United States
Environmental Protection
Agency
EPA/600/R 14/046 | May 2015 | www2.epa.gov/water-research
Peak Stress Testing
Protocol Framework
Office of Research and Development
Water Supply and Water Resources Division
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PEAK STRESS TESTING PROTOCOL
FRAMEWORK
Peak Wet Weather Flow Stress Testing
Contract No.: GS-10F-0227J
May 2015
Prepared for:
Environmental Protection Agency
US Environmental Protection Agency
National Risk Management Research Lab
26 West Martin Luther King Drive
Cincinnati, Ohio 45268
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Disclaimer
The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD)
National Risk Management Research Laboratory (NRMRL) Water Supply and Water Resources Division
(WSWRD), funded and managed this project through EPA Contract No. GS-10F-0227J. This report has
been both peer and administratively reviewed and approved for publication as an EPA document.
Mention of trade names or commercial products does not constitute endorsement or recommendation
for use of a specific product.
Questions concerning this document or its application should be addressed to:
Daniel J. Murray, Jr., P.E.
Water Supply and Water Resources Division
National Risk Management Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
26 West Martin Luther King Dr.
Cincinnati, OH 45268
513-569-7522
murray.dan@epa.gov
111
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Acknowledgments
Principal authors of this report were:
COM Smith, Inc.
Matt Crow
United States Environmental Protection Agency
Dan Murray
Contributions of the following individuals and organizations that assisted in reviewing this report are
gratefully acknowledged.
United States Environmental Protection Agency
Tony Tafuri
Mohamed Billah
Kevin Weiss
Ohio Water Environment Association
Dianne Sumego
Dale Kocarek
Tom Smith
Jason Tincu
IV
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Abstract
Treatment of peak flows during wet weather is a common challenge across the country for municipal
wastewater utilities with separate sanitary and/or combined sewer systems. Increases in wastewater flow
resulting from infiltration and inflow (I/I) during wet weather events can result in operational difficulties
for publically-owned treatment works (POTWs) and compromise proper treatment and compliance with
discharge permits or receiving water criteria. Thus, a need can exist for POTWs to increase peak wet
weather treatment capacity while protecting the functionality of sensitive unit treatment processes.
In order to assess the ability to capture and treat higher peak wet weather flow rates and greater volumes
of wet weather flows, POTWs are performing stress testing to demonstrate the capacity of existing unit
treatment processes and investigating ways to maximize overall treatment capacity. Communities around
the country are embarking on multi-year, capital-improvement programs to upgrade their wastewater
facilities for a variety of reasons, including aging infrastructure, regulatory requirements and increasing
populations. For these programs, treatment plant stress testing can help by assessing the maximum
capacity of existing POTWs that can be achieved through operational changes or cost-effective capital
improvements instead of larger capital investments in new treatment facilities. The goal of this stress
testing protocol framework report is to build upon knowledge of existing stress testing approaches and
procedures discovered during the course of the prior literature review. The main focus is to develop the
general equipment, steps, procedures, guidelines, etc. necessary to carry out stress testing for the
purpose of peak wet weather flow management. It is not intended to be a handbook, but rather a
framework document that outlines the general scheme in a single location.
One objective of this technical report is to recommend the application of the proposed protocol to pilot
testing by EPA ORD at POTWs that represent the diverse sizes and unit processes/treatment trains at
POTWs across the nation. This information is organized by geographic regions across the United States to
take into consideration varying climate, population and water quality concerns.
Another objective is to estimate the approximate level of effort and timeframe/schedule for the
completion of the general stress testing protocol and the recommended pilot testing of the protocol. This
includes an approximate cost for carrying out the stress testing program for the purpose of wet weather
flow management. Various sidestream treatments, solids handling considerations, and energy impacts
will not be covered in great detail in this document.
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Table of Contents
Front Material
Disclaimer iii
Acknowledgements iv
Abstract v
Section 1 - Introduction, Goal & Objectives, Document Purpose and Audience, and
Report Organization
1.1 Introduction 1-1
1.2 Goal & Objectives 1-1
1.3 Document Purpose and Audience 1-2
1.4 Report Organization 1-2
Section 2 - Stress Testing Methodologies
2.1 Peak Flow (Hydraulic) Stress Testing Considerations 2-2
2.2 Peak Load (Biological) Stress Testing Considerations
(Secondary Process) 2-3
2.2.1 Secondary Clarification 2-4
2.2.2 Polymer Addition to Secondary Clarifiers 2-6
2.3 Solids Handling Considerations 2-6
2.4 Energy Impacts 2-7
Section 3 - Wastewater Characteristics and Nutrient Considerations
3.1 Wastewater Characteristics 3-1
3.2 Nutrient Considerations 3-2
3.3 Internal Recycle and Sidestream Considerations 3-3
Section 4 - Modeling Considerations
4.1 Clarification Models 4-1
4.2 Activated Sludge Models 4-4
Section 5 - Protocol Framework
5.1 Protocol Schedule 5-1
5.2 Framework 5-2
5.3 Wet Weather Considerations 5-6
Section 6 - Applying the Proposed Protocol at POTWs
6.1 Application of Protocol 6-1
Section 7 - Stress Testing Program Cost
7.1 StressTesting Program Cost 7-1
7.2 Timeframe 7-2
Section 8 - Conclusions and Recommendations 8-1
Appendices
Appendix A References
Appendix B Modeling Processes
Appendix C Limitations of Activated Sludge Models
Appendix D Deficiencies Found During Stress Testing (Secondary Treatment)
VI
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Appendix E Peak Wet Weather Flow Literature Review
Figures
2.1 Underloaded Clarifier 2-4
2.2 Overloaded Clarifier with Respect to Thickening 2-5
2.3 Overloaded Clarifier with Respect to Thickening and
Clarification 2-5
3.1 Nitrogen Components in Municipal Wastewater 3-2
3.2 Phosphorus Components in Municipal Wastewater 3-3
4.1 CFD Particle Concentration Example 4-3
4.2 CFD Velocity Contour Example 4-3
4-3 BioWin Model Example 4-5
5-1 Example POTW Discharge Performance vs. Treatment
Requirements 5-3
5-2 Example Sate and Flux Point Analysis 5-6
6-1 Regions of the United States 6-2
Tables
1.1 POTW Size Classification 1-2
4.1 Clarification Parameters 4-1
5.1 Typical Stress Testing Schedule 5-2
6.1 POTW Recommendation Matrix 6-3
7.1 Approximate Stress Testing Budget 7-2
7.2 Timeframe for Protocol Development and Field Testing 7-3
Vll
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Acronyms
ADF Average Daily Flow
ASM1 Activated Sludge Model 1
ASM2 Activated Sludge Model 2
ASMS Activated Sludge Model 3
BNR Biological Nutrient Removal
BOD Biochemical Oxygen Demand
BOD5 Biochemical Oxygen Demand Five Day
CEPC Chemically Enhanced Primary Clarification
CFD Computational Fluid Dynamics
CRTC American Society of Civil Engineers Clarifier Research Technical Committee
CSO Combined Sewer Overflow
DO Dissolved Oxygen
DSS Dissolved Suspended Solids
ENR Enhanced Nitrogen Removal
e"KX Sludge Specific Settling Parameter and Concentration
ESS Effluent Suspended Solids
F/M Food to Mass Ratio
FSS Final Suspended Solids
g gram
gpm/sf gallons per minute per square feet
HPO High Purity Oxygen
I/I Infiltration/Inflow
MBR Membrane Bioreactor
MGD Million Gallons Per Day
ML Mixed Liquor
ml milliliter
MLSS Mixed Liquor Suspended Solids
POTW Publically Owned Treatment Works
RAS Return Activated Sludge
RASSS Return Activated Sludge Suspended Solids
RPM Revolution Per Minute
RSSS Return Sludge Suspended Solids
SBD Sludge Blanket Depth
sf square feet
SLR Solids Loading Rate
SOR Surface Overflow Rate
SPU Seattle Public Utilities
SS Suspended Solids
SSO Sanitary Sewer Overflow
SSVI Stirred Sludge Volume Index
TDS Total Dissolved Solids
TKN Total Kjeldahl Nitrogen
TMDL Total Maximum Daily Load
USEPA United States Environmental Protection Agency
VFD Variable Frequency Drive
V0 Empirical Sludge Settling Parameter
Vlll
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WAS Waste Activated Sludge
WERF/CRTC Water Environment Research Foundation/Clarifier Research Technical Committee
WWTP Wastewater Treatment Plant
IX
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Section 1
Introduction, Goal & Objectives, Document
Purpose, and Report Organization
1.1 Introduction
Treatment of peak flows during wet weather is a common challenge across the country for municipal
wastewater utilities with separate and/or combined sewer systems. Increases in wastewater flow resulting from
infiltration and inflow (I/I) during wet weather events can result in operational difficulties for publically-owned
treatment works (POTWs) and compromise proper treatment and compliance with discharge permits or
receiving water criteria. Thus, a need can exist for POTWs to increase peak wet weather capacity while
protecting the functionality of sensitive unit treatment processes.
In order to assess the ability to capture and treat higher peak flow rates and greater volumes of wet weather
flows, POTWs are performing stress testing to demonstrate the capacity of existing unit treatment processes and
investigating ways to maximize overall treatment capacity (WERF, 1999).
Communities around the country are embarking on multi-year, capital-improvement programs to upgrade their
wastewater facilities for a variety of reasons, including aging infrastructure, regulatory requirements and
increasing populations. For these programs, treatment plant stress testing can help by assessing the maximum
capacity of existing POTWs that can be achieved through operational changes or cost-effective capital
improvements instead of larger capital investments in new treatment facilities.
1.2 Goal & Objectives
The goal of this stress testing protocol framework is to build upon existing stress testing approaches and
procedures discovered during the course of the prior literature review (Task 1). The main focus is to develop the
general equipment, steps, procedures, guidelines, etc. necessary to carry out stress testing for the purpose of
peak wet weather flow management. It is not intended to be a handbook, but rather a framework document
that outlines the general scheme in a single location.
One objective of this technical plan is to recommend the application of the proposed protocol to pilot testing at
POTWs that represent the diverse sizes and unit processes/treatment trains at POTWs across the nation. This
information is organized by geographic regions across the United States to take into consideration varying
climate, population and water quality concerns. For the purpose of this document the following breakdown of
plant sizes will be used, as seen in Table 1.1.
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Table 1.1. POTW Size Classification
Classificatio Flow Range of the Facility
n (MGD)
Small
Medium
Large
Ito5
5 to 30
>30
Another objective is to estimate the approximate level of effort and timeframe/schedule for the completion of
the general stress testing protocol and the recommended pilot testing of the protocol. This includes an
approximate cost for carrying out the stress testing program for the purpose of wet weather flow management.
Various sidestream treatments, solids handling considerations, and energy impacts will not be covered in great
detail in this document.
1.3 Document Purpose and Audience
This is intended to serve as a guidance document and not a regulation for POTWs that are considering stress
testing for wet weather flow management. It is intended to covey the general steps that POTWs will need for
carrying out a stress testing program and considerations to keep in mind focusing on the main liquid flow
stream. Each treatment facility is unique with its own requirements, and deviations from the recommendations
in this document will be necessary. This guidance document does not change or substitute for any legal
requirement. It is not a rule, is not legally enforceable, and does not confer legal rights or impose legal
obligations.
1.4 Report Organization
This report is divided into eight sections: (1) Introduction, Goals and Objectives; (2) Stress Testing
Methodologies; (3) Wastewater Characteristics and Nutrient Considerations; (4) Modeling Considerations; (5)
Protocol Framework; (6) Applying the Proposed Protocol at POTWs; (7) Stress Testing Program Cost; and (8)
Conclusions and Recommendations.
Literature review references are included in Appendix A. The modeling process is included in Appendix B. The
limitations of activated sludge modeling are included in Appendix C. Common deficiencies found during stress
testing (secondary treatment) are included in Appendix D. Results from the Task 1 Literature Review have been
included in Appendix E
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Section 2
Stress Testing Methodologies
When embarking on stress testing, the dynamic character of the influent wastewater and plant operation should
be considered. All systems have a flow (hydraulic) limitation and a load (solids, etc.) limitation. The POTW will
need to determine and conduct the appropriate test. Additionally, both hydraulic flow and the concentration of
contaminants change on a diurnal, weekly, seasonal and long-term basis. Effluent quality requirements also
vary, most often on a seasonal basis. An understanding of these variations and identification of the critical
conditions are necessary for successful stress testing.
Peak Flows
Peak flows are usually triggered by a combination of precipitation, snowmelt and high groundwater table
(depending on the propensity of the collection system to infiltration and inflow or if the collection system is
combined). The biological process is typically designed to hydraulically pass the peak hour flows, as opposed to
effectively treat peak hour flows. However, depending on the geographic area, large incoming peak flows can
reduce a secondary system process mixed liquor temperature, which in turn slows the microbial activity and
may impact plant performance. Depending on the type of biological process (conventional activated sludge,
membrane bioreactors, attached growth, etc.) high flows may also wash out the active biomass within the
secondary systems (bioreactors), which will take the equivalent of at least one solids retention time (SRT) to
recover may impair effluent quality for a significant period.
Peak Loads
In addition to plant influent peak loads, significant peak loads are attributed to plant recycle streams from
sludge processing, which can have a considerable impact on the bioreactor loading, especially in terms of
nitrogen and phosphorus. Recycle streams can contribute up to 30 percent of the secondary system's nitrogen
load. Since sludge processing is typically not continuous, the POTWs need to evaluate the impact of intermittent
recycles on the biological process during the different operating conditions. The POTW should conduct an overall
mass balance to understand the magnitude of the various loads returned to the biological process. Based on the
peaks and their timing, the bioreactor sizing may need to be increased based on the maximum allowable oxygen
uptake rate under peak load conditions, or side stream treatment processes may need to be added to maintain
biological process performance.
Design Standards
Guidelines for clarifier design parameters such as these can be found in regulatory and regional standards:
• TR-i6 (Guides for the Design of Wastewater Treatment Works, NEIWPCC, 1998)
• lo-State Standards (Recommended Standards for Wastewater Facilities, 2004)
• WEF MOP on Clarifier Design (Water Environment Federation, Manual of Practice No. FD-8,
2006)
The standards provide for maximum allowable surface overflow rates, solids loading rates, weir loading rates,
side water depths, hydraulic residence time, and number of clarifier units. These resources should be consulted
prior to development of a testing program when requirements are specific to a particular plant, state or region.
Often the conservative nature of these standards allows for additional capacities that can be used for peak wet
weather events.
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The standards provide for maximum allowable surface overflow rates, solids loading rates, weir loading rates,
side water depths, hydraulic residence time, and number of clarifier units. These resources should be consulted
prior to development of a testing program when requirements are specific to a particular plant, state or region.
Often the conservative nature of these standards allows for additional capacities that can be used for peak wet
weather events.
2.1 Peak Flow (Hydraulic) Stress Testing Considerations
The purpose of stress testing is to evaluate the hydraulic performance characteristics and identify hydraulic
bottlenecks. Specifically, it can be used to identify the occurrence of short-circuiting, dead zones, and
density/thermal currents. The information generated allows strategies to be developed (e.g., baffling) for
improving clarifier hydraulics. This, in turn, will result in enhanced process efficiency.
Capacity of primary clarifiers as an individual unit operation is typically evaluated based on surface overflow rate
(SOR) and biochemical oxygen demand (BOD) removal criteria. Primary clarifier performance is dependent on
SOR and sludge withdrawal rates. Performance expectations are tied to the process capacity of the subsequent
biological systems and primary sludge handling systems.
Secondary clarifier performance is more complex to test because it is dependent upon SOR, mixed liquor
suspended solids (MLSS) concentrations and recycle ratios, aeration system performance, and sludge withdrawal
rates (Parker et al, 1999). Slug dye tests and the solids distribution/flow pattern tests are carried out during the
stress tests in an effort to better assess the hydraulic characteristics of the settling tank. Additionally, sludge
settling characteristic play a crucial role and is only as effective as upstream process. Episodes of bulking can be
normally attributed to excessive solids carryover from the aeration basins.
A desktop review or hydraulic calculation check (typically computer based) based on the plant hydraulic profile is
the first step in determining the existing system limitations and what is practical before embarking on a stress
testing program. Typical limitations examined as part of the desktop review include: approach velocities, recycle
rates, diurnal peak considerations and freeboard limitations.
Primary (Hydraulic Capacity)
For the primary clarifier tests the following methodology is typically used:
Hydraulic review conducted to identify any hydraulic limitations.
Samples of the clarifier influent and effluent at specified time intervals.
o Each sample is analyzed for total suspended solids (TSS); every third sample is analyzed for
BOD5.Typically these are collected typically every twenty minutes.
o The grab samples are used to determine the settleable solids (SS) in the clarifier influent and
effluent. Typically these are collected every hour.
o Consider performing dye testing of the clarifier to see if any short circuiting is occurring.
The sludge blanket level is measured with a sludge judge every hour at five locations along the clarifier
cross section to monitor the change in the sludge blanket profile during the test. Monitoring of blanket
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level prior to stress testing will help to establish baseline conditions and help uncover any anomalies
discovered during testing.
The supernatant from the effluent settleable solids sample is used to determine the dispersed solids
(DSS) in the clarifier effluent. The DSS concentration represents the minimum TSS concentration
achievable with an infinite clarifier.
The data from these tests are used to calculate the removal efficiency achieved as a function of flow. A graph of
flow versus TSS removal efficiency should be produced to determine the point of failure.
Secondary (Hydraulic Capacity)
For the secondary clarifier tests the following procedures apply:
Desktop analysis conducted to identify any hydraulic limitations.
Suggested target range of flows to the test clarifier. These targets represent total plant flows.
Samples of the clarifier effluent are collected typically every 20 minutes. Every sample is analyzed for TSS
and along with every third sample is filtered to analyze soluble CBOD or measured for BOD5. The data will
enable the POTW to determine clarifier performance as a function of hydraulic loading and allow for
calculation of the suspended solids in the effluent for a true measure of clarifier performance.
The sludge blanket level is measured with electronic sludge blanket measuring device or a sludge judge
at five locations along the clarifier cross section every hour to determine the changes in blanket level
profile during the test. These readings may be taken ahead of the suction arms or rakes to allow for the
highest blanket levels for the most accurate readings. The depth of the sludge blanket at the peak
hydraulic flow periods can have significant impact, since fluid velocity can re-suspend settled solids
causing spikes in effluent TSS.
A grab sample of the clarifier influent (mixed liquor) is collected every hour to determine the mixed
liquor TSS concentration, stirred sludge volume index (SSVI), and initial settling velocity (ISV).
The data from these tests are used to calculate the removal efficiency achieved as a function of flow. A graph of
flow versus TSS removal efficiency should be produced to determine the point of failure.
2.2 Peak Load (Biological) Stress Testing Considerations
(Secondary Process)
Peak load stress tests are carried out to determine the performance of a settling tank (typically for secondary
treatment processes) in terms of effluent suspended solids concentration for a variety of operating conditions.
Effluent suspended solids concentration is examined against parameters such as: SOR, solids loading rate (SLR),
return sludge suspended solids (RSSS) concentration, return activated sludge (RAS) flow rate, sludge settling
characteristics, dissolved suspended solids (DSS) concentration, and sludge blanket depth Gernant et al, 2009;
Penget al, 2007).
Before starting a peak load stress test, existing plant operational data should be examined in detail to
understand the limitations and operational constraints of the POTW. Laboratory data for a unit process are
especially critical for a fair comparison with stress testing results. One essential element of stress testing is that
a facility needs to have multiple settling tanks. This allows the plant to operate at normal performance levels
while also providing space to conduct the test. If the POTW has only one clarifier, the POTW will need to find a
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creative way to increase the SLR to the test tank, such as diverting a portion of the effluent to the tank influent
to increase SLR (Wahlberg, 2004).
2.2.1 Secondary Clarification
Secondary clarifiers have three functions:
Separate solids from mixed liquor and produce a clear effluent (clarification function).
Concentrate sludge to maintain MLSS in aeration tanks (thickening function).
Transfer thickened sludge to the collection point for pumping to the aeration tanks or to wasting
(conveyance function).
An under loaded clarifier is able to remove the flocculating particles from above the compression layer
(clarification function) and is also able to concentrate and remove solids without accumulation of a sludge
blanket (thickening function) as seen in Figure 2.1. (Daiger and Roper, 1985; Daiger, 1995)
Figure 2.1. Under loaded Clarifier
Drive Unit
Scum Trough Skimmer Arm
Influent Well
Water Surface Effluent Weir Plate
Effluent Trough—-
l
Scum Discharge —>P"
Sludge Collector
Mechanism
Influent
Sludge
Withdrawal Pipe
Key
Flow/pattern
Clarified zone • Compression layer
J Solids settling zone
A clarifier that is overloaded with respect to thickening will develop a sludge blanket that propagates upward
from the compression layer to the water surface as seen in Figure 2.2.
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Figure 2.2. Overloaded Clarifier with Respect to Thickening
Drive Unit
Scum Trough Skimmer Arm
Influent Well
Water Surface Effluent Weir Plate
Effluent Trough
Scum Discharge
Sludge Collector
Mechanism
Influent
-—Sludge
Withdrawal Pipe
Key
Flow pattern
Clarified zone • Compression layer
1 Solids settling zone
A clarifier that is overloaded with respect to thickening and clarification will develop a sludge blanket and will
also accumulate solids between the compression layer and the water surface as seen in Figure 2.3.
Figure 2.3 Overloaded Clarifier with Respect to Thickening and Clarification
Drive Unit
Effluent Trough—J
Scum Discharge —-
Sludge Collector
Mechanism
Scum Trough Skimmer Ann
f~*—
Influent Well
Water Surface Effluent Weir Plate
Influent
-—Sludge
Withdrawal Pipe
Key
Flow pattern
Clarified zone • Compression layer
j So//ds settling zone
Failure can occur by being operated at a prolonged overloaded condition, by flocculation problems, by poor tank
hydraulics or because of denitrification in a clarifier. (Jenkins et al, 2003)
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The SOR is a measurement of the volume of water, in gallons per day, rising up in one square foot of the clarifier
surface area. Essentially, this is the upward velocity. As the upward flow rate increases, a point is reached when
the water's velocity begins to carry suspended solids upward and solids loss occurs.
The other design standard, SLR, is a measurement of the amount of solids, in pounds per day, applied to one
square foot of clarifier surface area. As the pounds of solids per square foot increases, a point is reached when
settling rates decrease, resulting in solids loss. Hydraulic failure in the suspended growth treatment process is
determined when influent flow rates increase to an intensity or duration that leads to solids loss in the
secondary clarification units. Meeting these two design standards is recommended to prevent failure of the
secondary clarification units. (Benefield and Randall, 1985)
2.2.2 Polymer Addition to Secondary Clarifiers
Polymer use in a secondary clarification application has been practiced for a long time; however, available data
quantifying the benefits of this technique are scarce. Most of the reported applications address sludge settling
and foaming problems, such as those caused by filamentous bulking and Nocardia.
Polymer can improve a clarifier's performance in two interrelated but distinct ways. First, polymers effectively
increase zone settling velocity and decrease SSVI by creating larger and tighter agglomerates. This increases the
critical, allowable solids loading/overflow rate of the clarifier, as predicted by the flux theory. Secondly,
polymers facilitate flocculation and capture of the dispersed solids from the supernatant resulting in a lower
effluent TSS. This can be beneficial when inadequate flocculation opportunities exist in the secondary clarifiers
(e.g. lack of flocculating center well or functionally similar structures).
Before considering polymer as part of a stress testing program for wet weather flow management practices, the
POTW should conduct an analysis of the floe present in the secondary clarifier. If the floe present in the
secondary clarifier is already considered large and fairly tight, the addition of polymer may have little to no
effect on improved clarification.
In addition to TSS removal, the addition of polymer may have some impact on additional phosphorus removal
via enhanced settling. This is primarily in the form of inorganic phosphorus by means of chemical precipitation.
Commonly Alum, and Ferric have been used with each having alkalinity consumption, chemical sludge
production, and pH depression that must be balanced with discharge requirements. Care should be exercised in
selecting a polymer that neither inhibits nitrification nor contributes to effluent toxicity.
2.3 Solids Handling Considerations
Stress testing for solids processing systems is a challenging and often difficult task. This usually involves a large
number of operational variables to consider. Furthermore, process efficiency is often difficult to obtain in "real
time data" and many plants do not usually gather operational data beyond the solids mass entering and leaving
the system (Klein, 2008).
Solids processing systems are usually rated in terms of pounds of solids per day in conjunction with hydraulic
loading rate. This means that both hydraulic loading and feed solids concentration have to be taken into
account. Modifying the solids concentration is more complex than adjusting a pump set point. Making
adjustments during a test run is rarely possible; thus, a target feed concentration has to be selected weeks prior
to testing. This might require plant staff to modify plant operations to meet the selected target. Historical data
analysis and plant staff experience will usually indicate the most practical feed concentrations attainable.
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Influent flow increases during peak wet weather events. While the increased flow can result in reduced TSS
concentrations due to dilution, a high flow following a prolonged period of dry weather can also result in a first
flush event, especially in combined systems. Depending on the region and climate, considerations for the first
flush will introduce a noticeable higher solids loading content to the treatment facility from the collection
system, following a period of little to no rainfall.
Existing solids handling processes typically have sufficient capacity for single wet weather events. However,
back-to-back storm events can exceed the capacity of sludge processing units, such as digesters and storage
tanks. Before setting forth on a stress testing program, a thorough evaluation of the existing solids handling
train should be conducted with a review of historical performance (Newbigging et al, 2004). Since each POTW
and collection system is different, no one general rule will apply. During peak part of a storm to take into
account an elevated hydraulic loading rate, additional dewatering trains may be needed or additional storage to
act as a buffer to keep up with the influent flow rate.
As with the secondary treatment process, no one solution will work for all POTWs. A review of the solids
handling unit processes should be undertaken as part of the overall stress testing approach to understand the
current limitations. (Kalinske, 1973)
2.4 Energy Impacts
In recent decades energy costs have skyrocketed for many municipalities. The days of energy costs at 3.5 to 5
cents per kW*hr are practically gone. Municipalities are facing a renewed call to taking into account electrical
costs as part of an overall life cycle assessment to provide the best value to their rate payers, when considering
capital improvement and plant maintenance projects. Historically wet weather flows have been a small
percentage of the flows that POTW must treat, often in the range of 5% though depending on the shape of the
collection system and region of the country this number may be higher.
Any projects looking at addressing wet weather flows should be examining the energy impacts, for a holistic
approach of process treatment and equipment selection. Items such as process pump stations, aeration
blowers, etc. that can see variation numerous variations would be prudent to install energy saving devices, such
as variable frequency drives (VFD), to help reduce overall electrical consumption. Since the limit and extent of
wet weather flows is often a short time period of high intensity, a cost analysis should be undertaken to
determine the relative cost savings for these high but infrequent flow swings.
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Section 3
Wastewater Characteristics and Nutrient
Considerations
Wastewater comprises a number of different characteristics and nutrients that must be contended with as part
of the treatment process. This section is intended to give the reader an overview of characteristics and nutrient
considerations and how it relates to modeling and the field portion of a stress testing program. Having a keen
understanding of the various nutrient fractions (soluble and insoluble) will allow one to know what portion of
the treatment process during stress testing will be the most challenging and have a better opportunity for
removal of containments.
3.1 Wastewater Characteristics
Prior to the start of any testing program, a representative sample should be undertaken to understand all the
constituents that will be seen during the course of testing. Wastewater characteristics can change over time
with population shifts (e.g. leading to longer detention times in the collection systems) or with new industries
moving in the area with their associated discharges.
Daily influent plant data will be necessary. Commonly, a composite sampler is used to evaluate the flow over an
entire 24 hour sampling period. While one year of data collection is the minimum, three years of data is
recommended. Data should be plotted year-to-year to look at trends (e.g. seasonally or yearly). The type of daily
influent data needed is a function of the required level of treatment for the facility. The following should be
requested at a minimum from facilities providing treatment to the following standards:
Non-nitrifying facilities - Flow, wastewater temperature, BOD, TSS, VSS.
Nitrifying facilities - Flow, wastewater temperature, BOD, TSS, VSS, TKN, ammonia, total phosphorus,
alkalinity, and pH.
Biological nutrient removal (BNR) facilities - Flow, wastewater temperature, BOD, TSS, VSS, TKN,
ammonia, total phosphorus, orthophosphate, alkalinity, and pH.
Additional data (COD, nitrogen, phosphorus, etc.) will be necessary as part of the baseline conditions for the
wastewater characteristics. A summary of these constituents follows:
COD in wastewater influent can be divided into soluble and particulate fractions, as well as biodegradable
and inert fractions. An additional distinction can be made between colloidal and truly soluble COD.
Nitrogen in influent wastewater consists of soluble and particulate fractions, as well. Soluble nitrogen is
the sum of nitrate/nitrite, ammonia, and soluble organic nitrogen while particulate nitrogen is typically
organic. Biodegradable nitrogen is typically thought to comprise nitrate/nitrite, ammonia, and some
fraction of both soluble and particulate organic nitrogen.
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Phosphorus in influent wastewater can be either soluble or particulate. Soluble forms include
orthophosphate ('reactive" phosphorus) and polyphosphates, as well as some metal-phosphate
complexes. Particulate forms include organic phosphorus. For a thorough discussion of COD, N, and P
characteristics, please refer to WERF (2003).
Each part of the treatment process (primary and secondary treatment) is designed to address the specific
constituents (particulate and soluble components). The particulate forms are better suited to be handled in the
primary treatment process, where as the soluble components are more easily addressed in the secondary and
tertiary treatment processes.
3.2 Nutrient Considerations
Nitrogen Fractions
Nitrogen fractions can sometimes be estimated. For example, ammonia can be assumed to be 60 to 70 percent
of TKN in plant influent. In the event the facility has primary settling tanks, TKN, TSS and VSS concentrations in
the primary influent and effluent can be used to estimate the particulate fraction of TKN. A breakdown of the
typical nitrogen components in municipal wastewater can be seen in Figure 3.1.
Figure 3.1. Nitrogen Components in Municipal Wastewater
Biodegradable Soluble
Inorganic Nitrogen
Biodegradable Soluble f"~
Organic Nitrogen ~~1
Nonbiodegradable Soluble_J
Organic Nitrogen 1__
Biodegradable Particulate_J
Organic Nitrogen [__
Nonbiodegradable
Particulate Organic
Phosphorus Fractions
NITRATE &
NITRITF
Nitrate/nitrite
AMMONIA
Ammonia
;i[;ti ii ii SOLUBLE' ii ii ii ii i;
•:;;;;;;;;;;;;;;;;;;;;; \\.\\ ;i
SOLUBLE
PARTICULATE
R
PARTICULATE
GF-
filtered
TKN
f Total
TKN
J
Phosphorus is another major nutrient required for biological growth. Wastewater phosphorus is typically divided
into orthophosphate, polyphosphate and organic (both soluble and particulate) phosphorus. A breakdown of the
typical phosphorus components in municipal wastewater can be seen in Figure 3.2.
3-2
-------
Figure 3.2. Phosphorus Components in Municipal Wastewater
Soluble Acid Hydrolizable
Phosphorus
Soluble Organic Phosphorus
Soluble Reactive Phosphorus f
Particulate Reactive
Phosphorus
Particulate Organic
Phosphorus
Particulate Acid Hydrolizable
(Polyphosphates)
Phosphorus
SOLUBLE
SOLUBLE
SOLUBLE
PARTICULATE
PARTICULATE
3.3 Internal Recycle and Sidestream Considerations
During wet weather events, a reduced effectiveness of the secondary treatment process is expected, simply
because of the dilute nature of the influent. Prudence should be used in determining ways to reduce nutrient
loads/streams during wet weather events while keeping the treatment facility within discharge limits. Examples
of such considerations include:
RAS pumping is used to maintain solids inventory in the bioreactors. Frequently, RAS pumping systems
are designed for ultimate conditions and lack appropriate turndown capability for low flow conditions.
This situation would generate thin sludge. Depending on the wasting system's capabilities, the thing
sludge may impact the system's SRT and process performance. Special consideration should be given to
high RAS return flow rates from MBRs, which also carry high dissolved oxygen levels.
Dewatering of anaerobically digested sludge typically recycle 20 to 30 percent of their nitrogen load.
During wet weather events the bulk of this nitrogen would be lost to the effluent. Eliminating the bleed
through of this nitrogen load during wet weather events by means of sidestream treatment can reduce
nitrogen discharges during high flows and allow for reduced operating costs if implemented year round.
Introduction of influent at several locations along the length of bioreactors to minimizes the risk of
washout during storm flows. With fine-bubble aeration, distribution across the width of the aeration
tanks will need to be considered to avoid short circuiting. Note that this mode of operation (step
feeding) would approximate a complete mix reactor system and may favor the growth of filamentous
organisms.
In general, a full evaluation of the wet weather mode at a POTW will need to be conducted (including all
recycle streams) to identify any potential bottlenecks of nutrient removal. This would be conducted as part of
the baseline assessment, prior to the start of any field testing.
3-3
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Section 4
Modeling Considerations
4.1 Clarification Models
Clarification models are used to describe the behavior of solids, typically in primary or secondary clarifiers. The
models available in BioWin and GPS-X include: 1) point or ideal, 2) simple one-dimensional, 3) modified Vesilind,
and 4) double exponential. Basic overviews of these models follow:
The point or ideal model is based on a constant percent solids removal in the clarifier.
The simple one-dimensional, modified Vesilind and double exponential models are all one-dimensional
models that solve a series of equations describing solids behavior in a number of different clarifier "layers."
These models are based on standard solids flux analysis, which assumes that the mass flux of solids in the
clarifier is the sum of the gravity settling flux and the flux due to bulk movement.
The modified Vesilind and double exponential models modify the Vesilind approach(interface settling
velocity) in that a low settling velocity is estimated for low concentrations instead of having settling velocity
approach Vo (empirical sludge settling coefficient) at low solids concentrations. The double-exponential
approach has an additional e"KX term (exponential growth function) and the modified Vesilind has a
switching function. The simple one-dimensional is equivalent to the double exponential model. The
parameters that must be specified for each of these clarifier models are shown in Table 4-1.
Table 4.1. Clarification Parameters
Parameter to Be Specified
Area and depth
Underflow rate
Percent solids removal
Maximum Vesilind settling velocity
Vesilind hindered zone settling
Clarification switching function
Specific TSS cone, for height calculations
Maximum compactability constant
Maximum practical settling velocity
Flocculant zone settling parameter
Maximum non-settleableTSS
Non-settleable fraction
Yes
Yes
Yes
Clarifier Model
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Note: GPS-X allows the user to use the sludge volume index to calculate the maximum Vesilind settling velocity, the hindered
zone settling parameter, and the flocculant zone settling parameter.
4-1
-------
In addition, any of these clarifier models can be used with or without biological reactions. These biological
reactions can be described by any of the activated sludge models described in Section 4.2. Although, the same
biological model used to model biological growth and decay in aeration would be used to describe biological
reactions in a clarifier.
The general practice is to use a modified Vesilind or double exponential model for dynamic clarification
simulations.. This requires that good data on settling characteristics (SSVI data or data from column tests) are
available from baseline testing conditions. Using a point or ideal model (with constant percent solids removal)
during dynamic simulations will indicate increasing effluent TSS with increasing flows (which is the case when
one performs a stress testing program). There a number of correction factors and fine tuning of the modeling
necessary to predict clarifier overload (based on field testing data). Without these field data, the point and ideal
models are more appropriate for steady-state simulations.
Biological reactions should be modeled only when these are known to have an impact on the wastewater
treatment plant process. This can be evaluated by examining data on soluble species (COD or BOD, ammonia,
nitrate) before and after the clarifiers. POTW laboratory tests are driven by compliance monitoring of permitted
discharges thus not all information might not be readily available. If this is the case, additional time and cost
should be factored into the operating budget prior to collecting and analyzing data
Another useful tool in predicting clarifier behavior is the use of computational fluid dynamics (CFD). CFD allows
for detailed examination of multiple factors affecting flow within a proposed hydraulic design, specifically:
Creates more accurate scale-ups than with physical models.
Reveals configurations that improve hydraulic distribution and/or reduce head loss.
Identifies turbulent characteristics in the proposed design.
Provides quick assessments of hydraulic fixes in proposed or existing basins.
Details chemical reactions, hydraulic stress, physical barriers, mechanical movements, mass transfer, and
other factors affecting flow within an enclosed basin.
Typical outputs from CFD analysis can be seen in Figures 4.1 (particle concentration) and 4.2 (velocity contours).
4-2
-------
Figure 4.1. CFD Particle Concentration Example
alar o Contours of Particle Concentration in a Secondary Clarifier
de Concentration
7.544
8.000 (m)
Figure 4.2. CFD Velocity Contour Example
0.000
[m sM]
7.000 (m)
4-3
-------
4.2 Activated Sludge Models
Activated sludge models are used to model the microbiological growth and decay processes relevant to
biological treatment. These models are collections of process rate equations, which are solved to determine the
values of certain state variables. Examples of state variables are: the concentration of autotrophic biomass, the
concentration of soluble inert COD, and the concentration of ammonia. When stress testing is examining the
impact of nutrimental removal, coordination with activated sludge models will be necessary.
Three models developed by the International Water Association (IWA; formerly IAWPRC then IAWQ) Task Group
on Mathematical Modeling for Design and Operation of Biological Wastewater Treatment Processes are
commonly used as the basis of software and modeling efforts: Activated Sludge Model No. 1 (ASM1), Activated
Sludge Model No. 2d (ASM2d), and Activated Sludge Model No. 31. Additional simulation software is available,
BioWin developed by EnviroSim Associates and GPS-X developed by Hydromantis. Both BioWin and GPS-X have
their own default biological models: Activated Sludge/Anaerobic Digestion Model (ASDM) for BioWin and Mantis
for GPS-X. A comparison of the biological processes included in ASM1, ASM2d, ASMS, ASDM and Mantis is
shown in Appendix B. Limitations of each model are presented briefly in Appendix C.
In general, when selecting a biological treatment model, the user should start with the simplest model
appropriate to the simulation application. For example, if phosphorus does not need to be included in the
simulation, a COD- and N-only model would be sufficient for that application.
For the IWA activated sludge models, the general guidelines hold:
ASM1 should be used for COD- and N-removing biological treatment;
ASM2d should be used for COD-, N-, and P-removing biological treatment;
ASMS should be used for COD- and N- removing biological treatment or to incorporate other add-on
processes in a modular fashion, such as the Bio-P module from Reiger et al. (2001).
The BioWin ASDM can be used for COD-, N-, and P-removing biological treatment. An example of this type of
modeling can be seen in Figure 4.3. The GPS-X Mantis model is similar to ASM1, except that kinetic parameters
are temperature-dependent, aerobic denitrification is included and two additional growth processes (one
autotrophic and one heterotrophic) are introduced. Therefore, Mantis can be used for COD- and N-removing
biological treatment.
The IWA ASM models are described fully in a report by the IWA Task Group on Mathematical Modeling for Design and
Operation of Biological Wastewater Treatment entitled Activated Sludge Models ASM1, ASM2, ASM2d, and ASMS, published
by IWA Publishing in 2000 as part of their Scientific and Technical Report Series.
4-4
-------
Figure 4.3. BioWin Model
Example
n
•
N
-r
N
ij
r"
i
s
1
r~
'If
4-5
-------
Section 5
Protocol Framework
Carrying out a stress testing program can be a complicated procedure involving many steps and coordination
with multiple departments within a wastewater utility. This section is intended to convey the general steps that
a POTW will need to carryout for a stress testing program and considerations to keep in mind. It does not take
into account any of the specific details for modifications and temporary piping required for stress testing. Each
treatment facility is unique with its own requirements and deviations from this framework will likely be
necessary. Stress tests are carried out to determine the performance of a settling tank (both primary and
secondary) in terms of ESS concentration for a wide variety of operating conditions especially during peak wet
weather flows. Relationships between ESS concentration and SOR, SLR, RSSS concentration, RAS flow rate,
sludge settling characteristics, DSS concentration, and sludge blanket depth are examined in great detail. Slug
dye tests and solids distribution/flow pattern tests are also performed to assess the hydraulic characteristics of
the settling tank.
In 2001, Water Environment Research Foundation/American Society of Civil Engineers Clarifier Research
Technical Committee (WERF/CRTC) developed a stress testing protocol (Wahlberg, 2001). This protocol
framework is a variation of the WERF/CRTC approach. This modified approach was utilized in a number of the
case studies examined as part of the literature review performed previously.
5.1 Protocol Schedule
A typical stress test schedule is summarized in Table 5.1. This schedule is designed to be applied at three
different levels of SLR: low, medium, and high. The level of SLR is determined from the solids flux analysis. Each
SLR is replicated three times (once for each day). Measurement of flows, SS concentrations, and sludge blanket
height are conducted during each test. Four settling tests and SSVI measurements are carried out during each
test. Influent DSS, effluent DSS, ESS and FSS concentrations are determined during each test. A slug dye test is
carried out (either in the first or second replicate of each test) and a solids distribution/flow pattern test is
conducted (either in the second or third replicate of each test).
5-1
-------
Table 5.1. Typical Stress Testing Schedule
_ „-. _ _ ,. ,. Tests to be Conducted During Stress Tests
Day SLR Replicate Measurements
i
2
3
4
5
6
7
8
9
10
NA
High
Low
Low
High
Low
Medium
High
Medium
Medium
NA
i
i
2
2
3
i
3
2
3
Conduct flow,
suspended solids,
and sludge depth
with i hr intervals
Conduct flow,
suspended solids,
and sludge depth
with i minute
intervals
If sludge blanket
measurements are
not available on-
line, measurements
should be done
manually every 15-
30 minutes
Baseline condition assessment
Conduct 4 settling tests
using settling column
and 2 SSVI tests during
each test
Carry out influent DSS,
effluent DSS, ESS and
FSS concentrations
during each test
Slug dye
test
Slug dye
test
Slug dye
test
Continuous
dye/solids
distribution
tests
Continuous
dye/solids
distribution
tests
Continuous
dye/solids
distribution
tests
5.2 Framework
The WERF/CRTC protocol focuses on stress testing the secondary treatment process since primary treatment is
not typically considered the bottleneck in terms of process constraints. However, the protocol can be applied to
primary treatment for stress testing, especially if the facility receives CSO flows.
Considerations Prior to Testing
Prior to the start of testing, a clear failure point should be established early on during the stress testing plan
development. At the height of a wet weather event, removal efficiency will be impaired due to the peak flow
and loads being seen at the POTW. An acceptable value for failure might be the 7-day average permit value as
5-2
-------
opposed to the 30-day value. An example of POTW discharge performance versus treatment requirements can
be seen in Figure 5-1.
Figure 5.1. Example POTW Discharge Performance vs. Treatment Requirements
7 Day Average
30 Day Average
Jul'05 Aug'05 Sep'05 Oct'05 Nov'05 Dec'05 Jan'06 Feb'06 Mar'06 Apr'06 May'06 Jun'06
^^~ BOD Effluent ^^~T55 Effluent 30 Day Permit Limit ^^~7 Day Permit Limit
In many instances the 7-day permit value is taken as the point of failure, since peak wet weather influent flows
are experienced for a limited duration of time. Greater treatment performance is typically achievable for the
reminding monthly time period providing no operational abnormalities occur. Additionally, consideration for
back-to-back storm events should be considered. The clear definition of failure will tie into the regional
approach one should use when conducting stress testing. Prior to the start of stress testing, discussions with the
regional regulatory body should be undertaken to explain the reason and philosophy of stress testing since it can
lead to permit excursions. Often times events leading up to stress testing have been at the request or
suggestion of the regional regulatory body to address wet weather flows. This interactive dialogue will ensure
all parties are aware of the testing purpose, schedule, and results/potential impacts.
Following the test schedule, as shown in Table 5.1, will result with each stress test taking one day. It will involve
determining the SLRs applied during each stress test, with the SLRs derived from the solids flux analysis, based
on the results of the mixed liquor settling tests conducted prior to the stress tests.
One consideration would be incorporating the test schedule as it relates to daily diurnal flow periods to allow for
higher hydraulic loadings (peaks) than if testing is conducted by just taking clarification units off-line since the
peak of the daily influent flow to the facility is being taken into account. Simulation of wet weather flow
conditions is key, as described in Section 5.3.
Baseline Conditions and Setup
1. Obtain design data and blueprints of the entire secondary settling tank system, as well as the test
settling tank. Identify the sampling locations for influent, effluent and within the test tank. Become
5-3
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familiar with all the flow control systems for influent, effluent and RAS, as well as flow ranges that can
be applied. Review condition of the test tanks versus original manufacturer's design drawings and
evaluate the existing condition of the tank equipment (i.e., weir set points and elevations, sludge
withdrawal equipment, etc.).
Begin development of CFD, BioWin, or other models discussed in Section 4. The actual data collected
during the baseline condition assessment and field testing will be used to calibrate the model and will
allow numerous iterations to be performed after field testing has been completed to simulate proposed
modifications. Examples of such modifications include: installation of Stamford and peripheral baffles,
revisions to the RAS pumping system, and addition of center well EDI baffle arrangement.
2. The tests should be conducted when the plant is operating normally. Conducting the tests during
atypical (i.e., upset) conditions may lead to erroneous conclusions regarding plant capacity. Verify if
any outstanding maintenance issues or rate limiting steps need to be addressed and corrected prior to
commencement of testing.
3. Install various flow measuring devices.
• Install flow meters in the influent (or effluent), WAS (with totalizing), and RAS lines and
capture the output signals electronically. The tester may use a portable flow meter to
streamline data collection. Capturing flow measurements on both the influent and effluent is
not necessary. If only one flow stream can be measured due to funding constraints, consider
concentrating on WAS. Other flow streams can be determined by means of mass balance
calculations.
• Install suspended solids probes into the influent line, effluent launder and RAS line of the test
tank and automated sludge blanket monitoring device.
• Every time sampling is performed manually, it introduces a potential source of error and could
possibly skew results. The more automated the sampling process is, the more likely consistent
and accurate data collection will be obtained. Measurement of the sludge blanket depth in
the test tank manually by means of a sludge judge can introduce error since the sludge blanket
layer is being disturbed.
4. Determine a way to adjust the flow rate to the test tank to provide the three different SLRs. The tester
may adjust the test tank's influent flow rate by taking other clarifiers out of service gradually, by weir
adjustment, or by partial gate closing. The tester should keep the ratio of influent flow to RAS flow
constant during each SLR test condition.
An operator should be present at all times during the tests to collect samples and flow measurements.
The operator may also need to increase the RAS rate during the tests to prevent an excessive
accumulation of solids in the secondary clarifier.
Field Testing
During each stress test, keep the influent flow constant to the test tank for a period of three
theoretical hydraulic detention times prior to the start of testing.
A minimum of three hydraulic retention times should be passed before changing testing conditions.
This time period allows the clarification system to return to a steady state value.
5-4
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2. During each stress test and when the influent flow is constant, collect data from the suspended
solids probes, flow and sludge blanket height measuring devices at least at 1-minute intervals. If the
sludge blanket height is measured manually, then the measurements should be carried out at 15-30
minute intervals.
3. After a period of time that is equal to three theoretical hydraulic detention times, carry out (at least
once) influent DSS, effluent DSS, ESS, and FSS concentration tests.
4. During each stress test, conduct settling tests at four to six different suspended solids concentrations
to determine the V0 and k parameters of the Vesilind equation (zone settling velocity = V0*exp(-
k*Xt)).
5. During each stress test, conduct two SSVI tests, as described in Standard Methods for Water and
Wastewater (APHA, AWWA and WEF, 2005). In the SSVI test, mixed liquor is settled in a 1-liter
graduated cylinder for 30 minutes, as the contents of the graduated cylinder are stirred at one
revolution per minute (rpm).
6. During the first or second replicate of the stress test specified for each SLR and after three
theoretical hydraulic detention times have passed, carry out a slug dye test. (If constant flow for
three theoretical hydraulic detention times cannot be maintained, a slug dye test can be initiated
earlier). Samples should be collected until at least 90% of dye mass is recovered.
7. During the second or third replicate of the stress test specified for each SLR and after three
theoretical hydraulic detention times have passed, carry out continuous dye and suspended solids
distribution tests (if constant flow for three theoretical hydraulic detention times cannot be
maintained, a slug dye test can be initiated earlier). If a manual core sampler such as a sludge judge
is used, dye and suspended solids sampling can be done at the same time. If a portable hand-held
suspended solids analyzer is used, then the core sampler is employed for taking dye samples only,
and the suspended solids concentrations at different depths are determined using the electronic
device.
8. Continue the stress test at least for a period equal to one theoretical hydraulic detention time after
three theoretical hydraulic detention times have passed, and after completion of the entire slug dye,
continuous dye, and solids distribution tests.
9. The tests should only be interrupted if clearly excessive quantities of MLSS are observed going over
the secondary clarifier weirs. Interrupting the tests simply because blanket levels are rising or
effluent solids appear higher than normal will limit what can be learned from the test procedure.
Results should be based on quantified analytical values, rather than simply visual observations.
Data Interpretation
1. Perform solids flux and state point analysis graphs based on field tests for comparison to computer
models. This graphical tool allows designers and operators to graphically understand the dynamics
of an activated sludge clarifier. An example of a solids flux/state point analysis can be seen in Figure
5-2.
5-5
-------
Figure 5.2 . Example State and Flux Point Analysis
X
"5
CO
Overflow rate operating line
State point
Settling flux curve
Underflow rate operating line
1C
5 10 15
Solids Concentration (g/L)
An underflow rate operating line exceeding the settling curve will indicate a thickening failure. A
perfect balance of underflow rate (RAS pumping) and overflow rate should be accomplished for a
highly functional secondary clarifier.
2. Update CFD, BioWin, or other modeling software with field results to determine possible process
improvements that could be made to address wet weather flow conditions.
Typical deficiencies found during the course of stress testing and appropriate corrective actions, have been
included in Appendix D.
5.3 Wet Weather Considerations
The dilute nature of wet weather flow conditions can present a number of unique challenges for stress testing.
Simply taking clarifiers off-line can address the nature of the increased volume of flow for the test clarifiers;
however, it does not simulate the true nature of wet weather flow events.
Considerations for Simulating Peak Wet Weather Flow Conditions
Utilizing plant effluent as the make-up source flow is one way to address simulating dilute wet weather
influent flows. Although temperature effects from a peak wet weather event will not be simulated, it would
give a better indicator of performance for dilute influent than dry weather wastewater.
Peak flow concerns. During a wet weather event several unique influent wastewater characteristics can
result that will affect treatment plant performance. These includes: increased TSS loading, additional grit
loading from surface runoff, and elevated dissolved oxygen levels. Finding a way to simulate these
conditions will allow for a better overall stress testing program for peak wet weather flows.
Consider the season for stress testing. Variability in influent will occur during wet weather events; thus,
different operational strategies may be employed at a POTW to manage these flows (i.e., higher salinity
from snow and ice removal operations). Performing stress testing under different seasonal conditions will
5-6
-------
give the POTW a better understanding of the seasonal treatment process performance changes especially if
seasonal permit limits exist.
Potential new permit limits. With changing nitrogen and phosphorus control requirements it may be
beneficial to conduct stress testing with current and future requirements in mind. May and November have
been found to be the most limiting and challenging seasons for meeting nutrient limits. If budgetary
constraints only allow for limited testing, consideration of these months will give the best indicator of
limiting conditions and can be seasonally adjusted for summer months.
Considerations for Actual Peak Wet Weather Flow Conditions
Testing during actual wet weather flow is something each POTW will need to consider as an alternative to
simulating conditions. Testing during actual wet weather flow will give a better indicator of true
performance, eliminate artificially creating and managing numerous variables, and the rapid rate of flow
changes that operators must manage. If POTW has sufficient staff and resources to carry out such testing,
this will give a better indicator of overall wet weather performance.
5-7
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Section 6:
Applying the Proposed Protocol at POTWs
Treatment processes employed across the US vary significantly along with treatment plant size. Since a number
of municipalities are trying to negate capital costs of constructing new treatment trains, a need exists for having
a systematic way to conduct stress testing to maximize existing infrastructure to the extent possible. A number
of innovations in clarifier design and MLSS strategies have taken place since many of these facilities became
operational. A plant scale application of the proposed piloting measures will provide the tools for
accomplishing this need and help further refine the proposed protocol by applying it at numerous treatment
facilities across the nation.
6.1 Application of Protocol
A number of modified activated sludge processes have been developed over the years to meet specific
purposes. These adaptations of the activated sludge process, whether basin configuration, aeration
configuration, operating mode, or other proprietary configuration, present a tradeoff for items such as footprint,
hydraulic gradient, performance, reliability, flexibility, capital costs, operating costs, etc.
To examine a proper cross-section of POTWs that represents the diverse sizes and unit processes/treatment
trains across the nation, we considered the following characteristics:
Size of the facility. Larger treatment facilities tend to be located in metropolitan areas that often have
large diameter main interceptors as part of their collection systems. These large interceptors may
dampen the effects of wet weather and result in lower peaking factors to the treatment facilities.
Smaller to medium size facilities tend to have higher peaking factors as a result of the shorter time of
travel in the collection system. Therefore, a larger facility will often have more logistical challenges to
simulate peak wet weather flows for stress testing.
Regional location of the facility. Regional characteristics such as rainfall and climate differences,
population, and water quality concerns affect drivers for stress testing and permit limitations. It is very
difficult to have a "one size fits all" approach for stress testing.
Treatment process. Each treatment process is unique and certain approaches that will work for one
process will not necessarily translate into comparable results for another process (e.g., pure oxygen,
conventional activated sludge, BNR, etc.).
Combined or separate collection systems. Wastewater characteristics can differ between separate and
combined sewer systems. In addition, temperature influxes occurring from dilution in a combined
system can affect reaction kinetics for both primary and secondary treatment processes. This effect is
more pronounced in a combined system than in a separate collection system.
Age of the facility. Older facilities will tend to be located in areas that have more established collection
systems and experience a higher degree of I/I impacting the driver for stress testing.
To address the regional location of the facility, different geographic regions were adopted from the delineation
of the case studies presented in the literature study, Figure 6.1. The regions were based on the US Census
Bureau from the 2010 census.
6-1
-------
Recognizing regional differences allows for an easier reference for items such as climate, population and
receiving water characteristics, which are all drivers related to wet weather flow management and stress
testing at POTWs.
Figure 6.1. Regions of the United States
east
Recommendations for conducting the plant-scale applications of proposed stress testing protocol are shown in
Table 6.1. The matrix represents the different variables previously discussed for the wide ranging unit processes
and facility size common for that region.
6-2
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Table 6.1. POTW Recommendation List
Region
Flow Range
ofthe
Facility
(MGD)
Process
Combine
d System
Separate
Sanitary
System
Northeast
South
Mid-west
West
100 to 120
20 to 30
40 to 60
10 to 15
Activated sludge,
high pure oxygen
setup
Conventional
Activated sludge
Activated sludge,
BNR
Activated sludge,
BNR
X
X
X
X
The matrix shown above represents the different variables previously discussed for the wide ranging unit
processes and facility size common for that region. Since peak wet weather flows are experienced by all
POTWs, regardless of their location, it is important to look at development ofthe proposed protocol from a
regional perspective. Each region will have different weather patterns and water quality drivers that will
influence testing procedure and duration.
Typically speaking, larger more established and extensive combined collection systems are found in the
Northeast and the Mid-west with separate collection systems found in the South and the West. The Northeast
and Mid-west are predominated by larger treatment facilities for average plant size, partially stemming from the
combined collection system influence. The South is considered more rural with regional treatment facilities
spread out, with the West having a critical number of smaller treatment facilities in addition to larger facilities in
metropolitan areas. All of these regions employ a wide variety of treatment schemes, and certain approaches
that will work for one process will not necessarily translate into comparable results for another process. A
greater concentration of treatment plants in the Mid-West and the West incorporate some degree of nutrient
removal while a number of the treatment plants in the Northeast and the South have some variation of the
activated sludge process. The intention of this matrix is not to be a rigid structure, but rather a guideline for
selecting facilities that encompass this regional approach with variations in unit process considerations.
6-3
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Section 7
Stress Testing Cost and Timeframe
Development and conducting stress tests will involve a significant amount of time and expense to achieve the
desired results. This section covers the estimated expense and time for conducting stressing operations and
emphasis the application of proposed protocol outlined in Section 6.
7.1 Stress Testing Cost
The proposed protocol was used to determine labor, equipment and modeling requirements for developing the
cost of carrying out a stress testing protocol demonstration program. This cost focuses on carrying out tests
related to the secondary treatment process, which is typically the bottleneck in the treatment process as
discovered during the course of the previous literature review (Task 1). For plants that experience a significant
amount of CSO flow, this same basic principal can be applied to the primary treatment process.
To reiterate the test protocol previously discussed in Section 5, initial testing is conducted during baseline
conditions to generate the data that will be used during field and stress testing. Testing is typically performed
over a 10-day period as outlined in Table 5.1, from Section 5.
Baseline Conditions. Settling tests, influent DSS, effluent DSS/FSS/ESS concentrations, and flow and sludge
blanket verification tests will be carried out. These tests will be carried out by four persons, will take
approximately one day, and are used to establish the baseline conditions. Care should be taken that this is
performed during dry weather flow conditions.
Field and Stress Testing. The tests to be conducted include: stress tests, flow measurements, sludge
blanket measurements, settling tests, DSS/FSS/ESS testing, SSVI, MLSS, RASSS, slug dye test, continuous
dye/solids distribution tests, and modeling (CFD and/or BioWin).
These tests will be carried out over a nine day period, typically. For settling tests, three 1.5-meter columns
will be used for settling tests. Six different SS concentrations will be used to generate a settling flux curve for
stress tests. Up to six people will be needed during a 6-day period of the 9-day test period. Requirements
are reduced to approximately four people for the other three days.
Typical Equipment Requirements. Three 3.5"-diameter settling column with 1.5 m height equipped with 1
rpm-stirrer; two, 1- liter graduated cylinders, each, with a 1-rpm stirrer; three SS probes with data loggers;
four sludge blanket measuring devices; five sludge judges equipped with discharge ports placed 1 ft apart;
one fluorometer; one jar test apparatus; two magnetic flow meter (either portable or fixed), and one
Kemmerer sampler.
7-1
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Stress Testing Costs. Approximate costs associated with carrying out a stress testing program have
been included in Table 7-1. These costs will vary with complexity of the system being tested and the
level of modeling required.
Table 7.1. Approximate Stress Testing Budget
Labor Other Direct Costs (ODC)
Position
Project
Management
Field Engineers/
Operators
Project Engineer
CAD Technician
Administration
Total
Hours
50
320
100
40
32
542
Cost1
$7,000
$38,400
$13,500
$4,200
$1,900
$65,000
Laboratory
Equipment
Modeling Allowance
$7,000
$20,000
$12,000
Total Cost
$104,000
Note: 1 - Costs include loaded labor rate to account for all direct and indirect cost associated with personnel.
The costs developed in Table 7-1 represent an approximate cost for carrying out a stress test focusing on the
secondary treatment plant process based on previous experience. The costs listed above; do not include time
associated with permitting agency discussions nor the temporary piping modifications that might be required for
the field portion to simulate wet weather flows for stress testing. These types of discussions and modifications
will vary from location to location across the nation. Depending on the existing conditions at a facility,
modifications might be an additional $10,000, with time coordinating with permit agencies ranging from $5,000
to $10,000 depending on the level of involvement. Conducting a stress testing program for an entire plant
(similar to a rerating study) would involve a great deal more complexity and associated costs.
In addition, the level of modeling development can vary significantly since more complex BNR type processes
take more time to build and involve numerous more steps to ensure that acceptable effluent quality is
maintained. Depending on the model employed (as described previously in Section 4) and the process being
modeled, modeling costs can range from $8,000 to $25,000. The number of simulation runs significantly
impacts modeling costs.
POTWs that have adequate resources and staffing can perform a number of activities involved in stress testing
and can greatly reduce cost. Since each POTW is different, this is a judgment call each facility will have to
determine for development on their internal budget.
7.2 Timeframe
The timeframe for developing an all encompassing protocol will vary based on the particular process train for
which the protocol is being utilized. A number of factors ranging from internal recycle and sidestream
considerations, to sampling requirements and protocol, to test plans outlining specific roles and responsibilities,
to possibly BNR considerations, will need to be examined.
7-2
-------
A tentative timeframe for development of protocols in conjunction with actual field testing of the protocol at a
POTW has been outlined in Table 7.2.
Table 7.2. Timeframe for Protocol Development and Field Testing
Task
1
2
3
4
Duration
Component . , .
(months)
Review of POTW facility, tabulation of data needs, and baseline
condition assessment
Test Plan Development
Conducting Field Testing and Data Analysis
Report Findings and Recommendations
Total
2
1.5
1
2
6.5
This timeframe may need to be modified for facilities that have seasonal variations in their discharge permits
and seasonal variations in their wastewater characteristics as seen in northern climates. In these instances,
extending the testing period to multiple seasons may be appropriate. This is an item a POTW will need to
evaluate prior and consult historical data trending for their facility.
7-3
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Section 8
Conclusions and Recommendations
This report has outlined the proposed stress testing protocol to be employed at POTWs across the nation for a
wide range of unit processes. This has built upon the previous analysis of case studies from the literature review
and past experiences.
Coordination with regulatory agencies is considered key before embarking on such a program.
The implementation of a plant wide stress testing approach requires significant planning, up-front commitment
by decision-makers, potential design and flow modifications, and coordination with operations and laboratory
staff for the best chance of a successful outcome.
The use of process and hydraulic computer modeling are valuable engineering tools that streamline the
evaluation process and reduce demand on plant staff for on-site field stress testing. Modeling also enhances and
makes possible quick evaluations of various capital improvement and process optimization methods for
increasing flow through wastewater treatment facilities. Even though modeling cannot substitute for practical
field testing, one should always view the model results with some skepticism and not blindly accept the values.
Development of a standard stress testing protocol, which can be used by various POTWs serving effectively as a
"Go-By," would allow for a quicker comparison of results and techniques employed and aid smaller- to medium-
sized municipalities in their commitment to being stewards of the environment. Deviations in the protocol
would still be needed to account for the unique nature of each facility since there is not a "one size fits all
approach." Many POTWs that have not conducted a stress testing program would find this first-hand
information valuable in development of their wet weather program, once collection system alternatives have
been exhausted.
Many POTWs share a common driver stemming from increasing water quality concerns for receiving water
characteristics. TSS has been the traditional driver, but increasing nitrogen and phosphorus are playing an
increasing role even during wet weather events.
Aging infrastructure is another consideration. In some instances, many POTWs are faced with limited build out
capacity because of population encroachment at existing treatment facilities. Negating capital cost is a major
consideration solely to address peak wet weather flows, especially when examining smaller less funded
municipalities.
Through the use of conservative design standards (e.g., Ten States, etc.) there is untapped clarification capacity
at a majority of POTWs. POTWs can use this excess capacity to maximize use of their existing infrastructure as
part of their weather operational strategy.
8-1
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Appendix A
References
APHA, AWWA, and WEF (2005). Standard Methods for the Examination of Water and Wastewater.
21st Edition. American Public Health Association, Washington, D.C.
Benefield, L.D., and Randall, C.W. 1985. Biological Process Design for Wastewater Treatment. Prentice-Hall Inc.
Daigger, G.T., and Roper, R.E. (1985). "The relationship between SVI and activated sludge settling
characteristics." J Water Pollut Control Fed , 57, 859.
Daigger, G.T. 1995. "Development of Refined Clarifier Operating Diagrams Using an Updated Settling
Characteristics Database." Water Environ. Res. 67:95.
Gernant, Johanson, Lee, Jeyanayagam, Bertino, Albertson, Reinhold. Getting More Out of Secondary Clarifiers -
The Value of Full Scale Testing (2009). Proceedings of 82nd Annual Water Environment Federation
Conference and Exposition (WEFTEC 2009), Chicago, IL, 2-6 October 2009.
Jenkins, D., Richard, M.G., and Daigger, G.T. 2003. Manual on the Causes and Control of Activated Sludge
Bulking, Foaming, and Other Solids Separation Problems, 3rd edition, Lewis Publishers.
King County (2011). Wastewater Treatment Division. "CSO Control Program". Accessed April 5, 2011.
http://www.kingcountv.gov/environment/wastewater/CSO.aspx
Kalinske, A.A. 1973. Municipal Wastewater Treatment Plant Sludge and Liquid Sidestreams. Contract No. 68-01-
0324, Environmental Protection Agency, Washington, D.C.
Klein, Smith, Melcer (2008). Solids System Stress Testing and Process Optimization. Proceedings of 81st Annual
Water Environment Federation Technical Exhibition and Conference (WEFTEC 2008), Chicago, IL, 10-14
October 2008.
Newbigging, M., Dow, G., and Hill, J. (2004). Getting the solids out: optimization of solids
thickening and dewatering processes increases plant capacity. Proceedings of 77th
Annual Water Environment Federation Conference and Exposition (WEFTEC 2004), New Orleans, LA, 2-6
October 2004.
Parker D.S., Henry, H.Z., Kinnear, D.J., Wahlberg, E.J. (1999). Approach for diagnosing
secondary clarifier performance problems and prescribing improvements. Presented at the First annual
Water Environment Federation plant operations specialty conference: Maximizing the performance of
small- and medium-sized wastewater treatment plants, Milwaukee, Wisconsin, June 6-9, 1999.
Peng, Rigdon, Russell (2007). Sludge Blanket formation and its Effects on Secondary Clarifier Performance.
Journal of Environmental Engineering. July 2007.
Pitt, Niekerk, Garrett, Hildebrand, Bailey (2007). Management of Wet Weather Flow Conditions at BNR/ENR
Treatment Plants. Proceedings from the Annual Water Environment Federation Nutrient Removal
Conference, 2007, Baltimore, MD, 4-7 March 2007.
Reiger, L, Koch, G., Kuhni, M., Guger, W. and Siegrist, H., 2001. The EWWAG Bio-P module for activated sludge
model No. 3. Water Research, Vol. 35, No. 16, pp. 3887-3903.
Wahlberg, E.J. (1999). Establishing primary sedimentation tank and secondary clarifier Evaluation Protocols.
Research Priorities for Debottlenecking, Optimizing, and Rerating Wastewater Treatment Plants, Final
Report, Project 99-WWF-l. Water Environment Research Foundation, Alexandria, VA.
Wahlberg, E.J. (2001). WERF/CRTC Protocols for Evaluating Secondary Clarifier Performance; Project OO-CTS-1;
Water Environment Research Foundation, Alexandria, VA.
A-2
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WERF (1999). Research Priorities for Debottlenecking, Optimizing, and Rerating Wastewater Treatment Plants.
Project 99WWF1 Final Report. Water Environment Research Foundation, Alexandria, VA.
WERF (2003). Methods for Wastewater Characterization in Activated Sludge Modeling. Project 99WWF3 Final
Report. Water Environment Research foundation, Alexandria, VA.
A-3
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Appendix B
Modeling Processes
Biological Processes Included in Major Activated Sludge
Models
Process
ASM1
ASM2d
ASMS ASDM
Mantis
Non-Phosphate-Accumulating Heterotrophic Biomass Growth and Decay
Aerobic storage of readily
biodegradable substrate
Anoxic storage of readily
biodegradable substrate
Aerobic growth of non-phosphate-
accumulating heterotrophic biomass
Anoxic growth of non-phosphate-
accumulating heterotrophic biomass
Fermentation (performed under
anaerobic conditions by non-
phosphate-accumulating
heterotrophs)
Decay of non-phosphate-
accumulating heterotrophic biomass
Respiration of organics stored by non-
phosphate-accumulating heterotrophs
Yes
Yes
Yes
Yes1
Yes1
Yes
Yes
Yes
Yes
Yes
Yes
Yes2
Yes2
Yes3
Yes3
Yes
Yes
Yes
Yes
Yes
Yes
Autotrophic Biomass Growth and Decay
Aerobic growth of autotrophic
biomass (nitrification)
Decay of autotrophic biomass
Yes
Yes
Yes
Yes
Yes
Yes2
Yes4
Yes4
Yes
Yes
Phosphate-Accumulating Heterotrophic Biomass Growth and Decay
Storage of internal storage material by
phosphate-accumulating
heterotrophics
Aerobic storage of polyphosphate by
phosphate-accumulating
heterotrophics
Anoxic storage of polyphosphate by
phosphate-accumulating
heterotrophics
Yes
Yes
Yes
Yes
Yes
Yes
B-l
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Process
Aerobic growth of phosphate-
accumulating heterotrophics
Anoxic growth of phosphate-
accumulating heterotrophics
Decay/lysis of phosphate-
accumulating organisms
Lysis of cell internal storage material
Lysis of stored polyphosphate
Anoxic Methylotrophs
Growth of anoxic methylotrophs
Decay of anoxic methylotrophs
Hydrolysis
Aerobic hydrolysis of slowly
biodegradable substrate
Anoxic hydrolysis of slowly
biodegradable substrate
Anaerobic hydrolysis of slowly
biodegradable substrate
Ammonification of soluble organic
nitrogen
Hydrolysis of organic nitrogen
Hydrolysis of organic phosphorus
Metal Precipitation
Precipitation of ferric phosphate
Dissolution of ferric phosphate
Miscellaneous
Adsorption or flocculation of colloidal
organic material to particulate organic
material (occurring spontaneously
Assimilative denitrification of nitrate
or nitrite to ammonia for synthesis
ASM1
Yes
Yes
Yes
Yes
ASM2d
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
ASMS
Yes
Yes
ASDM
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Mantis
Yes
Yes
Yes
Yes
Source: IWA 2000 and BioWin and GPS-X help files.
B-2
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Notes:
1. Two processes are modeled here with ASM2d: growth on fermentable substrates and growth on fermentation products.
2. Both aerobic and anoxic decay (endogenous respiration) and respiration of organics stored by biomass is modeled.
3. Non-phosphate-accumulating heterotrophs can grow on complex readily-biodegradable organics, acetate, propionate, or methanol
under aerobic conditions, but complex readily-biodegradable organics, acetate or propionate under anoxic conditions. Ammonia is used
for the nitrogen source.
4. The growth and decay of ammonia-oxidizing and nitrite-oxidizing microorganisms are considered separately.
B-3
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Appendix C
Limitations of Activated Sludge Models
Limitations
ASM1
Assumes the system operates at constant temperature;
Assumes the pH is constant and near neutrality;
Does not contain kinetic equations that address nitrogen, phosphorus and
alkalinity limitations of heterotrophic growth;
Includes biodegradable soluble and particulate organic nitrogen - both of
which are difficult to measure;
Kinetics of ammonification are fast and don't affect model predictions;
Differentiates inert particulate COD based on origin (X, is from influent, XP is
from biomass decay), even though it is impossible to differentiate these two
fractions in reality;
Does not directly predict MLSS;
Lysis combined with hydrolysis and growth describes the lumped effects of
endogenous respiration of storage compounds, death, predation and
biomass lysis; it is difficult to evaluate the kinetic parameters for this
lumped process; and
Does not include processes that occur under anaerobic conditions.
ASM2d
Temperature is expected to be in the range of 10 to 25°C;
The wastewater should contain sufficient Mg2+ and K+;
pH should be near neutral; and
Processes with an overflow of acetate/fermentation products to the
aeration tank cannot be modeled.
ASMS
Developed based on experience for wastewater temperatures ranging from
of 8 to 23°C; model equations might not be valid outside this range;
Developed based on experience for wastewater pH ranging from 6.5 to 7.5;
model equations might not be valid outside this range;
Does not include any processes that describe biomass behavior under
anaerobic conditions;
Is not applicable to cases in which nitrite concentrations are elevated; and
Not applicable to activated sludge systems with very high loads or very
small (< 1 day) solids retention times (SRTs).
ASDM
Model equations are not published in the literature as are the IWA
activated sludge modelsl;
Substantially more complex (over 50 state variables and 60 process
equations) than the IWA models.
C-l
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Mantis
All of the ASM1 limitations apply.
Source: Limitations of ASM1, ASM2d and ASMS are taken from IWA 2000.
Note:
1. Although the BioWin model is based on a general model published by Barker and Dold (1997), the ASDM has evolved substantially since
then. The current model process equations are not included in the help files associated with BioWin, nor published in the literature.
C-2
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Appendix D
Deficiencies Found During Stress Testing
(Secondary Treatment)
Performance of a settling tank is measured by the quality of secondary effluent as determined by ESS
concentration and by the extent of thickening indicated by RSSS concentration. Before starting the test, the
water surface of the settling tank should be free of foam, scum and other floating material that may contribute
to high ESS concentrations. Approaches for overcoming the performance, as well as the design problems of
settling tanks, are discussed here.
Floating Sludge on the Settling Tank Water Surface
Floating sludge is caused by denitrification, growth of specific organisms, by the presence of poorly degradable
surfactants or nutrient limitations. The following discusses how to alleviate floating sludge.
Denitrification
Once it is clear that denitrification is occurring, the problem needs to be corrected before starting the stress
tests.
• Lower MCRT, if nitrification is not required or nitrification can be achieved at a lower MCRT. Reduce DO
in aeration basin.
• Increase the speed of scraper or hydraulic suction system. Speed should be increased gradually so that
floes at the bottom are not disturbed.
• Increase RAS flow rate.
• Decrease number of on-line clarifiers.
• Increase DO level in the last section of the aeration basin or mixed liquor channel leading to the settling
tank.
• Add hydrogen peroxide as an oxygen source in the center well of the settling tank (Richard, 2003).
Filamentous Organisms
• M. parvicella Foam
Foam due to M. parvicella is recognized under the microscope. This organism causes bulking and its
presence yields high SVI values (Jenkins et al., 2003). Foam on the settling tank water surface can
be eliminated by preventing or reducing the growth of M. parvicella in the aeration basin by
lowering sludge age, preventing aeration basin zones having low DO levels (such as less than 1
mg/L), avoiding intermittent aeration, or providing plug flow regime in anoxic and aerated zones.
Adding polyaluminum chloride has been shown to reduce the growth of this organism (Roels et al.,
2002). Eliminating foam trapping systems in the aeration basin also results in reduction of M.
parvicella growth. Once M. parvicella is eliminated from the aeration basin, the remaining foam on
the SST water surface can be hosed with water sprayers to the scum collection box.
Nocardia spp. Foam
Foam caused by Nocardia spp. can be differentiated from other foams through microscopic
examination. Even though they are filamentous organisms, they do not
D-l
-------
influence settleability of activated sludge. High sludge ages, high concentrations of oil and grease in
the influent, and higher temperature promotes its growth. Surface trapping also aggravates the
foaming problem in aeration basins. Foam can be removed from the water surface of the SST by
mechanical means such as scum collectors. Water spraying will help to direct the foam toward the
scum collection box. However, true elimination of foam from the settling tanks can be achieved by
eliminating it from the aeration basin. This can be accomplished by reducing sludge age, spraying
chlorine solution onto the foam on the surface of the aeration basin, physically removing foam
from the surface of the aeration basin, adding polymer into the return sludge line or mixed liquor
channel to flocculate Nocardia spp into activated sludge floes, or the use of anoxic selectors
(Jenkins et al., 2003).
• Type 1863 Foam
This foam can also be identified easily with microscopic examination. Type 1863 growth is caused
by low sludge age (usually less than 3-4 days combined with low aeration basin DO level) and also
by high influent oil and grease levels. It can be eliminated by increasing DO concentration and
sludge age and by reducing influent oil and grease levels.
• Other Types of Foams
White to gray foam at the start-up of the activated sludge process, which causes an increase in ESS
concentration, is a temporary situation. The gray foam caused by discharge from solids processing
systems can be eliminated by eliminating digester overflows and reducing polymer feed to
dewatering. Thick pasty or slimy grayish foams due to nutrient deficit conditions can be eliminated
by effluent concentrations of 1-2 mg/L for ammonia plus nitrate and for orthophosphate (Richard,
2003).
High Sludge Blankets
High sludge blankets can be caused by a number of factors including higher MLSS concentrations, higher flow
rates, lower RAS flow rates, deteriorating sludge settling characteristics, poor design of settling tanks, and poor
sludge removal mechanism. The roles of MLSS concentration, influent flow, RAS flow, sludge settling
characteristics, and settling tank surface area in causing high sludge blankets can be investigated using the State
Point Analysis.
Factors causing high sludge blankets are enumerated as follows:
• Higher MLSS Concentration
o An increase in MLSS concentration may overload the SST resulting in high sludge blankets.
Higher MLSS may be due to an inadequate amount of sludge wasting or excessive solids
discharge in the side streams combined with poor solids removal efficiency in the primary
settling tanks. There are several options to prevent overloaded conditions.
o If the state point, where the overflow rate and underflow rate operating lines intersect each
other, is below the sludge settling flux curve, increasing RAS flow rate may result in under
loaded condition and cause a decrease in sludge blanket depths.
o Recycle MLSS concentration temporarily in the influent to the settling tank by storing solids in
the aeration basin by applying a step-feed configuration and/or reducing aeration rate.
• Higher Flow Rates
o Higher flow rates cause higher SLRs, and if the solids removal rate is less than the solids
application rate, sludge blanket rises.
o Higher flow rates may be reduced by improving the poor design and/or operation of influent
distribution system and diurnal flows.
D-2
-------
o If unequal flow distribution is the cause, valves and gate positions can be adjusted to distribute
the flow equally.
o Peak flows temporarily cause overloaded condition during which solids are stored in the
settling tank, sludge blanket rises, and therefore, the MLSS concentration and SLR decrease.
The decrease in MLSS, in turn, may result in an underloaded or a critically loaded condition.
Then, the operator does not alter any operational conditions.
o However, if the settling tank is still overloaded despite a decrease in MLSS concentration, the
course of action depends on the location of the state point.
o If the state point is below the settling flux curve, the RAS flow rate can be increased to the
point that the settling tank becomes critically loaded or underloaded. If the state point is
above the settling curve, increasing the RAS flow rate will not remedy the situation. In that
case, put a spare settling tank online or expand the plant or provide flow equalization.
Lower RAS Flow Rate
Higher sludge blankets may also be due to a low RAS flow rate relative to influent flow rate. The
remedial actions to be taken include:
o Conduct a state point analysis to determine proper RAS flow rate. Applying a constant ratio of
influent flow to RAS flow may solve the problem.
o If one of the RAS pumps is not working properly, then the pump needs to be replaced or
repaired.
o The return sludge line may be clogged and should be cleaned to regain the proper RAS flow
rate.
Poor Sludge Settleability
Deterioration in settling and compaction characteristics of sludge may be mitigated by preventing
overgrowth of filamentous organisms or excessive production of exocellular polymers. Once the
identity of filamentous organisms is known, the conditions leading to their growth is determined.
By reversing those conditions or adding toxicants, filamentous organisms can be eliminated or
reduced in large numbers.
The sulfides can be reduced by its oxidation to sulfate by aeration or by precipitating with ferric
chloride. Other methods of filament control include installing aerobic, anoxic and anaerobic
selectors or adding toxicants such as chlorine, hydrogen peroxide and ozone. Polymers and
coagulants can be used to improve settleability of activated sludge. If a viscous bulking condition is
confirmed, it can be remedied by adding nutrients or by reducing F/M ratio and increasing DO
concentration.
Poor Sludge Removal Mechanism
High sludge blankets may result from the problems associated with sludge removal mechanisms.
These problems and their solutions are summarized below:
o In settling tanks equipped with suction sludge removal system, the seal between the rotating
arm with orifices and the underflow line may be worn out resulting in shortcircuiting of liquid
into the RAS line. If this is the case, seals need to be replaced.
o The orifices on the suction manifold may be plugged, resulting in less sludge removal.
o Drain the settling tank and clean orifices.
o In riser type suction sludge removal system, narrow riser tubes may be plugged easily.
o An inadequately designed scraper system may not effectively divert the sludge to the central
hopper.
D-3
-------
o The scraper speed may not be optimum. Adjust the scraper speed.
o Sludge collection may not be effective due to broken chains, flights or worn shoes in
rectangular settling tanks and uneven squeegees on plows, resulting in plows riding high on
one side of the tank and scraping the bottom on the opposite side in circular tanks. To address
these problems, take the settling tank out of service and drain it. Replace broken chains,
flights, and worn shoes in rectangular tanks, and adjust squeegees in circular tanks.
o Pumps are not running at proper capacity or sludge lines are clogged. Check pump suction line
for closed valves. If valves were closed, open them and recheck pump output. All valves are
open, backflush sludge lines to remove blockage. If blockage cannot be cleared, remove the
settling tank from service, drain it, and remove blockage from the sludge line.
D-4
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Appendix E
Peak Wet Weather Flow Literature Review
Table of Contents
Section 1: Introduction, Objective, and Report Organization E5
1.1 Introduction E5
1.2 Objectives E5
1.3 Report Organization E5
Section 2: Literature Review Methodology E6 •
Section 3: Overview of Stress Testing Considerations E7 •
3.1 Hydraulic Considerations E7
3.2 Biological Considerations E7
3.3 Solids Handling Considerations E8
Section 4: Stress Testing Strategies and Implementation E9
4.1 Northeast Region E10
4.1.1 Climate E10
4.1.2 Population E10
4.1.3 Receiving Water Considerations E10
4.1.4 Regional Stress Testing Examples E10
4.2 Midwest Region Ell
4.2.1 Climate Ell
4.2.2 Population E12
4.2.3 Receiving Water Considerations E12
4.2.4 Regional Stress Testing Examples E12
4.3 South Region E13
4.3.1 Climate E13
4.3.2 Population E13
4.3.3 Receiving Water Considerations E13
4.3.4 Regional Stress Testing Examples E14
4.4 West Region E14
4.4.1 Climate E14
4.4.2 Population E15
4.4.3 Receiving Water Considerations E15
4.4.4 Regional Stress Testing Examples E15
E-l
-------
Section 5: Case Studies E17
Case Study Criteria E17
Case Study #1 -Jones Island WastewaterTreatment Plant E18
Case Study #2 - North District Wastewater Treatment Plant E21
Case Study #3 - Mauldin Road Wastewater Treatment Plant E23
Case Study #4 - R.M. Clayton Water Reclamation Center E25
Case Study #5 -Alternative Case Study-City of Corona Plant No. 1 and No. 2 E27
Section 6: Engineering Approaches/Procedures Identified E29
Section 7: Evaluations and Conclusions E33
Appendix A: References E35
E-2
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Acronyms
ADF Average Daily Flow
ALCOSAN Allegheny County Sanitary Authority
ASM2 Activated Sludge Model 2
BNR Biological Nutrient Removal
BOD Biochemical Oxygen Demand
BODS Biochemical Oxygen Demand Five Day •
CEPC Chemically Enhanced Primary Clarification
CFD Computational Fluid Dynamics
CRTC American Society of Civil Engineers Clarifier Research Technical Committee
CSO Combined Sewer Overflow
DO Dissolved Oxygen •
DSS Dissolved Suspended Solids
ENR Enhanced Nitrogen Removal
ESS Effluent Suspended Solids
F/M Food to Mass Ratio
FSS Final Suspended Solids
g gram
gpm/sf gallons per minute per square feet
HPO High Purity Oxygen
IDEM Indiana Department of Environmental Management
I/I Infiltration/Inflow
MBR Membrane Bioreactor
MC4 Minimal Control Principal #4 •
MGD Million Gallons Per Day
ML Mixed Liquor
ml milliliter
MLSS Mixed Liquor Suspended Solids
MMSD Milwaukee Metropolitan Sewerage District
NDWWTP North District Wastewater Treatment Plant
POTW Publically Owned Treatment Works
PPCP Pharmaceuticals and Personal Care Products
PWD Philadelphia Water Department
E-3
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RAS Return Activated Sludge
RPM Revolution Per Minute
RSSS Return Sludge Suspended Solids
SBD Sludge Blanket Depth
sf square feet
SLR Solids Loading Rate
SOR Surface Overflow Rate
SPU Seattle Public Utilities
SS Suspended Solids
SSO Sanitary Sewer Overflow •
SSVI Stirred Sludge Volume Index
SVI Solids Volatile Index
IDS Total Dissolved Solids
TKN Total Kjeldahl Nitrogen
TMDL Total Maximum Daily Load
USEPA United States Environmental Protection Agency
WAS Waste Activated Sludge
WERF/CRTC Water Environment Research Foundation/Clarifier Research Technical Committee
WRC Water Reclamation Center
WWTP ... ...Wastewater Treatment Plant
E-4
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Section 1: Introduction, Objectives, and
Report Organization
1.1 Introduction
Treatment of peak flows during wet weather is a common challenge across the country for municipal
wastewater collection systems with separate or combined sewer systems. Increases in wastewater flow resulting
from infiltration and inflow (I/I) during wet weather events can result in operational difficulties for publically
owned treatment works (POTWs) and compromise proper treatment and compliance with discharge permits or
receiving water criteria. Thus, a need can exist for POTWs to increase peak wet weather capacity while
protecting the functionality of sensitive unit treatment processes.
In order to access the ability to capture and treat higher peak flow rates and greater volumes of wet weather
flow, POTWs are performing stress testing to demonstrate the capacity of existing treatment processing units
and investigating ways to maximize treatment capacity (WERF, 1999).
Communities around the country are embarking on multi-year, capital-improvement programs to upgrade their
wastewater and stormwater facilities for a variety of reasons, including aging infrastructure, regulatory
requirements, and increasing populations. For these programs, treatment plant stress testing can help by
assessing the maximum use of existing POTWs through operational changes or cost-effective capital
improvements that can potentially reduce larger capital investments in new treatment facilities.
1.2 Objectives
The goal of this literature review is to identify POTW stress testing approaches and procedures implemented
around the country. The main focus is on facilities that have conducted stress testing for the purpose of peak
wet weather flow management. Published literature for POTWs that performed stress testing for the purpose of
plant consolidations was examined in development of the case studies in this report.
One objective of this review was a comprehensive literature search and summary of published examples of
stress testing performed at POTWs. This information is organized by geographic regions across the United States
to take into consideration varying climate, population, and water quality concerns.
Another objective of the study was to evaluate key elements of a stress testing program for POTW managers to
consider when investigating peak wet weather treatment capacities. This includes a review of typical
deficiencies that need to be addressed to improve peak wet weather capacity. Lastly, summaries of case studies
identified during the course of the literature review are included to provide examples of stress testing programs
and conclusions.
1.3 Report Organization
The main report is divided into seven sections: (1) Introduction, Goals, and Objectives; (2) Literature Review
Methodology; (3) Overview of Stress Testing Considerations; (4) Stress Testing Strategies and Implementation;
(5) Case Studies; (6) Engineering Approaches/Procedures Identified and (7) Evaluation and Conclusions.
Literature review references are included in Appendix A.
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Section 2: Literature Review Methodology
A literature review was conducted in order to examine the scope of stress testing used in wet weather planning,
and to narrow the focus of the review to relevant case studies. This review included planning approaches
consistent with traditional wastewater facilities planning as well as innovative testing strategies that have been
implemented in the United States.
Sources of information included:
Engineering and scientific journals. This included:
ASCE Journal of Environmental Engineering
Journal of Water Science & Technology
Water Environment Research
Journal of Water Pollution Control Federation
Water Environment & Technology
Knovel Interactive Library and Ingentaconnect database. These sources were used for identification of
various manuals of practices, and published reference documents.
Various guidance documents. This included information from:
Water Environment Federation
Water Environment Research Foundation
Association of Metropolitan Sewer Agencies
National Association of Clean Water Agencies
Conference proceedings from various wet weather conferences. These included:
Water Environment Federation Technical Exhibition Conferences
Collection System Conferences
Nutrient Removal Conferences
Internal COM compilation library. These included past projects and consultation with senior
wastewater treatment design experts within COM.
Review of published literature was generally limited to the past 10 years through the various databases. For the
purpose of this report, we focused on information published since 2000. Information from this time period to
present is available more commonly in electronic format, thus providing a great amount of information for the
literature review.
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Section 3: Overview of Stress Testing
Considerations
The capacity of a wastewater treatment plant is more than the sum of its parts, yet having a good understanding
of the capacity of each part or process unit (including hydraulic), plays a crucial role in plant design and
operation. Most unit processes are adaptable to stress testing which is defined as the intentional operation of a
unit process to its point of failure. In some cases stress testing is straightforward and simple, e.g., flow is
increased through a process until the hydraulic capacity is exceeded (overflows, or back-ups). In other cases the
testing process is much more complex, such as when nutrient removal effects are examined (Nailor et al, 2006;
Pitt et al, 2007).
3.1 Hydraulic Considerations
Stress testing establishes performance under varying load conditions (Daigger and Buttz, 1998). More often the
focus is on primary and secondary capacity (clarification), but preliminary capacity (screening and grit removal)
needs to be examined as well. The purpose of this testing is to evaluate the hydraulic performance
characteristics and identify hydraulic bottlenecks. Specifically, it can be used to identify occurrence of short-
circuiting, dead zones, and density/thermal currents. The information generated allows strategies to be
developed (e.g., baffling) for improving clarifier hydraulics. This, in turn, will result in enhanced process
efficiency.
Capacity of primary clarifiers as an individual unit operation is typically evaluated based on surface overflow rate
(SOR) and biochemical oxygen demand (BOD) removal criteria. Primary clarifier performance is dependent on
SOR and sludge withdrawal rates. Performance expectations are tied to the process capacity of the subsequent
biological systems and primary sludge handling systems. Secondary clarifier performance is not only dependent
on SOR, but also mixed liquor suspended solids (MLSS) concentrations and recycle ratios, aeration system
performance, and sludge withdrawal rates, making it a much more complex system to test (Parker et al, 1999).
Slug dye tests and the solids distribution/flow pattern tests are carried out during the stress tests in an effort to
better assess the hydraulic characteristics of the settling tank.
A desktop review or hydraulic calculation check (typically computer based) based on the plant hydraulic profile is
the first step to determine the existing limitations and what is practical before embarking on a stress testing
program. Typical limitations examined as part of the desktop review include; approach velocities, recycle rates,
diurnal peak considerations, and freeboard limitations.
3.2 Biological Considerations
Stress tests are carried out to determine the performance of a settling tank (typically for secondary treatment
processes) in terms of effluent suspended solids concentration for a variety of operating conditions. Effluent
suspended solids concentration is examined against parameters such as: SOR, solids loading rate (SLR), return
sludge suspended solids (RSSS) concentration, return activated sludge (RAS) flow rate, sludge settling
characteristics, dissolved suspended solids (DSS) concentration, and sludge blanket depth (Gernant et al, 2009;
Peng et al, 2007).
Before starting a stress test, existing plant operational data must be examined in detail to understand the
limitations and operational constraints. Laboratory data for a unit process are especially critical for a fair
comparison with stress testing results. One essential element of stress testing is that a facility needs to have
multiple settling tanks so as to not impair continued treatment performance of the plant while still having a
settling tank available for testing purposes. It is difficult to conduct a stress test with a single clarifier unless
creative ways are found to increase the SLR to the test tank, such as diverting a portion of the effluent to the
tank influent to increase SLR (Wahlberg, 2004).
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3.3 Solids Handling Considerations
Stress testing for solids processing systems is a challenging and often difficult task. This usually involves a large
number of operational variables to consider. Furthermore, process efficiency is often difficult to obtain in "real-
time data" and many plants do not usually gather operational data beyond the solids mass entering and leaving
the system (Klein, 2008).
Solids processing systems are usually rated in terms of pounds of solids per day. This means that both hydraulic
loading and feed solids concentration have to be taken into account. Modifying the solids concentration is more
complex than adjusting a pump set point. It is rarely possible to make adjustments during a test run, thus a target
feed concentration has to be selected weeks prior testing. This might require plant staff to modify plant
operations to meet the selected target. Historical data analysis and plant staff experience will usually indicate the
most practical feed concentrations attainable.
During the course of this literature review, no examples were found focusing on solids handling stress testing for
the purpose of managing peak wet weather flows. Though influent flow increases during peak wet weather
events, total suspended solids (TSS) loading to the unit processes is typically reduced because of dilution. First
flush considerations will need to be examined however; the first flush and peak wet weather event may not
coincide. Depending on the region and climate, considerations for the first flush will introduce a noticeable higher
solids loading content to the treatment facility from the collection system, following a period of little to no rainfall.
Existing solids handling processes typically have sufficient capacity for single wet weather events. However, back-
to-back storm events can exceed the capacity of sludge processing units, such as digesters and storage tanks. A
review of the solids handling unit processes should be undertaken as part of the overall stress testing approach
(Newbigging et al, 2004).
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Section 4: Stress Testing Strategies and
Implementation
The results from the literature search were summarized into four regions of the United States (US) — Northeast,
Midwest, South, and West (see Figure 4.1). The delineation of geographic regions was adopted from the US
Census Bureau based on the 2010 census. For each region, climate, population, and receiving water
considerations were characterized to identify regional issues and drivers for wet weather flow management and
stress testing at POTWs (HowStuffWorks, 2011a-e). Regional examples of stress testing programs are included in
this section.
ifer* >
|r g Northeast
South
Figure 4.1. Regions of the United States
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4.1 Northeast Region
4.1.1 Climate
The Northeast climate is humid continental. Winters are cold with temperatures averaging 0-25°F, and snowfall
ranging from 32-100 inches. Summers are warm and humid, with temperatures averaging 65-80 F. "Noreaster"
storms in the winter provide steady, but light rain along the coast and spring and summer thunder storms
account for the remaining rainfall. Annual rainfall ranges from 32-64 inches.
4.1.2 Population
The Northeast region experienced the lowest population growth rate in the United States between the years
2000 and 2010 with a rate of approximately 3.2 percent (US Census Bureau, 2010). Growth rates were highest in
the northern states of Maine (4.2%) and New Hampshire (6.5%) and lowest in Pennsylvania (3.4%), New York
(2.1%), and Rhode Island (0.4%).
4.1.3 Receiving Water Considerations
Major water quality drivers in the Northeast include combined sewer overflows (CSOs), Total Maximum Daily
Load (TMDL) requirements, nutrient limitations, Pharmaceuticals and personal care products (PPCPs) in water
supplies, and beach closures after major storm events related to high fecal coliform counts. CSOs are prevalent
throughout the Northeast with the majority of cities implementing Long Term Control Plans and many cities
under consent decrees to reduce overflows. Approximately 772 cities serving 40 million people in the United
States are served by combined sewer systems (US EPA, 2010a). At least 40,000 sanitary sewer overflows (SSO's)
are estimated to occur throughout the United States (US EPA, 2010b). SSOs are caused by excessive runoff
entering the systems (l&l), excessive sewage flows, blockages, and/or mechanical failures in the system.
SSO and CSO control plan costs in major Northeast cities are estimated to be in the billions of dollars. TMDL
compliance is leading to the development of new approaches throughout the East. With sediment being the
most common TMDL, flow-based approaches in areas where erosion is a major problem are being developed.
After storm events, beach closures along the coast and inland rivers related to high fecal coliform counts are
major concerns.
4.1.4 Regional Stress Testing Examples
During the course of the literature review, three published examples of municipalities conducting stress testing
in the Northeast were identified and are summarized below.
4.1.4.1 Philadelphia, Pennsylvania
The Philadelphia Water Department (PWD) manages stormwater, drinking water, and wastewater within
Philadelphia. The PWD has embarked on a watershed-based methodology using a balanced "land-water-
infrastructure" approach to control CSOs. The PWD uses an integrated regional watershed planning approach
emphasizing adaptive management to appropriately balance each approach. Each component is balanced to
achieve an overall solution to control CSOs. Land is focused on source control, water on ecosystem restoration,
and infrastructure on capital improvement projects. The overall goal is to minimize the introduction of runoff
into the sewer system.
The PWD has implemented a Capital Improvements Program to construct CSO infrastructure to reduce CSOs.
Projects include storage, conveyance, and treatment facilities (Philadelphia Water Department, 2011).
In keeping with their long-term control plan strategy, PWD conducted stress testing at three of its wastewater
treatment facilities looking at both primary and secondary treatment processes. These were Southeast,
Southwest, and Northeast Water Pollution Control Plants which are of similar design and all underwent
secondary treatment plant expansion in the late 1970's. A series of stress tests was performed with varying
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SORs for primary treatment, and secondary clarification testing at constant SORs and 15-minute sampling
intervals for TSS and BOD5.
The results showed that a dramatic increase in primary treatment capacity was possible during peak flow
conditions. At the Southwest, Southeast, and Northeast treatment facilities, primary capacity was increased
from average flow conditions by 115 million gallons per day (MGD), 92 MGD, and 71 MGD, respectively.
Secondary treatment capacity was not noticeably increased due to current MLSS strategies and some
operational constraints. It was noted that by increasing primary sludge removal pumping rates during peak
flows, a reduced organic load would be transmitted downstream to the secondary treatment process. This
approach would allow for greater removal at all flow rates (Ferguson et al, 2000).
4.1.4.2 Allegheny County, Pennsylvania
A stress testing program was carried out by Allegheny County Sanitary Authority (ALCOSAN) (Pittsburgh, PA)
focusing on primary treatment performance as part of their initiative to reduce CSOs. The main focus of their
testing program was to maximize primary treatment capacity through existing process units. A sophisticated
approach was undertaken involving the use of a hydraulic model and a year-long test schedule to evaluate
various flow conditions. The results showed that the existing primary sedimentation tanks had an additional
hydraulic capacity of approximately 60 MGD, resulting in a peak surface over flow rate (SOR) of 3,100 gallons per
day per square foot (gpd/sf). All of this equated to an increase in primary treatment over the rated design value
of 540 MGD, provided this was accomplished without co-settling of waste activated sludge (WAS). Sampling and
laboratory analyses demonstrated similar primary treatment performance in terms of TSS and BOD removals at
typical dry weather SORs and peak SORs. Primary effluent TSS and BOD concentrations were essentially the
same during dry weather and wet weather conditions. Based on modeling, this method for mitigating CSOs in
the collection process aided in capturing of up to 65 percent of the CSOs in the collection system (Mehrotra,
2008).
4.1.4.3 Bergen County Utilities, New Jersey
The Bergen County Utilities provides treatment for several member communities served by older combined
collection systems with significant infiltration. At the treatment facility, flows above 160 MGD were not able to
be treated in order to protect the established biomass in the secondary treatment process. Restricting wet
weather flows to the treatment plant resulted in overflows upstream of the treatment facility. In order to
reduce the upstream overflows, a stress testing program was established that focused on the secondary
treatment process since there was a deficiency in treatment capacity compared to primary treatment.
The use of polymer addition as part of the secondary clarification process for peak wet weather flow conditions
was the focus of the stress testing program. A series of jar and field tests was conducted to determine the
optimum dosing rate for the selected SOR for clarifier loading. The results indicated that secondary treatment
capacity could be expanded by 48 MGD for excellent (effluent suspended solids (ESS) below 15 mg/L), and 80
MGD for acceptable removal (ESS between 30 to 40 mg/L) for short durations corresponding with a peak wet
weather event. This was the only documented occurrence in this literature review of this approach being taken
for increased secondary treatment capacity. Other documented cases in this literature review focused on
process optimization, such as MLSS concentration, pumping rates, etc. Economic information was not available
for predicted chemical and labor cost to determine a preliminary lifecycle cost for this approach (Patoczka,
1998).
4.2 Midwest Region
4.2.1 Climate
Climate in the Midwest is characterized as humid continental in the eastern portion, and semi-arid on the
western edge of the region. Winters are cold, with temperatures averaging 0-30°F and snowfall ranging from 10-
60 inches. Summers are warm, humid and wet, with average temperatures of 70-85°F. Rainfall is generally
heaviest in spring and summer months, averaging between 16-35 inches.
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4.2.2 Population -
Population growth in the Midwest is low, with a growth of 3.9 percent between 2000 and 2010 (United States
Census Bureau, 2010). Within the region, growth was highest in Missouri (7.0%), and Minnesota (7.8%), while
negative growth was experienced in Michigan (-0.6%).
4.2.3 Receiving Water Considerations
Major water quality drivers in the Midwest include CSOs, SSOs, nutrient limitations, and nonpoint source
pollution. Nonpoint source pollution attributed to urban and agricultural runoff is increasingly becoming a major
water quality driver in the region with impacts downstream and outside of the region in the Gulf of Mexico.
Contamination and water quality issues in the Great Lakes have been at the forefront of discussion in this region.
4.2.4 Regional Stress Testing Examples
During the course of the literature review, two published examples of municipalities conducting stress testing in
the Midwest region were identified and are summarized below.
4.2.4.1 Indianapolis, Indiana
Indianapolis has two major water issues revolving around water quality in waterways and occasional peak
demands exceeding water system capacity of their CSO program. Indianapolis is performing program
improvements as part of a consent decree with the United States Environmental Protection Agency (USEPA) and
Indiana Department of Environmental Management (IDEM) to reduce raw sewage overflows (CSOs). Efforts to
comply with the decree and to reduce peak water demands are highlighted in this section.
Raw Sewage Overflow Long Range Control Plan and Plant Stress Testing
To improve water quality and comply with a USEPA and IDEM consent decree, the City of Indianapolis
implemented a CSO long-term program aimed at reducing the occurrences of sewage overflows into waterways.
Currently, the White River and its tributaries do not meet Indiana state standards for dissolved oxygen, and
bacteria. During the late 1990's, a stress testing program was conducted to evaluate maximizing wet weather
flows to the existing treatment facilities, keeping with the minimum control principal #4 (MC4) of the nine
minimum control standards (US EPA, 1994). The results demonstrated that maximizing flow to existing
treatment facilities was a viable solution, allowing for an additional 10 to 30 MGD of additional capacity at the
City's numerous treatment facilities. The evaluation of maximum flow limits also took into consideration other
impacts on the facilities including reduced freeboard and additional stress on the concrete structures, limitations
of the plant internal piping system due to pipe age, and process equipment coming close to the end of its
operational lifespan. To date, no additional stress testing has been carried out, and results from the stress
testing program are being evaluated for inclusion into the wet weather operational scheme.
Based on the literature review, implementation of the long term control plan was expected to reduce overflows
from 45-80 times per year to two to four times per year. To reduce overflows, the plan contains multiple
components, mainly focusing on real-time controls and storage as the primary mitigation means. Major
components include construction of a deep tunnel to capture overflows for pump-back and treatment after peak
flows subside, new sewers to capture overflows and discharge to the tunnel, and separation of combined sewers
(City of Indianapolis, Department of Public Works, 2008, 2011 and 2006).
4.2.5.2 Milwaukee, Wisconsin
Milwaukee, Wisconsin, has undertaken multiple initiatives to address wet weather water quality issues. The
City's Office of Environmental Sustainability has developed a green program for the City which includes water
quality improvement. Milwaukee Metropolitan Sewerage District (MMSD) has initiated a $1 billion overflow
reduction plan to be completed in 2011 to reduce CSO and SSOs to receiving waters. Prior to initiating efforts to
reduce CSOs/SSOs, an average of 8 to 9 billion gallons of water in the sewer system was released to Lake
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Michigan per year. Additional benefits of the plan include a reduction in non-point source pollutants (MMSD,
2009).
The City's two main treatment facilities are Jones Island and South Shore, with peak treatment capacities of 330
and 450 MGD, respectively. As part of the 2020 Facilities Plan the South Shore facility is to expand biological
nutrient removal (BNR) to treat all flows from the existing 300 MGD capacity secondary treatment process.
Currently this project is beginning conceptual design, with activated sludge expansion and physical-chemical
processes (both ballasted and flocculation only, without bioenhancement) being considered as part of the main
process upgrade selection (Fandk and Smith, 2006).
In 2005 the Jones Island facility embarked on a stress testing program to investigate peak flow capabilities
resulting from numerous constraints in the secondary treatment process. After thorough investigation, it was
determined that poor settleability of the mixed liquor (ML) was the main capacity limiting factor. To address this
deficiency, a modification of the aeration system was employed to have a small portion of the aeration system
serve as biosolids storage during wet weather events. This process is referred to as tapered aeration and is
often used in storm events, sometimes in conjunction with the step-feed approach to lower the peak oxygen
demand in the aeration tanks and corresponding food to mass (F/M) ratio. The stress testing and subsequent
change in system operation resulted in full reinstatement of the secondary treatment capacity during wet
weather events.
The decision to restore performance to the Jones Island wastewater treatment plant (WWTP) was due to its
centralized location in the collection system and to protect process performance at the South Shore WWTP. The
South Shore WWTP has undergone upgrades in the past decade and features a biosolids program producing
Class A reusable biosolids. Any significant disruption to the plant could result in a loss of biomass affecting the
quality and market of this product. This market driving consideration is one of the many aspects that must be
balanced with operations and funding before a stress testing program is initiated (Marten et al, 2009).
4.3 South Region
4.3.1 Climate
Climate in the South is characterized as humid and sub-tropical. Winters are mild in the south, with little to no
snowfall, and average temperatures ranging 50-70°F. Summers are hot and humid, with average temperatures
ranging 80-90°F. Annual rainfall averages between 32-64 inches with significant rainfall events occurring both in
the summer and winter.
4.3.2 Population
The South region experienced the highest growth rate in the country of approximately 14.3 percent from the
year 2000 to 2010 (US Census Bureau, 2010). Growth was highest in Georgia (18.3%), Florida (17.6%), Texas
(20.6%), and lowest in Washington, D.C. (5.2%) and West Virginia (2.5%).
4.3.3 Receiving Water Considerations
Water quality is driven in the South by TMDLs, environmental resource permits, and tourism. Major TMDL
impairments in the South include nutrients, bacteria, and dissolved oxygen. Environmental resource permits are
drivers of water quality in Florida and other states, especially for the aquifer storage and recovery programs (US
Census Bureau, 2011). In Florida, environmental resource permits are required for projects involving
construction or a significant alteration to storm water or surface water management systems. Water related
tourism in the South is a major industry, and providing clean inland and ocean/gulf waters is essential to
maintaining that industry. CSOs are not prevalent in the South. In a few sub-regional areas, including Atlanta
and Columbus, Georgia, Nashville, Tennessee, and Louisville, Kentucky CSOs are water quality concerns.
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4.3.4 Regional Stress Testing Examples -
During the course of the literature review, two published examples of municipalities conducting stress testing in
the South region were identified and are summarized below.
4.3.4.1 Atlanta, Georgia
A CSO Consent Decree was issued in 1998 commiting the City of Atlanta to an accelerated program of activities
designed to improve water quality in metro Atlanta streams and the Chattahoochee and South Rivers. To this
end, the City has expedited ongoing sewer improvements, including an intensive evaluation and rehabilitation of
sewer pipe conditions; a grease management program; and a capacity certification program for new
development. Additionally, improvements to the main treatment facility have been scheduled relating to
increasing capacity for both daily and peak flow conditions (City of Atlanta, Department of Watershed
Management, 2011).
The R.M. Clayton Water Reclamation Center (WRC) is an activated sludge plant located in Northwestern Atlanta
and is one of the largest wastewater treatment facilities in the Southeastern United States. The WRC serves
portions of three counties in addition to most of the City of Atlanta, and provides advanced secondary treatment
for approximately 80 MGD of wastewater. The WRC was permitted to discharge 100 MGD of treated wastewater
into the Chattahoochee River on a maximum month basis.
The WRC required upgrades and improvements because the plant's service area was increasing in density (i.e.,
impervious area, and population). Besides flow increases, the plant needed to be expanded because it was
unable to pass all flows that it received through the entire treatment process. Primary effluent was blended with
secondary effluent when flows reached approximately 110 MGD, and flows in excess of 180 MGD were bypassed
with only screening and disinfection. The WRC expansion stress testing program was conducted to rerate the
plant for an annual average day flow of 120 MGD from its existing 103 MGD rating. This allowed an increase of
17 MGD of full secondary treatment capacity for all flow conditions, and thereby reducing primary effluent
blending during wet weather events (Camp, Dresser, McKee, 2002).
4.3.4.2 Miami, Florida
The Miami-Dade Water and Sewer Department is the regional water and wastewater utility providing service to
over thirty muncipalities in southeast Florida. In the late 1980's and early 1990's, enforcement action from the
United States EPA and Florida Department of Environmental Protection led to development of two separate
Consent Decrees resulting in an estimated $1 billion in improvements. Historically, to address SSOs, theMiami-
Dade Water and Sewer Department focused its efforts on plant improvements. Since 1990, an increasing effort
has been placed on improving its collection system to reduce large sources of I/I (Maimi-Dade County, Miami-
Dade Water and Sewer Department, 2006).
The North District Wastewater Treatment Plant is a 112 MGD treatment facility located in Miami, Florida. In the
late 1990's the facility conducted a stress testing program in an effort to maximize secondary treatment
capacity, mitigate SSOs in the collection system, and reduce the practice of blending wet weather discharges.
Results from the stress testing program revealed that an additional 80.5 MGD in secondary treatment capacity
could be realized if process improvements and additional baffling and weir arrangement were optimized
(Jimenez 2008).
4.4 West Region
4.4.1 Climate
Climate in the West is the most diverse of all the regions in the United States. It ranges from arid to semi-arid in
the southwestern portion, marine and Mediterranean along the coast, and highland in the mountain and
northern portions. Winters are typically cool to mild with average temperatures ranging from 30-40°F in the
mountain and Pacific Northwest areas, to 50-60°F in the southwest and along the coast. Summers are dry with
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low humidity, with average temperatures ranging from 50-70°F in the mountain and Pacific Northwest areas,
and 85-95°F in the southwest. The majority of precipitation falls along the coast or in areas of increased
elevation. Precipitation tends to occur with greater extent during the winter months in the form of rain in
coastal areas and snow in the mountain and northern areas. Average annual rainfall ranges from 8-15 inches in
the southwest, 16-64 inches along the coast, and as high as 96 inches in the Cascade Mountains. Snowfall can
range from 32-64 inches in the mountains.
4.4.2 Population
Between 2000 and 2010, population growth in the West was the second highest of all the other regions, with a
growth rate of approximately 13.8 percent (US Census Bureau, 2010). Growth rates were the highest in the
desert states of Arizona (24.6%) and Nevada (35.1 %); and lowest in the Rocky Mountain States of Wyoming
(14.1 %), and Montana (9.7%).
4.4.3 Receiving Water Considerations
Within the West, water quality is driven by achieving compliance with TMDLs, compliance with discharge
requirements, nonpoint source pollution, total dissolved solids (IDS) management, and to a lesser extent CSOs
concentrated in the northwest portion of the region. TMDLs have and continue to be adopted in sub-regions of
the West for both inland waters and oceans impacting both dry and wet weather discharges. Bacteria and
metals are the main TMDLs in the region. Nonpoint source pollution impacts both groundwater and surface
waters in the West and requires watershed-based management plans. Compliance with discharge requirements
has required innovative solutions to reduce discharge volumes and refinement in treatment processes. High TDS
or salinity levels are prevalent in western areas relying on water from the Colorado River and localized
groundwater basins. High TDS levels adversely impact groundwater and agriculture, as well as potentially limit
the application of recycled water for urban irrigation.
4.4.4 Regional Stress Testing Examples
During the course of the literature review, two published examples of municipalities conducting stress testing in
the West region were identified and are summarized below.
4.4.4.1 Seattle, Washington
Management of CSOs began in the 1970s for the greater Seattle area. There are two municipal utilities that
have significant involvement in the CSO program; King County (which owns and operates the main interceptors
and treatment facilities) and Seattle Public Utilities (SPU), which owns and operates the service lines inside
Seattle. Each entity has a significant number of outfalls that discharge CSOs to waterways in Puget Sound and
Lake Washington. Each utility can have a significant impact on the other, since the systems are interconnected,
and wastewater flow from SPU is conveyed to King County's treatment facilities. SPU's extraneous flow control
measures have focused on source reduction and use of green infrastructure to reduce impervious surfaces,
while King County has examined wet weather improvements to their treatment facilities.
Due to increased population density in the northeast sewer sub-basin and aging treatment facilities in the
greater Seattle area, King County conducted stress testing at the Renton Wastewater Treatment Plant in part to
establish the design criteria for the new Brightwater treatment facility currently under construction. Once
completed the Brightwater treatment facility will have a capacity to treat 14 MGD average daily flow, and 36
MGD for peak wet weather flow.
Stress testing was carried out at Renton Wastewater Treatment Plant to determine reasonable loadings for the
use of chemically enhanced primary clarification (CEPC) for peak wet weather flows. The use of the CEPC
process was of critical importance to mitigate and reduce influent TSS to the downstream membrane bioreactor
(MBR) process. High TSS and BOD loadings could clog and reduce the lifespan of the MBR equipment. Stress
testing showed that a design SOR of 3,600 gpd/sf for average flow conditions could be achieved with a peak
loading rate of 5,400 gpd/sf compared to the standard 1,200 gpd/sf employed at the existing King County
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treatment facilities. The design approach of additional BOD and TSS removal through CEPC resulted in reduced
BOD loading on the downstream MBR process, thereby, making the combination of processes economical at the
expanded project scale (King County, Combined Sewer Overflow Program, 2011).
4.4.4.2 Corona, California
During the course of this literature review, a small number of published studies were found for facilities in
California conducting stress testing for handling peak wet weather flows. The primary driver for one published
stress testing program was for plant consolidation to reduce capital expenditures. It is important to note that
effluent water quality requirements are among the most stringent in the nation in keeping with the Title 22,
Article 7 wastewater reuse strategy (California department of Public Health, 2011).
The City of Corona was facing a capacity shortage and examined numerous options to reduce capital
expenditures for plant expansion. The facility features a BNR process with a step-feed mode of operation and
total treatment capacity of 15.5 MGD. A stress testing program focusing on primary and secondary treatment
was undertaken with careful consideration of the BNR process. A revised version of the International Water
Association Activated Sludge Model2 (ASM 2) was used for process simulations (Daiger et al, 1998). The model
was necessary due to the various potential changes in the process flow schemes ranging from aeration
modifications, MLSS concentrations, and different flow split scenarios though the chemically enhanced
primaries. The end result was that an additional 3.5 MGD of capacity was achieved mainly through process
optimization, but with some equipment replacement necessary as part of equipment lifespan phasing (Roxburgh
etal, 2005).
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Section 5: Case Studies
Case Study Criteria
Prior to the start of the literature review, selection criteria were established to ensure a well rounded approach
was being utilized to identify key facilities as potential case study candidates. Characteristics used as selection
criteria included:
Size of the facility. Larger treatment facilities tend to be located in metropolitan areas that often have
large diameter main interceptors as part of their collection systems. These large interceptors dampen the
effects of wet weather and result in lower peaking factors to the treatment facilities. Smaller to medium
size facilities tend to have higher peaking factors as a result of shorter time of travel in the collection
system. Therefore, a wide range of plant sizes were sought to consider these peak flow differences.
Regional location of the facility. To provide a cross-section of treatment facilities across the United
States the literature search included a regional perspective. Regional characteristics such as rainfall and
climate differences, population, and water quality concerns affect drivers for stress testing. It is very
difficult to have a "one size fits all" approach for stress testing.
Treatment process. Each treatment process is unique and certain approaches that will work for one will
not necessarily translate into comparable results for another process (i.e., pure oxygen, conventional
activated sludge, BNR, etc.). Case studies encompassing different types were considered for easier
comparison of treatment facilities across the United States.
Peak wet weather flow handling problem definition. Only treatment facilities that performed stress
testing as part of wet weather flow management were considered candidates for case studies. Treatment
facilities that had performed stress testing for plant consolidation, although helpful, would not be
included for this detailed evaluation.
Loadings (both hydraulically and biologically). The results from the literature search identifying
treatment facilities that included both hydraulic and biological aspects of stress testing were considered
for case studies.
Combined or separate collection systems. Wastewater characteristics can differ between separate and
combined sewer systems. In addition, temperature influxes occurring from dilution in a combined system
can affect reaction kinetics for both primary and secondary treatment process. This effect is more
pronounced in a combined system than in a separate collection system. Case studies were considered
from both types of collection systems.
Age of the facility. Given the aging infrastructure in the United States, a cross-section of older,
established, and relatively modern plants was examined.
Level of documentation readily available. Facilities that had performed stress testing where limited
information was available were not selected for detailed evaluation.
The evaluation criteria were revisited once the literature review process was complete to ensure a well-rounded
selection had been achieved. To aid the reader in the selection of the case study candidates, Table 5.1 outlines the
selected case studies with the preceding criteria in mind.
E-17
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Table 5.1 Case Study Overview
Size of
the
Facility
(ADF)
Testing
Region Process for Wet
Weather
Only
Combined
System
Separate
Sanitary
System
Unique Features
Jones
Island
WWTP
RM
Clayton
WRC
Mauldin
Road
WWTP
North
District
WWTP
City of
Corona
Plant No.
11
and
No. 2
100
120
20
112
5.5/3
Midwest
South
South
South
West
Activated
sludge,
BNR
Activated
sludge,
BNR
Activated
sludge,
BNR
Activated
sludge,
i • u
n CTn
1 ' '&' '
pure
oxygen
setup
Activated
sludge,
BNR
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Original plant
commissioned in 1925;
Nutrient considerations
Restore secondary
capacity for increased
ADF and peak flow;
Simple cost-effective
solutions
High peaking factor of
8;
Extensive modeling for
process considerations;
Equalization basins
CFD Modeling;
Aquifer storage and
recovery considerations
Rerating for increasing
population density;
Process Modeling
E-18
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Case Study #1
Jones Island Wastewater Treatment Plant
Plant Description
The Jones Island Wastewater Treatment Plant (WWTP), which is owned by the Milwaukee
Metropolitan Sewerage District (MMSD), is a large advanced secondary treatment plant
constructed in phases over approximately 85 years of operation. Liquid treatment processes at
the plant include coarse screening, grit removal, primary clarification, activated sludge and
disinfection. The plant's design average daily flow capacity is 123 MGD, maximum daily flow
capacity is 300 MGD, and peak instantaneous flow capacity is 330 MGD. Jones Island is operated
by a private enterprise under contract to MMSD. Due to the cold climate, this plant receives a
portion of its wet weather flow from snow melt.
Disinfection
Facility
Jones Island
Location
Milwaukee, Wisconsin
(cold weather climate)
Collection System
Combined
Unique Features
BNR process
Plant commissioned in
1925
Sludge
Stress Testing Objectives/Goals
A hydraulic and process capacity and operations review of the Jones Island WWTP was completed to determine
the available wet weather flow capacity of the facility and to identify methods of improving the limited wet
weather treatment capacity and performance due to underperformance above 225 MGD.
Stress Testing Methodology
A desktop analysis was conducted to verify hydraulic capacity and identify specific bottlenecks in the treatment
process. Additionally, a computer-based process model was utilized to identify process capacity limitations that
might result in treatment deficiencies (Merlo et al, 2007).
Stress Testing Results
The study evaluated both the biological treatment and hydraulic capacity of the activated sludge systems, and
determined that secondary clarification was the most significant process bottleneck in terms of peak wet
weather capacity. The evaluation found that secondary treatment peak flow capacity varies from approximately
200 to over 330 MGD depending on the mixed liquor concentration and settling characteristics. The MLSS
settleability, as measured by stirred sludge volume index (SSVI), was highly variable, noting an historic average of
90 mL/gram (g), with values ranging from 44 to 174 mL/g. To evaluate the effect of sludge settleability on
secondary treatment capacity, the study used biological modeling. It was estimated that, with a MLSS
concentration of 2,200 mg/L, secondary treatment peak flow capacity would be:
E-19
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405 MGD at an SSVI of 72 mL/g (very good settling sludge).
295 MGD at an SSVI of 83 mL/g (good settling sludge).
The study noted that the Jones Island WWTP activated sludge systems experienced periods of poor ML
settleability, commonly referred to as episodes of bulking (both filamentous and non-filamentous). This
condition resulted in poor settling activated sludge that can decrease secondary clarifier capacity, and as a
result, the peak flow treatment capacity of the plant.
To address secondary treatment plant capacity, a number of corrective actions were implemented while keeping
the process in-service. These mainly consisted of low-cost capital improvements to optimize capacity during
peak wet weather flow periods. Six of the plant's thirty-two aeration basins were converted to biosolids storage
basins, serving as dual purpose units that automatically switch back after a storm event via pumping.
Additionally, other changes to the existing secondary clarifiers focusing on the sludge withdrawal and RAS
system were undertaken resulting in additional treatment capacity. The recommendations were incorporated
into the facility, with the end result restoring lost capacity .
E-20
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Case Study #2
North District Wastewater Treatment Plant
Plant Description
The North District Wastewater Treatment Plant (NDWWTP) is located in Miami, Florida. The
NDWWTP features conventional screening, primary clarification, and an activated sludge system
using a high purity oxygen (HPO) process, followed by secondary clarification and disinfection.
Treated effluent is discharged to the Atlantic Ocean by means of an ocean outfall or is pumped
under pressure to four deep injection wells.
High Purity Oxygen System
Pressure Flow
Oxygen 5'9nal Control Oxy9en
/ Valve
Vent
». /Waste Activated\
V Sludge /
Facility
North District
Location
Miami, Florida
Collection System
Separate
Unique Features
CFD Modeling
Aquifer Storage and
Recovery
Stress Testing Objectives/Goals
Two different secondary clarifier sidewall depths (12 feet (ft) and 20.5 ft) were being used as part of the
secondary treatment process. The secondary treatment process consisted of eight shallow and six deeper
clarifiers. The goal of stress testing was to verify if higher loading rates in the secondary clarification process
were possible through enhancements to the shallower clarifiers to address peak hour flows associated with wet
weather events in an effort to reduce capital expenditure cost.
Stress Testing Methodology
A series of field tests was conducted to determine secondary clarifier performance under peak flow conditions.
Mixed liquor settling and compression characteristics were determined by performing batch settling testing
using settling columns following the WERF/CRTC protocol (Walberg, 2004).
Secondary clarifier stress tests were conducted following the WERF/CRTC protocol (Wahlberg, 2004) to measure
the response of the test clarifiers to progressively increasing hydraulic and solids loadings. Some secondary
clarifiers were removed from service to increase loading to the test unit in service.
A computational fluid dynamics model (CFD) was compiled using the existing plant drawings. The field and
laboratory data were collected and used to calibrate the CFD model. Once the CFD model was properly
calibrated it was used to establish secondary clarification capacity for the shallow and deep clarifiers.
Additionally, the CFD model predicted flow velocity vectors and solids concentration through a two-dimensional
perspective of the clarifier, and aided in understanding the sludge blanket depth through the solids
concentration profile.
For the purpose of this testing, the 7-day ESS limit of 45mg/L in the plant discharge permit was used as the point
of failure.
E-21
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Stress Testing Results
A series of stress tests were conducted to evaluate performance for peak flow conditions. The shallow clarifiers
exceeded 45 mg/L after 8.75 hours (hrs) from the time the stress testing began, while the SOR and SLR reached
940 gpd/sf and 11.6 pounds per day per square foot (Ib/d/sf), respectively. In contrast, the deeper clarifiers
exceeded 40 mg/L 10.5 hrs into stress testing, with a SOR and SLR of 1,500 gpd/sf and 26.8 Ib/d/sf, respectively.
As anticipated, the deeper clarifiers demonstrated a greater loading capacity before exceeding the point of
failure than the shallow clarifiers. For both clarifiers, the sludge blanket depth (SBD) at the time the units reach
40 mg/L was 10.5 ft for the shallow unit, and 6 ft for the deeper unit.
Once field and laboratory analysis had been completed, this information was used to calibrate the CFD model.
Historical data for MLSS concentrations was entered into the CFD and a series of simulations were run with a
limit of 45 mg/L ESS concentration as the threshold value. The CFD model was run with increasing flow rates to
indicate maximum flow that could be maintained and still maintain regulatory compliance.
With the calibration complete, modifications to the CFD model were made to evaluate making hypothetical
improvements to the shallow clarifier to determine if additional capacity could be achieved. These modifications
included a larger center well, increased suction withdrawal capabilities, and modification to the effluent
launders. The CFD model indicated that if all of the proposed improvements were implemented, peak flow
capacity of the shallow clarifier could be increased to 24.5 MGD. This is an additional 8.5 MGD, or a 53 percent
increase in capacity in the secondary treatment process for this clarifier size.
E-22
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Case Study #3
Mauldin Road WastewaterTreatment Plant
Plant Description
The Mauldin Road Wastewater Treatment Plant (WWTP) is one of twelve wastewater treatment
plants owned and operated by Western Carolina Regional Sewer Authority in Greenville, South
Carolina. The Mauldin Road WWTP routinely treats a dry weather flow of 18 to 20 MGD, but
experiences very high wet weather flows of up to 160 MGD during 2-year storm events. This
equates to a peaking factor of 8, compared to dry weather conditions.
The primary cause of this high peaking factor is significant I/I from the collection system, in a
service area that is almost entirely built out. The plant has a rated dry weather treatment capacity
of 29 MGD and had been permitted to discharge a maximum of 70 MGD during wet weather
operating conditions under a tiered NPDES permit. The Mauldin Road WWTP has used a
combination of flow equalization and blending of primary effluent with tertiary effluent to treat
wet weather flows as high as 100 MGD while successfully meeting permit requirements.
Bypass
Facility
Mauldin Road
Location
Greenville, SC
Collection System
Separate
Unique Features
High peaking factor
Extensive process
modeling
Equalization basins
Emergency Storage
(earth lined]
Daily |—
I Equalization ~] tmer9
T Basin 1 Basin
LTLLJ
Lower Equalization
Basin
Waste \
Activated Sludge/
Waste Activated Sludge
Pumps
E-23
-------
Stress Testing Objectives/Goals
The goal of the stress testing was to verify if higher loading rates in the secondary clarification process were
possible through enhancements to the secondary clarifiers to address peak hour flows associated with wet
weather events. This would reduce capital cost expenditure for alternative wet weather treatment and reduce
the amount of primary and secondary effluent blended during peak wet weather events. This goal is
complicated by the fact that this facility has a significant portion of its flow recycled as part of the BNR/enhanced
nitrogen removal (ENR) approach (Hildebrand et al, 2004).
Stress Testing Methodology
A series of field tests was conducted to determine secondary clarifier performance under peak flow conditions.
Mixed liquor settling and compression characteristics were determined by performing batch settling testing
using settling columns equipped with slow speed mixers to minimize wall effects following the WERF/CRTC
protocol (Wahlberg, 2004). Mixed liquor flocculation parameters were determined following the protocol
described by Wahlberg et al. (1999).
Clarifier stress tests were conducted following the WERF/CRTC protocol (Wahlberg, 2004), to measure the
response of the test clarifiers to progressively increasing hydraulic and solids loadings. Some secondary clarifiers
were removed from service to increase loading to the test unit in service. Additional flow for simulating wet
weather events was available by utilizing an existing 35 MG equalization basin. The equalization basin features a
multi-cell design to accommodate both peak flow during storm events and as part of daily diurnal flow
management for protection of BNR/ENR bacteria population.
Additional lab testing was performed to gather pertinent data and was incorporated as part of a Biowin™ model.
The process model aspect was incorporated since not only could internal plant recycle flow be taken into
account, but additional information could be garnered in terms of wastewater strength (TSS and BOD
characteristics). This was especially true during the height of the storm event where there are dilution concerns
for the nitrifying bacteria and temperature effects.
Stress Testing Results
A series of stress tests was conducted to evaluate performance for peak flow conditions. The existing plant
loadings to the secondary clarifiers include an SOR and SLR of 1,000 gpd/sf and 25 Ib/d/sf, respectively. Stress
testing revealed that a SOR and SLR of 2,000 gpd/sf and 36 Ib/d/sf, respectively were possible while keeping the
effluent limitation of 15 mg/L TSS. In both instances the MLSS concentration was kept at 3,500 mg/L.
The results from the stress testing were used to calibrate the BioWin™ model. The model was used to predict
plant solids inventory in the secondary clarifiers and process performance for nitrogen and phosphorus removal.
The model was able to extrapolate that, during winter time peak flow conditions, the existing MLSS
concentration would be too low to cope with the increased dissolved oxygen content and lack of nitrifying
bacteria present in the dilute influent. The process model determined a minimum of 3,600 mg/L would be
necessary to ensure nitrifying conditions. Additional treatment modifications were simulated, including
modification of aeration basins for a five stage BNR process and installation of fine bubble diffusers for increased
oxygen transfer rates.
The stress testing concluded that flow through the secondary clarifiers could be doubled for peak flow
conditions while protecting the acclimated bacteria population for the BNR for nitrogen and phosphorus
removal.
E-24
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Case Study #4
R.M. Clayton Water Reclamation Center
Plant Description
The R.M. Clayton Water Reclamation Center (WRC) is an activated sludge plant located in
Northwestern Atlanta and is one of the largest wastewater treatment facilities in the southeastern
United States. The WRC serves portions of three counties in addition to most of the City of Atlanta,
and provides advanced secondary treatment for approximately 80 MGD of wastewater. The WRC
was permitted to discharge 100 MGD of treated wastewater into the Chattahoochee River on a
maximum month basis.
Chattahoochee
River
Facility
R.M. Clayton WRC
Location
Atlanta, GA
Collection System
Combined
Unique Features
BNR process
Restore process capacity
and limit blending
during wet weather
events
Stress Testing Objectives/Goals
The WRC required upgrades and improvements because the plant's service area was increasing in density (i.e.,
impervious area, and population). Besides flow increases, the plant needed to be expanded because it was
unable to pass all flows that it received through the entire treatment process. Primary effluent was blended with
secondary effluent when flows reached approximately 110 MGD, and flows in excess of 180 MGD were bypassed
with only screening and disinfection. The WRC expansion stress testing program was to rerate the plant for an
annual average day flow of 120 MGD from its current 103 MGD rating. This allowed an increase of 17 MGD of
full secondary treatment for all flow conditions, and limited blending during wet weather events.
Stress Testing Methodology
A state point analysis was conducted to determine the limiting solids flux and the corresponding underflow
concentration (Levesque et al, 2006). Some site specific settling tests were conducted, in conjunction with a
correlation between zone settling velocity and the SVI index developed by Wahlberg and Keinath (1988). To be
conservative, higher influent flows and only the circular portion of the settling tank thickening area were
considered.
Additionally, testing was conducted to determine sludge blanket depths and SS concentrations within the sludge
blanket at two settling tanks.
Stress Testing Results
An evaluation of the estimated oxygen demand from the historical primary effluent BOD5 and Total Kjeldahl
Nitrogen (TKN) concentrations resulted in estimated oxygen demand peaking factors of 1.25 for maximum
month, 1.43 for maximum week, and 2.03 for maximum day conditions. For rerating the plant, the aeration
system was designed to provide sufficient DO to meet both BOD5 removal and nitrification requirements at
maximum week oxygen demands while maintaining a DO concentration of 2.0 mg/L in the mixed liquor. These
oxygen demands are a direct function of the influent BOD5 and TKN loading parameters and effluent ammonia
E-25
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limit. Revisions to the aeration system consisted of new and refurbished blowers, as well as addition of fine
bubble diffusers to achieve the required DO concentration.
The next step was to determine whether or not the seals were leaking in the secondary clarification process. The
TowBro® sludge removal system had a rotating "doughnut" at the center to provide connection to the underflow
line. The "doughnut" had a 360° rotating seal. This seal was recently replaced at another settling tank and was
considered a possible source of leaks in other settling tanks. Clear supernatant from the sludge blanket could be
pumped through the leaking seal, creating a "short circuit" resulting in reduced RAS pumping capacity.
In the control tank, the average RAS and blanket SS concentrations were approximately 12,000 mg/L and 14,300
mg/L, respectively. The SS concentration in the RAS was 84 percent of the blanket SS concentration, indicating
that short-circuiting was not significant. On the other hand, the average RAS and blanket SS concentrations in
the test settling tank were 8,375 mg/L and 28,608 mg/L respectively. The RAS SS concentration in the test tank
represented only 29 percent of the blanket SS concentration. This indicated that significant short-circuiting was
taking place in the test tank. Furthermore, uniform SS concentration along the bottom of the test tank indicated
that the short-circuiting was not occurring along the bottom of the tank, but at the seal at the center column
"doughnut" ring. As a result of this study, seals were checked in other settling tanks and a majority was found to
be leaking. Replacement of seals led to significant reduction in sludge blanket levels.
The process modifications resulted in an additional 17 MGD available capacity in the secondary treatment train.
This in turn reduced the amount of effluent blending necessary during peak wet weather flows.
E-26
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Case Study #5 - Alternative Case Study
City of Corona Plant No. 1 and No. 2
Plant Description
The City of Corona (City) has three wastewater treatment plants that are currently rated for a total
capacity of 15.5 MGD. Plant No.1 features a step-feed activated-sludge process rated at 5.5 MGD.
Plant No. 2 utilizes a traditional activated sludge process for a capacity of 6 MGD. Plant No.3 is the
newest, featuring a MBR process rated for 1 MGD with a build out capacity of 3 MGD. Waste
secondary biosolids from Plant No. 3 are returned to the sewer system leading to Plant's No. 1 and
No. 2.
Stress Testing Objectives/Goals
The City has experienced continued population growth since the 1990's and wastewater flows were
approaching the rated capacities of the treatment facilities. Seeking alternatives to the large capital
costs of new treatment facilities, the City investigated rerating two of the existing plants (plants
No.l and No.2) to provide additional capacity and maximize existing assets. An operational testing
program (DTP) was developed to verify if treatment processes could be expanded through a
program of short-term and long-term capital improvements, and if operational modifications could be made to
the existing facilities. The goal was to provide an additional 3.5 MGD capacity from Plant No. 1 (3.0 MGD) and
Plant No. 2 (0.5 MGD). The focus of testing centered on Plant No. 1.
Stress Testing Methodology
Prior to the start of field testing a desktop review focusing on the hydraulic and biological capacity of unit
processes was conducted, revealing that primary sedimentation and secondary clarification were the limiting
process units. Field tests were conducted with the primary sedimentation and secondary clarifiers. Flow was
sent to these unit processes starting at the design rated flow, with all basins online, and increasing the flow by
0.5 MGD on consecutive days until proposed rerated flow was achieved with one basin offline. Influent and
effluent sampling was conducted at scheduled intervals during each day of testing, with sludge blanket levels
closely monitored. On-line turbidity monitoring was used as an indicator in the secondary clarification process.
The testing process involved three distinct steps. First, sampling along the length of the aeration basins for
dissolved oxygen (DO) and oxidation-reduction potential (ORP) for nutrient considerations. Second, optimization
of the BNR process using propriety modeling software based on the International Water Association Activated
Sludge Model 2 (ASM 2). This modeling examined flow split possibilities, lowering BOD loads to the aeration
basin, different DO concentrations, etc. The last step consisted of capacity testing to determine BNR
performance at higher influent flows up to the targeted goal.
Stress Testing Results
As a result of the testing, several recommendations were suggested for short-term and long-term
implementation, and operations. Recommendations are as follows:
Facility
City of Corona Plant
No.land No. 2
Location
Corona, CA
Collection System
Separate
Unique Features
BNR process
Stress testing for plant
re-rating study
Short-term
To operate at higher MLSS concentrations consistently, a new gravity belt thickener would provide
better reliability. Additionally, a polymer system could be added to the secondary clarifiers to improve
solids settling at high flows and loads.
E-27
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Installation of energy-dissipating baffling in the secondary clarifiers would improve performance.
Long-term
Aeration system capacity could be increased by enabling one of the standby blowers to be brought online
during high influent BOD loading conditions.
To enable plug-flow operations of the aeration basins should an aeration basin be taken off-line, an
internal recycle system could be installed for each basin. The extent of the recycle could be as high as 300
to 400 percent.
Operational
The RAS flow rate should be reduced as flows increase MLSS to around 3,400 mg/L and when MLSS
concentrations are higher.
Stress testing demonstrated that the existing plant capacity could be expanded by approximately 33 percent
through a sound testing process that examines all areas of a treatment facility. One of the key lessons learned
during this testing program was it is essential to have plant operators integrated into the project team. The plant
staff knowledge of existing plant operations can reduce the trial and error process and provide significant insight
to a stress testing program
E-28
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Section 6: Engineering Approaches/ Procedures
Identified
Stress tests are carried out to determine the performance of a settling tank in terms of ESS concentration for a wide
variety of operating conditions. Relationships between ESS concentration and SOR, SLR, RSSS concentration, RAS
flow rate, sludge settling characteristics, DSS concentration, and sludge blanket depth are examined in great detail.
Slug dye tests and solids distribution/flow pattern tests are also performed to assess the hydraulic characteristics of
the settling tank.
A stress testing protocol was developed by the Water Environment Research Foundation/The American Society of
Civil Engineers Clarifier Research Technical Committee (WERF/CRTC) (Wahlberg, 2004). During the course of this
literature review, the majority of the published studies use this criteria as part of their stress testing approach or a
derivation of this approach.
A typical stress test schedule is summarized in Table 6.1 (Wahlberg, 2004). The schedule is designed to have three
different levels of SLR to be applied: low, medium, and high. The level of SLR is determined from the solids flux
analysis. Each SLR is replicated three times (once for each day). Measurement of flows, SS concentrations, and
sludge blanket height are conducted during each test. Four settling tests and two stirred SVI are carried out during
each run. Influent DSS, effluent DSS, ESS and FSS concentrations are determined during each run. A slug dye test is
carried out (either in the first or second replicate of each run), and solids distribution/flow pattern test is
conducted (either in the second or third replicate of each run).
E-29
-------
Table 6-1. Typical Stress Testing Schedule
SLR Replicate . „ Tests to be Conducted During Stress Tests
Day Measurements
1
2
3
4
5
6
7
8
9
High
Low
Low
High
Low
Medium
High
Medium
Medium
1
1
2
2
3
1
3
2
3
Conduct flow,
suspended solids, and
sludge depth with 1
minute intervals
If sludge blanket
measurements are not
available on-line,
measurements should
be done manually
every 15-30 minutes
Conduct 4 settling tests
using settling column
and 2 stirred SVI tests
during each run
Carry out influent DSS,
effluent DSS, ESS and
FSS concentrations
during each run
Slug dye
test
Slug dye
test
Slug dye
test
Continuous
dye/solids
distribution
tests
Continuous
dye/solids
distribution
tests
Continuous
dye/solids
distribution
tests
The WERF/CRTC protocol focuses on stress testing the secondary treatment process, but is amenable for primary
stress testing as well although primary treatment is not typically considered the bottleneck in terms of process
constraints. For the purpose of this report, a condensed version of the stress testing protocol for secondary
treatment is presented for reference (WERF, 2009).
1. Obtain design data and blueprints of the entire secondary settling tank system, as well as the test settling
tank. Identify the sampling locations for influent, effluent, and within the test tank. Become familiar with
all the flow control systems for influent, effluent, and RAS, as well as flow ranges that can be applied.
2. Install flow measuring devices into the influent line (or effluent line) and RAS line, and capture the output
signals electronically. This can be by means of a portable flow meter.
3. Determine a way to change the flow rate to the test tank to provide three different SLRs. Influent flow
rate of the test tank can be changed by taking other clarifiers out of service gradually, by weir adjustment,
or by partial gate closing. Keep the ratio of influent flow to RAS flow constant during each SLR test
condition.
4. Install suspended solids probes into the influent line, effluent launder, and RAS line of the test tank, and
capture the signal from the probes electronically.
5. Install an automated sludge blanket monitoring device in the test tank. If this is not possible, measure the
sludge blanket depth in the test tank manually.
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6. The stress test schedule shown in Table 6-1 should be applied. Each stress test will take a day. Determine
the SLRs applied for each stress test. SLRs are determined from the solids flux analysis, based on the
results of the mixed liquor settling tests conducted prior to the stress tests.
7. During each stress test, keep the influent flow constant to the test tank for a period of three theoretical
hydraulic detention times.
8. During each stress test and when the influent flow is constant, collect data from the suspended solids
probes, flow, and sludge blanket height measuring devices at least at one minute intervals. If sludge
blanket height is measured manually, then the measurements should be carried out at 15-30 minute
intervals.
9. After a period of time that is equal to two theoretical hydraulic detention times, carry out (at least once)
influent DSS, effluent DSS, ESS, and FSS concentration tests.
10. During each stress test, conduct settling tests at four to six different suspended solids concentrations to
determine the V0 and k parameters of the Vesilind equation.
11. During each stress test, conduct two stirred sludge volume index (SSVI) tests, as described in Standard
Methods for Water and Wastewater (APHA, AWWA and WEF, 2005). In the SSVI test, mixed liquor is
settled in a one liter graduated cylinder for 30 minutes, and the content of the graduated cylinder is
stirred at one revolution per minute (rpm) during the settling.
12. During the first or second replicate of the stress test specified for a SLR and after three theoretical
hydraulic detention times have passed, carry out slug dye test (if constant flow for three theoretical
hydraulic detention times cannot be maintained, a slug dye test can be initiated earlier). Samples should
be collected until at least 90 percent of dye mass is recovered.
13. During the second or third replicate of the stress test specified for a SLR and after three theoretical
hydraulic detention times have passed, carry out continuous dye and SS distribution tests (if constant flow
for three theoretical hydraulic detention times cannot be maintained, a slug dye test can be initiated
earlier). If a manual core sampler such as a sludge judge is used, dye and SS sampling can be done at the
same time. If a portable hand-held SS analyzer is used, then the core sampler is employed for taking dye
samples only, and the SS concentrations at different depths are determined using the electronic device.
14. Continue the stress test at least for a period equal to one theoretical hydraulic detention time after three
theoretical hydraulic detention times have passed, and after completion of the entire slug dye,
continuous dye, and solids distribution tests.
A review of this protocol revealed several key parameters that are critical for treatment of wet weather flows.
Requirement
Existing information on the secondary clarifier system. Having a through understanding of the clarifier
dimensions, RAS and influent flow rates, and limitations of the MLSS properties for loading at various SLR
is critical.
Steady and constant flow to the test clarifier. Often to accommodate the peak flow rates for wet
weather, operational changes are made well in advance to ensure treatment/process train survivability.
This is often accomplished by tracking weather/satellite coverage.
Improvement
Increasing flow to the secondary clarification process is often accomplished by taking units off-line.
While this does increase flow to the test clarifier, it does not address the dilute nature of the influent
associated with wet weather conditions. Utilizing plant effluent as the make-up source flow could be one
way to address this issue. Although temperature effects from a peak wet weather event could not be
simulated it would give a better indicator of performance for dilute influent.
E-31
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A minimum of three hydraulic retention times should be passed before changing testing conditions.
This time period allows the clarification system to return to a steady state value.
All sampling should be performed by automatic means as much as practically possible. Every time
sampling is performed manually, it introduces a potential source of error and could possible skew results.
Same team members perform the same role on each day of stress testing. It is beneficial to have same
staff members perform the same function for each of the stress tests. This helps ensure repeatability of
results and comparison of different operating conditions.
In theory, other tests can be used to better understand hydraulic conditions inside the settling tanks. One
technique is the drogue current test. The drogue or a flow catcher is inserted into the clarifier to observe velocity
patterns. One could think of this as a "chip-float test" for a secondary clarifier similar to that which is commonly
used in open channel flow measurements.
During the course of the literature review, no examples of municipalities using this technique for a stress testing
were identified. This testing procedure is very labor intensive and offers little benefit over the use of the CFD
model to predict the velocity vectors.
Prior to the start of a stress testing program, coordination with the authorities having jurisdiction over the
discharge permit is recommended. During the course of stress testing there is potential to violate one or more
discharge limits while stress testing. Having all stakeholders aware of the procedure and schedule of stress testing
and potential reasons for a permit excursion will assist in gaining approval to perform the testing and acceptance
of the results.
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Section 7: Evaluations and Conclusions
This report has illustrated the state of knowledge and practice of stress testing at POTWs. Through the analysis of
real case studies, the report has identified actual plant improvements implemented to manage peak wet weather
flows.
The literature review shows stress testing is suited to assist POTWs in maximizing the use of their existing facilities
and assets. It is also recognized by regulatory agencies as an acceptable approach for municipalities to meet
challenges such as aging infrastructure, population growth and density changes, limited land for plant expansion,
increasing regulatory requirements, and revenue limited utilities. The implementation of a plant-wide stress testing
approach requires significant planning, up-front commitment by decision-makers, potential design and flow
modifications, and coordination with operations and laboratory staff for the best chance of a successful outcome.
The use of process and hydraulic computer modeling are engineering tools that have shown to streamline the
evaluation process, and can reduce demand on plant staff for on-site field stress testing. Modeling also enhances
and makes possible quick evaluations of various capital improvement and process optimization methods for
increasing flow through wastewater treatment facilities.
While most of the published stress testing cases reviewed as part of the literature search used similar techniques,
each case had subtle differences that account for site-specific conditions. A stress testing approach that worked for
one facility may not work for another facility due to the differences in flow availability, ability to simulate dilute
influent corresponding to a wet weather, and considerations of treatment process aspects.
A lesson learned from the different methodologies used by various POTWs undertaking stress testing is that all
aspects (hydraulic, biological, and to some degree solids handling) must be examined. Designers often incorporate
safety factors into the design of various unit processes to ensure water quality permit compliance through a wide
variety of conditions, including but not limiting to peak wet weather flows. These safety factors can vary from
design to design. The treatment process is only as effective as its weakest link.
The key objective for stress testing is to identify where treatment process design criteria do not reflect the available
capacity under actual operating conditions. Stress testing results can provide a method for POTW operators to
document and gain regulatory approval for re-rating treatment plant capacity while maintaining compliance with or
justifying modifications to discharge permit requirements.
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Appendix A: Peak Wet Weather Flow Literature
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