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Biological Nutrient Removal
Processes and Costs
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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.
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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.
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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
Ammonia-N
Nitrate-N
Particulate organic-N
Soluble organic-N
Common Removal Mechanism
Nitrification
Denitrification
Solids separation
None
Technology Limit (mg/L)
<0.5
1 -2
<1.0
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
Soluble phosphorus
Particulate phosphorus
Common Removal Mechanism
Microbial uptake
Chemical precipitation
Solids removal
Technology Limit (mg/L)
0.1
<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 poly-
hydroxyalkanoates (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
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chemical floes with phosphorus. These floes 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
• A2/O Process - MLE process preceded by an initial anaerobic stage; used to remove both TN
andTP
• 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 University of Cape Town (UCT) Process - A2/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., NO2" or NO3"). However,
anaerobic is a condition in which neither free nor combined oxygen is available (WEF and ASCE/EWRI, 2006).
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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.
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Exhibit 3. Comparison of Common BNR Configurations
Process
MLE
A2/O
Step Feed
Four-Stage Bardenpho
Modified Bardenpho
SBR
Modified UCT
Oxidation Ditch
Nitrogen Removal
Good
Good
Moderate
Excellent
Excellent
Moderate
Good
Excellent
Phosphorus Removal
None
Good
None
None
Good
Inconsistent
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)
Annapolis (MD)
Back River (MD)
Bowie (MD)
Cambridge (MD)
Cape Coral (FL)
Cox Creek (MD)
Cumberland (MD)
Frederick (MD)
Freedom District (MD)
Largo (FL)
Medford Lakes (NJ)
Palmetto (FL)
Piscataway (MD)
Seneca (MD)
Sod Run (MD)
Westminster (MD)
Treatment Process
Description
Bardenpho (4-Stage)
MLE
Oxidation Ditch
MLE
Modified Bardenpho
MLE
Step Feed
A2/O
MLE
A2/O
Bardenpho (5-stage)
Bardenpho (4-stage)
Step Feed
MLE
Modified A2/O
MLE-A2/0
Flow
(mgd)
13
180
3.3
8.1
8.5
15
15
7
3.5
15
0.37
1.4
30
20
20
5
Average Effluent Concentration
(mg/L)1
TN
7.1
7.6
6.6
3.2
1.0
9.7
7.0
7.2
7.8
2.3
2.6
3.2
2.7
6.4
9.2
5.3
TP
0.66
0.19
0.20
0.34
0.2
0.89
1.0
1.0
0.51
ND
0.09
0.82
0.09
0.08
0.86
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.
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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
MLE
Four-Stage Bardenpho
Three-Stage Bardenpho
SBR
RBC
Achievable TN Effluent Quality
1 0 mg/L
6 mg/L
6 mg/L
8 mg/L
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.
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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 site-
specific 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)
Aberdeen
Annapolis
Back River
Ballenger
Broadneck
Broadwater
Cambridge
Celanese
Centreville
Chesapeake Beach
Conococheague
Cox Creek
Cumberland
Denton
Dorsey Run
Emmitsburg
Frederick
Freedom District
Fruitland
Hagerstown
Havre DeGrace
Hurlock
Joppatowne
La Plata
Design
Capacity (mgd)
2.8
10
180
2.0
6.0
2.0
8.1
1.25
0.375
0.75
2.5
15
15
0.45
2.0
0.75
8.0
3.5
0.50
8.0
1.89
2.0
0.95
1.0
Treatment Process
MLE
Ringlace
MLE
Modified Bardenpho
Oxidation Ditch
MLE
Activated Sludge
Sequential step feed
SBR/Land Application
Oxidation Ditch
Carrousel
MLE
MLE
Biolac
Methanol
Overland
MLE
Activated Sludge
SBR
Johannesburg Process
MLE
Bardenpho
MLE
MLE
Completion
Date
Dec-98
Nov-00
Jun-98
Aug-95
1994
May-00
Apr-03
Jun-05
Apr-05
1992
Nov-01
May-02
Aug-01
Dec-00
1992
1996
Sep-02
1994
Jul-03
Dec-00
Nov-02
Aug-06
Jul-96
Jun-02
Total Capital BNR
Cost (2006$)1
$3,177,679
$14,687,326
$138,305,987
$2,891,906
$3,165,193
$6,892,150
$11,740,209
$7,424,068
$7,336,020
$2,158,215
$6,620,888
$11,466,657
$12,929,990
$4,203,767
$3,967,307
$2,562,722
$11,916,504
$1,462,798
$7,546,764
$11,190,344
$7,596,882
$5,200,000
$2,433,205
$4,952,150
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Exhibit 6. BNR Upgrade Costs for Maryland Wastewater Treatment Plants
Facilities with BNR
(as of 10/30/06)
Leonardtown
Little Patuxent
Marlay Taylor (Pine Hill Run)
Maryland City
Maryland Correctional
Institute
Mt. Airy
Northeast
Parkway
Patuxent
Piscataway
Pocomoke City
Poolesville
Princess Anne
Seneca
Sod Run
Taneytown
Thurmont
Western Branch
Westminster
Design
Capacity (mgd)
0.65
18
4.5
2.5
1.23
0.60
2.0
7.5
6.0
30
1.4
0.625
1.26
5.0
12
0.70
1.0
30
5.0
Treatment Process
Biolac
A2/O
Schreiber
Schreiber
Bardenpho
Activated Sludge
Activated Sludge
Methanol
Oxidation Ditch
MLE
Biolac
SBR
Activated Sludge
MLE
MLE
SBR
MLE
Methanol
Activated Sludge
Completion
Date
Oct-03
1994
Jun-98
1990
1995
Jul-99
Oct-04
1992
1990
Jul-00
Sep-04
Jan-05
2002
Dec-03
2000
Apr-00
Dec-96
Jul-95
Jan-01
Total Capital BNR
Cost (2006$)1
$2,811,448
$7,263,879
$4,986,641
$1,375,866
$2,703,932
$5,235,575
$4,225,029
$15,869,228
$2,106,763
$24,778,239
$3,924,240
$1,593,640
$4,311,742
$34,886,034
$21,999,198
$3,808,298
$3,122,264
$47,132,782
$5,274,444
Source: MDE (2006).
mgd = million gallons per day
1 Total capital BNR upgrade costs eligible for Maryland Department of the Environment 50% cost share
(http://www.mde.state.md.us/Programs/WaterPrograms/WQIP/wqip_bnr.asp) including engineering, pilot study, design, and
construction, updated to 2006 dollars using the ENR construction cost index assuming that the completion date represents
the original year dollars (2006 ENR index = 7910.81).
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Exhibit 7 shows BNR retrofit costs for wastewater treatment plants in Connecticut.
Exhibit 7. BNR Upgrade Costs for Connecticut Wastewater Treatment Plants
Facilities with BNR
Branford
Bridgeport East Phase 1
Bridgeport West Phase 1
Bristol Phase 1
Derby
East Hampton
East Windsor
Fairfield Phase 2
Greenwich
Led yard
Milford BB Phase 1
New Canaan
New Haven Phase 1
New London
Newtown
Norwalk Phase 1
Norwalk Phase 2
Portland
Seymour
Stratford Phasel
Thomaston
University of Connecticut
Waterbury
Design
Capacity
(mgd)
4.5
12
29
10.75
3.03
3.9
2.5
9
12
0.24
3.1
1.5
40
10
0.932
15
15
1
2.93
11.5
1.2
1.98
25
Treatment Process2
4-Stage Bardenpho
MLE*
MLE*
MLE*
MLE*
MLE*
MLE
4-Stage Bardenpho
MLE*
SBR
4-Stage Bardenpho
MLE
MLE*
MLE*
MLE*
MLE*
MLE
MLE
MLE*
4-Stage Bardenpho
SBR
MLE
4-Stage Bardenpho
Year Process
In Service
2003
2004
2004
2004
2000
2001
1996
2003
1996
1997
1996
2000
1997
2002
1997
1996
2000
2002
1993
1996
2001
1996
2000
Total Capital BNR
Cost (2006$)1
$3,732,049
$2,323,766
$2,640,643
$649,320
$3,513,514
$860,548
$1,407,617
$14,235,676
$703,809
$4,752,461
$1,407,617
$1,570,463
$11,134,336
$3,495,615
$1,436,601
$1,548,379
$7,042,287
$1,266,843
$379,597
$1,126,094
$1,451,708
$1,489,259
$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 $1.8 million.
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Despite this variability in costs, unit costs (i.e., total capital cost per mgd) generally decrease as the
size of the plant increases due to economies of scale. Exhibit 8 illustrates this relationship for the
Maryland and Connecticut facility upgrades presented in Exhibits 6 and 7 for three system size
categories.
Exhibit 8. Average Unit Capital Costs for BNR Upgrades at MD and CT Wastewater
Treatment Plants (2006$)1
Flow (mgd)
>0.1 -1.0
>1.0-10.0
>10.0
Cost/mgd
$6,972,000
$1,742,000
$588,000
Source: Based on MDE (2006) and CTDEP (2007).
mgd = million gallons per day
1 Calculated from cost information from Maryland Department of the Environment for 43 facilities and Connecticut
Department of Environmental Protection for 23 facilities; costs updated to 2006 dollars based on project completion date
using the ENR construction cost index (2006 index = 7910.81).
BNR systems for smaller facilities (i.e., flow less than 0.1 mgd) are usually pre-engineered, factory-,
or field-assembled package systems (Foess, et al., 1998). In most cases, chemical phosphorus
removal is preferred over biological removal because most small systems lack the operational
oversight necessary to achieve low phosphorus levels with biological treatment. In addition, small
systems will likely need effluent polishing filtration for added nitrogen removal (Foess, et al., 1998).
Exhibit 9 summarizes average BNR costs for small systems. The construction costs include all
required facilities for a new plant on a new site, including filtration. O&M costs include labor,
electricity, maintenance and repair materials, solids handling and disposal, administration labor,
laboratory analytical requirements, and chemical costs (Foess, et al., 1998).
Exhibit 9. Average BNR Costs for Small Systems: New Plants (2006$)1
System
4,000 gpd
10,000 gpd
25,000 gpd
50,000 gpd
100,000 gpd
MLE Process
Construction
O&M
$348,771
$37,263
$415,585
$43,515
$563,912
$60,553
$803,108
$81 ,636
$1,167,914
$122,699
4-Stage Process
Construction
O&M
$448,992
$64,353
$491,753
$70,604
$634,736
$90,462
$889,966
$117,551
$1,293,524
$162,169
3-Stage Process
Construction
O&M
$388,859
$44,005
$444,983
$51,360
$589,302
$69,133
$837,851
$93,403
$1,220,029
$142,066
SBR Process
Construction
O&M
$448,992
$34,321
$509,125
$41,799
$644,090
$60,185
$931,391
$82,862
$1,290,852
$122,577
Intermittent Process
Construction
O&M
$306,009
$34,321
$499,771
$41,799
$780,391
$60,185
$1,150,542
$82,862
$1,371,029
$122,577
10
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Exhibit 9. Average BNR Costs for Small Systems: New Plants (2006$)1
System
4,000 gpd
10,000 gpd
25,000 gpd
50,000 gpd
100,000 gpd
MLE and Deep Bed Filtration
Construction
O&M
$411,576
$45,231
$491,753
$52,340
$649,435
$71,217
$887,294
$93,036
$1,280,161
$136,550
Submerged Biofilter Process
Construction
O&M
$330,063
$23,902
$395,541
$29,909
$601,328
$50,379
$1,131,834
$74,036
(2)
(2)
RBC Process
Construction
O&M
$351,443
$25,006
$457,010
$31 ,747
$704,222
$53,198
$1,159,896
$75,385
$1,459,224
$109,584
Source: Foess, etal. (1998).
gpd = gallons per day
1 Construction costs updated from 1998 dollars using the ENR construction cost index (2006 index = 7910.81); O&M costs
updated from 1998 dollars using the Bureau of Labor Statistics consumer cost index (2006 index = 199.8).
2 Exceeded manufacturer's sizes.
Retrofit opportunities are more limited at smaller facilities; however two retrofit alternatives may
exist for nitrogen removal. The MLE process can be retrofitted by adding an anoxic basin upstream
of the existing influent point and adding recirculation pumping from the existing aeration basin to the
new anoxic basin. Also, deep-bed denitrification filters can be added downstream of an existing
package plant. The retrofit involves installation of new pumping facilities to pump secondary effluent
to the filters, methanol feed equipment, and chemical feed equipment (for phosphorus removal)
(Foess, et al, 1998). O&M costs represent only the incremental costs associated with the additional
equipment. Exhibit 10 summarizes these costs.
Exhibit 10. Average BNR Costs for Small Systems: Retrofits (2006$)1
System
4,000 gpd
10,000 gpd
25,000 gpd
50,000 gpd
100,000 gpd
Anoxic Tank for MLE Upgrade
Construction
O&M
$28,062
$14,832
$32,071
$15,445
$52,115
$16,425
$76,168
$22,922
$80,000
$21,100
Deep Bed Denitrification Filter
Construction
O&M
$145,655
$21 ,573
$161,691
$22,309
$196,434
$24,883
$217,815
$30,399
$213,000
$28,600
Source: Foess, etal. (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.
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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
O&M
$70.97/gpd
$7.86/gpd
$34.66/gpd
$3.70/gpd
$19.34/gpd
$2.10/gpd
$14.58/gpd
$1 .43/gpd
$8.50/gpd
$0.94/gpd
Retrofits
Construction
O&M
$16.25/gpd
$3.71/gpd
$7.25/gpd
$1.54/gpd
$3.72/gpd
$0.67/gpd
$2.20/gpd
$0.44/gpd
$1 .47/gpd
$0.25/gpd
Source: Foess, etal. (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%20Fleming-
GMB%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. McGrawHill: New York.
12
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United States Environmental Protection Agency
Office of Water
Washington, DC 20460
(4305T)
EPA-823-R-07-002
June 2007
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