United States
Environmental Protection
Agency
Case Studies on Implementing
Low-Cost Modifications to
Improve Nutrient Reduction at
Waste water Treatment Plants
DRAFT-Version 1.0
August 2015
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
Treatment Plants: DRAFT Version 1.0 August 2015
Acknowledgments
EPA gratefully acknowledges the efforts of the communities listed below and the time and assistance
these communities provided to enable the preparation of this document so that their experiences might
benefit communities at large:
City of Bozeman, Montana
City of Chinook, Montana
City of Flagstaff, Arizona
City of Layton, Florida
City of Montrose, Colorado
City of Tampa, Florida
City of Titusville, Florida
Florida Keys Aqueduct Authority
Hampden Township, Pennsylvania
Town of Crewe, Virginia
Town of Wolfeboro, New Hampshire
Victor Valley Wastewater Reclamation Authority, California
Tetra Tech, Inc. provided support to the U.S. Environmental Protection Agency for this document under
contract EP-C-12-055 TO 2. Victor D'Amato was the principal investigator and lead author, and Aileen
Molloy was the project manager.
U.S. Environmental Protection Agency Contributors
Technical Lead: Timothy Connor, Office of Wastewater Management,
Task Order Contract Officer: Katharine Dowell, Office of Wetlands, Oceans and Watersheds
Technical Advisors/Reviewers
Ellen Gilinsky, PhD, Office of Water Immediate David Pincumbe, Region 1
Office
Robert Freeman, Region 4
Lisa Larimer and Paul Shriner, Office of Science .-, , j -n ^ • •> ^ a ^ • +
, „ . , George Azevedo, Patrick Kuefler, Knsta
and I echnology A, T^. , n , ., T . n -
OJ McKim and Robert Newport, Region 5
Virginia Kibler, Office of Wastewater „. T ... „ 0
A . & Tina Laidlaw, Region 8
Management
^ \\T n r»«- f ™ +1 j r* j Brian Nickel, Region 10
Tom Wall, Office of Wetlands, Oceans and
Watersheds
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Case Studies on Implementing
Low-Cost Modifications to Improve
Nutrient Reduction at Wastewater
Treatment Plants
DRAFT - Version 1.0
August 2015
EPA-841-R-15-004
Prepared under Contract No. EP-C-12-055 TO 2
United States Environmental Protection Agency
Jointly supported by the
Office of Wetlands Oceans and Watersheds
Office of Science and Technology and
Office of Wastewater Management
1200 Pennsylvania Ave., NW
Washington, DC 20460
Available for download at (htt|)L//ww
Cover photo: Bozeman, Montana Water Reclamation Facility
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
Treatment Plants: DRAFT Version 1.0 August 2015
Contents
Acknowledgments 2
Executive Summary 9
Methodology 9
Results 10
1 Background 14
1.1 Scope and Purpose 14
1.1.1 Methodology 15
1.1.2 Case Studies 16
1.2 Related / Predecessor Guidance and Reports 17
1.3 How to Use this Document 20
2 Introduction 21
2.1 Nitrogen Removal 21
2.1.1 Nitrogen Removal Fundamentals 21
2.1.2 Nitrogen Removal Optimization Opportunities 24
2.2 Phosphorus Removal 28
2.2.1 Phosphorus Removal Fundamentals 28
2.2.2 Phosphorus Removal Optimization Opportunities 29
2.3 Nutrient Removal Attributes of Typical Wastewater Treatment Facilities and
Opportunities for Improvement 30
2.3.1 Activated Sludge 30
2.3.2 Lagoons 33
2.3.3 Trickling filter 34
2.3.4 Primary Treatment 34
2.3.5 Modified Discharge 35
2.4 Evaluating and Implementing Nutrient Reduction Improvements 36
3 Optimizing Nutrient Removal in Activated Sludge Systems 39
3.1 Aeration Modifications 48
3.2 Process Modifications/Operational Changes 49
3.3 Configuration Modifications 53
3.4 Ancillary Benefits of Enhanced Biological Nutrient Removal 53
4 Optimizing Nutrient Removal in Lagoon Systems 55
4.1 Controlled Discharge 58
4.2 Aeration Modifications 59
4.3 Conversion to Advanced Secondary Treatment 59
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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5 Optimizing Nutrient Removal in Trickling Filter Systems 60
5.1 Process Optimization 61
5.2 Conversion to Advanced Secondary Treatment 61
6 Other Nutrient Reduction Approaches 62
6.1 Post-Denitrification 62
6.2 Discharge Modification/Land Application 62
6.3 Chemical Treatment 63
6.3.1 Chemical Phosphorus Removal 63
6.3.2 Alkalinity Adjustment 64
6.3.3 Supplemental Carbon Addition 64
6.4 Emerging Nutrient Removal Approaches and Technologies 65
6.4.1 Discharge Reduction through Water/Nutrient Reuse 65
6.4.2 Nutrient Product Recovery and Reuse 65
6.4.3 Source Control/Separation 66
7 Conclusions and Recommendations 67
8 References 69
Appendix A List of Nutrient Reduction Resources
Appendix B Case Study Summary Documents
Bay Point, Florida - USBF Activated Sludge-Process Control and Mechanical
Modifications B-1
Bozeman, Montana - Activated Sludge-Process Control Modifications and Step Feed B-6
Chinook, Montana - Oxidation Ditch/Activated Sludge-Process Control and
Mechanical Modifications B-12
Crewe, Virginia - Oxidation Ditch Activated Sludge-Process Control and Mechanical
Modifications B-17
Flagstaff, Arizona - I FAS Activated Sludge-Process Control Modifications B-26
Hampden Township, Pennsylvania - Continuous-Flow Sequencing Reactor Activated
Sludge-Aeration Cycling B-29
Layton, Florida - Sequencing Batch Reactor-Process Control Modifications B-34
Montrose, Colorado - Activated Sludge-Aeration Control B-39
Tampa, Florida - Separate Stage Nitrification Activated Sludge-Operational
Modifications B-45
Titusville, Florida - A2/O with Secondary Anoxic and Wetland Discharge B-49
Victor Valley, California - Activated Sludge-Process Control and Mechanical
Modifications B-59
Wolfeboro, New Hampshire - Extended Aeration Activated Sludge-Cyclic Aeration B-66
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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Tables
Table 1 -1. Summary of TN Treatment Levels for Case Studies 17
Table 1-2. List of Related EPA Guidance and Reports 18
Table 1-3. Summary of Studies of Hypothetical Nutrient Removal Case Studies 19
Table 2-1. Nutrient removal performance (effluent concentration) for wastewater treatment
plants 30
Table 2-2. Average effluent TN and TP for various activated sludge process configurations 32
Table 2-3. Decision considerations for enhancing nitrogen removal at existing WWTPs 37
Table 3-1. Required conditions for biological nutrient removal (Daigger and Littleton, 2014) 39
Table 3-2. Functions of zones in BNR processes (Grady etal., 2011) 39
Table 3-3. Summary of Case Studies (basic, non-advanced treatment plants shaded in gray). 42
Table 3-4. Modifications Featured in Case Studies 43
Table 3-5. Case studies from Water Planet Company website 44
Table 3-6. Additional case studies identified in literature 45
Table 3-7. Recommended Parameters for Data Evaluation (USEPA, 2010) 47
Table 3-8. Potential sources of VFAs at municipal WWTPs (Jeyanayagam, 2005) 50
Table 3-9. Summary of Basic On-Line Instrumentation (USEPA, 2010) 51
Table 3-10. Recommended instrument locations for biological nutrient control (Tsuchihashi,
2008) 52
Table 4-1. Summary of main lagoon types and typical effluent concentrations 56
Table 4-2. Lagoon case studies in literature 57
Table 5-1. Trickling filter case studies in literature 60
Figures
Figure 2-1. Generalized secondary (activated sludge) liquid-phase treatment schematic 22
Figure 2-2. Generalized pre-anoxic zone nitrification/denitrification liquid-phase treatment
schematic 22
Figure 2-3. Generalized post-anoxic zone nitrification/denitrification liquid-phase treatment
schematic 23
Figure 2-4. Generalized single-reactor nitrification/denitrification liquid-phase treatment
schematic 23
Figure 2-5. Average WWTP energy use breakdown 24
Figure 2-6. Basic (non-advanced) Activated Sludge WWTP process schematic 30
Figure 4-1. Illustration of lagoon processes 57
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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Acronyms
AS
A2O
BAT
BMP
BNR
BOD
BODs
CMAS
COD
CSR
DO
EBPR
EPA
F/M ratio
gpd
HLR
HRT
I FAS
IMF
ISF
MLSS
MLE
NH4
N03/N02
NPDES
O&M
OLR
ORP
OWM
PE
RAS
RMF
RR
SA
SBR
activated sludge
anaerobic/anoxic/oxic process
best available technology
best management practice
biological nitrogen removal
biochemical oxygen demand
5-day biochemical oxygen demand
Complete Mix-Activated Sludge
chemical oxygen demand
continuously sequencing reactor
dissolved oxygen
enhanced biological phosphorus removal
U.S. Environmental Protection Agency
food-to-microorganism (F/M) ratio
gallons per day
hydraulic loading rate
hydraulic retention time
integrated fixed-film activated sludge
intermittent media filter
intermittent sand filter
mixed liquor suspended solids
Modified Ludzack-Ettinger
ammonium
nitrate/nitrite
National Pollutant Discharge Elimination System
operation and maintenance
organic loading rate
oxidation-reduction potential
EPA Office of Wastewater Management
population equivalents
return activated sludge
recirculating media filter
recirculation ratio
surface area
sequencing batch reactor
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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SCADA
sf
SRT
TKN
TMDL
TN
TP
TSS
VFA
VFD
WAS
WWTP
supervisory control and data acquisition
square feet
solids retention time
total Kjeldahl nitrogen
total maximum daily load
total nitrogen
total phosphorus
total suspended solids
volatile fatty acid
variable frequency drive
waste activated sludge
wastewater treatment plant
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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Executive Summary
This technical report supplements a number of recent guidance manuals and reports published
by EPA on nutrient removal at wastewater treatment plants (WWTPs) by providing useful
information to managers and operators of plants that may not be specifically designed for
nutrient removal or for those seeking to achieve even better treatment through relatively low-
cost modifications. It is intended to help fill gaps in published information about improving
nutrient reduction performance at existing WWTPs (generally activated sludge facilities with
basic treatment processes), using relatively low-cost techniques.
Although many published reports and papers address the nutrient removal performance of
WWTPs, this report is one of the first documented efforts to present empirical data via a
compendium of case studies of non-advanced1 WWTPs that have been optimized to improve
nutrient reduction without requiring costly infrastructure upgrades.
The economic implication of regulating nutrients is often perceived as an impediment to
progress. The results of this project suggest that opportunities for low-cost nutrient removal
optimization are common, particularly at basic activated sludge plants, which are the focus of
these case studies.
This report documents optimization techniques through empirical example at specific plants. It
does not address site-specific design, engineering or cost factors for other facilities. These
case studies are not appropriate for setting permit requirements; such requirements should be
established by permit directors in accordance with applicable CWA and state requirements.
METHODOLOGY
The U.S. Environmental Protection Agency (EPA) focused on identifying relevant case studies
to highlight in this report through the following main efforts:
• Internal EPA query to relevant Regional and state staff.
• Broad grey and white literature review.
• Review of existing EPA and other guidance documents.
• Query of selected industry practitioners.
• Supplemental search of Clean Water Needs Survey (CWNS) database.
From a master list of over 80 case studies, a total of 12 have been summarized for the project.
The main criteria for selecting case studies included:
• Responsiveness to project objectives: relatively basic (non-advanced) treatment plants
improving nitrogen or phosphorus reduction performance using low-cost techniques.
• Availability of monitoring and cost data.
• Representative of a range of scenarios (e.g., system types, geographies) and nutrient
optimization approaches.
1 For the purposes of this report, "non-advanced" means a WWTP that has not been designed as a nutrient removal
plant (e.g., originally no anaerobic selectors for TP removal or anoxic zones for denitrification).
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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Of the 12 selected case studies, seven fully meet the main criteria. Although the other five do
not meet all the specified criteria, they provide useful information that might help target
audiences understand nutrient reduction optimization approaches. Most of the other candidate
case studies were not selected for one or more of the following reasons:
• Lack of monitoring and/or cost data.
• WWTPs were found to be advanced plants and/or implemented improvements were
rebuilds for biological nutrient removal (BNR), requiring significant costs.
• Unable to complete follow-up with WWTP contacts within available time for collecting
study data.
EPA anticipated identifying a number of relevant, published case studies through this research.
However, despite extensive efforts to identify and develop relevant case studies, relatively few
met the aforementioned criteria. EPA concluded that the primary limitation in identifying
prospective case studies was that most efforts at improving small or non-advanced plants
appear to be unpublished or otherwise under documented. Most published literature focuses
instead on optimizing existing BNR systems. Nevertheless, the case studies that were
developed show that optimization of non-advanced WWTPs is feasible and cost-effective, and
provide useful information to support future efforts at other WWTPs.
As expected, the greatest number of potential case studies were identified for activated sludge
systems. EPA also attempted to identify case studies for lagoon and trickling filter systems, but
only a few examples of nutrient reduction optimization were uncovered; and most of them
included significant infrastructure modifications which disqualified them from the pool of case
studies, since they could not be considered "low cost" approaches.
Relevant data on approach, performance, and costs are summarized for the 12 case studies
that were developed for this project. These data were then used to develop broader technical
information to help treatment plant managers, operators, and others to improve the nutrient
reduction performance of their plants.
RESULTS
A number of modifications can be considered for improving nutrient removal at existing non-
advanced WWTPs, including (but not limited to) one or more of the following. Note that many of
the optimization activities described below are complimentary to one another and that control
system additions or modifications are needed for many applicable optimization activities.
Aeration modifications are changes to physical aeration equipment, controls, operation, and
function of equipment and aerated areas. They include installing energy efficient blowers,
variable frequency drives (VFDs), diffusers with improved distribution and oxygen transfer
efficiency (OTE), airflow meters, airflow control valves, and on/off cycling; and dissolved
oxygen (DO), ammonia, or oxidation reduction potential (ORP) control. Aeration
modifications are typically used to optimize anoxic conditions that support denitrification for
biological nitrogen removal. Creating anaerobic zones before aerated activated sludge
treatment can also support enhanced biological phosphorus removal (EBPR).
Process modifications include adjustments to process control characteristics, including
solids retention time (SRT), mixed liquor suspended solids (MLSS), food-to-microorganism
(F/M) ratio, and recycle/return rate. Physical process improvements might include adding
VFDs and/or return activated sludge (RAS) pumps for internal recycling; adding online
monitoring equipment for process control and optimization; or providing new screens or grit
removal equipment at the headworks to improve the performance of the treatment process.
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Configuration modifications are changes to, or the addition of, flowstreams within the
process or changes to the process configuration. They might include changes to channels;
manipulating gates; or modifying or adding piping, such as adding internal recycle lines or
step-feed provisions; and are frequently employed to create or enhance environments for
denitrification (e.g., by returning nitrate rich mixed liquor back to an anoxic zone).
Chemical modifications are the addition of, or changes to supplemental alkalinity and
organic carbon feed to support biological nitrogen removal.
Discharge modifications are made at the end of the treatment system to further reduce
nutrients prior to delivery to receiving surface waters. They generally use natural systems
and might include soil-based treatment systems or wetland assimilation discharge.
Specific characteristics of the case studies selected for this project are summarized in Table 1
(Summary of Case Studies) and Table 2 (Modifications Featured in Case Studies).
The results of this project illustrate that:
1. No- or low-cost activities can be implemented at existing WWTPs to significantly reduce
effluent nutrient discharges with minimal negative impacts on operations. In fact, in most
cases, the secondary impacts are overwhelmingly positive and include energy efficiency,
lower operational costs, and improved process stability. Although most of the case studies
did not specify the capital costs savings associated with their optimization approach over
alternative approaches, several did. Modifications at Crewe, Virginia had a capital cost of
$6,000, compared with an estimated upgrade cost of $800,000. Victor Valley spent $1.1 M
instead of $80M for a new treatment train. Two other case study contacts indicated that
optimization saved significant money versus more capital intensive alternatives.
2. Low-cost nutrient reduction improvements are most feasible for activated sludge plants,
where excess capacity (volumetric and/or aeration) can typically be leveraged to
facilitate nitrification and denitrification without requiring physical infrastructure
modifications. However, utilizing excess capacity may limit the ability of a WWTP to
increase its flow rate in the future without an expansion. For the case studies featured in
this project, only one contact indicated that their plant (Victor Valley, CA) needed to be
rerated as a result of their optimization efforts. EPA did not specifically ask WWTP
contacts about impacts on design capacity.
3. Low-cost nutrient reduction improvements, particularly for relatively basic treatment
systems, are underreported in the literature. EPA intends to identify additional case
studies and update this document in the future. EPA will also consider additional
activities to develop capacity and support nutrient reduction at such facilities.
4. Modestly improved phosphorus reduction often co-occurs as a result of improvements in
biological nitrogen removal. To achieve more significant phosphorus reductions, most
WWTPs opt for chemical precipitation, which is a well-established technology widely
adapted to different plant types and configurations. Enhanced biological phosphorus
removal (EBPR) generally requires significant physical infrastructure modifications at
existing plants (e.g., creation of anaerobic selector zones). Other opportunities for
reducing phosphorus discharges include control or side-stream treatment of return flows
and enhancing volatile acid production for driving EBPR in existing anaerobic selectors
(only applicable for an existing advanced treatment system). Soil- and plant-based
treatment systems are also particularly effective for reducing phosphorus, which is
removed from wastewaters by solid-phase sequestration.
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Table 1. Summary of Case Studies (basic, non-advanced treatment plants shaded in gray)
Case Study
Bay Point, FL
Bozeman, MT
Chinook, MT
Crewe, VA
Flagstaff, AZ
Hampden Twp.,
PA
Layton, FL
Montrose, CO
Tampa, FL
Titusville, FL
Victor Valley,
CA
Wolfeboro, NH
Design
Flow
(MGD)
0.054
5.2
0.5
0.5
6.0
5.69
0.066
4.32
96
6.75
13.8
0.6
WWTP Type
AS (MLE)
AS
AS (Oxidation
Ditch)
AS (Oxidation
Ditch)
AS (I FAS)
AS (CSR)
AS (SBR)
AS (Oxidation
Ditch)
AS (Separate
Stage)
AS (A2/O)
AS
AS (Extended
Aeration)
Modification Type
Aeration, chemical
Aeration,
configuration
Aeration
Aeration, chemical
Process
Configuration,
process
Aeration, process
Aeration
Aeration,
configuration
Discharge,
configuration,
process
Aeration, process
Aeration
Pre/post
TN(mg/l)1
6.33/3.99
17.8/10.5
20.3/5.44
7.85/3.63
14.0/8.5
4.66/3.64
7.88/3.33
Unk/14.7
18.62/13.82
5.67/0.94
8.93/6.83
6.32/1 .97
Pre/post
TP(mg/l)1
N/A
3.7/2.5
4.13/1.72
N/A
N/A
N/A
N/A
N/A
N/A
0.77/0.04
N/A
N/A
Capital
Costs
$170,365
$180,000
$81,000
$6,000
$10,000
Zero
$53,000
Zero
Zero
$2,240,000
$1,100,000
$116,000
Operational Costs/Savings
Savings not quantified
Zero
Energy savings more than
offset $1 ,000/yr in
maintenance
$1 7, 440/yr savings
$1 ,000/yr
Zero
$1 3, 500/yr savings
$34, 000/yr savings
$51 9, 900/yr savings
$45, 000/yr
1 0% savings
Savings not quantified
Notes:
AS = activated sludge; MLE = modified Ludzack Ettinger; IFAS = integrated fixed film activated sludge; SBR = sequencing batch reactor; N/A = not applicable; CSR = continuously
sequencing reactor.
1 Available flow data typically did not allow for quantification of pre- and post- optimization TN and TP loads (mass); therefore, concentration is used as the primary performance metric.
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Table 2. Modifications Featured in Case Studies
Modification
Aeration
Process
Configurati
Chemi
o
a
Aeration cycling
Mixer addition
Adjustable control
aeration
Equipment retrofit
Flow equalization
improvement
Recycle rate control
Side-stream control
Batch program
modifications
Predigestion of primary
sludge
Plug flow/series operation
Anoxic zone RAS bleed
Anaerobic zone VFA
addition
Alkalinity feed
improvements
Carbon product addition
Soil dispersal
Wetland discharge
'c
'o
Q.
re
m
V
V
V
V
Bozeman
V
V
V
Chinook
V
V
o
V
V
V
V
Flagstaff
V
V
V
Hampden
Township
V
V
V
c
o
V
V
Montrose
V
V
re
Q.
1
V
V
Titusville
V
V
V
0)
"re
0
V
V
V
Wolfeboro
V
V
V
13
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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1 Background
1.1 SCOPE AND PURPOSE
Impacts to water bodies across the United States from nutrient pollution are well-documented.
Various workgroups and workshops have been convened to review the scientific information,
evaluate tools to address nutrient pollution, identify barriers to progress, and outline next steps.
In a March 2011 memorandum to the states, tribes, and territories, the EPA Acting Assistant
Administrator for Water reiterated the need for action by stating:
"States, EPA, and stakeholders, working in partnership, must make greater progress in
accelerating the reduction of nitrogen and phosphorus loadings to our nation's waters"
(USEPA2011a).
One of the primary barriers to reducing nutrients is the cost of upgrading WWTPs to achieve
nutrient concentrations necessary to protect designated uses (i.e., recreation, aquatic life).
Much of the high cost associated with reducing nutrients to intermediate or low levels results
from building significant additional new infrastructure or facility retrofits and additional operation
and maintenance costs. The economic implication of regulating nutrients is often an impediment
to progress. Often overlooked in the current discussion is the opportunity to improve plant
performance largely using existing infrastructure.
This technical report is intended to help fill gaps in published information about improving
nutrient reduction performance at existing WWTPs (generally activated sludge facilities with
more basic treatment processes and less resources at their disposal), using relatively low-cost
techniques. Although many published reports and papers address the nutrient removal
performance of WWTPs, this report represents one of the first documented efforts to present
empirical data via a compendium of case studies of non-advanced WWTPs that have been
optimized to improve nutrient reduction without requiring costly infrastructure upgrades.
"Optimization" as used in this document is defined as an activity that results in an improvement
in the nutrient pollutant removal of an existing WWTP without requiring significant infrastructure
upgrades.
The availability of sufficient monitoring data has been and continues to be a limiting factor in
developing reliable information on low-cost nutrient optimization at non-advanced WWTPs.
Accordingly, this report represents an initial effort to collect and compile relevant data. EPA
intends to identify additional case studies and update this document in the future.
For the case studies developed for this report, at least two years of pre-optimization and two
years of post-optimization TN and/or TP concentration data were targeted. In most cases,
discharge monitoring report (DMR) data were used, which typically include one or two TN
and/or TP sampling events per month. In some cases, WWTPs provided a larger dataset (more
frequent sampling), but in several cases, pre-optimization data were limited (e.g., Montrose, CO;
Hampden Twp., PA; Blue Heron, FL). Graphical data presentations in the case studies generally
show all datapoints used in the analysis. Data summaries in the case studies and in the report
use arithmetic mean and standard deviation of all pre- and post- optimization date data
available to EPA. Where WWTPs implemented their optimization strategies slowly over a period
of time, optimization date was determined based on recommendations from the WWTP contact
and a visual interpretation of the time series data for that WWTP.
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A number of optimization options can be considered including (but not limited to) one or more of
the following:
• Implementing aeration changes such as cyclical aeration (primarily using existing tanks
and mechanical equipment), often supplemented with basic in-line monitoring
instrumentation and associated controls.
• Process control changes such as altering SRT, MLSS concentrations, and/or F/M ratios.
• Use of unused and/or existing tankage to create specialized zones (e.g., anoxic zones).
• Installation of baffles to create specialized zones within existing tanks.
• Piping and/or pumping changes to provide internal recycle or alter recycling rates.
• Carbon/volatile fatty acid (VFA) supplementation using existing or new source (e.g.,
waste or return sludge; septage, respectively) and repurposed fermentation reactors
(e.g., conversion of primary clarifiers or equalization basins).
• Nitrification improvements.
• New process modeling, operational training and staffing, and/or sampling.
This report documents optimization techniques through empirical example at specific plants. It
does not address site-specific design, engineering or cost factors for other facilities. These
case studies are not appropriate for setting permit requirements; such requirements should be
established by permit directors in accordance with applicable CWA and state requirements.
1.1.1 Methodology
To support the preparation of this report, examples of WWTPs that were successful in reducing
effluent nutrient concentrations at low cost were identified and, where appropriate, developed
into case studies. A number of sources were used to help identify case studies, including:
• Internal EPA query to relevant Regional and state staff.
• Broad grey and white literature review.
• Review of existing EPA and other guidance documents.
• Query of selected industry practitioners.
• Supplemental search of CWNS database.
The literature review identified the majority of the WWTP examples having sufficient data and
documentation to support a case study. However, many of the examples reported in the
literature featured WWTPs with a relatively high level of technical sophistication or consultant
support for nutrient reduction improvements. The CWNS database search was least effective,
given its relatively low-resolution data, which made it difficult to identify WWTPs that had
undergone low-cost optimization.
Case study information was tracked and characterized using a Microsoft Excel spreadsheet.
From a master list of over 80 case studies, a total of 12 have been summarized for the project.
The evaluation criteria used to select these 12 case studies included:
• Responsiveness to project objectives: relatively basic (non-advanced) treatment plants
improving nitrogen or phosphorus reduction performance using low-cost techniques.
• Availability of monitoring and cost data.
• Representative of a range of scenarios (e.g., system types, geographies) and nutrient
optimization approaches.
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Of the 12 selected case studies, seven fully meet the evaluation criteria. Although the other five
do not meet all of the specified criteria, they provide useful information that might help target
audiences understand nutrient reduction optimization approaches. Most of the other candidate
case studies that were not selected were disqualified early in the project for one or more of the
following reasons:
• Lack of monitoring and/or cost data.
• WWTPs were found to be advanced plants and/or implemented improvements were
rebuilds for biological nutrient removal (BNR), requiring significant costs.
• Incomplete follow-up with WWTP contacts.
EPA anticipated identifying a number of relevant, published case studies through this research.
However, despite extensive efforts to identify and develop relevant case studies, relatively few
met the aforementioned criteria. EPA concluded that the primary limitation in identifying
prospective case studies was that most efforts at improving small or non-advanced plants
appear to be unpublished or otherwise under documented. Most published literature focuses
instead on optimizing existing BNR systems. Nevertheless, these case studies show that
optimization of non-advanced WWTPs is feasible and cost-effective, and provide useful
information to support future efforts at other WWTPs.
As expected, the greatest number of potential case studies were identified for activated sludge
systems. EPA also attempted to identify case studies for lagoon and trickling filter systems, but
only a few examples of nutrient reduction optimization were uncovered and most of them
included significant infrastructure modifications which disqualified them from the pool of case
studies since they could not be considered "low cost" approaches.
1.1.2 Case Studies
Relevant data on approach, performance, and costs are summarized for the 12 case studies.
These data were then used to develop technical information to help treatment plant managers,
operators, and others to improve the nutrient reduction performance of their plants.
Table 1-1 lists the case studies and their corresponding pre- and post-optimization TN treatment
levels. All of the case studies listed are suspended growth (i.e., activated sludge) systems and,
although some were optimized for both TN and TP, all were optimized to improve nitrogen
reduction.
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Table 1-1. Summary of TN Treatment Levels for Case Studies
Bay Point, FL
Bozeman, MT
Chinook, MT
Crewe, VA
Flagstaff, AZ
Hampden Twp., PA
Layton, FL
Montrose, CO
Tampa, FL
Titusville, FL
Victor Valley, CA
Wolfeboro, NH
Pre-/Post-
Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Pre-Optimization
Post-Optimization
Level 1 1 Level 2 Level 3 Level 4 Level 5
>15mg/l 8-15mg/l 4-8 mg/l 2-4 mg/l <2 mg/l
6.33
3.99
17.8
10.5
20.3
5.44
7.85
3.63
14.0
8.50
4.66
3.64
7.88
3.33
unknown
14.7
18.62
13.82
5.67
0.94
8.93
6.83
6.32
1The treatment levels represent generally accepted ranges of effluent nutrient targets for wastewater treatment. Generally, most
resources refer to Level 1 as 'no nutrient treatment' or often 'basic secondary treatment.' The Agency used these as general
categories based on effluent data, where nutrient removal targets graduated for Level 2 at 8-15 mg N/L to the most stringent Level 5
from at <2 mg N/L.
The case studies are summarized and described further in Section 3 and provided as stand-
alone write-ups in Appendix B. Additionally, case study information is incorporated throughout
the document to illustrate and emphasize key points. Other case studies identified during the
literature review but not developed further for this project are identified as additional resources
where appropriate.
1.2 RELATED / PREDECESSOR GUIDANCE AND REPORTS
This report supplements a number of recent guidance manuals and reports published by EPA
on nutrient removal at WWTPs by providing useful information to managers and operators of
plants that may not be specifically designed for nutrient removal or for those seeking to achieve
even better treatment through relatively low-cost modifications.
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Table 1-2 provides a summary of EPA's recent documents and reports to help provide more
background information on wastewater treatment processes, design principles, and nutrient
removal technologies. Older EPA documents are cited where warranted throughout this report.
Additional nutrient reduction resources are listed in Appendix A.
Table 1-2. List of Related EPA Guidance and Reports
Document
Pub.
Year
Document ID
Brief Synopsis
Principles of Design and
Operations of Wastewater
Treatment Pond Systems
for Plant Operators,
Engineers, and Managers
2011
EPA/600/R-11/088
This manual provides an overview of
wastewater treatment pond systems and
discusses factors affecting treatment, process
design principles and applications, aspects of
physical design and construction, effluent
total suspended solids (TSS), algae, nutrient
removal alternatives, and cost and energy
requirements.
Nutrient Control Design
Manual
2010
EPA/600/R-10/100
This EPA design manual provides updated,
state-of-the-technology design guidance on
nitrogen and phosphorus control at municipal
WWTPs.
Nutrient Control Design
Manual State of
Technology Review
Report
2009
EPA/600/R-09/012
This document presents an extensive state-
of-the-technology review of nitrogen and
phosphorus control technologies and
techniques currently applied and emerging at
municipal WWTPs, including a description of
technologies and key design and operational
issues.
Municipal Nutrient
Removal Technologies
Reference Document,
Volume 1—Technical
Report
2008
EPA 832-R-08-006
This reference document includes technical
information (performance and costs) to assist
municipal decision makers and regional and
state regulators in planning for nutrient
removal from municipal wastewater.
Municipal Nutrient
Removal Technologies
Reference Document,
Volume 2—Appendices
2008
EPA 832-R-08-006
Appendices for Volume 1. Mostly includes
case study write-ups for WWTPs used to
inform document.
Biological Nutrient
Removal Processes and
Costs
2007
EPA-823-R-07-002
15-page fact sheet summarizing studies of
BNR performance and costs.
Advanced Wastewater
Treatment to Achieve Low
Concentration of
Phosphorus
2007
EPA910-R-07-002
EPA Region 10 presents observations of
advanced wastewater treatment installed at
23 municipalities in the U.S., achieving very
low phosphorus concentrations.
During the literature review, a number of documents were identified that included hypothetical
case studies; that is, the authors used modeling or engineering judgment to identify potentially
feasible nutrient reduction improvements that could be applied to existing plants and, in many
cases, estimated costs associated with the identified improvements. Although those "case
studies" did not meet the objectives of this project—which focused on empirical data based on
implemented improvements at actual treatment plants—information pertaining to the
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hypothetical studies can be valuable and the references are summarized in Table 1-3 (see
Section 8, References for full complete listings).
Table 1-3. Summary of Studies of Hypothetical Nutrient Removal Case Studies
Citation Data
Author
Camacho
CH2M Hill
Colorado
Foess
Jiang et al
JJ Environmental
Keplinger
Randall, et al
Tetra Tech
Tetra Tech
Tetra Tech
USEPA
USEPA
Year
1992
2010
2010
1998
2004
2015
2004
1999
2011
2013
2014
1987
2008
System Types
AS
X
X
X
X
X
X
X
X
X
X
X
X
Lag
X
X
X
X
X
TF
X
X
X
X
X
X
X
X
X
Nutrient
TN
X
X
X
X
X
X
X
X
TP
X
X
X
X
X
X
X
X
X
X
X
Scope
Chesapeake Bay Program compliance, with concept
designs and cost estimates for both specific and
generalized plants.
Concept designs and cost estimates for upgrading
specific and generalized plants in Utah to different
nutrient removal levels.
Concept designs and cost estimates for upgrading
generalized plants in Colorado to different nutrient
removal levels.
Cost comparison of advanced very small flow
generalized systems for Florida applications.
Concept designs and cost estimates for upgrading
generalized plants in Georgia to different nutrient
removal levels.
Concept designs and cost estimates for low-cost
upgrades to MLE processes for 20 specific facilities
in New England.
Concept designs and cost estimates for chemical
phosphorus precipitation for several small Texas
facilities.
Concept designs and cost estimates for low-cost
upgrades for 8 mg/l TN target for WWTPs in
Chesapeake Bay watershed.
Concept designs and cost estimates for upgrading
generalized plants in the state of Washington to
different nutrient removal levels.
Concept designs and cost estimates for upgrading
generalized plants in Ohio to different phosphorus
removal levels.
Concept designs and cost estimates for upgrading
specific plants in Lake Champlain watershed to
different phosphorus removal levels.
Concept designs for upgrading generalized plants in
Chesapeake Bay watershed to different phosphorus
removal levels.
Concept designs and cost estimates for upgrading
generalized plants in U.S. to different nutrient
removal levels; also includes nine actual activated
sludge BNR case studies.
Notes:
TN = total nitrogen; TP = total phosphorus; AS = activated sludge; Lag = lagoon; TF = trickling filter.
In the Scope column, "generalized plants" refers to plants with characteristics that were made up by the author (typically based on
representative characteristics of a set of plants in the study area), and "specific plants" use the characteristics of actual, identified
treatment plants in the associated conceptual design and costing exercises.
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In addition to these published efforts, several EPA Regions have pursued WWTP optimization
as important outreach elements of their National Pollutant Discharge Elimination System
(NPDES) and water infrastructure programs. For example, Region 1 has a master list of
WWTPs that are candidates for nutrient optimization. Region 4 has been assisting WWTPs in
optimizing for energy reduction for several years, noting nitrogen removal as a collateral benefit.
The Sustainable Infrastructure program of Region 9 sponsored a number of energy efficiency
evaluations at WWTPs, many of which included recommended process improvements that also
reduce nitrogen. State government agencies have also developed tools to help utilities optimize
nutrient removal at WWTPs (e.g., Wisconsin Pollutant Discharge Elimination System Program's
Phosphorus Operational Evaluation and Optimization Report Worksheet).
1.3 How TO USE THIS DOCUMENT
Although the primary focus of this report is on transferring technical information to WWTP
managers and operators, it is organized in a way that allows multiple audiences, including
regulators, policy makers, and nontechnical persons, to access information of interest.
Section 2 provides an introduction to nutrient removal in wastewater treatment, along with
descriptions of the nutrient reduction attributes of various common types of wastewater
systems. In addition to providing the technical background that supports the rest of the report,
that section also provides preliminary information about potential optimization techniques for
those readers seeking an overview rather than detailed technical information.
Sections 3 through 5 focus on the details of improving nutrient reduction for the three main
types of treatment systems considered: activated sludge, treatment lagoons, and trickling filters.
Section 3, the section about activated sludge, in particular, uses case studies to provide data
for, and examples of, nutrient reduction optimization efforts across the United States. These
sections are targeted to readers interested in the technical details associated with nutrient
reduction optimization, including WWTP technical managers and operators, technical outreach
specialists, and consultants.
Section 6 provides a brief introduction to other potential nutrient reduction strategies that are not
the main focus of this document, and Section 7 provides conclusions and recommendations for
practitioners and others.
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2 Introduction
2.1 NITROGEN REMOVAL
2.1.1 Nitrogen Removal Fundamentals
A thorough understanding of nitrogen removal during wastewater treatment is necessary to
recognize potential optimization opportunities. Those opportunities are introduced in Section
2.1.2 as a precursor to more detailed information and summaries of optimization case studies in
sections 3 through 5.
Nitrogen in municipal wastewater can come from multiple sources. Urine contains about 90
percent of the nitrogen excreted by humans, mostly from the breakdown of amino acids from
food. Food wastes and some industrial processes can also contribute significant amounts of
nitrogen to municipal wastewater influents. Nitrogen in food comes from amino acids in protein
and from purines, pyrimidines, free amino acids, vitamins, creatine, creatinine, and amino
sugars (Minnis 2006). Urea and organic nitrogen in wastewater influents are typically quickly
converted to ammonia under anaerobic conditions within sewer collection systems via a process
called "ammonification".
Removal of nitrogen during wastewater treatment is typically the result of natural biological
processes including uptake, biological nitrification and denitrification (generically termed
"biological nitrogen removal"), and anaerobic ammonia oxidation.
Biological (Cell) Uptake
Nitrogen is an essential component of all proteins. Therefore, all biological organisms require
nitrogen to grow.
Nitrogen comprises approximately 12 percent, by dry weight, of the cell mass of microbes
during wastewater treatment. Therefore, even in wastewater treatment systems not specifically
engineered for nitrogen reduction, a certain amount is removed by wasting biological solids, as
is typical in a biological wastewater treatment process. Those reductions are generally modest.
Nitrogen is also a primary macronutrient for plants, present in plant tissue in quantities from 1-6
percent, on a dry mass basis. Relatively few plants fix atmospheric nitrogen, so most rely on
nitrogen compounds in the soil (chemical fertilizer, manure, or wastewater or reclaimed effluent
dispersed into the soil near their root zone) to support their growth. Both oxidized and reduced
species of nitrogen can be taken up by plants, although amino acids and proteins can be built
only from ammonium (NhU), so oxidized species must first be reduced.
Biological Nitrogen Removal
"Biological nitrogen removal" (BNR) is the general term used to describe the 2-step nitrification-
denitrification process, which is the primary approach used to deliberately remove nitrogen
during municipal wastewater treatment.
Nitrification is the biological oxidation of ammonia to nitrate. Influent ammonia is first oxidized to
nitrite (NCb) by ammonia-oxidizing bacteria (AOB), then nitrite is oxidized further to nitrate (NOs)
by nitrite-oxidizing bacteria (NOB). Nitrification requires both oxygen and alkalinity to buffer
against a pH drop that can inhibit nitrifying bacteria. A portion of this lost alkalinity is recovered
in the subsequent denitrification process.
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Since AOB and NOB use inorganic carbon for cellular growth and synthesis rather than organic
carbon sources, they are classified as autotrophic organisms, and grow relatively slowly and
with a lower yield than the heterotrophic organisms responsible for biological oxygen demand
(BOD) removal. Temperature has a significant impact on the process kinetics and performance
of the nitrifying organisms; the rate and amount of nitrification generally decrease with a lower
temperature. Growing and maintaining a nitrifying biomass, therefore, requires a relatively long
aerobic solids retention time (SRT, or sludge age). The minimum SRT required for nitrification
increases with cooler temperatures to compensate for slower growth rates.
"Denitrification", the biochemical reduction of oxidized nitrogen—nitrate—to dinitrogen gas, is
much less sensitive to temperature, although it is still affected, and requires a relatively short
anoxic SRT. Denitrification is performed by heterotrophic bacteria and requires an organic
carbon source. Available carbon sources already present in wastewater or provided within the
treatment process include biodegradable soluble chemical oxygen demand (COD) in the influent
wastewater, biodegradable soluble COD from biological hydrolysis of particulates and colloids,
and the biodegradable soluble COD produced during endogenous decay of microbial cells.
Supplemental sources of carbon can also be added to the system if carbon is lacking or to
achieve higher levels of denitrification.
A generalized liquid-phase schematic diagram of a traditional secondary suspended growth
(i.e., activated sludge) treatment process is shown in Figure 2-1. There are three main types of
denitrification processes: 1) pre-anoxic denitrification (Figure 2-2), 2) post-anoxic denitrification
(Figure 2-3), and 3) single-reactor nitrification/denitrification (Figure 2-4). One or two of the
processes can be used within a secondary (i.e., biological) treatment process. The first two
involve the creation of dedicated unaerated or anoxic zones for denitrification. Single-reactor
nitrification and denitrification provide nitrification and denitrification in the same space. This
includes simultaneous nitrification/denitrification, which is promoted under low dissolved oxygen
(DO) conditions; cyclic processes where aeration is switched on and off; step-feed processes;
and others.
Influent Wastewater
Secondary Effluent
Secondary Treatment
(activated sludge)
Primary Settling (optional) Secondary Clarification
Figure 2-1. Generalized secondary (activated sludge) liquid-phase treatment schematic
Pre-AnoxicZone
Influent Wastewater
Secondary Effluent
Primary Settling (optional)
Secondary Clarification
Internal Recycle
Figure 2-2. Generalized pre-anoxic zone nitrification/denitrification liquid-phase treatment
schematic
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Post-AnoxicZone
Influent Wastewater
Primary Settling (optional)
Secondary Treatment
(activated sludge)
Secondary Effluent
Secondary Clarification
Supplemental
Carbon (optional)
Figure 2-3. Generalized post-anoxic zone nitrification/denitrification liquid-phase treatment
schematic
Influent Wastewater
Secondary Effluent
Primary Settling (optional)
Secondary Clarification
Internal Recycle (optional)
Figure 2-4. Generalized single-reactor nitrification/denitrification liquid-phase treatment schematic
Pre-anoxic denitrification typically relies on the carbon in the influent or primary clarifier effluent
to feed the denitrifying organisms that reduce nitrate, which is produced in the downstream
aerobic zone. It must, therefore, be returned to the pre-anoxic zone in the return activated
sludge (RAS) and/or internal recycle streams. In comparison, the post-anoxic zone follows the
aerobic zone and the carbon from endogenous decay is used for denitrification, which results in
a much lower nitrate/nitrite reduction rate than in the pre-anoxic zone. Carbon from external
sources can also be added to this zone to increase the denitrification rate.
Simultaneous and/or cyclic nitrification/denitrification are commonly used in systems with long
SRTs (20 days or more) and hydraulic retention times (HRT), such as oxidation ditches and
lagoons. Nitrification and denitrification rates are relatively slow, which is why longer SRTs are
required to achieve complete nitrification.
Anaerobic Ammonium Oxidation
Anaerobic Ammonium Oxidation (Sometimes called AnAmmOx, which is also a trademarked
process name, and is a type of deammonification process) is a natural biological process that
uses nitrite as the electron acceptor in the anaerobic oxidation of ammonium, producing N2 gas.
The first step in the process is the aerobic nitrification of part of the ammonia to nitrite. This
process has recently received considerable interest because it has the benefit of not requiring
an organic carbon source and using less air (and thus energy) than the traditional aerobic
nitrification followed by anoxic denitrification process described above, since only part of the
ammonium has to be oxidized to NO2. Thus far in practice, however, this process has primarily
been engineered for the treatment of high temperature and high ammonia sidestreams, such as
anaerobic digester dewatering recycle streams.
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2.1.2 Nitrogen Removal Optimization Opportunities
There are a variety of physical and operational modifications that can be made to a wastewater
treatment system to improve nitrogen removal. This document mainly focuses on operational
modifications and physical modifications that are relatively minor; significant infrastructure
modifications, like adding new reactors, are generally outside its scope. Some operational
changes might require equipment upgrades or modifications, but they are generally low-cost
compared with the more substantial upgrades associated with building new structures. Although
it is recognized that there is often overlap of and interdependency between categories of
potential modifications, optimization activities have been grouped into the following main
categories: 1) aeration, 2) process, 3) configuration, 4) chemical, and 5) discharge.
Aeration
As implied in Section 2.1.1, the oxidation-reduction (or redox) state of the treatment
environment is a major controlling factor for nitrogen removal processes with aerobic (or oxic)
conditions required for nitrification, and anoxic conditions required for denitrification.
Considering that many treatment plants that have never optimized their aeration systems over-
aerate, improving the control of aeration is often the lowest hanging fruit for a plant endeavoring
to improve nitrogen reduction. Reducing overall aeration has the added bonus of reducing
energy costs, often quite significantly, as aeration equipment typically has the single largest
energy demand of internal plant processes (see Figure 2-5). A number of utilities have stumbled
upon significantly improved nitrogen removal as a by-product of energy-efficiency efforts.
Screens
0,0%
Grit
1.4%
Wastewater Pumping
14.3%
Clarifters
3.2%
Lighting & Buildings
8.1%
Chlori nation
Aeration
54.1%
Be It Press
3.9%
Anaerobic Digestion
14,2%
Gravity Thickening
0.1%
Return Sludqe Pumping
0,5%
Electricity Requirements for Activated Sludge Wastewater
Derived from data from the Water Environment Energy Conseivation Task Force Energy Conservation in Wastewater TreaZvem
Source: WERF 2011.
Figure 2-5. Average WWTP energy use breakdown.
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Aeration modifications are changes to physical aeration equipment, controls, operation, and
function of equipment and aerated areas. They include installation of energy-efficient blowers,
variable frequency drives (VFDs) to provide adjustable control to air blowers or surface aerators,
diffusers with improved distribution and oxygen transfer efficiency (OTE), airflow meters, airflow
control valves, on/off cycling, the installation of DO, and ammonia or oxidation-reduction
potential (ORP) control.
Nitrification is a prerequisite for BNR, so if a WWTP is not fully nitrifying, optimization efforts are
probably limited. As previously indicated, nitrification requires a sufficient SRT (which translates
into a sufficient reactor volume) and sufficient aeration capacity to convert ammonia to nitrate.
Many existing WWTPs, particularly those featuring mechanically aerated aerobic processes like
activated sludge, already achieve significant nitrification for the following reasons:
1. Ammonia can be toxic to aquatic organisms; therefore, effluent ammonia limits have
been common for some time, and simple conversion of ammonia to nitrate generally
alleviates toxicity concerns.
2. Historically, WWTPs were not designed with energy efficiency as a top priority;
therefore, oversizing of aeration systems has generally been standard practice.
Likewise, aeration controls might not have been prioritized either in capital programs or
in ongoing performance evaluation.
3. Many conventional (i.e., non-advanced) and relatively small treatment plants use
activated sludge processes with relatively long SRTs and HRTs (long enough to affect
nitrification), since the increased volumetric capacity required generally only has minor
effects on the system footprint and because the increased volumetric and aeration
capacity provide internal buffering capability that might be important for plants that are
not staffed around the clock.
Many WWTPs nitrify and most that do have at least some excess aeration capacity under most
conditions. However, for those WWTPs, denitrification is often limited because of a lack of
proper conditions (i.e., nitrate, organic carbon, and anoxia).
The anoxic conditions required for denitrification can be created in several different ways in an
activated sludge system, provided that the system has some excess treatment capacity (even a
small amount). These include on/off cycling or throttling of aeration (for enhancing simultaneous
or phased denitrification within a single reactor), or the creation of dedicated anoxic and aerobic
zones by turning off the air to a portion of the aerated volume—typically at the front end of the
basin (to create a dedicated anoxic zone). Frequently, mixers are added to keep solids in
suspension or provide mixing in dedicated anoxic zones, or when air is turned down or cycled
off. Other modifications that improve the ability to modulate aeration include adjusting the pitch
angle of centrifugal blower blades and the use of synchronous blower motors.
Equipment upgrades that allow for adjustable aeration control include the use of VFDs on
positive displacement blowers to control aerator output and/or use of online monitoring tools to
inform aerator operational mode. Equipment replacement might also be beneficial in increasing
the efficiency and performance of the aeration system, as is the case with replacing aged
blowers and diffusers.
Some type of improved aeration control is the most common nitrogen removal optimization
technique at existing WWTPs, although it can often be supplemented with process, piping,
and/or chemical activities for enhanced effectiveness.
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Process
Process modifications include adjustments to process control characteristics. As previously
indicated, SRT is a particularly important process parameter for nitrification. Mixed liquor
suspended solids (MLSS) and food-to-microorganism (F/M) ratio are related parameters.
Internal recycle and RAS return rate can be particularly important for denitrification. Physical
process improvements can include the addition of VFDs and/or RAS pumps for improved
control of internal recycling; the addition of online monitoring equipment for process control and
optimization; or providing new screens or grit removal equipment at the headworks to improve
the reliability of the treatment process. Other examples of process modifications include flow
equalization improvements, optimizing internal mixed liquor recycle rates, modifying plant
recycle flow patterns, controlling sidestream flows, and adding the capability to ferment primary
sludge.
As previously indicated, denitrification is often limited because of a lack of proper conditions
(i.e., nitrate, organic carbon, anoxia). As highlighted above, providing anoxic conditions is
largely a function of aeration control. Although a WWTP might be nitrifying, it is critical to get the
nitrate into the anoxic environment, along with organic carbon, for denitrification. For this
reason, establishing anoxic conditions at the influent end of the process, where influent organic
carbon should be readily available, is generally preferred. With anoxic conditions and organic
carbon, treatment effectiveness depends largely on exposing nitrified mixed liquor to these
conditions, typically by internally recycling mixed liquor to the denitrification reactor. Adding or
improving the control of internal mixed liquor recycle systems is, therefore, an important process
control parameter for nitrogen removal. Likewise, it is important to minimize aeration occurring
within other unit processes and structures (e.g., influent and return channels) that may increase
DO carry-over into existing or new anoxic zones.
For systems that recycle mixed liquor for denitrification, the recycle rate can be optimized by
monitoring the nitrite and/or nitrate leaving the primary anoxic zones either by manually
sampling or using online monitoring to set the internal recycle (IR) rate. Only the amount of NOX
that can be denitrified needs to be returned to the primary anoxic zones. This can be an
automated process involving a feedback loop or use a manually set rate. The IR pumps will
need to be equipped with VFDs or multiple small pumps will need to be used to effectively
control the IR rate.
Note that increasing recycle rates, whether of mixed liquor or activated sludge, might add DO to
the anoxic zone. Tradeoffs, or unanticipated consequences, of activities like these are,
therefore, essential to consider when evaluating nitrogen optimization alternatives.
The availability of organic carbon in the anoxic zone can sometimes be a limitation for
denitrification, especially for single sludge and post-denitrification systems. Various options for
providing organic carbon are available, including some options internal to the plant that can be
enhanced through process modifications.
Primary sludge can be fermented to produce volatile fatty acids (VFAs) or available soluble
carbon for use in biological nutrient removal. Primary sludge fermentation can be accomplished
in the primary clarifier sludge blanket by modifying the primary sludge wasting rate to provide a
deeper blanket and longer residence time to allow fermentation, adding available soluble BOD
to the secondary treatment process influent. A portion of the primary sludge should be returned
to the influent of the primary clarifier to elutriate the VFAs in the sludge. Primary sludge can also
be fermented in a separate tank, typically a gravity thickener fermenter. Fermenting in a
separate reactor will involve a higher capital cost, but will also provide more carbon to the
process and more flexibility over where the carbon-rich stream is returned.
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Other process modifications may be important as well. For example, treatment processes
generally perform better, producing more consistent effluent, when the influent flow and load are
consistent or when the system is operated to minimize the impacts of the variations in the flows
and loads. Optimization could, therefore, include adding or improving influent flow equalization
or controlling or equalizing the plant return flows, such as the sludge dewatering return to avoid
spikes in nitrogen load and flow.
Configuration
Configuration modifications are changes to, or the addition of, flowstreams within the process or
changes to the process configuration. They might include changes to channels, manipulation of
gates or baffles, or modifying or adding piping, such as adding internal recycle lines or step-feed
provisions. Configuration modifications are distinguished from process modifications in that they
will require some (although usually minimal) new infrastructure. Process modifications use
existing infrastructure but might require new monitoring or control equipment.
Converting a complete mix reactor to a plug flow reactor can allow for the creation of aerobic
and anoxic zones to provide nitrification and denitrification. In some systems, a portion of the
RAS can be directed to the post-anoxic zone (sometimes called a "RAS bleed-off") to provide
carbon, improve denitrification, and lower the effluent TN. Step-feeding of influent can also be
implemented to provide a higher SRT by allowing higher MLSS in the front zones.
Modifications under this category could also include repurposing existing tankage, which can
include physical modifications, such as adding baffle walls and the like.
Chemical
Chemical modifications include the addition of alkalinity and supplemental carbon to improve
nitrification and denitrification, respectively. If low alkalinity is limiting nitrification, then alkalinity
can be added to the process (e.g., using lime) to improve nitrification. Performance can also be
improved by using inline monitoring and controls to maintain an optimum feed rate.
Supplemental carbon can be added, usually to a post-anoxic zone, to improve or speed up
denitrification. Supplemental carbon feed systems can be improved by selecting a more
effective carbon source or adding controls and monitoring to optimize the feed rate. As indicated
above and in the context of this technical document, nitrogen removal optimization using
chemical addition is supplemental to other aeration, process, and/or piping modifications.
Discharge
Discharge modifications are made at the end of the treatment system to further reduce nutrients
prior to delivery to receiving surface waters. They generally use natural systems and might
include land application or wetland assimilation discharge. This category of nitrogen removal
enhancement is typically independent of the other four approaches.
Modifying a WWTP discharge can be an effective way to reduce nitrogen delivery to surface
waters, although it might not be widely practical or affordable. Managers of plants having some
kind of alternative to a direct surface water discharge might have already considered
alternatives that could have lower impacts and help meet water quality objectives.
Two alternatives to direct discharge are addressed in this document, including land application
of effluent, which in most cases, uses the soil as a treatment process; and wetland assimilation
in which effluent is discharged into natural or man-made wetlands for further attenuation of
nutrients prior to receiving water delivery.
Nitrogen removal in both types of discharge is primarily through biologically mediated
nitrification and denitrification. Although, vegetative uptake can be a significant removal
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mechanism provided that biomass is harvested as needed to ensure that sequestered nitrogen
is permanently removed from the system.
2.2 PHOSPHORUS REMOVAL
2.2.1 Phosphorus Removal Fundamentals
A thorough understanding of phosphorus removal during wastewater treatment is necessary to
be able to recognize potential optimization opportunities. Those opportunities are introduced in
Section 2.2.2 as a precursor to more detailed information and summaries of case studies in
sections 3 through 5.
Phosphorus in municipal wastewater can come from multiple sources. Urine contains over 90
percent of the phosphorus excreted by humans. Dietary phosphorus is readily absorbed in the
small intestine and any excess is excreted into urine by the kidneys (Minnis 2006). Like
nitrogen, food wastes and some industrial processes can also contribute significant amounts of
phosphorus to municipal wastewater influents. Soluble phosphorus in wastewater is typically in
the form of orthophosphate (PCV3).
Removal of phosphorus during wastewater treatment is typically the result of natural biological
processes, including uptake and enhanced biological phosphorus removal (EPBR), although
many WWTPs will use metal salts to precipitate phosphorus to the solids (sludge) fraction. In
either case (biological or chemical treatment), phosphorus is removed by converting it to a solid,
so it partitions to the sludge.
Biological Uptake
Phosphorus is a constituent of nucleic acids, nucleotides, phospholipids, low phosphorus
starches (LPS), and teichoic acids in microbial cells. Phosphorus makes up approximately 2
percent, by dry weight, of the cell mass of microbes during wastewater treatment (not designed
for EBPR). Therefore, even in wastewater treatment systems not specifically engineered for
phosphorus reduction, a certain amount is removed (usually about 2 mg/l). These reductions
are generally modest, however, and rarely sufficient to meet water quality objectives or effluent
permit limits.
Phosphorus is also a primary macronutrient for plants, present in plant tissue, typically at
approximately 0.2 percent, on a dry mass basis. Phosphorus is especially important for plant
bioenergetics, for the conversion of light energy to chemical energy during photosynthesis. It is
also important in the activation of proteins and regulation of metabolic process. Phosphorus is
commonly a limiting factor for plant growth in many soils under most environmental conditions.
Enhanced Biological Phosphorus Removal
Specialized bacteria in activated sludge mixed liquors called "polyphosphate accumulating
organisms" (PAOs) can be used to biologically remove phosphorus from wastewater to levels
that might meet water quality objectives. PAOs require two stages for phosphorus removal. The
first stage is anaerobic, in which PAOs uptake VFAs from the organic carbon in the influent (or
added as a sidestream flow) and store it as polyhydroxyalkanoate (PHA) for later oxidation in an
aerobic zone. During this process, the PAOs also release phosphorus in the form of
orthophosphate under anaerobic conditions, which provides the energy required for the uptake
and storage of the VFAs. This first anaerobic stage is sometimes called an "anaerobic selector"
because it preferentially selects for the proliferation of PAOs.
The second stage takes place under aerobic (or oxic) conditions. In the aerobic stage, the
stored PHA is metabolized, providing energy for cell growth and the luxury uptake of soluble
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orthophosphate, which is stored as polyphosphates. The PAOs uptake and store more
phosphorus under aerobic conditions than is released under anaerobic conditions, providing a
net uptake and storage of phosphorus. This also provides the PAOs with a competitive
advantage over other organisms, allowing them to thrive under these conditions. The stored
phosphorus is then removed from the system with the waste sludge. If secondary clarifiers are
allowed to become anaerobic or the waste activated sludge (WAS) is treated in an anaerobic
digester, the PAOs can release stored phosphorus back into the process stream. Up to four
times as much phosphorus can be removed biologically using EBPR than conventional
activated sludge treatment.
Chemical Precipitation
Phosphorus can also be removed using chemical precipitation. The most common chemicals
used for the precipitation of phosphate are aluminum sulfate, ferric chloride, and ferrous
chloride. The precipitated phosphates must be removed by sedimentation and/or filtration. Note
that the use of metal salts for the precipitation of phosphorus will add to the sludge production of
the plant (EBPR generally does not increase sludge production appreciably). If the secondary
clarifiers are used for the removal of precipitants, inert solids will also be added to the activated
sludge process, decreasing the capacity for volatile solids or active biomass.
2.2.2 Phosphorus Removal Optimization Opportunities
EBPR can be added to an activated sludge treatment system by creating an anaerobic selector
zone at the front of the secondary treatment process. The anaerobic selector must be upstream
of the internal (nitrified) recycle if used in conjunction with a nitrification/denitrification process.
Soluble VFAs can be provided for EBPR through primary sludge fermentation, as described in
Section 2.1.2. Supplemental carbon can also be added to provide the VFAs needed for EBPR.
Unlike BNR, EBPR generally requires a dedicated anaerobic reactor, so some type of
partitioning and strict anaerobic conditions are required, which makes low-cost upgrades less
feasible for plants not originally designed with EBPR in mind.
For activated sludge and most other types of WWTPs, metal salts can be added to chemically
precipitate orthophosphate, which can then be removed with solids, during primary or secondary
clarification and/or tertiary filtration. Metal salts can be added upstream of the primary and/or
secondary clarifiers as well as at other points within the treatment system. Chemical
precipitation, however, can limit EBPR. To optimize EBPR, chemical precipitation of phosphorus
should be used as part of a tertiary treatment process. Chemical precipitation is the most
common technique to achieve higher levels of phosphorus removal in plants not designed for
EBPR. However, this technique is well-established and fully documented and described in
various references, so it is not a focus of this document.
As with nitrogen removal, modifying a WWTP discharge through land application or wetland
assimilation can be an especially effective way to reduce phosphorus delivery to surface waters,
although it may not be widely practical. Phosphorus removal in "natural" systems such as these
is typically the result of physiochemical immobilization reactions either in the soil matrix or in
solution in free surface wetlands (e.g., precipitation).
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2.3 NUTRIENT REMOVAL ATTRIBUTES OF TYPICAL
WASTEWATER TREATMENT FACILITIES AND
OPPORTUNITIES FOR IMPROVEMENT
Effluent nutrient characteristics vary considerably between different system types, different
geographical areas (e.g., temperature can play a significant role in biological nutrient removal
processes), and different influent wastewater, among other variables. Accordingly, average
effluent nutrient concentrations should only be considered ballpark values. Average effluent
nutrient concentrations for wastewater treatment systems are provided in Table 2-1.
Table 2-1. Nutrient removal performance (effluent concentration) for wastewater treatment plants.
Treatment System
Raw Wastewater1
Primary Treatment2
Activated Sludge (with no nutrient removal)1
Facultative Lagoon3
Trickling Filter2
Total Nitrogen (mg/l)
40
37
25
16
25
Total Phosphorus (mg/l)
7.0
6.2
5.6
4.2
5.8
Sources:
1Metcalf and Eddy 2004
2MetcalfandEddy1991
3 Metcalf and Eddy 1991; WEF 2003; USEPA 2011 b
2.3.1 Activated Sludge
Activated sludge is a suspended growth biological treatment process in which a large mass of
aerobic floe-forming microorganisms convert organic material and other constituents to gases or
assimilate them into cell tissue. Although activated sludge is conventionally defined to include
only aerobic process, the term can be used to describe systems that include anaerobic and
anoxic processes in addition to aerobic ones. The three basic elements of an activated sludge
process are the biological reactor(s), liquids-solids separation unit (secondary clarifierfor
conventional process or membranes in the case of membrane bioreactors), and a return stream
of the solids back to the reactor (Figure 2-6).
Primary Settling (optional)
Secondary Clarification
Influent Wastewater
Secondary Effluent
Secondary Treatment
(activated sludge)
Return Activated Sludge
Figure 2-6. Basic (non-advanced) Activated Sludge WWTP process schematic
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A portion of the solids is removed from the process or wasted in order to maintain an active,
growing biomass population, and to remove solids-associated constituents (like phosphorus).
Activated sludge processes that have been developed to include biological nitrogen and/or
phosphorus removal are commonly called "biological nutrient removal processes".
Biological nutrient removal is a well-documented set of processes for enhancing nitrogen and
phosphorus removal. As described in Section 2.1.1, BNR is a 2-step process, sequentially
requiring nitrification of ammonia to nitrate under aerobic (oxic) conditions, followed by the
anoxic (no free oxygen) denitrification of nitrate to dinitrogen gas, which is harmlessly released
to the atmosphere. A number of different process configurations have been used to affect
nitrification/denitrification in WWTPs. Upgrades of existing WWTPs for BNR is often practical,
although the extent to which TN can be reduced in WWTP effluents is a function of both pre-
upgrade process configuration and upgrade cost.
Phosphorus is removed biologically from wastewater by uptake into cell mass, which is then
wasted and disposed (or reused) as biosolids. EBPR is a process whereby bacterial cells are
triggered to uptake much larger amounts of orthophosphate than they would under normal
conditions (in very simple terms, EBPR is stimulated by anaerobic preconditioning followed by
aerobic conditions). Although EBPR can be retrofit into existing WWTPs, in many cases,
retrofits use chemical precipitation of phosphorus, which generally features lower capital costs
(but more expensive recurring costs for chemicals and for sludge disposal) and oftentimes
easier incorporation into existing WWTP process configurations.
Optimization opportunities for activated sludge processes are described in more detail in
Section 3.
Conventional (non-advanced) activated sludge treatment processes are typically designed and
operated with a focus on BOD and TSS removal, and sometimes nitrification. Nitrogen and
phosphorus removal in those processes predominantly occurs from the nutrients being
assimilated into the cell biomass during microbial (net) growth. Approximately 1 mg of
phosphorus removal and 5 mg of nitrogen removal can be expected per 100 mg of BOD
reduced in the system, although the ratio can vary depending on system characteristics and
other factors. In addition, solids handling and treatment processes, such as aerobic or
anaerobic digestion, often release some of these nutrients back into the solution during the
reduction of the biomass, which is then returned back to the treatment process via the solids-
handling sidestreams. As a result, overall nutrient removal in conventional activated sludge
treatment processes is typically relatively low (see Table 2-1). To address this limitation when
nutrient removal is required, conventional processes have been modified and new processes
developed for targeted biological removal of nitrogen and phosphorus from wastewater.
Phosphorus removal will depend in part on the carbon (typically reported as BOD) available and
its ratio to the phosphorus concentration in the influent. Similarly, nitrogen removal, which
typically involves both nitrification and denitrification, will also depend on the carbon available
(only the denitrification stage is carbon-dependent) and its ratio to the influent nitrogen (TKN)
concentration, as well as the concentration of recalcitrant organic nitrogen. Therefore, nutrient
removal performance will be a function of the influent wastewater characteristics and nutrient
concentrations. For the purposes of this discussion, the following "typical" average wastewater
influent concentrations have been assumed (Metcalf and Eddy 2014):
• BOD = 133-400 mg/L
• TKN = 23-69 mg/L
• Nonbiodegradable Soluble TKN = 1-2 mg/L
• TP = 3.7-11 mg/L
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Table 2-2 shows several commonly used activated sludge treatment processes and the effluent
nitrogen and phosphorus concentrations that can typically be achieved with each process. Note
that it might not be possible to achieve the same effluent concentrations using some of the
processes under higher TKN loadings (partly due to the higher nonbiodegradable soluble TKN
concentration, as well as limitations of individual configurations), in which case the achievable
effluent nitrogen concentration may exceed the upper end of the range reported in Table 2-2.
Table 2-2. Average effluent TN and TP for various activated sludge process configurations
Process
Configuration
Conventional
Treatment Processes
(Complete-Mix,
Extended Aeration,
Plug Flow)
Sequencing Batch
Reactor
Oxidation Ditch
dNOx™ Oxidation
Ditch
Low DO Oxidation
Ditch
Orbal™
MLE
Step-feed BNR
4-Stage Bardenpho
Post-Anoxic Zone with
Carbon Addition
Modified Bardenpho (5-
Stage)
A/O
A20
UCT
Application
BOD Removal, TSS
Removal, and Nitrification
Can be used for nitrogen
removal and phosphorus
removal or both
simultaneously depending
on volume
Cyclic
Nitrification/Denitrification
Cyclic
Nitrification/Denitrification
Simultaneous
Nitrification/Denitrification
Simultaneous
Nitrification/Denitrification
Pre-anoxic Denitrification
Nitrification/Denitrification
Nitrification/Denitrification
Applicable for various
nitrogen removal and
combined nitrogen and
phosphorus removal
process configurations
Can include supplemental
carbon feed to post-anoxic
zone
EBPR only, no nitrification
EBPR, denitrification
(high influent BOD/P ratio)
EBPR, denitrification
(low influent BOD/P ratio)
Achievable Effluent
TN
15-35mg/L
5-8 mg/L
5 mg/L
10 mg/L
3 mg/L
3 mg/L
6 -10 mg/L
5 mg/L
3 mg/L
3 mg/L
3 to 5 mg/L
15 -35 mg/L
6 -10 mg/L
6 -10 mg/L
Achievable Effluent
TP
4 -10 mg/L
0.5 -2.0 mg/L
4 -10 mg/L
4 -10 mg/L
4 -10 mg/L
4 -10 mg/L
4 -10 mg/L
4 -10 mg/L
4 -10 mg/L
NA (Dependent on
whether or not EBPR
is included)
0.5 -2.0 mg/L
0.5 -2.0 mg/L
1.0 -2.0 mg/L
0.5 -2.0 mg/L
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Process
Configuration
Johannesburg
EBPR with VFA
addition
Chemical Phosphorus
Removal
Application
EBPR, denitrification (see
Notes)
Used with multiple
configurations (see Notes)
Metal salts may be added
to secondary clarifiers or to
basins (see Notes)
Achievable Effluent
TN
5 mg/L
NA (Supplement to
various processes, TN
will depend on
configuration used)
NA (Supplement to
various processes, TN
will depend on
configuration used)
Achievable Effluent
TP
0.1 -0.5 mg/L
0.1 -0.2 mg/L
(soluble)
<0.1 mg/L
Source: Metcalf and Eddy/AECOM 2014.
Notes: These processes must typically also be combined with tertiary filters or membranes to achieve phosphorus concentrations
less than 0.5 mg/L, because of the effluent TP associated with effluent TSS.
TN = total nitrogen; TP = total phosphorus; MLE = Modified Lutzack Ettinger process; EBPR = enhanced biological phosphorus
removal; VFA = volatile fatty acids.
2.3.2 Lagoons
Lagoons have been used as low-cost wastewater treatment systems for many years,
particularly in relatively small and/or rural communities where sufficient land is available to site a
lagoon that generally has a larger footprint than more mechanically based treatment systems
(e.g., activated sludge).
Facultative lagoons are the most common type of lagoon used for WWTPs in the United States.
They typically feature aerobic conditions in the upper layer and anoxic or anaerobic conditions
toward the bottom, with the transition depth depending on the influence of wind-driven mixing.
Lagoon systems have not traditionally been designed to specifically remove nutrients, but
primarily owing to their long HRT and SRT and co-occurring aerobic and anoxic conditions,
often remove significant amounts of nitrogen with reductions of 40 percent common and
reductions of 90 percent or greater achievable in some facultative systems during warm weather
when biological reaction rates and other reduction processes (e.g., ammonia volatilization) are
highest.
Because of the relatively long residence time of facultative lagoons, it may be possible to
operate them to store wastewater in the winter (when nitrogen reduction is lowest) and
discharge in the summer (when nitrogen reduction is highest) in order to maximize TN
reductions. This type of operation is called a "controlled discharge" lagoon.
Various operating strategies can also be employed to optimize BNR in aerated lagoons,
particularly those with mechanical aeration, similar to the optimization strategies available for
activated sludge processes.
Phosphorus reductions are also widely variable, reported to range from 30-95 percent (Assenzo
and Reid 1966; Pearson 2005; Crites et al. 2006). The long residence times in lagoons afford
considerable capacity for storing phosphorus-rich sludge, much of which is generated via
physiochemical mechanisms such as adsorption, coagulation, and precipitation, although
biological uptake into algal and bacterial biomass is also important. Chemical precipitation has
been used to reliably meet effluent phosphorus concentrations of 1.0 mg/l, using ferric or
aluminum salts applied in batch or continuous modes.
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Because lagoon systems are often sited in areas with available land, land application or some
other type of modified discharge (e.g., wetland assimilation) might be a viable way to decrease
nutrient load delivery to surface waters.
2.3.3 Trickling filter
A "trickling filter" is a nonsubmerged, attached growth, aerobic, biological secondary treatment
process, in which wastewater is continuously or periodically distributed over rock or plastic
packing. A biofilm grows on the filter media that treats the wastewater as it flows over it. Air
moves through the media voids to provide oxygen for the treatment process either by natural
draft aeration or forced draft aeration.
Trickling filters are commonly used for BOD removal and sometimes for nitrification when
organic loading rates are low.
Nutrient reduction in WWTPs using trickling filters is typically modest, with average reductions of
10-30 percent TN and 8-12 percent TP (Metcalf and Eddy 1991). Nutrient reduction
optimization options for trickling filters are likewise limited. Process variables can be modified in
an effort to improve nitrogen removal, particularly if the trickling filters are designed to nitrify and
are equipped with internal recycle, the rate of which can be adjusted (generally increased, but
operational problems may occur if recycle rates are too high) to optimize denitrification.
A denitrification filter can be added after a nitrifying trickling filter to provide more significant total
nitrogen reductions, although supplemental carbon will often be needed and overall costs will be
significant. Chemical precipitation of soluble phosphate in a separate reactor or prior to the
secondary clarifier is typically the most feasible option for significant TP removal.
2.3.4 Primary Treatment
Primary treatment involves the removal of a portion of the solids from the raw wastewater by the
settling process using primary sedimentation tanks or clarifiers. Nutrient removal is generally
limited to whatever nitrogen and phosphorus is contained in the settled solids. Properly
designed primary clarifiers typically remove 50-70 percent of suspended solids and 25-40
percent of BOD in the influent. Primary settling has been reported to remove 5-10 percent of
influent TN and 10-20 percent of TP (Metcalf and Eddy 1991).
Settling can be improved by the addition of chemicals such as metal salts and polymer. Iron and
aluminum salts are often used in primary clarifiers to precipitate phosphorus, allowing it to be
removed with the settled solids.
A secondary treatment process designed for biological nutrient removal must be added for the
biological removal of nitrogen and phosphorus. Primary clarifiers can be designed and/or
operated to allow fermentation of the primary sludge providing soluble BOD for use in
denitrification or EBPR.
Existing primary treatment facilities can (and often do) land-apply primary effluent for secondary
and tertiary treatment in situ.
Note that the Clean Water Act requires secondary treatment, except for facilities that have
301 (h) variances, which are specific to marine discharges.
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2.3.5 Modified Discharge
Alternatives to direct surface water discharges of WWTP effluents are used for a number of
different reasons, with further attenuation of regulated pollutants—including nitrogen and
phosphorus—often a significant driver.
"Modified discharge" systems generally use some type of natural system to polish WWTP
effluent prior to the effluent entering a receiving surface water.
Land application is a broad term used to describe systems that discharge effluent (which may
be primary, secondary or tertiary treated) into a natural soil system for additional treatment and
dispersal into the receiving environment. In most cases, the effluent will eventually reach a
surface water. Land application encompasses many different system types. Some examples
include:
• Septic systems, which use a septic tank for primary treatment followed by a gravity-flow
subsurface drainfield.
• Cluster or community scale advanced treatment systems with surface (e.g., spray
irrigation) or subsurface (e.g., drip irrigation) soil dispersal systems.
• High-rate infiltration systems where effluent is applied to a basin where it percolates
rapidly through relatively coarse media.
In general, effluent dispersed in these soil-based treatment systems flows vertically through the
soil profile to the ground water table, where it then moves horizontally toward a receiving water.
The nutrient reduction performance of land application systems can vary widely (from virtually
no nutrient reduction to virtually complete nutrient removal) depending on the specific
characteristics of the pretreatment system, soil treatment system, and natural topographical and
hydrological conditions between the system and receiving water. Nutrient reduction
performance can be estimated where soil and hydrological conditions have been characterized
appropriately.
Like most other treatment system types, nitrogen reduction in land application systems is
predominantly through biologically mediated nitrification and dentrification. Phosphorus
reduction is primarily due to sorption and immobilization reactions between soluble phosphate
and soil particles. Physical filtration of solids-associated phosphorus also might occur.
Depending on the type of land application system, vegetative uptake of nutrients can also be a
significant removal mechanism (vegetation must be removed from the site for such removals to
be permanent). Where conditions are not conducive to sufficiently reduce nutrients prior to
impacting surface waters, "permeable reactive barriers" can be used to intercept and treat
nutrient plumes.
Wetland assimilation systems have been allowed in some states (e.g., Florida, Louisiana),
mainly in coastal regions where traditional soil-based treatment might not be practical (e.g., due
to insufficient space, high ground water table), but stringent nutrient limits still need to be met in
adjacent surface waters. In these systems, WWTP effluent is discharged into a natural,
restored, or man-made wetland that further reduces nutrients prior to dispersing effluent back
into receiving surface waters. Again biological nitrogen removal and phosphate sorption
processes dominate.
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Nutrient removal in modified discharge systems can be enhanced by considering the following:
• Nitrogen removal is predominantly through biological nitrification and denitrification,
which is most efficiently designed as a sequential process. For land application systems,
this means that effluent must first be nitrified under oxic conditions either prior to or
within the natural treatment system.
• Within a soil treatment system, oxic/nitrifying conditions can best be established by
applying effluent to the dispersal area periodically in small doses under pressure and
maintaining sufficient unsaturated soil between the dispersal depth and ground water
table (typically a minimum of 1 or 2 feet of unsaturated soil).
• Denitrification requires nitrates, anoxic conditions, and labile organic carbon. These
conditions typically predominate in wetland assimilation systems being fed with nitrified
effluent. Such conditions are more difficult to establish in land application systems,
particularly if nitrification is occurring in the soil treatment system. Although the saturated
zone (ground water) is typically anoxic, organic carbon is typically concentrated in
surficial, unsaturated soil layers. However, riparian areas that include the ground water-
surface water interface may provide conditions favoring denitrification.
• In land application systems, nutrient reduction is favored in fine textured soils versus
coarse textured soils (e.g., sands). Phosphorus reduction, in particular, requires sorption
sites as well as appropriate soil reactivity, both of which are going to be more favorable
in finely texture soils, such as clays. Reclaimed effluent can also be used to irrigate
crops which will take up nitrogen and phosphorous (although these nutrients will only be
removed if the vegetation is harvested); however, build up of salts in the soil may need
to be considered and/or mitigated.
Although considerations for optimizing the performance of existing modified discharge systems
have been presented, in the context of this report, modified discharge systems are primarily
presented as an approach to help existing WWTPs further reduce nutrients.
2.4 EVALUATING AND IMPLEMENTING NUTRIENT REDUCTION
IMPROVEMENTS
The results of the case studies featured in this project suggest that WWTPs with informed and
motivated managers or operators empowered to tweak operations can often identify ways to
improve nutrient removal with relative ease. Because treatment systems are unique, it can be
difficult to prescribe a single approach for no-cost or low-cost nutrient reduction for specific
situations. The case studies in this report offer WWTP operators, managers and others insights
into what could work for them. Table 2-3 provides some key questions and approaches for
nitrogen removal at the three major categories of WWTPs considered in this document:
activated sludge, lagoons, and trickling filters.
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Table 2-3. Decision considerations for enhancing nitrogen removal at existing WWTPs
WWTP type
Activated
Sludge
Lagoon
Trickling
Filter
Key questions to ask
Is there excess plant capacity?
- Is peak daily flow < 75% design
capacity?
- Are additional tanks/reactors available?
- Is flow equalization provided?
Is there excess aeration capacity?
- Can aeration be throttled?
- Does aeration system have automatic
control?
- Can contents be mixed without
aerating?
Are process parameters sufficient?
- Can nitrified liquor be returned to low
DO zone?
- Is alkalinity sufficient for full nitrification?
- Is carbon available to drive
denitrifi cation?
Is capacity available to store effluent?
Is the lagoon mechanically aerated? If so,
can it be controlled (see Activated Sludge
rows above)?
Is a nondischarge alternative available?
Does trickling filter currently nitrify?
Optimization efforts to consider
Create anoxic zone(s)
- On/off cycling for nitrification/denitrification
in single reactor
- Feed influent and internal recycle to
dedicated tank
- Denitrify in flow equalization with internal
recycle
Facilitate anoxic environments
- Maintain lower DO setpoint or dedicated
anoxic zone
- Install DO and/or ORP meters for auto
control
- Consider adding mixers
Modify process parameters as warranted
- Internal recycle to introduce nitrified liquor
to anoxic
- Add alkalinity
- Consider step-feed, pre-fermentation
additives
Control discharge to take advantage of
summer nutrient removal, while maintaining
receiving water standards
Create anoxic zones for enhanced BNR
Study alternative discharge methods
Add post-denitrification unit
Study alternative discharge methods
Opportunities for phosphorus optimization are more limited and come down to a couple key
questions:
1. For activated sludge, are reactors/tanks available or can the existing process be
segmented to provide an anaerobic selector reactor with an HRT of at least 30 minutes?
2. For lagoons, much like for nitrogen removal, is additional capacity available in order to
store effluent during the winter and discharge during summer, when algal growth and
phosphorous uptake and sequestration is highest?
3. For all systems, is it feasible to discharge either seasonally or year-round for land
application or to wetland assimilation?
If the answer to these questions is "no", then chemical precipitation is probably going to be the
most cost-effective way to increase phosphorus removal without major infrastructure
modifications.
In general, plant managers and operators should complete the following steps to screen,
evaluate, and implement nutrient reduction improvements.
1. First, look at WWTP influent nutrient sources and concentrations. Can any nutrients be
controlled at their source?
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2. Second, evaluate whether nutrients are being loaded to the WWTP through internal
recycle lines (particularly if the WWTP uses anaerobic digestion) and consider managing
these loads through sidestream control or treatment.
3. Then, identify existing unit processes, design parameters, and actual operating
conditions. For biological processes in particular, determine whether excess reactor or
aeration capacity exists. Note that plants with highly variable flows (e.g., l&l) or loading
may have excess capacity at most, but not all, times.
4. Compile TN and TP performance data and analyze process variables and other
important characteristics (e.g., time of year/temperature) to determine whether trends
are discernible.
5. Consider using quick field tests to analyze various nutrient species throughout the
biological treatment process at different times and under different conditions.
6. Use this document to determine potential broad areas where performance can be
optimized.
7. Change only one variable at a time, allow to reach steady-state, and document
performance implications.
In the early 1980s, based on experiences inspecting and troubleshooting WWTP performance,
EPA (1984) developed a framework for systematically improving system operation called the
Composite Correction Program (CCP). Much more recently, an updated, robust CCP was
developed for dischargers in the Grand River watershed in Canada (XCG Consultants 2010).
The process has been used to systematically improve nitrification at area WWTPs and reduce
ammonia discharges, and provides a useful framework for U.S.-based WWTPs seeking to
formally implement operation improvements such as nutrient reduction optimization at their
facilities. In addition to the CCP itself, the Canadian guidance provides a framework for
coordinating operator training, system auditing, and process modeling, all to support improved
performance. These systematic frameworks can be adapted by proactive WWTP staff to
evaluate optimization opportunities within all aspects of plant performance, including nutrient
removal.
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3 Optimizing Nutrient Removal in
Activated Sludge Systems
There are a variety of often simple and relatively inexpensive improvements or operational
changes that can be made at existing WWTPs to improve energy efficiency, provide or increase
biological nutrient removal, and reduce chemical costs.
A good understanding of the fundamental requirements for enhanced nitrogen and phosphorus
removal in WWTPs is important for understanding available optimization opportunities. Table
3-1 summarizes the conditions required for biological nitrogen and phosphorus removal, and
Table 3-2 summarizes the functions of anaerobic, anoxic, and aerobic (or oxic) zones in
biological nutrient removal systems.
Table 3-1. Required conditions for biological nutrient removal (Daigger and Littleton, 2014)
Biological nitrogen removal
An aerated (aerobic) zone with a sufficiently
long SRT and other environmental conditions
sufficient to allow the growth of nitrifying
bacteria.
An unaerated (anoxic) zone where dissolved
oxygen is excluded and to which sufficient
biodegradable organic matter is added
Recirculation of nitrate-containing liquid from the
aerated to the unaerated zone.
Biological phosphorus removal
An unaerated zone where dissolved oxygen,
nitrate, and nitrite are absent and other
environmental conditions sufficient to allow PAOs
to take up and store volatile fatty acids.
An aerated zone where appropriate environmental
conditions are provided to allow PAOs to
metabolize stored organic matter and grow.
Cycling of biomass between the unaerated and
aerated zones.
Feed of wastewater contain ing volatile fatty
acids (and also, possibly, readily biodegradable
organic matter) first to the unaerated zone.
Table 3-2. Functions of zones in BNR processes (Grady et al., 2011)
Zone
Anaerobic
Anoxic
Aerobic
Biochemical transformations
• Uptake and storage of VFAs by
PAOs with associated
phosphorus release
• Fermentation of readily
biodegradable organic matter by
heterotrophic bacteria
• De nitrification
• Alkalinity production
• Nitrification and associated
alkalinity consumption
• Metabolism of stored and
exogenous substrate by PAOs
• Metabolism of exogenous
substrate by heterotrophic
bacteria
• Phosphorus uptake
Functions
• Selection of PAOs
• Conversion of NO3-N
toN2
• Selection of
denitrifying bacteria
• Conversion of NH3-N
to N03-N
• Nitrogen removal
through gas stripping
• Formation of
polyphosphate
• Growth of nitrifiers
Growth of PAOs
Zone required for:
• Phosphorus
removal
• Nitrogen removal
• Nitrogen removal
• Phosphorus
removal
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In many cases, optimizing nitrogen reduction at non-advanced WWTPs focuses on maximizing
simultaneous nitrification and denitrification, rather than creating new, dedicated anoxic zones,
which may be infeasible and/or cost-prohibitive. Keys to effective simultaneous
nitrification/denitrification include (Daiggerand Littleton, 2014):
• An aerobic SRT that exceeds that needed for nitrification (considering the highest
expected loads and lowest expected temperature)
• Promoting on-uniform hydraulic flow patterns with the aeration and/or mixing systems
• Having the ability to effectively manage oxygen input
For the purposes of this report and the associated case studies, optimization activities have
been divided into five main categories. In addition to being referenced in the body of the report,
case study summaries are provided as stand-alone documents in Appendix B. Characteristics of
each case study are summarized in Table 3-3 and under the heading "Modification Type:"
• Aeration modifications include installation of efficient blowers or aerators, variable
frequency drives, on/off cycling, dissolved oxygen (DO) or oxidation-reduction potential
(ORP) control, and associated controls.
• Process modifications include adjustments to process control characteristics including
SRT, MLSS, F/M ratio, recycle/return rate, and associated controls.
• Configuration modifications include adding recycle lines, step feed provisions,
repurposing of tankage, and associated controls.
• Chemical modifications include use of metal salts to precipitate phosphorus, and
alkalinity and organic carbon addition to support biological nitrogen removal.
• Discharge modifications are those made at the end of the treatment system to further
reduce nutrients prior to delivery to receiving surface waters (e.g., land application,
wetland assimilation discharge, etc.).
Brief descriptions of each specific modification, as referenced in Table 3-4, are provided below.
Aeration
• Aeration cycling - includes on/off cycling of aeration, including the creation of dedicated
anoxic and oxic zones, and associated controls.
• Adjustable control aeration - use of variable frequency drives to control aerator output
and/or use of on-line monitoring tools to inform aerator operational mode.
• Mixer addition - addition of mixers to facilitate on/off cycling or maintain suspension of
solids when aerators are turned down.
• Equipment retrofit - replacement with energy efficient aeration equipment.
Process
• Flow equalization improvement - improving the influent flow to biological treatment
process to improve performance consistency.
• Recycle rate control - modifying internal mixed-liquor recycle rate to optimize
denitrification in primary anoxic zones.
• Sidestream control - modifying nutrient-rich internal plant return flows, such as sludge
dewatering returns.
• Pre-digestion of primary sludge - modifying primary sludge wasting rate to facilitate
biochemical oxygen demand (BOD) solubilization from settled sludge into secondary
process influent.
• Batch program modifications - changes to SBR program settings.
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Configuration
• Plug flow/series operation - conversion of complete mix reactor to plug flow to facilitate
oxic/anoxic zonation.
• Anoxic zone bleed - introduction of influent wastewater or return activated sludge (RAS)
into anoxic reactors to provide carbon for denitrification.
• Anaerobic zone VFA addition - introduction of RAS into anaerobic selector to provide
carbon for enhanced biological phosphorus removal (EBPR).
Chemical
• Alkalinity feed improvements - modifications to alkalinity control systems to facilitate
effective nitrification.
• Carbon product addition - addition of soluble BOD products to enhance denitrification or
EBPR.
Discharge
• Soil dispersal - conversion of a surface discharging system into a soil discharging
system.
• Wetland discharge - discharge into wetlands for further attenuation of nutrients prior to
receiving water delivery.
Note that chemical modifications and discharge modifications, because they are not necessarily
unique to activated sludge processes, are discussed in more detail in Section 6.
On average, the non-advanced WWTPs featured in these case studies were able to achieve
effluent TN reductions of greater than 50 percent (from 10.5 mg/l, pre-optimization, to 5.0 mg/l,
post-optimization, on average) and most realized net cost savings as a result of optimization
efforts.
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Table 3-3. Summary of Case Studies (basic, non-advanced treatment plants shaded in gray).
Case Study
Bay Point, FL
Bozeman, MT
Chinook, MT
Crewe, VA
Flagstaff, AZ
Hampden Twp.,
PA
Layton, FL
Montrose, CO
Tampa, FL
Titusville, FL
Victor Valley, CA
Wolfeboro, NH
Design
Flow
(MGD)
0.054
5.2
0.5
0.5
6.0
5.69
0.066
4.32
96
6.75
13.8
0.6
WWTP Type
AS (MLE)
AS
AS (Oxidation Ditch)
AS (Oxidation Ditch)
AS (I FAS)
AS (CSR)
AS (SBR)
AS (Oxidation Ditch)
AS (Separate Stage)
AS (A2/0)
AS
AS (Extended
Aeration)
Modification Type
Aeration, chemical
Aeration, configuration
Aeration
Aeration, chemical
Process
Configuration, process
Aeration, process
Aeration
Aeration, configuration
Discharge,
configuration, process
Aeration, process
Aeration
Pre/post
TN (mg/l)
6.33/3.99
17.8/10.5
20.3/5.44
7.85/3.63
14.0/8.5
4.66/3.64
7.88/3.33
Unk/14.7
18.62/13.82
5.67/0.94
8.93/6.83
6.32/1 .97
Pre/post
TP
(mg/l)
N/A
3.7/2.5
4.13/1.72
N/A
N/A
N/A
N/A
N/A
N/A
0.77/0.04
N/A
N/A
Capital
Costs
$170,365
$180,000
$81,000
$6,000
$10,000
Zero
$53,000
Zero
Zero
$2,240,000
$1,100,000
$116,000
Operational
Costs/Savings
Savings not quantified
Zero
Energy savings more
than offset $1 ,000/yr in
maintenance
$1 7, 440/yr savings
$1 ,000/yr
Zero
$1 3, 500/yr savings
$34,000/year savings
$51 9, 900/yr savings
$45,000/yr
10% savings
Savings not quantified
AS = activated sludge; MLE = modified Ludzack Ettinger; IFAS = integrated fixed film activated sludge; SBR = sequencing batch reactor; N/A = not applicable;
CSR = continuously sequencing reactor.
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Table 3-4. Modifications Featured in Case Studies.
Modification
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Additional case studies were identified during the literature review, but were not developed
further for this report as they did not meet one or more of the screening factors used in this
study. Important characteristics of these case studies are summarized in Table 3-5 and Table
3-6. Readers are encouraged to refer to these references for additional information.
Table 3-5. Case studies from Water Planet Company website2
Name
Amherst
Colchester-East
Hampton
Columbia Falls
Conrad
Hastings
Keene
Manhattan
Montague
Northfield
Palmer
Plainfield
Suffield
State
MA
CT
MT
MT
PA
NH
MT
MA
MA
MA
CT
CT
Flow
(MGD)
7.2
3.8
0.55
0.45
6
1.83
4.2
1.5
1.5
Type
AS-PF
AS-MLE
AS-MLE
Modified
Lagoon
AS-EA
AS
AS
AS-CM
AS
AS
AS-OD
Approaches
Lower F:M ratio and operation in
MLE/SBR mode, variable aeration
based on in-line ORP, DO instruments,
controlling ammonia removal to limit
alkalinity consumption
Minimized internal recycle flows and
bypassed influent around primary
clarifiers directly to pre-anoxic zones,
aeration cycling
Adjusted equalization tank flow and
internal recycle rates to convert
existing pre-anoxic tanks to fermenters
and enhance BPR
Aeration cycling
Conversion of equalization tank to
combination fermentation/EQ, added
digested sludge feed, aeration cycling
and decoupling of air lift and aeration,
monitoring and controls
Created fermentation zone for BPR by
closing aeration valves and activating
mixer in aeration cell
Increased RAS rate to 250% of the
forward flow
Conversion to sequenced aeration
mode involving the installation of motor
actuated RAS valve, aeration valves,
and DO/ORP monitors
Cyclic aeration, pre-fermentation of
septage for BOD/VFAs
Cyclic aeration
Drover™ process that recaptures BOD
lost during primary treatment, by
modifying primaries to pre-anoxic zone
Creation of pre-anoxic zone, pre-
fermentation in sludge holding tanks
TN
8 mg/l
8 mg/l
3.5 mg/l
X
50%
5 mg/l
X
75%
8 mg/l
2 mg/l
TP
0.5 mg/l
X
0.2 mg/l
0.75
mg/l
0.8 mg/l
0.5 mg/l
2 http://www.cleanwaterops.com/
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Name
Upton
Westfield
Windham
State
MA
MA
CT
Flow
(MGD)
0.4
6.1
5.5
Type
AS-EA
AS-PF
AS
Approaches
Aeration control to first two passes,
increased RAS rate to increase nitrate
return
Operate first third of plug flow reactor
as fermentation reactor fed with
sludge, in-line Ortho-P measurement to
control supplemental chemical dosing
Mixed liquor return to primary clarifiers
to create pre-fermentation zone for
BPR
TN
6 mg/l
TP
1 mg/l
X
Note: In TN and TP columns, reported optimized effluent concentration is listed where reported; percent reduction is listed where
reported; where no number is reported, an "x" indicates that plant was optimized to remove this nutrient.
AS = activated sludge; PF = plug flow; MLE = Modified Lutzack Ettinger process; OD = oxidation ditch; EA = extended aeration; CM
= completely mixed
Table 3-6. Additional case studies identified in literature.
Author
Block, et al.
Gangadharan, et
al.
Greene
JJ Environmental
Randall, et al.
Sadler, Stroud
Scheringer, et al
Solley, Barr
USEPA
USEPA
Winkler, et al
Young, et al
Year
2008
2012
2011
2015
1999
2007
2009
1999
2007b
2007a
2007
2011
System
AS
AS-BNR
AS-BNR
Various
various
AS-BNR
AS-BNR
AS
AS-BNR
AS-BNR
AS
AS-
OD/BNR
Location
Minneapolis,
MN
Chapel Hill,
NC
Various
New England
NY, PA, MD
and VA
North
Carolina
North
Carolina
Australia
US
US
Germany
Maryland
TN
X
X
X
X
X
X
X
X
X
X
TP
X
X
X
X
X
X
X
X
X
Summary
Upgrade to BPR by baffling existing
reactors
Convert existing BNR plant from plug
flow to step feed to reduce TN with no
capital costs
Multiple case studies in PowerPoint
presentation.
Paper study of 20 WWTPs in Upper
Long Island Sound watershed.
Paper study of 51 WWTPs in the
Chesapeake Bay watershed
Four case studies, relatively high level
systems
Chemical optimization, alternative
carbon sources, swing zones,
increased return flows, replaced
aerators, optimized blower operation,
control of digester supernatant
Zonation and aeration controls
Characterizes existing advanced
treatment plants achieving low TP
Summarizes costs for a variety of BNR
system retrofits and replacements
Significant upgrades to BNR using
existing structures
Review design and operation of two
high level advanced treatment plant
Note: In TN and TP columns, an "x" indicates that
AS = activated sludge; OD = oxidation ditch; BNR
plant was optimized to remove this nutrient.
= existing advanced nutrient removal
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Several key references in Table 3-6 were evaluated for general comparison with the empirical
data from the case studies associated with this project. Randall, et al. (1999) concluded that the
costs of nitrogen removal were plant specific and ranged from a savings of $0.79/lb to a cost of
$5.92/lb (for a target of 8 mg/l effluent TN), and that activated sludge systems, and oxidation
ditches in particular, were generally most cost-effective to optimize. They do advise that BNR
retrofits could impact design capacities and should be evaluated with this in mind.
JJ Environmental (2015) estimated 20-year life cycle costs of $0.36/lb to $3.85/lb effluent TN
reduced based on the results of BioWin modeling; however, it appears that there was no set
target effluent TN and in some cases, the modeled TN was very low (2 mg/l range). Therefore, it
could be concluded that their scoping-level report may focus on somewhat more extensive
retrofits. The empirical data presented in this report generally reflect efforts to address the most
cost-effective optimization opportunities, rather than the greatest reductions that could be
achieved without adding reactors.
Both reports conclude that low-cost optimization is feasible and cost-effective, and both reports
should be considered useful supplemental resources for professionals pursuing an optimization
strategy for nutrient reduction at their WWTPs.
USEPA (2010) provides useful information on optimization strategies for enhancing nutrient
removal. Two key points are to have a process for analyzing existing operations and identifying
tools to assist in an evaluation of optimization alternatives. Chapter 12 provides an existing
system analysis framework, consisting of the following tasks:
• Compile existing data
• Collect additional data (see Table 3-7)
o Optimize sampling and process monitoring to enable real time process control
and troubleshooting in influent, process and recycle flows. Also use portable test
kits as needed.
• Review and summarize data
• Evaluate relationships between key parameters
o Simulation models can be good tools for describing such relationships and
evaluating alternative strategies.
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Table 3-7. Recommended Parameters for Data Evaluation (USEPA, 2010)
Parameter1
Location(s)
Rationale
Flow
Influent, effluent, flow splits,
recycles
Essential for developing mass balances, which are
essential fora complete understanding of the treatment
system.
Ratio of Total
BOD/COD
Soluble1
BOD/COD
Influent, primary effluent,
effluent, anaerobic & anoxic
zone effluents
Can be used to evaluate substrate availability for
biological processes. High effluent BOD could indicate
activated sludge performance problem.
TSS, VSS
Primary effluent, secondary
effluent, final effluent
Important if phosphorus is removed chemically. Used to
calculate ISS, determine clarification efficiency, and
determine an accurate solids residence time (SRT) for the
bacteria.
DO
Aerobic, anaerobic, and
anoxic zones (multiple
locations recommended)
Minimum DO of 2.0 is usually needed to minimize oxygen
limitation of nitrification rates, which is important for low
SRT/HRT systems. DO should not be present in anoxic or
anaerobic zones.
PH
Influent, mixed liquors,
effluent
Should be above 6.5 and below 9.0 for biological nitrogen
removal. Low pH or wide swings in pH could mean
significant industrial component. Could affect BPR and
nitrification.
Alkalinity
Influent, primary effluent,
mixed liquor supernatants,
effluent
If effluent is below 50, there is probable nitrification
inhibition, and process is susceptible to large pH drops as
a result of nitrification or chemical addition for phosphorus
removal.
Temperature
Influent, mixed liquors of
reactors, effluent
Low temperatures can significantly reduce nitrification
rate. For the typical range between 10 and 25 °C, the rate
will drop by half for every 8 to 10 °C reduction in mixed
liquor temperature. Reactor temperatures are likely to be
significantly different from influent temperature because of
aeration.
NH3-N and/or
TKN
Influent, primary effluent,
reactor mixed liquors,
secondary effluent, effluent
Can be used to evaluate load to and performance of
biological nitrification kinetics.
Nitrate
Influent, reactor mixed
liquors, secondary effluent,
effluent
A check on nitrification, and can be used with TKN to
calculate denitrification.
Total
Phosphorus
Influent, primary &
secondary WAS, plant
effluent
Used to calculate phosphorus removal efficiency by
treatment processes.
Phosphate
Reactor mixed liquors,
primary & secondary
effluents, effluent
Used to determine release and uptake in reactors, release
in secondary clarifier, and phosphorus removal efficiency.
ORP
Anaerobic & anoxic reactor
mixed liquors
Measures the balance between oxidized and reduced
compounds present in solution. Will detect presence of
significant concentrations of oxidized compounds. Can
be used for automatic detection of excess electron
acceptors (DO, nitrate, and nitrite) in reactors.
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Parameter1
Location(s)
Rationale
VFA or rbCOD
Influent, primary effluent
Can be used to evaluate substrate availability for
enhanced biological phosphorus removal.
MLSS&
MLVSS test
and WAS test
MLSS & MLVSS: well-mixed
location in aeration basin
WAS: well-mixed and
representative sample from
the WAS pipe (may need
composite sample)
MLSS and WAS tests provide suspended solids
concentrations and can be used to determine percent
phosphorus in sludge. This information, in conjunction
with aeration basin volume and WAS flow, can be used to
calculate SRT. Maintaining SRT is critical for nitrification
and, sometimes, for enhanced biological phosphorus
removal.
1. BOD = biochemical oxygen demand
(5-day unless otherwise noted)
COD = chemical oxygen demand
TSS = total suspended solids
DO = dissolved oxygen
TKN = total Kjeldahl Nitrogen
VFA = volatile fatty acids
Total BOD/COD = unfiltered BOD/COD
Soluble BOD/COD = BOD/COD of filtrate from 0.45 urn pore size filter
VSS = volatile suspended solids
ORP = oxidation reduction potential
NH3-N = ammonia Nitrogen
rbCOD = readily biodegradable COD
WAS = waste activated sludge
MLSS = mixed liquor suspended solids
MLVSS = mixed liquor volatile suspended solids: Inorganic suspended solids (ISS) = MLSS - MLVSS
Source: WEF and ASCE (2006)
3.1 AERATION MODIFICATIONS
Aeration is one of the key operating parameters used to establish and control biological nutrient
removal, since it determines the operating environment in a particular treatment basin (aerobic
conditions for nitrification, anoxic conditions for dentrification, or anaerobic conditions for
EBPR). Aeration is often also the single largest power demand in a WWTP, so it is a key
operational consideration for energy efficiency. Therefore, aeration system modifications are
one of the more common nutrient removal improvements at existing WWTPs. Every case study
developed for this project that featured relatively basic (i.e., non-advanced) treatment included
some kind of aeration optimization as part of their portfolio of nutrient reduction improvements.
Some key points that emerge from the literature regarding aeration and dissolved oxygen levels
include:
• A minimum DO of 1.5 mg/l, or preferably 2.0, or greater is ideal for the initial (front-end)
oxic zones. These DO levels ensure optimum phosphorus uptake (where EBPR is
provided) and facilitate complete nitrification (provided other conditions are sufficient).
• If a dedicated anoxic zone is provided, zero DO is ideal for denitrification. The DO level
in the internal recycle flow must be kept to a minimum and some type of internal recycle
deoxygenation zone prior to mixing with the influent in an anoxic zone can be useful.
Additionally, where dedicated anoxic zones are used, uniform mixing within the anoxic
reactor is important.
• Likewise, it is important to maintain the integrity of the anaerobic zone for EBPR - a
separate reactor (with minimal back mixing) is typically needed and conditions must be
anaerobic.
• For simultaneous (single reactor) nitrification-denitrification processes, maintaining a low
DO following the initial 1.5-2.0 mg/l or greater oxic zone is important. In other words,
aeration should be tapered such that DO levels are highest at the influent end of the
aeration zone and lower downflow.
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Some of the most common aeration system improvements or upgrades include:
• individual zone airflow control,
• blower airflow modulation,
• aeration control feedback loops for DO, ORP or ammonia control,
• aeration on/off cycling or anoxic zone creation to allow for biological nutrient removal,
• mixer addition to keep solids in suspension when air is turned off or down, and
• replacement of old equipment with newer, more efficient equipment.
The Bozeman Water Reclamation Facility converted a conventional Complete Mix-Activated
Sludge (CMAS) system to a 4-zone plug flow reactor by using cyclic aeration and step feed in
the first two zones in order to meet new permit limits on effluent TN and TP discharge loads.
Total nitrogen was reduced from an annual average TN of 17.8 mg/L to 10.5 mg/L and total
phosphorus was reduced from an annual average TP of 3.7 mg/L to 2.5 mg/L. The original
configuration included a basin divided into four equally sized cells with the flow split and fed to
each cell from the outside wall and collected at the opposite wall shared by two cells, with all
cells aerated. Using the existing tank and dividing wall, the basin was converted to plug flow
with phased nitrification and denitrification by adding weir plates and using existing gates to
change the flow through the basin. By adding new aeration controls, ORP probes, and mixers to
the first two cells, the existing blowers and diffusers were used to implement cyclic aeration in
the first two zones, providing partial denitrification and EBPR, while still keeping the last two
zones sufficiently aerated to ensure consistent nitrification in order to meet the effluent ammonia
limit.
In Chinook, MT, staff used knowledge gained during a State-sponsored training session to
begin experimenting with on/off operation of the surface aerators (oxidation ditch rotors),
eventually adding mixers (primarily for energy efficiency at the time) and automatic DO controls
integrated with their SCADA system. Energy cost savings have more than paid for these
optimization efforts.
Plant staff in Crewe, VA employed a similar approach, first experimenting with simple on/off
aerator operation, and then advancing to DO-controlled, variable speed aeration, at a capital
cost of only $6,000.
The Wolfeboro Wastewater Treatment Facility implemented cyclic aeration to lower their effluent
TN. The existing aging ceramic diffusers were replaced with new diffusers with more efficient
oxygen transfer, reducing the airflow requirement. The old and oversized blowers were also
replaced with new lower horsepower blowers with VFDs and controllers. These equipment
upgrades provided improved process performance and energy savings. The upgraded aeration
controls include a timed cyclic aeration strategy and DO control based on readings from new
DO probes when the basin is aerated. New ORP probes were also installed for monitoring
system performance. Cyclic aeration provides nitrogen removal and also contributes to energy
savings, resulting in reduced operating costs.
3.2 PROCESS MODIFICATIONS/OPERATIONAL CHANGES
Process improvements for nutrient removal include operational changes without necessarily
requiring physical modifications to the existing facilities. Some of the key operational parameters
for biological nutrient removal include DO, alkalinity/pH, MLSS/SRT, sludge blanket depth, and
F/M ratio. Other more difficult to control variables that affect biological nutrient removal include
temperature and inhibitory compounds, both of which particularly impact nitrification.
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Some key points that emerge from the literature regarding process modifications and
operational changes include:
• Increasing internal recycle (IR) rate typically increases denitrification until the rate
becomes so high that recycled DO starts to inhibit denitrification. However, if the IR is
too low, then nitrates can become fully depleted in the anoxic zone(s) and
fermentation/release of phosphorus and foaming can occur. As a rule of thumb, the IR
should be 4-6 times the forward flow provided that the IR dissolved oxygen is less than
1.0 mg/l. A deoxidation zone (20-30 min. HRT is standard) can be provided for the IR to
lower the DO if necessary.
• RAS and WAS control is important for several reasons. Higher RAS rates can be used to
facilitate denitrification for plants without internal recycle capabilities; however, high RAS
rates can negatively impact EBPR. Controlling WAS rate is important for determining
MLSS/MLVSS concentrations and SRT, which is particularly important for nitrification
(anoxic SRT can also be important for denitrification). In general, RAS should be
controlled to maintain secondary clarifier sludge blankets at 1 foot or less. Dedicated
RAS controls/pumps for each clarifier can help with regard to maintenance of
appropriate sludge blanket depths.
• VFAs and readily biodegradable organic matter are important for EBPR and BNR. For
phosphorus removal, an rbCOD/TP ratio of 10-16 is typically targeted (Barnard 2006).
VFAs can be provided in-process by fermenting primary sludge within the sludge blanket
or in a separate reactor. Table 3-8 summarizes potential sources of VFAs at municipal
WWTPs.
Table 3-8. Potential sources of VFAs at municipal WWTPs (Jeyanayagam, 2005)
In-line sources
Off-line sources
• Fermentation in:
o Collection system
o Anaerobic zone of the bioreactor
o Primary clarifiers
• Fermentation in:
o Primary sludge fermenter
o Gravity thickener
o First stage of a two-phase anaerobic
digester
• Purchased acetic acid
• In general, it is important to maintain consistent operations and avoid frequent changes
in operation. Improvements can include flow or load equalization, especially for small
plants whose influents or internal process flows may have more variation. Adjusting SRT
and HRT in response to seasonal changes can be important for getting the most nutrient
removal out of a process. Another consistency-focused recommendation is to use flow-
or load-paced recycle flows (IR and RAS).
• For effective nitrification, pH should be maintained between 6.5 and 8.0 and effluent
alkalinity should be 80 mg/l as CaCOs or greater.
Improved sampling and monitoring can also provide valuable information for optimizing the
performance of a process by identifying non-optimum conditions in the system. A couple of
examples are presented below.
• If the DO is too high in an anoxic zone, then denitrification cannot occur until the DO is
exhausted. Excess DO can come from a variety of sources, including an internal recycle
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stream with a higher flow than can be denitrified, over-aeration of the aerobic zone, or air
entrainment in the influent and return streams caused by drops or free discharges into
the basin. Once identified, these can often be remedied by fairly simple operational
adjustments, such as turning the air down where the IR flow is collected and controlling
the IR flow so only the quantity that can be denitrified is returned.
• Basin DO and nutrient concentrations (including NOs-N, NO2-N, NHs-N, and PO4-P)
collected at different points throughout the basin can also be useful in assessing
performance and determining if there is additional potential capacity for denitrification.
For instance, if all the ammonia (NHs-N) has been nitrified at a point approximately two-
thirds of the way down a plug flow aeration basin, then there is potential for part of the
basin to be operated as an anoxic zone, or cycling the aeration in order to operate the
entire basin (or part of the basin) as an anoxic basin part of the time to provide some
denitrification. Field trials with additional monitoring would need to be conducted to
determine the best operating mode and the benefit of the alternate operation.
Table 3-9 provides a list of available on-line instruments along with measurement alternatives
and their advantages and disadvantages. Table 3-10 provides recommendations for uses and
locations for various online instruments.
Table 3-9. Summary of Basic On-Line Instrumentation (USEPA, 2010)
Analyte
Flow
TSS
Sludge blanket
monitor
DO
pH
ORP
Type of
Measurement
Mechanical
Pressure Drop
Magnetic
Reflective Sonic
Parshall Flume
Light scattering
(back scattered)
Light Adsorption
Ultrasonic
Microwave
Ultrasonic
TSS or Turbidity
Membrane
electrode
Galvanic
electrode
Optical probe
Electrode
Electrode
Advantages
Accurate
Low cost
No moving parts, no wear
No pressure drop, low
maintenance,
low cost
Simple, wide flow range
Better sensitivity, wider
measuring range
Less sensitive, smaller range,
inaccurate at low ranges
Insensitive to color
Insensitive to interference
Low maintenance
See TSS
Low cost
Durable, reliable
Durable, low maintenance,
reliable
Indicates true oxidizing
environment
(anaerobic, anoxic, or aerobic)
Disadvantages
Wear down
Highly dependent on installation,
pressure drop
High cost, inaccurate at low flow
Limited size of conduit, can't use
aggregate lined pipe, inaccurate at
low flow
Pressure drop, requires cleaning,
slow response
Needs effective cleaning system
Able to handle fouling better
without cleaning system
Fouling, background reading
required
High cost, only works for high TSS
High maintenance
Interference from hydrogen sulfide,
needs frequent calibration
Higher initial cost
Fouling
Indirect measurement
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Table 3-10. Recommended instrument locations for biological nutrient control (Tsuchihashi, 2008)
Location
Primary Effluent
Nitrate Recycle
Pre Anoxic (first
anoxic zone)
Aerobic zone
Pre (post anoxic zone)
(just upstream of
methanol addition)
Instrument
Ammonia
NOx, DO
DO, NOx,
possibly ORP
DO, pH, NOx
DO, NOx
Purpose
Monitor loading
Need to maintain no or minimal DO,
determine nitrate load on anoxic zone
Need to maintain no or minimal DO,
determine nitrate removal in anoxic
zone, ensure reducing conditions
Controls blowers, ensures proper
environment for organisms
Maintain no DO present and also use
nitrate concentration to pace methanol
or other carbon source addition
Comments
Helps in
troubleshooting
process upsets
Process monitoring
and optimization
Monitoring and
optimizing process
Energy consideration
and process
monitoring
Process optimization
and cost control
The effectiveness of preliminary treatment facilities (screens and grit removal equipment) can
also impact the performance and operation of downstream treatment processes, including the
ability to provide biological nutrient removal. Grit can accumulate in oxidation ditches, aeration
basins, or other reactors, decreasing their effective treatment capacity. Similarly, rags and
debris can plug or foul diffusers, mixers, and pumps, negatively impacting their performance as
well. So, improvements or upgrades to preliminary treatment facilities, including the addition of
fine screens or enhanced grit removal, can have the additional benefit of improving nutrient
removal efficiencies. This can also include operational changes, such as regularly cleaning
basins on a more frequent basis.
The Wildcat Hill WWTP in Flagstaff, AZ was able to reduce effluent TN by making operational
changes and adding process controls. A combined nitrate/ammonia probe was installed at the
end of the anoxic zone. The nitrate reading from the probe is used to control the internal recycle
flow, so the optimum amount of nitrate is returned to the anoxic zone, also minimizing the
amount of DO entering the anoxic zone. Primary clarifier sludge pumping was modified to
increase the solids detention time in the primary clarifier to get additional conversion of
particulate BOD to soluble BOD to provide more available carbon for denitrification in the
primary effluent. The return flow from the dewatering processes is also controlled to avoid
nitrogen loading spikes.
At the City of Layton (FL) Wastewater Treatment Plant, the effluent TN concentration was
reduced by changing the control of the Sequencing Batch Reactor cycle from a level batch
process to a timed batch process and adjusting the order and duration of aerobic and anoxic
operation, including adding new online monitoring probes, in order to improve effluent
consistency and optimize the fill, react, settle, and decant cycles.
At the Bay Point (FL) WWTP, the original manually cleaned static bar screen with large
openings and a high approach velocity was replaced with a tighter bar screen with an approach
channel and drying rack, which reduced the quantity of rags and debris passing into the
treatment process. This solved the problem of frequent clogging in the flow equalization pumps
and greatly reduced the build-up of debris on mixers and diffusers, improving performance and
operation.
Victor Valley, CA uses DO, ORP, alkalinity and sludge age to optimize their process for
simultaneous nitrification and denitrification.
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3.3 CONFIGURATION MODIFICATIONS
Piping and flow or configuration modifications can include changing where the RAS or internal
recycle is returned, splitting flows to go to more than one zone, providing sidestream treatment
for centrate or filtrate from solids handling processes, and changing the zone in the reactor
where backwash or dewatering streams are discharged. These may include physical
modifications to the existing facilities including adding or improving flow equalization, modifying
existing tankage, improving flow split mechanisms, and adding internal recycle lines.
Some key piping and configuration modifications include adding internal recycle capabilities
(largely discussed in the previous section) as well as minimizing the impact of internal loads
from solids handling systems and adding infrastructure to create dedicated redox zones.
The impact of recycle loads from solids processing can be managed by chemically precipitating
side streams (forTP reduction), chemical addition to solids processing feed, and minimizing
unaerated storage prior to sludge processing which can release phosphorus. Ammonia returned
from dewatering operations can also negatively impact BNR. Baffling can be used to build
aerobic/anoxic swing zones, to create high F:M conditions, and to approximate plug flow
conditions to effectively taper DO and minimize back-mixing.
At the Blue Heron Water Reclamation Facility in Titusville, Florida, a RAS denitrification stage
was added by creating a separate anoxic zone for just the RAS (also known as an exhauster
zone) before combining the RAS with the influent in the anaerobic zone. This allows the nitrate
to be removed from the RAS before it is introduced into the anaerobic zone. As a result, the
influent is introduced into a truly anaerobic zone, improving biological phosphorus removal. A
portion of the RAS was also sent to the front of the post-anoxic zone, which improved
denitrification. Note that these modifications are effective because the RAS has a large
equivalent endogenous oxygen demand even without an external carbon source or feed, in
large part due to the high MLSS of the RAS stream.
In Tampa, a gate was opened to recycle nitrified effluent into the pump station for a newly
created anoxic zone. Additionally, a portion of the influent was step fed around an initial
nitrification zone to provide BOD for the first internal anoxic zone.
At Bay Point, the RAS air lift system was decoupled from the air header feeding the activated
sludge and digester processes in order to allow for independent control as needed for effective
nitrogen removal.
3.4 ANCILLARY BENEFITS OF ENHANCED BIOLOGICAL
NUTRIENT REMOVAL
Improving nitrogen removal, particularly via biological nitrification and denitrification confers
multiple additional operation benefits. For example, upgrading aeration equipment (e.g., adding
VFDs) and improved aeration controls provide more efficient aeration, resulting in energy and
operational cost savings. Adding denitrification also reduces the amount of air required, further
reducing the energy requirement. Denitrification produces alkalinity and raises pH, recovering
some of the alkalinity consumed by nitrification and resulting in a more stable process,
potentially requiring less alkalinity to be added for nitrification and reducing the potential for
nitrite lock or incomplete nitrification. Providing an anoxic zone in front of the aerobic zones,
reduces the growth of filamentous bacteria and improves sludge settling. Since denitrification
consumes BOD, it has the ancillary benefit of decreasing the amount of air required compared
to a conventional nitrifying process, resulting in energy savings.
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Like BNR, EBPR provides secondary operational benefits to the secondary treatment process. It
is well documented that the creation of an anaerobic or anoxic selector zone in front of the
aerobic zone will decrease the growth of filamentous bacteria that cause sludge bulking,
improving settling and increasing biomass density. This is because the anaerobic and anoxic
conditions favor floe-forming bacteria over filamentous bacteria. When used with conventional
secondary clarifiers, the improved sludge settling characteristics allow the plant to be operated
at a higher MLSS, increasing treatment capacity in most cases. RAS chlorination can also be
used as a means to control filamentous growth. At plants where anaerobic or anoxic zones are
added in front of the aerobic zone, the amount of chlorine used for filamentous control can be
reduced or eliminated entirely. Therefore, some facilities include anaerobic or anoxic zones
primarily for filamentous control. In the case of biological nutrient removal facilities, the improved
settling characteristics provided by these zones and the reduced usage of chlorine for
filamentous control are additional ancillary benefits.
If alkalinity addition is required to maintain stable nitrification, denitrification will have the
ancillary benefit of reducing chemical usage and costs. Nitrification requires approximately 7.14
mg of alkalinity (as CaCOs) per mg of ammonia oxidized to nitrate. Denitrification produces 3.57
mg alkalinity (as CaCOs) per mg NOs-N (or NO2-N) reduced, recovering about half of the
alkalinity used in nitrification. Therefore, nitrification generally lowers pH, while denitrification
generally raises it. Denitrification helps recover alkalinity and keep the pH stable, preventing it
from dropping into a range that is inhibitory to nitrification (pH values below 7.0 can cause a
significant drop in ammonia oxidation rates).
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4 Optimizing Nutrient Removal in
Lagoon Systems
Lagoons have been used as low-cost wastewater treatment systems for many years,
particularly in relatively small and/or rural communities where sufficient land is available to site a
lagoon that generally has a larger footprint than more mechanically based treatment systems
(e.g., activated sludge).
Lagoons are typically characterized by their operating redox state, with the main types being
aerobic, anaerobic, and facultative lagoons.
Within the category of aerobic, lagoons are shallow "aerobic basins" (1-2 ft deep), aerated by
contact with the atmosphere (aided by wind) and daytime photosynthesis of algae; and "partial
mix aerobic", which are deeper aerobic lagoons that include some type of mechanical aeration
system (e.g., surface aerators, diffused air). In either case, aerobic lagoons typically have the
shortest hydraulic retention time of the three major types. For this reason, they are sometimes
called "high-rate" lagoons.
Anaerobic lagoons are deeper basins (typically over 15 ft deep) and are mainly used for treating
high-strength wastewaters, such as those from concentrated animal feeding operations
(CAFOs), food processing facilities, and other industrial process streams. Since they are of
limited applicability to municipal WWTPs, they are not addressed further in this document.
Facultative lagoons are generally 5-8 ft deep and represent the most versatile and common
type of lagoon used for WWTPs in the United States. As previously indicated, facultative
lagoons typically feature aerobic conditions in the upper layer and anoxic or anaerobic
conditions toward the bottom, with the transition depth depending on the influence of wind-
driven mixing.
Lagoon systems were traditionally used to remove organics (i.e., BOD) and suspended solids,
with their nutrient removal capability given little design consideration until recently.
Nevertheless, lagoon systems—even without special design provisions—are often surprisingly
effective in reducing total nitrogen.
As illustrated in Figure 4-1, influent TKN can be reduced via ammonia stripping to the
atmosphere, assimilation into biomass, biological nitrification/denitrification, and sedimentation
of insoluble organic nitrogen (USEPA 2011b). In facultative systems in particular, anoxic bottom
sediments can effectively denitrify nitrates that have been produced in upper layers. The long
retention time of these systems additionally favors relatively high levels of TN removal by
various mechanisms. TN reduction processes in lagoon systems may be affected by
temperature, DO concentration, pH, retention time, and wastewater characteristics. Alkalinity
changes and potential pH fluctuations resulting from the interaction of algae and HCOs' can be
important because they affect the speciation of ammonia, which is more volatile under alkaline
conditions.
Per EPA (2011b), "phosphorus removal in ponds occurs via physiochemical mechanisms such
as adsorption, coagulation, and precipitation. The uptake of P by organisms in metabolic
functions, as well as for storage, also contribute to its removal. Removal in wastewater ponds
has been reported to range from 30-95 percent (Assenzo and Reid 1966; Pearson 2005; Crites
et al. 2006)".
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EPA undertook a number of studies of facultative wastewater pond systems in the late 1970s.
The results verified the hypothesis that significant nitrogen removal occurs in pond systems.
Data from the studies are summarized in Table 4-1. Facultative lagoons have been documented
to achieve TN reductions ranging from 40-90 percent or greater, with higher reductions
associated with warmer weather, when volatilization and algae growth are highest.
In aerated ponds, nitrogen can be removed by assimilation into biomass (algae and bacteria),
biological nitrification/denitrification, and sedimentation of insoluble organic N. Volatilization can
also play a role, although pH is usually less than 8.0 and may not be favorable to large removals
by ammonia stripping.
Table 4-1. Summary of main lagoon types and typical effluent concentrations.
Lagoon Type
Facultative
Aerated
Effluent TN (mg/l)
Low
4
6
High
22
29
Average
13
17
Effluent TP (mg/l)
Low
1
High
5
Average
2.5
Mechanisms
Algae/wind provide DO in
surface
Denitrification in bottom
Variable seasonal Nhb
Controlled discharge
Summer nitrification
Shorter detention time
Source: Derived from data in EPA (2011b).
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riudg* or settled algae N T /
spring
Algal growth stage
summer fail
Source: USEPA, 2011b.
Figure 4-1. Illustration of lagoon processes
A more recent study of lagoon performance in Kansas from November 1997 to May 1999
concluded that a well-run lagoon could be expected to produce an effluent with a TN
concentration of 5-7 mg/l and TP concentration of 1.5 mg/l (Tate et al. 2002). Time series data
showed a more pronounced seasonal trend for TN than for TP.
Although no lagoon case studies were developed for this project, several were uncovered
during a literature review. Important characteristics of these case studies are summarized in
Table 4-2. Readers are encouraged to refer to these references for additional information,
although it is noted that both examples represent fairly significant upgrades, which might not be
warranted considering the frequently good overall nutrient removal performance of lagoon
systems.
Table 4-2. Lagoon case studies in literature.
Author
Hodgson and
Paspaliaris
Pattarkine, Chann,
and Tharp
Year
1996
2006
System
Lagoon
Lagoon
Location
Melbourne,
AUS
Ashland, MO
TN
X
X
TP
Improvements
Describes operation of "new style"
lagoon systems
Added internal separators to create
zones
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Operators of facultative lagoon systems endeavoring to optimize nutrient removal should
consider discharge methodologies as the primary approach. Two main options can be
considered:
1. Controlling the lagoon discharge to coincide with times when effluent nutrient
concentrations are lowest (as determined through spot or real-time sampling, nutrient
profiling, or best judgement) and/or when receiving water impacts will be lowest.
2. Using a nondischarge options, such as land application/soil treatment system.
For both facultative and aerobic lagoons, operators can also consider adding post-lagoon
treatment, which can vary from relatively passive constructed wetland systems (possibly
including both aerobic and anoxic/anaerobic sections) to post-denitrification facilities such as
biological filters. The latter option may be preferred where space is tight and the smaller
footprint afforded by a mechanical system are desired. Aerobic lagoons, particularly those with
mechanical aeration, may have available options for aeration control and development of
aerobic and anoxic zones similar to those described in Section 3 for activated sludge systems.
These options are discussed in more detail below, except for nondischarge (or modified
discharges) and post-denitrification, which are addressed in Section 6.
4.1 CONTROLLED DISCHARGE
Facultative lagoons have relatively long hydraulic retention times and accumulate solids over
long periods of time, both of which enhance nutrient removal. Most of the TN removal appears
to be by volatilization of ammonia across the large surface area at relatively high operating pH
(which is a result of algal respiration during daylight hours). The higher pH also facilitates
phosphorus precipitation (USEPA 2011b). TN removal in particular can be highly seasonal, with
removals up to 95 percent in the heat of summer, but down to 40 percent or less in winter
(USEPA 2011b; WEF 2003). Accordingly, because of the relatively long residence time of
facultative lagoons, these systems can be operated to hold water over the colder months and
discharge during the summer when nutrient concentrations are expected to be lowest. This type
of operation is called a "controlled discharge" lagoon.
Experience with controlled discharge lagoons is mostly from northern states that feature more
pronounced seasonal and climatic influences on algal growth. Controlled discharge lagoons
typically feature periodic, controlled discharge once or several times per year. A study of 49
controlled discharge ponds in Michigan indicated that discharge periods vary from less than 5
days to more than 31 days, and residence times were 120 days or greater (Pierce 1974). Ponds
of this type have operated satisfactorily in the north-central United States using the following
design criteria (USEPA 2011b):
• Overall organic loading: 20-25 Ib BOD5/ac/d
• Liquid depth: Not more than 6 ft for the first cell, not more than 8 ft for subsequent cells
• Hydraulic detention: At least 6 months of storage above the 2 ft liquid level (including
precipitation), but not less than the period of ice cover
• Number of cells: At least three for reliability, with piping flexibility for parallel or series
operation
Other fundamental design considerations include:
• Capacity of the lagoon to store wastewater for extended periods without infringing on
required freeboard levels, affecting mechanical devices, etc.
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• Ability of the lagoon effluent discharge structures to periodically discharge (i.e.,
gravity/demand versus pumped/controlled discharge system)
• Appropriate discharge schedule (new metering that may be required, etc.)
Winter storage typically aligns well with potential alternative end uses for effluent besides
discharge, such as land application. Vegetative growth and thus water demand will, of course,
be higher in the warmer months when lagoon effluent nutrient concentrations are expected to be
lowest.
The decision to convert to a controlled discharge operation must also include an analysis of the
impact of periodic discharge on receiving stream water quality standards. Some lagoons will
operate in a mode that is the opposite of that suggested for optimizing nutrient removal (i.e.,
they store and/or land-apply wastewater in the summer, and discharge in the winter). This is
due to higher demand for reclaimed water in the summer and low stream flows, and in turn less
dilution, leading to more stringent effluent limits for surface discharge in the summer. In fact,
hydrograph controlled release (HCR) systems represent a related strategy where discharges
are controlled to correspond to hydrologic conditions in the receiving environment when the
discharge is expected to have the least environmental impact.
Each situation is different and the specific context needs to be considered when developing a
plan for discharging lagoon effluent. Selecting a discharge schedule is very important and must
be determined well in advance. Conversion to controlled discharge operation will typically
require discharge permit modifications.
4.2 AERATION MODIFICATIONS
Various operating strategies can also be employed to optimize TN reduction in aerated lagoons,
particularly those with mechanical aeration, similar to those optimization strategies available for
activated sludge processes. See Section 3 for additional information.
4.3 CONVERSION TO ADVANCED SECONDARY TREATMENT
Biological nitrogen removal (nitrification/denitrification processes) represents the state-of-the-art
in terms of nitrogen reduction technologies for wastewater management. A number of different
processes (some proprietary, some not) that use suspended growth, attached growth (i.e., a
biological filter), or some combination thereof are available. Effluent TN concentrations of 5 mg/l
are typically achievable, with some systems reducing TN to 2 mg/l or less. Although lagoons
typically have a lot of space and "reactor volume" to allow for the creation of anoxic
(denitrification) and aerobic (nitrification) zones, conversion may be complicated by several
factors, including the need to add mechanical aeration and/or mixing equipment and recycle
pumps and piping and the difficulty in doing so in a large earthen pond.
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5 Optimizing Nutrient Removal in
Trickling Filter Systems
Trickling filters are most frequently used for BOD removal and, in some cases, nitrification. The
degree of treatment depends primarily on the organic loading of the system and the type of
aeration used.
Natural draft aerated filters are typically low rate filters, only able to handle low organic loading
rates due to the limited amount of air available. Trickling filters with forced draft aeration can be
operated at higher organic loading rates. Recirculation of the trickling filter effluent allows for
higher loading rates and improves performance and stability.
Nitrification can be achieved by using trickling filters with low organic loading rates. Multiple
trickling filters can be staged so that the first trickling filter removes BOD and has a higher
organic loading rate and a second trickling filter, loaded at a lower organic loading rate, is used
for nitrification. Trickling filters are typically followed by secondary clarifiers to settle the solids
resulting from the sloughing of the biofilm in the filters. Trickling filters have also been used as a
tertiary process for the nitrification of secondary (i.e., activated sludge) effluent.
Under typical operating conditions, trickling filters at WWTPs can reduce influent TN loads by
10-30 percent and TP loads by 8-12 percent (Metcalf and Eddy 1991). However, it should be
noted that many smaller (decentralized) attached growth systems are designed for relatively
high levels of nitrogen removal, typically achieved by recirculating nitrified effluent back to an
anoxic reactor (e.g., septic tank).
Although no trickling filter case studies were developed for this project, several were uncovered
during a literature review. Important characteristics of these case studies are summarized in
Table 5-1. Refer to these references for additional information.
Table 5-1. Trickling filter case studies in literature.
Author
Dai et al.
Dorias and
Baumann
Kardohely and
McClintock
Morgan et al.
Year
2013
1994
2001
1999
Location
Australia
Germany
Penn
State
Australia
TN
60%
15mg/l
TP
Improvements
Return nitrate-rich stream from secondary
clarifiers back to primaries
Denitrification in trickling filter plants by covering
filters for anoxic operation
Added BNR plant to blend effluent prior to
disposal or land application
Conversion to MLE-type BNR by adding
secondary reactors
As a general rule, trickling filter plant operators have limited opportunities to increase nutrient
reduction short of significant and costly infrastructure modifications. Systems with excess
nitrification capacity (flow and aeration) may have some opportunities to optimize aeration.
Conversion to BNR or the addition of a post-denitrification system can be effective, but
intensive.
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5.1 PROCESS OPTIMIZATION
As indicated above, nutrient reduction for typical trickling filter WWTPs is generally quite
modest. Likewise, nutrient reduction optimization opportunities are limited.
Some modest nitrogen reduction improvements can be achieved by optimizing the internal
recycle rate for systems with recycle capabilities. Provided that significant nitrification is
occurring, a higher recycle rate should result in opportunities for denitrification by 1) contacting
nitrified effluent with attached microorganisms, and 2) creating more anoxic sites by increasing
the hydraulic loading. However, this is a delicate balance, as operational problems (including
loss of nitrification) may occur if recycle rates are too high and the hydraulic capacity of the
trickling filter is exceeded and anaerobic conditions predominate.
Operators of trickling filters with forced-draft aeration can throttle aeration or use on/off aeration
controls to increase denitrification. However, controlling redox conditions in trickling filters is
more difficult than in activated sludge (and other suspended growth) systems, where inline
monitoring can be easily installed at representative reactor locations and feedback between
aeration controls and redox conditions is typically relatively consistent and easy to observe.
5.2 CONVERSION TO ADVANCED SECONDARY TREATMENT
Specially designed denitrification filters can be added after a nitrifying trickling filter to provide
more significant TN reductions, although some supplemental carbon addition will likely be
needed. Additionally, if the plant is not currently nitrifying, treatment may need to be added to
ensure consistent nitrification.
Trickling filter plants can be expanded to a more traditional activated sludge-type BNR process
by adding new BNR reactors after the trickling filter system. In this case, the existing trickling
filters are often used as "roughing filters" to decrease the organic load on the activated sludge
system and to remove toxic inhibitory compounds that decrease the performance of Ammonia-
Oxidizing Bacteria (AOB) and Nitrite-Oxidizing Bacteria (NOB). Although this strategy reduces
the oxygen requirements for the activated sludge system, it can have negative implications for
EBPR and denitrification by consuming soluble BOD in the influent.
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6 Other Nutrient Reduction Approaches
6.1 POST-DENITRIFICATION
In lieu of a complete conversion to BNR, "post electrification" can be implemented after
nitrification within a treatment system (see Section 2.1.1); however, this approach usually
requires the addition of an external source of carbon such as methanol, which typically requires
the installation of chemical feed equipment in addition to the carbon additive itself. Biological
filters are often used for post-denitrification in WWTPs, which may have relatively low levels of
operator oversight.
6.2 DISCHARGE MODIFICATION/LAND APPLICATION
As described in Section 2.3.4, removing a direct discharge and diverting effluent to a nutrient
polishing process, such as a land application system or wetland, can be a very effective
approach for reducing WWTP effluent nutrient loads. However, modifying a discharge can be a
difficult and, at times, infeasible, option. Some of its potential limitations include the following:
• In general, a significant amount of land is needed for these options. Therefore, WWTPs
in urban or other land-constricted areas, might find it difficult to acquire suitable land to
be used for effluent dispersal.
• Construction of a modified discharge system can be more expensive than other options
that may be available for reducing nutrient loads.
On the other hand, modifying the discharge has some significant potential benefits:
• Ability to phase in dispersal (nondischarge) capacity over time as land or other
resources become available.
• Ability to acquire effluent dispersal land that can be used to serve multiple community
purposes (e.g., recreation, food production, ecological enhancement, aquifer recharge).
• Ability to use decentralized systems to "shave" influent nutrient loads and treat/reuse
nutrients locally.
• Ability to make relatively small improvements to enhance nutrient removal at an existing
discharge. For example, many WWTP discharges include channels that convey effluent
to the main receiving water. Simply vegetating or adding appropriate filtration media to
the channel could provide significant nutrient reduction benefits without adversely
affecting existing operations.
Although the project team pursued several modified discharge case studies, only two of the final
case studies include this approach. For the Blue Heron Water Reclamation Facility in Titusville,
Florida, discharge to a restored wetland was an original feature of the design and permit, not a
retrofit or optimization effort. It has been, however, a very effective process for nutrient
polishing. The case study for Victor Valley Wastewater Treatment Facility also includes a land
application component, but again this was an original design component. Because many
alternative discharge systems are for small facilities, it proved especially difficult to obtain
project information in sufficient detail to support a full case study.
Nevertheless, the use of soil treatment systems is well-documented in the literature and
understanding of the nutrient reduction attributes of various types of systems continues to
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improve. Cost estimates can be developed using widely accepted guidance published by EPA,
the Water Environment Research Foundation (WERF), and others.
As described previously, modified discharge systems can be extremely effective in reducing
nutrients. The Blue Heron case study includes pre- and post-wetland nitrogen and phosphorus
performance data that shows reductions from 5.67 to 0.94 mg/l TN (83 percent TN reduction)
and 0.77 to 0.04 mg/l TP (95 percent TP reduction).
The states of Florida and Louisiana provide detailed permitting and other information about
wetland assimilation/discharge systems in their states.
Properly sized and sited land application systems can effectively reduce phosphorus loadings to
very low levels (virtually complete removal has been documented for many systems, but cannot
be reliably predicted without a site-specific analysis). Converting an existing point discharge to
land application, however, requires substantial amounts of land and can be quite expensive.
Land application systems that disperse secondary effluent to carbon-rich surficial soil horizons
under pressure can be an effective control strategy for nitrogen, compared with surface water
discharges, although again, a site-specific analysis must be conducted first to ensure that the
soil and site conditions are suitable and to determine appropriate design criteria.
6.3 CHEMICAL TREATMENT
Previous sections have primarily focused on BNR. Chemical addition can also be used directly
for phosphorus removal (by precipitating with metal salts), to support nitrification (by adding
alkalinity), or for both TN and TP removal (supplemental carbon addition).
6.3.1 Chemical Phosphorus Removal
Metal salts (typically alum or ferric chloride) can be added to chemically precipitate phosphorus,
which is subsequently removed and wasted with the primary sludge or WAS. The addition of
metal salts can also improve the settling characteristics of the primary sludge or secondary
sludge. Compared to WAS from an EBPR process—which can re-release soluble phosphorus if
exposed to anaerobic conditions—phosphorus that is precipitated with metal salts is less likely
to be released back into solution in the solids handling and treatment process.
Chemical precipitation using alum or other metal salts can be used to precipitate phosphorus
and is capable of achieving very low effluent TP levels, frequently down to 0.5 mg/l and
sometimes down to 0.1 mg/l or less when paired with highly efficient solids removal processes
(e.g., tertiary filtration). Process modifications for chemical precipitation are relatively simple and
the approach can be implemented at most treatment facilities. Primary disadvantages are
chemical costs and chemical sludge management.
Lagoons are relatively well-suited for chemical phosphorus removal in that their large volume
can provide for long-term storage of chemical sludge. Both batch and continuous chemical
dosing approaches can be used.
USEPA (2011b) describes a batch, in-pond chemical treatment (alum, ferric chloride, and lime)
in controlled-discharge ponds that was developed in Canada to meet a P requirement of 1 mg/L
for effluent discharge to the Great Lakes. Chemical additives were dosed to the pond by boat.
The costs for this method were reported to be reasonable and significantly less than those for
conventional phosphorus removal methods (although "conventional methods" are not defined).
USEPA (1992) reports that this approach has also been applied successfully in several
midwestern states.
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Studies of continuous in-pond precipitation of phosphorus were also conducted in Canada,
using ferric chloride and alum to successful maintain effluent TP concentrations below 1.0 mg/l,
although the use of lime was not able to consistently meet the effluent limit (USEPA 2011b).
Additionally, 37 pond systems in Michigan and Minnesota using chemical treatment to remove P
were studied (USEPA 1992). In general, facilities in both states were (and continue to be) able
to consistently meet a 1.0 mg/L effluent TP requirement with the majority using alum. Chemical
treatment has been applied to facultative and aerated lagoons both continuously and just prior
to the seasonal spring and fall discharges for controlled discharge systems. In Michigan,
phosphorus removal has been successful as long as the chemical precipitant is added at the
appropriate rate at the end of the pond system.
Chemical precipitation of soluble phosphate in a separate reactor or prior to the secondary
clarifier is typically the most feasible option for significant TP removal in trickling filter systems.
6.3.2 Alkalinity Adjustment
Nitrification, which consumes alkalinity, is also pH-dependent and inhibited under acidic
conditions. Therefore, sufficient alkalinity is required to prevent deleterious pH depression and
support stable nitrification. Some influent streams do not contain enough alkalinity to support
nitrification to the extent required to meet effluent limits, particularly when the influent water is
soft and/or the TKN concentrations are high. In these cases, alkalinity can be added in the form
of lime or a caustic solution in order to optimize nitrification and process performance. Further
optimization can be achieved by upgrading a manual chemical feed system to an automated
chemical feed system with a flow-paced or pH control loop.
6.3.3 Supplemental Carbon Addition
Carbon can be another limiting factor in both the denitrification process and EBPR, especially if
both nutrients are being removed biologically in the same single-sludge system. In these cases,
a supplemental carbon source can be added to improve denitrification and phosphorus removal.
Historically, a common supplemental carbon source used for denitrification has been methanol.
Denitrification using methanol requires a specific microbial population, so an acclimation period
is required, and methanol must be fed continuously to maintain the population. However,
because methanol is highly flammable, there are safety concerns associated with its storage
and use. Consequently, other carbon sources such as sodium acetate, sugar water, glycerol,
molasses, and proprietary products manufactured for use as supplemental carbon sources have
also been used for both denitrification and EBPR. These alternative carbon sources can be
used by common denitrifying heterotrophic bacteria and, therefore, do not need to be fed
continuously—another operational benefit compared to the use of methanol. Carbon feed
systems can be automated and controlled by flow-pacing, proportional to nitrate loading, and
anoxic effluent nitrate/nitrite feedback loops. Automated control methods reduce wasted
chemicals, which saves money and minimizes the amount of additional biomass that will be
generated from the additional carbon added to the system. This can also help prevent
overdosing of the carbon feed, which can lead to bleed-through of BOD into the effluent.
Operators at the WWTP in Crewe, Virginia, started adding lime for alkalinity control and
molasses as a supplemental carbon source. The plant later switched to a proprietary carbon
source called EnhanceBiop+N, a molasses product with added nutrients and minerals to improve
biological phosphorus removal. The plant saw increased biological removal of phosphorus,
while maintaining their level of nitrogen removal, which in turn reduced the amount of alum that
was needed to remove the remaining phosphorus. Using chemical addition along with modified
aerator controls, operators were able to significantly reduce the effluent TN concentrations from
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a pre-optimization concentration of 7.85 mg/l to 3.63 mg/l, while maintaining excellent
phosphorus removal (an average of 0.06 mg/l effluent TP).
6.4 EMERGING NUTRIENT REMOVAL APPROACHES AND
TECHNOLOGIES
Although beyond the technical scope of this report, various innovative approaches are available
to reduce nutrient loading of receiving waters associated with WWTP effluent discharges, and
deserve mention.
6.4.1 Discharge Reduction through Water/Nutrient Reuse
Similar to the land application alternatives previously described, removing the discharge or
reducing the volume of a discharge, particularly if the nutrients will be recycled, can be a very
effective approach for reducing nutrient loading to surface waters.
Water reuse, particularly where the reclaimed water will be used for irrigation (and thus
additional soil treatment and/or vegetative uptake of nutrients), can be an effective nutrient
removal approach. Unfortunately, many state standards for reclaimed water quality include strict
limits on nutrient concentrations, mostly in an attempt to limit biological growth/regrowth in
reclaimed water distribution piping networks and water use fixtures. This limitation can be
overcome, however, by limiting the extent of reclaimed water piping networks, siting reclamation
facilities closer to reuse areas using satellite and other decentralized reuse system approaches,
and by limiting such reclaimed water uses to irrigation as opposed to indoor water uses.
Employing a "fit-for-use" type of treatment approach, where nutrients are left in reclaimed water
so they can be recycled for irrigating vegetation, has multiple secondary benefits that are
consistent with EPA's mission:
• Offsets the use of inorganic fertilizers that contribute to nonpoint source loading and
require significant amounts of energy (and associated greenhouse gas emissions) to
produce.
• Reduces energy use for treatment of reclaimed water.
• Enhances landscapes that sequester carbon, produce food, reduce heat island effect,
improve physical and psychological health, and have other cascading benefits.
6.4.2 Nutrient Product Recovery and Reuse
Section 3.4.1 described an approach for reducing nutrient loading of surface waters by
leveraging reclaimed water systems in a way that recycles nutrients for purposes that result in
multiple benefits. Another approach to precluding the loading of WWTP-associated nutrients to
surface waters is to recover them during treatment and use the resulting product to offset the
use of other nutrient/fertilizer sources.
The advantage to the aforementioned fit-for-purpose treatment is that the energy and effort to
remove and concentrate nutrients into a product never have to be expended. Instead, treatment
is limited (which saves energy) and nutrients are provided in solution with reclaimed water.
Nutrient recovery during treatment is viable and gaining in popularity, particularly where existing
centralized infrastructure does not allow for widespread water and nutrient reuse. For example,
the Ostara's Pearl® process for the controlled production of struvite (which, uncontrolled,
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presents a significant operational problem in WWTPs) produces a slow-release fertilizer
containing nitrogen, phosphorus, and other essential plant growth nutrients.
6.4.3 Source Control/Separation
When considering how to control nutrient loading associated with wastewater treatment, it is
important to consider the source of nutrients in wastewater. In domestic (noncommercial,
nonindustrial) wastewaters, human urine contributes the majority of nitrogen and phosphorus.
Volumetrically, however, urine is a small fraction of the total wastewater flow (the vast majority
of the wastewater volume is water used for flushing, washing, etc.). As indicated in Section 2,
urine contains about 90 percent of the nitrogen excreted by humans, and unlike feces which are
high in biodegradable organic compounds and pathogens, urine is relatively low in pathogenic
organisms. Urine is self-disinfecting when held under natural alkaline conditions for a sufficient
period of time (Fewless, et al., 2011). Treated urine makes an excellent liquid fertilizer that is
typically diluted by a factor of 10 to 20 before application.
Urine diversion is indeed simple and practical and is being used as a nutrient control and
recovery technique in the United States (at a demonstration scale) and abroad. The main
challenges associated with urine diversion include:
• Difficulty procuring separating toilets (most of which are made and sold in Europe).
• Difficulty in providing dual plumbing systems (one for urine, one for blackwater) in
existing buildings.
• Current lack of capacity to manage treated urine (market for end product, institutional
arrangements for collecting and distributing product, etc.).
Related to source control, pretreatment at large dischargers, industrial facilities, or even within
the piping network, could also be considered to reduce the influent nutrient loading to the
WWTP.
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7 Conclusions and Recommendations
This technical report was prepared to help fill gaps in published information about improving
nutrient reduction performance at existing, non-advanced WWTPs using relatively low-cost
techniques. Although many published reports and papers address the nutrient removal
performance of WWTPs, this report represents one of the first documented efforts to present
empirical data via a compendium of case studies of non-advanced WWTPs that have been
optimized to improve nutrient reduction without requiring costly infrastructure upgrades.
The results of this project illustrate the following.
Optimization is often feasible and cost-effective
No- or low-cost activities can be implemented at existing WWTPs to significantly reduce
effluent nutrient discharges with minimal negative impacts on operations. In fact, in most
cases, the secondary impacts are overwhelmingly positive and include energy efficiency,
lower operational costs, and improved process stability.
Although most of the case studies did not specify the capital costs savings associated with
their optimization approach over alternative approaches, several did. Crewe's effort had a
capital cost of $6,000, compared with an estimated upgrade cost of $800,000. Victor Valley
spent $1.1M instead of $80M for a new treatment train. Two other case study contacts
indicated that optimization saved significant money versus more capital intensive
alternatives.
Some excess treatment capacity is ideal
Low-cost nutrient reduction improvements are most feasible for activated sludge plants,
where excess capacity (volumetric and/or aeration) can typically be leveraged to facilitate
nitrification and denitrification without requiring physical infrastructure modifications.
Aeration modifications (typically some kind of control of redox conditions or lowering of
average dissolved oxygen concentrations) represent the most common optimization
approach. However, these modifications are often supplemented with process modifications
(e.g., control of internal recycle rates, installation of inline monitoring equipment),
configuration modifications (e.g., adding internal recycle lines, step-feed provisions,
dedicated anoxic or anaerobic zonation), and chemical modifications (chemical phosphorus
precipitation, alkalinity addition, carbon supplementation).
Utilizing excess capacity may limit the ability of a WWTP to increase its flow rate in the
future without an expansion. For the case studies featured in this project, only one contact
indicated that their plant (Victor Valley, CA) needed to be rerated as a result of their
optimization efforts. It should be noted, however, that EPA did not specifically ask WWTP
contacts about impacts on design capacity.
Phosphorus removal is often complimentary to nitrogen removal
Modestly improved phosphorus reduction often co-occurs as a result of improvements in
biological nitrogen removal. To achieve more significant phosphorus reductions, most
WWTPs opt for chemical precipitation, which is a well-established technology widely
adapted to different plant types and configurations. Enhanced biological phosphorus
removal (EBPR) generally requires significant physical infrastructure modifications at
existing plants (e.g., creation of anaerobic selector zones).
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Other opportunities for reducing phosphorus discharges include control or side-stream
treatment of return flows and enhancing volatile acid production for driving EBPR in existing
anaerobic selectors (only applicable for an existing advanced treatment system). Soil- and
plant-based treatment systems are also particularly effective for reducing phosphorus, which
is removed from wastewaters by solid-phase sequestration.
Low-cost nutrient optimization is currently underreported
Low-cost nutrient reduction improvements, particularly for relatively basic treatment
systems, are underreported in the literature. In spite of extensive efforts at identifying and
developing relevant case studies, relatively few met the qualification criteria established by
EPA, typically due to insufficient monitoring or cost data, difficulty identifying prospective
case study plants (because of underreporting in the white and grey literature), and limited
responses from plant contacts during the time available for data collection for this study.
EPA concluded that the primary limitation in prospective case studies was that most efforts
at improving small or non-advanced plants are unpublished or otherwise under documented.
Most published literature focuses on optimizing existing biological nutrient removal systems.
EPA intends to identify additional case studies and update this document. EPA will also
consider additional capacity development activities.
Lagoon systems appear to have optimization opportunities
Although none of the case studies were for lagoon systems, it appears that low-cost
reduction of nutrient discharges associated with facultative lagoon systems should focus on
strategically timing discharges to coincide with times of low effluent nitrogen and
phosphorus concentrations. Nitrogen and phosphorus levels are typically lowest in the
summer due to algal nutrient assimilation and sequestration in sediments, and enhanced
volatilization of ammonia under conditions of elevated pH and temperature. Discharges can
also be timed, so nutrient discharges coincide with natural hydrologic conditions that
facilitate nutrient assimilation within the receiving environment.
Facultative lagoons unable to store water and control their discharge, as well as trickling
filters, are usually limited in their ability to reduce nutrients beyond baseline performance
without significant infrastructure modifications, which may include the addition of
pretreatment facilities to ensure effective nitrification or post-treatment denitrification
processes.
Other approaches can also be considered on a case-by-case basis
Other nutrient load reduction opportunities (which in many cases would be too intensive to
be considered "optimization") include removal or modifications to discharges (e.g., using
land application/soil-based treatment or constructed wetlands discharges), post-
denitrification, and nutrient reuse (water reuse for irrigation, nutrient product recovery, and
urine diversion).
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Appendix A
List of Nutrient Reduction Resources
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U.S. Environmental Protection Agency (EPA)
• EPA Office of Wastewater Management, Municipal Technologies website:
http://water.epa.gov/scitech/wastetech/mtb index.cfm.
Water Environment Research Foundation (WERF)
• WERF main website: http://www.werf.org.
• Nutrient Research at a Glance:
http://www.werf.Org/c/KnowledgeAreas/NutrientRemoval/Nutrients Research at a Glan
ce.aspx.
• Nutrient Management Compendium Documents:
http://www.werf.Org/c/KnowledgeAreas/NutrientRemoval/Nutrients Compendium.aspx.
Other Resources
• Water Environment Federation (WEF) Nutrient Knowledge Center:
http://www.wef.org/AWK/pages cs.aspx?id=1067.
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Appendix B
Case Study Summary Documents
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BAY POINT, FLORIDA
USBF ACTIVATED SLUDGE-PROCESS CONTROL AND MECHANICAL MODIFICATIONS
SYSTEM SUMMARY
Official Name: Bay Point Wastewater Treatment Plant (WWTP)
Location: 3116 Overseas Highway, MM15, Key West (Bay Point), FL 33040. Monroe County.
Florida Keys (latitude: 24° 37' 39" N; longitude: 81° 35' 40" W)
Permitted design flow: 0.054 MGD
Service area: The Bay Point system serves approximately 429 EDUs1 within the service area,
which includes the Bay Point subdivision and Blue Water RV park in the Saddlebunch Keys
System type: Activated sludge/Modified
Ludzack-Ettinger (MLE) and upflow sludge
blanket filtration (USBF) to Class V injection
wells
Initial year of operation: 2005
Upgrade type: Improved process controls
and minor mechanical modifications
Upgrade year of operation: 2008
Permitted effluent nitrogen limit: 12.5 mg/l,
monthly average TN; 10 mg/l, annual
average TN
Pre- and post-upgrade effluent nitrogen performance: 6.63 mg/l average, pre-upgrade;
3.99 mg/l average, post-upgrade
Permitted effluent phosphorus limit: 1.25 mg/l, monthly average TP; 1.0 mg/l, annual
average TP
Pre- and post-upgrade phosphorus performance: 0.47 mg/l average, pre-upgrade; 0.42 mg/l
average, post-upgrade
1 EDU = equivalent dwelling unit, which is the approximate number of residences served by the facility.
B-1
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Effluent Total Nitrogen
Pre-upgrade
Post-upgrade
Average
Concentration
6.63
3.99
Standard
Deviation
4.98
2.50
Effluent Total Phosphorus
Average
Concentration
0.47
0.42
Standard
Deviation
4.96
0.49
Units
mg/l
mg/l
[DECISION PROCESS
The Florida Keys Aqueduct Authority (FKAA) chose this approach as it appeared to be the most
economical way to consistently meet permitted nutrient requirements mandated by Section 6
of Chapter 99-395 of the Laws of Florida, which defines best available technology (BAT)
performance standards for wastewater treatment systems in the Florida Keys.
| SYSTEM OPTIMIZATION DESCRIPTION
The upgrades to Bay Point's WWTP consisted of minor modifications and improvements to
multiple system components including:
• Headworks
• Flow splitter box
• Air delivery system
• Alkalinity feed system
• Return activated sludge and digester
Headworks
The original headworks used a manually cleaned, static bar screen with very large openings
and an excessive approach velocity, which allowed most gross solids to pass. That system was
replaced with a tighter bar screen with an approach channel, and a drying rack. These
improvements prevented previously observed clogging of the flow equalization pumps and
buildup of debris on mixers and diffusers, improving overall system operation and process
performance.
Flow Splitter Box
A flow splitter box was installed to improve operational control of the flow splitting and dosing
of raw wastewater (originally, equalization pumps needed to be throttled, which could
exacerbate clogging). The new splitter box allows for continuous operation of the equalization
pumps, dosing a small amount of raw wastewater to the anoxic tank and returning some flow
back to the equalization tank, the ratio of which is controlled via adjustable gates.
Air Delivery System
The original system included two 10-HP positive displacement (PD) blowers controlled via six
adjustable timers, which was not adaptable to the changing loading and flow conditions
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routinely experienced at the size and type of this facility. Additionally, power consumption
was high and biological treatment could be improved with a more responsive air delivery
control system. The new air delivery system included the installation of variable frequency
drives (VFDs) for the blower motors, a programmable logic controller (PLC), a dissolved oxygen
(DO) analyzer, and a control panel.
The PLC-based system allows the operator to set the desired DO concentration while the VFDs
adjust blower output accordingly. The system also allows for timed operation of the blowers
to ensure complete operational control under any circumstances.
Alkalinity Feed System
Insufficient alkalinity in the influent wastewater required manual batch dosing of sodium
bicarbonate or hydrated lime to the flow equalization basin to ensure reliable nitrification.
Although that method worked in maintaining sufficient alkalinity, periodic problems with
overfeeding and underfeeding of chemicals caused other treatment issues. Accordingly, a
simple, permanent, duplex chemical feed system interlocked with the flow equalization
pumps was installed to provide reliable and flow-proportionate delivery of alkalinity.
Installation included two chemical feed pumps, poly tank with electric mixer, water supply,
control panel, wiring, and ancillary components.
Return Activated Sludge and Digester
The original piping configuration included interconnection between the main air header,
digester, and return activated sludge (RAS) piping. Therefore, adjustments made to process air
rates or digester levels caused fluctuations in the RAS and recycle rates, making control of the
biological nutrient removal process difficult. Accordingly, the RAS and digester functions were
isolated from the process blower header by installing two independent blowers—one for the
digester aeration and the second to supply air for returning RAS. One spare blower was
installed to be used as a backup for either system.
DISINFECTION EFFLUENT DISPOSAL
i 1 r
CHJCMNE CONUCI | |
UJf
I I
I I
J L
PROCESS FLOW DIAGRAM
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| COSTS AND OTHER IMPACTS
Capital costs: Approximately $170,365.
Operational costs: Not quantified, but significant labor cost savings were realized.
Additionally, operational costs for energy and chemicals have been reduced.
Technical assistance received or needed: Significant training was needed. From a regulatory
standpoint, operators at the Bay Point WWTP are required to have only a Florida Class C
WWTP operator's license (a Class C-licensed operator is required to know only basic
wastewater treatment techniques; nutrient removal is not introduced until Class B licensing).
The FKAA recognized early that it would be necessary to train operators to meet the
treatment standards required by their permits. FKAA currently has all in-house trained
operators except one who was trained outside FKAA.
PERFORMANCE
Pre- and post-upgrade TN and TP statistics are summarized below.
35.00
30.00
25.00
14.00
12.00
10.00
1/1/2005 1/1/2006 1/1/2007 1/1/2008 12/31/2008 12/31/2009 12/31/2010 12/31/2011 12/30/2012 12/30/2013
Date (m/d/yyyy)
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If cost were not an issue, adding automatic throttling valves to each side of the common
aeration header would help control aeration even more. Alternatively, installing a third blower
and replacing the common air header with two single headers, one for each treatment train,
would also allow independent control of aeration for each train. However, effluent quality is
currently excellent and the costs associated with further improvements are not justified at this
time.
Tom Pfiester. Florida Keys Aqueduct Authority, 3375 Overseas Highway, Marathon, FL 33050.
Phone: (305) 481-2015. Email: tpfiester@fkaa.com.
Keys Wastewater Plan: http://www.monroecounty-fl.gov/DocumentCenter/Home/View/478
Florida Keys Aqueduct Authority: http://www.fkaa.com/
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BOZEMAN, MONTANA
ACTIVATED SLUDGE-PROCESS CONTROL MODIFICATIONS AND STEP FEED
SYSTEM SUMMARY
Official Name: Bozeman Water Reclamation Facility (WRF)
Location: 2245 Springhill Road, Bozeman, Montana 59771 (latitude: 45° 43' 25" N; longitude:
111° 04' 08" W)
Permitted design flow: 5.2 MGD,
annual average; 6.5 MGD, peak
month
Service area: City of Bozeman
(population of approximately 36,000)
System type: Complete-mix
conventional activated sludge
Initial year of operation: 1985
Upgrade type: Conversion of
complete mix to plug-flow/step-feed
cyclic aeration
Upgrade year of operation: 2008
Permitted effluent nitrogen limit: 782 Ib/d TN (16.2 mg/l at annual average, 12.8 at peak
month flows) from June 1 to September 30 (daily maximum of 971 Ib/d); 864 Ib/d TN from
October 1 to May 31 (daily maximum of 1,072 Ib/d)
Pre- and post-upgrade effluent nitrogen performance: 2007 annual average of 17.8 mg/LTN
pre-upgrade; 2008 annual average of 10.5 mg/LTN post-upgrade
Permitted effluent phosphorus limit: 160 Ib/d TP (5.2 mg/l at annual average, 4.1 at peak
month flow) from June 1 to September 30 (daily maximum of 199 Ib/d); 170 Ib/d TP from
October 1 to May 31 (daily maximum of 211 Ib/d)
Pre- and post-upgrade phosphorus performance: 2007 annual average of 3.7 mg/l pre-
upgrade; 2008 annual average of 3.0 mg/L post-upgrade; 2009-2010 annual average of
2.5 mg/L
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RATIONALE AND DECISION PROCESS
The Bozeman WRF used a
complete-mix, conventional
activated sludge process
designed to handle an annual
average flow of 5.8 MGD and a
maximum monthly flow of
7.3 MGD. In 2007, when the
upgrades were made, its
average monthly flows were as
high as 7.7 MGD and influent
biochemical oxygen demand
(BOD) and total suspended
solids (TSS) concentrations
were higher than was typical,
because Bozeman had been
tightening its collection system
to eliminate infiltration and
inflow.
The project team's goal was to achieve interim compliance with new nutrient limits imposed
by the Montana Department of Environmental Quality. To do this, the team decided to modify
the existing activated sludge system so it would operate as a plug-flow process with phased
nitrification and denitrification.
[SYSTEM OPTIMIZATION DESCRIPTION
Phased nitrification and denitrification is an operating strategy in which one basin provides
both nitrification and denitrification sequentially by cycling the aeration system on and off.
When the aeration is on, the basin nitrifies the wastewater; when it is off, the basin denitrifies
it. Operators control the aerobic and anoxic times via a supervisory control and data
acquisition (SCADA) timer and an online oxidation-reduction potential (ORP) sensor.
The project team also split primary effluent between the first two cells of the aeration basin.
Operators use existing control gates to send 60 percent of primary effluent to Cell 1 and
40 percent to Cell 2. Only Cell 1 and Cell 2 shift between aerobic and anoxic conditions; Cell 3
and Cell 4 are aerated continuously to ensure that all remaining ammonia is completely
removed.
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Original Configuration Complete Mix Process Flow
PNDN = phased nitrification and electrification.
Modified Plugflow PNDN Flow
For this process change to succeed, the project team had to make some minor retrofits. For
example, the team added four submersible mixers in Cell 1 and Cell 2 to provide mixing during
anoxic periods (when the blowers are off). Those mixers had to be at least 4 feet above the
existing membrane diffusers. The team also upgraded some diffuser mounting brackets on the
basin floor. The new ones can withstand higher mixing velocities. In addition, the project team
fabricated new basin weir plates to facilitate the conversion to plug flow. Team members also
made fairly extensive changes to the existing SCADA blower controls to enable on/off
operations and implement ORP setpoint control.
The operators typically adjust the ORP setpoint so that a new aerobic cycle will begin after a
pH plateau has been maintained for about 10-15 minutes. They review the process weekly
and adjust the setpoint as needed. The ORP results showed a definite "nitrate knee" and were
used to control the anoxic cycle in the last 2 years of PNDN operation.
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| COSTS AND OTHER IMPACTS
Capital costs: Less than $180,000.
Operational costs: No increase or decrease in operating costs were noted.
Technical assistance received or needed: A consulting firm designed the upgrade and
modifications to the SCADA controls. They also provided construction oversight.
| PERFORMANCE
Process modifications have worked well under various flow and loading conditions. Effluent
ammonia levels remained steady, total effluent nitrogen dropped 40 percent, bulking
improved, and the solids volume index dipped slightly.
Ammonia. The process change did not compromise ammonia removal. Both before and after,
effluent ammonia levels averaged 0.22 mg/L—well below the plant's effluent ammonia limit
of 1.52 mg/L (30-day average).
TN. Despite water temperatures of about 52°F (11°C), the effluent TN concentration dropped
in less than a week—from 18.4 mg/L in December 2007 to 13.3 mg/L in January 2008.
Likewise, the aeration basins' pH rose almost immediately, enabling operators to quickly
establish the aeration cycles.
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Effluent TN concentration continued to improve as water temperature warmed, even though
lows increased. By summer 2008, the treatment plant was producing a final effluent that
contained 40 percent less TN than it had before the change.
TP. While influent phosphorus concentrations remained largely steady, effluent phosphorus
levels dipped from 3.7 to 2.5 mg/L.
Bulking. Average annual chlorine use dropped from 52 to 32 ton/yr (47 to 29 Mg/yr) because
operators did not have to chlorinate return activated sludge as often as before. Air cycling
reduced and limited the growth of filamentous organisms in the basins, reducing the need for
chemical treatment.
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Average Monthly
Concentration
Effluent Total Nitrogen
Effluent Total Phosphorus
Pre-Mixer
Upgrade
20.3
4.13
Post-Mixer
Upgrade
17.3
2.48
Post-ORP/LDO
Control Upgrade
5.44
1.72
Units
mg/l
mg/l
DECISION PROCESS
In 2004, mixers were added in the oxidation ditch to save on energy costs. In 2012, nitrogen
removal was required for permit reissuance. Shortly thereafter, staff received nutrient
removal training and applied their newfound knowledge to demonstrating how process
changes can significantly reduce nitrogen. The upgrades described were the most economical
way to consistently meet new permit requirements. A motivated, educated, empowered
staff—using upgraded monitoring equipment—achieved effective, consistent nitrogen
removal in a 1984-vintage oxidation ditch treatment plant that was modified in 2004 for
energy efficiency, but never designed for nutrient removal.
SYSTEM OPTIMIZATION DESCRIPTION
Improvements came about as a result of process
changes. A series of minor physical upgrades provided
tools that were used to support the process changes, but
were not the cause of the improvements. The biggest
capital expense was for energy savings equipment that
later proved to provide a dual benefit: nutrient removal
and energy savings. Process optimization proceeded in
four steps.
1. In 1984, a single oxidation ditch equipped with
dual aeration rotors was constructed to provide
TSS and BOD removal. The original installation
was designed for ammonia removal, not forTN or
TP removal.
2. In 2004, minor changes were made to improve
energy efficiency. As originally constructed, both
of the oxidation ditch rotors ran continuously. As
a result, the original equipment provided a surplus of dissolved oxygen (DO). To allow
for the cycling of the fixed-speed aeration equipment, rail-mounted mixers were
installed so the flow would continue to stay suspended and circle the oxidation ditch
with the rotors turned off. A DO probe was installed and integrated with the SCADA
system to maintain a DO setpoint of 4-5 mg/L by cycling the rotors on and off. At the
lower DO concentration resulting from the energy savings changes, incidental
improvements in nitrogen and phosphorus removal occurred.
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3. In 2012, Chinook staff attended a 2-day training class sponsored by the Montana
Department of Environmental Quality (DEQ). Using the knowledge they gained, staff
experimented with extended air-off cycle times. By allowing the DO in the ditch to
cycle between anoxic and oxic conditions, an immediate 50 percent improvement was
observed in nitrogen removal. No equipment was purchased; no funds were expended.
In fact, because of reduced rotor operating time, electrical costs were reduced. For
zero capital investment and at reduced operating expense, Chinook staff reduced TN
by 50 percent. And, as a result of the lower tank DO concentrations, some incidental
improvements in TP removal also occurred.
4. In 2013, an ORP probe was installed to provide improved process control. At the same
time, the old DO probe (2004 vintage) was replaced with a new LDO probe. Both
probes were integrated with the plant's SCADA system. Using the new
instrumentation, plant staff
have been able to maintain
optimal conditions for
biological nitrogen removal and
incidentally provide some level
of enhanced biological
phosphorus removal, while
enjoying additional energy
savings.
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SOLIDS FLOW STREAM
DEWATERING
SLUDGE aSPOSAL
FARM FIELDS
OR
SLUDGE
STORAGE
UKXIUM
DRYING
BED
SLUDGE
PUMPS
EFFLUENT
DISCHARGE TO
MILK RIVER
SANITARY LANDFILL
SEROSIC DIGESTERS
•4-
•«-
BLOWERS
SLUDGE STABILIZATION
| COSTS AND OTHER IMPACTS
Capital costs: Approximately $5,000 for ORP probe and integration with SCADA.
The energy savings improvements implemented in 2004 (i.e., mixers, DO probe, SCADA) cost
$68,200. In 2013, the DO probe was replaced with LDO equipment for $8,000.
Operational costs: Less than $l,000/year (oil and grease for mixers and 1-2 hours/year to
change the oil). Cost savings have been realized. The reduced electrical consumption more
than offsets the expense of cleaning, calibrating, and maintaining the ORP probe.
Technical assistance received or needed: In 2012, Chinook staff attended a 2-day training
class sponsored by the Montana DEQ. Using the knowledge they gained, staff felt empowered
to experiment with extended air-off cycle times and other process modifications.
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| PERFORMANCE
Pre- and post-upgrade TN and TP statistics are summarized in the chart below.
30.00 -
or TP (mg/l)
p i
5 c
1
1
*
*
Pre-optimization
+
•
* *
*
• TN HTP
Post-optimization
• ^
* * *
Sep-11 Dec-11 Apr- 12 Jul-12 Oct-12 Jan-13 May-13 Aug-13 Nov-13 Mar-14
Date (mmm-yy)
FUTURE IMPROVEMENTS
No improvements are planned at this time. Nitrogen removal is still a relatively new
requirement, so the plant is currently working on refining the process.
CONTACT INFORMATION
Eric Miller, P.O. Box 1177, Chinook, MT 59523.
Phone: (406) 357-2188. Email: chinookwwtp@gmail.com
OTHER RESOURCES
City of Chinook: http://www.cityofchinook.com/index.html
State of Montana MPDES Permits: http://deq.mt.gov/wqinfo/mpdes/maiorpermits.mcpx
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CREWE, VIRGINIA
OXIDATION DITCH ACTIVATED SLUDGE-PROCESS CONTROL MODIFICATIONS
SYSTEM SUMMARY
Official Name: Town of Crewe Wastewater Treatment Plant (WWTP)
Location: 370 Tyler Lane Court, Crewe, VA 23930 (latitude: 37° 11' 14" N; longitude: 78° 07'
23" W)
Permitted design flow: 0.5 MGD
Service area: Population of
2,386 over 2.0 square miles;
8 Wastewater Pump
Stations; 11 miles of
underground piping
System type: 3-channel
Orbal oxidation ditch
activated sludge; phosphorus
precipitation using alum
Initial year of operation:
1956 (trickling filter plant);
1997 (oxidation ditch
upgrade)
Upgrade type: Process
control modifications
Upgrade year of operation: 2007
Permitted effluent nitrogen limit: 9,137 Ib/yrTN, equivalent to a TN concentration of
6.0 mg/L at design flow of 0.5 MGD
Pre- and post-upgrade effluent nitrogen performance: Pre- and post-upgrade TN statistics are
summarized below.
Effluent Total Nitrogen
Pre-upgrade (2005-2006)
Post-upgrade (2007-2013)
Average Concentration
7.85
3.63
Units
mg/l
mg/l
Flow
Average
0.27
0.24
Units
MGD
MGD
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Permitted effluent phosphorus limit: 761 Ib/yr TP, equivalent to a TP concentration of
0.5 mg/L at design flow of 0.5 MGD
Pre- and post-upgrade phosphorus performance: 2005-2013 annual average of 0.06 mg/l
(effluent TP limits have never been an issue; however, the plant now uses fewer chemicals to
affect similar effluent concentrations)
| RATIONALE AND DECISION PROCESS
In 2007, regulatory changes in Virginia required the majority of wastewater treatment
facilities to significantly reduce the discharge of nitrogen and phosphorus in their final
effluents. While most facilities required significant physical upgrades to comply with the new
requirements, the town of Crewe operating staff instead evaluated their existing treatment
facility for optimizing nitrogen and phosphorus removal through operational modifications.
In discussing the challenge in 2006, staff estimated that upgrade costs were in excess of
$250,000, with the possibility of nearing $800,000 for anticipated equipment changes, which
included installing independently controlled means of delivering dissolved oxygen (DO) to
each oxidation ditch channel. This would require increasing the number of motors from two to
six, with variable frequency drives (VFDs) installed to control each motor independently.
In an effort to find an alternative, town staff began several years of operational
experimentation. Although the facility relies upon chemical precipitation to remove
phosphorus, plant staff made adjustments to several treatment process characteristics in an
attempt to reduce effluent TN levels. Although the facility was not designed for TN removal,
their oxidation ditch process does offer several operational control options to improve upon
the plant's nutrient removal performance.
The town first visited a number of different facilities to see how they achieved success and to
help formulate how the town might do things differently. Understanding that they could not
necessarily imitate the physical equipment used by other facilities (e.g., subsurface mixing in
an anaerobic/anoxic zone; independent control of DO to each zone; VFDs) their challenge was
how to best imitate the treatment by establishing and maintaining the proper environments
for nitrification and denitrification simultaneously, with minimal upgrades of existing
equipment.
| SYSTEM OPTIMIZATION DESCRIPTION
Beginning January 1, 2007, the facility began operating under the General Permit for Total
Nitrogen and Total Phosphorus Discharges and Nutrient Trading in the Chesapeake Bay
Watershed (9-VAC-25-82-70). Under the general permit, the Crewe WWTP has an annual
waste load allocation (WLA) of 9,137 pounds for TN and 761 pounds for TP. At design flow,
this WLA equals a TN concentration of 6.0 mg/L and TP 0.5 mg/L, respectively.
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— Waitewater Flow
-•• Return Activated Sludge
- Chemical Feed
- Waste Sludge Flow
Influent
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Bar Screen
Grit Removal
Pump Station
Oxi-Ditch a = #1 , b = #2, c = #3
Alum Feed Location
Splitter
Sec. #1
Sec. #2
CljC.T.
Cascade Steps
Chemical Feed Building
Sludge Pump Station
Aerobic Digestion
Belt Filter Press
Landfi
Effluent
Note: Not to Scale
Plant operating staff have put in a tremendous amount of effort to improve upon the plant's
nutrient removal performance. The following actions were taken to optimize the facility for
nitrogen removal.
Alkalinity Control
The nitrification process consumes about 7.14 pounds of alkalinity per pound of ammonia
converted to nitrate. Without sufficient alkalinity, process performance will decrease. Facility
staff add approximately 100 pounds of lime to the first channel of the oxidation ditch daily to
maintain enough alkalinity for nitrification.
Carbon Source for Denitrification
The denitrification process utilizes heterotrophic bacteria to convert nitrate to dinitrogen gas.
These organisms must have a readily available carbon source to effectively support their life
functions. Typically, in a pre-anoxic mode of denitrification (e.g., Modified Ludzack Ettinger
[MLE] process), influent carbon (BOD) is used to supply this food source. Additional BOD can
be added to increase nitrogen reduction if necessary. The town of Crewe add approximately
150 pounds of dried molasses daily to the first channel of the ditch to provide an additional
carbon source for denitrification.
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Dissolved Oxygen Control
The pre-upgrade operational strategy (during which the plant had no permit limit on TN, but a
permit limit on total Kjeldahl nitrogen [TKN]) was to keep dissolved oxygen levels in the
nitrification zone as high as possible by continuously running aeration on "high".
Improved control of DO levels and aeration rates can enhance nitrification/denitrification
performance by providing a controlled aerobic/anoxic environment. Typically, in an Orbal
oxidation ditch process, DO levels are kept at less than 0.5 mg/L in the outer channel and
between 1.0 - 3.0 mg/L in the inner channel(s). At the town of Crewe WWTP, whenever DO
levels in the oxidation ditch were lowered to attempt to improve denitrification, an increase in
effluent TKN occurred. Beginning in January 2007, plant operating staff decided to develop an
effective yet low cost alternative
to controlling the DO level in each
channel of the ditch to improve
denitrification while maintaining
effective TKN reduction
(nitrification).
One method of controlling DO in
an oxidation ditch that uses disc
aeration, is to add or remove
aeration discs. Plant staff
considered this method but were
concerned that mixing might be
negatively impacted, causing
settling or solids in the ditch.
Plant staff therefore decided to
experiment with operating the ditch aerators in an on/off operation mode. A 24-hour timer
with 15 minute on/off cycles was used in initial tests to determine the effectiveness of this
approach in maintaining the DO at the desired levels. The timer was wired to one of the two
available aeration motors. The timer cycled on/off effectively, but lacked the ability to alter
the speed of the motor. Plant performance improvements were marginal with this
modification.
A second test was initiated using the same timer to alter the speed of the motor in cycles. This
was accomplished by having the timer set-up to operate the aerators in "high mode"
normally. However, when the timer triggered a cycle, it changed the aerator speed from high
to low for the preset duration, returning to high mode upon completion of the cycle. Delay
timers were employed to stop and start the motors, limiting the impact the cycling would have
on the mechanical equipment.
After the second test, a DO probe was installed to monitor DO concentration of the mixed
liquor and send a signal to a controller used to adjust the disc aerators to high or low speed by
opening and closing relays in response to predetermined setpoints.
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Through trial and error, it was discovered that the most effective location for the DO probe
was in the third channel. Experience had shown that when DO levels fell below 3.0 mg/L in the
third channel, a significant increase in TKN levels occurred. Although the corresponding DO in
the first channel was low enough to increase denitrification and thus further reduce nitrates,
the increase in TKN resulted in no substantial change in total nitrogen levels. After further
experimentation, it was discovered that a setpoint range between 3.6-4.0 mg/L offered the
most balanced approach to controlling both total
nitrogen and TKN levels.
This operational method has provided the plant a
measure of control beyond a simple on/off approach to
limiting DO levels within the ditch. Total costs for the
system were approximately $6,000; however, it should
also be noted that the facility staff designed and
installed the system themselves, which resulted in a
significant cost savings to the town.
Process Monitoring
Increased process monitoring has been helpful in understanding plant performance and
establishing appropriate control strategies. Facility staff now perform additional process
control sampling and testing to characterize and monitor the treatment process. Nitrogen
profiling has been used to determine where adjustments are needed and whether
adjustments are effective in improving nutrient removal. Samples are analyzed in-house and
by an outside source. A good working relationship with an outside laboratory has proven to be
essential in the town's efforts to reduce nutrients. They are able to verify in-house results in a
timely fashion and allow the town to adjust the process based on the results.
Phosphorus Removal Enhancements
Back in 2006, during the process of assessing carbon sources for improving denitrification,
plant operational staff, after trying a number of products, discovered that the most "bang for
the buck" came from the use of molasses. Considerations in their choice of a product included
cost versus benefit, reliability of product availability and operator safety.
In June 2011, the plant operator was contacted by a
company developing molasses based products
specifically for the wastewater industry. They proposed a
free trial of "Enhance Bio-P", which is molasses with
micronutrient additives formulated to enhance biological
TP removal. The product was successful in consistently
reducing the plant's use of liquid alum by more than 50
percent. Anaerobic micro-environments within the
plant's two anoxic channels allow for biological
phosphorus reductions that are improved by the
Enhance Bio-P product.
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The plant repurposed a 5,000-gallon tank that was previously used to store magnesium
hydroxide which they stopped using in 2005. The system pumps the molasses product to the
influent pump station, where it mixes with influent flow immediately prior to the influent
being pumped to the first of the anoxic oxidation ditch channels. Addition of the Enhance Bio-
P product lowers the plant's TP concentration, prior to liquid alum addition from an average of
more than 1.00 mg/L to less than 0.50 mg/L, significantly lowering liquid alum demand and
associated costs.
| COSTS AND OTHER IMPACTS
Capital costs: $6,000 for DO control system.
Operational costs: Carbon source control costs are about $30,000 per year. However,
switching from molasses and alum to Enhance BioP and alum has resulted in an estimated
annual savings of $26,200.
Alkalinity addition is approximately $24/day or $8,760/year.
Technical assistance received or needed: Most technical support was provided by in-house
operational staff.
| PERFORMANCE DISCUSSION
Pre- and post-upgrade TN statistics are summarized on page 1. A graphical summary of
average annual effluent TN concentration is provided below (upgrades were implemented in
2007) and a monthly comparison between 2006 (pre-upgrade) and 2007 (post-upgrade) is
presented below that.
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10
9
8
7
1 5
1
1
3
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1 -
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20
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• Flow
04 2005 2006 2007 2008 2009 2010 2011 2012 2013 20
0.35
- 0.3
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O
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1
f
0.15 S?
- 0.1
0.05
- 0
14
Year
Total Nitrogen
Monthly Average Concentration (mgfl.)
Jan Fib Mar April May Junt July
| CHALLENGES
Several other operating factors are currently being addressed to potentially increase the
efficiency of TN removal.
Inflow and Infiltration (I/I)
The town has dealt with significant I/I problems, which may have directly impacted the ability
of the system to achieve optimal nutrient removal. The town is currently working diligently to
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address the I/I issues and increase the pumping and storage capacity of the plant. The town
has completed slip lining several problem pipeline areas within the collection system, which
has resulted in an immediate decrease in the amount of inflow and infiltration flow received
at the plant during rain events.
Internal Recycle
Typically, facilities operating for nitrogen removal use an internal recycle system to return a
much greater quantity of flow (2-4 Q), rich in nitrate nitrogen, to the anoxic zone for
conversion to dinitrogen gas. The current plant configuration does not include an internal
recycle, but the facility is performing a pilot test on using a temporary internal recycle system,
using a submersible pump and polyvinyl chloride (PVC) piping, to determine its effectiveness
in further reducing nitrogen levels. The pump recycles nitrates from the third channel
(aerobic) back to the first channel (anoxic) at approximately 130 GPM. Plant staff have
determined a 1:1 ratio works best to maximize nitrate removal.
Oxygen Addition into Anoxic Channel
Excessive agitation occurred where the
influent and return activated sludge (RAS)
entered the anoxic zone (first channel) of
the oxidation ditch. DO measurements
indicated a DO of 0.4 mg/L before and 0.8
mg/L after the influent discharge and 1.4
mg/L after the RAS discharge. This DO
must be used up by biological activity
before denitrification will occur. In an
attempt to decrease DO at this location,
the operating staff extended both the
influent and RAS piping below the water
surface in the first ditch. Results were immediate, decreasing DO by as much as 1.0 mg/L
where the influent and RAS enter the ditch.
Sidestream Flows
An often overlooked contributor of nutrients in plant influent results from internal plant
processes. If not managed properly, sidestream flows can significantly affect the plant's ability
to consistently remove nitrogen and phosphorus. Crewe's operating staff observed that,
whenever they decanted their aerobic digester and/or operated the belt filter press, an
increase in plant influent nitrate levels occurred. Currently, the aerobic digester is being
operated in an on/off mode (i.e., 2 hours on/1 hour off) to reduce the nitrates before they
enter the plant influent. This operational mode has resulted in an increase in ammonia and
TKN levels, but the process appears to be handling the load effectively. Plant staff are
continuing to monitor the nutrient levels to determine if any adjustment to the operational
strategy is required.
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| FUTURE IMPROVEMENTS
There are no plans for any other nutrient reduction upgrades at this time. Crewe is well-
positioned to generate modest capital returns by selling generated nutrient credits (through
the Virginia Nutrient Credit Exchange Association). With consistent TN averages well below
permit limits, even if the limit is decreased, Crewe's current process performance gives the
plant plenty of safety factor. The added benefit of cost-saving with regards to liquid alum use,
through its choice of carbon source addition, has Crewe in a great position with respect to TP
reduction.
| CONTACT INFORMATION
John Hricko, plant manager. Phone: (434)-645-9436. Email: hricko@hovac.com.
| OTHER RESOURCES
Town of Crewe: http://www.townofcrewe.com/
State of Virginia Department of Environmental Quality case study:
http://www.deq.state.va.us/Portals/0/DEQ/Water/WastewaterTreatment/crewe case study.
fidf
Treatment Plant Operator Magazine article:
http://www.tpomag.com/editorial/2009/09/top-performer-plant-a-little-creativity
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
Treatment Plants: DRAFT Version 1.0 August 2015
FLAGSTAFF, ARIZONA
IFAS ACTIVATED SLUDGE-PROCESS CONTROL MODIFICATIONS
SYSTEM SUMMARY
Official Name: Wildcat Hill Wastewater Treatment Plant (WWTP)
Location: 2800 North El Paso Road, Flagstaff, AZ 86004 (latitude: 35° 13'32"N; longitude: 111°
33'25"W)
Permitted design flow: 6.0 MGD
Service area: City of Flagstaff (2010 population of
66,067)
System type: Integrated fixed-film activated sludge
(IFAS) in Modified Ludzack-Ettinger (MLE)
configuration
Initial year of operation: 2010 (converted from
biotowers)
Upgrade type: Improved process controls
Upgrade year of operation: Improvements initiated in late 2013
Permitted effluent nitrogen limit: 10.0 mg/LTN (8.0 mg/LTN alert level) on 5 sample rolling
geometric monthly mean basis
Pre- and post-upgrade effluent nitrogen
performance: 14 mg/l pre-upgrade;
8.5 mg/l post-upgrade
Permitted effluent phosphorus limit:
None
Pre- and post-upgrade phosphorus
performance: N/A
RATIONALE AND DECISION PROCESS
Performance of the MLE process was insufficient to achieve consistent compliance with
Flagstaff's effluent TN limit during periods of high nitrogen loading. A 2013 plant evaluation
recommended steps to optimize performance of the MLE process and upgrades to improve
upon it. The relatively easily implemented instrumentation and operational improvements
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5.5 mg/i NH
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
Treatment Plants: DRAFT Version 1.0 August 2015
were implemented in late 2013 and 2014 while upgrades requiring more extensive design and
budget were targeted for 2015.
| SYSTEM OPTIMIZATION DESCRIPTION
A combined ammonia/nitrate probe (ISE type) was
installed in the effluent end of the anoxic zone. Nitrate
concentration is monitored and internal mixed liquor
recycle is adjusted as needed to maintain a nitrate level of
0.5-1.0 mg/L nitrate-N at that point in the process to avoid
overloading anoxic zones and further decreasing nitrogen
removal by unnecessarily decreasing anoxic detention
time.
Control of nitrogen sources (mainly sludge processing
recycle) to decrease loading spikes was essential. The new
nitrate probe indicated that the nitrate concentration at the
anoxic zone effluent was often greater than 1 mg/L,
indicating that more nitrate was being recycled than the
anoxic zone could effectively remove. Excessive nitrate
leaving the anoxic zone indicates either insufficiently anoxic
conditions or insufficient oxygen demand (due to insufficient
readily degradable carbon) at the anoxic zone.
Monitoring indicated that the BOD-to-nitrogen ratio in the primary effluent was low at times.
Therefore, the primary clarifier operation was modified to encourage greater hydrolysis
and/or fermentation of influent BOD. Pumping of settled sludge from the primary clarifiers
was modified to provide longer detention time for solids in the primaries to allow additional
conversion of particulate BOD to soluble BOD available for denitrification.
Oxidation-reduction potential profiling in the anoxic zone indicated that much of the zone was
too aerobic (oxidizing) to expect denitrification, likely due to excessive oxygen loading from
the internal mixed liquor recycle. Consequently, internal recycle rate control was modified.
Other operational changes included decreasing the rate of return from biosolids dewatering
processes as needed to manage nitrogen loading spikes.
| COSTS AND OTHER IMPACTS
Capital costs: Approximately $10,000 for ammonia/nitrate probe and installation.
Operational costs: Sensor cartridge replacement approximately $1,000 every 6 months. Probe
cleaning and calibration weekly.
Technical assistance received or needed: A consultant was hired to recommend modifications
to improve nutrient removal.
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| PERFORMANCE
Pre- and post-upgrade total nitrogen statistics are summarized below.
1 Parameter
April 2013
April 2014
Flow
Temp
Influent BOD
Primary Effluent BOD
Primary Effluent NHs-N
Final Effluent NOs
Final Effluent TN
3.3 mgd
18.3° C
595 mg/L
203 mg/L
22.1
12.5
14.0
3.9 mgd
18.4°C
498 mg/L
269 mg/L
32.6
7.0
8.5
FUTURE IMPROVEMENTS
Installation of additional ammonia and nitrate probes to allow continuous monitoring of
primary effluent and secondary effluent is underway. Addition of supplemental carbon
storage and feed are planned as well as the addition of an anoxic tank downstream of the
aeration basins to provide additional denitrification when necessary to decrease nitrogen to
less than the levels attainable with the MLE process.
CONTACT INFORMATION
Larry Lemke, 2800 North El Paso Road, Flagstaff, AZ 86004. Phone: (928) 526-2520. Email:
llemke@flagstaffaz.gov.
OTHER RESOURCES
City of Flagstaff: http://www.flagstaff.az.gov/index.aspx7NID=21
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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HAMPDEN TOWNSHIP, PENNSYLVANIA
CONTINUOUS-FLOW SEQUENCING REACTOR ACTIVATED SLUDGE-AERATION
CYCLING
SYSTEM SUMMARY
Official Name: Roth Lane Wastewater Treatment Plant (WWTP)
Location: 4200 Roth Lane, Mechanicsburg, PA 17050 (latitude: 40° 16' 27" N; longitude: 76°
58' 38" W)
Permitted design flow: 5.69 MGD, annual
average
Service area: Hampden Township, East
Pennsboro Township, Silver Spring Township, and
Camp Hill Borough in Cumberland County,
Pennsylvania
System type: Activated sludge (continuous-flow
sequencing reactor [CSR])
Initial year of operation: 1982
Upgrade type: Flexibility of series operation of
dual-train CSR system
Upgrade year of operation: 2010
Permitted effluent nitrogen limit (2014
compliance year): 114,558 Ib/yr TN (6.6 mg/l TN at permitted flow)
Pre- and post-upgrade effluent nitrogen performance: 4.66 mg/l (1.20 mg/l standard
deviation) TN during parallel operation (pre-upgrade); 3.64 mg/l (0.63 mg/l standard
deviation) TN during series operation (post-upgrade)
Permitted effluent phosphorus limit (2014 compliance year): 14,094 Ib/yr TP (0.81 mg/l TP at
permitted flow)
Pre- and post-upgrade phosphorus performance: 0.81 mg/l (0.28 mg/l standard deviation),
average June 2010-April 2014 (no TP removal improvements were made)
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Series Operation
Ammonia
Nitrite+Nitrate
TKN
TN
Average
Concentration
0.98
1.90
0.98
3.64
Standard
Deviation
0.62
0.48
0.62
0.63
Parallel Operation
Average
Concentration
0.67
2.73
0.67
4.66
Standard
Deviation
0.71
0.72
0.71
1.20
Units
mg/l
mg/l
mg/l
mg/l
| RATIONALE AND DECISION PROCESS
The Roth Lane WWTP has to comply with strict nitrogen and phosphorus removal
requirements associated with Pennsylvania's Chesapeake Bay Nutrient Reduction Strategy.
The facility's current National Pollutant Discharge Elimination System (NPDES) permit includes
TN and TP effluent mass load limits, which are enforced on a 12-month "compliance year"
basis from Oct. 1 through Sept. 30 of the following year. The flexibility of series operation of
the existing CSR system helped lower sludge wasting, power consumption, and chemical use
and could be easily implemented using existing infrastructure at the facility.
| SYSTEM OPTIMIZATION DESCRIPTION
The Roth Lane WWTP discharges treated effluent to Sears Run under NPDES Permit No.
PA0080314. The WWTP includes the following treatment processes:
• Screening
• Grit removal
• Grease removal
• CSR activated sludge process
• Chemical phosphorus removal
• Final clarification
• Filtration
• Ultraviolet (UV) disinfection
• Effluent discharge to Sears Run
Roth Lane has two CSRs that are normally operated in parallel (i.e., as separate treatment
trains). Each reactor is equipped with a rotating aeration bridge with membrane tube
diffusers—which are mounted on retrievable rack assemblies and suspended from the
bridge—and stationary membrane tube diffusers above the floor on retrievable rack
assemblies attached to the tanks' walls. Aeration in each CSR basin is supplied by three
positive displacement blowers, each equipped with variable-frequency drives. For each
bioreactor, one of the blowers is dedicated to the stationary diffusers and the second blower
is dedicated to the rotating diffusers. The third blower serves as a redundant standby blower
for either set of diffusers.
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
Treatment Plants: DRAFT Version 1.0 August 2015
A small plant upgrade in 2008 added a third clarifier and a distribution box between the
aeration units, which incidentally provided staff with the flexibility to run the CSR basins in
series. In addition to improving nitrogen removal, series operation reduced waste sludge
volumes by about 40 percent.
Additional upgrades in 2010 added a process control system capable of continuously
monitoring dissolved oxygen (DO) and nitrate. Signals are sent to a programmable logic
controller (PLC) to establish process phasing through oxic, anoxic, and anaerobic cycles. A
proportional-integral-derivative (PID) control loop is used to modulate the blower speed in
each reactor to maintain the DO setpoint. To allow operational flexibility, the PLC enables the
user to adjust the DO setpoint and stage timers for each phase.
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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| COSTS AND OTHER IMPACTS
Capital costs: None.
Operational costs: None.
Technical assistance received or needed: None.
PERFORMANCE
As expected, TN removal was more efficient during series operation (figure below), with the
improvements associated with enhanced denitrification (table above, which shows lower
nitrate+nitrite concentrations during series operation).
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Measurement Date (mm/dd/yyyy)
| CHALLENGES
The Roth Lane system has significant problems with infiltration and inflow (I/I) and rain events
that push flow above about 5 MGD, which require the staff to return to a parallel operational
mode.
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| FUTURE IMPROVEMENTS
The Roth Lane WWTP is currently expanding to accommodate additional flows and loadings
from a nearby municipality. As part of the expansion project, denitrification filters with
methanol addition will be installed to enhance nitrogen removal at the projected design flows
and loadings.
| CONTACT INFORMATION
Diane Fox, Superintendent. Jeffrey Klahre, Operations Supervisor. Hampden Township, Public
Works—Wastewater Division, 4200 Roth Lane, Mechanicsburg, PA 17050. Phone: (717) 761-
7963. Email: DFox@hampdentownship.us; JKIahre@hampdentownship.us.
| OTHER RESOURCES
Hampden Township Wastewater Division:
http://www.hampdentownship.us/township-department/public-works-wastewater-division/
Shawwa, A.R., and D.C. Shope. 2013. Dynamic modeling of cyclic aeration process for
biological nutrient removal. Water Environment and Technology 25(7):30-34.
Schreiber Continuously Sequencing Reactor:
http://www.schreiberwater.com/CSRAeration.shtml
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LAYTON, FLORIDA
SEQUENCING BATCH REACTOR-PROCESS CONTROL MODIFICATIONS
SYSTEM SUMMARY
Official Name: City of Layton Wastewater Treatment Plant (WWTP)
Location: 67711 Overseas Highway, Long Key, FL 33001. Monroe County. Florida Keys
(latitude: 24° 49' 16.5593" N; longitude: 80° 49' 14.4679" W)
Permitted design flow: 0.066 MGD, monthly average
Service area: Approximately 350 EDUs1,
including Long Key State Park
System type: Sequencing batch reactor
(SBR)
Initial year of operation: 2007
Upgrade type: Process control
modifications
Upgrade year of operation: 2009
Permitted effluent nitrogen limit:
12.5 mg/l TN, monthly average; 10 mg/l TN, annual average
Pre- and post-upgrade effluent nitrogen performance: Pre- and post-upgrade TN statistics are
summarized below
Pre-upgrade
Post-upgrade
Influent Total Nitrogen
Average Concentration
89.3
64.1
Effluent Total Nitrogen
Average
Concentration
7.88
3.33
Standard
Deviation
4.26
1.87
Units
mg/l
mg/l
Permitted effluent phosphorus limit: 1.25 mg/l TP, monthly average; 1.0 mg/l TP, annual
average
Pre- and post-upgrade phosphorus performance: 0.58 mg/l TP, average 2007-2013 (no TP
removal improvements were made)
1 EDU = Equivalent Dwelling Unit, which is the approximate number of residences served by the facility.
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| RATIONALE AND DECISION PROCESS
The plant was not consistently meeting permitted effluent TN limits. Therefore, Layton's
approach focused on improving the control of their SBR system to achieve much more
consistent effluent TN concentrations. Based on Florida Keys Aqueduct Authority (FKAA)
experience with other similar SBR systems, operations staff were aware that programming
adjustments to the control system could allow for better control of conditions during the
batch cycle by mixing only for the fill cycle and then cycling blowers on and off as needed to
ensure consistent nitrification-denitrification. Improved controls are supplemented by real-
time dissolved oxygen (DO) and oxidation-reduction potential (ORP) monitors.
[SYSTEM OPTIMIZATION DESCRIPTION
As indicated above, the focus of FKAA was to improve the control of the SBR cycle to maximize
nitrification-denitrification. Biological nitrogen removal is a sequential process, first requiring
aerobic conditions for converting ammonia and organic nitrogen to nitrate (nitrification) and
then anoxic conditions to convert nitrate to harmless dinitrogen gas (denitrification). The
aerobic conditions needed for nitrification can be maintained by actively aerating the mixed
liquor (the contents of the reactor), while anoxic conditions are induced by suspending the
mixed liquor using submerged mixers, with no aeration.
The original SBR wastewater facility was put into operation in 2007 and cost approximately
$5.7 million. Upgrades to the City of Layton WWTP consisted mainly of reprogramming of the
SBR control scheme. The original manufacturer of the SBR did not provide sufficient
operational control over the "fill" and "react" cycles in each batch process to facilitate optimal
nitrification and denitrification.
Each batch starts at bottom
water level (BWL). At BWL, the
tank is at a predefined depth
that is established by the
elevation of the fixed-hood
decanter; this elevation cannot
be adjusted. Next, the fill valve
is opened and raw influent is
pumped into one of the
reactors. Raw influent pumping
is controlled by floats in the
collection system lift stations.
This is important, because it can
control the batch time, which had been targeted to be 4 hours, but could be longer if flows
were insufficient or shorter if there was a hydraulic surge (e.g., from a storm event). Each
batch includes a fill cycle, react cycle, settle cycle, decant/waste activated sludge (WAS) cycle,
and idle cycle.
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Under the original setup, oxic (aeration and mixing) and anoxic (only mixing) timers alternated
while raw influent was pumped in during the fill cycle. This process repeated until a float was
tripped at approximately 13 feet of liquid depth, which triggered the fill valve to close and oxic
(aerated and mixed) and anoxic (just mixed) conditions to alternate for 45-60 minutes,
followed by a settling cycle (no aeration or mixing) for another 45-60 minutes. Then the
decant valve opened and clarified effluent would be decanted down to the BWL while the unit
remained in an idle/fill cycle waiting for the float to trip at 13 feet and the batch process to
repeat.
Process control optimization consisted of modifying the programming to a timed batch rather
than a level batch process. Using a fixed timed batch of 6 hours, operational control of all
360 minutes was implemented to ensure a more consistent effluent. Within the current
6-hour batch are 3 hours for filling and 3 hours for the react, settle, decant, and WAS cycles
(and the idle cycle, if necessary). During the first 180 minutes, mixed fill is employed for
45 minutes. For the remaining 135 minutes, blowers are controlled using on/off timers to
affect almost complete nitrification. During the subsequent react cycle, only mixing is used —
although there is an option to aerate if necessary. The operator can then set the settling cycle
for between 45-60 minutes, followed by decant and idle cycles.
| COSTS AND OTHER IMPACTS
Capital costs: Approximately $53,000 for new online monitoring of DO, ORP, and total
suspended solids (TSS) probes in each SBR. DO and ORP are monitored to quickly determine
the oxidation state during anoxic or aerobic cycles. The TSS probe is used strictly as a time-
saving factor, as a surrogate for laboratory mixed liquor suspended solids (MLSS) analysis.
Operational costs: The biggest change was in sludge hauling and lab testing. Once nutrient
removal was optimized, the plant was able to cut down on laboratory discovery sampling.
They also were able to operate proactively by focusing on dewatering and biosolids removal.
For the City of Layton WWTP, this equated to approximately $12,000-15,000 in savings
(compared with an annual $80,000 budget), not including labor.
Technical assistance received or needed: Significant training was needed. Layton used the
not-for-profit Florida Rural Water Association (FRWA) in a joint effort with their technical staff
on specific nutrient removal training and spends approximately $4,000 annually on
microscopy and microbiological training through a company called Environmental Leverage.
From a regulatory standpoint, operators at the Layton WWTP are only required to have a
Florida Class C WWTP operator's license (a Class C-licensed operator is required to know only
basic wastewater treatment techniques; nutrient removal is not introduced until Class B
licensing). The FKAA recognized early that it would be necessary to train operators to meet the
treatment standards required by their permits. FKAA currently has all in-house trained
operators except one who was trained outside FKAA.
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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PERFORMANCE
Pre- and post-upgrade TN statistics are summarized on page 1. A graphical illustration of
effluent TN and TP concentrations versus time and a summary of average annual effluent
concentrations for nitrogen species are provided below (upgrades were implemented in early
2009).
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Date (m/d/yyyy)
2008
TN
TON
TKN
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29
76
11
36
39
2009
4.29
1.36
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2.38
2010
4.37
1.50
2.00
0.50
2.31
2011
2.72
1.51
1.73
0.30
0.87
2012
3.01
1.53
1.71
0.18
1.27
2013
3.44
1.48
1.89
0.42
1.35
mg/l,
mg/l,
mg/l,
mg/l,
mg/l,
average
average
average
average
average
Ugg
effluent
effluent
effluent
effluent
effluent
concentration
concentration
concentration
concentration
concentration
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| FUTURE IMPROVEMENTS
TN removal could be improved even further by outfitting the blowers with variable frequency
drives (VFD) coupled with the DO control system. However, effluent quality is currently
excellent and the costs associated with further improvements are not justified at this time.
| CONTACT INFORMATION
Tom Pfiester. Florida Keys Aqueduct Authority, 3375 Overseas Highway, Marathon, FL 33050.
Phone: (305) 481-2015. Email: tpfiester@fkaa.com.
| OTHER RESOURCES
City of Layton: http://www.citvoflayton.com/
Keys Wastewater Plan: http://www.monroecountv-fl.gov/DocumentCenter/Home/View/478
Florida Keys Aqueduct Authority: http://www.fkaa.com/
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MONTROSE, COLORADO
ACTIVATED SLUDGE —AERATION CONTROL
[SYSTEM SUMMARY
Official Name: Montrose Wastewater
Treatment Plant (WWTP)
Location: 3315 North Townsend
Avenue, Montrose, CO 81402 (latitude:
38° 30' 41.44" N; longitude: 107° 55'
11.74" W)
Permitted design flow: 4.32 MGD
(expanded from 2.88 MGD in 2008)
Service area: Shown at right
System type: Extended aeration
activated sludge/oxidation ditch
Initial year of operation: 1984;
expansion in 2008
Upgrade type: Aeration control
Upgrade year of operation: 1997
Permitted effluent nitrogen limit:
Total inorganic nitrogen (TIN) —
report only (limits expected after
2020)
Pre- and post-upgrade effluent
nitrogen performance:
14.7 ± 4.3 mg/l TIN, post-upgrade
(2012-2014)
Permitted effluent phosphorus limit: N/A
Pre- and post-upgrade phosphorus performance: 3.0 ± 0.8 mg/l TP, post-upgrade
(2012-2014)
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DECISION PROCESS
The project was mainly initiated as a way to cut energy costs; however, improved nitrogen
removal has proven to be a major benefit.
[SYSTEM OPTIMIZATION DESCRIPTION
The extended aeration process, which is used at the Montrose facility, uses low organic
loading and high detention time. Extended aeration can use aeration tanks of various shapes
including square, rectangular, and "endless" ditch, often called an oxidation ditch. Extended
aeration systems use high mean cell residence times (MCRTs), also known as sludge age,
which typically range from about 15-30 days. However, certain conditions might require that
values outside that range be used. When the higher MCRTs are used, autotrophic bacteria
feed on inorganic matter and grow slowly on compounds such as ammonia (NHs) and convert
it first to nitrite (NC>2~), then to nitrate (NOs). Almost all extended aeration systems nitrify,
especially in summer, whether intended or not.
On/off aeration has been used since initial experiments with activated sludge, and is
commonly used in package treatment plants today. However, many package plant systems are
destined to fail because they are operated with short periods of "on" and short periods of
"off"—such as 15 minutes on and 15 minutes off. Half hour on/half hour off is also common,
as are similar variations. However, when used properly, on/off aeration can be very effective,
especially when the air is left off long enough to affect complete denitrification.
At the time of the optimization effort, the Montrose WWTP operated two ditches, each with
1.4 MG volume, and two 68-foot diameter clarifiers. Aeration was provided by six brush
aerators each powered by 75-hp motors. The facility also has an aerobic sludge digestion
system with large-bubble diffused aeration system.
/75 hp Brush Aerators
\
Pumps
Influent
Bar Screen
Clarifiers
Oxidation Ditches
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The oxidation ditch had been well-operated
and controlled so that, from January 1996 to
November 1997, the monthly average effluent
BOD was 2.55 mg/l, TSS was 3.75 mg/l, and NH3
was 0.34 mg/l. The operators tried to maintain
a dissolved oxygen (DO) level between
3.0-4.0 mg/l based on recommendations from
consultants and the literature, which indicated
that relatively high DO levels were required for
nitrification. However, even at the high DOs, it
appears that the oxidation ditch still allowed
denitrification to occur in the ditch because of the good effluent suspended solids
concentrations. High clarifier denitrification rates would have caused increased suspended
solids concentrations in the effluent.
Indeed, oxidation ditches with brush aerators
are known for nitrifying and denitrifying in the
same tank by carrying a low DO down the tank,
ahead of the next brush aerator. As the mixed
liquor travels around the tank, it provides for
sections with zero DO ahead of the next brush.
However, maintaining low DO sections of the
reactor would be more difficult if the DO
following the brushes ranged as high as
3-4 mg/l. So, in November 1997, staff began
turning off the brush aerator nearest to the
influent to see if power could be saved by carrying a lower DO around the entire tank and
maintaining an anoxic section all the way to the next brush aerator. The ditches are currently
being operated at a DO of less than 1 mg/l.
Aerobic digesters can also play an important part in the nitrification/denitrification process.
Most aerobic digesters are aerated 24 hours per day unless the operator is trying to decant a
clear supernatant during the thickening process. Continuous aeration can completely oxidize
the available carbon as well as nitrify all of the available nitrogen. Thus, high DO is common as
is very low pH in the range of 4-5 units. Even if the air is turned off for a period of time during
decanting, there is so little available oxygen demand that denitrification is minimal. Starting in
January 1997, a daily off-period was provided from about 9:00 a.m. to 3:00 p.m. daily rather
than just during decanting.
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| COSTS AND OTHER IMPACTS
Capital costs: None.
Operational costs: The combined data indicates that on/off aeration in the oxidation ditch and
in the aerobic digesters accounted for about 29,000 kWh/month and about $2,825/month
savings. Pre-optimization energy costs for the wastewater system averaged $4,161 for
60,968 kWh/month to process 1.66 MGD; post-optimization costs averaged $2,979 for
53,810 kWh/month to process 1.54 MGD (1995-1999 costs).
Technical assistance received or needed: Consultants advised the Montrose staff on energy
and nutrient optimization strategies.
| PERFORMANCE
The plant staff has always produced an effluent with extremely good quality. Prior to on/off
being initiated, the effluent BOD, TSS, and ammonia were 2.55 mg/L, 3.75 mg/L, and
0.34 mg/L, respectively.
Results after on/off was initiated showed substantial improvement in TSS and ammonia. The
TSS dropped by 36 percent while the ammonia dropped by almost 68 percent. Effluent nitrate
concentrations varied from below 5 mg/l to approximately 10 mg/l. Recent time series data
are plotted in the chart below.
The operators had theorized that on/off aeration during digestion would not only save money,
but would also reduce the amount of nitrate returned and increase the pH of the supernatant
returned to the aeration tank. The nitrate concentration in the supernatant can easily
approach 30 mg/L or more at the beginning of the off cycle. No actual data is available to
support the observation that digested sludge quality has remained as good, or been better
than before relative to sludge dewatering. However, there have been no noticeable problems
with sludge dewatering. Nor have there been problems meeting Class B biosolids regulations
as the specific oxygen uptake rate (SOUR) typically runs 0.7-0.8, or about half of that allowed.
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centrati
t-»
U"i
Y<
A
A
r
-*
0 -
9/14/2011
••*>«••»-!
••«• ***••• ^C» ••••>*
4/1/2012
10/18/2012
5/6/2013 11/22/2013
Date (m/d/vyyy)
A TIN BTN «TP -"NH3
-»»•
6/10/2014
12/27/2014
7/15/2015
FUTURE IMPROVEMENTS
Since the original optimization efforts, the only improvements made at the Montrose WWTP
have been the variable frequency drives (VFDs) installed on the rotors. VFDs are operational in
two of the ditches now, and the third should be running by the end of 2015. The cost for the
VFDs was about $30,000 per ditch. The plant is also installing DO, oxidation-reduction
potential, and TSS probes in the ditches. This project should also be completed by the end of
the year.
By experimenting with rotor speeds and rotor submergence depths, it has been determined
that an anticipated effluent limit of 15 mg/l TN could be met some of the time, but not all of
the time. Upgrades would need to be made to meet the anticipated phosphorus limit of
1.0 mg/l.
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CONTACT INFORMATION
Allen Coriell, Superintendent, City of Montrose Wastewater Treatment Plant. Phone: (970)
240-1452. Email: acoriell@ci.montrose.co.us.
OTHER RESOURCES
City of Montrose Wastewater Treatment:
http://www.citvofmontrose.org/160/Wastewater-Treatment
Schuyler, R.G., A. Coriell, and M. Carrano. On/Off Aeration Energy Savings.
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TAMPA, FLORIDA
SEPARATE STAGE NITRIFICATION ACTIVATED SLUDGE-OPERATIONAL
MODIFICATIONS
SYSTEM SUMMARY
Official Name: Howard F. Curren Advanced Wastewater Treatment Plant
Location: 2400 Guy North Verger Boulevard, Tampa, FL 33605 (latitude: 27° 55'33"N;
longitude: 82° 26'06"W)
Permitted design flow: 96 MGD annual average
(actual flow: 57 MGD, annual average)
Service area: City of Tampa (2010 population of
335,709)
System type: Multi-stage:
Stage 1 High-purity oxygen activated sludge for
carbonaceous BOD removal
Stage 2 Dissolved aeration activated sludge for
nitrification
Stage 3 Biological filters for denitrification
Initial year of operation: 1977
Upgrade type: Modified operational strategy
Upgrade year of operation: Improved operation initiated July 1, 2013
Permitted effluent nitrogen limit: 3.0 mg/LTN
annual average, 3.75 mg/LTN monthly average,
4.5 mg/L TN weekly average, 6.0 mg/L TN single
sample
Pre- and post-upgrade effluent nitrogen
performance: Stage 2 bioreactor effluent:
18.62 mg/L 10-month average pre-upgrade;
13.82 mg/L 10-month average post-upgrade;
denitrification filter effluent remained below
3.0 mg/L annual average throughout
Permitted effluent phosphorus limit: None
Pre- and post-upgrade phosphorus performance: N/A
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DECISION PROCESS
A 2013 preliminary engineering study for replacement of the aeration system at the
nitrification tanks included modeling the tanks in several operating modes. Some of the
modeled modes considered the excess capacity available at the current flow compared to the
design flow. A key observation was that complete nitrification could be achieved with less
aerobic volume than was being used. By temporarily operating a portion of the nitrification
tanks in a low-dissolved oxygen (DO) or no-DO mode, anoxic conditions could be generated
and some denitrification achieved. Any denitrification achieved at the nitrification tanks
decreases the nitrogen removal required at the denitrification filters, allowing the feed of
methanol as supplemental carbon source for denitrification to be decreased.
SYSTEM OPTIMIZATION DESCRIPTION
Upgrade was through operational changes only. Aeration
was decreased in initial zones (1 and 2 of 6) in each of the
three operating nitrification tanks to create an initial low-
DO region for simultaneous nitrification and denitrification.
DO concentrations were decreased from 2.5 mg/L to less
than 0.5 mg/L, average.
The nitrate content of return activated sludge (RAS) was
low so internal recycle of nitrates into the low-DO region
was accomplished by opening a gate to allow a portion of
the nitrification tanks effluent (approximately 35 percent)
to flow into the stage 2 influent pump station. Also, a
portion of the plant influent (approximately 20 percent) was diverted around the stage 1
activated sludge process to provide influent BOD as a carbon source for denitrification.
A 4 mg/L decrease in nitrate nitrogen out of the second stage (pre-optimization average of
17 mg/L NOs-N lowered to 13 mg/L NOs-N) and into the denitrification filters allowed the
operators to decrease the feed of methanol as supplemental carbon source by approximately
31 percent. The decrease in methanol feed resulted in chemical savings and decreased sludge
production at the denitrification filters.
COSTS AND OTHER IMPACTS
Capital costs: None.
Operational costs: Internal recycle pumping increased energy required at stage 2 influent
pump station by approximately 50 hp (37.3 KW). At $0.07/KWhr, the additional energy cost is
approximately $22,900. The methanol feed was decreased by an average of 1,487 gallons per
day for an annual chemical savings of approximately $542,800. Net savings is approximately
$519,900.
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Technical assistance received or needed: A consultant was hired to recommend modifications
to improve nutrient removal.
PERFORMANCE
Pre- and post-upgrade total nitrogen statistics are summarized in the chart below.
S 25.00
u
c
S 20.00
Pre-Upgrade
Post-Upgrade
a IE j- * + r"» •». » * * * "
N03-N • NH3-N
FUTURE IMPROVEMENTS
Installation of a new aeration in the nitrification tanks will allow the fourth nitrification tank to
be placed into service, thereby providing additional bioreactor volume that can be converted
to anoxic or low-DO conditions to increase the nitrogen removal capacity at stage 2. Low
power mixers will be added at zones 1 and 2 of each nitrification tank to maintain solids in
suspension without aeration in those zones, allowing aeration in zones 1 and 2 to be
eliminated entirely to approach true anoxic conditions for improved denitrification. The new
aeration system will include controls to provide the flexibility for cyclical or on/off aeration in
any of the six zones of each nitrification tank to maximize stage 2 denitrification. Dedicated
recycle pumps will be added in zone 6 of each nitrification tank to provide more efficient
(lower head) internal recycle than is achieved using the stage 2 influent pump station for
recycle.
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| CONTACT INFORMATION
Rory Jones, Wastewater Design, 306 East Jackson Street 6N, Tampa, FL 33602. Phone: (813)
274-7045. Email: Rory.Jones@ci.tampa.fi.us.
| OTHER RESOURCES
City of Tampa: http://www.tampagov.net/wastewater
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TITUSVILLE, FLORIDA
A2/C> WITH SECONDARY ANOXIC AND WETLAND DISCHARGE
[SYSTEM SUMMARY
Official Name: City of Titusville Blue Heron
Water Reclamation Facility (WRF) and Wetland
Location: 4800 Deep Marsh Road, Titusville, FL
32780 (latitude: 28° 32' 58" N; longitude: 80° 51'
41" W)
Permitted design flow: 6.75 MGD, average daily
flow
Service area: City of Titusville
System type: Anaerobic/anoxic/oxic (A2/0)
process with secondary anoxic zones
Initial year of operation: 1996
Upgrade type: Process optimization and
discharge into constructed/restored wetland
(part of original design)
Upgrade year of operation: 1996
Permitted effluent nitrogen limit: 6 mg/l TN,
annual average from plant; 1.6 mg/l TN, annual
average from wetland
Pre- and post-upgrade effluent nitrogen
performance: Wetland influent and effluent TN
statistics are summarized below for the years
2009 through 2013
Permitted effluent phosphorus limit: 1.75 mg/l
TP, annual average from plant; 0.16 mg/l TP,
annual average from wetland
Pre- and post-upgrade phosphorus performance: Wetland influent and effluent total
phosphorus statistics are summarized below for the years 2009 through 2013
CITY OF
TITUSVII.LI:
BLUE
HERON
WATER
RECLAMATION
f.MTUTIkS
& WETLANDS
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Wetland Influent Concentration
Total Nitrogen
Total
Phosphorus
Average
Concentration
5.67
0.77
Standard
Deviation
2.28
0.66
Wetland Effluent Concentration
Average
Concentration
0.94
0.04
Standard
Deviation
0.41
0.03
Units
mg/l
mg/l
| RATIONALE AND DECISION PROCESS
The Blue Heron Water Reclamation Facility was originally designed to achieve Florida Class III
Surface Water requirements (i.e., Fish Consumption, Recreation, Propagation and
Maintenance of a Healthy, Well-Balanced Population of Fish and Wildlife) through a
combination of biological nutrient removal (BNR), water reuse, and, for water not reused,
treatment using constructed wetlands prior to discharge into the Addison Canal, which is a
tributary of the nutrient-impaired St. Johns River estuary.
The original BNR process was installed in 1996 and initially optimized by plant staff in
collaboration with the technology vendor through 1997. The treatment wetland was a feature
of the initial plant design, and the reuse system was put into operation several years later. This
case study discusses initial BNR optimization efforts, as well as the treatment wetland, which
further reduces nutrients prior to the receiving water.
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| SYSTEM OPTIMIZATION AND PERFORMANCE DESCRIPTION
Initial BNR Process Optimization
During start-up and initial process optimization efforts, modifications were successfully
applied to the A2/0 process to consistently meet stringent permitted effluent limits of 5 mg/l
BOD, 5 mg/l TSS, 3 mg/l TN, and 1 mg/l TP without the addition of metal salts for precipitating
phosphorus or a supplemental carbon source to facilitate denitrification. These initial
optimization improvements included the return activated sludge (RAS) denitrification stage
modification for improved phosphorus uptake and removal, and the RAS bleed-off
improvement for increasing denitrification within the secondary anoxic zones.
The Blue Heron WRF utilizes a 2-train A2/0 (anaerobic/anoxic/oxic) process, which is
configured with anaerobic selectors, primary anoxic zones, oxidation ditches equipped with
mechanical brush aerators, secondary anoxic zones, and a reaeration basin followed by
secondary clarifiers. The unaerated stages of the process (the anaerobic and anoxic stages) are
each equipped with submersible mixers to suspend the mixed liquor and facilitate biological
processes. Each oxidation ditch is also equipped with submersible wall pumps that return
nitrified mixed liquor to the primary anoxic stages where denitrification occurs.
The RAS Denitrification Stage modification—also known as the Block and Hong Process-
refers to the strategy of staging the introduction of RAS and influent into the anaerobic
selector. As depicted in the diagram below, RAS only is introduced to the first stage of the
anaerobic selector, while the denitrified RAS and influent are mixed in the second and
subsequent stages. This modification ensures that ideal anaerobic conditions are maintained
and that the availability of substrate for the phosphorus-removing organisms within the
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anaerobic selector is maximized in the subsequent stages. In most biological phosphorus
removal processes, sludge collected in the secondary clarifiers and containing nitrates is
combined with influent wastewater and fed into a selector. The introduction of nitrates from
the RAS into the selector compromises the phosphorus removal process by inhibiting the
metabolism of organisms responsible for enhanced biological phosphorus removal by allowing
other organisms to consume soluble carbon substrates (i.e., BOD), thereby lowering the
soluble BOD:P ratio below optimal levels and compromising biological phosphorus removal.
Influent
Wastewater
Anaerobic
Anoxic
Internal Recycle
Clarifier
Effluent
RAS Bleed off
Into Secondary Anoxic Zone
Return Activated Sludge
Phosphorous Rich Waste Sludge
There were several periods when the effluent phosphorus concentration was unusually high.
When the plant was initially started up, the influent pumping scheme delivered influent to the
biological process in surges. For example, the influent pumps might have fed the process for 5
minutes at a relatively high flow rate and then been inactive for a one-half hour or more. This
"all-or-nothing" feeding sequence appeared to result in hydraulic surges through the
anaerobic selector as well as a discontinuous organic loading to the organisms responsible for
taking up high levels of phosphorus. The amount of phosphorus uptake and removal was
compromised, as documented in the effluent phosphorus measurements during plant start-up
(August and September 1996). In October 1996, the influent pumping scheme was modified to
minimize influent flow surges, which greatly improved the performance of the phosphorus
removal system. Influent wastewater was directed into the Blue Heron WRF by gravity flow
from the South Master Pump Station, resulting in a relatively consistent influent flow rate to
the plant. The pumps in this basin activate only when the influent flow is high and the water
level within the station exceeds a preset limit.
In the summer of 1997, the return activated sludge (RAS) bleed-off improvement for nitrogen
removal was being tested and optimized. For several periods during that time, too little RAS
was being returned to the first stage of the anaerobic selector. As a result, the amount of
phosphorus released in the anaerobic selector was not sufficient and did not adequately
promote enhanced phosphorus uptake in the oxic stage of the process. At high RAS bleed-off
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rates and, therefore, correspondingly lower RAS return rates to the head of the biological
plant, phosphorus uptake was temporarily compromised.
12.00-
6.00
—.—Influent TP
D Effluent TF
to
M
O
1
i
10.00
8.00
5.00
2.00
0.00
5/96
5/23/964
12/1/96
Influent Surging
3/11
/97
RAS
6/19/97
Bleed-off
1 9/27/97
Testing)
1/5/98
4/15/98 7/24
0.00
Date
The treatment process at the Blue Heron WRF includes secondary anoxic zones for
supplemental denitrification, required to meet the stringent effluent TN limit of 3 mg/l as N.
The efficiency of typical secondary anoxic reactors is often low due to the lack of a readily
degradable carbon source in the mixed liquor following oxic treatment. The RAS bleed-off
improvement allows for a portion of the RAS to be introduced directly into the secondary
anoxic zones, as indicated in the flow schematic above, to increase the extent and rate of
denitrification. Unique to the A2/0 process, the RAS bleed-off improvement increases the
biomass and degradable carbon to stimulate denitrification.
Influent and effluent nitrogen profiles for the Blue Heron WRF indicate that denitrification via
the primary and secondary anoxic zones has been effective and, after the start-up period,
consistently averaged below 3 mg/L (monthly average basis). In June 1997, the RAS bleed-off
system was being tested and the resulting average effluent TN exceeded 3 mg/l. Once
optimized, the RAS bleed-off system has resulted in lower effluent TN than did the process
without the RAS bleed-off.
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tal Nitrogen (mg/l)
N) hO CO CO 4^ 4^ Ul
O (Jl O (Jl O Ul O
H 15
3 10
;C
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Blue Heron Wetland Treatment System
The Blue Heron Wetland Treatment System was designed so that any effluent not used in the
reclaimed water distribution system can be diverted to the wetland area. The man-made
wetlands, including berms and upland area, cover almost 300 acres and can process around
6.75 million gallons a day (MGD) of wastewater: 4.0 MGD of wastewater coming from the
city's Blue Heron WRF and 2.75 MGD coming from the Osprey/North facility. The wetlands are
designed for influent characteristics of 5 mg/l CBOD, 5 mg/LTSS, 6 mg/LTN, and 1.75 mg/LTP.
The Blue Heron Wetland Treatment System was designed as a flow-through system that uses
visible and microscopic aquatic plants (macrophytes and phytoplankton) to remove nutrients
from the treatment plant effluent. The wetland is divided into seven cells: one pond cell, three
deep (2'-4') marsh cells, and three shallow marsh cells. The cells are separated by earthen
berms. Water flows by gravity through the seven cells to a collection system along the south
side of the site, which drains to the Addison Canal. Flows from the wetland discharge to the
Addison Canal, which is a tributary of the St. Johns River.
,
CELL No, 3
DEEP MARSH
44. B6 Ac.
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CELL No. 2
DEEP MARSH
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DEEP MARSH
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PRDCESS FLOW
The berms are designed to maintain the water levels so that target vegetation species receive
adequate water supply. The berms also provide storage for a 100-year 24-hour storm event.
The average detention time for water in the wetland is approximately 60 days. The minimum
detention time is 14 days. Removing or adding flash boards to the weir structures located in
each cell controls water depth and internal flow routing.
The Blue Heron wetland area is open to the public and a popular site for bird watching and
photography enthusiasts and is listed on the Great Florida Birding Trail. It is often included as a
field trip for the renowned annual Space Coast Birding and Wildlife festival. The total distance
around the perimeter is 2.8 miles. The predominant plant species found in the wetland include
Arrowhead, Bladderwort, Bulrushes, Duckweed, Naiad, Pickerelweed, Sand Cordgrass,
Seashore Paspalum, Smartweed, Spikerush, and Watergrass.
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Some of the birds observed at the wetland include:
• Blue Heron
• Great, Common, and Snowy Egret
• Moore Hen
• Mourning Dove
• Green Heron
• Anhinga
• Pied-Billed Grebe
• Great Blue Heron
• Red-Shouldered Hawk
White Ibis
Mottled Duck
Purple Gallinule
Yellow-and Black-Crowned Night
Heron
Northern Shrike
Northern Mockingbird
Red-Winged Blackbird
Common Crackle
Pre- and post-upgrade TN and TP statistics are summarized on page 1. Detailed nutrient
removal data for the wetlands discharge are presented in the graphs below.
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Jul-09 Jan-10 Aug-lO Feb-11 Sep-11 Apr-12 Oct-12 May~13 Mov-13 Jun-14
Month-Year
§
I 2.00
• EfflyentTP
O.OO - '=
Jun-08 Oec-08 Joi-09
Jan-10 Aug-10 Feb-11 Sep-11 Apr-12 Oct-12 May-13 Nov-13 Jun-14
Month-Year
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| COSTS AND OTHER IMPACTS
Capital costs: $2.24 million (1996 costs) for wetland.
Operational costs: Approximately $45,000 annually for the wetlands, which includes
monitoring responsibilities (water quality sampling, vegetation monitoring, soil sampling,
detritus sampling, and denitrification studies), operational responsibilities (inspection, water-
level adjustment, vegetation maintenance, evapotranspiration monitoring, annual reporting,
permitting assistance, public education), and maintenance responsibilities (weir structures,
control of invasive plants).
Technical assistance received or needed: Process supplier provided start-up and optimization
services for the BNR system. No particular technical assistance has been required for the
wetland.
| CHALLENGES
If the system could be designed over again, plant staff would like to have the ability to
recirculate the water within the wetland (back to front) to achieve an even higher level of
nutrient removal.
| FUTURE IMPROVEMENTS
Aeration upgrade for the Blue Heron WRF treatment unit. No plans for wetland system
improvements.
| CONTACT INFORMATION
Matt Hixson. City of Titusville Water Reclamation, 4800 Deep Marsh Road, Titusville, FL 32780.
Phone: (321) 567-3891. Fax: (321) 383-5646. Email: matt.hixson@Titusville.com.
| OTHER RESOURCES
City of Titusville: http://www.titusville.com/
Domestic wastewater wetland sites in Florida:
http://www.dep.state.fl.us/water/wastewater/dom/wetsites.htm
Blue Heron Wetlands: http://www.dep.state.fl.us/water/wastewater/dom/wetheron.htm
D'Amato, V.A., M. Hixson, and S.N. Hong. 1998. Environmental Protection through Innovative
Wastewater and Sludge Treatment Strategies in Florida. In Proceedings of Water Environment
Federation 71st Annual Conference and Exposition, Vol. 1, Pt. II. Municipal Wastewater
Treatment, Session 2. Biological Nutrient Removal, Orlando, Florida, October 1998, pp. 487-
498.
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VICTOR VALLEY, CALIFORNIA
ACTIVATED SLUDGE-PROCESS CONTROL AND MECHANICAL MODIFICATIONS
SYSTEM SUMMARY
Official Name: Victor Valley Wastewater Reclamation Authority (WRA)
Location: 20111 Shay Road, Victorville, CA 92394 (latitude: 34° 37' 14.76"; longitude: 117° 21'
26.47" W)
Permitted design flow: 13.8 (originally 18) MGD
Service area: Victor Valley WRA's four member agencies: the Town of Apple Valley, the City of
Victorville, the City of Hesperia, and San Bernardino County Service areas 42 and 64 (including
Spring Valley Lake and Oro Grande). Population of approximately 400,000.
System type: Conventional activated sludge
Initial year of operation: 1981
Upgrade type: Improved process controls and
mechanical modifications
Upgrade year of operation: 2007-2008 (additional
upgrades in 2013)
Permitted effluent nitrogen limit: 10.3 mg/l TN,
monthly average
Pre- and post-upgrade effluent nitrogen
performance: 8.93 mg/l TN average, pre-upgrade;
6.83 mg/l TN average, post-upgrade
Permitted effluent phosphorus limit: N/A
Pre- and post-upgrade phosphorus performance:
N/A
Average Average Standard
Concentration Concentration Deviation
Average Standard
Concentration Deviation
Units
Ammonia
Nitrite
Nitrate
TKN
TN
26.6
39.2
0.84
0.54
7.55
1.43
8.93
0.91
0.40
1.45
0.84
1.06
0.26
0.22
5.30
1.28
6.83
0.37
0.13
1.72
0.57
1.58
mg/l
mg/l
mg/l
mg/l
mg/l
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| DECISION PROCESS
Around 2006, Victor Valley WRA's regulatory authority, the Lahontan Regional Water Quality
Control Board, suggested a 6.0 mg/l TN effluent discharge standard for the facility. A
subsequent feasibility report recommended the addition of a treatment train at an estimated
capital cost of about $80 million with no increase in treatment capacity. At that time, a
director of operations was hired to determine how the WRA could improve the efficiency of
their existing operation and address about 10 years of deferred maintenance.
| SYSTEM OPTIMIZATION DESCRIPTION
The Victor Valley WRA Shay Road plant includes two mechanically cleaned 5mm X 20 mm bar
screens and aerated grit removal with cyclone separators for headworks. From the
headworks, wastewater flows through primary clarifiers (four parallel trains) and then onto
secondary treatment (with the option to use flow equalization to mitigate wet weather flows
and diurnal peaks). Eight secondary treatment basins are aerated by centrifugal blowers. Six
secondary clarifiers are used and waste activated sludge (WAS) is sent to dissolved air
flotation thickeners and then to anaerobic digesters. Secondary effluent is typically treated
with alum and polymer prior to tertiary filtration in either traveling bridge or moving bed
filters, followed by chlorine disinfection and dechlorination using bisulfite. The disinfected and
dechlorinated effluent is either reclaimed for irrigation (at the plant or nearby golf course),
industrial process water, or other beneficial purposes, or it is discharged directly to the
Mojave River. Additionally, secondary effluent can be discharged directly to any one of six
percolation ponds, which have a combined surface area of about 13 acres.
In the 2007-2008 time frame, the WRA performed upgrades to primary clarifiers including
improved grease removal, aeration basin rebuilds (replacing existing soft diffusers),
installation of high-speed turbo blowers, and a multitude of operational changes, including
switching from an extended aeration activated sludge operation to conventional activated
sludge and adding recirculation pumps, dissolved oxygen (DO) probes, and oxidation-
reduction potential (ORP) sensors. This first phase of optimization cost approximately
$1.1 million over 3 years and allowed the WRA to be able to meet their new nitrogen limits of
8.3 mg/l nitrate (revised from the originally suggested 6.0 mg/l TN).
Subsequent operational and process control modifications included optimizing wasting rates
by targeting a sludge volume index (SVI), which improved settling and process stability. Staff
also continued to upgrade the monitoring system, including integrating DO and ORP sensors
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Case Studies on Implementing Low-Cost Modifications to Improve Nutrient Reduction at Wastewater
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into their supervisory control and data acquisition (SCADA) system. They also converted from
engine-driven blowers to electric blowers for energy efficiency.
The Victor Valley VWRA is currently into the third phase of process optimization, which
includes installing membrane diffusers and rebuilding the air distribution system. They are
also rebuilding their existing aeration basins to allow for better control of aeration/redox
conditions using baffle walls to facilitate tapered aeration. Their current effluent discharge
limits are 10.3 mg/ITN.
In summary, plant staff currently monitor and control DO, ORP, alkalinity, and sludge age
(typically 8-15 days), which varies based on the time of year and the temperature of the
water. The facility's current operation uses a process referred to as "plug-flow extended
aeration", although the plant is designed so the operators can also use step-feed, contact
stabilization, or conventional aeration treatment. Process objectives include:
• Using simultaneous nitrification and denitrification in the aeration zones for nitrate
removal. This is achieved by profiling the DO concentration in the aerobic zones of the
aeration basins, which is necessary as the anoxic recycle pump capacity is limited and
the anoxic zones do not have spare volumetric capacity to deal with much more
recycle.
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• Operating the facility at the minimum sludge age to achieve both nitrification and
denitrification. This limits the biomass carried in the aeration basins and, therefore,
reduces the solids load to the clarifiers. The most critical process conditions are
maximum month loadings during winter conditions, which result in decreased aerobic
solids retention time (SRT) values that make nitrification during winter months the
controlling factor. Simulation modeling indicates that the current configuration of
aeration basin volume and DO profile will continue to result in compliant effluent as
flow increases provided a minimum sludge age of around 7 days is maintained.
Maintaining a 7-day SRT as flows increase requires that the total reactor biomass be
increased proportionally. That increase in biomass can be accomplished by raising the
mixed liquor suspended solids (MLSS) concentration or by adding additional reactor
volume. Although SRT can be increased by raising the MLSS concentration, doing so
results in increased solids loading on the secondary clarifiers. When a clarifier is loaded
beyond capacity, the TSS concentration from the clarifiers to the downstream filters
can increase to a very high level, which will result in plugging up the filters.
| COSTS AND OTHER IMPACTS
Capital costs: Phases 1 and 2: approximately $1.1 million over 3 years. Phase 3 (ongoing):
approximately $1.2 million.
Operational costs: Operational cost-savings of approximately 10 percent.
Other implications: Process changes caused some loss in hydraulic capacity, from the original
rating of 18 MGD to about 14 MGD now.
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| PERFORMANCE
Pre- and post-upgrade TN and T statistics are summarized in the "System Summary. Time
series data are presented in the chart below.
.a
1
Optimization period 2007-2008
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Measurement Date (mm/dd/yy)
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Nitrate concentrations in ground water monitoring wells continue to fall with less loading to
the percolation ponds (see the figure below).
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FUTURE IMPROVEMENTS
Into phase 3 now, as described above, which includes adding high-speed turbo blowers,
installing membrane diffusers, and rebuilding the air distribution system. They are also
rebuilding the aeration basins to control loading/time of year with baffle walls to facilitate
tapered aeration.
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| CONTACT INFORMATION
Logan Olds, General Manager, Victor Valley Wastewater Reclamation Authority. Phone: (760)
246-8638. Email: lolds@vvwra.com.
Gilbert Perez, Director of Operations, Victor Valley Wastewater Reclamation Authority. Email:
gperez@vvwra.com.
| OTHER RESOURCES
Victor Valley Wastewater Reclamation Authority: http://vvwra.com/
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WOLFEBORO, NEW HAMPSHIRE
EXTENDED AERATION ACTIVATED SLUDGE —CYCLIC AERATION
[SYSTEM SUMMARY
Official Name: Wolfeboro Wastewater Treatment Facility (WWTF)
Location: 46 Filter Bed Road, Wolfeboro, NH 03894 (latitude: 43° 35' 33" N; longitude: 71° 13'
06" W)
Permitted design flow: 0.6 MGD
Service area: Town of Wolfeboro,
population of approx. 6,000
System type: Activated sludge
(extended aeration)
Initial year of operation: 1975
Upgrade type: Cyclic aeration
Upgrade year of operation: 2007
Permitted effluent nitrogen
limit: 10 mg/l TN in rapid
infiltration basin monitoring wells
Pre- and post-upgrade pond
effluent nitrogen performance:
6.32 mg/l TN pre-upgrade;
1.97 mg/l TN post-upgrade
Permitted effluent phosphorus limit: N/A
Pre- and post-upgrade phosphorus performance: 0.71 mg/l (no upgrades were made to TP
removal processes)
| RATIONALE AND DECISION PROCESS
The Wolfeboro WWTF was initially built in the 1970s as a temporary facility to replace a
primitive system that discharged to a tributary of Lake Winnipesaukee. The temporary plant
was built as an interim measure until the Winnipesaukee River Basin Program (WRBP) could
be planned and implemented. The WRBP is a regional treatment system that collects
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wastewater from many communities on the western side of the lake. Wolfeboro on the
eastern side of the lake is not yet served.
In 1975, the extended aeration activated sludge WWTF initially treated an average daily flow
of approximately 200,000 gallons per day (gpd) in the summer and 100,000 gpd in the winter.
Initially, the treated effluent discharged to Front Bay. A land application, ground water
recharge effluent spray irrigation system was brought online in 1978. The effluent spray
system initially consisted of a 93-million-gallon storage reservoir (which accepts effluent flow
from the WWTF) and approximately 140 acres of land divided into five spray fields.
In the early 2000s, the WWTF was approaching its 30-year anticipated lifespan and the
summer resort town was under threat of an administrative order due to limited effluent
disposal capacity (winter storage limitations and spray capacity) and a noncompliant sludge
composting area (that could not meet new siting/permitting requirements). In general, the
aging WWTF needed significant equipment upgrades to meet New Hampshire Department of
Environmental Services (NHDES) requirements, which were eventually negotiated in an
administrative order by consent (AOC) issued in the early 2000s.
The AOC outlined actions needed to improve effluent management and address the residuals
disposal issues. Wolfeboro commissioned several facility and disposal option evaluations. The
proposed solutions to the WWTF and effluent disposal issues led to a new discharge permit
with more stringent limits on total nitrogen (10 mg/l) and ammonia (5 mg/l) and triggers for
actions to improve overall plant performance. The action levels tied the town to the potential
need for a major WWTF upgrade. In December 2007, the town's wastewater consultant issued
a Basis of Design Report that recommended a major upgrade that included a new sequencing
batch reactor (SBR) at a cost of $15 million.
Wolfeboro needed to determine whether a new plant was inevitable or it was feasible to
rehabilitate the existing WWTF in a low-cost manner to comply with the proposed permit
limits and extend its useful life another 10-20 years. The town also chose to construct rapid
infiltration basins (RIBs) to address the effluent disposal capacity issues. Because of the cost of
the effluent disposal project (approximately $7 million), the town wanted to explore low-
capital cost WWTP upgrades that would help avoid or delay the costs of a major capital
upgrade anticipated to meet the new permit limits. The use of cyclic aeration was chosen as
the alternative method.
| SYSTEM OPTIMIZATION DESCRIPTION
While the various facility and disposal option evaluations were being conducted, Wolfeboro
began an effort to update several pieces of process equipment to keep the aging
infrastructure operational.
The WWTF had struggled to maintain an adequate dissolved oxygen (DO) concentration in the
aeration process. To increase oxygen transfer, the antiquated ceramic dome diffusers were
replaced. Wolfeboro and Woodard & Curran, the plant's contract operator/consultant,
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Spray Fields
RIB Site Rapid Infiltration Basins
Effluent Storage Reservoir (ESR)
Effluent P.S.
Effluent Disposal
Pun
Spray S
Cyclic Aeration Process Control Added
No. 4
Secondary Clarifiers (2)
Waste Biosolids Thickened Biosolids
Storage, Aerobic for Off-site Disposal
Mill Street Influent P.S
avlor Home Influent P.S
Septage
Holdini
Septage Receive
Digestion and
Sludge Storage
Tank
decided that it would be most cost-effective to retain the current air distribution piping and
replace only the diffusers. The improvement in DO became apparent immediately upon start-
up with the new diffusers, resulting in improved DO transfer and steady DO concentrations in
the tanks. The improvements enabled one of the three aeration blowers to be turned off for
all but the highest flow and loading conditions.
During a peer review meeting to discuss the facility upgrade evaluation, the idea of cyclic
aeration was proposed as an alternative to a major plant upgrade. Based on the discussions
and the fact that some capital upgrades were already underway, the town and its contract
operator/consultant pursued the cyclic aeration alternative on a trial basis to improve the
efficiency and performance of the activated sludge system.
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With new, more efficient d iff users, the next improvement was to replace the system's aging
and oversized blowers, which would increase reliability, save energy costs, and improve
process performance. The replacement blowers were chosen to save on power costs and
provide improved controls for process optimization techniques like cyclical aeration. As a part
of the new blower installation, new blower programmable logic controllers (PLCs) and variable
frequency drives (VFDs) were installed. During a follow-up phase of the upgrade, online DO
and oxidation-reduction potential (ORP) instrumentation was installed in the two main
aeration tanks. The probes were connected to a HACH SC1000 terminal to give operators
continuous access to the probes' readings.
With the risk of mechanical equipment failure reduced, the upgrade made it possible to
deviate from conventional aeration methods and experiment with cyclical aeration. The
blowers could now be controlled based on DO levels and/or timers. ORP readings are used to
monitor performance of the cyclic aeration system.
In July 2007, with the aeration
upgrades completed, Woodard
& Curran began implementation
of the new process control
strategy (using cyclical aeration
as a means for optimizing
activated sludge treatment and
efficiency). Based on their
experience with cyclic aeration
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at other locations in New England, operational staff settled on an operator-adjustable aeration
cycle that typically runs at 45 minutes on and 1 hour off. The cycle timing is adjusted
seasonally to ensure treatment efficiencies. When the blowers are on, the PLC controls the
aeration tank's DO from becoming too high by decreasing the output of the blowers and will
increase the blowers output when the DO is too low.
Cyclical aeration saves operating costs by reducing blower run times, and it can provide
process benefits including a healthier, more stable mixed liquor and improved nutrient
removal.
I COSTS AND OTHER IMPACTS
Capital costs:
Capital Project
New Aeration Diffusers ( and RAS/WAS valve project)
Upgraded Aeration Blowers
Online Plant Process Instrumentation
Year
2006
2007
2008
Cost
$48,000
$50,000
$18,000
Operational costs: Operation and maintenance costs have decreased since the upgrade,
mainly due to the decreased energy usage in the aeration process. Reducing the blower
horsepower and incorporating cyclical aeration into the process has reduced the energy usage
by up to 60 percent, saving thousands of dollars a year in energy costs. Additionally, the
cyclical aeration process reduces the average run time of the blowers, potentially extending
the capital lifespan of the equipment.
Monitoring the additional instrumentation and managing the cyclical aeration requires a small
amount of additional labor, but that time is very well invested and provides returns in a
smoother process that performs better and requires less troubleshooting.
The primary cost-savings metric has been the ability to meet the permit requirements and
improve plant safety and reliability. Currently, the need for a major capital upgrade (the
$15-million SBR project) has been not been triggered in the AOC and Wolfeboro can continue
to pay down the bonds from the discharge upgrade without the need for another major
capital request.
Technical assistance received or needed: The upgrades at the Wolfeboro WWTP have made
the facility more intuitive to operate. The new blowers are set to automatic setpoints and run
times and do not require sustained attention. The new HACH process instrumentation allows
for more transparent system performance and enables the operator to optimize the
processes.
Site operators were required to participate in additional operator training classes on cycling
air systems. Woodard & Curran brought in biological treatment process experts to work with
staff and evaluate system operation.
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PERFORMANCE DISCUSSION
Pre- and post-upgrade total nitrogen statistics are summarized on page 1. A time-series plot of
average annual effluent concentrations for ammonia and nitrate is provided below (for
storage pond effluent).
* Ammonia
A Nitrate-N
— — Approximate Upgrade Date
7/2/2006 10/10/2006 1/18/2007 4/28/2007 8/6/2007 11/14/2007 2/22/2008 6/1/2008 9/9/2008 12/18/2008
CHALLENGES
With the aging plant and the likelihood that a major upgrade would be needed, there was an
initial perception that funding minor equipment upgrades was not worth the investment since
replacement equipment would most likely need to be replaced again when a new system was
installed. So, a major hurdle was resolved when the town of Wolfeboro agreed to invest in
incremental capital improvements even though the full WWTF evaluation had not been
completed.
A technical issue was to ensure efficient nitrification during colder weather. To address this,
Woodard & Curran has been experimenting with the use of temporary biomedia tubes. They
are 3-inch round tubes perforated, weighted, and filled with fixed media to which the
biogrowth can adhere. During the winter period, the fixed-film media tubes are added to each
active tank to increase SRT.
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| FUTURE IMPROVEMENTS
Performance could be further improved in the facility by integrating more instrumentation
and supervisory control and data acquisition (SCADA) process control. The plant currently
lacks automation for the majority of its processes. The aeration upgrades have highlighted the
positive effects that automation has on treatment performance and operational costs. Further
automation and controls would provide the operators with more control over the process,
allowing further optimization of the activated sludge process and other processes throughout
the plant. The SCADA upgrades would enhance treatment reliability but would not necessarily
reduce costs.
| CONTACT INFORMATION
David Ford, Director, Wolfeboro Department of Public Works, 9 Union Street, P.O. Box 629,
Wolfeboro, NH 03894. Phone: (603) 569-8176. Email: pwdirector@wolfeboronh.us.
Russ Howe, Plant Manager, Woodard & Curran, 46 Filter Bed Road, Wolfeboro, NH 03894.
Phone: (603) 569-3185. Email: rhowe@woodardcurran.com.
| OTHER RESOURCES
Town of Wolfeboro Wastewater Treatment Facility:
http://www.wolfeboronh.us/pages/wolfeboronh water/wastewater
Town of Wolfeboro Water and Sewer Utilities:
http://wolfeboronh.us/Pages/WolfeboroNH Water/index
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