This Fact Sheet was prepared to provide 

information on the types of biological 

nutrient removal technologies, nutrient 

removal efficiencies, and the associated 

costs for small and large municipal systems. 
Biological Nutrient Removal Processes and Costs 

Nitrogen and phosphorus are the primary causes of cultural eutrophication (i.e., nutrient enrichment 

due to human activities) in surface waters. The most recognizable manifestations of this 

eutrophication are algal blooms that occur during the summer. Chronic symptoms of over-enrichment 

include low dissolved oxygen, fish kills, murky water, and depletion of desirable flora and fauna. In 

addition, the increase in algae and turbidity increases the need to chlorinate drinking water, which, in 

turn, leads to higher levels of disinfection by-products that have been shown to increase the risk of 

cancer. Excessive amounts of nutrients can also stimulate the activity of microbes, such as Pfisteria, 

which may be harmful to human health (U.S. EPA, 2001). 

Approximately 25% of all water body impairments are due to nutrient-related causes (e.g., nutrients, 

oxygen depletion, algal growth, ammonia, harmful algal blooms, biological integrity, and turbidity) 

(U.S. EPA, 2007). In efforts to reduce the number of nutrient impairments, many point source 

dischargers have received more stringent effluent limits for nitrogen and phosphorus. To achieve 

these new, lower effluent limits, facilities have begun to look beyond traditional treatment 

technologies. 

Description 

Biological nutrient removal (BNR) removes total nitrogen (TN) and total phosphorus (TP) from 

wastewater through the use of microorganisms under different environmental conditions in the 

treatment process (Metcalf and Eddy, 2003). 

Nitrogen Removal 

Total effluent nitrogen comprises ammonia, nitrate, particulate organic nitrogen, and soluble organic 

nitrogen. The biological processes that primarily remove nitrogen are nitrification and denitrification 

(Jeyanayagam, 2005). During nitrification ammonia is oxidized to nitrite by one group of autotrophic 

bacteria, most commonly Nitrosomonas (Metcalf and Eddy, 2003). Nitrite is then oxidized to nitrate 

by another autotrophic bacteria group, the most common being Nitrobacter. 

Denitrification involves the biological reduction of nitrate to nitric oxide, nitrous oxide, and nitrogen 

gas (Metcalf and Eddy, 2003). Both heterotrophic and autotrophic bacteria are capable of 

denitrification. The most common and widely distributed denitrifying bacteria are Pseudomonas 

species, which can use hydrogen, methanol, carbohydrates, organic acids, alcohols, benzoates, and 

other aromatic compounds for denitrification (Metcalf and Eddy, 2003). 

In BNR systems, nitrification is the controlling reaction because ammonia oxidizing bacteria lack 

functional diversity, have stringent growth requirements, and are sensitive to environmental 

conditions (Jeyanayagam, 2005). Note that nitrification by itself does not actually remove nitrogen 

from wastewater. Rather, denitrification is needed to convert the oxidized form of nitrogen (nitrate) 

to nitrogen gas. Nitrification occurs in the presence of oxygen under aerobic conditions, and 

denitrification occurs in the absence of oxygen under anoxic conditions. 

1
 
Exhibit 1 summarizes the removal mechanisms applicable to each form of nitrogen. 

Exhibit 1. Mechanisms Involved in the Removal of Total Nitrogen 

Form of Nitrogen Common Removal Mechanism Technology Limit (mg/L) 

Ammonia-N Nitrification <0.5 

Nitrate-N Denitrification 1 – 2 

Particulate organic-N Solids separation <1.0 

Soluble organic-N None 0.5 – 1.5 

Source: Jeyanayagam (2005). 

Note that organic nitrogen is not removed biologically; rather only the particulate fraction can be 

removed through solids separation via sedimentation or filtration. 

Phosphorus Removal 

Total effluent phosphorus comprises soluble and particulate phosphorus. Particulate phosphorus can 

be removed from wastewater through solids removal. To achieve low effluent concentrations, the 

soluble fraction of phosphorus must also be targeted. Exhibit 2 shows the removal mechanisms for 

phosphorus. 

Exhibit 2. Mechanisms Involved in the Removal of Total Phosphorus 

Form of Phosphorus Common Removal Mechanism Technology Limit (mg/L) 

Soluble phosphorus Microbial uptake 

Chemical precipitation 0.1 

Particulate phosphorus Solids removal <0.05 

Source: Jeyanayagam (2005). 

Biological phosphorus removal relies on phosphorus uptake by aerobic heterotrophs capable of 

storing orthophosphate in excess of their biological growth requirements. The treatment process can 

be designed to promote the growth of these organisms, known as phosphate-accumulating organisms 

(PAOs) in mixed liquor (WEF and ASCE/EWRI, 2006). Under anaerobic conditions, PAOs convert 

readily available organic matter [e.g., volatile fatty acids (VFAs)] to carbon compounds called polyhydroxyalkanoates (PHAs). PAOs use energy generated through the breakdown of polyphosphate 

molecules to create PHAs. This breakdown results in the release of phosphorus (WEF and 

ASCE/EWRI, 2006). 

Under subsequent aerobic conditions in the treatment process, PAOs use the stored PHAs as energy 

to take up the phosphorus that was released in the anaerobic zone, as well as any additional 

phosphate present in the wastewater. In addition to reducing the phosphate concentration, the process 

renews the polyphosphate pool in the return sludge so that the process can be repeated 

(Jeyanayagam, 2005). 

Some PAOs use nitrate instead of free oxygen to oxidize stored PHAs and take up phosphorus. These 

denitrifying PAOs remove phosphorus in the anoxic zone, rather than the aerobic zone 

(Jeyanayagam, 2005). 

As shown in Exhibit 2, phosphorus can also be removed from wastewater through chemical 

precipitation. Chemical precipitation primarily uses aluminum and iron coagulants or lime to form 

2
 
 

chemical flocs with phosphorus. These flocs are then settled out to remove phosphorus from the 

wastewater (Viessman and Hammer, 1998). However, compared to biological removal of 

phosphorus, chemical processes have higher operating costs, produce more sludge, and result in 

added chemicals in sludge (Metcalf and Eddy, 2003). When TP levels close to 0.1 mg/L are needed, 

a combination of biological and chemical processes may be less costly than either process by itself. 

Process 

There are a number of BNR process configurations available. Some BNR systems are designed to 

remove only TN or TP, while others remove both. The configuration most appropriate for any 

particular system depends on the target effluent quality, operator experience, influent quality, and 

existing treatment processes, if retrofitting an existing facility. BNR configurations vary based on the 

sequencing of environmental conditions (i.e., aerobic, anaerobic, and anoxic)1

 and timing 

(Jeyanayagam, 2005). Common BNR system configurations include: 

•	 Modified Ludzack-Ettinger (MLE) Process – continuous-flow suspended-growth process 

with an initial anoxic stage followed by an aerobic stage; used to remove TN 

2 •	 A /O Process – MLE process preceded by an initial anaerobic stage; used to remove both TN 

and TP 

•	 Step Feed Process – alternating anoxic and aerobic stages; however, influent flow is split to 

several feed locations and the recycle sludge stream is sent to the beginning of the process; 

used to remove TN 

•	 Bardenpho Process (Four-Stage) – continuous-flow suspended-growth process with 

alternating anoxic/aerobic/anoxic/aerobic stages; used to remove TN 

•	 Modified Bardenpho Process – Bardenpho process with addition of an initial anaerobic zone; 

used to remove both TN and TP 

•	 Sequencing Batch Reactor (SBR) Process – suspended-growth batch process sequenced to 

simulate the four-stage process; used to remove TN (TP removal is inconsistent) 

• Modified Un 2 	 iversity of Cape Town (UCT) Process – A /O Process with a second anoxic 

stage where the internal nitrate recycle is returned; used to remove both TN and TP 

•	 Rotating Biological Contactor (RBC) Process – continuous-flow process using RBCs with 

sequential anoxic/aerobic stages; used to remove TN 

•	 Oxidation Ditch – continuous-flow process using looped channels to create time sequenced 

anoxic, aerobic, and anaerobic zones; used to remove both TN and TP. 

Although the exact configurations of each system differ, BNR systems designed to remove TN must 

have an aerobic zone for nitrification and an anoxic zone for denitrification, and BNR systems 

designed to remove TP must have an anaerobic zone free of dissolved oxygen and nitrate. Often, 

sand or other media filtration is used as a polishing step to remove particulate matter when low TN 

and TP effluent concentrations are required. Sand filtration can also be combined with attached 

growth denitrification filters to further reduce soluble nitrates and effluent TN levels (WEF and 

ASCE/EWRI, 2006). 

Choosing which system is most appropriate for a particular facility primarily depends on the target 

effluent concentrations, and whether the facility will be constructed as new or retrofit with BNR to 

achieve more stringent effluent limits. New plants have more flexibility and options when deciding 

1

 Anoxic is a condition in which oxygen is available only in the combined form (e.g., NO - or NO - 2 3 ). However, 

anaerobic is a condition in which neither free nor combined oxygen is available (WEF and ASCE/EWRI, 2006). 

3
 
which BNR configuration to implement because they are not constrained by existing treatment units 

and sludge handling procedures. 

Retrofitting an existing plant with BNR capabilities should involve consideration of the following 

factors (Park, no date): 

• Aeration basin size and configuration 

• Clarifier capacity 

• Type of aeration system 

• Sludge processing units 

• Operator skills 

The aeration basin size and configuration dictates which BNR configurations are the most economical 

and feasible. Available excess capacity reduces the need for additional basins, and may allow for a 

more complex configuration (e.g., 5-stage Bardenpho versus 4-stage Bardenpho configuration). The 

need for additional basins can result in the need for more land if the space needed is not available. If 

land is not available, another BNR process configuration may have to be considered. 

Clarifier capacity influences the return activated sludge (RAS) rate and effluent suspended solids, 

which in turn, affects effluent TN and TP levels. If the existing facility configuration does not allow for 

a preanoxic zone so that nitrates can be removed prior to the anaerobic zone, then the clarifier should 

be modified to have a sludge blanket just deep enough to prevent the release of phosphorus to the 

liquid. However, if a preanoxic zone is feasible, a sludge blanket in the clarifier may not be necessary 

(WEF and ASCE/EWRI, 2006). The existing clarifiers also remove suspended solids including 

particulate nitrogen and phosphorus, and thus, reduce total nitrogen and phosphorus concentrations. 

The aeration system will most likely need to be modified to accommodate an anaerobic zone, and to 

reduce the DO concentration in the return sludge. Such modifications could be as simple as removing 

aeration equipment from the zone designated for anaerobic conditions or changing the type of pump 

used for the recycled sludge stream (to avoid introducing oxygen). 

The manner in which sludge is processed at a facility is important in designing nutrient removal 

systems. Sludge is recycled within the process to provide the organisms necessary for the TN and TP 

removal mechanisms to occur. The content and volume of sludge recycled directly impacts the 

system’s performance. Thus, sludge handling processes may have to be modified to achieve optimal 

TN and TP removal efficiencies. For example, some polymers in sludge dewatering could inhibit 

nitrification when recycled. Also, because aerobic digestion of sludge produces nitrates, 

denitrification and phosphorus uptake rates may be lowered when the sludge is recycled (WEF and 

ASCE/EWRI, 2006). 

Operators should be able to adjust the process to compensate for constantly varying conditions. BNR 

processes are very sensitive to influent conditions which are influenced by weather events, sludge 

processing, and other treatment processes (e.g., recycling after filter backwashing). Therefore, 

operator skills and training are essential for achieving target TN and TP effluent concentrations. 

Performance 

Exhibit 3 provides a comparison of the TN and TP removal capabilities of common BNR 

configurations. Note that site-specific conditions dictate the performance of each process, and that 

the exhibit is only meant to provide a general comparison of treatment performance among the 

various BNR configurations. 

4
 
Exhibit 3. Comparison of Common BNR Configurations 

Process Nitrogen Removal Phosphorus Removal 

MLE Good None 

A2

/O Good Good 

Step Feed Moderate None 

Four-Stage Bardenpho Excellent None 

Modified Bardenpho Excellent Good 

SBR Moderate Inconsistent 

Modified UCT Good Excellent 

Oxidation Ditch Excellent Good 

Source: Jeyanayagam (2005). 

The limit of technology (LOT), at least for larger treatment plants, is 3 mg/L for TN and 0.1 mg/L for 

TP (Jeyanayagam, 2005). However, some facilities may be able to achieve concentrations lower than 

these levels due to site-specific conditions. 

Exhibit 4 provides TN and TP effluent concentrations for various facilities using BNR processes. 

Exhibit 4. Treatment Performance of Various BNR Process Configurations 

Treatment Plant (State) 

Treatment Process 

Description 

Flow 

(mgd) 

Average Effluent Concentration 

(mg/L)1 

TN TP 

Annapolis (MD) Bardenpho (4-Stage) 13 7.1 0.66 

Back River (MD) MLE 180 7.6 0.19 

Bowie (MD) Oxidation Ditch 3.3 6.6 0.20 

Cambridge (MD) MLE 8.1 3.2 0.34 

Cape Coral (FL) Modified Bardenpho 8.5 1.0 0.2 

Cox Creek (MD) MLE 15 9.7 0.89 

Cumberland (MD) Step Feed 15 7.0 1.0 

Frederick (MD) A2

/O 7 7.2 1.0 

Freedom District (MD) MLE 3.5 7.8 0.51 

Largo (FL) A2

/O 15 2.3 ND 

Medford Lakes (NJ) Bardenpho (5-stage) 0.37 2.6 0.09 

Palmetto (FL) Bardenpho (4-stage) 1.4 3.2 0.82 

Piscataway (MD) Step Feed 30 2.7 0.09 

Seneca (MD) MLE 20 6.4 0.08 

Sod Run (MD) Modified A2

/O 20 9.2 0.86 

Westminster (MD) MLE-A2

/O 5 5.3 0.79 

Sources: EPA (2006); Gannett Fleming (no date); Park (no date). 

mgd = million gallons per day
 

ND = no data 

1

 Represents the average of average monthly values from 2003 to 2006, where available. 

5
 
LOT levels (i.e., TN less than 3 mg/L and TP less than 0.1 mg/L) have not been demonstrated at 

treatment plants with capacities of less than 0.1 mgd (Foess, et al., 1998). BNR for TN removal may 

be feasible and cost effective. However, BNR for TP removal is often not cost effective at small 

treatment plants (Keplinger, et al., 2004). Therefore, performance data for TP removal at small 

treatment plants is limited. Exhibit 5 summarizes the TN levels achievable with various BNR 

configurations. 

Exhibit 5. BNR Performance for Small Systems (Less than 0.1 mgd) 

BNR Process Achievable TN Effluent Quality 

MLE 10 mg/L 

Four-Stage Bardenpho 6 mg/L 

Three-Stage Bardenpho 6 mg/L 

SBR 8 mg/L 

RBC 12 mg/L 

Source: Foess et al. (1998). 

Operation and Maintenance 

For BNR systems to result in low TN and TP effluent concentrations, proper operation and control of 

the systems is essential. Operators should be trained to understand how temperature, dissolved 

oxygen (DO) levels, pH, filamentous growth, and recycle loads affect system performance. 

Biological nitrogen removal reaction rates are temperature dependent. Nitrification and 

denitrification rates increase as temperature increases (until a maximum temperature is reached). In 

general, nitrification rates double for every 8 to 10°C rise in temperature (WEF and ASCE/EWRI, 

2006). The effect of temperature on biological phosphorus removal is not completely understood 

(WEF and ASCE/EWRI, 2006), although rates usually slow at temperatures above 30°C 

(Jeyanayagam, 2005). 

DO must be present in the aerobic zone for phosphorus uptake to occur. However, it is important not 

to over-aerate. DO concentrations around 1 mg/L are sufficient. Over-aeration can lead to secondary 

release of phosphorus due to cell lysis, high DO levels in the internal mixed liquor recycle (which 

could reduce TP and TN removal rates), and increased operation and maintenance (O&M) costs 

(Jeyanayagam, 2005). 

There is evidence that both nitrification and phosphorus removal rates decrease when pH levels drop 

below 6.9. Nitrification results in the consumption of alkalinity. As alkalinity is consumed, pH 

decreases. Thus, treatment plants with low influent alkalinity may have reduced nitrification rates 

(WEF and ASCE/EWRI, 2006). Glycogen-accumulating organisms may also compete with PAOs at 

pH values less than 7. 

Filamentous growth can cause poor settling of particulate nitrogen and phosphorus in final clarifiers. 

However, many conditions necessary to achieve good BNR rates, such as low DO, longer solids 

retention times, good mixing, also promote filament growth (Jeyanayagam, 2005). Therefore, 

operators may need to identify the dominate filaments present in the system so that they can design 

strategies to target their removal (e.g., chlorinating recycle streams, chemical addition as polishing 

step) while still maintaining nutrient removal rates. 

6
 
 

 

Nitrogen and phosphorus removal efficiencies are a function of the percentage and content of the 

mixed liquor recycle rate to the anoxic zone and the RAS recycle rate to the anaerobic zone (WEF 

and ASCE/EWRI, 2006). The mixed liquor recycle stream supplies active biomass that enables 

nitrification and denitrification. Optimizing the percentage and content of this recycle stream results 

in optimal TN removal. The RAS recycle rate should be kept as low as possible to reduce amount of 

nitrates introduced to the anaerobic zone because nitrates interfere with TP removal. In addition, the 

type of pump used to recycle the activated sludge is important to avoid aeration and increased DO 

concentrations in the anaerobic zone (WEF and ASCE/EWRI, 2006). 

Costs 

BNR costs differ for new plants and retrofits. New plant BNR costs are based on estimated influent 

quality, target effluent quality, and available funding. Retrofit costs, on the other hand, are more sitespecific and vary considerably for any given size category. Retrofit costs are based on the same 

factors as new plants, in addition to the layout and design of the existing treatment processes. 

Exhibit 6 provides capital costs to upgrade wastewater treatment plants in Maryland with BNR. 

These costs represent retrofits of existing facilities. 

Exhibit 6. BNR Upgrade Costs for Maryland Wastewater Treatment Plants 

Facilities with BNR 

(as of 10/30/06) 

Design 

Capacity (mgd) Treatment Process 

Completion 

Date 

Total Capital BNR 

Cost (2006$)1 

Aberdeen 2.8 MLE Dec-98 $3,177,679 

Annapolis 10 Ringlace Nov-00 $14,687,326 

Back River 180 MLE Jun-98 $138,305,987 

Ballenger 2.0 Modified Bardenpho Aug-95 $2,891,906 

Broadneck 6.0 Oxidation Ditch 1994 $3,165,193 

Broadwater 2.0 MLE May-00 $6,892,150 

Cambridge 8.1 Activated Sludge Apr-03 $11,740,209 

Celanese 1.25 Sequential step feed Jun-05 $7,424,068 

Centreville 0.375 SBR/Land Application Apr-05 $7,336,020 

Chesapeake Beach 0.75 Oxidation Ditch 1992 $2,158,215 

Conococheague 2.5 Carrousel Nov-01 $6,620,888 

Cox Creek 15 MLE May-02 $11,466,657 

Cumberland 15 MLE Aug-01 $12,929,990 

Denton 0.45 Biolac Dec-00 $4,203,767 

Dorsey Run 2.0 Methanol 1992 $3,967,307 

Emmitsburg 0.75 Overland 1996 $2,562,722 

Frederick 8.0 MLE Sep-02 $11,916,504 

Freedom District 3.5 Activated Sludge 1994 $1,462,798 

Fruitland 0.50 SBR Jul-03 $7,546,764 

Hagerstown 8.0 Johannesburg Process Dec-00 $11,190,344 

Havre DeGrace 1.89 MLE Nov-02 $7,596,882 

Hurlock 2.0 Bardenpho Aug-06 $5,200,000 

Joppatowne 0.95 MLE Jul-96 $2,433,205 

La Plata 1.0 MLE Jun-02 $4,952,150 

7
 
 

Exhibit 6. BNR Upgrade Costs for Maryland Wastewater Treatment Plants 

Facilities with BNR 

(as of 10/30/06) 

Design 

Capacity (mgd) Treatment Process 

Completion 

Date 

Total Capital BNR 

Cost (2006$)1 

Leonardtown 0.65 Biolac Oct-03 $2,811,448 

Little Patuxent 18 A2

/O 1994 $7,263,879 

Marlay Taylor (Pine Hill Run) 4.5 Schreiber Jun-98 $4,986,641 

Maryland City 2.5 Schreiber 1990 $1,375,866 

Maryland Correctional 

Institute 1.23 Bardenpho 1995 $2,703,932 

Mt. Airy 0.60 Activated Sludge Jul-99 $5,235,575 

Northeast 2.0 Activated Sludge Oct-04 $4,225,029 

Parkway 7.5 Methanol 1992 $15,869,228 

Patuxent 6.0 Oxidation Ditch 1990 $2,106,763 

Piscataway 30 MLE Jul-00 $24,778,239 

Pocomoke City 1.4 Biolac Sep-04 $3,924,240 

Poolesville 0.625 SBR Jan-05 $1,593,640 

Princess Anne 1.26 Activated Sludge 2002 $4,311,742 

Seneca 5.0 MLE Dec-03 $34,886,034 

Sod Run 12 MLE 2000 $21,999,198 

Taneytown 0.70 SBR Apr-00 $3,808,298 

Thurmont 1.0 MLE Dec-96 $3,122,264 

Western Branch 30 Methanol Jul-95 $47,132,782 

Westminster 5.0 Activated Sludge Jan-01 $5,274,444 

Source: MDE (2006). 

mgd = million gallons per day

1

 Total capital BNR upgrade costs eligible for Maryland Department of the Environment 50% cost share 

(http://www.mde.state.md.us/Programs/WaterPrograms/WQIP/wqip_bnr.asp) including engineering, pilot study, design, and 

construction, updated to 2006 dollars using the ENR construction cost index assuming that the completion date represents 

the original year dollars (2006 ENR index = 7910.81). 

8
 
Exhibit 7 shows BNR retrofit costs for wastewater treatment plants in Connecticut. 

Exhibit 7. BNR Upgrade Costs for Connecticut Wastewater Treatment Plants 

Facilities with BNR 

Design 

Capacity 

(mgd) Treatment Process2 

Year Process 

In Service 

Total Capital BNR 

Cost (2006$)1 

Branford 4.5 4-Stage Bardenpho 2003 $3,732,049 

Bridgeport East Phase 1 12 MLE* 2004 $2,323,766 

Bridgeport West Phase 1 29 MLE* 2004 $2,640,643 

Bristol Phase 1 10.75 MLE* 2004 $649,320 

Derby 3.03 MLE* 2000 $3,513,514 

East Hampton 3.9 MLE* 2001 $860,548 

East Windsor 2.5 MLE 1996 $1,407,617 

Fairfield Phase 2 9 4-Stage Bardenpho 2003 $14,235,676 

Greenwich 12 MLE* 1996 $703,809 

Ledyard 0.24 SBR 1997 $4,752,461 

Milford BB Phase 1 3.1 4-Stage Bardenpho 1996 $1,407,617 

New Canaan 1.5 MLE 2000 $1,570,463 

New Haven Phase 1 40 MLE* 1997 $11,134,336 

New London 10 MLE* 2002 $3,495,615 

Newtown 0.932 MLE* 1997 $1,436,601 

Norwalk Phase 1 15 MLE* 1996 $1,548,379 

Norwalk Phase 2 15 MLE 2000 $7,042,287 

Portland 1 MLE 2002 $1,266,843 

Seymour 2.93 MLE* 1993 $379,597 

Stratford Phase1 11.5 4-Stage Bardenpho 1996 $1,126,094 

Thomaston 1.2 SBR 2001 $1,451,708 

University of Connecticut 1.98 MLE 1996 $1,489,259 

Waterbury 25 4-Stage Bardenpho 2000 $22,074,225 

Source: CT DEP (2007). 

mgd = million gallons per day

1

 Total capital BNR upgrade projects financed by the Clean Water Fund through 2006, updated to 2006 dollars using the 

ENR construction cost index assuming that the year in service date represents the original year dollars (2006 ENR index = 

7910.81).

2

 Treatment process with an “*” are designed to meet interim TN limits of 6 – 8 mg/L; all other facilities designed to meet TN
 

limits of 3 – 5 mg/L. 

Site-specific factors such as existing treatment system layout and space availability may cause costs 

to vary significantly between treatment plants with the same design capacities implementing the 

same BNR configuration. For example, the La Plata and Thurmont wastewater treatment plants in 

Maryland both have design capacities of 1 mgd and upgraded to a modified Ludzack-Ettinger (MLE) 

BNR system. However, total capital costs to retrofit the La Plata facility ($5.0 million) exceed those 

for the Thurmont facility ($3.1 million) by more than $1.8 million. 

9
 
Despite this variability in costs, unit costs (i.e., total capital cost per mgd) generally decrease as the 

size of the plant increases due to economies of scale. Exhibit 8 illustrates this relationship for the 

Maryland and Connecticut facility upgrades presented in Exhibits 6 and 7 for three system size 

categories. 

Exhibit 8. Average Unit Capital Costs for BNR Upgrades at MD and CT Wastewater 

Treatment Plants (2006$)1 

Flow (mgd) Cost/mgd 

>0.1 – 1.0 $6,972,000 

>1.0 – 10.0 $1,742,000 

>10.0 $588,000 

Source: Based on MDE (2006) and CTDEP (2007). 

mgd = million gallons per day

1

 Calculated from cost information from Maryland Department of the Environment for 43 facilities and Connecticut 

Department of Environmental Protection for 23 facilities; costs updated to 2006 dollars based on project completion date 

using the ENR construction cost index (2006 index = 7910.81). 

BNR systems for smaller facilities (i.e., flow less than 0.1 mgd) are usually pre-engineered, factory-, 

or field-assembled package systems (Foess, et al., 1998). In most cases, chemical phosphorus 

removal is preferred over biological removal because most small systems lack the operational 

oversight necessary to achieve low phosphorus levels with biological treatment. In addition, small 

systems will likely need effluent polishing filtration for added nitrogen removal (Foess, et al., 1998). 

Exhibit 9 summarizes average BNR costs for small systems. The construction costs include all 

required facilities for a new plant on a new site, including filtration. O&M costs include labor, 

electricity, maintenance and repair materials, solids handling and disposal, administration labor, 

laboratory analytical requirements, and chemical costs (Foess, et al., 1998). 

Exhibit 9. Average BNR Costs for Small Systems: New Plants (2006$)1 

System 4,000 gpd 10,000 gpd 25,000 gpd 50,000 gpd 100,000 gpd 

MLE Process 

Construction $348,771 $415,585 $563,912 $803,108 $1,167,914 

O&M $37,263 $43,515 $60,553 $81,636 $122,699 

4-Stage Process 

Construction $448,992 $491,753 $634,736 $889,966 $1,293,524 

O&M $64,353 $70,604 $90,462 $117,551 $162,169 

3-Stage Process 

Construction $388,859 $444,983 $589,302 $837,851 $1,220,029 

O&M $44,005 $51,360 $69,133 $93,403 $142,066 

SBR Process 

Construction $448,992 $509,125 $644,090 $931,391 $1,290,852 

O&M $34,321 $41,799 $60,185 $82,862 $122,577 

Intermittent Process 

Construction $306,009 $499,771 $780,391 $1,150,542 $1,371,029 

O&M $34,321 $41,799 $60,185 $82,862 $122,577 

10
 
Exhibit 9. Average BNR Costs for Small Systems: New Plants (2006$)1 

System 4,000 gpd 10,000 gpd 25,000 gpd 50,000 gpd 100,000 gpd 

MLE and Deep Bed Filtration 

Construction $411,576 $491,753 $649,435 $887,294 $1,280,161 

O&M $45,231 $52,340 $71,217 $93,036 $136,550 

Submerged Biofilter Process 

Construction $330,063 $395,541 $601,328 $1,131,834 (2) 

O&M $23,902 $29,909 $50,379 $74,036 (2) 

RBC Process 

Construction $351,443 $457,010 $704,222 $1,159,896 $1,459,224 

O&M $25,006 $31,747 $53,198 $75,385 $109,584 

Source: Foess, et al. (1998). 

gpd = gallons per day

1

 Construction costs updated from 1998 dollars using the ENR construction cost index (2006 index = 7910.81); O&M costs 

updated from 1998 dollars using the Bureau of Labor Statistics consumer cost index (2006 index = 199.8). 2

 Exceeded manufacturer’s sizes. 

Retrofit opportunities are more limited at smaller facilities; however two retrofit alternatives may 

exist for nitrogen removal. The MLE process can be retrofitted by adding an anoxic basin upstream 

of the existing influent point and adding recirculation pumping from the existing aeration basin to the 

new anoxic basin. Also, deep-bed denitrification filters can be added downstream of an existing 

package plant. The retrofit involves installation of new pumping facilities to pump secondary effluent 

to the filters, methanol feed equipment, and chemical feed equipment (for phosphorus removal) 

(Foess, et al., 1998). O&M costs represent only the incremental costs associated with the additional 

equipment. Exhibit 10 summarizes these costs. 

Exhibit 10. Average BNR Costs for Small Systems: Retrofits (2006$)1 

System 4,000 gpd 10,000 gpd 25,000 gpd 50,000 gpd 100,000 gpd 

Anoxic Tank for MLE Upgrade 

Construction $28,062 $32,071 $52,115 $76,168 $80,000 

O&M $14,832 $15,445 $16,425 $22,922 $21,100 

Deep Bed Denitrification Filter 

Construction $145,655 $161,691 $196,434 $217,815 $213,000 

O&M $21,573 $22,309 $24,883 $30,399 $28,600 

Source: Foess, et al. (1998). 

gpd = gallons per day

1

 Construction costs updated from 1998 dollars using the ENR construction cost index (2006 index = 7910.81); O&M costs 

updated from 1998 dollars using the Bureau of Labor Statistics consumer cost index (2006 index = 199.8). 

Similar to large facilities, unit costs for smaller facilities also tend to decrease as flow increases. 

Exhibit 11 summarizes average unit costs across all treatment processes for new plants and retrofits 

based on the cost information in Exhibits 9 and 10. 

11
 
Exhibit 11. BNR Unit Costs for Small Systems (2006 Dollars) 

Component 4,000 gpd 10,000 gpd 25,000 gpd 50,000 gpd 100,000 gpd 

New Plants 

Construction $70.97/gpd $34.66/gpd $19.34/gpd $14.58/gpd $8.50/gpd 

O&M $7.86/gpd $3.70/gpd $2.10/gpd $1.43/gpd $0.94/gpd 

Retrofits 

Construction $16.25/gpd $7.25/gpd $3.72/gpd $2.20/gpd $1.47/gpd 

O&M $3.71/gpd $1.54/gpd $0.67/gpd $0.44/gpd $0.25/gpd 

Source: Foess, et al. (1998). 

gpd = gallons per day

1

 Construction costs updated from 1998 dollars using the ENR construction cost index; O&M costs updated from 1998 

dollars using the Bureau of Labor Statistics consumer cost index.
 

References 

Connecticut Department of Environmental Protection (CTDEP). 2007. Nitrogen Removal Projects 

Financed by the CWF through 2006. Provided by Iliana Ayala June 13, 2007. 

Foess, G.W., P. Steinbrecher, K. Williams, G.S. Garrett. 1998. Cost and Performance Evaluation of 

BNR Processes. Florida Water Resources Journal: December 1998. 

Gannett Fleming. No date. Refinement of Nitrogen Removal from Municipal Wastewater Treatment 

Plants. Prepared for the Maryland Department of the Environment. Online at 

http://www.mde.state.md.us/assets/document/BRF%20Gannett%20FlemingGMB%20presentation.pdf. 

Jeyanayagam, Sam. 2005. True Confessions of the Biological Nutrient Removal Process. Florida 

Water Resources Journal: January 2005. 

Keplinger, K.O., J.B. Houser, A.M. Tanter, L.M. Hauck, and L. Beran. 2004. Cost and Affordability 

of Phosphorus Removal at Small Wastewater Treatment Plants. Small Flows Quarterly. Fall 

2004, Volume 5, Number 4. 

Maryland Department of the Environment (MDE). 2006. BNR Costs and Status BNR Project Costs 

Eligible for State Funding. Provided by Elaine Dietz on October 31, 2006. 

Park, Jae. No date. Biological Nutrient Removal Theories and Design. Online at 

http://www.dnr.state.wi.us/org/water/wm/ww/biophos/bnr_removal.htm. 

U.S. EPA. 2007. National Section 303(d) List Fact Sheet. Online at 

http://iaspub.epa.gov/waters/national_rept.control. 

U.S. EPA. 2006. Permit Compliance System (PCS) Database. Online at 

http://www.epa.gov/enviro/html/pcs/adhoc.html. 

U.S. EPA. 2001. Memorandum: Development and Adoption of Nutrient Criteria into Water Quality 

Standards. Online at http://oaspub.epa.gov/waters/national_rept.control#TOP_IMP. 

Water Environment Federation (WEF) and American Society of Civil Engineers 

(ASCE)/Environmental and Water Resources Institute (EWRI). 2006. Biological Nutrient 

Removal (BNR) Operation in Wastewater Treatment Plants. McGraw Hill: New York. 

12
 
United States Environmental Protection Agency
 

Office of Water
	

Washington, DC 20460
	

(4305T)
	

EPA-823-R-07-002
	

June 2007