $302 $766 

& 1 mg/L 

40 mg/L TN, 5 

mg/L TP 

3-stage processes (e.g., 

UCT, VIP) 

$2.05 $436 $925 

40 mg/L TN, 5 

mg/L TP 

5-stage Bardenpho, no 

filter 

$2.19 $452 $975 

40 mg/L TN, 5 

mg/L TP 

Modified UCT with 

fermenter and filter 

TN+TP, 5 mg/L $2.33 $456 $1014 

& 0.5 mg/L 

40 mg/L TN, 5 

mg/L TP 

5-stage Bardenpho with 

filter 

$2.45 $455 $1040 

40 mg/L TN, 5 

mg/L TP 

PID with chemical 

addition, clarifier, and filter 

$0.83 $259 $456 

TN+TP, 5 mg/L 

& 0.1 mg/L 40 mg/L TN, 5 

mg/L TP 

SBR with chemical 

addition and filter 

$1.87 $387 $834 

40 mg/L TN, 5 

mg/L TP 

Nitrification with 

1-point chemical addition 

and denitrificiation filter 

$0.75 $448 $626 

TN+TP, 3 mg/L 

& 0.1 mg/L 40 mg/L TN, 5 

mg/L TP 

5-stage Bardenpho with 

chemical addition and filter

$2.48 $477 $1070 

Notes: 

A/O = anoxic/oxic 

PID = phased isolation ditch 

SBR = sequencing batch reactor 

TN = total nitrogen as N 

TP = total phosphorus as P 

UCT = University of Cape Town process 

VIP = Virginia Initiative process 

For combined TN and TP technologies (TN+TP), first target number is for TN, second for TP. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-21

Technology Selection and Permit Limits 

Chapter 5 of the document describes how to evaluate options for nutrient removal retrofits. 

The technologies are summarized in Tables 5-1 through 5-4. The first consideration is to 

determine what the candidate processes should be, which depends on the target concentration 

of the nutrient and the existing facility. 

The review of data sources performed in preparing this document identified six technologies 

that meet low (less than 3 mg/L) TN concentration limits with good reliability. These are the 

4-stage Bardenpho process with a conventional filter; the step-feed AS process with a 

conventional filter; concentric (phased) oxidation ditches; and two types of denitrification 

filters, one downflow and the other upflow. 

The technologies identified for achieving mid-level TN removal concentrations (between 3 

and 8 mg/L) are the A2

/O, biological aerated filter, the MLE process, sequencing batch 

reactors, cyclic AS, biological aerated filters, IFAS, moving bed biofilm reactor, 3-stage 

Westbank, 4-stage Bardenpho and post-aeration anoxic with methanol (known as the Blue 

Plains process). The nitrogen removal technologies mentioned in the preceding paragraph 

were also identified as being able to reliably meet TN concentrations of 8 mg/L or higher 

without the use of filters. 

The technologies that meet low (less than 0.1 mg/L) TP concentration limits with good 

reliability are membrane filters, proprietary high-performance filters (Trident, Dual Sand D2, 

Advanced Filtration System, and land application through infiltration bed. The emerging 

technologies include the prioprietary Blue PRO and CoMag process. 

The additional technologies identified for meeting mid-level TP removal concentrations 

(between 0.1 and 0.5 mg/L) are chemical precipitation with conventional filter, 3-stage 

Westbank with conventional filter, chemical precipitation with tertiary clarifier and 

conventional filter, modified UCT process with conventional filter, PIDs with conventional 

filter, 5-stage Bardenpho with chemical and conventional filter, and step-feed AS with 

conventional filter. The conventional filters include sand filters, deep bed anthracite filters, 

dual-media filters, and other traditional filters. 

The technologies that meet TP concentrations between 1.0 and 0.5 mg/L are chemical 

precipitation, A/O with conventional filter preferred, 5-stage Bardenpho, the proprietary 

PhoStrip process with conventional filter preferred, and sequencing batch reactors. 

The technologies that meet concentrations of 3 mg/L or less of TN and 0.1 mg/L or less of 

TP are step-feed AS plus a tertiary clarifier with chemical addition and a high-performance 

filter, denitrification filter with chemical addition and a high-performance filter, membrane 

bioreactors with chemical addition, and the use of land application associated with any of the 

three technologies listed above. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

ES-22 Executive Summary 

Additional technologies that meet TN concentrations of less than 3 mg/L and achieve TP 

concentrations between 0.1 and 0.5 mg/L were identified. These are step-feed AS with a 

conventional filter, 5-stage Bardenpho with a conventional filter, PID with a conventional 

filter, denitrification filters plus chemical phosphorus removal and conventional filtration, 4-

stage Bardenpho process plus chemical phosphorus removal with conventional filtration and 

denitrification, and AS plus chemical phosphorus removal with conventional filtration. 

The technologies identified as meeting TN concentrations between 3 and 8 mg/L and 

meeting TP concentrations between 0.5 and 1 mg/L are sequencing batch reactors with 

conventional filtration, the modified UCT process, the A/O process, the PhoStrip II process, 

3-stage Westbank, step-feed AS, PIDs, and 5-stage Bardenpho, as well as the following 

technologies with chemical phosphorus removal added: A2/O, biological aerated filter, the 

MLE process, cyclic AS, biological aerated filters, IFAS, moving bed biofilm reactor, 4-

stage Bardenpho, and post-aeration anoxic with methanol (known as the Blue Plains 

process). Sequencing batch reactors and the 3-stage Westbank process can also achieve these 

limits if accompanied by chemical phosphorus removal. 

Factors in Upgrading Existing Facilities, Decision Matrix, and 

Sensitivity Analysis 

Factors to be considered in upgrading existing facilities include wastewater characteristics, 

site constraints, existing solids-handling facilities, wet-weather flows, automation and 

sensors, and sustainability. These factors are listed in Chapter 5, Tables 5-5 through 5-7. 

Among other factors, the wastewater characteristics determine the need for fermenters for 

VFA production for use in biological nutrient removal. Fermenters can produce enough 

carbon to supplement the wastewater for both biological phosphorus removal and 

denitrification. An external carbon supply should be avoided for long-term sustainability, 

where feasible. 

Site constraints could determine whether it is feasible to add new processes or whether there 

is only enough space to modify the existing facilities. Thus, such constraints are a key factor 

in making decisions regarding upgrades. 

Existing on-site sludge-handling facilities have a significant impact on selecting technologies 

and eventually sizing the selected technologies. One needs to determine whether a sidestream treatment is needed or whether there is a need to oversize the main treatment 

processes to handle recycle loadings from solids thickening, digestion, and dewatering. For 

biological phosphorus removal processes, aerobic thickening and digestion are 

recommended. For facilities with anaerobic digestion, proper recycle loads should be 

incorporated into process sizing and selection. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Executive Summary ES-23

Wet-weather flows are a significant factor in selecting technologies, as well as in 

incorporating an equalization basin. The size of the basin and its location are usually based 

on the site-specific flow conditions. 

Automation and the use of online sensors are significant factors in managing energy, labor, 

and chemicals. Cost savings from reduced energy use is a key justification for the 

investment, as well as a reliable process control measure. Online sensors monitor dissolved 

oxygen, ammonia nitrogen and nitrate nitrogen, ortho-phosphate, oxidation reduction 

potential, and some specific parameters of local interest. Supervisory control and data 

acquisition (SCADA) systems are usually recommended for monitoring and, in some cases, 

for process control and optimization. 

Regulators and municipalities evaluating nutrient removal technologies should consider 

sustainability as a key criterion in selecting a technology. One of the great challenges in the 

wastewater sector is to provide reasonable cost treatment that will enable reuse of the 

effluent, as well as recovery of the nutrients, and thereby reduce the overall demand on water 

resources. The costs of nutrient removal are affected significantly by the selection of a 

technology. Each technology requires a certain level of energy consumption, requires a 

particular amount of treatment chemicals, and has implications for the potential reuse of 

effluent and biosolids. 

The process evaluation tables (decision matrices) in Chapter 5 show specific criteria that 

project proponents can develop on the basis of stakeholder input. These criteria and the 

weighting factors are based on the site-specific conditions of the facility and the priority of 

the project. Typical criteria include site requirements and the room for future plant 

expansion, costs (capital and life-cycle costs), the efficiency and sustainability of treatment, 

and flexibility for accommodating future changes in wastewater characteristics. 

Sustainability factors include the energy (electrical) consumption, the external carbon source 

(e.g., methanol) requirement, the potential for methane gas recovery and power generation, 

the sludge reuse potential, odors, and public perceptions. It is recommended that a sensitivity 

analysis be conducted in anticipation of increased costs associated with rising energy costs, 

increased biosolids management costs, uncertainties in waste characteristics, uncertainties in 

regulations, and other costs, assuming increases of 50 percent or 100 percent. Alternatives 

should be compared with respect to these factors in making the final technology selection. 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-1 

CHAPTER 1: Introduction and History 

1.1 Overview 

Wastewater treatment plants (WWTPs) that use conventional biological treatment processes 

designed to meet secondary treatment effluent levels do not remove nitrogen or phosphorus 

to any substantial extent. Retrofitting the conventional secondary biological treatment 

processes is an increasingly popular way to achieve nitrogen and phosphorus reductions. This 

approach typically relies on modifications to biological treatment processes so that the 

bacteria in these systems also convert nitrate nitrogen to inert nitrogen gas and trap 

phosphorus in the biosolids that are removed from the effluent. 

This document presents information about available technologies that can be used to remove 

nitrogen and phosphorus from municipal wastewater. Descriptions of the technologies are 

presented, along with data that show the cost and reliability of the performance that was 

achieved for specific applications of the technology. One of the purposes of the resources 

presented in this document is to help regulators develop appropriate discharge permit limits 

with a full understanding of available technologies, the reliability of the technologies, and the 

ability of plants that are retrofitted with such technologies to meet their permit limits in a 

sustainable way. Another key objective of the document is for this information to be 

available for assimilation by the regulated community so that plant operators and other 

decisionmakers, including rate payers, can be better informed about this topic. 

Information about many of the technologies described in this document was obtained by 

developing a series of nine case studies. Additional information about these and other 

technologies was gathered from the literature, including publications of the Water 

Environment Federation, Water Environment Research Foundation, other organizations, 

recent reports from Maryland and Connecticut, and data from about 20 municipalities that 

participated voluntarily by providing data during the case study development process. One of 

the unique aspects of this document is that full-scale plant performance data were collected, 

and this information is expressed on the same statistical basis for easy comparison. The data 

are accompanied by a discussion of the key factors that contribute to the performance 

reliability of various process systems. These factors include wastewater characteristics; 

process parameters; environmental parameters such as temperature, chemistry, and biology; 

and operating parameters. Cost information is presented, as are factors that should be 

considered when selecting a retrofit or new upgrade process technology for removing 

nitrogen, phosphorus, or both. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

1-2 Chapter 1: Introduction and History

1.2 Background of Nitrogen and Phosphorus Removal in the 

United States 

The Federal Water Pollution Control Act of 1965 began a series of environmental legislative 

reforms that initiated a consistent approach to pollution control based on water quality and 

beneficial use goals (USEPA 1993). During the late 1960s and throughout the 1970s, much 

effort was devoted to reducing phosphorus in wastewater effluents (Rast and Thornton 1996). 

For example, the implementation of phosphorus removal at the Blue Plains WWTP in 

Washington, D.C, resulted in dramatic water quality improvement in the Potomac River. 

The 1972 Amendments to the Federal Water Pollution Control Act, known as the Clean 

Water Act (CWA), established the foundation for wastewater discharge control in the United 

States. With respect to nonpoint source pollution, section 208 of the CWA Amendments of 

1972 stipulated that “substate watersheds in which nonpoint source pollution control, along 

with the control of point source discharges by required technologies, was to be addressed by 

a watershed water quality plan” (National Academy of Sciences 1999). Since 1972, the 

U.S. Environmental Protection Agency (EPA) has awarded $44.6 billion under the sewagetreatment construction grants program. The 1987 CWA Amendments authorized an 

additional $18 billion, total, for the construction grants program through 1990 and the State 

Revolving Fund (SRF) program through 1994. Of this amount, $9.6 billion is authorized for 

continuation of construction grants, and at least $8.4 billion is for use as capitalization grants 

to set up the SRFs (USEPA 1998). 

Although nearly all WWTPs provide a minimum of secondary treatment, conventional 

secondary biological treatment processes do not remove the phosphorus and nitrogen to any 

substantial extent. Tertiary treatment can remove nitrogen and phosphorous through carefully 

designed chemical reactions that generate easily isolated products such as precipitates and 

gases, though it is considered a costly technology (Carberry 1990). 

Advanced treatment technologies can be extensions of conventional secondary biological 

treatment to remove nitrogen and phosphorus. Biological treatment processes called 

biological nutrient removal (BNR) can also achieve nutrient reduction, removing both 

nitrogen and phosphorus. Most of the BNR processes involve modifications of suspendedgrowth treatment systems so that the bacteria in these systems also convert nitrate nitrogen to 

inert nitrogen gas and trap phosphorus in the solids that are removed from the effluent 

(USEPA 2004b). 

In addition to conventional tertiary treatment, there are half-tertiary treatment methods that 

remove some minerals and organic material, such as the method used by the WWTP in 

Orange County, California. In addition, constructed wetlands are now being looked at as a 

cost-effective and technically feasible approach to treating wastewater. In 2004 there were 

approximately 1,000 constructed wetlands in operation in the United States. Constructing 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-3

wetlands is often less expensive than building traditional wastewater treatment facilities. In 

addition, wetlands have low operation and maintenance (O&M) costs and can handle 

fluctuating water levels (USEPA 2004a). 

The following discussion provides some examples of nutrient removal technologies that have 

been used or are currently in use in the United States. The discussion is not intended to be an 

exhaustive history of nutrient removal from municipal wastewater. EPA recognizes the 

substantial efforts being undertaken in many parts of the country to reduce nutrients, 

including many mid-continent locations, Long Island Sound, Puget Sound, and numerous 

other estuaries on the Atlantic, Gulf of Mexico, and Pacific coasts. 

1.2.1 Lake Tahoe 

One of the earliest tertiary treatment plants was introduced by South Lake Tahoe, California, 

and Stateline, Nevada, to protect Lake Tahoe. The threat to the lake became serious in the 

late 1950s when the population along the south shore of the 21-mile-long lake increased 

sixfold in addition to an increase in the number of tourists. Septic tanks were used for many 

years before the construction of WWTPs in the 1960s (Lake Tahoe Environmental Education 

Coalition 2005). 

The South Tahoe Public Utility District was formed in 1950 with the intention of treating 

sewage from the south shore, including Nevada’s casinos. Later in that decade, the Nevada 

side formed its own district, and the South Tahoe Public Utility District took responsibility 

for sewage from the city of South Lake Tahoe and from Eldorado County (Lake Tahoe 

Environmental Education Coalition 2005). 

By 1965 an innovative advanced tertiary treatment plant, which treated sewage to drinking 

water standards, was installed. Effluent from the plant is not discharged to Lake Tahoe; 

instead, it is pumped through a 27-mile pipeline over Luther Pass into a storage reservoir in 

Alpine County. All effluent from the city of South Lake Tahoe and Eldorado County areas 

has been exported through this system since April 1968 (Lake Tahoe Environmental 

Education Coalition 2005). 

Once the export system was completed, the treatment plant reduced its treatment to advanced 

secondary treatment, which disinfected the treated effluent water but left trace amounts of 

nitrogen and phosphorus. This effluent water is stored in a reservoir near Woodfords during 

the winter and released for use by ranchers to irrigate their pastures and alfalfa crops in the 

summer (Lake Tahoe Environmental Education Coalition 2005). 

On California’s northern and western shores, two local jurisdictions collect all the sewage in 

the sanitary sewers and pump it to Truckee. The Tahoe City Public Utility District and the 

North Tahoe Public Utility District cooperated under a Joint Powers Agreement to build a 
Municipal Nutrient Removal Technologies Reference Document September 2008 

1-4 Chapter 1: Introduction and History

joint sewerage facility in 1968 to treat the sewage of both districts (Lake Tahoe 

Environmental Education Coalition 2005). 

The Tahoe Regional Planning Compact was adopted in 1969 when the California and 

Nevada legislatures agreed to create the Tahoe Regional Planning Agency (TRPA) to protect 

Lake Tahoe (Tahoe Regional Planning Agency 1980). The compact, as amended in 1980, 

defines the purpose of the TRPA: 

To enhance governmental efficiency and effectiveness of the Region, it is 

imperative there be established a Tahoe Regional Planning Agency with the 

powers conferred by this compact including the power to establish 

environmental threshold carrying capacities and to adopt and enforce a 

regional plan and implementing ordinances which will achieve and maintain 

such capacities while providing opportunities for orderly growth and 

development consistent with such capacities (Tahoe Regional Planning 

Agency 1980). 

In 1978 the Tahoe-Truckee Sanitation Agency built a state-of-the-art tertiary treatment plant 

in Truckee. Since then, the Tahoe City and North Tahoe Public Utility Districts have 

collected all sewage between D.L. Bliss State Park at Emerald Bay and Kings Beach and 

have transported it to the Tahoe-Truckee Sanitation Agency plant through pipelines. The 

tertiary-treated effluent from this plant is injected into the ground (Lake Tahoe 

Environmental Education Coalition 2005). 

1.2.2 Great Lakes 

The Great Lakes region led the nation in developing successful nutrient control and 

contaminant cleanup strategies during the 1970s. The deterioration of Lake Erie in the 1960s 

due to eutrophication prompted bilateral actions by Canada and the United States to sign the 

first Great Lakes Water Quality Agreement in 1972. The Agreement outlined abatement 

goals for reducing phosphorus loads primarily from laundry detergents and municipal sewage 

effluent. In response to this commitment, Canada and Ontario enacted legislation and 

programs for controlling point sources. Between 1972 and 1987, Canada and Ontario 

invested more than $2 billion in sewage treatment plant construction and upgrading in the 

Great Lakes Basin (Environment Canada 2006). 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-5

The 1978 Agreement between the United States and Canada on Great Lakes Water Quality, 

which followed the original 1972 Agreement, contains the following text: 

3. Nutrients 

(a) Phosphorus 

The concentration should be limited to the extent necessary to 

prevent nuisance growths of algae, weeds and slimes that are or 

may become injurious to any beneficial water use. (Specific 

phosphorus control requirements are set out in Annex 3.) 

ANNEX 3 

Control of Phosphorus 

1. The purpose of the following programs is to minimize 

eutrophication problems and to prevent degradation with regard to 

phosphorus in the boundary waters of the Great Lakes System. The 

goals of phosphorus control are: 

(a) Restoration of year-round aerobic conditions in the bottom 

waters of the Central Basin of Lake Erie; 

(b) Substantial reduction in the present levels of algal bio-mass to 

a level below that of a nuisance condition in Lake Erie; 

(c) Reduction in present levels of algal biomass to below that of a 

nuisance condition in Lake Ontario including the International 

Section of the St. Lawrence River; 

(d) Maintenance of the oligotrophic state and relative algal 

biomass of Lakes Superior and Huron; 

(e) Substantial elimination of algal nuisance growths in Lake 

Michigan to restore it to an oligotrophic state; and 

(f) The elimination of algal nuisance in bays and in other areas 

wherever they occur. 

2. The following programs shall be developed and implemented to 

reduce input of phosphorus to the Great Lakes: 

(a) Construction and operation of municipal waste treatment 

facilities in all plants discharging more than one million 

gallons per day to achieve, where necessary to meet the 

loading allocations to be developed pursuant to paragraph 3 

below, or to meet local conditions, whichever is more 

stringent, effluent concentrations of 1 milligram per liter total 
Municipal Nutrient Removal Technologies Reference Document September 2008 

1-6 Chapter 1: Introduction and History

phosphorus maximum for plants in the basins of Lake 

Superior, Michigan, and Huron, and of 0.5 milligram per liter 

total phosphorus maximum for plants in the basins of Lakes 

Ontario and Erie. 

(b) Regulation of phosphorus introduction from industrial 

discharges to the maximum practicable extent. 

(c) Reduction to the maximum extent practicable of phosphorus 

introduced from diffuse sources into Lakes Superior, 

Michigan, and Huron; and a substantial reduction of 

phosphorus introduced from diffuse sources into Lakes 

Ontario and Eric, where necessary to meet the loading 

allocations to be developed pursuant to paragraph 3 below, or 

to meet local conditions, whichever is more stringent. 

(d) Reduction of phosphorus in household detergents to 0.507 

percent by weight where necessary to meet the loading 

allocations to be developed pursuant to paragraph 3 below, or 

to meet local conditions, whichever is more stringent. 

(e) Maintenance of a viable research program to seek maximum 

efficiency and effectiveness in the control of phosphorus 

introductions into the Great Lakes (CIESIN 1995). 

As reported in the 1995 biennial progress report, all the U.S. and Canadian open-water 

phosphorus target levels have been achieved through combined efforts to improve the 

performance of sewage treatment plants, to reduce levels of phosphorus in detergents, and to 

implement agricultural best management practices. Current loads are clearly below the target 

loads of the 1978 Agreement for Lakes Superior, Huron, and Michigan and are at or near 

target limits for Lakes Erie and Ontario. Lake Erie still experiences brief periods of anoxia in 

some areas of its central basin. The 1997 State of the Great Lakes Report reviewed nutrient 

data since 1994 and concluded that no appreciable change had occurred in the nutrient status 

of the lakes and that the lakes continued to meet the targets for phosphorus reduction in the 

agreement. Continuing success is attributed to implementing a number of programs to control 

soil erosion and sedimentation, as well as other forms of nonpoint source control (USEPA 

2006). 

1.2.3 The Occoquan Reservoir and the Chesapeake Bay 

The Occoquan Reservoir serves as a drinking water supply for a service area of more than 1 

million people in the northern Virginia suburbs of Washington, D.C. The Occoquan Policy

was adopted by the Virginia State Water Control Board in 1971. This Policy commissioned 

the development of a regional treatment plant, the Upper Occoquan Sewage Authority 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-7

(UOSA) Advanced Wastewater Treatment (AWT) Plant. When it went online in 1978, the 

UOSA AWT Plant replaced 11 conventional secondary treatment plants in Fairfax and Prince 

William counties. The quality of effluent from these plants was variable, and no provisions 

were made for the removal of nutrients from the discharges. In contrast, the regional UOSA 

AWT Plant included monthly average nutrient limits of 0.1 milligrams per liter (mg/L) for 

Phosphorus and 1.0 mg/L for unoxidized nitrogen measured as total Kjeldahl nitrogen (TKN) 

(Occoquan Monitoring Lab 2008 and Randall 2008). Developed to protect the drinking water 

supply (Occoquan Reservoir) for the Virginia suburbs of Washington, D.C., this plant 

currently treats 50 MGD and is the largest indirect sewage-to-drinking water system in the 

world. It is unique in that, although originally designed to remove nitrogen by ion exchange 

removal of NH4-N, most of the nitrogen is discharged in the form of nitrates to a stream that 

enters the reservoir to reduce the release of phosphorus from the sediments, thereby helping 

control algae blooms and protecting water quality (Randall 2008). 

In the late 1970s and early 1980s, Congress funded scientific and estuarine research in the 

Chesapeake Bay, pinpointing three areas, including nutrient enrichment, as requiring 

immediate attention. The Chesapeake Bay Program was created in 1983 after the signing of 

The Chesapeake Bay Agreement of 1983 (Chesapeake Bay Program 1983). The Chesapeake 

Bay Program is a unique regional partnership between the states of Maryland, Pennsylvania, 

and Virginia; the District of Columbia; the Chesapeake Bay Commission; EPA; a tristate 

legislative body; and citizen advisory groups. It has led and directed the restoration of the 

Chesapeake Bay since 1983 (Chesapeake Bay Program 2005). However, the Chesapeake Bay 

2006 Health and Restoration Assessment reports show that the bay’s overall health remains 

degraded, despite significant advances in restoration efforts by program partners through 

newly focused programs, legislation, and funding (Chesapeake Bay Program 2007). 

Since the signing of the Chesapeake Bay Agreement of 1983, program partners have adopted 

two additional bay agreements, the 1987 Chesapeake Bay Agreement and Chesapeake 2000 

(Chesapeake Bay Program 2000). The main goals of Chesapeake 2000 are to continue efforts 

to achieve and maintain the 40 percent nutrient reduction goal agreed to in 1987; to adopt 

goals for tributaries south of the Potomac River; and to “correct the nutrient and sediment 

related problems in the Chesapeake Bay and its tidal tributaries sufficiently to remove the 

Bay and the tidal portions of its tributaries from the list of impaired waters under the CWA” 

(Chesapeake Bay Program 2000). 

EPA has calculated that nitrogen and phosphorus discharges from all sources must be 

drastically reduced beyond current levels and that municipal WWTPs, in particular, will have 

to reduce nitrogen discharges by 72 percent (Chesapeake Bay Foundation 2002). On 

March 21, 2003, the program partners agreed to reduce nutrient pollution by more than twice 

as much as was accomplished since coordinated bay restoration efforts began nearly 20 years 

ago. The District of Columbia and states that border the bay have agreed to reduce the 

amount of nitrogen discharged from the current 275 million pounds to no more than 
Municipal Nutrient Removal Technologies Reference Document September 2008 

1-8 Chapter 1: Introduction and History

175 million pounds per year and to reduce the amount of phosphorus discharged from the 

current 18.8 million pounds to no more than 12.8 million pounds per year (Chesapeake Bay 

Program 2003). EPA Region 3 has established a nutrient criteria team to implement the 

National Nutrient Strategy issued by EPA for the Mid-Atlantic region (Chesapeake Bay 

Program 2003). 

1.3 NPDES Permitting 

The National Pollutant Discharge Elimination System (NPDES) aids in the attainment of 

water quality standards by regulating point sources that discharge into the surface 

waterbodies. Effluent limitations are the primary mechanism in NPDES permits for 

controlling discharges of pollutants to receiving waters. When developing effluent limitations 

for an NPDES permit, a permit writer must consider limits based on both the technology 

available to control the pollutants (i.e., technology-based effluent limits) and limits that are 

protective of the water quality standards of the receiving water (i.e., water quality-based 

effluent limits, or WQBELs). A permit writer may find that a discharge causes, has the 

reasonable potential to cause, or contributes to an exceedance of a water quality standard and 

that technology-based effluent limits are not adequate to ensure that water quality standards 

will be attained in the receiving water. In such cases, CWA section 303(b)(1)(c) and the 

NPDES regulations at 40 CFR 122.44(d) require that the permit writer develop more 

stringent WQBELs designed to ensure that water quality standards are attained. Developing 

appropriate effluent limits in NPDES permits is a vital component of the water quality 

standards-to-permits process. 

For more information about the permitting process, see the following documents and Web 

sites: 

EPA’s National Pollutant Discharge Elimination System (NPDES) Web site at 

http://www.epa.gov/npdes

EPA’s NPDES Permit Writers’ Manual 

http://cfpub.epa.gov/npdes/writermanual.cfm?program_id=45

1.3.1 Watershed-based Permitting and Water Quality Trading 

To assist in determining how various permitted discharges affect attainment of water quality 

criteria on a watershed basis, EPA has developed watershed-based permitting and water 

quality trading as innovative tools that use a watershed approach. EPA expects that 

watershed-based permitting and water quality trading will be useful tools for implementing 

WQBELs for nitrogen and phosphorus in NPDES permits. 

Watershed-based permitting is an approach to NPDES permitting in which permits are 

designed to attain watershed goals that reflect consideration of all sources/stressors in a 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-9

watershed or basin. These permits are developed through a watershed planning framework to 

communicate with stakeholders and to integrate permit development with monitoring, water 

quality standards, total maximum daily load (TMDL), nonpoint source, source water 

protection, and other programs. The ultimate goal of watershed-based permitting is to 

develop and issue NPDES permits that consider the entire watershed, not just an individual 

point source discharger. 

Water quality trading is a voluntary market-based approach that, if used in certain 

watersheds, might achieve water quality standards more efficiently and at lower cost than 

traditional approaches. For a given pollutant, costs for controlling point source discharges 

compared to costs for controlling nonpoint source runoff often vary significantly in a 

watershed, creating the impetus for water quality trading. Through water quality trading, 

facilities that face higher pollutant control costs to meet their regulatory obligations can 

purchase pollutant reduction credits from other sources that can generate the reductions at a 

lower cost, thus achieving the same or better overall water quality improvement. Trading also 

can provide ancillary environmental benefits like flood control, riparian improvement, and 

habitat. 

For more information on new tools developed by EPA that can assist with meeting water 

quality goals, see the following documents and Web sites: 

Watershed-based Permitting Policy Statement and related information

http://www.epa.gov/npdes/pubs/watershed-permitting-policy.pdf

Watershed-Based National Pollutant Discharge Elimination System (NPDES) Permitting 

Implementation Guidance 

http://www.epa.gov/npdes/pubs/watershedpermitting_finalguidance.pdf

Watershed-based National Pollutant Discharge Elimination System (NPDES) Permitting 

Technical Guidance http://www.epa.gov/npdes/pubs/watershed_techguidance_entire.pdf

Water quality based permitting and related information 

http://cfpub.epa.gov/npdes/wqbasedpermitting/wspermitting.cfm

EPA’s Water Quality Trading Web site http://www.epa.gov/owow/watershed/trading.htm

Trading policy statement http://www.epa.gov/owow/watershed/trading/tradingpolicy.html

Water Quality Trading Assessment Handbook 

http://www.epa.gov/owow/watershed/trading/handbook

Water Quality Trading Toolkit for Permit Writers 

http://www.epa.gov/waterqualitytrading/WQTToolkit.html
Municipal Nutrient Removal Technologies Reference Document September 2008 

1-10 Chapter 1: Introduction and History

1.4 References 

Carberry, J.B. 1990. Environmental Systems and Engineering. Saunders College Publishing, 

Orlando, FL. 

Chesapeake Bay Foundation. 2002. Reducing Nitrogen and Phosphorus from Wastewater 

Treatment Facilities. http://www.cbf.org/site/PageServer?pagename=

resources_facts_nutrient_red_ww. July 2002. Accessed October 3, 2007. 

Chesapeake Bay Program. 1983. The Chesapeake Bay Agreement of 1983. 

http://www.chesapeakebay.net/pubs/1983ChesapeakeBayAgreement.pdf. Revised 

March 4, 1996. Accessed October 3, 2007. 

Chesapeake Bay Program. 1987. 1987 Chesapeake Bay Agreement. 

http://www.chesapeakebay.net/pubs/199.pdf. December 15, 1987. Accessed October 

3, 2007. 

Chesapeake Bay Program. 2000. Chesapeake 2000.

http://www.chesapeakebay.net/pubs/chesapeake2000agreement.pdf. June 28, 2000. 

Accessed October 3, 2007. 

Chesapeake Bay Program. 2003. Bay Criteria: Defining Restored Bay Water Quality. 

http://meso.spawar.navy.mil/Newsltr/Refs/AWQC_DO_WC_CA_Chesapeake.pdf. 

June 4, 2003. Accessed October 3, 2007. 

Chesapeake Bay Program. 2005. Overview of the Bay Program.

http://www.chesapeakebay.net/overview.htm. Revised February 20, 2008. Accessed 

October 3, 2007. 

Chesapeake Bay Program. 2007. 2006 Bay Health & Restoration Assessment Details Bay’s 

Degraded Water Quality, Restoration Efforts Throughout Watershed.

http://www.chesapeakebay.net/newsassessment041807.htm. Revised February 14, 

2008. Accessed October 3, 2007. 

CIESIN (Center for International Earth Science Information Network). 1995. Environmental 

Treaties and Resource Indicators (ENTRI): 1978 Agreement Between the United 

States and Canada on Great Lakes Water Quality. 

http://sedac.ciesin.org/entri/texts/bi-lateral/2.6X-USXCan-Gt.Lks-H2O.html. 

Amended October 16, 1983. Accessed October 3, 2007. 

Environment Canada. 2006. Great Lakes Water Quality Agreement. 

http://www.on.ec.gc.ca/greatlakes/default.asp?lang=En&n=FD65DFE5-1. 

December 12, 2006. Accessed October 3, 2007. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 1: Introduction and History 1-11

Lake Tahoe Environmental Education Coalition. 2005. Lake Tahoe Report #87: Export of 

Tahoe’s Treated Sewage Protects the Lake. 

http://4swep.org/resources/LakeTahoeReport/087.html. Revised February 3, 2005. 

Accessed May 13, 2008. 

National Academy of Sciences. 1999. New Strategies for America’s Watersheds. National 

Academy of Sciences, Washington, DC. 

Occoquan Watershed Monitoring Lab. 2008. Information on the Occoquan Policy. 

http://www.owml.vt.edu/aboutowml.htm. Accessed August 5, 2008. 

Randall, C.W. 2008. Personal communication through peer review comments on draft 

document. Received May 2008. 

Rast, W., and J.A. Thornton. 1996. Trends in Eutrophication Research and Control. 

Hydrological Processes 10:295–313. 

Tahoe Regional Planning Agency. 1980. Tahoe Regional Planning Compact (Public Law 96-

551-Dec. 19, 1980). http://www.trpa.org/documents/about_trpa/Bistate_Compact.pdf. 

December 19, 1980. Accessed October 3, 2007. 

USEPA (U.S. Environmental Protection Agency). 1993. Nitrogen Control Manual.

EPA/625/R-93/010. U.S. Environmental Protection Agency, Washington, DC. 

USEPA (U.S. Environmental Protection Agency). 1998. EPA Announces Guidance on State 

Revolving Funds for Sewage Treatment.

http://www.epa.gov/history/topics/cwa/02.htm. Revised September 21, 2007. 

Accessed October 3, 2007. 

USEPA (U.S. Environmental Protection Agency). 2004a. Constructed Treatment Wetlands.

EPA 843-F-03-013. U.S. Environmental Protection Agency, Office of Water. 

http://www.epa.gov/owow/wetlands/pdf/ConstructedW.pdf. August 2004. Accessed 

October 3, 2007. 

USEPA (U.S. Environmental Protection Agency). 2004b. Primer for Municipal Wastewater 

Treatment Systems. EPA 832-R-04-001. http://www.epa.gov/OWM/primer.pdf. 

September 2004. Accessed October 3, 2007.

USEPA (U.S. Environmental Protection Agency). 2006. United States Great Lakes Program 

Report on the Great Lakes Water Quality Agreement. http://www.epa.gov/glnpo/

glwqa/usreport/part4.html# EXCESSIVE%20NUTRIENT%20LOADINGS. March 9, 

2006. Accessed October 3, 2007. 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-1

CHAPTER 2: Treatment Technologies 

2.1 Overview 

This chapter presents information about available technologies that can be used to remove 

nitrogen and phosphorus from municipal wastewater. The technologies are presented along 

with performance data that show the variability of the effluent concentrations. The purpose of 

this chapter is to help permit writers develop appropriate discharge permit limits with a full 

understanding of available technologies, their variability, and their ability to meet the 

proposed limits in the most sustainable way. The same information is made available to 

municipal planners and engineers to assist in their preparation of compliance plans for new 

discharge permits. 

The information in this document was obtained from many sources, including publications of 

the Water Environment Federation (WEF), Water Environment Research Foundation 

(WERF), and other organizations; reports from the states of Maryland and Connecticut; and 

data from 30 municipal treatment facilities that participated voluntarily in 2006 and 2007. 

The full-scale performance data are expressed on the same statistical basis for easy 

comparison. The data are accompanied by a discussion of the key factors that contribute to 

the performance variability and reliability of various systems. These factors are wastewater 

characteristics; process parameters; environmental parameters such as temperature, 

chemistry, and biology; and operating parameters. It should be emphasized that the 

performance data for these facilities reflect differences in operating philosophy, permit 

limitations, temperature, influent conditions, flow conditions, and the relative plant load 

compared to design. Thus, the documented performance does not necessarily represent 

optimum operation of the technologies presented. 

Following this chapter, Chapter 3 provides a detailed discussion about the case studies on 

nine facilities. Cost information is presented in Chapter 4, and design factors that should be 

considered when selecting a retrofit or a new upgrade process technology are discussed in 

Chapter 5. 

2.2 Nitrogen Removal Processes 

2.2.1 Nitrogen Species in Wastewater 

For the purposes of this document, only biological nitrogen removal is considered, along 

with physical removal by filtration of solids containing incorporated nitrogen. Other 

physical-chemical processes, such as breakpoint chlorination, ion exchange, and air stripping, 

are not included because they are not usually feasible for municipalities because of technical, 

regulatory, and cost considerations. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-2 Chapter 2: Treatment Technologies 

The biological removal of nitrogen is carried out through a three-step process: (1) the 

conversion of ammonia from organic nitrogen by hydrolysis and microbial activities, called 

ammonification; (2) the aerobic conversion of ammonia to nitrate by reacting the ammonia 

with oxygen in a process called nitrification; and (3) the conversion of nitrate to nitrogen gas 

by reacting the nitrate with organic carbon under anoxic conditions in a process called 

denitrification. The nitrification process is accompanied by the destruction of alkalinity 

(e.g., bicarbonate, HCO3

-

, is neutralized to carbonic acid, H2CO3). Alkalinity is recovered as 

part of the denitrification process with the generation of hydroxide. 

The chemical equations involved in the biological conversion of nitrogen are as follows: 

1. Formation of ammonia from organic nitrogen by microorganisms (ammonification): 

 Organic nitrogen Æ NH4

+ 

2. Nitrification to nitrite by Nitrosomonas species and other autotrophic bacteria genera: 

 NH4

+

 + 3/2 O2 + 2HCO3

- Æ NO2-

 + 2H2CO3 + H2O 

3. Nitrification to nitrate by Nitrobacter species and other autotrophic bacteria genera: 

 NO2

-

 + 1⁄2 O2 Æ NO3

-

4. Denitrification by denitrifying microorganisms with no oxygen present: 

 NO3

-

 + organic carbon Æ N2 (g) + CO2 (g) + H2O + OHThe stoichiometry for nitrification shows 4.57 grams of oxygen per gram of ammonia 

nitrogen and consumption of 7.14 grams of alkalinity as measured as calcium carbonate 

(CaCO3) per gram of ammonia nitrogen. The stoichiometry for denitrification shows at least 

2.86 grams of chemical oxygen demand (COD) required per gram of nitrate nitrogen or 1.91 

grams of methanol per gram of nitrate nitrogen. This includes carbon incorporated into 

biomass production (Metcalfe & Eddy, 2003, p 621). Alkalinity is generated as 3.57 grams 

per gram of nitrate nitrogen denitrified (WEF and ASCE, 2006, pp. 39 and 71, respectively). 

Nitrification operates on ammonia nitrogen and most of the total Kjeldahl nitrogen (TKN). 

Dissolved organic nitrogen (DON) compounds are typically complex nitrogen-containing 

molecules that constitute a portion of TKN but are difficult to break down. In most localities, 

the average DON concentration ranges from 0.5 to 2 milligrams per liter (mg/L) (Demirtas et 

al. 2008); however, higher DON concentrations can occur in some locations because of 

industrial sources or natural components in the background. High DON concentrations 

negatively affect the ability of a plant to achieve a low final total nitrogen (TN) level, even 

with installation of the processes described in this chapter. 

A reasonable solution to address the presence of DON that is based on science and 

technology is needed. Pagilla (WERF 2007) characterized DON in wastewater and reported 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-3

that it could be grouped by molecular weight (MW). The DON compound group with an 

MW of less than 1,000 Dalton (Da) includes urea, amino acids, DNA, peptides, and various 

synthetic compounds. The DON group with an MW ranging between 1,000 and 1,000,000 

Da includes fulvic acids. High-MW DON includes humic substances. Biological wastewater 

treatment has been considered effective in removing low-MW DON, while high-MW DON 

is considered refractory to this kind of treatment. Sedlak and Pehlivanoglu-Mantas (2006) 

indicated that a portion of the recalcitrant DON could have a slow degradation rate, and thus 

the effect on the receiving stream water would be low. Pagilla (WERF 2007) proposed a 

short-term test protocol to assess the bioavailability of DON using algae and bacteria. More 

research is needed to determine the sources and fate of DON in wastewater treatment 

processes and the method by which bioavailability can be demonstrated. 

2.2.2 Nitrogen Removal Factors 

Carbon Source 

Many have reported from their research the quantity of carbon, in proportion to the nitrogen 

present in the wastewater, necessary to reduce the amount of nitrogen. They have included 

the COD-to-TKN ratio, the readily biodegradable COD (rbCOD)-to-TKN ratio, and volatile 

fatty acids (VFAs). A biochemical oxygen demand (BOD)-to-TKN ratio of 4 or greater is 

sufficient for biological nitrogen removal to occur (Neethling et al. 2005; WEF and ASCE 

1998; Lindeke et al. 2005). Normal domestic wastewater contains a sufficient COD-to-TKN 

ratio to remove 65 to 85 percent of the nitrogen in a single-pass process like the modified 

Ludzack-Ettinger (MLE) (Barnard 2006) or in attached-growth systems like biological 

aerated filters (BAFs) (Stephenson et al. 2004) and moving-bed biofilm reactors (MBBRs) 

(Rusten et al. 2002). Simultaneous nitrification and denitrification (SND) in oxidation 

ditches, such as the Orbal oxidation ditch system, has shown up to 90 percent nitrogen 

removal in the extended-aeration mode. When combined with a pre-anoxic zone, the 

oxidation ditch can produce an average TN concentration of less than 3.1 mg/L with an 85th

percentile value of 4 mg/L (Barnard 2006). The 4-stage Bardenpho process using a carousel 

aeration basin produced an average TN concentration of 1.9 mg/L, where plant sludge was 

hauled away to another plant for processing (deBarbadillo et al. 2003). Processes such as the 

4-stage Bardenpho can fully utilize influent carbon and meet most limits except for very low 

targets. This means that they will have cost savings via reduced energy use and no need for 

supplemental carbon, which results in no additional sludge production. 

When it is determined that an additional carbon source is needed to achieve the desired level 

of nitrogen removal, there are two types of sources—in-plant and external. In-plant sources 

include primary effluent, which can be step-fed to the activated-sludge process, and 

fermentation of primary sludge to obtain VFAs and other readily used carbon compounds. 

Step-feeding reduces the needed tank footprint due to lower hydraulic and sludge detention 

times; reduces or eliminates the need for an additional carbon source; and, as an extra 

advantage, provides the wastewater treatment plant (WWTP) operators the ability to better 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-4 Chapter 2: Treatment Technologies 

handle wet-weather flows. Successful operation, however, requires oxygen control, an 

alkalinity supply, and good instrumentation incorporated into the system. 

In New York, Chandran et al. (2004) were able to achieve 4 mg/L in a four-pass system with 

additional carbon in a limited anoxic zone volume. McGrath et al. (2005b) reported threeway split feed with anoxic, swing, and aeration zones that were 35 percent, 15 percent, and 

50 percent of the activated sludge tank volume, respectively. During the last 12 months of the 

study, effluent TN ranged from 3 to 7.25 mg/L, with an annual average of 5 mg/L. Tang et al. 

(2004) developed a model to obtain the optimal step-feed pattern to use in four plants in the 

Los Angeles County Sanitation Districts. They found that for a system with two anoxic 

stages, feeding approximately two-thirds of the flow to the first stage and one-third of the 

flow to the second stage provided the best overall removal. As the number of anoxic stages 

increases, less is put into the first stage, but the first stage still receives 40 to 50 percent of 

the total flow. In general, use of more than four anoxic stages provides minimal benefits. 

Less primary effluent and return activated sludge (RAS) should be fed to the last stage 

because any ammonia or nitrate that is not treated in the last stage passes through the 

secondary clarifiers and increases the TN concentration in the overall discharge. If the BODto-TKN ratio is low, however, a higher percentage of the flow (compared to typical BOD-toTKN ratios) should be fed to the later stages. Alternatively, equal distribution of flow among 

the splits, with no flow to the last section, has also been found to work well (Metcalf & Eddy 

2003). 

An in-plant alternative to step-feeding is fermentation. Fermenters are usually associated 

with plants that perform biological phosphorus removal. It is also noted, however, that when 

high nitrate concentrations are returned to the anaerobic zone, denitrification occurs, 

consuming some of the VFAs that the fermenter produced. Fermenters can therefore be used 

to provide a source of carbon for plants that perform denitrification. Stinson et al. (2002) 

reported on a pilot study performed by the New York City Department of Environmental 

Protection that compared denitrification rates achieved by feeding methanol, acetate, and 

fermentate. The results from the pilot plants operating in the step-feed activated-sludge mode 

indicate that denitrification rates for fermentate and acetate were similar at 0.15 to 0.30 mg of 

nitrate nitrogen per milligram of volatile suspended solids (VSS) per day (mg N/mg VSSday), whereas denitrification rates using methanol were significantly lower at 0.04 to 0.08 mg 

N/mg VSS-day. Higher denitrification rates resulted in less carbon source addition and 

therefore less sludge production. 

Methanol is most often used as an external carbon source because of its relatively low cost. 

Methanol, however, is corrosive and combustible and therefore requires special handling to 

meet Occupational Safety and Health Administration (OSHA) regulations and NFPA-820: 

Standard for Fire Protection in Wastewater Treatment and Collection Facilities. Some plants 

are considering alternatives to methanol because of these flammability and explosion 

concerns. Other chemicals, such as MicroC (distributed by Environmental Operating 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-5

Solutions, Inc.) can be used. The company that manufactures MicroC recently introduced 

MicroCG, which replaces the 5 to 6 percent of methanol in MicroC with glycerin.

Communities with a nearby supplemental carbon source, such as molasses or brewery waste, 

can consider the opportunity to use such sources. The advantage of using an external carbon 

source is that the operation is independent of the nature and volume of the influent 

wastewater, giving the operators much more flexibility. The disadvantage is that the 

additional carbon source will increase capital costs due to the construction of chemical 

storage, feed pumps, and piping and will require substantial operating costs for purchasing 

the chemicals and handling the extra sludge generated because of the addition. The carbon 

source is typically added to the separate-stage anoxic zone or denitrification filters. A study 

by deBarbadillo et al. (2005) presented external carbon source doses for denitrification 

filters. Typical methanol doses are between 2.5 and 3 times the amount of nitrate nitrogen to 

be removed on a mass basis (WEF and ASCE 2006, p. 75). 

Number of Anoxic Zones 

A single anoxic basin with an internal recycle stream can achieve reasonable rates of TN 

removal in the range of 6 to 8 mg/L. An example of such a process is the MLE process 

(described in more detail later). The internal recycle returns nitrates produced by nitrification 

in the aeration basin to the anoxic zone for denitrification. With the anoxic zone at the 

beginning of the process, carbon source addition is not usually necessary because domestic 

wastewater typically provides enough carbon to achieve 65 to 85 percent removal (Barnard 

2006). 

In general, the denitrification rate increases with increasing internal recirculation, up to a 

maximum of 500 percent (WEF and ASCE 2006). Denitrification in wastewaters with BODto-TKN ratios less than 4-to-1 or COD-to-TKN ratios less than 10-to-1 typically is not 

benefited by high internal recirculation. This is because such wastewater has insufficient 

carbon to support an elevated denitrification rate. 

Having two anoxic zones allows lower TN effluent concentrations to be achieved because 

more of the nitrates produced after nitrification in the aeration basin can be treated by an 

internal recycle to the first anoxic zone or by flowing through the second anoxic zone. 

Aeration is usually recommended after the second anoxic zone to both strip nitrogen gas 

formed in the anoxic zone and to decrease the possibility of denitrification in the secondary 

clarifiers. This has the effect of reducing nitrogen gas release in the secondary clarifier, 

which could result in rising sludge. In addition, maintaining aerobic conditions prevents 

phosphorus release in the secondary clarifiers. Adding a carbon source to the second anoxic 

zone can further increase denitrification by ensuring that sufficient carbon is available for the 

process to occur. Barnard (2006) has reported that processes involving two anoxic zones, 

such as the 4- or 5-stage Bardenpho process, can achieve TN concentrations between 2.5 and 

3.5 mg/L. Clearwater, Florida, operates the Marshall Street and Northeast Advanced 
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2-6 Chapter 2: Treatment Technologies 

Pollution Control Facilities. Both plants are achieving annual average TN levels below 3 

mg/L––2.32 and 2.04 mg/L, respectively––without a supplementary carbon source. 

Temperature 

Temperature affects the rate of both nitrification and denitrification. At lower temperatures, 

the nitrification and denitrification rates decrease, leading to poorer performance in the 

winter if operational changes are not made to compensate for the decreased kinetic rates. 

Nitrification can occur in wastewater temperatures of 4 to 35 degrees Celsius (oC). Typical 

wastewater temperatures range between 10 and 25 °C (WEF and ASCE 2006, p. 41). The 

nitrification rate doubles for every 8 to 10 °C rise in temperature, meaning that in areas that 

experience a wide range of temperatures between winter and summer, nitrification rates 

could differ by a factor of 4 over the course of a year (WEF and ASCE 2006, p. 42). 

Denitrification is also subject to temperature, although to a lesser extent than nitrification. On 

the basis of a wastewater temperature range of 10 to 25 °C, the denitrification rate would be 

expected to vary by a factor of only 1.5 (WEF and ASCE 2006, p. 73). Alternative carbon 

sources should be explored to determine if an additional carbon supply could provide better 

denitrification performance in cold weather than others. Alternatively, external carbon might 

not be needed at all during warm weather conditions, because the process might be able to 

meet the treatment objectives with the available carbon in the wastewater. 

Alkalinity 

Alkalinity is consumed as part of the nitrification process because hydrogen ions are created 

when ammonia nitrogen is converted to nitrate nitrogen. Denitrification restores a portion of 

the alkalinity during the conversion of nitrate nitrogen to nitrogen gas. The nitrification 

process consumes 7.14 grams of alkalinity as calcium carbonate (CaCO3) per gram of 

ammonia nitrogen removed (WEF and ASCE 2006, p. 39). Denitrification produces 

3.57 grams of alkalinity as CaCO3 per gram of nitrate nitrogen removed (WEF and ASCE 

2006, p. 71). Therefore, to convert 1 gram of ammonia nitrogen to 1 gram of nitrogen gas, 

approximately 3.6 grams of alkalinity as CaCO3 is consumed. Nitrification can generally 

occur at pH values between 6.5 and 8.0 standard units (s.u.). The recommended minimum 

alkalinity in the secondary effluent is 50 mg/L as CaCO3, although as much as 100 mg/L is 

suggested by standard design manuals (WEF and ASCE 2006, p. 43). If the alkalinity will be 

below the recommended levels, chemical addition might be necessary. Sodium hydroxide or 

lime is added at some plants to maintain acceptable alkalinity and pH levels. 

Solids Retention Time 

The solids retention time (SRT) must be long enough to maintain nitrification. The 

microorganisms responsible for nitrification have a much slower growth rate than other 

heterotrophic bacteria. Therefore, doubling of the nitrification microorganism population 

requires 10 to 20 times longer than for other heterotrophic bacteria (WEF and ASCE 2006, p. 

40). Maintaining longer SRTs can also reduce the amount of energy required for mixing if 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-7

tank sizes and liquid volumes can be reduced. In addition, energy can be saved if 

denitrification can be achieved in an SND application (Barnard et al. 2004; Kang et al. 2006). 

Some plants with mechanical aerators and/or concentric ditches operating with the same 

SRTs have a lower aerator energy-to-volume ratio. The lower energy requirement is because 

of SND, which results in reduced oxygen needs. For example, Barnard (2004) reported that 

the aerator energy density was decreased by more than half when the SRT was increased 

from 8 days to 24 days. Concurrently, the SND rate increased from 30 percent at an 8-day 

SRT to 70 percent at a 24-day SRT. The optimum SRT for a given WWTP depends on 

several factors, including wastewater temperature, dissolved oxygen (DO) concentration in 

the aerobic zone, pH, alkalinity, inhibition from chemicals, and variations in hydraulic flow 

and organic load. In general, denitrifying microorganisms are relatively slow growing and 

can be subject to washout at high flows in the bioreactor or clarifier. Therefore, secondary 

clarifier design that assures maximum retention of the slower growing microorganisms can 

be critical to maintaining reliable nitrogen removal performance. 

Hydraulic Retention Time 

Hydraulic retention time (HRT) affects both nitrification and denitrification. The aerobic 

zone(s) of single nitrification/denitrification processes must be large enough to allow most of 

the carbonaceous BOD (CBOD) to be consumed before nitrification can begin. The size of 

the anoxic zone(s) must be sufficient to allow denitrification to occur without consuming the 

entire carbon source that might be needed for biological phosphorus removal. Anoxic zones 

are typically 35 to 50 percent of the secondary treatment process by volume. Because of the 

uncertainty in the required volume, some plants are designed with swing zones that can be 

operated in an anoxic or aerobic mode. Generally, swing zones can be operated as aerobic 

during the summer and anoxic during the winter, when low temperatures lower the 

denitrification rates. However, sufficient aerobic detention time must be maintained for 

adequate nitrification. To test whether the HRT is limiting, two samples can be collected at 

the same time from the end of the anoxic zone. One sample can be filtered and analyzed for 

nitrates immediately, while the second sample can be filtered and analyzed for nitrates after 

30 minutes. If the second sample has a lower nitrate concentration than the first, the HRT 

limits denitrification. If there is little difference between the two samples, it is more likely 

that lack of an adequate carbon source is limiting the denitrification (Tang et al. 2004). 

Dissolved Oxygen 

Nitrification requires the presence of sufficient DO, and the rate of nitrification can be 

limited when the DO concentration is too low to support a sufficiently high oxygen transfer 

rate. In municipal activated sludge systems with HRTs of 6 to 8 hours, the nitrification rate is 

maximized when the DO concentration is 2 mg/L or greater (WEF and ASCE 2006, p 42). 

Fixed-film systems, such as the MBBR or the integrated fixed-film activated sludge (IFAS) 

system, might need higher DO concentrations to prevent the biomass attached to the media 

from becoming anaerobic, which could lead to poor system performance. 
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2-8 Chapter 2: Treatment Technologies 

The presence of DO inhibits some desirable biological processes, particularly denitrification. 

DO concentrations of 0.2 mg/L or greater decrease the denitrification rate in the anoxic zone 

(WEF and ASCE 2006, p 183). This is because the energy expended breaking NO3 to obtain 

oxygen is greater than that from using molecular oxygen, so most microorganisms 

preferentially use DO when it is available rather than NO3. Denitrification is made most 

efficient when DO is kept low throughout the anoxic zone. Thus, low DO levels in the 

internal recycle or RAS lines can effectively reduce the required HRT of the anoxic zone. It 

is recommended that DO concentrations in return flows be limited to about 1 mg/L (WEF 

and ASCE 2006, p.182). 

Additional Design Considerations to Enhance Nitrogen Removal 

Additional design considerations that can help make plant operation easier include 

accounting for return flow and loads as well as external loads in the basis of the design, 

incorporating a supervisory control and data acquisition (SCADA) system and other online 

monitoring for process control, and including allowances for flexibility in the design. If the 

return flow and loadings from side-streams such as filter backwash or sludge handling are 

included in the basis of the design, the plant will be able to treat the loadings that the 

processes receive, rather than requiring a portion of the standard design safety factor to 

account for these return streams. In addition, if the plant receives septage, landfill leachate, or 

similar hauled or direct waste streams, those loads should be accounted for in the basis of 

design. Accounting for all these loadings might lead to larger tank sizes, blowers, or 

chemical storage facilities. 

Online monitoring allows the recording of real-time process information that can be used to 

make operating decisions. SCADA allows automatic control of the process in conjunction 

with online monitoring. The SCADA system can help in process optimization, and it can 

result in operational savings in situations where too much air is being added by automatically 

adjusting the amount of air delivered to the process. 

Operational flexibility is gained by installing swing zones in anticipation of uncertainties in 

the future wastewater characteristics and operating conditions. The swing zone should be 

equipped with both mixers and aerators, which can be operated as either an anoxic or aerobic 

zone, depending on conditions at the plant (which might vary seasonally as well as diurnally 

between the day and night hours). The swing zone is known as a performance enhancer as 

well as an electricity saver when mixers are used instead of aerators during low-flow periods. 

This flexibility in design increases costs but can contribute to the long-term compliance and 

sustainability of biological nutrient removal as the wastewater and flow characteristics at the 

facility will change in the future. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-9

2.2.3 Nitrogen Removal Technologies 

Nitrification/Denitrification 

Some facilities are required to remove only ammonia-N or TKN, with no current 

requirements to remove nitrate or nitrite. In such instances, the biological conversion of 

ammonia (or TKN) to nitrate is readily accomplished by increasing the SRT in the biological 

system, increasing the HRT, and ensuring that there is sufficient oxygenation to accomplish 

the conversion. System designs in the past incorporated a two-sludge strategy: first, BOD 

was removed in an activated-sludge reactor with a clarifier, and then a second activatedsludge system was run in series to accomplish nitrification. Current practice is to do both 

BOD removal and nitrification in a single sludge system, especially in retrofit situations, 

thereby saving land requirements by avoiding a new set of clarifiers and making the 

operation simpler. Significant energy savings is also anticipated in a single sludge system due 

to a reduced volume requiring aeration. McClintock et al. (1988, 1992) and Randall et al. 

(1992) report a reduction of 20 percent in volume requiring aeration, with a 40 percent or 

greater reduction in sludge production in single sludge systems compared to separate 

systems. 

All the technologies described below except the denitrifying filters can be used to accomplish 

nitrification; if nitrate removal is not required, that portion need not be implemented. This 

could result in savings of capital or operation and maintenance (O&M) costs. During the 

planning and design of a nitrification retrofit, it would be logical to leave space for future 

denitrification in case future permits require it. In addition, there are advantages to 

accomplishing some denitrification, if it is supported by influent BOD, because doing so 

results in reduced overall sludge generation. 

Denitrification Filters 

Denitrification filters are usually placed after the secondary treatment process. One of their 

advantages is that in addition to providing nitrogen removal, they act as an effluent filter. 

Denitrification filters have a more compact area compared to other add-on denitrification 

processes. The filters can be operated in downflow or upflow configurations. As the process 

is performing denitrification after most of the BOD has been removed from the wastewater, a 

carbon source, such as methanol, must be supplied. 

There are two general types of denitrifying filter. One is an adaptation of conventional deepbed filters, as provided by Leopold Co. and Severn-Trent. These employ sand, gravel, 

anthracite, or other filter media in some combination, at a depth of 8 to 12 feet (WEF and 

ASCE 2006). They are typically operated in a downflow mode (with water directed down 

from the top of the bed); this means that nitrogen gas generated within the bed could be 

trapped between the grains. Thus, the beds require periodic bumping, whereby the flow of 

water or air is directed upward in the bed. This releases the trapped nitrogen gas. Less 

frequently, a full backwash is performed to remove accumulated solids. The backwash is 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-10 Chapter 2: Treatment Technologies 

usually returned to the head of the plant or to the secondary treatment process. Existing sand 

filters can be retrofitted to become denitrification filters if there is sufficient space to increase 

the bed depth. Conventional denitrification filters are typically loaded at 2 to 3 

gallons/min/ft2, with removal to 1 to 2 mg/L nitrate nitrogen. 

The second general type of denitrifying filter is exemplified by the Biofor system from 

Infilco/Degremont and the Biostyr system from Kruger/Veolia. They are operated in an 

upflow configuration and use plastic media for microbial attachment. These types of filters 

are often paired with biological activated filters (BAFs), which accomplish BOD removal 

and nitrification. Because they are operated in upflow mode, pumping is required, which 

increases operating costs. The plastic media can be hydraulically loaded at 8 to 

9 gallons/min/ft2, with removal to 1 to 1.5 mg/L nitrate nitrogen. However, the plastic media 

are not as efficient at capturing solids as the conventional granular media. A biological 

upflow filter is illustrated in Figure 2-1. 

Secondary Treatment 

Process

Secondary 

Clarifier

(if used)

Effluent

Influent

RAS (if used)

WAS

Methanol or other 

carbon source

Denitrifying 

Filter

Filter Backwash

Figure 2-1. Denitrifying filter process. 

The advantage of both types of denitrifying filters is that they can accomplish complete 

nitrate removal with a very small footprint in a building, and possibly no additional footprint 

if an older conventional filter can be retrofitted. A disadvantage of this system is that an 

additional carbon source (typically methanol) is usually required, with associated increased 

sludge generation. In certain cases, additional pumping and electrical costs might be 

required. The denitrifying filter process is illustrated in Figure 2-2. 
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Chapter 2: Treatment Technologies 2-11

Effluent Influent

Recirculation

Anaerobic Zone

Aerobic Zone

Main flow 

direction

Figure 2-2. Biological upflow filter. 

Modified Ludzack-Ettinger Process 

The MLE consists of an anoxic basin upstream of an aerobic zone. An internal recycle carries 

nitrates created during the nitrification process in the aerobic zone along with mixed liquor to 

the anoxic zone for denitrification. RAS is mixed with the influent to the anoxic zone. The 

extent of denitrification is tied to the mixed liquor recycle flow; higher recycle rates increase 

denitrification. Because only recycled nitrate has the opportunity to be denitrified, the MLE 

alone cannot achieve extremely low final nitrogen concentrations. The maximum 

denitrification potential is approximately 82 percent at a 500 percent recycle rate (WEF and 

ASCE 2006). TN effluent concentrations typically range from 5 to 8 mg/L (Barnard 2006). 

Actual denitrification might be limited by other factors, such as carbon source availability, 

process kinetics, and anoxic or aerobic zone sizes. Furthermore, oxygen recycled from the 

aerobic zone can negatively affect the denitrification rate in the anoxic zone (WEF and 

ASCE 2006). Performance factors include limitations due to the single anoxic zone and the 

internal recycle rate that returns nitrates to the anoxic zone. Selection factors include the 

possibility of constructing walls in existing basins to create an anoxic zone; additional 

pumping, piping, and electricity to accommodate the internal recycle; and the possible need 

for an additional carbon source to promote denitrification. The modified Ludzack-Ettinger 

process is illustrated in Figure 2-3. 
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2-12 Chapter 2: Treatment Technologies 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

WAS

Figure 2-3. Modified Ludzack-Ettinger process. 

Cyclically Aerated Activated Sludge 

In a cyclically aerated activated-sludge system, the aeration system is programmed to turn off 

periodically, allowing denitrification and nitrification to occur in the same tank. This 

conversion is thus easy to achieve, with little or no capital expenditure required. The 

cyclically aerated activated-sludge system can be used to retrofit existing plants if sufficient 

SRTs can be maintained to allow nitrification to occur. The length of the cycle time depends 

on the loading rate and the target limit, with the HRT being 2 to 4 times the cycle time (Ip et 

al. 1987). If the aerobic SRT time is sufficient to achieve nitrification, the cyclic process can 

reduce TN in the effluent. Aeration can be provided by diffusers or surface aerators. 

Sequencing batch reactors (SBRs) and oxidation ditches can be designed to operate as 

cyclically aerated activated-sludge systems (WEF and ASCE 1998). The cyclically aerated 

activated-sludge process is illustrated in Figure 2-4. The range of TN effluent concentrations 

found at the case study facilities for this study was 3.1 mg/L to 10.4 mg/L (see Table 2-1 in 

Section 2.5). 

Anoxic/Aerobic Tank 

(Aeration on Timer)

Secondary 

Clarifier

Influent Effluent

RAS

WAS

Figure 2-4. Cyclically aerated activated-sludge process. 
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Chapter 2: Treatment Technologies 2-13

Four-Stage Bardenpho Process 

The 4-stage Bardenpho process involves an anoxic zone, followed by an aerobic zone (with 

an internal recycle to the first anoxic zone), which is followed by a second anoxic zone and a 

small aerobic zone. The first two tanks are similar to the MLE process. With pumps and zone 

sizes typically set large enough to accommodate a 400 percent internal recycle rate, the first 

anoxic zone accomplishes the bulk of the denitrification. The second anoxic zone removes 

nitrates from the first aerobic zone that are not recycled to the first anoxic zone. A carbon 

source, such as methanol, might need to be added to the second anoxic zone to achieve good 

denitrification. The second aerobic zone removes the nitrogen gas from the wastewater 

before the wastewater enters the secondary clarifiers. By aerating, the possibility of 

denitrification in the clarifier is removed. The sludge tends to settle better, and overall 

operation of the secondary clarifier is improved (WEF and ASCE 2006). 

The Bardenpho process has been used in numerous underlying configurations, including plug 

flow, complete mix, and oxidation ditch reactors; some configurations have used existing 

oxidation ditches for the first two basins and additional constructed tanks for the secondary 

basins (WEF and ASCE 2006). Selection factors include a large process footprint involving 

several large basins; retrofit of existing basins is possible but unlikely. Additional piping, 

pumping, and electricity are needed for the internal recycle streams. Although the available 

carbon source might be adequate, it is likely that the second anoxic zone will require 

supplemental carbon, with the associated generation of additional sludge and increased O&M 

costs. Performance factors include the presence of two anoxic zones and the internal recycle 

rate, which runs as high as five times of the influent flow rate. The 4-stage Bardenpho 

process is illustrated in Figure 2-5. The range of TN effluent concentrations found in the 

literature evaluated for this study was 3.5 mg/L to 12.1 mg/L (see Table 2-1 in Section 2.5). 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anoxic 

Tank

Aeration Tank

(Optional) Methanol 

Addition

WAS

Figure 2-5. Four-stage Bardenpho process. 
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2-14 Chapter 2: Treatment Technologies 

Oxidation Ditch Processes 

Oxidation ditch processes use channels in a loop to provide continuous circulation of 

wastewater and activated sludge. Surface or submerged aerators provide aeration, and anoxic 

zones are made possible by judicious placement of the aerators. As mentioned earlier, 

oxidation ditches can also be operated as cyclically aerated activated-sludge systems. As an 

alternative to internal anoxic zones, anoxic basins can be constructed before or after an 

existing ditch. If there are aerobic zones downstream of anoxic zones, provision should be 

made for an internal recycle or an external anoxic basin to allow denitrification of any 

additional nitrates. Selection factors include a potentially large footprint. Compared to an 

existing system that is not denitrifying, electricity usage could be reduced because some 

aerators would be turned off to create anoxic zones. With ditches, no additional piping and 

pumping are needed unless external basins are used. Such basins might be needed to achieve 

very low nitrogen levels. In such cases, an additional carbon source would likely be needed 

for the denitrification reactor. Performance can be increased by using automatic DO controls 

that are capable of turning blowers on or off as necessary to maintain the desired set points. 

The oxidation ditch process is illustrated in Figure 2-6. The range of TN effluent 

concentrations for processes evaluated for this study were 6 to 10 mg/L as TN. Sen et al. 

(1990) reported less than 4 mg/L TN after optimizing aeration. 

Secondary 

Clarifier

Influent Effluent

Flow Aerators

WAS

RAS

Figure 2-6. Oxidation ditch process. 

Fixed Film Processes 

Fixed-film, or attached growth systems are possible alternatives to suspended growth systems 

for nutrient removal. The medium can either be in a packed configuration, such as in a 

trickling filter, or suspended, such as in an IFAS or MBBR system. Fixed films provide an 

advantage for slow-growing bacteria, such as those involved in nitrification and 

denitrification processes, because the attachment gives those organisms longer residence 

times in the reactor. Fixed-film systems are also less prone to washout or toxic upsets. 
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Chapter 2: Treatment Technologies 2-15

Trickling filters can be used as an add-on process to provide nitrification. Additional 

information on trickling filters can be found in Wastewater Engineering: Treatment and 

Reuse (Metcalf & Eddy 2003). Use of a fixed-film carrier media allows the growth of 

additional biomass compared to a similarly sized activated sludge system, which can result in 

increased nitrification and denitrification in the given volume. Carrier media can be 

retrofitted into existing suspended-growth systems, provided that sufficient volume is 

available in the reactors. Separate aerobic and anoxic zones could be required if both 

nitrification and denitrification are desired. 

Integrated Fixed-Film Activated Sludge 

IFAS systems are hybrids that have attached growth media included in an activated-sludge 

basin. The media for attached growth can be a fixed type or a floating type. Fixed media 

include rope or some other stranded material (example brand names include Ringlace® and 

Bioweb®). Floating media can include sponges (Captor®, Linpor®), packing material, such 

as the saddles that would be used in packed towers for air stripping or gas absorption; or 

plastic media similar to those used in an MBBR (Kaldnes®, Hydroxyl®, or Bioprotz®) 

(Copithorn 2007; Welander and Johnson 2007). Floating media can be free-floating or 

retained in the basin by enclosing them in cages or installing screens. 

IFAS systems usually have higher treatment rates and generate sludges with better settling 

characteristics and lower mass than activated-sludge systems. Because the media can be held 

in zones within the overall basin, there is the possibility of having nitrification and 

denitrification occur in the same basin, in separate anoxic and aerobic zones. To obtain very 

low nitrogen concentrations in the effluent, two distinct anoxic zones should be provided so 

that one can be used for final polishing. Alternatively, separate zones can be eliminated with 

tight control of DO. Selection factors include a medium footprint using the existing 

activated-sludge system, with construction within the aeration basin to install the selected 

method of media retention. No additional piping and pumping are typically needed, as long 

as the flow pattern is set to obtain sequential aerobic/anoxic conditions. If no additional 

pumping is needed, electrical costs are likely to be reduced for zones no longer being aerated. 

If a carbon source needs to be fed to secondary anoxic zones to maintain denitrifying activity, 

some additional sludge will be generated, and O&M costs will increase. 

The IFAS system reduces the footprint significantly by providing additional surface area for 

attached growth to occur within the same basin area as a comparable activated-sludge 

process. As such, a retrofit IFAS is frequently an alternative to adding tank capacity to an 

activated-sludge system (Johnson et al. 2005). A fine screen (3 to 6 mm) is recommended 

upstream of the secondary process to prevent material like hair from interfering with the 

surface area of the medium. To promote nitrification and denitrification, a higher DO 

concentration might be required in the aerobic zone when compared to an extended-aeration 

process. The extra oxygen could be required to penetrate the biomass growth on the medium 

so that it does not become anaerobic. Aeration is often done by diffused air (fine or coarse 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-16 Chapter 2: Treatment Technologies 

bubble), as mechanical aeration could interfere with the fixed-film media (Johnson et al. 

2005). Higher RAS rates tend to promote better TN removal. The IFAS process is illustrated 

in Figure 2-7. The range of TN effluent concentrations at plants with IFAS technologies in 

the literature evaluated as part of this study was 2.8 mg/L to 17.0 mg/L (see Table 2-1 in 

Section 2.5). 

Anoxic 

Tank

Aerobic Tank

Secondary 

Clar ifier

Influent Effluent

Tanks contain f ixed or floating 

media for a ttached growth

Waste

Sludge

Medi a re tention sieves 

(if necess ary) RAS

Figure 2-7. Integrated fixed-film activated sludge process. 

Moving-Bed Biofilm Reactor 

An MBBR consists of small plastic media (carrier elements) in an anoxic or aerobic zone that 

allow attached growth to occur. The plastic is typically polyethylene with a specific gravity 

slightly less than 1.0. The carrier elements from most manufacturers are shaped like cylinders 

or wheels with internal and external fins. These shapes provide a high surface area per unit 

volume that is protected from shear forces, allowing better biofilm growth. The MBBR 

process can be retrofitted into an existing activated-sludge basin. This process combines the 

technologies of activated sludge and biofilm processes and is frequently used for upgrading 

an existing plant, especially when space is limited. Such high-rate biofilm processes are 

highly efficient in removing organic and nitrogen loads. 

MBBRs can be used within separate aerobic and anoxic zones. Slow-speed, submersible 

mixers are used in anoxic zones; air is supplied in aerobic zones by coarse bubble diffusers 

because fine bubbles tend to coalesce in the plastic media. A sieve is used to retain the media 

in the designated basin. MBBR technology does not involve any return flows and does not 

rely on suspended growth to provide additional treatment. Selection factors include a 

medium footprint (however, including a recycle stream would increase the footprint), with 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-17

construction of only screens for retaining the carrier elements needed in the basins. 

Depending on how the system is being operated and the target limit, there might or might not 

be a need for an additional carbon source or post-reactor treatment; if no additional carbon is 

needed, no additional sludge will be generated, reducing O&M costs. The MBBR process is 

illustrated in Figure 2-8. The range of TN effluent concentrations for MBBR found in the 

literature evaluated as part of this study was 2.8 mg/L to17 mg/L (see Table 2-1 in Section 

2.5). 

Figure 2-8. Moving-bed biofilm reactor process. 

Membrane Bioreactor 

The membrane bioreactor (MBR) consists of anoxic and aerobic zones followed by a 

membrane that filters the solids from the mixed liquor, taking the place of secondary 

clarifiers. The membranes can be immersed in the final activated-sludge basin, or they can be 

set up in a separate vessel. The membranes can function in an inside-out mode, where only 

clean water exits the membranes, or in an outside-in manner, where only clean wastewater 

can enter the membranes. Usually wastewater must be pumped through the membranes, but 

in certain circumstances gravity feed can be used. By removing the need for settling, MBRs 

can operate at higher mixed liquor suspended solids (MLSS) concentrations (8,000 to 18,000 

mg/L) than a comparable activated-sludge system (2,000 to 5,000 mg/L). The membranes are 

typically configured as hollow fiber tubes or flat plates. The membranes can either be 

immersed in the final tank, or they can be operated as a separate, stand-alone unit. If they are 

operated as a separate unit, an internal recycle returns a portion of the solids retained by the 

membranes to the anoxic zone. 

Anoxic 

Tank 

Aerobic Tank 

Secondary 

Clarifier 

Influent Effluent 

Tanks contain suspended plastic 

media for attached growth 

 Waste 

Sludge 

Media retention 

sieves 
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2-18 Chapter 2: Treatment Technologies 

Biofouling of the membranes is typically the most significant operational issue. Some 

manufacturers schedule a relax period or a back pulse to reduce the growth. The membranes 

must be cleaned periodically (quarterly to semiannually) using citric acid or sodium 

hypochlorite, depending on the type of fouling and the system manufacturer’s 

recommendations. In addition, reducing the sludge age tends to reduce the formation of 

extracellular polymers, which tend to be the major cause of fouling. MBR system selection 

factors include a smaller footprint than conventional activated-sludge systems, with 

construction in existing basins or clarifiers. Lower effluent nitrogen is obtained with second 

anoxic zones, potentially accompanied by carbon source addition. As mentioned earlier, 

additional pumping is likely to be required to get water through the membrane, thus requiring 

additional electricity. The additional chemicals result in more sludge formation, but MBR 

systems overall provide less sludge formation than conventional systems because of their 

higher SRTs and higher sludge concentrations. New facilities are being designed and built to 

meet limits of 3 mg/L TN. The MBR process is illustrated in Figure 2-9. 

Anoxic 

Tank

Aerobic

Tank

Membrane 

Filter

Influent Effluent

Retained Biomass

Waste

Sludge

Membrane 

Pump

RAS

Aerobic

Tank

Anoxic 

Tank

(a) 

Figure 2-9. MBR process. (a) External filter in lieu of clarifier 
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Chapter 2: Treatment Technologies 2-19

Anoxic 

Tank

Aerobic

Tank Membrane 

Filter

Effluent

Influent

Waste

Sludge

Membrane 

Pump

Aerobic

Tank

Anoxic 

Tank

(b) 

Figure 2-9. MBR process. (b) In-tank filter 

Step-Feed Activated Sludge 

The difficulty with carrying out full nitrification/denitrification with a sequence of aerobic 

and anoxic reactors is that little carbon is available to maintain microbial populations in the 

secondary reactors. As described earlier, this problem can be overcome by feeding 

supplemental carbon. Alternatively, feed can be provided directly to each anoxic zone to 

ensure that sufficient carbon is available. This step-feed strategy has the additional advantage 

of providing a means for the plant to better handle wet-weather events by reducing the solids 

loading to the clarifiers. The biomass inventory is maintained preferentially more in the first 

section and then less in the second section and the succeeding sections in proportion to the 

flow split going into each section. The last section maintains the least amount of biomass in 

inverse proportion to the flow and thus the MLSS concentration is the lowest. The solids 

loading rate to the clarifiers thus will be the lowest for the flow. The microorganisms in the 

later stages treat not just the fresh feed but also anything coming from upstream zones. The 

selection factors are a relatively large footprint for the basins, with in-basin retrofit of 

additional piping and possibly pumping. Extra head could be required, depending on the 

hydraulic configuration of the system. If additional pumping is required, electrical costs will 

be increased. However, no additional chemicals should be required because the food needs of 

the secondary microorganisms are accounted for. The step-feed activated sludge process is 

illustrated in Figure 2-10. The range of TN effluent found in the literature evaluated as part 

of this study was 1.0 mg/L to 14.0 mg/L (see Table 2-1 in Section 2.5). 
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2-20 Chapter 2: Treatment Technologies 

Secondary 

Clarifier

Effluent

Influent

RAS

WAS

Aerobic 

Tank

Anoxic 

Tank

Aerobic 

Tank

Anoxic 

Tank

Aerobic 

Tank

Anoxic 

Tank

Figure 2-10. Step-feed activated sludge process. 

Biodenitro Process 

The Biodenitro process, a variation of the oxidation ditch, consists of two oxidation ditches 

side by side. Influent is fed alternately to the ditches, allowing anoxic and aerobic zones to 

form for nitrification and denitrification. The ditches are alternately aerated and not aerated, 

and mixing is maintained by the flow in the ditches. The ditches periodically switch modes, 

and the overall result is that the water passes through multiple aerobic and anoxic zones 

before discharge. Because oxidation ditches require large footprints, having multiple ditches 

requires a great deal of land. Some pumping might be required, depending on the hydraulic 

profile. Because the influent is continually switched, the need for additional food is typically 

reduced or eliminated. To accomplish low effluent nitrogen concentrations, an external 

anoxic zone following the oxidation ditches might be needed. The Biodenitro process is 

illustrated in Figure 2-11. The range of TN effluent concentrations for a phased isolation 

ditch (PID) found at the case study facilities for this study was 1.8 mg/L to 7.0 mg/L and in 

the literature evaluated as part of this study was 1.6 mg/L to 5.4 mg/L (see Table 2-1 in 

Section 2.5). 
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Chapter 2: Treatment Technologies 2-21

Secondary 

Clarifier

Influent Effluent

RAS

WAS

Alternating 

Aerobic/Anoxic Tanks

Flows switch when tanks 

alternate Tank 1-Anoxic

Tank 2-Aerobic

Figure 2-11. Biodenitro process. 

Schreiber Process

The patented Schreiber countercurrent aeration process can provide nitrification and 

denitrification in one basin. The wastewater enters a circular basin equipped with a rotating 

bridge that provides mixing. Aeration is provided by fine-bubble diffusers attached to the 

bridge. Should sequencing between aerobic and anoxic conditions be required, the aeration 

can be turned off while the bridge continues to keep the tank mixed. The system footprint is 

approximately that of a conventional activated-sludge system but with additional equipment 

built in. Because it includes alternating aerobic/anoxic conditions in one tank, very low 

effluent TN concentrations are possible, when optimized. The performance could be further 

enhanced with additional anoxic zones downstream of the primary reactor. In the standard 

Schreiber configuration, the need for additional food is typically reduced or eliminated 

because everything is done in one tank. The Schreiber countercurrent aeration process is 

illustrated in Figure 2-12. The TN effluent concentrations for the Schreiber process in the 

literature evaluated as part of this study was 8.0 mg/L (see Table 2-1 in Section 2.5). 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-22 Chapter 2: Treatment Technologies 

Secondary 

Clarifier

Influent Effluent

Waste

Sludge

RAS

Rotating 

Bridge

Aeration/Anoxic/

(Anaerobic) Tank

Aeration on timer

Figure 2-12. Schreiber countercurrent aeration process. 

Sequencing Batch Reactor 

Facilities with small or intermittent flows might benefit from using an SBR. Such reactors 

are filled over the course of time, and then the contents are processed under the conditions 

deemed necessary to achieve the required treatment. In the case of nitrogen removal, the 

conditions would include both aerobic and anoxic time. Multiple SBR units allow for 

continuous feeding of wastewater and batch processing. SBRs typically have four phases: a 

fill phase, during which mixing is maintained; a react phase with alternating aerobic and 

anoxic cycles; a settle phase, during which mixing is turned off so the microorganisms can 

settle; and a decant phase, when the effluent is drained. Depending on the length of time 

needed for filling, reacting, and decant, multiple SBR units allow for receiving wastewater 

continuously. The footprint for SBRs can be small, depending on the required number of 

reaction vessels. The need for additional food is typically low because everything is done in 

one vessel. Very low effluent nitrogen can be obtained through the use of multiple aeration 

and anoxic steps, with food added if needed. An SBR is illustrated in Figure 2-13. The range 

of TN effluent concentrations for the SBR found in the literature evaluated as part of this 

study was 1.6 mg/L to 13.6 mg/L (see Table 2-1 in Section 2.5). 
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Chapter 2: Treatment Technologies 2-23

Influent Effluent

Waste

Sludge

SBR under 

aerobic/anoxic 

conditions 

Cycle times 

determined based 

on conditions

Figure 2-13. Sequencing batch reactor. 

Sidestream Processes 

One issue that needs to be decided at plants where low TN is required in the effluent is 

whether and how to treat recycle nitrogen loads coming from sludge processing and other 

activities and deciding whether to include a separate treatment of those streams. These 

streams typically contain high concentrations of ammonia nitrogen and would make 

treatment in the main biological treatment system difficult to manage when the sludge 

handling processes send large loads. 

Typical solutions for high sidestream loads would be equalization; diurnal control of the 

sidestream flow so that it is fed during lower mainstream load periods; or dedicating one 

mainstream train to handle the sidestream. Often, a better solution is to treat the sidestream 

before recycle. The following systems have been proposed for such sidestream nitrogen 

treatment and have been implemented at some places in the United States and Europe. 

InNitri Process—Nitrification 

The ammonia-laden water is treated in a separate nitrification reactor before recycling to the 

plant headworks. This therefore reduces the ammonia-nitrogen load in the recycle stream. 

The recycle stream then provides a seed of nitrifying bacteria to the main reactor. Such a 

supply would often not be available in the mainstream because of having to maintain too low 

a SRT so that the nitrifiers would wash out of the system (Philips and Kobylinski 2007). This 

constant feed of nitrifiers is thus beneficial for facilities that must nitrify or achieve low 

effluent concentrations year-round. The sidestream reactor size can be small, and it can be 

operated at an elevated temperature compared to the main reactor. By seeding that main 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-24 Chapter 2: Treatment Technologies 

system with nitrifiers, the main SRT can be reduced, which can thus reduce overall capital 

and O&M costs. The process has been piloted in the United States in Arizona (Warakomski 

et al. 2007). The process is illustrated in Figure 2-13A. 

Figure 2-13A. InNitri process. 

Bio-Augmentation Batch Enhanced (BABE)—Nitrification 

The BABE process is a variation on the InNitri process. In BABE, the reactor is a batch 

system that is fed batches of return sludge from the main activated sludge system along with 

the sidestream. This batch reactor is operated both aerobically and anoxically and therefore 

both nitrifies and denitrifies the sidestream. This means that alkalinity lost during the 

nitrification process will be partially recovered during denitrification, and the sidestream will 

not require neutralization before reintroduction to the mainstream. The effluent of the batch 

reactor contains nitrifiers, which will enhance the population in the main system. Full-scale 

testing has been done in the Netherlands (Philips and Kobylinski 2007). The process is 

illustrated in Figure 2-13B. 

There are a number of other bio-augmentation processes that have been developed in Europe 

and elsewhere. Examples include the Mainstream Autotrophic Recycle Enabling Enhanced 

N-removal (MAUREEN), the Bio-augmentation R (regeneration) (BAR) process, the 

Aeration Tank 3 (AT-3) Process, and the biofilm activated sludge innovative nitrification 

(BASIN) process (Parker and Wanner 2007). The BAR and AT-3 processes have been 

proven in demonstration scale, while others show promise for the future. 

Activated Sludge 

Tank 

Secondary 

Clarifier 

Influent Effluent 

RAS 

WAS 

Nitrification Recycled Sidestream 

Reactor 
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Chapter 2: Treatment Technologies 2-25

Figure 2-13B. BABE process. 

Nitritation–Denitritation 

A recently developed alternative to conventional nitrification/denitrfication is the process of 

nitritation, where only nitrite is produced aerobically. This process is sold under the 

proprietary name SHARON, or Single-reactor High-activity Ammonia Removal over Nitrite. 

The process is run at elevated temperatures (30–35 °C) at lower SRTs to favor growth of 

ammonia oxidizers (such as Nitrosomonas sp.) over nitrite oxidizers (such as Nitrobacter

sp.). Denitrifiers are then encouraged to convert the nitrite to nitrogen gas. By not oxidizing 

all the way to nitrate, oxygen and energy usage is reduced (Warakomski et al. 2007). 

Methanol can be used for dentrification, if necessary. The process is used at several locations 

in Europe and, as of 2007, was being installed at the New York City Ward Island Water 

Pollution Control Facility. The SHARON process is illustrated in Figure 2-13C. 

Activated Sludge 

Tank 

Secondary 

Clarifier 

Influent Effluent 

RAS 

WAS 

BABE Recycled Sidestream 

Reactor 

Bio-augmentation 
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2-26 Chapter 2: Treatment Technologies 

Figure 2-13C. SHARON process 

Another process for converting ammonia to nitrite uses a newly discovered group of 

autotrophic microorganisms that can anaerobically oxidize ammonia using nitrite. This is 

called the ANAMMOX process, for Anaerobic Ammonia Oxidation. These microorganisms 

are favored by elevated temperatures (above 35 °C), and grow very slowly. In this case, a 

strategy for obtaining nitrite is to use the first step of the SHARON process to produce 

nitrite, then oxidizing a bypassed ammonia stream with the nitrite. ANAMMOX systems 

have been implemented in Europe, in particular in Rotterdam (Warakomski et al. 2007). The 

ANAMMOX process is presented in Figure 2-13D. Two fixed-film processes using similar 

strategies to SHARON and ANAMMOX—the Oxygen Limited Aerobic NitrificationDenitrification (OLAND) and Completely Autotrophic Nitrogen Removal Over Nitrite 

(CANON)—are under development (Stensel 2006). 

Activated Sludge 

Tank 

Secondary 

Clarifier 

Influent Effluent 

RAS 

WAS 

Recycled 

Sidestream Nitritation 

Reactor 

(Aerobic) 

Denitritation 

Reactor 

(Anoxic) 

Heat 

Exchanger 
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Chapter 2: Treatment Technologies 2-27

Figure 2-13D. ANAMMOX process. 

2.3 Phosphorus Removal Processes 

Phosphorus can be removed from wastewater by biological uptake by microorganisms and by 

chemical precipitation with a metal cation. Depending on the target concentration, a plant 

process might employ both technologies. Such a combined approach might be of particular 

benefit if the target concentration is very low and the starting concentration is high. In such a 

case, biological removal is used to remove the bulk of the phosphorus, and chemical 

polishing follows to achieve the final concentration; such an approach tends to reduce sludge 

formation. 

2.3.1 Biological Phosphorus Removal 

Biological Phosphorus Species 

The reactions involved in biological phosphorus removal are as follows (Pattarkine and 

Randall 1999): 

1. Anaerobic biological phosphorus release by Phosphate Accumulating Organisms 

(PAOs): 

 PAOs + stored polyphosphate + Mg++ + K+

+ glycogen + VFA Æ 

 PAOs + stored biopolymers + Mg++ + K+

 +_CO2 + H2O + PO4

3- (released) 

Activated Sludge 

Tank 

Secondary 

Clarifier 

Influent Effluent 

RAS 

WAS 

Nitritation Recycled Sidestream 

Reactor 

(Aerobic) 

Denitritation 

Reactor 

(Anoxic) 

Ammonia Bypass 
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2-28 Chapter 2: Treatment Technologies 

2. Aerobic biological phosphorus uptake by PAOs: 

 PAOs + stored biopolymers + Mg++ + K+ + O2 (or NO3) + PO4

3- Æ 

 PAOs + stored polyphosphate + Mg++ + K+

 +glycogen + CO2 + H2O 

Biological phosphorus removal works by encouraging the growth of phosphate-accumulating 

organisms (PAOs), which are then subjected first to anaerobic conditions and then to aerobic 

conditions. Under anaerobic conditions, the microbes break the high-energy bonds in 

internally accumulated polyphosphate, resulting in the release of phosphate (PO4

3-) and the 

consumption of organic matter in the form of volatile fatty acids (VFAs) or other easily 

biodegraded organic compounds. VFAs include short-chained carboxylic acids such as 

acetic, proprionic, butyric, and valeric, among others. When the microbes are then put under 

aerobic conditions, they take up phosphate, forming internal polyphosphate molecules. This 

luxury uptake results in more phosphate being included in the cells than was released in the 

anaerobic zone, so the total phosphate concentration in solution is reduced. When the microorganisms are wasted, the contained phosphate is also removed. 

Beyond the luxury uptake of phosphate, microorganisms also remove phosphate as part of 

their normal BOD removal. A small amount of phosphorus is removed in the conventional 

activated-sludge process during BOD removal. This amount is typically 1.5 to 2 percent on a 

dry weight basis (WEF and ASCE 1998). 

When these organisms enriched in polyphosphate are wasted, the contained phosphate is also 

removed. PAOs that are exposed to an anaerobic environment followed by an aerobic zone 

can exhibit phosphorus removal levels 2.5 to 4 times higher than those for conventional 

activated-sludge systems (WEF and ASCE 1998). Thus, the fraction of phosphorus in wasted 

dry solids could be at least 8 percent or more. 

A sufficient supply of VFAs is the key to removing phosphorus biologically. Barnard et al. 

(2005) reported that when using a mixture of acetic and propionic acids produced by on-site 

fermentation, the COD-to-TP ratio in the plant influent could be as low as 8. As the 

phosphorus permit limits have been lowered in recent years, chemical polishing, often 

combined with better filtration processes, has become necessary. A discussion of significant 

factors that affect the phosphorus removal process follows. 

Biological Phosphorus Removal Factors 

Volatile Fatty Acid Availability in Wastewater 

A key factor in determining the cost-effectiveness of biological phosphorus removal is the 

relative amount of organic material that can be used by the PAOs. That is because if VFAs or 

rbCOD is not present in a sufficient ratio to ortho-phosphorus, the process becomes less 

reliable and phosphorus removal can be reduced. 
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Chapter 2: Treatment Technologies 2-29

For facilities where retrofitting is being considered, the data most likely available are COD 

and BOD. Literature indicates that a COD-to-TP ratio of 45 and a BOD-to-TP ratio of 20 are 

the minimum values needed to meet a 1 mg/L TP limit (McGrath 2005a; WEF and ASCE 

2006). If data are available about rbCOD, literature shows that good biological phosphorus 

removal was observed at rbCOD-to-TP ratios of about 15 (Barnard et al. 2005). The rbCOD 

can be converted to short-chain VFAs in the anaerobic zone and then used by PAOs 

(Lindecke et al. 2005). Finally, if VFAs have been determined, it has been found that a 

minimum VFA-to-TP ratio of at least 4 is recommended to obtain good biological 

phosphorus removal (Neethling et al. 2005). 

In-plant generation of VFAs is made possible in many different ways at existing facilities––

adding a new fermenter (for either primary or RAS), converting a sludge thickener, or 

returning supernatant from an existing anaerobic digester. Fermenters can be single-stage or 

two-stage. They are optimized by adding a mixer that allows VFAs in the solids to enter the 

anaerobic zone directly. A source of elutriating water, either primary effluent or final 

effluent, is fed to the fermenter to flush out the VFAs produced and sent to the anaerobic 

zone. The VFA production from fermenting primary sludge typically yields 0.066 to 

0.15 g VFA/g total solids (both expressed as COD), although values up to 0.3 g VFA/g solids 

have been reported (Barnard et al. 2005). Complete-mix fermenters are typically designed 

with an HRT of 6 to 12 hours, an SRT of 4 to 8 days, and a solids concentration between 

1 and 2 percent (WEF and ASCE 2006, p 326). 

Temperature 

Although the biological phosphorus removal process is not significantly affected by 

temperature, the fermentation process is slower at low temperatures (Lindeke et al. 2005). 

Therefore, lower phosphorus removal might occur in the winter because of reduced VFA 

production in the plants that use fermenters. The temperature effect was reported as directly 

influencing the sludge age needed for adequate generation of VFAs (Baur et al. 2002). At 24 

°C, a 1-day sludge age was sufficient. A 4-day sludge age was required to generate sufficient 

VFAs at 14 °C. 

At the upper end of the temperature range, the performance of PAOs showed reduced 

phosphorus uptake activity above 30 °C and seriously inhibited activity at 40 °C (Panswad et 

al. 2003; Rabinowitz et al. 2004). At temperatures above 30 °C, glycogen-accumulating 

organisms (GAOs) were reported as a detriment to enhanced biological phosphorus removal 

(EBPR) (Barnard 2006). 

Solids Retention Time 

Very good biological phosphorus removal performance was reported when SRT values of 

16 and 12 days were provided for wastewater at 5 °C and 10 °C, respectively. The system 

performance was not affected when the SRT was varied between 16 and 24 days at 5 °C. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-30 Chapter 2: Treatment Technologies 

Similarly, varying the SRT between 12 and 17 days at 10 °C did not affect the biological 

phosphorus removal (Erdal et al. 2002). 

Secondary Phosphorus Release 

Secondary release is of concern in EBPR plants. Biological phosphorus removal occurs in a 

two-step process, in which phosphorus is released in the anaerobic zone by PAOs and then 

taken up by the same PAOs in the aerobic zone. The microorganisms favored in the 

anaerobic zone are capable of absorbing more phosphorus in the aerobic zone than was 

released in the anaerobic zone, leading to a net reduction in phosphorus when these 

microorganisms are removed from the process through settling and wasting. However, if the 

PAOs are put under anaerobic conditions following phosphorus update, there will be 

unintended secondary release of phosphorus. Anaerobic conditions can arise in secondary 

clarifiers with long SRTs, inside the tertiary filters, and in some sludge-handling operations. 

In addition, anaerobic conditions can arise in the activated sludge unit if aeration is not 

maintained at a sufficiently high level or if the extent of oxygen tapering through a plug flow 

system is too great. The released phosphorus is often returned to the head of the secondary 

process, via the RAS from the secondary clarifiers or supernatant/filtrate from sludgehandling operations. This recirculated phosphorus thus increases the load on the secondary 

process and decreases the overall biological phosphorus removal that can be achieved. 

Secondary phosphorus release can be reduced by minimizing the amount of time that mixed 

liquor or return sludge is held before recirculation, reducing return flows from sludgehandling operations, and treating the sludge-handling return streams before introduction to 

the secondary process. In some cases, a small dose of alum is added to the tertiary filter to 

minimize the secondary release (see the Clark County, Nevada, Case Study in Chapter 3). 

Sidestream treatment has been proposed for some large facilities, including facilities in New 

York (Constantine 2006) and Washington, D.C. (Constantine 2005). 

Nitrates in Return Streams 

Nitrates in the return streams, such as RAS or internal recycle lines, can negatively affect 

biological phosphorus removal. The nitrates cause consumption of the VFAs needed for 

biological phosphorus removal and introduce a source of oxygen that is used before 

phosphorus release, minimizing the amount of biological phosphorus that can be removed. 

Because 1 mg/L nitrate-N is the equivalent of 2.86 mg/L DO, and nitrate does not have a 

maximum concentration like DO, nitrate has the potential to cause even more disruption to 

biological phosphorus removal than DO. In a step-feed system, McGrath et al. (2005) 

determined that 6 mg/L of nitrate nitrogen was the upper limit tolerable for successful EBPR 

in one particular full-scale operation. If sufficient rbCOD is available, higher concentrations 

of return stream nitrate could possibly be tolerated with adequate phosphorus removal.
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Chapter 2: Treatment Technologies 2-31

Dissolved Oxygen in Return Streams 

Similar to nitrates, DO entering the anaerobic zone negatively affects biological phosphorus 

removal. The recycled DO essentially reduces the anaerobic HRT, because it needs to be 

reduced to zero before anaerobic metabolic activities for biological phosphorus removal can 

occur. Because the PAOs are facultative aerobes, they will use any available oxygen present 

in the anaerobic zone to aerobically metabolize VFAs. This results in a reduced anaerobic 

phosphorus release and, thus, reduced phosphorus uptake in the aerobic zone. In addition, 

heterotrophic aerobic organisms present in the anaerobic zone will compete for the VFAs, 

further reducing PAO anaerobic metabolism. If DO sources in the return stream cannot be 

minimized, additional VFAs might be required (Benisch 2004). 

2.3.2 Chemical Phosphorus Removal 

Chemical Phosphorus Species 

Current chemical phosphorus removal design is based on equilibrium precipitation theory 

(WEF 1997, 1998; USEPA 1987a, 1987b). The chemical precipitate of ortho-phosphorus is 

carried by treatment with a trivalent metal cation, typically ferric ion (Fe3+) or aluminum 

(Al3+). Ferric ion is typically supplied in the form of ferric chloride (FeCl3). Aluminum is 

supplied as alum (aluminum sulfate). When a source like waste pickle liquor is available, the 

ferrous ion (Fe2+) can be used as the metal cation. The precipitation reaction depends on the 

various phosphate species (e.g., H2PO4

1-,, HPO42-) being converted to PO43-, with the 

consumption of alkalinity (or OH-, hydroxide ion). This means that sufficient alkalinity must 

be present for the chemical precipitation reaction to be completed. 

1. Conversion of phosphate species to phosphate ion: 

 HnPO4

(3-n)- + nOH- Æ PO43- + n H2O 

2. Chemical phosphorus removal by alum (aluminum sulfate): 

 Al2 (SO4)3 Æ 2 Al3+ + 3 (SO4

2-) 

 Al3++ PO4

3- Æ AlPO4 (s) 

3. Chemical phosphorus removal by ferric chloride: 

 FeCl3 Æ Fe3+ + 3 Cl-

 Fe3+ + PO4

3- Æ FePO4 (s) 

4. Ferrous ion conversion to ferric ion: 

 2Fe2+ + 1⁄2 O2 + 2H+ Æ 2Fe3+ + H2O 
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2-32 Chapter 2: Treatment Technologies 

Chemical addition occurs in primary clarifiers (when present) or in the secondary process, in 

the aeration basin or upstream of the secondary clarifiers, or in tertiary clarifiers or other 

treatment processes. Feeding chemicals to the primary clarifiers usually requires less 

chemical use than feeding to the secondary or tertiary process. Feeding chemicals at both the 

primary and secondary clarifiers results in less chemical use than feeding to the secondary 

process alone, allowing some phosphorus to be removed in the primary clarifiers and 

polishing to occur in the secondary process. 

Recent research shows that in addition to equilibrium precipitation, sorption is a predominant 

phenomenon. The factors that promote sorption, such as sorbent characteristics, variable 

stoichiometry, alkalinity, mixing at point of dosage, diffusion, and time-based kinetics, are 

processes that must be considered in designing systems to achieve a low total phosphorus (TP) 

concentration (Smith et al. 2007). 

Chemical Phosphorus Removal Factors 

Phosphorus Species at Application Point 

Phosphorus in the raw wastewater is found in three forms—organically bound phosphorus, 

polyphosphate, and orthophosphate. Organically bound phosphorus can be settled in the 

primary clarifiers or transformed into orthophosphates by the microorganisms in the 

secondary process. Polyphosphates are soluble and pass through the primary clarifiers. In the 

secondary process, the polyphosphates are converted biologically into orthophosphates. 

Orthophosphates are also soluble and will pass into the final effluent if they are not removed 

biologically or chemically. Orthophosphates readily form a precipitate following the addition 

of metal salts, forming a floc that can then be settled or filtered from the wastewater. 

Metal-to-Phosphorus Ratio 

In general, the molar ratio of the metal to influent phosphorus concentration (Al to P or Fe to 

P) increases as the target phosphorus effluent concentration decreases. The curve is flatter at 

higher effluent phosphorus concentrations, but it becomes particularly steep as the target 

concentration decreases below 0.5 mg/L (WEF and ASCE 1998). For a given amount of 

phosphorus to be removed, the amount of chemical that needs to be fed is less for plants that 

employ tertiary chemical treatment following biological phosphorus removal, as compared to 

plants that use one- or two-point chemical phosphorus removal. When simultaneous 

precipitation is practiced, the MLSSs will contain a high phosphorus concentration, and it 

could easily exceed the concentration for an EBPR sludge. 

Choice of Chemical 

Alum and ferric chloride are often used for chemical phosphorus removal. The choice of 

which chemical to use should be made on the basis of jar testing and chemical costs. The 

impact on downstream processes is another factor to consider. Alum sludge can be more 

difficult to thicken and dewater than sludge from ferric chloride. Ferric chloride is more 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-33

corrosive and requires special piping. In addition, ferric chloride can cause problems with 

ultraviolet disinfection if the chemical is baked onto the quartz sleeves. Ferrous sulfate is a 

viable alternative if a source is available. The presence of other metals or species that react 

with phosphate can cause simultaneous precipitation of multiple metal phosphates or 

complexation effects that would thus require higher aluminum or ferric doses than expected. 

Feed Point Location 

Chemicals for phosphorus removal can be added to the primary clarifiers, the secondary 

treatment system (either to the aeration basin or upstream of the secondary clarifiers), or the 

tertiary treatment process. The chemical should be well mixed with the wastewater at the 

feed location to form a good floc. If the chemical is not fed to a well-mixed location, an inline static mixer might be needed. 

Phosphorus is a micronutrient and is needed by the microorganisms in the aeration basin to 

remove BOD and nitrogen. Care must be taken not to remove too much phosphorus in the 

primary clarifiers. A general rule of thumb is that the phosphorus in the influent to the 

activated-sludge basin should be 1 to 1.5 percent of the BOD to be removed. For example, if 

200 mg/L of BOD is to be removed in the aeration basin, at least 2 to 3 mg/L of phosphorus 

is required in the primary effluent. This rule-of-thumb ratio, however, can vary according to 

sludge age and the chemical dosage. If too much phosphorus is removed in the primary 

clarifiers, the operation of the aeration basin might be negatively affected. 

Number of Feed Points 

Alum or ferric chloride can be fed at one, two, or more locations in the plant. Single-point 

application works well for plants that need to achieve moderate phosphorus removal 

concentrations (approximately 0.5 mg/L). Two feed points, which might be either primary 

and secondary clarifiers or secondary clarifiers and tertiary treatment, can achieve lower 

phosphorus concentrations and can use less chemicals than dosing in one location. Several of 

the tertiary treatment processes operate more efficiently if the TP concentration in the 

secondary effluent is less than 1 mg/L. For plants that do not have the ability to remove 

phosphorus biologically, two chemical feed points might be required because the tertiary 

treatment process typically requires alum or ferric chloride to form a floc before removal. 

Three feeding locations might be appropriate for plants that have a relatively high TP raw 

influent concentration (greater than 6 mg/L), which might occur in communities with 

significant commercial and industrial discharges. 

Mixing Requirements 

Rapid mixing is required when the chemical is added to the wastewater to allow the 

molecules to react. In addition, the density and viscosity of the metal salts are larger than that 

of the wastewater, which allows the chemical to settle to the bottom. If rapid mixing 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-34 Chapter 2: Treatment Technologies 

conditions do not exist at the chemical injection location, an in-line static mixer might be 

required. 

After the initial rapid mixing, gentle mixing is required to allow larger flocs to form. Usually, 

the movement of the wastewater through the treatment plant is sufficient for the floc 

formation. However, turbid areas of the plant might break up the floc, which would decrease 

phosphorus removal through settling or filtration. 

pH 

Phosphorus solubility varies between iron and aluminum ions. Theoretically, the lowest 

effluent phosphorus concentration achievable with iron salts is 0.07 mg/L at a pH of 

approximately 6.9 to 7.0 s.u. Recent laboratory results indicate that uniformly high removal 

may occur at pH values between 5.5 and 7 (Smith 2007). Aluminum salts are theoretically 

capable of achieving effluent phosphorus concentrations down to 0.01 mg/L in a pH range of 

6.6 to 7.2 s.u. (Kang et al. 2001; WEF and ASCE 1998). The actual effluent phosphorus 

concentration achieved and the optimum pH range will be site-dependent because of other 

chemical reactions that occur in the wastewater. Recent research indicates that the highest 

phosphorus removal efficiency occurs at a pH of between 5.5 and 7.0 s.u. However, the pH 

of the wastewater does not have a significant effect on chemical phosphorus removal (Smith 

et al. 2007). Alum and ferric chloride are acidic and therefore capable of lowering the 

effluent pH. If sufficient alkalinity is not available to adequately buffer the wastewater, pH 

adjustment might be needed. 

Suspended Solids Removal 

Capturing the floc formed during chemical precipitation plays an important role in 

phosphorus removal. Sludge at WWTPs that use biological phosphorus removal contains 4.5 

percent phosphorus on a dry weight basis compared to 1.5 percent phosphorus for plants that 

use chemicals (USEPA 1987a). The phosphorus in the suspended solids in the effluent will 

require additional removal of the total suspended solids (TSS) in the final effluent in meeting 

low phosphorus limits. Tertiary processes like clarifiers or filters are more efficient at 

capturing solids. Feeding metal salts in conjunction with the tertiary process improves 

phosphorus removal. 

Sludge Handling 

Chemical phosphorus removal generates additional sludge. Phosphorus can be released from 

the chemical sludge if the sludge is exposed to or stored in an anaerobic environment. Return 

flows, such as supernatant or water from dewatered sludge, can contain high levels of 

phosphorus. Maintaining aerobic conditions in the sludge is recommended. If this is not 

feasible, treatment of the sludge return flows should be considered. 
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Chapter 2: Treatment Technologies 2-35

Recent analysis of phosphorus precipitation (Smith et al. 2007) suggests possible 

thermodynamic and kinetic factors in achieving extremely low (less than 0.01 mg/L) effluent 

phosphate concentrations. These factors include pH, metal (aluminum or iron) used for 

precipitation, the presence of additional metals, materials that can complex with the 

phosphate such as COD and TSS, alkalinity, degree of mixing, and process time. 

2.3.3 Phosphorus Removal Technologies 

Enhanced Biological Phosphorus Removal Technologies 

Fermentation 

If biological phosphorus removal is desired, the process is carried out under anaerobic 

conditions with VFAs providing the carbon source for the microorganisms. If the influent has 

too low a VFA concentration for adequate phosphorus removal (greater than approximately 

4-to-1 mass of VFAs to mass of phosphorus in the influent to be removed) the concentration 

must be increased. The VFAs can be supplied from an outside source or can be formed by 

on-site fermentation of primary sludge or RAS. Fermenting sludge to generate VFAs has the 

additional advantage of reducing the amount of sludge to be disposed of. Fermenting the 

primary sludge is preferred to using secondary sludge because doing so reduces the amount 

of phosphate released from the sludge. Fermentation is particularly helpful for large plants in 

cold climates and for plants where the strength of the wastewater tends to be low. In some 

situations, fermentation also occurs in the collection system, as well as in the anaerobic zone. 

Fermentation is illustrated in Figure 2-14. 

Anaerobic Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

WAS

RAS + 

VFA

Fermentation 

of Primary 

Sludge or RAS

RAS to Fermenter 

(optional)

Primary Sludge to 

Fermenter (optional)

Figure 2-14. Fermentation process. 

Anaerobic/Oxic (A/O), or Phoredox, Process 

The anaerobic/oxic (A/O) process consists of an anaerobic zone upstream of an aerobic zone. 

The RAS enters the head of the anaerobic zone with the influent. In the anaerobic zone, 
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2-36 Chapter 2: Treatment Technologies 

PAOs release phosphorus, which is subsequently taken up in the aerobic zone. One potential 

problem for A/O operation is that any nitrates recycled from the aerobic zone of side streams 

can inhibit anaerobic growth (selection) of PAOs. To reduce this effect, the anaerobic zone is 

often split into an anoxic chamber for nitrate denitrification and a series of anaerobic zones 

for phosphorus release. The process has a medium-sized footprint and is relatively easy to 

retrofit into an existing basin by installing baffle walls and mixers to produce an anaerobic 

zone. If sufficient VFAs are available, an additional carbon source is not needed. Because 

there is some additional pumping, there are some additional electrical costs; however, less 

sludge is generated under anaerobic conditions. To obtain extremely low phosphorus (less 

than 0.1 mg/L), chemical addition should be examined. The A/O (Phoredox) process is 

illustrated in Figure 2-15. The range of TP effluent concentrations evaluated in the literature 

reviewed for this study was 0.025 mg/L to 2.3 mg/L (see Table 2-5 in Section 2.5 for details). 

Anaerobic Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

WAS

Figure 2-15. A/O (Phoredox) process. 

Oxidation Ditch 

Oxidation ditches were discussed earlier in the Nitrogen Removal section. The design and 

operation of an oxidation ditch for phosphorus removal is much the same, with the 

requirement that an anaerobic zone be established. The anaerobic zone (sometimes called a 

selector) can be set up within the ditch or as an external tank upstream of the ditch. The 

oxidation ditch does not necessarily need to be operated with anoxic zones, although doing 

so can aid in the partial recovery of alkalinity. As with the A/O process, additional carbon in 

the form of VFAs is needed only if sufficient rbCOD is not already present in the influent. To 

obtain very low phosphorus (under 0.1 mg/L), additional carbon is required. The carbon 

should be added upstream of the secondary clarifier to avoid depleting that nutrient from the 

biological process. Lower TP concentrations can be achieved by close monitoring and 

regulation of the anaerobic zone flow and DO levels. An oxidation ditch with an anaerobic 

zone is illustrated in Figure 2-16. TP effluent concentrations range between 1 and 2 mg/L 

with the anaerobic zone. See Concentric Oxidation Ditch elsewhere in this chapter for its 

performance. 
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Chapter 2: Treatment Technologies 2-37

Secondary 

Clarifier

Influent Effluent

Flow Aerators

Anaerobic 

Zone 

(Selector)

WAS

RAS

Figure 2-16. Oxidation ditch with anaerobic zone. 

Physical-Chemical Technologies 

Chemical Addition 

Aluminum or iron salts––most commonly alum (aluminum sulfate) and ferric chloride––can 

be added to precipitate phosphorus in the secondary clarifiers or tertiary filters. Adding 

chemicals results in a limited capital investment initially, but chemical costs and the 

additional sludge generated increase the O&M costs of the process. Chemicals can be added 

at a number of locations in the wastewater treatment process; the most common are the 

primary clarifiers, the secondary basins just before the secondary clarifiers, and upstream of 

the tertiary filters. Care must be taken in the first two cases not to add so much chemical as to 

take the phosphorus below the concentration needed to sustain the secondary treatment 

microorganisms. 

Chemical addition allows the facility to obtain extremely low effluent concentrations because 

more chemical can always be used with no significant impact on the microorganisms. 

However, using more chemicals creates a greater volume of sludge. Filtration can be used to 

remove fine precipitate particles and achieve lower concentrations. The footprint for 

chemical addition is small, and only limited piping and pumping are required (see Chapter 5 

for more details). This means the capital costs are low, but the operating costs can be high for 

chemicals and sludge disposal. The phosphorus chemical/filter process is illustrated in Figure 

2-17. The range of TP effluent concentrations from combined chemical addition followed by 

filtration from the case studies and evaluated in the literature reviewed for this study was 0.1 

mg/L to 2.3 mg/L (see Table 2-5 in Section 2.5 for details). 
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2-38 Chapter 2: Treatment Technologies 

Aerobic Tank Secondary 

Clarifier

Effluent

Secondary 

Influent

RAS

WAS

Filter

Chemical Addition 

(Alum or Ferric) 

(optional)

Chemical Addition 

(Alum or Ferric) 

(optional)

Filter Backwash

Primary 

Clarifier

Chemical Addition 

(Alum or Ferric) 

(optional)

Figure 2-17. Phosphorus chemical/filter process. 

Chemical Removal with Conventional Filtration Technologies 

Sand filters are used to polish effluents by removing suspended solids. To the extent that 

phosphate is associated with suspended solids, sand filters can provide effective removal. 

Rapid mixing should occur before entering the filter to ensure that the chemicals are 

distributed throughout the secondary effluent, allowing floc to form. Descriptions of the 

some of the filtration alternatives are provided below. They have particular application 

downstream of chemical addition and when the discharge standard is below 0.1 mg/L. 

Filters can be used to capture phosphorus in the solid phase. Adding chemicals can increase 

the effectiveness of the filters by precipitating the soluble phosphorus into a solid form that 

can be captured by the filters. Several varieties of filters can be used in multiple 

configurations. Filter media can range from mono-media, such as sand, to multimedia, such 

as sand with anthracite or gravel. Standard and deep-bed filters are available. Cloth filters, 

which use cloth to capture the solids rather than sand, can also be used. Filters can be 

operated in series to improve removal. For example, a deep-bed filter could be operated in 

series with a standard bed filter. A clarifier is used to remove solids from the filter backwash 

before the treated backwash is returned to the head of the plant. 

Filter Media 

Media within the filters trap particles between the pores. The smaller the pore spaces, the 

smaller the particles that can be removed. Descriptions of the most common filter media 

follow. 

Sand Filters 

Sand is typically used in single-medium filters. The size and uniformity of the sand depend 

on the application and characteristics of the secondary effluent at the individual plant. The 

uniformity of the sand is important because varying sizes can provide smaller pores for 
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Chapter 2: Treatment Technologies 2-39

greater filtration; however, during backwashing the media can become stratified, with the 

smaller particles toward the top of the filter. Large solids particles in the secondary effluent 

might fill the pores between the small particles, decreasing the amount of water that can pass 

through the filter, increasing head loss, and leading to more frequent backwash cycles. 

Dual-Media Filters 

Dual-media filters typically use anthracite in combination with the sand. Anthracite can 

adsorb organic compounds in addition to filtering solids. It is softer than sand, and abrasion 

during backwashing can decrease the size of the anthracite particles. These smaller particles 

could cause blinding of the filter by trapping larger solids on the surface of the media, which 

would block the passage of the water if the anthracite is not removed during the backwash 

cycle. Other media combinations in dual-media filters include activated carbon and sand, 

resin beds and sand, and resin beds and anthracite. 

Multimedia Bed Filters 

Multimedia beds can contain anthracite, sand, and garnet or ilmenite. Garnet and ilmenite 

have a higher density than sand, allowing them to settle to the bottom of the filter after 

backwashing. In addition, garnet and ilmenite can have a smaller pore size than sand, 

allowing the smallest solids to be trapped by the garnet before the wastewater exits the filter 

when operated in a downflow mode. 

Backwash Methods 

Filters can be backwashed using water, air, or a combination of air and water. Filters can be 

backwashed using water alone. Air backwashing can be used as part of the backwashing 

process, but it is usually not effective in removing solids particles without the use of water. 

Descriptions of various backwash protocols are provided below. 

The water acts to fluidize the bed, thereby releasing the particles trapped between the pores. 

The most effective way to backwash a bed is to force the filter media to rub together. This 

assists in removing the secondary effluent particles, which can adhere to media because of 

the biological nature of the solids, which can be sticky. Backwashing the filter using air 

before or during the water backwash cycle allows scouring of the media. 

Air and water can be used simultaneously to backwash the filter. This process reduces the 

amount of water required to backwash the filter and also requires less time, reducing the 

volume of backwash water generated per cycle. Simultaneous air and water backwash can 

produce a cleaner filter, which will extend the run time of the filter compared to water alone 

or air scour followed by water. In addition, stratification does not occur with simultaneous air 

and water backwashing, so the smaller media grains do not accumulate at the surface of the 

filter bed. The smaller grains at the filter surface can become clogged by large particles in the 

wastewater, which reduces the filter run time. In an unstratified bed, the media grain sizes are 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-40 Chapter 2: Treatment Technologies 

distributed uniformly throughout the filter, allowing the suspended solids to be trapped 

throughout the filter depth, further increasing the run time of the filter. 

Filtered effluent can be used to backwash and remove the solids trapped in the filter media. 

The filter typically needs to be backwashed at a high rate, approximately 15 to 20 gpm/ft2, 

for at least 15 to 20 minutes. Backwashing generates a large volume of reject water to be 

returned to the head of the plant. The slug of backwash water, which contains solids and 

phosphorus, can negatively affect the operation of the treatment plant. To minimize the 

impact of the filter backwash, the water can be stored in an equalization basin and slowly fed 

back to the head of the plant; treatment with chemicals to remove some of the solids is also 

possible. 

Bed Depth 

The media depth can vary with the type of filter selected. Traveling bridge and pulse bed 

filters have relatively shallow bed depths of 10 to 12 inches. Standard beds typically contain 

approximately 2 feet of filter media, while deep beds contain approximately 4 to 6 feet of 

media. The deeper the bed, the longer the filter can operate before backwashing. However, 

deeper beds require a higher backwash rate to release the solids trapped in the media. 

Gravity Filters 

Wastewater is applied to the top of the media bed, allowing the water to flow downward by 

gravity. The wastewater is then collected in underdrains that allow the wastewater to enter 

but retain the filter media. Once a set amount of time passes or a predetermined head loss is 

measured in the filter, a backwash cycle is initiated. The filter is backwashed by either water 

or an air/water combination that removes the trapped solids from the media. The solids are 

directed to the head of the plant, while the filter media are retained. After the backwash cycle 

is complete, the filter is placed back into service. (Usually, the entire filter must be taken out 

of service to be backwashed.) A supply of filtered effluent (stored in a clear well) to 

backwash the filter might be required. If the production rate of the filters that remain in 

service is greater than the required backwash rate, the clear well might not be needed. 

Alternatively, some filters are divided into two to four cells, which allows only one cell to be 

taken out of service at a time while the remaining cells continue to produce water that can be 

used to backwash the out-of-service cell. 

If the sand or alternate medium contains a variety of particle sizes, the smaller grains might 

accumulate at the top of the filter. The smaller grains have smaller pore sizes, which might be 

filled or blinded by the wastewater at a faster rate than the larger grains. This accumulation 

might lead to more frequent backwashing of the filters. Using an air/water backwash system, 

rather than water alone, might minimize this problem by not fully fluidizing the bed. 
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Chapter 2: Treatment Technologies 2-41

Moving-Bed Filter 

A moving-bed filter cleans a portion of the granular media continuously so that operation 

does not need to stop to perform a backwash cycle. The filter can operate in a downflow or 

upflow mode. In the downflow mode, wastewater enters the top of the filter and flows 

downward through the media. The solids filtered from the wastewater are drawn downward 

with the sand. An airlift pump transfers the solids and sand to the top of the filter, where a 

filter washbox is located. The sand is separated from the solids by gravity. The cleaned sand 

is returned to the top of the filter, and the solids are returned to the plant headworks or 

directed to disposal. 

In an upflow moving-bed filter, the wastewater enters through the bottom of the filter and is 

pumped upward through the sand. Solids captured in the sand move downward and are 

airlifted to a reject compartment through the center of the filter. The turbulence created by 

the air lift pumps separates the solids from the sand. The clean sand is separated from the 

solids by gravity. The solids are directed to the headworks of the plant, and the clean sand is 

deposited on top of the filter. The advantage of an upflow filter is that the wastewater 

encounters the sand containing the most solids first and passes through the cleanest sand 

before it exits the filter over a weir. 

Pulsed-Bed Filters 

Pulsed-bed filters have a relatively shallow bed containing approximately 10 inches of sand 

or alternate medium. Secondary effluent is applied to the top of the filter. Diffusers are 

located on the top of the bed surface. Once the water level reaches the air mix probe, air is 

supplied to the diffusers, suspending the larger particles above the bed; filtration continues 

during this process. As operation continues, solids might settle onto the surface of the bed, 

causing a rise in the water level to the pulse mix probe. When the bed is pulsed, effluent is 

not discharged from the filters. Air that is trapped in the underdrains is released by backwash 

pumps and travels upward through the clogged media. The solids are released at the top of 

the bed and suspended by the diffusers on the media surface. The effluent valve is then 

reopened, and operation of the filters can continue. After a set number of pulses, a full 

backwash of the filter occurs to remove the trapped solids from the filter. The pulses are 

designed to extend the operation of the filter and decrease the number of backwash cycles 

required compared to a conventional filter. 

Traveling Bridge/Automatic Backwash Filter 

The automatic backwash filter or traveling bridge filter has a relatively shallow sand depth of 

12 inches. The width of the unit is typically fixed at approximately 16 feet. The length of the 

traveling bridge filter is determined by the amount of surface area required for a given 

application. Wastewater is applied to the top of the sand and filters downward through the 

filter. The head loss across the filter is relatively low at less than 4.9 feet (WEF and ASCE 

1998, pp. 16–19). A traveling bridge and backwash hood move along the filter to backwash 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-42 Chapter 2: Treatment Technologies 

one cell of the filter at a time. This allows the remaining filter cells to stay in operation while 

a portion of the filter is backwashed. The rate of backwash water generated is less because of 

the shallow sand bed and smaller area backwashed when compared to a conventional sand 

filter. In a conventional filter, the backwash might need to be stored to allow the solids to be 

fed slowly back to the head of the process to avoid slug loads. A filter then removes solids 

that pass through the tertiary clarifier. The traveling bridge filter produces a relatively 

constant amount of backwash when the filter is in operation. 

Cloth Filters 

Cloth filters, as the name implies, use specially designed cloth to filter the wastewater, rather 

than sand or other granular media. The cloth panels are installed vertically inside a steel or 

concrete tank. The wastewater submerges the cloth panels and travels horizontally through 

them. Solids accumulate on the outside of the cloth panels, while filtered water is collected 

on the inside of the panels and directed to the effluent chamber. The solids on the outside of 

the cloth form a mat, and the water level in the filter rises. When the water reaches a preset 

level, the filter is backwashed by liquid suction. The cloth filters are rotated during the 

backwash process. Two cloth filter panels are backwashed at a time, allowing the other 

panels to continue filtering water, thereby eliminating the need to have a tank to store flows 

for backwashing the filter. The backwashed solids are directed to the headworks of the plant. 

Larger solids settle to the bottom of the basin, from which they are periodically pumped out 

and directed to the headworks of the plant or the solids-handling process. 

Cloth filters can operate at a higher hydraulic loading rate than granular media filters, 

resulting in a smaller footprint. The backwash rate is also reduced because there is no need to 

fluidize the bed as required with granular media filters. Cloth filters are usually installed in 

small plants (average flows less than 5 to 10 MGD); however, the Orange County, Florida, 

WWTP uses cloth filters at an average annual flow of 29.5 MGD and plans to expand to 

accommodate 43 MGD. 

Tertiary Clarification with Filtration 

The practice of adding tertiary clarifiers upstream of filters can further achieve low solids 

concentrations and thus low phosphorus effluent levels. Scott and Laurence (2007) 

performed a pilot study using the combination of tertiary clarification followed by filtration 

to obtain effluent concentrations in a municipal wastewater consistently below 0.05 mg/L, 

and sometimes below 0.01 mg/L, when using alum doses of 75 mg/L with polymer addition. 

Tertiary clarifiers that could be used include solids contact clarifiers, upflow buoyant-media 

clarifiers, tube clarifiers, plate clarifiers, and a second set of secondary clarifiers. To improve 

performance through the tertiary clarifiers, a coagulant, such as alum or ferric chloride, and a 

polymer can be added upstream of the unit. 
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Chapter 2: Treatment Technologies 2-43

Solids contact clarifiers mix the secondary effluent with coagulants and previously settled 

solids, forming larger floc. After mixing in the center of the clarifier, the wastewater moves 

outward to the settling zone, where the solids move downward to the bottom and the treated 

water exits the unit over a weir. Periodically, solids are removed from the clarifier for 

treatment and disposal. A filter then removes solids that pass through the tertiary clarifier. 

The upflow buoyant-media clarifier mixes the coagulant and polymer with the secondary 

effluent, allows flocculation to occur, and provides clarification. A tube clarifier has inclined 

tubes in a portion of the clarifier. The water flows up through the tubes, and solids flow 

downward to the bottom of the clarifier, from which they are pumped out. Similarly, a plate 

clarifier has inclined plates installed in a portion of the clarifier. The water flows upward 

between the plates, and solids settle onto the plates and slide down to the bottom of the 

clarifier. For the tertiary clarification process to be successful, the velocity through the unit 

must be low enough to allow the solids in the secondary effluent to settle. 

Membrane Filtration Technologies 

Membrane filters can be used externally to remove suspended solids or can be incorporated 

into the activated-sludge process as an MBR. MBR systems employ a suspended-growth 

biological reactor, from which effluent is passed through a membrane filter. By so doing, 

suspended solids are effectively removed from the effluent. Phosphate is retained in the 

reactor as polyphosphate taken up by the microorganisms (if biological phosphate removal is 

operating) or as chemically generated suspended solids. Crawford et al. (2006) examined 

phosphorus removal at several U.S. plants, including one in Traverse City, Michigan. In all 

configurations, the membrane is associated with the final aerobic step, and microorganisms 

are recirculated with a portion of the membrane concentrate. If chemicals are needed to 

achieve low effluent phosphorus, they can be added before the membrane to precipitate 

whatever the microorganisms have not taken up. At the Hyrum WWTP in Utah, an MBR 

produced effluent concentrations of 0.07 mg/L for the annual average. The facility uses an 

aluminum salt for coagulation and chemical phosphorus removal. Another MBR facility at 

Lone Tree Creek, Colorado, achieves an annual average of 0.027 mg/L. 

Specialty Filter Descriptions 

Dynasand D2 Advanced Filtration System 

The Dynasand D2 advanced filtration system consists of deep bed and standard bed upflow 

filters in series. The deep bed filter contains coarse sand and uses a proprietary process called 

continuous contact filtration, which allows coagulation, flocculation, and separation to occur 

in the filter. The standard bed filter is filled with a finer sand mix. Both filters are 

continuously backwashed. The backwash water from the filters is treated in a lamella gravity 

settler, a high-rate gravity plate settler, to remove solids before being returned to the head of 

the plant. Alum is added upstream of both filters. The process is depicted in Figure 2-18. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-44 Chapter 2: Treatment Technologies 

Figure 2-18. Parkson Dynasand D2 advanced filter system. 

Actiflo 

Actiflo is a sand-ballasted flocculation process. Metal salt and polymer are added upstream 

of the coagulation tank. The pH is adjusted to optimize phosphorus removal on the basis of 

the wastewater characteristics at the specific site and the type of polymer used. The 

wastewater is then mixed with fine sand and polymer. The fine sand, referred to as 

microsand, provides a large surface area to which the formed floc can attach; it also increases 

the sedimentation rate by acting as ballast. The solids are settled in a clarifier equipped with 

lamellar tubes. The microsand is recovered in a cyclonic separator and returned to the 

process. A sand filter can follow the Actiflo process to capture additional solids and further 

reduce the effluent phosphorus concentration. 

DensaDeg 

DensaDeg is a high-rate solids contact clarification process that consists of a reactor zone, a 

presettling/thickening zone, and a clarification zone. Metal salts are mixed with the influent 

to the process, and the pH is adjusted to optimize phosphorus removal based on the 

wastewater characteristics at the specific site. The wastewater then enters the base of the 

reactor and is mixed with sludge returned from the solids contact clarifier. The reactor tank 

contains a turbine and draft tube that promote floc formation and separate the solids. Polymer 

is added to increase the sludge density. In the presettling/thickening zone, the sludge settles 

to the bottom because of the increased density and continues to thicken. Sludge is returned to 

the reactor zone or removed from the process for further treatment. In the clarification zone, 

Dynasand 

Deep Bed 

Filter 

Dynasand 

Standard 

Bed 

Filter 

Final 

Effluent 

Lamella 

Filter 

Alum 

Alum 

Reject 

Water 

Secondary 

Effluent 

2nd filter 

scour 

water 1st

filter 

scour 

water
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Chapter 2: Treatment Technologies 2-45

the supernatant flows up through the settling tubes as the effluent from the process. 

Additional phosphorus can be removed by following the process with filtration. 

Enhanced Biological Phosphorus Removal with Filtration Technologies 

Chemical addition, filtration, or both can be used to polish effluent from biological 

phosphorus removal processes. Drury et al. (2005) reported chemical polishing of EBPR 

effluent ahead of the tertiary filter at Clark County, Nevada. An upper feed limit was 

established at 10 mg/L alum to prevent plugging in the filter, while the effluent phosphorus 

concentration was regularly as low as 0.1 mg/L. At Durham, Oregon, Stephens (2004) 

reported effluent concentrations as low as 0.07 mg/L with chemical polishing and filtration. 

Any of the biological phosphorus removal technologies described previously could be 

combined with chemical addition, conventional filtration, membrane filtration (external to 

the activated-sludge process), or the specialty filtration processes mentioned earlier. 

Emerging Technologies 

The following processes have undergone testing, treating a portion of the flow at a minimum 

of one plant. Full-scale data are not available for these processes, although the processes are 

being considered for upgrades at several facilities. 

CoMag Process 

The CoMag process consists of ballasted flocculation, solids contact clarification, and highgradient magnetic separation. The flocculation tank has three compartments. In the first 

compartment, wastewater is mixed with a metal salt and the pH is adjusted to optimize 

phosphorus removal on the basis of the wastewater characteristics at the specific site. Then 

fine magnetic particles are added to increase the density of the floc. In the third compartment, 

polymer is added to increase flocculation. The wastewater enters a solids contact clarifier. 

Most of the solids are returned to the flocculation tank. The remaining solids are wasted from 

the system. A magnet separator captures solids that passed through the clarifier by attracting 

the magnetic particles that were added in the second compartment of the flocculation tank. 

The effluent is then sent to the disinfection process for the treatment plant. The backwash 

from the magnet separator is mixed with the wasted sludge, which passes through a magnetic 

ballast recovery system to minimize the loss of the magnetic ballast. The remaining sludge is 

sent to the sludge-handling system for the plant. The recovered magnetic ballast is returned 

to the flocculation tank (Tozer 2007). The CoMag process is illustrated in Figure 2-19. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-46 Chapter 2: Treatment Technologies 

Figure 2-19. CoMag process. 

Blue PRO Process 

The Blue PRO process consists of addition of a chemical, typically ferric chloride to form 

ferric phosphate precipitate. This step is followed by a proprietary pre-reactor zone and 

moving-bed filter. The process uses a Centra-flo continuous backwashing filter. Unlike most 

filters, which rely on trapping solids between the media particles to remove phosphorus, the 

Blue PRO filtration system contains a bed of hydrous ferric oxide-coated media, in which 

ferric phosphate and other pollutants are filtered. The abrasion of the sand particles against 

one another in the moving bed filter exposes new adsorption sites on the media. The process 

can be operated in a dual-stage mode with two Blue PRO filtration systems in series. In the 

Blue PRO-CEPT system, the reject from the filters is returned to the head of the plant. The 

process is illustrated in Figure 2-20. 

Magnetic 

Separator 

Clarifier 

Separator 

Water

Magnetic 

Ballast 

Recovery 

Fresh 

Magnetic 

Ballast 

Sludge 

Processing 

Chemical 

Treatment 

To 

Disinfection

Metal Salt + Polymer 

pH Control 

Feed 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-47

Figure 2-20. Blue-PRO process. 

Trident HS 

The Trident HS system consists of two clarification processes followed by filtration. Metal 

salts and polymer are added upstream of the tube clarifier, which contains a recycle flow of 

precipitated solids to decrease the variation of the influent quality entering the unit. From the 

tube clarifier, additional polymer is added before the wastewater enters an adsorption 

clarifier. The adsorption clarifier consists of a buoyant-media bed to remove additional solids 

before filtration. The media in the clarifier do not have any adsorption properties. The unit is 

an upflow filter containing coarse media. The accumulated solids are flushed from the 

clarifier using air and water from the tube clarifier. A mixed-media gravity filter follows the 

two-stage clarification process for applications designed to meet phosphorus concentrations 

of less than 0.1 mg/L. The filter is backwashed using air and water simultaneously. The 

process is depicted in Figure 2-21. 

Figure 2-21. U.S. Filter Trident process. 

Tube 

Clarifier 

Adsorption 

Clarifier 

Mixed 

Media Filter 

Final 

Effluent Influent 

Al/Fe 

Polymer 

Polymer 

Fe3+

Moving Bed Filter 1 

Pre-reactor 1 

Secondary 

Effluent 

Fe3+

Pre-reactor 2 

Moving Bed Filter 2 

Reject 

Effluent 

Reject 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-48 Chapter 2: Treatment Technologies 

Infiltration Basin/Land Application 

Use of an infiltration basin or land application is the state of the science in phosphorus 

removal technology. Phosphorus is further removed through land application, before 

discharge to surface water. In addition, some soils are capable of adsorbing phosphorus, 

thereby extending the limit of technology for phosphorus removal. Land application requires 

a large amount of available space. A system designed for the Brighton, Michigan, plant 

employs approximately 25 acres/MGD treated. In that system, after biological treatment in an 

oxidation ditch, chemical treatment to 0.2 mg/L, and tertiary clarification, the wastewater is 

sprayed onto the land and collected through a system of underdrains. The treated water is 

then discharged to a surface water. The treated water has a phosphorus concentration of less 

than 0.01 mg/L at all times. The soil should be tested before constructing a land application 

system to verify the adsorption capacity of the soil and predict the length of time that the 

field can be used. In the case of Brighton, the design flowrate for the infiltration basin is 2.54 

gal/day/ft2

. As designed, the soil had a cation exchange capacity of 2.0 milliequivalents/100 

g; with the initial phosphorus content in the soil, the mini-column test results presented in the 

engineering report indicate an estimated useful life of the soil of at least 40 years. 

2.4 Nitrogen and Phosphorus Removal Processes 

2.4.1 Nitrogen and Phosphorus Removal Factors 

The removal factors for processes that achieve both nitrogen and phosphorus removal are a 

combination of the removal factors for processes that target removal of only one of the 

nutrients. 

Balancing the various factors might be necessary to achieve adequate TN and TP removal. 

For instance, nitrates in the RAS stream might need to be minimized to promote biological 

phosphorus removal in the anaerobic zone. This is more likely to be an issue at plants with a 

single anoxic zone because the nitrates created after nitrification in the aeration zone do not 

have an opportunity to denitrify. To avoid this problem, the RAS could be held in an 

equalization tank before return to the anaerobic zone. The equalization zone might also 

reduce the DO in the RAS, which would further improve the performance of the anaerobic 

zone. The retention time, however, would have to be limited to prevent secondary release of 

phosphorus. Alternatively, the RAS could be diverted to the anoxic zone rather than to the 

anaerobic zone. 

Plants achieving both nitrogen and phosphorus removal must be designed and operated to 

avoid the release of phosphate without either VFAs or nitrates present. Anaerobic or anoxic 

zones that are too big remove carbon and nitrates, resulting in the release of phosphate that is 

not taken up in the aerobic zone. Similarly, if nitrate removal has been enhanced, there might 

not be any way to prevent phosphorus release in the lower portions or deep in the sludge 

blanket of the secondary clarifier. If that happens in the sludge, the release might not 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-49

immediately affect the effluent directly, but it could result in a buildup of phosphorus in the 

secondary treatment system that would eventually raise the final phosphorus concentration. 

This trade-off between nitrate and biological phosphorus removal requires well-considered 

design, analysis, and process control. As an alternative, nitrate removal can be made extremely 

efficient in the secondary process, and chemical precipitation and filtration can be used 

subsequently to remove phosphorus to an extremely low level. 

There are several other factors to be aware of in a combined system. In general, the COD-toTKN ratio determines which zones need to be large and which should be small. Oxygen in the 

feed to anaerobic or anoxic zones should be minimized because it will inhibit those reactions. 

Temperature can also affect the processes: under cold conditions, fermentation might not 

provide sufficient VFAs for the less temperature-sensitive phosphate uptake reaction, and 

denitrification can be slowed. The result would be reduced phosphorus and nitrogen removal. 

Because phosphate removal and nitrification consume alkalinity while denitrification supplies 

alkalinity, it could be that some alkalinity will have to be supplied to allow all processes to 

proceed normally. In addition, phosphorus is a necessary nutrient for microbial growth. If TP is 

reduced too low before a biological process like denitrification, the growth of the 

microorganisms could be inhibited, thereby requiring the addition of supplemental phosphorus. 

The biological removal of phosphorus and of nitrogen compete for available carbon at certain 

plants with two anoxic zones, such as those in the 5-stage Bardenpho process. The available 

carbon in the influent or return side streams can be used for biological phosphorus removal 

and nitrification. However, some plants might require methanol or an alternative carbon 

source to be added to the second anoxic zone. Plants that are required to meet low effluent 

phosphorus limits (less than 0.1 mg/L) will likely require an alternative carbon source and 

tertiary filtration. 

An evaluation comparing the 5-stage Bardenpho process and the 4-stage Bardenpho process 

might be warranted in situations that would require methanol to be fed for nitrogen removal, 

as well as alum or ferric chloride for additional phosphorus removal. In the 4-stage 

Bardenpho process, phosphorus could be removed only chemically. Depending on the 

operation of the aerobic zones, sufficient carbon could be available at the second anoxic zone 

to eliminate the need to feed methanol. The 4-stage Bardenpho process would have no 

additional sludge from the nitrogen removal process because methanol would not be added; 

however, there would be more sludge from the chemical phosphorus removal. By using 

chemical phosphorus removal only, some of the balancing act described previously between 

biological phosphorus and nitrogen removal would not be necessary, making operation of the 

plant easier. If methanol addition can be avoided, it can save some costs and O&M problems 

associated with its handling. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-50 Chapter 2: Treatment Technologies 

2.4.2 Nitrogen and Phosphorus Removal Technologies 

Anaerobic/Anoxic/Oxic 

The anaerobic/anoxic/oxic (A2O) process consists of an anaerobic zone, an anoxic zone, and an 

aerobic zone. An internal recycle stream returns nitrates from the aerobic zone to the anoxic 

zone, as in the MLE process. RAS is recycled to the head of the anaerobic zone along with the 

secondary influent. This process thus allows for simultaneous nitrogen and phosphorus removal. 

With the inclusion of the anoxic zone, the concentration of nitrates in the return sludge is reduced 

(compared to the A/O process), meaning that the anaerobic process is more efficient. The 

existing activated-sludge basin can be modified to include the anaerobic and anoxic zones, 

assuming sufficient volume remains in the aerobic zone to perform nitrification. With two 

recycle streams, some piping and pumping are needed, but extra head might not be required 

depending on the hydraulics of the plant. With only one anoxic zone and if VFAs are sufficient, 

there might not be a need to supplement the carbon. With A2

O, as with all combined nitrogenphosphorus biological systems, some phosphorus is taken up in the anoxic zone by the PAOs, 

and the sludge residence time in each zone must be sufficient to allow complete phosphate 

release or uptake. The clarifier also must be operated to regularly waste solids to avoid release of 

phosphate by the endogenous respiration of PAOs. The A2O process is illustrated in Figure 2-22. 

The ranges for TN and TP removal technologies evaluated in this study are 7.3 mg/L to 9.0 mg/L 

and 0.025 mg/L to 0.98 mg/L, respectively (see Tables 2-1 and 2-5). These results are for effluent 

from a system such as that shown in Figure 2-22. Another facility using A2O followed by 

chemical addition and filtration achieved TP concentrations of 0.13 mg/L annual average (see 

Table 2-2 in Section 2.5). 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anaerobic 

Tank

WAS

Figure 2-22. A2O process. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-51

Five-Stage Bardenpho Process 

The 5-stage Bardenpho process is similar to the 4-stage Bardenpho process with the 

exception that an anaerobic zone is added ahead of the 4-stage system. The internal recycle 

from the first aerobic zone to the first anoxic zone remains in place. RAS is returned to the 

head of the anaerobic zone and is fed with the secondary influent (or primary effluent if 

primary settling is implemented). Methanol might need to be fed to the second anoxic zone to 

provide a carbon source for denitrification. As with the 4-stage Bardenpho, the footprint is 

large and internal construction is needed during retrofits to define the zones. Extra head is not 

typically needed; however, nitrate recycle between the first aerobic zone and anoxic zone 

might require additional pumping. The second anoxic zone could require additional carbon in 

the form of methanol or as VFAs if such are being added to support phosphorus uptake. For 

extremely low phosphorus concentrations, chemical addition, preferably in a downstream 

system so as to not remove all phosphate from the biological system, could be used. The 

5-stage Bardenpho process is illustrated in Figure 2-23. The ranges of TN and TP effluent 

concentrations from the case studies that used 5-stage Bardenpho with chemical addition 

were 0.87 mg/L to 5.59 mg/L in TN and 0.06 mg/L to 1.09 mg/L in TP (see Table 2-3 in 

Section 2.5 for details). 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Effluent

Influent

RAS

Nitrified Recycle

Anoxic 

Tank

Aerobic Tank

(Optional) Methanol 

Addition

WAS

Anaerobic Tank

Figure 2-23. Five-stage modified Bardenpho process. 

University of Cape Town Process 

The University of Cape Town (UCT) process, a variation of the Phoredox process, consists 

of anaerobic, anoxic, and aerobic zones. An internal recycle returns nitrates from the aerobic 

zone to the head of the anoxic zone. A second internal recycle returns wastewater from the 

anoxic zone to the head of the anaerobic zone. RAS is directed to the head of the anoxic zone 

to minimize the amount of nitrates entering the anaerobic zone. The intent of the design is to 

keep the concentration of VFAs and the phosphate-accumulating reactions high, without 

competition from denitrification reactions using the VFAs. This process has a medium-sized 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-52 Chapter 2: Treatment Technologies 

footprint and could be set up in existing basins, depending on the volume available. As with 

other processes in this section, no additional head would be needed, but there would be 

extensive piping and pumping for the recycle streams. If sufficient VFAs are present, no 

supplemental carbon sources are required. Achieving a very low phosphate concentration 

requires downstream chemical precipitation and filtration. The UCT process is illustrated in 

Figure 2-24. The TN and TP effluent concentrations found in the literature for the UCT 

process with filtration were 8.9 mg/L to 10.0 mg/L in TN and 0.3 mg/L in TP (see Tables 2-5 

and 2-8 in Section 2.5 for details). 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anaerobic 

Tank

WAS

Mixed Liquor 

Recycle

Figure 2-24. University of Cape Town process. 

Modified University of Cape Town Process 

The modified UCT process, another variation of the Phoredox process, has an anaerobic zone 

followed by two anoxic zones and an aerobic zone upstream of the secondary clarifiers. The 

two anoxic zones in series are designed to operate such that no nitrates are returned to the 

anaerobic zone. The nitrates from the aerobic zone are returned to the head of the second 

anoxic zone, while a second internal recycle returns flow from the end of the first anoxic 

zone to the head of the anaerobic zone. RAS is directed to the head of the first anoxic zone. 

This process has a medium-sized footprint and could be set up in existing basins if sufficient 

volume is available. As with other processes in this section, no additional head is needed, but 

extensive piping and pumping are needed for the recycle streams. If sufficient VFAs are 

present, no supplemental carbon sources are required. Achieving a very low phosphate 

concentration requires downstream chemical precipitation and filtration. The modified UCT 

process is illustrated in Figure 2-25. The ranges of TN and TP effluent concentrations found 

in the literature for the modified UCT process with VFA addition were 5.0 mg/L to 6.0 mg/L 

in TN and 0.1 mg/L to 2.7 mg/L in TP (see Table 2-8 in Section 2.5 for details). 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-53

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anaerobic 

Tank

WAS

Mixed Liquor 

Recycle

Anoxic 

Tank

Anoxic 

Tank

Figure 2-25. Modified University of Cape Town process. 

Virginia Initiative Process 

The Virginia Initiative process (VIP) is similar to the modified UCT process and is another 

variation of the Phoredox process. The nitrates from the aerobic zone are returned to the head 

of the first anoxic zone, instead of the second anoxic zone as is done with the modified UCT 

process. The second return is from the end of the second anoxic zone to the head of the 

anaerobic zone. RAS continues to enter the head of the first anoxic zone. The VIP process 

allows for additional denitrification and thus minimizes the introduction of nitrate to the 

anaerobic zone. Nitrate in the anaerobic zone would interfere with phosphorus release, and so 

would reduce the opportunity for subsequent phosphorus uptake in the aerobic zone. The VIP 

process is operated in a high-rate mode, allowing for small tank volumes, which require less 

space than other similar processes. This process has a medium-sized footprint and could be 

set up in existing basins. As with other processes in this section, no additional head is 

needed, but extensive piping and pumping are needed for the recycle streams. As with the 

other processes, if sufficient VFAs are present, no supplemental carbon sources are required. 

Achieving a very low phosphate concentration requires downstream chemical precipitation and 

filtration. The VIP is illustrated in Figure 2-26. The ranges of TN and phosphorus effluent 

concentrations found in the literature for the VIP were 3.0 mg/L to 10.0 mg/L in TN and 0.19 

mg/L to 5.75 mg/L in TP; for the VIP with VFA addition the ranges were 5.0 mg/L to 10.0 

mg/L in TN and 0.6 mg/L to 0.8 mg/L in TP (see Tables 2-1, 2-2, and 2-3 in Section 2.5 for 

details). 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-54 Chapter 2: Treatment Technologies 

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anaerobic 

Tank

WAS

Mixed Liquor 

Recycle

Anoxic 

Tank

Anoxic 

Tank

Figure 2-26. Virginia Initiative process. 

Johannesburg Process 

The Johannesburg process, another variation of the Phoredox process, consists of anaerobic, 

anoxic, and aerobic tanks in series. An internal recycle returns nitrates from the end of the 

aerobic zone to the head of the anoxic zone. An anoxic zone on the RAS line allows 

denitrification to occur, reducing the amount of nitrates that enter the anaerobic zone. 

Denitrification in the anoxic tank on the RAS line can be limited by a lack of carbon, which 

can be overcome by bringing sludge from the end of the anaerobic zone to the RAS-line 

anoxic zone. The dedicated anoxic zone allows a smaller footprint than some other systems. 

As with other processes in this section, no additional head is needed, but extensive piping 

and pumping are needed for the recycle streams. If sufficient VFAs are present, no 

supplemental carbon sources are required. Achieving a very low phosphate concentration 

requires downstream chemical precipitation. The Johannesburg process is illustrated in 

Figure 2-27. The range of TN for one application of the Johannesburg process was 2.03 mg/L 

to 11.44 mg/L TN (see Table 2-1 in Section 2.5 for details). 

Anoxic 

Tank

Aerobic Tank Secondary 

Clarifier

Influent Effluent

RAS

Nitrified Recycle

Anaerobic 

Tank

WAS

Anoxic 

Tank

Figure 2-27. Johannesburg process. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-55

Biodenipho Process 

The PID, or Biodenipho, process is similar to the Biodenitro process, with the exception that 

an anaerobic tank is placed upstream of the two oxidation ditches, which are operated in a 

cyclical manner to promote denitrification and nitrification. The result of phasing (or cycling) 

is that organic carbon in the wastewater is used for both denitrification and biological 

phosphorus removal. If sufficient carbon is present, no supplemental source is required. The 

RAS is directed to the anaerobic zone. As with the Biodenitro process, the footprint of the 

system can be large. Achieving a very low phosphate concentration might require 

downstream chemical precipitation and filtration. The Biodenipho process is illustrated in 

Figure 2-24. The ranges of TN and TP effluent concentrations found in the literature for the 

Biodenipho process were 1.78 mg/L to 7.02 mg/L in TN and 0.09 mg/L to 1.99 mg/L in TP 

(see Table 2-3 in Section 2.5 for details). 

Secondary 

Clarifier

Influent Effluent

RAS

Anaerobic 

Tank

WAS

Alternating 

Aerobic/Anoxic Tanks

Flows switch when tanks 

alternate Tank 1-Anoxic

Tank 2-Aerobic

Figure 2-28. Biodenipho (phased isolation ditch) process. 

Blue Plains Process 

The Blue Plains process was a retrofit to the existing nitrification activated-sludge process at 

the Washington, DC, facility. A new anoxic zone was created inside the aeration tank with an 

HRT of 0.8 hour from the nominal 3.3 hours in the total basin. The design sludge age was 13 

days. The existing return activated-sludge system remained unchanged in this retrofit. 

Methanol was fed directly into this new anoxic zone for a target nitrogen concentration of 7.5 

mg/L (Kang et al. 1992; Sadick et al. 1998). Phosphorus is removed by ferric chloride 

addition and tertiary filtration. The Blue Plains process is depicted in Figure 2-29. The TN 

and TP effluent concentrations found in the literature for the Blue Plains process were 7.5 

mg/L in TN and 0.12 mg/L in TP (see Table 2-1 in Section 2.5 for details). 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-56 Chapter 2: Treatment Technologies 

Aerobic Zone 

Nitrification

Secondary 

Clarifier

Effluent 

to 

Tertiary 

Fitler

Effluent from 

First Stage---

High Rate 

Activated Sludge

RAS

WAS

Methanol Feed

Aerobic Zone 

Nitrification

Anoxic Zone 

Denitrification

FeCl3 addition

Figure 2-29. Blue Plains process. 

Westbank Process 

The Westbank process is a modification of a five-stage Bardenpho, with elimination of both 

the second anoxic zone and the reaeration zone. The process uses a step-feed arrangement for 

distributing primary effluent and fermenter supernatant (VFA-enriched) to the anaerobic and 

anoxic zones, as shown in Figure 2-30. The process consists of a small pre-anoxic zone, 

followed by an anaerobic zone, an anoxic zone, and an aerobic zone. The pre-anoxic zone 

minimizes the DO and nitrates entering the anaerobic zone, thereby maximizing the release 

of phosphorus. RAS is fed to the anoxic zone. Primary effluent is divided between the preanoxic zone (to denitrify the RAS), anaerobic zone (to stimulate phosphorus release), and 

anoxic zone (to stimulate denitrification). The direct feeding of the primary effluent to the 

anoxic zone increases the denitrification rate, thereby reducing the required size of the anoxic 

zone compared to that in a 5-stage Bardenpho system. The fermenter supernate, containing 

VFAs, is fed directly to the anaerobic zone. An internal recycle at a flow ratio of up to 600 

percent directs the nitrates from the aerobic zone to the anoxic zone for denitrification. In the 

specific case of Kelowna, British Columbia, the effluent from the Westbank process is 

gravity filtered. The TN and TP effluent concentrations found during the case study period 

for the Westbank process with filtration ranged from 2.7 mg/L to 5.8 mg/L in TN, with an 

average of 4.4 mg/L in TN, and were 0.05 mg/L to 1.88 mg/L in TP, with an average of 0.14 

mg/L TP (see Tables 2-1 and 2-2 in Section 2.5 for details). 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-57

Figure 2-30. Westbank process (Kelowna, British Columbia). 

2.5 Full-Scale Nutrient Removal Process Cases 

2.5.1 Nitrogen Removal Matrix and Variability Data 

The variability or reliability of nitrogen control technologies is summarized in Table 2-1. For 

plants for which a full year of daily nitrogen removal data were obtained, a statistical 

summary is presented for the annual average, maximum month, maximum week, and 

maximum day. These correspond to the points at 50, 92, 98, and 99.7 percent, respectively, 

when plotted on probability paper (based on the number of data points). This summary is the 

first known in the literature to compare full-scale technologies on an equal basis. Monthly, 

weekly, and daily maximums may be set by regulatory authorities for facilities that discharge 

to particularly sensitive waters. If TN effluent data were collected weekly because that was 

all that the permit required, the daily maximum value was not available. The performance 

levels shown from reported sources are documented in descending order for the technologies 

reported. It should be emphasized that, for this table as well as Tables 2-5 and 2-8, the 

performance results reflect specific operating philosophy, permit limitations, temperature, 

influent conditions, flow conditions, and the relative plant load compared to design. Thus, 

they do not necessarily represent optimum operation of the technologies presented. Most of 

the selected periods appeared to be typical; however, climate and weather variations could 

significantly affect performance. Similarly, the performance results reflected in the curves in 

Figures 2-31, 2-33, 2-34, 2-35, and 2-36 reflect site-specfic situations and do not necessarily 

represent optimum operation of the technologies. 

To Secondary 

Clarifier 
2-58 Chapter 2: Treatment Technologies Table 2-1. Process Performance Data: Nitrogen Removal—Plant Effluent Nitrogen removal Variability (mg/L) 

TN (ppm) Technology Plant effluent observed value or range (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) Johannesburg 2.03 to 11.44 1.66/21 7.86 10.41 11.57 13.28 Hage

A2O 7.3 to 9.0a -- -- -- -- -- MaryVIP 6.12a -- 6.12 -- -- -- Neet

10 Step-feed AS 1.80/27 6.70 8.62 9.82 13.05 Cum

IFAS 4.9 to 11.3a -- -- -- -- -- Mast

McQ

MBBR 5.8 to 6.8a -- -- -- -- -- TäljeMLE 2.2 to 15a 1.00/23 4.35 5.54 6.13 7.76 Lees

Wes

4-stage Bardenpho 3.5 to 12.1a -- -- -- -- -- MarySchreiber system 8a -- -- -- -- -- MaryBlue Plains process 7.5a -- -- -- -- -- Was

(Kan(Sad

PIDs, clarifiers, aerobic digestion 1.8 to 7.0 0.51/14 3.67 4.46 5.87 -- North 

Caro

PIDs 1.4 to 11.3 1.81/42 4.2 7.3 11.3 -- JeweSBR 1.6 to 13.6 2.31/50 4.59 6.84 10.68 14.35 ThomCyclic on-off 3.1 to 10.4 1.17/25 4.59 6.15 7.62 8.64 RidgWestbank 2.7 to 5.8 0.51/12 4.38 4.9 5.84 -- Kelo

Colu

5 Step-feed AS 3.7 to 7.4 0.63/12 5.25 6.15 8.01 -- Fairf
Chapter 2: Treatment Technologies 2-59 Table 2-1. Process Performance Data: Nitrogen Removal—Plant Effluent (continued) Nitrogen removal Variability (mg/L) 

TN (ppm) Technology Plant effluent observed value or range (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) Biological aerated filters 1.4 to 6.8 2.24/62 3.61e 7.13e 9.80a 13.91a Ches

Concentric oxidation ditch 1.6 to 5.4 0.95/32 3.0 4.24 5.29 6.46 HamStep-feed AS 1 to 14 1.48/57 2.58 4.30 5.89 9.16 Pisca5-stage Bardenpho 1.24 to 4.29 0.35/16 0.86/42 2.32 2.04 3.10 3.10 3.75 3.90 4.36 5.44 Clea

Clea

3 Denitrification filters 0.47 to 3.76 0.36/16 2.14 2.77 3.13 4.25 John

Caro

 Denitrification filter 0.13 to 6.50 0.56/28 1.71 2.61 3.90 -- Lee 

Denitrifying activated sludge 0.4 to 10.4 0.59/36 1.63 2.46 4.22 -- Wes

Mary

Notes: A2O = anaerobic/anoxic/oxic AS = activated sludge COV = coefficient of variation IFAS = integrated fixed-film activated sludge MBBR = moving-bed biofilm reactor MLE = modified Ludzak-Ettinger PID = phased isolation ditch SBR = sequencing batch reactor VIP = Virginia Initiative process Performance periods are listed in Attachment at end of References section. a Data obtained from literature; data were not reviewed as part of project. b George Miles & Buhr, LLC, and Gannett Fleming 2004. c Retrofit application. d Case study plant is explained in more detail in Chapter 3. e Values are based on 8 months of data, rather than 12 months. f Clearwater, Florida, has two facilities––Marshall Street (MS) and Northeast (NE) The data reflect specific operating philosophy, permit limitations, influent conditions, flow conditions and the relative plant loadings compared to their desi

they do not necessarily represent optimum operation of the technologies presented. Source: Table format adapted from WEF and ASCE 1998. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-60 Chapter 2: Treatment Technologies 

Figure 2-31 shows nitrogen removal technologies that have been demonstrated to produce 

low-range TN average results. Four technologies are shown in this group: denitrification 

filter, 5-stage Bardenpho, concentric oxidation ditch with a high recirculation rate, and stepfeed activated sludge. All of these technologies produced effluent that was under 3 mg/L TN 

for an annual average, and each facility exhibited low variability. In addition, the monthly 

maxima (92 percent) points for all five technologies were under 5 mg/L; therefore, it is 

conceivable that all of these technologies could meet that value for a monthly maximum 

limit. 

Table 2-2, as well as Tables 2-3, 2-4, 2-6 and 2-7, presents information about the case study 

facilities included in Figures 2-31, 2-33, 2-34, 2-35, and 2-36, respectively. These tables 

show the number of data points used in developing the referenced curves, temperature 

information, and average flow compared to design load. This number of data points show 

that the curve is based on a full year. The loading shows what fraction of the overall capacity 

was in use during the selected year. The temperature shows the range over which the plant 

was operated during the year. The reader is cautioned that these curves represent actual 

performance but not necessarily the optimal performance level for the given treatment 

technologies. This also means that the coefficient of variation (COV) could be higher or 

lower for different actual applications. Additional details, including permitted discharge 

limits, are presented in Chapter 3. 

1

6

2

3

7

5

4

0.1

1

10

Percent Less Than or Equal To

Nitrogen, mg/L

1---Denitrifying AS---Western Branch, MD 6- Step Feed AS -Piscataway, MD

2 - Five-stage Bardenpho - Northeast Clearwater, FL 3 - Denitrification Filter-Johnston Co. NC

7 - Oxidation Ditch w/ IR-Hammonton, NJ 5 - Five-stage Bardenpho - Marshall St.,Clearwater, FL 

4-Denite Filter-Lee County, FL

0.05 0.1 0.5 1 2 5 10 3020 40 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Figure 2-31. Monthly average frequency curves for TN: low-range removal. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-61

Table 2-2. Detailed curve information for Case Study Facilities in Figure 2-31 

Plant 

# Data 

points 

Min 

temp 

Max 

temp 

Ave. temp Ave. flow 

MGD 

Design 

flow MGD 

Western Branch, 

MD 

236 51 °F 76 °F 64 °F 19.3 30 

Lee Co., FL 261 -- -- 30.4 °C* 3.2 5.0 

Central Johnston 

Co., NC 

249 13.1 °C 27.9 °C 20.4 °C 4.1 7.0 

Marshall St., 

Clearwater, FL 

260 22.1 °C 31.1 °C 27.5 °C 5.5 10.0 

* Only one available in July 2006. 

The annual average and the maximum month concentrations, respectively, are shown for 

each facility: 

1 = Denitrifying activated sludge: 1.63 mg/L and 2.46 mg/L with a COV of 36 percent 

at Western Branch, Maryland 

2 = 5-stage Bardenpho: 2.04 mg/L and 3.10 mg/L with a COV of 42 percent at the 

Clearwater, Florida, northeast plant 

3 = Denitrification filter: 2.14 mg/L and 2.77 mg/L with a COV of 16 percent at the 

Central Johnston County, North Carolina, plant 

4= Denitrification filter: 1.71 mg/L and 2.61 mg/L with a COV of 28 percent at the Lee 

County, Florida, plant 

5 = 5-stage Bardenpho: 2.32 mg/L and 3.1 mg/L with a COV of 16 percent at the 

Clearwater, Florida, Marshall Street plant 

6 = Step-feed activated sludge (AS): 2.58 mg/L and 4.30 mg/L with a COV of 57 

percent at the Piscataway, Maryland, plant 

7 = Concentric oxidation ditch with internal recycle: 3.0 mg/L and 4.24 mg/L with a 

COV of 32 percent at the Hammonton, New Jersey, plant 

A brief summary of the selected facilities that meet the low annual nitrogen limit of 3 mg/L 

is presented below. The 4-stage and 5-stage Bardenpho processes can achieve low effluent 

nitrogen concentrations because of the presence of two anoxic zones, allowing nitrates 

created in the first aeration basin to be denitrified. A review of literature results suggests that 

a 4-stage Bardenpho should do as well as or better than a 5-stage Bardenpho in nitrogen 

removal (deBarbadillo et al. 2003), but no facility was available for this investigation. A high 

internal recycle rate (four times the average flow) is also important with this process. A stepfeed activated sludge process has proven reliable in nitrogen removal because of a new 

understanding of increased denitrification at the anoxic zone in the presence of primary 

effluent as a natural carbon source. Denitrification filters can also be used to achieve low 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-62 Chapter 2: Treatment Technologies 

effluent nitrogen levels. The process requires that methanol or an alternative carbon source 

be provided because the secondary treatment process usually has removed most of the BOD, 

which is needed to convert nitrates to nitrogen gas. In a concentric oxidation ditch, SND has 

been successful. 

Clearwater, Florida 

Clearwater, Florida, operates two 5-stage (one anaerobic zone followed by two anoxic and 

two aerobic zones) Bardenpho WWTPs, the Marshall Street and Northeast advanced 

pollution control facilities. The second anoxic zone provides an opportunity to denitrify the 

nitrates created in the aeration zone, allowing lower TN effluent concentrations to be 

achieved when compared to a plant with only one anoxic zone. Some 5-stage Bardenpho 

plants require that a carbon source, such as methanol, be fed to the second anoxic zone (the 

fourth reactor zone). However, neither of the plants in Clearwater requires methanol addition 

to achieve average TN concentrations below 3 mg/L. This is because of high internal 

recirculation, which aids efficient use of the carbon source in accomplishing denitrification 

(WEF 1998). The internal recycle rate is 400 percent of the average flow (4Q), which is 

relatively high. The RAS rate is approximately 90 percent at the Marshall Street plant and 

110 percent at the Northeast plant. The food-to-microorganism ratios are relatively low––

0.024 and 0.04––at the Northeast and Marshall Street plants, respectively. 

Johnston County, North Carolina 

The Central Johnston County Regional WWTP in North Carolina employs biological 

nitrogen removal through an MLE configuration followed by denitrification filters supplied 

by F.B. Leopold to achieve an annual average TN effluent concentration of 2.06 mg/L. The 

biological treatment consists of an anoxic zone followed by an aerobic zone. An internal 

recycle returns some of the nitrates from the aerobic zone to the anoxic zone for 

denitrification, as in the MLE process. Methanol is fed upstream of the denitrification filters 

at a methanol-to-nitrate ratio of 4.5 lb to 1 lb. 

Piscataway, Maryland 

The Piscataway Advanced Wastewater Treatment Plant in Accokeek, Maryland, has two 

step-feed activated sludge treatment trains to remove TN. The first train consists of five sets 

of anoxic and aerobic basins in series, with the capability to feed primary effluent to the first 

four anoxic zones. There are four sets of anoxic and aerobic zones in series. Primary effluent 

can be fed to the first three anoxic zones. RAS enters the first anoxic zone in both trains. The 

annual average TN concentration in the effluent from the Piscataway plant is 2.58 mg/L. 

Methanol or an alternative carbon source is not needed because primary effluent can be fed 

to each anoxic zone, with the exception of the last zone in each train. In addition, internal 

recycles are not necessary because of the multiple anoxic/aerobic zones in series. Sodium 

hydroxide is added upstream of the step-feed activated sludge process to increase the 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-63

alkalinity of the wastewater. This is necessary since alkalinity is consumed as part of the 

nitrification process but only a portion is returned following denitrification. 

Hammonton, New Jersey 

The Hammonton WWTP has a concentric oxidation ditch consisting of three channels 

(Figure 2-32). The outer and middle channels are operated at a low DO concentration to 

promote anoxic conditions. Surface aerators provide aeration and mixing to limit anaerobic 

conditions. The inner channel is operated at a DO concentration of 2 to 2.5 mg/L to promote 

nitrification. The plant uses automatic DO control. If the DO is outside the programmed 

range, the number of aerators operating in each channel is automatically adjusted. If the DO 

is too high, an aerator is shut down; if the DO in the channel is too low, an aerator turns on. 

As with other oxidation ditches, the operating level of the tanks can be adjusted to provide 

additional HRT, as needed, within the operating limits of the aerators. The external RAS rate 

is 100 percent of the average flow (1Q). The unique aspect of the oxidation ditch is the 

internal recycle that directs mixed liquor from the inner channel to the outer channel. In the 

summer the internal recycle is 400 percent of the average flow (4Q). Because of lower 

temperatures in the winter, which affect the kinetics of the nitrification and denitrification 

processes, the internal recycle is increased to 500 percent of the average flow (5Q). 

Figure 2-32. Concentric oxidation ditch. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-64 Chapter 2: Treatment Technologies 

Lee County, Florida 

The Fiesta Village Advanced Treatment Facility employs oxidation ditches followed 

denitrification filters to achieve an annual average concentration of 1.38 mg/L with a COV of 

40 percent. Methanol is fed upstream of the denitrification filters at a dosage of 3 lb methanol 

per lb of nitrate. 

Western Branch, Maryland 

This facility employs a unique denitrifying activated-sludge stage following two upstream 

activated-sludge stages, making a three-stage system overall. Methanol is fed upstream of the 

denitrifying stage. The annual average effluent concentration was 2.52 mg/L, with COV of 

36 percent. 

The next group of technologies is shown in Figure 2-33. These facilities have different TN 

permit limits, and some provide TN removal on a voluntary basis; thus, a wide variation is 

shown in their performance. These technologies could be extended to achieve effluent 

concentrations in the range of 3 to 8 mg/L on an annual average basis. Annual averages and 

the maximum month concentrations were as follows: 

1 = Phased isolation ditch (Biodenipho or PID): 3.67 mg/L and 4.46 mg/L, COV of 

14 percent, at North Cary, North Carolina 

2 = Biological aerated filter (BAF): 3.61 mg/L and 7.13 mg/L, COV of 62 percent, at 

Cheshire, Connecticut 

3 = Phased isolation ditch: 4.2 mg/L and 7.3 mg/L, COV of 42 percent, at Jewett City, 

Connecticut 

4 = Westbank: 4.38 and 4.9, COV of 12 percent, at Kelowna, British Columbia 

5 = Modified Ludzak-Ettinger (MLE): 4.35 mg/L and 5.54 mg/L, COV of 23 percent, at 

Westminster, Maryland 

6 = Cyclic on-off activated sludge (AS): 4.59 mg/L and 6.15 mg/L, COV of 25 percent, 

at Ridgefield, Connecticut 

7 = Sequencing batch reactor (SBR): 4.59 mg/L and 6.84 mg/L, COV of 50 percent, at 

Thomaston, Connecticut 

8 = Step-feed activated sludge (AS): 6.70 mg/L and 8.62 mg/L, COV of 27 percent, at 

Cumberland, Maryland 

9 =Johannesburg: 7.86 mg/L and 10.41 mg/L, COV of 21 percent, at Hagerstown, 

Maryland 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-65

1

6

2

7

3

8

4

9

5

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

1 - Phased Isolation Ditch--North Cary, NC 6 - Cyclic On-Off-Ridgefield, CT

2 - BAF-Cheshire, CT 7 - SBR-Thomaston, CT

3 - Phased Isolation Ditch - Jewett City, CT 8 - Step Feed AS-Cumberland, MD

4 - Westbank - Kelowna, BC 9 - Johannesburg--Hagerstown, MD

5 - MLE-Westiminster, MD

0.05 0.1 0.5 1 2 5 10 20 4030 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Figure 2-33. Monthly average frequency curves for TN—mid-range removal. 

Table 2-3. Detailed curve information for Case Study Facilities in Figure 2-33 

Plant # Data 

points 

Min 

temp 

Max 

temp Ave. temp Ave. flow MGD 

Design 

flow MGD 

North Cary, NC 155 16 °C 27 °C 21 °C 7.0 12.0 

Kelowna, BC 52 13 °C 22 °C 17.3 °C 8.5 10.6 

Low variability was shown by the North Cary PID and the Westbank process, with COVs of 

14 percent and 12 percent, respectively. Common elements of these two facilities were two 

anoxic zones and a good carbon supply for denitrification, preceded by an anaerobic zone for 

phosphorus removal. Although the other systems would meet an annual average limit of 5 

mg/L, they would not meet a monthly maximum limit of 5 mg/L because of higher annual 

averages or higher COVs. 

Some facilities have to meet ammonia nitrogen limitations without meeting a TN limit. In 

general, removal of ammonia nitrogen involves ensuring enough sludge residence time and 

hydraulic residence time to allow nitrification to occur in all seasons covered by the permit. 

Figure 2-30 reflects facilities that have been used to meet ammonia limitations, all of which 

are discussed further in the case studies in Chapter 3. The figure shows that all the facilities 

met their ammonia limits, ranging between 0.5 and 2.0 mg/L at all times, with relatively low 

COVs. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-66 Chapter 2: Treatment Technologies 

Monthly Average Frequency Curves for Ammonia-Nitrogen

1

4

2 

5

3.

0.01

0.1

1

10

Percent Less Than or Equal To

Nitrogen, mg/L

1 - Step Feed AS-Noman Cole, VA 4 - Modified UCT-Kalispell, MT

2 - Plug Flow A.S.-Central Johnston Co., NC 5 - Phased Isolation Ditch-North Cary, NC

3 - A/O - Clark Co, NV

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Figure 2-34. Monthly average frequency curves for ammonia nitrogen. 

Table 2-4. Detailed curve information for Case Study Facilities in Figure 2-34 

Plant # Data 

points 

Min 

temp 

Max 

temp 

Ave. 

temp 

Ave. flow 

MGD 

Design 

flow MGD 

Clark Co., NV 365 17 °C 30 °C 25 °C 98 110 

North Cary, NC 249 16 °C 27 °C 21 °C 7.0 12.0 

Noman Cole, 

Fairfax Co., VA 

365 14 °C 28 °C 21 °C 47.4 67 

Central Johnston 

Co., NC 

249 13.1 °C 27.9 °C 20.4 °C 4.1 7.0 

Kalispell, MT 12 8.5 °C 19.6 °C 14.3 °C 2.9 3.1 

* Only one available in July 2006. 

2.5.2 Phosphorus Removal Matrix and Variability Data 

Table 2-5 shows variability data for phosphorus removal technologies from 14 selected 

facilities. The figures following the table show variability data from well-operated full-scale 

facilities for a full year’s operation. The results are shown in the order of descending effluent 

phosphorus concentration. 
Chapter 2: Treatment Technologies 2-6

7

Table 2-5. Process Performance Data: Phosphorus Removal—Plant Effluent Phosphorus removal Variability (mg/L) P (ppm) Technology Range of values observed (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) 

2 1 A2O with VFA addition, chemical addition, tertiary clarifier, and filtrationa 0.025 to 0.98 0.044/33 0.132 0.18 0.646 0.98 Durham

VIP 0.19 to 5.0c -- 0.40 1.75 3.6 7.5 VIP, NeA/O 0.03 to 0.43 0.12/50 0.24 0.36 0.44 0.75 GeneseUCT with filter 0.3c -- -- -- -- -- Penticto

Barnard

0.5 Westbank with fermenter and filters 0.05 to 1.88 0.03/21 0.14 0.20 0.25 -- Kelowna

Chemical addition and flocculating clarifiers 0.07 to 0.23 0.01/14 0.09 0.11 -- -- ChelseaPhoStrip < 0.1 Ortho-Pc -- -- -- -- -- Truckee 

Barnard 

Modified UCT with fermenter 0.03 to 0.37 0.023/19 0.12 0.15 0.31 0.36 KalispelTertiary clarifier with chemical addition and filtersa 0.026 to 0.24 0.036/63 0.058 0.12 0.17 0.23 McMinnA/O with chemical addition to filters 0.03 to 2.3 0.03/30 0.10 0.17 0.41 0.56 Clark CoStep-feed AS with fermenter and filter 0.02 to 0.26 0.02/21 0.09 0.12 0.16 0.26 Fairfax, 

0.1 MBR 0.011 to 0.554 0.075/107 0.070 0.17 0.29 0.54 Hyrum, 

Denitrification filter with alum 0.02 to 1.34 0.05/35 0.102 0.19 0.39 -- Lee Cou
2-68 Chapter 2: Treatment Technologies Table 2-5. Process Performance Data: Phosphorus Removal—Plant Effluent (continued) Phosphorus removal Variability (mg/L) P (ppm) Technology Range of values observed (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) 

5-stage Bardenpho oxidation ditch with chemical addition and filters 0.02 to 0.078 0.011/34 0.031 0.061 0.078 -- Pinery WMBR 0.01 to 0.083 0.0074/27 0.027 0.038 0.053 -- Lone TrEBPR with high rate solids contact clarifier, chemical addition, and filters to 0.02c -- 0.01 0.02 -- -- BreckenChemical addition, tertiary clarifiers, filter, infiltration basin 0.01 (monthly averages) 0/0 0.01 0.01 0.01 0.01 Brighton

Notes: A2O = anaerobic/anoxic/oxic A/O = anoxic/oxic AS = activated sludge EBPR = enhanced biological phosphorus removal MBR = membrane reactor UCT = University of Cape Town process VFA = volatile fatty acid VIP = Virginia Initiative process Performance periods are listed in Attachment at end of References section. a Seasonal permit limit of 0.07 mg/L; results based on only seasonal data. b Subject of case study described in Chapter 3. c Data obtained from literature; data were not reviewed as part of report. d Retrofit applications. The data reflect specific operating philosophy, permit limitations, influent conditions, flow conditions and the relative plant loadings compared to their desi

they do not necessarily represent optimum operation of the technologies presented. Source: Table format adapted from WEF and ASCE 1998.
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-69

Figure 2-35 shows the best-performing TP removal processes, with the lowest phosphorus 

concentration in the effluent. All of these plants added chemicals to achieve their respective 

effluent phosphorus concentrations. Beginning with the lowest, the land application of 

tertiary effluent in Brighton, Michigan, had a flat line at 0.01 mg/L. The next lowest used 

MBR in Lone Tree Creek, Colorado, which had an annual average concentration of 0.027 

mg/L, and a 5-stage Bardenpho with tertiary clarifier and Trident filter in Pinery, Colorado, 

which had an annual average of 0.031 mg/L. The annual average and maximum month 

concentrations are shown below with COVs. 

1 = Land application: 0.01 mg/L for annual average and maximum month with COV of 

0 percent, in Brighton, Michigan 

2 = Biofor, DensaDeg, and MBR: 0.01 mg/L annual average and 0.02 mg/L monthly 

maximum, in Breckenridge, Colorado 

3 = MBR: 0.027 mg/L and 0.038 mg/L, COV of 27 percent, in Lone Tree Creek, 

Colorado 

4 = Five-stage Bardenpho, tertiary clarifier, Trident filter: 0.031 mg/L and 0.061 mg/L, 

COV of 34 percent, in Pinery, Colorado 

5 = Tertiary clarifier/filter: 0.058 mg/L and 0.12 mg/L, COV of 63 percent, in 

McMinnville, Oregon (seasonal limit) 

6 = MBR: 0.07 mg/L and 0.17 mg/L, COV of 107 percent, in Hyrum, Utah 

7 = Denite filter: 0.10 mg/L and 0.19 mg/L, COV of 35 percent, in Lee County, Florida 

Note that COV did increase to 107 percent and 63 percent for Hyrum and McMinnville. This 

was caused by changing conditions, especially temperature and water chemistry, through the 

year, which affected the precipitation/membrane filtration process in Hyrum and the effects 

of the seasonal limit for McMinnville. Although these two facilities can meet the 0.1 mg/L 

TP limit on an annual average basis, they might not meet the monthly maximum. 

The second group of TP removal processes included EBPR with chemical addition and 

tertiary filters––Fairfax County, Virginia, and Clark County, Nevada. The annual average 

concentrations were 0.09 mg/L at both, and both were very reliable with the maximum month 

concentration below 0.2 mg/L. More description of selected facilities follows. 

Achieving TP concentrations of less than 0.1 mg/L usually requires the use of chemicals 

(alum or ferric chloride) and filtration. Alum has a lower solubility limit than ferric chloride 

and is usually used in low-effluent phosphorus applications. Filtration can be provided by 

sand filters, denitrification filters, or membranes. For the phosphorus to be captured by 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-70 Chapter 2: Treatment Technologies 

1

2

3

4

5

6

7

0.001

0.01

0.1

1

10

Percent Less Than or Equal To

Total Phosphorus, mg/L

1 - Chem Addn + Tert Clarifiers + Land Application-Brighton, MI

2 - Biofor, DensaDeg, and MBR-Breckenridge, CO (only Ann. Ave. and Max Month available)

3 - MBR-Lone Treek Creek, CO

4 - 5 Stage Bardenpho w chemical and filter, Pinery, CO

5 - Tert Clarifier + Chem Addn + Filter-McMinnville, OR

6 - MBR + Chem Addn-Hyrum, UT

7-Denite filter Lee County, FL

0.05 0.1 0.5 1 2 5 2010 30 40 50 60 70 80 90 95 98 99 99.5 99.95 99.9

Figure 2-35. Monthly average frequency curves for TP—low-end removal. 

Table 2-6. Detailed curve information for Case Study Facility in Figure 2-35 

Plant 

# Data 

points 

Min 

temp 

Max 

temp 

Ave. temp Ave. flow 

MGD 

Design flow 

MGD 

Lee Co., FL 261 -- -- 30.4 °C* 3.2 5.0 

* Only one available in July 2006 

either filtration method, a sufficiently sized floc must be formed through coagulation with 

alum or other chemicals. To achieve effluent concentrations below 0.03 mg/L, more than one 

filtration step is required. For example, the Iowa Hill Reclamation Facility in Breckenridge, 

Colorado, has a DensaDeg high-rate solids clarifier (manufactured by Infilco Degremont) 

that uses sand particles to remove phosphorus floc, followed by additional chemical addition 

and a deep-bed sand filter. The lowest phosphorus concentrations were observed at the 

Brighton Environmental Control Facility in Brighton, Michigan, which uses chemical 

addition, claricones, and land application to an infiltration bed. The mechanism for 

phosphorus removal in the infiltration bed is adsorption to the soil particles. The water is 

collected from the infiltration beds by underdrains before discharge to the surface water. The 

process is very reliable, with a COV of 0 percent. It requires a large area, however, which 

might not be available in most places, particularly for retrofit applications. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-71

McMinnville, Oregon 

Phosphorus is removed through one-point chemical addition using alum and aluminum 

chlorohydrate upstream of the tertiary clarification, which is then followed by filtration. The 

seasonal average effluent concentration achieved is 0.058 mg/L. The permit limit applies 

from May through October. During the rest of the year, no phosphorus limit is in place and 

effluent concentrations are significantly higher. TP is not measured from November through 

April. Orthophosphorus concentrations as high as 3 mg/L have been recorded during this 

period. 

Hyrum, Utah 

The Hyrum WWTP has an MBR manufactured by Enviroquip/Kubota. Phosphorus is 

removed by adding alum to the activated-sludge tank. The floc that is formed does not pass 

through the flat-plate membranes, which take the place of the clarifiers and filtration in a 

conventional plant. The annual average effluent phosphorus concentration is 0.07 mg/L. The 

COV is relatively high at 107 percent. 

Pinery Water, Colorado 

The Pinery Water plant removes phosphorus biologically in the five-stage Bardenpho 

process, which is followed by a clarification and filtration process called Trident, 

manufactured by U.S. Filter. The Trident process involves tube clarification, adsorption 

clarification, and multimedia filtration. Alum is fed upstream of the tube clarifier to promote 

flocculation. The plant meets a monthly average permit limit of 0.05 mg/L. 

Breckenridge, Colorado 

The Iowa Hill Water Reclamation Facility achieves low effluent phosphorus concentrations, 

beginning with an anaerobic selector upstream of the activated-sludge process. A BAF 

follows. Then the wastewater enters a high-rate solids contact clarifier known as the 

Densadeg process, manufactured by Infilco-Degremont. The process involves flocculation 

and plate settling. The sand is recycled and reused in the flocculation process. The 

wastewater then passes through a Dynasand filter, manufactured by Parkson. The plant has a 

daily phosphorus limit of 0.05 mg/L and has reported effluent values below 0.01 mg/L. 

Brighton, Michigan 

The Brighton Environmental Control Facility’s process consists of secondary treatment, 

claricones, and infiltration basins. The secondary treatment process is an oxidation ditch, 

which provides little biological phosphorus removal. Ferric chloride is added to provide 

some chemical removal. Additional ferric chloride and a polymer are added to remove 

phosphorus in the claricones. The wastewater is then applied to a rapid infiltration bed. The 

phosphorus is adsorbed to the soil particles in the infiltration bed. The wastewater is then 

collected in underdrains for discharge to the surface water. Soil adsorption provides a very 

reliable treatment system, as long as the capacity of the soil is not exceeded. Although the 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-72 Chapter 2: Treatment Technologies 

process is capable of meeting a 0.01 mg/L TP effluent concentration, it requires a large 

amount of space, which might not be available in most areas. Furthermore, the soil in the 

vicinity of the treatment plant needs to allow water infiltration (sandy soils are needed; clay 

is not suitable). The land requirement for this facility was 25 acres per million gallons of 

capacity, which was sufficient to provide treatment for 40 years. 

Figure 2-36 shows the mid-level phosphorus removal processes. The Piscataway Advanced 

Wastewater Treatment Plant (1) in Accokeek, Maryland, feeds alum and polymer to 

secondary clarification followed by a gravity mixed-media filter to remove phosphorus. The 

secondary treatment process is a step-feed activated sludge process. The annual average final 

effluent phosphorus concentration was 0.09 mg/L, and the maximum month was 0.20 mg/L. 

Lee County, Florida, achieved an annual average concentration of 0.102 mg/L with alum 

addition through a denitrification filter acting as a filter. 

Treatment plants in Kalispell, Montana (2) and Kelowna, British Columbia (5) use EBPR 

with fermenters, producing annual average concentrations of 0.12 mg/L and 0.139 mg/L, 

respectively. Neither facility used chemicals, and yet both were very reliable in their 

performance, with maximum month concentrations of 0.15 mg/L and 0.2 mg/L, respectively. 

The COVs for Kalispell and Kelowna were very low at 19 percent and 12 percent, 

respectively. 

The next facility removed phosphorus by using EBPR without fermenters with some alum 

addition. Clearwater, Florida (numbers 3 and 6 in Figure 2-36) added alum to the 5-stage 

Bardenpho facility and achieved effluent concentrations of 0.132 mg/L and 0.21 mg/L as the 

annual average and the maximum month, respectively. 
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Chapter 2: Treatment Technologies 2-73

1

6

2

7

3

8

4

9

5

10

0.01

0.1

1

10

Percent Less Than or Equal To

Total Phosphorus, mg/L

1 - Step Feed w/ Fermenter-Piscataway, MD 6- Five-stage Bardenpho-Northeast, Clearwater, FL

2 - EBPR w/ VFA Addn + Filters-Kalispell, MT 7 - Denitrification Filters + Chem Addn-Johnston Co., NC

3 - Five-stage Bardenpho-Marshall St., Clearwater, FL 8 - A/O--Genesee Co., MI

4 - A2O with VFA, chemical, and filter-Durham, OR 9 - Phased Isolation Ditch-North Cary, NC

5 - Westbank--Kelowna, BC 10 - Triple sludges---Western Branch, MD. 

0.05 0.1 5040 0.5 1 2 5 302010 60 70 80 90 95 98 99 99.5 99.95 99.9

Figure 2-36. Monthly frequency curves for TP removal—mid-range removal. 

Table 2-7. Detailed curve information for Case Study Facilities in Figure 2-36 

Plant 

# Data 

points 

Min 

temp 

Max 

temp 

Ave. 

temp 

Ave. flow 

MGD 

Design 

flow MGD 

Western Branch, MD 285 51 °F 76 °F 64 °F 19.3 30 

Central Johnston Co., NC 52 13.1 °C 27.9 °C 20.4 °C 4.1 7.0 

Marshall St, Clearwater, FL 260 22.1 °C 31.1 °C 27.5 °C 5.5 10.0 

Kalispell, MT 104 8.5 °C 19.6 °C 14.3 °C 2.9 3.1 

Kelowna, BC 51 13 °C 22 °C 17.3 °C 8.5 10.6 

North Cary, NC 155 16 °C 27 °C 21 °C 7.0 12.0 

* Only one available in July 2006 

The next group of TP removal processes included EBPR without fermenters or chemical 

addition––Genesee County, Michigan; North Cary, North Carolina; Central Johnston County, 

North Carolina, and Western Branch, Maryland. Their annual average effluent concentrations 

were 0.24 mg/L, 0.38 mg/L, and 0.26 mg/L, respectively. The variability increased to varying 

degrees without fermenters at these facilities, and thus the maximum month concentrations 

were 0.36 mg/L, 1.0 mg/L, and 0.64 mg/L, respectively. No chemicals were added at these 

facilities, and the results were therefore more variable. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-74 Chapter 2: Treatment Technologies 

In summary, chemical precipitation with alum or ferric chloride has been proven to produce 

effluent concentrations below 1 mg/L in TP since the 1970s in the Great Lakes region based 

on the International Joint Commission agreement between the United States and Canada. 

Chemical precipitation followed by filtration likewise has been proved to produce effluent 

concentration between 0.5 and 0.1 mg/L in TP. If the permittee desires the lowest possible 

concentration with low variability, the following processes appear suitable––MBR, land 

application using infiltration beds, or step-feed activated sludge with fermenters. In addition, 

chemical addition with special filters is a technology that is emerging, and it is anticipated 

that full-scale data could be available in the near future. These special filters include the 

Dynasand D2 advanced filtration process, the Trident process, the Blue PRO filtration 

system, and the CoMag process. The second group, EBPR with chemical addition, appears 

suitable for compliance with a 0.5 mg/L limit. If the permittee desires EBPR only, EBPR 

with fermenters appears to be most suitable. EBPRs without fermenters offer higher 

variability but will meet the 1.0 mg/L permit limit. 

2.5.3 Combined Nitrogen and Phosphorus Removal Matrix and 

Variability Data 

To achieve TN concentrations below 3 mg/L and TP concentrations of less than 0.3 mg/L 

usually requires chemical addition and filtration for phosphorus removal because few plants 

are able to achieve effluent concentrations below 0.3 to 0.4 mg/L biologically. Therefore, the 

options for meeting simultaneous nitrogen and phosphorus removal are the same as those for 

TN removal, as long as chemical addition and filtration are added for phosphorus removal. In 

the extreme cases where the phosphorus limit is near 0.01 mg/L, Husband et al. (2005) 

reported the need to supplement phosphorus in support of full denitrification. A study by 

deBarbadillo et al. (2006) also recommended a phosphoric acid feed system if the growth 

rate of denitrifying heterotrophic organisms might be limited during high-nitrate loading 

periods. During the pilot testing, they observed that a filter influent orthophosphorus-tonitrate nitrogen ratio of 0.02 or higher is safe. 

North Cary, North Carolina 

The North Cary WWTP in North Carolina is a PID. The Biodenipho process from Veolia 

Water/Krueger has two oxidation ditches, which are automatically controlled to operate in 

alternating aerobic and anoxic modes. An external anaerobic tank for phosphorus removal is 

configured upstream of the ditches. When aeration is not required, mixers are available to 

keep the mixed liquor in suspension. The anaerobic tank upstream of the ditch has a 2-hour 

detention time. The isolation ditch is operated in a 240-minute cycle. For the first 30 minutes, 

both ditches operate in an aerobic mode. For the next 90 minutes, ditch 1 operates in an 

anoxic mode while ditch 2 continues in an aerobic zone. A second 30-minute cycle follows 

with both ditches operating in an aerobic zone. In the last 90 minutes, ditch 1 continues to 

operate in an aerobic mode, while ditch 2 operates in an anoxic mode. This alternating 

strategy allows nitrification to occur, followed by denitrification while in the anoxic mode. 
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Chapter 2: Treatment Technologies 2-75

The low TN concentrations are achieved with a final anoxic tank that follows the PIDs; it is 

the second anoxic tank, as in the 5-stage Bardenpho process. The wastewater is then 

reaerated before the secondary clarifiers to prevent denitrification, causing rising sludge. The 

RAS rate is 55 percent of the average flow (0.55Q). The RAS enters a holding tank with a 

30-minute detention time to minimize the amount of nitrates that enter the anaerobic tank. 

The sludge age is approximately 16 to 18 days. The secondary effluent passes through a 

deep-bed sand filter. Methanol can be fed to aid in denitrification, if needed; however, 

methanol has never been fed. The plant has the ability to add alum or ferric chloride to aid in 

phosphorus removal, but this has not been necessary for the past 2 years. The annual 

averages were 3.55 mg/L in TN and 0.38 mg/L in TP in the case study. The maximum month 

averages were 4.46 mg/L in TN and 1.06 mg/L in TP, without any chemical addition. 

McDowell Creek, North Carolina 

The McDowell Creek WWTP in North Carolina uses a modified UCT process. Sugar waste 

is added upstream of the anaerobic zone to provide an additional source of carbon for 

phosphorus removal. To supplement the influent alkalinity, lime is also added to ensure that 

nitrification rates will not be limited. The process has two internal recycle streams––from the 

second anoxic zone to the anaerobic zone and from the aeration zone to the first anoxic zone. 

The internal recycle from the end of the anoxic zone to the head of the anaerobic zone 

minimizes the nitrates that are returned. This eliminates competition of the nitrates with the 

PAOs for the VFAs, thereby improving biological phosphorus removal. The anoxic-toanaerobic recycle is returned at a rate of 70 percent of the average flow (0.7Q), while the 

aerobic-to-anoxic recycle has a return rate of 320 percent of average flow (3.2Q). The RAS is 

returned to the first anoxic zone at a rate of 90 percent of average flow (0.9Q). The RAS 

enters at the head of the anoxic zone to minimize the presence of nitrates in the anaerobic 

zone. 

A deep-bed sand filter follows the secondary clarifiers; it is designed to provide an 

opportunity for additional denitrification by microorganisms under anoxic conditions. The 

water passes through the filter, but no chemicals are added because of the ability to meet the 

loading limits biologically. Alum is fed to the belt filter press filtrate, which returns to the 

plant headworks. If the primary influent phosphorus concentration is elevated, additional 

alum can be fed to the primary clarifiers. A second feed point is available upstream of the 

secondary clarifiers, but it is not used. The McDowell Creek WWTP is achieving an annual 

average TP concentration of 0.1 mg/L biologically through VFA addition, with minimal 

chemical addition. The secondary effluent and final effluent TP values are approximately 

equal (the filter has little effect on the phosphorus concentration). 

Clearwater, Florida 

Clearwater, Florida, operates two 5-stage Bardenpho WWTPs, the Marshall Street and 

Northeast advanced pollution control facilities. The second anoxic zone provides an 
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2-76 Chapter 2: Treatment Technologies 

opportunity to denitrify the nitrates created in the aeration zone, allowing lower TN effluent 

concentrations to be achieved when compared to a plant with only one anoxic zone. Some 5-

stage Bardenpho plants require that a carbon source, such as methanol, be fed to the second 

anoxic zone (the fourth reactor zone). However, neither of the plants in Clearwater requires 

methanol addition to achieve average TN concentrations below 3 mg/L. The internal recycle 

rate is 400 percent of the average flow (4Q), which is relatively high. The RAS rate is 

approximately 90 percent at the Marshall Street plant and 110 percent at the Northeast plant. 

The food-to-microorganism ratios for these plants were relatively low––0.024 and 0.04 at the 

Northeast and Marshall Street plants, respectively. 

Phosphorus is removed biologically in the anaerobic zone. Alum is fed to the second anoxic 

zone (the fourth reaction basin) for the dual purpose of phosphorus polishing and meeting the 

state’s additional requirement to meet low-level trihalomethane concentrations in the reuse 

water. In addition, a gravity sand filter is used to remove phosphorus tied to suspended solids 

in the final effluent. The annual average concentrations were 2.32 mg/L in TN and 0.11 mg/L 

in TP at the Marshall Street plant and 2.04 mg/L in TN and 0.20 mg/L in TP at the Northeast 

plant. Both plants operated with high efficiency and low variability. 

Central Johnston County, North Carolina 

The Central Johnston County Regional WWTP in North Carolina employs biological 

nitrogen removal followed by F.B. Leopold denitrification filters to achieve an annual 

average TN effluent concentration of 2.14 mg/L. The biological treatment consists of an 

MLE, with an anoxic zone followed by an aerobic zone. An internal recycle returns some of 

the nitrates from the aerobic zone to the anoxic zone for denitrification, as in the MLE 

process. Methanol is fed upstream of the denitrification filters using a methanol-to-nitrates 

ratio of 4.5 to 1. 

No chemicals are added for phosphorus removal, although it is likely that some phosphorus 

is removed through the denitrification filters. The annual average concentrations were 

2.14 mg/L in TN and 0.26 mg/L in TP. The maximum month concentrations were 2.77 mg/L 

in TN and 0.64 mg/L in TP, a very reliable nitrogen removal but less for TP. 

Table 2-3 summarizes technologies to achieve both TN and TP removal. The results are 

shown in the order of descending effluent phosphorus concentration. COVs for TN and TP 

removal displayed similar patterns to plants that perform only TN or TP removal. That is, 

COVs were lower for TN and higher for TP removal. Nitrogen removal performance was 

efficient and reliable. COVs were below 50 percent for the 5-stage Bardenpho, denitrification 

filters, and the PID with anoxic tank. COVs for phosphorus, however, went high as the target 

concentration approached the critical level of 0.1 mg/L or below. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-77

Research Needs 

More research is needed to identify the cause of high variability when phosphorus 

concentrations are low and to develop strategies to improve performance. The solubility 

products of phosphorus compounds are known to be dependent on other chemical species 

and the pH in the wastewater. In addition, Banisch et al. (2007) reported nonreactive 

phosphorus in the effluent in the range of 0.01 to 0.12 mg/L. More research is needed to 

determine the minimal level of phosphorus needed to support healthy growth of nitrogenremoval organisms in facilities where removal of both nitrogen and phosphorus is required. 

2.6 Summary 

On the basis of an extensive review of the literature and actual data from 30 full-scale 

facilities, this chapter has presented a compilation of full plant performance on the same 

statistical basis. The following paragraphs summarize major findings on nutrient removal 

technologies. 

2.6.1 Performance and Variability 

A probability plot of actual data from full-scale operation is a good representation of 

performance and allows easy comparison of one technology against another on a consistent 

basis. Full-scale data generally follow a normal distribution. The coefficient of variation, or 

COV, is a good measure of statistical variability. The lower the COV, the more reliable the 

performance; conversely, higher COV values indicate more variability in performance. 
2-78 Chapter 2: Treatment Technologies Table 2-8. Process Performance Data: Nitrogen and Phosphorus Removal—Plant Effluent Nitrogen and phosphorus removal Variability (mg/L) N (ppm) Technology Range of values observed (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) R

Johannesburg TN: 2.03 to 11.44 TP: 0.19 to 8.3 1.66/21 0.96/145 7.86 0.66 10.41 2.49 11.57 3.9 13.28 8.3 Hager

Maryla

10 UCT TN: 8.9 to 10a TP: 0.3 WEF a

1998 

IFAS TIN: 5.6 to 11.3a TP: 0.2 to 1.7a Broom

McQu

VIP TN: 3 to 10 TP: 0.19 to 5.75 RabinoVIP with VFA addition TN: 5 to 10 TP: 0.6 to 0.8 NeethStep-feed with fermenter TN: < 5.0; 3 to 13 TP: < 0.3; 0.1 to 5 1.48/57 0.08/89 2.59 0.09 4.30 0.20 5.89 0.31 9.16 0.52 PiscatBiodenipho/PID TN: 1.78 to 7.02 TP: 0.09 to 1.99 0.93/14 0.27/64 3.67 0.38 4.46 1.06 5.87 1.45 6.79 1.78 North 

5 Modified UCT with VFA addition TN: 5 to 6a TP: 0.1 to 2.7a 0.10 0.25 0.75 3.75 McDow

NC Neeth
Chapter 2: Treatment Technologies 2-79

Table 2-8. Process Performance Data: Nitrogen and Phosphorus Removal—Plant Effluent (continued) Nitrogen and phosphorus removal Variability (mg/L) N (ppm) Technology Range of values observed (mg/L) Std dev./ COV % Annual average (50%) Max. month (92%) Max. week (98%) Max. day (99.7%) R

High rate, nitrification, and denitrification activated sludge TN: 0.4 to 10.4 TP: 0.05 to 1.88 0.59/36 0.27/62 1.63 0.43 2.46 0.89 4.22 0.99 Weste

Hyatts

TN: 1.24 to 4.29 TP: 0.06 to 0.37 0.36/16 0.07/40 2.32 0.13 3.1 0.21 3.75 0.26 4.29 0.46 Clearw

5-stage Bardenpho with chemical addition for P removal TN: 0.87 to 5.59 TP: 0.06 to 1.09 0.86/42 0.16/82 2.04 0.20 3.10 0.44 3.90 0.63 5.44 1.07 Clearw

Denitrification filters with chemical addition TN: 0.13 to 6.50 TP: 0.02 to 1.34 0.56/28 0.05/35 1.71 0.102 2.61 0.19 3.70 0.39 Fiesta 

Lee C

Denitrification filters with chemical addition TN: 0.84 to 3.13 TP: 0.1 to 1.01 0.36/16 0.09/62 2.14 0.26 2.77 0.64 3.13 1.01 -- -- Johnst

Notes: IFAS = integrated fixed-film activated sludge PID = phased isolation ditch UCT = University of Cape Town process VFA = volatile fatty acids VIP = Virginia Initiative process Performance periods are listed in table at end of References section a Data obtained from literature; data were not reviewed as part of report. b Subject of case study described in Chapter 3. c Retrofit application. The data reflect specific operating philosophy, permit limitations, influent conditions, flow conditions and the relative plant loadings compared to their desi

they do not necessarily represent optimum operation of the technologies presented. Source: Table format adapted from WEF and ASCE 1998. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

2-80 Chapter 2: Treatment Technologies 

2.6.2 Nitrogen Removal Technologies 

Four technologies were identified to meet the low concentration limit with low variability. 

These are the 4- and 5-stage Bardenpho processes, the step-feed activated sludge process, 

concentric oxidation ditches, and denitrification filters with carbon sources. COVs for these 

processes are below 50 percent, and the technologies achieve annual average TN effluent 

concentrations below 3 mg/L. Seven technologies were identified for mid-level nitrogen 

removal between concentrations of 3 and 8 mg/L. The key factors that contribute to efficient 

and reliable nitrogen removal are an adequate supply of a carbon source (internal or 

external), flexibility in design (such as the number of anoxic/aerobic zones), temperature, 

alkalinity balance, and sludge age. DON has been reported in varying concentrations. The 

DON is a critical variable for determining TN standards because the chemicals have limited 

availability for biological removal.

2.6.3 Phosphorus Removal Technologies 

Biological phosphorus removal (without filters or chemical addition) achieved an annual 

average effluent concentration of 0.26 mg/L with a COV of 35 percent. Excellent 

performance with low variability was reported for biological phosphorus removal processes, 

which achieved mean effluent concentrations down to 0.12 mg/L with a COV of 19 percent 

and 0.14 mg/L with a COV of 12 percent in another plant. The key performance factors 

included a sufficient supply of VFAs from an on-site fermenter, temperature, control of 

secondary phosphorus release, and good filtration. The practical TP limit of 0.1 mg/L is 

reached consistently with chemical addition and filtration. However, variability increases 

significantly at the target limit of 0.1 mg/L and below. The COVs increase to 93 percent for a 

plant achieving an annual average of 0.058 mg/L and to 107 percent for a facility meeting an 

annual average mean 0.07 mg/L. Special filters have proved effective in achieving low 

concentrations below 0.03 mg/L. They include the Trident filter from U.S. Filter, the 

Dynasand D2 advanced filtration system from Parkson, and membrane filtration processes 

from various manufacturers. The only technology evaluated that meets a TP effluent 

concentration of 0.01 mg/L at all times is land application of tertiary effluent through soil. 

The COV at the Brighton, Michigan, plant employing this technology is 0 percent in this 

case. 

2.6.4 Combined Nitrogen and Phosphorus Removal Technologies 

The TN and TP removal data show the same trends as the plants that performed only TN or 

TP removal. The nitrogen removal is as reliable as that described earlier, whereas the 

phosphorus removal shows higher COVs, indicating potential interaction between the 

solubility product at pH and other chemical species that occur throughout the year. Another 

factor is the nonreactive form of phosphorus in the wastewater, which might vary during the 

year. More research is needed to determine these chemical interactions. More research is also 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 2: Treatment Technologies 2-81

needed to establish the minimum phosphorus concentration needed to support nitrogen 

removal in the same process. 

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Conference on Water Quality, Vancouver, June 1998. 

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Sedlak, D.L., and E. Pehlivanoglu-Mantas. Characterization of DON: Moving Beyond Bulk 

Parameters. In Proceedings of WERF Workshop BNR: How Low Can We Go and 

What Prevents Us from Going Lower?, March 2006. 

Sen, D., C. Randall, and T. Grizzard. 1990. Biological Nitrogen and Phosphorus Removal in 

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1990. U.S. Environmental Protection Agency, Chesapeake Bay Program. 
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Chapter 2: Treatment Technologies 2-91

Attachment 1: Locations Providing Data 

Location Data performance period 

Brighton, Michigan January through December 2005 

Chelsea, Michigan October 2005 through September 2006 

Clark County, Nevada January through December 2006 

Clearwater, Florida: Northeast October 2005 through September 2006 

Clearwater, Florida: Marshall Street October 2005 through September 2006 

Cumberland, Maryland January through December 2005 

Cheshire, Connecticut April through November 2006 

Durham, Oregon January through December 2004 

Enfield, Connecticut September 2005 through August 2006 

Jewett City, Connecticut September 2005 through August 2006 

New Haven, Connecticut September 2005 through August 2006 

Ridgefield, Connecticut September 2005 through August 2006 

Thomaston, Connecticut September 2005 through August 2006 

Hagerstown, Maryland January through December 2005 

Genesee County, Michigan October 1999 through September 2000 

Hammonton, New Jersey January through December 2005 

Hyrum, Utah November 2005 through September 2006 

Central Johnston County, Smithfield, 

North Carolina 

October 2005 through September 2006 

Kalispell, Montana July 2005 through June 2006 

Fairfax County, Virginia January through December 2006 

Piscataway, Maryland January through December 2005 

McMinnville, Oregon January 2005 through October 2006 

Kelowna, British Columbia January 2005 through December 2005 

North Cary, North Carolina October 2005 through September 2006 

Pinery Water, Colorado January through December 2004 

Western Branch, Marlboro, Maryland January through December 2006 

Westminster, Maryland October 2005 through September 2006 

Lee County, Florida January through December 2006 

Breckenridge, Colorado January through December 2003 

Lone Pine Tree, Colorado January through December 2006 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-1

CHAPTER 3: Case Studies and Reliability Factors 

3.1 Introduction and Overview 

The objective of the case studies was threefold: to study selected technologies that remove 

nitrogen, phosphorus, or both to low concentrations; to identify factors that contribute to 

reliability; and to identify factors that contribute to costs. Nine facilities were selected as case 

studies, and the results are summarized in this chapter. (The complete case studies are 

presented in Volume II, Appendix A). 

In selecting the case study locations, consideration was given to ensuring that there was a 

variety of technologies that achieved low effluent concentration in either ammonia nitrogen 

and phosphorus or both total nitrogen (TN) and total phosphorus (TP), a variety of locations 

in cold- and warm-weather regions, and a variety of sludge-handling processes. The chapter 

is organized to present the specific nutrient removal technologies selected, followed by 

factors that influence the reliability of performance and costs, in both capital and operation 

and maintenance (O&M), at these facilities. 

To establish a simple and yet sound statistical method by which plant performance and data 

can be presented and compared, the coefficient of variation (COV) was used in this study. 

The statistical background is included in Volume II, Appendix B. 

3.1.1 Permit Limits for the Case Study Facilities 

Table 3-1 summarizes the nutrient permit requirements for all the case study facilities. For 

the U.S. facilities, these values come from the National Pollutant Discharge Elimination 

System (NPDES) permits issued by the facility’s state regulatory authority, and they were 

current as of the writing of this manual in summer 2007. For the facility in Kelowna, British 

Columbia, the relevant limits were as given by Environment Canada. 

Table 3-1 demonstrates that there was a wide variety in discharge seasons and numeric limits 

for the case study facilities. The facilities have standards ranging from annual averages to 

daily maximums. Total maximum daily loads (TMDLs) were used in some locations to 

develop load limits, while straight concentration limits were used as the basis for 

technologies in Florida. It is notable that limits for maximum week and maximum month 

were specified in one case, while the limits for maximum day and maximum month were 

specified in another. In another case, limits were set for maximum week, maximum month, 

and entire quarter. Clearwater, Florida, has limits for maximum week and maximum month 

and an annual limit. 
3-2 Chapter 3 - Case Studies and Reliability Factors Table 3-1. Discharge permit limits and performance data summary Permit limits and performance results( mg/L) Quarterly or annual Month Week 

Plant and location Nutrient removal processes Annual average COV for annual average Avg permit limits Max (q) Avg (a) result Avg permit limits Max month result Avg permit limits Max week result 

Noman M. Cole Pollution Control Plant; Fairfax County, Virginia Step-feed AS with dual media & deep bed filter + ferric chloride TP = 0.09 NH4-N = 0.12 TN = 5.25 TP: 21% NH4-N: 14% TN: 12% No quarterly or annual limits -- TP = 0.18 Sum NH4- N = 1.0 Win NH4-N = 2.2 TN = report TP = 0.12 NH4-N = 0.135 TN = 6.0 TP = 0.27 Sum NH4- N = 1.5 Win NH4-N = 2.7 TN = report TP = 0.16 NH4-N =0.29 TN = 8.01 Kalispell Advanced Wastewater Treatment, Kalispell, Montana Mod. UTC with fermenter and up flow sand filter (no chemical for P) TP = 0.121 NH4-N = 0.07 TP: 19 % NH4-N: 0% No quarterly or annual limits -- TP=1.0 NH4-N =1.4 TP=0.15 NH4-N = 0.07 No weekly limits -- Clark County Water Reclamation District, Nevada A/O process with active primary for VFA with dual media filters + alum TP = < 0.09 NH4-N = 0.12 TP: 30% NH4-N: 14% No quarterly or annual limits -- No monthly average limits TP = 0.12 NH4-N = 0.15 No weekly limits -- Central Johnston County Smithfield, North Carolina Plug flow AS (anoxic, aeration) with denitrification filter and methanol (No chem. for P) TP = 0.26 NH4-N = 0.44 TN = 2.14 TP: 62% NH4-N: 12% TN: 16 % TP = 2.0 (Q) TN = 3.7** TP = 0.47 (Q Max) TN = 2.14 (Av) TP=1.0 Sum NH4- N = 2.0 Win NH4-N = 4.0 TP = 0.64 NH4-N = 0.54 Sum NH4- N = 6.0 Win NH4-N = 12.0 NH4-N = 0.86 North Cary, North Carolina Oxidation ditch (Biodenipho) with upflow sand filter (No chemical for P) TP=0.38 Ammonia N = 0.08 TN=3.7 TP: 64% Ammonia N: 102% TN: 14 % TP = 2.0 (Q) TN = 3.94b (A) TP = 0.57 (Q max) TN = 3.7 (Av) Sum NH4- N = 0.5 Win NH4-N = 1.0 NH4-N = 0.34 No weekly limits -- 
Chapter 3 - Case Studies and Reliability Factors 3-3 Table 3-1. Discharge permit limits and performance data summary (continued) Permit limits and performance results( mg/L) Quarterly or annual Month Week 

Plant and location Nutrient removal processes Annual average COV for annual average Avg permit limits Max (q) Avg (a) result Avg permit limits Max month result Avg permit limits Max week result 

Kelowna, British Columbia, Canada 3-stage Westbank with fermenters (No chemical for P) TP=0.139 TN=4.38 TP: 21% TN: 12% TP = 0.25 (A) TP = 0.139 (Av) No monthly average limit TP = 0.20 NH4-N =1.01 TN = 4.9 No weekly limits -- Marshall Street Water Reclamation Facility, Clearwater, Florida 5-stage Bardenpho with sand filter + alum TP=0.132 NH4- N=0.038 TN=2.32 TP: 40% NH4-N: 18% TN: 16% TP = 1.0 (A) TN = 3.0 (A) TP = 0.132 (Av) TN = 2.32 (Av) TP = 1.25 TN = 3.75 TP = 0.21 TN = 3.1 TP = 1.5 TN = 4.5 TP = 0.26 TN = 3.75 Lee County, Florida AS with denitrification filter and methanol addition + alum TP=0.102 TN=1.57 TP: 35% TN: 28% TP = 0.5 (A) TN = 3.0 (A) TP = 0.08 (Av) TN = 1.57 (Av) TP = 0.5 TN=3 TP = 0.19 TN = 2.61 TP = 0.75 TN=4.5 TP = 0.39 TN = 3.70 Western Branch, Upper Marlboro, Maryland Three AS with methanol addition + alum TP = 0.47 NH4-N = 0.22 TN = 1.70 TP: 62% NH4-N: 163% TN: 36% TP = 0.3c (A) TN = 4.0c (A) -- TP = 1.0 TN = 3.0 (Apr-Oct) NH4-N = 1.5 (Apr-Oct), NH4-N = 5.5 (Nov-Mar) -- TN = 4.5 (Apr-Oct) -- Notes: A/O = anoxic/oxic AS = activated sludge COV = coefficient of variation: one standard deviation divided by the mean NH4-N = ammonia nitrogen NL = No limit TP = total phosphorus TN = total nitrogen Sum = Summer Win = Winter Chem. = chemical a Daily maximum permit limits were derived from a wasteload alloc

maxima at design flow b Annual average permit limit derived from a wasteload allocation o

at design flow c Annual average limits based on 30 MGD annual load following insupgrades 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-4 Chapter 3 - Case Studies and Reliability Factors

The permit for Kelowna, British Columbia, also includes reliability criteria for phosphorus 

removal, which specified limits for the maximum day, 99th percentile, 90th percentile, and 

annual average, as follows: 

 Maximum day 2.0 mg/L TP 

 99th percentile 1.5 mg/L 

 90th percentile 1.0 mg/L 

 Annual average 0.25 mg/L 

These Canadian limits are comparable to statistics as applied at U.S. facilities, as shown in 

Table 3-2. For example, the Canadian 90th percentile limit is slightly less stringent than the 

U.S. maximum month limit for data collected on a daily basis. 

Table 3-2. Statistical comparison of Canadian and U.S. permit limits 

Statistical 

percentile 

U.S. term––averaging or 

maximum period 

Canadian term––percent less 

than value 

50 Annual average 50th percentile 

80 Maximum quarter -- 

90 -- 90th percentile 

92.3 Maximum month -- 

98.1 Maximum week -- 

99 -- 99th percentile 

99.7 Maximum day -- 

3.2 Summary of Case Studies 

Table 3-1 above shows the location of and technologies used in each of the case studies. 

The highlights of each case study are presented in order, from facilities with the lowest 

permit limit to those with the highest permit limit. All facilities were required to remove 

varying levels of phosphorus. Two facilities had ammonia nitrogen permit limits but no TN 

limits, while the other seven facilities had TN limits, as shown in Table 3-1. This chapter 

presents the nitrogen and phosphorus removal facilities first, followed by the facilities 

required to remove ammonia nitrogen and phosphorus. 

3.2.1 Total Nitrogen and Phosphorus Removal at Low Concentration 

Limits (3 mg/L or less in TN) 

Four case studies fell into this group, and they are characterized as having multiple, distinct, 

anoxic zones with sufficient carbon supply. 
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Chapter 3 - Case Studies and Reliability Factors 3-5

Case Study No. 1: Western Branch, WSSC, Maryland 

This facility has a three-stage activated-sludge system followed by tertiary filtration. The 

third activated-sludge stage was added to provide denitrification using methanol as the 

carbon source. The permit includes a monthly average ammonia limit of 1.5 milligrams per 

liter (mg/L) in summer (April–October) and 5.5 mg/L in winter (November–March). The TN 

limits are 3 mg/L as a monthly average and 4.5 mg/L as a weekly average, which both apply 

during the summer (April–October). An annual average TN limit of 4.0 mg/L is expected to 

go into effect in the future. The phosphorus limit for the future is 0.3 mg/L at the design flow 

of 30 million gallons per day (MGD). 

The Western Branch Wastewater Treatment Plant (WWTP) is part of the Washington 

Suburban Sanitary Commission (WSSC) system, and it is in Upper Marlboro, Maryland. The 

plant employs three separate activated-sludge systems in series to accomplish biochemical 

oxygen demand (BOD) removal (high-rate activated sludge, or HRAS), nitrification 

(nitrification activated sludge, or NAS), and denitrification (denitrification activated sludge, 

or DNAS). The return activated sludges (RAS) are kept separate to facilitate the growth of 

specific types of microorganisms in each system by allowing for differing sludge residence 

times. After grit removal and screening, the wastewater goes directly to the HRAS. The 

effluent is filtered prior to ultraviolet (UV) disinfection. Waste activated sludges (WASs) 

from the three systems are combined, thickened by dissolved air flotation (DAF), dewatered 

by centrifuge, and burned in two multiple-hearth incinerators. 

The plant does not have primary settling. Alum was added to remove phosphorus. The process 

recycle water from the DAF, centrifuge, and incinerator is returned to the headworks. The plant 

treated an annual average flow of 23.0 MGD, and the maximum month flow was 26.6 MGD. 

Western Branch WWTP, WSSC

Monthly Average Frequency Curves for Total Phosphorus

0.001

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Plant Influent HRAS Influent HRAS Effluent

NAS Effluent DNAS Effluent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.43 mg/L

Std. Dev. = 0.27 mg/L

C.O.V. = 62%

Figure 3-1. Western Branch WWTP: Monthly frequency curves for TP. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-6 Chapter 3 - Case Studies and Reliability Factors

Western Branch WWTP, WSSC

Monthly Average Frequency Curves for Ammonia-Nitrogen

0.001

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia-Nitrogen, mg/L

Combined Raw Influent HRAS Influent HRAS Effluent

NAS Effluent DNAS Effluent Final Effluent

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.13 mg/L

Std. Dev. = 0.22 mg/L

C.O.V. = 163%

Figure 3-2. Western Branch WWTP: Monthly frequency curves for ammonia nitrogen. 

The performance was efficient and reliable during the evaluation period for TN but moderate 

for ammonia nitrogen and TP. The average concentrations were 1.63 mg/L in TN with a 

COV of 36 percent, 0.22 mg/L in ammonia nitrogen with a COV of 163 percent, and 0.43 

mg/L in TP with a COV of 62 percent. 

Several factors contributed to this performance at Western Branch, including the following: 

1. Wastewater characteristics. Because this facility uses a separate stage for 

denitrification with methanol and alum for phosphorus removal, the wastewater 

characteristics were not a concern. 

2. Three activated-sludge systems in series provided redundancy and thus added 

reliability for TN removal, but a large footprint was required for the bioreactors and 

clarifiers. 

3. The first two stages provided BOD removal and nitrification in two separate stages in 

series. The nitrate in the effluent from the second stage (NAS) averaged 16.5 mg/L 

with a COV of 12 percent. Note that this is high because denitrification was not 

designed to occur in this stage. The methanol dosage averaged 2.5 lb/nitrate nitrogen 

entering the DNAS tank. 
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Chapter 3 - Case Studies and Reliability Factors 3-7

Western Branch WWTP, WSSC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Combined Raw Influent HRAS Influent NAS Effluent

DNAS Effluent Final Effluent

0.05 0.1 0.5 1 2 5 2010 4030 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 1.63 mg/L

Std. Dev. = 0.59 mg/L

C.O.V. = 36%

Figure 3-3. Western Branch WWTP: Monthly frequency curves for TN. 

4. The control strategy included daily testing of key parameters and adjustment of the 

methanol dosage, when needed. 

5. Alum was fed to the DNAS tanks for phosphorus removal at an average dosage of 10 

mg/L with good results. 

6. Dissolved oxygen (DO) probes were installed for continuous monitoring in the first 

two stages. The signals from the probes were used to control the air valves and thus 

control the air flow to the basins. The plant also has online sensors for suspended 

solids and sludge blanket levels in the secondary clarifiers. 

7. Recycling of phosphorus has been minimized at the facility. The sludge is thickened 

under aerobic conditions, which reduces phosphorus release. In addition, the biosolids 

are incinerated rather than digested, which also reduces phosphorus recycle because 

most of it stays with the ash. 

8. No special procedures are in place for managing wet-weather flows. 

Case Study No. 2: Fiesta Village WWTP in Lee County, Florida 

This facility has an oxidation ditch followed by denitrification filters with a methanol feed 

system. The permit contains an annual average TN limit of 3 mg/L and TP limit of 0.5 mg/L, 

with higher values for monthly and weekly averages. In addition, the permit specifies a daily 

limit of 5 mg/L in total suspended solids (TSS) for water reuse. 
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3-8 Chapter 3 - Case Studies and Reliability Factors

The facility was an extended-aeration oxidation ditch, and four denitrification filters were 

added along with three screw pumps. Alum was used for the TSS control required for water 

reuse, which aided phosphorus removal at the same time. The sludge was digested 

aerobically, stored, and hauled away for processing at another county facility. 

Fiesta Village WWTP, Lee Co., FL

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 5040 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.102 mg/L

Std. Dev. = 0.035 mg/L

C.O.V. = 35%

Figure 3-4. Lee County, Florida: Monthly frequency curves for TP. 

Fiesta Village WWTP Lee Co., FL

Monthly Average Frequency Curves for Nitrogen

0.100

1.000

10.000

100.000

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Total N Final Effluent - Total N

Secondary Effluent NO3-N

0.05 0.1 0.5 1 2 5 10 3020 40 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 1.71 mg/L

Std. Dev. = 0.48 mg/L

C.O.V. = 28%

Figure 3-5. Lee County, Florida: Monthly frequency curves for TN. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-9

The performance was efficient and reliable. The plant achieved a TN concentration of 1.71 

mg/L with a COV of 28 percent and TP concentration of 0.1 mg/L with a COV of 35 percent. 

A number of key factors contributed to this high performance: 

1. Denitrification is accomplished in two processes in series: first in the oxidation ditch, 

which has a target nitrate nitrogen concentration of 3 to 3.5 mg/L, followed by the 

denitrification filter, with a minimal dose of methanol feed to remove additional 

nitrate nitrogen. 

2. The high level of denitrification in the oxidation ditch is a unique accomplishment at 

this plant, achieved by carefully controlling the brush aerators. The standard 

procedure calls for turning off one brush aerator during the day and two during the 

night, which provides anoxic zone(s) within the ditch comprising approximately 25 to 

50 percent of the tank volume. The location of these aerators varies, depending on the 

season. In addition, the clarifiers operate with a denitrifying sludge blanket depth 

between 2.5 ft and 3.5 ft. 

3. The denitrifying filters then bring the nitrate nitrogen below 3 mg/L with a methanol 

feed rate of 1.9 lb/lb nitrate nitrogen, which is low. The dosage is determined on the 

basis of daily performance. This process is compact with a small footprint and, thus, 

is preferred at a location with limited space. 

4. Alum was fed at an average dosage of 8.9 mg/L at a mass basin Al/TP ratio of 2.32 to 

reach an effluent concentration of 0.1 mg/L 

5. Recycle loads are minimal primarily due to the fact that aerobically digested sludge is 

hauled off-site for processing at another facility. 

6. During wet-weather periods, a normal mode of operation is maintained. Under 

extreme peak flow conditions, however, the biomass inventory is protected from 

surges by shutting off a number of aerators. 

Case Study No. 3: Central Johnston County, North Carolina 

This facility has a plug-flow activated-sludge system followed by a denitrification filter with 

a methanol feed. The permit includes a monthly ammonia limit of 2 mg/L in summer and 4 

mg/L in winter, and an annual average TP limit of 1 mg/L. In addition, the TN limit is set on 

the basis of a load limit of 56,200 lb per year, which is equivalent to 3.7 mg/L on an annual 

average basis at the design flow of 7 MGD. 

This facility was retrofitted from conventional activated sludge to enhanced biological 

phosphorus removal (EBPR), followed by a separate-stage denitrification filter for nitrogen 

removal. The plant treated 5.17 MGD during the maximum month and had an annual average 

of 4.12 MGD during the evaluation period. There is no primary settling, and the WAS is 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-10 Chapter 3 - Case Studies and Reliability Factors

aerobically digested before dewatering by a belt filter press off-site. After UV disinfection, 

the reclaimed water is stored for reuse. 

Johnston County, NC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 2010 504030 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 0.26 mg/L

Std. Dev. = 0.164 mg/L

C.O.V. = 62%

Figure 3-6. Johnston County, North Carolina: Monthly average frequency curves for TP. 

Johnston County, NC

Monthly Average Frequency Curves for Ammonia-N

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent Ammonia-N Final Effluent Ammonia-N

0.05 0.1 0.5 1 2 5 2010 30 40 50 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0.44 mg/L

Std. Dev. = 0.055 mg/L

C.O.V. =12%

Figure 3-7. Johnston County, North Carolina: Monthly average frequency curves for ammonia 

nitrogen. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-11

Johnston County, NC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent TN Final Effluent TN

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 2.14 mg/L

Std. Dev. = 0.36 mg/L

C.O.V. =16%

Figure 3-8. Johnston County, North Carolina: Monthly average frequency curves for TN. 

The performance was highly efficient and reliable for nitrogen removal and biological 

phosphorus removal. The TN COV for the monthly average was very reliable at 16 percent at 

a mean concentration of 2.14 mg/L. The COV for ammonia nitrogen was also very reliable at 

12 percent at a mean concentration of 0.44 mg/L. The COV for TP was 62 percent at an 

average of 0.26 mg/L on a monthly basis. TP during the maximum month was 0.64 mg/L at 

this COV, meeting the permitted limit of 1 mg/L. 

Several distinct factors were noted at this facility: 

1. The wastewater characteristics are favorable for nutrient removal. The BOD-to-TP 

ratio and BOD-to-total Kjeldahl nitrogen (TKN) ratio were 55.1 and 10, respectively, 

where 25 and 4 are reported as adequate in the literature (Lindeke et al. 2005; 

Neethling et al. 2005; WEF and ASCE 1998). 

2. A significant amount of denitrification was accomplished in the clarifier, with a deep 

sludge blanket of 3 to 4 feet. (This can be referred to as a denitrification blanket.) As 

a result, the RAS flow rate was reduced significantly. Note that all these procedures 

were developed and optimized by plant personnel. 

3. Biological phosphorus removal was successfully achieved in the retrofitted activated 

sludge without any chemical addition. 

4. Methanol was added before the denitrification filters. The methanol was fed at a 

controlled dosage with online monitoring of nitrate, by a Hach probe. The average 

dosage of methanol for the year was relatively low at 14 mg/L. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-12 Chapter 3 - Case Studies and Reliability Factors

5. This process is compact with a small footprint and, thus, is preferred at a location 

with limited space. 

6. The recycle loads are minimized because only aerobic digestion is available on-site. 

The sludge is dewatered off-site, so there is no return flow from the dewatering 

process. 

7. Wet-weather flows are managed without major difficulties. Even during Tropical 

Storm Alberto in June 2006, the plant operated normally. As a result of Alberto, flow 

increased from 3.4 to 10.5 MGD, following 10 inches of precipitation in a 24-hour 

period. Under extreme weather conditions like a hurricane, the plant would shut down 

part of the aeration basins and protect the sludge inventory. This could last for up to a 

day without causing adverse effects on the biomass at the plant. 

Case Study No. 4: Marshall Street Advanced WWTP, Clearwater, Florida 

The Clearwater, Florida, facility uses a 5-stage Bardenpho process followed by a sand filter 

with alum feed. This facility was retrofitted from conventional activated sludge to the 5-stage 

Bardenpho process and tertiary filtration. 

The NPDES permit includes annual limits of 3 mg/L for TN and 1 mg/L for TP. Monthly 

average limits are 3.75 mg/L for TN and 1.25 mg/L for TP. In addition, 

dichlorobromomethane and dibromochloromethane are limited to 24 and 46 μg/L, 

respectively, on an annual average basis. 

The plant was designed to treat 10 MGD. It treated 6.85 MGD during the maximum month 

with an annual average of 5.45 MGD for the evaluation year. The wastewater is settled in the 

primary tanks, treated at the Bardenpho process, and filtered through rapid sand filters before 

chlorination. The WAS is thickened by rotary-drum thickeners and then digested along with 

the primary sludge in anaerobic digesters before dewatering by a belt filter press. The 

biological treatment process selected is a classic 5-stage Bardenpho design with a long 

hydraulic retention time (HRT, 20 hours) and a long sludge retention time (SRT, 25 days) 

with an anaerobic zone for denitrification followed by two separate alternative aerobic and 

anoxic zones for nitrification and phosphorus removal. No external carbon source is used. 

The performance was highly efficient and reliable for nitrogen and phosphorus removal. The 

COVs were 16 percent for the monthly averages at a mean concentration of 2.32 mg/L TN. 

The COV for phosphorus was 40 percent at an average concentration of 0.13 mg/L. This 

process has a large footprint and thus requires a large site. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-13

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.132 mg/L

Std. Dev. = 0.052 mg/L

C.O.V. = 40%

Figure 3-9. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average frequency 

curves for TP. 

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 0.038mg/L

Std. Dev. = 0.007 mg/L

C.O.V. = 18%

Figure 3-10. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average frequency 

curves for ammonia nitrogen. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-14 Chapter 3 - Case Studies and Reliability Factors

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 9999.5 99.9599.9

Mean = 2.32 mg/L

Std. Dev. = 0.38 mg/L

C.O.V. = 16%

Figure 3-11. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average frequency 

curves for TN. 

Contributing factors at this facility include the following: 

1. Thorough operating guidelines were developed and followed by the plant personnel. 

2. This facility had favorable wastewater characteristics in BOD, TKN, and TP. The 

BOD-to-TP ratio was 37.5 and the BOD-to-TKN ratio was 6.7. Both ratios are higher 

than what is considered adequate, 25 and 4, respectively. 

3. Process controls were automated with online probes for DO, nitrate, ortho-phosphate, 

and oxidation-reduction potential (ORP). The controls were based on readings in the 

second anoxic zone for nitrate, DO, and ORP. The strategy developed by plant 

personnel includes a minimum target ORP at –60 millivolts and nitrate nitrogen at 0.5 

mg/L. DO levels are then adjusted by automatic controls to stay in this region. In 

addition, plant personnel use turbidity and conductivity meters for effluent 

monitoring. Turbidity is for reuse monitoring, as required by Florida, and 

conductivity is used as an early warning indicator for potential seawater intrusion to 

the sewer system to protect the water reuse customers. 

4. Alum is added to meet the trihalomehthane requirements of Florida for water reuse. 

The average dosage was 27 mg/L as alum, or at the aluminum-to-phosphorus ratio of 

1.6 on a molar basis. 

5. Recycle flows from dewatering are mixed with the plant influent and treated together. 

The recycle loadings from dewatering were moderate at 30 percent for phosphorus 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-15

and 14 percent for nitrogen. Thickening by mechanical equipment helped in 

minimizing recycle loads in nitrogen and phosphorus. Anaerobic digestion also 

contributed to the increase in recycle loads of these nutrients to moderate levels. 

6. Wet-weather flows are handled with the normal mode of operation. 

3.2.2 Total Nitrogen and Phosphorus Removal at Mid-Level 

Concentration Limits (3 to 6 mg/L) 

Three case studies fell into this group, and they are characterized as having one anoxic zone 

with or without external carbon sources. 

Case Study No. 5: North Cary, North Carolina 

The North Cary, North Carolina, facility uses a phased isolation oxidation ditch (PID) 

(Biodenipho) with an upflow sand filter. The NPDES permit requires ammonia limits of 

0.5 mg/L for summer and 1.0 mg/L in winter, 3.94 mg/L for TN on an annual average, and 

2 mg/L on a quarterly basis. 

This facility was expanded, replacing the Schreiber process with a PID, in 1997. The design 

flow was 12 MGD and treated 8.7 MGD in the maximum month and 7 MGD as the annual 

average of this evaluation period. The wastewater is treated at this PID, filtered by an upflow 

Dynasand filter, and disinfected by UV irradiation before discharge. The facility has a 2–

million-gallon (MG) equalization basin online and has additional storage capacity of 7 MG in 

the old facility on-site. The sludge is aerobically digested and shipped to another facility for 

dewatering and drying. 

North Cary, NC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 5040 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.379 mg/L

Std. Dev. = 0.241 mg/L

C.O.V. = 64%

Figure 3-12. North Cary, North Carolina: Monthly average frequency curves for TP. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-16 Chapter 3 - Case Studies and Reliability Factors

North Cary, NC

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia-N

0.05 0.1 5040 0.5 1 2 5 10 20 30 60 70 80 90 95 98 9999.5 99.9599.9

Mean = 0.081 mg/L

Std. Dev. = 0.082 mg/L

C.O.V. = 102%

Figure 3-13. North Cary, North Carolina: Monthly average frequency curves for ammonia 

nitrogen. 

North Cary, NC

Monthly Average Frequency Curves for Total Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Total Nitrogen Final Effluent - Total Nitrogen

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 3.67 mg/L

Std. Dev. = 0.512 mg/L

C.O.V. = 14%

Figure 3-14. North Cary, North Carolina: Monthly average frequency curves for TN. 

The performance was efficient and reliable. The COVs were 102 percent for ammonia at a 

mean concentration of 0.08 mg/L, 64 percent for TP at a mean concentration of 0.38 mg/L, 

and 14 percent for TN at a mean concentration of 3.67 mg/L. This was achieved without any 

chemical addition. The quarterly average TP was 0.57 mg/L at this COV and met the limit of 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-17

2 mg/L. The maximum month ammonia nitrogen concentration was 0.34 mg/L, well within 

the permit limit of 0.5 mg/L in summer. 

This process provides two anaerobic selector zones ahead of the oxidation ditch, which 

alternate between anoxic and aerobic cycles to maximize nitrification and denitrification 

using the carbon present in the wastewater. The oxidation ditch is followed by two anoxic 

zones and one reaeration zone. 

Several key factors were noted at this facility: 

1. Wastewater characteristics were favorable. The BOD-to-TP ratio and BOD-to-TKN 

ratio were 31.6 and 4.3, respectively. Both were adequate in accordance with the 

literature. 

2. Reliability is ensured with two anaerobic zones ahead of and two anoxic zones after 

the ditch. The automatic process logic control of cycle times is based on online DO 

measurements. 

3. Separate means of providing aeration and mixing in the ditch allow optimal control of 

aerobic and anoxic cycles for best performance. 

4. Recycle loads are minimized by having only aerobic digestion on-site; there are no 

anaerobic processes. The digested sludge is transported off-site for processing, and 

therefore no recycled loads go back to the facility. 

5. Wet-weather operation follows the normal mode of operation. The PID process 

design allows the cycle time to be adjusted based on incoming flow. When the system 

is in storm mode the plant is switched to sedimentation, thereby preventing solids 

washout. 

6. Equalization basins with a total volume of 9 MG are available. During Tropical Storm 

Alberto in June 2006, all the equalization basins were filled, the PID operated in 

storm mode for a short duration, and the facility was still able to comply with all 

NPDES permit limits. 

Case Study No. 6: Kelowna, British Columbia 

The Kelowna, British Columbia, facility uses a 3-stage Westbank process with a fermenter 

and a dual-media filter. The Canadian Ministry of Environment permit requires a TN limit of 

6 mg/L and 2.0 mg/L on a daily basis, or 1.5 mg/L on the 99th percentile, or 1 mg/L on the 

90th percentile, and 0.25 mg/L on an annual basis for TP. 

This facility was retrofitted from the existing 5-stage Bardenpho to the 3-stage Westbank 

process. The design capacity is 10.5 MGD, and the facility treated 7 MGD as the annual 

average for the evaluation year. The wastewater is settled in primary tanks, treated at the 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-18 Chapter 3 - Case Studies and Reliability Factors

bioreactor, filtered through dual-media filters, and disinfected by UV. The primary sludge is 

fermented and stored before dewatering and composting off-site. The secondary sludge is 

thickened by DAF and stored. Combined primary and secondary sludge is then dewatered by 

centrifuge for off-site composting. The primary effluent can be sent to the equalization basin, 

which has a capacity equivalent to approximately 7.5 percent of the average design flow. 

The performance was efficient and reliable. The COV was 12 percent for the monthly 

average data at a mean concentration of 4.38 mg/L TN. The COV for TP was 21 percent at 

the mean concentration of 0.139 mg/L. The maximum month average was 0.20 mg/L TP and 

4.9 mg/L TN at these low COVs. 

Some of the unique features of this facility are as follows: 

1. The wastewater characteristics were favorable but not sufficient year-round. The 

BOD-to-TP ratio was 26.6 and found to be somewhat low for a cold-region facility. 

The BOD-to-TKN ratio was 5.3, which was adequate for what is recommended in the 

literature. 

2. The unique features of this facility include the fermenter for volatile fatty acid (VFA) 

production, which ensured a reliable and efficient operation of the EBPR. The design 

and operation of fermenter technology evolved from this facility. Current operation is 

based on 5-day sludge age year-round, and the predominant VFA species were acetic 

acid and propionic acid, both of which are desirable for a good EBPR. 

Kelowna, BC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 0.139 mg/L

Std. Dev. = 0.03 mg/L

C.O.V. = 21%

Figure 3-15. Kelowna, British Columbia: Monthly average frequency curves for TP. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-19

Kelowna, BC

Monthly Average Frequency Curves for Ammonia-N

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent Ammonia-N Final Effluent Ammonia-N

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.575 mg/L

Std. Dev. = 0.26 mg/L

C.O.V. =45%

Figure 3-16. Kelowna, British Columbia: Monthly average frequency curves for ammonia 

nitrogen. 

Kelowna, BC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent TN Final Effluent TN

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 4.38 mg/L

Std. Dev. = 0.51 mg/L

C.O.V. =12%

Figure 3-17. Kelowna, British Columbia: Monthly average frequency curves for TN. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-20 Chapter 3 - Case Studies and Reliability Factors

 The annual production of VFA is equivalent to 10 mg/L in the influent to the 

bioreactor from the fermenters. Other sources of VFA in the plant included centrate 

from dewatering, primary effluent, and influent, all of which added up to the 

equivalent of 7 mg/L as an average. The target VFA was 4–8 mg VFA/mg for soluble 

phosphorus removed. The VFA-rich fermenter supernatant (17 mg/L) was directly 

discharged to the anaerobic zone, ensuring a steady feed of VFA to the phosphorusaccumulating organisms (PAOs). 

3. A step-feed of influent is split equally for both the anaerobic and anoxic zones. This 

process has evolved from the 5-stage Bardenpho, with the modifications resulting in 

shorter HRT (10 hours) and SRT (10 days). The HRT of the anaerobic zones was 

reduced from 3 hours to 1 hour with the direct addition of VFA and primary effluent. 

4. Like the Central Johnston County, North Carolina, and Lee County, Florida, facilities, 

this facility used a denitrification blanket in the clarifier. The average depth ranged 

between 2 and 3 feet. Nitrate reduction in the RAS blanket up to 6 mg/L did not 

create rising sludge concerns. This allowed minimal potential for nitrate return to the 

anaerobic zone. 

5. The denitrification was controlled by continuously monitoring ORP at the end of the 

anoxic zone. These data were fed into the computer system to ensure that a sufficient 

amount of primary effluent was diverted to the anoxic zone to meet the nitrate load 

from the nitrified internal recycle flow. The average was approximately 50 percent of 

the primary effluent. 

6. The Kelowna plant operates under extremely low temperatures. A monthly average 

low temperature of 13 degrees Celsius (°C) was observed during the months of 

January and February. A high temperature of 22 °C was recorded in August. 

7. The facility has an especially flexible design. Of the 21 bioreactor cells, 17 have dual 

equipment––mixers and diffusers. This allows each cell to have the ability to be 

operated as a swing zone, which was expensive but offers great flexibility for changes 

in wastewater and permit requirements. 

8. Recycle loads from dewatering were minimized by maintaining separate processes for 

secondary sludge and primary sludge. No sludge digestion was practiced. The total 

recycle loads from dewatering were only 13 percent in TP and 0.1 percent in TN. 

9. Wet-weather flows were managed under the normal mode of operation, using the 

equalization basin. The collection system was separated, and the seasonal variation 

was not very significant. The maximum month flow was 10 percent higher than the 

average flow. The total basin capacity was equivalent to 7.5 percent of the influent 

design flow rate. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-21

Case Study No. 7: Fairfax County, Virginia 

The Noman M. Cole facility in Fairfax County, Virginia, uses a step-feed activated-sludge 

process with a tertiary filter with ferric chloride addition. The NPDES permit limits are 

1.0 mg/L for ammonia nitrogen in summer and 2.2 mg/L in winter, and 0.18 mg/L for TP on 

a monthly basis. The facility is not required to report TN, but it started a voluntary program 

in anticipation of the future limit as a part of the Chesapeake Bay Initiative. 

This facility was expanded from the existing step-feed activated-sludge plant in 2002. The 

new design capacity of the plant is 67 MGD, and the annual average flow was 47 MGD 

during the evaluation year. The wastewater is settled in the primary tanks, treated by 

activated sludge with three to four feed points, settled again in the tertiary clarifier, and 

filtered before disinfection by chlorination and dechlorination. The secondary effluent is 

equalized up to 13.2 MG. The tertiary filters consist of the old, dual-media units and the new, 

deep mono-media filters. The headworks area also has retention basins with a capacity of 5.7 

MG and an equalization basin with 4 MG, which together are equivalent to 15 percent of the 

average flow. The secondary sludge is thickened by DAF. The primary sludge is fermented 

in the gravity thickeners, which operate at an HRT of less than a day and a sludge age of 3 

days. The fermented primary sludge and thickened secondary sludge were mixed together for 

dewatering by centrifuge, followed by incineration. 

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Primary Influent Primary Effluent Secondary Effluent

Tertiary Clarifier Influent Tertiary Clarifier Effluent Final Effluent

0.05 0.1 0.5 1 2 5 10 20 504030 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.086 mg/L

Std. Dev. = 0.018 mg/L

COV = 21%

Figure 3-18. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly average 

frequency curve for TP. 
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3-22 Chapter 3 - Case Studies and Reliability Factors

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 20 4030 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.12 mg/L

Std. Dev. = 0.016 mg/L

COV = 14%

Figure 3-19. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly average 

frequency curve for ammonia nitrogen. 

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Monthly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 5.25 mg/L

Std. Dev. = 0.63 mg/L

COV = 12%

Figure 3-20. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly average 

frequency curve for TN. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-23

The plant performed well with good reliability. The COV was 21 percent for TP on a 

monthly basis at a mean concentration of 0.09 mg/L. The COV for ammonia nitrogen was 

14 percent at a mean concentration of 0.12 mg/L N. The COV was 12 percent for TN on a 

monthly average at a mean concentration of 5.25 mg/L. This facility included three feed 

locations with a 40, 40, and 20 percent flow split in the old plant and four feed points with a 

25 percent split in the new plant, with an anoxic zone in each feed point, which provides an 

opportunity to denitrify rapidly with carbon present in wastewater. 

Contributing factors for this group of facilities include the following:

1. The wastewater characteristics were favorable. The BOD-to-TP ratio and BOD-toTKN ratio were 29.5 and 5.4, respectively. 

2. The step feed is unique in providing a dedicated carbon supply and three anoxic 

cycles for better nitrogen removal. Caustic soda is fed to supplement the alkalinity 

deficiency in the wastewater. 

3. Phosphorus removal is achieved in three steps: biological removal in activated sludge, 

chemical removal in tertiary clarifier, and then tertiary filters. 

4. Primary sludge is fermented in gravity thickeners with an SRT of 3 days and HRT of 

approximately 1 day. The units are operated between the hours of 8 a.m. and 2 p.m., 

when the greatest amount of ammonia reaches the plant. The VFA production was 

equivalent to 10 mg/L by chemical oxygen demand in the primary effluent. The 

VFAs consist of 33 percent acetic acid, 49 percent propionic acid, and 18 percent 

others. VFAs produced in this way are fed to the anoxic zones of the activated-sludge 

process. 

5. Secondary sludge is thickened by DAF, thus minimizing the release of phosphorus 

and ammonia. 

6. The plant design already includes recycle flows and loadings. They include 

10 percent in BOD, 19 percent in TSS and 23 percent in TP. All tanks and processes 

are sized to include these loadings, which is a conservative approach. Lime is added 

to the cake up to 13 percent by weight to minimize these recycle loads back to the 

plant. 

7. Wet-weather operation follows the normal mode of operation. The facility manages 

the wet-weather flow in four distinct steps: at the retention basins first, then 

equalization at the headworks, step-feed activated sludge, and finally equalization of 

the secondary effluent. Because the facility is a step-feed facility, the process is more 

stable than that at other plants. The equalization basins were designed to divert flow 

at 1.6 times the average flow and then again in the secondary effluent before the 

tertiary clarifiers. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-24 Chapter 3 - Case Studies and Reliability Factors

3.2.3 Phosphorus Removal Low Level (less than 0.1 mg/L) 

There are two case studies for this level of performance, one with both biological phosphorus 

removal and chemical phosphorus removal (Clark County, Nevada) and the other 

characterized as having biological phosphorus removal (Kalispell, Montana).

Case Study No. 8: Clark County, Nevada 

The Clark County, Nevada, facility uses an anoxic and then oxygenated (anoxic/oxic, or 

A/O) process with primary sludge thickening for VFA production and tertiary filters with 

alum feed. This facility was expanded from 88 MGD to 110 MGD in 1995 for the A/O 

process to meet daily load limits on nutrients. The limits are equivalent to an ammonia limit 

of 0.56 mg/L and a phosphorus limit of 0.19 mg/L at 110 MGD. The facility treated 95 MGD 

during the evaluation period. Clark County made these voluntary efforts to reduce 

phosphorus during the evaluation period to the lowest possible concentration. 

The facility has two plants––the Central Plant (CP) and the Advanced Waste Treatment Plant 

(AWT). The wastewater is settled at primary tanks, treated in the A/O process, filtered, and 

disinfected by UV irradiation and sent either to reclaimed-water users or to Las Vegas Wash 

and eventually to Lake Meade. The AWT has tertiary clarifier ahead of its filter, whereas the 

CP does not. The tertiary filter at the AWT does not have air scour, whereas the CP filter has 

air scour capability. Ferric chloride is added into the primary clarifiers for odor control, while 

alum is added to both the tertiary clarifiers and tertiary filters. The secondary sludge is 

thickened by DAF, stored together with primary sludge, and dewatered by a belt filter press. 

The cake is sent to a landfill. 

The facility produced low concentration phosphorus and ammonia in the effluent with good 

reliability. The COV was 30 percent on the average concentration of 0.099 mg/L in TP and 

14 percent on the average ammonia concentration of 0.12 mg/L. The daily limits were met in 

accordance with the permit, at 0.22 mg/L for TP and 0.57 mg/L for ammonia nitrogen. 

Contributing factors for this facility include the following:

1. The wastewater characteristics were favorable. The BOD-to-TP ratio was 29.8 for the 

year and ranged from 26.5 to 34.2 on a monthly basis. 

2. VFA generation was achieved in the primary settling tanks. The practice was based 

on thickening the sludge up to 5 percent total solids (TS). The upper limit was 

6 percent; and higher percentages should be avoided. 

3. The A/O process parameters were optimized for the SRT and RAS flow rate 

seasonally. The clarifier operated with a minimal sludge blanket—less than 6 inches. 

The automation and computer controls include DO control at 0.5 mg/L using multiple 

probes on all nine basins with WAS management on a daily basis. The critical time of 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-25

the year is when the temperature rises above 110 degrees Fahrenheit (°F) and oxygen 

transfer becomes less efficient. 

4. Biological phosphorus removal by the A/O process followed by chemical addition 

reduced overall chemical sludge generation. 

5. Waste secondary sludge was thickened in the DAF unit, thereby minimizing 

phosphorus release because of anaerobic conditions. 

6. Alum was added to the tertiary clarifier in the AWT and alum was added again to the 

tertiary filter to minimize secondary release of phosphorus in the filters. 

7. Recycle loads were minimized by daily processing of all sludge (a no storage policy), 

ferric addition to the filtrate from dewatering, DAF thickening of the WAS, and not 

digesting sludge on-site. 

8. The plant follows the normal operating procedures during wet-weather events in 

August and September. 

Clark Co. Water Reclamation Plant - Las Vegas, NV

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

0.05 0.1 0.5 1 2 5 10 20 504030 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0.0995 mg/L

Std. Dev. = 0.0297 mg/L

COV = 30%

Figure 3-21. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly average 

frequency curve for TP. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-26 Chapter 3 - Case Studies and Reliability Factors

Clark Co. Water Reclamation Facility

Monthly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 9999.5 99.9599.9

CP Mean =15.4 mg/L

Std. Dev. = 0.54 mg/L

COV = 3.5%

Figure 3-22. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly average 

frequency curve for TN. 

Clark Co. Water Reclamation Facility

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

CP Mean =0.12 mg/L

Std. Dev. = 0.0116 mg/L

COV = 14%

Figure 3-23. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly average 

frequency curve for ammonia nitrogen. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-27

Case Study No. 9: Kalispell, Montana 

The Kalispell, Montana, facility uses a modified University Cape Town (UCT) process with 

a fermenter and an upflow sand filter. The NPDES permit specifies a monthly limit of 1 

mg/L in TP and an ammonia limit of 0.14 mg/L N. The facility was designed for 3 MGD and 

treated 2.8 MGD during this evaluation year. The wastewater is settled in the primary tanks, 

treated in the modified UCT process, filtered, and then disinfected using ultraviolet radiation 

before discharge. The primary sludge is fermented before anaerobic digestion. The secondary 

sludge is thickened by DAF. The thickened secondary sludge and digested primary sludge 

are dewatered together by a belt filter press and then trucked off-site for composting. 

The plant operated very efficiently and reliably. The COV was 19 percent on a monthly 

average basis at the mean concentration of 0.12 mg/L TP. The COV for ammonia nitrogen 

was 0 percent, below detection at all periods. The effluent concentrations in the maximum 

month were 0.15 mg/L for TP and 0.07 mg/L for ammonia nitrogen. 

The unique features of this facility are as follows: 

1. The wastewater characteristics were favorable. The BOD-to-TP ratio was 55, much 

higher than the value of 25 reported favorable in the literature. No nitrogen removal is 

required at this facility. 

2. A 2-stage fermenter was installed to ensure a definite supply of VFAs in anticipation 

of low winter temperatures and increased flows during the spring snowmelts and 

rainfalls. The fermenter took the primary sludge and operated at an SRT of 5 days and 

an HRT of 7 to 21 hours. The unique system allows independent control of HRT and 

SRT. The HRT is controlled by adjusting fermenter volume, while the SRT is 

controlled by a function of the mass of solids lost from the primary clarifier and the 

volume of fermented sludge wasted daily from the fermenter. The VFA production 

was estimated to be equivalent to 18 mg/L at 20 °C and 13 mg/L at 13 °C. 

3. The bioreactor has been optimized for SRT and HRT, and the RAS is now designed 

to maintain a target mixed liquor suspended solids level in the aeration basin. A noblanket policy has been applied to the clarifier operation. 

4. The WAS is thickened in DAF units and thus kept aerobic to minimize release of 

phosphorus, before it is dewatered by belt filter press. Although this facility nitrified 

fully down to the detection limit (at 0.07 mg/L), denitrification was not required and 

thus was not practiced. The COV for ammonia nitrogen was 0 percent at the mean 

concentration of 0.07 mg/L as nitrogen. The COV was 31 percent at the mean 

concentration of TN of 10.6 mg/L. 

5. Both digester supernatant and filtrate from dewatering returned to the plant’s 

headworks. The ortho-phosphorus in these streams averaged 6 percent of the plant’s 

influent TP. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-28 Chapter 3 - Case Studies and Reliability Factors

6. The equalization basin was sized to store 12.5 percent of the plant influent. The 

normal mode of operation is maintained during wet-weather periods. 

Figure 3-24. Kalispell, Montana: Monthly average frequency curve for TP. 

Figure 3-25. Kalispell, Montana: Monthly average frequency curve for ammonia nitrogen. 

0.01

0.1

1

10

100

Raw Influent Final Effluent

0.05 6 0.1 0.5 1 2 5 2010 30 40 50 0 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.121 mg/L

Std. Dev. = 0.023 mg/L

C.O.V. = 19%

Kalispell, MT

Monthly Average Frequency Curves for Total Phosphorus

Percent Less Than or EqualTo

T

otal Phosphorus, mg/

L

Kalispell, MT

Monthly Average Frequency Curves for Ammonia Nitrogen

0.0

0.1

1.0

10.0

100.0

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent Final Effluent

0.05 60 70 80 90 95 0.1 50 0.5 1 2 5 40302010 98 99 99.5 99.9599.9

Mean < 0.07 mg/L

Std. Dev. = 0.00 mg/L

C.O.V. = 0%
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-29

Figure 3-26. Kalispell, Montana: Monthly average frequency curve for TN. 

3.3 Reliability Factors 

3.3.1 Wastewater Characteristics 

BOD-to-TP and BOD-to-TN Ratios 

The BOD-to-TP ratios ranged between a low of 26.6 at Kelowna and a high of more than 50 

at Kalispell on average for the year. There were monthly variations in these ratios at each 

facility because of weather conditions and the sewer system. The monthly variations in 

Fairfax County, Virginia, are shown in Table 3-3. To make the extent of variations clearer, 

the BOD-to-TP (or BOD-to-TKN) ratios were normalized by dividing the monthly values by 

the average of those values. This shows that the relative variation in values was small, 

between 90 percent and 110 percent of the average. The BOD-to-TP ratio remained between 

28 and 33.5 in the plant influent and 24.4 and 33.8 in the primary effluent. 

The BOD-to-TKN ratios ranged between a low of 4.3 at North Cary to a high of 10 at the 

Central Johnston County, North Carolina. The BOD-to-TKN ratio varied between 4.6 and 6.1 

at Fairfax, Virginia, as shown below. 

Table 3-3 shows that the BOD-to-TP ratio in the primary effluent went down as low as 24 in 

some months at the Fairfax, Virginia. The North Cary and Central Johnston plants, as well as 

the Fairfax County plant, decided to use EBPR and installed fermenters to ensure reliable 

operation. At the Clark County, Nevada, facility, the BOD-to-TP ratio was noted as a low 29 

for the annual average and the primary settling tanks were converted to a thickener/fermenter 

for VFA generation. 

Kalispell, MT

Monthly Average Frequency Curves for Total Nitrogen (Goal)

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 10.6 mg/L

Std. Dev. = 3.31 mg/L

C.O.V. = 31%
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-30 Chapter 3 - Case Studies and Reliability Factors

Table 3-3. Monthly variation of wastewater characteristics at Fairfax County, Virginia 

Date INF 

BOD-to-TP 

Normalized 

by avg 

PE 

BOD-to-TP 

Normalized 

by avg 

INF 

BOD/TKN 

Normalized 

by avg 

Jan-06 33.1 1.1 29.8 1.0 6.1 1.1 

Feb-06 33.7 1.1 29.5 1.0 5.4 1.0 

Mar-06 28.3 0.9 27.4 1.0 5.3 1.0 

Apr-06 28.2 0.9 27.1 0.9 5.5 1.0 

May-06 27.2 0.9 26.8 0.9 4.7 0.9 

Jun-06 28.9 1.0 24.4 0.9 5.5 1.0 

Jul-06 28.8 1.0 26.1 0.9 5.9 1.1 

Aug-06 29.4 1.0 28.1 1.0 4.6 0.9 

Sep-06 31.5 1.0 32.4 1.1 5.0 0.9 

Oct-06 33.5 1.1 33.8 1.2 4.9 0.9 

Nov-06 32.2 1.1 32.0 1.1 5.4 1.0 

Dec-06 28.2 0.9 26.3 0.9 5.4 1.0 

Avg 30.3 28.7 5.3 

The low temperatures encountered by the case study facilities during the study period were 

10 °C and 13 °C at the Kalispell, Montana, and Kelowna, British Columbia, facilities, 

respectively. Much warmer conditions occurred at plants in Florida. 

At those facilities that depend on methanol feed for nitrogen removal or chemical feed for 

phosphorus removal, however, these characteristics in BOD (i.e., BOD-to-TP or BOD-toTKN ratio) were not as critical in design or operation as at the other facilities for reliable 

performance. 

3.3.2 Fermenter and VFA Generation 

Three facilities had dedicated fermenters; one used primary settling tanks for VFA 

generation. The reliability of EBPR was ensured by having a fermenter in the facility. 

• Kalispell, Montana, had a 2-stage fermenter with a variable SRT and HRT up to an 

SRT of 3–5 days with an HRT of 24 hours or less. 

• Kelowna, British Columbia, and Fairfax County, Virginia, had gravity thickeners 

converted with an SRT of 3 to 5 days. The HRT averaged approximately 1 day (a 

24-hour period). 

• Clark County, Nevada, had primary settling tanks successfully serving as thickeners 

and fermenters at total solids concentrations of 5 percent. 

The other facilities relied on VFAs found in the influent or generated from in-plant recycles. 

The goal was to produce VFA of 18 mg/L in the influent to bioreactors so that a VFA-tophosphorus ratio of 4 or higher could be maintained. Among these facilities, Kalispell and 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-31

Kelowna had biological phosphorus removal (no chemicals) and produced an annual average 

of 0.12 and 0.13 mg/L with COVs of 19 and 21 percent, respectively. Other facilities 

achieved lower concentrations but required chemical addition, filtration, or both. 

3.3.3 Bioreactor Design and Process Parameters 

The case studies demonstrated a range of process parameters that were designed in 

accordance with industry standards or were uniquely developed by plant personnel. These 

parameters are summarized below. 

1. SRT and HRT: The bioreactor systems for the case study plants have a wide range of 

SRTs, from 5 days and more than 40 days in warm months to between 9 and 60 days 

in cold months. The Kelowna system was optimized for nutrient removal through the 

use of a denitrifying sludge blanket; when the system was operated in step-feed mode, 

shorter SRTs and HRTs were possible than in other systems. The HRTs ranged from 

10 hours in Kelowna to 20 hours in Clearwater, Florida. 

2. Denitrifying sludge blanket in secondary clarifier: A denitrifying blanket is 

maintained in three facilities. Denitrifying blankets ensured good phosphorus 

removal, alkalinity recovery, and shortening the anoxic zone in the bioreactor. Two 

facilities, however, maintain a no-blanket policy; the concern is secondary release of 

phosphorus (Kalispell and Clark County). 

3. Step-feed mode: For both nitrogen and phosphorus removal, primary effluent was a 

good source of carbon, and thus the step-feeding of primary effluent accelerated the 

biological activities and reduced the size of the anoxic zone requirement. The feed 

locations varied from two at Kelowna to four at Fairfax, Virginia. 

4. Flexible design/swing zones: The reliability of the biological process increases 

significantly in facilities with built-in swing zones. Kelowna and Clearwater have 

swing zones, and they allow adjustment on the basis of changing wastewater 

characteristics as well as the treatment objective. Kelowna evolved from the classic 

5-stage Bardenpho to a 3-stage Westbank process using flexible design. It treats 

70 percent more flow in the same original tanks. Clearwater has the flexibility to 

operate in either 5-stage Bardenpho mode for nitrogen and phosphorus removal or 

4-stage Bardenpho mode for nitrogen removal. North Cary has the flexibility of 

separate mixing and aeration in the oxidation channel and thus can operate in 

accordance with wastewater flow and demand (summer vs. winter, or wet vs. dry 

days). 

5. Carbon source: All the facilities use the internal carbon present in the wastewater, 

although some facilities have fermenters to increase VFA production. Three facilities 

use external carbon sources to support denitrification: two (Central Johnston County 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-32 Chapter 3 - Case Studies and Reliability Factors

and Lee County) use denitrifying filters and one (Western Branch) uses denitrifying 

activated sludge. 

6. Automatic controls and sensors: In all facilities, DO control is universally practiced. 

All activated sludge plants have a network of DO probes and use the resulting data in 

making daily decisions on operation and control. Nitrate and ORP sensors are used in 

the facilities that remove nitrogen. Nitrate probes are in used in Central Johnston 

County, Clearwater, and Kelowna. An ORP probe is used in Clearwater and 

Kelowna. Standard procedures include target values for ORP, nitrate nitrogen, and 

DO for best nitrogen removal in these facilities. 

7. WAS management: Sludge blanket meters are used in Clearwater, Florida, and Clark 

County, Nevada. At the Clark County facility, the plant computer (following preprogrammed operating criteria) makes daily WAS decisions for each of the nine 

activated-sludge units. At other facilities, samples are analyzed and operating 

decisions are made accordingly, with excellent results. 

8. Denitrification filter: Central Johnston County, North Carolina, and Lee County, 

Florida, both employ downflow denitrification filters with supplemental carbon 

supplied by an automated methanol feed system. Both facilities pump the filter 

effluent to the next treatment process. Both systems have good reliability for both 

nitrogen removal and phosphorus removal to low levels via filtration. 

3.3.4 Secondary Sludge Thickening 

Secondary sludge thickening is one of the key factors in maintaining EBPR and minimizing 

the negative effect of recycle loads on the facility. For those facilities with EBPR, all but one 

uses DAF or a rotary-drum thickener and avoid phosphorus release. 

3.3.5 Sludge Digestion 

Sludge digestion is a critical element in reducing the recycle loads to the facility. Three 

facilities digest sludge aerobically: Central Johnston County, North Carolina; Lee County, 

Florida; and North Cary, North Carolina. The plants thereby minimize phosphorus and 

nitrogen release that would otherwise occur under anaerobic conditions. The Kalispell, 

Montana, facility digests primary sludge, but not secondary sludge, anaerobically. The 

remaining facilities digest sludge anaerobically and thus increase the recycle loads to the 

main plant. Two of these facilities, Kelowna and Fairfax County, have the capability to add 

lime to the cake and thus reduce the recycle of phosphorus. Kelowna eventually ceased lime 

addition when the recycle loads become low enough to manage. 

3.3.6 Recycle Flows and Loads 

All facilities with tertiary filters have provisions to equalize backwash water through existing 

or new tanks. The filtrate from dewatering is sent back to the main plant and could impose 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-33

significant nutrient loads. Recycle loads were observed to range between 13 and 30 percent 

of TP and TKN, respectively. 

The best cases for recycle loads were found at the Central Johnston County, North Carolina, 

and Lee County, Florida, facilities, where the secondary sludge is aerobically digested and 

sent away for off-site processing. These facilities do not practice primary settling. 

The next-best case was found at the Kelowna, British Columbia, facility, where the 

secondary sludge is thickened aerobically with DAF and stored separately from the primary 

sludge. The primary sludge is fermented and stored for joint dewatering with the thickened 

secondary sludge. The final cake is sent away for composting. The recycle loads at Kelowna 

were the lowest of all the case study facilities at 13 percent of the influent TP load and 

0.1 percent of the influent TN load. 

The next best case was Western Branch, Maryland, where all the sludge is thickened 

aerobically by DAF, dewatered by centrifuge, and burned in incinerators. It is notable that 

recycle loads are received from dewatering back to the main plant but not from secondary 

sludge handling. 

The next best case was found at the North Cary, North Carolina, facility, where the 

secondary sludge is digested aerobically and dewatered. 

The next best case was found at Fairfax County, Virginia, where the secondary sludge is 

thickened aerobically while the primary sludge is fermented; the fermented primary sludge 

and the thickened secondary sludge are combined for dewatering. Lime can be added to 

reduce the phosphorus recycle load. The plant was designed to handle recycle loads of 13 

percent of influent TP and BOD and 30 percent of influent TSS. 

The next best case was found at the Clark County, Nevada, facility, where the secondary 

sludge is thickened aerobically and stored with the fermented primary sludge; they are 

subsequently dewatered together. Ferric chloride is added to the sludge feed. 

Two facilities operate anaerobic sludge digestion and thus increase the opportunity for 

recycling ammonia nitrogen and phosphorous to the plant headworks. The Kalispell, 

Montana, facility digests primary sludge only before dewatering, and the Clearwater, Florida, 

facility digests both primary and thickened secondary sludge. 

3.3.7 Wet-Weather Flow Management 

Equalization Basin 

Three facilities have some form of flow equalization. Fairfax County, Virginia, has two 

basins: one at the headworks area and one at the secondary effluent. They have capacities 

representing 11 percent and 20 percent of influent daily flow, respectively. North Cary, North 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-34 Chapter 3 - Case Studies and Reliability Factors

Carolina, has a large-capacity basin at the headworks. All told, the plant could equalize 

28 percent of the influent daily flow by using the basin and existing unused tanks. Kelowna, 

British Columbia, has a basin with a capacity of 7.5 percent of the daily primary effluent 

flow. These basins ensure steady operation of the facility under high-flow conditions by 

reducing hydraulic surges in the settling tanks, providing stable chemical feed, and 

maintaining reliable process controls, where automated. 

Process Design 

Three plants have processes that have inherent advantages during high-flow periods. Fairfax 

County, Virginia, and Kelowna, British Columbia, both have step-feed activated-sludge 

systems, which means that high flows can be distributed through the aeration basins better 

than with a conventional feed plant. North Cary, North Carolina, has a PID, which offers the 

operators the ability to adjust the cycle time during wet-weather flow conditions. 

Mode of Operation 

Under emergency conditions, plant personnel can shut down portions of the aeration basins 

and thereby prevent solids from overloading the clarifiers, potentially causing a loss of 

biomass. The recovery back to normal operation is thereby ensured with the secured biomass 

inventory. The shutdown can be accomplished manually or by preprogrammed modes of 

operation. 

3.3.8 Tertiary Filters 

All facilities have tertiary filters, which help the facilities meet low TP concentration limits. 

Central Johnston County and Lee County, Florida, have denitrification filters, which differ 

from the traditional filters used at other locations. The filters at the other locations include 

rapid sand filters (two locations); dual-media filters with anthracite and sand (four locations); 

a deep mono-media filter (one location), and a dual sand filter (one location). More details 

can be found in Chapter 2 of this manual. 

3.3.9 Tertiary Clarifier 

Two facilities have tertiary clarifiers. Fairfax County, Virginia, has a tertiary clarifier before 

the tertiary filters. Ferric chloride is added to the clarifier to aid settling. The sludge from this 

process is recycled to the plant headworks. At Clark County, Nevada, the AWT plant 

operates a tertiary clarifier ahead of the sand filter. Alum is fed to the clarifier to aid settling. 

The sludge from this clarifier is thickened before being recycled to the headworks. For both 

facilities, the use of tertiary clarification provides added reliability for the subsequent tertiary 

filter operations. 

Table 3-4 presents the wastewater characteristics and treatment processes of the case study 

facilities. 
Chapter 3 - Case Studies and Reliability Factors 3-35

Table 3-4. Case study facilities’ treatment processes Facility 

Fairfax, 

VA 

Kalispell, 

MT 

Clark 

Co., NV 

Central 

Johnston, NC

N. Cary, 

NC 

Kelowna, 

BC 

Clearwater, 

FL 

Wastewater characteristicsBOD-to-TP ratio 29.5 54.9 29. 55.1 31.6 26.6 37.5 BOD-to-TKN ratio 5.4 5.7 5.5 10. 4.3 5.3 6.7 Temperature in oC 15–25 10–20 20–29 14–27 16–27 13–22 23–31 Equalization basin– percent of influent 

flow 11.5–20 58 7.5 

Equalization basin location Raw/SE Raw Raw BioreactorsBioreactor type Step-feed 

AS 

Mod. 

UCT A/O AS denit. filter Oxid. ditch Westbank Bardenpho 

d

SRT in bioreactor, days 16–19 8–12 5–9 7-9 11–14 6–9 25–44 Fermenter SRT, days 3 4–5 primary No No 3–6 No Swing zones yes yes yes yes Mixing/aeration separated in 

bioreactor yes yes 

Denitrification blanket yes yes yes Denitrification filter yes External carbon source Methanol MAlkalinity supplement NaOH Online DO probe yes yes yes yes yes yes yes Online NO3 probe yes yes yes Online ORP probe yes yes Online ortho-P probe yes yes Tertiary TreatmentTertiary clarifier yes yes Tertiary filters Tertiary conventional sand filter yes yes Tertiary dual-media filter yes yes yes Tertiary deep mono-media filter yes Tertiary dual sand filter yes 
3-36 Chapter 3 - Case Studies and Reliability Factors Table 3-4. Case study facilities’ treatment processes (continued) Facility Fairfax, VA Kalispell, MT Clark Co., NV Central Johnston, NC N. Cary, NC Kelowna, BC Clearwater, FL 

Sludge Handling Primary settling / yes or no yes yes yes NO NO yes yes Secondary sludge thickening – aerobic DAF DAF DAF DAF Rotary drum Sec sludge gravity thickening/holding yes Separate secondary/primary sludge storage yes No dewatering on-site yes On-site aerobic sludge digestion yes yes On-site anaerobic primary sludge digestion Yes On-site anaerobic prim+sec. sludge dig. Yes Yes No on-site sludge digestion yes yes yes Recycle Streams chemical treatment of filtrate/centrate lime FeCl3 lime Filtrate/centrate recycle point Filter Backwash recycle point primary HDWK HDWK 

Notes: BOD-to-TP = biochemical oxygen demand-to-total phosphorus ratio BOD-to-TKN = biochemical oxygen demand-to-total Kjeldahl nitrogen ratio DAF = dissolved-air flotation unit DO = dissolved oxygen HDWK = headworks ORP = oxidation-reduction potential SE = Secondary effluent SRT = solids retention time 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-37

3.4 Cost Factors 

The costs incurred for the studied facilities were analyzed to allow comparison on a 

consistent basis. Both capital and O&M costs were considered. 

3.4.1 Capital Costs 

Capital costs incurred for nutrient removal upgrades were provided by plant personnel at the 

studied facilities. If the upgrades were part of an overall program of upgrades at the plant 

(e.g., expansion, installation of UV disinfection), the best estimates were obtained of what 

was done for secondary treatment, tertiary filtration, and applicable sludge handling. The 

costs incurred in the past were updated on the basis of the Engineering News-Record’s 

Construction Cost Index, as provided in the U.S. Department of Agriculture Web site (USDA 

2007). The capital costs for the plants are shown in Table 3-5. The land cost was not included 

in this evaluation. 

The capital costs incurred for the biological nutrient removal (BNR) projects were allocated 

among phosphorus removal, nitrogen removal, and BOD removal. The allocation was done 

as follows: 

1. If costs for particular pieces of equipment specifically for one nutrient could be 

obtained, that cost was allocated entirely to that nutrient. For example, the cost of the 

fermenter used at Kalispell was known and attributed entirely to phosphorus removal. 

Another example is denitrificaiton filters entirely for nitrogen removal. 

2. If particular pieces of equipment could be allocated to two nutrients, that was done 

with an even split. For example, the mixers used at Kalispell were just for anoxic 

tanks, where nitrogen and BOD would be removed. In other cases, the installation 

was clearly for both phosphorus and nitrogen removal. 

3. For all instances where no equipment breakout was available or the equipment would 

go toward all treatment, the costs were allocated by the percentages 12 percent to 

phosphorus, 48 percent to nitrogen, and 40 percent to BOD. This allocation was based 

on an estimate-of-cost breakdown for Kelowna and provided a consistent basis for 

comparing relative costs. For denitrifying filters, all the BOD percentage was attributed 

to nitrogen, making the split 12 percent phosphorus and 88 percent nitrogen. 

4. For Clearwater (Marshall Street), the cost fractions were 17, 63, and 20 percent for 

phosphorus, nitrogen, and BOD, respectively. In that instance, the total cost of the 

BNR upgrades was known, as were the volumes of the various vessels. Those 

volumes were attributed to phosphorus or nitrogen removal (fermenter to phosphorus, 

anoxic basins to nitrogen), while the aeration basin was apportioned 10 percent for 

phosphorus, 50 percent for nitrogen, and 40 percent for BOD. The fraction of the total 

volume for each nutrient was used for the cost apportionment. 
3-38 Chapter 3 - Case Studies and Reliability Factors Table 3-5. Case study facilities’ capital costs Facility Fairfax, VA Kalispell, MT Clark County, NV Central Johnston, NC North Cary, NC Kelowna, BC Clearwater, FL Le

Design flow MGD 67 3 110 7 12 10.5 10 Influent P (mg/L) 6.4 4.1 5.8 5.8 7.7 6.0 5 Influent N (mg/L) 34.6 39.6 30.3 31.2 56.4 28.8 44.8 Influent NH3-N (mg/L) 24.3 26.8 28 45.5 21.3 28 Capital attributed to P (2007 dollars) $0 $1,190,000 $65,450,000 $889,000 $4,089,600 $6,818,000 $5,019,000 Capital attributed to N (2007 dollars) $71,600,000 $4,310,000 $56,980,000 $2,400,000 $16,358,000 $27,273,000 $18,600,000 $13Capital attributed to other (2007 dollars) $0 $3,600,000 $98,570,000 $0 $13,632,000 $22,728,000 $5,905,000 $11Electrical attributed to P (kWh/yr) 3,360,000 389,000 * 1,843,000 377,400 884,000 931,000 Electrical attributed to P (dollars/yr) $185,000 $17,500 * $103,200 $17,400 $41,500 $102,400 Electrical attributed to N (kWh/yr) 18,060,000 1,077,000 * 4,169,000 2,558,000 4,100,000 4,623,000 1Electrical attributed to N (dollars/yr) $993,000 $48,500 * $233,500 $118,000 $193,000 $509,000 Electrical attributed to other (kWh/yr) 0 1,045,600 * 588,000 0 919,000 2,075,000 2Electrical attributed to other (dollars/yr) $0 $47,100 * $33,000 $0 $43,200 $230,000 Chemical attributed to P (dollars/yr) (alum, ferric) $393,000 $5400 * $0 $0 $0 $7,400 Chemical attributed to N (dollars/yr) (Methanol, pH control caustic as needed) $250,000 $0 * $53,100 $0 $0 $0 Sludge attributed to P (tons/yr) (alum/ferric or biological uptake sludge) 730 0 * 0 90 0 5.8 
Chapter 3 - Case Studies and Reliability Factors 3-39 Table 3-5. Case study facilities’ capital costs (continued) Facility Fairfax, VA Kalispell, MT Clark County, NV Central Johnston, NC North Cary, NC Kelowna, BC Clearwater, FL Le

Sludge attributed to P (dollars/yr) $393,000 $0 * $0 $17,900 $0 $1,500 Sludge attributed to N (methanol conversion) (tons/yr) 0 0 * 60 1702 0 0 Sludge attributed to N (dollars/yr) $0 $0 * $15,100 $341,000 $0 $0 Total O&M attributed to P (dollars/yr) $970,000 $17,500 $3,004,000 $103,200 $35,300 $40,300 $109,800 Total O&M attributed to N (dollars/yr) $1,243,000 $57,800 $3,852,000 $301,700 $230,000 $109,000 $509,000 Annualized capital for P (6%, 20 years) $0 $168,100 $5,710,000 $51,000 $357,000 $595,000 $438,000 Annualized capital for N (6%, 20 years) $6,240,000 $323,600 $4,970,000 $302,700 $1,426,000 $2,378,000 $1,620,000 $1Unit O&M for TP ($/lb) $1.07 $0.49 $1.81 $1.48 $0.23 $0.27 $1.37 Unit capital For TP ($/lb) $0.00 $2.84 $3.43 $0.73 $2.28 $3.97 $5.38 Unit total for TP ($/lb) $1.07 $3.33 $5.24 $2.21 $2.51 $4.24 $6.76 Unit O&M for TN ($/lb) $0.29 $0.19 $0.43 $0.49 $0.41 $0.14 $0.75 Unit capital for TN ($/lb) $1.47 $1.46 $0.55 $0.49 $2.54 $3.05 $2.40 Unit total for TN ($/lb) $1.76 $1.65 $0.98 $0.98 $3.36 $3.19 $3.15 Unit capital cost ($/gal/day) $1.07 $3.03 $2.01 $0.58 $2.84 $3.25 $2.95 

Note: Cost data available per unit operation; boldface type indicates actual dollar amounts. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-40 Chapter 3 - Case Studies and Reliability Factors

The apportioned costs are detailed in each case study and summarized in Table 3-5. Table 3-

5 also presents the annualized capital cost for phosphorus and nitrogen, which was done at a 

6 percent interest rate for 20 years. 

3.4.2 Operation and Maintenance Costs 

The case studies considered only three categories of O&M costs: power/electrical usage, 

chemical usage, and the cost of disposing of extra sludge generated by the nutrient removal 

processes. All other costs, including labor, were excluded from consideration. Labor was 

excluded for three reasons: 

1. Labor and energy costs are highly sensitive to local conditions, such as the prevailing 

wage rate, the relative strength of the local economy, the presence of unions, and so 

on; thus, they would only confound comparison of the inherent cost of various 

technologies. 

2. For most processes, the incremental extra labor involved in carrying out nutrient 

removal is recognized but not significant, in view of the automatic controls and 

supervisory control and data acquisition (SCADA) system that came with the 

upgrades. For example, if an activated-sludge system is now to be operated to remove 

nutrients, the amount of monitoring, checking of basins, and making process 

decisions is very similar. Adding an entirely new process could require additional 

labor hours per week, but that is very site-specific and difficult to determine and 

compare. With nutrient removal upgrades, many facilities added automation features 

along with SCADA system and thus the need for additional labor is minimal. 

3. Additional labor-hour determination is difficult because the plants were largely 

unable to break down which extra personnel were employed because of nutrient 

removal and related overtime costs. 

Electrical costs were estimated on the basis of direct plant data or the power draw of the 

blowers/aerators, mixers, and pumps used for treatment. The power draw, in kilowatt-hours 

per year (kWh/year), was allocated among phosphorus, nitrogen, and BOD removal. The 

allocation was done in the same manner as capital allocation: 

1. If the piece of equipment was for one nutrient, those kWh/yr were allocated entirely 

to that nutrient. That was the case for mixers in an anaerobic tank or a fermenter. 

2. For aeration blowers, the allocation of kWh was based on the oxygen demand by 

PAOs, nitrifiers, and standard heterotrophic organisms. In this manner, the allocation 

10 percent for phosphorus, 50 percent for nitrogen, and 40 percent for BOD was 

developed. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-41

For other pieces of equipment, the distribution of kWh was the same as the main capital 

distribution: 12 percent for phosphorus, 48 percent for nitrogen, and 40 percent for BOD. 

The electrical cost for each nutrient was determined by multiplying the kWh/year by the 

average electrical cost, as supplied by plant personnel. This average cost covers the actual 

consumption charge (kWh) and the peak demand charge imposed by the local utility. 

If a plant had to add chemicals for nutrient removal, the cost and amount of the chemicals 

were supplied for the case study. The chemicals used by these municipalities were alum and 

ferric chloride for phosphorus removal and methanol for denitrification. One plant (Noman 

M. Cole in Fairfax County, Virginia) also used sodium hydroxide as needed to control the pH 

following nitrification. 

If a plant generated additional sludge because of nutrient removal, the cost of disposing of 

that additional sludge was included. In general, if an anaerobic or anoxic process is used for 

nutrient removal, the overall amount of sludge should go down compared to a fully aerobic 

process, and those processes inherently generate less sludge. Additional sludge would be 

generated if phosphorus was removed chemically or if methanol (or other external carbon 

source) was added for denitrification. The sludge yield of 0.2 lb/lb BOD was used for 

methanol feed, where applicable. 

Table 3-5 presents the annual O&M estimates for nutrient removal at the case study facilities. 

3.4.3 Unit Costs 

The annualized capital costs and annual O&M costs were normalized by dividing by the 

mass of nutrient removed during the study period. These normalized costs became the Unit 

O&M and Unit capital costs presented in Table 3-5, with the cost expressed as dollars per 

pound of nutrient removed. The calculation was done for TP and TN. As a comparison, the 

unit capital cost for the BNR expansion was calculated as the dollars per gallon per day 

capacity. The following observations can be made about the results in the table: 

1. The unit O&M cost for TP ranged from $0.23 per pound for North Cary, $0.27 per 

pound in Kelowna, and $0.49 per pound in Kalispell to $1.81 for Clark County at the 

high end. The low cost in North Cary is because of fully functioning biological 

phosphorus removal. The low costs for Kelowna and Kalispell are because of those 

plants having fully functioning biological phosphorus removal aided by a fermenter 

(so chemicals are not needed). The trade-off for the low operating costs was higher 

capital costs ($3.05 per pound phosphorus and $1.48 per pound phosphorus for 

Kelowna and Kalispell, respectively). The capital cost at North Cary was moderate at 

$2.35 per pound. 

2. The capital cost range for TP was $0 for the Noman M. Cole plant in Fairfax County 

to high-end costs of $3.05 per pound for Kelowna, $3.27 per pound in Western 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-42 Chapter 3 - Case Studies and Reliability Factors

Branch, and $3.87 per pound in Lee County. The costs for installing phosphorus 

removal processes at Noman M. Cole, an older plant, were fully depreciated and were 

not available. 

3. The unit O&M for nitrogen removal is low, with costs ranging from $0.14 to $0.99 

per pound removed. Higher costs are associated with having to add chemical, while 

the plants with the lowest costs for nitrogen removal (Kelowna and Kalispell) are able 

to effect total nitrification and denitrification without chemical addition. 

4. The capital cost for nitrogen removal is low, ranging from $0.49 to $3.87 per pound, 

with the lowest capital cost for TN removal at Central Johnston County, where the 

retrofit was based substantially on existing facilities with addition of denitrification 

filters. High-value locations involved major additions, and they included Lee County, 

Western Branch, and Kelowna, at $3.87, $3.27, and $3.05, respectively. 

5. The overall unit capital cost for these plants on a flow basis ranged from $0.58/gallon 

per day (gpd) for Central Johnston County to $3.25/gpd for Kelowna. 

The factors affecting capital costs are as follows: 

1. The treatment to be added: The major factor is the availability of existing facilities. In 

those facilities with available site and tanks, much can be retrofitted without 

significant cost. If a new biological treatment is added, however, it adds to higher 

capital cost. Chemical treatment requires less capital. 

2. Built-in flexibility in the design: Some plants include fermenters for EBPR or swing 

zones with two types of equipment and two modes of operation for nitrogen removal, 

such as the 4-stage and 5-stage Bardenpho process. The added flexibility increases 

capital costs but allows operators to potentially reduce O&M costs through reductions 

in energy or chemical usage. 

3. Equalization basin: Basins are expensive capital items but could reduce O&M costs 

because of lower peak demands for power and chemical. 

4. Treatment of recycle flows: Equipment and buildings for lime or ferric addition are 

expensive. 

5. The amount of automation: Depending on the size of the facility, this expense could 

be expensive but could be beneficial in saving O&M costs. 

6. Separate storage tanks for the secondary sludge from the primary sludge: This is an 

added cost at EBPR facilities to consider for control of recycle loads. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-43

The factors affecting the O&M costs are the following: 

1. Use of external supplemental carbon: If methanol must be purchased, that will tend to 

push the costs up, compared to use of in-stream carbon generated by a fermenter or 

fed to the anoxic zone by a step-feed mechanism. 

2. Use of chemical for phosphorus removal: As with methanol addition, if the plant has 

to use chemical phosphorus removal to meet the discharge limit, it will tend to have a 

higher cost than if biological phosphorus removal can be used. Ideally, the biological 

phosphorus removal will use site-generated VFAs for the carbon source in the 

anaerobic zone; if such are not available, chemical addition will be required, at 

greater cost. 

3. Alkalinity Addition for nitrification. Caustic soda is added to supplement alkalinity in 

those areas with soft water (low alkalinity). 

4. Use of a fermenter: In some cases, sufficient VFAs exist in the influent and in-plant 

recycle stream so that the EBPR does not need additional VFAs. If some must be 

added, they are typically generated by fermenting primary solids. This fermentation 

can take place in a dedicated unit that, therefore, has associated operating costs, or in 

some cases, the fermentation can occur in a sludge thickener, with less electrical use 

than a dedicated unit. 

5. Automation: Automation adds cost but could save O&M costs by optimizing power 

usage and chemical usage. 

3.5 Summary 

The performance and costs of the treatment processes at the nine case study facilities are 

summarized in the sections that follow. 

3.5.1 Discharge Permits 

The discharge permits are diverse in the number of discharge seasons, allowable nutrient 

discharge limits, and averaging period. Some of these diverse permit requirements are based 

on TMDLs; others are based on a state policy for water reuse (such as Florida’s), or a 

regional goal (such as the goals for the Chesapeake Bay and the Long Island Sound). 

An in-depth evaluation was made of full-scale plant performance, and the performance was 

presented using a simple statistical basis for comparison and interpretation. The coefficient of 

variation, or COV, represents one standard deviation divided by the mean. With this, all 

performance data for one year were presented for these selected facilities. 

All nine facilities provided one year of data, and the monthly average values were used in the 

summary as the base. Weekly averages were also used where a permit limit was specified. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-44 Chapter 3 - Case Studies and Reliability Factors

3.5.2 Phosphorus Removal 

The discussion below summarizes the highlights of efficiency and reliability related to 

phosphorus removal that was accomplished at the facilities. Four facilities removed 

phosphorus biologically, without chemical addition. Three facilities used chemicals to 

supplement biological phosphorus removal. Two facilities removed phosphorus primarily 

with chemical addition. The first four facilities relied on biological phosphorus removal 

followed by tertiary filters. They met the permit limits and performed efficiently and reliably. 

• Kalispell, Montana: 0.12 mg/L TP at a COV of 19 percent 

• Kelowna, British Columbia: 0.14 mg/L TP at a COV of 21 percent 

• Central Johnston County, North Carolina: 0.26 mg/L TP at a COV of 62 percent 

• North Cary, North Carolina: 0.38 mg/L TP at a COV of 64 percent 

Both Kalispell and Kelowna have fermenters, operate year-round, and show very high 

reliability. Two facilities without fermenters produced good effluent but at an increased 

COV, or reduced reliability compared to those with fermenters. 

With small amounts of chemical addition, biological phosphorus removal was taken to even 

lower effluent concentrations and increased reliability, as follows: 

• Clark County, Nevada: 0.09 mg/L TP at a COV of 30 percent 

• Fairfax, Virginia: 0.09 mg/L TP at a COV of 21 percent 

• Clearwater, Florida, Marshall Street: 0.13 mg/L TP at a COV of 40 percent 

Clark County, Nevada, used primary settling tanks as VFA generators, and Fairfax County, 

Virginia, had a fermenter online. Both showed lower COVs than the others at varying 

chemical dosages. Clearwater, Florida, fed alum to remove trihalomethanes for water reuse 

purposes, and Clark County, Nevada, added alum to push the limit of the existing facility. 

Two facilities used chemicals for phosphorus removal: 

• Lee County, Florida: 0.10 mg/L TP at a COV of 35 percent 

• Western Branch, Maryland: 0.43 mg/L TP at a COV of 62 percent 

These facilities remove nitrogen in an add-on process with methanol, and thus a decision had 

been made to remove phosphorus with chemical addition instead of retrofitting the existing 

processes for biological phosphorus removal. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-45

The factors for reliable operation include the following: 

1. Favorable wastewater characteristics: The BOD-to-TP ratio in the influent and 

primary effluent. 

2. Fermenters that ensured an adequate supply of carbon: Ensured reliability during 

cold months and wet-weather periods. Kalispell, Montana, and Kelowna, British 

Columbia, achieved the highest reliability, followed by Fairfax County, Virginia, and 

Clark County, Nevada, which use active primary settling for sludge thickening and 

VFA production, both with chemical addition. 

3. Flexible design: Flexible design with swing zones in anticipation of changing 

conditions in weather and wastewater characteristics, including 

− Equipping aeration tanks with both mixers and aerators for separate control, like 

North Cary, North Carolina 

− Compartmentalizing the basin and creating swing zones, like Kalispell, 

Montana, and Kelowna, British Columbia 

− Adding multiple feed points for carbon feed, like Fairfax County, Virginia, for 

step-feed 

− Placing a tertiary clarifier ahead of the tertiary filter to reach a low 

concentration limit 

− Having multiple chemical feed points 

4. Flow equalization for stable operation: The facilities have equalization available, 

with the amount of storage varying based on their sewer systems. Kelowna has an 

equalization basin with volume 7.5 percent of the design flow, based on a relatively 

tight sewer system that has low infiltration. Fairfax County, Virginia, has two basins 

with a total volume of 31 percent of the design flow. 

5. Wet weather: Storm flows were handled favorably with step-feed activated sludge at 

Fairfax County, Virginia, and the PID at North Cary, North Carolina. In addition, 

North Cary had the largest basins to store wet-weather flows––a 7-MG retention 

basin and a 2-MG equalization basin for the 12-MGD facility). The operational 

controls in many facilities included shutting off a section of the aeration tanks during 

peak flow periods and protecting the biomass inventory to prevent solids washout. 

This emergency shutoff mode of operation lasted up to 24 hours but was followed by 

a successful restart, meeting all permit limits. 

6. Recycle loads were minimized by

− Keeping secondary sludge aerobic until dewatering; thickening, storing, and 

aerobic digestion—Central Johnston County, North Carolina; Lee County, 

Florida; North Cary, North Carolina; and Kalispell, Montana. 
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3-46 Chapter 3 - Case Studies and Reliability Factors

− Treating filtrate with lime or ferric chloride—Fairfax County, Virginia, and 

Clark County, Nevada. Kelowna, British Columbia, no longer adds lime. 

− No sludge digestion on-site—Kelowna, British Columbia, and Clark County, 

Nevada. 

7. Operating denitrification blanket in secondary clarifiers: Four facilities have 

developed operating procedures to maximize the denitrification of RAS in the 

clarifier and have made it a permanent practice for successful biological phosphorus 

removal: Kelowna, British Columbia; Central Johnston County, North Carolina; Lee 

County, Florida; and Clearwater, Florida. 

8. Automation of process controls with sensors: Some facilities have multiple sensors, 

with automated process controls using the resulting signals. Commonly monitored 

parameters were DO, ORP, and nitrate in the wastewater and the sludge 

blanket. Clark County, Nevada, used the daily results to control the blowers and the 

sludge age. Other facilities rely on a daily check of critical parameters and take steps 

to optimize the operation with efficiency and achieve a high degree of reliability. 

3.5.3 Nitrogen Removal 

For nitrogen removal, the following paragraphs summarize the efficiency and reliability 

accomplished at the case study facilities. 

Two facilities that are required to remove only ammonia nitrogen met the permit limits and 

performed efficiently and reliably: 

• Kalispell, Montana: 0.07 mg/L ammonia nitrogen at a COV of 0 percent 

• Clark County, Nevada: 0.12 mg/L ammonia nitrogen at a COV of 14 percent 

Seven facilities are required to remove TN. These facilities met the permit limits and 

performed efficiently and very reliably: 

• Central Johnston County, North Carolina: 2.14 mg/L TN at a COV of 16 percent 

• Lee County, Florida: 1.71 mg/L TN at a COV of 28 percent 

• Clearwater, Florida, Marshall Street: 2.32 mg/L TN at a COV of 16 percent 

• North Cary, North Carolina: 3.7 mg/L TN at a COV of 14 percent 

• Kelowna, British Columbia: 4.38 mg/L TN at a COV of 12 percent 

• Western Branch, Maryland: 1.63 mg/L TN at a COV of 36 percent 

• Noman Cole, Fairfax County, Virginia: 5.25 mg/L TN at a COV of 12 percent 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-47

Reliability factors for these performances included the following: 

1. Favorable wastewater characteristics: BOD-to-TKN ratios of above 4 in all cases. 

For add-on processes with methanol addition, however, this ratio was not a concern. 

Methanol dosage at three such facilities depended directly on the nitrate nitrogen 

coming into the add-on process. 

2 Adequate supply of carbon: Central Johnston County, North Carolina; Western 

Branch, Maryland; and Lee County, Florida, need a separate stage for denitrification 

and all feed methanol. Other plants use carbon present in their wastewater. 

3. Flexible design: This ensures added reliability in anticipation of variations in the 

weather and in wastewater characteristics. 

− Swing zones at Kelowna, British Columbia 

− Separate control of mixing and aeration at North Cary, North Carolina 

− Multiple carbon feed points (step-feed) at Fairfax County, Virginia; Kelowna, 

British Columbia; and Clearwater, Florida 

4. Flow equalization: The same principles that apply to phosphorus removal also apply 

to nitrogen removal. The backwash water from the denitrification filter is sent to a 

basin for equalization. In Central Johnston County, North Carolina, the backwash 

water is stored in filter backwash reclaim tanks before it is pumped to the headworks. 

In Lee County, the backwash goes to a mud well before it is returned to the 

headworks. 

5. Recycle loads: The best strategy is to minimize recycle loads: 

− Keep sludge handling aerobic in thickening, storing, and stabilization 

− Process all sludge each day: Clark County, Nevada 

− Provide no sludge digestion on-site: Kelowna, British Columbia; Clark County, 

Nevada; and Western Branch, Maryland 

− Treat the recycle loads: lime addition at Fairfax County, Virginia, and Kelowna, 

British Columbia, and ferric chloride at Clark County, Nevada 

− It is important to note that Fairfax County, Virginia, accounted for the recycle 

loads in their retrofit or expansion design 

6. Wet-weather flows: The step-feed and PID offer the best process protection against 

wet-weather flows. North Cary, North Carolina, has the largest storage capability, as 

described above. The operational controls in many facilities included shutting off a 

section of aeration tanks during the peak flow periods and protecting the biomass 

inventory to prevent solids washout. This emergency shutoff mode of operation lasted 

up to 24 hours but was followed by a successful restart in meeting all permit limits. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-48 Chapter 3 - Case Studies and Reliability Factors

7. Denitrification blanket in secondary clarifier: A denitrification blanket ensures good 

nitrate removal and alkalinity recovery; it also optimizes the anoxic zone in the 

bioreactor, allowing the anoxic basin size to be reduced. 

8. Automatic controls with multiple sensors: Some facilities have multiple sensors, and 

process controls were based on them. They include DO, ORP, and nitrate in the 

wastewater and sludge blanket. Clark County, Nevada, uses these data to control the 

blowers and the sludge age. Clearwater, Florida, uses these data on DO, ORP, and 

nitrate. The target limits are set for these three parameters at the anoxic zone for 

reliable nitrogen removal. 

3.5.4 Costs for Capital and O&M 

Capital Cost 

The capital cost at the case study facilities for nutrient removal was estimated on the basis of 

consultation with the owners because all the upgrades were a part of an overall expansion. 

The estimate was obtained for secondary treatment, tertiary clarification and filtration, and 

applicable sludge handling. The capital costs incurred for nutrient removal were then 

allocated among phosphorus removal, nitrogen removal, and BOD removal. There is a wide 

variation in costs because of the type and age of the existing facility. 

1. Unit cost for treatment capacity: This cost varied from a low of $0.58 to a high of 

$3.25/gpd capacity according to the existing facility and the age of construction. The 

variation was based on the capability of the existing facility, the desired level of 

treatment, and when the plant modifications were carried out. 

2. Unit cost for TP removal: This cost varied from zero to a high of $5.38 per pound of 

TP removed. 

3. Unit cost for TN removed: This cost varied from a low of $0.49 to a high of $387 per 

pound TN removed. 

O&M Costs 

This study included three categories of O&M costs: power/electrical usage, chemical usage, 

and cost of disposing of extra sludge generated by nutrient removal processes. Labor was 

excluded because of the high variation in geographic location, difficulty in allocating the 

fraction for nutrient removal, and an understanding that the staff size did not change after 

upgrade. 

1. Unit cost for phosphorus: This cost ranged from a low of $0.23 to $0.1.81 per pound 

TP removed. 

2. Unit cost for TN removal: This cost ranged from a low of $0.14 to a high of $0.99 per 

pound TN removed. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 3 - Case Studies and Reliability Factors 3-49

Contributing factors include the following: 

1. The biological phosphorus removal facilities added capital cost, especially where 

fermenters were installed, but ensured a high degree of reliability in performance. The 

O&M cost was lower than that for the other processes. 

2. Flexibility in design, such as the swing zone and alternate mode of operation, added 

costs (and redundant equipment and structures) but provided added capability to 

handle changing wastewater characteristics and new regulations. 

3. Equalization basins added costs but provided a higher degree of treatment for 

captured flows in the basin. 

4. Separate storage tanks for secondary sludge and primary sludge added costs but 

ensured more reliable performance, especially at EBPR facilities. 

5. Treatment of recycle flows with lime or ferric chloride added costs but resulted in 

more reliable performance. 

Total Annual Cost 

The annual costs were estimated on the basis of a 6 percent interest rate for 20 years. 

1. The unit cost for phosphorus removal ranged from a low of $1.07 to a high of $6.76 

per pound TP removed. 

2. The unit cost for TN removed ranged from a low of $0.98 to a high of $4.78 per 

pound TN removed. 

The contributing factors described in the above section on Capital and O&M costs all apply 

to the total cost. 

3.6 References 

Lindeke, D., and B, James. 2005. The Role and Production of VFAs in a Highly Flexible 

BNR Plant. In Proceedings of WEFTEC, 2005. 

McBean, E., and F. Rovers. 1998. Statistical Procedures for Analysis of Environmental 

Monitoring Data & Risk Assessment. Prentice Hall, Upper Saddle River, NJ. 

Neethling, J.B., B. Bakke, M. Benisch, A. Go, H. Stephens, H.D. Stensel, and R. Moore. 

2005. Factors Influencing the Reliability of Enhanced Biological Phosphorus 

Removal. Water Environmental Research Federation (WERF) Report 01-CTS-3. IWA 

Publishing, London. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

3-50 Chapter 3 - Case Studies and Reliability Factors

USDA (U.S. Department of Agriculture). 2007. Price Indexes and Discount Rates. 

U.S. Department of Agriculture, Natural Resources Conservation Service. 

http://www.economics.nrcs.usda.gov/cost/priceindexes/index.html. Accessed May 15, 

2007. 

USEPA (U.S. Environmental Protection Agency). 1988. Retrofitting POTWs for Phosphorus 

Removal in the Chesapeake Bay Drainage Basin. U.S. Environmental Protection 

Agency, Washington, DC. 

USEPA (U.S. Environmental Protection Agency). 1993. Manual: Nitrogen Control. 

EPA-625-R-93-010. U.S. Environmental Protection Agency, Washington, DC. 

WEF and ASCE (Water Environment Federation and American Society of Civil Engineers). 

1998. Design of Municipal Wastewater Treatment Plants. WEF Manual of Practice 

No. 8, Volume II, 4th ed. American Society of Civil Engineers, Reston, VA. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-1

CHAPTER 4: Cost Factors 

This chapter summarizes the cost factors to be considered in evaluating alternative 

technologies for upgrades at existing facilities and, more broadly, provides context on cost 

for the entire document. Costs vary widely, depending on a number of factors, including the 

target concentrations for nitrogen and phosphorus, as well as the existing facilities’ suitability 

to be used for upgrades. 

As also discussed in Chapter 5, there are three general types of possible plant upgrades to be 

considered for costs: retrofitting existing facilities with additional piping and equipment; 

adding a new process or technology to an existing treatment train; or expanding the existing 

plant, possibly with an entirely different technology. Section 4.1 describes costs reported at 

selected facilities in the United States, as well as costs incurred by facilities included in the 

cases studies provided in Chapter 3. Section 4.2 describes the results of estimating costs for 

12 retrofit alternatives for nutrient removal, with the estimates generated using 

CAPDETWorks software. Finally, Section 4.3 describes the estimated costs for 20 expansion 

alternatives for nutrient removal, again with the estimates generated using CAPDETWorks. 

In all cases, capital and operation and maintenance (O&M) costs are provided as available. In 

addition, Section 4.3 provides a breakdown of the relative fraction of various components of 

O&M costs, i.e., labor, chemical, and energy costs. 

4.1 Modifying Existing Facilities 

4.1.1 Literature Review 

Modifying or retrofitting existing facilities is often the least costly and most environmentally 

favorable approach for wastewater treatment facilities that are required to implement nutrient 

removal. Maryland (MDE 2004) and Connecticut (CTDEP 2006) have both implemented 

programs to assist treatment plants in upgrading processes for nutrient removal. Note that the 

construction industry indices of Engineering News-Record were 7,298.25 for September 

2004 and 7,700 for July 2006 for comparative purposes. Table 4-1 shows the costs for 

various technologies used in the upgrades. For each technology, the following are shown: the 

size of the facility (in million gallons per day [MGD]), the capital cost (in dollars per gallon 

per day [gpd] capacity), the O&M cost per MGD day treated, and the O&M unit cost for 

removal of 1 pound of nitrogen. The upgrades cost between $0.22 and $5.20 per gpd 

capacity. The Maryland data consist of projected costs to upgrade from the current treatment 

process. The O&M costs from Maryland, where available, are based on only the projected 

increase in electricity and chemical usage. Specifically, labor costs and any anticipated 

increases in sludge-handling or disposal costs are not included in the Maryland data. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-2 Chapter 4: Cost Factors

Table 4-1. Upgrade costs for Maryland, Connecticut, and others 

Treatment 

processes Location 

Actual 

effl. TN 

(ppm) 

Initial 

TN 

conc. 

(ppm) 

New 

TN 

goal 

(ppm) 

Flow 

(MGD)

Capital 

Cost 

$/gpd 

capacity 

O&M 

Cost 

$/MG 

treated 

O&M 

Cost 

$/lb TN 

removed

Cyclic on/off Ridgefield, CT -- 9.6 5.1 1 $0.20 $111 $0.60 

Added upflow 

denitrification filter 

Cheshire, CT 6 7 5 3.5 $1.65 $136 $1.05 

MLE Æ

4-stage 

Bardenpho 

Seneca, MD 

16 8 3 20 $0.21 $63 $1.51 

MLE Æ

4-stage 

Bardenpho 

Freedom, MD 

8 8 3 3.5 $0.99 -- -- 

Step-feed AS Æ

4-stage 

Bardenpho 

Cumberland, 

MD 7.3 8 3 15 $1.10 $122 $2.94 

MLE Æ added 

denite. filter 

Baltimore, MD 9 8 3 180 $1.39 -- -- 

MLE Æ added 

denitrification filter 

Cox Creek, 

MD 16 8 3 15 $1.74 $104 $2.50 

A2O Æ

5-stage 

Bardenpho + 

denitification filter 

Frederick, MD 

23 6.5 3 7 $1.41 -- -- 

Post-MBBR for 

TN removala

Broomfield, 

CO 8 8 3 8 $1.70 -- -- 

On/off aerationa Broomfield, 

CO -- 8 3 8 $1.00 -- -- 

IFASa Broomfield, 

CO -- 8 3 8 $0.85 -- -- 

Lagoon Æ

4-stage 

Bardenpho 

Hurlock, MD 

-- 15 3 1.5 $4.12 -- -- 

MLE Æ MBR/5-

stage Bardenphoa

Las Virgenes 

Calabasas, CA -- 42 1 16 $5.20 -- -- 

Notes: 

A2

O = anaerobic/anoxic/oxic MBBR = moving-bed biofilm reactor 

AS = activated sludge MBR = membrane biological reactor 

IFAS = integrated fixed-film activated sludge MLE = modified Ludzack-Ettinger 

MG = million gallons TN = total nitrogen 

a

Stephenson and Mohr 2005 

O&M cost data are presented for facilities that had such information included in the studies reviewed. 

The Broomfield, Colorado, and Las Virgenes Municipal Water District (in California) capital 

costs are also based on projections. The Connecticut data are based on actual costs to upgrade 

the plant. The O&M costs for the Connecticut plants are an estimate of the actual costs 

associated with nitrogen removal. The O&M costs represent the total O&M costs for nitrogen 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-3

removal, not the incremental increase from the upgrades. Specifically, the Ridgefield, 

Connecticut, O&M costs include electricity, sludge-handling and disposal, labor, 

maintenance, and administrative costs associated with nitrogen removal at the plant. No 

chemicals are added at this facility. O&M costs for Cheshire, Connecticut, include electricity, 

chemicals, and laboratory costs associated with nitrogen removal at the plant. 

4.1.2 Case Studies 

Table 4-2 presents the cost results for the upgrades done at the facilities examined in the case 

studies found in Chapter 3 and Appendix A (in Volume II) of this document. The table shows 

the flow, the capital cost in dollars per gallon of treatment capacity, the O&M cost in dollars 

per million gallons (MG) treated, and the unit O&M costs for removal of a pound of 

nitrogen, phosphorus, or ammonia nitrogen, as applicable. The capital costs were determined 

by totaling the capital expenditures for nutrient removal technologies (costs brought to 

present worth by using cost indices) and then dividing that total by the gpd capacity of the 

plant. The construction index of the Engineering News-Record was 7,959.17 in July 2007. 

The O&M costs for the case studies included power/electricity usage, chemical usage, and 

sludge disposal costs associated with the nutrient being removed (total nitrogen [TN], total 

phosphorus [TP], or ammonia nitrogen). Labor costs were not included in the O&M costs. 

The O&M costs for nitrogen, ammonia, and phosphorus for a year were divided by the 

pounds of those substances removed over the year to obtain the unit costs. Similarly, the total 

O&M dollars expended over the year were divided by the total flow processed at the plant 

over the year to obtain the dollars per MG treated. 

Table 4-2. Upgrade costs for case studies 

Process Location 

Design 

flow 

(MGD) 

Target 

Concentration 

(annual 

average) 

Initial 

Concentration 

(annual 

average) 

$/gpd 

capacity $/MG 

O&M 

$/lb 

TN 

O&M 

$/lb 

TP 

O&M

$/lb 

NH3

Step-feed AS 

with tertiary 

clarifier and 

filter 

Fairfax 

County, 

VA 67 

AmmoniaN+TP, 

1 mg/L & 

0.18 mg/L 

(TN 5.2 mg/L 

voluntary) 

18.9 mg/L 

ammoniaN, 6.4 mg/L 

TP $1.07 $106 $0.46 $1.07 -- 

Modified 

UCT with 

fermenter + 

tertiary filter 

Kalispell, 

MT 3 

AmmoniaN+TP, 1.4 

mg/L & 1 

mg/L 

24 mg/L 

ammoniaN, 4 mg/L 

TP 

$3.03 $108 -- $0.49 $0.22 

A/O with 

tertiary 

clarifier + 

tertiary filter 

Clark 

County, 

NV 100 

AmmoniaN+TP, 0.6 

mg/L & 0.2 

mg/L 

27 mg/L 

AmmoniaN, 5.8 mg/L 

TP 

$2.01 $183 -- $1.81 $0.50 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-4 Chapter 4: Cost Factors

Table 4-2. Upgrade costs for case studies (continued) 

Process Location 

Design 

flow 

(MGD) 

Target 

Concentration 

(annual 

average) 

Initial 

Concentration 

(annual 

average) 

$/gpd 

capacity $/MG 

O&M 

$/lb 

TN 

O&M 

$/lb 

TP 

O&M

$/lb 

NH3

Plug flow AS 

with 

denitrification 

filter 

Johnston 

County, 

NC 7 

TN+TP, 3.7 

mg/L & 1 

mg/L 

31.2 mg/L 

TN, 5.8 

mg/L TP $0.58 $221 $0.48 $1.48 -- 

Key factors affecting the costs presented in Tables 4-1 and 4-2 are summarized below. 

Capital costs depend largely on the facilities available to meet the new requirements. Making 

a simple change in mode of operation from continuous aeration to the cyclic on-off mode of 

activated sludge was the lowest-cost conversion (Ridgefield, Connecticut). Conversion of the 

Freedom, Maryland, modified Ludzack-Ettinger (MLE) to a 4-stage Bardenpho required a 

mid-level expense of approximately $1.00/gpd capacity. Adding denitrification filters at Cox 

Creek, Maryland, was more expensive at $1.30 to $1.74/gpd capacity, but the upgrade had a 

small footprint. Adding fixed-film media at Broomfield, Colorado (Stephenson and Mohr 

2005) was an option at a similar cost, also with a small footprint. Adding a fermenter for 

biological phosphorus removal at Kalispell, Montana, brought the cost above $3.00/gpd 

capacity, but the benefits were reliable operation and low O&M costs. Flexible design also 

tended to increase the capital costs. An example of this is the Marshall Street plant in 

Clearwater, Florida, which included having the flexibility of two modes of operation––

operating as a 4-stage or a 5-stage Bardenpho––at the same facility. Another example of 

designed flexibility is North Cary, North Carolina, where mixers and aerators are in the same 

basin for better control in the phased isolation ditch (PID). 

Costs for flow equalization basins were included for Fairfax, North Cary, and Kelowna, and 

could not readily be separated from other plant costs. Typically, equalization basins add 

$1.00 to $2.00 per gallon capacity in capital costs, depending on location, hydraulics, and 

control devices. Equalization basins give the plant operators capabilities to better manage and 

treat recycle flow streams, as well as to handle wet-weather flows. 

4.2 Retrofit Process Cost Models 

As discussed in detail in Chapter 5, readers of this document seeking to select a retrofit 

process technology to achieve nutrient removal requirements should first determine which 

processes are technically suited for the specific objectives at their facility. After candidate 

technologies have been identified, the readers should assess the estimated capital and O&M 

costs. They should use this cost information as a factor in selecting the process, along with 

other factors such as reliability, sustainability, and environmental considerations discussed in 

Chapter 5. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-5

This section provides planning-level cost estimates for 12 retrofit treatment alternatives, 

including several alternatives involving the anoxic/oxic (A/O) process, also known as the 

Phoredox process. These alternatives were deemed the most likely retrofit options to be used 

at existing plants because of footprint size, ease of installation in an operating facility, ability 

to meet treatment objectives, and overall costs. Each alternative was evaluated at three 

different flow rates––1 MGD, 5 MGD, and 10 MGD. This range was deemed to cover the 

flow rate range of most readers of this manual. Note that the cost rankings represented in the 

cost curves will likely hold for larger and smaller flows. Ultimately, readers should find the 

process scenarios that best match their candidate technologies to obtain a cost estimate. Three 

alternatives not included in these estimates are membrane biological reactors, land 

application for nutrient removal, and special filters like the Blue PRO; these technologies are 

new and thus have no software-based model for estimating costs. A reader seeking cost 

estimates for these processes should contact the process vendors. 

4.2.1 CAPDETWorks 

The cost estimates for the scenarios were developed using CAPDETWorks software, version 

2.1. The U.S. Environmental Protection Agency (EPA) and U.S. Army Corps of Engineers 

originally developed this software as a planning tool; Hydromantis Corporation now 

maintains and updates (2006). The software works as follows. The user generates a process 

layout involving a number of unit operations. The user can also define input variables, 

including wastewater flow rate, wastewater influent quality, and desired effluent quality or 

other performance coefficients. Alternatively, the user can choose to use default values 

developed by Hydromantis. The software then calculates the required sizes of the unit 

operations and uses cost-curve models from the software’s database to estimate the capital, 

labor, chemical, and energy costs that would be incurred. 

The cost functions included in software version 2.1 (the most recent available during 

document preparation) were updated in 2000. The model uses several standard indices to 

update costs to current dollars: the Engineering News-Record (ENR) Construction Cost 

Index, the Marshall & Swift Index, and the Pipe Index. Values were obtained from a U.S. 

Department of Agriculture Web site (USDA 2007) that transcribes historical values of these 

indices. The values used for the indices are shown in Table 4-3, along with other cost factors 

used in all CAPDETWorks runs. 

A number of authors have used the CAPDET model to provide planning-level costs. Wright 

et al. (1988) found that the program provides construction cost estimates that are within plus 

or minus 20 percent of the actual costs. Overall, they showed that feasible alternatives can be 

compared using estimations from CAPDETWorks because the software includes all the items 

needed for process assessment except land and building costs. In general, the rankings of 

alternatives by cost will not change significantly for plants of different sizes. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-6 Chapter 4: Cost Factors

To further verify the validity of CAPDETWorks, examples were run with the model to match 

what was installed at two of the case study locations discussed in Chapter 3, the Marshall 

Street facility in Clearwater, Florida, and the Kalispell Advanced Wastewater Treatment 

facility in Kalispell, Montana. The model was used to cost the biological treatment systems, 

tertiary filters, and any fermenters/digesters used to provide volatile acids for phosphorus 

treatment. Primary treatment, preliminary treatment, and sludge handling were not included 

in the costing. Because CAPDETWorks does not include a fermenter unit operation, two 

thickeners were used as an approximation. 

Table 4-3. Cost factors used in CAPDETWorks estimates 

Indices used to 

update costs 

Value Date 

ENR 7,940 May 2007 

Marshall & Swift 1,360 2007 

Pipe Index 732 2007 

Cost factors Value Unit 

Electricity $0.08 kW-hr 

Labor $25.00 Person-hour 

Alum $0.41 lb as Al2(SO4)3-14H2O 

The design and actual influent flow rate, influent constituents, and other basic plant 

parameters were inputs for the software, along with the known plant layouts. The model was 

then run to obtain estimates of capital cost for installing the plant, as well as O&M estimates 

for power consumption, chemical usage (if any), and additional sludge generation (if any). 

Those were the only components of O&M included in the case studies. In the event that 

current operating flow was well below the design flow, as was the case for Clearwater, a 

second run was done with the current flow to obtain operating costs. 

Life-cycle costs were calculated for the CAPDETWorks results by first annualizing the 

capital cost at 20 years at 6 percent interest. The annualized capital cost was then added to 

the annual O&M cost to obtain a total annual cost. This cost was then divided by the annual 

flow to get the life-cycle cost per MG treated. The model results were then compared with 

the results presented in the case studies developed for this manual. All capital costs for both 

the software estimates and the actual capital costs were updated to 2007 dollars. 

For electrical costs, the total electricity used in the unit operations cited was used rather than 

the breakdowns for phosphorus and nitrogen used in the case studies; this is because the 

CAPDETWorks software provides total electricity, including that attributable to biochemical 

oxygen demand (BOD) removal. Chemical costs in such cases were both nil. 

Table 4-4 presents the results of the modeling versus case study results comparison for both 

Clearwater and Kalispell. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-7

For Clearwater, the estimated capital cost from CAPDETWorks was close to the actual 

capital cost, within 17 percent. The O&M costs are similarly close, within 6 percent. 

For Kalispell, the estimated capital cost is within 2 percent of the actual cost. The O&M costs 

are 29 percent off. The difference here might be that CAPDETWorks designed a smaller 

anoxic zone relative to the rest of the system than Kalispell actually uses; that would account 

for higher blower usage and, thus, the higher electrical costs. 

Overall, these results indicate that CAPDETWorks would provide reasonably accurate cost 

estimates for comparative purposes during planning. 

Table 4-4. Comparison of CAPDETWorks and actual costs for Clearwater, Florida, and 

Kalispell, Montana 

Cost Clearwater: 5-stage Bardenpho Kalispell: University of Cape Town 

CAPDETWorks Actual 

% 

Difference CAPDETWorks Actual 

% 

Difference

Total 

capital $35,541,000 $29,500,000 17 $8,903,000 $9,100,000 2 

O&M 

electrical $640,000 $610,000 5 $159,000 $113,000 29 

O&M 

chemical $0 $74,000 -- $0 $0 -- 

O&M 

sludge $0 $0 -- $0 $0 -- 

Total 

O&M $640,000 $684,000 6 $159,000 $113,000 29 

Certain input assumptions were common to all model runs, and they are presented in Table 

4-5. Beyond these values, various model components had default parameters, which are 

shown as the scenarios are discussed below. All capital cost results include a percentage of 

total unit costs to account for engineering, site preparation, electrical and control installation, 

and building costs. O&M results include operations labor, maintenance labor (both costed at 

$25/hour), maintenance materials, chemicals (alum for selected phosphorus removal 

processes, methanol for selected nitrogen removal processes), and energy. This is in contrast 

to the Maryland study, the Connecticut study, and the case studies, in which labor and 

maintenance material costs were excluded. This means that the CAPDETWorks O&M 

estimates will be higher than those for similar systems in those studies. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-8 Chapter 4: Cost Factors

4.2.2 Retrofit Phosphorus Removal Technologies 

Five retrofit plans for implementing phosphorus removal were evaluated. All five involved 

modifying or making additions around an existing biological treatment system that was not 

accomplishing phosphorus removal. 

1. Alum addition at one point upstream of an existing conventional activated-sludge 

process, 0.5 parts per million (ppm) TP target, no filter 

2. Alum addition at two points, both upstream and downstream of an existing 

conventional activated-sludge process, 0.1 ppm TP target, sand filter downstream 

Table 4-5. Cost model influent wastewater parameters 

Parameter Value 

Average flow (MGD) 1, 5, 10 

TSS (mg/L) 220 

% volatile solids 75% 

BOD (mg/L) 220 

Soluble BOD (mg/L) 80 

COD (mg/L) 500 

Soluble COD (mg/L) 300 

TKN (mg/L) 40 

Soluble TKN (mg/L) 25 

Ammonia nitrogen (mg/L) 22 

TP (mg/L) 5 

pH (standard units) 7.6 

Nitrite/nitrate (mg/L) 0.0 

Temperature, summer 23 oC 

Temperature, winter 10 oC 

Notes: 

BOD = biochemical oxygen demand 

COD = chemical oxygen demand 

TKN = total Kjeldahl nitrogen 

TP = total phosphorus 

TSS = total suspended solids 

3. Refitting an existing conventional activated-sludge system to an A/O (Phoredox) or 

University of Cape Town (UCT) process, with a fermenter added to generate volatile 

fatty acids to support biological phosphorus uptake, with a target of 0.5 mg/L. The 

A/O CAPDET model was used to represent all such processes. 

4. Refitting an existing conventional activated-sludge system to an A/O configuration, 

with a fermenter added to generate volatile fatty acids and a sand filter added to aid 

removal of phosphorus-containing solids, with a target of 0.5 mg/L 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-9

5. Refitting an existing conventional activated-sludge system to an A/O configuration, 

with a fermenter and sand filter added, as well as alum addition, with a target of 

0.1 mg/L 

In these calculations, the following assumptions were made with respect to planning cost 

estimates: 

1. The cost to convert an existing activated-sludge process to A/O would incur 

25 percent of the cost of a new A/O unit with the same capacity; this allows for the 

addition of walls and baffles to set off anaerobic zones. 

2. As a fermenter module has yet to be established for CAPDETWorks, the capital and 

operating costs of a gravity thickener, adjusted upward by 50 percent, were used 

instead. A thickener operated anaerobically is a good approximation of a fermenter; 

the 50 percent allowance includes the pipes and mixers that would be required for a 

fermenter. 

3. Post-secondary chemical treatment was assumed to require an alum dose four times 

the dose that would be suggested by stoichiometry; for example, if the stoichiometric 

dose was 0.5 mg/L, a dose of 2 mg/L of alum as Al was used. 

4. A sand filter was assumed for tertiary filtration used in retrofits 4 and 5. 

5. It was assumed that no additional aeration capacity would be needed for retrofitting 

an existing activated-sludge reactor to A/O. 

Table 4-6 presents the alum doses used in the costing for retrofits 1, 2, and 5. 

Figures 4-1 through 4-3 are graphs of the O&M costs (in dollars per MG treated), capital 

costs (in dollars per gpd capacity), and life-cycle costs (in dollars per MG treated) for these 

scenarios. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD average influent flow. 

The life-cycle costs were determined by summing the O&M dollars per MG treated and the 

capital costs annualized to 20 years at 6 percent per MG treated. 

From these figures, the following observations can be made: 

1. The overall lowest cost option for O&M, as well as for life-cycle, is installation of a 

fermenter with a retrofit of the existing process to A/O. The lowest capital cost 

alternative is one-point chemical addition, but the retrofit/fermenter alternative is the 

second-lowest capital alternative. The estimated O&M for A/O plus fermentation is 

$5 to $25 per MG treated, while the capital ranges between $0.19 and $0.32 per gpd 

capacity. 

2. The lowest operating costs are associated with not using chemical, i.e., the 

alternatives of A/O with a fermenter and A/O with a fermenter and filter. These 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-10 Chapter 4: Cost Factors

alternatives are two of the three lowest-life-cycle-cost alternatives, with the low 

capital cost for one-point chemical addition also resulting in a low life-cycle cost. The 

highest operating cost was associated with doing two chemical additions because both 

have elevated doses. These high operating costs result in that alternative’s having the 

highest life-cycle costs as well. 

Table 4-6. Process parameters for phosphorus removal scenarios 

Process Costed items 

Alum dose 

(mg/L as Al) 

1 Alum feed 2.4 

2 Alum feed (2 points) with sand filter 2.6; 2.4 

3 A/O conversion, fermenter N/A 

4 A/O conversion, fermenter, Sand filter N/A 

5 A/O conversion, fermenter, sand filter, alum feed 

(post-secondary treatment) 1.3 

Notes: 

A/O = anoxic/oxic 

N/A = not available 

3. If a TP level of 0.1 mg/L or lower in the effluent is required, use of post-secondary 

chemical treatment with a filter might be needed. In that instance, these results 

indicate that using biological phosphorus removal with a fermenter to get the 

concentration down to between 0.5 and 1 mg/L can save resources because doing so 

reduces the required post-secondary chemical dose. 

4. The cost for using alum in the second feed point was conservative, because regular 

biological uptake was not excluded in the dosage estimation. This means that actual 

costs for post-secondary chemical phosphorus removal will be lower. 

5. A tertiary clarifier might be applicable to aid the tertiary filtration process, but it was 

not included in these retrofit cost curves. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-11

0

50

100

150

200

250

300

0 2 4 6 810 12

Flow (MGD)

O&M Costs ($/MG treated)

Fermenter Retrofit, 0.5 ppm TP target, no filter

Fermenter+Sand Filter Retrofit, 0.5 ppm target

1 point addition alum addition , 0.5 ppm TP target, no filter

Fermenter+Filter+1 point Alum addition, 2 ppm dose, 0.1 ppm TP target

2 pt alum addition, 0.1 ppm TP target, filter

Figure 4-1. O&M costs for retrofit phosphorus removal technologies ($/MG treated). 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 2 4 6 810 12

Flow (MGD)

Capital costs ($/gpd capacity)

1 point addition alum addition , 0.5 ppm TP target, no filter

Fermenter Retrofit, 0.5 ppm TP target, no filter

2 pt alum addition, 0.1 ppm TP target, filter

Fermenter+Sand Filter Retrofit, 0.5 ppm target

Fermenter+Filter+1 point Alum addition, 2 ppm dose, 0.1 ppm TP target

Figure 4-2. Capital costs for retrofit phosphorus removal technologies ($ per gpd capacity). 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-12 Chapter 4: Cost Factors

$0.00

$50.00

$100.00

$150.00

$200.00

$250.00

$300.00

$350.00

$400.00

$450.00

$500.00

0 2 4 6 810 12

Flow (MGD)

Life-cycle Costs ($/MG treated)

Fermenter Retrofit, 0.5 ppm TP target, no filter

1 point addition alum addition , 0.5 ppm TP target, no filter

Fermenter+Sand Filter Retrofit, 0.5 ppm target

Fermenter+Filter+1 point Alum addition, 2 ppm dose, 0.1 ppm TP target

2 pt alum addition, 0.1 ppm TP target, filter

Figure 4-3. Life-cycle costs for retrofit phosphorus removal technologies ($/MG treated). 

4.2.3 Retrofit Nitrogen Removal Technologies 

Four retrofit plans for implementing nitrogen removal were evaluated. All four involved 

modifying or making additions around an existing biological treatment system that was not 

accomplishing phosphorus removal. 

1. Installation of additional tank capacity for an oxidation ditch to allow sufficient 

residence time in an anoxic zone for denitrification to occur 

2. Retrofitting an existing activated-sludge system as a modified Lutzak-Ettinger 

(MLE), with anoxic and aerobic zones and internal recirculation 

3. Retrofitting an existing activated-sludge system to be a step-feed system to provide 

sufficient biochemical oxygen demand (BOD) to the anoxic zones to allow 

denitrification to occur 

4. Installation of a denitrifying filter, along with a methanol system 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-13

In these calculations, the following assumptions were made with respect to planning cost 

estimates: 

1. The cost to convert an existing activated-sludge process to MLE or step-feed would 

incur 33 percent of the cost of a new such unit with the same capacity; this allows for 

the addition of walls and baffles to set off anoxic zones, along with additional piping 

and recirculation pumping. The same ratio was applied to install additional secondary 

clarification and blower capacity. 

2. Methanol was assumed to be needed only for the denitrifying filter, at a dose of 3 mg 

methanol per mg nitrate nitrogen. 

3. The target TN concentration was assumed to be 3 mg/L. 

Table 4-7 presents the items costed for these plans, as well as the methanol dosage if 

applicable. 

Table 4-7. Process parameters for nitrogen removal scenarios 

Process Costed items 

Methanol dose 

(lb/lb NO3) 

1 Tanks, mixers N/A 

2 MLE conversion, filter N/A 

3 Step-feed AS conversion, filter N/A 

4 Denitrifying filter 3 

Notes: 

AS = activated sludge 

MLE = modified Ludzack-Ettinger 

N/A = not available 

Figures 4-4 through 4-6 are graphs of the capital costs (in dollars per gpd capacity), O&M 

costs (in dollars per MG treated), and life-cycle costs (in dollars per MG treated) for these 

nitrogen treatment plants. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD 

average influent flow. The life-cycle costs were determined by summing the O&M dollars 

per MG treated and the capital costs annualized to 20 years at 6 percent per MG treated. 

From these figures, the following observations can be made: 

1. The lowest-cost option is the installation of additional tank capacity for an existing 

oxidation ditch because this had both the lowest capital and O&M costs and therefore 

the lowest life-cycle cost. It should be noted, however, that a great deal of land area 

could be required for this alternative. 

2. The denitrifying filter had clearly the highest operating cost, because of the methanol 

requirement. The other processes use organic matter in the wastewater to support 

denitrification, which gives a large cost savings. The high operating cost for 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-14 Chapter 4: Cost Factors

denitrifying filters led to that alternative’s having the highest life-cycle cost. The 

advantage of denitrifying filters, however, is that they have very small footprints. 

3. The step-feed and MLE alternatives had similar capital, O&M, and life-cycle costs. 

Technically, each has advantages and disadvantages, as described elsewhere in this 

manual. 

4. None of these alternatives employed a post-secondary filter. Such a filter could be 

useful in achieving low TN concentrations through removal of solids containing 

nitrogen. For example, an effluent with a total suspended solids (TSS) at 10 mg/L 

could contain 0.5 mg/L (i.e., 5 percent) TN in a filterable form. Therefore, in meeting 

an overall effluent limit of 3 mg/L TN, a tertiary filter could be helpful. 

$0.00

$0.20

$0.40

$0.60

$0.80

$1.00

$1.20

$1.40

$1.60

0 2 4 6 810 12

Flow (MGD)

Capital Cost per gpd

Denit Filter 3 ppm TN Step Feed Retro--1/3 of Step Feed/PFR on reactor MLE Retro--1/3 of MLE Extra basins for PID

Figure 4-4. Capital costs for retrofit nitrogen removal scenarios. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-15

$0.00

$50.00

$100.00

$150.00

$200.00

$250.00

$300.00

$350.00

$400.00

$450.00

$500.00

0 2 4 6 8 10 12

Flow (MGD)

O&M Costs per MG treated

Denit Filter 3 ppm TN Step Feed Retro--1/3 of Step Feed/PFR on reactor Extra basins for PID MLE Retro--1/3 of MLE

Figure 4-5. O&M costs for retrofit nitrogen removal scenarios. 

$0.00

$100.00

$200.00

$300.00

$400.00

$500.00

$600.00

$700.00

$800.00

$900.00

0 2 4 6 810 12

Flow (MGD)

Life Cycle Costs ($/MG treated)

Denit Filter 3 ppm TN Extra basins for PID Step Feed Retro--1/3 of Step Feed/PFR on reactor MLE Retro--1/3 of MLE

Figure 4-6. Life-cycle costs for retrofit nitrogen removal scenarios. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-16 Chapter 4: Cost Factors

4.2.4 Combined Nitrogen and Phosphorus Scenarios 

The following three retrofit alternatives for achieving combined nitrogen and phosphorus 

removal to very low levels (3 ppm TN, 0.1 ppm TP) were evaluated. All the technologies 

used a combination of biological nutrient removal to achieve nitrification/denitrification and 

some phosphorus removal, plus chemical phosphorus removal to polish the effluent to 0.1 

ppm TP. All scenarios included a tertiary filter (or denitrification filter). 

1. Oxidation ditch retrofitted with additional tanks (as was done above for nitrogen 

removal), with one-point alum addition for phosphorus removal, plus a tertiary 

clarifier and a tertiary sand filter. 

2. A nitrifying activated-sludge reactor retrofitted with two-point alum addition for 

phosphorus removal plus a denitrification filter. 

3. Conversion of an activated-sludge system to a 5-stage Bardenpho with chemical 

addition for polishing phosphorus and a tertiary filter 

In these calculations, the following assumptions were made with respect to planning cost 

estimates: 

1. The cost to convert an existing activated-sludge process to a 5-stage Bardenpho 

would incur 50 percent of the cost of a new such unit with the same capacity. This 

allows for the addition of walls and baffles to set off anoxic zones, installation of 

additional tank capacity, and additional piping and recirculation pumping. The same 

ratio was applied to install additional secondary clarification and blower capacity, as 

well as material and energy costs. 

2. Methanol was assumed to be needed only for the denitrifying filter, at a dose of 3 mg 

methanol per mg nitrate-nitrogen. 

3. The target TN concentration was assumed to be 3 mg/L; the target TP concentration, 

0.1 mg/L. 

Table 4-8 presents the items costed for these plans, as well as the methanol dosage if 

applicable. 

Table 4-8. Process parameters for nitrogen removal scenarios 

Process Costed items 

Methanol dose 

(lb/lb NO3) 

1 Basins, mixers, alum addition, tertiary 

clarifier, tertiary filter N/A 

2 Alum addition, denitrifying filter 3 mg/mg NO3-N 

3 5-stage Bardenpho conversion, alum 

addition, tertiary filter N/A 

Note: N/A = not available 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-17

Figures 4-7 through 4-9 are graphs of the capital costs (in dollars per gpd capacity), O&M 

costs (in dollars per MG treated), and life-cycle costs (in dollars per MG treated) for these 

alternatives. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD average influent 

flow. The life-cycle costs were determined by summing the O&M dollars per MG treated and 

the capital costs annualized to 20 years at 6 percent per MG treated. 

From these figures, the following observation can be made: 

1. The lowest-cost alternative on a life-cycle basis is the oxidation ditch modifications. 

This is so because it has the best combination of reduced operating costs (because of 

accomplishing biological nutrient removal) and capital costs. The lowest-capital-cost 

option was the use of chemical phosphorus removal and a denitrifying filter, but that 

alternative has the highest operating costs; the lowest operating costs were attached to 

the ditch additions. 

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

0 2 4 6 810 12

Flow (MGD)

Capital Cost ($/gpd capacity)

5-Stage/Chem P Retro = 1/2 old 5 Stage ---3 NO3/0.3 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P N&P PID Retro 

Figure 4-7. Capital costs for retrofit nitrogen plus phosphorus removal technologies. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-18 Chapter 4: Cost Factors

$0.00

$100.00

$200.00

$300.00

$400.00

$500.00

$600.00

$700.00

$800.00

$900.00

0 2 4 6 810 12

Flow (MGD)

O&M Costs ($/MG treated)

5-Stage/Chem P Retro = 1/2 old 5 Stage ---3 NO3/0.3 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P N&P PID Retro 

Figure 4-8. O&M costs for retrofit nitrogen plus phosphorus removal technologies. 

$0.00

$200.00

$400.00

$600.00

$800.00

$1,000.00

$1,200.00

$1,400.00

0 2 4 6 810 12

Flow (MGD)

Life-Cycle Costs ($/MG treated)

5-Stage/Chem P Retro = 1/2 old 5 Stage ---3 NO3/0.3 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P N&P PID Retro 

Figure 4-9. Life-cycle costs for retrofit nitrogen plus phosphorus removal technologies. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-19

4.3 Expansion Process Cost Models 

There are situations where a retrofit might not be adequate and the owners of the wastewater 

treatment operation must consider a plant expansion. As discussed in detail in Chapter 5, 

project teams seeking to select a new process technology to meet nutrient removal 

requirements should first determine which processes are technically suited for the specific 

objectives at their facility. Once candidate technologies have been identified, the project team 

members should assess the estimated capital and O&M costs. They should use this cost 

information as a factor in making the process selection, along with other factors such as 

reliability, sustainability, and environmental considerations. 

This section provides planning-level cost estimates for 20 treatment scenarios. The scenarios 

were selected because they are most representative of the likely treatment process candidates 

to be considered, on the basis of what has been observed from the case studies and other 

evaluations conducted for this document. As was done for the retrofit cases above, each 

scenario was evaluated using CAPDETWorks at three different flow rates––1 MGD, 5 MGD, 

and 10 MGD. This range was deemed to cover the flow rate range of most readers of this 

manual. All basic process assumptions used in the retrofit section also apply for these 

expansion cases. 

4.3.1 Phosphorus Removal Technologies 

Eight scenarios for implementing phosphorus removal were evaluated: 

1. Alum addition at one point upstream of an existing conventional activated-sludge 

process, 0.5 ppm TP target, no filter 

2. Alum addition at two points, both upstream and downstream of an existing 

conventional activated-sludge process, 0.1 ppm TP target, sand filter downstream 

3. A/O process with biological treatment to 1 ppm TP, no extra equipment 

4. A/O process with biological treatment to 0.5 ppm TP, fermenter included to supply 

volatile fatty acids 

5. A/O process with biological treatment to 0.5 ppm TP, fermenter and sand filter 

included 

6. A/O process with biological treatment to 0.1 ppm TP, fermenter, chemical addition 

downstream of bioreactor, and sand filter included 

7. Modified UCT (3-stage) process with biological treatment to 0.5 ppm TP, fermenter 

and sand filter included 

8. Five-stage Bardenpho process with biological treatment to 0.5 ppm TP, sand filter 

included 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-20 Chapter 4: Cost Factors

Table 4-9 presents process parameters for these scenarios, including the hydraulic residence 

time (HRT) in the bioreactor, if applicable, and the chemical dosage of alum. 

Figures 4-10 through 4-12 are graphs of the O&M costs (in dollars per MG treated), capital 

costs (in dollars per gpd capacity), and life-cycle costs (in dollars per MG treated) for these 

scenarios. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD average influent flow. 

The life-cycle costs were determined by summing the O&M dollars per MG treated and the 

capital costs annualized to 20 years at 6 percent per MG treated. Figures 4-13 and 4-14 

present the percentage breakdown of the total O&M costs for the eight technologies for 1 

MGD and 10 MGD, respectively. 

Table 4-9. Process parameters for phosphorus removal scenarios 

Process Costed items 

HRT 

(hr) 

Alum dose 

(mg/L as Al) 

1 Alum feed N/A 2.4 

2 Alum feed (2 points) N/A 2.6; 2.4 

3 A/O, clarifier, blower 12.6 N/A 

4 A/O, clarifier, fermenter, Blower 13.1 N/A 

5 A/O, clarifier, fermenter, filter, blower 13.1 N/A 

6 A/O, clarifier, fermenter, filter, blower, alum feed 13.1 2.4 

7 UCT, clarifier, filter, fermenter, blower 12.0 N/A 

8 5-stage Bardenpho, clarifier, filter, blower 15.1 N/A 

Notes: 

A/O = anoxic/oxic 

N/A = not available 

UCT = University of Cape Town process 

From these figures, the following observations can be made: 

1. The lowest-cost options are to implement one-point and two-point alum addition for 

an existing biological treatment system. This can be done if no additional nitrogen 

removal is required. 

2. The A/O costs are fairly close together, with higher overall costs incurred as 

additional units are added, as well as when chemical addition is included. These costs 

include installation of a new A/O biological treatment system (reactor plus clarifier). 

If an existing reactor/clarifier can be modified to an A/O system by installing walls in 

the basin, adding an anaerobic tank, and so forth, the capital costs for these 

alternatives would be substantially reduced. 

3. The costs for including post-secondary chemical phosphorus removal are 

conservative, because regular biological phosphorus uptake was not accounted for. 

This means the required dose would be lower, thereby reducing chemical costs. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-21

4. The 3-stage modified UCT process and 5-stage Bardenpho process have the highest 

overall costs because of process complexity, as well as conservative inclusion of a 

number of recycle pumps (4 Q for internal recirculation and 3 Q for return-activated 

sludge and other back-up design elements). These processes should be considered 

when substantial nitrogen removal is also required. 

5. As shown by processes 5 and 6, all biological removal processes, whether new or 

existing, can be enhanced for phosphorus removal by installing a sand filter, alum 

addition, or both. As discussed elsewhere in this document, including a sand filter or 

alum addition might have overall treatment benefits beyond nutrient removal, 

including enhanced wastewater reuse potential, TSS control, and reduction of 

trihalomethane formation. 

6. The UCT process costs can be assumed to be representative of other 3-stage 

processes, such as the Virginia Initiative process (VIP), anaerobic/anoxic/oxic 

process (A2

O), and Westbank bioreactor because all involve three stages with varying 

amounts of mixing and aeration in the different stages. 

7. Figures 4-13 and 4-14 indicate that the fraction of the total cost attached to labor 

expenses decreases with larger facilities; associated increases in the fraction of the 

cost are attached to energy and other components. In particular, the 1 MGD labor 

costs range from 40 to 50 percent of the total except for the two simple chemical 

addition technologies, while the 10 MGD labor costs range from 15 to 25percent of 

the total. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-22 Chapter 4: Cost Factors

$0.00

$100.00

$200.00

$300.00

$400.00

$500.00

$600.00

$700.00

$800.00

$900.00

0 2 4 6 8 10 12

Flow (MGD)

O&M Costs ($/MG treated)

5 Stage Bardenpho to 0.5 ppm TP w filter Mod UCT to 0.5 ppmTP w ferm+filt

A/O w fermenter, filter, and alum addition to 0.1 ppm TP A/O to 0.5 ppm TP w fermenter+filter

A/O to 0.5 ppm TP w Fermenter A/O to 1 ppm TP No ferm, no chem, no filter

1 point addition alum addition , 0.5 ppm TP target, no filter 2 pt alum addition, 0.1 ppm TP target, filter

Figure 4-10. O&M costs for expansion phosphorus removal technologies ($/MG treated). 

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

$3.50

$4.00

$4.50

0 2 4 6 810 12

Flow (MGD)

Capital costs ($/gpd capacity)

5 Stage Bardenpho to 0.5 ppm TP w filter Mod UCT to 0.5 ppmTP w ferm+filt

A/O w fermenter, filter, and alum addition to 0.1 ppm TP A/O to 0.5 ppm TP w fermenter+filter

A/O to 0.5 ppm TP w Fermenter A/O to 1 ppm TP No ferm, no chem, no filter

2 pt alum addition, 0.1 ppm TP target, filter 1 point addition alum addition , 0.5 ppm TP target, no filter

Figure 4-11. Capital costs for expansion phosphorus removal technologies. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-23

$0.00

$200.00

$400.00

$600.00

$800.00

$1,000.00

$1,200.00

$1,400.00

$1,600.00

$1,800.00

$2,000.00

0 2 4 6 810 12

Flow (MGD)

Life-cycle Costs ($/MG treated)

5 Stage Bardenpho to 0.5 ppm TP w filter Mod UCT to 0.5 ppmTP w ferm+filt

A/O w fermenter, filter, and alum addition to 0.1 ppm TP A/O to 0.5 ppm TP w fermenter+filter

A/O to 0.5 ppm TP w Fermenter A/O to 1 ppm TP No ferm, no chem, no filter

2 pt alum addition, 0.1 ppm TP target, filter 1 point addition alum addition , 0.5 ppm TP target, no filter

Figure 4-12. Life-cycle costs for expansion phosphorus removal technologies. 

1 MGD P Technologies

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

5 Stage

Bardenpho

Westbank A/O with

Chem P

A/O with

Fermenter

and Filter

A/O with

Fermenter

A/O no

ferm,

chem, or

filter

Plain AS, 2

alum feed,

.1 ppm out,

filter

1 alum

feed, .5

ppm out,

no filter

Energy

Chemical 

O&M Materials

Labor

Figure 4-13. Component percentages of total O&M costs for 1 MGD expansion phosphorus 

removal technologies. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-24 Chapter 4: Cost Factors

10 MGD P Technologies

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

5 Stage

Bardenpho

Westbank A/O with

Chem P

A/O with

Fermenter

and Filter

A/O with

Fermenter

A/O no

ferm,

chem, or

filter

Plain AS, 2

alum feed,

.1 ppm out, 

filter

1 alum

feed, .5

ppm out, no

filter

Energy

Chemical

Materials

Labor

Figure 4-14. Component percentages of total O&M costs for 10 MGD expansion phosphorus 

removal technologies. 

4.3.2 Nitrogen Removal Technologies 

The following eight scenarios for implementing nitrogen removal were evaluated. All the 

technologies were selected specifically for TN removal, i.e., including denitrification. If a 

facility is required to remove only ammonia or total Kjeldahl nitrogen (TKN), all systems 

outlined below except the denitrifying filter will accomplish that task. In that case, the 

denitification system need not be operated. The project team might, however, wish to design 

the implementation to allow for future denitrification should the permit eventually change. 

1. PID, 5 ppm TN target 

2. Denitrifying filter, 3 ppm TN target 

3. Step-feed, 5 ppm TN target (1 ppm TP) 

4. MLE, 5 ppm TN target, without sand filter 

5. Sequencing batch reactor (SBR), 5 ppm TN target 

6. Three-stage UCT 5 ppm TN target (1 ppm TP) 

7. Four-stage Bardenpho, 5 ppm TN target (biological phosphorus removal not 

supported) 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-25

8. Five-stage Bardenpho process, 5 ppm TN target with biological treatment to 0.5 ppm 

TP 

Table 4-10 presents process assumptions for these scenarios, including the HRT in the 

bioreactor (if applicable) and methanol dosage (if applicable). 

Figures 4-15 through 4-17 are graphs of the O&M costs (in dollars per MG treated), capital 

costs (in dollars per gpd capacity), and life-cycle costs (in dollars per MG treated) for these 

nitrogen scenarios. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD average 

influent flow. The life-cycle costs were determined by summing the O&M dollars per MG 

treated and the capital costs annualized to 20 years at 6 percent per MG treated. Figures 4-18 

and 4-19 present the percentage breakdown of the total O&M costs for the eight technologies 

for 1 MGD and 10 MGD, respectively. 

Table 4-10. Process parameters for nitrogen removal scenarios 

Process Costed items HRT (hr) 

Methanol dose 

(lb/lb NO3) 

1 PID, clarifier, sand filter 8.8 N/A 

2 Denitrification filter N/A 3 

3 Step-feed AS, clarifier, filter, blower 12.4 N/A 

4 MLE reactor, clarifier, blower 16.8 N/A 

5 SBR, sand filter, blower 9 (total cycle time) N/A 

6 UCT, clarifier, blower 12.1 N/A 

7 4-stage Bardenpho, clarifier, blower 14.5 N/A 

8 5-stage Bardenpho, clarifier, blower 15.1 N/A 

Notes: 

AS = activated sludge 

MLE = modified Ludzack-Ettinger 

N/A = not available 

PID = phased isolation ditch 

SBR = sequencing batch reactor 

UCT = University of Cape Town process 

From these figures, the following observations can be made: 

1. The lowest-cost options are the oxidation ditch and the denitrification filter. The 

denitrification filter has the advantage of a very small footprint for inclusion within 

an existing system. 

2. Step feed also has major advantages as a retrofit technology with an existing 

activated-sludge system because it can be accomplished largely by redirecting flows 

without needing to build substantial additional tank capacity. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-26 Chapter 4: Cost Factors

3. The 3-, 4-, and 5-stage processes had the highest overall costs because of the number 

of tanks and process complexity. The 3-stage process costs can be assumed to be 

representative of all 3-stage processes––UCT, A2O, VIP, Westbank, or others. 

4. Figures 4-18 and 4-20 indicate that the fraction of the total cost attached to labor 

expenses decreases with larger facilities; the associated increases in the fraction of the 

cost are attached to energy and other components. In particular, the 1 MGD labor 

costs range from 40 to 50 percent of the total for all technologies except for the 

denitrification filter that has comparatively low other costs, so that labor is a large 

fraction of the total, while the 10 MGD labor costs range from 15 to 25 percent of the 

total. 

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

$3.50

$4.00

0 2 4 6 810 12

Flow (MGD)

Capital Cost per gpd

5 Stage 5N 0.5 P 4 Stage Bardenpho 5 N 5 P 3 Stage generic - 5 N 1 P SBR 5 N 1 P

MLE to 5 ppm TN, no filter Step Feed 5 N 1 P Denit Filter 3 ppm TN PID 5 ppm TN target

Figure 4-15. Capital costs for expansion nitrogen removal scenarios. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-27

$0.00

$100.00

$200.00

$300.00

$400.00

$500.00

$600.00

$700.00

$800.00

$900.00

0 2 4 6 8 10 12

Flow (MGD)

O&M Costs per MG treated

5 Stage 5N 0.5 P 4 Stage Bardenpho 5 N 5 P 3 Stage generic - 5 N 1 P SBR 5 N 1 P

MLE to 5 ppm TN, no filter Step Feed 5 N 1 P Denit Filter 3 ppm TN PID 5 ppm TN target

Figure 4-16. O&M costs for expansion nitrogen removal scenarios. 

$0.00

$200.00

$400.00

$600.00

$800.00

$1,000.00

$1,200.00

$1,400.00

$1,600.00

$1,800.00

0 2 4 6 8 10 12

Flow (MGD)

Life Cycle Costs ($/MG treated)

5 Stage 5N 0.5 P 4 Stage Bardenpho 5 N 5 P 3 Stage generic - 5 N 1 P SBR 5 N 1 P

MLE to 5 ppm TN, no filter Step Feed 5 N 1 P Denit Filter 3 ppm TN PID 5 ppm TN target

Figure 4-17. Life-cycle costs for expansion nitrogen removal scenarios. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-28 Chapter 4: Cost Factors

1 MGD N Technologies

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

5 Stage

Bardenpho--

N

4 Stage

Bardenpho--

5stage with

Anaerobic

zeroed out

MLE SBR I (with

Anaerobic

phase)

Step

Feed/PFR

Denite Filter

(alone)

PID 3 Stage

generic

Energy

Chemical 

O&M Materials

Labor

Figure 4-18. Component percentages of total O&M costs for 1 MGD expansion nitrogen 

removal technologies. 

$0.00

$200.00

$400.00

$600.00

$800.00

$1,000.00

$1,200.00

$1,400.00

$1,600.00

$1,800.00

0 2 4 6 8 10 12

Flow (MGD)

Life Cycle Costs ($/MG treated)

5 Stage 5N 0.5 P 4 Stage Bardenpho 5 N 5 P 3 Stage generic - 5 N 1 P SBR 5 N 1 P

MLE to 5 ppm TN, no filter Step Feed 5 N 1 P Denit Filter 3 ppm TN PID 5 ppm TN target

Figure 4-19. Life-cycle costs for expansion nitrogen plus phosphorus removal technologies. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-29

10 MGD N Technologies

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

5 Stage

Bardenpho-

-N

4 Stage

Bardenpho-

-5stage

with

Anaerobic

zeroed out

MLE SBR I (with

Anaerobic

phase)

Step

Feed/PFR

Denite

Filter

(alone)

PID 3 Stage

generic

Energy

Chemical

Materials

Labor

Figure 4-20. Component percentages of total O&M costs for 10 MGD expansion nitrogen 

removal technologies. 

4.3.3 Combined Nitrogen and Phosphorus Scenarios 

The following four scenarios for implementing combined nitrogen and phosphorus removal 

to very low levels (3 ppm TN, 0.1 ppm TP) were evaluated. All the technologies used a 

combination of biological nutrient removal to achieve nitrification/denitrification and some 

phosphorus removal, plus chemical phosphorus removal to polish the effluent to 0.1 ppm TP. 

All scenarios included a tertiary filter (or denitrification filter), and all except the SBR 

included a tertiary clarifier to help get solids (and thus TN and TP) lower. 

1. PID with dual (two-point) chemical addition plus tertiary clarifier plus tertiary filter 

2. SBR with chemical plus tertiary filter 

3. Nitrification with chemical plus denitrification filter 

4. Five-stage Bardenpho with chemical plus tertiary filter 

Table 4-11 presents process assumptions for these scenarios, including the HRT in the 

bioreactor and alum dose for the chemical phosphorus removal. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-30 Chapter 4: Cost Factors

Table 4-11. Process parameters for combined nitrogen and phosphorus removal 

scenarios 

Process Costed items 

HRT 

(hr) 

Alum dose 

(mg/L as Al) 

1 PID, clarifier, dual alum additions, tertiary 

clarifier, sand filter, blower 13 2.0; 1.0 

2 SBR, alum addition, sand filter, blower 15 (total cycle 

time) 2.08 

3 Nitrifying AS, clarifier, dual alum addition, 

denitrification filter, blower 11.8 2.6, 4.8 

4 5-stage Bardenpho, clarifier, alum addition, 

sand filter, blower 15.5 2.1 

Notes: 

AS = activated sludge 

PID = phased isolation ditch 

SBR = sequencing batch reactor 

Figures 4-21 through 4-23 are graphs of the O&M costs (in dollars per MG treated), capital 

costs (in dollars per gpd capacity), and life-cycle costs (in dollars per MG treated) for these 

nitrogen scenarios. The costs were estimated at 1 MGD, 5 MGD, and 10 MGD average 

influent flow. The life-cycle costs were determined by summing the O&M dollar per MG 

treated and the capital costs annualized to 20 years at 6 percent per MG treated. Figures 4-24 

and 4-25 present the percentage breakdown of the total O&M costs for the four technologies 

for 1 MGD and 10 MGD, respectively. 

From these figures, the following observations can be made: 

1. The lowest-cost option was the oxidation ditch, with the other technologies close in 

cost. 

2. In comparing the nitrification with dual alum additions and denitrifcation filter 

(which is very dependent on chemicals) with the 5-stage Bardenpho, the capital costs 

for the Bardenpho are greater than those for the nitrification system, but the operating 

costs are much lower, with the result that the total costs (as reflected in the life-cycle 

graph) are similar. 

3. Overall, if chemical phosphorus removal is to be used to attain 0.1 ppm or lower, 

either early chemical phosphorus removal or an efficient biological phosphorus 

removal system should be employed so that the dose in the final stage is reasonable. 

4. As Figures 4-24 and 4-25 indicate, the fraction of the total cost attached to labor 

expenses decreases with larger facilities, with the associated increases in the fraction 

of the cost attached to energy and other components. In particular, the 1 MGD labor 

costs range from 40 to 50 percent of the total for all technologies, while the 10 MGD 

labor costs range from 15 to 25 percent of the total. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-31

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

$3.50

$4.00

$4.50

$5.00

0 2 4 6 8 10 12

Flow (MGD)

Capital Cost ($/gpd capacity)

5Stage Barden- 3 NO3 0.1 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P PID w chemical + tert clar + tert filt SBR w chem+filt

Figure 4-21. Capital costs for expansion nitrogen plus phosphorus removal technologies. 

$0.00

$200.00

$400.00

$600.00

$800.00

$1,000.00

$1,200.00

$1,400.00

0 2 4 6 810 12

Flow (MGD)

O&M Costs ($/MG treated)

5Stage Barden- 3 NO3 0.1 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P PID w chemical + tert clar + tert filt SBR w chem+filt

Figure 4-22. O&M costs for expansion nitrogen plus phosphorus removal technologies. 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-32 Chapter 4: Cost Factors

$0.00

$500.00

$1,000.00

$1,500.00

$2,000.00

$2,500.00

0 2 4 6 810 12

Flow (MGD)

Life-Cycle Costs ($/MG treated)

5Stage Barden- 3 NO3 0.1 P Nitrification/Chem P/Denite Filter--3 NO3/0.1 P PID w chemical + tert clar + tert filt SBR w chem+filt

Figure 4-23. Life-cycle costs for expansion nitrogen plus phosphorus removal technologies. 

1 MGD N+P Technologies

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Nitrification/Chem

P/Denite Filter--3

NO3/0.1 P

5-Stage

Barden/Chem P--3

NO3/0.1 P

PID with chemical +

tert clar + tert filter

SBR w chem + filt

Energy

Chemical

Materials

Labor

Figure 4-24. Component percentages of total O&M costs for 1 MGD expansion nitrogen plus 

phosphorus removal technologies. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-33

10 MGD N+P

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Nitrification/Chem

P/Denite Filter--3

NO3/0.1 P

5-Stage Barden w

Chem P---3

NO3/0.1 P

PID w chemical +

tert clar + tert filt

SBR w chem + filt

Energy

Chemical

Materials

Labor

Figure 4-25. Component percentages of total O&M costs for 10 MGD expansion nitrogen plus 

phosphorus removal technologies. 

4.4 Discussion of Cost Factors 

For a project team trying to estimate costs for upgrading existing facilities, the first step is to 

obtain site-specific data. On the basis of the data, the project team can determine what 

modifications are available and their associated costs. 

The cost estimates given in this chapter are for only the technologies directly involved in 

nutrient removal; however, an overall program to upgrade a plant to allow for nutrient 

removal will likely include other costs not reflected in this analysis. The ability of a plant to 

adequately handle nutrients will be enhanced by flow equalization, proper management of 

recycle flows, wise selection of sludge-handling processes, and allowance for variable 

conditions in the influent wastewater and the weather. To the extent that these considerations 

are included in a plant’s process design, the overall costs could be substantially affected. 

However, most of these plant-wide processes will be the same for all nutrient removal 

technologies, so inclusion is not an issue for planning-level process selection. 

In addition, site-specific sludge-handling practices affect the overall costs. Decisions about 

sludge handling could be made on the basis of more than the process upgrades for nutrient 

removal in some instances, or they could be a part of the nutrient removal project. In 

particular, the cost of thickening, stabilization (digestion), and dewatering might favor one 

alternative over another. Aerobic digestion of secondary sludge would be desired for the 
Municipal Nutrient Removal Technologies Reference Document September 2008 

4-34 Chapter 4: Cost Factors

biological phosphorus removal alternative, whereas it would not be critical for the chemical 

phosphorus removal alternative. Anaerobic digestion might not be favored for nitrogen 

removal if another alternative exists. 

O&M costs are dependent on the nutrient removal technology selected, the flexibility of the 

operating mode, and whether the systems for controlling power and chemical feed are 

automated or manual. Operating the exact number of aerobic zones at optimal dissolved 

oxygen control levels would be critical in power management. Chemical dosage for methanol 

or coagulating chemical is a key factor at such facilities. The labor cost included in this 

section uses the national average rate of $25 per hour. Regional labor cost variations can be 

expected. 

As shown in Figures 4-13, 4-14, 4-18, 4-19, 4-24, and 4-25, the distribution of the O&M 

costs among the various components will vary depending on facility size, in addition to 

natural variations based on the selected technology, the characteristics of the influent 

wastewater, and the required level of treatment. In general, smaller facilities will have a 

larger fraction of the total O&M costs devoted to labor compared to that fraction for larger 

facilities. This is because, essentially, a 10-MGD facility will not require 10 times the 

staffing of a 1-MGD facility but will require 10 times the energy and chemical expenditure. 

Thus, the overall fraction devoted to staffing decreases, and the relative fraction devoted to 

other components increases. For the technologies examined in this study, the labor costs for 

1-MGD facilities are generally around 40 to 50 percent of the total, while the costs for 10-

MGD facilities are generally around 15 to 25 percent of the total. These are general trends, 

and local conditions could greatly affect the final fractions. 

4.5 Summary 

In upgrading existing facilities, readers are encouraged to evaluate all feasible alternatives 

and determine the best plan in accordance with the success criteria established early in the 

planning phase. Key factors in the decisionmaking include the proposed permit limits, the 

availability of an existing facility for modification, site requirements, costs in capital and 

O&M, sustainability in energy, chemical and sludge management, and potential odor or other 

environmental factors. For analyzing costs, the reader can examine what has happened at 

potentially similar facilities, as described in Section 4.1. From the case studies developed for 

this manual, as well as surveys carried out in Maryland and Connecticut, it was found that 

costs could vary greatly, depending on the nature of the existing facility, the required 

upgrade, and other site-specific factors. The costs for modification ranged from a low of 

$0.20 to a high of $5.25 per gpd capacity. 

Planning-level capital, O&M, and life-cycle cost curves for 12 retrofit and 20 expansion 

alternatives were developed using CAPDETWorks. These curves provide an estimate of the 

costs; exact costs will likely be different because of local conditions. In general, unit costs 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 4: Cost Factors 4-35

are higher for smaller facilities than for larger facilities because of economies of scale. The 

relative fraction of the O&M costs for the expansion alternatives were also determined for 

1-and 10-MGD plants. In general, labor represents 40 to 50 percent of the O&M costs for 

1-MGD facilities and 15 to 25 percent of the O&M costs for 10-MGD facilities. 

The cost curves were developed so that project teams can directly compare costs of selected 

alternatives. Project teams following the selection matrix process described in Chapter 5 can 

use the curves for capital, O&M, or life-cycle costs in their evaluation of alternatives. 

4.6 References 

CTDEP (Connecticut Department of Environmental Protection). 2006. Annual Report of the 

Nitrogen Credit Advisory Board, 2006 

Hydromantis Corporation. 2004. CAPDETWorks User’s Manual, v 2.1. Hydramantis 

Corporation, Hamilton, Ontario. 

MDE (Maryland Department of the Environment). 2004. Refinement of Nitrogen Removal 

from Municipal Wastewater Treatment Plants. Maryland Department of the 

Environment, Baltimore, MD. 

Stephenson, R., and J. Mohr. 2005. Nutrient Removal in an Uncertain Regulatory 

Environment. In Proceedings of the Water Environment Federation’s 78th Annual 

Technical and Educational Conference, Washington, DC, October 29–November 2, 

2005, pp. 518–532. 

USDA (U.S. Department of Agriculture). 2007. Price Indexes and Discount Rates. 

U.S. Department of Agriculture, Natural Resources Conservation Service. 

http://www.economics.nrcs.usda.gov/cost/priceindexes/index.html. Accessed May 15, 

2007. 

Wright, D.G., G.G. Patry, C.E. Letman, and D.R. Woods. 1988. A Procedures for Estimating 

the Capital Cost of Ontario Wastewater Treatment Plant Using CAPDET. Canadian 

Journal of Civil Engineering 15:799–806. 

September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 5: Upgrading Existing Facilities 5-1

CHAPTER 5: Upgrading Existing Facilities 

The topic of upgrading existing facilities has become increasingly important because many 

publicly owned treatment works (POTWs) in the United States are considering the addition 

of nutrient removal technologies. This chapter presents important issues to be considered in 

upgrading existing facilities, including evaluation of available technologies, general selection 

factors, design and operational factors, and finally selection of the appropriate technology 

using a decision matrix methodology. 

5.1 General Approach to Upgrading 

In addition to the important topics presented in this chapter for assessing appropriate nutrient 

removal technologies, readers are encouraged to review relevant technical references on the 

subject of upgrading wastewater treatment plants, such as WEF Manual of Practice No. 28, 

Upgrading and Retrofitting Water and Wastewater Treatment Plants (WEF 2004); EPA’s 

Handbook: Retrofitting POTWs for Phosphorus Removal in the Chesapeake Bay Drainage 

Basin (1987); EPA’s Nitrogen Control Manual (1993) and, specifically on nutrient removal, 

WEF Manual of Practice No. 30, Biological Nutrient Removal Operation in Wastewater 

Treatment Plants (WEF 2005). 

5.1.1 Success Criteria 

One of the key tasks of the project team evaluating a facility upgrade is to establish success 

criteria for the project early in the planning stage. In doing so, the team should seek input 

from all stakeholders in the permittee’s organization. The stakeholders might include elected 

officials; utility administrators; and representatives from operation, maintenance, and 

technical services at the facility. They might also include representatives from environmental 

organizations, citizens groups, and the public. The selection criteria might include 

sustainability, cost-effectiveness, ease of operation and maintenance (O&M), project 

schedule, and site requirements. Sustainability implies a simultaneous focus on economic, 

social, and environmental performance. A technology selected on the basis of sustainability 

has reliably lower energy use, lower production of sludge, lower use of added chemicals, and 

generally lower use of resources compared to other technologies. Another selection criterion 

to be considered is the potential to use the treated wastewater for reuse applications. For 

nutrient removal technologies, selection considerations include energy usage, chemical 

usage, and generation of biosolids. The project team can establish a rating formula by which 

each topic is assigned a weighting factor. A final score can thereby be obtained for each 

alternative evaluated.
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5-2 Chapter 5: Upgrading Existing Facilities 

5.1.2 Facility Planning 

The project team has three main tasks to carry out: 

1. Assess the existing facility and its ability to meet permit requirements. 

2. Identify needed upgrades. 

3. Develop and evaluate feasible alternatives. 

Choices in upgrading the plant might include modifying existing facilities, adding new 

processes in parallel or in series, or replacing existing processes with new ones. Critical 

issues to be considered by the project team include agreement on design loadings for the 

planning period based on mass balance and energy balances (where applicable), the degree of 

flexibility for future uncertainties in regulation or influent wastewater characteristics (or 

both), operation during construction, interim permit compliance, and the phasing of the 

construction in the future. Related project issues include the schedule, safety, site 

requirements, the potential for odor, and the costs of the alternatives. 

The project team typically presents a recommended implementation plan, from previously 

agreed-upon selection criteria, to the approval authority. The team can invite the public to 

participate in the evaluation. 

5.2 Available Technologies 

WEF Manual of Practice No. 8, Design of Municipal Wastewater Treatment Plants (WEF 

and ASCE 1988), Volume II, Chapter 15, defines an integrated process as a method that 

combines biological and chemical or physical unit operations and processes to reduce 

concentrations of nitrogen and phosphorus in plant effluent to below levels that would be 

attainable solely by synthesis in a typical secondary treatment facility. The manual, referred 

to as MOP 8, includes design guidelines. It also identifies inherent limitations in biological 

processes, which stem from variables such as influent wastewater characteristics, methods of 

solids handling, and the biological population dynamics of mixed cultures within a given 

treatment facility. For example, an insufficient supply of volatile fatty acids (VFAs) during 

the year can decrease denitrification and phosphorus removal. 

Decreased VFAs are expected with cold wastewater temperatures and wet-weather events. 

Nutrients, particularly ammonia and phosphorus, can be released because of prolonged 

sludge storage and digestion. High nutrient concentrations can also be found in return flows 

from the other sludge-handling processes. Secondary release of nutrients in secondary 

clarifiers and tertiary filters can occur, and the nutrients are capable of overloading the 

biological process when returned upstream of the sensitive unit process. Upsets from slug 

discharges from industries can also negatively affect the treatment plant by organically or 

hydraulically overloading the biological process or introducing pollutants that are toxic to the 
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Chapter 5: Upgrading Existing Facilities 5-3 

microorganisms. Because of these biological limitations, the biological system often needs to 

be supplemented by a physical process, such as final filtration, to remove additional 

phosphorus and nitrogen-bearing solids; chemical addition, to precipitate additional 

phosphorus; or an external carbon source for denitrification. Chemical coagulants and 

flocculants are added to wastewater to remove phosphorus separately or concurrently with 

nitrogen removal. 

The project team can consider technologies on the basis of target concentrations, as shown in 

Tables 5-1 through 5-4, which are adapted from similar tables in Chapter 2. These tables 

show the levels of treatment possible by using various technologies for the removal of total 

nitrogen (TN), total phosphorus (TP), or both. While the lists are reasonably comprehensive, 

a number of technologies, particularly emerging technologies, are not included in the tables. 

Such technologies might be available and appropriate for a given application and should 

therefore be considered, where applicable. 

Table 5-1. Process list: TP 

Concentration Technology 

Up to 1 mg/L All 

1.0 to 0.5 mg/L Chemical precipitation 

A/O; filter preferred 

 5-stage Bardenpho 

PhoStrip; filter preferred 

SBR 

0.1 to 0.5 mg/L Chemical precipitation with filter 

3-stage Westbank with filter 

Chemical precipitation with tertiary clarifier and filter 

Modified UCT with filter 

PID with filter 

5-stage Bardenpho with chemical and filter 

Step-feed AS with filter 

0.1 mg/L and below Membrane filter 

High-performance filter: 

Trident 

Dynasand D2 Advanced Filtration System 

Blue PRO 

 CoMag 

Land application/infiltration bed 

Notes: 

A/O = anoxic/oxic 

AS = activated sludge 

PID = phased isolation ditch 

SBR = sequencing batch reactor 

UCT = University of Cape Town process 

“filter” = conventional filter, such as a sand filter, deep bed anthracite filter, dual-media filter, or one of the other traditional filters 

described in Chapter 2 
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5-4 Chapter 5: Upgrading Existing Facilities 

Table 5-2. Process list: TN 

Concentration Technology 

 8 mg/L or higher All; no filter required 

 3 to 8 mg/L A2O; filter preferred 

 MLE 

 SBR 

 Cyclic activated sludge 

 BAF 

 IFAS 

 MBBR 

 3-stage Westbank 

 4-stage Bardenpho 

 Post-aeration anoxic with methanol (Blue Plains process) 

3 mg/L and below 4-stage Bardenpho; filter required 

 PID with filter 

Denitrification AS with filter 

 Step-feed AS with filter 

 

Denitrification filter: 

Tetra filter 

Leopold filter 

 Biostyr filter 

Notes: 

A2

O = anaerobic/anaerobic/oxic 

AS = activated sludge 

BAF = biological aerated filter 

IFAS = integrated fixed-film activated sludge 

MBBR = moving-bed biofilm reactor 

MLE =modified Ludzack-Ettinger 

PID = phased isolation ditch 

“filter” = conventional filter, such as sand filter, deep bed anthracite filter, dual-media filter, or one of the other traditional filters 

described in Chapter 2 

Table 5-3. Process list: ammonia nitrogen 

Concentration Technology 

All Add aeration and clarifier 

 IFAS 

 MBBR 

 A/O 

 MLE 

Oxidation ditch 

 SBR 

 3-stage Westbank 

Notes: 

IFAS/MBBR = integrated fixed-film activated sludge/moving-bed biofilm reactor 

A /O = anoxic/oxic 

MLE = Modified Ludzack-Ettinger 

SBR = sequencing batch reactor 
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Chapter 5: Upgrading Existing Facilities 5-5 

Table 5-4. Process list: TN plus TP 

TN concentration TP concentration Technology 

3 to 8 mg/L > 1 mg/L SBR; filter preferred 

 Modified UCT 

 A/O 

 PhoStrip II 

 3-stage Westbank 

 Step-feed AS 

 PID 

 5-stage Bardenpho 

 Table 5-2 alternatives for TN 3 to 8 mg/L plus 

chemical phosphorus removal 

3 to 8 mg/L 0.5 to 1 mg/L SBR; filter required 

 Modified UCT 

 A/O 

 PhoStrip II 

 3-stage Westbank 

 Step-feed AS 

 PID 

 5-stage Bardenpho 

 Table 5-2 alternatives for TN 3 to 8 mg/L plus 

chemical phosphorus removal 

3 mg/L or less 0.1 to 0.5 mg/L Step-feed AS; filter required 

 5-stage Bardenpho; filter required 

 PID; filter required 

 Denitrification filter 

 Table 5-2 alternatives for TN 3 mg/L or below plus 

chemical phosphorus removal and filtration 

3 mg/L or less 0.1 mg/L or less Step-feed AS + tertiary clarifier with chemical and 

filter 

 

Denitrification filter with chemical and additional filter 

Ammonia removal process 

Phospshorus process 

Upstream process 

 MBR with chemical 

 Land application from above three 

Notes: 

A/O = anoxic/oxic

AS = activated sludge 

PID = phased isolation ditch 

MBR = membrane bioreactor 

SBR= sequencing batch reactor 

UCT = University of Cape Town process 

“filter” = conventional filter, such as a sand filter, deep bed anthracite filter, dual-media filter, or one of the other traditional 

filters described in Chapter 2 
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5-6 Chapter 5: Upgrading Existing Facilities 

After the project team identifies the processes that are capable of meeting the desired target 

levels for nitrogen, phosphorus, or both, they can review the descriptions of the processes 

and schematics in Chapter 2. Capital and O&M costs can be found in Chapter 4. Section 5.3, 

Technology Selection Criteria, and Section 5.4, Design and Operational Factors in Nutrient 

Removal, can then be reviewed. Using all this information and the individual conditions at 

the plant, the project team can narrow down the list of processes to be evaluated for 

implementation and then conduct an evaluation as described in Section 5.5, Finalizing 

Process Selection, to select the process that will be used to upgrade the plant. 

5.3 Technology Selection Criteria 

In developing technology alternatives, the project team should consider appropriate factors 

for design and operation. These include site constraints; reliability of the technology; capital 

and O&M costs; and sustainability, including energy usage, chemical usage, and additional 

sludge protection. A detailed discussion on reliability and contributing factors is presented in 

Chapter 2 of this document, and cost considerations are provided in Chapter 4. This chapter 

presents process selection factors for the project team to consider. 

Tables 5-5 and 5-6 contain factors that affect the selection of feasible technologies for 

nitrogen and phosphorus removal, respectively. Table 5-7 summarizes the selection factors 

for technologies that remove both TN and TP. These tables present key factors, arranged in a 

matrix that should be considered in selecting the best technology for a given location. The 

factors in the tables represent general trends. However, some of the factors might not be 

applicable in specific situations. For example, in warm-weather locations, a building might 

not be required, despite a Yes for that technology. 

For the same discharge limit, the constraints could favor one technology over another. Some 

of the processes can be used in series. For example, to achieve low TN levels, a 

denitrification filter might be used following the 4-stage Bardenpho process. Both the 4-stage 

Bardenpho and filtration lines would need to be reviewed in Table 5-5 to determine the site, 

wastewater, and operational factors that apply to these processes. Similarly, to achieve low 

TP effluent concentrations, a 5-stage Bardenpho process could be used in combination with 

chemical precipitation and filtration. All three lines for these processes would need to be 

reviewed in Table 5-6 to determine the applicable selection factor values. Descriptions of the 

columns under each factor category are provided below. 

The following three sections (5.3.1, 5.3.2, and 5.3.3) discuss selection factors to be 

considered for achieving nitrogen and phosphorus removal, as well as nitrogen plus 

phosphorus removal. 

• Footprint. The Footprint column refers to the relative amount of space that a process 

requires, which depends on the number of tanks and the required size of each process. 

General size indications of small, medium, and large are provided in the tables. 
Chapter 5: Upgrading Existing Facilities 5-7 Table 5-5. Technology selection matrix: nitrogen removal Site factors Wastewater factors Opera

Process Footprint Building needed Construction in existing aeration basin Piping and pumping Extra head needed Secondary process recycle streams Additional carbon source needed Extra electricity Denitrification filters Small Yes No Yes Yes No Yes Yes MBBR Medium No Maybe1 No No No Maybe No SBR/cyclic activated sludge Medium No Yes No Maybe No Maybe Yes MLE/3-stage Westbank Medium No Maybe Yes No Yes Maybe Yes Phased isolation ditch Large No Maybe No No No Maybe Yes 4-stage Bardenpho Large No Maybe Yes No Yes Maybe Yes A2O Medium No Maybe Yes No Yes Maybe Yes Anoxic zone following aeration(Blue Plains) 

Medium No Maybe No Maybe No Yes Yes 

Biological aerated filtration 

Medium Maybe No Yes Yes Yes Maybe Maybe 

Step-feed AS Large No Yes Yes No Yes No No IFAS Medium No Maybe1 No No Yes Maybe Maybe Notes: 1Installation of media retention screens, as needed. 

Additional carbon source and chemical needs: To obtain sufficient carbon for anoxic reaction, use either external source (methanol) or step-feed activated 

Construction needs: External filters require extra space and could require a building depending on conditions. 

Adjustments in basins include walls to section off anoxic basins and, potentially, piping to accommodate step-feed activated sludge. Additional sludge generation is partially dependent on the need for addition of a carbon source. Selection factor designations are general guidelines and may not apply for all site-specific conditions.
5-8 Chapter 5: Upgrading Existing Facilities Table 5-6. Technology selection matrix: phosphorus removal Site factors Wastewater factors Ope

Process Footprint Building needed Construction in existing aeration basin Piping and pumping Extra head needed Secondary process recycle streams Additional carbon source needed Extra electricity C A/O Medium No Yes No No No Maybe Yes MSBR/CAS Medium No Yes No Maybe No No Yes VIP/modified UCT Medium No Maybe Yes No Yes Maybe Yes MChemical precipitation Small Yes No Yes No No No Minimal Tertiary clarifier/tertiary filter Medium Maybe No Maybe Yes Yes No Yes PhoStrip Small No No Yes No Yes No Yes 5-stage Bardenpho Large No Maybe Yes No Yes Maybe Yes M3-stage Westbank Medium No Maybe Yes No Yes Maybe Yes MEBPR with VFA addition and filters Medium Maybe (Filters) Maybe Yes Yes Yes Yes Yes YMBR Small Yes Yes Yes Yes No Maybe Yes MPhased isolation ditch Large No Maybe No No No No Yes Membrane filter Small Maybe No Yes Yes Yes No Yes CoMag Medium Maybe Maybe Yes Yes Yes No Yes Infiltration basin Large No No No Yes No No Minimal Notes: CAS = cyclic activated sludge; VIP = Virginia Initiative process; EBPR = enhanced biological phosphorus removal; VFA = volatile fatty acids See “Tertiary clarifier/tertiary filter” row for information that applies to specialty filters. Selection factor designations are general guidelines and may not apply for all site-specific conditions.
Chapter 5: Upgrading Existing Facilities 5-9 Table 5-7. Technology selection matrix: nitrogen and phosphorus removal Site factors Wastewater factors Oper

Process Footprint Building needed Construction in existing aeration basin Piping and pumping Extra head needed Secondary process recycle streams Additional carbon source needed Extra electricity 

A

Step-feed with selector Medium No Yes Yes No No No No IFAS with selector Medium No Maybe1 No No Maybe Maybe Maybe MA2O Medium No Yes Yes No Yes Maybe Yes MSBR/CAS Medium No Yes No Maybe No Maybe Maybe MVIP/modified UCT Medium No Maybe Yes No Yes Maybe Yes MPhostrip II Medium No No Yes No Yes No Yes 3-stage Westbank Medium No Maybe Yes No Yes Maybe Yes M5-stage Bardenpho Large No Maybe Yes No Yes Maybe Yes MEBPR with VFA addition Medium No Maybe Yes Maybe No Yes Yes YMBR Small Yes Yes Yes Yes Yes Maybe Yes MChemical precipitation Small Yes No Yes No No No Minimal Filtration Medium Maybe No Maybe Yes Yes No Yes Denitrification filter Small Yes No Yes Yes No Yes Yes YPhased isolation ditch Large No Maybe No No No No Yes Notes: 1Installation of media retention screens, as needed. 

Additional carbon source: To obtain sufficient carbon for anoxic reaction, use external source (methanol) or step-feed activated sludge. 

Construction needs: Adjustments in basins include walls to section off anoxic basins and, potentially, piping to accommodate step-feed activated sludge.

Selection factor designations are general guidelines and may not apply for all site-specific conditions
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5-10 Chapter 5: Upgrading Existing Facilities 

• Building Needed. This column indicates whether the process should be placed in a 

building to prevent operational problems at low temperatures. An entry of Maybe in 

the tables indicates that for plants in the north (with low winter temperatures) the 

equipment probably needs to be placed in a building, while for plants in more 

moderate climates it might not. 

• Construction in Existing Aeration Basin. The existing aeration basin might require 

modifications (construction) to implement the process. Processes that are external to 

the secondary treatment process, such as filters, would not involve construction in the 

existing aeration basin. Several of the processes require a particular ratio of 

anaerobic, anoxic, and aerobic zones to function properly. Construction within the 

existing aeration basin would involve building walls, baffles, or both to create such 

zones. Replacement of the aeration devices with mixers would also be needed in the 

anaerobic and anoxic zones. A table entry of Maybe indicates that modifications 

might be needed, depending on the size of the existing aeration basins. For example, 

if there is space to create an anoxic zone within the existing aeration basin and 

sufficient volume remains to perform nitrification in the aerobic zone, the answer is 

Yes. Conversely, if insufficient volume remains for the aerobic zone, the anoxic zone 

would be constructed ahead of the existing aeration basin and minimal changes would 

be made, with the exception of allowing flow to enter the aeration basin from the 

newly constructed anoxic zone. 

• Piping and Pumping. The term piping and pumping refers to whether the process 

involves return lines, requires more than one feed line (in the case of step-feed 

applications), or requires additional pumping equipment, or both. In the case of 

processes that use filters (for nitrogen or phosphorus removal), the column entry is 

Yes because the filter backwash is returned upstream of the secondary process. Piping 

and pumping are needed for all processes that involve new internal recycle lines. An 

entry of Maybe indicates that internal recycles might be needed, depending on the 

existing process. For example, an existing modified Ludzack-Ettinger (MLE) process 

that is being upgraded to a 4-stage Bardenpho might not need a new internal recycle, 

depending on the existing capacity, whereas an existing extended-aeration plant 

would require the installation of a new internal recycle line. 

• Extra Head Needed. The hydraulics for the proposed process should always be 

reviewed to verify that the wastewater flows through the plant as designed. Certain 

processes require additional head, which likely needs to be provided by pumps, for 

the wastewater to enter and pass through the process, and therefore additional land is 

required. Processes that require additional biological tanks likely require little 

additional head, whereas add-on processes might require pumping. 

• Secondary Process Recycle Streams. This column identifies the processes that have 

internal recycle lines that require additional land. To achieve low effluent nitrogen 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 5: Upgrading Existing Facilities 5-11 

limits, internal recycle lines are usually required to denitrify the nitrates created 

during nitrification. Phosphorus removal technologies, unless paired with nitrogen 

removal processes, usually have only a return activated sludge (RAS) line. 

• Additional Carbon Source Needed. Some processes require an additional carbon 

source to function. For example, denitrification filters require methanol or an 

alternative carbon source, such as MicroC, for the process to function correctly 

because most of the biochemical oxygen demand (BOD) has been removed from the 

wastewater after secondary treatment. Adequate carbon is needed for biological 

nutrient removal (BNR) (USEPA 1993). A table entry of Maybe indicates that 

whether additional carbon is needed depends on the available carbon source (in the 

form of BOD or VFAs) in the influent to the process at the plant where the upgrades 

are being evaluated. Some plants might require additional carbon sources, whereas 

others might not. 

• Additional Electricity. An increase in electricity usage is expected in cases where the 

upgrade requires the addition of pumps because of increased hydraulic head, 

additional reactor volume (to aerate or provide mixing), or new or increased return 

rates. A table entry of Maybe indicates that electricity usage depends on the existing 

process at the plant and the type of upgrade being evaluated. 

• Chemicals Needed. This column is included under Operation Factors to emphasize 

the cost associated with chemical addition. If VFAs are generated by using an on-site 

fermenter, no additional chemical cost would be expected at the plant, although 

electricity usage might increase somewhat because of the power required to operate 

the mixer. Other chemicals that might need to be added include caustic soda or lime 

for alkalinity control and metal salts, such as alum or ferric chloride, for phosphorus 

removal.

• Additional Sludge. Additional sludge generation is typically associated with chemical 

addition. If the process or the site-specific conditions at the plant being retrofitted 

require chemical addition, additional sludge will be generated compared to the 

amount generated by the existing process. Plants that do not need chemical addition 

are unlikely to generate significant additional sludge, unless an expansion is being 

performed in conjunction with the plant upgrades. All additional sludges will 

typically incur additional disposal costs. 

5.3.1 Nitrogen Removal 

Site Factors 

Footprint 

The footprint of the selected process is important in any retrofit because space is typically 

limited. For retrofitting an existing activated-sludge system, an additional footprint could be 
Municipal Nutrient Removal Technologies Reference Document September 2008 

5-12 Chapter 5: Upgrading Existing Facilities 

avoided with the MLE process. MLE is a single-stage, two-basin system in which the anoxic 

zone precedes the aerobic zone, with recycling of aerobic mixed liquor to allow the nitrate 

arising in the aerobic zone to be denitrified. This configuration has the advantage of not 

requiring an additional carbon source for the anoxic process because the anoxic 

microorganisms are mixed with the full-strength waste from the primary clarifier. The 

disadvantages of the process are twofold: the overall nitrogen removal is limited by the 

mixed-liquor recycling rate, and the oxygen included in the recycle stream reduces 

denitrification (WEF 2006). Thus, MLE is suitable where the site is limited and the level of 

treatment required is in the mid-range between 7.5 and 8 mg/L for TN. 

The 3-stage Westbank process is similar to the 5-stage Bardenpho without the last two zones. 

In addition, the 3-stage Westbank process has the capability to feed a portion of the primary 

effluent to the head of the process, the anaerobic zone, and/or the anoxic zone. This step-feed 

approach results in a smaller anoxic zone compared to prior practice because of a steady 

supply of readily biodegradable carbon for denitrification. 

Denitrification filters provide the smallest footprint for a newly installed process, and they 

are a proven option for achieving low nitrogen concentrations. The technology involves 

passing secondary effluent through a deep-bed filter that contains denitrifying organisms. 

The process has the additional advantage of acting as a filter for removing suspended solids. 

Implementation requires capital expenditures for equipment, building, and costs associated 

with a pumping station where available hydraulic head is limited. In addition, denitrification 

filters require an external carbon source, typically methanol, which results in a significant 

chemical cost. The effluent TN concentrations can be below 3 mg/L; they are less than 2 

mg/L at some facilities. 

For ammonia removal, using an integrated fixed-film activated sludge (IFAS) technology or 

a moving-bed biofilm reactor (MBBR) technology offers a small footprint as a retrofit into 

aeration basins or as an add-on process at existing facilities (Copithorn 2007). 

Building Needed 

Most of the retrofit systems for nitrogen removal do not need additional buildings because 

the systems use existing aeration basins or construction of external anoxic zones. Filters, 

such as denitrifying filters and upflow biological aerated filters, could be housed in buildings 

in cold-weather locations. Certain manufacturers use biological upflow filters in integrated 

systems designed to achieve nitrification and denitrification in a single unit. In addition, 

upflow filters can be operated aerobically to achieve nitrification (WEF and ASCE 2006). 

Construction in Existing Aeration Basin 

Most of the retrofit technologies make use of existing aeration basins because of site 

limitations and the need to increase capacity in existing tanks. Where needed, partitions are 

installed to create zones, and screens are installed to hold microbial carrier media, if required. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 5: Upgrading Existing Facilities 5-13 

Such media are used in both the MBBR and IFAS systems (Johnson et al. 2005; McQuarrie 

et al. 2004). Example installations include denitrification by IFAS using sponge media in 

Fairfield, Connecticut; ringlace in Annapolis, Maryland; and BioWeb in Windsor Locks, 

Connecticut. Nitrification examples include Kaldnes media in Broomfield, Colorado, and 

Cheyenne, Wyoming (Loader 2007). Readers are encouraged to review the process 

descriptions for MBBR and IFAS in Chapter 2. 

The phased isolation ditch (PID) is appropriate only if an oxidation ditch is already present 

but would require construction in the existing aeration basin. Filtration units, such as 

denitrification filters and biological aerated filters, are usually not constructed in existing 

aeration basins. The MLE, 3-stage Westbank, and 4-stage Bardenpho processes would likely 

require construction in the aeration basin to install baffle walls and internal recycle lines. If 

sufficient volume is available, anoxic and anaerobic zones (in the case of the 3-stage 

Westbank and 4-stage Bardenpho process) could be constructed in the existing aerobic tank. 

The step-feed activated-sludge system would require constructing anoxic basins and feed 

points within the aeration basin. If sufficient volume is not available, some of the alternating 

anoxic and aerobic basins could be constructed outside the existing aeration basin. 

Piping and Pumping/Extra Head Needed/Secondary Process Recycle Streams 

Most of the nitrogen removal technologies involve secondary recycles. Exceptions include 

the MBBR, denitrifying filters, sequencing batch reactor (SBR), cyclic activated sludge 

(CAS), anoxic zone following aeration, and PID. The piping and pump size for the secondary 

recycle depend on the design flow, but they must be sufficient to maintain the needed 

nutrient concentrations in each zone so that nitrate is removed to the target level (WEF and 

ASCE 2006). The step-feed activated-sludge and 3-stage Westbank processes would also 

require piping for the multiple feed points to the anoxic zones. 

Pumping would be required in plants with denitrifying filters because sufficient head 

following the aeration basins is not usually available. The filters could be constructed as a 

new add-on process such as the Tetra Filter or Leopold filter or as a retrofit of existing 

granular media filters into biological anoxic filters (BAFs). 

Wastewater Factors 

Additional Carbon Source Needed 

With the exception of the step-feed activated-sludge process, any of the biological nitrogen 

removal processes could need additional carbon, depending on the primary effluent 

characteristics at the specific plant. The carbon source could be methanol or a VFA source 

inside or outside the plant. VFAs can be added directly in the form of acetic acid, brewery 

waste, molasses, or other sugar forms. A fermenter could also be used to generate VFAs onsite from primary sludge, RAS, or the supernatant from the anaerobic digesters. Additional 

discussion is presented in Section 5.4 of this chapter, Design and Operational Factors in 
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5-14 Chapter 5: Upgrading Existing Facilities 

Biological Nutrient Removal. Methanol or MicroC would need to be added for the 

denitrification filter to function properly. 

Operational Factors 

Additional Electricity 

If internal recycle lines are not included in the existing process, electricity costs would likely 

increase when implementing biological nitrogen removal. Electricity costs would also 

increase with the use of a denitrification filter or biological aerated filter because of the 

pumping that would be required. 

Chemicals Needed 

All TN removal applications require adequate influent alkalinity concentrations for 

nitrification to occur. The denitrification process restores some of the alkalinity consumed 

during nitrification, but there is a net loss of alkalinity through the TN removal process. In 

soft-water regions, nitrification consumes alkalinity, lowers pH, and impairs biological 

processes. Adequate alkalinity is maintained by adding lime or caustic when influent 

alkalinity levels are low. Maintaining a target pH between 6.2 and 6.5 standard units (s.u.) 

will minimize the cost of adding lime or caustic soda in nitrifying facilities. Chapter 2 

presents more details on alkalinity. 

As mentioned previously, a carbon source in the form of methanol or VFAs might also be 

required for biological nitrogen removal. The filter options would also require the addition of 

methanol or an alternative carbon source for nitrogen removal.

Additional Sludge 

Upgrades that require the addition of methanol would produce additional sludge compared to 

the existing process. If chemical addition is not needed, the sludge yield would not be 

expected to change significantly. 

5.3.2 Phosphorus Removal Plant Factors 

Site Factors 

Footprint 

Chemical addition of metal salts, such as ferric chloride or alum, requires a minimal footprint 

for installing chemical feed pumps and a storage tank. The capital cost is low, but O&M 

costs are high because of ongoing chemical costs. Chemical feed doses increase 

exponentially as the phosphorus effluent goal approaches 0 mg/L; the feed dose curve is 

particularly steep in the range of 0 to 0.3 mg/L. The minimum phosphorus concentration that 

can be achieved economically through chemical addition is dependent on the wastewater 

characteristics and the effectiveness of the clarifiers. Polymer addition is practiced in many 

plants to aid settling, with minimal site requirements. Chemical addition can increase the 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Chapter 5: Upgrading Existing Facilities 5-15 

effectiveness of other phosphorus removal technologies, such as tertiary clarifiers and filters. 

Therefore, chemical addition is rarely used as the sole phosphorus removal technology to 

achieve low-level phosphorus limits (less than 0.2 mg/L). 

Membrane bioreactors (MBRs) have a relatively small footprint because of the higher mixedliquor suspended solids (MLSS) concentration in the aeration basin when compared to 

conventional plants that rely on clarifiers to remove solids. The pores in the membranes form 

a physical barrier to particles/floc that is larger than the pore size (and any biological growth 

that forms on the membrane). Clarifiers require floc of sufficient size and density to settle so 

the floc can be removed from the flow stream. The higher MLSS concentration in the MBR 

system allows more biomass to be treated in a smaller tank. 

The SBR and CAS systems have smaller footprints than the other activated-sludge 

alternatives because clarifiers are not needed to settle the mixed liquor. The settling occurs in 

the same tank as the reaction phase. 

The anaerobic/oxic (A/O) process consists of three anaerobic cells upstream of an aerated 

zone. This process might require little additional footprint in a retrofit application if sufficient 

volume capacity is available in the existing aeration basin. Otherwise, additional tankage 

would need to be constructed for the anaerobic cells. A fermenter might be required if 

additional VFAs are needed to meet biological phosphorus removal requirements. 

Alternatives that include the 5-stage Bardenpho process would require a large footprint 

because two anoxic zones would be needed in addition to the anaerobic zone. The 5-stage 

Bardenpho process is usually geared more toward removal of both phosphorus and nitrogen, 

rather than phosphorus removal alone. However, the process denitrifies the nitrates produced 

during nitrification, reducing the amount of nitrates in the RAS, which increases the 

effectiveness of the anaerobic zone. The 5-stage Bardenpho process with chemical addition 

and filtration would require a very large footprint. The PID also has a relatively large 

footprint because two oxidation ditch tanks would need to be constructed. The infiltration 

bed would require the largest footprint among all the technologies, but it also provides the 

highest level of removal with low O&M costs. 

Buildings 

Buildings are usually required for chemical feed equipment and filtration processes in areas 

of the country that experience low winter temperatures. The chemical needs to be protected 

to avoid freezing, which can decrease the efficiency or make feeding the chemicals difficult. 

Blowers that provide air to the aeration tanks would need to be installed in a building for ease 

of maintenance as well as for noise control. The fermenter, anaerobic, anoxic, and aerobic 

tanks typically would not need to be covered or constructed inside a building. 
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5-16 Chapter 5: Upgrading Existing Facilities 

Construction within Aeration Basins 

Anaerobic zones could be constructed in existing aeration basins if sufficient capacity would 

remain for the removal of BOD and ammonia nitrogen (if needed). SBRs are usually 

constructed at new plants rather than as a retrofit alternative because the tank is usually 

deeper than a typical aeration basin and extensive piping changes would be required. 

Chemical feed equipment, fermenters, and filters would all be constructed outside the 

aeration basin. Piping to connect any of the chemical feed pumps, fermenter tank, or filter 

backwash system to the aeration basins would be needed, but construction within the aeration 

basins would be minimal. The PhoStrip process treats a portion of the settled mixed liquor 

from the secondary clarifier. Therefore, construction within the aeration basin would not be 

necessary, but modifications to the RAS system would be needed. 

Conversion to an MBR system could involve significant construction within the aeration 

basins. In addition to installing the membranes, a portion of the aeration basin might be 

converted to anaerobic and anoxic zones to promote biological phosphorus removal and 

alkalinity recovery. If sufficient aerobic capacity is not available at the existing plant, the 

anaerobic and anoxic zones, as well as the membrane tank, could be constructed outside the 

aeration basins. 

Piping and Pumping/Extra Head/Secondary Recycle Streams 

The 5-stage Bardenpho process, MBRs, and filters all have return streams. Internal return 

streams are part of the 5-stage Bardenpho and MBR processes, and backwash is returned 

during cleaning of the filters. Filter backwash can be returned to the head of the plant or to 

the secondary process. Phosphorus concentrations in the filter backwash could be high. 

Returning the backwash to an equalization basin to allow the flow to be slowly blended with 

the raw influent might improve the phosphorus removal efficiency by not introducing a slug 

of phosphorus all at once. Alternatively, sidestream treatment of the filter backwash, which 

could involve settling the solids before returning the flow, could also improve phosphorus 

removal at the plant. 

Under most circumstances MBRs require additional head to draw wastewater through the 

membranes at the end of the process. Certain manufacturers might be able to provide a 

gravity-fed system in special situations, depending on the hydraulics of the rest of the plant. 

Pumping is generally required at plants that select filtration to achieve phosphorus removal. 

Wastewater Factors 

Additional Carbon Source Needed 

In biological phosphorus removal, a sufficient concentration of VFAs is required. VFAs are 

present in sufficient quantity in the wastewater in many locations but need to be 

supplemented in other locations. Where needed, VFAs can be added directly from an 
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Chapter 5: Upgrading Existing Facilities 5-17 

external source in the form of acetic acid, brewery waste, molasses, or other sugar forms. A 

fermenter could also be installed to generate VFAs on-site from primary sludge, RAS, or the 

supernatant from the anaerobic digesters. Carbon sources would not be necessary for process 

options that rely on chemical phosphorus removal or filtration, unless they are used in 

conjunction with biological phosphorus removal. Additional discussion is presented in 

Section 5.4 of this chapter, Design and Operational Factors in Biological Nutrient Removal. 

Operational Factors 

Additional Electricity 

Anaerobic zones and fermenters require mixers that use a relatively small amount of 

electricity. Electricity usage increases would be expected with processes that have an internal 

recycle, such as the Virginia Initiative process (VIP) and 5-stage Bardenpho, whereas a 

significant decrease would be expected from a reduced aeration volume in the tank. The 

MBR process requires additional electricity for aeration and pumping. Chemical phosphorus 

removal would require the addition of pumps; however, the pumps would be relatively small, 

and electricity usage would be expected to be minimal. 

Chemicals Needed 

Phosphorus removal technologies could require adding metal salts like alum or ferric 

chloride, particularly in plants required to meet low phosphorus limits. Because of the 

possibility of an upset in the biological phosphorus process, many plants incorporate 

chemical phosphorus removal as a back-up system to ensure that permit limits can be met. At 

facilities that must meet an extremely low phosphorus limit, two or more feed points for 

coagulants and polymer addition would be expected. 

The PhoStrip process treats only a portion of the RAS. Following the anaerobic tank, 

chemical precipitation, using lime, removes the phosphorus from the stripper overflow before 

returning the rest of the flow to the primary clarifier. Because only a portion of the 

wastewater is treated through the PhoStrip process, the amount of chemical needed is 

significantly less than what would be required in a secondary treatment process.

Additional Sludge 

Upgrades that incorporate chemical phosphorus removal would be expected to generate 

additional sludge, especially at facilities that are required to feed a high dose of chemicals to 

reach a low level of phosphorus, such as below 0.5 mg/L and especially below 0.2 mg/L. 

Biological phosphorus removal, however, would not generate additional sludge. In some 

circumstances, less sludge might be generated if the coagulant was fed to the primary 

clarifiers, thereby reducing the BOD and total suspended solids (TSS) loadings to the 

secondary process. 
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5-18 Chapter 5: Upgrading Existing Facilities 

5.3.3 Nitrogen and Phosphorus Removal Plant Factors 

Most of the processes listed in Table 5-7 were included in Table 5-5 or 5-6 but without 

certain minor modifications (e.g., step-feed and IFAS include selectors in Table 5-7, but not 

those in Table 5-5). The selector would consist of an anaerobic tank to promote biological 

phosphorus removal. As with the A/O process, the anaerobic tank could be constructed in the 

aeration tank if sufficient capacity was available to accommodate the anoxic and aerobic 

zones needed for treatment. No building would be required for the anaerobic tank. No 

internal recycles are associated with the selector beyond the RAS line. 

The PhoStrip II process is similar to the PhoStrip process, except a pre-stripper tank is 

upstream of the stripper tank to remove nitrates before they enter the anaerobic zone. 

As noted in Table 5-4, chemical precipitation, tertiary filtration, or both could be used to 

provide phosphorus removal at the end of any of the processes designed solely for nitrogen 

removal. Therefore, readers should consult Table 5-5 in conjunction with Table 5-7 to 

identify other feasible upgrade alternatives for plants attempting to achieve both nitrogen and 

phosphorus removal. Adding chemical and physical phosphorus removal technologies, as 

opposed to biological removal, might be particularly attractive at a plant that already is 

removing nitrogen. 

5.4 Design and Operational Factors in Nutrient Removal 

The following factors are critical in assessing existing facilities and developing feasible 

alternatives: 

• Influent wastewater characteristics 

• Sources of biodegradable carbon 

• Impact of wet-weather flows 

• Managing sludge-handling processes 

• Recycle flows 

• Supervisory control and data acquisition (SCADA) requirements and sensors 

• Staffing requirements 

• Training needs 

• Pilot testing 

The project team members are encouraged to consider all these factors and incorporate them 

in developing feasible technology alternatives. 
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Chapter 5: Upgrading Existing Facilities 5-19 

Influent wastewater characteristics are important to consider because the technology being 

considered might not operate properly if the plant cannot provide the conditions required. For 

example, if the BOD-to-TP ratio is too low in the primary effluent, biological phosphorus 

removal will not occur consistently at the plant without a supplemental carbon source. For 

plants that need to increase biodegradable carbon, considering possible sources is important. 

Such sources could include chemical addition or in-plant generation by constructing 

fermenters. 

Permit limits for nitrogen, phosphorus, or both must be met during dry- and wet-weather 

conditions. Wet-weather flows can be significantly higher than average dry-weather 

conditions for plants with combined collection systems or separated sewers that have high 

rates of inflow and infiltration. Biological systems can lose solids during elevated flow 

periods at the plant. Biological processes rely on sufficient biomass to remove nitrogen, 

phosphorus, or both. Chemical and physical processes are also less efficient under elevated 

hydraulic conditions. Considering alternative operation strategies will minimize the 

possibility of exceeding permit limits because of wet-weather events. For example, the North 

Cary Water Reclamation Facility in North Cary, North Carolina, was able to comply with all 

permit limits when 8 inches of rain fell during Tropical Storm Alberto in 2006 by diverting 

water to equalization and operating the BioDenipho process in the storm mode. For more 

information on this plant, see the North Cary Case Study in Volume II, Appendix A. 

The return flows from sludge handling can contain significant amounts of nitrogen and 

phosphorus, which can organically overload the removal process and cause poor performance 

and the possibility that the plant’s permit limits will be exceeded. Reviewing methods to 

minimize or treat the return of nitrogen and phosphorus and incorporating them into the plant 

design will improve the operation of the selected process. In addition, recycle flows from 

other processes, such as internal recycles, RAS, and filter backwash, can also affect the 

nitrogen and phosphorus removal process, particularly if biological removal has been 

selected. For example, high concentrations of dissolved oxygen in the RAS can negatively 

affect anaerobic and anoxic zones. It takes time for the microorganisms to use the dissolved 

oxygen, turning part of an anaerobic tank into an anoxic zone or a portion of the anoxic tank 

into an aeration zone. The smaller effective volume could decrease the efficiency of the 

process. 

Operation of the selected process could be optimized by incorporating SCADA and 

operational sensors that can be programmed to make changes using real-time analyses. 

Optimization could include diurnal variations in the incoming wastewater or occasional 

changes in the influent wastewater characteristics. If the operational strategies are 

programmed, the plant can react to changes much more quickly, instead of waiting for results 

from the laboratory, which could be hours or days old. For example, online monitoring can 

automatically adjust the feed rate of coagulant to be fed for chemical phosphorus removal or 

the methanol dose in denitrification filters. The alternative would be to wait for the 
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5-20 Chapter 5: Upgrading Existing Facilities 

laboratory results to show that the effluent concentrations were elevated for several days and 

then have to feed more chemicals to ensure that the monthly permit limit was met. 

A plant upgraded to provide nutrient removal cannot function properly without enough staff 

to operate the processes and perform maintenance activities. In addition, it is important that 

the staff be trained to operate the upgraded processes properly. 

Pilot testing of a limited number of upgrade process alternatives is highly recommended to 

verify that the process will operate as anticipated on the wastewater at a given location. Many 

of the processes depend on the proper influent wastewater characteristics and water 

chemistry to provide the predicted nitrogen or phosphorus concentrations. By incorporating 

pilot testing into the selection or design phase, accommodations can be made to amend the 

influent wastewater, if required. For example, if it is found during pilot testing that VFAs are 

deficient upstream of a biological nitrogen removal process, a fermenter can be incorporated 

into the design and constructed as part of the upgrade instead of incorporating it as an 

expensive retrofit after most of construction has been completed. 

A key issue is to understand the interrelationship among the liquid treatment and solidshandling processes and to quantify and size all unit processes properly. The mass balance and 

energy balance should include all recycle loads for the selected technology. Preparing a mass 

balance during the planning stage can contribute greatly to a successful upgrade project. The 

mass balance needs to include all the nutrient inputs and outputs for each unit process. The 

return flows from sludge-handling processes and filter backwashes that are quantified as part 

of the mass balance can then be incorporated into the basis of design for the upgraded plant. 

This approach will ensure that the processes are designed to handle the nutrient loadings 

from these sources. 

Another key question is how to manage the uncertainty in the future––changing regulations 

and the need for more stringent nutrient removal, variable wastewater characteristics, and 

phasing of future population growth in the service area. Design flexibility might be needed in 

anticipation of future uncertainties. It can be incorporated by constructing additional process 

tanks or swing zones in the secondary process. The swing zone could be between the 

anaerobic and anoxic zones, the anaerobic and aerobic zones, or the anoxic and aerobic 

zones, depending on the secondary process selected. For example, a swing zone between the 

anoxic and aerobic zones would contain both mixers and diffusers. If it was determined that 

additional residence time was needed in the anoxic zone for denitrification, the mixers in the 

swing zone could be operated. If additional aeration was needed for nitrification, the 

diffusers could be operated. Depending on the season and influent wastewater characteristics, 

the swing zone could be operated in either mode. Switching between modes on the basis of 

the wastewater characteristics would be relatively easy to do through the use of automated 

controls. The following sections provide additional information on the critical factors in 

design, operation, or in many cases both. 
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Chapter 5: Upgrading Existing Facilities 5-21 

5.4.1 Influent Wastewater Characteristics 

The evaluation begins with a review of the wastewater data described below, if available. If 

few or no data are available, data should be collected, especially at the influent, primary 

effluent, other in-plant sources, and secondary effluent if tertiary processes are being 

considered. It is recommended that at least a full year of data be collected to quantify 

seasonal variations. 

Biological Phosphorus Removal 

Data needed include the BOD-to-TP, VFA-to-TP, and readily biodegradable chemical 

oxygen demand (rbCOD)-to-P ratios in the plant influent and primary effluent. A BOD-to-TP 

ratio of 25 or higher at the bioreactor influent is desirable as a general guide when there are 

no data for the influent to the bioreactor. At the wastewater treatment plant in Durham, 

Oregon, the performance of the enhanced biological phosphorus removal (EBPR) was 

unstable at a BOD-to-TP ratio of 20 or below at the aeration influent (Neethling et al. 2005). 

A VFA-to-P ratio of 4.3 was observed. At the McDowell Creek, North Carolina, facility, the 

BOD-to-TP ratio remained above 30 and had favorable EBPR performance. The COD-to-TP 

ratio should exceed 8 in the plant influent (Tremblay et al. 2005), while the rbCOD-to-P ratio 

should exceed 11 (Barnard et al. 2005) or 18 with a fermenter (Barnard 2006). The important 

point to monitor is at the influent to the bioreactor, not at the plant influent. An assessment 

should be made to determine whether the supply of VFAs is sufficient year-round, especially 

in cold months when VFA production is lowest in the sewer system and incidental sources at 

the wastewater treatment plant. The decision to install a fermenter should be based on this 

assessment. As Chapter 3 of this document notes, the benefits of having a fermenter on-site 

were documented in Kalispell, Montana, and Kelowna, British Columbia. The phosphorus 

removal at those plants was achieved strictly by a biological process using the modified 

University of Cape Town (UCT) process and 3-stage Westbank process, respectively. The 

reliability graphs indicate low coefficient of variation (COV) values at both plants. 

Chemical Phosphorus Removal 

Jar testing is recommended to determine the design parameters for chemical phosphorus 

removal. Jar tests can be conducted to compare coagulants, such as lime, ferric chloride, and 

alum, to determine which one provides the most efficient and cost-effective removal. The 

tests should be conducted on the wastewater from the location where the chemical will be 

added. Mixing, flocculation, and settling times that approximate the conditions that will be 

found at the chemical feed point and settling location should be selected. Several doses of 

each coagulant should be tested, and the quantity of sludge produced from each should be 

compared before selecting which one will be used at the plant. Analyzing the clear liquid for 

TP will aid in determining which coagulant provides the best removal. Polymer can be 

combined with the coagulant testing to determine whether performance can be improved. 

The jar tests can also be used to determine the recommended chemical feed dose by focusing 

on the value that provided the best result. For example, if alum was selected as the coagulant 
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5-22 Chapter 5: Upgrading Existing Facilities 

and a dose of 10 mg/L as aluminum produced the clearest effluent during the comparison of 

coagulants, performing additional jar tests at 6, 8, 10, 12, and 14 mg/L as aluminum would 

help to identify the best feed dose for use in the design. 

Titrations can be conducted to determine the alkalinity of the solids produced during the jar 

testing. The pH, TSS, and volatile suspended solids in the settled precipitate can also be 

analyzed to estimate the amount of chemical sludge that will be produced in the upgraded 

plant. This information can be used to ensure that sufficient sludge-handling capacity is 

included in the design for the upgraded plant. 

Chemical addition can also be used to remove phosphorus from recycle flows, such as RAS, 

filter backwash, or return from sludge-handling processes. Lime, alum, or ferric chloride 

could be used in this application. Jar tests for these chemical could be performed. Sludge 

quantities produced from chemical addition could be estimated by analyzing the settled solids 

for alkalinity, pH, ortho-phosphorus, ammonia nitrogen, and TSS. 

Nitrogen Removal 

Useful data include the BOD-to-total Kjeldahl nitrogen (TKN) and COD-to-TKN ratios in 

the plant influent and primary effluent. Both ratios are important factors to achieve low TN 

effluents. Low ratios might limit the nitrogen removal efficiency. Grady et al. (1999) 

reported a BOD-to-TKN ratio in the range of 6.8 to 9.5 for near-complete nitrogen removal. 

Most wastewater has a COD-to-TKN ratio of 8 to 10 after primary sedimentation. At low 

ratios, more reliance on endogenous respiration is needed to denitrify. When the COD-toTKN ratio falls below 9 and high nitrogen removal is required, providing some form of 

methanol, acetate, or other carbon source is necessary. Alternatively, fermentation would 

produce supplemental carbon to increase the BOD-to-TKN ratio to promote nitrogen 

removal. When the ratio is expected to be low consistently throughout the year or 

periodically (from cold weather, for example), decisions need to be made regarding the 

source, feed equipment, and storage tanks that will be used to supply carbon. 

Temperature Impacts 

The impact of temperature on biological removal processes varies depending on the location 

of the plant in the country. Usually, areas that experience cold winters need to incorporate 

operational flexibility to successfully treat wastewater at low temperatures. On the other 

hand, areas of the country that experience wastewater temperatures greater than 30 oC could 

experience inhibition of the biological phosphorus removal process and might need to 

consider using chemical phosphorus removal during such periods. For nitrifying organisms, 

the upper limit is 33 o

C, beyond which the nitrification rate falls off quickly (USEPA 1993). 

The most critical low-temperature impact occurs during spring wet-weather events when the 

snow and ice melts increase the TSS and BOD in the influent wastewater and at the same 

time lower the water temperature. Lower wastewater temperatures result in a reduction of the 
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Chapter 5: Upgrading Existing Facilities 5-23 

VFAs from the sewer system and in-plant process sources. With fermenters on-site, both 

Kalispell, Montana, and Kelowna, British Columbia, reported full EBPR operation at 

temperatures as low as 8 oC, as shown in Chapter 3 of this document. Other plants 

experienced poor biological performance and used chemical addition as a standard procedure 

when low wastewater temperatures were recorded. 

Generally, biological processes operate with a longer sludge age at low temperatures, 

especially when nitrification and denitrification are required as part of the treatment process. 

If the process cannot be operated at a long enough sludge age to support nitrification and 

denitrification, alternative processes should be considered, such as converting the existing 

process to IFAS or MBBR. Generally, low temperatures do not adversely affect EBPR. 

5.4.2 Sources of Biodegradable Carbon 

This section discusses how to use wastewater characteristics in selecting a carbon source. 

Maximize In-plant Sources 

Using primary settling tanks, an anaerobic digester, a thickener, or sludge storage tanks can 

produce a significant amount of VFAs. A primary settling tank has been used successfully as 

a VFA generator in many locations. At the Clark County, Nevada, facility, the primary 

sludge is thickened up to 5 percent to produce VFAs in the range of 35 to 45 mg/L. The 

upper solids limit was established at 6 percent. However, the VFA production from primary 

tanks varies from plant to plant. Thickeners and sludge storage tanks are rich sources of 

VFAs. The Genesee County, Michigan, facility relies on these two sources for a year-round 

supply of VFAs. 

Find Local Commercial or Industrial Sources 

Local and industrial sources can provide a good supply of readily biodegradable waste 

materials. At the McDowell Creek plant owned by Charlotte-Mecklenburg Utility in North 

Carolina, high-level BOD water from a soft drink manufacturer is delivered directly to an 

anaerobic zone. The waste has a BOD of 40,000 to 130,000 mg/L (Neethling 2005). 

Install Fermenters 

Installing fermenters by converting existing thickeners or by adding a new fermenter can 

ensure an adequate supply of VFAs for cold months and wet periods. The reliability of EBPR 

depends on a continuous supply of VFAs. The Kelowna, British Columbia, facility converted 

its existing thickeners. The Kalispell, Montana, facility designed a two-stage fermenter 

system. A new generation of fermenters provides improved mixing and effective controls. 

The minimal solids retention time was 2.5 days to generate sufficient VFAs at the McDowell 

Creek Plant (Tremblay 2005). The fermenters were also an additional carbon source for 

nitrogen removal. The feed points can be prioritized between phosphorus removal and 

nitrogen removal on the basis of the performance of both. For example, the VFAs from the 
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5-24 Chapter 5: Upgrading Existing Facilities 

fermenter can be sent to either the anaerobic zone or the anoxic zone, depending on the need 

at the time. In Kelowna, British Columbia, VFAs are sent to both zones, thereby reducing the 

size of the anoxic zone significantly. While the fermenter requires land to build on, the 

reduction in the anoxic zone can result in an overall decreased land requirement. The critical 

factor to note is the improved reliability of the BNR process when operated in conjunction 

with a fermenter. 

Temperature Impacts 

The impact of temperature on biodegradable carbon sources varies depending on the location 

of the plant in the country. Temperature has a greater effect on areas that experience cold 

winter temperatures. Lower wastewater temperatures result in a reduction of the VFAs from 

the sewer system and in-plant process sources. Fermenters in the northern portions of the 

country need to be sized such that sufficient VFAs are produced to support BNR at the 

lowest wastewater temperatures anticipated. 

5.4.3 Impact of Wet-Weather Flows 

This section describes how the management of wet-weather flows can affect process 

selection. Although this issue is not usually severe in municipalities with separate storm 

sewers, it is an important issue in communities with combined sewer systems or collection 

systems that have high inflow and infiltration rates. The peaking factor and permit conditions 

are usually established for the critical discharge season. 

Two factors are noteworthy in selecting technologies. First, certain technologies handle wetweather flows better than those based on activated sludge and clarification. They include 

denitrification filters, fixed-film processes such as MBBR, and step-feed activated sludge. 

Second, off-line storage of peak flows can be a critical requirement. 

During wet-weather events, equalization basins can be used to store as much water as 

possible, if they are available. If equalization basins are not part of the existing plant, it is 

recommended that they be included as part of the upgrade project. The equalization basin can 

be operated as an on-line process in that all wastewater flows through the basin as part of 

normal operation. Alternatively, the equalization basin could be an off-line process, such that 

flow would be diverted to the equalization basins only when needed during wet-weather 

events. The equalization basin could be located in the sewer system, at the headworks, or 

following the primary clarifiers or secondary clarifiers. Placement after the secondary 

clarifiers would be recommended only if the nutrient removal processes to be protected from 

elevated hydraulic flow rates are tertiary clarifiers or filters. The size of the equalization 

basin should be based on the flow patterns that the plant experiences (including diurnal flow 

variations and wet-weather volumes received). The permit limits also should be considered 

when sizing the equalization basins: more equalization volume would be required at plants 
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Chapter 5: Upgrading Existing Facilities 5-25 

that need to meet a daily nitrogen or phosphorus limit, while plants with monthly or yearly 

limits might be able to use smaller equalization basins. 

How the plant is operated during wet-weather events depends on the specific unit processes 

available. For plants that would like to continue the normal mode of operation during wetweather events, converting the existing facilities to a step-feed activated-sludge or IFAS 

system is a possible alternative. Both processes protect the MLSS, so they are designed to 

maintain the biomass inventory in the process train during elevated hydraulic flow periods. 

Another alternative would be to construct new facilities to address wet-weather flows. For 

example, additional secondary clarifiers could be constructed to decrease the surface 

overflow rates during wet-weather events. Step-feed activated-sludge, CAS, PID, or IFAS 

systems could be constructed as new facilities to handle wet-weather flows. The PID would 

need to be operated in storm mode, which can be controlled by a SCADA system, similar to 

the operating mode used at the North Cary Water Reclamation Facility in North Cary, North 

Carolina. Storm mode operation includes allowing flow to enter and leave only the outer ring 

of the ditch. Wastewater within the middle and inner channels continues to circulate, but it is 

not discharged to the clarifier until after the wet-weather event has ended, thereby retaining 

the MLSS, which are then available to repopulate the outer channel. New storage tanks could 

also be used to store recycle flows during wet-weather events. 

The existing or upgraded facilities could also be operated using a wet-weather mode specific 

to the plant. For example, the return of recycle flows could be temporarily suspended during 

wet-weather events, or the aeration zone(s) could be shut down for up to 24 hours. These are 

common practices used to protect secondary clarifiers from solids overloading. In addition, 

temporarily shutting down the sludge-handling processes is an option, if feasible. 

5.4.4 Managing Sludge-Handling Processes 

Managing sludge-handling processes is one of the key factors to consider in selecting a 

nutrient removal process and making the upgrade successful. As described below, both 

quantity and quality are of concern in sludge management and recycle flows in a nutrient 

removal process. 

• Quantity. Upgrading from the normal secondary process to the advanced treatment 

level might increase the sludge quantity (and alter the quality) significantly. Adequate 

capacity should be incorporated into the design. Chemical phosphorus removal 

increases the quantity of sludge. The quantity could double if the target phosphorus 

concentration falls below 0.5 mg/L and could triple if the target falls below 0.2 mg/L 

because of the increased capture of fine particles through tertiary filtration (USEPA 

1987). No increases in sludge, however, were reported from conversion to biological 

phosphorus removal when operated at a long sludge age. Similarly, no additional 

increase in sludge production was reported for conversion to nitrogen removal, except 
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5-26 Chapter 5: Upgrading Existing Facilities 

in the case where methanol or another carbon source was added. In both examples, 

the sludge age was a key parameter. The biological sludge quantity is inversely 

proportional to the design sludge age (WEF and ASCE 1998). The sludge-handling 

processes might need to be sized up to handle the increased quantity of sludge from 

the upgrades. 

• Quality. In terms of quality of sludge and recycle, significant changes can be 

expected. They include changes in thickening and dewatering due to changes in the 

composition of the sludge from the new technology. For example, the chemical 

sludge produced from phosphorus removal from iron salts is easier to settle, thicken, 

and dewater (USEPA 1987) than from traditional activated sludge processes. Tertiary 

filters, however, capture fine particles and make sludge thickening, dewatering, and 

digestion more challenging. BNR sludge with a long sludge age tends to digest well 

but results in significant recycle loads to the main treatment train (see the next 

section). In some BNR facilities, primary settling and anaerobic digestion are 

eliminated, and thus the sludge quality is changed drastically; aerobic thickening and 

aerobic digestion reduce the recycle loadings back to the main train, and the digested 

sludge might require different chemicals for dewatering. 

5.4.5 Recycle Flows 

Recycle flows can introduce significant variability in the treatment plant flows and loads. It 

has been reported that a total of 15 percent to 50 percent of the phosphorus removed 

originates in the recycle flows (WERF 2005). The recycle flows to be evaluated are flows 

from sludge-handling facilities and backwash water from tertiary filters and tertiary clarifiers 

because those flows can carry nutrients that are not removed with the sludge back upstream 

in the treatment process. 

Anaerobic digestion releases high concentrations of ammonia. High levels of phosphorus can 

also be released at an EBPR plant. Return flows can upset the carbon-to-nitrogen ratio or the 

carbon-to-phosphorus ratio required for effective BNR processes. This is particularly true at 

regional facilities that handle sludge from multiple plants or at facilities that receive large 

volumes of septage. 

In many cases, the magnitude of loadings from recycle flows can be minimized. Following 

are recommendations for properly handling recycle flows. 

The first recommendation to minimize the recycle impact is to keep the sludge aerobic for 

EBPR upgrades. This means keeping the sludge aerobic at each step and all the way through 

the sludge-handling processes––digestion, thickening, and storage. Two examples are noted 

in Chapter 3. All waste-activated secondary sludge (WAS) is kept aerobic at the Kelowna, 

British Columbia, facility, while the primary sludge is separately fermented and stored before 

dewatering. No digestion is provided at the Kelowna plant because the sludge is shipped off-
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Chapter 5: Upgrading Existing Facilities 5-27 

site for composting. The recycle loads contained 13 percent TP and 0.1 percent TKN. When 

lime was added before dewatering, the phosphorus load to the EBPR process was further 

reduced to 6 percent. When sludge is digested, the recycle loads increase. At the Kalispell, 

Montana, facility, the secondary sludge is thickened using dissolved air flotation, while the 

primary sludge is anaerobically digested. The primary and secondary sludges are kept 

separate until the time of dewatering. This operation is noteworthy in avoiding a potential 

increase in recycled phosphorus loads if these two sludges were to be combined and stored 

for any time. 

The second recommendation is to plan for recycle loading from anaerobic digestion and 

dewatering, especially from BNR facilities. The recycle phosphorus loads were reported to 

be high in the facilities with anaerobic processes––25 percent of the total influent phosphorus 

loading at the Durham, Oregon, facility and the McDowell Creek, North Carolina, facility 

and 50 percent at the Lower Reedy Creek, Florida, facility (WEF 2005). Jones and Takacs 

(2004) reported a modeling technique that can be used to estimate recycle loads in such 

cases. They reported recycle loadings of 35 percent to 50 percent in phosphorus and 

20 percent to 30 percent in ammonia nitrogen from the anaerobic digester operating at a 

20-day sludge age in their modeling when the volatile suspended solids destruction was 

assumed to be 35 percent. These authors used the General Activated Sludge–Digestion 

Model (ASDM), which was developed and implemented in BioWin 2.1. They also identified 

the variables that can reduce recycle loads on the basis of the digestion detention time, 

equilibrium chemistry, and formation of struvite in the system. Tang et al. (2004) reported 

recycle loads of 50 percent for ammonia nitrogen in comparison to the plant influent loads at 

the Valencia Water Reclamation Plant in the Los Angeles County Sanitation District, where 

both primary and secondary sludge were anaerobically digested. 

At non-EBPR plants, the recycle loads are less than those from EBPR plants; they are 

typically less than 15 percent. Anaerobic digestion is the main source of ammonia and 

phosphorus recycle back to the main plant. Under a normal operating schedule, this load can 

be managed successfully by carefully controlling the operating schedule and implementing 

certain strategies. These include flow equalization to avoid shock loadings, filter backwash to 

an equalization basin with sufficient storage capacity, and proper controls. At nitrogen 

removal plants, it was not suggested that the nitrogen in recycle be removed. Ammonia 

nitrogen is high in anaerobic digester supernatant and in the liquid from dewatering such 

sludge. 

For EBPR plants, it is suggested that WAS be kept separate from primary sludge and be kept 

aerobic, as described earlier. The following guidelines are suggested for biosolids thickening, 

stabilization, and storage facilities: 

• For sludge thickened by dissolved air flotation or other mechanical thickeners (rotary 

belt thickeners), aerobic digestion is preferred. 
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5-28 Chapter 5: Upgrading Existing Facilities 

• Storage tanks that have aeration capability and are large enough to hold biosolids are 

recommended. 

• An equalization basin for recycle flows might be needed. 

• The dewatering schedule might need to be modified to alter when the recycle loads 

are returned from the day shift to a later shift. 

• Rerouting the feed point to the bioreactor to maximize VFAs and other beneficial 

ingredients, that is, step feed to the anaerobic and anoxic zones, or to one of them, 

depending on the priority at the time, should be considered. 

• Adding lime to the centrifuge to minimize the amount of recycle load should be 

considered. 

The centrifuge at the Noman Cole plant in Fairfax County, Virginia, was designed to add 

lime to reduce recycle loads at the dosage of up to 13 percent of the solids loading. To 

remove most of the soluble phosphorus, the Kelowna, British Columbia, facility fed lime to 

centrifuges to a pH of 9. This practice ended in 2006. The net increase of phosphorus in 

recycle loads was dramatic, while the TSS loads were reduced as illustrated below: 

• Soluble COD: 417 to 700 mg/L, a 67 percent gain 

• Soluble P: 34 to 137 mg/L, a 405 percent gain 

• Total P: 109 to 200 mg/L, an 82 percent gain 

• Ammonia nitrogen: 15 to 21 mg/L, a 39 percent gain 

• TKN: 52 to 68 mg/L, a 30 percent gain 

• TSS: 1,031 to 765 mg/L, a 24 percent decrease 

The Kelowna plant determined that the increase in recycle loads in both nitrogen and 

phosphorus was manageable because of the availability of VFAs from the fermenter, 

wastewater characteristics, and other process control parameters in place. 

The third recommendation is that facilities should consider converting anaerobic digestion to 

aerobic digestion as part of long-term planning, where it is found to be cost-effective. 

Consideration should also be given to the potential loss of green energy when deciding how 

to act on this recommendation. 

For drying, composting, or other Class A products, additional guidelines might apply. 

In addition, the following guidelines apply for certain facilities with tertiary clarifiers and 

tertiary filters: 
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Chapter 5: Upgrading Existing Facilities 5-29 

• Consider adding an equalization basin for filter backwash water so that the flow can 

be distributed evenly throughout the 24-hour period, thereby controlling the power 

usage and chemical feed rate evenly. Most facilities send this water to the headworks 

or equalization basin, thereby reducing shock loads to the rest of the plant. 

• Add thickener for tertiary clarifier sludge. Because the waste sludge is dilute with a 

low solids concentration, thickening of the tertiary sludge is recommended. 

All the above should be incorporated into the design basis (flow and loadings), and each unit 

process should be evaluated for efficient and reliable performance. 

5.4.6 SCADA Requirements and Sensors 

Most BNR facilities have adopted automated control systems, which include dissolved 

oxygen controllers operating with a programmable logic controller (PLC) and associated 

SCADA. In other facilities more specific controls, based on other sensors for nitrate, 

oxidation-reduction potential (ORP), and flow have been adopted. Each facility has 

developed specific programs on the basis of its permit limits, the skill levels of personnel, 

and the technology employed at the facility. In other facilities, more specific controls using a 

combination of sensors have been installed. Available sensors included ammonia nitrogen, 

nitrate and nitrite nitrogen, ortho-phosphorus, and ORP (Weerapperuma and De Silva 2004; 

Demko et al. 2007). New tools included the sludge blanket monitor, TSS and turbidity 

meters, and total organic carbon analyzers. Some of these tools have been installed at BNR 

facilities for accurate monitoring of performance and also as an early warning system for 

toxic shocks that might be entering the plant. No facility has employed these data in actual 

control of treatment processes, however. At the North Cary, North Carolina, plant, the 

oxidation ditch operates in anoxic and aerobic cycles in alternating phases. The exact 

decision on phasing is made on the basis of preset control logic, which is site-specific and 

fully automated. Most facilities install PLCs with a human-machine interface and multiple 

screens connected to the main computer system, flow meters, level switches, online sensors, 

and system alarms. 

5.4.7 Staffing Requirements 

Most facilities maintained the same level of staffing after the BNR processes were installed. 

No new hires were reported for the BNR upgrades. The existing staff was trained on the 

operation of the BNR process and related new equipment and instrumentation. 

5.4.8 Training Needs 

In the upgrade projects and case studies included in this document, training in new process 

operation and monitoring was provided by the project consultant or the manufacturer of the 

process, when applicable, during the design and construction period. During startup, more 

detailed training was provided as a part of the construction contract. Follow-up training 
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5-30 Chapter 5: Upgrading Existing Facilities 

included seminars and workshops offered by state agencies and professional organizations 

like the Water Environment Federation (WEF), its local affiliates, and other organizations. 

Training in process controls and information technology was provided mostly in-house. 

Laboratory personnel received training from equipment suppliers, where applicable. 

Additional training could be obtained by attending specialty courses on the subject. 

5.4.9 Pilot Testing 

In many of the upgrade projects, before selecting a technology, the project team went through 

a preliminary engineering evaluation and conducted pilot testing of the favored alternative to 

verify the concept with the actual wastewater to be treated and to develop sizing data for the 

process. The duration of the pilot testing depended on the seasonal changes in wastewater 

characteristics, the technologies selected, whether the facility was new versus well 

established, the number of parameters to operate under, and the specific objectives 

established. The typical duration ranged from 6 months to 1 year for a biological process. At 

the Blue Plains facility in Washington, D.C., pilot testing of the process lasted 6 months. For 

physical and chemical processes, however, pilot testing requires far less time. 

This pilot testing can be carried out at the planning stage or delayed until the design phase of 

the project to accommodate the overall schedule. 

5.5 Finalizing Process Selection 

The project team’s final step is to evaluate feasible alternatives and determine the 

recommended plan. The project team can compare the potential alternatives and select the 

best process in accordance with the success criteria established at the beginning of the 

project. 

The reader can obtain information on general cost estimation from Chapter 4 of this 

document for appropriate technologies being considered. Alternatively, a cost estimate based 

on upgrading the individual plant to the processes being considered could be performed as 

part of the evaluation. 

The accuracy of the cost estimate will vary depending on the level of detail provided in the 

evaluation. If the cost estimate is based on cost curves or costs from similar facilities or 

technologies with very little consideration of local conditions, the cost estimate might be 

accurate to only within approximately 50 percent. If more detailed studies such as soil 

borings, preliminary engineering design drawings, and outline specifications are prepared, 

the estimate will be more accurate. Non-cost criteria should include those items of most 

concern to the public. At a minimum, odor, traffic, noise, air emissions, dust, water quality, 

wetland infringement, and other environmental impacts need to be assessed. 
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Chapter 5: Upgrading Existing Facilities 5-31 

Two examples are shown below. Both examples include a weighting factor to indicate the 

relative importance of each criterion being considered. In Table 5-8, the alternatives are 

assigned values from 1 to 4 (there are four alternatives), with 1 the most favored alternative 

for the given criteria. Therefore, the most favored alternative is the one with the lowest 

number at the bottom of the table. Once this basic rating is complete, a sensitivity analysis 

can be carried out by changing the weighting factors for one or more criteria items, e.g., 

increasing the cost of energy by a factor of 50 percent or 100 percent. The process can be 

repeated for another parameter, such as the future regulatory requirements or biosolids 

handling. The new ranking can be compared among these alternatives. 

Table 5-8. Decision matrix example 1 

Alternatives 

Criteria for comparison 

Weighting 

factor A B C D 

Costs (capital and O&M) 1 2 1 4 3 

Reliability 1 3 1 2 4 

Efficiency 1 1 2 4 3 

Expandability 1 2 3 1 4 

Ease of O&M 2 3 1 4 2 

Environmental impacts—chemical use 3 4 1 3 2 

Efficient land use 1 2 1 4 3 

Energy use 2 2 1 4 3 

Future regulatory requirements 1 4 3 2 1 

Employee and public health and safety 2 2 3 4 1 

Market stability for biosolids reuse/disposal 1 1 4 2 3 

TOTAL -- 41 28 52 39 

Note: The rating scale is 1 to 4. Smaller numbers represent a more favorable alternative. No two alternatives may have the 

same score for the same category. Totals are determined by summing the score for each criterion times the weighting factor, 

which should be adjusted for the local situation. The alternative with the lowest total is the best overall alternative by this

measure. 

In the second example, an alternate scale is used to evaluate the alternatives. It is based on 

assigning a number from 1 to 10 for each criterion, with 1 being the least favored and 10 the 

most favored. The weighting factor is multiplied by the score to determine the total points. 

Table 5-9 shows an example from another case (WEF 2004). The bottom line figures are then 

compared to select the recommended alternative. The highest scorer is the best alternative in 

this method. The same sensitivity analysis described under the first example can be carried 

out for the cost of energy, or other parameters of choice, and compared again. 
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5-32 Chapter 5: Upgrading Existing Facilities 

Table 5-9. Decision matrix example 2 

Option 1 Option 2 Option 3 

Criteria Weight Score Points Score Points Score Points 

Site requirement 8 5 40 5 40 4 32 

Capital cost 6 3 18 5 30 4 24 

O&M cost 6 3 18 4 24 4 24 

Reliability 8 4 32 5 40 5 40 

Flexibility 10 3 30 4 40 3 30 

Use of existing facilities 6 3 18 4 24 4 24 

Sludge production 8 2 16 3 24 3 24 

Public perception 7 3 21 3 21 3 21 

Total points 193 243 219 

Rank 3 1 2 

The project team can choose which method to employ and the criteria to be included in 

making the final technology selection. 

Once a technology is selected, the next step is to prepare a conceptual design. The design 

should include a demolition plan, a site plan with major pipes and facilities, a hydraulic 

profile, process flow and mass balances, general drawings with building and major 

equipment footprints, one-line electrical drawings, a basic instrumentation and control 

philosophy and block diagram, and architectural renderings. A construction schedule should 

also be developed to ensure that the existing plant can continue to operate and meet permit 

limits until the upgrades are placed on-line. The recommended plan is then presented for 

public approval and implementation. 

5.6 Summary 

As more municipalities are required to meet stringent nutrient load limitations to protect 

receiving waterbodies, upgrading existing facilities with sustainable technologies is an 

important challenge, as well as an opportunity. This chapter presented the general approaches 

to upgrading existing facilities, explained how to set success criteria for the upgrade, and 

provided tables that can be used to screen feasible alternatives, along with selection factors 

and design and operational factors that can assist in identifying the right technology for the 

municipality. 

Planning for process upgrades includes projecting future loads, assessing existing 

capabilities, preparing a mass balance that includes all return and recycle flows and loads, 

developing the needed expansion and upgrade that should incorporate flexibility into the 

operation of the plant to account for future uncertainties, evaluating feasible alternatives, and 

selecting the recommended plan. The success criteria might include sustainability, cost-
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Chapter 5: Upgrading Existing Facilities 5-33 

effectiveness, ease of O&M, project schedule, and site requirements. The sustainability 

factors could include energy usage, chemical usage, and the recycling of biosolids. 

A list of technologies capable of meeting the selected target effluent range for nitrogen 

and/or phosphorus can be developed. Technology selection factors, including the following, 

should then be reviewed for each alternative: 

• Site factors 

– Footprint 

– Need for a building 

– Possibility of construction in the existing aeration basin 

– Piping and pumping requirements 

– Need for additional head 

– Presence of secondary process recycle streams 

• Wastewater factors 

– Need for additional carbon source 

• Operation factors 

– Extra electricity usage 

– Need for chemical addition 

– Generation of additional sludge 

The next step is to identify and evaluate feasible technologies on the basis of design and 

operational factors and cost factors like the following: 

• Wastewater characteristics in the influent, primary effluent, and recycle streams 

• Carbon source management, internal and external 

• How to manage wet-weather flows 

• How to manage recycle flows 

• How to design and operate sludge-handling processes 

• How much automation and control is needed 

• Staffing and training needs 

Important considerations include how to incorporate flexibility in anticipation of 

uncertainties and changing conditions in wastewater characteristics and regulations. Factors 

considered might include flow equalization, the number of swing zones, alternative modes of 

operation, and safety factors. 
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5-34 Chapter 5: Upgrading Existing Facilities 

The project team should select the recommended process in accordance with the established 

success criteria. The final process might include a combination of the following, in parallel or 

in series: 

• Replace the existing process with a new process. 

• Convert the existing process to a new process. 

• Add a new process to the existing process. 

The recommended process option should accompany an implementation plan that includes an 

overall schedule, funding, a construction and operational plan, costs, and startup procedures. 

The success of the upgrades will ensure full compliance with the new permit with good 

reliability in the most sustainable way (in energy usage, chemical usage, and recycle of 

biosolids). 

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