Municipal Nutrient Removal Technologies
Reference Document
Volume 1 — Technical Report
U.S. Environmental Protection Agency
Office of Wastewater Management, Municipal Support Division
Municipal Technology Branch
EPA832-R-08-006 • September 2008
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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:
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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.
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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.
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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
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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.
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 LakeTahoe 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 Fermenterand 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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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
Denitrifi cation
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 Gary, 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 forTN: 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 forTP 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 forTN 3-7
Figure 3-4. Lee County, Florida: Monthly frequency curves for TP 3-8
Figure 3-5. Lee County, Florida: Monthly frequency curves forTN 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 forTP 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 forTN 3-14
Figure 3-12. North Gary, North Carolina: Monthly average frequency curves forTP 3-15
Figure 3-13. North Gary, North Carolina: Monthly average frequency curves for
ammonia nitrogen 3-16
Figure 3-14. North Gary, North Carolina: Monthly average frequency curves forTN 3-16
IV
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September 2008 Municipal Nutrient Removal Technologies Reference Document
Figure 3-15. Kelowna, British Columbia: Monthly average frequency curves forTP 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 forTN 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
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
VI
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Municipal Nutrient Removal Technologies Reference Document
Acronyms/Abbreviations
ANAMMOX
ASCE
A/O
A2/O
AS
ATS
BABE
BAF
BAR
BASIN
BNR
BOD
BOD-to-TKN
BOD-to-TP
BPR
CANON
CAS
CBOD
COD
COV
CP
CWSRF
DAF
DO
DON
EBPR
ENR
Aluminum sulfate (or Alum)
anaerobic ammonia oxidation
American Society of Civil Engineers
anoxic/oxic
anaerobi c/anaerobi c/oxi c
activated sludge
Aeration Tank 3 process
bio-augmentation batch enhanced
biological aerated filter
bioaugmentation reaeration
biofilm activated sludge innovative nitrifiation
biological nutrient removal
biochemical oxygen demand
biochemical oxygen demand-to-total Kjeldahl nitrogen ratio
biochemical oxygen demand-to-total phosphorus ratio
biological phosphorus removal
completely autotrophic nitrogen removal over nitrite
cyclic activated sludge
carbonaceous biochemical oxygen demand
chemical oxygen demand
coefficient of variation
central plant
Clean Water State Revolving Fund
dissolved-air flotation unit
dissolved oxygen
dissolved organic nitrogen
enhanced biological phosphorus removal
Engineering News-Record
Vll
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Municipal Nutrient Removal Technologies Reference Document
September 2008
EPA
FeCl3
FFS
GAO
GPD
HDWK
HCO3"
H2CO3
HRT
IFAS
I&C
kWh/year
MAUREEN
MBBR
MBR
MG
MGD
mg/L
LE
MLSS
MW
N
NH4
NH4-N
NL
NPDES
OLAND
ORP
O&M
PAO
Environmental Protection Agency
ferric chloride
fixed-film systems
glycogen accumulating organism
gallons per day
headworks
bicarbonate
carbonic acid
hydraulic retention time
integrated fixed-film activated sludge
instrumentation and control
kilowatt-hours per year
mainstream autotrophic recycled enhanced N-removal
moving-bed biofilm reactor
membrane bioreactor
million gallons
million gallons per day
milligrams per liter (equivalent to parts per million)
modified Ludzack-Ettinger
mixed liquor suspended solids
molecular weight
nitrogen
ammonium
ammonia nitrogen
no limit
National Pollutant Discharge Elimination System
oxygen limited aerobic nitrification-denitrification
oxidation-reduction potential
operation and maintenance
phosphate accumulating organisms
Vlll
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September 2008
Municipal Nutrient Removal Technologies Reference Document
3-
PID
PLC
PO4
POTWs
RAS
rbCOD
rDON
SCADA
SBR
SE
SND
SF
SHARON
SRT
TMDL
TKN
TN
TP
TRPA
TSS
UCT
UV
VFA
VIP
vss
WAS
WEF
WERF
WQBEL
WWTP
phased isolation ditch
programmable logic controller
phosphate
publicly owned treatment works
return-activated sludge
readily biodegradable carbonaceous oxygen demand
refractory dissolved organic nitrogen
supervisory control and data acquisition
sequencing batch reactor
secondary effluent
simultaneous nitrification and denitrification
subsurface flow
single reactor high-activity ammonia removal over nitrate
solids retention time
total maximum daily load
total Kjeldahl nitrogen
total nitrogen
total phosphorus
Tahoe Regional Planning Agency
total suspended solids
University of Cape Town
ultraviolet
volatile fatty acids
Virginia Initiative process
volatile suspended solids
waste activated sludge
Water Environment Federation
Water Environment Research Foundation
water quality-based effluent limit
wastewater treatment plant
IX
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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
Executive Summary ES-1
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Municipal Nutrient Removal Technologies Reference Document September 2008
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.
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 (A2O) process, literature report, 7.3 to 9.0 mg/L, no location or
COV available
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Municipal Nutrient Removal Technologies Reference Document September 2008
• 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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
• 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 Westbcmk 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
• 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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 Gary, 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 less-
biodegradable 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,
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 year-
round, 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 single-
sludge 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.
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 Gary, 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 Gary has a storm mode of operation, which allows quick adjustment of the
cycle time and thus affords protection of the sludge inventory from washouts.
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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
Gary (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
Case Study Locations
Kelowna, British Columbia (Canada)
Clear-water, 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)
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 (step-
feed 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 Gary (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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Cosfs
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
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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
TN,
5.0 mg/L
TN,
3 mg/L or less
TN, 1 mg/L
9.6 mg/L TN
7 mg/L
TN
8 mg/L TN
8 mg/L TN
8 mg/L TN
15 mg/L TN
8 mg/L TN
8 mg/L TN
6.5 mg/L TN
40 mg/L TNd
40 mg/L TNd
40 mg/L TNd
40 mg/L TNd
42 mg/L TNe
Cyclic on/off
aeration3
Denitrification
filterb
MLE^4-stage
Bardenpho3
MLE^4-stage
Bardenpho3
MLE^4-stage
Bardenpho3
Lagoon->4-
stage
Bardenpho3
Denitrification
filterb
Denitrification
filterb
5-stage
Bardenpho +
denitrification
filterb
Phased
oxidation ditch3
MLE retrofit3
Step-feed
retrofit3
Denitrification
filterb
5-stage
Bardenpho with
MBR and
reverse
osmosisb
Ridgefield,
CT
Cheshire,
CT
Seneca,
MD
Freedom,
MD
Cumber-
land, MD
Hurlock,
MD
Baltimore,
MD
Cox
Creek, MD
Frederick,
MD
CW
CW
CW
CW
Las
Virgenes,
Calabasas,
CA
1
3.5
20
3.5
15
1.5
180
15
7
10
10
10
10
16
$0.20
$1.65
$0.21
$0.99
$1.10
$4.12
$1.39
$1.74
$1.41
$0.47
$0.71
$0.65
$0.71
$5.20
$111
$136
$63
—
$122
—
$104
$44
$82
$91
$156
—
—
—
—
—
—
—
$157
$164
$245
$324
ES-14
Executive Summary
-------
September 2008
Municipal Nutrient Removal Technologies Reference Document
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
TP,
0.5 mg/L
TP,
0.1 mg/L
5 mg/L TP
5 mg/L TP
5 mg/L TP
5 mg/L TP
5 mg/L TP
Fermenter
retrofit, no filterb
1 -point chemical
addition,
no filterb
Fermenter +
sand filter
retrofit15
Fermenter +
sand filter +
1 -point chemical
addition13
2-point chemical
addition + filter"
CW
cw
CW
cw
cw
10
10
10
10
10
$0.18
$0.03
$0.44
$0.47
$0.29
$7
$91
$25
$106
$215
$50
$98
$130
$218
$283
Ammonia-N + TP limits
Ammonia-
N+TP, 1.5
mg/L & 1 mg/L
Ammonia-
N+TP, 1.5
mg/L & 1 mg/L
Ammonia-
N+TP, 1.5
mg/L & 1 mg/L
Ammonia-
N+TP, 1.4
mg/L & 1 mg/L
Ammonia-
N+TP,
1 mg/L & 0.1 8
mg/L
Ammonia-
N+TP, 0.6
mg/L & 0.2
mg/L
37 mg/L
Ammonia-N,
10 mg/L TP
37 mg/L
Ammonia-N,
10 mg/L TP
37 mg/L
Ammonia-N,
10 mg/L TP
24 mg/L
Ammonia-N, 4
mg/L TP
18. 9 mg/L
Ammonia-N,
6.4 mg/L TP
27 mg/L
Ammonia-N,
5.8 mg/L TP
Cyclic on/off
aeration3 for
ammonia-N
IFASa for
ammonia-N
MBBRb for
nitrification/
denitrification
Modified UCT
with fermenter
and sand filter0
Step-feed AS
with dual-media
and deep-bed
filter + 1 -point
chemical
addition0
A/0 with VFA
and dual media
filters and chemi-
cal addition3
Broomfield
,co
Broomfield
,co
Broomfield
,co
Kalispell,
MT, case
study
Fairfax,
VA, case
study
Clark Co.,
NV, case
study
8
8
8
3
67
100
$1.00
$0.85
$1.70
$3.03
$1.07
$2.01
$108
$106
$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
fermenters3
Kelowna,
BC, case
study
10.5
$3.25
$77
Executive Summary
ES-15
-------
Municipal Nutrient Removal Technologies Reference Document
September 2008
Table ES-1. Cost estimates for retrofit technologies (continued)
Target
concentration
(annual
average)
TN+TP, 3.9
mg/L & 2 mg/L
TN+TP, 3.7
mg/L & 1 mg/L
TN+TP, 3 mg/L
& 1 mg/L
TN+TP, 3 mg/L
& 1 mg/L
TN+TP, 3 mg/L
& 0.5 mg/L
TN+TP,
3 mg/L &
0.1 mg/L
Initial
concentration
(annual
average)
56 mg/L TN,
7.7 mg/L TP
3 1.2 mg/L TN,
5.8 mg/L TP
24 mg/L TN,
3.7 mg/L TP
28 mg/L TN, 5
mg/L TP
33. 2 mg/L TN,
3.8 mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
Technologies
(conversion or
add-on
indicated by
footnote)
Oxidation ditch
with sand filter3
Plug flow AS
with
denitrification
filter0
3-stage activated
sludge with
chemical
addition13
5-stage
Bardenpho with
sand filter +
chemical
addition3
Denitrification
filter + chemical
addition13
PID retrofit with
1 -point chemical
addition, clarifier,
and filter3
5-stage
Bardenpho
retrofit with
chemical
addition3
Nitrification/
chemical
addition/
denitrification
filter retrofit15
Location
North
Gary, NC,
case study
Central
Johnston
Co., NC
case study
Western
Branch,
MD, case
study
Clearwater
,FL
(Marshall
Street)
case study
Lee Co.,
FL (Fiesta
Village)
case study
CW
CW
CW
Flow
rate
(MGD)
12
7
30
10
5
10
10
10
Capital
$/gpd
capacity
$2.84
$0.58
$1.73
$2.95
$2.79
$0.89
$1.30
$0.75
O&M
$/MG
treated
$60
$221
$165
$242
$265
$199
$256
$448
Life-cycle
$/MG
treated
$411
$566
$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
ES-16
Executive Summary
-------
September 2008 Municipal Nutrient Removal Technologies Reference Document
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
0 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.
Executive Summary ES-17
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Municipal Nutrient Removal Technologies Reference Document September 2008
• 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.
ES-18 Executive Summary
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 MOD. 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.
Executive Summary ES-19
-------
Municipal Nutrient Removal Technologies Reference Document
September 2008
Table ES-2. Cost estimates for expansion technologies for 10 MGD
Nutrient/
target
concentration
TN, 5 mg/L
TP, 1 mg/L
TP, 0.5 mg/L
TP, 0.1 mg/L
TN+TP, 5 mg/L
& 1 mg/L
TN+TP, 5 mg/L
& 0.5 mg/L
TN+TP, 5 mg/L
&0.1 mg/L
TN+TP, 3 mg/L
&0.1 mg/L
Initial
concentration
40 mg/L TN
40 mg/L TN
40 mg/L TN
40 mg/L TN
5 mg/L TP
5 mg/L TP
5 mg/L TP
5 mg/L TP
5 mg/L TP
5 mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
40 mg/L TN, 5
mg/L TP
Technology
PID
MLE
4-stage Bardenpho
Denitrification filter
A/O, no additional
equipment
1 -point chemical addition
A/O with fermenter
A/O with fermenter and
sand filter
2-point chemical addition
with filter
A/O with fermenter, filter,
and chemical addition
Step-feed
SBR
3-stage processes (e.g.,
UCT, VIP)
5-stage Bardenpho, no
filter
Modified UCT with
fermenter and filter
5-stage Bardenpho with
filter
PID with chemical
addition, clarifier, and filter
SBR with chemical
addition and filter
Nitrification with
1 -point chemical addition
and denitrificiation filter
5-stage Bardenpho with
chemical addition and filter
Capital
$/gpd
capacity
$0.63
$1.61
$2.17
$0.71
$1.21
$0.03
$1.26
$1.52
$0.29
$1.55
$1.36
$1.94
$2.05
$2.19
$2.33
$2.45
$0.83
$1.87
$0.75
$2.48
O&M $/MG
treated
$122
$309
$453
$156
$280
$91
$290
$308
$215
$389
$299
$302
$436
$452
$456
$455
$259
$387
$448
$477
Life-cycle
$/MG
treated
$273
$695
$971
$324
$568
$98
$590
$670
$283
$758
$625
$766
$925
$975
$1014
$1040
$456
$834
$626
$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.
ES-20
Executive Summary
-------
September 2008 Municipal Nutrient Removal Technologies Reference Document
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, IF AS, 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.
Executive Summary ES-21
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Municipal Nutrient Removal Technologies Reference Document September 2008
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, IF AS, 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 side-
stream 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.
ES-22 Executive Summary
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Executive Summary ES-23
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Chapter 1: Introduction and History 1-1
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 sewage-
treatment 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 suspended-
growth 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
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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
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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).
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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
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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
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(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 MOD 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
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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)(l)(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
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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 fmalguidance.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/waterqualitvtrading/WQTToolkit.html
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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 (ENTPJ): 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-l.
December 12, 2006. Accessed October 3, 2007.
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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.
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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
Chapters.
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.
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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 -> NH4+
2. Nitrification to nitrite by Nitrosomonas species and other autotrophic bacteria genera:
NH4+ + 3/2 O2 + 2HCO3" -» NO2" + 2H2CO3 + H2O
3. Nitrification to nitrate by Nitrobacter species and other autotrophic bacteria genera:
NO2" + Vi O2 -» NO3"
4. Denitrification by denitrifying microorganisms with no oxygen present:
NO3" + organic carbon -» N2 (g) + CO2 (g) + H2O + Off
The 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
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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
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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 three-
way 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 BOD-
to-TKN ratio is low, however, a higher percentage of the flow (compared to typical BOD-to-
TKN 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 VSS-
day), 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
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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 BOD-
to-TKN ratios less than 4-to-l or COD-to-TKN ratios less than 10-to-l 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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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 (°C). 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 (CaCOs) per gram of
ammonia nitrogen removed (WEF and ASCE 2006, p. 39). Denitrification produces
3.57 grams of alkalinity as CaCOs 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 CaCOs 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 CaCOs, 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
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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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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 NOs to obtain
oxygen is greater than that from using molecular oxygen, so most microorganisms
preferentially use DO when it is available rather than NOs. 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. SCAD A 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.
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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 activated-
sludge 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 deep-
bed 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
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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 rng/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.
Filter Backwash
Influent
Secondary Treatment
Process
Methanol or other
carbon source
Denitrifying
Filter
Effluent
WAS
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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Influent
Main flow
direction
Effluent
Recirculation
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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Nitrified Recycle
Influent
Anoxic
Tank
Aerobic Tank
Secondary
Clarifier
Effluent
RAS
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
Figure 2-4. Cyclically aerated activated-sludge process.
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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).
Nitrified Recycle
(Optional) Methanol
Addition
Influent
Tank
Aerobic Tank
Tank
Secondary
Clarifier
Effluent
RAS
WAS
Figure 2-5. Four-stage Bardenpho process.
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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.
Flow
Aerators
WAS
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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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
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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 IF AS process is illustrated
in Figure 2-7. The range of TN effluent concentrations at plants with IF AS 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).
Tanks contain fixed or floating
media for attached growth
Influent
Secondary
Clar if ier
Effluent
Anoxic
Tank
Aerobic Tank
7
Mediaretention sieves
(if necessary)
RAS
Waste
Sludge
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
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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 tol? mg/L (see Table 2-1 in Section
2.5).
Tanks contain suspended plastic
media for attached growth
Influent
•
Secondary
Clarifier
Effluent
Anoxic
Tank
Aerobic Tank
Media retention
sieves
Figure 2-8. Moving-bed biofilm reactor process.
Waste
Sludge
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.
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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.
Influent
Anoxic
Tank
Aerobic
Tank
Anoxic
Tank
Aerobic
Tank
Membrane
Filter
Retained Biomass
Membrane
Pump
RAS
Waste
Sludge
(a)
Figure 2-9. MBR process, (a) External filter in lieu of clarifier
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Influent
Anoxic
Tank
Aerobic
Tank
Anoxic
Tank
Membrane
Pump
Effluent
Aerobic
Tank
Waste
Sludge
(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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Influent
Anoxic
Tank
Aerobic
Tank
Anoxic
Tank
Aerobic
Tank
Anoxic
Tank
Aerobic
Tank
RAS
WAS
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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Alternating
Aerobic/Anoxic Tanks
Flows switch when tanks
alternate
Tank 1-Anoxic
Influent
Secondary
Clarifier
Effluent
Tank 2-Aerobic
RAS
WAS
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).
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Rotating
Bridge
Effluent
Waste
Sludge
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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Influent
SBR under
aerobic/anoxic
conditions
Cycle times
determined based
on conditions
Effluent
Waste
Sludge
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
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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.
Influent
Activated Sludge
Tank
Secondary
Clarifier
Effluent
RAS
Nitrification
Reactor
WAS
Recycled Sidestream
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.
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Influent
Activated Sludge
Tank
Secondary
Clarifier
Effluent
RAS
WAS
Bio-augmentation
Figure 2-13B. BABE process.
BABE
Reactor
Recycled Sidestream
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.
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Influent
A A »
Activated Sludge
Tank
Secondary
Clarifier
Effluent
RAS
WAS
Denitritation
Reactor
(Anoxic)
Nitritation
Reactor
(Aerobic)
Recycled
Sidestream
Heat
Exchanger
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 Nitrification-
Denitrification (OLAND) and Completely Autotrophic Nitrogen Removal Over Nitrite
(CANON)—are under development (Stensel 2006).
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Influent
Activated Sludge
Tank
Secondary
Clarifier
Effluent
RAS
Denitritation
Reactor
(Anoxic)
Nitritation
Reactor
(Aerobic)
WAS
Recycled Sidestream
Ammonia Bypass
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 ->
PAOs + stored biopolymers + Mg++ + K+ +_CO2 + H2O + PO43" (released)
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2. Aerobic biological phosphorus uptake by PAOs:
PAOs + stored biopolymers + Mg++ + K+ + O2 (or NO3) + PO43" -»
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 (PO43") 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 micro-
organisms 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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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 FtRT 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.
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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 sludge-
handling 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 sludge-
handling 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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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
(A13+). Ferric ion is typically supplied in the form of ferric chloride (FeCls). 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., H^PO^"', 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)" + nOFT -» PO43" + n H2O
2. Chemical phosphorus removal by alum (aluminum sulfate):
A12 (SO4)3 -» 2 A13+ + 3 (SO42-)
A13++ PO43' -» A1PO4 (s)
3. Chemical phosphorus removal by ferric chloride:
FeCl3 -» Fe3+ + 3 Cl'
Fe3+ + PO43' -» FePO4 (s)
4. Ferrous ion conversion to ferric ion:
2Fe2+ + V2 O2 + 2H+ -» 2Fe3+ + H2O
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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 floe 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
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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 floe. If the chemical is not fed to a well-mixed location, an in-
line 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 floe 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
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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 floes to form. Usually,
the movement of the wastewater through the treatment plant is sufficient for the floe
formation. However, turbid areas of the plant might break up the floe, 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 floe 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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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-l 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.
Influent
RAS +
VFA
Anaerobic
Aerobic Tank
Secondary
Clarifier
Effluent
RAS
RAS to Fermenter
(optional) WAS
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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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).
Influent
Anaerobic
Aerobic Tank
RAS
— 1 — 7^\
Secor
Clar
idary
ifier
Effluent
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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Flow
Aerat ors
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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Chemical Addition
(Alum or Ferric)
(optional)
Filter Backwash
Primary
Clarifier
Secondary
Influent
Chemical Addition
(Alum or Ferric)
(optional)
Aerobic Tank
i ,
Secondary
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 floe 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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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
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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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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
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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 MOD); however, the Orange County, Florida,
WWTP uses cloth filters at an average annual flow of 29.5 MOD 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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Solids contact clarifiers mix the secondary effluent with coagulants and previously settled
solids, forming larger floe. 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.
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Alum
Secondary 1
Effluent
Dynasand
Deep Bed
Filter
Reject
Water
^
Alum
1st \
filter\
scour \
water ^-'
Final
Dynasand
Standard
Bed
Filter
*
+
s
*
*
+
s
s
+
/Lamella /
/ Filter /
Effluent
\
l
\
\
\
\
\
\
\ 2nd filter
scour
water
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 floe 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 floe 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,
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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 high-
gradient 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 floe. 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.
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Municipal Nutrient Removal Technologies Reference Document
September 2008
Metal Salt +
pH Control
Feed
Fresh
To
Disinfection
Magnetic
Ballast
Magnetic
Ballast
Recovery
Separator
Water
Sludge
Processing
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.
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Secondary
Effluent
Moving Bed Filter 1
Moving Bed Filter 2
Pre-reactor 1
Fe
3+
Pre-reactor 2
7 Effluent
Fe
3+
Reject
Reject
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.
Polymer
Polymer
Al/Fe
Influent
wwwww:
OKftKftKft
•I
#K#K#K;
In1 in 51 1
Effluent
Tube
Clarifier
Adsorption
Clarifier
Mixed
Media Filter
Figure 2-21. U.S. Filter Trident process.
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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
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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-to-
TKN 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.
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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 A2O, as with all combined nitrogen-
phosphorus 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).
Nitrified Recycle
Influent
Anaerobic
Tank
Anoxic
Tank
Aerobic Tank
Secondary
Clarifier
Effluent
RAS
WAS
Figure 2-22. A2O process.
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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).
Nitrified Recycle
Anaerobic Tank
Influent
(Optional) Methanol
Addition
Aerobic Tank
Anoxic
Tank
Aerobic Tank
Anoxic
Tank
Secondary
Clarifier
Effluent
RAS
WAS
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
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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).
Nitrified Recycle
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).
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Nitrified Recycle
Influent
Anaerobic
Tank
Anoxic
Tank
Anoxic
Tank
Aerobic Tank
Secondary
Clanfier
Effluent
Mixed Liquor
Recycle
WAS
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 VTP 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).
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Nitrified Recycle
Influent
Anaerobic
Tank
Anoxic
Tank
Mixed Liquor
Recycle
Anoxic
Tank
Aerobic Tank
Secondary
Clanfier
Effluent
RAS
WAS
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).
Nitrified Recycle
Figure 2-27. Johannesburg process.
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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).
Alternating
Aerobic/Anoxic Tanks
Flows switch when tanks
alternate
Tank 1-An oxic
Influent
Anaerobic
Tank
X
IP
tii
/
Tank 2
r
Secondary
Clanfier
Effluent
RAS
WAS
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).
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Effluent from
First Stage-
High Rate
Activated Sludge
Aerobic Zone
Nitrification
Anoxic Zone
Denitrification
Aerobic Zone FeC13 addltlon
Nitrification
Secondary
Clarifier
Effluent
to
Tertiary
Filler
Methanol Feed
RAS
WAS
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 pre-
anoxic 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).
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PimpedSypcBs
Pttmcf y Effluent
V=A
To Secondary
Clarifier
A ->•-*=•'•- -•'-
6Q ntemd Recycle
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.
Chapter 2: Treatment Technologies
2-57
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Table 2-1. Process Performance Data: Nitrogen Removal—Plant Effluent
TN
(ppm)
10
5
Nitrogen removal
Technology
Johannesburg
A2O
VIP
Step-feed AS
I FAS
MBBR
MLE
4-stage Bardenpho
Schreiber system
Blue Plains process
PIDs, clarifiers, aerobic
digestion
PIDs
SBR
Cyclic on-off
Westbank
Step-feed AS
Plant effluent
observed value
or range
(mg/L)
2.03 to 11. 44
7.3to9.0a
6.12a
4.9 to 11. 3a
5.8to6.8a
2.2to15a
3.5to12.1a
8a
7.5a
1.8 to 7.0
1.4 to 11.3
1.6 to 13.6
3.1 to 10.4
2.7 to 5. 8
3.7 to 7.4
Variability (mg/L)
Std dev./
COV %
1.66/21
~
~
1 .80/27
~
~
1 .00/23
~
~
0.51/14
1.81/42
2.31/50
1.17/25
0.51/12
0.63/12
Annual
average
(50%)
7.86
~
6.12
6.70
~
~
4.35
~
~
3.67
4.2
4.59
4.59
4.38
5.25
Max.
month
(92%)
10.41
~
~
8.62
~
~
5.54
~
~
4.46
7.3
6.84
6.15
4.9
6.15
Max.
week
(98%)
11.57
~
~
9.82
~
~
6.13
~
~
5.87
11.3
10.68
7.62
5.84
8.01
Max.
day
(99.7%)
13.28
~
~
13.05
~
~
7.76
~
~
~
~
14.35
8.64
~
~
Reference
Hagerstown, Maryland
Maryland Reportb
Neethling 2005
Cumberland, Maryland
Masterson 2004
McQuarrie 2004
Taljemark et al. 2004
Leesburg
Westminster, Maryland
Maryland Report
Maryland Report
Washington DCC
(Kangetal. 1992)
(Sadicketal. 1998)
North Gary, North
Carolina01
Jewett City, Connecticut0
Thomaston, Connecticut0
Ridgefield, Connecticut0
Kelowna, British
Columbia001
Fairfax, Virginia001
-------
Table 2-1. Process Performance Data: Nitrogen Removal—Plant Effluent (continued)
TN
(ppm)
3
Nitrogen removal
Technology
Biological aerated filters
Concentric oxidation ditch
Step-feed AS
5-stage Bardenpho
Denitrification filters
Denitrification filter
Denitrifying activated
sludge
Plant effluent
observed value
or range
(mg/L)
1.4 to 6.8
1 .6 to 5.4
1 to 14
1.24 to 4.29
0.47 to 3. 76
0.13 to 6.50
0.4 to 10.4
Variability (mg/L)
Std dev./
COV %
2.24/62
0.95/32
1 .48/57
0.35/16
0.86/42
0.36/16
0.56/28
0.59/36
Annual
average
(50%)
3.61e
3.0
2.58
2.32
2.04
2.14
1.71
1.63
Max.
month
(92%)
7.1 3e
4.24
4.30
3.10
3.10
2.77
2.61
2.46
Max.
week
(98%)
9.80a
5.29
5.89
3.75
3.90
3.13
3.90
4.22
Max.
day
(99.7%)
13.91a
6.46
9.16
4.36
5.44
4.25
-
~
Reference
Cheshire, Connecticut0
Hammonton, New Jersey
Piscataway, Maryland0
Clearwater, Florida-MSdf
Clearwater, Florida-NEf
Johnston County, North
Carolina °'d
Lee County, Florida01
Western Branch,
Maryland01
Notes:
A2O = anaerobic/anoxic/oxic
AS = activated sludge
COV = coefficient of variation
I FAS = 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.
aData obtained from literature; data were not reviewed as part of project.
"George Miles & Buhr, LLC, and Gannett Fleming 2004.
0 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.
'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 design at these facilities. Thus,
they do not necessarily represent optimum operation of the technologies presented.
Source: Table format adapted from WEF and ASCE 1998.
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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 step-
feed 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.
0.05 0.1
0.5 1
10
95
99.5 99.9 99.95
20 30 40 50 60 70 80
Percent Less Than or Equal To
^^—1— Denitrifying AS—Western Branch, MD •.•mmmm:,Q. 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
Figure 2-31. Monthly average frequency curves for TN: low-range removal.
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Table 2-2. Detailed curve information for Case Study Facilities in Figure 2-31
Plant
Western Branch,
MD
Lee Co., FL
Central Johnston
Co., NC
Marshall St.,
Clearwater, FL
#Data
points
236
261
249
260
Min
temp
51 °F
~
13.1 °C
22.1 °C
Max
temp
76 °F
~
27.9 °C
31.1 °C
Ave. temp
64 °F
30.4 °C*
20.4 °C
27.5 °C
Ave. flow
MGD
19.3
3.2
4.1
5.5
Design
flow MGD
30
5.0
7.0
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 step-
feed 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
Chapter 2: Treatment Technologies
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 Ib to 1 Ib.
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
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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).
MSWBEPJHKt'-Q"
mmj
mwi
Figure 2-32. Concentric oxidation ditch.
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 Ib methanol
per Ib 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 Gary, 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
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100
• 1 - Phased Isolation Ditch-North Gary, NC
•-•2-BAF-Cheshire, CT
"3 - Phased Isolation Ditch - Jewett City, CT
-4 - Westbank - Kelowna, BC
= 5 - MLE-Westiminster, MD
—6 - Cyclic On-Off-Ridgefield, CT
7 - SBR-Thomaston, CT
8 - Step Feed AS-Cumberland, MD
— - 9 - Johannesburg-Hagerstown, MD
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
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
North Gary, NC
Kelowna, BC
#Data
points
155
52
Min
temp
16 °C
13 °C
Max
temp
27 °C
22 °C
Ave. temp
21 °C
17.3 °C
Ave. flow
MGD
7.0
8.5
Design
flow MGD
12.0
10.6
Low variability was shown by the North Gary 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.
Chapter 2: Treatment Technologies
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September 2008
10
0.01
0.05 0.1
0.5 1
10
95
98 99 99.5 99.9 99.95
20 30 40 50 60 70 80 90
Percent Less Than or Equal To
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 Gary, NC
3 - A/0 - Clark Co, NV
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
Clark Co., NV
North Gary, NC
Noman Cole,
Fairfax Co., VA
Central Johnston
Co., NC
Kalispell, MT
#Data
points
365
249
365
249
12
Min
temp
17°C
16 °C
14 °C
13.1 °C
8.5 °C
Max
temp
30 °C
27 °C
28 °C
27.9 °C
19.6 °C
Ave.
temp
25 °C
21 °C
21 °C
20.4 °C
14.3°C
Ave. flow
MGD
98
7.0
47.4
4.1
2.9
Design
flow MGD
110
12.0
67
7.0
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.
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Chapter 2: Treatment Technologies
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Table 2-5. Process Performance Data: Phosphorus Removal—Plant Effluent
p
(ppm)
2
1
0.5
0.1
Phosphorus removal
Technology
A2O with VFA addition,
chemical addition, tertiary
clarifier, and filtration3
VIP
A/0
UCT with filter
Westbank with fermenter and
filters
Chemical addition and
flocculating clarifiers
PhoStrip
Modified UCT with fermenter
Tertiary clarifier with chemical
addition and filters3
A/O with chemical addition to
filters
Step-feed AS with fermenter
and filter
MBR
Denitrification filter with alum
Range of values
observed
(mg/L)
0.025 to 0.98
0.19to5.0c
0.03 to 0.43
0.3C
0.05 to 1.88
0.07 to 0.23
< 0.1 Ortho-Pc
0.03 to 0.37
0.026 to 0.24
0.03 to 2.3
0.02 to 0.26
0.011 to 0.554
0.02 to 1.34
Variability (mg/L)
Std dev./
COV %
0.044/33
-
0.12/50
-
0.03/21
0.01/14
-
0.023/19
0.036/63
0.03/30
0.02/21
0.075/107
0.05/35
Annual
average
(50%)
0.132
0.40
0.24
-
0.14
0.09
-
0.12
0.058
0.10
0.09
0.070
0.102
Max.
month
(92%)
0.18
1.75
0.36
-
0.20
0.11
-
0.15
0.12
0.17
0.12
0.17
0.19
Max.
week
(98%)
0.646
3.6
0.44
-
0.25
-
-
0.31
0.17
0.41
0.16
0.29
0.39
Max.
day
(99.7%)
0.98
7.5
0.75
-
-
-
-
0.36
0.23
0.56
0.26
0.54
-
Reference
Durham, Oregond
VIP, Neethling 2005
Genesee County, Michigan01
Penticton, British Columbia
Barnard, 2006
Kelowna, British Columbia13'01
Chelsea, Michigan
Truckee Meadows, Nevada
Barnard et al. 2006
Kalispell, Montana13
McMinnville, Oregon3
Clark County, Nevada"
Fairfax, Virginia13'01
Hyrum, Utah
Lee County, Florida13
-------
Table 2-5. Process Performance Data: Phosphorus Removal—Plant Effluent (continued)
p
(ppm)
Phosphorus removal
Technology
5-stage Bardenpho oxidation
ditch with chemical addition
and filters
MBR
EBPR with high rate solids
contact clarifier, chemical
addition, and filters
Chemical addition, tertiary
clarifiers, filter, infiltration basin
Range of values
observed
(mg/L)
0.02 to 0.078
0.01 to 0.083
to 0.02C
0.01
(monthly
averages)
Variability (mg/L)
Std dev./
COV %
0.011/34
0.0074/27
-
0/0
Annual
average
(50%)
0.031
0.027
0.01
0.01
Max.
month
(92%)
0.061
0.038
0.02
0.01
Max.
week
(98%)
0.078
0.053
-
0.01
Max.
day
(99.7%)
-
-
-
0.01
Reference
Pinery Water, Colorado
Lone Tree Creek, Colorado
Breckenridge, Colorado
Brighton, Michigan
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.
0 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 design at these facilities. Thus,
they do not necessarily represent optimum operation of the technologies presented.
Source: Table format adapted from WEF and ASCE 1998.
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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
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10
o
.c
Q.
I
0.
0.1
0.01
0.001
0.05 0.1
0.5 1
10 20 30 40 50
70 80
90 95
99 99.5 99.9 99.95
Percent Less Than or Equal To
1 - Chem Addn + Tert Clarifiers + Land Application-Brighton, Ml
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
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
Lee Co., FL
#Data
points
261
Min
temp
-
Max
temp
-
Ave. temp
30.4 °C*
Ave. flow
MGD
3.2
Design flow
MGD
5.0
* Only one available in July 2006
either filtration method, a sufficiently sized floe 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 floe, 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.
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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 floe 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
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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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Municipal Nutrient Removal Technologies Reference Document
0.01
0.05 0.1
10
20 30 40 50 60 70 80 90
Percent Less Than or Equal To
95
99 99.5 99.9 99.95
"1 - Step Feed w/ Fermenter-Piscataway, MD
-2 - EBPR w/VFA Addn + Filters-Kalispell, MT
3 - Five-stage Bardenpho-Marshall St., Clearwater, FL
•4 - A2O with VFA, chemical, and filter-Durham, OR
»5- Westbank-Kelowna, BC
::!:6- Five-stage Bardenpho-Northeast, Clearwater, FL
7 - Denitrification Filters + Chem Addn-Johnston Co., NC
-8 - A/O-Genesee Co., Ml
9 - Phased Isolation Ditch-North Gary, NC
-10 - Triple sludges—Western Branch, MD.
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
Western Branch, MD
Central Johnston Co., NC
Marshall St, Clearwater, FL
Kalispell, MT
Kelowna, BC
North Gary, NC
#Data
points
285
52
260
104
51
155
Min
temp
51 °F
13.1 °C
22.1 °C
8.5°C
13 °C
16 °C
Max
temp
76 °F
27.9 °C
31.1 °C
19.6 °C
22 °C
27 °C
Ave.
temp
64 °F
20.4 °C
27.5 °C
14.3 °C
17.3 °C
21 °C
Ave. flow
MGD
19.3
4.1
5.5
2.9
8.5
7.0
Design
flow MGD
30
7.0
10.0
3.1
10.6
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 Gary, 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.
Chapter 2: Treatment Technologies
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Municipal Nutrient Removal Technologies Reference Document September 2008
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-to-
nitrate nitrogen ratio of 0.02 or higher is safe.
North Gary, North Carolina
The North Gary 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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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-to-
anaerobic 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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Municipal Nutrient Removal Technologies Reference Document September 2008
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.
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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 nitrogen-
removal 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.
Chapter 2: Treatment Technologies 2-77
-------
Table 2-8. Process Performance Data: Nitrogen and Phosphorus Removal—Plant Effluent
N
(ppm)
10
5
Nitrogen and phosphorus removal
Technology
Johannesburg
UCT
I FAS
VIP
VIP with VFA addition
Step-feed with
fermenter
Biodenipho/PID
Modified UCT with VFA
addition
Range of values
observed
(mg/L)
TN: 2.03 to 11. 44
TP: 0.1 9 to 8.3
TN:8.9to 10a
TP: 0.3
TIN:5.6to11.3a
TP:0.2to1.7a
TN:3to10
TP: 0.1 9 to 5.75
TN:5to10
TP: 0.6 to 0.8
TN:<5.0;3to 13
TP:<0.3;0.1 to 5
TN: 1.78 to 7.02
TP: 0.09 to 1.99
TN: 5 to 6a
TP:0.1 to2.7a
Variability (mg/L)
Std dev./
COV %
1 .66/21
0.96/145
1 .48/57
0.08/89
0.93/14
0.27/64
Annual
average
(50%)
7.86
0.66
2.59
0.09
3.67
0.38
0.10
Max.
month
(92%)
10.41
2.49
4.30
0.20
4.46
1.06
0.25
Max.
week
(98%)
11.57
3.9
5.89
0.31
5.87
1.45
0.75
Max. day
(99.7%)
13.28
8.3
9.16
0.52
6.79
1.78
3.75
Reference
Hagerstown,
Maryland
WEF and ASCE
1998
Broomfield, CO
McQuarrie 2004
Rabinowitz 2004
Neethling 2005
Piscataway, MD
North Gary, NCb
McDowell Creek,
NC
Neethling 2005
P
(ppm)
2
1
0.5
0.1
-------
Table 2-8. Process Performance Data: Nitrogen and Phosphorus Removal—Plant Effluent (continued)
N
(ppm)
Nitrogen and phosphorus removal
Technology
High rate, nitrification,
and denitrification
activated sludge
5-stage Bardenpho with
chemical addition for P
removal
Denitrification filters
with chemical addition
Denitrification filters
with chemical addition
Range of values
observed
(mg/L)
TN: 0.4 to 10.4
TP: 0.05 to 1.88
TN: 1.24 to 4. 29
TP: 0.06 to 0.37
TN: 0.87 to 5.59
TP: 0.06 to 1.09
TN: 0.13 to 6.50
TP: 0.02 to 1.34
TN: 0.84 to 3. 13
TP:0.1 to 1.01
Variability (mg/L)
Std dev./
COV %
0.59/36
0.27/62
0.36/16
0.07/40
0.86/42
0.16/82
0.56/28
0.05/35
0.36/16
0.09/62
Annual
average
(50%)
1.63
0.43
2.32
0.13
2.04
0.20
1.71
0.102
2.14
0.26
Max.
month
(92%)
2.46
0.89
3.1
0.21
3.10
0.44
2.61
0.19
2.77
0.64
Max.
week
(98%)
4.22
0.99
3.75
0.26
3.90
0.63
3.70
0.39
3.13
1.01
Max. day
(99.7%)
4.29
0.46
5.44
1.07
Reference
Western Branchb
Hyattsville, MD
Clear-water, FL-MSb
Clearwater, FL-NE
Fiesta Village13
Lee County, FL
Johnston Co., NCbc
P
(ppm)
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.
0 Retrofit application.
The data reflect specific operating philosophy, permit limitations, influent conditions, flow conditions and the relative plant loadings compared to their design at these facilities. Thus,
they do not necessarily represent optimum operation of the technologies presented.
Source: Table format adapted from WEF and ASCE 1998.
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Municipal Nutrient Removal Technologies Reference Document September 2008
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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
needed to establish the minimum phosphorus concentration needed to support nitrogen
removal in the same process.
2.7 References
Barnard, J. 2006. Biological Nutrient Removal: Where We Have Been, Where We Are
Going. In Proceedings of the Water Environment Federation's 79th Annual Technical
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Carroll, J.R., P. Pitt, and A. vanNiekerk. 2005. Optimization of Nitrification/Denitrification
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Details: Full-Scale Optimization of the EBPR Process at the City of Las Vegas
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2-90 Chapter 2: Treatment Technologies
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Attachment 1: Locations Providing Data
Location
Brighton, Michigan
Chelsea, Michigan
Clark County, Nevada
Clearwater, Florida: Northeast
Clearwater, Florida: Marshall Street
Cumberland, Maryland
Cheshire, Connecticut
Durham, Oregon
Enfield, Connecticut
Jewett City, Connecticut
New Haven, Connecticut
Ridgefield, Connecticut
Thomaston, Connecticut
Hagerstown, Maryland
Genesee County, Michigan
Hammonton, New Jersey
Hyrum, Utah
Central Johnston County, Smithfield,
North Carolina
Kalispell, Montana
Fairfax County, Virginia
Piscataway, Maryland
McMinnville, Oregon
Kelowna, British Columbia
North Gary, North Carolina
Pinery Water, Colorado
Western Branch, Marlboro, Maryland
Westminster, Maryland
Lee County, Florida
Breckenridge, Colorado
Lone Pine Tree, Colorado
Data performance period
January through December 2005
October 2005 through September 2006
January through December 2006
October 2005 through September 2006
October 2005 through September 2006
January through December 2005
April through November 2006
January through December 2004
September 2005 through August 2006
September 2005 through August 2006
September 2005 through August 2006
September 2005 through August 2006
September 2005 through August 2006
January through December 2005
October 1999 through September 2000
January through December 2005
November 2005 through September 2006
October 2005 through September 2006
July 2005 through June 2006
January through December 2006
January through December 2005
January 2005 through October 2006
January 2005 through December 2005
October 2005 through September 2006
January through December 2004
January through December 2006
October 2005 through September 2006
January through December 2006
January through December 2003
January through December 2006
Chapter 2: Treatment Technologies
2-91
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Chapter 3 - Case Studies and Reliability Factors 3-1
-------
Table 3-1. Discharge permit limits and performance data summary
o
-a'
0)
Plant and
location
Noman M. Cole
Pollution
Control Plant;
Fairfax County,
Virginia
Kalispell
Advanced
Wastewater
Treatment,
Kalispell,
Montana
Clark County
Water
Reclamation
District, Nevada
Central
Johnston
County
Smithfield,
North Carolina
North Gary,
North Carolina
Nutrient
removal
processes
Step-feed AS
with dual media
& deep bed filter
+ ferric chloride
Mod. UTC with
fermenter and
up flow sand
filter (no
chemical for P)
A/O process with
active primary
for VFA with
dual media filters
+ alum
Plug flow AS
(anoxic,
aeration) with
denitrification
filter and
methanol (No
chem. for P)
Oxidation ditch
(Biodenipho)
with upflow sand
filter
(No chemical
forP)
Permit limits and performance results( mg/L)
Annual
average
TP = 0.09
NH4-N =
0.12
TN = 5.25
TP = 0.121
NH4-N =
0.07
TP = <
0.09
NH4-N =
0.12
TP = 0.26
NH4-N =
0.44
TN = 2.14
TP=0.38
Ammonia
N = 0.08
TN=3.7
COV for
annual
average
TP: 21%
NH4-N:
14%
TN: 12%
TP: 19%
NH4-N: 0%
TP: 30%
NH4-N:
14%
TP: 62%
NH4-N:
12%
TN: 16%
TP: 64%
Ammonia
N: 102%
TN: 14%
Quarterly or annual
Avg
permit
limits
No
quarterly or
annual
limits
No
quarterly or
annual
limits
No
quarterly or
annual
limits
TP = 2.0
(Q)
TN = 3.7**
TP = 2.0
(Q)
TN = 3.94b
(A)
Max (q)
Avg (a)
result
TP = 0.47
(Q Max)
TN = 2.14
(Av)
TP = 0.57
(Q max)
TN = 3.7
(Av)
Month
Avg
permit
limits
TP = 0.18
Sum NH4-
N = 1.0
Win NH4-N
= 2.2
TN =
report
TP=1.0
NH4-N
=1.4
No
monthly
average
limits
TP=1.0
Sum NH4-
N = 2.0
Win NH4-N
= 4.0
Sum NH4-
N = 0.5
Win NH4-N
= 1.0
Max
month
result
TP = 0.12
NH4-N =
0.135
TN=6.0
TP=0.15
NH4-N =
0.07
TP = 0.12
NH4-N =
0.15
TP = 0.64
NH4-N =
0.54
NH4-N =
0.34
Week
Avg
permit
limits
TP = 0.27
Sum NH4-
N = 1.5
Win NH4-N
= 2.7
TN = report
No weekly
limits
No weekly
limits
Sum NH4-
N = 6.0
Win NH4-N
= 12.0
No weekly
limits
Max
week
result
TP = 0.16
NH4-N
=0.29
TN = 8.01
NH4-N =
0.86
Max day
Permit
limit
No daily
limits
No daily
limits
TP= 0.22a
NH4-N =
0.64a
No daily
limits
No daily
limits
Max day
TP = 0.22
NH4-N =
0.57
-------
Table 3-1. Discharge permit limits and performance data summary (continued)
Plant and
location
Kelowna, British
Columbia,
Canada
Marshall Street
Water
Reclamation
Facility,
Clearwater,
Florida
Lee County,
Florida
Western
Branch,
Upper Marlboro,
Maryland
Nutrient
removal
processes
3-stage
Westbankwith
fermenters
(No chemical
forP)
5-stage
Bardenpho with
sand filter +
alum
AS with
denitrifi cation
filter and
methanol
addition + alum
Three AS with
methanol
addition + alum
Permit limits and performance results( mg/L)
Annual
average
TP=0.139
TN=4.38
TP=0.132
NH4-
N=0.038
TN=2.32
TP=0.102
TN=1.57
TP = 0.47
NH4-N =
0.22
TN = 1.70
COV for
annual
average
TP: 21%
TN: 12%
TP: 40%
NH4-N:
18%
TN: 16%
TP: 35%
TN: 28%
TP: 62%
NH4-N:
163%
TN: 36%
Quarterly or annual
Avg
permit
limits
TP = 0.25
(A)
TP= 1.0
(A)
TN = 3.0
(A)
TP = 0.5
(A)
TN = 3.0
(A)
TP = 0.3C
(A)
TN = 4.0C
(A)
Max (q)
Avg (a)
result
TP = 0.139
(Av)
TP = 0.132
(Av)
TN = 2.32
(Av)
TP = 0.08
(Av)
TN = 1.57
(Av)
Month
Avg
permit
limits
No
monthly
average
limit
TP= 1.25
TN = 3.75
TP = 0.5
TN=3
TP= 1.0
TN = 3.0
(Apr-Oct)
NH4-N =
1.5
(Apr-Oct),
NH4-N =
5.5
(Nov-Mar)
Max
month
result
TP = 0.20
NH4-N
=1.01
TN=4.9
TP = 0.21
TN = 3.1
TP = 0.19
TN=2.61
Week
Avg
permit
limits
No weekly
limits
TP= 1.5
TN = 4.5
TP = 0.75
TN=4.5
TN = 4.5
(Apr-Oct)
Max
week
result
TP = 0.26
TN = 3.75
TP = 0.39
TN = 3.70
Max day
Permit
limit
TP = 2.0
TN =6.0
(daily max)
TP = 2.0
TN = 6
(daily max)
TP= 1.0
TN=6
(daily max)
Max day
TP = 0.25
TN = 5.8
TP = 0.37
TN = 4.29
TP = 0.27
TN = 5.98
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 allocation on the basis of daily
maxima at design flow
b Annual average permit limit derived from a wasteload allocation on the basis of annual average
at design flow
c Annual average limits based on 30 MGD annual load following installation of Maryland-funded
upgrades
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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
99th percentile
90th percentile
Annual average
2.0 mg/L TP
1.5mg/L
1.0 mg/L
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
50
80
90
92.3
98.1
99
99.7
U.S. term — averaging or
maximum period
Annual average
Maximum quarter
~
Maximum month
Maximum week
~
Maximum day
Canadian term — percent less
than value
50th percentile
-
90th percentile
~
~
99th percentile
-
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.
3-4
Chapter 3 - Case Studies and Reliability Factors
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Municipal Nutrient Removal Technologies Reference Document
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.
100
10
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Total Phosphorus
0.1
0.01
0.001
: Mean = 0.43 mg/L
EStd. Dev. = 0.27 mg/L
I C.O.V. = 62%
0.05 0.1 0.5 1
10
20 30 40 50 60 70 80 90
Percent Less Than or Equal To
95 98 99 99.5 99.999.95
e Plant Influent
o NAS Effluent
n HRAS Influent
A DNAS Effluent
x HRAS Effluent
x Final Effluent
Figure 3-1. Western Branch WWTP: Monthly frequency curves for TP.
Chapter 3 - Case Studies and Reliability Factors
3-5
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Municipal Nutrient Removal Technologies Reference Document
September 2008
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Ammonia-Nitrogen
_, 1°-
0)
§ i
0) '
z
| 0.1 -
o
E
0.01 -
n nn-i -
1 1 1 x x x
n
O
° ° A^-£^
- —
ii iii ii iii
1 1 i
0 0 0 0
A— •*•*""*
•£~X~>T X
1
3 O
< *
O
3
Std. Dev. - 0.22 mg/L
C.O.V. = 163%
i i i i i i
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
o Combined Raw Influent n HRAS Influent x HRAS Effluent
o MAS Effluent A DMAS Effluent x Final Effluent
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 Ib/nitrate nitrogen
entering the DNAS tank.
3-6
Chapter 3 - Case Studies and Reliability Factors
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100 •
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Total Nitrogen
0)
5
81
10
0.1
-c 8 9 t 9 8
8 i
A A A
_Mean= 1.63 mg/L
; Std. Dev. = 0.59 mg/L
: C.O.V. = 36%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
o Combined Raw Influent n HRAS Influent o MAS Effluent
A DMAS Effluent x Final Effluent
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.
Chapter 3 - Case Studies and Reliability Factors
3-7
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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.
100
u>
E
o
.c
Q.
t/>
O
.E
Q.
10
0.01
Fiesta Village WWTP, Lee Co., FL
Monthly Average Frequency Curves for Total Phosphorus
Mean = 0.102 mg/L
E Std. Dev. = 0.035 mg/L
: C.O.V. = 35%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 3-4. Lee County, Florida: Monthly frequency curves for TP.
100.000
u>
E
c"
HI
O)
o
10.000
1.000
0.100
Fiesta Village WWTP Lee Co., FL
Monthly Average Frequency Curves for Nitrogen
- Mean = 1.71 mg/L
- Std. Dev. = 0.48 mg/L
-C.O.V. = 28%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
» Raw Influent - Total N x Final Effluent - Total N
• Secondary Effluent NO3-N
Figure 3-5. Lee County, Florida: Monthly frequency curves for TN.
3-8
Chapter 3 - Case Studies and Reliability Factors
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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 Ib/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 Ib 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
Chapter 3 - Case Studies and Reliability Factors 3-9
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Municipal Nutrient Removal Technologies Reference Document
September 2008
aerobically digested before dewatering by a belt filter press off-site. After UV disinfection,
the reclaimed water is stored for reuse.
100 n
d 10-
sphorus, m
0
.c
Q.
s
£ 0.1 -
n m -
Johnston County,
Monthly Average Frequency Curves 1
„ ..•***
V V
ii iii i i iii
x-*-"r""
1 1 1
NC
or Total Phosphorus
K—-"^
Mean = 0.26 mg/L
Std. Dev. = 0.164 mg/L
C.O.V. - 62%
I I I I I I I I
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent x Final Effluent
Figure 3-6. Johnston County, North Carolina: Monthly average frequency curves for TP.
Johnston County, NC
Monthly Average Frequency Curves for Ammonia-N
_, 10-
0)
£
=
O>
&
§ 1 -
n 1 .
»• • »
,
.„ M- V.
m x— * '
» » * *
» »
^ — i/g Moan _ n AA mn/l
Std. Dev. = 0.055 mg/L
C.O.V. =12%
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
» Raw Influent Ammonia-N
x Final Effluent Ammonia-N
Figure 3-7. Johnston County, North Carolina: Monthly average frequency curves for ammonia
nitrogen.
3-10
Chapter 3 - Case Studies and Reliability Factors
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September 2008
Municipal Nutrient Removal Technologies Reference Document
100 •
Johnston County, NC
Monthly Average Frequency Curves for Total Nitrogen
10
u>
E
HI
ra
o
0.1
: Mean = 2.14 mg/L
I Std. Dev. = 0.36 mg/L
-C.O.V. =16%
0.050.1 0.5 1 2
10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent TN x Final Effluent TN
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.
Chapter 3 - Case Studies and Reliability Factors
3-11
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 |ig/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.
3-12 Chapter 3 - Case Studies and Reliability Factors
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September 2008
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100
O>
E
10
0.1
0.01
Marshall Street Advanced WWT
Monthly Average Frequency Curves
_^-x-*~r
• 3fca- * *
ii iii ii iii
^*^~
I I I
3 Clearwater, FL
For Total Phosphorus
» •
* ~* Mean = 0.132 mg/L
Std. Dev. = 0.052 mq/L
C.O.V. - 40%
I I I I I I I I
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 3-9. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average frequency
curves for TP.
100-
Marshall Street Advanced WWTP Clearwater, FL
Monthly Average Frequency Curves for Ammonia Nitrogen
O>
E
5
O>
10
0.01
» »
E Mean = 0.038mg/L
: Std. Dev. = 0.007 mg/L
: C.O.V. = 18%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
» Raw Influent - Ammonia-N
x Final Effluent -Ammonia N
Figure 3-10. Marshall Street Advanced WWTP, Clearwater, Florida: Monthly average frequency
curves for ammonia nitrogen.
Chapter 3 - Case Studies and Reliability Factors
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Municipal Nutrient Removal Technologies Reference Document
September 2008
100-
Marshall Street Advanced WWTP Clearwater, FL
Monthly Average Frequency Curves for Nitrogen
10
D)
E
5
D)
O
0.1
• »
: Mean = 2.32 mg/L
- Std. Dev. = 0.38 mg/L
C.O.V. = 16%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90
Percent Less Than or Equal To
95 989999.5 99.999.95
• Raw Influent - Ammonia-N
x Final Effluent -Total N
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
3-14
Chapter 3 - Case Studies and Reliability Factors
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September 2008
Municipal Nutrient Removal Technologies Reference Document
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 Gary, North Carolina
The North Gary, 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.
100-
North Gary, NC
Monthly Average Frequency Curves for Total Phosphorus
ra
E
10
$
o
0.1
0.01
! Mean = 0.379 mg/L
1 Std. Dev. = 0.241 mg/L
! C.O.V. = 64%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 3-12. North Gary, North Carolina: Monthly average frequency curves for TP.
Chapter 3 - Case Studies and Reliability Factors
3-15
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Municipal Nutrient Removal Technologies Reference Document
September 2008
North Gary, NC
100 -I
10 -
5 1-
O)
s.
z
0.1 -
n m .
moniniv Mveraae i-requer
* — * • • *
,
-J^^"*"*" *"
*^-~*^^
cv curves T
» « « « — •
Jlf*% — X — X
ar Ammonia Niiroaen
» »
•x
.,
Mean = n DR1 mg/l —
Std. Dev. - 0.082 mg/L
C.O.V. = 102%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N
x Final Effluent - Ammonia-N
Figure 3-13. North Gary, North Carolina: Monthly average frequency curves for ammonia
nitrogen.
100-
North Gary, NC
Monthly Average Frequency Curves for Total Nitrogen
10
Ul
E
HI
ra
o
0.1
0.01
: Mean = 3.67 mg/L
-Std. Dev. = 0.512 mg/L
"C.O.V. = 14%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
» Raw Influent - Total Nitrogen x Final Effluent - Total Nitrogen
Figure 3-14. North Gary, 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
3-76
Chapter 3 - Case Studies and Reliability Factors
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 MOD, and the facility treated 7 MOD as the annual
average for the evaluation year. The wastewater is settled in primary tanks, treated at the
Chapter 3 - Case Studies and Reliability Factors 3-17
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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.
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Kelowna, BC
Monthly Average Frequency Curves for Ammonia-N
_, 10-
0)
E
c
0)
2 1 -
n 1 -
» * *
;,; X Xj*»Jfc
^p^^--*^"^
^^^K**^— -
•3K*'"X^
^ 4
r^
Mean - 0.575 rny/L
Std. Dev. - 0.26 mg/L
C.O.V. =45%
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
99 99.5 99.999.95
Raw Influent Ammonia-N
x Final Effluent Ammonia-N
Figure 3-16. Kelowna, British Columbia: Monthly average frequency curves for ammonia
nitrogen.
100-
Kelowna, BC
Monthly Average Frequency Curves for Total Nitrogen
0)
c"
0
D)
O
: Mean = 4.38 mg/L
; Std. Dev. = 0.51 mg/L
C.O.V. =12%
0.1
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent TN x Final Effluent TN
Figure 3-17. Kelowna, British Columbia: Monthly average frequency curves for TN.
Chapter 3 - Case Studies and Reliability Factors
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 phosphorus-
accumulating 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 FtRT (10 hours) and SRT (10 days). The FtRT 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.
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Municipal Nutrient Removal Technologies Reference Document
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 MOD, and the annual average flow was 47 MOD
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
: Mean = 0.086 mg/L
-3d. Dev. = 0.018 mg/L
-COV = 21%
30 40 50 60 70 80
Percent Less Than or Equal To
• Raw Influent D Primary Influent
X Tertiary Clarifier Influent * Tertiary Clarifier Effluent
I Primary Effluent
k Final Effluent
i Secondary Effluent
Figure 3-18. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly average
frequency curve for TP.
Chapter 3 - Case Studies and Reliability Factors
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September 2008
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Monthly Average Frequency Curves for Ammonia Nitrogen
Mean = 0.12 mg/L
~d. Dev. = 0.016 mg/L
-COV=14%
0.05 0.1 0.5 1 2 5 10
20 30 40 50 60 70 80
Percent Less Than or Equal To
99.5 99.9 99.95
> Raw Influent - Ammonia N X Final Effluent - Ammonia N
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
O)
E
- Mean = 5.25 mg/L
~Std. Dev. = 0.63 mg/L
_COV=12%
0.05 0.1 0.5 1 2 5
10 20 30 40 50 60 70 80 90
Percent Less Than or Equal To
95 98 99 99.5 99.9 99.95
• Raw Influent - TKN
X Final Effluent - Total N
Figure 3-20. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia: Monthly average
frequency curve for TN.
3-22
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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-to-
TKN 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 VF A 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.
Chapter 3 - Case Studies and Reliability Factors 3-23
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 MOD to 110 MOD 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 MOD. The facility treated 95 MOD
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
3-24 Chapter 3 - Case Studies and Reliability Factors
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September 2008
Municipal Nutrient Removal Technologies Reference Document
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
100-
10
0.1
0.01
: Mean = 0.0995 mg/L
:Std. Dev. = 0.0297 mg/L
; COV = 30%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
Figure 3-21. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly average
frequency curve for TP.
Chapter 3 - Case Studies and Reliability Factors
3-25
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Municipal Nutrient Removal Technologies Reference Document
September 2008
Clark Co. Water Reclamation Facility
Monthly Average Frequency Curves for Nitrogen
IUU -|
_, 10-
0)
£
c
0)
1
= 1 -
n 1 -
. • • » • *
» » » *
CP Mean =15.4 mg/L —
Strl DPV = n 54 mg/l
rnv = 3 R%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
Raw Influent - TKN
x Final Effluent - Total N
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
10-
D)
% 1-
o
*J
z
0.1 -
rim -
* * * * * '
» »
CP Mean =0.12 mg/L
Std. Dev. = 0.01 16 mg/L
COV 14% N
« *
0.05 0.1 0.5 1 2
10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
989999.5 99.999.95
• Raw Influent - TKN
x Final Effluent - Total N
Figure 3-23. Clark County Water Reclamation Plant, Las Vegas, Nevada: Monthly average
frequency curve for ammonia nitrogen.
3-26
Chapter 3 - Case Studies and Reliability Factors
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 no-
blanket 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.
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Municipal Nutrient Removal Technologies Reference Document
September 2008
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.
Kalispell, MT
Monthly Average Frequency Curves for Total Phosphorus
f *
-*—*—» » *
: Mean = 0.121 mg/L =
: Std. Dev. = 0.023 mg/L :
: C.O.V. = 19% :
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5
Percent Less Than or EquaTo
* Raw Influent
Final Effluent
Figure 3-24. Kalispell, Montana: Monthly average frequency curve for TP.
Kalispell, MT
Monthly Average Frequency Curves for Ammonia Nitrogen
AmmtaiB
0.1
- Mean < 0.07 mg/L
; Std. Dev. = 0.00 mg/L
: C.O.V. = 0%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
X Final Effluent
Figure 3-25. Kalispell, Montana: Monthly average frequency curve for ammonia nitrogen.
3-28
Chapter 3 - Case Studies and Reliability Factors
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September 2008
Municipal Nutrient Removal Technologies Reference Document
Kalispell, MT
Monthly Average Frequency Curves for Total Nitrogen (Goal)
— Mean = 10.6 mg/L
= Std. Dev. =3.31 mg/L
IC.O.V. = 31%
20 30 40 50 60 70 80
Percent Less Than or Equal To
» Raw Influent X Final Effluent
99.5 99.9 99.95
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 Gary 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 Gary 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.
Chapter 3 - Case Studies and Reliability Factors
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September 2008
Table 3-3. Monthly variation of wastewater characteristics at Fairfax County, Virginia
Date
Jan-06
Feb-06
Mar-06
Apr-06
May-06
Jun-06
Jul-06
Aug-06
Sep-06
Oct-06
Nov-06
Dec-06
Avg
INF
BOD-to-TP
33.1
33.7
28.3
28.2
27.2
28.9
28.8
29.4
31.5
33.5
32.2
28.2
30.3
Normalized
by avg
1.1
1.1
0.9
0.9
0.9
1.0
1.0
1.0
1.0
1.1
1.1
0.9
PE
BOD-to-TP
29.8
29.5
27.4
27.1
26.8
24.4
26.1
28.1
32.4
33.8
32.0
26.3
28.7
Normalized
by avg
1.0
1.0
1.0
0.9
0.9
0.9
0.9
1.0
1.1
1.2
1.1
0.9
INF
BOD/TKN
6.1
5.4
5.3
5.5
4.7
5.5
5.9
4.6
5.0
4.9
5.4
5.4
5.3
Normalized
by avg
1.1
1.0
1.0
1.0
0.9
1.0
1.1
0.9
0.9
0.9
1.0
1.0
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-to-
TKN 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 FtRT 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-to-
phosphorus ratio of 4 or higher could be maintained. Among these facilities, Kalispell and
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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. SRTcmdHRT: 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 Gary 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
Chapter 3 - Case Studies and Reliability Factors 3-31
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 pre-
programmed 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 Gary, 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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 Gary, 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 Gary, North
Chapter 3 - Case Studies and Reliability Factors 3-33
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 Gary, 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.
3-34 Chapter 3 - Case Studies and Reliability Factors
-------
Table 3-4. Case study facilities' treatment processes
Facility
Fairfax,
VA
Kalispell,
MT
Clark
Co., NV
Central
Johnston, NC
N. Gary,
NC
Kelowna,
BC
Clearwater,
FL
Lee Co.
FL
Western
Branch, MD
Wastewater characteristics
BOD-to-TP ratio
BOD-to-TKN ratio
Temperature in °C
Equalization basin- percent of influent
flow
Equalization basin location
29.5
5.4
15-25
11.5-20
Raw/SE
54.9
5.7
10-20
29.
5.5
20-29
55.1
10.
14-27
31.6
4.3
16-27
58
Raw
26.6
5.3
13-22
7.5
Raw
37.5
6.7
23-31
34.8
4.0
30
89.7 (COD/TP)
13.9(COD/TN)
13-23
Bioreactors
Bioreactor type
SRT in bioreactor, days
Fermenter SRT, days
Swing zones
Mixing/aeration separated in
bioreactor
Denitrification blanket
Denitrification filter
External carbon source
Alkalinity supplement
Online DO probe
Online NOs probe
Online ORP probe
Online ortho-P probe
Step-feed
AS
16-19
3
yes
NaOH
yes
Mod.
UCT
8-12
4-5
yes
yes
A/0
5-9
primary
yes
AS
denit. filter
7-9
No
yes
yes
Methanol
yes
yes
Oxid. ditch
11-14
No
yes
yes
Westbank
6-9
3-6
yes
yes
yes
yes
yes
yes
yes
Bardenpho
25^4
No
yes
yes
yes
yes
yes
yes
AS/
denit. filter
24-59
No
yes
yes
yes
Methanol
yes
Triple AS
6-60
yes
Methanol
yes
Tertiary Treatment
Tertiary clarifier
Tertiary filters
Tertiary conventional sand filter
Tertiary dual-media filter
Tertiary deep mono-media filter
Tertiary dual sand filter
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
-------
Table 3-4. Case study facilities' treatment processes (continued)
Facility
Fairfax,
VA
Kalispell,
MT
Clark
Co., NV
Central
Johnston, NC
N. Gary,
NC
Kelowna,
BC
Clearwater,
FL
Lee Co.,
FL
Western
Branch, MD
Sludge Handling
Primary settling / yes or no
Secondary sludge thickening -
aerobic
Sec sludge gravity
thickening/holding
Separate secondary/primary sludge
storage
No dewatering on-site
On-site aerobic sludge digestion
On-site anaerobic primary sludge
digestion
On-site anaerobic prim+sec. sludge
dig.
No on-site sludge digestion
yes
DAF
Yes
yes
DAF
Yes
yes
yes
DAF
yes
yes
NO
yes
NO
yes
yes
yes
DAF
yes
yes
yes
Rotary drum
Yes
NO
Yes
NO
DAF
yes
Recycle Streams
chemical treatment of
filtrate/centrate
Filtrate/centrate recycle point
Filter Backwash recycle point
lime
primary
FeCb
HDWK
HDWK
lime
HDWK
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
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 Kali spell 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.
Chapter 3 - Case Studies and Reliability Factors 3-37
-------
Table 3-5. Case study facilities' capital costs
o
-a'
0)
Facility
Design flow MGD
Influent P (mg/L)
Influent N (mg/L)
Influent NH3-N (mg/L)
Capital attributed to P
(2007 dollars)
Capital attributed to N
(2007 dollars)
Capital attributed to
other (2007 dollars)
Electrical attributed to P
(kWh/yr)
Electrical attributed to P
(dollars/yr)
Electrical attributed to N
(kWh/yr)
Electrical attributed to N
(dollars/yr)
Electrical attributed to
other (kWh/yr)
Electrical attributed to
other (dollars/yr)
Chemical attributed to P
(dollars/yr) (alum, ferric)
Chemical attributed to N
(dollars/yr) (Methanol,
pH control caustic as
needed)
Sludge attributed to P
(tons/yr) (alum/ferric or
biological uptake sludge)
Fairfax, VA
67
6.4
34.6
$0
$71,600,000
$0
3,360,000
$185,000
18,060,000
$993,000
0
$0
$393,000
$250,000
730
Kalispell, MT
3
4.1
39.6
24.3
$1,190,000
$4,310,000
$3,600,000
389,000
$17,500
1,077,000
$48,500
1,045,600
$47,100
$5400
$0
0
Clark
County, NV
110
5.8
30.3
26.8
$65,450,000
$56,980,000
$98,570,000
*
*
*
*
*
*
*
*
*
Central
Johnston,
NC
7
5.8
31.2
28
$889,000
$2,400,000
$0
1,843,000
$103,200
4,169,000
$233,500
588,000
$33,000
$0
$53,100
0
North Gary,
NC
12
7.7
56.4
45.5
$4,089,600
$16,358,000
$13,632,000
377,400
$17,400
2,558,000
$118,000
0
$0
$0
$0
90
Kelowna, BC
10.5
6.0
28.8
21.3
$6,818,000
$27,273,000
$22,728,000
884,000
$41,500
4,100,000
$193,000
919,000
$43,200
$0
$0
0
Clearwater,
FL
10
5
44.8
28
$5,019,000
$18,600,000
$5,905,000
931,000
$102,400
4,623,000
$509,000
2,075,000
$230,000
$7,400
$0
5.8
Lee County,
FL
5
3.85
33.2
27.2
$480,000
$13,460,000
$11,630,000
62,700
$7,500
1,911,500
$192,000
2,162,000
$259,000
$34,600
$83,900
37.5
Western
Branch, MD
30
3.7
23.9
19.6
$2,464,000
$49,583,000
$84,906,000
262,400
$26,200
8,865,000
$886,500
16,066,000
$1,607,000
$107,000
$425,000
72.9
-------
Table 3-5. Case study facilities' capital costs (continued)
Facility
Sludge attributed to P
(dollars/yr)
Sludge attributed to N
(methanol conversion)
(tons/yr)
Sludge attributed to N
(dollars/yr)
Total O&M attributed to
P (dollars/yr)
Total O&M attributed to
N (dollars/yr)
Annualized capital for P
(6%, 20 years)
Annualized capital for N
(6%, 20 years)
Unit O&M for TP($/lb)
Unit capital For TP($/lb)
Unit total for TP($/lb)
Unit O&M for TN($/lb)
Unit capital for TN($/lb)
Unit total for TN($/lb)
Unit capital cost
($/gal/day)
Fairfax, VA
$393,000
0
$0
$970,000
$1,243,000
$0
$6,240,000
$1.07
$0.00
$1.07
$0.29
$1.47
$1.76
$1.07
Kalispell, MT
$0
0
$0
$17,500
$57,800
$168,100
$323,600
$0.49
$2.84
$3.33
$0.19
$1.46
$1.65
$3.03
Clark
County, NV
*
*
*
$3,004,000
$3,852,000
$5,710,000
$4,970,000
$1.81
$3.43
$5.24
$0.43
$0.55
$0.98
$2.01
Central
Johnston,
NC
$0
60
$15,100
$103,200
$301,700
$51,000
$302,700
$1.48
$0.73
$2.21
$0.49
$0.49
$0.98
$0.58
North Gary,
NC
$17,900
1702
$341,000
$35,300
$230,000
$357,000
$1,426,000
$0.23
$2.28
$2.51
$0.41
$2.54
$3.36
$2.84
Kelowna, BC
$0
0
$0
$40,300
$109,000
$595,000
$2,378,000
$0.27
$3.97
$4.24
$0.14
$3.05
$3.19
$3.25
Clearwater,
FL
$1,500
0
$0
$109,800
$509,000
$438,000
$1,620,000
$1.37
$5.38
$6.76
$0.75
$2.40
$3.15
$2.95
Lee County,
FL
$21,900
14.1
$8,300
$64,000
$284,200
$41,900
$1,174,000
$1.77
$1.16
$2.93
$0.91
$3.87
$4.78
$2.79
Western
Branch, MD
$32,400
839
$372,000
$165,000
$1,311,000
$215,000
$4,324,000
$0.78
$1.01
$1.79
$0.99
$3.27
$4.26
$1.73
&Q
S"
a
a
a,
rs
S"
Note: Cost data available per unit operation; boldface type indicates actual dollar amounts.
-------
Municipal Nutrient Removal Technologies Reference Document September 2008
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.
3-40 Chapter 3 - Case Studies and Reliability Factors
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 Ib/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 Gary, $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 Gary 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 Gary 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
Chapter 3 - Case Studies and Reliability Factors 3-41
-------
Municipal Nutrient Removal Technologies Reference Document September 2008
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.
3-42 Chapter 3 - Case Studies and Reliability Factors
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Chapter 3 - Case Studies and Reliability Factors 3-43
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Municipal Nutrient Removal Technologies Reference Document September 2008
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.
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 Gary, 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 Gary, North Carolina. In addition,
North Gary 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 Gary, North Carolina; and Kalispell, Montana.
Chapter 3 - Case Studies and Reliability Factors 3-45
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Municipal Nutrient Removal Technologies Reference Document September 2008
- 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
3-46 Chapter 3 - Case Studies and Reliability Factors
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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 Gary, 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 PUD offer the best process protection against
wet-weather flows. North Gary, 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.
Chapter 3 - Case Studies and Reliability Factors 3-47
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Municipal Nutrient Removal Technologies Reference Document September 2008
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 perpound
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.
3-48 Chapter 3 - Case Studies and Reliability Factors
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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 ofWEFTEC, 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 Ol-CTS-3. IWA
Publishing, London.
Chapter 3 - Case Studies and Reliability Factors 3-49
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Municipal Nutrient Removal Technologies Reference Document September 2008
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. RetrofittingPOTWs 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.
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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 [MOD]), the capital cost (in dollars per gallon
per day [gpd] capacity), the O&M cost per MOD 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.
Chapter 4: Cost Factors 4-1
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Municipal Nutrient Removal Technologies Reference Document
September 2008
Table 4-1. Upgrade costs for Maryland, Connecticut, and others
Treatment
processes
Cyclic on/off
Added upflow
denitrification filter
MLE^
4-stage
Bardenpho
MLE^
4-stage
Bardenpho
Step-feed AS ->
4-stage
Bardenpho
MLE -* added
denite. filter
MLE -* added
denitrification filter
A2O^
5-stage
Bardenpho +
denitification filter
Post-MBBR for
TN removal3
On/off aeration3
I FAS3
Lagoon ->
4-stage
Bardenpho
MLE -» MBR/5-
stage Bardenpho3
Location
Ridgefield, CT
Cheshire, CT
Seneca, MD
Freedom, MD
Cumberland,
MD
Baltimore, MD
Cox Creek,
MD
Frederick, MD
Broomfield,
CO
Broomfield,
CO
Broomfield,
CO
Hurlock, MD
Las Virgenes
Calabasas, CA
Actual
effl. TN
(ppm)
~
6
16
8
7.3
9
16
23
8
~
~
~
~
Initial
TN
cone.
(ppm)
9.6
7
8
8
8
8
8
6.5
8
8
8
15
42
New
TN
goal
(ppm)
5.1
5
3
3
3
3
3
3
3
3
3
3
1
Flow
(MGD)
1
3.5
20
3.5
15
180
15
7
8
8
8
1.5
16
Capital
Cost
$/gpd
capacity
$0.20
$1.65
$0.21
$0.99
$1.10
$1.39
$1.74
$1.41
$1.70
$1.00
$0.85
$4.12
$5.20
O&M
Cost
$/MG
treated
$111
$136
$63
~
$122
~
$104
~
~
~
~
~
~
O&M
Cost
$/lb TN
removed
$0.60
$1.05
$1.51
~
$2.94
~
$2.50
~
~
~
~
~
~
Notes:
A2O = anaerobic/anoxic/oxic
AS = activated sludge
IFAS = integrated fixed-film activated sludge
MG = million gallons
"Stephenson and Mohr 2005
MBBR = moving-bed biofilm reactor
MBR = membrane biological reactor
MLE = modified Ludzack-Ettinger
TN = total nitrogen
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
4-2
Chapter 4: Cost Factors
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September 2008
Municipal Nutrient Removal Technologies Reference Document
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 $\Q 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
Step-feed AS
with tertiary
clarifierand
filter
Modified
UCT with
fermenter +
tertiary filter
A/O with
tertiary
clarifier +
tertiary filter
Location
Fairfax
County,
VA
Kalispell,
MT
Clark
County,
NV
Design
flow
(MGD)
67
3
100
Target
Concen-
tration
(annual
average)
Ammonia-
N+TP,
1 mg/L &
0.1 8 mg/L
(TN 5.2 mg/L
voluntary)
Ammonia-
N+TP, 1.4
mg/L & 1
mg/L
Ammonia-
N+TP, 0.6
mg/L & 0.2
mg/L
Initial
Concen-
tration
(annual
average)
18. 9 mg/L
ammonia-
N, 6.4 mg/L
TP
24 mg/L
ammonia-
N, 4 mg/L
TP
27 mg/L
Ammonia-
N, 5.8 mg/L
TP
$/gpd
capa-
city
$1.07
$3.03
$2.01
$/MG
$106
$108
$183
O&M
$/lb
TN
$0.46
-
-
O&M
$/lb
TP
$1.07
$0.49
$1.81
O&M
$/lb
NH3
~
$0.22
$0.50
Chapter 4: Cost Factors
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September 2008
Table 4-2. Upgrade costs for case studies (continued)
Process
Plug flow AS
with
denitrification
filter
Location
Johnston
County,
NC
Design
flow
(MGD)
7
Target
Concen-
tration
(annual
average)
TN+TP, 3.7
mg/L & 1
mg/L
Initial
Concen-
tration
(annual
average)
31 .2 mg/L
TN, 5.8
mg/L TP
$/gpd
capa-
city
$0.58
$/MG
$221
O&M
$/lb
TN
$0.48
O&M
$/lb
TP
$1.48
O&M
$/lb
NH3
-
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 Gary, 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 Gary, 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
Chapters.
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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.
Chapter 4: Cost Factors 4-5
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September 2008
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
ENR
Marshall & Swift
Pipe Index
Cost factors
Electricity
Labor
Alum
Value
7,940
1,360
732
Value
$0.08
$25.00
$0.41
Date
May 2007
2007
2007
kW-hr
Person-hour
lbasAI2(S04)3-14H20
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.
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Chapter 4: Cost Factors
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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
Total
capital
O&M
electrical
O&M
chemical
O&M
sludge
Total
O&M
Clearwater: 5-stage Bardenpho
CAPDETWorks
$35,541,000
$640,000
$0
$0
$640,000
Actual
$29,500,000
$610,000
$74,000
$0
$684,000
%
Difference
17
5
-
-
6
Kalispell: University of Cape Town
CAPDETWorks
$8,903,000
$159,000
$0
$0
$159,000
Actual
$9,100,000
$113,000
$0
$0
$113,000
%
Difference
2
29
-
-
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.
Chapter 4: Cost Factors
4-7
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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
Average flow (MGD)
TSS (mg/L)
% volatile solids
BOD (mg/L)
Soluble BOD (mg/L)
COD (mg/L)
Soluble COD (mg/L)
TKN (mg/L)
Soluble TKN (mg/L)
Ammonia nitrogen (mg/L)
TP (mg/L)
pH (standard units)
Nitrite/nitrate (mg/L)
Temperature, summer
Temperature, winter
Value
1,5, 10
220
75%
220
80
500
300
40
25
22
5
7.6
0.0
23 °C
10 °C
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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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
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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
1
2
3
4
5
Costed items
Alum feed
Alum feed (2 points) with sand filter
A/O conversion, fermenter
A/O conversion, fermenter, Sand filter
A/O conversion, fermenter, sand filter, alum feed
(post-secondary treatment)
Alum dose
(mg/L as Al)
2.4
2.6; 2.4
N/A
N/A
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.
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Flow (MOD)
—X—Fermenter Retrofit, 0.5 ppm TP target, no filter
1 ^ 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).
Flow(MGD)
-D— 1 point addition alum addition , 0.5 ppm TP target, no filter
—X—Fermenter Retrofit, 0.5 ppm TP target, no filter
• 2 pt alum addition, 0.1 ppm TP target, filter
1 ^ 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).
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$500.00
$450.00
$0.00
Flow (MGD)
-Fermenter Retrofit, 0.5 ppm TP target, no filter
•1 point addition alum addition , 0.5 ppm TP target, no filter
Fermenter+Sand Filter Retrofit, 0.5 ppm target
-Fermenter+Filter+1 point Alum addition, 2 ppm dose, 0.1 ppm TP target
•2 pt alum addition, 0.1 ppm TP target, filter
Figure 4-3. Life-cycle costs for retrofit phosphorus removal technologies ($/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
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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
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
1
2
3
4
Costed items
Tanks, mixers
MLE conversion, filter
Step-feed AS conversion, filter
Denitrifying filter
Methanol dose
(Ib/lb N03)
N/A
N/A
N/A
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
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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.
$1.60
$1.40
$1.20
$1.00
o
i
$0.80 --
$0.60
$0.40
$0.20
$0.00
-X
10
12
Flow(MGD)
- Denit Filter 3 ppm TN -B-Step Feed Retro-1/3 of Step Feed/PFR on reactor -A- MLE Retro-1/3 of MLE —X—Extra basins for PID
Figure 4-4. Capital costs for retrofit nitrogen removal scenarios.
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$500.00
$450.00
$400.00
$350.00
$300.00
$250.00
$200.00
08
O
$150.00
$100.00
$50.00
$0.00
Flow (MOD)
-Denit Filter 3 ppmTN
Step Feed Retro-1/3 of Step Feed/PFR on reactor -A-Extra basins for PID -B-MLE Retro--1/3 of MLE
Figure 4-5. O&M costs for retrofit nitrogen removal scenarios.
$900.00
$800.00
$0.00
Flow (MOD)
•Denit Filter 3 ppmTN -*- Extra basins for PID —•— Step Feed Retro-1/3 of Step Feed/PFR on reactor -A-MLE Retro-1/3 of MLE
Figure 4-6. Life-cycle costs for retrofit nitrogen removal scenarios.
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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
1
2
3
Costed items
Basins, mixers, alum addition, tertiary
clarifier, tertiary filter
Alum addition, denitrifying filter
5-stage Bardenpho conversion, alum
addition, tertiary filter
Methanol dose
(Ib/lb NO3)
N/A
3 mg/mg NO3-N
N/A
Note: N/A = not available
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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.
Flow (MGD)
-A-5-Stage/Chem P Retro = 1/2 old 5 Stage —3 NO3/0.3 P -^Nitrification/Criem P/Denite Filter-3 NO3/0.1 P ^^N&P PIP Retro
Figure 4-7. Capital costs for retrofit nitrogen plus phosphorus removal technologies.
Chapter 4: Cost Factors
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$900.00
Flow(MGD)
-^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 PIP Retro
Figure 4-8. O&M costs for retrofit nitrogen plus phosphorus removal technologies.
$1,400.00
$1,200.00
$0.00
Flow (MOD)
-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 PIP Retro
Figure 4-9. Life-cycle costs for retrofit nitrogen plus phosphorus removal technologies.
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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
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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
1
2
3
4
5
6
7
8
Costed items
Alum feed
Alum feed (2 points)
A/O, clarifier, blower
A/O, clarifier, fermenter, Blower
A/O, clarifier, fermenter, filter, blower
A/O, clarifier, fermenter, filter, blower, alum feed
UCT, clarifier, filter, fermenter, blower
5-stage Bardenpho, clarifier, filter, blower
HRT
(hr)
N/A
N/A
12.6
13.1
13.1
13.1
12.0
15.1
Alum dose
(mg/L as Al)
2.4
2.6; 2.4
N/A
N/A
N/A
2.4
N/A
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.
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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 (A2O), 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.
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$900.00
$800.00
$100.00
$0.00
Flow (MOD)
—«—5 Stage Bardenpho to 0.5 ppm TP wfilter —[
^^~A/O wfermenter, filter, and alum addition to 0.1 ppm TP ^^
~»* A/OtoO.5 ppmTPwFermenter ^
• " 1 point addition alum addition , 0.5 ppm TP target, no filter —
— Mod UCT to 0.5 ppmTP wferm+filt
A/O to 0.5 ppm TP wfermenter+filter
™A/O to 1 ppm TP No ferm, no chem, no filter
- -2 pt alum addition, 0.1 ppm TP target, filter
Figure 4-10. O&M costs for expansion phosphorus removal technologies ($/MG treated).
$4.50
$4.00
$0.50
$0.00
Flow (MOD)
-5 Stage Bardenpho to 0.5 ppm TP wfilter
•A/O wfermenter, filter, and alum addition to 0.1 ppm TP
A/O to 0.5 ppm TP w Fermenter
- 2 pt alum addition, 0.1 ppm TP target, filter
1—Mod UCT to 0.5 ppmTP wferm+filt
* A/O to 0.5 ppmTP wfermenter+filter
^~A/O to 1 ppm TP No ferm, no chem, no filter
> • 1 point addition alum addition , 0.5 ppm TP target, no filter
Figure 4-11. Capital costs for expansion phosphorus removal technologies.
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$2,000.00
$1,800.00
$1,600.00
£• $1,400.00
S
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10 MGD P Technologies
5 Stage Westbank A/O with A/O with A/O with WO no Plain AS, 2 1 alum
Bardenpho Chem P Fermenter Fermenter ferm, alum feed, feed, .5
and Filter chem, or .Ippmout, ppmout.no
filter filter filter
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)
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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
1
2
3
4
5
6
7
8
Costed items
PID, clarifier, sand filter
Denitrification filter
Step-feed AS, clarifier, filter, blower
MLE reactor, clarifier, blower
SBR, sand filter, blower
UCT, clarifier, blower
4-stage Bardenpho, clarifier, blower
5-stage Bardenpho, clarifier, blower
HRT (hr)
8.8
N/A
12.4
16.8
9 (total cycle time)
12.1
14.5
15.1
Methanol dose
(Ib/lb N03)
N/A
3
N/A
N/A
N/A
N/A
N/A
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.
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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 MOD 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 MOD labor costs range from 15 to 25 percent of the
total.
$4.00
$3.50
$0.00
Flow(MGD)
-5 Stage 5N 0.5 P
MLE to 5 ppm TN, no filter
-4 Stage Bardenpho 5 N 5 P
-Step Feed 5 N 1 P
3 Stage generic - 5 N 1 P
-Denit Filter 3 ppm TN
-SBR5N1 P
PID 5 ppm TN target
Figure 4-15. Capital costs for expansion nitrogen removal scenarios.
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$900.00
$800.00
$0.00
Flow (MOD)
-5 Stage 5N 0.5 P
MLE to 5 ppm TN, no filter
-4 Stage Bardenpho 5 N 5 P
-Step Feed 5 N 1 P
3 Stage generic - 5 N 1 P
-Denit Filter 3 ppmTN
-SBR5N 1 P
PID 5 ppm TN target
Figure 4-16. O&M costs for expansion nitrogen removal scenarios.
$0.00
Flow (MOD)
-5 Stage 5N 0.5 P
MLE to 5 ppm TN, no filter
-4 Stage Bardenpho 5 N 5 P
-Step Feed 5 N 1 P
3 Stage generic - 5 N 1 P
-Denit Filter 3 ppmTN
-SBR5N 1 P
PID 5 ppmTN target
Figure 4-17. Life-cycle costs for expansion nitrogen removal scenarios.
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100% n
90% -
80%
60% -
:
n
R
1MGD N Technologies
—
5 Stage 4 Stage MLE SBR I (with Step Denite Filter PID 3 Stage
Bardenpho-- Bardenpho-- Anaerobic Feed/PFR (alone) generic
N Sstage with phase)
Anaerobic
zeroed out
D Chemical
• O&M Materials
D Labor
Figure 4-18. Component percentages of total O&M costs for 1 MGD expansion nitrogen
removal technologies.
$1,800.00
$1,600.00
$0.00
Flow (MGD)
-5 Stage 5N 0.5 P
MLE to 5 ppm TN, no filter
-4 Stage Bardenpho 5 N 5 P
-Step Feed 5 N 1 P
3 Stage generic - 5 N 1 P
-Denit Filter 3 ppmTN
-SBR5N 1 P
PID 5 ppm TN target
Figure 4-19. Life-cycle costs for expansion nitrogen plus phosphorus removal technologies.
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10 MGD N Technologies
0%
5 Stage 4 Stage
Bardenpho- Bardenpho-
-N -Sstage
with
Anaerobic
zeroed out
MLE SBRI (with Step Denite
Anaerobic Feed/PFR Filter
phase) (alone)
PID
3 Stage
generic
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.
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Table 4-11. Process parameters for combined nitrogen and phosphorus removal
scenarios
Process
1
2
3
4
Costed items
PID, clarifier, dual alum additions, tertiary
clarifier, sand filter, blower
SBR, alum addition, sand filter, blower
Nitrifying AS, clarifier, dual alum addition,
denitrification filter, blower
5-stage Bardenpho, clarifier, alum addition,
sand filter, blower
HRT
(hr)
13
1 5 (total cycle
time)
11.8
15.5
Alum dose
(mg/L as Al)
2.0; 1.0
2.08
2.6,4.8
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.
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$5.00
$4.50
$0.00
Flow (MOD)
-5StageBarden-3NO30.1 P -«-Nitrification/Chem P/Denite Filter-3 NO3/0.1 P PID w chemical + tert clar + tert fill SBR wchem+filt
Figure 4-21. Capital costs for expansion nitrogen plus phosphorus removal technologies.
$1,400.00
$1,200.00
$1,000.00
$800.00
o
08
o
$400.00
$200.00
$0.00
Flow (MOD)
SStage Barden-3 NO30.1 P —•—Nitrification/Chem P/Denite Filter-3 NO3/0.1 P PID w chemical + tert clar + tert fill SBR wchem+filt
Figure 4-22. O&M costs for expansion nitrogen plus phosphorus removal technologies.
Chapter 4: Cost Factors
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$2,500.00
$2,000.00
- $1,500.00
$1,000.00
$0.00
Flow (MGD)
-SStage Barden-3 NO30.1 P —•— Nitrification/Chem P/Denite Filter-3 NO3/0.1 P PID w chemical + tertclar + tert fill SBR w chem+filt
Figure 4-23. Life-cycle costs for expansion nitrogen plus phosphorus removal technologies.
100%
80%
50%
^n%
0%
1 MGD N+P Technologies
-
U
-
Nitrification/Chem 5-Stage PID with chemical + SBR w chem + filt
P/Denite Filter-3 Barden/Chem P-3 tert clar + tert filter
NO3/0.1 P NO3/0.1 P
D Energy
D Chemical
• Materials
D Labor
Figure 4-24. Component percentages of total O&M costs for 1 MGD expansion nitrogen plus
phosphorus removal technologies.
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100% -
90%
80%
70%
m%
n%
10 MGD N+P
Nitrification/Chem 5-Stage Barden w PID w chemical + SBR wchem + fill
P/Denite Filter-3 Chem P— 3 tert clar + tert fill
N03/0.1 P N03/0.1 P
D Energy
D Chemical
• Materials
D 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
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Municipal Nutrient Removal Technologies Reference Document September 2008
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
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September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Chapter 4: Cost Factors 4-35
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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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5.1.2 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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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
Up to 1 mg/L
1.0 to 0.5 mg/L
0.1 to 0.5 mg/L
0.1 mg/L and below
Technology
All
Chemical precipitation
A/O; filter preferred
5-stage Bardenpho
PhoStrip; filter preferred
SBR
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
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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Table 5-2. Process list: TN
Concentration
8 mg/L or higher
3 to 8 mg/L
3 mg/L and below
Technology
All; no filter required
A2O; filter preferred
MLE
SBR
Cyclic activated sludge
BAF
I FAS
MBBR
3-stage Westbank
4-stage Bardenpho
Post-aeration anoxicwith methanol (Blue Plains process)
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:
A2O = anaerobic/anaerobic/oxic
AS = activated sludge
BAF = biological aerated filter
I FAS = 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
I FAS
MBBR
A/0
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
5-4
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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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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.
5-6 Chapter 5: Upgrading Existing Facilities
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Table 5-5. Technology selection matrix: nitrogen removal
Process
Denitrification
filters
MBBR
SBR/cyclic
activated
sludge
MLE/3-stage
Westbank
Phased
isolation ditch
4-stage
Bardenpho
A20
Anoxic zone
following
aeration(Blue
Plains)
Biological
aerated
filtration
Step-feed AS
IFAS
Site factors
Footprint
Small
Medium
Medium
Medium
Large
Large
Medium
Medium
Medium
Large
Medium
Building
needed
Yes
No
No
No
No
No
No
No
Maybe
No
No
Construction
in existing
aeration
basin
No
Maybe1
Yes
Maybe
Maybe
Maybe
Maybe
Maybe
No
Yes
Maybe1
Piping
and
pumping
Yes
No
No
Yes
No
Yes
Yes
No
Yes
Yes
No
Extra
head
needed
Yes
No
Maybe
No
No
No
No
Maybe
Yes
No
No
Secondary
process
recycle
streams
No
No
No
Yes
No
Yes
Yes
No
Yes
Yes
Yes
Wastewater
factors
Additional
carbon
source
needed
Yes
Maybe
Maybe
Maybe
Maybe
Maybe
Maybe
Yes
Maybe
No
Maybe
Operation factors
Extra
electricity
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Maybe
No
Maybe
Chemicals
needed
Yes
Maybe
Maybe
Maybe
Maybe
Maybe
Maybe
Yes
Maybe
No
Maybe
Add'l
sludge
Yes
Maybe
Maybe
Maybe
Maybe
Maybe
Maybe
Yes
Maybe
No
Maybe
Notes:
Installation 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 sludge.
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.
-------
Table 5-6. Technology selection matrix: phosphorus removal
Process
A/0
SBR/CAS
VIP/modified
UCT
Chemical
precipitation
Tertiary
clarifier/tertiary
filter
PhoStrip
5-stage
Bardenpho
3-stage
Westbank
EBPR with VFA
addition and
filters
MBR
Phased
isolation ditch
Membrane filter
CoMag
Infiltration basin
Site factors
Footprint
Medium
Medium
Medium
Small
Medium
Small
Large
Medium
Medium
Small
Large
Small
Medium
Large
Building
needed
No
No
No
Yes
Maybe
No
No
No
Maybe
(Filters)
Yes
No
Maybe
Maybe
No
Construction
in existing
aeration
basin
Yes
Yes
Maybe
No
No
No
Maybe
Maybe
Maybe
Yes
Maybe
No
Maybe
No
Piping
and
pumping
No
No
Yes
Yes
Maybe
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
No
Extra
head
needed
No
Maybe
No
No
Yes
No
No
No
Yes
Yes
No
Yes
Yes
Yes
Secondary
process
recycle
streams
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
No
No
Yes
Yes
No
Waste water
factors
Additional
carbon
source
needed
Maybe
No
Maybe
No
No
No
Maybe
Maybe
Yes
Maybe
No
No
No
No
Operation factors
Extra
electricity
Yes
Yes
Yes
Minimal
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Minimal
Chemicals
needed
C add'n
Maybe
No
Maybe
No
No
No
Maybe
Maybe
Yes
Maybe
No
No
No
No
Chem
P rem
Maybe
Maybe
Maybe
Yes
Yes
Yes
Maybe
Maybe
Maybe
Yes
Maybe
Yes
Yes
Yes
Add'l
Sludge
No
No
No
Yes
Yes
Yes
Maybe
Maybe
Maybe
Yes
Maybe
Yes
Yes
Yes
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.
-------
Table 5-7. Technology selection matrix: nitrogen and phosphorus removal
Process
Step-feed with
selector
1 FAS with
selector
A^O
SBR/CAS
VIP/modified
UCT
Phostrip II
3-stage
Westbank
5-stage
Bardenpho
EBPR with
VFA addition
MBR
Chemical
precipitation
Filtration
Denitrification
filter
Phased
isolation ditch
Site factors
Footprint
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Large
Medium
Small
Small
Medium
Small
Large
Building
needed
No
No
No
No
No
No
No
No
No
Yes
Yes
Maybe
Yes
No
Construction
in existing
aeration
basin
Yes
Maybe1
Yes
Yes
Maybe
No
Maybe
Maybe
Maybe
Yes
No
No
No
Maybe
Piping
and
pumping
Yes
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Maybe
Yes
No
Extra
head
needed
No
No
No
Maybe
No
No
No
No
Maybe
Yes
No
Yes
Yes
No
Secondary
process
recycle
streams
No
Maybe
Yes
No
Yes
Yes
Yes
Yes
No
Yes
No
Yes
No
No
Wastewater
factors
Additional
carbon
source
needed
No
Maybe
Maybe
Maybe
Maybe
No
Maybe
Maybe
Yes
Maybe
No
No
Yes
No
Operation factors
Extra
electricity
No
Maybe
Yes
Maybe
Yes
Yes
Yes
Yes
Yes
Yes
Minimal
Yes
Yes
Yes
Chemicals
needed
C
Add'n
No
Maybe
Maybe
Maybe
Maybe
No
Maybe
Maybe
Yes
Maybe
No
No
Yes
No
Chem P
Rem
Maybe
Maybe
Maybe
Maybe
Maybe
Yes
Maybe
Maybe
Maybe
Yes
Yes
Yes
Maybe
Maybe
Add'l
sludge
Maybe
Maybe
Maybe
Maybe
Maybe
Yes
Maybe
Maybe
Maybe
Yes
Yes
Yes
Yes
Maybe
Notes:
Installation 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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Municipal Nutrient Removal Technologies Reference Document September 2008
• 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
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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
Chapter 5: Upgrading Existing Facilities 5-11
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Municipal Nutrient Removal Technologies Reference Document September 2008
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.
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Such media are used in both the MBBR and IF AS systems (Johnson et al. 2005; McQuarrie
et al. 2004). Example installations include denitrification by IF AS 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 IF AS 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 on-
site 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
Chapter 5: Upgrading Existing Facilities 5-13
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Municipal Nutrient Removal Technologies Reference Document September 2008
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
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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 mixed-
liquor 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/floe that is larger than the pore size (and any biological growth
that forms on the membrane). Clarifiers require floe of sufficient size and density to settle so
the floe 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.
Chapter 5: Upgrading Existing Facilities 5-15
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Municipal Nutrient Removal Technologies Reference Document September 2008
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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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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Municipal Nutrient Removal Technologies Reference Document September 2008
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 IF AS 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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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
Gary Water Reclamation Facility in North Gary, 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 Gary 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 SCAD A 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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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 solids-
handling 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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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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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-to-
TKN 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 °C 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 °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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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 °C, 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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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. Perm enters 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 wet-
weather 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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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 wet-
weather events, converting the existing facilities to a step-feed activated-sludge or IF AS
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 IF AS
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 Gary Water Reclamation Facility in North Gary, 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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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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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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• 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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• 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 Gary, 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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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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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
Criteria for comparison
Costs (capital and O&M)
Reliability
Efficiency
Expandability
Ease of O&M
Environmental impacts — chemical use
Efficient land use
Energy use
Future regulatory requirements
Employee and public health and safety
Market stability for biosolids reuse/disposal
TOTAL
Weighting
factor
1
1
1
1
2
3
1
2
1
2
1
-
Alternatives
A
2
3
1
2
3
4
2
2
4
2
1
41
B
1
1
2
3
1
1
1
1
3
3
4
28
C
4
2
4
1
4
3
4
4
2
4
2
52
D
3
4
3
4
2
2
3
3
1
1
3
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.
Chapter 5: Upgrading Existing Facilities
5-31
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Municipal Nutrient Removal Technologies Reference Document
September 2008
Table 5-9. Decision matrix example 2
Criteria
Site requirement
Capital cost
O&M cost
Reliability
Flexibility
Use of existing facilities
Sludge production
Public perception
Total points
Rank
Weight
8
6
6
8
10
6
8
7
Option 1
Score
5
3
3
4
3
3
2
3
Points
40
18
18
32
30
18
16
21
193
3
Option 2
Score
5
5
4
5
4
4
3
3
Points
40
30
24
40
40
24
24
21
243
1
Option 3
Score
4
4
4
5
3
4
3
3
Points
32
24
24
40
30
24
24
21
219
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-
5-32
Chapter 5: Upgrading Existing Facilities
-------
September 2008 Municipal Nutrient Removal Technologies Reference Document
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.
Chapter 5: Upgrading Existing Facilities 5-33
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Municipal Nutrient Removal Technologies Reference Document September 2008
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).
5.7 References
Barnard, J. 2006. Biological Nutrient Removal: Where We Have Been, Where We Are
Going. In Proceedings of the Water Environment Federation's 79th Annual Technical
and Educational Conference, Dallas, TX, October 21-25, 2006.
Barnard, J., A. Shaw, and D. Lindeke, 2005. Using Alternative Parameters to Predict Success
for Phosphorus Removal in WWTPs. In Proceedings of the Water Environment
Federation's 78th Annual Technical and Educational Conference, Washington, DC,
October 29-November 2, 2005.
Copithorn, R. 2007. Design and Operational Issues Associated with Integrated Fixed Film
Activated Sludge (IFAS). Nutrient Removal Workshop: Doing More with Less, Water
Environment Federation/IWA, Baltimore, MD.
deBarbadillo, D., A. Shaw, and C.L. Wallis-Lage. 2005. Evaluation and Design of Deep Bed
Denitrification Filters: Empirical Design parameters vs. Process Modeling. In
Proceedings of the Water Environment Federation's 78th Annual Technical and
Educational Conference, Washington, DC, October 29-November 2, 2005.
Demko, M., F. Coughehenour, A. Santos, and S. Jeyannayagam. 2007. Analyzing Analyzers:
An Evaluation of OnOline Nutrient Analyzers at the City of Plant City Wastewater
Reclamation Facility. In Proceedings, Specialty Conference on Nutrient 2007, Water
Environment Federation and International water Association, PP 298-310, 2007.
Grady, C.P.L., G.T. Daigger, and H.C. Lim. 1999. Biological Wastewater Treatment. Marcel
Dekker, Inc., New York.
5-34 Chapter 5: Upgrading Existing Facilities
-------
September 2008 Municipal Nutrient Removal Technologies Reference Document
Johnson, T., C. Wallis-Lage, A. Shaw, and J. McQuarrie. 2005. IF AS Options—Which One
Is Right for Your Project? In Proceedings of the Water Environment Federation's
78th Annual Technical and Educational Conference, Washington, DC, October 29-
November2, 2005.
Jones, R., and I. Takacs. 2004. Modeling the Impact of Anaerobic Digestion on the Overall
Performance of Biological Nutrient Removal Wastewater Treatment Plants. In
Proceedings of the Water Environment Federation's 77th Annual Technical and
Educational Conference, New Orleans, LA, October 2-6, 2004.
Kang, S.J., P. Horvatin, and L. Briscoe. 1985. Full Scale Biological Phosphorus Removal
Using A/O Process in a Cold Climate. In Proceedings of the International Conference
on Management Strategies for Phosphorus in the Environment, July 1985.
Loader, K. 2007. IF AS and MBBR Solutions to Plant Upgrade Problems: Cheyenne,
Wyoming Case Studies. In Proceedings, Workshop B: Nutrient 2007, Water
Environment Federation and International Water Association, March 2007.
McQuarrie, J., K. Rutt, and J. Seda. 2004. Observations from the First Year of Full Scale
Operation—The IFAS/BNR Process at the Broomfield Wastewater Reclamation
Facility in Broomfield, CO. In Proceedings of the Water Environment Federation's
77th Annual Technical and Educational Conference, New Orleans, LA, October 2-6,
2004.
Neethling, J.B., and A. Gu. 2007. Phosphorus Speciation Provides Direction to Produce 10
ug/1. 2007. Nutrient Removal Specialty Conference, Baltimore, 2007.
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 Ol-CTS-3. IWA
Publishing, London.
Pehlivanoglu, D., andD. Sedlak. 2004. Bioavailability of Wastewater Derived Organic
Nitrogen to the Alga Selenastrum Capricornutum. Water Research 38:3189-3196.
Pehlivanoglue, D., and D. Sedlak. 2006. Wastewater Derived Dissolved Organic Nitrogen:
Analytical Methods, Characterization, and Effects—A Review. Critical Reviews in
Environmental Science and Technology 36(3):261-285.
Tang, C.-C., P. Prestia, R. Kettle, D. Chu., B. Mansell, J. Kuo, R. Horvatin, and J. Stahl.
2004. Start-up of a Nitrification/Denitrification Activated Sludge Process with a High
Ammonia Side-Stream. In Proceedings, Water Environment Federation, WEFTEC,
2004.
Chapter 5: Upgrading Existing Facilities 5-35
-------
Municipal Nutrient Removal Technologies Reference Document September 2008
Tremblay, S., H. Hilger, J. Barnard, C. deBarbadillo, and P. Goins. 2005. Phosphorus
Accumulating Organisms Utilization of Volatile Fatty Acids Produced by
Fermentation of Anaerobic Mixed Liquor. In Proceedings of the Water Environment
Federation's 78th Annual Technical and Educational Conference, Washington, DC,
October 29-November 2, 2005.
USEPA (U.S. Environmental Protection Agency). 1989. RetrofittingPOTWs for Phosphorus
Removal in the Chesapeake Bay Drainage Basin. EPA/625/6-89/020. U.S.
Environmental Protection Agency, Washington, DC.
USEPA (U.S. Environmental Protection Agency). 1993. Nitrogen Control. EPA/625/R-
93/010. U.S. Environmental Protection Agency, Office of Water, Washington, DC.
Weerapperuma, D., and V. De Silva. 2004. On-line Analyzer Applications for BNR Process
Control. In Proceedings of the Water Environment Federation's 7 7th Annual
Technical and Educational Conference, New Orleans, LA, October 2-6, 2004.
WEF (Water Environment Federation). 2004. Upgrading and Retrofitting Water and
Wastewater Treatment Plants. WEF Manual of Practice No. 28. WEFPress,
Alexandria, VA.
WEF (Water Environment Federation). 2005. Biological Nutrient Removal Operation in
Wastewater Treatment Plants. WEF Manual of Practice No. 30.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers).
1998. Design of Municipal Wastewater Treatment Plants. WEF Manual of Practice
No. 8, Volume II, 4th ed. pp 15-1 through 15-114. American Society of Civil
Engineers, Reston, VA.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers)
Environmental and Water Resources Institute. 2006. Biological Nutrient Removal
(BNR) Operation in Wastewater Treatment Plants. WEF Manual of Practice No. 29.
WEFPress, Alexandria, VA.
Welander, T., and C. Johnson. 2007. Upgrading Activated Sludge Plants for Enhanced
Nitrogen Removal Using MBBR-based Technology. In Proceedings, Workshop B:
Doing More with Less. Nutrient Removal 2007, Water Environment
Federation/International Water Association, March 2007.
5-36 Chapter 5: Upgrading Existing Facilities
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Municipal Nutrient Removal Technologies
Reference Document
Volume 2 — Appendices
U.S. Environmental Protection Agency
Office of Wastewater Management, Municipal Support Division
Municipal Technology Branch
EPA832-R-08-006 • September 2008
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September 2008 Municipal Nutrient Removal Technologies Reference Document
Appendix A: Case Studies
Appendix A provides detailed case studies with information from nine wastewater treatment
facilities selected for their excellent performance and varying technologies. Two facilities
were chosen because of their denitrification technologies, two were chosen because of their
phosphorus removal technologies, and an additional five facilities were included because of
both nitrogen and phosphorus removal technologies.
Denitrification
• Central Johnston County, North Carolina
• Lee County, Florida
Phosphorus removal
• Kalispell, Montana (biological phosphorus)
• Clark County, Nevada (biological phosphorus and chemical phosphorus)
Nitrogen and phosphorus removal
• Kelowna, British Columbia (biological nitrogen and phosphorus)
• Marshall Street in Clearwater, Florida (biological N and chemical phosphorus)
• Noman Cole in Fairfax County, Virginia (biological nitrogen and chemical
phosphorus)
• North Gary, North Carolina (biological nitrogen and phosphorus)
• Western Branch in Upper Marlboro, Maryland (three separate activated-sludge
systems operated in series)
Appendix A: Case Studies A-l
-------
Acknowledgements
EPA and the authors would like to acknowledge the commitment, ingenuity and leadership
demonstrated by the owners and personnel at the plants represented by the data reported in
this document. The case studies in this document represent significant accomplishments
made by the leaders of the facilities and their dedicated personnel. EPA recognizes their
cooperation and assistance in providing information on their facilities. Permit compliance
was achieved under all conditions, even under tropical storm conditions in North Carolina
and under an extreme heat wave in Nevada. Some plants (Fairfax County, Virginia, and
Clark County, Nevada), recognized as environmental leaders in their regions, are providing
levels of treatment that go beyond their permit requirements. Central Johnston County, North
Carolina, retrofitted an existing aeration system for biological phosphorus removal and
nitrogen removal and developed the denitrification sludge blanket; Kelowna, British
Columbia, and Lee County, Florida, made similar modifications. Kalispell, Montana,
developed ways to minimize recycle loads from its sludge-handling processes while
producing the lowest phosphorus concentration achieved entirely by a biological process.
Clark County, Nevada, has a Process Today's Sludge Today policy. Clearwater, Florida,
developed a control strategy for nitrogen removal on the basis of three sensors, producing a
low nitrogen concentration in the effluent. Kalispell, Montana, is a good example of sound
technical analysis carried out daily by the plant personnel in optimizing the phosphorus
removal with the best reliability.
-------
Central Johnston County Wastewater Treatment
Plant
Smithfield, North Carolina
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Central Johnston County Wastewater Treatment Plant (WWTP) is in Smithfield, North
Carolina. The facility is designed for a capacity of 7 million gallons per day (MOD), and it
processed an average of 4.12 MGD during the evaluation period, October 2005 to September
2006.
The plant was selected as a case study because it achieves a high level of biological nitrogen
and phosphorus removal through a unique plug-flow, activated-sludge (AS) process
retrofitted to the existing facility, followed by a new stand-alone denitrification filter process.
The relevant National Pollutant Discharge Elimination System (NPDES) permit limits for the
facility are shown in Table 1.
Table 1. NPDES permit limits
Parameter
BOD5, 4/1-10/31
BOD5, 11/1-3/31
TSS
Ammonia-Nitrogen,
4/1-10/31
Ammonia-N
11/1-3/31
Total phosphorus
Total nitrogen
Annual
loading (Ib)
56,200a
Quarterly
(mg/L)
2
Monthly average
(mg/L)
5
10
30
2
4
1
Weekly average
(mg/L)
7.5
15
45
6
12
~
Notes:
BOD5= biochemical oxygen demand
mg/L = milligrams per liter
P = phosphorus
TSS = total suspended solids
a Equivalent to 3.7 mg/L at 5 MGD
Appendix A
Central Johnston County, NC • Wastewater Treatment Plant - 1
-------
Nutrient Removal Technology Assessment Case Study September 2008
Plant Process
The plant layout is shown in Figure 1, and the process schematic is shown in Figure 2. After
bar screens, wastewater flows first to anoxic basin 5, then to aerobic basin 4 or 6. The flow
then goes to aerobic basin 1, 2, or 3 before secondary clarification and going through the
denitrifying filters. Following ultraviolet disinfection, the water is discharged to the Neuse
River. Biosolids are aerobically digested, dewatered, and hauled to a landfill.
Basis of Design and Actual Flow
Flow
The design flow for the facility is 7 MGD. The average flow for the study period was 4.12
MGD, while the maximum month flow during the study period was 5.17 MGD during June
2006. The maximum month flow occurred when Tropical Storm Alberto subjected North
Carolina to very heavy rains.
Loadings
Plant loadings were as follows:
Anoxic basin 5: 1 million gallons (MG), or 4.8 hours
Aerobic basin-large: 1 MG, or 4.8 hours
Aerobic basin-small, 1 and 2: 0.55 MG, or 1.9 hours
Aerobic basin-small, 3: 0.34 MG, or 1.2 hours
Total hydraulic retention time (FtRT): 11.5 hours
Internal recirculation rate: 8,000-12,000 gallons per minute (gpm), or four times the
influent flow rate
Secondary clarifier: 6.7 hours, or 412 gallons per day per square foot (gpd/ft2)
Denitrification filter, hydraulic loading rate: 3 gpm/ft2
Plant influent and effluent average results for the period October 2005 to September 2006 are
shown in Table 3.
Table 4 presents plant monthly averages for process parameters.
2 - Central Johnston County, NC • Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
3
o
>
«8
3
O
O
O
to
c
0)
o
O)
Appendix A
Central Johnston County, NC • Wastewater Treatment Plant - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Alum. Mg{OH)z
Hani
Influent
LEGEND:
Pi*]
El
FBW
FBWR
NRCY
Flow Meier
Pumps
Filler Backwash
Filler Backwash
Return
Nitrified Recycle
RAS Return Activated
Sludge
WAS Waste Activated
Sludge
Alternate Flow
Plant Effluent
to Neuse River
UNDERDRAW
Figure 2-4
Figure 2. Central Johnston County WWTP process schematic.
4 - Central Johnston County, NC • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Table 3. Influent and effluent averages
Parameter
Flow (MGD)
Influent TP (mg/L)
Effluent TP (mg/L)
Influent BOD (mg/L)
Effluent BOD (mg/L)
Influent TSS (mg/L)
Effluent TSS (mg/L)
Influent NH4-N (mg/L)
Effluent NH4-N (mg/L)
Influent TN (mg/L)
Effluent TN (mg/L)
Average
4.12
5.8
0.26
320
3
328
1.21
28
0.44
31.2
2.14
Max
month
5.17
8.5
0.64
386
4.59
419
1.47
34.4
0.54
42.7
2.77
Max
month vs.
avg.
25%
46%
140%
20%
32%
27%
13%
27%
22%
37%
30%
Max
week
6.2
13.6
1.01
497
5.2
564
1.8
37.4
0.86
63.1
3.13
Sample
method/frequency
-
Weekly/composite
Weekly/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Notes:
BOD = biochemical oxygen demand
mg/L = milligrams per liter
NH4-N = ammonia measured as nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
Appendix A
Central Johnston County, NC • Wastewater Treatment Plant - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 4. Monthly averages for plant process parameters
Month
Oct 2005
Nov 2005
Dec 2005
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sep 2006
MLSS
(mg/L)
2,527
2,445
2,650
2,686
2,452
2,643
2,679
2,417
2,300
2,378
2,448
2,574
Sludge age
(d)
8.1
7.9
8.5
8.6
7.9
8.5
8.6
7.8
7.4
7.6
7.9
8.3
HRT
(hr)
23.1
13.9
15.9
15.1
16.4
23.6
27.9
23.5
19.4
23
25.1
21.6
Temperature
(°C)
23
19
17
16
14
16
18
20
24
26
27
25
Notes:
HRT = hydraulic retention time
MLSS = mixed liquor suspended solids
Performance Data
Figures 3 and 4 present reliability data for removal of total phosphorus (TP). The removal is
good, with the effluent TP averaging 0.26 mg/L and a medium coefficient of variation (COV)
of 62 percent.
6 - Central Johnston County, NC • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
100
Johnston County, NC
Monthly Average Frequer
. * • »=3*:
V V
ii ill i i ill
icy Curvesl
^*-*""rn
i i i
or Total Phosphorus
— ^
«•— "^
Mean = 0.26 mg/L
Strl Dev = n 164 mg/l
C.O.V. - 62%
I I I I I I I I
10
O)
E
of
5
o
1
0.1
0.01
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 3. Monthly average frequency curves for TP.
100
Johnston County, NC
Weekly Average Frequency Curves for Total Phosphorus
0)
E
Q.
01
O
.c
Q.
$
O
0.1
. Mean = 0.268 mg/L
: Std. Dev. = 0.208 mg/L
: c.o.v. = 78%
0.01
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 4. Weekly average frequency curves for TP.
Figures 5 and 6 present reliability data for ammonia nitrogen removal. Removal of ammonia
nitrogen is very good, with a mean effluent of 0.44 mg/L and a very low COV of 12 percent.
Appendix A
Central Johnston County, NC • Wastewater Treatment Plant - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100
10
D)
E
c"
01
0)
p
0.1
Johnston County, NC
raoniniy Average i-rec
.» w
A 'Jk ' •*•
uency ourv
• * * *
esior Ammoma-iM
» »
— yg M»UM - n 44 mij/l
Std. Dev. - 0.055 mg/L
C.O.V. =12%
0.050.1 0.5 1 2
5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent Ammonia-N
x Final Effluent Ammonia-N
Figure 5. Monthly average frequency curves for ammonia nitrogen.
100-
Johnston County, NC
Weekly Average Frequency Curves for Ammonia-N
"S
c"
01
0)
p
10
0.1
- Mean = 0.44 mg/L
"Std. Dev. =0105 mg/L
" C.O.V. =24%
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent Ammonia-N
x Final Effluent Ammonia-N
Figure 6. Weekly average frequency curves for ammonia nitrogen.
Figures 7 and 8 present reliability data for removal of total nitrogen (TN). Between the
anoxic portion of the AS system and the denitrification filter, the plant gives outstanding TN
removal, with effluent TN of 2.14 mg/L and a COV of 1 percent.
8 - Central Johnston County, NC • Wastewater Treatment Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Johnston County, NC
100 H
_, 10-
0)
£
C
O)
Z 1 -
n 1 -
mommy Average i-reqi
• » * * '
., »K — Y~
fl *' *'
ency ourve
_± i » • ~
sror loiai Niirogen
. A
* *
Rtrl DPV = n 3R mg/l
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
Raw Influent TN
x Final Effluent TN
Figure 7. Monthly average frequency curves for TN.
100
10
Johnston County, NC
Weekly Average Frequency Curves for Total Nitrogen
c"
01
D)
O
: Mean = 2.14 mg/L
I Std. Dev. = 0.48 mg/L
- C.O.V. =23%
0.1
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent TN
x Final Effluent TN
Figure 8. Weekly average frequency curve for TN.
Appendix A
Central Johnston County, NC • Wastewater Treatment Plant - 9
-------
Nutrient Removal Technology Assessment Case Study September 2008
Reliability Factors
This facility is unique in two areas: (1) biological phosphorus removal and nitrogen removal
in a plug-flow, AS process and (2) separate-stage denitrification filters. The results are
excellent. The plant achieves a phosphorus mean concentration of 0.26 mg/L with a COV of
62 percent without any chemical addition and a TN concentration of 2.14 mg/L with a COV
of only 16 percent. The key factors for this exceptional performance are briefly discussed
below.
In terms of wastewater characteristics, the BOD-to-TP ratio is high, with an average value of
55.1. This means that no additional food is required to support anaerobic phosphorus release.
The BOD-to-TN ratio is high at 10, when 5 or greater would be recommended.
The plant uses a plug-flow, AS process with anoxic and aerobic basins in series. This was a
retrofit design that the plant personnel implemented. Some unique features of this process are
an anoxic basin with a long detention time, followed by a two-stage aerobic stage in series
and, at the same time, the flexibility of operating parallel trains, such as during high-flow
periods. The base mode of operation includes a long detention time at the anoxic basin (1
MG in basin 5), followed by an equal-size first aerobic basin (1 MG, basin 4 or 6) and then a
smaller basin (either basin 3 or basins 1 and 2 combined). The internal recirculation from
aerobic zone to the anoxic zone in the head area is up to four times the influent flow rate.
A unique operational strategy developed at the plant calls for a low return activated-sludge
(RAS) flow rate and a deep sludge blanket in the clarifiers. The clarifiers are operated with
3 to 4 feet of blanket, while RAS is maintained at only 10 to 25 percent of the flow rate. In
addition, the controlling parameter is mixed-liquor suspended solids (MLSS), ranging
between 1,700 mg/L in summer and 2,400 mg/L in winter. There is no separate tank for
volatile fatty acid generation. This practice has proven to provide full nitrification and a
significant degree of denitrification in the retrofitted AS process. The average nitrate-
nitrogen in the secondary effluent was 4 to 8 mg/L, leaving the denitrification filter to polish
the effluent.
The plant uses denitrification filters manufactured by Leopold with a down-flow pattern and
an automated system to control the methanol feed. The package includes a nitrate probe by
Hach and a dosage-control algorithm by Leopold. The process is economical and efficient in
denitrification. This is a compact process with a small footprint.
Another unique feature of this plant is that there is no primary settling and therefore all
sludge produced is aerobic sludge. The sludge is pumped to the dewatering facility 5 miles
away for dewatering with a cationic polymer. The filtrate is returned to the head of the plant
for further processing.
10- Central Johnston County, NC • Wastewater Treatment Plant Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Recycle loads are minimal because only aerobic digestion occurs on-site.
Wet-weather flows are managed with a normal mode of operation. The plant operated
normally during a tropical storm in June 2006, when the flow increased from less than
4 MGD to more than 10.5 MGD in 3 days. Under extreme conditions such as a hurricane, the
plant would shut down part of the aeration basin and protect the sludge inventory.
Cosf Factors
Capital Costs
The main upgrades of the plant for biological nutrient removal (BNR) were implemented in
2000, when the existing aeration basins were reconfigured to allow an anoxic/anaerobic/
aerobic series, and in 2005, when denitrifying filters were installed. The total cost for those
upgrades, which were largely done by plant personnel, was $3.76 million. The components
were updated to a total of $4.056 million in 2007 dollars using the Engineering News-Record
Capital Cost Index (ENR CCI) index (USDA 2007).
It was assumed that 50 percent of the 2000 upgrade and 12 percent of the 2005 upgrade could
be attributed to phosphorus removal, while 50 percent of the 2000 upgrade and 88 percent of
the 2005 upgrade could be attributed to nitrogen removal. This attribution of the 2005
upgrade was based on the bulk of those capital improvements being for the denitrifying filter.
The capital expenditure in 2007 dollars that could be attributed to phosphorus removal was
$889,000. The annualized capital charge (20 years at 6 percent) was $77,500 for phosphorus
removal.
The capital expenditure in 2007 dollars that could be attributed to nitrogen removal was
$2.4 million. The annualized capital charge (20 years at 6 percent) was $210,000 for nitrogen
removal.
The total capital attributed to BNR in 2007 dollars was $4.056 million. For the 7-MGD
facility, the capital expenditure for BNR was $0.58/gpd capacity.
Operation and Maintenance Costs
The plant uses biological phosphorus removal to achieve the limit, while using methanol
addition to complete the nitrogen removal. This means that the costs for phosphorus removal
are all electrical, while the costs for nitrogen removal are electrical plus methanol. A
summary of the electrical calculations is provided in an attachment at the end of this case
study. The total electrical usage for phosphorus removal, assumed to be 30 percent of the
total used, was 1,842,000 kilowatt-hours per year (kWh/yr). When the average electrical rate
of $0.056/kWh was applied, the cost for phosphorus removal was $103,000 for the year. The
total electrical usage for nitrogen removal was 4,170,000 kWh/yr, or $233,000.
Appendix A Central Johnston County, NC • Wastewater Treatment Plant - 11
-------
Nutrient Removal Technology Assessment Case Study September 2008
The plant adds methanol at the rate of 83.1 gpd, at a cost of $1.75/gallon. This is equivalent
to $53,000/yr for nitrogen removal.
Because of the methanol addition, an incremental amount of sludge is generated. The volume
of methanol added is equivalent to 547 Ib/day after accounting for the density of methanol,
which is 0.79 g/cm3. The chemical oxygen demand (COD) of the methanol is 1.5 Ib COD/lb
methanol, and the yield of volatile suspended solids (VSS) on methanol was assumed to be
0.4 Ib VSS/lb COD (McCarty et al. 1969). The plant generated 328 Ib sludge/day from
methanol addition, or 59.9 ton sludge/yr. Assuming $200/ton for sludge disposal, the
incremental amount for sludge addition attributed to nitrogen removal is $12,000.
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 69,900 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal is $1.48, while the unit capital cost is
$0.73/lb of phosphorus removed.
During the same period, the plant removed 619,000 Ib of TN. With the results above, the unit
O&M cost for TN removal is $0.49, while the capital cost is $0.49/lb of TN removed.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the unit capital and unit O&M costs. Thus, the life-cycle
cost for phosphorus removal is $2.21/lb phosphorus removed, the life-cycle cost for ammonia
nitrogen removal is $1.02/lb nitrogen removed, and the life-cycle cost for TN removal is
$0.98/lb TN removed.
Cost-Effectiveness of the Denitrification Filter
The cost-effectiveness of the denitrification filter was evaluated separately for this plant.
From filter influent and effluent data collected during a filter stress test in 2007, the filter on
the average removes 3.5 mg/L nitrate-nitrogen. At a flow rate of 4.12 MGD, the filter
removed 43,900 Ib of nitrate-nitrogen during a year. Using the costs established above—
$53,000 for methanol for the year and $12,000 for additional sludge disposal costs from
methanol addition—the O&M cost per pound of nitrate removed in the denitrification filters
is $65,000/43,900 = $1.48/lb nitrate-nitrogen removed.
Assessment of Magnitude of Costs and Main Factors
The life-cycle costs for phosphorus removal and full nitrification are extremely low,
considering the phosphorus reduction level the plant has achieved. The main factors
contributing to this achievement are the maximum use of existing facilities, good biological
phosphorus removal, and efficient control with automation and many online sensors.
12 - Central Johnston County, NC • Wastewater Treatment Plant Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Assessment of magnitude of costs and main cost factors: The magnitude of cost at this
facility is very low, mainly because of the availability of existing facilities and the original
operating strategies of the plant personnel in maximizing both nitrogen and phosphorus
removal at the retrofitted AS process. The new denitrification filters, therefore, use a minimal
amount of methanol. In addition, no chemical is used to remove phosphorus. These factors
make both the capital cost and O&M costs of this plant very low.
Discussion
Reliability factors: The plant achieves excellent performance at the mean concentration of
2.14 mg/L of TN with a COV of 16 percent. This is mainly because the plant has two
separate-stage denitrification processes with an external carbon source at the second stage, or
dentrification filter. Operational strategies developed by the plant personnel achieved a
significant amount of denitrification in the AS process, followed by a separate-stage
polishing with an automated feed strategy using an online nitrate probe. For phosphorus
removal, the mean concentration of 0.26 mg/L is excellent, while the COV is moderate at
62 percent. This low a level is remarkable for an entirely biological phosphorus removal
process. Note that the denitrification filter by Leopold uses a down-flow process and
therefore removes suspended solids concurrently with nitrogen removal.
Cost factors: Three key factors are identified in achieving a high level of BNR at a low cost
at this facility: (1) the maximum use of an existing AS process with minimal retrofit costs;
(2) development of an original operating strategy to maximize BNR in the retrofitted AS
process; and (3) a separate-stage denitrification with minimal methanol feeding. This
combination of biological phosphorus removal and a down-flow denitrification filter in series
resulted in a reliable, low-cost solution for both nitrogen and phosphorus removal.
Summary
This facility removes both nitrogen and phosphorus exceptionally well and reliably. The two-
stage biological processes in series offer the highest efficiency in nutrient removal at
minimum costs. The source of wastewater is typical residential customers in the suburb of a
large metropolitan area. The BOD-to-TP ratio averages 55.1. The retrofitted AS process
consists of an anoxic stage with a 4.8-hour residence time, followed by an aerobic stage in
two tanks with a residence time of 11.5 hours. The operating strategy developed at this
facility is unique because the sludge blanket at the clarifiers is 3 to 4 feet deep and the RAS
flow rate is maintained at a low (10-25 percent) portion of the plant flow. The second-stage
denitrification filters then remove the remaining nitrogen with a methanol feed.
The design and operation result in a high level of removal—an effluent TN concentration of
2.14 mg/L with a COV of only 19 percent and an effluent TP concentration of 0.26 mg/L
with a COV of 62 percent.
Appendix A Central Johnston County, NC • Wastewater Treatment Plant - 13
-------
Nutrient Removal Technology Assessment Case Study September 2008
The costs of removal were very low for both capital and O&M. The life-cycle cost for
removal of TP was $2.21/lb of TP removed, while the life-cycle cost for TN removal was
$0.98/lb of TN removed, including the cost for methanol. The capital cost for the flow
capacity was low at $0.58/gpd capacity.
Acknowledgments
The authors are grateful for the significant assistance and guidance provided by Haywood
Phthisic, III, director of Utilities in Johnston County, North Carolina. This case study would
not have been possible without Mr. Phthisic's prompt response, with well-deserved pride in
the facility and its operation. Thanks are extended to Johnston County for participating in this
case study for the U.S. Environmental Protection Agency.
References and Bibliography
McCarty, P., L. Beck, and P. St. Amant. 1969. Biological denitrification of wastewater by
addition of organic materials. In Proceedings of the 24th Purdue Industrial Waste
Conference, Lafayette, IN.
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.
14 - Central Johnston County, NC • Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Attachment: Electrical Cost Calculation
Electrical
Anoxic/Anaerobic Mixers
HP
10
15
15
Blowers
150
100
Filter Pumps
150
Total Draw
Methanol
Number
3
1
1
2
2
3
83.1
1.75
145.425
53,080.125
Power
draw
(kW)
22.38
11.19
11.19
223.8
149.2
335.7
gal/day
cost/gal
cost/day
cost/yr
kWh/day
24
24
24
24
24
24
kWh
draw/day
537.12
268.56
268.56
5,371.2
3,580.8
8,056.8
kWh
196,048.8
98,024.4
98,024.4
1,960,488
1,306,992
2,940,732
6,600,310
%P
70
70
70
30
30
20
%N
30
30
30
70
70
60
ForP
137234.2
68617.08
68617.08
588146.4
392097.6
588146.4
1,842,859
ForN
58,814.64
29,407.32
29,407.32
1,372,342
914,894.4
1,764,439
4,169,304
Appendix A
CentralJohnston County, NC • Wastewater Treatment Plant - 15
-------
-------
Fiesta Village Advanced Wastewater Treatment
Plant
Lee County, Florida
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
This plant was selected as a case study because it is a good example of the use of the
denitrification filter process. The plant consists of an extended air oxidation ditch process
followed by denitrification filters with methanol feed. Phosphorus removal is achieved with
alum feed to the secondary effluent. Nitrogen and phosphorus are being removed
successfully down to 3 and 0.1 milligrams per liter (mg/L), respectively.
The Fiesta Village Advanced Wastewater Treatment Plant is in Lee County, Florida. It is
permitted for 5 million gallons per day (MGD) capacity, and in 2006 it processed an average
of 3.16 MGD. The plant is designed to send 2.0 MGD (annual average) into a slow-rate,
public-access reuse system for irrigation of golf courses and residential developments. It has
the potential for future reuse expansion to 3.158 MGD. Any water not reused, including
stormwater flow, is permitted for a surface water discharge to the Caloosahatchee River.
The relevant National Pollutant Discharge Elimination System (NPDES) permit limits for the
facility are shown in Tables 1 and 2.
Table 1. NPDES permit limits
Parameter (mg/L
unless stated)
BOD5
TSS
Total nitrogen
Total phosphorus
Annual
average
20
20
3
0.5
Monthly
average
25
30
3
0.5
Weekly average
40
45
4.5
0.75
Daily
maximum
60
60
6
1
Notes:
BOD = biochemical oxygen demand.; TSS = total suspended solids
Table 2. Reuse water permit limits
Parameter (mg/L
unless stated)
BOD5
TSS
Residual chlorine
Annual
average
20
Monthly
average
30
Weekly average
45
Daily
maximum
60
5
1 (minimum)
Appendix A
Lee County, FL • Advanced Wastewater Treatment Plant - 1
-------
Nutrient Removal Technology Assessment Case Study September 2008
Treatment Processes
Figures 1 and 2 present the plant layout and process flow for the Fiesta Village Facility. The
plant is an extended-aeration oxidation ditch facility, and the treatment process includes an
odor control system, primary bar manual/mechanical screening, aerated grit removal, two
oxidation ditches, two clarifiers, two aerobic digesters, three screw lift pumps, four
denitrification filters, dual chlorine contact chambers, effluent transfer pumping station,
chemical feed equipment, sulfur dioxide dechlorination, post-re-aeration, a reuse storage
tank, and a high-service reuse/effluent pump station.
Basis of Design and Actual Flow
The design flow for the facility is 5 MGD. The average flow for the study period was
3.16 MGD, while the maximum month flow during the study period was 4.14 MGD during
July 2006. The peak day flow recorded was 5.78 MGD.
Design loadings:
Biochemical oxygen demand (BOD): 240 mg/L
Total suspended solids (TSS): 268 mg/L
Total Kjeldahl nitrogen (TKN): 37 mg/L
Total nitrogen (TN): 38.2 mg/L
Total phosphorus (TP): 7.3 mg/L
Alkalinity: 284 mg/L as calcium carbonate (CaCO3)
Oxidation ditch—437 ft long x 80 ft wide x 12 ft deep, or 3 million gallons (MG), each
Anoxic zone: one aerator turned off, or 25 percent by volume
Aerators: 60 hp, four each per oxidation ditch
Hydraulic retention time (HRT): 28.8 hours
Mixed liquor suspended solids (MLSS): 3,500 mg/L
Mixed liquor volatile suspended solids (MLVSS): 2,500 mg/L
Mean cell residence time: 30 days
Food to microorganism ratio: 0.1:0.4 Ib BOD/lb MLVSS
Waste activated sludge (WAS): 0.06 MGD, each, or 6,500 Ib/day, each
Dissolved oxygen (DO): 0.5-2.0 mg/L in aerobic zone and 0.1-0.5 mg/L in anoxic
zone
Secondary clarifiers—diameter of 90 ft (each, and there are two)
Volume: 0.665 MG (each)
Surface area: 5,538 ft2 (each) and surface loading rate = 600-1,200 gpd/ft2
Blanket depth: less than 3 ft
Return activated sludge (RAS)—rate at 100 percent of plant influent, or 3.5 MGD (3 each)
Denitrification filter—10 ft x 40 ft, 4 cells each
Hydraulic loading rate: 2.2 gpm/ft2 at design
2 - Lee County, FL • Advanced Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
0)
15
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-------
Nutrient Removal Technology Assessment Case Study
September 2008
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£
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- Lee County, FL • Advanced Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Aerobic Digestion
- Diameter: 39 ft, 16 ft deep
- Volume: 0.143 MG, 2 each
- Disc diffusers
- Loading rate: 0.01-0.02 Ib VSS/ ft3 day
- DO: 1-3 mg/L
Sludge age: 5-40 days
- Digester temperature: less than 30 degrees Celsius (°C)
Plant Parameters
Overall plant influent and effluent average results for the period January 2006 to December
2006 are shown in Table 3.
Table 3. Fiesta Village influent and effluent averages
Parameter (mg/L
unless stated)
Flow (MGD)
Influent TP
Effluent TP
Influent BOD
Effluent BOD
Influent TSS
Effluent TSS
Influent NH4-N
Effluent NH4-N
Secondary
Effluent NO3-N
Influent TN
Effluent TN
Average
value
3.16
3.85
0.102
134
1.37
199
0.72
27.2
0.13
2.9a
33.2
1.71
Maximum
month
4.14
4.58
0.19
167
2.95
261
1.17
34.5
0.2
3.0a
50.6
2.61
Max
month vs.
ave.
31%
18%
85%
24%
116%
31%
61%
27%
50%
7%
53%
53%
Maximum
week
4.26
-
0.39
179
5.2
348
1.48
-
0.28
3.9a
-
3.90
Sample
method/frequency
Monthly/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Monthly/composite
Daily/composite
Daily/composite
Monthly/composite
Daily/composite
Notes:
BOD = biochemical oxygen demand
Max month vs. average = (max month - average)/average x
NH4-N = ammonia measured as nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
a Jan-April 2007
100
Table 4 presents plant monthly averages for the process parameters, as available.
Appendix A
Lee County, FL • Advanced Wastewater Treatment Plant - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 4. Monthly averages for plant process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
MLSS
(mg/L)
3,578
3,807
4,085
3,845
3,510
3,564
3,571
3,480
3,495
3,509
3,775
4,204
Sludge age
(d)
37
39
35
24
33
28
32
36
34
37
59
41
HRT
(hr)
48
44
46
50
55
47
35
44
39
49
49
50
Temperature
(°C)
-
-
-
-
-
-
30.4
-
-
-
-
-
Performance Data
Figure 4 presents reliability data for the removal of TP. The removal is good, with an effluent
TP average of 0.1 mg/L and a medium coefficient of variation (COV) of 35 percent.
100-
ra
E
to"
E
o
10
0.01
Fiesta Village WWTP, Lee Co., FL
Monthly Average Frequency Curves for Total Phosphorus
Mean = 0.102 mg/L
E Std. Dev. = 0.035 mg/L
: C.O.V. = 35%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 4. Monthly average frequency curves for TP.
6 - Lee County, FL • Advanced Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Figure 5 presents reliability data for ammonia nitrogen removal. The removal of ammonia
nitrogen is very good, with a mean effluent of 0.134 mg/L and a low COV of 40 percent.
100 •
Fiesta Village WWTP Lee Co., FL
Monthly Average Frequency Curves for Ammonia Nitrogen
E Mean = 0.134mg/L
:Std. Dev. = 0.054 mg/L
:C.O.V. = 40%
0.01
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N
x Final Effluent - Ammonia N
Figure 5. Monthly average frequency curves for ammonia nitrogen.
Figure 6 present reliability data for removal of TN. Nitrogen is removed in two steps at this
facility. The oxidation ditch takes nitrate-nitrogen down to an average of 3 mg/L, and then
the denitrification filter takes it down to an annual average of 1.45 mg/L. at a COV of 28
percent.
100.000 -
Fiesta Village WWTP Lee Co., FL
Monthly Average Frequency Curves for Nitrogen
O)
E
10.000
1.000
0.100
:Mean = 1.71 mg/L
I Std. Dev. = 0.48 mg/L
- C.O.V. = 28%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Total N x Final Effluent - Total N
• Secondary Effluent NO3-N
Figure 6. Monthly average frequency curves for nitrogen.
Appendix A
Lee County, FL • Advanced Wastewater Treatment Plant - 7
-------
Nutrient Removal Technology Assessment Case Study September 2008
Reliability Factors
This facility is unique in three ways: separate-stage denitrification using methanol; alum feed
to the oxidation ditch effluent prior to the secondary clarifiers for chemical phosphorus
removal; and filtration of effluent with the same denitrification filters. The facility is also
unusual in that it has no primary settling and thus all sludge generated is kept aerobic before
it is disposed of off-site at another county facility.
The results are excellent. The plant achieved a TN concentration of 1.71 mg/L with a COV of
28 percent and a total phosphorus (TP) concentration of 0.1 mg/L with a COV of 35 percent.
The key factors contributing to this performance are described below.
The key reason for excellent denitrification is the use of two processes in series—the first in
the oxidation ditch for most of the removal, followed by polishing at the denitrification filter.
The oxidation ditch is operated with the target nitrate-nitrogen concentration of 3.0 to
3.5 mg/L and ammonia nitrogen at 0.2 mg/L in the secondary effluent. This target removal is
accomplished under the current loading conditions by turning one of four brush aerators off
during the day and two off during the night, thereby maintaining 25 percent and then
50 percent of the volume, respectively, as an anoxic zone. The DO concentration in the
oxidation ditch is adjusted using the remaining brush aerators. The oxidation ditch is
operated with a long SRT (30-40 days) and HRT (20-30 hours). In addition, another unique
operating plan includes the denitrification blanket in the clarifiers. The sludge blanket depth
is maintained at between 2.5 and 3.5 feet.
The denitrification filters then brings the nitrate-nitrogen to below 2 mg/L, with a low
methanol feed rate of 129 Ib per day. The methanol-to-nitrate-nitrogen ratio averaged
1.9 pounds of methanol per pound of nitrate present, or 2.4 Ib per pound of nitrate removed.
The plant measures nitrate-nitrogen in the effluent in adjusting the methanol feed rate, which
is steady year-round.
Alum was fed at the average dosage of 8.9 mg/L as aluminum, or at the aluminum-to-TP
ratio of 2.31, in achieving a low concentration of 0.1 mg/L for the year.
Recycle loads are minimal at this facility because aerobically digested sludge is hauled away
to another facility for final sludge processing.
During wet-weather periods, a normal mode of operation is maintained. Under extreme peak
flow conditions, the clarifiers are protected from surges by shutting off a number of brush
aerators.
8 - Lee County, FL • Advanced Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Costs
Capital Costs
The main upgrades of the plant for biological nitrogen removal (BNR) occurred in 1984,
when Phase 1, consisting of the east oxidation ditch, east clarifier, denitrifying filter, and
major structures for the west ditch and west clarifier were installed; in 1986, when Phase 2
improvements were installed; and in 2002, when equipment for the west oxidation ditch and
west clarifier was installed. Table 5 presents the costs for those improvements (Voorhees et
al. 1987; TKW Online 2007), along with capital cost updates based on the Engineering
News-Record Capital Cost Index (ENR CCI). The ENR CCI, compiled by McGraw-Hill,
provides a means of updating historical costs to account for inflation, thereby allowing
comparison of costs on an equal basis. From a Web site provided by the U.S. Department of
Agriculture (USDA 2007), the ENR index for 1984 was 4,146; for 1986, 4,295; for 2002,
6,538; and for May 2007, 7,942.
Table 5. Plant improvement costs
Phase 1
Denite Filter
Controls
Phase 2
Phase 3
TOTAL
Year
1984
1984
1984
1986
2002
Original cost
$6,505,833
$930,059
$441,323
$1,200,000
$6,800,000
2007 cost
$12,462,452
$1,781,604
$845,390
$2,218,952
$8,260,263
$25,568,661
%P
2%
12%
2%
0%
0%
-
%N
50%
88%
50%
50%
50%
-
%other
48%
0%
48%
50%
50%
-
Pcost
$249,249
$213,792
$16,908
$0
$0
$479,949
N cost
$6,231 ,226
$1,567,811
$422,695
$1,109,476
$4,130,132
$13,461,340
The table also shows the percentage of capital cost for each unit that was attributed to
phosphorus or nitrogen removal; the rest of the capital cost was attributed to other treatment,
particularly biochemical oxygen demand (BOD) and total suspended solids (TSS) removal
and disinfection. Because the plant is not doing biological phosphorus removal, it was
assumed that only 2 percent of the Phase 1 cost plus 2 percent of the cost of controls could be
attributed to phosphorus removal for the alum addition system. Because the denitrification
filters remove solids, including aluminum phosphate precipitate, it was assumed that 12
percent of that cost could be attributed to phosphorus.
On the basis of DO usage, it was assumed that 50 percent of the cost of Phases 1, 2, and 3
could be attributed to nitrogen removal. It was assumed that 88 percent of the cost of the
denitrification filters could be attributed to nitrogen removal. To be consistent with other case
studies in this document, it was assumed that 50 percent of the control costs could be
attributed to nitrogen removal.
Appendix A
Lee County, FL • Advanced Wastewater Treatment Plant - 9
-------
Nutrient Removal Technology Assessment Case Study September 2008
The above analysis resulted in a total of $480,000 in capital attributed to phosphorus removal
and $13,461,000 attributed to nitrogen removal, in 2007 dollars. The annualized capital
charge for phosphorus removal (20 years at 6 percent) was $42,000. The annualized capital
charge for nitrogen removal was $1,174,000.
The total capital attributed to nutrient removal, in 2007 dollars, was $13.9 million. For the
5-MGD facility, this means the capital expenditure per gallon of treatment capacity was
$2.79.
Operation and Maintenance Costs
The plant uses chemical phosphorus removal and BNR, with extensive use of alum for the
former and methanol as a supplemental carbon source for the latter. This means that the cost
for phosphorus removal is essentially all for chemicals and for the disposal of the resulting
sludge, while the cost for nitrogen removal is electrical (for the aeration basins), chemical
(for the methanol), and for the disposal of the extra sludge resulting from methanol addition.
A summary of the electrical calculations is provided in the Attachment. It was assumed that
some of the electricity for the blowers could be attributed to phosphorus removal, to account
for mixing alum in the ditch. The total electrical usage for nitrogen removal was 1,911,000
kilowatt-hours (kWh). When the average electrical rate of $0.12/kWh (including demand
charges) was applied, the cost of electricity for nitrogen removal was $229,000.
Alum is applied for both phosphorus removal and TSS reduction to meet the permit
requirements for water reuse. The average amount of alum applied over the period was
151 gallons/MG of flow; assuming $0.66/gallon, the cost of alum was $115,400. It was
assumed that 30 percent of the alum cost was attributed to phosphorus removal, bringing the
chemical cost for phosphorus removal to $34,600.
Methanol is applied at the denitrification filter to promote nitrate removal. The total amount
of methanol added over the study period was 47,000 Ib. Assuming a cost of $0.27/lb (cost of
methanol for another case study plant), the chemical cost for nitrogen removal was $12,500.
The alum added (8.9 mg/L as Al) was assumed to entirely convert to aluminum hydroxide
sludge; at the average flow of 3.16 MOD, this was 677 Ib of aluminum sludge per day, or
124 dry tons/year. The plant trucks its sludge at an average cost of $0.048/gallon. Assuming
a concentration of 2 percent solids, the 124 dry tons of alum sludge is equivalent to
1,486,000 gallons of sludge. Assuming 30 percent of the sludge is associated with
phosphorus removal, the cost for phosphorus sludge disposal was $21,700.
The 47,000 Ib/yr of methanol has a chemical oxygen demand (COD) of 1.5 Ib COD/lb
methanol, or 70,750 Ib COD/yr. The typical yield of volatile suspended solids (VSS) on
methanol is 0.4 Ib VSS/lb COD, giving 28,300 Ib sludge/yr, or 14.2 tons sludge/yr from
10 - Lee County, FL • Advanced Wastewater Treatment Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
methanol addition. At a solids concentration of 2 percent, this means an additional 708 tons,
or 170,000 gal/yr, of liquid sludge to haul to other Lee County plants for treatment and
disposal. The total hauled during 2006 was 7,520,000 gallons, meaning the methanol sludge
was approximately 2.2 percent of the total. At the plant's average disposal charge of
4.9 cents/gallon, the total cost for nitrogen removal sludge was $8,300.
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 36,100 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal is $1.77, while the unit capital cost is
$1.16/lb of phosphorus removed. If the plant were operating at full capacity (5 MOD), the
unit O&M cost for phosphorus removal would be $1.34, with the unit capital cost $0.73/lb of
phosphorus removed.
During the evaluation period, the plant removed 303,000 Ib of TN. With the results above,
the unit O&M cost for nitrogen removal is $0.91, while the capital cost is $3.87/lb of TN
removed. If the plant were operating at full capacity, the unit O&M and capital costs would
be $0.57 and $2.45, respectively, per pound of TN removed.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the unit capital and unit O&M costs. Thus, the life-cycle
cost for phosphorus removal is $2.93/lb of phosphorus removed, the life-cycle cost for TN
removal is $4.78/lb of TN removed, and the life-cycle cost for ammonia nitrogen removal is
$5.57/lb of nitrogen removed. For full-capacity operations, the costs would be $2.07/lb for
phosphorus, $3.02/lb for TN, and $3.52/lb for ammonia nitrogen.
Assessment of magnitude of costs: The capital cost of $2.79 per gpd capacity is on the high
side, but the O&M costs are moderate because of the low electrical costs but high chemical
costs.
Discussion
Reliability factors: The performance has been very reliable in nitrogen and phosphorus
removal. Nitrogen removal was achieved very reliably by having two processes in series for
denitrification. Most of the removal was accomplished by the optimal use of the oxidation
ditch system, where denitrification was achieved in anoxic zones of various sizes, as well as
in the denitrifying sludge blanket in the clarifiers. The polishing of nitrate was accomplished
at the denitrification filters with minimal dosage of methanol. Phosphorus removal was
accomplished by alum addition before the secondary clarifiers, followed by the same
denitrification filters, making the process both efficient and reliable.
Appendix A Lee County, FL • Advanced Wastewater Treatment Plant - 11
-------
Nutrient Removal Technology Assessment Case Study September 2008
Cost factors: Costs for both methanol and alum are low because of the optimal use of the
existing facilities. Costs are also low because sludge is not processed on-site.
Summary
The Fiesta Village facility is an advanced wastewater treatment plant with an oxidation ditch
followed by secondary clarifiers and four dedicated denitrification filters. The performance
was highly efficient and reliable for the year studied. Nitrogen removal was achieved
biologically to the mean concentration of 1.44 mg/L with a COV of 27 percent. Many factors
contributed to this high result, including maximum use of the oxidation ditch for
denitrification, thereby reducing the load to the denitrifcation filters. The personnel at the
facility are credited for developing daily operating procedures for the control parameters and
implementing them consistently. Using denitrifying blankets in the clarifiers and maintaining
flexible anoxic zones in the oxidation ditch are two unique features of the operation in
achieving effluent nitrate-nitrogen concentration of 3 mg/L as a monthly average. The
methanol usage was minimal at the average dosage of 1.9 Ib per pound of nitrate applied,
compared to 3 Ib in the literature.
Acknowledgments
The authors of this report acknowledge with gratitude the significant assistance and guidance
provided by Tom Hill, Lee County utilities deputy director; Dennis Lang, chief operator at
the Fiesta Village Facility; and Jon Meyer, Utilities Operations Manager. This report would
not have been possible without their prompt response with well-deserved pride in their
facility and operation. EPA acknowledges Lee County, Florida, for its participation in this
case study.
References and Bibliography
TKW Online. 2007. http://www.tkwonline.com/enviromental.html. Accessed July 15, 2007.
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.
Voorhees, J.R., W.G. Mendez, and E.S. Savage. 1987. Produce an AWT Effluent for Florida
Waters, Environmental Engineering Proceedings (EE Div) ASCE, Orlando, Florida,
July 1987.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers).
1998. Design of Municipal Wastewater Treatment Plants. Manual of Practice No. 8,
Figure 11.7, Net sludge production versus solids retention time.
12 - Lee County, FL • Advanced Wastewater Treatment Plant Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Attachment: Electrical and Chemical Costs
Electrical
Hp
Aerator
60
RAS pump
30
WAS pump
7.5
Total draw
Alum cost
% for P removal
Alum cost for P
Methanol cost
Number
8
3
2
$115,338
30
$34,616
$12,735
kW
Power draw hours/day
358.08 24
67.14 24
11.19 24
(all for N
removal
kWh
draw/day
8593.92
1611.36
268.56
kWh
draw/year
3136781
588146.4
98024.4
3822952
for P for N
%P %N
2 50 62735.6 1568390.4
0 50 0 294073.2
0 50 0 49012.2
62735.6 1911475.8
Appendix A
Lee County, FL • Advanced Wastewater Treatment Plant - 13
-------
-------
Kalispell Advanced Wastewater Treatment
Kalispell, Montana
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Kalispell Wastewater Treatment Plant (WWTP) is an advanced wastewater treatment
facility in Kalispell, Montana. Kalispell is in the northwestern part of the state, near Glacier
National Park. The area is subjected to extreme weather conditions, with temperatures
ranging from 95 degrees Fahrenheit (°F) in the summer to -30 °F in the winter.
This facility was selected as a case study because of good biological phosphorus removal and
nitrification using a modified University of Cape Town (UCT) process with the fermenter
technology in a cold region.
The facility began operating in October 1992 to protect Flathead Lake, the largest freshwater
lake west of the Mississippi River. The plant has received a national first place and two
Region 8 first place Operations and Maintenance Excellence Awards from the U.S.
Environmental Protection Agency (EPA), a Commendation of Excellence Award from the
Flathead Basin Commission, and a System of the Year Award from Montana Rural Water
Systems. In addition, the processes for nitrogen removal was designed and implemented as a
voluntary initiative.
Kalispell has experienced a significant increase in population since the facility was
constructed. The city plans to expand the plant over the next several years to accommodate
growth. The expansion will add to or replace some units and modify others to continue the
concept of treatment without using chemicals. The plant is designed with expansion planned
for the flows and loads shown in Table 1.
Table 1. Design flow and loads
Year
2000
2008
Flow
(MGD)
2.5
3.0
BOD5
(mg/L)
216
216
TSS
(mg/L)
259
260
TKN
(mg/L)
25
25
TP
(mg/L)
4.5-6.5
4.5-6.5
Notes:
BOD5 = biochemical oxygen demand
MGD = million gallons per day
TKN = total Kjeldahl nitrogen
TSS = total suspended solids
TP = total phosphorus
Appendix A
Kalispell, MT • Advanced Wastewater Treatment - 1
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Nutrient Removal Technology Assessment Case Study
September 2008
The National Pollutant Discharge Elimination System (NPDES) permit limits for the plant
are shown in Table 2.
Table 2. NPDES permit limits
Parameter
BOD5
TSS
Total P
Ammonia nitrogen
7-day average
(mg/L)
15
15
-
-
30-day average
(mg/L)
10
10
1.0
1 .4 (sufficient to meet stream limits)
Treatment Processes
Wastewater treatment at the Kalispell WWTP begins with flow entering the plant through a
36-inch-diameter pipe from the city's system. The influent flows through the headworks and
is pumped to two rectangular primary clarifiers by five low-head lift pumps. Primary clarifier
effluent then flows into the bioreactor, which consists of 11 tanks in series. During periods of
high flow, primary effluent is directed to the equalization basin. Flow from the equalization
basin is then returned to the primary clarifiers during periods of lower influent flow.
The system at Kalispell is unique because it is based on the modified UCT process with
additional flexibility provided by swing zones that can be operated in several different
modes. Four zones (anaerobic, first and second anoxic, and aerobic) are created for solids
and nutrient removal. Depending on the chemistry and biology, the plant personnel can
determine the optimum number of anaerobic zones and, thus, the subsequent anoxic zones.
Bioreactor effluent flows to two circular, center-drive secondary clarifiers and then through
an effluent deep-bed sand filter, with an up-flow, continuous backwash design. The filtered
effluent then flows through an ultraviolet disinfection system and is re-aerated before it is
discharged to Ashley Creek.
The solids process train in the plant starts with the primary sludge that is removed from the
primary clarifiers by two primary sludge pumps to the completely mixed fermenter. Primary
sludge is pumped to the fermenter at timed intervals—typically at 4.8 minutes per hour. The
target solids concentration in the fermenter is 12,000 milligrams per liter (mg/L). Waste
fermented sludge flows to the gravity thickener; two pumps return the fermenter supernatant
to the bioreactor. The fermenter has a volume of 118,000 gallons, a hydraulic retention time
of 7 to 21 hours, and a mixing power of 0.06 horsepower (FTP) per 1,000 gallons. The solids
retention time (SRT) is designed to be 4 to 5 days.
2 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
Sludge from the gravity thickener is pumped to the primary digester and then to the two
secondary digesters. Digested primary sludge is pumped to two belt filter presses. Secondary
sludge is pumped as return activated sludge (RAS) to the bioreactor. The RAS is pumped by
two RAS pumps to two dissolved air flotation (DAF) thickeners. DAF filtrate is wasted back
to the bioreactor, and the thickened sludge from the DAF is pumped via two DAF float
pumps to two belt filter presses, where it is mixed with digested primary sludge just before
the presses. The DAF sludge is not anaerobically digested to avoid re-release of accumulated
phosphorus. The belt press cake is trucked to a composting operation. Digester supernatant
and the filtrate from belt press are returned to the headworks.
Figure 1 shows the overall process flow diagram. Figure 2 shows details of the biological
reactor and how RAS can be directed to one of three cells depending on operating conditions.
The fermenter supernatant also can be directed to any of the first three cells as conditions
warrant.
Appendix A Kalispell,MT • Advanced Wastewater Treatment - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
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4 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
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Appendix A
Kalispell, MT • Advanced Wastewater Treatment - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Plant Parameters
Overall plant influent and effluent average results for the period July 2005 through June 2006
are shown in Table 3.
Table 3. Influent and effluent averages
Parameter
(mg/L unless
stated)
Flow (MGD)
Influent TP
Effluent TP
Influent BOD
Effluent BOD
Influent TSS
Effluent TSS
Influent NH4-N
Effluent NH4-N
Influent TKN
Effluent TKN
Influent TN
Effluent TN
Average
2.95
4.11
0.12
226.36
<4
225.17
1.21
24.35
<0.07
39.28
0.63
39.6
10.6
Maximum
month
3.45
4.88
0.15
282
<4
326
2.9
29.4
<0.07
47
1.26
48.0
19.9
Max
month vs.
avg.
17%
19%
25%
25%
0%
45%
140%
21%
0%
20%
100%
21%
86%
Maximum
week
4.04
5.2
0.31
428
5.8
680
4.1
-
-
-
-
-
-
Sample
method/frequency
-
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Notes:
BOD = biochemical oxygen demand
Max month vs. average = (max month - average)/average x 100
MGD = million gallons per day
NH4-N = ammonia measured as nitrogen
TKN = total Kjeldahl nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
6 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Table 4 presents plant monthly averages for process parameters.
Table 4. Monthly averages for plant process parameters
Month
July 2005
Aug. 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
MLSS
(mg/L)
2,586
2,517
2,625
2,659
2,637
2,808
2,744
2,757
2,657
2,568
2,536
2,529
Sludge age
(d)
10
8
11
12
10
11
10
9
9
9
9
9
HRT
(hrs)
13
14
13
14
15
15
12
13
14
11
13
11
Water temp
(°C)
18.9
20
18.6
17.1
14.9
12.1
11.4
10.8
10.9
12.3
14.8
16.7
Notes:
HRT = hydraulic retention time
MLSS = mixed liquor suspended solids
Performance Data
This section provides information about the operational performance of nutrient removal at
the plant. Figures 3 and 4 present reliability plots for monthly average and weekly average
phosphorus. For the monthly average data, the facility has a very low coefficient of variation
(COV) of 19 percent, with standard deviation of 0.023 mg/L and a mean of 0.121 mg/L for
the 12-month period. The COV is defined as the standard deviation divided by the mean, and
it is a measure of a system's reliability. The lower the COV, the less the data are spread and
the higher the reliability. Variation is slightly higher on a weekly basis, with a COV of
41 percent. Overall, the facility is highly reliable at removing phosphorus. This is remarkable
in comparison to many other facilities, which have reported poor reliability for biological
phosphorus removal.
Appendix A
Kalispell, MT • Advanced Wastewater Treatment -1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Kalispell, MT
Monthly Average Frequency Curves for Total Phosphorus
O)
» » »
^= Mean = 0.121 mg/L :
: Std. Dev. = 0.023 mg/L '-
: C.O.V. = 19% :
0.05 0.1 0.5 1 2
10 20 30 40 50 60 70 80 90
Percent Less Than or Equal To
95 98 99 99.5 99.9 99.95
* Raw Influent
X Final Effluent
Figure 3. Monthly average frequency curves for TP.
Kalispell, MT
Weekly Average Frequency Curves for Total Phosphorus
100.00
10.00
»»*• * *
1.00
0.10-
0.01
ZMean = 0.121 mg/L
- Std. Dev. = 0.050 mg/L
" C.O.V. = 41%
0.05 0.1 0.5 1 2
10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
99 99.5 99.9 99.95
• Raw Influent
X Final Effluent
Figure 4. Weekly average frequency curves for TP.
8 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Figure 5 presents the reliability plot for monthly average ammonia nitrogen. The facility
reports only a monthly result for nitrogen compounds, which precludes generating a
reliability plot for weekly data. For the period of July 2005 to June 2006, the plant routinely
produced effluent ammonia nitrogen below a detection level of 0.07 mg/L. This is
remarkable for a cold-region operation with an average water temperature of 8 degrees
Celsius (°C) on cold days. The plant's successful operating strategy has been to maintain
sufficient biomass during the winter, i.e., 2,700 parts per million (ppm) of mixed liquor
suspended solids (MLSS) vs. 2,500 ppm in the summer. The higher biomass in winter allows
the process to overcome the greatly slowed growth of nitrifiers under cold conditions.
Kalispell, MT
Monthly Average Frequency Curves for Ammonia Nitrogen
100.0
10.0
I 1.0
0.1
0.0
II III II III
I I I
» *
* Std. Uev. = 0.00 mg/L —
I I I I I I I I
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
99 99.5 99.999.95
» Raw Influent
X Final Effluent
Figure 5. Monthly average frequency curves for ammonia nitrogen.
Appendix A
Kalispell, MT • Advanced Wastewater Treatment - 9
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Nutrient Removal Technology Assessment Case Study
September 2008
Figure 6 presents the reliability plot for monthly TN. The plant personnel set a design goal of
900 Ib/day as TN (36 mg/L at 3 million gallons per day [MOD]), but this is not a permit
limit. For the period of July 2005 to June 2006, the plant produced an effluent with an
average TN of 10 mg/L, with more than 90 percent of that in the form of nitrate.
Kalispell, MT
Monthly Average Frequency Curves for Total Nitrogen (Goal)
20 30 40 50 60 70 80
Percent Less Than or Equal To
• Raw/Influent XFinal Effluent
Figure 6. Monthly average frequency curves for TN (goal).
Reliability Factors
The plant has a permit limit for phosphorus of 1 ppm year-round monthly average; for
ammonia nitrogen, it has a permit limit of 1.4 ppm monthly average to meet all stream
requirements. However, the plant has an operational policy to achieve the maximum nutrient
reduction without needing to add chemicals to precipitate phosphate or to support
denitrification.
The key factor in the facility's success is generating sufficient volatile fatty acids (VFAs).
The plant routinely meets its target of 18 mg/L VFAs at 20 °C and 13 mg/L VFAs at 13 °C in
the anaerobic zones. This means that the VFA-to-total phosphorus (TP) ratio ranges
seasonally between 1.5 and 6. The yearly average ratio is 3.5. The plant uses a two-stage
fermenter to generate VFAs from primary sludge and produces around 200 mg/L VFAs in
winter and 450 mg/L VFAs in summer under the sludge age of 4 to 5 days and an FIRT of 7
to 21 hours. Unique design allows separate control of the SRT and FIRT at this facility.
Thickened fermented sludge is transferred to the anaerobic digesters, while the supernatant is
pumped to the first anaerobic cell in the biological nutrient removal (BNR) system (Emrick
10 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
and Abraham 2002; Natvik et al. 2003). The result is that the plant obtains effluent TP
concentrations averaging 0.12 mg/L over the year with a low COV.
Another factor in the facility's success is that the plant personnel monitor each cell in the
biological reactor for nutrient concentration, pH, and suspended solids and take actions as
needed. Personnel do the monitoring by daily analyzing grab and composite samples rather
than by using online sensors. The hands-on approach and daily attention to system
performance prevent problems from becoming uncontrolled, while giving the operators a
stake in the plant performance rather depending on the computer. Adjustments that can be
made include solids wasting rate, recycle points, and which cells are aerobic or anoxic.
The flexibility in the process design is another valuable feature at Kalispell because the plant
personnel can change the effective volumes of the anaerobic, anoxic, and aerobic zones by
independently adjusting the conditions in each reactor cell as conditions warrant. The
bioreactor is optimized for SRT and HRT at varying temperatures.
Another important operating practice is that of not maintaining sludge blankets in the
secondary clarifiers (No Blanket Policy). This has helped the plant to achieve healthy biology
with sufficient sludge age and excellent phosphorus removal because maintaining an
inventory of sludge that has accumulated phosphorous maintains the chance that some of that
phosphorous will eventually be released. In the summer the sludge age is maintained at
between 8 and 10 days with an MLSS of 2,500 ppm. In winter the MLSS is increased to
2,700 ppm to ensure full nitrification under cold weather conditions.
Although this facility nitrified fully down to the detection limit (0.07 mg/L), the
denitrification was not required and therefore 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.
Recycle loads were kept low at this facility. Secondary sludge was kept aerobic until
dewatering, and the digester supernatant was kept at a minimum. The results were that the
ortho-phosphorus returning to the headworks was measured at 6 percent of the influent TP
load.
Wet-weather flows were managed through the equalization basin, which can store
12.5 percent of the influent flow. No special mode of operation was required at this facility.
Appendix A Kalispell, MT • Advanced Wastewater Treatment -11
-------
Nutrient Removal Technology Assessment Case Study September 2008
Costs
Capital Costs
The plant was upgraded for BNR in 1992, when the system was set up as an 11-cell modified
UCT with swing zones. The modifications for BNR were part of an overall upgrade program
that cost a total of $13.5 million—$9.94 million in construction costs and $3.56 million in
indirect costs. The elements involved in BNR that were included in the 1992 expansion are
shown in Attachment 1. They included additional tanks, tank coatings, a supervisory control
and data acquisition (SCADA) system, mixers, pumps, blowers, a fermenter, and a secondary
sludge thickener. As shown in Attachment 1, these costs were attributed to removal of
phosphorus, removal of nitrogen, or removal of non-nutrients, specifically biochemical
oxygen demand (BOD). For units where the purpose could be fixed on one nutrient (e.g., a
fermenter, which is only for phosphorus removal), the cost was attributed entirely to that
nutrient. For the anoxic zone mixers, the cost was evenly divided between nitrogen and BOD
removal because they are removed equally in those zones during denitrification. For the
aeration zones and where units could not be specified for nutrients, the distribution was
12 percent for phosphorus, 48 percent for nitrogen, and 40 percent for BOD, which is the
ratio at which those three removal processes take up oxygen on a molar basis during aeration.
The total of the construction costs for the BNR units was $4.2 million. Because the total
indirect costs on the $9.9 million construction were $3.56 million, the indirect costs
attributed to BNR were $1.51 million by ratio. These costs were allocated to phosphorus,
nitrogen, and BOD removal using the 12/48/40 formula, resulting in $749,000 for
phosphorus removal, $2.71 million for nitrogen removal, and $2.26 million for BOD
removal, all in 1992 dollars.
These capital cost results were updated to 2007 dollars using the Engineering News-Record's
Construction Cost Index (ENR CCI). The ENR CCI, compiled by McGraw-Hill, provides a
means of updating historical costs to account for inflation, thereby allowing comparison of
costs on an equal basis. From a Web site provided by the U.S. Department of Agriculture,
the ENR index for 1992 was 4,985, while the ENR index for May 2007 was 7,942 (USDA
2007). Multiplying the above results by the ratio 7,942/4,985 obtained the result of
$1.19 million for phosphorus removal, $4.31 million for nitrogen removal, and $3.60 million
for BOD removal in 2007 dollars.
These results were annualized using the interest rate formula for determining a set of annual
payments for a present value, given an interest rate and payback period. For this and all other
case studies for this document, a 6 percent interest rate and 20-year payback was assumed,
resulting in a multiplication factor of 0.0872. The annualized capital cost for phosphorus
removal was thus $101,500, while the annualized capital cost for nitrogen removal was
$376,000. This annualized capital cost for nitrogen removal was used for later unit cost
estimates for TN.
12 - Kalispell, MT • Advanced Wastewater Treatment Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
As shown in Attachment 1, the total capital charge for the BNR removal system was
$5.7 million in 1992 dollars, which updated to $9.1 million in 2007 dollars. For this 3-MGD
facility, the total capital cost for BNR removal was $3.03/gallon of treatment capacity.
Operation and Maintenance Costs
In all case studies prepared for this document, the O&M costs considered were for electricity,
chemicals, and sludge disposal. Labor costs for operation and maintenance were specifically
excluded for three reasons:
1. Labor 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 other factors;
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
SCADA system that accompany most upgrades.
3. Most facilities were unable to break down which extra personnel were employed
because of nutrient removal and related overtime costs, making labor cost
development difficult.
The Kalispell plant uses an entirely biological process to achieve both nitrogen and
phosphorus limits; therefore, the only significant operating cost is electrical use for mixers,
pumps, and operating the fermenter. Attachment 2 shows a summary of the power use
calculations. The power use attributed to phosphorus removal was 389,000 kilowatt-hours
(kWh); using the average electrical rate of $0.045/kWh (which included all demand charges),
the electrical cost for phosphorus removal was $17,500. The power usage attributed to
nitrogen removal was 1,077,000 kWh, and at the average electrical rate, the electrical cost for
nitrogen removal was $48,500.
Unit Costs for Nitrogen and Phosphorus Removal
In the evaluation period, the plant removed 35,700 Ib of phosphorus. With the results above,
the unit O&M cost for phosphorus removal was $0.49/lb, while the annualized unit capital
cost for phosphorus removal was $2.84.
In the evaluation period, the plant removed 258,000 Ib of TN. With the results above, the unit
O&M cost for TN removal was $0.19/lb of TN, while the annualized unit capital cost for TN
removal was $1.46.
Appendix A Kalispell, MT • Advanced Wastewater Treatment -13
-------
Nutrient Removal Technology Assessment Case Study September 2008
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the annualized unit capital and unit O&M costs. Thus, the
life-cycle cost for phosphorus removal was $3.33/lb and the life-cycle cost for TN removal
was$1.64/lb.
Assessment of magnitude of costs: The capital cost of $3.03/gpd capacity is relatively high,
but the O&M costs are very low. One of the key factors is that chemicals are not used for
nutrient removal, saving both those costs and costs that would be attributed to additional
sludge generation.
Summary
The Kalispell Advanced WWTP has proven to successfully provide enhanced biological
phosphorus removal in a cold-climate region of the United States. The reliability of the
facility is good, with a mean effluent concentration of 0.12 mg/L as TP and a COV of
19 percent monthly average, or a COV of 41 percent weekly average. Ammonia nitrogen
removal reliability is outstanding, with a mean concentration at or below the detection limit
of 0.07 mg/L and a COV of 0 percent on a monthly average basis.
Reliability factors include a science-based control strategy, in-house generation of sufficient
VFAs in the fermenter, and diligent monitoring and timely control of key process parameters
by plant personnel. Removal costs for both phosphorus and nitrogen were shown to be
reasonable, with O&M costs for both being largely driven by electricity usage and relatively
low capital costs.
A ckn o wledgments
The authors of this report are grateful to Joni Emrick, water resource manager, and Curt
Konecky of the Kalispell Advanced WWTP for their guidance and assistance in preparing
this case study. This case study report would not have been possible without their prompt
response with well-deserved pride in the facility and its operation. The authors also wish to
thank the city of Kalispell for its participation.
References and Bibliography
Emrick, J., and K.Abraham. 2002. Long-term BNR Operations—Cold in Montana! In
Proceedings of the Water Environment Federation 75th Annual Technical Exhibition
& Conference, Chicago, IL, September 28-October 2, 2002.
Natvik, O., B. Dawson, J. Emrick, and S. Murphy. 2003. BNR "Then" and "Now"—A Case
Study—Kalispell Advanced Wastewater Treatment Plant. In Proceedings of the
14 - Kalispell, MT • Advanced Wastewater Treatment Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Water Environment Federation 76th Annual Technical Exhibition & Conference,
Los Angeles, California, October 11-15, 2003.
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.
Appendix A Kalispell, MT • Advanced Wastewater Treatment -15
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Attachment 1: Capital Costs
Tanks
Tank coats
SCADA
Mixers
Ret/Sup pumps
Blowers
Fermenter
Thickener
Primary sludge pump
Piping
Site work
Total
Indirects
Total capital
Updated to 2007
Annualized
Updating factors
1992ENRCCI
May 2007 ENRCCI
A/P (6%, 20 years)
$1,300,000
$75,000
$1,000,000
$43,000
$175,000
$155,000
$45,000
$35,000
$80,000
$500,000
$800,000
$4,208,000
$1,505,526
$5,713,526
$9,102,673
4,985
7,942
0.0872
%P
12%
12%
12%
0%
12%
0%
100%
100%
10%
12%
12%
12%
%N
48%
48%
48%
50%
48%
50%
0%
0%
50%
48%
48%
48%
%BOD
40%
40%
40%
50%
40%
50%
0%
0%
40%
40%
40%
40%
$P
$156,000
$9,000
$120,000
$0
$21,000
$0
$45,000
$35,000
$8,000
$60,000
$96,000
$550,000
$180,663
$730,663
$1,164,078
$101,508
$N
$624,000
$36,000
$480,000
$21,500
$84,000
$77,500
$0
$0
$40,000
$240,000
$384,000
$1,987,000
$722,653
$2,709,653
$4,316,963
$376,439
$BOD
$520,000
$30,000
$400,000
$21,500
$70,000
$77,500
$0
$0
$32,000
$200,000
$320,000
$1,671,000
$602,211
$2,273,211
$3,621,633
$315,806
16 - Kalispell, MT • Advanced Wastewater Treatment
Appendix A
-------
Attachment 2: Electrical Costs
Horsepower
Mixers
3
7.5
Ret Pumps
10
4
Blowers
200
Super Pumps
7.5
Fermenter
5
15
10
Volts
460
460
460
460
460
460
460
460
460
Gravity Thickener
2
460
Amps
4
10
5.82
6.7
220
9.7
6.8
27
14
3.1
Primary Clarifier Sludge Pump
5
460
6.8
VA
1,840
4,600
2,677.2
3,082
101,200
4,462
3,128
12,420
6,440
1,426
3,128
Number
5
4
1
1
2
2
2
2
1
1
1
kW
Power draw
9.2
18.4
2.6772
3.082
202.4
8.924
6.256
24.84
6.44
1.426
3.128
hours/day
24
24
24
24
24
24
24
24
24
24
24
kWh
draw/day
220.8
441.6
64.2528
73.968
4,857.6
214.176
150.144
596.16
154.56
34.224
75.072
kWh/yr
Rate
Totals
kWh
draw/year
80,592
161,184
23,452.27
26,998.32
1,773,024
78,174.24
54,802.56
217,598.4
56,414.4
12,491.76
27,401.28
2,512,133
0.045
113,046
%P
12%
12%
12%
12%
0%
12%
100%
100%
100%
100%
12%
$/kWh
%N
48%
48%
48%
48%
50%
48%
0%
0%
0%
0%
48%
$/yr
ForP
9,671.04
19,342.08
2,814.273
3,239.798
0
9,380.909
54,802.56
217,598.4
56,414.4
12,491.76
3,288.154
389,043.4
P
17,506.95
ForN
38,684.16
77,368.32
11,257.09
12,959.19
886,512
37,523.64
0
0
0
0
13,152.61
1,077,457
N
48,485.57
-------
-------
Clark County Water Reclamation Facility
Las Vegas, Nevada
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Clark County Water Reclamation Facility (WRF) is in Las Vegas, Nevada. This facility
was selected as a case study because of the anoxic/oxic (A/O) process for biological
phosphorus removal with alum feed.
Originally commissioned in 1956, the facility was enhanced with biological nutrient removal
(BNR) in 1995 during an 88-million gallon per day (MGD) expansion. The plant has
obtained a very high level of phosphorus removal following a series of facility upgrades.
With the expansion, the facility essentially operates as two plants—the Advanced Waste
Treatment Plant (AWT) and the Central Plant (CP)—with separate discharges available. The
expansion allowed the plant to gain nitrification capabilities for the entire plant flow, in both
the CP and the AWT. Although the facility initially used and still uses chemical treatment to
meet standards, it has also implemented the A/O process to provide biological phosphorus
removal. The facility is designed for an average flow of 100 MGD and averaged 95 MGD
during the 2006 calendar year.
The relevant National Pollutant Discharge Elimination System (NPDES) permit limits are
listed in Table 1.
Table 1. Clark County WRF NPDES permit limits
Parameter
BOD
TSS
TP
Total NH4-N
30-day avg.
(mg/L)
30
30
-
-
7-day avg.
(mg/L)
45
45
-
-
30-day avg.
(Ib/day)
37,530
37,530
-
-
Daily wasteload
allocation (Ib/day)
-
-
173
502
Notes:
BOD = biochemical oxygen demand
NH4-N = ammonia measured as nitrogen
P = phosphorus
TSS = total suspended solids
Appendix A
Clark County, NV • Water Reclamation Facility - 1
-------
Nutrient Removal Technology Assessment Case Study September 2008
The wasteload allocation is an arrangement in which the Nevada Division of Environmental
Protection set an overall load on the Las Vegas Wash from Clark County, the city of Las
Vegas, and Henderson, Nevada. The allocations for Clark County translate into 0.21
milligrams per liter (mg/L) total phosphorus (TP) and 0.6 mg/L for ammonia nitrogen at 100
MOD.
Basis of Design and Flow Schematic
Primary settling tanks: 818 gallons per day per square foot (gpd/ft2) at annual average flow
and 1,309 gpd/ft2 at peak hour
Activated sludge: nine basins
Hydraulic capacity per basin 10 MOD
Total volume per basin 2.13 MG
Hydraulic retention time 5.1 hours
Sludge age 5-9 days
Secondary clarifier: 710 gpd/ft2 at annual average flow
A flow sheet for the CP is presented in Figure 1 for the entire facility. The main difference
between the AWT and the CP is that the AWT employs tertiary clarifiers in advance of the
tertiary filters, as shown in Figure 2. In both plants, influent is treated in the primary settling
tanks with ferric chloride added as enhancement, then through A/O biological reactors. The
A/O process provides biological phosphorus removal and nitrification, along with some
degree of denitrification. From there, the wastewater is dosed with alum for additional
phosphorus removal and then treated in a tertiary clarifier/filter combination in the AWT or
in just a tertiary filter in the CP. When the clarifiers were first installed in the 1980s, filter
technology was such that they needed protection from high solids that would make operation
and maintenance (O&M) difficult; the CP uses an air-water, scour-backwash system so that
such protection is not vital to continued good operation. The effluent is filtered and
disinfected by ultraviolet (UV) radiation and then either sent to reclaimed water customers or
discharged to the Las Vegas Wash and the Lake Meade Wetlands.
The secondary sludge is thickened by dissolved air flotation (DAF). The primary sludge is
thickened to 5 percent solids in the settling tanks and then sent to the same holding tank with
the thickened secondary sludge. They are dewatered together by belt filter press for
landfilling.
2 - Clark County, NV • Water Reclamation Facility Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
_ .
Primary
Treatment
III
f*
I
BPR
Sludge
III
*-
t
i
M-
k
CPT Tertiary
Facility
AWT Tertiary
Liquid
Solid
Recycle
Dewatering
Figure 1. Plant flow schematic.
Alum
8-13 mg/L
Backwash
Excess Flows
AWT
Secondary
Effluent ^
CENTRAL
PLANT
Discharge to
east side of Las
Vegas Wash
To Headworks
Backwash Clarifier
Discharge to
west side of Las
Vegas Wash
Alum
4-10 mg/L
Figure 2. Tertiary processes.
Appendix A
Clark County, NV • Water Reclamation Facility - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Influent Flow 110 MOD
Influent Suspended Solids 299 mg/L
Influent Biochemical Oxygen Demand 294 mg/L
Grit Removal
Tank
Ferric Chloride
Polymer
Primary
Settling Tanks
Forced
Thickened
Sludge Holding
Tank
Aeration
Basins
Return
Activated
Sludge
Ferric Chloride
Polymer
I dissolved Air
Flotation
Thickener
Secondary
Settling Tanks
Waste
Activated
Sludge
Sludge
Cake To
Landfill
Dual Media
Filters
Ultraviolet
Light Disinfection
Sodium
Hypochlorite
To Reclaimed
Water Customers
Discharge to Las Vegas Wash and Lake Mead Wetlands
Figure 3. Clark County WRF CP flowsheet schematic.
4 - Clark County, NV • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Plant Data
Table 2 presents average plant data for the 2006 calendar year. The data show outstanding
removal of nutrients, biochemical oxygen demand (BOD), and suspended solids. The facility
easily meets all of its permit limits.
Table 2. 2006 average CP water quality data
Parameter
Flow (MGD)
Influent TP (mg/L)
Effluent TP (mg/L)
Influent BOD (mg/L)
Effluent BOD (mg/L)
Influent TSS (mg/L)
Effluent TSS (mg/L)
Influent NH4-N (mg/L)
Effluent NH4-N (mg/L)
Influent TKN (mg/L)
Effluent TKN (mg/L)
Influent NO3/NO2 (mg/L)
Effluent NO3/NO2 (mg/L)
Influent TN (mg/L)
Effluent TN (mg/L)
Average
98
5.8
0.1
357
<2
366
<5
26.8
0.08
46
0.69
0.18
15.3
30.3
15.2
Max
month
101.4
7.0
0.17
390
4.75
413
10
28.8
0.31
53
1.02
0.46
16.4
34.5
16.6
Max month
vs. avg.
3.5%
20%
73%
9%
1 37%
13%
1 00%
7%
300%
14%
47%
155%
7%
14%
7%
Max
week
102.3
7.5
0.41
445
7
456
21
30
1.22
75
2.3
0.8
16.5
37.6
16.7
Sample method/
frequency
-
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
-
-
Notes:
BOD = biochemical oxygen demand
NH4-N = ammonia measured as nitrogen
NO3/NO2= nitrate + nitrite
TKN = total Kjeldahl nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
Appendix A
Clark County, NV • Water Reclamation Facility - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 3 presents plant monthly average plant process parameters.
Table 3. CP monthly average plant process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
MLSS
(mg/L)
2,902
3,422
3,684
3,732
3,147
3,499
3,166
3,057
2,425
2,441
2,760
2,535
Sludge age
(d)
9
9
8
7
6
5
5
5
6
7
8
8
HRT
(hr)
5.43
5.45
5.31
5.10
5.06
5.82
5.73
5.80
6.32
6.31
6.42
6.49
Temperature
(°C)
20
20
24
26
28
29
29
29
28
26
24
20
Notes:
HRT = hydraulic retention time
MLSS = mixed liquor suspended solids
Performance Data
Figures 4 and 5 present reliability plots for weekly average and monthly average TP. The
plant operation provides outstanding performance in TP removal: the average effluent
concentration is under 0.1 mg/L and the coefficient of variation (COV) is low at 30 percent.
This means that the data have a low standard deviation relative to the mean and, therefore,
that the plant will routinely produce effluent with TP below 0.2 mg/L through the course of
the year.
6 - Clark County, NV • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
100
Clark Co. NV Water Reclamation Plant
Weekly Average Frequency Curves for Total Phosphorus
0.01
:CP Mean = 0.1 mg/L
; Std. Dev. = 0.042 mg/L
-COV = .
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
A Final Effluent
Figure 4. Weekly average frequency curves for TP.
Clark Co. Water Reclamation Plant - Las Vegas, NV
Monthly Average Frequency Curves for Total Phosphorus
100
=d 10
O)
in
O
S. 1
O
Q.
15
0.01
* — » * * *
: Mean = 0.0995 mg/L
: Std. Dev. = 0.0297 mg/L
; COV = 30%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
Figure 5. Monthly average frequency curves for TP.
Appendix A
Clark County, NV • Water Reclamation Facility - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Figures 6 and 7 present reliability plots for the weekly average and monthly average total
nitrogen (TN) for the facility. TN removal is not required under the permit, and therefore it is
limited. The effluent TN averages 15.2 mg/L with a standard deviation of 0.6 mg/L.
Clark Co. Nevada Water Reclamation Facility
Weekly Average Frequency Curves for Nitrogen
100
10
0.1
- Mean = 15.2mg/L
Std. Dev. = 0.67 mg/L
~ COV = 4%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - TKN
X Final Effluent - Total N
Figure 6. Weekly average frequency curves for nitrogen.
8 - Clark County, NV • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
100
10
0)
c"
01
0)
0.1
Clark Co. Water Reclamation Facility
Monthly Average Frequency Curves for Nitrogen
» » • * » *
• •
> »
Mean =15.2 mg/L —
Std Dev - 0 6 mg/l
rnv = 4%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
989999.5 99.999.95
• Raw Influent - TKN
x Final Effluent - Total N
Figure 7. Monthly average frequency curves for nitrogen.
Figures 8 and 9 present reliability plots for weekly average and monthly average ammonia
nitrogen for the plant. Ammonia is routinely removed to near the detection level in the plant,
with a mean of 0.05 mg/L and a very low COV of 22 percent.
Appendix A
Clark County, NV • Water Reclamation Facility - 9
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Clark Co. Nevada Water Reclamation Facility
Weekly Average Frequency Curves for Ammonia Nitrogen
100-
10
0.1
0.01
" Mean = 0.049 mg/L
"Std. Dev. = 0.023 mg/L
~ COV =49%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9 99.95
Percent Less Than or Equal To
• Raw Influent-Ammonia-N XFinal Effluent-Ammonia N
Figure 8. Weekly average frequency curves for ammonia nitrogen.
100 •
Clark Co. Water Reclamation Facility
Monthly Average Frequency Curves for Ammonia Nitrogen
10
c
HI
D)
o
0.1
0.01
= CP Mean =0.048 mg/L
:Std. Dev. = 0.011 mg/L
" COV = 22%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N x Final Effluent - Ammonia N
Figure 9. Monthly average frequency curves for ammonia nitrogen.
10 - Clark County, NV • Water Reclamation Facility
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Reliability Factors
Several factors have contributed to efficient and reliable operation at this facility. The
effluent concentration was low at 0.09 mg/L in TP with a COV of 30 percent and 0.05 mg/L
in ammonia nitrogen with a COV of 22 percent.
One key is the wastewater characteristics and in-plant generation of volatile fatty acids
(VFAs). The BOD-to-TP ratio averaged 29.8 for the year and ranged from an average 26.5 to
34.2 monthly. Furthermore, this facility generated additional VFAs by operating primary
settling tanks as fermenters. Typical operating parameters included thickening the primary
sludge to 5 percent total solids, thereby generating enough VFAs to maintain 35 to 45 mg/L
of VFA in the primary effluent. Thickening primary sludge to 6 percent total solids was
found excessive and detrimental to both odor-control and clarification purposes.
The biological process was originally a conventional process, which was later converted to
an A/O process by adding aeration controls to ensure sufficient dissolved oxygen (DO) in the
aerobic zones. The DO set point is 2.4 mg/L to meet an instantaneous minimum DO of
2.0 mg/L. The optimal sludge age ranged from 5 days in summer at 29 degrees Celsius (°C)
to 9 days in winter at 20 °C. The average secondary effluent concentration showed an
average of 0.7 mg/L as TP, 0.1 mg/L ammonia nitrogen, and 15 mg/L in TN, with a return
activated sludge (RAS) flow ranging from 45 to 60 percent. The clarifiers are operated with a
minimal blanket (less than 6 inches) to prevent secondary release of phosphorus. Secondary
release of phosphorus is of concern at this plant because of the generally high temperatures
increasing biological activity.
Another factor is the successful polishing of the biological process effluent for phosphorus
by the tertiary clarifiers and filters. The AWT has a tertiary clarifier ahead of tertiary filters
and performs better than the CP when the biological phosphorus removal process is upset and
carries elevated levels of suspended solids. The tertiary clarifier acts as an added line of
defense for the filters and maintains steady effluent quality ahead of the filters. At the AWT,
alum addition can go up to 15-16 mg/L without a having an adverse effect on the filters. The
CP, however, does not have a tertiary clarifier, and the alum dosage is limited to 10-12 mg/L
before the filters become blinded by solids. Note that filters at the CP have an air-water
backwash capability and therefore work well under these operating conditions.
Another key to successful removal of phosphorus is having multiple chemical feeding points.
Ferric chloride is fed to the primary settling tanks with the primary purpose of removing
suspended solids and a resulting side benefit of removing some phosphorus. The dosage of
ferric chloride averages 10-12 mg/L. Alum is added as described above to polish residual
phosphorus ahead of the tertiary filters.
Appendix A Clark County, NV • Water Reclamation Facility - 11
-------
Nutrient Removal Technology Assessment Case Study September 2008
Another key to successful phosphorus removal is minimal recycle of in-plant phosphorus
loads. Waste activated sludge (WAS) is thickened in a dissolved air floatation (DAF)
process, and the combined primary sludge (0.7 MOD) and WAS sludge (1.15 MOD) are
dewatered daily at the belt filter press with ferric chloride and polymer addition. This
operation minimizes the release of phosphorus and prevents odor generation. The key
operational activity here is the daily dewatering of all sludge. Reduction in odors is also
aided by processing the sludge daily, which is accomplished by plant personnel working two
10-hour shifts and processing all sludge generated at the plant. This practice ensures a
minimal amount of odor generation at the plant and the minimum recycle of phosphorus
loadings back to the treatment processes. The TP in the filtrate from dewatering ranges
between 100 and 300 mg/L. The TP in the recycle flows is in the range of 20 to 25 percent of
the influent total.
The final line of defense is the tertiary filters. They were designed to operate at 5 gpm/ft2
during dry-weather peak flows and have performed well. The maintenance dosage of alum is
fed into tertiary filters to prevent secondary release from biological solids. They average
6 mg/L at the AWT and 4 mg/L at the CP. The long-term average soluble phosphorus leaving
the filters is less than 0.02 mg/L.
A benefit of having biological phosphorus removal followed by chemical polishing is
reduction in chemical sludge. Over the years, the plant has observed a decrease in total
sludge production. In 1997 the average sludge production was approximately 600 wet tons
per day. In 2007 even with increased flows, the sludge production is approximately 400 wet
tons per day.
Another key in the successful operation of the plant was automating the process monitoring
and controls. Two distinct functions are automated at this plant. One is that the decisions on
WAS from nine separate trains are made and carried out by a program developed in-house
using a mixed liquor suspended solids (MLSS) probe. The other is automatic blower control
in the aerobic zones. The head section of the aerobic zone receives the maximum supply of
air, while the latter section of the zone is controlled by a program with a set point of 2.4
mg/L DO using multiple probes.
The blowers are a key part of the process and require redundancy. The operating philosophy
is to provide a minimum of 0.5 mg/L DO at all times, even during the peak hot period of the
day. The plant experienced a DO deficit during a week of air temperatures at 113 degrees
Fahrenheit (°F) (45 °C), which was detrimental to the biological treatment process.
Another key is good redundancy, achieved by running nine separate treatment processes in
parallel. If one train experiences an upset condition, operators can supply good seed MLSS
from one of the other trains.
12 - Clark County, NV • Water Reclamation Facility Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Alternative Processes Considered
Because the plant is almost at capacity (100 MOD versus 110 MOD), expansion plans are
being pursued. For the AWT, a pilot program is underway for membrane filtration of
secondary effluent. Three different membranes are being evaluated concurrently. If the
evaluations are successful, the membrane filter could replace both the tertiary clarifier and
the dual media filters.
Cosfs
Capital Costs
The plant has undergone a number of upgrades and renovations since the original
commissioning of the AWT in 1982. Those total costs were updated to 2007 dollars using the
Engineering News-Record Construction Cost Index for construction costs and the Consumer
Price Index for the applicable years (USDA 2007). The resulting capital costs, the attributed
percentages for phosphorus and nitrogen removal, and the resulting total capital costs are
shown in Table 4.
Table 4. Upgrade capital costs and resulting phosphorus and nitrogen removal
Capital
AWTDes
AWT Const
CP Expan Design
CP Expan Const
CP Filters Design
CP Filters Const
Central Plant S.
Sec. Design
Central Plant S.
Sec. Const
Central Plant S.
Sec. Design
Central Plant S.
Sec. Const
TOTAL
Year
1982
1982
1994
1994
2002
2002
2003
2003
2005
2005
Amount
$2,800,000
$28,000,000
$2,000,000
$29,000,000
$4,200,000
$27,600,000
$3,790,000
$39,,304,293
$1,901,098
$19,218,993
$157,814,384
Updated cost
$5,956,103
$58,137,516
$2,770,875
$42,588,388
$4,794,056
$33,526,950
$4,230,603
$46,625,048
$1,998,417
$20,499,227
$221,000,000
%P
50%
50%
12%
12%
50%
50%
12%
12%
12%
12%
-
%N
0%
0%
48%
48%
0%
0%
48%
48%
48%
48%
-
P removal
$2,978,051
$29,068,758
$332,505
$5,110,607
$2,397,028
$16,763,475
$507,672
$5,595,006
$239,810
$2,459,907
$65,452,819
N removal
$0
$0
$1,330,020
$20,442,426
$0
$0
$2,030,689
$22,380,023
$959,240
$9,839,629
$56,982,027
The capital expenditure in 2007 dollars that could be attributed to phosphorus removal was
$65.4 million. The annualized capital charge (20 years at 6 percent) was $5.71 million for
phosphorus removal.
Appendix A
Clark County, NV • Water Reclamation Facility - 13
-------
Nutrient Removal Technology Assessment Case Study
September 2008
The capital expenditure in 2007 dollars that could be attributed to TN removal was
$57 million. The annualized capital charge (20 years at 6 percent) was $4.97 million for TN
removal. This same expenditure could be attributed to ammonia nitrogen removal.
The total capital attributed to BNR in 2007 dollars was $221 million. For the 110-MGD
facility, the capital expenditure per gallon of BNR treatment capacity was $2.01.
Operation and Maintenance Costs
The Clark County plant uses a combination of biological and chemical phosphorus removal
to achieve the limit. This means that costs for phosphorus removal are distributed among
primary treatment (adding ferric chloride), secondary treatment (aeration basins, mixers, and
pumps), tertiary treatment (chemical addition and filtration), solids dewatering, and
laboratory testing. Costs for each of those components of wastewater treatment are shown in
Table 5, with the percentages of the costs that were attributed to TP and TN removal and the
final values.
Table 5. Component costs and resulting phosphorus and nitrogen removal
Component
Primary
Secondary
Tertiary
Solids dewatering
Lab
Other
TOTAL
Total op. costs
$1,877,685
$5,829,302
$3,967,135
$3,957,135
$1,529,827
$3,875,144
$19,979,131
% for P
12%
12%
12%
50%
10%
0%
-
% for N
48%
48%
0%
0%
10%
0%
-
PO&M
$225,322
$699,516
$476,056
$1,450,019
$152,983
$0
$3,003,896
NO&M
$901,289
$2,798,065
$0
$0
$152,983
$0
$3,852,337
Unit Costs for Nitrogen and Phosphorus Removal
In 2006 the plant removed 1,663,000 Ib of phosphorus. With the results shown in Tables 3
and 4, the unit O&M cost for phosphorus removal is $1.81/lb, and the unit capital cost is
$3.43/lb of phosphorus removed.
In 2006 the plant removed 8,994,000 Ib of nitrogen. With the results shown in Tables 3 and
4, the unit O&M cost for nitrogen removal is $0.43/lb and the capital cost is $0.55/lb of TN
removed.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the unit capital and unit O&M costs. Thus, the life-cycle
cost for phosphorus removal is $5.24/lb phosphorus removed and while the life-cycle cost for
nitrogen removal is $0.98/lb nitrogen removed.
14 - Clark County, NV • Water Reclamation Facility
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Assessment of Magnitude of Costs and Main Factors
The life-cycle costs for phosphorus removal and full nitrification are on the high side, for
achieving an extremely low level of phosphorus and ammonia nitrogen by upgrading existing
facilities.
Discussion
Reliability factors: Three major factors contribute to a reliable performance in phosphorus
removal and nitrification: (1) multiple chemical feeds to the system, (2) good biological
phosphorus removal with in-plant VFA generation and full nitrification, and (3) good tertiary
filters in suspended solids removal. This combination of chemical, biological, and physical
processes in series provides a reliable operation with exceptionally low concentrations of
phosphorus at 0.09 mg/L with a low COV of 30 percent, while the ammonia nitrogen
concentration is at 0.05 mg/L with an even lower COV of 22 percent average monthly.
Cost factors: This plant is an example of exceeding the original design capacity with retrofit
upgrades, which results in significant cost savings. The capital cost for phosphorus removal
and complete nitrification is estimated to be low at $2.01/gpd capacity. The unit costs for
capital and O&M were $5.43/lb of phosphorus removed and $1.38/lb of nitrogen removed.
The unit costs for O&M were $1.84/lb of phosphorus removed and $0.51/lb of nitrogen
removed.
Summary
The Clark County plant operation has been successful in reducing effluent phosphorus to the
limit of technologies at the existing plant using a combination of biological and chemical
treatment processes in series with good reliability. The plant is almost at capacity and yet has
produced effluent far below the discharge limits. The mean TP concentration was 0.099
mg/L for the year with a COV of less than 30 percent, at either the AWT or CP. The
technique of using several different technologies in series to achieve the treatment objective
works, especially when operation is done with computer control and the system has been
designed with a reasonable amount of robustness to allow for repairs and routine
maintenance. The instrumentation technician on staff is a unique and valuable member of the
team at this facility. The costs of operation are also reasonable: life-cycle costs are $5.24/lb
and $0.98/lb for phosphorus and nitrogen removal, respectively.
Acknowledgments
The authors are grateful for the significant assistance and guidance provided by Dr. Douglas
Drury, deputy general manager, and Danielle Fife at the Clark County WRF. This case study
Appendix A Clark County, NV • Water Reclamation Facility - 15
-------
Nutrient Removal Technology Assessment Case Study September 2008
would not have been possible without their prompt response with well-deserved pride in their
facility and its operation. EPA thanks Clark County for participating in this case study.
References and Bibliography
Drury, D. 2005. Phosphorus—How Low Can You Go? In Proceedings of the Water
Environment Federation, 78th Annual Conference, Washington, DC, October 29-
November2, 2005, pp 1125-1134.
Drury, D. 2006. Clark County, NV—Using BPR with Chemical Polishing to Achieve TP
<0.1 mg/L at a 100-MGD Plant. In Proceedings of the Water Environment
Federation, 79th Annual Conference, Dallas, TX, October 21-25, 2006.
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.
16 - Clark County, NV • Water Reclamation Facility Appendix A
-------
Kelowna Wastewater Treatment Plant
Kelowna, British Columbia, Canada
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Kelowna Wastewater Treatment Plant (WWTP) is in the province of British Columbia in
western Canada. This plant was selected as a case study because of its cold-weather
biological nutrient removal (BNR) with a five-stage Bardenpho process, which has been
retrofitted into a new, three-stage Westbank process.
A BNR process, as depicted in Figure 1, was commissioned in 1982 and was operated
successfully through the 1980s. Optimization was ongoing, and an understanding of the BNR
removal mechanisms and pathways was developed, tested, and documented in Kelowna and
through other worldwide research programs.
Stam O\ef1o
uv E'l'jsnrt
WA
i
•
AnaerofcJc
1
r
L A-CMC f&c&c
Asrcbic AnQxfc Aaratfc
6Q intsird tecyde
Figure 1. Kelowna five-stage Bardenpho process.
The Canadian Ministry of Environment (MOE) permit requirements, shown in Table 1,
include biochemical oxygen demand (BOD5)-total, total suspended solids (TSS), total
nitrogen (TN), and total phosphorus (TP) limits. The plant's overall performance is shown in
Table 2.
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 1. Permit requirements for effluent quality
MOE permit requirements
BOD5-total
TSS
TN
TP
Maximum
99th percentile
90th percentile
Annual average (added in 1988)
Daily limits
(mg/L)
8
7
6
2.0
1.5
1.0
0.25
Table 2. Influent and effluent averages
Parameter
(mg/L unless
stated)
Flow (MGD)
Influent TP
Effluent TP
Influent COD
Effluent COD
Effluent BOD
Influent TSS
Effluent TSS
Influent NH4-N
Effluent NH4-N
Influent TKN
Effluent TKN
Influent TN
Effluent TN
Average
8.5
6.0
0.14
626
32
2.5
389
1.2
21.3
0.57
28.8
2.0
28.8
4.38
Maximum
month
8.8
7.4
0.20
747
36
3.8
472
1.6
23.1
1.0
33
2.98
33
4.9
Max month
vs. avg.
3.4%
23%
42%
19%
10%
48%
21%
42%
8.3%
76%
14%
49%
14%
12%
Maximum
week
8.9
9.1
0.25
910
38
5.7
532
2.4
27.6
1.13
38.4
3.5
38.4
5.84
Sample
method/frequency
-
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Grab/monthly
Notes:
BOD = biochemical oxygen demand
COD = chemical oxygen demand
Max month vs. average = (max month - average) / average x
MGD = million gallons per day
NH4-N = ammonia measured as nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
100
2 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Treatment Processes
As the load on the facility increased, it became clear that the five-stage process with a
22-hour hydraulic retention time (HRT) design far exceeded the HRT necessary to meet
effluent discharge requirements for both TP (0.25 milligrams per liter [mg/L]) and TN (6.0
mg/L). Process developments led to implementing a high-rate BNR process that was initially
tested at the Kelowna facility. The first full-scale implementation was at the Westbank
WWTP 20 miles southwest of the Kelowna plant. Details of the basis for plant design are
provided in Attachment 1.
Figure 2 depicts a shorter HRT process, and in 1994 the Kelowna facility was retrofitted in
this mode of operation. In effect, the last two stages (anoxic and aerobic) were bypassed and
made redundant. Later, the bypassed modules were retrofitted as two additional, smaller
Westbank-type modules.
Punped &rtDO5s
Anoidc
i-teno! ftecMele
Figure 2. The Westbank three-stage process.
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
The Kelowna WWTP layout, as depicted in Figure 3, was implemented with the following
process elements:
The liquid train includes
• Screening
• Grit removal
• Primary sedimentation
• Three-stage Westbank BNR
configuration
• Secondary clarifiers
• Dual media filters
• UV disinfection
• Flow and load equalization
The solids train includes
• Primary sludge fermenter
• Air flotation for waste activated
sludge (WAS) thickening
• Centrifuge
• Hauling to compost facility
LIQUID STREAM
SCREENING GRIT REMOVAL PRIMARY
SEDIMENTATION
BIOREACTOR
SECONDARY
CLARIFICATION
SOLIDS STREAM
DEWATERING
FERMENTER STORAGE
ma
FILTRATION UV DISINFI
STORAGE THICKENER
COMPOSTING
Figure 3. Kelowna WWTP 2005 configuration.
4 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
The Westbank process configuration employs a step-feed strategy for distributing primary
effluent and fermenter supernatant (volatile fatty acids [VFA]-enriched) to the specific areas
in the process where they are required. The logic described in the sections below was
applied.
Return Sludge and Pre-anoxic Zone
The Kelowna secondary clarifier design included sidewall depths of 4 meters (m) or greater.
Additionally, the original secondary clarifiers in Kelowna were designed with side-outlet
stilling wells to reduce turbulence under the center inlet well. Floor sloping enabled sludge
and helical scrapers to convey sludge to the center of the clarifier for collection and return to
the bioreactor.
Typical return activated sludge (RAS) rates of 75 percent of the influent flow (Q) maintained
sludge blankets of 0.5 to 0.75 m, which, when concentrated to three times the mixed liquor
suspended solids (MLSS) concentrations, demonstrated significant denitrification potential.
Nitrate reductions in the RAS blanket of up to 6 mg/L have not caused rising sludge
concerns; thus, the Kelowna secondary clarifiers have been operated since 1982 as anoxic
denitrification zones and included in the overall process strategy.
With control of nitrates in the return sludge stream within the clarifier, there is minimal
potential for nitrate return to the anaerobic zone. As an added protection, the original five-
stage design included a small pre-anoxic zone for denitrification of any residual RAS nitrates
before entering the anaerobic zone.
Given the limited potential for nitrate recycle in the return sludge, the amount of primary
effluent required for RAS denitrification is greatly reduced. Plant personnel therefore
developed plans to step-feed primary effluent to both the anaerobic zone (to stimulate
phosphorus release) and the anoxic zones (to stimulate denitrification).
As a result of step-feeding the primary effluent to the main anoxic zone, the suspended solids
concentration increases significantly in the pre-anoxic and anaerobic zones. With 50 percent
primary effluent diversion, the suspended solids concentration is approximately 50 percent
higher than MLSS concentrations in the aerobic zones.
With only a small amount of primary effluent added to the RAS entering the pre-anoxic zone,
a very high denitrification rate ensures that no nitrate breaks through to the anaerobic zone.
The sizing of the pre-anoxic zone in Kelowna is less than 1 percent of bioreactor volume.
Appendix A Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 5
-------
Nutrient Removal Technology Assessment Case Study September 2008
Anaerobic Zone
It has been well documented that the anaerobic zone requires consistent and sufficient VFA
loadings to stimulate phosphorus release and uptake. The amount of VFA required has been
documented as 4-8 kg VFA/kg soluble phosphorus removed.
At the Kelowna facility, a primary sludge fermenter was included in the original Bardenpho
design, and it had a proven track record of consistent VFA production in the range required
for good phosphorus removal. Therefore, the VFA-rich fermenter supernatant is discharged
directly to the anaerobic zone, ensuring a steady feed of VFA to the phosphorus
accumulation organisms (PAO).
It was established that with the side-stream VFA addition, the process performed better when
the HRT of the anaerobic zones was reduced from 3 hours to 1 hour. This might have been
the result of reduction of secondary release of phosphorus in the larger anaerobic cells.
With a Westbank configuration, the primary effluent step-feed to the anoxic zone is adjusted
to complete two tasks:
• Primary effluent containing some VFA is added to the anaerobic zone along with the
supernatant from the side-stream, primary-sludge fermenter. The combination of the
two meets the total VFA requirements of the process.
• Primary effluent is step-fed to the anoxic zone to complete denitrification.
Under normal operating conditions, a portion of the primary effluent (approximately
50 percent) is required in the anoxic zone to complete denitrification, and the remainder is
fed through the pre-anoxic zone to the anaerobic zone.
Anoxic Zone
The main anoxic zone requires a variable chemical oxygen demand (COD) load to control
the denitrification process. Therefore, a portion of the primary effluent is pumped directly to
the anoxic zone to stimulate denitrification. Using this technique, denitrification rates in the
anoxic zone are greatly increased, the anoxic zones are reduced to 16-21 percent of
bioreactor volume, and the amount of primary effluent step-feed to the anoxic zone is
controlled.
Control of the denitrification rate can be achieved by monitoring the oxidation-reduction
potential (ORP) at the end of the anoxic zone 24 hours a day. This information can be fed
into the computer system and sufficient primary effluent diverted to the anoxic zone to meet
the nitrate load from the nitrified internal recycle flow.
6 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Aerobic Zone
The remaining volume (up to 75 percent) of the bioreactor is allocated for nitrification. This
zone is sized on the basis of the nitrifier growth rate of the activated sludge during the coldest
anticipated wastewater temperatures, and it controls the solids retention time (SRT) in the
bioreactor.
One advantage of a step-fed configuration is the decrease in anaerobic and anoxic zone
HRT—approximately 25 percent of the bioreactor. The reduced un-aerated fraction results in
reducing the un-aerated decay rates for nitrifying bacteria. With shorter time spent under
anoxic conditions, the net nitrifier growth rate increases. This is one reason for the reduced
SRT normally used by plant operators in the Westbank configuration.
Table 3 provides a 2005 monthly summary of bioreactor operating parameters for HRT,
SRT, temperature, MLSS, and percentage of bioreactor volume in service. Throughout 2005,
one of the small modules was not required. In addition, the highest monthly MLSS was
2,803 mg/L, or approximately 80 percent of the design MLSS. It could be expected that an
additional 20 percent load could be treated using the three operational bioreactors.
Table 3. Bioreactor operating parameters
Month
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
HRT
(hr)
11.1
11.1
11.4
11.6
11.3
10.9
10.9
10.8
10.9
11.1
11.5
11.5
SRT
(days)
8.9
8.8
8.2
8.1
8.0
6.7
6.0
5.8
6.0
7.0
7.4
7.5
Temp
(°C)
13.1
13.0
13.8
15.6
18.1
19.4
21.1
22.0
20.9
19.3
16.8
14.3
MLSS
(mg/L)
2,562
2,761
2,803
2,486
2,238
2,414
2,301
1,992
1,901
2,142
2,451
2,899
Bioreactor
in service
84%
84%
84%
84%
84%
84%
84%
84%
84%
84%
84%
84%
Internal Nitrified Recycle Rates
Depending on the desired effluent nitrate concentration, the aerobic/anoxic configuration
commonly uses four to six times the Q for internal recycle flows. With controlled primary
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
effluent diversion to the anoxic zone, effluent nitrate concentrations in the 3.0 to 4.5 mg/L
range can consistently be achieved.
The dissolved oxygen (DO) concentration in the aerobic zone can be reduced to between
1.0 to 2.0 mg/L with little impact on nitrifier growth rate, which is maximized at DO
concentrations of 2.0 mg/L.
Three important advantages of reduced DO have assisted Kelowna operations:
• Reduced recycle of DO to the anoxic zone requires less primary effluent to initiate
and complete denitrification.
• Reduced DO concentrations have reduced the endogenous release of nutrients.
• Reduced DO has reduced the proliferation of foam-producing organisms.
Supplemental Alum and Lime Addition
The Kelowna facility is equipped with a supplemental alum dosing system that is automated
with an online analyzer. This system has been provided to help the biological phosphorus
removal system achieve an annual average TP of 0.25 mg/L. The alum can be used if
equipment maintenance or process issues disrupt effective phosphorus removals. As shown
in Table 4, alum additions in 2005 were limited to 5 days.
The 1994 expansion included a lime system for controlling dissolved phosphorus in the
centrifuge centrate return stream. The option of adding lime was terminated in March 2005
because of the strong bio-phosphorus removal performance in the bioreactor.
Table 4. Supplemental alum usage
2005
6/29/2005
6/30/2005
12/20/2005
12/21/2005
12/21/2005
Alum
(Ib/d)
500
500
150
150
200
Metals and Other Cations in Activated Sludge
Under normal operating conditions, the heavy-metal load to the Kelowna sewer system is
typical of domestic sewage only. On rare occasions, however, discharges have disrupted both
nitrogen and phosphorus removal. Throughout 2005, there were no such occasions, and
8 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Table 5 shows typical metal concentrations found in the BNR sludge. With these
concentrations of heavy metals, it could be expected that the nitrifier growth rate would be
normal.
Table 5. Metals and other cations in activated sludge
Metal/Cation
Aluminum
Antimony
Arsenic
Barium
Beryllium
Bismuth
Cadmium
Calcium
Chromium
Cobalt
Copper
Iron
Lead
Lithium
Magnesium
Unit
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
%
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
%
Value
6,914
1.7
1.9
236
0.11
12.27
1.40
1.16
Mil
3.41
768
4,085
16.85
2.37
1.08
Metal/Cation
Manganese
Mercury
Molybdenum
Nickel
Phosphorus
Potassium
Selenium
Silver
Sodium
Strontium
Thallium
Tin
Vanadium
Zinc
Zirconium
Unit
M9/9
M9/9
M9/9
M9/9
%
%
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
M9/9
Value
96.5
0.90
6.88
16.47
3.9
1.54
4.34
11.07
2,446
122.8
0.309
3.78
7.18
288
29.7
VFA Sources—Fermenter, Influent Sewage, Centrifuge
The primary sludge fermenter returns the overflow (supernatant) directly to the anaerobic
zone of the bioreactor. Table 6 identifies the flows and concentrations of various parameters.
As the data show, a significant amount of VFA is produced in the fermenter supernatant
stream.
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 9
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 6. Fermenter supernatant return to anaerobic zone
Month
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Flow
(mL/d)
1.56
1.56
1.56
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
SRT
(d)
5.3
5.5
5.4
5.6
4.7
3.2
3.4
2.7
2.7
3.2
4.6
5.8
Solids
(%)
6.9%
6.5%
6.6%
5.6%
6.4%
5.7%
6.2%
6.7%
6.5%
5.8%
5.5%
5.6%
Ammonia
(mg/L)
18.7
18.1
18.6
19.5
15.8
15.7
14.4
16.4
16.5
18.6
19.3
20.5
Soluble
phosphorus
(mg/L)
5.62
6.39
7.06
7.68
7.61
6.95
7.85
7.16
6.82
7.43
7.92
7.85
Soluble
COD
(mg/L)
358
407
427
538
632
583
640
611
575
582
603
614
Suspended
solids
(mg/L)
142
170
169
180
166
190
212
208
227
232
198
260
Total
VFA
(mg/L)
116
131
140
196
225
236
254
242
229
222
216
227
VFA
(kg/d)
181
204
218
305
351
368
393
375
355
344
334
351
Samples of the fermenter supernatant are sent off-site monthly for analysis in a gas
chromatography (GC) analyzer to determine the concentration of various fractions of VFA.
Table 7 lists the various fractional concentrations. The most desirable fraction for favoring
the growth of PAOs is a combination of acetic and propionic acids stimulating phosphorus
release/uptake. As the data show, these two acids are the most prevalent form of VFA in the
fermenter supernatant.
Table 7. Fermenter VFA fractions
Month
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Acetic
(mg/L)
55.5
65
109
154
137
121
178
209
124
165
97
122
Propionic
(mg/L)
37.0
26.1
26.2
57.8
123
64.5
155
105
104
105
122
130
Isobutyric
(mg/L)
2.4
2.2
1.9
1
1.7
2.6
1.7
3.8
4.8
1.9
1
3
Butyric
(mg/L)
9.9
9.7
9
26.8
26.5
21.2
32
24.9
16.8
2.7
27.3
33.3
Isovaleric
(mg/L)
1.9
2.1
3.1
1
1.9
1.1
2
3.4
3.7
1.7
1
2.7
Valeric
(mg/L)
2.4
2.6
3.3
9.9
12.5
6.3
18.3
9.3
7.4
9
10
14.6
10 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
VFAs are also found in the influent sewage and centrifuge centrate. Only a limited amount of
sampling has been performed on these two sources. Table 8 lists the available data on
influent, primary effluent, and centrifuge centrate VFA concentrations.
Given the limited number of samples, an estimate of the sources of VFA that feed the
Kelowna anaerobic zone is as follows:
• Primary sludge fermenter
• 50 percent of primary effluent
Table 8. Other VFA sources
Average 315 kg/d
Average 252 kg/d
Date
May 8, 2007
May 10, 2007
May 15, 2007
June 8, 2006
June 15, 2006
June 22, 2006
May 8, 2007
May 10, 2007
May 8, 2007
Centrifuge
centrate
(mg/L)
401
285
415
281
215
281
Primary
effluent
(mg/L)
15
20
Influent
sewage
(mg/L)
8
Flow
rate
est. 130m3/d
est. 130m3/d
est. 130m3/d
est. 130m3/d
est. 130m3/d
est. 130m3/d
est. 36 ML/d
est. 36 ML/d
est. 32 ML/d
VFA
(kg/d)
52
37
54
37
28
37
540
720
256
Centrifuge
The primary fermented and thickened waste activated sludge are combined at the centrifuge
for dewatering and off-site composting. The key operating parameters for the centrifuge are
included in Table 9. The first four months of 2005 included lime addition to the centrate to a
level that saw the pH rise above 9.0. This effectively precipitated most of the soluble
phosphorus to low levels. In May 2005 the operations staff stopped adding lime to the
centrate because the bio-phosphorus removal efficiencies in the bioreactor were such that the
return phosphorus load was effectively removed biologically and the assistance provided by
lime addition was not required.
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 11
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 9. Centrifuge centrate return to plant influent
Flow
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Flow
(m3/d)
133.7
113.6
124.5
117.3
118.7
128.8
143.6
124.5
118.7
128.5
123.8
135.3
Ammonia
(mg/L)
14.3
15.3
15.3
17.5
16.7
23.1
28.5
22.4
23.5
17.6
20.9
21.4
TP
(mg/L)
118
70
160
91
225
235
165
200
235
200
173
170
Soluble P
(mg/L)
11
23
55
47
173
159
161
164
148
96
118
84
TKN
(mg/L)
43
46
68
54
63
54
60
55
61
95
66
95
Soluble COD
(mg/L)
318
376
452
522
827
667
783
726
599
632
779
593
TSS
(mg/L)
1,105
1,115
861
1,045
270
320
1,001
520
1,135
1,084
854
939
Performance Data for Nitrogen Removal
Overall plant influent and final filtered effluent average results for the 2005 calendar year are
shown in Table 10. The operators at the Kelowna facility have found that to maximize
biological phosphorus removal, the SRT needs to be just enough to complete nitrification.
If a small amount of ammonia remains in the effluent (0.2-0.5 mg/L), biological phosphorus
removal appears to work at top efficiency. Table 10 shows the monthly averages in 2005,
achieved as a result of this strategy. Tables 11 and 12 show the nitrogen concentrations at
various stages in the process.
12 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Table 10. Nitrogen removal
Month
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Influent
flow
(ML/d)
33.2
32.1
31.5
30.8
32.3
32.8
33.0
33.5
33.4
32.3
31.2
31.9
Influent
TKN
(mg/L)
30.6
30.5
27.0
32.5
24.0
24.7
27.0
33.0
27.5
27.8
30.7
31.1
Effluent
TN
(mg/L)
4.64
4.90
4.40
4.49
4.12
3.21
3.53
4.39
4.45
4.89
4.66
4.78
Nitrogen
removal
(%)
84.8
83.9
83.7
86.1
81.3
87.0
86.9
86.6
83.8
82.4
84.8
84.6
Effluent
nitrates
(mg/L)
2.10
1.93
2.20
2.65
2.21
1.99
2.08
2.53
2.80
2.67
2.45
2.18
Effluent
ammonia
(mg/L)
0.85
1.01
0.51
0.48
0.51
0.07
0.44
0.40
0.52
0.50
0.67
0.96
Table 11. Nitrate profile—annual average of grab samples taken at 8:00 a.m. (mg/L)
Anaerobic
zone
0.02
End
Anoxic
0.2
25%
aerobic
1.1
50%
Aerobic
1.9
End
aerobic
2.8
Secondary
clarifier
2.5
Return
sludge
0.13
Filter
effluent
2.6
Table 12. Ammonia profile—annual average of grab samples taken at 8:00 a.m. (mg/L)
Primary
effluent
19.44
Anaerobic
Zone
9.6
End
anoxic
3.19
25%
aerobic
2.12
50%
aerobic
1.31
End
aerobic
0.05
Secondary
clarifier
0.29
Return
sludge
0.26
Filter
effluent
0.23
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 13
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Nutrient Removal Technology Assessment Case Study
September 2008
Figures 4 and 5 show monthly frequency curves for effluent TN and ammonia.
100-
Kelowna, BC
Monthly Average Frequency Curves for Total Nitrogen
D)
E
c"
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September 2008
Nutrient Removal Technology Assessment Case Study
Performance Data for Phosphorus Removal
Overall plant influent and final filtered effluent average results for the 2005 calendar year are
shown in Table 14. As the data show, biological removal of soluble phosphorus is operating
at near maximum capability.
Table 14. Phosphorus removal
Date
Jan 2005
Feb2005
Mar 2005
Apr 2005
May 2005
Jun 2005
Jul 2005
Aug 2005
Sept 2005
Oct 2005
Nov 2005
Dec 2005
Influent flow
(ML/d)
33.2
32.1
31.5
30.8
32.3
32.8
33.0
33.5
33.4
32.3
31.2
31.9
Influent
TP
(mg/L)
5.9
5.95
5.5
7.35
5.67
5.4
6.05
6.1
6.3
6.35
5.03
6.15
Effluent
TP
(mg/L)
0.13
0.16
0.16
0.13
0.19
0.11
0.12
0.10
0.10
0.12
0.13
0.21
Phosphorus
removal
(%)
97.8%
97.3%
97.1%
98.2%
96.6%
97.9%
98.0%
98.3%
98.4%
98.1%
97.4%
96.5%
Effluent
soluble P
(mg/L)
0.04
0.04
0.04
0.04
0.05
0.03
0.03
0.03
0.02
0.02
0.02
0.06
Table 15 shows the soluble phosphorus concentrations at various stages in the process.
Table 15. Ortho-phosphorus profile—annual average of grab samples taken at
8:00 a.m. (mg/L)
Primary
Effluent
4.26
Anaerobic
zone
14.9
End
anoxic
2.54
25%
aerobic
cell
0.18
50%
aerobic
0.01
End
aerobic
0.01
Secondary
clarifier
0.02
Return
sludge
1.91
Filter
effluent
0.03
The soluble phosphorus load to the aerobic zone is quite low because of the moderate release
of phosphorus in the anaerobic zone and the significant phosphorus uptake in the anoxic zone
for most of the year.
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 15
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Nutrient Removal Technology Assessment Case Study
September 2008
100
10
Kelowna, BC
Monthly Average Frequency Curves for Total Phosphorus
I
0.1 -:
0.01
Mean = 0.139 mg/L
;Std. Dev. = 0.03 mg/L
:C.O.V. = 21%
0.050.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent x Final Effluent
Figure 6. Monthly average frequency curves for TP.
Reliability Factors
The Kelowna plant has achieved a high degree of reliability in the biological removal of
nitrogen and phosphorus in a cold climate. The mean effluent concentrations were 0.14 mg/L
in TP with a low coefficient of variation (COV) of 21 percent and 4.38 mg/L in TN with a
low COV of 12 percent.
The key operating principles applied at the Kelowna site include the following:
• Anaerobic zone sizing was reduced from 3 hours to 1 hour for optimal operation
when a primary sludge fermenter was used to produce a constant, side-stream VFA
source.
• Secondary clarifiers with a bottom-central draw-off are used to significantly reduce
nitrates in the return sludge.
• The secondary clarifier RAS rate is adjusted to remove nitrates and prevent excessive
phosphorus release.
• A small pre-anoxic zone for final denitrification of RAS before entering the anaerobic
zone prevents excessive phosphorus release before the anaerobic zone.
• When a portion of the primary effluent was introduced directly to the anoxic zone,
rapid denitrification occurred and anoxic zone sizing could be reduced.
16 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
• Simultaneous nitrification/denitrification occurred when submerged turbine aerators
were used, thereby improving the overall nitrate removal.
• DO in the range of 1.0 to 2.0 mg/L produced the best combined TN and TP removals.
• Sufficient SRT is maintained to just achieve full nitrification. A small amount of
ammonia in the effluent is acceptable.
• Online effluent monitoring of nutrients provides valuable information to the plant
operators.
If there is a soluble phosphorus breakthrough to the effluent, the online effluent
analyzer that collects and analyzes samples every 15 minutes for ammonia, nitrates,
and ortho-phosphorus provides a signal to the process computer, which can
automatically turn the supplemental alum-dosing upstream of the secondary clarifiers
on or off.
• Flow and load equalization volume equivalent to 7.5 percent of daily flow helps to
stabilize the nutrient removal processes.
• With a 6Q recycle, the fourth and fifth stages in the five-stage Bardenpho mode were
not required to meet TN and TP permit requirements.
• Computer control systems monitor, operate, and alarm all equipment on-site. This
provides 24-hour-a-day, consistent process control.
• The anoxic zone is removing significant amounts of dissolved phosphorus. This
appears to be stimulated by the addition of primary effluent and the higher
denitrification rates.
• Recycle loads from dewatering were minimized by maintaining separate processes for
secondary sludge and primary sludge. No sludge digestion was practiced in Kelowna.
The total recycle loads from dewatering were only 13 percent in TP and 0.1 percent in
TN.
• Wet-weather flows were managed under the normal mode of operation, using the
equalization basin. The sewer system was separated, and the seasonal variation in
flow was not very high. The maximum month flow was 10 percent higher than the
average flow. The total basin equalization capacity was 7.5 percent of the design
average flow.
All these operating principles have been put into effect because of the flexibility of process
layout, the built-in swing zones, and the leadership of the plant personnel in research and
process optimization.
Appendix A Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 17
-------
Nutrient Removal Technology Assessment Case Study September 2008
Costs
Treatment Plant Expansions
This section provides a design summary of the Kelowna facility expansions, from the 1980
expansion and Bardenpho upgrade (from conventional, high-rate activated sludge) through
two additional upgrades to the Westbank process—each with higher loadings than the
original Bardenpho bioreactor.
Expansion of 1969 Kelowna WWTP
The Kelowna WWTP was converted in 1980 from secondary treatment to nutrient removal.
The following facilities from the previous 1969 expansion were incorporated into the design:
• Two influent comminutors
• Two grit channels
• Raw sewage lift station
• Three primary clarifiers
• Short HRT activated-sludge process (converted to flow equalization)
• Two secondary clarifiers (converted to sludge fermenters in Phase 2)
• Sludge thickener
1980 Five-stage Bardenpho
The 1980 Bardenpho five-stage design made the Kelowna WWTP the first full-scale facility
designed for nutrient removal in North America. The unique and highly flexible bioreactor
had two trains, each with 22 cells for anaerobic, anoxic, and aerobic service. Of the 22 cells,
17 were swing zones with either anoxic or aerobic configurations. This design enabled
complete flexibility in operating the nitrifying and denitrifying components of the process.
The original design was commissioned with a high priority on reliability. Consequently, a
very conservative HRT/SRT was used to ensure complete nitrification and denitrification to
facilitate a TN below 6.0 mg/L. Through extended optimization, it became clear that the long
HRT/SRT was not necessary to achieve the required effluent nitrogen standards.
The preexisting sludge thickener was put into service for primary sludge only with
supernatant returning to the influent works. Thus it provided sufficient rapidly degradable
COD to stimulate phosphorus removal and denitrification. Through extended optimization, it
became clear that the on-site thickener (later called afermenter) was producing sufficient
VFA to reduce the anaerobic zone from three cells to a single cell.
The capital cost for the 1980 conversion to the Bardenpho configuration was 12.5 million
Canadian dollars (CDN$).
18 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
Westbank Process Configuration
On the basis of the full-scale operation of the five-stage process, a more compact process was
developed and initially tested at Kelowna. Then a full-scale version was designed and
constructed at the Westbank WWTP site across the lake from Kelowna.
The Westbank configuration uses a step-feed primary effluent strategy to split the primary
effluent (COD) for denitrification in the anoxic zones. It also ensures anaerobic conditions
for phosphorus release in the anaerobic zone.
Using a primary sludge fermenter with direct discharge to the anaerobic zone provides a
consistent VFA source, and primary effluent is added to the anaerobic zone only if additional
VFA load to the anaerobic zone is required.
The high-rate Westbank process was implemented in two phases. The first phase involved
breaking up the five-stage process into two intermediate-sized bioreactors and two smaller
bioreactors. The second phase added more capacity upstream and downstream to the original
bioreactor.
The objective of the second-phase expansion was to fully develop the capacity of the original
bioreactor with the new high-rate process. The plant was again re-rated upward to an average
dry weather flow of 10.6 million gallons per day (MGD) (40 ML/d).
The principal change to the process involved a controlled diversion of primary effluent to
enhance the denitrification rate in the main anoxic zone. The addition of primary effluent
directly to the anoxic zone allowed smaller anoxic zones and facilitated adjustment to the
denitrification rate. Combined with the smaller anaerobic zone previously developed in the
1980s, the aerobic fraction of the process was increased from 55 percent to 71 percent.
The capital cost of the 1992 Phase 2 conversion was approximately CDN$6.2 million. The
capital cost of the 1994 Phase 3 conversion was approximately CDN$20.75 million.
Canadian-U.S. Dollar Exchange
To calculate the capital and operation and maintenance (O&M) costs in U.S. dollars,
Canadian-to-U.S. dollar exchange rate values were required. Table 16 presents the average
Canadian-to-U.S. dollar exchange rates in the 3 years that capital improvements were made,
along with the current exchange rate for calculating O&M costs (Oanda Corporation 2007).
Appendix A Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 19
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Nutrient Removal Technology Assessment Case Study
September 2008
Table 16. Average Canadian to U.S. dollar
exchange rate value
Year
1980
1992
1994
2007
1 Canadian $ = x
U.S. $
0.86
0.83
0.73
0.94
Table 17 presents the assumed split of the capital cost among phosphorus removal, nitrogen
removal, and other, which is BOD removal. It was assumed that 12 percent of the upgrades
could be attributed to phosphorus removal, while 48 percent of the upgrades could be
attributed to nitrogen removal. The balance of the upgrades could be attributed to BOD
removal or other activities required by permit (e.g., filters for suspended solids). This meant
that the capital expenditure in 2007 dollars that could be attributed to phosphorus removal
was US$6.8 million. The annualized capital charge (20 years at 6 percent) was US$595,000
for phosphorus removal.
Table 17. Split of capital cost between phosphorus, nitrogen, and other
Capital
year
1980
1992
1994
Totals
CDN$
$12,500,000
$6,200,000
$20,750,000
$39,450,000
us$
$10,750,000
$5,146,000
$15,147,500
$31,043,500
US$ present
worth
$26,375,193
$8,198,502
$22,245,090
$56,818,785
%
other
40%
40%
40%
%P
12%
12%
12%
%N
48%
48%
48%
Phosphorus
$3,165,023
$983,820
$2,669,411
$6,818,254
Nitrogen
$12,660,093
$3,935,281
$10,677,643
$27,273,017
The capital expenditure in 2007 dollars that could be attributed to nitrogen removal was
US$27.2 million. The annualized capital charge (20 years at 6 percent) was US$2.38 million.
for nitrogen removal. This same expenditure could be attributed to ammonia nitrogen
removal.
The total capital attributed to BNR in 2007 dollars was US$34 million. For the 10.6 MOD
(40 ML/day) facility, this means the capital expenditure per gallon of BNR treatment
capacity was US$3.25.
Operation and Maintenance Costs
The plant uses both biological phosphorus and nitrogen removal, with minimal use of alum
and no use of supplemental carbon sources. This means that costs for nutrient removal are
essentially all electrical. A summary of the electrical calculations is provided in Attachment
2. The total electrical usage for phosphorus removal was 884,000 kilowatt-hours per year
20 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
(kWh/yr). When the average electrical rate of US$0.047/kWh was applied, the cost for
phosphorus removal was US$41,500 for the year. The total electrical usage for nitrogen
removal was 4,100,000 kWh/yr, or US$193,000.
Unit Costs for Nitrogen and Phosphorus Removal
During the 1-year case study period, the plant removed 150,000 Ib of phosphorus. With the
results above, the unit O&M cost for phosphorus removal is US$0.27 and the unit capital
cost is US$3.97/lb of phosphorus removed.
During the same period, the plant removed 781,000 Ib of TN. With the results above, the unit
O&M cost for TN removal is US$0.14 and the unit capital cost is US$3.05/lb of ammonia
removed.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the unit capital and unit O&M costs. Thus, the life-cycle
cost for phosphorus removal is US$4.24/lb phosphorus removed and the life-cycle cost for
nitrogen removal is US$3.19/lb TN removed.
Assessment of magnitude of costs: The costs are shown to be on the high side in capital cost
and very low in O&M costs. This reflects the innovative technologies used at the plant,
which resulted in increasing the treatment capacity while still using the existing facilities.
Summary
The Kelowna, British Columbia, plant's retrofit of the original five-stage Bardenpho process
into the three-stage Westbank process has provided excellent reliability in both nitrogen and
phosphorus removal, especially for this cold-weather region. The phosphorus removal is
achieved biologically to the mean concentration of 0.14 mg/L with a low COV of 21 percent.
The nitrogen removal is achieved biologically to the mean concentration of 4.38 mg/L with
an extremely low COV of 12 percent without using an external carbon source. The Kelowna
plant is one of the best-performing BNR plants in North America. Many factors have
contributed to this remarkable achievement. They include flexibility in design for
bioreactors, adequate VFA production in separate fermenters, online monitoring and
automatic controls, and the plant personnel developing optimal operating strategies.
Key factors include downsizing the anoxic zones; maintaining 2- to 3-foot-deep blankets in
the secondary clarifier for added denitrification, thereby downsizing the pre-anoxic zone;
simultaneous nitrification and denitrification; DO controls in the range of 1 to 2 mg/L in the
aerobic zone; maintaining a short sludge age of about 10 days, a short HRT of about
11 hours, and sufficient internal recirculation for denitrification at 6Q; and a computer
control system. Recycle loads from sludge handling were minimized by maintaining separate
Appendix A Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 21
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Nutrient Removal Technology Assessment Case Study September 2008
processes for secondary sludge and primary sludge. No sludge digestion was practiced, and
thus the total recycle loads were 13 percent in TP.
The capital cost was moderately high at US$3.25 per gallon per day, and the O&M costs
were extremely low at US$0.28/lb of phosphorus removed and $ US$0.29/lb of nitrogen
removed. The capital cost reflects added costs for flexible flow patterns with multiple swing
zones for both anoxic and aerobic zones, fermenters, and tertiary filters. The O&M costs are
low because of efficient use of power and no chemical addition for either nitrogen or
phosphorus removal. The life-cycle costs are low at US$3.19/lb of nitrogen and US$4.25/lb
of phosphorus removed.
As a result of the continuous improvements, the Kelowna plant treats 70 percent more flow
than the original plant did using the same bioreactor tanks.
A ckn o wledgments
The authors are grateful to Earth Tech Canada, Ltd., and Gerry Stevens, Earth Tech Canada,
Ltd., for the bulk of the information contained in this case study. Mr. Stevens worked for
Kelowna through implementing and optimizing the BNR process until 1990. He helped to
obtain records and analyze and present technical data about the innovations achieved at the
plant. Mr. Stevens' expertise in BNR technologies in general and his accomplishments in
Kelowna, British Columbia, are recognized and appreciated.
The authors are also grateful to the city of Kelowna and members of the WWTP staff for
participating in this study. They include Jim White, plant manager; Sheila Carey, lab
supervisor; and Marj Van de Mortel, lab technician.
References/Bibliography
Barnard, J.L., G.M. Stevens, andPJ. Leslie. 1984. Design Strategies for Nutrient Removal
Plant. IAWPRC 1984 Post-Conference Sessions, Paris, France.
Oanda Corporation. 2007. FXHistory: historical currency exchange rates and FX Converter
Results, http://www.oanda.com/convert/fxhistory.
http://www.oanda.com/convert/classic.
Stevens, G.M., J.L. Barnard, L. Forty, and C. Cameron. 1996. Verification of anoxic
phosphorus uptake in a full-scale plant. In Proceedings of Water Environment
Federation 69th Annual Technical Exhibition & Conference.
Stevens, G.M., J.L. Barnard, andB. Rabinowitz. 1997. Optimizing Biological Nutrient
Removal in Anoxic Zones. Australia BNR 3 Conference, 1997.
22 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
Stevens, G.M., C. Cameron, S. Hunt, and S. Carey. 2002. Operational Experiences with
Sludge Fermenters. In Proceedings of Water Environment Federation 75th Annual
Technical Exhibition & Conference, Chicago, IL, September 28-October 2, 2002.
Stevens, G.M., M.K. Fries, and J.L. Barnard. 1995. Biological Nutrient Removal Experience
at Kelowna, British Columbia. In Proceedings of Water Environment Federation 68th
Annual Technical Exhibition & Conference.
Stevens, G.M., andW.K. Oldham. 1992. Biological Nutrient Removal Experience at
Kelowna British Columbia. European Conference on Nutrient Removal from
Wastewater. University of Leeds, Wakefield, U.K.
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.
Wilson, A.W., G.M. Stevens, and P. Do. 1992. Retrofitting Biological Nutrient Removal
Processes at Existing Wastewater Treatment Facilities. European Conference on
Nutrient Removal from Wastewater. University of Leeds, Wakefield, U.K., 1992.
Appendix A Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 23
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Nutrient Removal Technology Assessment Case Study
September 2008
Attachment 1: Design Basis
Design Flows and Loads
Flow Data
Sewered Population
Flow per Capita
Base Infiltration
BCTWP Industrial Effluent
Average Daily Flow
Maximum Month Flow
Maximum Daily Flow
Peak Hourly Flow
BOD, TSS, TKN, TP Loads
BOD
Average Daily Unit Load
Allowance for BCTWP
Average Daily Total
Maximum Month Unit Load
Maximum Month Total
Maximum Week Unit Load
Maximum Week Total
TSS
Average Daily Unit Load
Allowance for BCTWP
Average Daily Total
Maximum Month Unit Load
Maximum Month Total
Maximum Week Unit Load
Maximum Week Total
TKN
Average Daily Unit Load
Allowance for BCTWP
Average Daily Total
Maximum Month Unit Load
Maximum Month Total
Maximum Week Unit Load
Maximum Week Total
TP
Average Daily Unit Load
Allowance for BCTWP
Average Daily Total
Maximum Month Unit Load
Maximum Month Total
Maximum Week Unit Load
Maximum Week Total
WASTEWATER TEMPS
Summer
Winter
Units
per
L/c.d
ML/d
ML/d
ML/d
ML/d
ML/d
ML/d
kg/c.d
kg/d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
kg/d
kg/c.d
kg/d
kg/c.d
kg/d
°C
°C
1980
Bardenpho
upgrade
Phase 1
56,000
400
1.0
22.5
25.1
28.7
34.8
0.080
4,480
0.095
5,320
0.105
5,880
0.080
4,480
0.100
5,600
0.120
6,720
0.015
840
0.017
952
0.019
1,064
0.003
168
0.003
168
0.004
224
20
10
Design value
Stage 1
Phase 2
64,000
400
1.0
27.5
31.0
35.5
43.0
0.080
5,120
0.095
6,080
0.105
6,720
0.080
5,120
0.100
6,400
0.120
7,680
0.015
960
0.017
1,090
0.019
1,215
0.003
192
0.003
192
0.004
256
20
10
Phase 3
95,000
400
1.0
1.0
40.0
44.0
50.0
69.0
0.080
200
7,800
0.095
9,225
0.105
10,175
0.080
20
7,620
0.100
9,520
0.120
1 1 ,420
0.015
10
1,435
0.017
1,625
0.019
1,815
0.003
5
290
0.003
290
0.004
385
20
10
24 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Process Design Data
Raw Sewage Pumping
Station 1
Number of Units
Capacity
Station 2
Number of Units
Capacity
Comminutor
Number of Units
Mechanical
Manual
Capacity per unit
Bar Screen
Number of Units
Mechanical
Manual
Capacity per unit
Grit Removal
Number of Units
Capacity per Unit
Primary Clarifiers
Number of Units
Length
Width
SWD, 1-3
SWD, 4-6
SWD, 7-10
PeakOFR, 1 out of service
Primary Flow Equalization
Fraction of Average Flow
Volumes
NE Trunk
Existing Tanks
Future Primary Clarifiers
New Equalization Tanks
Primary Sludge Fermenters
SRT, avg
Number of Units
Dimensions
Diameter
SWD, 1-2
SWD, 3-4
Units
L/s
L/s
ML/d
ML/d
ML/d
m
m
m
m
m
m3/m2-d
percent
m3
m3
m3
m3
d
m
m
m
1980
Bardenpho
upgrade
Phase 1
6
380
2
—
16.0
2
16.0
3
27.4
6.1
2.2
2.5
62.7
8.4
1,200
700
7
1
17
4.5
-
Design value
Stage 1 -upgrade
Phase 2
6
380
1
1
75.0
1
75.0
4
27.4
6.1
2.2
2.5
94.1
6.9
1,200
700
-
-
5
1
15
3.5
-
Phase 3
3
440
1
1
75.0
1
75.0
6
27.4
6.1
2.2
2.5
91.1
7.50
-
700
1,150
1,200
5
2
15
3.5
-
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 25
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Process Design Data
Bioreactors
Basic Design Parameters
SRT, Summer
SRT, Winter
Bioreactor
Existing Modules 1 and 2
No. of Anaerobic Cells
Anaerobic Volume
No. of Anoxic Cells
Anoxic Volume
No. of Aerobic Cells
Aerobic Volume
No. of Anaerobic Stirrers
Anaerobic Stirrer hp
No. of Swing Zone Mixers
Swing Zone Mixers hp
Bioreactor
Modified Modules 1 and 4
No. of Anaerobic Cells
No. of Anaerobic Stirrers
Anaerobic Stirrer hp
Anaerobic Volume
No. of Anoxic Cells
No. of Anoxic Stirrers
Anaerobic Stirrer hp
Anoxic Volume
No. of Aerobic Cells
Aerobic Volume
No. of Anaerobic Stirrers
Anaerobic Stirrer hp
No. of Aerobic Mixers
Aerobic Mixer hp
No. of Aerobic Mixers
Aerobic Mixer hp
No. of Swing Zone Mixers
Swing Zone Mixers hp
Units
d
d
m3
m3
m3
m3
m3
m3
1980
Bardenpho
upgrade
Phase 1
15
20
3
1,365
6-10
3,640
9-13
5,005
3
5
19
7.5/15
Design value
Stage 1 -upgrade
Phase 2
12
15
1
1
5
225
2
2
5
680
4
1,820
1
2.5
1
40
1
30
3
7.5/15
Phase 3
10
12
1
1
5
225
2
2
5
680
4
1,820
1
2.5
1
40
1
30
3
7.5/15
26 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Process Design Data
Modified Modules 2 and 3
No. of Anaerobic Cells
Anaerobic Volume
No. of Anoxic Cells
Anoxic Volume
No. of Aerobic Cells
Aerobic Volume
No. of Anaerobic Stirrers
Anaerobic Stirrer hp
No. of Aerobic Mixers
Aerobic Mixer hp
No. of Aerobic Mixers
Aerobic Mixer hp
No. of Swing Zone Mixers
Swing Zone Mixers hp
Blowers
No. of Blowers
Size
Units
m3
rr,3
m3
hp
1980
Bardenpho
Upgrade
Phase 1
4
100
Design Value
Stage 1 -Upgrade
Phase 2
1
455
3
1,365
10
4,550
1
5
2
40
7
7.5/15
4
100
Phase 3
1
455
3
1,365
10
4,550
1
5
2
40
2
30
5
7.5/15
4
250
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 27
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Process Design Data
Secondary Clarifiers
Clarifiers
Number
Dimensions
Diameter
SWD
RAS Pumps
Number
Capacity
Maximum RAS Flow
WAS Pumps
Number
Capacity
Filtration
PeakOFR, 1 unit out of service
Existing Units
Number
Area per Unit
New Units
Number
Area per Unit
Ultraviolet Disinfection
Dosage
Transmissivity
Number of Lamps
Arrangement
Number of Channels
Banks per Channel
Racks per Bank
Lamps per Rack
WAS Thickening
Design Load, Peak
DAF Units
Number
Area per Unit
Dewatering
PS Flow, peak
WAS Flow, Peak
Centrifuges
Number
Capacity
Units
m
m
L/s
L/s
L/s
rr,2
rr,2
mWs/cm2
percent
kgTSS/d
rr,2
m3/d
m3/d
L/s
1980
Bardenpho
Upgrade
Phase 1
3
26
4.5
6
80
240
2
8
290
4
64
chlorine
2
18.9
none
none
none
Design Value
Stage 1 -Upgrade
Phase
2
4
26
4.5
8
80
320
3
12
290
4
64
chlorine
2
18.9
none
none
none
Phase 3
5
26
4.5
9
80
400
4
12
290
4
64
1
96
48
65
1,152
2
3
24
8
4,615
3
18.9
90
195
2
4.7
28 - Kelowna, British Columbia, Canada • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Attachment 2: Electrical Cost
Electrical cost
Anoxic/Anaerobic mixers
kW
power
HP Number draw
hours/
day
kWh
draw/
day
kWh %BOD
draw/
year
%P
%N
forP
draw
forN
draw
Anaerobic mixer
5
2.5
2
1
7.46
1.865
24
24
179.04
44.76
65,349.6
16,337.4
0
0
100
100
0
0
65
16
349.6
337.4
0
0
Fermenter rake mechanism drive
5
1
3.73
24
89.52
32,674.8
0
100
0
32
674.8
0
Anoxic mixers
5
2.5
Blowers
250
8
2
1.25
29.84
3.73
233.125
Swing zone stirrers — 19 available,
7.5
15
Aerobic zone
40
30
15
Recirculation
20
15
Filter pumps
7.5
10
9
10
mixers
5
5
11
pump
2
1
4
1
50.355
111.9
149.2
111.9
123.09
29.84
11.19
22.38
7.46
24
24
24
716.16
89.52
5,595
can go either anoxic
24
24
24
24
24
24
24
24
24
1,208.52
2,685.6
3,580.8
2,685.6
2,954.16
716.16
268.56
537.12
179.04
261,398.4
32,674.8
2,042,175
(7.5 hp) or aerobic
441,109.8
980,244
1,306,992
980,244
1,078,268.4
261,398.4
98,024.4
196,048.8
65,349.6
0
0
45
(15
0
45
45
45
45
0
0
0
0
0
0
10
hp)
0
10
10
10
10
0
0
50
50
100
100
45
100
45
45
45
45
100
100
50
50
204
98
130
98
0
0
217.5
0
024.4
699.2
024.4
107,826.84
98
32
0
0
024.4
674.8
261,398.4
32,674.8
918,978.75
441,109.8
441,109.8
588,146.4
441,109.8
485,220.78
261,398.4
98,024.4
98,024.4
32,674.8
Appendix A
Kelowna, British Columbia, Canada • Wastewater Treatment Plant - 29
-------
-------
Marshall Street Water Reclamation Facility
Clearwater, Florida
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Marshall Street Water Reclamation Facility (WRF) in Clearwater, Florida, is designed
for a capacity of 10 million gallons per day (MOD). This facility was selected as a case study
because it has achieved low levels of nitrogen and phosphorus in the effluent using the five-
stage Bardenpho process. The plant processed an average of 5.48 MGD during the evaluation
period, October 2005 through September 2006. Some of the reclaimed water is sent for reuse
(irrigation); the remainder is discharged under a permit via Stevenson's Creek to Clearwater
Harbor. The WRF uses a five-stage Bardenpho process to remove both total nitrogen (TN)
and total phosphorus (TP) to below 3 milligrams per liter (mg/L) and 1 mg/L on an annual
average, respectively.
The relevant National Pollutant Discharge Elimination System (NPDES) permit limits for the
facility are shown in Table 1.
Table 1. NPDES permitted discharge limits
Parameter
BOD5
TSS
TN
TP
Dichlorobromo-methane
Dibromochloro-methane
Annual
average
5 mg/L
5 mg/L
3 mg/L
1 mg/L
24 ug/L
46 pg/L
Monthly
average
6.25 mg/L
6.25 mg/L
3.75 mg/L
1 .25 mg/L
Report
Report
Weekly
average
7.5 mg/L
7.5 mg/L
4.5 mg/L
1 .5 mg/L
-
-
Notes:
|jg/L = micrograms per liter
BOD5 = biochemical oxygen demand
TSS = total suspended solids
TN = total nitrogen
TP = total phosphorus
Plant Process
Figures 1 and 2 present a plant layout and a process flow diagram for the Marshall Street
WRF.
Appendix A
Marshall Street, Clearwater, FL • Water Reclamation Facility - 1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
2 - Marshall Street, Clearwater, FL • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
DECHLORINATION
/DISSOLVED
OXYGEN
CONTACT ^ BOOST
sli r
t
T
( i
t
t
1 i
I
I
T
k*
TO WAS
1 40 INTERNAL RECYCLE (IR)
2ND ANOXIC REAERATION
TANKS ZONE
:j-
t
1 'T T
O
o
o
o
o
o
o
o
o
t
o
o
o
~o t
i
T 1 M T 1 I
B--*
! TM TT M
o
I-,
<^~~~-,
o
gs
< s
• z
£ ?
*~ m
3J ||
"H '
£ |
a "
|
>-
1
OOO
0
0
0
t°t
0
0
*
t
Tt t
TT T
o
o
t°t
0
0
T H T T
*
n
in rm A
re
b)
ra
o
I/)
8
Q.
LL
t)
ra
£
S2
re
O)
Appendix A
Marshall Street, Clearwater, FL • Water Reclamation Facility - 3
-------
Nutrient Removal Technology Assessment Case Study September 2008
The plant uses a five-stage Bardenpho biological nutrient removal (BNR) process. The liquid
train consists of the following components: an on-site influent pumping station with three
variable-rate, dry-pit pumps; preliminary treatment consisting of two mechanically cleaned
fine-bar screens, a four-unit vortex-cyclonic grit removal system with associated grit
classifier, and an influent flow measurement via a 36-inch Parshall flume with an ultrasonic
flow meter; primary treatment consisting of sedimentation in four 49,370-gallon rectangular
basins and four 52,960-gallon rectangular basins; a biological treatment process consisting of
a five-stage Bardenpho BNR process that includes three 250,000-gallon fermentation basins,
three 333,000-gallon first anoxic reactors, 13 aeration basins or nitrification reactors (three
363,170-gallon basins, and ten 127,160-gallon basins), four 280,000-gallon second anoxic
basins, and four 63,000-gallon re-aeration basins; four 100-foot-diameter secondary
clarifiers; four return-activated sludge pumps; an intermediate effluent pumping station using
three 60-inch-diameter Archimedes screw lifts and three centrifugal pumps; polishing
filtration consisting of 12 rapid-sand, pulsed-filtration, gravity-type automatic backwash
filters with a total surface area of 4,320 square feet; an effluent disinfection system using
gaseous chlorination and a 315,000-gallon, dual-channel chlorine contact basin. Alum is
added before the effluent reaches the polishing filters to aid in total suspended solids (TSS)
removal and thereby reduce trihalomethane (THM) formation potential. Also on-site is a
5-million-gallon (MG) reclaimed water storage tank and accompanying high-service pumps.
Chlorinated effluent from the chlorine contact basin is directed to the Master Reuse System
or to a 315,000-gallon dechlorination basin that uses flow-paced sulfur dioxide to eliminate
the remaining chlorine residual. It then flows through a 100,000-gallon re-aeration basin and
finally through a 48-inch-diameter outfall pipe that discharges to Stevenson's Creek, 20 feet
from shore.
Waste sludge from the primary clarifiers is pumped to one 930,000-gallon anaerobic digester.
Waste sludge from the secondary clarifiers is pumped to two 108,000-gallon-per-day (gpd)
rotary drum thickeners equipped with polymer injection, then to the anaerobic digester. The
digested sludge is then directed to a 127,000-gallon sludge blend tank. The blended sludge is
dewatered using two 2-meter belt filter presses.
Basis of Design and Actual Flow
Flow
The design flow for the facility is 10 MGD; the average flow for the study period was
5.48 MGD, and the maximum month flow during the study period was 6.85 MGD during
September 2006.
4 - Marshall Street, Clearwater, FL • Water Reclamation Facility Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Loadings
Plant design loadings and equipment parameters are as follows:
Average day 10MGD
Peak day 15 MOD
Primary settling tanks: 4 each at 49,370 gallons, 4 each at 52,960 gallons
The plant operates four units regularly.
Activated- sludge
Fermentation basins: 3 each at 250,000 gal
First anoxic basin: 3 each at 333,000 gal
Aerobic basin: 3 each at 367,000 gal
Aerobic basin: 10 each at 127,000 gal
Anoxic basin: 4 each at 280,000 gal
Re-aerobic basin: 4 each at 63,000 gal
Total hydraulic retention time (FtRT): 20 hours
Design mixed liquor suspended solids (MLSS): 4,000 mg/L
Return activated sludge (RAS) rate: 80-120 percent
Internal recycle rate: 400-600 percent
Food-to-microorganism (F-to-M) ratio: 0.05
Mean cells residence time (MCRT): 25-40 days
Secondary clarifier: 4 each, diameter = 100 ft at 12.5-ft depth
Surface loading rate: 318 gpd/ft2 at average daily flow (ADF)
Detention time: 7 hours at ADF
The plant operates three units regularly.
Rapid sand, pulsed filter: 12 each, 12 ft by 30 ft, or a total of 4,320 sf
ADF capacity: 2 MOD each
Peak capacity: 28 MOD
Hydraulic loading rate: 3.8 gpm/sf at ADF
4.5 gpm/sf at peak
Sludge thickener—Carter rotary drum
Capacity: 2 each, 75 gpm
Thicken sludge: Waste-activated sludge (WAS) at 4-6 percent
Volume: 15,552gpd
Appendix A Marshall Street, Clearwater, FL • Water Reclamation Facility - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Anaerobic digester
Primary digester:
Diameter = 85 ft, volume = 0.93 million gallons
Digesters—The sludge-heating system and gas-mixing system were not operational in
the primary digester from October 2005 through September 2006 because the primary
digester system was being rebuilt. During that period, all sludge was pumped directly
to the blending tank for dewatering. The primary digester was back online in January
2007.
Dewatering—The primary sludge and WAS are blended with polymer for dewatering with an
Andritz belt filter press. The cake is hauled away by truck.
Plant Parameters
Overall plant influent and effluent average results for the period October 2005 to September
2006 are shown in Table 2.
Table 2. Influent and effluent averages
Parameter
(mg/L unless stated)
Flow (MGD)
Influent TP
Effluent TP
Influent BOD
Effluent BOD
Influent TSS
Effluent TSS
Influent NH4-N
Effluent NH4-N
Influent Total N
Effluent Total N
Average
value
5.48
5.0
0.13
188
2.3
231
0.89
28.0
0.036
28.0
2.32
Maximum
month
6.85
5.53
0.21
234
4.1
277
1.11
32
0.045
32
3.1
Max
month vs.
avg.
25%
10%
62%
24%
78%
20%
24%
16%
25%
16%
35%
Maximum
week
7.62
6.35
0.26
263
5.3
317
1.6
34.0
0.062
34.0
3.75
Sample
method/frequency
-
Weekly/composite
Weekly/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Daily/composite
Notes:
BOD = biochemical oxygen demand
Max month vs. average = (max month - average) / average x
NH4-N = ammonia measured as nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
100
6 - Marshall Street, Clearwater, FL • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Table 3 presents the plant's monthly averages for the Bardenpho process parameters.
Table 3. Monthly averages for plant process parameters
Month
Oct 2005
Nov 2005
Dec 2005
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sep 2006
MLSS
(mg/L)
3,979
4,106
4,181
4,425
4,094
3,951
3,857
3,340
3,704
4,205
3,701
3,921
Sludge age
(d)
51
44
44
36
27
25
34
31
41
34
37
36
HRT
(hr)
27
28
30
30
28
28
27
28
29
26
25
22
Temperature
(°C)
29.3
27.2
24.6
23.8
23
25
27
28
30
30
31
30
Notes:
HRT = hydraulic retention time
MLSS = mixed liquor suspended solids
Performance Data
Figures 3 and 4 present reliability data for TP removal. The removal is good, with the
effluent TP averaging 0.13 mg/L and having a medium coefficient of variation (COV) of 40
percent. The COV is defined as the standard deviation divided by the mean, and it is a
measure of the reliability of a system. The lower the COV, the less the data are spread and so
the higher the reliability.
Appendix A
Marshall Street, Clearwater, FL • Water Reclamation Facility - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100-
Marshall Street Advanced WWTP Clearwater, FL
Monthly Average Frequency Curves for Total Phosphorus
0)
tfi
5
o
.c
Q.
01
O
$
10
0.1- =
0.01
Mean = 0.132 mg/L
E Std. Dev. = 0.052 mg/L
: C.O.V. = 40%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent x Final Effluent
Figure 3. Monthly average frequency curves for TP.
100-
Marshall Street Advanced WWTP Clearwater, FL
Weekly Average Frequency Curves for Total Phosphorus
10
3
o
0.1 - =
0.01
-3r
. Mean = 0.132 mg/L
E Std. Dev. = 0.058 mg/L
I C.O.V. = 44%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
» Raw Influent x Final Effluent
Figure 4. Weekly average frequency curves for TP.
Figures 5 and 6 present reliability data for ammonia nitrogen removal. Removal of ammonia
nitrogen is very good, with a mean effluent of 0.038 mg/L and a very low COV of 18
percent.
8 - Marshall Street, Clearwater, FL • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Marshall Street Advanced WWTP Clearwater, FL
Monthly Average Frequency Curves for Ammonia Nitrogen
=! 10-
E
* 1
CO
o
E 0.1-
nm .
^ 4 4 * * *
y itl Jk "I1 1 1|P"
ir Mr 3c ^
t ^
Mean - 0.038mq/L
Std. Dev. - 0.007 mg/L
p o \/ - 1 R°/
«— -*
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N x Final Effluent - Ammonia N
Figure 5. Monthly average frequency curves for ammonia nitrogen.
100-
Marshall Street Advanced WWTP Clearwater, FL
Weekly Average Frequency Curves for Ammonia Nitrogen
d 10
O)
E
5
O)
2 1
ro
'c
o
E
§ 0.1
0.01
_ Mean = 0.036 mg/L
: Std. Dev. = 0.0075 mg/L
IC.O.V. = 21%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N
x Final Effluent - Ammonia N
Figure 6. Weekly average frequency curves for ammonia nitrogen.
Figures 7 and 8 present reliability data for removal of TN. With the two anoxic stages, the
plant gives outstanding TN removal, with effluent TN of 2.32 mg/L and a COV of 16
percent.
Appendix A
Marshall Street, Clearwater, FL • Water Reclamation Facility - 9
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100
10
c
HI
u>
o
Marshall Street Advanced WWTP Clearwater, FL
Monthly Average Frequency Curves for Nitrogen
^ • » » • *
**, M/ .
T . 'I III " |T|
»—*——*
^ * *• "*• — '
^ «
i «c
K
Mean = 2.32 mg/L
C.O.V. = 16%
0.1
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N x Final Effluent - Total N
Figure 7. Monthly average frequency curves for nitrogen.
100
10
u>
E
c"
HI
u>
o
0.1
Marshall Street Advanced WWTP Clearwater, FL
Weekly Average Frequency Curves for Nitrogen
: Mean = 2.32 mg/L
I Std. Dev. = 0.43 mg/L
C.O.V. = 19%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N x Final Effluent - Total N
Figure 8. Weekly average frequency curves for nitrogen.
Reliability Factors
This facility's design is unique in several ways. The plant has multiple treatment processes in
series to provide efficiency and reliability in meeting nitrogen and phosphorus limits. They
include primary settling, a five-stage Bardenpho process for biological nitrogen and
10-Marshall Street, Clearwater, FL • Water Reclamation Facility
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
phosphorus removal, and tertiary filtration. In addition, some chemical removal of
phosphorus can be obtained when alum is added before the tertiary filters for THM control.
The results are excellent: the plant achieves a phosphorus mean concentration of 0.13 mg/L
with a COV of 40 percent and a TN mean concentration of 2.32 mg/L with a COV of only
16 percent as a monthly average. The plant's maximum average week results were good,
with the maximum average week phosphorus at 0.26 mg/L versus the weekly standard of
1.5 mg/L, the maximum average week ammonia nitrogen at 0.062 mg/L, and the maximum
average week TN at 3.75 mg/L versus the weekly standard of 4.5 mg/L. These results are
well within the normal range of variation from average for a wastewater treatment process, as
reflected in the low to very low COVs shown in Figures 3, 5, and 7. As shown in Table 2, the
fractions by which the monthly effluent maxima exceeded the corresponding annual averages
(62 percent, 25 percent, and 35 percent for TP, ammonia nitrogen, and TN, respectively)
were consistent with or better than the literature suggestion of 63 percent (Brandao et al.
2005). The key factors for this exceptional performance are discussed below.
Wastewater characteristics: The BOD-to-TP ratio was favorable, with an average value of
37.5, and ranged monthly between 31 and 44. A ratio of 20 is recommended in the literature
(WEF and ASCE 1998). The average BOD-to-total Kjehldahl nitrogen (TKN) ratio was
6.7 and ranged monthly between 6.1 and 7.6. Both ratios are favorable for BNR. The soluble
BOD-to-ammonia nitrogen ratio has been in the range of 4 to 5, less than what was originally
recommended (6). It should be noted that on weekdays 160,000 gal/day of filtrate from the
belt filter presses is returned to the head of the plant; this filtrate contains 51 mg/L of TP and
131 mg/L of ammonia nitrogen. These loads amount to 30 percent of the influent TP and
14 percent of influent ammonia, with the effective minimum BOD-to-TP and BOD-to-TN
ratios dropping to 24 and 5.3, respectively. The soluble BOD-to-ammonia nitrogen ratio
similarly drops to 3.7. Despite these recycle stream loads and the low BOD-to-ammonia
nitrogen ratio, no adverse effect was reported under the operating parameters developed at
this facility.
Primary settling tanks: The plant regularly operates four tanks out of the eight available, and
the efficiencies in removal are typical—30 percent in BOD and 50 percent in TSS.
Activated sludge: The five-stage Bardenpho process at the facility is a typical design. It
includes a fermentation zone, followed by the first anoxic and aerobic zones in series, a
second anoxic zone, and the re-aeration zone. The typical internal recirculation of MLSS to
the first anoxic zone from the second aerobic zone is five times the influent flow rate. Some
unique features of this process are two separate anoxic zones, each with long detention times
of approximately 1.5 hours, long sludge age ranging between 30 and 50 days, and high water
temperature.
Appendix A Marshall Street, Clearwater, FL • Water Reclamation Facility - 11
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Phosphorus removal far exceeds the permit requirement with good reliability and is achieved
by two processes: first by the Bardenpho process as the primary process, then later as a side
benefit to alum addition, which is done primarily to reduce TSS and so reduce potential THM
formation. The Marshall Street WRF has to meet a limit on dichlorobromomethane of
22 micrograms per liter (|ig/L) and a dibromochloromethane limit of 34 |ig/L to meet
Florida's state requirements for water reuse. The typical dosage of alum is 27 mg/L, or 2.4
mg/L as aluminum (Al). This dosage is equivalent to an Al-to-TP ratio of 1.6 on a molar
basis in the plant influent. For the effluent concentration the plant produces, this ratio is
considered low for a strictly chemical removal process.
Nitrogen removal has been excellent with good reliability. No external carbon source is used.
The use of two anoxic zones with an internal recirculation flow rate of five times the influent
flow rate has been found to be sufficient to produce low nitrogen concentrations (WEF and
ASCE 1998). It is also noteworthy that the plant maintains a sludge blanket in the secondary
clarifiers. The depth ranges between 2 and 3 feet and is a part of the TN removal strategy and
the biological phosphorus removal strategy.
Another key operational factor is the automated process control system, which uses
Chemscan and supervisory control and data acquisition (SCADA). These programs monitor
online at the second anoxic zone nitrate-nitrogen, dissolved oxygen (DO), oxidation-
reduction potential (ORP), and ortho-phosphorus to optimize nitrogen removal. Table 4 lists
the sensors used at the Marshall Street facility. The minimum ORP is set at -60 millivolts
(mV), and the nitrate-nitrogen is set at a minimum of 0.5 mg/L. The DO is adjusted on the
basis of these two parameters. In addition, the system monitors MLSS and the sludge blanket
in the secondary clarifiers. The plant also has monitors for turbidity, in accordance with the
permit, and conductivity, to monitor for salts that could intrude by means of seawater and
adversely affect irrigation reuse. All the automation and controls have contributed to an
efficient phosphorus removal and full denitrification with good reliability.
Table 4. Probe and sensor suppliers
Parameter
Dissolved oxygen
MLSS
Nitrate-nitrogen
Ammonia nitrogen
Clarifier sludge blanket depth
PH
Oxidation-reduction potential (ORP)
Ortho-phosphorus
Turbidity
Supplier(s)
Hach, Royce
Hach
Chemscan
Chemscan
Hach, Royce
Hach
Hach
Chemscan
Hach
12 - Marshall Street, Clearwater, FL • Water Reclamation Facility
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
In addition, this plant has the flexibility of operating as a four-stage Bardenpho process,
thereby providing additional tank volume dedicated to nitrogen removal. Under this mode of
operation, the phosphorus removal is achieved primarily by alum addition.
Secondary clarifiers: The plant regularly operates three out of four units at the current flow.
One practice to note is the maintenance of the sludge blanket at between 2 and 3 feet, which
is monitored with the new blanket monitors installed in 2002.
Tertiary filter: The tertiary filter is an original Zimpro filter with air and water backwash
provisions. The system is effective in suspended solids removal: the effluent TSS averages
1 mg/L or less. This, in turn, is a key to achieving the low phosphorus concentration in the
final effluent.
Recycle flows from dewatering and thickening go back to the primary clarifier influent. The
returns are controlled to flow uniformly around the clock and avoid a shock loading to the
treatment processes. No adverse impact has been observed under this practice at this facility.
Another key parameter to note is the long sludge age maintained at this plant. Because of this
long sludge age at warm temperature ranges, a sludge yield of around 0.25-0.4 Ib volatile
suspended solids (VSS) per Ib of BOD removed has been reported. This is consistent with
Manual of Practice No. 8 (WEF and ASCE 1998). This low yield naturally contributes to a
low cost in sludge handling.
Cosfs
Capital Costs
The main upgrade of the plant for BNR occurred in 1988 when the basins were reconfigured
for the five-stage Bardenpho process. The upgrade then cost $16.8 million, which was
updated to $29.5 million in 2007 dollars using the Engineering News-Record (USD A 2007).
The upgrade included additional tanks or dividing walls, mixers, pumps, blowers/aerators
and tertiary filtration.
It was assumed that 17 percent of the upgrade was attributed to phosphorus removal, while
63 percent of the upgrade was for nitrogen removal. This allocation was done in consultation
with plant personnel and was based on the fraction of the secondary system volume that
could be attributed to phosphorus or nitrogen removal. Specifically, all anaerobic tank
volume plus 10 percent of the volume of the aerobic tanks (based on oxygen usage) was
attributed to phosphorus removal, while all anoxic tank volume plus 50 percent of the aerobic
tanks (based on oxygen usage) was attributed to nitrogen removal. The balance of the
upgrade was attributed to BOD removal or other activities required by permit. The tertiary
filters were installed to meet the requirements for surface water discharge under reuse rule
Appendix A Marshall Street, Clearwater, FL • Water Reclamation Facility - 13
-------
Nutrient Removal Technology Assessment Case Study September 2008
62-610 in Florida. This meant that the capital expenditure in 2007 dollars that was attributed
to phosphorus removal was $5.02 million. The annualized capital charge (20 years at
6 percent) was $438,000 for phosphorus removal.
The capital expenditure attributed to nitrogen removal was $10.6 million in 2007 dollars. The
annualized capital charge (20 years at 6 percent) was $1.6 million for nitrogen removal.
The total capital expenditure attributed to BNR was $29.5 million in 2007 dollars. For the
10-MGD facility, the capital expenditure per gallon of BNR treatment capacity was $2.95.
Operation and Maintenance Costs
In all case studies prepared for this document, the O&M costs considered were for electricity,
chemicals, and sludge disposal. Labor costs for O&M were specifically excluded for three
reasons:
1. Labor 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 other factors;
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
SCADA system that accompany most upgrades.
3. Most facilities were unable to break down which extra personnel were employed
because of nutrient removal and related overtime costs, making labor cost
development difficult.
CAPDETWorks was used to provide a relative comparison of labor costs compared to power
costs. CAPDETWorks is a software package developed by Hydromantis Corporation
(Hamilton, Ontario, Canada). It is used to estimate conceptual capital and operating cost
estimates for wastewater treatment facilities. It is based on work originally done by EPA and
the U.S. Army Corps of Engineers. Two flow scenarios were run for a model consisting of a
five-stage Bardenpho reactor, a secondary clarifier, a tertiary filter, and an anaerobic
digester: (1)5.5 MGD to mimic the current flow at the plant and (2) 10 MGD to match the
design flow. For 5.5 MGD, the CAPDET electrical cost estimate using the plant's overall
average rate of $0.11 per kilowatt-hour (kWh) was $960,000, while the O&M labor cost at an
average regional rate of $35/hour was $540,000. For a 10-MGD facility, the CAPDET
electrical cost estimate was $1.7 million, while the labor cost was $680,000. By comparison,
as shown below, the Marshall Street facility's electrical cost for similar equipment at an
average flow of 5.5 MGD was $840,000, including electrical costs for BOD removal.
14 - Marshall Street, Clearwater, FL • Water Reclamation Facility Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
The plant uses both biological phosphorus and nitrogen removal, with minimal use of alum
and no use of supplemental carbon sources. The plant could use minimal chemicals because
the ratios of influent BOD to TP and influent BOD to TN were both very high (37.6 and 6.7,
respectively). This means that costs for nutrient removal are essentially all electrical. A
summary of the electrical use calculations is provided in the Attachment. The specific
electrical usage for phosphorus removal was 931,000 kWh per year (kWh/yr). The average
electrical rate for the plant was $0.11/kWh, and it was based on the cost per kWh plus a
demand charge plus a Florida-required fuel surcharge. When that rate was applied, the cost
for phosphorus removal was $102,400 for the year. The total electrical usage for nitrogen
removal was 4,620,000 kWh/yr, or $509,000. The electrical usage for BOD removal in the
system was 2,091,000 kWh/yr, or $230,000.
Alum is applied as an effluent-polishing step primarily for reducing THM formation
potential; however, some phosphorus removal does occur with alum addition. The total cost
of alum used over the evaluation period was $74,000. On the basis of the dosage of alum and
the possible removal that could occur, it was assumed that 10 percent of the alum could be
attributed to phosphorus removal; the chemical cost for phosphorus removal was therefore
$7,400. All the alum added (2.4 mg/L as Al) was assumed to convert to aluminum hydroxide
sludge; at the average flow of 5.48 MOD, this was 317 Ib of aluminum sludge per day, or
58 dry tons/yr. Assuming that phosphorus removal accounted for 10 percent of the sludge
and using the plant's cost of sludge disposal of $253/dry ton, the chemical sludge cost for
phosphorus removal was $1,463.
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 81,200 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal was $1.37 per pound, while the
annualized unit capital cost was $5.39/lb of phosphorus removed. At design flow, the
annualized capital would drop to $2.95/lb of phosphorus removed.
During the evaluation period, the plant removed 428,000 Ib of TN. With the results above,
the unit O&M cost for TN removal was $1.18/lb, while the annualized unit capital cost is
$3.79/lb of nitrogen removed. At design flow, the annualized capital cost would drop to
$2.07/lb of TN removed.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the annualized unit capital and unit O&M costs. Thus, the
life-cycle cost for phosphorus removal was $6.76/lb phosphorus removed, while the life-
cycle cost for TN removal was $4.97/lb nitrogen removed, all at current flows. At design
flows, assuming the O&M costs increase proportionally to flow and loadings, the life-cycle
costs would be $4.32/lb of phosphorus removed and $3.25/lb of TN removed.
Appendix A Marshall Street, Clearwater, FL • Water Reclamation Facility - 15
-------
Nutrient Removal Technology Assessment Case Study September 2008
Assessment of magnitude of costs: The capital cost of $2.95/gpd capacity is relatively high,
but the O&M costs remain low. One of the key factors is that no methanol is purchased
because of the use of the incoming carbon source for both nitrogen and phosphorus removal
with the five-stage Bardenpho process.
Discussion
Reliability factors: The treatment processes at the Marshall Street plant represent a traditional
layout for the original five-stage Bardenpho process for both biological nitrogen and
phosphorus removal—one anaerobic zone, two anoxic zones with a high rate of internal
recirculation, an aeration zone in between, and the final re-aeration zone. This is
accomplished with a conservative design basis—a long HRT, a long sludge age, and a low
clarifier loading rate in a warm-temperature region. Another key is the automated controls
the plant personnel use, which are based on online monitoring with multiple sensors and
process control parameters for the Bardenpho process. In addition, good primary settling
tanks and efficient tertiary filters added reliability along with alum addition for effluent THM
reduction. This process as operated by the plant personnel has proven to be efficient and
reliable in meeting the permit limits of 3 mg/L for nitrogen and performing significantly
better than the limit of 0.2 mg/L in phosphorus.
Cost factors: The costs are relatively high for capital but low for O&M. This plant was
designed with conservative design parameters, at $2.95/gpd capacity. The O&M costs are
low at $1.37/lb of phosphorus removed and $1.18/lb of TN removed. The main reasons for
these low costs are efficient operation of the biological processes and no need for an external
carbon source (e.g., methanol). Even though the power cost in Florida, compared to that of
other states, is high at $0.11/ kWh, the overall O&M cost is relatively low. In addition, the
alum addition is at a reduced dosage and thus the cost impact is low because the Bardenpho
process removes a significant amount of phosphorus biologically. All these costs are based
on the plant's current flow. As the plant flow increases to the full design loadings, these unit
costs would be expected to decrease.
Summary
The Marshall Street WRF is an advanced wastewater treatment plant with a five-stage
Bardenpho process that meets the effluent discharge limit for nitrogen and exceeds that for
phosphorus. The reliability has been excellent in achieving low concentrations—0.13 mg/L
in phosphorus with a COV of 40 percent and 2.32 mg/L in nitrogen with a COV of 16
percent monthly average. The cost for this facility is considered high with a capital cost at
$2.95/gpd capacity, but the O&M costs are low. The unit costs are low at $6.76/lb of
phosphorus removed and $4.97/lb of TN removed.
16 - Marshall Street, Clearwater, FL • Water Reclamation Facility Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Key contributing factors for reliability include favorable wastewater characteristics,
conservative design with multiple processes in series, good operating procedures for the
Bardenpho process developed by the plant personnel, and automation with online sensors and
control devices.
Key contributing factors to facility costs include a conservative design originally, an efficient
operation without an external carbon source, and optimization of energy and chemical usage,
while minimizing sludge production from the biological process.
A ckn o wledgments
The authors are grateful for the significant assistance and guidance provided by John
Milligan, superintendent of the Wastewater Environmental Technologies Division,
Clearwater; Tom Nietzel, coordinator of the Wastewater Environmental Technologies
Division, Clearwater; and Jeff Borden, chief operator of the Marshall Street WRF. This case
study would not have been possible without their prompt response with well-deserved pride
in the facility and its operation. EPA thanks Clearwater, Florida, for participating in this case
study.
References and Bibliography
Brandao, D., G.T. Daigger, M. O'Shaughnessy, and T.E. Sadick. 2005. Comprehensive
Assessment of Performance Capabilities of Biological Nutrient Removal Plants
Operating in the Chesapeake Bay Region. In Proceedings of the Water Environment
Federation, 78th Annual Conference, Washington, DC, October 29-November 2,
2005.
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.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers).
1998. Design of Municipal Wastewater Treatment Plants. Manual of Practice No. 8,
Figure 11.7, Net sludge production versus solids retention time. Water Environment
Federation, Alexandria, VA.
Appendix A Marshall Street, Clearwater, FL • Water Reclamation Facility - 17
-------
Attachment: Electrical Use and Chemical Costs
Ferment basin
mixers
1st anoxic mixers
Aerator
Pumps — internal
recycle
2nd anoxic mixers
Filter lift pumps
Total draw kWh/yr
Horse-
power
10
7.5
400
50
7.5
50
#
6
9
2
3
12
1
kW
power
draw
44.76
50.355
596.8
111.9
67.14
37.3
hours/
day
24
24
24
24
24
24
kWh
draw/day
1,074.24
1,208.52
14,323.2
2,685.6
1,611.36
895.2
kWh
draw/year
392,097.6
441,109.8
5,227,968
980,244
588,146.4
326,748
7,629,566
% BOD
0%
0%
40%
0%
0%
0%
%P
100%
0%
10%
0%
0%
5%
%N
0%
1 00%
50%
1 00%
1 00%
0%
Usage for
BOD
0
0
2,091,000
0
0
0
2,091,187
Usage
forP
392,097.6
0
522,796.8
0
0
16,337.4
931,231.8
Usage
forN
0
441,109.8
2,613,984
980,244
588,146.4
0
4,623,484
*£! *™°°
% for P
Alum
cost for
P
10
$7,400
-------
Noman M. Cole, Jr., Pollution Control Plant
Fairfax County, Virginia
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
This facility was selected for as case study because it employs a step-feed activated-sludge
strategy with tertiary filters and a ferric chloride feed.
The Noman M. Cole, Jr., Pollution Control Plant serves the area of Fairfax County, Virginia,
in the Washington, D.C., metropolitan area. The plant was originally placed in operation in
1970. The average wastewater treatment capacity of the plant was 18 million gallons per day
(MGD) when commissioned; this has risen to 67 MGD after a series of successful
expansions. Biological nutrient removal (BNR) was added in 2002 as part of a 13-MGD
expansion.
The Virginia Pollutant Discharge Elimination System (VPDES) permit limits for the Noman
M. Cole, Jr., Pollution Control Plant are shown in Table 1.
Table 1. VPDES permit limits
Parameter
CBOD
TSS
Ammonia-N (April-Oct)
Ammonia-N (Nov-Mar)
Total N
Total P
Monthly
average
(mg/L)
5
6
1.0
2.2
Report
0.18
Monthly
average
(Ib/day)
2,790
3,348
559
-
-
101
Weekly average
(mg/L)
8
9
1.5
2.7
Report
0.27
Weekly
average
(Ib/day)
4,464
5,020
836
-
-
150
Notes:
CBOD = carbonaceous biochemical oxygen demand
N = nitrogen
P = phosphorus
TSS = total suspended solids
Appendix A
Fairfax County, VA. • Noman M. Cole, Jr., Pollution Control Plant - 1
-------
Nutrient Removal Technology Assessment Case Study September 2008
Treatment Processes
The facility uses a step-feed strategy to distribute organic matter throughout the biological
treatment basins. Following primary settling, the flow goes to a set of nine aeration basins
that are operated in anaerobic, aerobic, or anoxic modes. The activated-sludge process was
designed for a normal detention time of 8.9 hours with up to five feed points into the basin.
Feed is typically distributed to three anaerobic or anoxic points in the system. Polymer can be
added to aid secondary clarification. The facility uses ferric chloride and polymer to polish
the secondary effluent, primary-to-tertiary clarification, and filtration. The final effluent is
chlorinated/dechlorinated before discharge to Pohick Creek, a tributary to the Potomac River.
The primary sludge is fermented in the gravity thickeners at a sludge residence time (SRT) of
3 days and a hydraulic retention time (HRT) of less than 24 hours. The secondary sludge is
thickened at the dissolved air flotation (DAF) units. The fermented primary sludge and
thickened secondary sludge are mixed together for dewatering by centrifuge, followed by
incineration. Lime can be added to the dewatering process to minimize recycle loads of
nutrients.
Figure 1 shows the plant's flow schematic. The secondary system consists of nine parallel
aeration basins—six small (1.67 million gallon [MG] total volume each) and three large
(4.89 MG total volume each). Figure 2 shows how the step-feed works in the larger basins.
The feed can be provided at five anoxic zones through each basin, although in practice only
four (A, C, D, and E) receive feed. The smaller basins have three points for step-feeding
primary effluent. Under normal circumstances, the flow split between zones A, B, and C in
the smaller basins is 40 percent, 40 percent, and 20 percent, respectively, while the larger
basins, zones A, C, D, and E, each get 25 percent of the flow. Other design information on
the facility is provided in Table 2 and the attachment.
2 - Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
2
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Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
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4 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Table 2. Facility design data
Secondary tanks
Volume
Anoxic volume
HRT (average)
SRT at maximum month loading
(last MLSS 4,400 mg/L)
Gravity thickeners (2)
Volume
SRT
HRT
Tertiary clarifier
Diameter
Hydraulic loading rate
Tertiary filters
Number
Media type
Depth
Design loading rate, gpm/sf
Dimensions
Tanks 1-6
1.67MG
3.5-5 MG
8.9 hr
1 8 days
Tanks 7-9
4.89 MG
5. 1-7.3 MG
8.9 hrs
1 8 days
0.146 MG each
3 days
> 12 hours
152ft
735 gpd/sf (average flow)
Monomedia
8
Anthracite
5ft
2.9
30 ft x 17 ft x2 cells
Gravity filters
10
Garnet/sand/anthracite
2.25ft
2.6
30 ft x 30 ft x 2 cells
Notes:
gpd/fs = gallons per day per square foot
HRT = hydraulic retention time
MG = million gallons
MLSS = mixed liquor suspended solids
SRT = solids retention time
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Plant Parameters
Overall plant influent and effluent average results for the 2006 calendar year are shown in
Tables.
Table 3. Influent and effluent averages
Parameter
(mg/L unless stated)
Flow (MGD)
Influent TP (mg/L)
Effluent TP (mg/L)
Influent BOD (mg/L)
Effluent BOD (mg/L)
Influent TSS (mg/L)
Effluent TSS (mg/L)
Influent NH4-N (mg/L)
Effluent NH4-N (mg/L)
Influent TKN (mg/L)
Effluent TKN (mg/L)
Effluent N03/N02 (mg/L)
Average
value
47.4
6.39
0.09
189
2.0
225
1.0
18.9
0.12
34.6
0.9
4.35
Maximum
month
51.4
7.06
0.12
205
2.0
253
2.2
22.5
0.15
40.4
1.12
5.03
Max
month
vs. Avg.
8%
10%
33%
8%
0%
12%
120%
19%
25%
17%
26%
16%
Maximum
week
54.4
8.16
0.16
305
2.0
353
3.06
24.8
0.29
48.1
1.6
6.41
Sample
method/frequency
Daily
Composite/daily
Composite/daily
Composite/daily
Composite/daily
Composite/daily
Composite/daily
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Composite/weekly
Notes:
TP = total phosphorus
BOD = biochemical oxygen demand
TSS = total suspended solids
TKN = total Kjeldahl nitrogen
NH4-N = ammonia measured as nitrogen
NO3 = nitrate
NO2= nitrite
NH4-N = ammonia measured as nitrogen
NO3 = nitrate
NO2 = nitrite
6 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Table 4 presents plant monthly average plant process parameters.
Table 4. Monthly averages for plant process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
MLSSa
(mg/L)
3,626
3,267
3,390
2,851
3,142
2,784
2,383
3,139
3,192
2,922
2,403
2,852
Sludge age/mean cell
residence time
(d)
18
19
19
19
18
18
17
17
17
16
16
18
HRT
(hr)
9.2
9
9.2
10
9.8
8.8
8.3
8
7.8
8.2
9.4
10
Temperature
(°C)
17.9
15.3
17.4
19.5
20.6
22.5
23.8
25.7
25.4
23.5
21.2
19.4
MLSS is the combined average of last pass (C-PASS for AST 1 -6, F-PASS for AST 7-9).
Table 5. Monthly average BOD/TP and BOD/TKN ratios
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
Influent BOD/TP
33.1
33.7
28.3
28.2
27.2
28.9
28.8
29.4
31.5
33.5
32.2
28.2
Primary
effluent
BOD/TP
29.8
29.5
27.4
27.1
26.8
24.4
26.1
28.1
32.4
33.8
32.0
26.3
Influent BOD/TKN
6.1
5.4
5.3
5.5
4.7
5.5
5.9
4.6
5.0
4.9
5.4
5.4
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Performance Data
This section provides information about the operational performance of nutrient removal at
the plant. Figures 3 and 4 present the facility's 2006 monthly and weekly reliability data for
phosphorus removal. The average phosphorus effluent concentration was 0.09 mg/L with a
coefficient of variation (COV) of 21 percent on a monthly average basis. The COV is defined
as the standard deviation divided by the mean and is a measure of the reliability of a system.
The lower the COV, the less the data are spread and the higher the reliability. The
phosphorus concentration exhibited a low COV of 28 percent for the weekly averages. The
plant's performance in 2006 was excellent: the weekly average never exceeded even the
monthly limit. The secondary effluent exhibited an average of 0.7 mg/L for the year. These
figures demonstrate that both the tertiary clarifier with chemical addition and tertiary filters
are key factors in meeting the permit limit at all times. Note also that the primary influent
contains higher total phosphorus (TP) than the raw influent because of internal recirculation
flows at the facility.
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Monthly Average Frequency Curves for Total Phosphorus
z Mean = 0.086 mg/L
-Std. Dev. = 0.018 mg/L
-COV = 21%
20 30 40 50 60 70 80
Percent Less Than or Equal To
• Raw Influent D Primary Influent • Primary Effluent
X Tertiary Clarifier Influent X Tertiary Clarifier Effluent A Final Effluent
90 95 98 99 99.5
& Secondary Effluent
Figure 3. Monthly average frequency curves for TP.
8 - Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Tertiary Influent
Mean = 0.85 mg/L
. _Std. Dev. = 0.35 mg/L
E COV = 42%
Secondary Effluent
Mean = 0.74 mg/L
0.1 -Std. Dev. = 0.37 mg/L
COV = 50%
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Weekly Average Frequency Curves for Total Phosphorus
Tertiary Effluent
Mean = 0.36 mg/L
Std. Dev. = 0.118 mg/L
COV = 33%
Final Effluent
Mean = 0.09 mg/L
Std. Dev. = 0.025 mg/L
COV = 29%
30 40 50 60 70 80
Percent Less Than or Equal To
98 99 99.5
* Raw Influent
X Tertiary Clarifier Influent
D Primary Influent
X Tertiary Clarifier Effluent
• Primary Effluent
A Final Effluent
& Secondary Effluent
Figure 4. Weekly average frequency curves for TP.
Figures 5 and 6 present the 2006 monthly and weekly reliability data for ammonia nitrogen
removal. The weekly effluent ammonia concentration averaged 0.12 mg/L, with a standard
deviation of 0.035, giving a COV of 29 percent. The plant's performance in 2006 was
excellent: the weekly average never exceeded 0.3 mg/L, compared to the monthly standard of
1 mg/L during the summer months.
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 9
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Nutrient Removal Technology Assessment Case Study
September 2008
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Monthly Average Frequency Curves for Ammonia Nitrogen
= Mean = 0.12 mg/L
~d. Dev. = 0.016 mg/L
-COV=14%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90 95
Percent Less Than or Equal To
99 99.5 99.9 99.95
• Raw Influent - Ammonia N X Final Effluent - Ammonia N
Figure 5. Monthly average frequency curves for ammonia nitrogen.
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Weekly Average Frequency Curves for Ammonia Nitrogen
O)
E
i Mean =0.12 mg/L
13d. Dev. = 0.035 mg/L
- COV = 29%
0.05 0.1 0.5 1 2
10 20 30 40 50 60 70 8(
Percent Less Than or Equal To
99 99.5 99.9
Jaw Influent - Ammonia-N X Final Effluent - Ammonia N
Figure 6. Weekly average frequency curves for ammonia nitrogen.
10 - Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Figures 7 and 8 present the 2006 monthly and weekly reliability data for total nitrogen (TN)
removal. The weekly effluent TN averaged 5.12 mg/L, with a standard deviation of
1.02 mg/L, giving a COV of 20 percent.
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Monthly Average Frequency Curves for Nitrogen
- Mean = 5.25 mg/L
IStd. Dev. = 0.63 mg/L
_COV=12%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90
Percent Less Than or Equal To
99 99.5 99.9 99.95
« Raw Influent - TKN
X Final Effluent - Total I
Figure 7. Monthly average frequency curves for nitrogen.
Noman M. Cole Pollution Control Plant - Fairfax County, VA
Weekly Average Frequency Curves for Nitrogen
O)
E
- Mean = 5.12 mg/L
Std. Dev. = 1.02 mg/L
~ COV = 20%
0.05 0.1 0.5 1
20 30 40 50 60 70 80 90 95 98 99 99.5
Percent Less Than or Equal To
» Raw Influent - TKN
X Final Effluent - Total N
Figure 8. Weekly average frequency curves for nitrogen.
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 11
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Nutrient Removal Technology Assessment Case Study September 2008
Reliability Factors
The plant has a permit limit for phosphorus of 0.18 mg/L as a year-round monthly average
and monthly average ammonia nitrogen limits of 1.0 mg/L for the summer months and
2.2 mg/L for the winter months. The plant personnel have a policy of operating the plant such
that these limits are seldom even approached much less exceeded. The overall reliability was
good, with COVs of 21 percent for TP at the mean concentration of 0.09 mg/L, 14 percent
for ammonia nitrogen at the mean concentration of 0.12 mg/L, and 12 percent for TN at the
mean concentration of 5.25 mg/L for the monthly average.
A key factor in the high reliability of this step-feed plant is the care that operating staff take
to ensure that any process problems do not become uncontrollable. Attention to operating
details and taking appropriate and timely actions in response to plant performance data go a
long way toward attaining good plant performance. It has been found that encouraging
operating staff to use field test kits (e.g., Hach kits) to determine nitrogen and phosphorus
concentrations provides a number of benefits, including allowing staff to take immediate
action to fine-tune chemical addition and any adjustments to the biological system rather than
waiting for laboratory results. It also results in a sense of ownership of the test data because
they did the tests themselves. The plant has an operator for the secondary system on duty
24 hours a day, 7 days a week, and there is daily interaction between operators and engineers
to review the process. A BioWin model is also used to run scenarios.
Phosphorus removal is achieved in three steps—biological removal in activated sludge,
chemical removal in a tertiary clarifier, and then tertiary filters. McGrath et al. (2005)
reported that biological phosphorus removal occurs when low nitrates cause the first
unaerated zone to become anaerobic. Thus, the amount of nitrate returns through return
activated sludge could directly affect biological removal. When nitrate levels go above
6 mg/L in the secondary effluent, biological phosphorus removal is greatly reduced. This is
why the main removal mechanism for phosphorus is chemical addition followed by tertiary
clarification and filtration. This sequence of operations ensures sufficient phosphorus
removal, especially with chemical addition under close control by plant operators. Under
current operating conditions, the operators treat any removal of phosphorus in the biological
system as a bonus.
Primary sludge was fermented in gravity thickeners with an SRT of 3 days and an HRT of
less than 24 hours. The volatile fatty acids (VFA) production was equivalent to 10 mg/L in
chemical oxygen demand (COD) in the primary effluent, and the VFAs consisted of 33
percent acetic acid, 49 percent propionic acid, and 18 percent others (McGrath et al. 2004).
The secondary sludge was thickened at the DAF unit, thereby preventing release of
phosphorus and ammonia.
12 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
Using step-feed is the primary means by which nitrogen removal through multiple anoxic
zones is achieved. In the smaller biological reactors, the flow is split among three passes on a
40 percent, 40 percent, and 20 percent basis, with each pass having an anoxic zone and an
oxic zone. Thus, the flow entering the first pass goes through three sets of anoxic/oxic zones,
while the flow from the second pass goes through two sets of zones. In the larger basins, feed
is sent to four points on the basis of 25 percent each. The system offers reliable operation
because it allows using the carbon in the wastewater for denitrification rather than having to
add a supplemental carbon source like methanol. Avoiding the need for supplemental carbon
ensures a more economical operation because there is no need for additional feed pumps,
storage tanks, and distribution and control equipment or additional sludge handling.
Recycle loads went to the primary influent, and they averaged 10 percent in biochemical
oxygen demand (BOD), 19 percent in total suspended total suspended solids (TSS) and
23 percent in TP. All processes were sized to treat these recycle flows, including lime
addition to the dewatering to minimize recycle loads.
The wet-weather operation included four distinct steps—retention basin (5.7 MG) first, then
equalization at the headworks (4 MG), step-feed activated sludge, and finally equalization of
secondary effluent (13.2 MG). The step-feed makes the process more stable than that at other
plants. The holding capacity at the headworks area was equivalent to 15 percent of the design
flow rate, a significant factor for good reliability.
Finally, the reliability of the plant is enhanced by a well-designed and maintained control and
monitoring system, supplemented by field testing. The dissolved oxygen probes are
frequently calibrated and maintained, and the plant's supervisory control and data acquisition
(SCADA) system is well designed. An instrument technician is available on-site and ensures
proper maintenance at this facility.
Cosfs
Capital Costs
The main upgrades of the plant for BNR occurred in 1979, when the Advanced Wastewater
Treatment (AWT) plant was installed for phosphorus removal, and in 1997, when the
aeration basins were retrofitted for step-feed operation to accomplish nitrogen removal. The
AWT is a chemical phosphorus-removal facility that includes mixing and reaction tanks with
filtration. The step-feed retrofit consisted of piping modifications and tank additions and
filtration.
The costs for installation of the AWT facility were not available; however, they would have
been typical of retrofits where chemical is added before tertiary clarifiers and filters because
such facilities would be used for normal BOD/TSS removal. This means that the capital
Appendix A Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 13
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Nutrient Removal Technology Assessment Case Study September 2008
expenditure for a retrofit for chemical phosphorus removal is fairly low because all that
would be needed would be storage tanks, pumps, and controls, with many of those possibly
available by reusing existing equipment.
Plant personnel provided the estimate that the capital expenditure in 1997 that could be
attributed to nitrogen removal is $52.5 million. This estimate was updated to 2007 dollars
using the Engineering News-Record Construction Cost Index (ENR CCI). The ENR CCI is
compiled by McGraw-Hill and provides a means of updating historical costs to account for
inflation, thereby allowing comparison of costs on an equal basis. From a Web site provided
by the U.S. Department of Agriculture, the ENR index for 1997 was 5,826, while the ENR
index for May 2007 was 7,942 (USDA 2007). Multiplying the above results by the ratio
7,942/5,826 obtained the result of $71.6 million in 2007 dollars.
This result was annualized using the interest rate formula for determining a set of annual
payments for a present value, given an interest rate and payback period. For this and all other
case studies for this report, a 6 percent interest rate and 20-year payback were assumed,
resulting in a multiplication factor of 0.0872. The annualized capital cost for nitrogen
removal was $6.2 million. This annualized capital for nitrogen removal was used for later
unit cost estimates for TN and ammonia nitrogen.
The total capital attributed to BNR in 1997 dollars was $52.5 million, which was adjusted to
$71.6 million in 2007 dollars using the ENR index. For this 67-MGD facility, this means the
capital expenditure per gallon of BNR treatment capacity is $1.07.
Operation and Maintenance Costs
In all case studies prepared for this document, the O&M costs considered were for electricity,
chemicals, and sludge disposal. Labor costs for operation and maintenance were specifically
excluded for three reasons:
1. Labor 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 other factors;
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
SCADA system that accompany most upgrades.
3. Most facilities were unable to break down which extra personnel were employed
because of nutrient removal and related overtime costs, making labor cost
development difficult.
The Noman M. Cole, Jr., plant uses primarily chemical phosphorus removal and biological
nitrogen removal. This means that the primary O&M costs for phosphorus removal are for
14 - Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
electricity, chemicals, and sludge disposal, while the primary O&M costs for nitrogen
removal are for electricity. Chemical sludge is recycled to the plant headworks, but it
contributes to the eventual primary sludge.
The Attachment lays out the electrical usage for the plant. The entire electrical usage for
phosphorus removal lies in the AWT portion of the plant, at 280,000 kilowatt-hours (kWh)
per month, or 3,360,000 kWh/yr. Using the average electrical rate of $0.055/kWh, which
includes all demand charges, the cost of electricity for phosphorus removal is $185,000. The
power usage for nitrogen removal was 18,059,000 kWh/hr. At the average electrical rate, the
cost of electricity for nitrogen removal is $993,300.
Plant personnel estimated that chemical (ferric chloride) usage for phosphorus removal cost
$l,076/day. In addition, plant personnel estimated that the ferric chloride generated an
additional 2 dry tons of primary sludge per day, which cost an additional $l,076/day for
disposal. This meant that the additional cost for phosphorus removal for chemical and sludge
disposal totaled $785,500/yr. Over the evaluation period, plant personnel used an estimated
$250,000 worth of caustic for pH adjustment, which is needed for nitrogen removal.
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 909,600 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal was $1.07/lb, while the annualized unit
capital cost for phosphorus removal was $0.
During the evaluation period, the plant removed 4,240,000 Ib of TN. With the results above,
the unit O&M cost for TN removal was $0.29/lb of TN, while the annualized unit capital cost
for TN removal was $1.47.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle cost is the sum of the annualized unit capital and unit O&M costs. Thus, the
life-cycle cost for phosphorus removal was $1.07/lb and the life-cycle cost for TN removal
was$1.76/lb.
Assessment of magnitude of costs: The capital cost of $1.07/gpd capacity is low because of
the existing facility before the upgrade. The O&M cost for phosphorus removal is high due to
chemical use to reach a low concentration limit, while the O&M cost for nitrogen removal
are in the middle range, compared with those for other facilities.
Summary
The Noman M. Cole, Jr., plant retrofit to a step-feed strategy has provided excellent
reliability in meeting both nitrogen and phosphorus limits. The COVs were 21 percent for
Appendix A Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant-15
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Nutrient Removal Technology Assessment Case Study September 2008
TP at the annual average of 0.09 mg/L, 14 percent for ammonia nitrogen at the average
concentration of 0.12 mg/L, and 12 percent for TN at the average concentration of
5.25 mg/L. The phosphorus removal is achieved primarily by chemical addition followed by
tertiary filters. The nitrogen removal is achieved with multiple anoxic zones in the process. In
addition, the step-feed provides operational benefits during wet-weather conditions because
the strategy allows the operators to distribute the increased flows throughout the aeration
basins in steps, thereby protecting the clarifiers from added solids loadings during high-flow
periods. Removal costs for both phosphorus and nitrogen were reasonable, with low capital
at $1.07/gpd capacity, and O&M costs at $1.07/lb TP removed and $1.77/lb TN removed.
Acknowledgments
The authors are grateful for the significant help and guidance provided by Michael McGrath,
operations director, and Roger Silverio, process engineer, at the Noman M. Cole, Jr.,
Pollution Control Plant. This case study would not have been possible without their prompt
response with well-deserved pride in the facility and its operation. The authors also
acknowledge Fairfax County for its participation in this case study.
References and Bibliography
McGrath, M, K. Gupta, and G.T. Daigger. 2005. Operation of a Step-Feed BNR Process for
Both Biological Phosphorus and Nitrogen Removal. In Proceedings of Water
Environment Federation 78th Annual Technical Exhibition & Conference,
Washington, DC, October 29-November 2, 2005.
McGrath, M., S. Shero, and J. Wleton, 2004. Fermentation for Improving Nutrient Removal
at a Virginia Wastewater Facility. In Proceedings of Water Environment Federation,
October 2004.
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 2007.
16 - Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Attachment: Facility Design Information
Design Flow
Minimum flow
Average daily flow
Peak instantaneous flow
Peak process flow
Design Average Loadings
BOD
TSS
TKN
TP
Retention Basin 1 (QQ1)
Retention QQ1
Quantity
Type
Volume
Retention basin pumps
Quantity
Type
Capacity
Large
Small
Screen Building (B1)
Bar screens
Quantity
Total channel width
Opening size
RAW Wastewater Pump Station (B)
RAWwastewater pumps
Quantity
Type
Speed
A-1
A-2
A-3
A-4
A-5
26.8 MGD
67.0 MGD
134.0 MGD
107.2 MGD
118,000 Ib/d
126,000lb/d
21,000 Ib/d
4,100 Ib/d
1
Open
5.7 MG
Submersible
3,300 gpm at 27 ft
350 gpm at 27 ft
3
8ft
3/4 in
Vertical, centrifugal
Adjustable
Two-speed
Constant
Adjustable
Constant
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 17
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Nutrient Removal Technology Assessment Case Study
September 2008
Capacity
A-1
A-2
A-3
A-4
A-5
RAW Wastewater/EQ Tank Pump Station (B2)
Equalization tank pumps
Quantity
Type
Capacity, each
Raw wastewater pumps
Quantity
Type
Capacity, each
Equalization Tanks (B3)
Equalization tanks
Quantity
Type
Dimensions, each
Volume, each
Flash Mix Tanks (C1)
Quantity
Dimensions
Volume, each
Detention time
Primary Settling Tanks (C)
Primary settling tanks
Quantity
Type
Size
Weir length, each
Weir loading
Hydraulic overflow rate
Primary influent odor control scrubber
Quantity
Type
Depth of packing
Cross section area
Capacity
20,500 gpm at 30 ft TDH
19,165gpmat30ftTDH
20,700 gpm at 30 ft TDH
20,700 gpm at 30 ft TDH
18,500 gpm at 30 ft TDH
Submersible
6,544 gpm at 84 ft TDH
Submersible
9,682 gpm at 47 ft TDH
4
Concrete
200 ft long X 100 ft wide X 27 ft Deep (SWD)
4MG
30 ft LX 18 ft WX 10 ft SWD
40,400 gallons
1.74 minutes at average daily flow
8
Rectangular
139 ft L X 45 ft WX 10 SWD
120ft
69,800 gpd/linear foot at average daily flow
1,340 gpd/ft2 at average daily flow
1
Packed bed
12ftmin
19.6ft2
5,000 CFM
18 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Scrubber recirculation pump
Quantity
Type
Capacity, each
Horsepower
Small Activated Sludge Tanks 1 TO 6 (D)
Small activated sludge tanks
Quantity
Number of passes, each
Size, each pass
Volume, each tank
Total volume
Total anoxic volume
HRT @ average flow
SRT @ max mo load,
& last pass MLSS OF 4,400
1
Vertical wet pit centrifugal
100gpmat30ftTDH
2
6
3
182 ft L X 30 ft WX 13.6 ft SWD
1.67MG
10.0MG
3.5to5.0MG
8.9 hours
18 days
Mixers
Quantity
Type
Horsepower, each
78
Submersible, mast-mounted
4 HP
Process oxygen requirements
BNR operation
Average
Maximum month
Maximum day
Nitrification only operation
Average
Maximum month
Maximum day
48,000 Ib/d
51,000 Ib/d
71,400 Ib/d
70,800 Ib/d
76,200 Ib/d
115,000 Ib/d
Diffused aeration equipment
Type
Large Activated Sludge Tanks 7 TO 9 (D1)
Large activated sludge tanks
Quantity
Number of passes, each
9-in porous flexible membrane
Full floor coverage
3
6
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 19
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Nutrient Removal Technology Assessment Case Study
September 2008
Size, each pass
Volume, each tank
Total volume
Total anoxic volume
HRT @ average flow
SRT @ max mo load,
& last pass MLSS of 4,400
2at165ftl_X18ftWX22ft SWD
4 at 165 ft L X 36 ft WX 22 ft SWD
4.89 MG
14.7MG
5.1 to7.3MG
8.9 hours
18 days
Mixers
Quantity
Type
Horsepower, each
57
Vertical turbine
Platform-mounted
24 at 3 HP
12 at 5 HP
9 at 7.5 HP
12 at 15 HP
PE channel mixers
Quantity
Type
Horsepower, each
18
Submersible, mast-mounted
2.5 HP
Process oxygen requirements
BNR operation
Average
Maximum month
Maximum day
Nitrification only operation
Average
Maximum month
Maximum day
88,200 Ib/d
95,700 Ib/d
149,000 Ib/d
101,000 Ib/d
108,000 Ib/d
164,000 Ib/d
Diffused aeration equipment
Type
AST dewatering pumps
Quantity
Large
Small
9-in porous flexible membrane
Full floor coverage
2
2
20 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Type
Capacity, each
Large
Small
Horsepower, each
Large
Small
Blower Building (E1)
Small AST aeration blowers
Quantity
Type
Capacity, each
Horsepower, each
Submersible
2,025gpmat25ftTDH
75 gpm at 60 ft TDH
25 HP
5 HP
Multistage centrifugal
16,OOOSCFMat8.0psi
800 HP
Clarifiers 12-15 RAS pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
Single-passage screw impeller, centrifuge
Adjustable
4,400 gpm at 28 ft TDH
50 HP
WAS pumps
Quantity
Type
Capacity, each
Blower Building (E2)
Aeration blowers
Quantity
Small AST blowers
Large AST blowers
Type
Capacity
Small AST blowers
Large AST blowers
Horizontal centrifugal
510 gpm at 30 ft TDH
2
4
Multistage centrifugal
17,500at8.0psi
14,000 at 12.7 psi
Horsepower, each
Small AST blowers
Large AST blowers
800 HP
1,250 HP
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 21
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Nutrient Removal Technology Assessment Case Study
September 2008
Clarifiers 5-8 RAS pumps
Quantity
Type
Capacity
Horizontal centrifugal
6,500 gpm at 37 ft TDH
Clarifiers 16-17 RAS pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
Secondary Clarifiers 5 to 8 (F)
Quantity
Type
Diameter
Sidewater depth
Hydraulic overflow rate
Solids loading rate
Secondary Clarifiers 12 to 17 (F1)
Secondary Clarifiers
Quantity
Type
Dimensions, each
Hydraulic overflow rate
Solids loading rate
Single-passage screw impeller, centrifuge
Adjustable
4,400 gpm at 28 ft TDH
50 HP
4
Circular
145ft
14.75ft
540 gpd/ft2 at peak process flow
31 Ip/d/ft2 at peak process flow
Rectangular chain & flight
260 ft L X 55 ft WX 16 ft SWD
540 gps/ft2 at peak process flow
31 If/d/ft2 at peak process flow
Secondary clarifier dewatering pumps
Quantity
Type
Capacity, each
Horsepower, each
Chlorination Facility (G)
SPH pumps
Quantity
Type
Capacity, each
Submersible
500 gpm at 50 ft TDH
15HP
Vertical turbine
3,100 gpm at 216 ft TDH
SPH strainers
Quantity
Type
Capacity, each
Automatic, self-cleaning
1,050 gpm
22 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
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September 2008
Nutrient Removal Technology Assessment Case Study
Sodium hypochlorite feed pumps
Quantity
Large NaOCI pumps
Small NaOCI pumps
Type
Capacity
Large NaOCI pumps
Small NaOCI pumps
Sodium hypochlorite storage tanks
Quantity
Dimensions, each
Volume, each
Chemical Feed Building (S)
Caustic feed pumps
Quantity
Type
Control
Capacity, each
Typical dose
Caustic storage tanks
Quantity
Dimensions, each
Volume, each
Polymer feed pumps
Quantity
Type
Speed
Capacity, each
Typical dose
Polymer transfer pump
Quantity
Type
Capacity
4
2
Tubular diaphragm chemical metering
200 gph max
50 gph max
11.5ftdiaX15.5fthigh
12,000 gallons
Tubular diaphragm chemical metering
Adjustable stroke & speed
420 gph max
11 mg/L as CACO3 for PH control
12 ft diameter X 19 ft high
16,000 gallons
12
Progressing cavity
Adjustable
250 gph max
0.5-1.0 mg/L
1
Progressing cavity
80 gpm
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 23
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Nutrient Removal Technology Assessment Case Study
September 2008
Polymer mixing, aging, and storage tanks
Quantity
Dimensions, each
Volume
7 ft dia X 7 ft high
2,000 gallons
Chemical feed pumps for primary settling tank odor control
Quantity
Caustic
Sodium hypochlorite
Type
Capacity
Caustic
Sodium hypochlorite
1
1
Eccentric lobe peristaltic
8.6 gpm
7.0 gmp
Sodium hypochlorite storage tank (exist)
Quantity
Dimensions, each
Volume
Equalization Basins 2 & 3 (QQ2 & QQ3)
Equalization basins
Type
Volume
Basin QQ2
Basin QQ3
1
12 ft dia X 19 ft high
16,000 gallons
Concrete-lined, open
7.4 MG
5.8 MG
Wash water return pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
ASE Pump Station (BB)
ASE pumps
Quantity
Type
Capacity
Adj speed
Constant speed
Constant speed
Submersible
Constant
600 gpm at 50 ft TDH
20 HP
Vertical turbine
2 at 29,400 gpm
1 @ 22,600 gpm
2@ 16,000 gpm
24 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
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Nutrient Removal Technology Assessment Case Study
Tertiary Clarifiers (CC)
Tertiary clarifiers
Quantity
Type
Nominal inside diameter
Hydraulic overflow rate
Tertiary clarifier dewatering pumps
Quantity
Type
Speed
Capacity, each
Horsepower
Tertiary Clarifiers (CC1)
Flow distribution structure mixer
Quantity
Type
Horsepower
Tertiary clarifier
Quantity
Type
Diameter
Hydraulic loading rate
Tertiary clarifier dewatering pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
TCE Pump Station (CC)
Tertiary clarifier effluent pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
Foreign Sludge Incinerator Building (KK)
Ferric chloride pumps
Quantity
Type
4
Octagonal
148ft
735 gpd/ft2 at average flow
Horizontal centrifugal
Constant
2,400gpmat50ftTDH
50 HP
1
Vertical turbine, platform-mounted
15 HP
1
Circular
152ft
735 gpd/ft2 at average flow
1
Horizontal centrifugal
Constant
1,000gpmat21 ftTDH
15HP
Vertical turbine
Adjustable
22,700 gpm at 35 ftTDH
300 HP
Tubular diaphragm chemical metering
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 25
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Control
Capacity
Typical dose
Polymer feed pumps
Quantity
Type
Speed
Capacity
Typical dose
Monomedia Filter Building (FF)
Monomedia filters
Quantity
Type
Media type
Number cells, each
Dimensions, each cell
Media depth
Design loading rate
Backwash pump
Quantity
Type
Capacity
Gravity Filter Building (DD)
Gravity filters
Quantity
Media type
Dimensions, each cell
Media depth
Design loading rate
Backwash pump
Quantity
Type
Capacity
Gravity filter effluent pumps
Quantity
Constant speed
Adj speed
Type
Adjustable stroke & speed
200 gpm max
25-30 mg/L
Progressing cavity
Adjustable
2.0 gpm max
0.1-0.2 mg/L
8
Center gullet
Anthracite
2
30ftl_X17ftW
5ft
2.9 gpm/ft2 at average daily flow with
all units in service
1
Vertical turbine
20,400 gpm
10
Anthracite/sand
30ftLX30ftW
2.25ft
2.6 gpm/ft2 at average daily flow with
all units in service
1
Vertical turbine
18,000 gpm
2
2
Vertical turbine
26 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Capacity, each
Constant speed
Adj speed
Backwash Effluent Tanks (EE)
Quantity
Dimensions, each
Volume, each
Reaeration Tank (HH)
Quantity
Dimensions
Volume
APW Pump Station (HH1)
Advanced plant water pumps
Quantity
Type
Speed
Capacity, each
Horsepower, each
Blended Sludge Storage Tanks (R1/R2)
Odor control scrubber system
Quantity
Type
Chemicals treated
Capacity, each
Depth of bedding
Cross-sectional area
22,500 gpm
27,000 gpm
85 ft L X 20 ft W X 11.3 ft SWD
144,000 gallons
1
72 ft L X 70 ft WX 22 ft SWD
830,000 gallons
Vertical turbine
Adjustable
4,400 gpm at 212 ft TDH
300 HP
1
Two-stage, packed-bed wet type
NH3, H2S
5,000 cfm
7ft
19.6ft2
Chemical feed pumps for odor control
Quantity
Caustic
Sodium hypochlorite
Sulfuric acid
Type
Capacity, each
Chemical storage tanks
Chemical
Quantity
Dimensions, each
Volume, each
2
2
2
Tubular diaphragm, chemical metering
23gph
NAOH
1
6ftdiaX10fthigh
2,100 gallons
Appendix A
Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 27
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Chemical
Quantity
Dimensions, each
Volume, each
Chemical
Quantity
Dimensions, each
Volume, each
Degritting Building (H1)
Cyclone separators
Quantity
Capacity, each
Grit classifiers
Quantity
Capacity, each
Primary Sludge Thickeners (J1/J2)
Gravity thickeners
Quantity
Type
Diameter
Sidewater depth
Flotation Thickeners (Q1/Q2)
DAF thickeners
Quantity
Type
Size
Capacity, each
Sludge Storage (R1/R2)
Sludge storage tanks
Quantity
Diameter
Sidewater depth
Volume, each
Sludge Dewatering (K3)
Centrifuge
Quantity
Type
Sludge loading, each
With lime
Excluding lime
Sludge feed concentration (percent)
NAOCL
1
4ftdiaX11 ft 7 in high
1,000 gallons
H2S04
1
38 indiaX82 in long
400 gallons
465 gpm at 12 psi
108ft7hr
4
Circular
50ft
10ft
Rectangular
40.2ftl_x12ftWx12ftSWD
960 gpm
367,000 gallons
Bowl and scroll conveyor
5,351 Ib/hr
4,730 Ib/hr
28 - Fairfax County, VA • NomanM. Cole, Jr., Pollution Control Plant
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Minimum 3.00%
Maximum 6.00%
Minimum cake solids concentration 29.00%
Minimum solids capture 95.00%
Capacity, each, based on 3.5% solid feed
95% solids capture, 29% cake solid 60 dry tons per day
Sludge Incineration (K1/K2)
Incinerators Nos. 1 &2
Quantity 2
Type Multiple hearth
Capacity, each 45 dry tons per day
Incinerators Nos. 3 & 4
Quantity 2
Type Multiple hearth
Capacity, each 92 dry tons per day
Appendix A Fairfax County, VA • Noman M. Cole, Jr., Pollution Control Plant - 29
-------
-------
North Gary Water Reclamation Facility
North Gary/ North Carolina
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The North Gary, North Carolina, Water Reclamation Facility (WRF) is a 12-million-gallon-
per-day (MGD) facility that included biological nutrient removal (BNR) as part of a 1997
expansion. This facility, which was a replacement/expansion of the 4-MGD Schreiber
process on the same site, was selected as a case study because of its phased isolation ditch
(PID) technology with tertiary filters.
The WRF does not have primary settling and uses the PID technology or the BioDenipho
process by Kruger. The facility uses two pairs of oxidation ditches with anaerobic selectors
ahead of the ditches and a second anoxic zone following the ditches. Each pair of ditches is
operated in an aerobic/anoxic sequencing mode or phases. The effluent from the ditches goes
to two 130-foot-diameter clarifiers. Before discharge to Crabtree Creek, effluent is passed
through an upflow Dynasand filter by Parkson and ultraviolet disinfection and is aerated. The
original Schreiber tank was converted into a 7-million-gallon (MG) equalization basin in
addition to a 2-MG equalization basin at the headworks area, and the stored water is drained by
gravity to the influent pump station for subsequent treatment. Sludge is thickened and
aerobically digested before it is transported to the South Gary WRF for dewatering and drying
for final disposal.
The relevant permit limits that the North Carolina Department of Environment and Natural
Resources (NCDENR) established for the plant are shown in Table 1. Compliance limits are
primarily for the monthly averages shown for carbonaceous biochemical oxygen demand
(CBOD), total suspended solids (TSS), and ammonia nitrogen. Additional limits are specified
for the quarterly limit for total phosphorus (TP) and for the annual maximum limit of
144,000 Ib for total nitrogen (TN), which is equivalent to 3.94 milligrams per liter (mg/L) as
nitrogen.
A distinguishing feature of the BioDenipho process is the alternating flow pattern and
process conditions (aerobic and anoxic) occurring within the oxidation ditches. This
operating strategy allows nitrogen and CBOD removal to occur within the active process
volume, eliminating the need for internal recycle pumping. The operation is executed by a
programmable logic controller (PLC)-based system that coordinates the operation of the
mechanical process equipment and controls the phase lengths within each ditch. The PLC
system allows both manual and automatic control of the treatment process. The PLC panel
Appendix A North Gary, NC • Water Reclamation Facility - 1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 1. NCDENR permit limits
Parameter
CBOD
TSS
NH3-N
TN
TP
Conforms
Summer limits
(mg/L)
4.1
30
0.5
-
-
-
Winter limits
(mg/L)
8.2
30
1.0
-
-
-
Quarterly limits
(mg/L)
-
-
-
-
2.0
200/1 00 ml
Annual limits
-
-
-
144, 000 Ib (max)3
-
-
Notes:
NH3-N = ammonia nitrogen
a Equivalent to 3.94 mg/L as TN for 12 MGD
also includes preprogrammed operational modes, such as the stormwater mode to address
infiltration/inflow (I/I) concerns. For example, automatic or manual activation of the
stormwater mode incorporates a sedimentation phase into the BioDenipho process to prevent
solids washout during severe rain events. This innovation allows reduction of the required
size of the secondary clarifiers or eliminates the requirement for redundant clarifiers.
Plant Design and Process Parameters
A schematic for the North Gary WRF is shown in Figure 1. To ensure economical and
efficient treatment, the system also controls the aeration equipment by automatic dissolved
oxygen (DO) control. DO probes continuously monitor and report residual DO levels within
the oxidation ditches to the PLC panel that controls the aeration equipment to meet, but not
exceed, the current oxygen demand. This eliminates costly and wasteful over-aeration that
can compromise process stability and operational budgets. Table 2 and Attachment 1 present
relevant design data for the facility and Attachment 2 presents a plant operating process
diagram. The sludge residence time (SRT) for an oxidation ditch was 12 days at 12 degrees
Celsius (°C).
Table 2. Facility design data
Units
Anaerobic selectors
Oxidation ditch
Secondary anoxic zone
Reaeration zone
Clarifiers
Number
4 each train
2 each train
3 each train
1 each train
2 each
Volume
0.093MGx4 = 0.372MG
1 .5 MG x 2 = 3 MG
0.111 MG x 3 = 0.333 MG
0.111 MG
130 ft diameter
Note: MG = million gallons
2 - North Gary, NC • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
an
o
i
ce
re
E
0)
0)
u
o
a:
re
O
01
Appendix A
North Gary, NC • Water Reclamation Facility - 3
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Table 3 presents operational results for the October 2005 to September 2006 period. Table 4
presents plant monthly average plant process parameters.
Table 3. Influent and effluent averages
Parameter
(mg/L unless stated)
Flow (MGD)
Influent TP (mg/L)
Effluent TP (mg/L)
Influent BOD (mg/L)
Effluent BOD (mg/L)
Influent TSS (mg/L)
Effluent TSS (mg/L)
Influent NH4-N (mg/L)
Effluent NH4-N (mg/L)
Influent TKN (mg/L)
Effluent TN (mg/L)
Average
value
7.0
7.7
0.38
244
0.8
366
1.0
45.5
0.08
56.4
3.67
Maximum
month
8.71
9.2
1.06
271
1.26
418
1.47
49.4
0.34
62.2
4.46
Max
month
vs. avg.
24%
20%
180%
11%
50%
14%
45%
8%
316%
10%
21%
Maximum
week
10.8
11.1
1.45
296
1.84
594
2.28
53.5
1.03
65.6
5.87
Sample
method/frequency
-
Composite, 3x/week
Composite, 3x/week
Composite, 5x/week
Composite, 5x/week
Composite, 5x/week
Composite, 5x/week
Composite, 5x/week
Composite, 5x/week
Composite, 3x/week
Composite, 3x/week
Note:
TKN = total Kjeldahl nitrogen
BOD = biochemical oxygen demand
Table 4. Monthly averages for plant process parameters
Month
Oct 2005
Nov 2005
Dec 2005
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sep 2006
MLSS
(mg/L)
2,665
2,628
2,736
2,672
2,720
2,692
2,661
2,625
2,700
2,713
2,709
2,685
Sludge age
(days)
13.1
13.8
13.0
13.3
12.8
13.3
12.6
13.5
11.3
12.3
12.6
12.1
HRT
(hours)
28
29
26
27
27
29
27
28
21
25
25
24
Temperature
(°C)
23
20
19
18
16
18
19
21
24
26
27
26
Notes:
HRT = hydraulic retention time
MLSS = mixed liquor suspended solids
4 - North Gary, NC • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Plant Performance
This section of the case study provides information about the operational performance of
nutrient removal at the facility. Figures 2 and 3 present monthly and weekly reliability data
for ammonia nitrogen removal. These data cover the period of October 2005 through
September 2006. Note that the apparent outlier values are from the period in June 2006 when
the plant's service area was subjected to nearly 8 inches of rain in a 24-hour period from
Tropical Storm Alberto. Note also that despite that upset, the plant still met the monthly limit
of 0.5 mg/L for ammonia nitrogen. Overall, ammonia nitrogen oxidation was complete, with
a mean of 0.06 mg/L and a 31 percent coefficient of variation (COV) for non-tropical storm
months.
100
10
ra
E
HI
ra
2
0.1
0.01
North Gary, NC
Monthly Average Frequency Curves for Ammonia Nitrogen
..,-»•"*•
vt^^Hl^TK
X^-"*^
> *
*
— — •
Mean = 0.081 mg/L —
C.O.V. = 102%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N
x Final Effluent - Ammonia-N
Figure 2. Monthly average frequency curves for ammonia nitrogen.
Appendix A
North Cory, NC • Water Reclamation Facility - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100
10
North Gary, NC
Weekly Average Frequency Curves for Ammonia Nitrogen
« • » * ••*•••••*•
x — x — x xxjpaan***'*''^
; '
X
X *_
ja****' "
Mean ~ 0 81 mg/L —
C.O.V. - 174%
0.1
0.01
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Ammonia-N x Final Effluent - Ammonia-nitrogen
Figure 3. Weekly average frequency curves for ammonia nitrogen.
Figures 4 and 5 present monthly and weekly reliability data for TP removal. Phosphorus
removal was completely by biological means and worked well, with a monthly mean of
0.38 mg/L and a COV of 64 percent. This removal was sufficient to meet the facility's
quarterly limit of 2 parts per million (ppm).
100
u>
E
o
.c
Q.
t/>
O
.E
Q.
1
O
10
0.1
0.01
North Gary, NC
Monthly Average Frequency Curves for Total Phosphorus
E Mean = 0.379 mg/L
: Std. Dev. = 0.241 mg/L
! C.O.V. = 64%
0.05 0.1 0.5 1 2
10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 4. Monthly average frequency curves for TP.
6 - North Gary, NC • Water Reclamation Facility
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
100
ra
E
$
o
0.01
North Gary, NC
Weekly Average Frequency Curves for Total Phosphorus
Mean = 0378 mg/L
Std. Dev. = 0.273 mg/L
C.O.V. = 72%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
• Raw Influent
x Final Effluent
Figure 5. Weekly average frequency curves for TP.
Figures 6 and 7 present reliability data for removal of TN at the facility. TN removal was
excellent, with the effluent mean 3.7 mg/L with a COV of 14 percent on a monthly average
basis, including the period with heavy precipitation caused by the tropical storm.
North Gary, NC
Monthly Average Frequency Curves for Total Nitrogen
10-
5 1-
O)
o
z
0.1 -
nn-i .
* * — • • » »
• * * *
<* <* —
•*. A '
Std Dev ~ 0 51° mg/L
C.O.V. = 14%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
• Raw Influent - Total Nitrogen x Final Effluent - Total Nitrogen
Figure 6. Monthly average frequency curves for TN.
Appendix A
North Gary, NC • Water Reclamation Facility - 1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100
10
North Gary, NC
Weekly Average Frequency Curves for Total Nitrogen
0.1
0.01
yfc-^^l^ 'M.
u^wttttttKM
mMim
,
^w — t™"^™
BM*™r^i^'K
Mean - 3.67 mg/L
SM D°v - 0 7"1 mg/L
C.O.V. - 21%
0.05 0.1 0.5 1
10 20 30 40 50 60 70 80 90 95 989999.5 99.999.95
Percent Less Than or Equal To
» Raw Influent - Ammonia-N x Final Effluent - Ammonia-nitrogen
Figure 7. Weekly average frequency curves for TN.
Reliability Factors
The performance was efficient and reliable for entirely biological phosphorus and nitrogen
removal at North Gary. The COVs were 102 percent for ammonia nitrogen at the mean
concentration of 0.08 mg/L, 64 percent for total phosphorus at the mean concentration of
0.38 mg/L, and 14 percent for total nitrogen at the mean concentration of 3.67 mg/L.
The following points summarize the factors affecting the reliability of the North Gary WRF:
• The BioDenipho process at North Gary is a flexible process with regard to varying
wastewater strength and flow rate. The reliability is achieved through well-controlled
oxidation of ammonia and subsequent denitrification in two distinct anoxic steps. The
anoxic cycle phase in the ditch can be adjusted from 60 minutes to 90 minutes, for
example, during a low-flow period, while it can be reversed during a high-flow
period. The rotors are controlled to provide sufficient oxygen to maintain the DO
concentration at 1 to 1.5 mg/L in the ditch, while mixers keep the organisms in
suspension during the anoxic phase. This flexibility to control mixing separately from
aeration is one of the keys to this plant's reliability. The low DO in the ditch effluent
ensures good denitrification in the second anoxic step to reach the desired nitrogen
level in the effluent. No external carbon source is needed to meet the permit limit.
8 - North Gary, NC • Water Reclamation Facility
Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Another key reliability factor is the automated control system, which consists of
sensors and DO controllers operating with the PLC and associated supervisory
control and data acquisition (SCADA) system. The exact phasing decision is made on
the basis of the preset control logic, which is site-specific and fully automated.
A key reliability factor for biological phosphorus removal is the feed point of the
influent. The influent is fed to the second anaerobic selector, while return activated
sludge is fed to the first selector to ensure that the returning nitrate from the clarifier
will be denitrified in the first selector zone. The second, third, and fourth selector
zones thus become anaerobic and allow full energy exchange for polyphosphate-
accumulating organisms (PAOs). The wastewater exhibited a favorable ratio of
biochemical oxygen demand (BOD) to TP, greater than 30 as an average. The plant
performance has been proven reliable through this process (WEF and ASCE 1998).
A key reliability factor for nitrogen removal is the three phases of anoxic cycles. The
first is in the anaerobic selector before the ditch, the second is in the ditch, and the
third is in the anoxic zone after the ditch. These multiple opportunities to denitrify in
the presence of BOD in the wastewater are unique and ensure good removal of
nitrogen. The wastewater exhibited a favorable BOD/TKN ratio of 5 as an average,
which is adequate for good denitrification (USEPA 1993).
Training is another key factor for achieving high reliability. Online monitoring and
automatic controls make training easy but require continuous maintenance by the
plant personnel.
Less power is used because of the maximum use of nitrate during the anoxic phase
and the prevention of over-aeration during the oxic phase. Pumping of oxidized
effluent to 3 to 4 times the discharge (Q) is not required to reach the same level of
denitrification.
Tertiary filters are effective in suspended solids removal.
Recycle loads are minimized; aerobic digestion occurs on-site, and the digested
sludge is shipped away for processing at another facility.
Wet-weather flows are handled in two ways: The equalization basins have a total of
9 MG storage, or 75 percent of the influent design flow; the PID has a storm mode in
the process control, under which the program switches into a sedimentation phase,
thereby preventing solids washout. These helped manage high flows during the June
2006 event, when Tropical Storm Alberto brought high flows to the plant. All the
storage volume was used, and the PID went into the storm mode for a short duration.
The plant treated all flows and complied with the permit.
Appendix A North Gary, NC • Water Reclamation Facility - 9
-------
Nutrient Removal Technology Assessment Case Study September 2008
Costs
Capital Costs
The main upgrade of the plant for BNR was in 1997 when the ditches were installed. The
upgrade then cost $25 million. The upgrade included additional ditches, the selector, pumps,
aerators, and tertiary filtration.
Because all phosphorus and nitrogen removal is biological, the capital costs were attributed
to different removal processes on the basis of the amount of oxygen used during biological
treatment, which is 12 percent for TP removal, 48 percent for nitrogen removal, and
40 percent for other (i.e., BOD removal). This means that the capital expenditure attributed to
TP removal was $3 million, and the expenditure attributed to nitrogen removal was
$12 million.
These capital cost results were updated to 2007 dollars using the Engineering News-Record
Construction Cost Index (ENR CCI). The ENR CCI is compiled by McGraw-Hill and
provides a means of updating historical costs to account for inflation, thereby allowing
comparison of costs on an equal basis. From a Web site provided by the U.S. Department of
Agriculture, the ENR index for 1997 was 5,826, while the ENR index for May 2007 was
7,942 (USDA 2007). Multiplying the above results by the ratio 7,942/5,826 obtained the
result of $4.09 million for phosphorus removal and $16.9 million for nitrogen removal in
2007 dollars.
The total capital expenditure attributed to BNR in 2007 dollars was $34.1 million. For the
12-MGD facility, the capital expenditure per gallon of BNR treatment capacity was $2.84.
Operation and Maintenance Costs
In all case studies prepared for this document, the O&M costs considered were for electricity,
chemicals, and sludge disposal. Labor costs for O&M were specifically excluded for three
reasons:
1. Labor costs are highly sensitive to local conditions, such as the prevailing wage rate,
the relatively strength of the local economy, the presence of unions, and other factors;
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 automatic controls and SCADA
system that accompany most upgrades.
3. Most facilities were unable to break down which extra personnel were employed
because of nutrient removal and related overtime costs, making labor cost development
difficult.
10 - North Gary, NC • Water Reclamation Facility Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
The plant uses an entirely biological phosphorus removal process to achieve the limit;
therefore, the primary operating cost is electrical use for operating the mixers, pumps, and
selector. Attachment 3 presents the electrical cost calculations for one train; the second train
is a duplicate. Power usage was attributed on the basis of discussions with plant personnel,
who suggested 5 percent for phosphorus removal and 95 percent for nitrogen removal, except
for units that could be entirely attributed to phosphorus or nitrogen (i.e., anaerobic mixers for
phosphorus, anoxic mixers for nitrogen). From this, the total power usage attributed to
phosphorus removal was 377,000 kilowatt-hours per year (kWh/yr). When calculated using
the average electrical cost of $0.056/kWh (which includes all demand charges), the cost for
power for phosphorus removal was $17,400. The total power usage attributed to nitrogen
removal was 2,558,000 kWh/yr; applying the electrical unit price, the cost for power for
nitrogen removal was $118,000.
The sludge generated during the process is transported to another town of Gary facility for
disposal. From consultation with plant personnel, the sludge generated (4.91 tons/day) was
attributed at 5 percent to phosphorus removal and 95 percent to nitrogen removal. The cost
for the plant to send the sludge out for treatment was $200/ton. The cost for sludge disposal
for phosphorus removal was $17,900, while the sludge disposal for nitrogen removal was
$341,000.
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 156,000 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal was $0.23/lb, while the annualized unit
capital cost for phosphorus removal was $2.28.
During the evaluation period, the plant removed 1,121,000 Ib of total nitrogen. With the
results above, the unit O&M cost for total nitrogen removal was $0.41/lb of TN, while the
annualized unit capital cost for TN removal was $1.27.
Life-Cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the annualized unit capital and unit O&M costs. Thus, the
life-cycle cost for phosphorus removal was $2.51/lb and the life-cycle cost for TN removal
was$1.68/lb.
Assessment of magnitude of costs: The capital cost of $2.84 per gallon per day (gpd) capacity
is relatively high, but the O&M costs are very low. One of the key factors is that chemicals
are not used for nutrient removal, saving both those costs as well as costs that would be
attributed to additional sludge generation.
Appendix A North Gary, NC • Water Reclamation Facility - 11
-------
Nutrient Removal Technology Assessment Case Study September 2008
Summary
The North Gary facility is unique in that it provides reliable nutrient removal by means of a
PID process followed by tertiary filters. The phosphorus removal is achieved entirely by a
biological process with a mean concentration of 0.38 mg/L with a COV of 64 percent. The
nitrogen removal is also achieved entirely by a biological process with a mean of 3.67 mg/L
with an extremely low COV of 14 percent. The process is flexible enough to accommodate
varying flow conditions and the wastewater characteristics through the year, including the
severe rain caused by Tropical Storm Alberto in June 2006. Automatic controls incorporated
into the plant ensure reliable operation and control through these operating periods. The
wastewater characteristics are favorable to both nitrogen and phosphorus removal, and no
external carbon sources are needed with this PID process.
The capital cost is relatively high at $2.84/gpd capacity as a new facility but compares well
with others, which normally exceed $3/gpd. The O&M costs are estimated at $1.26/lb of TP
removed and $0.41/lb of TN removed. These costs are remarkably low, reflecting the
inherent advantages of this unique treatment process. The total costs were $2.21/lb of TP
removed and $2.92/lb of TN removed.
A ckn o wledgments
The authors are grateful for the significant assistance and guidance that Chris Parisher, North
Gary WRF superintendent, provided. This case study would not have been possible without
his prompt response with well-deserved pride in the facility and its operation. The authors
also wish to thank the town of Gary for its participation.
References and Bibliography
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. April 3, 2007.
USEPA (U.S. Environmental Protection Agency). 1993. Nitrogen Control Manual.
EPA/625/R-93/010. U.S. Environmental Protection Agency, Washington, DC.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers).
1998. Design of Municipal Wastewater Treatment Plants. Manual of Operation No. 8,
4th ed. Water Environment Federation, Alexandria, VA.
12 - North Gary, NC • Water Reclamation Facility Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Attachment 1: Key Design Parameters
SECTION 5: KEY DESIGN PARAMETERS
Design Basis:
Annual Average Daily Flow
Peak Daily Flow
BOD
TSS
TKN
NH;N'
TP
TN
Temp
Influent
10
20
250
300
35
—
7
—
12/27
Expected Secondary
Effluent
—
-
<10
<10
—
<0.5/1.0
2
6
—
Unit
MGD
MGD
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
2C
'Summer/Winter
system parameters:
Unit
Number of Trains
Anaerobic Selector
Physical Parameters
Number of Stages/Train
Volume per Stage
Length per Stage
Width per Stage
SWD
Equipment/Stage
Mixers
Number
Model
HP
RPM
2
4
0.093
35.3
17.6
20.0
1
POP-I
4.9
180
MG
ft
ft
ft
HP
RPM
Appendix A
North Gary, NC • Water Reclamation Facility - 13
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Attachment 2: Operating Stages of the BioDenipho
Process
•••••
Tank
Dernriticalion
ENlLKKIt
'am
Nilrrtitir.tr
Effluent
Tank
PtiaseE(2.SO-3j(X)h)
lnHU8nt
PtiBBa F (aSO-4j
-------
Attachment 3: Electrical Costs
Electrical 1 train
kW
Hp Number Power draw
Anaerobic Mixers
4.9 4 14.6216
Rotors
60 4 179.04
Main mixers
9 4 26.856
Anoxic mixers
6.5 3 14.547
Reaeration blower
20 1 14.92
Clarifer drive
1 1 0.746
Total for 1 train
Total for 2 trains TRAINS
kWh kWh
hours/day draw/day draw/year
24 350.9184 128,085.2
15.12 2,707.085 988,086
8.88 238.4813 87,045.67
24 349.128 127,431.7
24 358.08 130,699.2
24 17.904 6,534.96
Rate
Totals
% for P
100
5
5
0
5
5
0.05
% for N
0
95
95
100
95
95
forP
128,085.2
49,404.3
4,352.283
0
6,534.96
326.748
188,703.5
377,407
forP
$17,361
forN
0
938,681.65
82,693.384
127,431.72
124164.24
6,208.212
1,279,179.2
2,558,358.4
forN
$117,684
-------
-------
Western Branch Wastewater Treatment Plant
Upper Marlboro, Maryland
Nutrient Removal Technology Assessment Case Study
Introduction and Permit Limits
The Western Branch Wastewater Treatment Plant (WWTP) was selected as a case study
because of a unique feature—three separate activated-sludge systems operating in series to
remove nutrients.
The Western Branch WWTP is part of the Washington Suburban Sanitary Commission
(WSSC), and it is in Upper Marlboro, Maryland. It is permitted for a flow of 30 million
gallons per day (MOD); in 2006 it processed an average of 19.3 MOD. The plant is permitted
to discharge to the Western Branch of the Patuxent River.
The relevant National Pollutant Discharge Elimination System (NPDES) permit limits for the
facility are shown in Table 1.
Table 1. NPDES permit limits
Parameter
BOD5
4/1-10/31
BOD5
11/1-3/31
TSS
Total phosphorus
Total nitrogen
Ammonia-N 4/1-10/31
Ammonia-N 11/1-3/31
Annual
loading
(mg/L)
0.3
4.0
Monthly average
(mg/L)
9
30
30
1.0
3.0
1.5
5.5
Weekly average
(mg/L)
14
45
45
N/A
4.5
N/A
N/A
Notes:
BOD = biochemical oxygen demand
mg/L = milligrams per liter
N/A = not applicable
TSS = total suspended solids
Note that 0.3 mg/L TP and 4 mg/L TN on an annual load basis will be required after completion of enhanced
nutrient removal upgrades funded by Maryland.
"Total nitrogen and total phosphorus are based on a design flow of 30 MGD.
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 1
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Plant Process
Figures 1 is an overall process flow diagram, and Figure 2 is a detailed liquid side process
flow diagram for the Western Branch Facility. The plant has three separate activated-sludge
systems in series: a high-rate activated-sludge (FtRAS) system, intended primarily for BOD
removal; a nitrification activated-sludge (NAS) system, for conversion of ammonia nitrogen
to nitrate; and a denitrification activated-sludge (DNAS) system, for conversion of nitrate to
nitrogen gas. The return activated sludge for each system is kept separated to allow for
independent setting of sludge residence times. The system does not include primary settling,
and grit removal and screenings are provided ahead of the FIRAS. The effluent is filtered
prior to ultraviolet (UV) disinfection. Waste activated sludge from the three systems is
mixed, thickened by dissolved air flotation (DAF), dewatered by centrifuge, and incinerated
in two multiple-hearth incinerators. Process water from the DAF, centrifuge, and incinerator
air scrubbers is returned to the headworks.
RAW
SEWAGE )
PRELIMINARY
TREATMENT
WASTE
ACTIVATED
SLUDGE
HIGH RATE
ACTIVATED
SLUDGE
FLOATATION
THICKENING
FILTRATION
NITRIFICATION
DENITRIFI-
CATION
TO
WESTERN
BRANCH
CHLORINE
CONTACT & POST-
AERATION
(FUTURE UV
DISINFECTION)
ASH TO
1 DISPOSAL
J (LANDFILL)
CENTRIFUGE
DEWATERING
INCINERATION
Figure 1. Western Branch WWTP process flow.
2 - Western Branch, MD • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
FLOW SPLIT STRUCTURE
(FOUR WEIRS WITH GATES)
PARSHALL FLUMES
GRIT/SCREEN UNITS
HIGH RATE AERATION
HIGH RATE CLARIFIERS
NITRIFICATION AERATION
NITRIFICATION
CLARIFIERS
DENITRIFICATION
REACTORS
NITROGEN
STRIPPING
CHANNELS
DENITRIFICATION
CLARIFIERS
CD
FROM RAW WWPS
11
CO
CD
o
CD
TO GRAVITY FILTERS AND UV
Figure 2. Western Branch WWTP liquid process flow.
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 3
-------
Nutrient Removal Technology Assessment Case Study September 2008
Basis of Design and Actual Flow
Flow
The design flow for the facility is 30 MGD. The average flow for the study period was
19.3 MGD (23.0 MGD including recycles), while the maximum month flow during the study
period was 26.5 MGD (including recycles) during November 2006.
Loadings
Plant design based on the following:
Plant influent: BOD 200 mg/L
TSS 200 mg/L
HRAS effluent: BOD 60 mg/L
NAS effluent: BOD 20 mg/L
TKN 2 mg/1 L
Nitrate-nitrogen 15-30 mg/L
Process Size Detention time VLR
(hours) BOD Ib/kcf/d
HRAS 3.35 (0.84 MG, 4 each) 2.68 112
NAS 6.89 (1.72 MG, 4 each) 5.51 16
DNAS -Anoxic 3.35 (0.84 MG, 4 each) 2.68
- Stripping/reaeration = 0.68 MG (0.28 MG, 2 each) 0.45
- TKN loading rate = 5.5 Ib TKN/kcf/d
Sludge age = 5-10 days
- RAS = 100% of plant flow
Methanol reed rate =100 mg/L
- Alum feed point is the Stripping/reaeration channel
Clarifiers
HRAS
NAS
DNAS
120 x 80x13 ft, 4 each
150 x 80x1 1.5 ft, 4 each
Diameter- 160 ft, 4 each
Overflow rate
781 gpd/ft2
625 gpd/ft2
373
SLR
34 Ib/ft2/d
27.4
16.3
Note:
SLR = sludge loading rate and is based on a mixed liquor suspended solids concentration of 3,000 mg/L.
Tertiary filters-gravity filters, with air-water backwash capability
- 30 ft x 30 ft, 11 each, total area 9,900 ft2
- Filter bottom = Leopold clay tiles
- Media-20 inches of anthracite, 8 inches of sand, 12 inches of gravel
Hydraulic loading rate = 2.1 gpm/ft2
4 - Western Branch, MD • Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Plant Parameters
Overall plant influent and effluent average results for the period January 2006 to December
2006 are shown in Table 2.
Table 2. Influent and effluent averages
Parameter
(mg/L unless
stated)
Flow incl recycle
(MGD)
Influent TP
Effluent TP
Influent COD
Effluent COD
Effluent BOD
Influent TSS
Effluent TSS
Influent NH4-N
Effluent NH4-N
Influent Total N
Effluent Total N
Average
value
23.0
3.70
0.43
332
16.1
2.69
222
1.23
19.6
0.22
23.9
1.63
Maximum
month
26.5
4.22
0.89
417
25.8
3.94
282
2.28
22.3
0.93
28.7
2.46
Max
month vs.
ave.
15%
14%
89%
26%
60%
46%
27%
85%
14%
323%
20%
45%
Maximum
week
30.9
5.57
0.99
641
38.6
6.08
400
4.60
25.1
3.41
44.8
4.22
Sample
method/frequency
Twice weekly/
composite
Five times weekly/
composite
Twice weekly/
composite
Five times weekly/
composite
Five times weekly/
composite
Twice weekly/
composite
Five times weekly/
composite
Twice weekly/
composite
Five times weekly/
composite
Twice weekly/
composite
Five times weekly/
composite
Notes:
BOD = biochemical oxygen demand
Max month vs. average = (max month - average)/average x 100
NH4-N = ammonia measured as nitrogen
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 5
-------
Nutrient Removal Technology Assessment Case Study
September 2008
Tables 3, 4, 5, and 6 present plant monthly averages for the process parameters, as available.
Table 3. Monthly averages for HRAS process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
HRAS MLSS
(mg/L)
4,710
4,232
3,808
3,798
4,208
5,454
4,028
4,306
5,545
4,066
3,431
4,017
HRAS sludge age
(d)
1.9
1.8
1.9
1.7
2.7
7.3
1.8
1.9
2.7
1.7
0.9
2.0
HRAS HRT
(hr)
3.4
3.3
3.6
3.7
3.8
3.5
3.5
3.8
3.5
3.4
3.0
3.6
Table 4. Monthly averages for NAS process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
NAS MLSS
(mg/L)
4,264
3,800
3,617
2,794
3,644
3,706
3,523
4,286
4,987
4,806
4,212
5,117
NAS sludge age
(d)
34.8
29.9
46.6
34.7
24.3
21.4
72.5
65.6
84.6
79.7
34.4
43.2
NAS HRT (hr)
7.1
6.7
7.4
7.7
7.7
7.2
7.3
7.9
7.1
6.9
6.2
7.3
6 - Western Branch, MD • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Table 5. Monthly averages for DMAS process parameters
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
DMAS MLSS
(mg/L)
5,006
4,329
3,541
3,818
2,795
3,427
4,201
3,192
3,939
3,968
4,081
4,990
DMAS sludge age
(d)
32.6
24.4
17.8
8.8
5.8
11.9
23.3
10.9
58.4
18.9
40.2
17.0
DMAS HRT
(hr)
3.4
3.3
3.6
3.7
3.8
3.5
3.5
3.8
3.5
3.4
3.0
3.6
Table 6. Monthly averages for influent temperature
Month
Jan 2006
Feb 2006
Mar 2006
Apr 2006
May 2006
June 2006
July 2006
Aug 2006
Sept 2006
Oct 2006
Nov 2006
Dec 2006
Temperature
(°F)
58.5
56.3
57.5
61.9
64.4
68.2
72.4
73.6
71.4
67.7
63.9
61.1
Temperature
(°C)
14.7
13.5
14.2
16.6
18.0
20.1
22.4
23.1
21.9
19.8
17.7
16.2
Performance Data
Figure 3 presents reliability data for the removal of total phosphorus (TP). The removal is
good, with the effluent TP averaging 0.43 mg/L, and a coefficient of variation (COV) of
62 percent.
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 7
-------
Nutrient Removal Technology Assessment Case Study
September 2008
100
10
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Total Phosphorus
0.1
0.01
0.001
e g 9 9 J
o " 9 9
X J £ A
.£-•»-*-»— i
3 § =
i x
Std. Dev. 0.27 mg/L I
C.O.V. - 62%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
o Plant Influent a HRAS Influent x HRAS Effluent
o MAS Effluent £. DMAS Effluent x Final Effluent
Figure 3. Monthly average frequency curves for TP.
Figure 4 presents reliability data for ammonia nitrogen removal. Removal of ammonia
nitrogen is very good, with a mean effluent of 0.13 mg/L and a high COV of 163 percent.
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Ammonia-Nitrogen
10-
0)
O) 1
z
1 0.1-
o
E
0.01 -
n nn-i -
.
8 S 1 * x £
o
O
° ° A^-^*
^
ii iii ii iii
x 1 i
0 0 0 0
^***~~f
•£^* x
1
3 O
K X
o
3
Std. Dev. - 0.22 mg/L
C.O.V. = 163%
i i i i i i
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
o Combined Raw Influent n HRAS Influent x HRAS Effluent
o MAS Effluent A DMAS Effluent x Final Effluent
Figure 4. Monthly average frequency curves for ammonia nitrogen.
8 - Western Branch, MD • Wastewater Treatment Plant
Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Figure 5 presents reliability data for the removal of total nitrogen (TN). The plant gives
outstanding total nitrogen removal, with effluent TN of 1.63 mg/L and a COV of 36 percent.
100-
Western Branch WWTP, WSSC
Monthly Average Frequency Curves for Total Nitrogen
10
D>
c"
o
o
0.1
6 ft R
A A A
:Mean= 1.63 mg/L
; Std. Dev. = 0.59 mg/L
: C.O.V. = 36%
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.999.95
Percent Less Than or Equal To
o Combined Raw Influent n HRAS Influent o MAS Effluent
A DMAS Effluent x Final Effluent
Figure 5. Monthly average frequency curves for TN.
Reliability Factors
This facility is unique in three ways: (1) three separate activated-sludge systems operated in
series with dedicated clarifiers and RAS lines for biochemical oxygen demand (BOD)
removal, nitrification, and denitrification with methanol feed; (2) chemical phosphorus
removal; and (3) tertiary filtration. The facility also is unusual in that it has no primary
settling. All sludge generated is biological and chemical sludge combined, which is
incinerated after thickening by DAF and dewatering by centrifugation.
The results were excellent. The plant achieved a TN concentration of 1.63 mg/L with a COV
of 36 percent and a TP concentration of 0.43 mg/L with a COV of 62 percent. Many factors
accounted for this performance, and the key factors are presented below.
Wastewater characteristics: Because this facility uses a separate stage for denitrification, the
use of an external carbon source (methanol) is a requirement. In addition, phosphorus
removal is designed to be achieved with alum feed. The typical ratio for characterizing the
adequacy of BOD is not applicable because the plant does not rely on internal carbon sources
for biological removal of nitrogen or phosphorus.
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 9
-------
Nutrient Removal Technology Assessment Case Study September 2008
The plant added a new process for nitrogen removal as the third step of the treatment train
and called it DeNitrifying Activated Sludge, or DNAS, with separate clarifiers. The existing
plant had a two-stage activated-sludge process before the current expansion: high-rate
activated-sludge, or HRAS, for BOD removal and nitrifying activated sludge, orNAS. Both
had separate aeration basins and dedicated clarifiers. The first two stages provided effluent
with good BOD removal and full nitrification. The average concentrations in NAS effluent
were 16.5 mg/L in nitrate-nitrogen with a COV of 12 percent and 1 mg/L in ammonia
nitrogen. Note that nitrate-nitrogen is high because denitrification was not designed for. The
third step, DNAS, proved effective in nitrogen removal. The control strategy included daily
testing of key parameters, as well as adjustment of the dosage on an as needed basis. No
online sensors are used in the DNAS basin.
A comparison of design vs. actual parameters follows:
Parameters Design Actual
HRAS HRT (hours) 2.68 3.0-3.8
HRAS Sludge age (days) 0.9-7.3
NAS HRT (hours) 5.51 6.7-7.9
NAS sludge age (days) 21-84
DNAS HRT (hours) 2.68 3.0-3.8
DNAS sludge age (days) 5-10 5-58
Another key feature of the plant is chemical phosphorus removal. Alum is added to the
stripper/reaeration channel of the DNAS process at an average concentration of 10 mg/L and
has proven effective. The tertiary filter is another key in providing reliability in nitrogen and
phosphorus removal.
Methanol is added to the DNAS tanks at an average rate of 1,165 gpd to provide sufficient
carbon for denitrification to occur. The methanol dosage is approximately 2.5 Ib per pound of
nitrate entering the DNAS tanks. Nitrate is checked by chemical testing three times a day to
allow methanol dosage adjustment. The sludge generated is settled out with the rest of the
DNAS sludge, mixed with the HRAS and NAS sludge, and thickened in the DAF units.
The facility employs online monitoring of dissolved oxygen (DO) in the HRAS and NAS
basins, with one DO probe per reactor cell. The probe signals are used to control air valves
and thus control the air feed to the basins. The plant also has online suspended solids probes
in the aeration basins, which are used for monitoring, as well as sludge blanket monitors in
the DNAS clarifiers.
Another key feature of the plant is that there is no primary settling. All sludge comes from
the three biological systems, and the sludge is thickened aerobically at DAFs before
dewatering and incineration. The recycle loads of nitrogen and phosphorus, therefore, remain
low because there is no anaerobic digestion.
10 - Western Branch, MD • Wastewater Treatment Plant Appendix A
-------
September 2008
Nutrient Removal Technology Assessment Case Study
Wet-weather operation: Normal operating procedures are followed. No off-line storage is
available.
Cosfs
Capital Costs
The plant was constructed in three phases. Phase 1, carried out in the early 1970s, included a
dual sludge system for achieving BOD removal and nitrification, as well as filters that
accomplish both nitrogen and phosphorus removal. The Phase 1 construction was sized for
15 MOD. Phase 2, completed in the late 1970s, consisted of additional tanks and filters to
bring the dual sludge system to 30 MGD. Chemical addition for phosphorus removal was
installed temporarily in the late 1980s, but not as a capital expense. Phase 3, carried out in the
early 1990s, added a third sludge system for denitrification, along with making the alum
addition system for phosphorus removal permanent. Table 5 shows the costs of those
improvements, along with capital cost updates based on the Engineer ing News-Record
Capital Cost Index (ENR CCI). The ENR CCI, which is compiled by McGraw-Hill, provides
a means of updating historical costs to account for inflation, thereby allowing comparison of
costs on an equal basis. From a Web site provided by the U.S. Department of Agriculture
(USDA 2007), the ENR index for 1973 was 1,895; for 1976, 2,401; for 1991, 4,835; and for
May 2007, 7,942.
Table 5. Plant improvement costs
Phase 1
Phase 2
Phase 3
Total
Year
1973
1976
1991
Original
cost
$15,000,000
$7,500,000
$30,000,000
2007 cost
$62,865,435
$24,808,413
$49,278,180
$136,952,028
%P
0%
0%
5%
%N
20%
30%
60%
%
other
80%
70%
35%
Phosphorus
cost
$0
$0
$2,463,909
$2,463,909
Nitrogen
cost
$12,573,087
$7,442,524
$29,566,908
$49,582,519
The table also shows the percentage of capital cost for each phase that was attributed to
phosphorus or nitrogen removal; the rest of the capital cost was attributed to other treatment,
particularly BOD and TSS removal. Because the plant does not do biological phosphorus
removal, it was assumed that only 5 percent of the Phrase 1, 2, and 3 costs could be attributed
to phosphorus removal, which is a portion of the costs for filtration, plus the alum addition
system. Nitrification was installed during both Phase 1 and Phase 2, but Phase 1 included
additional activities not included in Phase 2, such as incineration and disinfection systems.
Thus, 15 percent of the Phase 1 cost was attributed to nitrogen removal, whereas 30 percent
of the Phase 2 costs were attributed to nitrogen removal. Since a large part of Phase 3 was the
denitrification unit, it was assumed that 60 percent of the Phase 3 costs were for nitrogen
removal.
Appendix A
Western Branch, MD • Wastewater Treatment Plant - 11
-------
Nutrient Removal Technology Assessment Case Study September 2008
The above analysis resulted in a total of $6,850,000 in capital attributed to phosphorus
removal and $41,500,000 attributed to nitrogen removal, in 2007 dollars. The annualized
capital charge for phosphorus removal (20 years at 6 percent) was $598,000. The annualized
capital charge for nitrogen removal was $3,620,000.
The total capital attributed to nutrient removal, in 2007 dollars, was $48.4 million. For the
30-MGD facility, this means the capital expenditure per gallon of treatment capacity was
$1.73.
Operation and Maintenance Costs
The plant uses chemical phosphorus removal and biological nitrogen removal, with extensive
use of alum for the former and methanol as a supplemental carbon source for the latter. This
means that the cost for phosphorus removal is essentially all chemical and for the disposal of
the resulting sludge, with a small amount of electricity; the cost for nitrogen removal is
electrical (for the aeration basins), chemical for the methanol, and for the disposal of the
extra sludge resulting from methanol addition. A summary of the electrical calculations is
provided in the Attachment. When the average electrical rate of $0.10/kWh (including
demand charges) was applied, the cost of electricity for nitrogen removal was $229,000.
The average amount of alum applied for phosphorus removal over the period was
14.4 gallons per MG of flow, or 502 tons; at a cost of $212.25/ton, the cost of alum was
$106,000. This cost was entirely attributed to phosphorus removal.
Methanol is applied at the DNAS to promote nitrate removal. The total amount of methanol
added over the study period was 425,000 gallons. At an average cost of $1.00/gallon, the
chemical cost for nitrogen removal was $425,000.
The alum added (9.5 mg/L as alum, or 0.86 mg/L as aluminum) was assumed to all convert
to aluminum hydroxide sludge; at the average flow of 19.2 MGD, this was 400 Ib of
aluminum sludge per day, or 73 dry tons/year. The plant's average cost of disposal,
considering trucking and incineration, was $440/dry ton. This made the cost of sludge for
phosphorus removal $32,400.
The 425,000 gal/yr (2.8 million Ib/yr) of methanol has a chemical oxygen demand (COD) of
1.5 Ib COD/lb of methanol, or 4.2 million Ib COD/yr. The typical yield of volatile suspended
solids (VSS) on methanol is 0.4 Ib VSS/lb of COD, giving 1.7 million Ib sludge/yr, or 839
tons sludge/yr from methanol addition. At a cost of $440/dry ton, this made the cost of sludge
for nitrogen removal $372,000.
12 - Western Branch, MD • Wastewater Treatment Plant Appendix A
-------
September 2008 Nutrient Removal Technology Assessment Case Study
Unit Costs for Nitrogen and Phosphorus Removal
During the evaluation period, the plant removed 213,000 Ib of phosphorus. With the results
above, the unit O&M cost for phosphorus removal is $0.78, while the unit capital cost is
$1.01/lb of phosphorus removed.
During the evaluation period, the plant removed 1.32 million Ib of total nitrogen. With the
results above, the unit O&M cost for nitrogen removal is $0.99, while the capital cost is
$3.27/lb of TN removed.
Life-cycle Costs for Nitrogen and Phosphorus Removal
The life-cycle costs are the sum of the unit capital and unit O&M costs. Thus, the life-cycle
cost for phosphorus removal is $1.78/lb of phosphorus removed, and the life-cycle cost for
TN removal is $4.27/lb of TN removed.
Assessment of magnitude of costs: The capital cost of $1.73 per gpd capacity is about average
for the case studies. The capital for phosphorus removal is low, whereas the capital for
nitrogen removal is high because of the use of the separate third stage for nitrogen removal.
The O&M costs for phosphorus removal are low, whereas those for nitrogen removal are
high because of the large amounts of chemical use with associated sludge generation.
Discussion
Reliability factors: This facility has a unique feature—three activated-sludge systems for
biological treatment for nitrogen removal and chemical addition for phosphorus removal,
followed by tertiary filtration. The reliability was excellent: the average concentrations were
1.63 mg/L in TN with a COV of 36 percent and 0.43 mg/L in TP with a COV of 62 percent.
For nitrogen removal, the third process, DNAS, relies on the external carbon source (in this
case methanol), and the dosage was reasonable at 2.5 Ib per pound of nitrate-nitrogen
applied. The high level of nitrate in the NAS was noted. Chemical phosphorus removal was
consistent in meeting the current limits.
Many factors have contributed to this reliable performance. The first key factor is the three
separate processes in series—BOD and ammonia removal in the first two activated-sludge
systems, followed by a separate activated-sludge system to denitrify with an independent
supply of carbon. The fluctuations in wastewater or operating parameters and thus
performance in one stage possibly can be balanced by the succeeding processes to achieve
overall reliability in the plant's performance. An increased reliability for nitrogen removal
was achieved through the use of an external carbon source; thus, the performance was not
dependent on favorable wastewater characteristics. In addition, operating all four trains
Appendix A Western Branch, MD • Wastewater Treatment Plant - 13
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Nutrient Removal Technology Assessment Case Study September 2008
(30-MGD capacity) while having a 19.3-MGD average influent flow contributed to excellent
performance.
Note, however, that this unique system required a significant amount of land for aeration and
clarification tanks; separate sludge return systems; and associated control equipment to
operate, maintain, and monitor.
The cost for capital was low at $1.73 per gpd capacity as an upgrade. The O&M costs for
phosphorus and nitrogen removal were $0.78/lb and $0.99/lb, respectively. The life-cycle
cost for nutrient removal was $1.78/lb for phosphorus and $.4.27/lb for nitrogen.
Summary
The Western Branch WWTP is an advanced facility with a unique, multiple-system
activated-sludge system followed by tertiary filtration. The facility was expanded and
upgraded to meet new requirements with the maximum use of existing technologies. The
latest upgrade included a third activated-sludge system for nitrogen removal, or DNAS.
The nitrogen removal was efficient and reliable at the mean concentration of 1.63 mg/L in
TN with a COV of 36 percent. The phosphorus removal was also efficient and reliable at the
mean concentration of 0.43 mg/L with a COV of 62 percent.
Many factors have contributed to this reliable performance. The first key factor is the three
separate processes operating in series—BOD and ammonia removal in the first two
activated-sludge systems, followed by a separate activated-sludge system to denitrify with an
independent supply of carbon. The fluctuations in wastewater and/or operating parameters
and thus performance in one stage were balanced by the succeeding processes to ensure the
overall reliability of the plant's performance. Performance was also enhanced by operating
all four treatment trains (30-MGD capacity) while the influent flow was only 19.3 MGD.
Phosphorus removal was achieved by adding chemicals to the DNAS.
Capital costs for the upgrade were low at $1.73 per gpd capacity. The O&M costs for
phosphorus and nitrogen removal were $0.78/lb and $0.99/lb, respectively, and the life-cycle
cost for nutrient removal was $1.78/lb for phosphorus and $.4.27/lb for nitrogen.
Key contributing factors for reliability include the inclusion of a separate third stage for
denitrification. The separate stage with substantial methanol feed is able to provide a high
degree of denitrification. That extra volume also provides further dampening of wastewater
fluctuations, resulting in a very consistent effluent quality.
14 - Western Branch, MD • Wastewater Treatment Plant Appendix A
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September 2008 Nutrient Removal Technology Assessment Case Study
A separate stage for denitrification not only increases capital costs for the equipment but also
necessitates the use of significant amounts of methanol to effect the needed denitrification.
Phosphorus removal costs are reasonable with the use of alum for precipitation.
A ckn o wledgments
The authors acknowledge with gratitude the significant assistance and guidance provided by
Robert Buglass, principal scientist for WSSC, and Nick Shirodkar, Plant Engineering
supervisor at the Western Branch facility. This report would not have been possible without
their prompt response with well-deserved pride in their facility and operation. EPA
acknowledges the WSSC for its participation in this case study.
References and Bibliography
TKW Online. 2007. http://www.tkwonline.com/enviromental.html. Accessed July 15, 2007.
USDA (U.S. Department of Agriculture). 2007. Price Indexes and Discount Rates. Natural
Resources Conservation Service.
http://www.economics.nrcs.usda.gov/cost/priceindexes/index.html.
Voorhees, J.R., W.G. Mendez, and E.S. Savage. 1987. Produce an AWT Effluent for Florida
Waters, Environmental Engineering Proceedings. (EE Div) ASCE, Orlando, Florida,
July 1987.
WEF (Water Environment Federation) and ASCE (American Society of Civil Engineers)
1998. Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8,
Figure 11.7, Net sludge production versus solids retention time.
Appendix A Western Branch, MD • Wastewater Treatment Plant - 15
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Attachment: Electrical Costs
HRAS/NAS blowers
Raw pumps
Denite mixers
ID fans
Final RAS pumps
Stripping channel
blowers
Centrifuge
Air lift pump blowers
Total draw
HP
1,500
250
20
75
100
200
300
60
Number
1
2
16
1
4
2
1
6
kW
Power draw
1,119
373
238.72
55.95
298.4
298.4
223.8
268.56
hours/
day
24
24
24
24
24
24
24
24
kWh
draw/day
26,856
8,952
5,729.28
1,342.8
7,161.6
7,161.6
5,371.2
6,445.44
kWh
draw/year
9,802,440
3,267,480
2,091,187.2
490,122
2,613,984
2,613,984
1,960,488
2,352,585.6
25,192,270.8
%P
0%
5%
0%
0%
0%
0%
5%
0%
%N
50%
20%
70%
10%
0%
20%
5%
50%
PkWh
0
163,374
0
0
0
0
98,024.4
0
261,398.4
N kWh
4901220
653496
1463831.04
49012.2
0
522796.8
98024.4
1176292.8
8864673.24
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September 2008 Municipal Nutrient Removal Technologies Reference Document
Appendix B: Reliability, Variability, and Coefficient of
Variation
When operating a treatment facility, the objective is to regularly produce an effluent that
meets the discharge standards specified in the permit. Such regularity can be difficult to
obtain because the measured effluent concentration of all constituents will vary. Some
variations will be due to process upsets caused by weather conditions, accidents, and
equipment failure. Others will be due to natural variations in influent conditions, as well as
natural variability in laboratory measurements, sampling, and flow. In selecting a process,
one possible criterion is finding one that has a higher probability of regularly producing a
high-quality effluent and thereby keeps the facility well within permit compliance. The
reliability reflects the overall performance of the facility in regularly meeting the treatment
objectives, exclusive of extraordinary events like process upsets. Evaluating reliability or
variability allows for screening of technologies by an assessment of how well a system might
perform daily.
The variability of a data set can be represented by the coefficient of variation (COV). The
COV is one standard deviation divided by the mean, expressed as a percentage.
Figure B-l illustrates the meaning and determination of COV. By definition, a normally
distributed population of data, such as measurements of total phosphorus in an effluent,
results in a straight line when plotted on probability paper, as shown in Figure B-l. The mean
of the data set falls at the 50 percent position, while one standard deviation from the mean
can be found at plus or minus 34 percent, or at the 84 percent and 16 percent positions
(McBean and Rovers 1998). This means that if the data are normally distributed, 68 percent
of the results will have values within one standard deviation above or below the mean value.
For the given period of evaluation, the slope of the line represents the reliability, or COV
(i.e., the steeper the slope, the less reliable the performance; conversely, the flatter the slope,
the higher the reliability). For example, Figure B-l, which shows effluent phosphorus for the
Noman M. Cole Pollution Control Plant in Fairfax County, Virginia, indicates that the COV
is 21 percent for the monthly averages for total phosphorus.
Note that the calculated reliability is a function of the data-averaging period. For the same
year, COVs can be determined for the monthly average concentrations as well as the weekly
average concentrations. In the example above, the COV of the Fairfax County facility is
higher for the weekly averages, while the mean value is practically the same—29 percent on
the weekly average, as compared to 21 percent on the monthly average. The same can be true
with the COVs on the basis of a daily maximum at 45 percent.
For the purposes of this document, COVs are primarily based on monthly averages for
consistent interpretation and easy comparison. When necessary because of the permit
Appendix B: Reliability and Coefficient of Variation B-l
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Municipal Nutrient Removal Technologies Reference Document
September 2008
requirements, however, COVs with reference to the weekly averages are added. The decision
to select a given averaging period is important and should be based on the permit conditions.
0.1
0.01
M
M
°t
C
onthly-
ean = 0.086 mg/L
d. Dev. = 0.0179 m
OV = 21%
n/l
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-S5s-
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^^
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^
^^
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Weekly-
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Std. De\
CO
V - t
O.Of
=
9%
56 mg/L
0.0248 mg/L
0.05 0.1
0.5 1
10 20 30 40 50 60 70 80 86
Percent Less Than or Equal To
92.3 95 98.1 99 99.7 99.9 99.95
Figure B-1. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia—daily frequency
curves for total phosphorus.
The overall reliability of a facility increases with the increase in the number of processes
installed in series, as shown in Figure B-2. For example, the reliability of a tertiary treatment
facility would be higher than that of a secondary treatment facility. The designer of a facility
can select multiple processes in series to increase the reliability of the entire treatment
system. For example, the following data from the Noman Cole facility show total phosphorus
concentration and COVs at each step of the treatment system:
• Secondary effluent: 0.74 mg/L at COV of 50 percent
• Tertiary clarifier effluent: 0.36 mg/L at COV of 33 percent
• Tertiary filter effluent: 0.09 mg/L at COV of 29 percent
The decision to add a particular level of reliability depends on the proposed permit limit and
the degree of safety to be incorporated.
B-2
Appendix B: Reliability and Coefficient of Variation
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September 2008
Municipal Nutrient Removal Technologies Reference Document
"5)
Primary Effluent
Mean = 4.88 mg/L
Std. Dev. = 0.55 mg/L
COV= 11%
Tertiary Effluent
Mean = 0.36 mg/L
Std. Dev. = 0.118 mg/L
COV = 33%
Secondary Effluent
Mean = 0.74 mg/L
Std. Dev. = 0.37 mg/L
COV = 50%
nal Effluent
= 0.09 mg/L
Std. Dev. = 0.025 mg/L
COV = 29%
0.01
0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80
Percent Less Than or Equal To
95
99.5 99.9 99.95
» Raw Influent • Primary Effluent A Secondary Effluent X Tertiary Effluent A Final Effluent
Figure B-2. Noman M. Cole Pollution Control Plant, Fairfax County, Virginia—weekly average
frequency curves for total phosphorus.
Reference
McBean, E.A. and F.A. Rovers 1998. Statistical Procedures for Analysis of Environmental
Monitoring Data and Risk Assessment. Prentice Hall PTR.
Appendix B: Reliability and Coefficient of Variation
B-3
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