September 2008 Municipal Nutrient Removal Technologies Reference Document 

Appendix A: Case Studies A-1

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 Cary, North Carolina (biological nitrogen and phosphorus) 

• Western Branch in Upper Marlboro, Maryland (three separate activated-sludge 

systems operated in series) 
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. 
Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 1

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 (MGD), 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 

Annual 

loading (lb)

Quarterly 

(mg/L) 

Monthly average 

(mg/L) 

Weekly average 

(mg/L) 

BOD5, 4/1–10/31 5 7.5 

BOD5, 11/1–3/31 10 15 

TSS 30 45 

Ammonia-Nitrogen, 

4/1–10/31 2 6 

Ammonia-N 

11/1–3/31 4 12 

Total phosphorus 2 1 -- 

Total nitrogen 56,200a 

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 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

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 (HRT): 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 3 Figure 1. Central Johnston County WWTP layout. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

Figure 2. Central Johnston County WWTP process schematic. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 5 

Table 3. Influent and effluent averages 

Parameter Average 

Max 

month 

Max 

month vs. 

avg. 

Max 

week 

Sample 

method/frequency 

Flow (MGD) 4.12 5.17 25% 6.2 -- 

Influent TP (mg/L) 5.8 8.5 46% 13.6 Weekly/composite 

Effluent TP (mg/L) 0.26 0.64 140% 1.01 Weekly/composite 

Influent BOD (mg/L) 320 386 20% 497 Daily/composite 

Effluent BOD (mg/L) 3 4.59 32% 5.2 Daily/composite 

Influent TSS (mg/L) 328 419 27% 564 Daily/composite 

Effluent TSS (mg/L) 1.21 1.47 13% 1.8 Daily/composite 

Influent NH4-N (mg/L) 28 34.4 27% 37.4 Daily/composite 

Effluent NH4-N (mg/L) 0.44 0.54 22% 0.86 Daily/composite 

Influent TN (mg/L) 31.2 42.7 37% 63.1 Daily/composite 

Effluent TN (mg/L) 2.14 2.77 30% 3.13 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 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

Table 4. Monthly averages for plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(d) 

HRT 

(hr) 

Temperature 

(°C) 

Oct 2005 2,527 8.1 23.1 23 

Nov 2005 2,445 7.9 13.9 19 

Dec 2005 2,650 8.5 15.9 17 

Jan 2006 2,686 8.6 15.1 16 

Feb 2006 2,452 7.9 16.4 14 

Mar 2006 2,643 8.5 23.6 16 

Apr 2006 2,679 8.6 27.9 18 

May 2006 2,417 7.8 23.5 20 

June 2006 2,300 7.4 19.4 24 

July 2006 2,378 7.6 23 26 

Aug 2006 2,448 7.9 25.1 27 

Sep 2006 2,574 8.3 21.6 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 7 

Johnston County, NC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 0.26 mg/L

Std. Dev. = 0.164 mg/L

C.O.V. = 62%

Figure 3. Monthly average frequency curves for TP. 

Johnston County, NC

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 50 0.5 1 2 5 302010 40 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.268 mg/L

Std. Dev. = 0.208 mg/L

C.O.V. = 78%

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

Johnston County, NC

Monthly Average Frequency Curves for Ammonia-N

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent Ammonia-N Final Effluent Ammonia-N

0.05 0.1 0.5 1 2 5 40302010 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.44 mg/L

Std. Dev. = 0.055 mg/L

C.O.V. =12%

Figure 5. Monthly average frequency curves for ammonia nitrogen. 

Johnston County, NC

Weekly Average Frequency Curves for Ammonia-N

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent Ammonia-N Final Effluent Ammonia-N

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.44 mg/L

Std. Dev. = 0105 mg/L

C.O.V. =24%

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 9 

Johnston County, NC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent TN Final Effluent TN

0.05 0.1 50403020 0.5 1 2 5 10 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 2.14 mg/L

Std. Dev. = 0.36 mg/L

C.O.V. =16%

Figure 7. Monthly average frequency curves for TN. 

Johnston County, NC

Weekly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent TN Final Effluent TN

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 2.14 mg/L

Std. Dev. = 0.48 mg/L

C.O.V. =23%

Figure 8. Weekly average frequency curve for TN. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

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 nitratenitrogen 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 11 

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. 

Cost 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

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 lb/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 lb COD/lb 

methanol, and the yield of volatile suspended solids (VSS) on methanol was assumed to be 

0.4 lb VSS/lb COD (McCarty et al. 1969). The plant generated 328 lb 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 lb 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 lb 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 lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 13 

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 twostage 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Central Johnston County, NC z Wastewater Treatment Plant Appendix A 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Central Johnston County, NC z Wastewater Treatment Plant - 15 

Attachment: Electrical Cost Calculation 

Electrical 

Anoxic/Anaerobic Mixers 

HP Number 

Power 

draw 

(kW) kWh/day

kWh 

draw/day kWh %P %N For P For N 

10 3 22.38 24 537.12 196,048.8 70 30 137234.2 58,814.64

15 1 11.19 24 268.56 98,024.4 70 30 68617.08 29,407.32

15 1 11.19 24 268.56 98,024.4 70 30 68617.08 29,407.32

Blowers 

150 2 223.8 24 5,371.2 1,960,488 30 70 588146.4 1,372,342 

100 2 149.2 24 3,580.8 1,306,992 30 70 392097.6 914,894.4 

Filter Pumps 

150 3 335.7 24 8,056.8 2,940,732 20 60 588146.4 1,764,439 

Total Draw 6,600,310 1,842,859 4,169,304 

 

Methanol 83.1 gal/day 

 1.75 cost/gal 

 145.425 cost/day 

 53,080.125 cost/yr 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 1

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) 

Annual 

average 

Monthly 

average Weekly average

Daily 

maximum 

BOD5 20 25 40 60 

TSS 20 30 45 60 

Total nitrogen 3 3 4.5 6 

Total phosphorus 0.5 0.5 0.75 1 

Notes: 

BOD = biochemical oxygen demand.; TSS = total suspended solids 

Table 2. Reuse water permit limits 

Parameter (mg/L 

unless stated) 

Annual 

average 

Monthly 

average Weekly average

Daily 

maximum 

BOD5 20 30 45 60 

TSS 5 

Residual chlorine 1 (minimum) 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A

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 lb BOD/lb MLVSS 

Waste activated sludge (WAS): 0.06 MGD, each, or 6,500 lb/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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 3 Figure 1. Treatment process flow diagram Fiesta Village Advanced Wastewater Treatment Plant. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A Figure 2. Site plan Fiesta Village Advanced Wastewater Treatment Plant. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 5

Aerobic Digestion 

- Diameter: 39 ft, 16 ft deep 

- Volume: 0.143 MG, 2 each 

- Disc diffusers 

- Loading rate: 0.01–0.02 lb 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) 

Average 

value 

Maximum 

month 

Max 

month vs. 

ave. 

Maximum 

week 

Sample 

method/frequency 

Flow (MGD) 3.16 4.14 31% 4.26 

Influent TP 3.85 4.58 18% -- Monthly/composite 

Effluent TP 0.102 0.19 85% 0.39 Daily/composite 

Influent BOD 134 167 24% 179 Daily/composite 

Effluent BOD 1.37 2.95 116% 5.2 Daily/composite 

Influent TSS 199 261 31% 348 Daily/composite 

Effluent TSS 0.72 1.17 61% 1.48 Daily/composite 

Influent NH4-N 27.2 34.5 27% -- Monthly/composite 

Effluent NH4-N 0.13 0.2 50% 0.28 Daily/composite 

Secondary 

Effluent NO3-N 

2.9a

 3.0a 7% 3.9a Daily/composite 

Influent TN 33.2 50.6 53% -- Monthly/composite 

Effluent TN 1.71 2.61 53% 3.90 Daily/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 

a

 Jan–April 2007 

Table 4 presents plant monthly averages for the process parameters, as available. 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A 

Table 4. Monthly averages for plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(d) 

HRT 

(hr) 

Temperature 

(°C) 

Jan 2006 3,578 37 48 -- 

Feb 2006 3,807 39 44 -- 

Mar 2006 4,085 35 46 -- 

Apr 2006 3,845 24 50 -- 

May 2006 3,510 33 55 -- 

June 2006 3,564 28 47 -- 

July 2006 3,571 32 35 30.4 

Aug 2006 3,480 36 44 -- 

Sept 2006 3,495 34 39 -- 

Oct 2006 3,509 37 49 -- 

Nov 2006 3,775 59 49 -- 

Dec 2006 4,204 41 50 -- 

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. 

Fiesta Village WWTP, Lee Co., FL

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 5040 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.102 mg/L

Std. Dev. = 0.035 mg/L

C.O.V. = 35%

Figure 4. Monthly average frequency curves for TP. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 7

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. 

Fiesta Village WWTP Lee Co., FL

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 60 70 80 90 95 98 0.1 50403020 0.5 1 2 5 10 99 99.5 99.9599.9

Mean = 0.134mg/L

Std. Dev. = 0.054 mg/L

C.O.V. = 40%

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. 

Fiesta Village WWTP Lee Co., FL

Monthly Average Frequency Curves for Nitrogen

0.100

1.000

10.000

100.000

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Total N Final Effluent - Total N

Secondary Effluent NO3-N

0.05 0.1 0.5 1 2 5 10 3020 40 50 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 1.71 mg/L

Std. Dev. = 0.48 mg/L

C.O.V. = 28%

Figure 6. Monthly average frequency curves for nitrogen. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A 

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 lb per day. The methanol–to–nitrate-nitrogen ratio averaged 

1.9 pounds of methanol per pound of nitrate present, or 2.4 lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 9

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 

Year Original cost 2007 cost %P %N %other P cost N cost 

Phase 1 1984 $6,505,833 $12,462,452 2% 50% 48% $249,249 $6,231,226

Denite Filter 1984 $930,059 $1,781,604 12% 88% 0% $213,792 $1,567,811

Controls 1984 $441,323 $845,390 2% 50% 48% $16,908 $422,695

Phase 2 1986 $1,200,000 $2,218,952 0% 50% 50% $0 $1,109,476

Phase 3 2002 $6,800,000 $8,260,263 0% 50% 50% $0 $4,130,132

TOTAL $25,568,661 -- -- -- $479,949 $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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A 

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 lb. 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 MGD, this was 677 lb 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 lb/yr of methanol has a chemical oxygen demand (COD) of 1.5 lb COD/lb 

methanol, or 70,750 lb COD/yr. The typical yield of volatile suspended solids (VSS) on 

methanol is 0.4 lb VSS/lb COD, giving 28,300 lb sludge/yr, or 14.2 tons sludge/yr from 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 11

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 lb 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 MGD), 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 lb 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Lee County, FL z Advanced Wastewater Treatment Plant Appendix A 

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 lb per pound of nitrate applied, 

compared to 3 lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Lee County, FL z Advanced Wastewater Treatment Plant - 13

Attachment: Electrical and Chemical Costs 

Electrical for P for N

 

kW kWh kWh %P %N 

Hp Number Power draw hours/day draw/day draw/year 

Aerator 

60 8 358.08 24 8593.92 3136781 2 50 62735.6 1568390.4

RAS pump 

30 3 67.14 24 1611.36 588146.4 0 50 0 294073.2

WAS pump 

7.5 2 11.19 24 268.56 98024.4 0 50 0 49012.2

Total draw 3822952 62735.6 1911475.8

 

Alum cost $115,338 

% for P removal 30 

Alum cost for P $34,616 

Methanol cost $12,735 (all for N 

removal 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 1 

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 (o

F) in the summer to –30 oF 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 

Flow 

(MGD) 

BOD5 

(mg/L) 

TSS 

(mg/L) 

TKN 

(mg/L) 

TP 

(mg/L) 

2000 2.5 216 259 25 4.5-6.5 

2008 3.0 216 260 25 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 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

The National Pollutant Discharge Elimination System (NPDES) permit limits for the plant 

are shown in Table 2. 

Table 2. NPDES permit limits 

Parameter 

7-day average 

(mg/L) 

30-day average 

(mg/L) 

BOD5 15 10 

TSS 15 10 

Total P -- 1.0 

Ammonia nitrogen -- 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 (HP) per 1,000 gallons. The solids 

retention time (SRT) is designed to be 4 to 5 days. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 3 

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Kalispell, MT z Advanced Wastewater Treatment Appendix A Figure 1. Kalispell’s advanced WWTP. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 5 Figure 2. Kalispell biological reactor detail. 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

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) Average 

Maximum 

month 

Max 

month vs. 

avg. 

Maximum 

week 

Sample 

method/frequency 

Flow (MGD) 2.95 3.45 17% 4.04 -- 

Influent TP 4.11 4.88 19% 5.2 Composite/weekly 

Effluent TP 0.12 0.15 25% 0.31 Composite/weekly 

Influent BOD 226.36 282 25% 428 Composite/weekly 

Effluent BOD < 4 < 4 0% 5.8 Composite/weekly 

Influent TSS 225.17 326 45% 680 Composite/weekly 

Effluent TSS 1.21 2.9 140% 4.1 Composite/weekly 

Influent NH4-N 24.35 29.4 21% -- Grab/monthly 

Effluent NH4-N < 0.07 < 0.07 0% -- Grab/monthly 

Influent TKN 39.28 47 20% -- Grab/monthly 

Effluent TKN 0.63 1.26 100% -- Grab/monthly 

Influent TN 39.6 48.0 21% -- Grab/monthly 

Effluent TN 10.6 19.9 86% -- 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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 7

Table 4 presents plant monthly averages for process parameters. 

Table 4. Monthly averages for plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(d) 

HRT 

(hrs) 

Water temp 

(°C) 

July 2005 2,586 10 13 18.9 

Aug. 2005 2,517 8 14 20 

Sept 2005 2,625 11 13 18.6 

Oct 2005 2,659 12 14 17.1 

Nov 2005 2,637 10 15 14.9 

Dec 2005 2,808 11 15 12.1 

Jan 2006 2,744 10 12 11.4 

Feb 2006 2,757 9 13 10.8 

Mar 2006 2,657 9 14 10.9 

Apr 2006 2,568 9 11 12.3 

May 2006 2,536 9 13 14.8 

June 2006 2,529 9 11 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

0.01

0.1

1

10

100

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0.121 mg/L

Std. Dev. = 0.023 mg/L

C.O.V. = 19%

Kalispell, MT

Monthly Average Frequency Curves for Total Phosphorus

Percent Less Than or Equal To

Total Phosphorus, mg/L

Figure 3. Monthly average frequency curves for TP. 

Kalispell, MT

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.10

1.00

10.00

100.00

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 10 3020 5040 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.121 mg/L

Std. Dev. = 0.050 mg/L

C.O.V. = 41%

Figure 4. Weekly average frequency curves for TP. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 9

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

0.0

0.1

1.0

10.0

100.0

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 40302010 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean < 0.07 mg/L

Std. Dev. = 0.00 mg/L

C.O.V. = 0%

Figure 5. Monthly average frequency curves for ammonia nitrogen. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

Figure 6 presents the reliability plot for monthly TN. The plant personnel set a design goal of 

900 lb/day as TN (36 mg/L at 3 million gallons per day [MGD]), 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)

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 10 403020 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 10.6 mg/L

Std. Dev. = 3.31 mg/L

C.O.V. = 31%

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 HRT of 7 

to 21 hours. Unique design allows separate control of the SRT and HRT 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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 11

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 13

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 lb 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 lb 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

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. 

Acknowledgments 

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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kalispell, MT z Advanced Wastewater Treatment - 15

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

16 - Kalispell, MT z Advanced Wastewater Treatment Appendix A

Attachment 1: Capital Costs 

 %P %N %BOD $P $N $BOD 

Tanks $1,300,000 12% 48% 40% $156,000 $624,000 $520,000

Tank coats $75,000 12% 48% 40% $9,000 $36,000 $30,000

SCADA $1,000,000 12% 48% 40% $120,000 $480,000 $400,000

Mixers $43,000 0% 50% 50% $0 $21,500 $21,500

Ret/Sup pumps $175,000 12% 48% 40% $21,000 $84,000 $70,000

Blowers $155,000 0% 50% 50% $0 $77,500 $77,500

Fermenter $45,000 100% 0% 0% $45,000 $0 $0

Thickener $35,000 100% 0% 0% $35,000 $0 $0

Primary sludge pump $80,000 10% 50% 40% $8,000 $40,000 $32,000

Piping $500,000 12% 48% 40% $60,000 $240,000 $200,000

Site work $800,000 12% 48% 40% $96,000 $384,000 $320,000

Total $4,208,000 $550,000 $1,987,000 $1,671,000

 

Indirects $1,505,526 12% 48% 40% $180,663 $722,653 $602,211

Total capital $5,713,526 $730,663 $2,709,653 $2,273,211

 

Updated to 2007 $9,102,673 $1,164,078 $4,316,963 $3,621,633

Annualized $101,508 $376,439 $315,806

Updating factors 

1992 ENR CCI 4,985 

May 2007 ENR CCI 7,942 

A/P (6%, 20 years) 0.0872 
Appendix A Kalispell, MT z Advanced Wastewater Treatment - 17 Attachment 2: Electrical Costs kW kWh kWh %P %N 

Horsepower Volts Amps VA Number Power draw hours/day draw/day draw/year Mixers 3 460 4 1,840 5 9.2 24 220.8 80,592 12% 48% 

7.5 460 10 4,600 4 18.4 24 441.6 161,184 12% 48% 

Ret Pumps 10 460 5.82 2,677.2 1 2.6772 24 64.2528 23,452.27 12% 48% 

4 460 6.7 3,082 1 3.082 24 73.968 26,998.32 12% 48% 

Blowers 200 460 220 101,200 2 202.4 24 4,857.6 1,773,024 0% 50% Super Pumps 7.5 460 9.7 4,462 2 8.924 24 214.176 78,174.24 12% 48% Fermenter 5 460 6.8 3,128 2 6.256 24 150.144 54,802.56 100% 0% 

15 460 27 12,420 2 24.84 24 596.16 217,598.4 100% 0% 10 460 14 6,440 1 6.44 24 154.56 56,414.4 100% 0% 

Gravity Thickener 2 460 3.1 1,426 1 1.426 24 34.224 12,491.76 100% 0% Primary Clarifier Sludge Pump 5 460 6.8 3,128 1 3.128 24 75.072 27,401.28 12% 48% 

kWh/yr 2,512,133 

Rate 0.045 $/kWh 

Totals 113,046 $/yr 

Appendix A Clark County, NV z Water Reclamation Facility - 1

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 

30-day avg. 

(mg/L) 

7-day avg. 

(mg/L) 

30-day avg. 

(lb/day) 

Daily wasteload 

allocation (lb/day) 

BOD 30 45 37,530 -- 

TSS 30 45 37,530 -- 

TP -- -- -- 173 

Total NH4-N -- -- -- 502 

Notes: 

BOD = biochemical oxygen demand 

NH4-N = ammonia measured as nitrogen 

P = phosphorus 

TSS = total suspended solids 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Clark County, NV z Water Reclamation Facility Appendix A 

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 

MGD. 

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 MGD 

 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 3

Figure 1. Plant flow schematic. 

Figure 2. Tertiary processes. 

Floc 

Basin 

Filters

(no air scour) 

U.V.

EQ

Basin 

Filters

(with air scour) 

U.V. Discharge to 

west side of Las 

Vegas Wash 

Discharge to 

east side of Las 

Vegas Wash 

Backwash

Excess Flows

AWT 

Alum 

8-13 mg/L 

To Headworks

Secondary Sludge

Effluent 

Alum 

4-10 mg/L 

CENTRAL 

PLANT 

To Headworks Sludge

Tertiary Clarifiers

Gravity Thickener

Backwash Clarifier

Primary 

Treatment 

CPT Tertiary 

Facility 

AWT Tertiary 

Facility 

GT DAFT 

Dewatering 

BPR 

Activated 

Sludge 

Liquid 

Solid 

Recycle 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Clark County, NV z Water Reclamation Facility Appendix A 

Figure 3. Clark County WRF CP flowsheet schematic. 

Influent Flow 110 MGD 

Influent Suspended Solids 299 mg/L 

Influent Biochemical Oxygen Demand 294 mg/L 

Bar 

Screen 

Holding 

Bin 

Screenings 

Grit Removal 

Tank Grit 

To 

Landfill 

Primary 

Settling Tanks 

Ferric Chloride 

Polymer 

Aeration 

Basins 

Thickened 

Sludge Holding 

Tank 

Forced 

Return 

Activated 

Sludge 

Dissolved Air 

Flotation 

Thickener 

Ferric Chloride 

Polymer 

Waste 

Activated 

Sludge 

Dual Media 

Filters 

Secondary 

Settling Tanks 

Sludge 

Cake

Filter 

Press 

To 

Landfill 

Discharge to Las Vegas Wash and Lake Mead Wetlands 

To Reclaimed 

Water Customers 

Sodium 

Hypochlorite 

Ultraviolet 

Light Disinfection 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 5

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 Average 

Max 

month 

Max month 

vs. avg. 

Max 

week 

Sample method/ 

frequency 

Flow (MGD) 98 101.4 3.5% 102.3 -- 

Influent TP (mg/L) 5.8 7.0 20% 7.5 Daily/composite 

Effluent TP (mg/L) 0.1 0.17 73% 0.41 Daily/composite 

Influent BOD (mg/L) 357 390 9% 445 Daily/composite 

Effluent BOD (mg/L) < 2 4.75 137% 7 Daily/composite 

Influent TSS (mg/L) 366 413 13% 456 Daily/composite 

Effluent TSS (mg/L) < 5 10 100% 21 Daily/composite 

Influent NH4-N (mg/L) 26.8 28.8 7% 30 Daily/composite 

Effluent NH4-N (mg/L) 0.08 0.31 300% 1.22 Daily/composite 

Influent TKN (mg/L) 46 53 14% 75 Daily/composite 

Effluent TKN (mg/L) 0.69 1.02 47% 2.3 Daily/composite 

Influent NO3/NO2 (mg/L) 0.18 0.46 155% 0.8 Daily/composite 

Effluent NO3/NO2 (mg/L) 15.3 16.4 7% 16.5 Daily/composite 

Influent TN (mg/L) 30.3 34.5 14% 37.6 -- 

Effluent TN (mg/L) 15.2 16.6 7% 16.7 -- 

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 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Clark County, NV z Water Reclamation Facility Appendix A 

Table 3 presents plant monthly average plant process parameters. 

Table 3. CP monthly average plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(d) 

HRT 

(hr) 

Temperature 

(°C) 

Jan 2006 2,902 9 5.43 20 

Feb 2006 3,422 9 5.45 20 

Mar 2006 3,684 8 5.31 24 

Apr 2006 3,732 7 5.10 26 

May 2006 3,147 6 5.06 28 

June 2006 3,499 5 5.82 29 

July 2006 3,166 5 5.73 29 

Aug 2006 3,057 5 5.80 29 

Sept 2006 2,425 6 6.32 28 

Oct 2006 2,441 7 6.31 26 

Nov 2006 2,760 8 6.42 24 

Dec 2006 2,535 8 6.49 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 7

Clark Co. NV Water Reclamation Plant

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 60 70 80 90 95 98 99 99.5 99.9599.9 0.1 5040302010 0.5 1 2 5

CP Mean = 0.1 mg/L

Std. Dev. = 0.042 mg/L

COV = 43%

Figure 4. Weekly average frequency curves for TP. 

Clark Co. Water Reclamation Plant - Las Vegas, NV

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

0.05 0.1 0.5 1 2 5 10 20 504030 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0.0995 mg/L

Std. Dev. = 0.0297 mg/L

COV = 30%

Figure 5. Monthly average frequency curves for TP. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Clark County, NV z Water Reclamation Facility Appendix A 

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

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 20 30 5040 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 15.2mg/L

Std. Dev. = 0.67 mg/L

COV = 4%

Figure 6. Weekly average frequency curves for nitrogen. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 9

Clark Co. Water Reclamation Facility

Monthly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 60 70 80 90 95 98 0.1 50403020 0.5 1 2 5 10 9999.5 99.9599.9

Mean =15.2 mg/L

Std. Dev. = 0.6 mg/L

COV = 4%

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Clark County, NV z Water Reclamation Facility Appendix A 

Clark Co. Nevada Water Reclamation Facility

Weekly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 60 70 80 90 95 98 0.1 0.5 1 2 5 10 50403020 99 99.5 99.9 99.95

Mean = 0.049 mg/L

Std. Dev. = 0.023 mg/L

COV =49%

Figure 8. Weekly average frequency curves for ammonia nitrogen. 

Clark Co. Water Reclamation Facility

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9599.9

CP Mean =0.048 mg/L

Std. Dev. = 0.011 mg/L

COV = 22%

Figure 9. Monthly average frequency curves for ammonia nitrogen. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 11

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Clark County, NV z Water Reclamation Facility Appendix A 

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 MGD) and WAS sludge (1.15 MGD) 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 13

Alternative Processes Considered 

Because the plant is almost at capacity (100 MGD versus 110 MGD), 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. 

Costs 

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 Year Amount Updated cost %P %N P removal N removal 

AWT Des 1982 $2,800,000 $5,956,103 50% 0% $2,978,051 $0

AWT Const 1982 $28,000,000 $58,137,516 50% 0% $29,068,758 $0

CP Expan Design 1994 $2,000,000 $2,770,875 12% 48% $332,505 $1,330,020

CP Expan Const 1994 $29,000,000 $42,588,388 12% 48% $5,110,607 $20,442,426

CP Filters Design 2002 $4,200,000 $4,794,056 50% 0% $2,397,028 $0

CP Filters Const 2002 $27,600,000 $33,526,950 50% 0% $16,763,475 $0

Central Plant S. 

Sec. Design 

2003 $3,790,000 $4,230,603 12% 48% $507,672 $2,030,689

Central Plant S. 

Sec. Const 

2003 $39,,304,293 $46,625,048 12% 48% $5,595,006 $22,380,023

Central Plant S. 

Sec. Design 

2005 $1,901,098 $1,998,417 12% 48% $239,810 $959,240

Central Plant S. 

Sec. Const 

2005 $19,218,993 $20,499,227 12% 48% $2,459,907 $9,839,629

TOTAL $157,814,384 $221,000,000 -- -- $65,452,819 $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. 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Clark County, NV z Water Reclamation Facility Appendix A 

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 Total op. costs % for P % for N P O&M N O&M 

Primary $1,877,685 12% 48% $225,322 $901,289 

Secondary $5,829,302 12% 48% $699,516 $2,798,065 

Tertiary $3,967,135 12% 0% $476,056 $0 

Solids dewatering $3,957,135 50% 0% $1,450,019 $0 

Lab $1,529,827 10% 10% $152,983 $152,983 

Other $3,875,144 0% 0% $0 $ 0 

TOTAL $19,979,131 -- -- $3,003,896 $3,852,337 

Unit Costs for Nitrogen and Phosphorus Removal 

In 2006 the plant removed 1,663,000 lb 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 lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Clark County, NV z Water Reclamation Facility - 15

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 
Nutrient Removal Technology Assessment Case Study September 2008 

16 - Clark County, NV z Water Reclamation Facility Appendix A 

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–

November 2, 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. 
Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 1

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. 

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Table 1. Permit requirements for effluent quality 

MOE permit requirements 

Daily limits 

(mg/L) 

BOD5-total 8 

TSS 7 

TN 6 

TP 

 Maximum 

 99th percentile 

 90th percentile 

 Annual average (added in 1988) 

 2.0 

 1.5 

 1.0 

 0.25 

Table 2. Influent and effluent averages 

Parameter 

(mg/L unless 

stated) Average 

Maximum 

month 

Max month 

vs. avg. 

Maximum 

week 

Sample 

method/frequency 

Flow (MGD) 8.5 8.8 3.4% 8.9 -- 

Influent TP 6.0 7.4 23% 9.1 Composite/weekly 

Effluent TP 0.14 0.20 42% 0.25 Composite/weekly 

Influent COD 626 747 19% 910 Composite/weekly 

Effluent COD 32 36 10% 38 Composite/weekly 

Effluent BOD 2.5 3.8 48% 5.7 Composite/weekly 

Influent TSS 389 472 21% 532 Composite/weekly 

Effluent TSS 1.2 1.6 42% 2.4 Composite/weekly 

Influent NH4-N 21.3 23.1 8.3% 27.6 Grab/monthly 

Effluent NH4-N 0.57 1.0 76% 1.13 Grab/monthly 

Influent TKN 28.8 33 14% 38.4 Grab/monthly 

Effluent TKN 2.0 2.98 49% 3.5 Grab/monthly 

Influent TN 28.8 33 14% 38.4 Grab/monthly 

Effluent TN 4.38 4.9 12% 5.84 Grab/monthly 

Notes: 

BOD = biochemical oxygen demand 

COD = chemical oxygen demand 

Max month vs. average = (max month – average) / average x 100 

MGD = million gallons per day 

NH4-N = ammonia measured as nitrogen 

TN = total nitrogen 

TP = total phosphorus 

TSS = total suspended solids 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 3

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. 

Figure 2. The Westbank three-stage process. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

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

Figure 3. Kelowna WWTP 2005 configuration. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 5

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 fivestage 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 7

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 

HRT 

(hr) 

SRT 

(days) 

Temp 

(°C) 

MLSS 

(mg/L) 

Bioreactor 

in service 

Jan 2005 11.1 8.9 13.1 2,562 84% 

Feb 2005 11.1 8.8 13.0 2,761 84% 

Mar 2005 11.4 8.2 13.8 2,803 84% 

Apr 2005 11.6 8.1 15.6 2,486 84% 

May 2005 11.3 8.0 18.1 2,238 84% 

Jun 2005 10.9 6.7 19.4 2,414 84% 

Jul 2005 10.9 6.0 21.1 2,301 84% 

Aug 2005 10.8 5.8 22.0 1,992 84% 

Sept 2005 10.9 6.0 20.9 1,901 84% 

Oct 2005 11.1 7.0 19.3 2,142 84% 

Nov 2005 11.5 7.4 16.8 2,451 84% 

Dec 2005 11.5 7.5 14.3 2,899 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 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

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 

Alum 

(lb/d) 

6/29/2005 500 

6/30/2005 500 

12/20/2005 150 

12/21/2005 150 

12/21/2005 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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 9

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 Unit Value Metal/Cation Unit Value 

Aluminum μg/g 6,914 Manganese μg/g 96.5 

Antimony μg/g 1.7 Mercury μg/g 0.90 

Arsenic μg/g 1.9 Molybdenum μg/g 6.88 

Barium μg/g 236 Nickel μg/g 16.47 

Beryllium μg/g 0.11 Phosphorus % 3.9 

Bismuth μg/g 12.27 Potassium % 1.54 

Cadmium μg/g 1.40 Selenium μg/g 4.34 

Calcium % 1.16 Silver μg/g 11.07 

Chromium μg/g 17.77 Sodium μg/g 2,446 

Cobalt μg/g 3.41 Strontium μg/g 122.8 

Copper μg/g 768 Thallium μg/g 0.309 

Iron μg/g 4,085 Tin μg/g 3.78 

Lead μg/g 16.85 Vanadium μg/g 7.18 

Lithium μg/g 2.37 Zinc μg/g 288 

Magnesium % 1.08 Zirconium μg/g 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Table 6. Fermenter supernatant return to anaerobic zone 

Month 

Flow 

(mL/d) 

SRT 

(d) 

Solids 

(%) 

Ammonia

(mg/L) 

Soluble 

phosphorus

(mg/L) 

Soluble

COD 

(mg/L) 

Suspended 

solids 

(mg/L) 

Total 

VFA 

(mg/L)

VFA 

(kg/d)

Jan 2005 1.56 5.3 6.9% 18.7 5.62 358 142 116 181 

Feb 2005 1.56 5.5 6.5% 18.1 6.39 407 170 131 204 

Mar 2005 1.56 5.4 6.6% 18.6 7.06 427 169 140 218 

Apr 2005 1.55 5.6 5.6% 19.5 7.68 538 180 196 305 

May 2005 1.55 4.7 6.4% 15.8 7.61 632 166 225 351 

Jun 2005 1.55 3.2 5.7% 15.7 6.95 583 190 236 368 

Jul 2005 1.55 3.4 6.2% 14.4 7.85 640 212 254 393 

Aug 2005 1.55 2.7 6.7% 16.4 7.16 611 208 242 375 

Sept 2005 1.55 2.7 6.5% 16.5 6.82 575 227 229 355 

Oct 2005 1.55 3.2 5.8% 18.6 7.43 582 232 222 344 

Nov 2005 1.55 4.6 5.5% 19.3 7.92 603 198 216 334 

Dec 2005 1.55 5.8 5.6% 20.5 7.85 614 260 227 351 

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 

Acetic 

(mg/L) 

Propionic 

(mg/L) 

Isobutyric 

(mg/L) 

Butyric 

(mg/L) 

Isovaleric 

(mg/L) 

Valeric 

(mg/L) 

Jan 2005 55.5 37.0 2.4 9.9 1.9 2.4 

Feb 2005 65 26.1 2.2 9.7 2.1 2.6 

Mar 2005 109 26.2 1.9 9 3.1 3.3 

Apr 2005 154 57.8 1 26.8 1 9.9 

May 2005 137 123 1.7 26.5 1.9 12.5 

Jun 2005 121 64.5 2.6 21.2 1.1 6.3 

Jul 2005 178 155 1.7 32 2 18.3 

Aug 2005 209 105 3.8 24.9 3.4 9.3 

Sept 2005 124 104 4.8 16.8 3.7 7.4 

Oct 2005 165 105 1.9 2.7 1.7 9 

Nov 2005 97 122 1 27.3 1 10 

Dec 2005 122 130 3 33.3 2.7 14.6 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 11

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 Average 315 kg/d 

• 50 percent of primary effluent Average 252 kg/d 

Table 8. Other VFA sources 

Date 

Centrifuge 

centrate 

(mg/L) 

Primary 

effluent 

(mg/L) 

Influent 

sewage 

(mg/L) 

Flow 

rate 

VFA 

(kg/d) 

May 8, 2007 401 est. 130 m3/d 52 

May 10, 2007 285 est. 130 m3/d 37 

May 15, 2007 415 est. 130 m3/d 54 

June 8, 2006 281 est. 130 m3/d 37 

June 15, 2006 215 est. 130 m3/d 28 

June 22, 2006 281 est. 130 m3/d 37 

May 8, 2007 15 est. 36 ML/d 540 

May 10, 2007 20 est. 36 ML/d 720 

May 8, 2007 8 est. 32 ML/d 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Table 9. Centrifuge centrate return to plant influent 

Flow 

Flow 

(m3/d) 

Ammonia 

(mg/L) 

TP 

(mg/L) 

Soluble P 

(mg/L) 

TKN 

(mg/L) 

Soluble COD 

(mg/L) 

TSS 

(mg/L) 

Jan 2005 133.7 14.3 118 11 43 318 1,105 

Feb 2005 113.6 15.3 70 23 46 376 1,115 

Mar 2005 124.5 15.3 160 55 68 452 861 

Apr 2005 117.3 17.5 91 47 54 522 1,045 

May 2005 118.7 16.7 225 173 63 827 270 

Jun 2005 128.8 23.1 235 159 54 667 320 

Jul 2005 143.6 28.5 165 161 60 783 1,001 

Aug 2005 124.5 22.4 200 164 55 726 520 

Sept 2005 118.7 23.5 235 148 61 599 1,135 

Oct 2005 128.5 17.6 200 96 95 632 1,084 

Nov 2005 123.8 20.9 173 118 66 779 854 

Dec 2005 135.3 21.4 170 84 95 593 939 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 13

Table 10. Nitrogen removal 

Month 

Influent 

flow 

(ML/d) 

Influent 

TKN 

(mg/L) 

Effluent 

TN 

(mg/L) 

Nitrogen 

removal 

(%) 

Effluent 

nitrates 

(mg/L) 

Effluent 

ammonia 

(mg/L) 

Jan 2005 33.2 30.6 4.64 84.8 2.10 0.85 

Feb 2005 32.1 30.5 4.90 83.9 1.93 1.01 

Mar 2005 31.5 27.0 4.40 83.7 2.20 0.51 

Apr 2005 30.8 32.5 4.49 86.1 2.65 0.48 

May 2005 32.3 24.0 4.12 81.3 2.21 0.51 

Jun 2005 32.8 24.7 3.21 87.0 1.99 0.07 

Jul 2005 33.0 27.0 3.53 86.9 2.08 0.44 

Aug 2005 33.5 33.0 4.39 86.6 2.53 0.40 

Sept 2005 33.4 27.5 4.45 83.8 2.80 0.52 

Oct 2005 32.3 27.8 4.89 82.4 2.67 0.50 

Nov 2005 31.2 30.7 4.66 84.8 2.45 0.67 

Dec 2005 31.9 31.1 4.78 84.6 2.18 0.96 

Table 11. Nitrate profile—annual average of grab samples taken at 8:00 a.m. (mg/L) 

Anaerobic 

zone 

End 

Anoxic 

25% 

aerobic 

50% 

Aerobic 

End 

aerobic 

Secondary 

clarifier 

Return 

sludge 

Filter 

effluent 

0.02 0.2 1.1 1.9 2.8 2.5 0.13 2.6 

Table 12. Ammonia profile—annual average of grab samples taken at 8:00 a.m. (mg/L) 

Primary 

effluent 

Anaerobic 

Zone 

End 

anoxic 

25% 

aerobic

50% 

aerobic 

End 

aerobic

Secondary 

clarifier 

Return 

sludge 

Filter 

effluent 

19.44 9.6 3.19 2.12 1.31 0.05 0.29 0.26 0.23 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Figures 4 and 5 show monthly frequency curves for effluent TN and ammonia. 

Kelowna, BC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent TN Final Effluent TN

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 4.38 mg/L

Std. Dev. = 0.51 mg/L

C.O.V. =12%

Figure 4. Monthly frequency curves for effluent TN. 

Figure 5. Monthly frequency curves effluent ammonia. 

Kelowna, BC

Monthly Average Frequency Curves for Ammonia-N

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent Ammonia-N Final Effluent Ammonia-N

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.575 mg/L

Std. Dev. = 0.26 mg/L

C.O.V. =45%
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 15

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 

Influent flow 

(ML/d) 

Influent 

TP 

(mg/L) 

Effluent 

TP 

(mg/L) 

Phosphorus 

removal 

(%) 

Effluent 

soluble P 

(mg/L) 

Jan 2005 33.2 5.9 0.13 97.8% 0.04 

Feb 2005 32.1 5.95 0.16 97.3% 0.04 

Mar 2005 31.5 5.5 0.16 97.1% 0.04 

Apr 2005 30.8 7.35 0.13 98.2% 0.04 

May 2005 32.3 5.67 0.19 96.6% 0.05 

Jun 2005 32.8 5.4 0.11 97.9% 0.03 

Jul 2005 33.0 6.05 0.12 98.0% 0.03 

Aug 2005 33.5 6.1 0.10 98.3% 0.03 

Sept 2005 33.4 6.3 0.10 98.4% 0.02 

Oct 2005 32.3 6.35 0.12 98.1% 0.02 

Nov 2005 31.2 5.03 0.13 97.4% 0.02 

Dec 2005 31.9 6.15 0.21 96.5% 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 

Anaerobic 

zone 

End 

anoxic 

25% 

aerobic 

cell 

50% 

aerobic 

End 

aerobic 

Secondary 

clarifier 

Return 

sludge 

Filter 

effluent 

4.26 14.9 2.54 0.18 0.01 0.01 0.02 1.91 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

16 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Kelowna, BC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.999.95

Mean = 0.139 mg/L

Std. Dev. = 0.03 mg/L

C.O.V. = 21%

Figure 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 17

• 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

18 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

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 a fermenter) 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$). 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 19

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). 
Nutrient Removal Technology Assessment Case Study September 2008 

20 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Table 16. Average Canadian to U.S. dollar 

exchange rate value 

Year 1 Canadian $ = × U.S. $ 

1980 0.86 

1992 0.83 

1994 0.73 

2007 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 CDN$ US$ 

US$ present 

worth 

% 

other %P %N Phosphorus Nitrogen 

1980 $12,500,000 $10,750,000 $26,375,193 40% 12% 48% $3,165,023 $12,660,093

1992 $6,200,000 $5,146,000 $8,198,502 40% 12% 48% $983,820 $3,935,281

1994 $20,750,000 $15,147,500 $22,245,090 40% 12% 48% $2,669,411 $10,677,643

Totals $39,450,000 $31,043,500 $56,818,785 $6,818,254 $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 MGD 

(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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 21

(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 lb 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 lb 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 
Nutrient Removal Technology Assessment Case Study September 2008 

22 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

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. 

Acknowledgments 

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, and P.J. 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, and B. Rabinowitz. 1997. Optimizing Biological Nutrient 

Removal in Anoxic Zones. Australia BNR 3 Conference, 1997. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 23

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., and W.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. 
Nutrient Removal Technology Assessment Case Study September 2008 

24 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Attachment 1: Design Basis 

Design Flows and Loads 1980 

Bardenpho 

Design value 

 Units upgrade Stage 1 

Flow Data Phase 1 Phase 2 Phase 3 

 Sewered Population per 56,000 64,000 95,000 

 Flow per Capita L/c.d 400 400 400 

 Base Infiltration ML/d 1.0 1.0 1.0 

 BCTWP Industrial Effluent ML/d 1.0 

 Average Daily Flow ML/d 22.5 27.5 40.0 

 Maximum Month Flow ML/d 25.1 31.0 44.0 

 Maximum Daily Flow ML/d 28.7 35.5 50.0 

 Peak Hourly Flow ML/d 34.8 43.0 69.0 

BOD, TSS, TKN, TP Loads 

 BOD 

 Average Daily Unit Load kg/c.d 0.080 0.080 0.080 

 Allowance for BCTWP kg/d 200 

 Average Daily Total kg/d 4,480 5,120 7,800 

 Maximum Month Unit Load kg/c.d 0.095 0.095 0.095 

 Maximum Month Total kg/d 5,320 6,080 9,225 

 Maximum Week Unit Load kg/c.d 0.105 0.105 0.105 

 Maximum Week Total kg/d 5,880 6,720 10,175 

 TSS 

 Average Daily Unit Load kg/c.d 0.080 0.080 0.080 

 Allowance for BCTWP kg/d 20 

 Average Daily Total kg/d 4,480 5,120 7,620 

 Maximum Month Unit Load kg/c.d 0.100 0.100 0.100 

 Maximum Month Total kg/d 5,600 6,400 9,520 

 Maximum Week Unit Load kg/c.d 0.120 0.120 0.120 

 Maximum Week Total kg/d 6,720 7,680 11,420 

 TKN 

 Average Daily Unit Load kg/c.d 0.015 0.015 0.015 

 Allowance for BCTWP kg/d 10 

 Average Daily Total kg/d 840 960 1,435 

 Maximum Month Unit Load kg/c.d 0.017 0.017 0.017 

 Maximum Month Total kg/d 952 1,090 1,625 

 Maximum Week Unit Load kg/c.d 0.019 0.019 0.019 

 Maximum Week Total kg/d 1,064 1,215 1,815 

 TP 

 Average Daily Unit Load kg/c.d 0.003 0.003 0.003 

 Allowance for BCTWP kg/d 5 

 Average Daily Total kg/d 168 192 290 

 Maximum Month Unit Load kg/c.d 0.003 0.003 0.003 

 Maximum Month Total kg/d 168 192 290 

 Maximum Week Unit Load kg/c.d 0.004 0.004 0.004 

 Maximum Week Total kg/d 224 256 385 

WASTEWATER TEMPS 

 Summer °C 20 20 20 

 Winter °C 10 10 10 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 25

Process Design Data 1980 

Bardenpho 

Design value 

 Units upgrade Stage 1–upgrade 

 Phase 1 Phase 2 Phase 3 

 

Raw Sewage Pumping 

 Station 1 

 Number of Units 6 6 

 Capacity L/s 380 380 

 Station 2 

 Number of Units 3 

 Capacity L/s 440 

 

Comminutor 

 Number of Units

 Mechanical 2 

 Manual -- 

 Capacity per unit ML/d 16.0 

Bar Screen 

 Number of Units

 Mechanical 1 1 

 Manual 1 1 

 Capacity per unit ML/d 75.0 75.0 

 

Grit Removal 

 Number of Units 2 1 1 

 Capacity per Unit ML/d 16.0 75.0 75.0 

 

Primary Clarifiers 

 Number of Units 3 4 6 

 Length m 27.4 27.4 27.4 

 Width m 6.1 6.1 6.1 

 SWD, 1-3 m 2.2 2.2 2.2 

 SWD, 4-6 m 2.5 2.5 2.5 

 SWD, 7-10 m 

 Peak OFR, 1 out of service m3/m2.d 62.7 94.1 91.1 

 

Primary Flow Equalization 

 Fraction of Average Flow percent 8.4 6.9 7.50 

 Volumes 

 NE Trunk m3 1,200 1,200 -- 

 Existing Tanks m3 700 700 700 

 Future Primary Clarifiers m3 -- 1,150 

 New Equalization Tanks m3 -- 1,200 

Primary Sludge Fermenters 

 SRT, avg d 7 5 5 

 Number of Units 1 1 2 

 Dimensions 

 Diameter m 17 15 15 

 SWD, 1-2 m 4.5 3.5 3.5 

 SWD, 3-4 m -- -- -- 
Nutrient Removal Technology Assessment Case Study September 2008 

26 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Process Design Data 1980 

Bardenpho 

Design value 

 Units upgrade Stage 1–upgrade 

 Phase 1 Phase 2 Phase 3 

 

Bioreactors 

Basic Design Parameters 

 SRT, Summer d 15 12 10 

 SRT, Winter d 20 15 12 

Bioreactor 

 Existing Modules 1 and 2 

 No. of Anaerobic Cells 3 

 Anaerobic Volume m3 1,365 

 No. of Anoxic Cells 6–10 

 Anoxic Volume m3 3,640 

 No. of Aerobic Cells 9–13 

 Aerobic Volume m3 5,005 

 

 No. of Anaerobic Stirrers 3 

 Anaerobic Stirrer hp 5 

 No. of Swing Zone Mixers 19 

 Swing Zone Mixers hp 7.5/15 

Bioreactor 

 Modified Modules 1 and 4 

 No. of Anaerobic Cells 1 1 

 No. of Anaerobic Stirrers 1 1 

 Anaerobic Stirrer hp 5 5 

 Anaerobic Volume m3 225 225 

 No. of Anoxic Cells 2 2 

 No. of Anoxic Stirrers 2 2 

 Anaerobic Stirrer hp 5 5 

 Anoxic Volume m3 680 680 

 No. of Aerobic Cells 4 4 

 Aerobic Volume m3 1,820 1,820 

 

 No. of Anaerobic Stirrers 1 1 

 Anaerobic Stirrer hp 2.5 2.5 

 No. of Aerobic Mixers 1 1 

 Aerobic Mixer hp 40 40 

 No. of Aerobic Mixers 1 1 

 Aerobic Mixer hp 30 30 

 No. of Swing Zone Mixers 3 3 

 Swing Zone Mixers hp 7.5/15 7.5/15 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 27

Process Design Data 1980 

Bardenpho 

Design Value 

 Units Upgrade Stage 1–Upgrade 

 Phase 1 Phase 2 Phase 3 

 Modified Modules 2 and 3 

 No. of Anaerobic Cells 1 1 

 Anaerobic Volume m3 455 455 

 No. of Anoxic Cells 3 3 

 Anoxic Volume m3 1,365 1,365 

 No. of Aerobic Cells 10 10 

 Aerobic Volume m3 4,550 4,550 

 

 No. of Anaerobic Stirrers 1 1 

 Anaerobic Stirrer hp 5 5 

 No. of Aerobic Mixers 2 2 

 Aerobic Mixer hp 40 40 

 No. of Aerobic Mixers 2 

 Aerobic Mixer hp 30 

 No. of Swing Zone Mixers 7 5 

 Swing Zone Mixers hp 7.5/15 7.5/15

 

Blowers 

 No. of Blowers 4 4 4 

 Size hp 100 100 250 
Nutrient Removal Technology Assessment Case Study September 2008 

28 - Kelowna, British Columbia, Canada z Wastewater Treatment Plant Appendix A 

Process Design Data 1980 

Bardenpho 

Design Value 

 Units Upgrade Stage 1–Upgrade 

 Phase 1 Phase 

2 

Phase 3 

 

Secondary Clarifiers 

Clarifiers 

 Number 3 4 5 

 Dimensions 

 Diameter m 26 26 26 

 SWD m 4.5 4.5 4.5 

RAS Pumps 

 Number 6 8 9 

 Capacity L/s 80 80 80 

 Maximum RAS Flow L/s 240 320 400 

WAS Pumps 

 Number 2 3 4 

 Capacity L/s 8 12 12 

 

Filtration 

 Peak OFR, 1 unit out of service 290 290 290 

 Existing Units 

 Number 4 4 4 

 Area per Unit m2 64 64 64 

 New Units 

 Number 1 

 Area per Unit m2 96 

 

Ultraviolet Disinfection 

 Dosage mWs/cm2 chlorine chlorine 48 

 Transmissivity percent 65 

 Number of Lamps 1,152 

 Arrangement 

 Number of Channels 2 

 Banks per Channel 3 

 Racks per Bank 24 

 Lamps per Rack 8 

 

WAS Thickening 

 Design Load, Peak kgTSS/d 4,615 

 DAF Units 

 Number 2 2 3 

 Area per Unit m2 18.9 18.9 18.9 

 

Dewatering 

 PS Flow, peak m3/d none none 90 

 WAS Flow, Peak m3/d none none 195 

 Centrifuges 

 Number 2 

 Capacity L/s none none 4.7 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Kelowna, British Columbia, Canada z Wastewater Treatment Plant - 29

Attachment 2: Electrical Cost 

Electrical cost 

Anoxic/Anaerobic mixers

kW kWh kWh %BOD %P %N for P for N

HP Number 

power 

draw 

hours/ 

day 

draw/

day

draw/

year draw draw

Anaerobic mixer 

5 2 7.46 24 179.04 65,349.6 0 100 0 65,349.6 0

2.5 1 1.865 24 44.76 16,337.4 0 100 0 16,337.4 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 8 29.84 24 716.16 261,398.4 0 0 100 0 261,398.4

2.5 2 3.73 24 89.52 32,674.8 0 0 100 0 32,674.8

Blowers 

250 1.25 233.125 24 5,595 2,042,175 45 10 45 204,217.5 918,978.75

Swing zone stirrers—19 available, can go either anoxic (7.5 hp) or aerobic (15 hp) 

7.5 9 50.355 24 1,208.52 441,109.8 0 0 100 0 441,109.8

15 10 111.9 24 2,685.6 980,244 45 10 45 98,024.4 441,109.8

Aerobic zone mixers 

40 5 149.2 24 3,580.8 1,306,992 45 10 45 130,699.2 588,146.4

30 5 111.9 24 2,685.6 980,244 45 10 45 98,024.4 441,109.8

15 11 123.09 24 2,954.16 1,078,268.4 45 10 45 107,826.84 485,220.78

Recirculation pump 

20 2 29.84 24 716.16 261,398.4 0 0 100 0 261,398.4

15 1 11.19 24 268.56 98,024.4 0 0 100 0 98,024.4

Filter pumps 

7.5 4 22.38 24 537.12 196,048.8 0 50 50 98,024.4 98,024.4

10 1 7.46 24 179.04 65,349.6 0 50 50 32,674.8 32,674.8

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 1 

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 (MGD). This facility was selected as a case study 

because it has achieved low levels of nitrogen and phosphorus in the effluent using the fivestage 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 

Annual 

average 

Monthly 

average 

Weekly 

average 

BOD5 5 mg/L 6.25 mg/L 7.5 mg/L 

TSS 5 mg/L 6.25 mg/L 7.5 mg/L 

TN 3 mg/L 3.75 mg/L 4.5 mg/L 

TP 1 mg/L 1.25 mg/L 1.5 mg/L 

Dichlorobromo-methane 24 μg/L Report -- 

Dibromochloro-methane 46 μg/L Report -- 

Notes: 

μg/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. 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A Figure 1. Marshall Street WRF layout. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 3 Figure 2. Marshall Street WRF process flow diagram.
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 5

Loadings 

Plant design loadings and equipment parameters are as follows: 

Average day 10 MGD 

Peak day 15 MGD 

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 (HRT): 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 MGD each 

Peak capacity: 28 MGD 

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: 15,552 gpd 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

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) 

Average 

value

Maximum 

month 

Max 

month vs. 

avg. 

Maximum 

week

Sample 

method/frequency 

Flow (MGD) 5.48 6.85 25% 7.62 -- 

Influent TP 5.0 5.53 10% 6.35 Weekly/composite 

Effluent TP 0.13 0.21 62% 0.26 Weekly/composite 

Influent BOD 188 234 24% 263 Daily/composite 

Effluent BOD 2.3 4.1 78% 5.3 Daily/composite 

Influent TSS 231 277 20% 317 Daily/composite 

Effluent TSS 0.89 1.11 24% 1.6 Daily/composite 

Influent NH4-N 28.0 32 16% 34.0 Daily/composite 

Effluent NH4-N 0.036 0.045 25% 0.062 Daily/composite 

Influent Total N 28.0 32 16% 34.0 Daily/composite 

Effluent Total N 2.32 3.1 35% 3.75 Daily/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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 7

Table 3 presents the plant’s monthly averages for the Bardenpho process parameters. 

Table 3. Monthly averages for plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(d) 

HRT 

(hr) 

Temperature 

(°C) 

Oct 2005 3,979 51 27 29.3 

Nov 2005 4,106 44 28 27.2 

Dec 2005 4,181 44 30 24.6 

Jan 2006 4,425 36 30 23.8 

Feb 2006 4,094 27 28 23 

Mar 2006 3,951 25 28 25 

Apr 2006 3,857 34 27 27 

May 2006 3,340 31 28 28 

June 2006 3,704 41 29 30 

July 2006 4,205 34 26 30 

Aug 2006 3,701 37 25 31 

Sep 2006 3,921 36 22 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.132 mg/L

Std. Dev. = 0.052 mg/L

C.O.V. = 40%

Figure 3. Monthly average frequency curves for TP. 

Marshall Street Advanced WWTP Clearwater, FL

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.132 mg/L

Std. Dev. = 0.058 mg/L

C.O.V. = 44%

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 9

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 0.038mg/L

Std. Dev. = 0.007 mg/L

C.O.V. = 18%

Figure 5. Monthly average frequency curves for ammonia nitrogen. 

Marshall Street Advanced WWTP Clearwater, FL

Weekly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 0.036 mg/L

Std. Dev. = 0.0075 mg/L

C.O.V. = 21%

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

Marshall Street Advanced WWTP Clearwater, FL

Monthly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 2.32 mg/L

Std. Dev. = 0.38 mg/L

C.O.V. = 16%

Figure 7. Monthly average frequency curves for nitrogen. 

Marshall Street Advanced WWTP Clearwater, FL

Weekly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 9999.5 99.9599.9

Mean = 2.32 mg/L

Std. Dev. = 0.43 mg/L

C.O.V. = 19%

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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 11

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

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 (μg/L) and a dibromochloromethane limit of 34 μg/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), oxidationreduction 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 Supplier(s) 

Dissolved oxygen Hach, Royce 

MLSS Hach 

Nitrate-nitrogen Chemscan 

Ammonia nitrogen Chemscan 

Clarifier sludge blanket depth Hach, Royce 

pH Hach 

Oxidation-reduction potential (ORP) Hach 

Ortho-phosphorus Chemscan 

Turbidity Hach 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 13

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 lb volatile 

suspended solids (VSS) per lb 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. 

Costs 

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 (USDA 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 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 15

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 MGD, this was 317 lb 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 lb 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 lb 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 lifecycle 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

16 - Marshall Street, Clearwater, FL z Water Reclamation Facility Appendix A

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Marshall Street, Clearwater, FL z Water Reclamation Facility - 17

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. 

Acknowledgments 

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. 
18 - Marshall Street, Clearwater, FLz Water Reclamation Facility Appendix A

Attachment: Electrical Use and Chemical Costs 

Horsepower # 

kW 

power 

draw 

hours/

day 

kWh 

draw/day 

kWh 

draw/year % BOD %P %N 

Usage fo

BOD 

Ferment basin mixers 10 6 44.76 24 1,074.24 392,097.6 0% 100% 0% 1st anoxic mixers 7.5 9 50.355 24 1,208.52 441,109.8 0% 0% 100% Aerator 400 2 596.8 24 14,323.2 5,227,968 40% 10% 50% 2,091,00Pumps—internal recycle 50 3 111.9 24 2,685.6 980,244 0% 0% 100% 2nd anoxic mixers 7.5 12 67.14 24 1,611.36 588,146.4 0% 0% 100% Filter lift pumps 50 1 37.3 24 895.2 326,748 0% 5% 0% Total draw kWh/yr 7,629,566 2,091,18

 Alum 

use $74,000 

 % for P 10 Alum 

cost for 

P 

$7,400 
Appendix A Fairfax County, VA. z Noman M. Cole, Jr., Pollution Control Plant - 1

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 

Monthly 

average 

(mg/L) 

Monthly 

average 

(lb/day) 

Weekly average 

(mg/L) 

Weekly 

average 

(lb/day) 

CBOD 5 2,790 8 4,464 

TSS 6 3,348 9 5,020 

Ammonia-N (April–Oct) 1.0 559 1.5 836 

Ammonia-N (Nov–Mar) 2.2 -- 2.7 -- 

Total N Report -- Report -- 

Total P 0.18 101 0.27 150 

Notes: 

CBOD = carbonaceous biochemical oxygen demand 

N = nitrogen 

P = phosphorus 

TSS = total suspended solids 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 3 Figure 1. Noman M. Cole, Jr., Pollution Control Plant process flow. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A Figure 2. Noman M. Cole, Jr., Pollution Control Plant step-feed detail. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 5

Table 2. Facility design data 

Secondary tanks Tanks 1–6 Tanks 7–9 

Volume 1.67 MG 4.89 MG 

Anoxic volume 3.5–5 MG 5.1–7.3 MG 

HRT (average) 8.9 hr 8.9 hrs 

SRT at maximum month loading 

(last MLSS 4,400 mg/L) 

18 days 18 days 

Gravity thickeners (2) 

Volume 0.146 MG each 

SRT 3 days 

HRT > 12 hours 

Tertiary clarifier 

Diameter 152 ft 

Hydraulic loading rate 735 gpd/sf (average flow) 

Tertiary filters Monomedia Gravity filters 

Number 8 10 

Media type Anthracite Garnet/sand/anthracite 

Depth 5 ft 2.25 ft 

Design loading rate, gpm/sf 2.9 2.6 

Dimensions 30 ft x 17 ft x 2 cells 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 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

Plant Parameters 

Overall plant influent and effluent average results for the 2006 calendar year are shown in 

Table 3. 

Table 3. Influent and effluent averages 

Parameter 

(mg/L unless stated) 

Average 

value 

Maximum 

month 

Max 

month 

vs. Avg. 

Maximum 

week 

Sample 

method/frequency 

Flow (MGD) 47.4 51.4 8% 54.4 Daily 

Influent TP (mg/L) 6.39 7.06 10% 8.16 Composite/daily 

Effluent TP (mg/L) 0.09 0.12 33% 0.16 Composite/daily 

Influent BOD (mg/L) 189 205 8% 305 Composite/daily 

Effluent BOD (mg/L) 2.0 2.0 0% 2.0 Composite/daily 

Influent TSS (mg/L) 225 253 12% 353 Composite/daily 

Effluent TSS (mg/L) 1.0 2.2 120% 3.06 Composite/daily 

Influent NH4-N (mg/L) 18.9 22.5 19% 24.8 Composite/weekly 

Effluent NH4-N (mg/L) 0.12 0.15 25% 0.29 Composite/weekly 

Influent TKN (mg/L) 34.6 40.4 17% 48.1 Composite/weekly 

Effluent TKN (mg/L) 0.9 1.12 26% 1.6 Composite/weekly 

Effluent NO3/NO2 (mg/L) 4.35 5.03 16% 6.41 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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 7

Table 4 presents plant monthly average plant process parameters. 

Table 4. Monthly averages for plant process parameters 

Month 

MLSSa

(mg/L) 

Sludge age/mean cell 

residence time 

(d) 

HRT 

(hr) 

Temperature 

(°C) 

Jan 2006 3,626 18 9.2 17.9 

Feb 2006 3,267 19 9 15.3 

Mar 2006 3,390 19 9.2 17.4 

Apr 2006 2,851 19 10 19.5 

May 2006 3,142 18 9.8 20.6 

June 2006 2,784 18 8.8 22.5 

July 2006 2,383 17 8.3 23.8 

Aug 2006 3,139 17 8 25.7 

Sept 2006 3,192 17 7.8 25.4 

Oct 2006 2,922 16 8.2 23.5 

Nov 2006 2,403 16 9.4 21.2 

Dec 2006 2,852 18 10 19.4 a

 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 Influent BOD/TP

Primary 

effluent 

BOD/TP Influent BOD/TKN 

Jan 2006 33.1 29.8 6.1 

Feb 2006 33.7 29.5 5.4 

Mar 2006 28.3 27.4 5.3 

Apr 2006 28.2 27.1 5.5 

May 2006 27.2 26.8 4.7 

June 2006 28.9 24.4 5.5 

July 2006 28.8 26.1 5.9 

Aug 2006 29.4 28.1 4.6 

Sept 2006 31.5 32.4 5.0 

Oct 2006 33.5 33.8 4.9 

Nov 2006 32.2 32.0 5.4 

Dec 2006 28.2 26.3 5.4 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

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

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Primary Influent Primary Effluent Secondary Effluent

Tertiary Clarifier Influent Tertiary Clarifier Effluent Final Effluent

0.05 0.1 0.5 1 2 5 10 20 504030 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.086 mg/L

Std. Dev. = 0.018 mg/L

COV = 21%

Figure 3. Monthly average frequency curves for TP. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 9

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Primary Influent Primary Effluent Secondary Effluent

Tertiary Clarifier Influent Tertiary Clarifier Effluent Final Effluent

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Final Effluent

Mean = 0.09 mg/L

Std. Dev. = 0.025 mg/L

COV = 29%

Tertiary Effluent

Mean = 0.36 mg/L

Std. Dev. = 0.118 mg/L

Secondary Effluent COV = 33%

Mean = 0.74 mg/L

Std. Dev. = 0.37 mg/L

COV = 50%

Tertiary Influent

Mean = 0.85 mg/L

Std. Dev. = 0.35 mg/L

COV = 42%

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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 20 4030 50 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.12 mg/L

Std. Dev. = 0.016 mg/L

COV = 14%

Figure 5. Monthly average frequency curves for ammonia nitrogen. 

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Weekly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia N

0.05 0.1 0.5 1 2 5 10 20 30 5040 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 0.12 mg/L

Std. Dev. = 0.035 mg/L

COV = 29%

Figure 6. Weekly average frequency curves for ammonia nitrogen. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 11

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

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 5.25 mg/L

Std. Dev. = 0.63 mg/L

COV = 12%

Figure 7. Monthly average frequency curves for nitrogen. 

Noman M. Cole Pollution Control Plant - Fairfax County, VA

Weekly Average Frequency Curves for Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - TKN Final Effluent - Total N

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 98 99 99.5 99.9 99.95

Mean = 5.12 mg/L

Std. Dev. = 1.02 mg/L

COV = 20%

Figure 8. Weekly average frequency curves for nitrogen. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 13

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. 

Costs 

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 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

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 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 15

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 

$1,076/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 $1,076/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 lb 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 lb 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 
Nutrient Removal Technology Assessment Case Study September 2008 

16 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 17

Attachment: Facility Design Information 

Design Flow 

 Minimum flow 26.8 MGD 

 Average daily flow 67.0 MGD 

 Peak instantaneous flow 134.0 MGD 

 Peak process flow 107.2 MGD 

Design Average Loadings 

 BOD 118,000 lb/d 

 TSS 126,000 lb/d 

 TKN 21,000 lb/d 

 TP 4,100 lb/d 

Retention Basin 1 (QQ1) 

 Retention QQ1 

 Quantity 1 

 Type Open 

 Volume 5.7 MG 

 Retention basin pumps 

 Quantity 4 

 Type Submersible 

 Capacity 

 Large 3,300 gpm at 27 ft 

 Small 350 gpm at 27 ft 

Screen Building (B1) 

 Bar screens 

 Quantity 3 

 Total channel width 8 ft 

 Opening size 3/4 in 

RAW Wastewater Pump Station (B) 

 RAW wastewater pumps 

 Quantity 5 

 Type Vertical, centrifugal 

 Speed 

 A-1 Adjustable 

 A-2 Two-speed 

 A-3 Constant 

 A-4 Adjustable 

 A-5 Constant 

 
Nutrient Removal Technology Assessment Case Study September 2008 

18 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Capacity 

 A-1 20,500 gpm at 30 ft TDH 

 A-2 19,165 gpm at 30 ft TDH 

 A-3 20,700 gpm at 30 ft TDH 

 A-4 20,700 gpm at 30 ft TDH 

 A-5 18,500 gpm at 30 ft TDH 

RAW Wastewater/EQ Tank Pump Station (B2) 

 Equalization tank pumps 

 Quantity 3 

 Type Submersible 

 Capacity, each 6,544 gpm at 84 ft TDH 

 

 Raw wastewater pumps 

 Quantity 2 

 Type Submersible 

 Capacity, each 9,682 gpm at 47 ft TDH 

Equalization Tanks (B3) 

 Equalization tanks 

 Quantity 4 

 Type Concrete 

 Dimensions, each 200 ft long X 100 ft wide X 27 ft Deep (SWD) 

 Volume, each 4 MG 

Flash Mix Tanks (C1) 

 Quantity 2 

 Dimensions 30 ft L X 18 ft W X 10 ft SWD 

 Volume, each 40,400 gallons 

 Detention time 1.74 minutes at average daily flow 

Primary Settling Tanks (C) 

 Primary settling tanks 

 Quantity 8 

 Type Rectangular 

 Size 139 ft L X 45 ft W X 10 SWD 

 Weir length, each 120 ft 

 Weir loading 69,800 gpd/linear foot at average daily flow 

 Hydraulic overflow rate 1,340 gpd/ft2

 at average daily flow 

 

Primary influent odor control scrubber 

 Quantity 1 

 Type Packed bed 

 Depth of packing 12 ft min 

 Cross section area 19.6 ft2

 Capacity 5,000 CFM 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 19

 

Scrubber recirculation pump 

 Quantity 1 

 Type Vertical wet pit centrifugal 

 Capacity, each 100 gpm at 30 ft TDH 

 Horsepower 2 

Small Activated Sludge Tanks 1 TO 6 (D) 

 Small activated sludge tanks 

 Quantity 6 

 Number of passes, each 3 

 Size, each pass 182 ft L X 30 ft W X 13.6 ft SWD 

 Volume, each tank 1.67 MG 

 Total volume 10.0 MG 

 Total anoxic volume 3.5 to 5.0 MG 

 HRT @ average flow 8.9 hours 

 SRT @ max mo load, 18 days 

 & last pass MLSS OF 4,400 

 

 Mixers 

 Quantity 78 

 Type Submersible, mast-mounted 

 Horsepower, each 4 HP 

 

 Process oxygen requirements 

 BNR operation 

 Average 48,000 lb/d 

 Maximum month 51,000 lb/d 

 Maximum day 71,400 lb/d 

 Nitrification only operation 

 Average 70,800 lb/d 

 Maximum month 76,200 lb/d 

 Maximum day 115,000 lb/d 

 

 Diffused aeration equipment 

 Type 9-in porous flexible membrane 

 Full floor coverage 

Large Activated Sludge Tanks 7 TO 9 (D1) 

 Large activated sludge tanks 

 Quantity 3 

 Number of passes, each 6 
Nutrient Removal Technology Assessment Case Study September 2008 

20 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Size, each pass 

 2 at 165 ft L X 18 ft W X 22 ft SWD 

 4 at 165 ft L X 36 ft W X 22 ft SWD 

 Volume, each tank 4.89 MG 

 Total volume 14.7 MG 

 Total anoxic volume 5.1 to 7.3 MG 

 HRT @ average flow 8.9 hours 

 SRT @ max mo load, 18 days 

 & last pass MLSS of 4,400 

 

 Mixers 

 Quantity 57 

 Type Vertical turbine 

 Platform-mounted 

 Horsepower, each 24 at 3 HP 

 12 at 5 HP 

 9 at 7.5 HP 

 12 at 15 HP 

 

 PE channel mixers 

 Quantity 18 

 Type Submersible, mast-mounted 

 Horsepower, each 2.5 HP 

 

 Process oxygen requirements 

 BNR operation 

 Average 88,200 lb/d 

 Maximum month 95,700 lb/d 

 Maximum day 149,000 lb/d 

 Nitrification only operation 

 Average 101,000 lb/d 

 Maximum month 108,000 lb/d 

 Maximum day 164,000 lb/d 

 

 Diffused aeration equipment 

 Type 9-in porous flexible membrane 

 Full floor coverage 

 

 AST dewatering pumps 

 Quantity 

 Large 2 

 Small 2 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 21

 Type Submersible 

 Capacity, each 

 Large 2,025 gpm at 25 ft TDH 

 Small 75 gpm at 60 ft TDH 

 Horsepower, each 

 Large 25 HP 

 Small 5 HP 

Blower Building (E1) 

 Small AST aeration blowers 

 Quantity 4 

 Type Multistage centrifugal 

 Capacity, each 16,000 SCFM at 8.0 psi 

 Horsepower, each 800 HP 

 

 Clarifiers 12–15 RAS pumps 

 Quantity 5 

 Type Single-passage screw impeller, centrifuge 

 Speed Adjustable 

 Capacity, each 4,400 gpm at 28 ft TDH 

 Horsepower, each 50 HP 

 

 WAS pumps 

 Quantity 4 

 Type Horizontal centrifugal 

 Capacity, each 510 gpm at 30 ft TDH 

Blower Building (E2) 

 Aeration blowers 

 Quantity 

 Small AST blowers 2 

 Large AST blowers 4 

 Type Multistage centrifugal 

 Capacity 

 Small AST blowers 17,500 at 8.0 psi 

 Large AST blowers 14,000 at 12.7 psi 

 

 Horsepower, each 

 Small AST blowers 800 HP 

 Large AST blowers 1,250 HP 

 
Nutrient Removal Technology Assessment Case Study September 2008 

22 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Clarifiers 5–8 RAS pumps 

 Quantity 5 

 Type Horizontal centrifugal 

 Capacity 6,500 gpm at 37 ft TDH 

 

 Clarifiers 16–17 RAS pumps 

 Quantity 2 

 Type Single-passage screw impeller, centrifuge 

 Speed Adjustable 

 Capacity, each 4,400 gpm at 28 ft TDH 

 Horsepower, each 50 HP 

Secondary Clarifiers 5 to 8 (F) 

 Quantity 4 

 Type Circular 

 Diameter 145 ft 

 Sidewater depth 14.75 ft 

 Hydraulic overflow rate 540 gpd/ft2

 at peak process flow 

 Solids loading rate 31 lp/d/ft2

 at peak process flow 

Secondary Clarifiers 12 to 17 (F1) 

 Secondary clarifiers 

 Quantity 6 

 Type Rectangular chain & flight 

 Dimensions, each 260 ft L X 55 ft W X 16 ft SWD 

 Hydraulic overflow rate 540 gps/ft2

 at peak process flow 

 Solids loading rate 31 lf/d/ft2

 at peak process flow 

 

 Secondary clarifier dewatering pumps 

 Quantity 2 

 Type Submersible 

 Capacity, each 500 gpm at 50 ft TDH 

 Horsepower, each 15 HP 

Chlorination Facility (G) 

 SPH pumps 

 Quantity 4 

 Type Vertical turbine 

 Capacity, each 3,100 gpm at 216 ft TDH 

 

 SPH strainers 

 Quantity 3 

 Type Automatic, self-cleaning 

 Capacity, each 1,050 gpm 

 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 23

 Sodium hypochlorite feed pumps 

 Quantity 

 Large NaOCl pumps 4 

 Small NaOCl pumps 2 

 

 Type Tubular diaphragm chemical metering 

 Capacity 

 Large NaOCl pumps 200 gph max 

 Small NaOCl pumps 50 gph max 

 

 Sodium hypochlorite storage tanks 

 Quantity 4 

 Dimensions, each 11.5 ft dia X 15.5 ft high 

 Volume, each 12,000 gallons 

Chemical Feed Building (S) 

 Caustic feed pumps 

 Quantity 4 

 Type Tubular diaphragm chemical metering 

 Control Adjustable stroke & speed 

 Capacity, each 420 gph max 

 Typical dose 11 mg/L as CACO3 for PH control 

 

 Caustic storage tanks 

 Quantity 3 

 Dimensions, each 12 ft diameter X 19 ft high 

 Volume, each 16,000 gallons 

 

 Polymer feed pumps 

 Quantity 12 

 Type Progressing cavity 

 Speed Adjustable 

 Capacity, each 250 gph max 

 Typical dose 0.5–1.0 mg/L 

 

 Polymer transfer pump 

 Quantity 1 

 Type Progressing cavity 

 Capacity 80 gpm 

 
Nutrient Removal Technology Assessment Case Study September 2008 

24 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Polymer mixing, aging, and storage tanks 

 Quantity 2 

 Dimensions, each 7 ft dia X 7 ft high 

 Volume 2,000 gallons 

 

 Chemical feed pumps for primary settling tank odor control 

 Quantity 

 Caustic 1 

 Sodium hypochlorite 1 

 Type Eccentric lobe peristaltic 

 Capacity 

 Caustic 8.6 gpm 

 Sodium hypochlorite 7.0 gmp 

 

 Sodium hypochlorite storage tank (exist) 

 Quantity 1 

 Dimensions, each 12 ft dia X 19 ft high 

 Volume 16,000 gallons 

Equalization Basins 2 & 3 (QQ2 & QQ3) 

 Equalization basins 

 Type Concrete-lined, open 

 Volume 

 Basin QQ2 7.4 MG 

 Basin QQ3 5.8 MG 

 

 Wash water return pumps 

 Quantity 2 

 Type Submersible 

 Speed Constant 

 Capacity, each 600 gpm at 50 ft TDH 

 Horsepower, each 20 HP 

ASE Pump Station (BB) 

 ASE pumps 

 Quantity 5 

 Type Vertical turbine 

 Capacity 

 Adj speed 2 at 29,400 gpm 

 Constant speed 1 @ 22,600 gpm 

 Constant speed 2 @ 16,000 gpm 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 25

Tertiary Clarifiers (CC) 

 Tertiary clarifiers 

 Quantity 4 

 Type Octagonal 

 Nominal inside diameter 148 ft 

 Hydraulic overflow rate 735 gpd/ft2

 at average flow 

 

 Tertiary clarifier dewatering pumps 

 Quantity 2 

 Type Horizontal centrifugal 

 Speed Constant 

 Capacity, each 2,400 gpm at 50 ft TDH 

 Horsepower 50 HP 

Tertiary Clarifiers (CC1) 

 Flow distribution structure mixer 

 Quantity 1 

 Type Vertical turbine, platform-mounted 

 Horsepower 15 HP 

 

 Tertiary clarifier 

 Quantity 1 

 Type Circular 

 Diameter 152 ft 

 Hydraulic loading rate 735 gpd/ft2

 at average flow 

 

 Tertiary clarifier dewatering pumps 

 Quantity 1 

 Type Horizontal centrifugal 

 Speed Constant 

 Capacity, each 1,000 gpm at 21 ft TDH 

 Horsepower, each 15 HP 

TCE Pump Station (CC) 

 Tertiary clarifier effluent pumps 

 Quantity 3 

 Type Vertical turbine 

 Speed Adjustable 

 Capacity, each 22,700 gpm at 35 ft TDH 

 Horsepower, each 300 HP 

Foreign Sludge Incinerator Building (KK) 

 Ferric chloride pumps 

 Quantity 4 

 Type Tubular diaphragm chemical metering 
Nutrient Removal Technology Assessment Case Study September 2008 

26 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Control Adjustable stroke & speed 

 Capacity 200 gpm max 

 Typical dose 25–30 mg/L 

 

 Polymer feed pumps 

 Quantity 6 

 Type Progressing cavity 

 Speed Adjustable 

 Capacity 2.0 gpm max 

 Typical dose 0.1–0.2 mg/L 

Monomedia Filter Building (FF) 

 Monomedia filters 

 Quantity 8 

 Type Center gullet 

 Media type Anthracite 

 Number cells, each 2 

 Dimensions, each cell 30 ft L X 17 ft W 

 Media depth 5 ft 

 Design loading rate 2.9 gpm/ft2 at average daily flow with 

 all units in service 

 Backwash pump 

 Quantity 1 

 Type Vertical turbine 

 Capacity 20,400 gpm 

Gravity Filter Building (DD) 

 Gravity filters 

 Quantity 10 

 Media type Anthracite/sand 

 Dimensions, each cell 30 ft L X 30 ft W 

 Media depth 2.25 ft 

 Design loading rate 2.6 gpm/ft2

 at average daily flow with 

 all units in service 

 Backwash pump 

 Quantity 1 

 Type Vertical turbine 

 Capacity 18,000 gpm 

 

 Gravity filter effluent pumps 

 Quantity 

 Constant speed 2 

 Adj speed 2 

 Type Vertical turbine 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 27

 Capacity, each 

 Constant speed 22,500 gpm 

 Adj speed 27,000 gpm 

Backwash Effluent Tanks (EE) 

 Quantity 3 

 Dimensions, each 85 ft L X 20 ft W X 11.3 ft SWD 

 Volume, each 144,000 gallons 

Reaeration Tank (HH) 

 Quantity 1 

 Dimensions 72 ft L X 70 ft W X 22 ft SWD 

 Volume 830,000 gallons 

 

APW Pump Station (HH1) 

 Advanced plant water pumps 

 Quantity 4 

 Type Vertical turbine 

 Speed Adjustable 

 Capacity, each 4,400 gpm at 212 ft TDH 

 Horsepower, each 300 HP 

Blended Sludge Storage Tanks (R1/R2) 

 Odor control scrubber system 

 Quantity 1 

 Type Two-stage, packed-bed wet type 

 Chemicals treated NH3, H2S 

 Capacity, each 5,000 cfm 

 Depth of bedding 7 ft 

 Cross-sectional area 19.6 ft2

 

 Chemical feed pumps for odor control 

 Quantity 

 Caustic 2 

 Sodium hypochlorite 2 

 Sulfuric acid 2 

 Type Tubular diaphragm, chemical metering 

 Capacity, each 23 gph 

 

 Chemical storage tanks 

 Chemical NAOH 

 Quantity 1 

 Dimensions, each 6 ft dia X 10 ft high 

 Volume, each 2,100 gallons 

 
Nutrient Removal Technology Assessment Case Study September 2008 

28 - Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant Appendix A 

 Chemical NAOCL 

 Quantity 1 

 Dimensions, each 4 ft dia X 11 ft 7 in high 

 Volume, each 1,000 gallons 

 

 Chemical H2S04

 Quantity 1 

 Dimensions, each 38 in dia X 82 in long 

 Volume, each 400 gallons 

Degritting Building (H1) 

Cyclone separators 

Quantity 6 

Capacity, each 465 gpm at 12 psi 

Grit classifiers 

Quantity 3 

Capacity, each 108 ft3

/hr 

Primary Sludge Thickeners (J1/J2) 

Gravity thickeners 

Quantity 4 

Type Circular 

Diameter 50 ft 

Sidewater depth 10 ft 

Flotation Thickeners (Q1/Q2) 

DAF thickeners 

Quantity 3 

Type Rectangular 

Size 40.2 ft L x 12 ft W x 12 ft SWD 

Capacity, each 960 gpm 

Sludge Storage (R1/R2) 

Sludge storage tanks 

Quantity 2 

Diameter 

Sidewater depth 

Volume, each 367,000 gallons 

Sludge Dewatering (K3) 

Centrifuge 

Quantity 4 

Type Bowl and scroll conveyor 

Sludge loading, each 

With lime 5,351 lb/hr 

Excluding lime 4,730 lb/hr 

Sludge feed concentration (percent) 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Fairfax County, VA z Noman M. Cole, Jr., Pollution Control Plant - 29

 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 North Cary, NC z Water Reclamation Facility - 1 

North Cary Water Reclamation Facility 

North Cary, North Carolina 

Nutrient Removal Technology Assessment Case Study 

Introduction and Permit Limits 

The North Cary, North Carolina, Water Reclamation Facility (WRF) is a 12-million-gallonper-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 Cary 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 lb 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 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - North Cary, NC z Water Reclamation Facility Appendix A

Table 1. NCDENR permit limits 

Parameter 

Summer limits 

(mg/L) 

Winter limits 

(mg/L) 

Quarterly limits 

(mg/L) Annual limits 

CBOD 4.1 8.2 -- -- 

TSS 30 30 -- -- 

NH3-N 0.5 1.0 -- -- 

TN -- -- -- 144,000 lb (max)a

TP -- -- 2.0 -- 

Coliforms -- -- 200/100 mL -- 

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 Cary 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 Number Volume 

Anaerobic selectors 4 each train 0.093 MG x 4 = 0.372 MG 

Oxidation ditch 2 each train 1.5 MG x 2 = 3 MG 

Secondary anoxic zone 3 each train 0.111 MG x 3 = 0.333 MG 

Reaeration zone 1 each train 0.111 MG 

Clarifiers 2 each 130 ft diameter 

Note: MG = million gallons 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 3 Figure 1. North Cary WRF process schematic. 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - North Cary, NC z Water Reclamation Facility Appendix A

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) 

Average 

value 

Maximum 

month 

Max 

month 

vs. avg. 

Maximum 

week 

Sample 

method/frequency 

Flow (MGD) 7.0 8.71 24% 10.8 -- 

Influent TP (mg/L) 7.7 9.2 20% 11.1 Composite, 3x/week 

Effluent TP (mg/L) 0.38 1.06 180% 1.45 Composite, 3x/week 

Influent BOD (mg/L) 244 271 11% 296 Composite, 5x/week 

Effluent BOD (mg/L) 0.8 1.26 50% 1.84 Composite, 5x/week 

Influent TSS (mg/L) 366 418 14% 594 Composite, 5x/week 

Effluent TSS (mg/L) 1.0 1.47 45% 2.28 Composite, 5x/week 

Influent NH4-N (mg/L) 45.5 49.4 8% 53.5 Composite, 5x/week 

Effluent NH4-N (mg/L) 0.08 0.34 316% 1.03 Composite, 5x/week 

Influent TKN (mg/L) 56.4 62.2 10% 65.6 Composite, 3x/week 

Effluent TN (mg/L) 3.67 4.46 21% 5.87 Composite, 3x/week 

Note: 

TKN = total Kjeldahl nitrogen 

BOD = biochemical oxygen demand 

Table 4. Monthly averages for plant process parameters 

Month 

MLSS 

(mg/L) 

Sludge age 

(days) 

HRT 

(hours) 

Temperature 

(°C) 

Oct 2005 2,665 13.1 28 23 

Nov 2005 2,628 13.8 29 20 

Dec 2005 2,736 13.0 26 19 

Jan 2006 2,672 13.3 27 18 

Feb 2006 2,720 12.8 27 16 

Mar 2006 2,692 13.3 29 18 

Apr 2006 2,661 12.6 27 19 

May 2006 2,625 13.5 28 21 

June 2006 2,700 11.3 21 24 

July 2006 2,713 12.3 25 26 

Aug 2006 2,709 12.6 25 27 

Sep 2006 2,685 12.1 24 26 

Notes: 

HRT = hydraulic retention time 

MLSS = mixed liquor suspended solids
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 5

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. 

North Cary, NC

Monthly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia-N

0.05 0.1 5040 0.5 1 2 5 10 20 30 60 70 80 90 95 98 9999.5 99.9599.9

Mean = 0.081 mg/L

Std. Dev. = 0.082 mg/L

C.O.V. = 102%

Figure 2. Monthly average frequency curves for ammonia nitrogen. 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - North Cary, NC z Water Reclamation Facility Appendix A

North Cary, NC

Weekly Average Frequency Curves for Ammonia Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia-nitrogen

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 0.81 mg/L

Std. Dev. = 0.142 mg/L

C.O.V. = 174%

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). 

North Cary, NC

Monthly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 302010 5040 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.379 mg/L

Std. Dev. = 0.241 mg/L

C.O.V. = 64%

Figure 4. Monthly average frequency curves for TP. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 7

North Cary, NC

Weekly Average Frequency Curves for Total Phosphorus

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Final Effluent

0.05 0.1 0.5 1 2 5 5040302010 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0378 mg/L

Std. Dev. = 0.273 mg/L

C.O.V. = 72%

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 Cary, NC

Monthly Average Frequency Curves for Total Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Total Nitrogen Final Effluent - Total Nitrogen

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 3.67 mg/L

Std. Dev. = 0.512 mg/L

C.O.V. = 14%

Figure 6. Monthly average frequency curves for TN. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - North Cary, NC z Water Reclamation Facility Appendix A

North Cary, NC

Weekly Average Frequency Curves for Total Nitrogen

0.01

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Raw Influent - Ammonia-N Final Effluent - Ammonia-nitrogen

0.05 0.1 0.5 1 2 5 10 50403020 60 70 80 90 95 9899 99.5 99.9599.9

Mean = 3.67 mg/L

Std. Dev. = 0.781 mg/L

C.O.V. = 21%

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 Cary. 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 Cary WRF: 

• The BioDenipho process at North Cary 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 9

• 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 polyphosphateaccumulating 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - North Cary, NC z Water Reclamation Facility Appendix A

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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 11

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 Cary 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 lb 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 lb 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - North Cary, NC z Water Reclamation Facility Appendix A

Summary 

The North Cary 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. 

Acknowledgments 

The authors are grateful for the significant assistance and guidance that Chris Parisher, North 

Cary 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 Cary 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A North Cary, NC z Water Reclamation Facility - 13

Attachment 1: Key Design Parameters 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - North Cary, NC z Water Reclamation Facility Appendix A

Attachment 2: Operating Stages of the BioDenipho 

Process 

Figure 8. Operating stages for the BioDenipho process (WEF and ASCE 1998).
Appendix A North Cary, NC z Water Reclamation Facility - 15 Attachment 3: Electrical Costs Electrical 1 train kW kWh kWh % for P % for N for P 

Hp Number Power draw hours/day draw/day draw/year Anaerobic Mixers 4.9 4 14.6216 24 350.9184 128,085.2 100 0 128,085.2 

Rotors 60 4 179.04 15.12 2,707.085 988,086 5 95 49,404.3 Main mixers 9 4 26.856 8.88 238.4813 87,045.67 5 95 4,352.283 Anoxic mixers 6.5 3 14.547 24 349.128 127,431.7 0 100 0 Reaeration blower 20 1 14.92 24 358.08 130,699.2 5 95 6,534.96 Clarifer drive 1 1 0.746 24 17.904 6,534.96 5 95 326.748 Total for 1 train 188,703.5 Total for 2 trains TRAINS 377,407 

Rate 0.05 for P 

Totals $17,361 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 1

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 (MGD); in 2006 it processed an average of 19.3 MGD. 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 

Annual 

loading 

(mg/L) 

Monthly average 

(mg/L) 

Weekly average 

(mg/L) 

BOD5

4/1–10/31 9 14 

BOD5

11/1–3/31 30 45 

TSS 30 45 

Total phosphorus 0.3 1.0 N/A 

Total nitrogen 4.0 3.0 4.5 

Ammonia-N 4/1–10/31 1.5 N/A 

Ammonia-N 11/1–3/31 5.5 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. 

a

Total nitrogen and total phosphorus are based on a design flow of 30 MGD. 
Nutrient Removal Technology Assessment Case Study September 2008 

2 - Western Branch, MD z Wastewater Treatment Plant Appendix A

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 (HRAS) 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 HRAS. 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. 

Figure 1. Western Branch WWTP process flow. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 3

Figure 2. Western Branch WWTP liquid process flow. 

DENITRIFICATION 

CLARIFIERS 

HIGH RATE AERATION 

HIGH RATE CLARIFIERS 

NITRIFICATION AERATION 

NITRIFICATION 

CLARIFIERS 

DENITRIFICATION 

REACTORS 

NITROGEN 

STRIPPING 

CHANNELS 

TO GRAVITY FILTERS AND UV 

FLOW SPLIT STRUCTURE FROM RAW WWPS 

(FOUR WEIRS WITH GATES) 

PARSHALL FLUMES 

GRIT/SCREEN UNITS 

“C” Train 

“A” Train 

“B” Train 

“Numbers” 

Train 

Common RAS line 

Common RAS line 
Nutrient Removal Technology Assessment Case Study September 2008 

4 - Western Branch, MD z Wastewater Treatment Plant Appendix A

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/l L 

 Nitrate-nitrogen 15–30 mg/L 

Process Size Detention time VLR 

 (hours) BOD lb/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 lb 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 Size Overflow rate SLR 

HRAS 120 x 80 x 13 ft, 4 each 781 gpd/ft2 34 lb/ft2/d 

NAS 150 x 80 x 11.5 ft, 4 each 625 gpd/ft2 27.4 

DNAS Diameter – 160 ft, 4 each 373 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
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 5

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) 

Average 

value 

Maximum 

month 

Max 

month vs. 

ave. 

Maximum 

week 

Sample 

method/frequency 

Flow incl recycle 

(MGD) 

23.0 26.5 15% 30.9 

Influent TP 3.70 4.22 14% 5.57 Twice weekly/ 

composite 

Effluent TP 0.43 0.89 89% 0.99 Five times weekly/ 

composite 

Influent COD 332 417 26% 641 Twice weekly/ 

composite 

Effluent COD 16.1 25.8 60% 38.6 Five times weekly/ 

composite 

Effluent BOD 2.69 3.94 46% 6.08 Five times weekly/ 

composite 

Influent TSS 222 282 27% 400 Twice weekly/ 

composite 

Effluent TSS 1.23 2.28 85% 4.60 Five times weekly/ 

composite 

Influent NH4-N 19.6 22.3 14% 25.1 Twice weekly/ 

composite 

Effluent NH4-N 0.22 0.93 323% 3.41 Five times weekly/ 

composite 

Influent Total N 23.9 28.7 20% 44.8 Twice weekly/ 

composite 

Effluent Total N 1.63 2.46 45% 4.22 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 
Nutrient Removal Technology Assessment Case Study September 2008 

6 - Western Branch, MD z Wastewater Treatment Plant Appendix A

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 

HRAS MLSS 

(mg/L) 

HRAS sludge age 

(d) 

HRAS HRT 

(hr) 

Jan 2006 4,710 1.9 3.4 

Feb 2006 4,232 1.8 3.3 

Mar 2006 3,808 1.9 3.6 

Apr 2006 3,798 1.7 3.7 

May 2006 4,208 2.7 3.8 

June 2006 5,454 7.3 3.5 

July 2006 4,028 1.8 3.5 

Aug 2006 4,306 1.9 3.8 

Sept 2006 5,545 2.7 3.5 

Oct 2006 4,066 1.7 3.4 

Nov 2006 3,431 0.9 3.0 

Dec 2006 4,017 2.0 3.6 

Table 4. Monthly averages for NAS process parameters 

Month 

NAS MLSS 

(mg/L) 

NAS sludge age 

(d) NAS HRT (hr) 

Jan 2006 4,264 34.8 7.1 

Feb 2006 3,800 29.9 6.7 

Mar 2006 3,617 46.6 7.4 

Apr 2006 2,794 34.7 7.7 

May 2006 3,644 24.3 7.7 

June 2006 3,706 21.4 7.2 

July 2006 3,523 72.5 7.3 

Aug 2006 4,286 65.6 7.9 

Sept 2006 4,987 84.6 7.1 

Oct 2006 4,806 79.7 6.9 

Nov 2006 4,212 34.4 6.2 

Dec 2006 5,117 43.2 7.3 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 7

Table 5. Monthly averages for DNAS process parameters 

Month 

DNAS MLSS 

(mg/L) 

DNAS sludge age 

(d) 

DNAS HRT 

(hr) 

Jan 2006 5,006 32.6 3.4 

Feb 2006 4,329 24.4 3.3 

Mar 2006 3,541 17.8 3.6 

Apr 2006 3,818 8.8 3.7 

May 2006 2,795 5.8 3.8 

June 2006 3,427 11.9 3.5 

July 2006 4,201 23.3 3.5 

Aug 2006 3,192 10.9 3.8 

Sept 2006 3,939 58.4 3.5 

Oct 2006 3,968 18.9 3.4 

Nov 2006 4,081 40.2 3.0 

Dec 2006 4,990 17.0 3.6 

Table 6. Monthly averages for influent temperature 

Month 

Temperature 

(

o

F) 

Temperature 

(

o

C) 

Jan 2006 58.5 14.7 

Feb 2006 56.3 13.5 

Mar 2006 57.5 14.2 

Apr 2006 61.9 16.6 

May 2006 64.4 18.0 

June 2006 68.2 20.1 

July 2006 72.4 22.4 

Aug 2006 73.6 23.1 

Sept 2006 71.4 21.9 

Oct 2006 67.7 19.8 

Nov 2006 63.9 17.7 

Dec 2006 61.1 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

8 - Western Branch, MD z Wastewater Treatment Plant Appendix A

Western Branch WWTP, WSSC

Monthly Average Frequency Curves for Total Phosphorus

0.001

0.01

0.1

1

10

100

Percent Less Than or Equal To

Total Phosphorus, mg/L

Plant Influent HRAS Influent HRAS Effluent

NAS Effluent DNAS Effluent Final Effluent

0.05 0.1 50 0.5 1 2 5 2010 4030 60 70 80 90 95 98 99 99.5 99.95 99.9

Mean = 0.43 mg/L

Std. Dev. = 0.27 mg/L

C.O.V. = 62%

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

0.001

0.01

0.1

1

10

100

Percent Less Than or Equal To

Ammonia-Nitrogen, mg/L

Combined Raw Influent HRAS Influent HRAS Effluent

NAS Effluent DNAS Effluent Final Effluent

0.05 0.1 0.5 1 2 5 302010 40 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 0.13 mg/L

Std. Dev. = 0.22 mg/L

C.O.V. = 163%

Figure 4. Monthly average frequency curves for ammonia nitrogen. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 9

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. 

Western Branch WWTP, WSSC

Monthly Average Frequency Curves for Total Nitrogen

0.1

1

10

100

Percent Less Than or Equal To

Nitrogen, mg/L

Combined Raw Influent HRAS Influent NAS Effluent

DNAS Effluent Final Effluent

0.05 0.1 0.5 1 2 5 2010 4030 50 60 70 80 90 95 98 99 99.5 99.9599.9

Mean = 1.63 mg/L

Std. Dev. = 0.59 mg/L

C.O.V. = 36%

Figure 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. 
Nutrient Removal Technology Assessment Case Study September 2008 

10 - Western Branch, MD z Wastewater Treatment Plant Appendix A

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, or NAS. 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 lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 11

Wet-weather operation: Normal operating procedures are followed. No off-line storage is 

available. 

Costs 

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 MGD. 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 Engineering 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 

Year 

Original 

cost 2007 cost %P %N 

% 

other

Phosphorus 

cost 

Nitrogen 

cost 

Phase 1 1973 $15,000,000 $62,865,435 0% 20% 80% $0 $12,573,087

Phase 2 1976 $7,500,000 $24,808,413 0% 30% 70% $0 $7,442,524

Phase 3 1991 $30,000,000 $49,278,180 5% 60% 35% $2,463,909 $29,566,908

Total $136,952,028 $2,463,909 $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. 
Nutrient Removal Technology Assessment Case Study September 2008 

12 - Western Branch, MD z Wastewater Treatment Plant Appendix A

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 lb 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 lb/yr) of methanol has a chemical oxygen demand (COD) of 

1.5 lb COD/lb of methanol, or 4.2 million lb COD/yr. The typical yield of volatile suspended 

solids (VSS) on methanol is 0.4 lb VSS/lb of COD, giving 1.7 million lb 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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 13

Unit Costs for Nitrogen and Phosphorus Removal 

During the evaluation period, the plant removed 213,000 lb 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 lb 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 lb 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 
Nutrient Removal Technology Assessment Case Study September 2008 

14 - Western Branch, MD z Wastewater Treatment Plant Appendix A

(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. 
September 2008 Nutrient Removal Technology Assessment Case Study 

Appendix A Western Branch, MD z Wastewater Treatment Plant - 15

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. 

Acknowledgments 

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. 
16 - Western Branch - MD z Wastewater Treatment PlantAppendix A

Attachment: Electrical Costs 

kW kWh kWh %P %N 

HP Number Power draw 

hours/ 

day draw/day draw/year 

HRAS/NAS blowers 1,500 1 1,119 24 26,856 9,802,440 0% 50% 

Raw pumps 250 2 373 24 8,952 3,267,480 5% 20% 

Denite mixers 20 16 238.72 24 5,729.28 2,091,187.2 0% 70% 

ID fans 75 1 55.95 24 1,342.8 490,122 0% 10% 

Final RAS pumps 100 4 298.4 24 7,161.6 2,613,984 0% 0% 

Stripping channel 

blowers 200 2 298.4 24 7,161.6 2,613,984 0% 20% 

Centrifuge 300 1 223.8 24 5,371.2 1,960,488 5% 5% 

Air lift pump blowers 60 6 268.56 24 6,445.44 2,352,585.6 0% 50% 

Total draw 25,192,270.8 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Appendix B: Reliability and Coefficient of Variation B-1

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-1 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-1. 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-1, 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 
Municipal Nutrient Removal Technologies Reference Document September 2008 

B-2 Appendix B: Reliability and Coefficient of Variation 

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. 

0.01

0.1

1

Percent Less Than or Equal To

Total Phosphorus, mg/L

0.05 0.1 0.5 1 2 5 302010 5040 60 70 80 92.3 95 98.1 99 99.7 99.9599.9

DailyMean = 0.086 mg/L

Std. Dev. = 0.039 mg/L

COV = 45%

WeeklyMean = 0.086 mg/L

Std. Dev. = 0.0248 mg/L

COV = 29%

MonthlyMean = 0.086 mg/L

Std. Dev. = 0.0179 mg/L

COV = 21%

86

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. 
September 2008 Municipal Nutrient Removal Technologies Reference Document 

Appendix B: Reliability and Coefficient of Variation B-3

0.01

0.1

1

10

Percent Less Than or Equal To

Total Phosphorus, mg/L

Raw Influent Primary Effluent Secondary Effluent Tertiary Effluent Final Effluent

0.05 0.1 0.5 1 2 5 40302010 50 60 70 80 90 95 98 99 99.5 99.95 99.9

Final Effluent

Mean = 0.09 mg/L

Std. Dev. = 0.025 mg/L

COV = 29%

Tertiary Effluent

Mean = 0.36 mg/L

Std. Dev. = 0.118 mg/L Secondary Effluent COV = 33%

Mean = 0.74 mg/L

Std. Dev. = 0.37 mg/L

COV = 50%

Primary Effluent

Mean = 4.88 mg/L

Std. Dev. = 0.55 mg/L

COV = 11%

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.