United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-93/007
September 1993
Manual
Combined Sewer Overflow
Control
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EPA/625/R-93/007
September 1993
Manual
Combined Sewer Overflow
Control
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Cincinnati, Ohio
Printed on Recycled Paper
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Notice
This document has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
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Contents
Page
Chapter 1 Introduction 1
Purpose 1
Scope and Organization 1
Chapter 2 Introduction to CSO Control Technologies 3
Unique Aspects of CSOs 3
Major Technologies To Be Addressed 5
Other Control Methods 6
References 11
Chapter 3 Process Selection 13
Performance Goals 13
Data Requirements for Design of CSO Controls 22
Technology Selection 30
References 34
Chapter 4 CSO Control Technologies 37
In-System Controls/ln-Line Storage 37
Off-line Near-Surface Storage/Sedimentation 43
Deep Tunnel Storage 55
Coarse Screening 65
Swirl/Vortex Technologies 69
Disinfection 81
References 89
Chapter 5 Costs for CSO Control Technologies 93
Sources of Cost Data 93
Construction Costs for CSO Controls 93
O&M Costs for CSO Controls 94
References 95
HI
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List of Figures
Figure Page
3-1 Approximate relationship between performance of CSO storage and storm size used
as design basis—percent capture 15
3-2 Approximate relationship between performance of CSO storage and storm size used
as design basis—number of overflows 16
3-3 Relationship between percent capture and number of overflows for CSO storage control 17
3-4 Illustration of use of monitoring data to characterize first flush 21
3-5 Illustration of knee-of-the-curve analysis 22
3-6 Probability distribution of storm event volumes 26
3-7 Illustration of intensity-duration curve 27
3-8 CSO settling velocity—typical test equipment and sample results 28
3-9 Effect of particle settling velocity and hydraulic loading rate on TSS removal efficiency 31
3-10 Cost optimization of storage-treatment 34
4-1 Example of a vortex valve 38
4-2 Example of a wirbeldrossel and wirbelvalve 40
4-3 Example of an inflatable dam 41
4-4 Example of a motor-operated gate regulator 41
4-5 Example of an elastomeric tide gate 42
4-6 Flow schematic for typical storage/sedimentation facility, Newport, Rhode Island 47
4-7 Plan and profile for a typical rectangular storage/sedimentation facility 49
4-8 Examples of tangential inlet configuration 58
4-9 The E-15 dropshaft 59
4-10 The D-4 dropshaft 60
4-11 Cutting cycle for a typical tunnel boring machine 62
4-12 Tunnel system dewatering pump station 64
4-13 Catenary-type mechanically cleaned bar screen 67
4-14 Example EPA swirl concentrator 70
4-15 Example Fluidsep vortex separator 71
4-16 Example Storm King hydrodynamic separator 72
4-17 Particle settling velocities for grit and organic material in still water 73
4-18 Typical gradation for grit and organic material 73
4-19 Settling velocity profiles of combined and sanitary wastewaters 74
4-20 Head vs. discharge per linear foot of weir length for a circular weir 76
4-21 Common layouts for swirl/vortex installations 77
4-22 Graphical representations of log Yt/Y0 versus log CT 82
4-23 Relationship between GT and bacterial kill 83
4-24 Schematic of typical liquid sodium hypochlorite system 86
4-25 Diffuser types 87
5-1 Construction costs for CSO controls 94
5-2 O&M costs for CSO controls 95
IV
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List of Tables
Table Page
3-1 Reported Performance of CSO Treatment Technologies 18
3-2 Summary of Peak Hourly Rainfall Characteristics 20
3-3 Rainfall Characteristics for Six U.S. Locations 27
4-1 Typical Overflow Rates for Primary Settling Tanks 45
4-2 Typical Detention Times for Primary Settling Tanks 45
4-3 Typical Rectangular Tank Dimensions for Storage/Sedimentation Facilities 50
4-4 Examples of Floor Slopes for Storage/Sedimentation Facilities 50
4-5 Typical Dimensions of Circular Storage and Storage/Sedimentation Tanks 51
4-6 Advantages and Disadvantages of Deep Tunnel Excavation Methods 61
4-7 Chicago TARP Mainstream Pumping Station Capacities 65
4-8 Examples of Bar Screen Installations at CSO Control Facilities 69
4-9 Comparison of Design Details for Swirl/Vortex Devices 79
4-10 Examples of Swirl/Vortex Chamber Construction 79
4-11 Examples of Sodium Hypochlorite Disinfection Systems at CSO Control Facilities 85
5-1 Cost Equations for CSO Control Technology 94
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Acknowledgments
This manual is the product of the efforts of many individuals. Gratitude goes to each person involved
in the preparation and review of the document.
Authors
Donald Walker, Jonathan Golden, and David Bingham of Metcalf & Eddy, Inc., Wakefield, MA, and
Eugene Driscoll of Hydroqual, Inc., Mahwah, NJ, were the principal authors of this manual.
Technical Contributors
The following individuals provided invaluable technical assistance during the development of this
manual:
Richard Field, U.S. EPA, Office of Research and Development, Storm and Combined
Sewer Program, Edison, NJ
Michael Brown, Doctoral Candidate, Cranfield Institute of Technology, School of Water
Sciences, Woldingham, Surrey, UK
Joseph Mauro, U.S. EPA, Office of Water, Municipal Support Division, Washington, DC
Peer Reviewers
The following individuals peer reviewed this manual:
Michael Domenica, Massachusetts Water Resources Authority, Boston, MA
Kenneth Pew, Northeast Ohio Regional Sewer District, Cleveland, OH
Robert Pitt, University of Alabama - Birmingham
Editorial Reviewers and Document Production
Heidi Schultz, Eastern Research Group, Inc. (ERG), Lexington, MA, provided editorial review and
produced this manual.
Technical Direction and Coordination
Daniel Murray, U.S. EPA, Office of Research and Development, Center for Environmental Research
Information, Cincinnati, OH, coordinated the preparation of this manual and provided technical
direction throughout its development.
VI
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Chapter 1
Introduction
Overflows from combined sewers during storm events
result in the discharge to receiving waters of untreated
sanitary sewage, which also may contain pre-treated
industrial wastewaters and untreated stormwater.
Combined sewer overflows (CSOs) contain pollutants
that are present in the domestic and industrial
wastewaters, as well as those in the urban stormwater
runoff that enters the combined sewer system.
In many cases, these discharges have an adverse
effect on receiving water quality and attainment of
designated uses. In addition, since CSOs are point
source discharges of untreated sanitary sewage, they
require National Pollutant Discharge Elimination System
(NPDES) discharge permits. In recent years, there has
been an enhanced regulatory focus on CSOs and their
control, and communities with combined sewer systems
are being called upon to develop and implement programs
for control of CSOs.
Control technology applicable to CSOs has many
aspects in common with treatment of domestic
wastewaters in publicly owned treatment works (POTWs).
However, a number of unique aspects of CSOs must be
considered both for overall control program design and
for effective design and operation of treatment units.
Purpose
This manual provides information to assist in selecting
and designing control measures for reducing pollutant
discharges from CSOs. The manual will be useful for
municipal public works staff, design engineers, and
regulatory agency staff tasked with the development
and review of facility plans and long-term CSO control
programs.
Scope and Organization
The manual provides design information for six CSO
control technologies that were selected because they
represent the most commonly employed techniques
used in programs developed to date. Additional control
techniques are identified and described, but detailed
design guidance for these techniques is beyond the
scope of this manual.
Chapter 2 discusses the characteristics of CSOs and
the special considerations these features impose on the
development of effective designs for control systems
and unit processes. It describes the technologies for
which design guidance is provided, and the basis for
technology selection. Finally, Chapter 2 identifies and
discusses other potential CSO control techniques that
are not addressed by the manual.
Chapter 3 presents process selection considerations, and
describes how the six control technologies for which
design details are provided relate to each other and to
potential control programs for an overall system. The
first section discusses a variety of alternative design
goals that may be imposed by a regulatory agency or
otherwise adopted for design, and describes how to
convert such general requirements to a specific design
for a control system or treatment unit. Factors that
influence the performance of each selected control
technology are identified, and the advantages and
disadvantages associated with each technology are
discussed.
Combined sewer systems with their associated overflow
points are relatively complex. Hydraulic conditions are
highly variable due to the intermittent and variable
characteristics of rainfall. The quality characteristics of
CSO flows, and hence their treatability, also can vary
significantly from location to location and from storm to
storm at a given location. As a result, effective design
programs usually require application of computer
models, appropriate analysis and interpretation of rain
gage records, and quality and treatability characterizations
of the CSOs. The second section of Chapter Three
describes the data requirements for addressing these
elements and for developing the information that forms
the basis for the design details presented in Chapter 4.
Chapter 4 provides design details, rules of thumb, and
examples of practice for the following control
technologies:
• In-System Controls/ln-Line Storage
• Off-line Near-Surface Storage/Sedimentation
• Deep Tunnel Storage
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• Coarse Screening and/or operational features from reported operating
• Swirl/Vortex Technologies units also are Provided'
D's'nf t'o Chapter 5 summarizes currently available capita! and
operation and maintenance (O&M) cost information
For each of these technologies, Chapter 4 summarizes relating to the selected control technologies. The
process theory, design parameters, operational information provided is suitable for preliminary indications
requirements, and important related aspects (e.g., of the general order of costs associated with the control
solids handling, odor control). Examples of design measures, and the need for site-specific estimates for
developing accurate local costs.
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Chapter 2
Introduction to CSO Control Technologies
Unique Aspects of CSOs
The design criteria and performance capabilities for
most unit operations at POTWs generally are well
established, based on the existing wealth of knowledge
regarding municipal wastewater flow and load
characteristics, and decades of operating experience at
thousands of treatment facilities. This same municipal
sewage is an important component of the overflows
from combined sewers during storms. Although mixed
with surface stormwater runoff in varying degrees
during CSO events, both the quality and treatability
characteristics of CSO discharges and municipal sewage
are relatively similar. Differences are more in degree
than in kind. As a result, experience with the treatment
of municipal wastewater provides an important and
useful basis for the design and operation of CSO
treatment technologies.
A number of important differences must be recognized,
however, because they have a significant influence on
the effort needed to develop effective designs for CSO
control units. The differences derive from several unique
aspects of CSOs; the influences they have on the
design of CSO control programs and individual control
units are discussed briefly below. Specific considerations
are addressed in greater detail in other sections of the
manual, where appropriate.
Performance Goals Are Not Uniform
CSO control goals may be governed by receiving water
quality-based and/or technology-based requirements,
depending on the policies of state and federal permitting
authorities. A considerably wider range of potential
performance requirements may be applied for a CSO
area compared to the standard "secondary" or
"advanced" treatment objectives that apply in the case
of most POTWs. Chapter 3 provides examples of
commonly encountered CSO performance goals. In
addition, while the technology and design basis for
providing secondary treatment at a POTW is well
established, CSO control concepts and design details
are influenced by the specific performance goal that is
applied. Additional data and system analysis is required,
in relation to POTW design, because the development
of controls that meet a selected performance goal must
consider the variability of CSO flows and the frequency
of extreme events.
System Characteristics Are Site Specific
The characteristics of combined sewer systems are site
specific, and have an important influence on the
elements of an overall control program, the control
technologies that are most appropriate, and design
features of individual control units.
Many CSO control programs include clever modifications
or adaptations of features of the existing combined
sewer collection/conveyance system. For example,
relatively minor piping changes or regulator modifications
can significantly affect design of end-of-pipe control
units. The design, maintenance, and operation of
regulators can have an important effect on the CSO
flows delivered to a control technology.
Typically, a combination of management practices and
CSO control technologies is required to meet the CSO
program goal for a given community. Simply applying a
control technology at a particular overflow point usually
is not appropriate; effective control programs require
consideration of the system as a whole.
An important factor contributing to the complexity of
CSO control is the existence of multiple overflow
locations, which may discharge to different receiving
water bodies. Multiple overflows may be consolidated
for treatment or they may be addressed separately. In
some cases, the most effective design approach may
completely eliminate a particular CSO that discharges
at a sensitive location, by means of piping system
modifications to reroute the affected flows.
Often a number of possible alternatives exist for a given
community. The design of cost-effective CSO control
programs thus may involve modifications to the
collection system to relocate or consolidate overflows,
or to reduce overflows by expanding hydraulic
conveyance capacity at critical locations. Such
modifications will influence the number and design
capacity of any end-of-pipe CSO treatment technologies
that may be required to meet performance goals. As a
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result, site-specific features of a local combined sewer
system impose an additional level of design considerations
than the typical POTW design program.
Design Basis Selection Is Not Standardized
Rainfall, which produces flows in the combined sewer
system, is the factor that determines when and where
overflows will occur, and the rates, volumes, and
durations of the episodes. Rainfall amounts vary from
year to year, storm event to storm event, and hour to
hour during individual events.
Effective CSO control requires that an appropriate
design condition be selected from among the wide array
of naturally occurring conditions. In some cases, a
predetermined "design storm" may have been
designated. While this can be used to provide a design
basis for a particular CSO treatment unit, it leaves a
number of design issues unanswered. First, there is
some ambiguity in converting a designated design
storm to CSO control system design, depending on
whether the control technology design is keyed to
volume or to flow rate. In cases where only a return
period is used in the definition (e.g., 10-year storm,
1-year storm), a wide variation in the volumes and rates
will occur depending on the arbitrarily selected duration
associated with the recurrence interval. Even in the
case of a more complete designation, as in the 1-year,
6-hour design storm, ambiguity exists. While this
defines a volume, it does not represent a volume for a
complete storm event, so inferences on performance of
storage units will be uncertain. A rate of flow can be
extracted from this design condition in a number of
ways, none of which provide a confident basis for
design of a technology unit based on flow rates and
peak flows.
The principal issue to address in selecting an
appropriate design basis for a treatment technology can
be stated as follows. While an arbitrarily selected design
storm condition provides a convenient starting point to
size an individual control unit and develop design
details, it provides no information on the overall level of
performance that will be provided. This approach tacitly
assumes that the designated design storm is, in fact,
the appropriate basis for meeting control objectives, and
that control units based on it will provide acceptable
levels of performance. Consider situations where
control performance goals are stated in other terms (see
Chapter 3). If a CSO control program is to be designed
to capture or treat some percentage of the total
combined sewer flow, or to limit untreated overflows to
some specified number per year, then the designer is
required to develop, by appropriate analyses of local
rainfall and collection system characteristics, a
site-specific determination of the appropriate design
condition to apply. Rainfall characteristics and analysis
issues are discussed further in Chapter 3.
In summary, the design storm approach comes from
flood control and urban drainage planning, where peak
flow is of concern for flood damage risk control. Smaller
storms are not an issue. For pollution control, even
small storms can result in standards violations and
environmental harm, especially due to bacterial
contamination. As a consequence, the effect of control
measures on the annual frequency and volume of all
CSO events is more important than conditions
associated with a particular peak flow.
CSO Flows Are Intermittent and Highly
Variable
While analysis and interpretation of rainfall records can
provide useful information for identifying an appropriate
design basis, drainage area and conveyance system
characteristics impose site-specific influences on the
flow rates and volumes that will occur during any storm
event at locations in the system where control
technologies will be applied. Each combined sewer
system is unique, and most will be of sufficient extent
and complexity that deriving the necessary information
on combined sewer flows and CSOs usually requires
the application of simulation models such as EPA's
Storm Water Management Model (SWMM).
Data requirements for the proper application of such
models can be substantial, and are discussed in further
detail in Chapter 3. Data requirements include
information on physical features of the sewershed and
combined sewer system, such as areas, pipe sizes,
slopes, regulator and pump station design, and
operating parameters. In addition, accurate information
on the characteristics of a number of individual storms
and the flows they produce at various locations in the
system is required to calibrate the model. This, in turn,
requires flow monitoring, usually at multiple locations
and for an adequate number of storm events to permit
an acceptable mode! calibration to be made.
Thus, a considerable study effort often is required
simply to identify the flow regime that will prevail at the
point of application of a control technology. The
characteristics of the variable flows at one or more
control locations must be developed by applying the
calibrated model to a sufficiently long sequence of
rainfall events to provide representative results.
Additional analysis then is required to derive
appropriate design parameters for the treatment
technology to be evaluated or applied, based on the
pattern of variable flows projected to be the inflows to
the unit.
Facilities for the control of CSOs must be capable of
performing under a wide range of flow and load
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conditions. CSO facilities have influent hydrographs that
often are sharply peaked along with periods of no flow,
so that the concept of an "average flow" often used in
POTW design has little physical meaning to CSO
facilities. Peak flow rate is an important design
parameter for a number of technologies, and the
designer must select an appropriate value from the
range of extreme values that will apply at the control
location. The range in actual flow values must be
developed by the simulation model, however, insight to
the inherent variability and recurrence frequency of
extreme values is provided by the rainfall characteristics
summarized in Table 3-2 of Chapter 3.
CSO Quality and Testability Are Site Specific
Pollutant concentration levels in CSOs are site specific
and influenced by the strength of dry weather flows, the
age and condition of the collection system, and the
amount of infiltration/inflow relative to the sanitary flow.
The fact that CSO quality characteristics are site
specific limits the confidence with which data from other
sites can be applied. A number of the quality features
also influences treatability parameters. For example,
the settling characteristics of solids in CSOs to be
treated by sedimentation technology are influenced by
local system features, so monitoring and treatability
testing is advisable to develop control unit design
parameters.
Higher pollutant concentrations may be associated with
the initial peak flows, depending on factors such as the
size and slope of the piping system, the time interval
between storms, the drainage area characteristics and
response, and the solids accumulation in the collection
system. The existence and/or magnitude of high initial
loadings can influence the design of a control unit, as
discussed in Chapter 3.
Performance Data on CSO Controls Is Limited
Due to the highly variable nature of CSO flows, the
relationships between pollutant removals and design
parameters, such as hydraulic loading rates and
detention times, can be difficult to establish with
reliability. Thus, while many studies on pilot-scale or
one-of-a-kind installations have been published in the
past, a comprehensive data base covering performance
and design criteria for the most commonly employed
CSO control measures remains incomplete.
Contributing to this limitation is the cost and difficulty of
implementing effective monitoring programs to develop
operating data on existing CSO facilities. Unlike at
dry-weather POTWs, where operating data generally
can be collected at the operator's convenience,
collecting useful data at CSO facilities requires a
concerted effort by personnel to be available on short
notice and at the odd hours during which storm events
often occur. In addition, because of the substantial
variability in applied flows and pollutant loads,
monitoring programs to characterize performance must
extend over a sufficiently long period of time to reliably
determine performance level.
Because of the variability in flow and quality at any site,
the differences between sites, and the technical difficulty
and cost of developing comprehensive performance
monitoring data, CSO control technologies represent a
situation where theoretical prediction of flow and quality
(based on models calibrated against limited data sets)
may provide a more accurate basis for determining
design parameters and performance characteristics.
Major Technologies To Be Addressed
This section identifies six CSO control technologies for
which detailed design guidance is provided in Chapter
4. The technologies were selected based on the fact
that they are currently in wide use and have been
demonstrated to be effective in reducing CSO flows
and/or pollutant loads.
Separately, these control technologies will almost never
be sufficient to satisfy the needs of a comprehensive
CSO control program. For example, coarse screening
is rarely considered the only technology to be applied
at an overflow point; its more common use is in
providing pretreatment for any or all of the other control
technologies. In cases where bacterial levels produced
by CSOs cause designated use impairment, disinfection
is a necessary component of a CSO control system that
may include any of the other technologies. In fact,
disinfection of CSOs often requires some level of solids
reduction by one of the other technologies for maximum
effectiveness and reliability. System-wide CSO control
may well require application of different basic control
technologies or combinations at different overflow
locations.
A comprehensive control plan addressing the
characteristics of the combined sewer system and
overflows, which identifies the impact of CSOs on
receiving water uses and establishes performance
goals for the CSO control program, will provide the basis
for selecting and locating appropriate technologies (or
technology combinations) in the system.
A comprehensive control program design also should
consider other control methods, such as those
discussed below. Any methods that apply should be
incorporated because some will affect the design basis
of a control technology, and many will be necessary to
assure continuing effective operation of a selected
treatment technology.
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A comprehensive CSO control program is likely to
incorporate one or more of the following technologies,
for which design details are provided in Chapter 4.
In-System Controls/ln-Line Storage
This technology seeks to optimize the use of existing
storage capacity in the collection system, and maximize
the conveyance of combined flows to the POTW.
In-system controls typically are less costly than other,
more capital-intensive technologies such as off-line
storage/sedimentation, and are attractive because they
utilize the existing facilities most efficiently. However,
they are not normally sufficient to provide the complete
degree of control required. The application of in-system
controls, and the feasibility of in-system storage is very
site specific. A variety of common regulator types, along
with control strategies and operational issues pertaining
to in-system controls, are addressed below.
Near-Surface Off-Line Storage/Sedimentation
This technology reduces overflow quantity and frequency
by storing all or a portion of the CSO that occurs during
storm events. In designs providing sedimentation, flow
in excess of the tank volume passes through the units,
receiving some measure of solids separation. For
smaller storms, the tanks may provide 100-percent
capture. Stored flows are returned to the interceptor for
conveyance to the POTW once system capacity is
available. In some cases, flows may be conveyed to a
CSO treatment facility. This manual reviews basic
sedimentation theory as applied to CSO control
facilities, and presents design criteria, examples of
design details, and control strategies for storage/
sedimentation facilities.
Deep Tunnel Storage
This technology provides storage and conveyance of
storm flows in large tunnels constructed well below the
surface. Tunnels can provide large storage volumes
with relatively minimal disturbance to the ground
surface, which can be very beneficial in congested
urban areas. The components of tunnel systems as
they relate to CSO control are described below.
Geotechnical aspects of tunnel construction are noted,
but are not addressed in detail in this manual.
Coarse Screening
This technology provides coarse solids removal, as well
as a degree of floatables removal. Coarse screening
typically is provided upstream of other control
technologies, such as storage facilities or vortex units
that are applied as off-line treatment units (rather than
as in-line regulator/degritter units). Aspects of coarse
screening as they relate to CSO applications and design
are presented below.
Swirl/Vortex Technologies
These devices provide flow regulation and solids
separation by inducing a swirling motion within a vessel.
Solids are concentrated and removed through an
underdrain, while clarified effluent passes over a weir at
the top of the vessel. The swirl/vortex devices described
in this manual include the U.S. Environmental Protection
Agency (EPA) swirl concentrator and commercial vortex
separators.
Conceptually, the EPA swirl concentrator is designed to
act as an in-line regulator device. In addition to flow
routing or diversion, it removes heavy solids and
floatables from the overflow. The commercial vortex
separators are based on the same concept as the EPA
swirl concentrator, but include a number of design
modifications intended to improve solids separation.
Two commercial designs, the Fluidsep vortex separator
and the Storm King hydrodynamic separator have been
applied as off-line treatment units. Each type of
swirl/vortex unit has a different configuration of depth/
diameter ratio, baffles, pipe arrangements, and other
details designed to maximize performance. The basis
of design for each type of swirl/vortex unit, and
examples of design details and control strategies, are
reviewed in this manual.
Disinfection
This process inactivates or destroys microorganisms in
overflows, most commonly through contact with
chlorine, although a variety of disinfection technologies
are available without chlorine. Some of the more
common technologies include gaseous chlorine, liquid
sodium hypochlorite, chlorine dioxide, ultraviolet
radiation, and ozone. For disinfection of CSOs, liquid
sodium hypochlorite is the most common technology.
This manual focuses on the design of liquid sodium
hypochlorite disinfection systems.
Other Control Methods
A variety of practices and control techniques can be
utilized to supplement the application of a control
technology at a CSO discharge location. An overall
control program should consider all possibilities and
utilize any that apply for the local situation.
Implementing a locally appropriate combination of the
practices discussed below can enhance performance of
the control technology applied and, in some cases, may
reduce the design size and cost of the basic control unit,
while maintaining the targeted performance level.
Control "practices" fall into one of three categories:
• Practices that restrict the rate and/or volume of
stormwater runoff that enters the combined sewer
system.
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• Pollution prevention practices that reduce the
quantity of pollutants that enter the system.
• Operation and maintenance practices for the combined
sewer system that improve its ability to contain wet
weather flows and deliver them to the POTW.
Examples of such control practices are described below.
A number of treatment technologies available for control
of CSOs are not addressed in this manual. Examples
include dissolved air floatation, fine screens and
microstrainers, high rate filtration, and biological
treatment. These technologies have been tested on a
demonstration scale and found to be capable of
effective CSO control. Detailed discussion of these
technologies is outside the scope of this manual;
however, a brief discussion of each is provided later in
this section.
Flow Control Practices
Both infiltration of ground water into a combined sewer
system, and direct inflow of surface stormwater runoff
to the system can significantly influence the magnitude
and frequency of CSOs, and the size and cost of control
technologies. Reducing the quantity of infiltration and
inflow (l&l) will make additional system capacity
available to contain wet weather flows and reduce the
magnitude of the CSOs reaching the control technology
application points.
Two examples of flow control practices are infiltration
and inflow control, described below:
• Infiltration control: Sources of infiltration include
ground water entering the collection system through
defective pipe joints, cracked or broken pipes, and
manholes as well as footing drains and springs.
Infiltration flow rates tend to be relatively constant
and result in lower volumes than inflow contributions.
Infiltration problems usually are not isolated, and
often reflect a more general sewer system deterioration.
Extensive sewer rehabilitation typically is required to
effectively remove infiltration. The rehabilitation effort
often must include house laterals, which are normally
a significant source. Infiltration control will generally
have a much smaller impact on CSO reduction than
will control applied to inflow.
• Inflow control: Combined sewer systems were
designed to drain stormwater effectively and to
convey sanitary sewage. For a large percentage of
storm events, the surface runoff flows are much
greater than the sanitary sewage flows in the
combined system. CSO control efforts can be
assisted either by diverting some of the surface runoff
inflows from the combined sewer system to an
alternate surface drainage system or to ground water
via infiltration devices, or by retarding the rate at
which these flows are permitted to enter the system.
Inflow of surface runoff can be retarded by using
special gratings or restricted outlet pipes to modify
catch basin inlets to restrict the rate at which surface
runoff is permitted to enter the conveyance system.
Inlet flow restrictions may be designed to produce
acceptable levels of temporary ponding on streets or
parking lot surfaces, allowing all runoff to eventually
enter the system at the inflow point, but reducing the
peak flow rates that the combined sewer system
experiences. Flow detention to delay the entry of
runoff into the collection system by storing it
temporarily and releasing it at a controlled rate also
can be accomplished by roof-top storage under
appropriate site conditions.
Peak flow rates in downstream segments of the
collection system and at overflow points also may be
regulated by the installation of flow-restricting devices
at suitable locations in upstream portions of the
combined sewer system. A variety of commercial
flow-restricting devices are available, such as the vortex
valves described in Chapter 4.
Eliminating the direct connection of roof drains to the
CSO collection system and causing this runoff to reach
the system inlets by overland flow patterns (preferably
via unpaved or vegetated areas) also can retard inflows.
When site conditions permit, some surface runoff flows
may be prevented from entering the combined system
by diverting them via overland flow to pervious areas or
to separate storm drains. When these outlets are not
available, excess surface runoff flows may be diverted
to more favorable locations in the combined system, a
technique referred to as flow-slipping (WPCF, 1989).
Also, depending on site conditions, it may be possible
to intercept some of the surface runoff flows by using
infiltration devices, and divert them to ground water. In
coastal communities, repair and adequate maintenance
of tide gates may prevent flow intrusion caused by
diurnal tide cycles.
The implementation ("retrofit") of l&l controls in
developed areas on a scale necessary to substantially
control CSOs is difficult and may be impractical. Flow
control techniques, however, may be useful and
practical in selected problem areas for addressing
specific segments of an overall system. Control
measures that address l&l to avoid increasing
stormwater flows in a combined system are most
effectively implemented in areas currently being
developed, where their use can be required as a
condition for development or reconstruction.
-------
Pollution Prevention Practices
Pollution prevention measures include source controls
and other actions within a drainage basin that reduce
the amount of stormwater-related pollution entering the
combined sewer system. Source controls usually do not
require large capital expenditures, but they are
generally labor intensive, and ongoing operation and
maintenance costs can be high.
Controlling the use of problem materials such as
de-icing salt, fertilizers, and pesticides can reduce
pollutants entering the system. Product bans or
substitutions (e.g., for plastic fast-food packaging) can
prevent particular problem pollutants from being
generated. Pollution prevention also can include
increased frequency of solid waste collection, and
programs that enhance the environmentally responsive
disposal of bulk refuse, home renovation debris, and
household hazardous wastes.
Other control measures in this category are designed to
minimize accumulation of pollutants on streets and
other tributary land areas and in catchbasins.
Implementing these measures decreases pollutant
loadings to combined sewers by preventing or reducing
their entry.
Examples of pollution prevention practices applicable to
comprehensive control programs for reducing pollutant
discharges from CSOs are discussed below.
Street Cleaning
Street litter can be a significant source of certain
pollutants (e.g., floatables) entering receiving waters via
CSOs. An extensive monitoring program conducted by
the City of New York (NYC DEP, 1993) concluded that
the major portion (perhaps 95 percent or more) of the
floatables in CSOs originate as street litter. Litter on
streets can be removed by mechanical or manual street
cleaning. Street sweeping often is considered a
practical best management practice (BMP) for CSO
pollution control. The effectiveness of street sweeping
depends on the rainfall frequency, sweeping frequency,
and other factors such as street density and the
prevalence of curbside parking. A major impediment to
street cleaning in densely populated cities is cars
parked adjacent to the curb; therefore, enforced parking
regulations are an essential component of a street
cleaning program.
Public Education Programs
Education methods may consist of developing public
announcements, advertising, stenciling street drain
inlets, and distributing information with water/sewer
bills. An important aspect of a public education program
for CSO control is to encourage the proper disposal of
sanitary and personal hygiene items disposed of through
household toilets. The New York City floatables study
(NYC DEP, 1993) determined that although these items
accounted for only about 5 percent of floatable materials
discharged by CSOs, they include the more
objectionable items which cause the greatest public
concerns and result in beach closings.
Anti-litter campaigns can reduce the amount of street
litter and floatables that originate from CSO. Since it is
unrealistic to anticipate widespread enforcement of
anti-litter ordinances, the effectiveness of such
programs depends on public education. Citizen action
or education programs instituted to focus on specific
issues such as those identified above, also will raise
public awareness of the problems associated with
CSOs and the justification for the broader control
programs.
Recycle Programs
Crankcase oil, paints, cleaning agents, chemicals and
other household wastes, as well as leaves and grass
clippings sometimes are disposed of in catchbasins or
street inlets. Proper disposal of these materials should
be addressed in public education programs, but an
essential element of an effective program is the
availability of suitable disposal mechanisms. Recycle
programs that establish disposal locations and/or
collection schedules must be organized by the
municipal agency, though they may utilize commercial
establishments to implement components of such
programs (e.g., acceptance of waste oil by service stations).
Fertilizer and Pesticide Control
Fertilizers and pesticides washed off the ground during
storms contribute to runoff pollutant levels. Controlling
use of these chemicals on municipal lands can help
reduce the pollutant load. For example, in urban areas
an important source of fertilizer and pesticide runoff that
can readily be controlled is the park systems. Individual
homeowner use of these chemicals is not likely to be a
major source of pollution in urban areas served by
combined sewers. Where homeowners do use
fertilizers and/or pesticides, a public education program
is required to address this issue.
Soil Erosion Control
Controlling soil erosion is important because soil
particles carry nutrients and metals as well as contribute
sediment. The principal areas of potential concern in
urban areas served by combined sewers are public
parks and construction sites. Problems in public parks
may require limitations on the type of use, regrading,
and/or revegetation of eroding areas.
-------
Commercial/Industrial Runoff Control
Certain commercial and industrial sites can be
responsible for disproportionate contributions of some
pollutants (e.g., grit, oils, grease, and toxic materials) to
the combined sewers. Sources of potential concern
include gasoline stations, railroad yards, freight loading
areas, and parking lots. In specific cases where
significant pollutant loadings to the combined sewer
system are contributed by well-defined locations of
limited area, pretreatment of runoff from these areas
may be a practical and effective control measure.
Pretreatment measures can be required as part of a
community's sewer use regulations. Examples of
pretreatment measures include oil/water separators for
problem service stations, or use of modified catch-basin
designs to enhance retention of oil and grease or solids.
Procedures for detecting and locating illicit connections
to separate storm drains by testing for specific chemical
tracers (U.S. EPA, 1993) can be applied to CSOs to
identify commercial or industrial sources contributing
substantial levels of problem pollutants.
Operation and Maintenance Practices
Operation and maintenance (O&M) program activities
that focus on combined sewer system components
(regulators, tide gates, pump stations, sewer lines, and
catchbasins), can significantly influence the level of
control applied to CSOs.
Regulator/Tide Gate Maintenance
Because of the debris normally present in combined
sewage, especially at times of storm flows, regulators
continually accumulate materials that cause clogging
and blockages. This is a particular problem with
static-passive regulators.
The majority of unnecessary overflows in passive
regulators are caused by trash blocking the entrance
orifice to the interceptor. Other causes of unnecessary
diversions at regulators include weir plates or dams that
are improperly set, damaged, or broken off. Improperly
operating tide gates allow receiving water to enter the
combined sewer system, and reduce the storage and
flow capacity of downstream interceptors that otherwise
would be available during wet weather. Trash in
combined sewer flows and/or trash and timber in the
receiving water body can cause a tide gate to remain
partially open. Corroded or warped gates, or
deteriorated gate gaskets are also common causes of
improper operation.
Frequent inspection of CSO regulators and tide gates
will assure that as much of the wet weather flow as
system capacity permits is retained in the system. The
size and characteristics of the combined sewer system
will influence the schedule, but where practical, an
inspection several times per month, and after every storm,
will ensure that necessary repairs and maintenance to
clear debris and obstructions are performed in a timely
manner.
Sensors to detect the presence of water in overflow
lines during dry weather periods (resulting from faulty
regulator or tide gate operation) are used to signal the
need for maintenance action.
Maintenance programs that prevent the systematic
overflow of unnecessary quantities of wet weather flows
can influence the required design size and cost of
treatment technologies. Furthermore, effective O&M for
collection system elements is necessary to ensure that
CSO controls operate in accordance with conditions for
which the design is based.
Pump Station Maintenance
Proper operation and maintenance will ensure that
pump stations transfer the design flows. Inadequate
pumping capacity may result from changes in the
upstream area that cause flows to exceed the original
design basis, or from mechanical defects that cause
substandard performance. Inadequate capacity results
in back-up of dry weather flow in the sewer system,
possibly contributing to dry weather overflows and
resulting in reduced storage and hydraulic flow capacity
for wet weather flows. This contributes to CSO at
overflow points upstream of the pump station, and also
prevents the wastewater treatment facility from
maximizing treatment of wet weather flows.
Removal of Sewer Line Obstructions
Blockages in interceptor sewers can cause back-ups
that create excess CSO overflows upstream of the
blockage. A common cause of sewer line obstruction is
the deposition of solids, which deplete the in-system
storage capacity otherwise available for wet weather
flow and reduce the flow-carrying capacity of the sewers
(U.S. EPA, 1984a). Other system obstructions may result
from roots growing into pipes and from collapsed pipes.
Removal of flow obstructions may include maintenance
activities to remove and prevent accumulations of
debris in parts of the system that experience flow
restrictions. Where flow obstruction is the result of
sediment accumulations, sewer flushing may be an
effective control measure. A maintenance program of
periodically flushing sewers during dry weather to
convey settled materials to dry weather treatment
facilities minimizes the buildup of such sediments. In
cases where a section of the conveyance system
routinely accumulates sediment deposits at a
substantial rate, design and installation of a permanent
flushing station or an in-line grit chamber may be a
cost-effective approach.
-------
Severe situations may require the application of sewer
cleaning measures to physically clean a segment of the
conveyance system. Hydraulic, mechanical, or manual
devices may be required to remove solids or resuspend
solids into the waste flow and carry them out of the
collection system (U.S. EPA, 1984b).
Knowledge of the area and collection system assists in
determining the parts of the system where particular
problems can occur, and guides development of an
effective inspection program. For example, collapsed
pipes occur most frequently in areas with old sewer
lines or with current or recent construction activity.
Excessive infiltration also is most likely to be a problem
in these areas, as well as where water tables are high.
The potential for obstructions caused by solids
deposition is greatest where velocities are low during
dry weather and for small- and average-sized storms.
These conditions exist where lines are oversized for the
flows they normally convey and/or where gradients are
flat. As a result, solids deposition obstructions likely will
recur where they are present. Lines with a history for
acquiring sediment deposits should be scheduled for
cleaning on a regular basis.
Catchbasin Cleaning
In many communities, catchbasin cleaning is performed
infrequently and is targeted towards maintaining proper
drainage system performance rather than pollution
control. Regular cleaning of catchbasins (once or twice
per year) can remove accumulated sediment and debris
that ultimately could be discharged from CSOs, thus
providing some degree of pollution control. This
technique applies for true catchbasins, that is inlet
chambers that provide a sump for retention of sediment
and debris. The technique does not apply for simple
drain inlets.
Other Control Technologies
Physical treatment technologies other than those
identified earlier in this chapter and that have been
applied to CSOs include dissolved air floatation, fine
screens and microstrainers, dual-media high rate
filtration, and biological treatment. Sewer separation
also is considered a control technology here and is
included in the discussion that follows. Although the
technologies have not been widely applied in CSO
control applications, they have been tested on a
demonstration scale and are effective in CSO control. A
brief synopsis of these technologies as they apply to
CSO control is presented below.
Sewer Separation
Separation is the conversion of a combined sewer
system into separate stormwater and sanitary sewage
collection systems. This alternative, historically considered
the ultimate answer to CSO pollution control, has been
reconsidered in recent years because of cost and the
major disruptions to traffic and other daily community
activities associated with separation. Separate stormwater
runoff also contains pollutants (sediments, organic
matter, bacteria, metals, oils, floatables, etc.), which
continue to be discharged to the receiving waters.
Several potential benefits of sewer separation might
warrant its consideration in specific cases. These
include:
• Eliminating CSOs and preventing untreated sanitary
sewage from entering the receiving waters during
wet weather periods. Sanitary sewage is a more
objectionable source of some pollutants, such as
TSS, sanitary floatables, and bacteria.
• Reduced volume of flow to be treated at the POTW,
thus reducing O&M costs, by eliminating surface
runoff inflows during wet weather periods.
• Reduced infiltration and excess flow to a POTW if
new sanitary sewers are constructed to replace old
combined sewers.
• Reducing upstream flooding as well as overflows in
cases where the existing combined sewers are
undersized and back up frequently during storm
events.
• Being more effective and economical than treatment
facilities for remote segments of a combined sewer
system serving relatively small areas.
Dissolved Air Floatation
Dissolved air floatation (DAF) removes solids by
introducing fine air bubbles to wastewater. Air bubbles
attach to solid particles suspended in the liquid, causing
the solids to float to the surface where they can be
skimmed off. This technology has been tested in CSO
applications (U.S. EPA, 1972a, 1975a, 1977, 1979a). A
major advantage of DAF is its relatively high overflow
rate and short detention time, which results in reduced
facility size compared to conventional sedimentation.
Oil and grease also are more readily removed by DAF.
Operating costs for DAFs are high due to a large energy
demand, and skilled operators are required for its
operation.
Fine Screens and Microstrainers
These devices remove solids through capture on screen
media. They have been tested for use in CSO control
(U.S. EPA, 1970, 1971, 1973, 1974a). The most
common fine screening devices are rotary drum and
rotary disk devices. In the rotary drum screen, media is
mounted on a rotating drum. Flow enters the end of the
drum and passes out through the filter media. Drum
rotational speed usually is adjustable. Solids retained
10
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on the inside of the drum are backwashed to a collection
trough. Filter media aperture size typically ranges from
15 to 600 microns. The rotary disk screen media is
mounted on a circular frame placed perpendicular to the
flow. Flow passes through the bottom half of the rotating
disk, which is submerged. Solids retained on the disk
are directed to a discharge launder using spray water.
One form of static screens features wedge-shaped steel
bars, with the flat part of the wedge facing the flow.
These "wedge-wire" screens typically have openings
ranging from 0.01 to 0.05 in. These screens require
daily maintenance to prevent clogging (Metcalf & Eddy,
Inc., 1991).
Screens are subject to blinding from grease and "first
flush" solids loads. A means for providing high-pressure
backwash, and collecting and conveying backwash
solids typically is required. Effective cleaning of screens
after storm events using high-pressure steam or
cleaning agents is required to maintain performance.
Removal efficiencies may be increased by decreasing
media aperture size, but smaller apertures are more
likely to blind. Coarse screening and disinfection facilities
often are provided in conjunction with microstrainers.
Filtration
Dual-media high-rate filtration has been piloted for
treatment of CSO flows (U.S. EPA, 1972b, 1979b). A
two-layer bed, consisting of coarse anthracite particles
on top of less coarse sand, was used. After backwash,
the less dense anthracite remains on top of the sand.
Filtration rates of 16 gal/f^/min or more were utilized
resulting in substantially smaller area requirements
compared with sedimentation. Demonstration test
systems included pretreatment by fine-mesh screens.
The use of chemical coagulants improved performance
considerably. Filtration is more appropriately applied
after pretreatment provided by fine screening.
Operation may be automated, but tends to be O&M
intensive.
Biological Treatment
Biological treatment processes have been tested in
CSO control applications (U.S. EPA, 1974b, 1975b,
1981a,b). Although they have potential to provide a high
quality effluent, disadvantages of biological treatment of
CSOs include:
• The biomass used to break down the organic material
and assimilate nutrients in the combined sewage
must be kept alive during dry weather, which can be
difficult except at an existing treatment plant.
• The land requirements for these types of processes
can preclude their consideration in an urban area.
• Operation and maintenance can be costly and
facilities require highly skilled operators.
Some biological treatment technologies are utilized in
CSO control as elements of a wastewater treatment
plant. Pump-back or bleed-back flows from CSO
storage facilities commonly receive secondary
treatment at the treatment plant, once wet weather flows
have subsided. In a treatment plant that has maximized
the wet weather flows that are accepted, flows are
sometimes split, with only a portion of the primary
treated flows receiving secondary treatment, to avoid
process upset. The split flows are blended and
disinfected for discharge.
References
When an NTIS number is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
703-487-4650
Metcalf & Eddy, Inc. 1991. Wastewater engineering:
treatment, disposal, reuse. 3rd edition. New York:
McGraw-Hill.
NYC DEP. 1993. New York City Department of
Environmental Protection. Citywide floatables study:
characterization and quantification of sources of
floatable materials. Report by HydroQual, Inc.
February.
U.S. EPA. 1970. U.S. Environmental Protection Agency.
Microstraining and disinfection of combined sewer
overflows. Report no. 11023 EVO 06/70 (NTIS PB
195 674). (U.S. EPA called Federal Water Quality
Administration in 1970). June.
U.S. EPA. 1971. U.S. Environmental Protection Agency.
Demonstration of rotary screening for combined
sewer overflows. Report no. 11023 FDD 07/71 (NTIS
PB 206 814). Portland, OR. July.
U.S. EPA. 1972a. U.S. Environmental Protection
Agency. Screening/floatation treatment of combined
sewer overflows. Report no. 11020 FDC 01/72 (NTIS
PB 215 695). January.
U.S. EPA. 1972b. U.S. Environmental Protection
Agency. High-rate filtration of combined sewer
overflows. Report no. 110EYI 04/72 (NTIS PB 211
144). Cleveland, OH. April.
U.S. EPA. 1973. U.S. Environmental Protection Agency.
Microstraining and disinfection of combined sewer
overflows: phase II. EPA/R2-73/124 (NTIS PB 219
879). January.
11
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U.S. EPA. 1974a. U.S. Environmental Protection
Agency. Microstraining and disinfection of combined
sewer overflows: phase III. EPA/670/2-74/049 (NTIS
PB 235 771). August.
U.S. EPA. 1974b. U.S. Environmental Protection
Agency. Combined sewer overflow treatment by the
rotating biological contactor process. EPA/670/
2-74/P50 (NTIS PB 231 251). June.
U.S. EPA. 1975a. U.S. Environmental Protection
Agency. Treatment of combined sewer overflow by
dissolved air floatation. EPA/600/2-75/033 (NTIS PB
248 186). September.
U.S. EPA. 1975b. U.S. Environmental Protection
Agency. Biological treatment of combined sewer
overflow at Kenosha, Wisconsin. EPA/670/2-75/019
(NTIS PB 242 126). April.
U.S. EPA. 1977. U.S. Environmental Protection Agency.
Screening/floatation treatment of combined sewer
overflows: volume I—bench scale and pilot plant
investigations. EPA/600/2-77/069a (NTIS PB 272
834). August.
U.S. EPA. 1979a. U.S. Environmental Protection
Agency. Screening/floatation treatment of combined
sewer overflows: volume If—full-scale operation
Racine, Wisconsin. EPA/600/2-79/106a (NTIS PB
80-130693). August.
U.S. EPA. 1979b. U.S. Environmental Protection
Agency. Dual process high-rate filtration of raw
sanitary sewage and combined sewer overflows.
EPA/600/2-79/015 (NTIS PB 296 626/AS). April.
U.S. EPA. 1981 a. U.S. Environmental Protection
Agency. Evaluation of a treatment lagoon for
combined sewer overflow. EPA/600/2-81/196 (NTIS
PB 82-105214).
U.S. EPA. 1981b. U.S. Environmental Protection
Agency. Joint dry/wet weather treatment of municipal
wastewater at Clatskanie, Oregon. EPA/600/2-81/061
(NTIS PB 81-187262).
U.S. EPA. 1984a. U.S. Environmental Protection
Agency. Procedures for estimating dry-weather sewage
in-line pollutant deposition—phase II. EPA/600/2-84/
020 (NTIS PB 84-141480).
U.S. EPA. 1984b. U.S. Environmental Protection
Agency. Dry weather deposition and flushing for
combined sewer overflow pollution control. EPA/600/
2-79/133 (NTIS PB 80-118524). '
U.S. EPA. 1993. U.S. Environmental Protection Agency.
Investigation of inappropriate pollutant entries into
storm drainage systems: a user's guide. EPA/600/
R-92/238 (NTIS PB 93-131472). January.
WPCF. 1989. Water Pollution Control Federation.
Combined sewer overflow pollution abatement:
manual of practice no. FD-17. Alexandria, VA.
12
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Chapter 3
Process Selection
Performance Goals
While the overall objective of CSO control programs is
to protect or improve water quality and designated uses
of water bodies that receive CSO discharges, selecting
a treatment process and determining size or capacity
involves a focus on a more specific performance target.
Individual technologies are more effective for controlling
some pollutants or pollutant classes than others.
Depending on the designated use of the receiving
water, the specific pollutant classes that directly affect
that use assume primary importance in developing
control plans. Therefore, the first criterion in selecting
thr most appropriate technology to apply in a specific
situation is its control effectiveness for pollutants that
directly affect the protected use. Some overlap exists,
but the following list of uses, CSO pollutants, and
control technologies generally indicates the basic type
of control technology to consider.
Considerations In Technology Selection
Use Pollutants
Swimming
Shellfish
Aquatic
life/fin fish
Pathogens,
bacteria
Pathogens,
bacteria
Solids, BOD,
COD, metals,
toxic organtas
Aesthetics Solids, floatables
Disinfection,
storage/sedimentation,
fine screens
Disinfection,
storage/ sedimentation,
fine screens
Storage/sedimentation,
fine screens
Screens, swirl/vortex,
storage/sedimentation
Where the objective of CSO control is to mitigate a
specific water quality problem in an area immediately
influenced by a CSO, the foregoing considerations
should have a major influence on treatment process
selection. However, in many cases CSO control
requirements are addressed in a more general manner.
Cause and effect relationships between CSO and water
quality impacts often are difficult to establish. The
intermittent and highly variable nature of CSO and the
complexities of many receiving waters impose significant
demands on monitoring data requirements. In addition,
impacts on aquatic life resulting from long-term
sediment pollutant buildup and/or an already degraded
receiving water may not be obvious, or able to be related
to CSO in a definitive way. In such cases, performance
goals for CSO control may be governed by state or
other regulations, and expressed as one of a number of
alternative technology-based goals.
Examples of commonly used technology-based
performance goals are:
• Percent capture: That a specified percentage of flow
be captured and/or treated.
• Overflow frequency: Reducing the number of
untreated CSOs per year to a specified number.
• Treatment level: Specifying the pollutant removal
efficiency of the CSO controls; often specified as the
equivalent of primary treatment.
• First flush: Providing capture and/or treatment of
some portion of a total overflow, determined to
contain a major fraction of the pollutant load.
• Knee-of-the-curve: Basing the size of a control unit
on cost effectiveness (i.e., where significant
increases in cost produce marginal improvements in
performance).
Selection of the general control strategy and specific
performance goals may depend on local, state, or
federal regulations, a community's long-term CSO
control plan, or requirements of a permitting authority.
In practice, a combination of technology and water
quality considerations may be used in developing a
CSO control plan. Water quality considerations may be
used to guide the general approach, identify the
pollutant types of major concern at a particular location
(and hence the appropriate technologies), address
spatial issues such as consolidation and/or relocation of
outfalls, and similar issues. Specific performance goals,
such as those listed above, then would be used to
develop the design basis for individual treatment units
or systems.
Implementing any strategy requires an estimate of the
CSO flows or volumes that must be used as a design
13
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condition in order to meet the selected performance
goal. These estimates require information on local
rainfall characteristics and an appropriate engineering
analysis of the system layout and hydraulic
characteristics. In most cases, an appropriate analysis
may require development of a monitoring program and
application of computer models, in order to adequately
assess the operation of the collection system and/or the
water quality impacts in the receiving water body.
Computer models such as EPA's Storm Water
Management Model (SWMM) (U.S. EPA, 1988) are
commonly applied to simulate the hydraulic behavior of
a combined sewer system, and develop the information
necessary to design CSO controls.
All strategies require an estimate of the design flow to
use in developing the design details discussed in
Chapter 4. If a design storm has been specified (for
example, the 1-year, 6-hour event), then an analysis
would be made to determine the CSO flows that would
be delivered to a planned control unit by that rainfall
condition. However, if the performance goal specifies a
desired result (e.g., four overflows per year), then the
system analysis to be performed must, in effect,
develop both the appropriate "design storm" to apply,
and the CSO flows and volumes to be used for detailed
design.
Data requirements for system modeling analysis and
process selection and design are presented later in this
chapter. General design considerations that relate to
each of the possible performance goals for a CSO
control are presented below.
Design for Percent Capture
This goal is properly defined as capture for subsequent
treatment. It is used to define performance goals for
storage technology. When applied to off-line storage for
CSO, this performance goal is used to define the
additional storage volume required to capture a
specified percentage of either the volume of current
CSOs or the volume of wet weather flows in the
combined sewer system, usually considered on a
system-wide annual average basis. This assumes that
all captured flow is returned to the collection system
during dry weather for treatment at a POTW. The
required storage used for design considers the storage
provided by both the collection system and additional
storage devices.
This performance goal could also be applied to
treatment units or combination storage-treatment
systems located at individual outfall points (e.g.,
sedimentation, swirl/vortex, screens) if the specified
percentage is considered to be the percentage of the
total combined sewage volume treated by the unit at
flow rates consistent with design criteria.
The percent capture by a storage unit could be
expressed in an individual case as the percentage of
current discharges from an overflow point. In the
general case, however, it is usually and more
appropriately considered to be a percentage of the total
wet weather combined sewage flows. This definition
imposes more uniform requirements for different CSO
systems, and allows the designer flexibility to optimize
the use of both in-line and off-line storage. Percent
capture is most appropriately considered as a long-term
average (or in a year of average rainfall).
An indication of the approximate total storage
requirements for different capture efficiencies can be
developed by extracting from the rainfall record the
percentage of total rainfall volume resulting from storm
events that are equal or smaller than a selected design
storm volume. Figure 3-1 shows the relationship
between design storm size used as a basis for storage
design and the percentage of the combined sewage
flows that will be captured, based on 42-year rainfall
records at six locations. The performance pattern is
similar for all locations. As the size of the design storm
increases, the effect of regional differences in rainfall on
percent capture become progressively smaller. Since a
design storm (for example, the 1-year, 6-hour storm)
can be expressed as a volume, the approximate
performance of a storage control using it as a design
basis can be estimated from the rainfall record.
A critical assumption for this estimate relates to the
ability to physically place the storage at the locations
where it is required. The results apply directly for the
analysis of a single overflow point. If applied for a
system with multiple overflows, the tacit assumption is
that the required storage capacity is appropriately
distributed among the overflow locations. Existing
systems with portions that are undersized, or otherwise
sensitive to "kicking-off" an overflow at particular
locations during very small storms would generate more
overflow events than the screening analysis suggests.
However, where these situations are associated with
relatively small drainage areas, the number of overflow
occurrences may be disproportionately high, but the
effect on the estimate of the overall percent captured
may be minor.
The tacit assumption is that the aggregate of all storm
events equal to or smaller than a selected storm size
that accounts for a particular percentage of total rain
volume also estimates the corresponding percentage of
the total wet weather flow volumes. For example, if 75
percent of the total amount of rainfall is delivered by
storms equal to or smaller than 1 inch, then about 74
percent of the total wet weather flow volume will be
provided by the runoff from storms of this size. Thus,
storage capacity sufficient to contain the runoff from a
1-inch storm would retain, in such a case, about 75
14
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DESIGN STORM SIZE (Inch**)
Figure 3-1. Approximate relationship between performance of CSO storage and storm size used as design basis—percent capture.
percent of the wet weather flows. The absolute amount
will vary with the physical size and other features of the
drainage area and collection system, and will be site
specific. Details must be developed by an appropriate
model or other analysis, but the required storage
capacity inferred from the area rainfall characteristics
would include all forms of storage that may be provided
by the overall system. This would include capacity
resulting from both in-system storage and off-line basins
at the overflow points (as well as any natural depression
storage that attenuates the rainfall reaching the
combined sewers).
Maximization of in-system storage accordingly would
reduce off-line storage capacity required to produce a
desired capture percentage, because the required
storage volume also would be accounted for, in part, by
the wet weather flows that the conveyance system
retains and delivers to the POTW as wet weather flows.
An off-line storage basin is required to provide capacity
only for the excess over capacity provided by other
features of the overall system.
The captured volume usually is sent to the POTW
where it is treated before ultimate discharge. At this
point it is a wet weather-related component of the
treatment plant flow. The ability of the POTW to process
captured flows that are returned to the system, and the
treatment for those flows, are important elements of the
overall control program. An important influence on the
performance of CSO storage units is the rate at which
they can be emptied following a storm. A very large
storage volume may be provided, but if it cannot be
substantially emptied during the interval between most
storms, it has the practical effect of providing a much
smaller "effective" volume.
Two factors may restrict the emptying rate. One is the
hydraulic capacity of the interceptors to which captured
flows are returned for conveyance to the POTW. If
infiltration into the combined sewer system is great
enough that flows in the line remain high for extended
periods following a storm, the ability to remove captured
CSOs may be significantly restricted. The other factor
relates to the capacity of the POTW and its ability to
accept increased flows on a relatively consistent basis.
If a large storage volume is provided, and a substantial
percentage of wet weather flow is captured and then
returned to the system over extended periods during dry
weather, the result will be an increase in the average
dry weather flow to the POTW. This flow could be
significant.
The approximate ratio between annual volumes of
sanitary sewage and stormwater in a combined sewer
area is site specific. The sanitary flow from the area is
determined by the wastewater generated (gallons per
capita), and by the population density (persons per
acre). Stormwater runoff depends principally on the
amount of rainfall and how impervious the area is. In
areas such as San Francisco with annual precipitation
in the order of 15 or 20 inches, the annual sanitary
volume generated is perhaps 3 to 5 times greater than
the stormwater volume. In areas with 60 inches of
15
-------
annual precipitation (e.g., the Gulf Coast), the ratio is
about 1 and the annual volumes generated are about
equal. A large percentage of CSO areas are in regions
with about 30 to 40 inches of annual precipitation, and
annual sanitary volumes may be about 2 times the
stormwater volume generated. Storage controls that
provide a high percentage of CSO capture and
distribute the returned flow to the system over much of
the dry weather periods can significantly increase the
"average" dry weather flow delivered to the POTW for
treatment. There also will be additional sludge and grit
generated by CSO control. This is an element of CSO
control planning that should be properly considered.
Design To Reduce Overflow Frequency
Overflow frequency usually is defined as the number of
overflows per year as a long-term average. Overflow
frequency is higher in wet years and lower in dry years.
This performance goal may be applied to a specific
discharge, or treated as a value applied to the combined
sewer system as a whole.
A preliminary indication of the design storm size used
as a basis for CSO storage design can be developed
by analyzing the rainfall record to determine the number
of storm events that have larger volumes than the
design storm. CSO storage units normally are not
emptied until after the end of a storm because available
system capacity is utilized by wet weather flows, so
event volumes that exceed the design storm produce
an overflow. Figure 3-2 illustrates the approximate
relationship between the number of overflows
(expressed as an annual average), and the design
storm size for which storage is provided, based on
analysis of rainfall records. Results are plotted using a
log scale to assist in reading the low values.
A relationship exists between percent capture and
number of overflows for any site using storage as the
basic approach to control. Figure 3-3 illustrates the
"equivalence" between performance goals that specify
percent capture and number of overflows. The plot
combines the results for all six locations shown
separately on Figures 3-1 and 3-2. Of particular interest
is the indication that very large storage volumes and
capture efficiencies are required to reduce the number
of overflows to 4 or 6 per year. Independent support for
the relationship indicated is provided by results from a
CSO study for the city of Windsor, Ontario (Mahood and
Zukovs, 1993). A model analysis performed using the
complete simulation model, STORM, was used to
define the relationship between CSO capture and
overflow frequency, and the results compare quite well
with the general relationship indicated by Figure 3-3.
Design to comply with this performance goal would
require continuous rainfall-flow modeling to verify and
refine the above approximations. Parts of the collection
system with marginal excess capacity produce a
disproportionate number of overflows compared with
the system as a whole.
DESIGN STORM SIZE v« OVERFLOWS per YEAR
100
e
s
—«"••-*•-•
1- -
1 0 - -: :
U.
O
UJ
O
e
ui
j..—j....j—.4 .—i >'-.?•'• 4—^-...*"..|....j....4-—J—-T™j—*f™4—-i—*••"•*•
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1 _.>....;....4..»i...4...4...H....«.k..i....; • • '• - '- • '• '-
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Newark NJ
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Portland ME "1~
Portland OR _L.
Loultvill* KY i
Chicago IL —r
Attana QA
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: : : : : i : : : : i ; ! i ! : : :
1 1.5 2 2.5 3
DESIGN STORM SIZE (Inch**)
3.5
Figure 3-2. Approximate relationship between performance of CSO storage and storm size used as design basis—number of
overflows.
16
-------
COMBINED RELATIONSHIPS FOR 6 CITIES
Newark NJ , Portalnd ME , Portland OR, Louisville KY. Chicago IL, Atlanta GA
100
2L
(0
o
10
I 1
« *
30 40 50 60 70 80 90
% of Total Wet Weather How Volume Captured by Storage
Figure 3-3. Relationship between percent capture and number of overflows for CSO storage control.
100
This performance goal could be applied to CSO control
systems that provide treatment to CSOs, but continue
to discharge them at the overflow point. In such a case,
it would be necessary to establish that an "overflow"
referred to an untreated discharge. In addition, for
treatment units that operate as flow-through devices,
the variable inflows produce short duration peaks, some
amount of which may be bypassed. Since these peaks,
at times, occur at more than one period during the same
storm event, agreement as to what constitutes an
"overflow evenf is important to develop an appropriate
design.
Design To Provide a Specified Treatment
Level
Treatment technologies such as screens, sedimentation,
and swirl/vortex designs are applied at individual
discharge locations to remove pollutants from all or a
portion of the combined sewer overflows, without
modifying the overflow volume discharged at that point.
Combination surface storage/sedimentation units are
an exception because they return captured flow to the
POTW for additional treatment. Also, the underflow (foul
sewer discharge) from swirl/vortex units is retained in
the conveyance system. However, for pure treatment
technologies, essentially all flows are discharged at the
overflow point in question, but pollutants are reduced or
removed from these flows in accordance with the design
size and treatment efficiency of the unit or process
selected.
Any treatment device has an inherent performance
capability dictated by its design principles and the
removal mechanisms it employs. The pollutant mass
removal efficiency it can apply to the flows delivered to
it, even under optimum operating conditions, will vary
for different pollutants. For example, removals by a
particular unit could be nearly 100 percent for floatables,
essentially zero for soluble nutrients, and anywhere
between 5 and 50 percent for total suspended solids
(TSS). Partitioning between dissolved/paniculate fractions,
particle sizes and settling velocity distributions, and
settleable solids characteristics are all site specific and
extremely important for design. Selection of the treatment
technology to be adopted should consider the pollutants
of concern for the outfall location and the average
removal the type of unit is capable of providing.
A receiving water impact analysis can determine a
specific treatment level, which is used to define a
treatment level performance goal. An alternate application
of this performance goal is to specify a technology-
based treatment level. A common requirement is that
17
-------
CSO control provide the equivalent of primary treatment.
Because of the variability of CSO discharge flows, when
this performance goal is applied a design storm flow
rate also is specified and treatment units are designed
for flows up to those produced by the design storm.
Reduced performance is accepted for extreme events.
Estimates of performance characteristics for many
types of treatment units can be based on experience
with their performance as unit processes in a POTW.
The relationship is not absolute because of differences
between CSOs and sanitary sewage. The major factor
for CSOs is the intermittent and highly variable nature
of the influent flows and pollutant concentrations. Other
factors, such as the reported tendency for CSO solids
to have higher settling velocities and proportionally
greater quantities of floatables, also may have an
important bearing on design details.
Design/performance relationships could also be based
on units that treat CSOs. The difficulty is that
performance data on CSO treatment units is limited,
and because the characteristics of CSOs vary
substantially, the results for one site may not apply very
well for other CSO sites. Table 3-1 summarizes the
range in reported performance for CSO treatment units.
Also, effectively monitoring intermittent, short-duration
events, with variable and usually high flow rates and
heavy particles, is difficult and the number of separate
Table 3-1. Reported Performance of CSO Treatment Technologies
Percent Reduction
Technology TSS BOD
events monitored often is quite limited. Further, results
from individual studies often are reported in such a way
as to make it difficult or questionable to generalize
sufficiently for transfer to other situations. Because of
these considerations, the Table 3-1 results are best
used as a general indication of approximate performance
capabilities.
Treatment units that employ sedimentation are designed
on the basis of overflow rates and detention times,
which in turn are derived from particle settling velocities.
A study of the relationship between solids removal
performance and overflow rates for POTWs suggests
that the performance expected from a given design
overflow rate may also be affected by other factors
(WEF, 1992). Since CSO quality and treatability tend to
be highly site specific, the design of treatment units for
CSO control should consider local analysis, including
bench- and pilot-scale tests where reliable determination
of performance is required. Determining the actual
settling velocity distribution of the solids in the CSO will
provide a more accurate assessment of the appropriate
design overflow rate. Procedures to consider for local
sampling and testing programs are further discussed
later in this chapter.
Design of a CSO treatment-type control to meet a mass
removal performance goal must consider two elements
in combination. One is the efficiency of pollutant
Remarks
Source
Sedimentation
Without chemicals
Chemical assisted
20-60
68
30
68
General characteristics
General characteristics
U.S. EPA, 1977a
U.S. EPA, 1977a
Storage/Sedimentation
Cottage Farm site 45 erratic
Cottage Farm site—later data 46 28
Prison Point site 34 14
Chippewa Falls 18-70 22-24
Columbus, OH—Whittier St. 15-45 15-35
Swirl Concentrator/Regulator
Washington, DC 33 83
Decatur, IL (0-87) (0-72)
avg 37 avg 34
General data 40-60 25-60
Screens
Microstrainers 50-95 10-50
Drum Screens 30-55 10-40
Disc Screens 10-45 5-20
Static Screens 5-25 0-20
General characteristics
Average data for 1988-1992
Average data for 1988-1992
General characteristics
General characteristics
Test data8
Test data"*
General characteristics'"
U.S. EPA, 1974
MWRA, 1993
MWRA, 1993
U.S. EPA, 1974
U.S. EPA, 1974
Washington, DC, 1992
BGMAandCMT, 1987
U.S. EPA, 1977a
U.S. EPA, 1977a
U.S. EPA, 1977a
U.S. EPA, 1977a
U.S. EPA, 1977a
a 18% attributed to flow diversion, 15% to treatment by unit. Average flow was 32% of design peak; observed peak was 70% of design peak.
b No information was provided on removal by diversion or by treatment. Removals stated are presumed to be due to combination of treatment
and diversion.
0 49 observations during four storm events. Individual range 0-87% TSS and 0-72% BOD reduction.
18
-------
removal applied to the flows that are delivered to a unit
for treatment, as discussed above. The other is the
percentage of total flow delivered to and processed by
the unit. When the performance goal is defined as the
equivalent of primary treatment and a design storm size
is specified, both of the required design conditions are
defined. However, when a treatment level goal is
defined only in terms of an overall pollutant mass
removal percentage, then the design analysis also must
determine the equivalent of the design storm size to be
used as a basis for the treatment unit hydraulic design.
A trial and error assessment of alternative design sizes
using a SWMM analysis may be required to make this
determination. However, since treatment unit sizing is
significantly influenced by peak flow rates, a summary
of the frequency pattern of hourly rainfall intensities,
extracted by analysis of rain gage data, can be helpful
in identifying the most appropriate range of conditions
to explore by the model analysis. Table 3-2 summarizes
rainfall intensity characteristics for six locations in the
United States. For each of a range of hourly intensities
(inches/hour) the table lists the following information
derived from a 42-year record and converted to an
average annual basis:
• The upper segment indicates the average number of
storm events per year that include one or more hours
in which the intensity is greater than the intensity
shown at the end of each row.
• The middle segment breaks down this total, and
indicates that as the reference intensity increases,
the number of "peak" hours in any event becomes
increasingly restricted to only one or two individual
hours.
• The lower segment lists the percentage of the total
rainfall volume contained in all the hours of rainfall
that have intensities equal to or lower than the
reference value.
Practical considerations usually restrict SWMM model
runs to limited time spans. Multi-year simulations rarely
are feasible, and for complex systems practical
simulation periods may be several months. Reliable
estimates of the annual average number of untreated
overflows or the percentage of total volume treated may
not, therefore, be provided by the model simulation and
limit its ability to suitably evaluate alternate design
sizes. However, the model output can be analyzed/
interpreted to identify the magnitude of the flow rates
(and rainfall intensity) that will produce an overflow, or
a peak condition at which the unit will be bypassed. The
information on the overall pattern of rainfall characteristics
listed in Table 3-2, can be used to determine the
frequency at which the flagged condition (an overflow
or a bypass) would occur as a long-term average, and
the fraction of the total volume associated with that
condition. In cases where the initial design selection
either was inadequate or excessive in terms of the
applicable performance goal, the rainfall relationships
indicated in the table, coupled with the information
derived from the model simulation, can be used to
provide guidance on appropriate adjustments to the
initially selected design parameters.
Design To Capture First Flush
The concept indicated by the term "first flush," is that in
the early stages of a storm runoff or combined sewer
overflow event, a relatively small percentage of the total
flow contains a disproportionately large percentage of
the total pollutant mass associated with the overall
storm event.
Significant first flush effects are most likely to be present
with small catchments, flat slopes, low impervious
fractions, relatively simple conveyance system networks,
and lines with low dry weather flow velocities that permit
solids to accumulate in-line. For larger drainage areas,
and complex piping networks, an array of separate first
flush conditions, may occur but reach the downstream
location at which control is to be applied at staggered
intervals that substantially attenuate and blend the
small-scale effects. In addition, the design and capacity
of the conveyance and treatment system may
effectively retain that portion of many of the storms in
which the first flush effect is present. Regulator
configuration also may influence first flush solids
loadings in CSO flows. If the regulator that diverts flow
to the CSO control facility is a side weir or high-level
outlet, much of the heavy bed-load of grit associated
with the first flush may not be carried over to the CSO
facility.
Monitoring data can be used to characterize the first
flush. One method of estimating first flush volumes was
employed in the design of first-flush tanks in Decatur,
Illinois (BGMA and CMT, 1987). A sampling program
established "baseline" average dry weather flow (DWF)
concentrations for biochemical oxygen demand (BOD),
TSS, and volatile suspended solids (VSS). The first
flush was defined to start when the CSO facility influent
concentrations rose above the baseline level, and
continued until the concentrations returned to the baseline
level. The first flush volume was then estimated by plotting
flow on the same axis as the pollutant concentration. A
sample plot is illustrated in Figure 3-4. The shaded area
indicates the volume of the first flush. The limits of the
shaded area under the flow curve (Q) correspond to the
duration in which the pollutant concentration was
greater than the baseline concentration.
A reasonably large event sampling program is desirable
to characterize the potential significance of first flush
effects at the point in the system where a control
measure will be located. Various intensities/durations or
19
-------
Table 3-2. Summary of Peak Hourly Rainfall Characteristics
Intensity
for
bypass Atlanta,
(in/hr) GA
Average Number of
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85
1.00
1.15
1.25
Average Number of
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85
1.00
1.15
1.25
Percent
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85
1.00
1.15
1.25
1 hr
20.5
24.0
20.9
16.6
13.0
9.2
6.4
4.5
2.6
2.0
1.4
0.9
Louisville,
KY
Storms Per Year Having
63.8
52.8
34.2
22.9
15.8
10.8
7.0
4.8
2.8
2.0
1.4
1.0
Chicago,
IL
Portland,
ME
1 or More Hours With a Greater Intensity
63.6 51 .2
49.1 35.5
27.3 18.9
17.0 11.8
10.9 8.1
6.9 6.0
4.9 3.9
3.1 2.9
1.6 1.9
0.9 1 .3
0.6 0.9
0.5 0.8
53.9
37.5
17.7
9.2
5.2
2.9
1.6
0.8
0.5
0.3
0.2
0.1
Newark,
NJ
57.2
42.1
22.1
12.3
8.0
5.3
3.2
2.1
1.2
0.5
0.3
0.2
Portland,
OR
57.3
28.0
5.B
1.4
0.3
0.1
0.1
0.0
0.0
0.7
0.5
0.4
Intensity
for by-
pass
(in/hr)
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85.
1.00
1.15
1.25
Storms Per Year With Indicated Number of Hours of Greater Intensity
2 hr >2 hr
14.6 28.2
12.1 16.7
7.8 5.5
4.5 1 .8
2.0 0.8
1 .4 0.2
0.6
0.3
0.1
of Total Rainfall Volume
37.7
56.2
75.1
84.5
89.8
93.1
95.3
96.6
97.8
98.6
99.1
99.4
1 hr
20.0
23.4
17.9
12.9
8.5
5.9
4.4
2.8
1.5
0.8
0.6
0.4
Associated
2 hr >2 hr
15.1 28.2
11.9 13.8
5.5 3.9
2.9 1.2
1.8 0.6
0.9 0.2
0.5 0.1
0.3
0.1
0.1
With Hours Having
44.8
63.7
80.9
88.4
92.6
95.2
96.8
97.8
98.7
99.2
99.5
99.7
1 hr 2 hr
13.7 13.3
17.9 8.4
12.0 4.5
8.7 2.2
6.5 1.2
4.9 0.9
3.1 0.6
2.4 0.4
1.6 0.3
1.1 0.2
0.8 0.1
0.7 0.1
>2hr
24.0
9.2
2.4
0.9
0.5
0.3
0.2
0.1
1 hr
13.6
14.1
10.1
6.3
3.9
2.3
1.5
0.7
0.5
0.3
0.1
0.1
2 hr >2 hr
10.2 29.6
7.8 15.4
3.7 4.0
1.7 1.2
1 .0 0.4
0.4 0.1
0.1 0.1
1 hr 2 hr >2 hr
15.3 11.4 30.1
17.5 90 15.6
13.3 4.8 4.0
8.6 2.6 1.1
6.2 1.3 0.5
4.5 0.6 0.3
2.8 0.3 0.2
1.9 0.2 0.1
1.1
0.4
0.3
0.2
1 hr 2 hr >2 hr
19.7 12.9 24.1
15.3 5.9 6.8
4.6 0.7 0.5
1.3
0.3
0.1
0.1
0.0
0.0
0.6
0.5
0.4
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85
1.00
1.15
1.25
Equal or Lower Intensity
43.3
64.3
79.8
86.6
90.5
93.1
94.9
96.1
97.3
98.1
98.7
99.0
53.9
73.7
89.2
94.6
97.1
98.3
99.0
99.3
99.6
99.8
99.9
99.9
48.8
68.4
84.8
91.3
94.6
96.5
97.7
98.6
99.2
99.6
99.8
99.8
73.2
91.0
98.5
99.8
99.9
99.9
99.9
99.9
100.0
100.0
100.0
100.0
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.85
1.00
1.15
1.25
-------
TYPICAL FIRST FLUSH VOLUME GRAPH
BOD, TSS, or VSS Concentration
Sample Time - minutes
*188mg/LtorBOD
291 mg/LtorTSS
203 rng/L tor VSS
Figure 3-4. Illustration of use of monitoring data to
characterize first flush.
more events should be sampled, because variations
from event to event are expected. In addition, when
such monitoring may be associated with reduced-scale
pilot testing, an effort should be made to ensure that the
influent samples used to characterize a first flush
represent conditions at the location where the full-scale
controls will be installed.
Whether a consistent and significant first flush effect
exists at the point in the system at which control
measures are to be applied is highly site specific, and
only by appropriate monitoring can its presence and
magnitude be determined. However, where appropriate
monitoring can demonstrate and adequately characterize
a first flush, then more cost-effective design of CSO
controls may be possible. Where the first flush effect is
significant and occurs reasonably consistently, some
control units may have smaller design sizes for a given
level of mass removal performance than otherwise
would be the case. The design capacity of storage units
would have a direct relationship to the presence of a
first flush; however, the design of treatment-type units
cannot be related to first flush in any practical manner,
unless applied in relation to integral equalization storage
that may be incorporated in the design.
A number of existing CSO facilities are designed to
capture the more concentrated combined flows that
may occur during the initial stages of a storm event. The
key to designing such a facility is to define the limits of
the first flush. First flush effects vary substantially from
storm to storm, and sampling of combined flows
resulting from a range of storm durations and intensities
is required to indicate the duration of the first flush. For
example, at the Massachusetts Water Resources
Authority (MWRA) Cottage Farm facility, sampling
indicated that the concentration of pollutants dropped
off after 2 hours following activation of the facility (U.S.
EPA, 1977b). At Bannockburn, Scotland, sampling
indicated that BOD concentrations dropped to 65
percent of peak in 30 minutes, and 50 percent of peak
in 60 minutes (Henderson et al., 1981).
Sizing for the first flush is complicated by the fact that,
depending on the system configurations, part of the first
flush may not even enter the CSO facility. Sampling at
the Bannockburn facility indicated that the initial flow
into the facility had the highest pollutant concentrations.
The "rising leg" of the first flush, therefore, must have
been carried down the interceptor. At the MWRA
Cottage Farm facility, the downstream interceptor was
sized for 2.1 times the mean dry weather flow.
Depending on the timing of the storm, the interceptor
may have available capacity to carry part of the first
flush.
Design Based on Knee-of-the-Curve
The term "knee-of-the-curve" reflects the fact that
as control units become larger, the incremental
improvements in performance become progressively
smaller in relation to incremental increases in design
size and cost. This pattern is generally true, but often is
intensified in the case of CSO control because the size
of increasingly rare events (volumes and flows) grows
logarithmically, and these rare events have an important
influence at the high end of the performance scale.
The knee-of-the-curve technique is widely used to
establish the design size of a specific control measure
or an overall CSO control system, by identifying the
point at which costs increase disproportionately for only
a marginal improvement in performance. For any site,
cost will be directly related to design size, and cost
estimates, developed for units with a range of design
capacities, can be developed and plotted against the
corresponding performance levels for a formal
knee-of-the curve assessment. Usually some degree of
subjectivity involving best professional judgment is used
to select the "knee," and it may be influenced by a
variety of legitimate non-technical factors.
The knee-of-the-curve concept may be applied to the
assessment of alternative design sizes for a particular
control, when a particular type of control measure has
been identified as the preferred approach. In a case
where a CSO control assessment is made on a
system-wide basis and includes consideration of
alternate technologies as well as design sizes, a cost
performance relationship developed to examine the
knee of the curve may result in a pattern with
discontinuities, rather than the smooth curves that
usually apply for the assessment of a selected
technology.
21
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For a simple illustration, consider a case where storage
technology has been selected for CSO control. For
either surface storage or tunnel storage, the cost generally
will increase relatively uniformly as performance level
increases. However, space availability or other site
constraints may place an upper limit on the maximum
size and hence performance level (percent capture) that
surface storage could provide, so that performance
levels greater than this would require the use of tunnel
storage. Examples of site constraints other than space
availability that have been found to influence the
placement of CSO control structures include:
• Ownership (including multiple ownership) of site.
• The presence of other utilities on site.
• Historic landmarks.
• Traffic concerns.
• Ground-water conditions.
• Zoning and other land use issues.
• Neighborhood resident concerns with visual impact
of proposed structures and/or odor problems.
For situations where the maximum size of one control
technology is constrained, a display of the overall cost
curve showing the relationship over a broad range of
performance levels shows a discontinuous step at the
technology break point, when unit storage costs for
tunnels are significantly greater than for surface storage
units. Figure 3-5 illustrates the cost-performance
relationships for a knee-of-the-curve design assessment.
Data Requirements for Design of CSO
Controls
Several classes of data are important for designing
CSO control systems or individual treatment units so
that they achieve a specified performance goal.
• Information on the combined sewage and CSO
volumes and flow rates used to design control units
normally is developed using a simulation model such
t
Size & Cost
\
Surface Storage
Size Limit for Surface
'Storage Based on Available Space
Performance •
Figure 3-5. Illustration of knee-of-the-curve analysis.
as SWMM. Data requirements for this model are
discussed below in the section on modeling analysis.
« Although the model can accept all available hourly
sequences of rainfall contained in a rain gage record,
it is rarely practical to utilize the entire record. An
appropriate year, season, month, or storm event must
be selected. Analyzing a rainfall record for the study
area can provide information to guide this selection.
Rainfall data sources and analysis techniques are
discussed below in the section on rainfall analysis.
e Most combined sewer systems are unique. Quality
characteristics, including pollutant particle size and
settling velocity distributions, tend to be site specific.
Appropriate CSO monitoring and bench-scale
treatability tests to establish treatability parameters
can be important for the design of effective CSO
controls. This issue is discussed below in the section
on treatability analysis.
Modeling Analysis
Design of CSO control systems and individual units
requires a determination of the combined sewage flow
rates and volumes produced by rainfall on the drainage
area. Most collection and conveyance systems are
sufficiently large and/or complex that computations
using a computer model are necessary. EPA's Storm
Water Management Model (SWMM) is a common
choice to simulate the behavior of a combined sewer
system and approximate the characteristics of the
associated CSOs. SWMM simulates the complex
time-varying physical process of rainfall onto land of
varying characteristics, the conversion of rainfall to
runoff, and the collection and transport of mixed
stormwater runoff and sanitary sewage through the
collection system. It can develop hydrographs for
selected nodes within a combined sewer system. Both
hydraulic and pollutant routing are performed, and the
model can be used for both single storm and long-term
(continuous) simulation.
Single storm simulation provides a detailed assessment
of the sewer system and overflow characteristics during
individual storm events. This analysis mode can provide
detailed predictions of flow and pollutant concentrations
and can illustrate the manner in which control strategies
or design alternatives affect these flows and concentrations.
The continuous simulation mode performs a long-term
analysis based on an annual or seasonal rainfall record,
and is used to develop CSO flow and load statistics for
an existing system as well as to simulate the effects of
different control options. It provides information to
assess the CSO problem and the cost effectiveness of
abatement options. Both types of simulation are key to
understanding the behavior of a combined sewer
system, designing controls, and projecting CSO impacts
on the receiving wateis.
22
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Detailed guidance on applying SWMM or other
simulation models is provided in separate documents or
by contacting EPA's Center for Exposure Assessment
Modeling (CEAM), Athens, GA. The data required to
perform an analysis using the SWMM model are
discussed below.
Structure and Input Data
For convenience and computational manageability,
SWMM is constructed in modules or "blocks." Included
are modules that manage information and data, and
modules that perform modeling and evaluation tasks.
The Runoff and Transport Block modules determine the
combined sewer system data requirements and relate
to the design of CSO controls.
The Runoff Block is designed to simulate the quantity
and quality of runoff from a drainage area. It utilizes a
rainfall input file to produce hydrographs and
pollutographs at selected locations in the system. These
serve as inputs to the Transport Block, which routes
them through the combined sewer system. Input data
for this block includes:
• Meteorological data: Consists of rainfall and snowfall
amounts, wind speeds, and air temperatures. All data
should be local to the drainage area since storm
variations throughout the drainage area can affect the
patterns of runoff.
• Surface quantity data: Consists of characteristics that
define the drainage area, which includes information
on subcatchment areas, land slope, sewer inlet
locations, infiltration, evaporation, depression storage,
width of overland flow, and the land surface
roughness. The model uses these factors (which may
be developed through evaluating aerial photographs,
topographic maps, field testing, or literature) to
determine the amount and rate of runoff flow.
• Surface quality data: Includes information for
calculating the amount and type of pollutants carried
by the overland runoff flows. For a fully implemented
Runoff Block, surface quality data relating to erosion,
street dirt build-up and rain water washoff, street
cleaning, snowplowing, and catch basin cleaning are
needed. This information can be provided by
literature, field data, and/or through calibration of
observed pollutant loading data.
The Runoff Block is based on segmentation of the
drainage area into an appropriate number of
subcatchments. The number is determined by the
features of the drainage area and by the layout and
characteristics of the collection system. Manning's
equation is used to calculate the maximum rate of
runoff from each subcatchment area. The maximum
subcatchment runoff is reduced by an infiltration and
evaporation factor to produce a volumetric loading rate
for use as input to the Transport Block.
The Transport Block routes inputs from the array of
locations where flows enter the collection system, and
simulates the quantity and quality of flow at locations of
interest. Inputs, generated by the Runoff Block or by
other means, are entered into the network at designated
inlet manholes. The calculated flows and concentrations
define the characteristics of specific overflows, or may
be applied as input for another SWMM block, such as
Storage/Treatment, or for a separate receiving water
quality model. Information required by this block
includes:
• Transport data: Describes the physical characteristics
of the sewer system, which is conceptualized as a
network of conduits connected by non-conduit nodes
representing manholes or regulators. Conduits are
described by shape or element type, dimensions,
length, slope, roughness, and number of adjacent
lines (barrels). Transport data are acquired from
as-built drawings and field inspections of the sewer
system.
• Quality data: Describes pertinent characteristics of
pollutants the model routes through the sewer
system. The required data for each pollutant
simulated includes first-order decay rate, specific
gravity, and solids particle size distributions. These
parameters allow the Transport Block to determine
pollutant concentrations after decay, scour, and
deposition processes are accounted for in the model.
• Internal storage data: Input as an individual element
of the drainage system. Geometry, depth-discharge
relationships, and initial pollutant concentrations of a
storage unit are all required data. A storage unit may
be used to simulate processes such as sedimentation
or chlorination.
• Infiltration data: Used to assess the amount of flow
and pollutants entering the sewer system from
leaking pipes and joints. Infiltration flow may come
from ground water, rain water, residual moisture, and
base dry weather infiltration. Monthly degree day
data is used to determine infiltration from residual
moisture. These data are most useful when obtained
directly from the study area. Historical data from a
near-by study area, estimates by local professionals,
and estimates based on country-wide observations
are useful alternatives when direct data are not
available.
Additional Transport Block data requirements include
subarea identification parameters, observed flow data,
and flow estimating data. Subareas are subdivisions of
the drainage area identified according to zoning
classifications. Subareas may not correspond to
subcatchments defined in the Runoff module. Land use,
23
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subarea acreage, water usage, population density,
dwelling unit density, housing values, average income,
and process flows are data required by the Transport
Block to estimate average daily sewage flow and
pollutant concentrations. This average sewage flow also
is corrected for hourly and daily fluctuations in dry
weather flows and concentrations. The Transport Block
includes a dry-weather flow model that may be used to
estimate both sanitary sewage flow and pollutant
concentrations from the drainage area.
Transport of wet- and dry-weather flow by the Transport
Block is based on representation of flow as a kinematic
wave. Disturbances are propagated only in the
downstream direction and backwater effects are limited
to only a single conduit. Surcharging is modeled by
storing excess flow at the upstream non-conduit until
capacity is freed to accept the stored flow. Equations for
gradually varied, unsteady flow are used by Transport
to calculate the velocity and depth of flow along the
sewer system. This procedure is not appropriate for
analysis of system segments that consistently
experience significant surcharge conditions. In such
cases, the Extran (Extended Transport) Block must be
used.
Practical Application and Calibration
Calibration of the model is the only practical way to
assure that the substantial array of input parameter
values assigned accurately represents the system
being modeled. Model calibration consists of adjusting
selected input parameters so that the computed output
matches observed monitoring results at key points in
the system. On completion, a calibrated model should
provide a good match between predicted and observed
results, when applied to data sets that were not used in
the calibration step. When this criterion is met, the
model is considered validated or verified.
Model calibration requires synoptic rainfall and flow
measurements for a number of storm events. Rainfall
data should be obtained from one or more gages
located in or close to the drainage area. Flow monitoring
should be conducted at representative locations in the
system. Monitoring overflows at the largest CSO points
in systems with a number of different CSO locations is
advisable. Even the best model predictions are only
approximations of reality, and emphasis on the largest
CSOs will help to secure the best accuracy for the most
significant discharges, and thus the best overall
estimate of CSO volumes and pollutant loads for the
system as a whole. In addition to the overflows, flow
monitoring stations should be located at one or more
points in the collection system upstream of the
regulators. This is necessary to calibrate the model
parameters defining the drainage area features that
control the conversion of rainfall to flows in the collection
system.
An adequate number of storm events (usually 5 to 10)
should be monitored and used in the calibration. This is
important because of possible malfunctions in different
flow sensors at different times, and also to compensate
for variations in rainfall distribution over the drainage
area. The point rainfall measurement at the rain gage
provides only an approximation of the distribution over
the entire drainage area, so variations in individual
events should be expected.
The availability of a sufficient number of different storm
events also will facilitate and improve the model
calibration. Many users prefer to base the calibration on
the model's ability to reproduce the overall results from
a number of different events, rather than attempt to
match instantaneous flow measurements.
Flow measurements almost always will be made using
automatic sensing units, but it is important to provide
operator inspection and supervision. Sensors tend to
clog and generate erroneous readings, which can
seriously impede model calibration efforts if left
undetected.
In addition to timely maintenance of the flow meters,
operator supervision also can assure that the location
being monitored is not surcharged at the time of flow
measurement. The existence of a surcharged condition
is essential information for reliable application of the
simulation model.
For lines that surcharge, the Extran Block must be used,
rather than the normal Transport Block. This adds
complexity to the model analysis, not so much in the
computations, but in the model's sensitivity to the
accuracy of the input parameters assigned. Where
Extran is employed, a better definition of ground truth
(pipe diameters, slopes, lengths, etc.) is required.
Applying the model is more cumbersome for systems
with many regulators and overflow points. Modeling
large, complex systems also imposes significant
demands on available computer memory for input and
output files. This is a serious consideration for model
analyses performed on PC platforms. The practical
effect is to impose a limit on the length of rainfall record
that can be analyzed.
For large or complex systems, practical maximum
record lengths are a year or less, and in some cases
could be less than a 6-month simulation. When the
period analyzed is short, it is important that the most
appropriate period of the rain record be selected for
analysis, so that the model provides representative
projections of system or control performance.
Independent analysis of rain data can assist in selecting
an appropriate portion of the overall rainfall record.
24
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Many applications of the SWMM model do not
independently compute the surface runoff quality as
above, and then combine it with the sanitary sewage
quality characteristics to define the quality of the CSOs.
A common approach is to directly assign values for
quality characteristics of the combined sewage and
CSOs, based on local CSO monitoring data or the
quality of wet-weather inflows to the POTW. When an
average concentration value is assigned for all storms
(or a concentration correlated with storm size when the
data so indicates), the model will not reproduce
individual event quality, but provides acceptable
estimates of pollutant loads averaged over the analysis
period. Many analysts believe that such projections
have equal or better reliability, because of the
uncertainties associated with the use of the build-up/
wash-off model routines and the difficulty commonly
encountered in calibrating this model component.
Rainfall Analysis
Precipitation is the driving force that mobilizes and
transports pollutants via CSOs to receiving waters.
Evaluating the precipitation characteristics of an area
can assist in analyzing issues such as the estimation of
CSO pollutant loads, the water quality impacts they
produce, and the assessment of control strategies.
When the basic analysis is performed using a simulation
model such as SWMM, separate analysis of the rainfall
record can assist in selecting the most appropriate
rainfall periods or events to use in the model analysis.
Rainfall is highly variable, and excursions of individual
events above typical or average values of parameters
such as intensity, duration, and volume, can be
considerable. The greater the magnitude of rainfall
volume or intensity, the more larely it occurs. Statistical
analysis of rainfall records can define the frequency
distribution (or probability of occurrence) of magnitudes
of the various rainfall parameters. Such probabilities
often are expressed in terms of return period or
recurrence interval.
Hourly rainfall records for rain gages in the United
States are available from the National Weather
Service's (NWS's) National Climatic Data Center
(NCDC) in Asheville, North Carolina. Each record is
identified by a unique 6-digit number, consisting of a
2-digit state code followed by a 4-digit gage number.
The information in a gage record includes the location,
latitude and longitude, elevation of the measurement
site, and the depth of hourly rainfall recorded (in 0.01
inches). The date and hour are recorded for each depth
in the record.
The NWS generates regional summary reports from
time to time, but for all NWS gages, the basic data are
readily available in electronic format from both the
NCDC and commercial sources. The electronic data
include records from both previously and currently
operating stations in the 50 states. These records
include daily rainfall data for over 25,000 stations,
hourly data for over 5,500 stations, and 15-minute data
from over 2,700 stations. Approximately 8,000 rain
gages currently operate across the country. The
increased use of personal computers over the past
decade has allowed interested parties to conveniently
obtain and analyze the actual data for a particular
location, avoiding the need to rely on published
summaries.
A long-term sequence of rainfall data can be analyzed
in a number of different ways to develop relatively
concise characterizations, which may then be used
for engineering purposes. Common methods for
characterizing rainfall include total volumes, event
statistics, return period/volume curves, and intensity-
duration frequency curves. Each method is described
below.
Total Volumes
A common basis for describing rainfall is the total
volume of rainfall occurring each year. The NWS
publishes annual totals as well as deviations from the
average for each rain gage in its network. Definitions of
wet- and dry-year rainfall can be made by comparing a
particular year's rainfall to the long-term average.
Monthly totals and averages also can be computed in
the same way to examine seasonal differences.
Evaluating annual or seasonal rainfall totals is a
common basis for selecting a specific time period to use
in detailed simulation modeling.
Event Statistics
Information may be developed on the characteristics of
individual storm "events" for a site. If the sequence of
hourly volumes is grouped into separate events, then
each event may be characterized by its duration,
volume, average intensity, and the time interval
between successive events. The event data can be
analyzed using standard statistical procedures to
determine the mean and standard deviation, as well as
probability distributions and recurrence intervals. A
computer program, "SYNOP," performs a statistical
analysis of the rainfall data in a NWS record (U.S. EPA,
1989). It segregates the hourly rainfall values into
independent storm events, and determines the
parameters of each event (volume, duration, average
intensity, and interval since the preceding storm). The
array of individual event values is analyzed to determine
the mean and standard deviation or coefficient of
variation for selected stratifications of the data set,
producing results sorted by month and by year, as well
as for the entire period of record. Outputs include
information on the frequency distribution and recurrence
25
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period for the event parameter values. This report also
presents summaries of rainfall statistics developed for
rain gages in different areas of the country.
Return Period/Volume Curves
The frequency of occurrence for a given magnitude of
a storm event parameter such as volume may be shown
as a plot of its probability distribution, as illustrated in
Figure 3-6. The plot indicates that approximately 10
percent (90th percentile) of the storm events in the
42-year record used for the analysis, deposited about
1.5 inches or more of rain. If the statistical analysis
indicates an average of 60 storm events per year, this
would mean that six storms per year had a rainfall
volume equal or greater than 1.5 inches. Furthermore,
six events per year averages one event per 2 months,
so the 1.5-inch rain event could be characterized as the
storm-event volume with a 2-month return period, or the
"2-month storm."
Note that this 2-month storm definition is not related to
duration; the 252 storms equal or greater than 1.5
inches during the 42-year period (10 percent x 42 year
x 60 per year) would have a range of durations. The
return period-volume relationship is different than
corresponding relationships that are based on rainfall
intensity-duration (e.g., 1-year, 6-hour storm) curves
provided by other design storm determinations. The
relationships from a storm "evenf analysis are more
appropriate for the assessment of storage requirements
because events, by definition, provide storm volumes
that are typically followed by dry periods that average
several days, during which a CSO storage unit is
emptied.
The specific values for storm parameters vary regionally,
but the event summary and distribution plot (Figure 3-6)
illustrates the nature of the storm event statistics
produced by any rain gage. Table 3-3, which lists
summary event statistics for rain gages in different
regions of the country, illustrates both the general
similarity as well as the regional differences in rainfall
characteristics.
Intensity-Duration Frequency Curves
Curves of this nature are another way of characterizing
the variable rainfall at a site. A typical set of
intensity-duration frequency curves is illustrated by
Figure 3-7. For this record, a 60-minute duration
(1-hour) rainfall intensity of about 1 inch/hour occurs
once every year. One-hour intensities that are higher
than this occur more rarely (e.g., 2 inches/hour every
10 years). Note also that rainfall with an average
intensity of 1 inch/hour also may occur over longer
durations, at less frequent recurrence intervals.
A major use of these families of rainfall characterization
curves is to design hydraulic structures (e.g., storm
drains, culverts), where short-duration peak flows must
Storm Event Volumes
10
o
111
o
OT
Louisville, KY
N = 2910
Median = 0.42 inch
cov = 0.99
Mean = 0.61 inch
.1
1 5 10 20 30 50 70 80 90 95
Percent of Storms Equal or Less Than
Figure 3-6. Probability distribution of storm event volumes.
99
99.9 99.99
26
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Table 3-3. Rainfall Characteristics for Six U.S. Locations3
Annual Averages
Event Rainfall Statistics Mean Storm Event
Storm Event Volumes
for Three Recurrence
Intervals'1 (in)
Number Rain
City of Stormsb Volume (in)
Atlanta, GA
Louisville, KY
Portland, ME
Newark, NJ
Chicago, IL
Portland, OR
66
69
64
64
58
72
47.4
42.0
41.9
41.8
33.4
34.2
Volume
(in)
0.71
0.61
0.66
0.65
0.57
0.47
Intensity
(in/hr)
0.112
0.092
0.065
0.076
0.095
0.034
Duration
(hr)
9.4
9.5
12.5
11.1
9.1
15.7
a Based on 42 years of records, from 1949 through 1990.
Storm events greater than 0.1 inches with a minimum of 6 dry hours to separate storm events.
c Delta is the average interval between the midpoint of storm events.
The tabulation of recurrence interval volumes indicates rainfall volumes for events that recur on
Delta0
(days)
5.55
5.34
5.79
5.76
6.29
5.08
average 3-,
Return Period
3 mo 6 mo 1 yr
2.0
1.6
1.8
1.7
1.5
1.4
6-, and
2.6
2.1
2.3
2.2
2.1
1.8
3.2
2.6
2.8
2.8
2.6
2.3
12-month intervals.
10
or
x
2
•^*
fc
UJ
o
$ ~
3 YR. STORU
10 YR. STORM
\ YR. STORU
2 YR. STORU
NOTE:
1 IN/HR.-25.4 mm/hr.
10 100 200
DURATION (MIN.)
Rgure 3-7. Illustration of intensity-duration curve (Moffa, 1990).
be considered to avoid local flooding. They also may be
applied to the design of CSO treatment units where
peak flow is a relevant design parameter.
These curves are developed by analyzing an hourly
rainfall record in such a way as to compute a running
sum of volumes for consecutive hours equal to the
duration of interest. The set of volumes for that duration
are then rank ordered, and based on the length in years
of the record, the recurrence interval for any rank/value
is determined. This rainfall analysis procedure is used
to calculate the local value for a design storm such as
a 1-year, 6-hour design condition.
Treatability Tests
CSO treatment technologies such as sedimentation
basins and swirl/vortex units remove particulate pollutants
by employing design parameters and unit configurations
that enhance the separation of solids by settling.
Therefore, the settling velocity of the pollutants in the
CSOs to be treated has an important influence on
performance and should be considered in developing
design details.
Bench-scale tests can be employed to determine the
typical range of CSO settling velocities. A number of
variations of the procedure and equipment are used for
these tests. Figure 3-8 is a schematic illustration of several
settling test column designs. Test procedures vary with the
design of the test apparatus and are described briefly
below for the units illustrated. Results obtained from either
procedure can be analyzed to determine the probability
distribution of particle settling velocities in the CSO sample
tested, and displayed as illustrated on the plot shown in
the lower portion of Figure 3-8.
For the 8-in-diameter by 5-ft-tall test column with
multiple sample ports, the column is filled to the top with
a sample of the water to be tested, and then stirred to
distribute the settleable solids uniformly throughout the
column. A sample at time zero is analyzed to establish
initial conditions. Samples are then withdrawn from
each port at selected time intervals. They are analyzed
to determine the concentration of TSS that remains at
the sample location after the elapsed time interval.
For the other test unit, clean water is used to fill the
column up to the level of the butterfly valve at the upper
end. A 500-ml sample is then added at the top of the
cylinder. After the sample has been properly stirred, the
butterfly valve is opened and particles in the sample
settle through the body of clean water. Water samples
containing settled suspended solids are collected from
the bottom cone at various elapsed times ranging from
1 to 60 minutes, and analyzed for TSS. For a 60-cm
settling distance, the test results will provide the
27
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8"0
(J) 65mm
1
1
I
i
*
1
5fj
i
i
IXX
SAMPLING PORT (TYP)
~~ii
rj
—t
xxxxxx
r
r
r
r
-Sample 500ml
• Butterfly valve
— Plexiglass cylinder
- Clean water
i —Sample collection point
TYPICAL SETTLING COLUMNS
Initial TSS = -165 mg/L
Rutland, Vermont CSO
1000-
99.9 99 95 90 80 50 20 10 5
PERCENT EQUAL OR GREATER
0.1
Figure 3-8. CSO settling velocity—typical test equipment and sample results.
28
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distribution of particle settling velocities in the range of
1 to 60 cm/min (2 to 120 ft/hr).
TSS normally is selected as the pollutant analyzed, but
other pollutants of concern also could be selected to provide
information on the settleability of BOD, for example.
A number of different approaches have been used to
analyze, summarize, and interpret settling column test
results. A procedure that provides useful results
recognizes that particulates are present in combined
sewer overflows in a range of particle sizes and settling
velocities, and that it is appropriate to characterize
settling properties in terms of a settling velocity
distribution. Such distributions define the cumulative
percentages of solids having settling velocities greater
than a specified value. Test data can be analyzed to
develop this information by following these steps:
1. For each measured concentration (each sample port
and sample time), the difference between the
concentration and the amount present initially is
used to compute a percent removal.
2. Since each combination of depth to a sample point,
and elapsed time between the start of the test and
the time the sample is taken, reflects a specific
distance settled in a specific interval of time, each
sample corresponds to a settling velocity (ft/hr).
Each such value corresponds to a percent removed
value computed from the measured concentration.
The results can be interpreted as the percentage of
TSS in the sample that has settling velocities equal
to or greater than the value represented by the
settling distance and sample time.
3. The results then are plotted on graph paper with a
probability scale to indicate the frequency distribution
of pollutant settling velocities in the sample. CSO
settling velocity characteristics developed in this
manner are shown in Figure 3-8.
Test results can be displayed on probability plots to
illustrate the range in settling velocities of the
suspended solids (or other pollutants) in the CSOs.
Results from different samples at a particular site or
from different CSOs also can be summarized and
compared by listing the median (50 percent) and/or the
90th percentile settling velocity.
For example, settling tests conducted on CSO samples
at Washington, DC, indicated that for 11 samples during
eight events, the median settling velocity ranged
between about 1 and 30 ft/hr. For these same samples,
10 percent of the particles had settling velocities in the
range 10 to 300 ft/hr (Washington, DC, 1992).
Median CSO settling velocities at other locations were
measured in the range of 5 to 50 ft/hr in one city and
estimated from particle settling velocity data to be in the
range 50 to 100 ft/hr in another (Dufresne-Henry, Inc.,
1992). Both of these results can be considered to fall in
the same general range as the Washington data, given
the inherent variability. Several tests on New York City
CSOs, in contrast, provided median settling velocities
in the range of 1 to 3 ft/hr (NYC DEP, 1993). However,
the CSO quality at this location is suspected to be
significantly influenced by ground-water infiltration.
The variability in available data from different locations
emphasizes the importance of local testing to determine
design parameters for CSO controls that are based on
sedimentation. The significant variability exhibited by
different samples at the same site indicates the
importance of (a) testing a sufficient number of samples
(at least 6 to 12 in most cases) to define a representative
range of conditions for the site, and (b) testing samples
from representative parts of a storm.
The latter is particularly important if the overflow has a
significant first flush effect, because particle sizes and
settling velocity are significantly different in different
parts of the overflow event. Part of the wide variation in
median settling velocities in the Washington, DC, data
set is attributed to this factor.
Care is required in collecting samples used in settling
column tests to ensure that results provide representative
settling characteristics of the flows to be treated.
Samples should be collected from an appropriate point
in the collection/overflow system. In addition, sampling
equipment and procedures should be checked to
assure that the sample itself properly collects the solids
that are present. Sample intake locations and intake
velocities must be established to capture the heavier
solids present in most CSO flows.
Settling velocity test results can guide design decisions
by determining the removal efficiency that a CSO
control unit will provide for particles with selected
settling velocities, when the unit processes CSO flows at
the design flow rate (or other applied rates of interest).
For sedimentation basins, a relationship between
steady state removal efficiency and the hydraulic
loading rate and particle settling velocity is provided by
the following equation (Fair and Geyer, 1954).
where:
R = fraction of initial solids removed (R x 100 =
percent removal)
n = a parameter that provides a measure of the
degree of turbulence or short-circuiting, which
tends to reduce removal efficiency
vs = settling velocity of particles
Q/A = rate of applied flow divided by surface area of
basin (an "overflow velocity," often designated
the overflow rate)
29
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The upper plot in Figure 3-9 illustrates the relationship
between the design size of a sedimentation basin
(expressed as an overflow rate, gpm/ft2 or ft/hr) and
removal efficiencies for a range of particle settling
velocities, as provided by the above equation (U.S.
EPA, 1986).
For swirl/vortex units, the design capacity and
performance do not relate directly to an overflow rate
based on surface area, as is the case for sedimentation
basins. An indication of the relationships of interest as
they apply to a swirl unit based on the EPA design is
provided by the lower plot in Figure 3-9. This plot is
adapted from information developed from a series of
experiments in Sweden on a 1.17-m diameter swirl
concentrator, supplemented with data from two full
actual operating concentrators (Lygren and Damhaug,
1986).
Figure 3-9 summarizes the interrelationship between
removal efficiency, applied flow rate, and particle
settling velocity. Comparing these relationships with
local test data that defines the distribution of settling
velocities in the CSO flows to be treated, will permit
estimates of expected performance relative to a
particular size fraction at any design flow selected for
evaluation. In addition, overall performance estimates
for a selected unit design size (over the range of
projected CSO flows) may be developed by
incorporating the relationships in a simulation model
(e.g., the Storage-Treatment Block of SWMM). In this
case, the continuous distribution of settling velocities
can be incorporated by dividing the distribution into
three to five size fractions and assigning an average
settling velocity to each. The performance analysis
would be applied independently to each of the fractions.
Design decisions could then be based either on
projected results for a selected size fraction, or on the
combined results for all fractions.
Technology Selection
Factors That Influence Technology Selection
Selecting a specific control technology, or as is often the
case, the combinations of technologies to be incorporated
in a comprehensive CSO control program, requires an
engineering evaluation that considers a variety of
elements. The following elements usually have a major
influence on technology selection:
• The layout and hydraulic characteristics of the
combined sewer system and overflow points.
• The number and size of overflow locations, the
distance they are separated, and the feasibility of
consolidating multiple overflows at a point at which a
control technology can be applied.
• The magnitude (volumes, peak flow rates) that will
occur at individual overflow control locations, which
influences the size requirements for specific
technologies.
• Available space and other siting constraints in the
vicinity of CSO locations.
• The performance goal that is applicable for the
system as a whole and/or for specific overflow
locations.
• The ability of a particular technology to provide
effective control of specific pollutants that are
contributing to designated use impairment.
Such a comprehensive evaluation, addressing the
system as a whole and identifying control needs at
specific CSO locations, and site features that influence
the applicability of individual technologies, is essential
to develop an effective CSO control program. Surveys
are necessary to establish characteristics of the study
area that are pertinent to technology selection. In
addition, the hydraulic characteristics of the combined
sewer/CSO system must be characterized by a
monitoring/modeling effort. Performance/applicability
factors that relate to individual control technologies also
must be evaluated.
The principal performance factors that relate to the
applicability of the individual CSO control technologies
addressed in Chapter 4 are discussed below.
In-System Controls/ln-Line Storage
All CSO control programs should incorporate in-system
controls appropriate for the features of the local
combined sewer system. Both the measures that are
applicable and the degree of control they will provide
are variable and site-specific. However, the increase in
in-line storage and reduction in overflows produced by
in-system controls generally will be cost effective, since
they represent modifications of existing facilities. Many
of the applicable measures will require relatively minor
engineering and cost commitments.
To the extent that the existing collection system can be
adapted to increase the retention of wet weather flows,
it will reduce the required design capacity and cost of
other control technologies that must be applied. This
technology, therefore, is essential and should be a
component of all control plans.
Increases in the utilization of the available hydraulic
capacity of the existing combined sewer system for
containing wet weather flows will increase the risk of
street and basement flooding. This situation must be
carefully considered in establishing the maximum in-line
storage that can be safely achieved, and in many cases
will require a model analysis to define acceptable
conditions. Even with model projections as a guide,
30
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SEDIMENTATION BASIN
adapted from Rg 7 [EPA 1986]
1
DC
9
100 I -^
80
60
40 -;
20
Hydraulic Loading Rate
gal/day /sq ft
for short-circuit factor n > 3
NOTE : 1 cm/sec = 118 Whr
I
50 100 '150
Particle Settling Velocity (feet per hour)
200
§
o
tr
CO
I
100
80
6°
40
20
SWIRL CONCENTRATOR
adapted from Fig 2 [Lygren 1986]
NOTE : 1 cm/sec = 118 ft/hr
50 100 150
Particle Settling Velocity (feet per hour)
200
Figure 3-9. Effect of particle settling velocity and hydraulic loading rate on TSS removal efficiency.
31
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weirs should be raised in incremental stages with
suitable follow-up monitoring. Increased collection
system storage also will increase the potential for
accumulating sediment deposits. Operation and
maintenance efforts can increase for the effective
application of in-system controls, because effective
performance relies on continuing reliable operation of
regulators, tide gates, and pump stations, and on
preventing sediment deposits from accumulating to
problem levels.
Storage—General
Storage technology is probably the most favorable
control technology to consider as a general rule. For the
volumes captured at a CSO location, storage removes
all pollutants (pathogens, floatable and dissolved
pollutants, solids) from discharge at that location. When
captured flows are delivered to the POTW, treatment is
provided at that point and discharge occurs at the plant
outfall. For discharges at the overflow point produced
by storms in excess of the storage capacity,
sedimentation in the storage unit provides a degree of
treatment, and where disinfection may be required, all
or part of the storage volume may be utilized to provide
contact time.
However, despite the theoretical attractiveness of
storage technology, a number of factors limit its
applicability. For relatively high levels of control, storage
volumes are large, with corresponding costs. Cost
effectiveness may be very poor if storage technology is
applied separately to multiple CSO locations, or to
CSOs with relatively small discharges. Consolidation of
a number of existing CSOs for treatment by a single
storage unit may involve substantial modifications to the
collection system and significant disruption of the
service area during installation. Storage units are often
difficult to site because of the need for adequate space
and other constraints imposed by land use in the area.
When all CSO flows captured by storage are returned
to the interceptor for conveyance to and treatment by
an existing POTW, the treatment plant loadings
increase, as do the solids residual and sludge handling
requirements. The ability of the POTW to respond to
changes in operation required to accommodate the
implementation of CSO storage technology must be
adequately addressed by the control program studies.
These factors apply to both surface and tunnel storage
technologies. Selection factors specific to each of these
technologies are discussed below.
Off-Line Near-Surface Storage/Sedimentation
This technology balances control provided by storage
to capture CSOs and return them to the POTW for
treatment and direct discharge. It may limit the furnished
storage to a capacity that is feasible based on local site
constraints and cost considerations. However, for CSO
volumes that are not captured, it provides effective
sedimentation treatment by utilizing efficient settling
basin design features. This control technology provides
effective reductions in suspended solids and associated
pollutants, is very effective for control of floatables, and
provides contact time for disinfection when control of
bacterial levels is required. CSO flows captured by
storage will result in an increase in flows and solids that
must be processed by the POTW.
While less costly than tunnel storage up to certain
storage requirements, surface storage often is difficult
to site. Combination surface storage/sedimentation
designs may permit more efficient utilization of available
space and optimize the level of control that can be
applied at a particular location. The overall performance
level that can be achieved depends on the proportion
of the CSOs that are captured and returned to the
POTW versus treated by sedimentation and discharged
at the overflow location.
The sedimentation unit should produce reductions in
pollutants such as suspended solids and BOD
equivalent to primary treatment. This control should
completely remove floatables and effectively disinfect
for bacteria.
Deep Tunnel Storage
Deep tunnels are the most costly storage capacity, but
they are less constrained by considerations of surface
space availability and construction impacts to the
community. Large storage volumes are more readily
effected than in the case of surface storage, and tunnel
designs also can provide conveyance of captured
volumes to the eventual treatment/discharge location.
Geotechnical conditions influence the feasibility of
applying this technology at any site, and consolidation
and other collection system modifications may require
significant effort to deliver CSOs to the tunnel. This will
be site specific and depend on the tunnel alignment and
its location with respect to the existing overflows. As
with any storage technology, the ability of the POTW to
process the captured flows must be considered in the
technology selection process. This may have particular
importance in the case of tunnel storage, because of
the ability to capture very large percentages of the wet
weather flows.
In low-lying areas with little topographical relief, using
tunnels to control CSO may serve the additional
purpose of reducing flooding problems.
Coarse Screening
The control provided by this technology is limited to
removal of large objects, and its principal use is to
32
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protect downstream control units. It is addressed in
Chapter 4 because it is commonly included with a
variety of control technologies as part of the overall
control system.
Other than the removal of debris and floatable materials,
coarse screens accomplish little in the way of removal
of other pollutants. As such, they would not normally be
considered as a stand-alone CSO control. There may
be exceptions, as in a case of relatively small CSOs
discharging to segments of water bodies where
aesthetic appearance and floatables are the only
significant water quality impact.
Proper maintenance and frequent cleaning of these
screens is important both to ensure effective operation
and to avoid restrictions to flow, which might induce
upstream flooding.
Swirl/Vortex Technologies
These controls are attractive primarily because of their
relatively low cost and space requirements in relation to
the degree of treatment they can provide. They are
applied at individual overflow points to provide some
level of treatment before direct discharge.
They provide effective removal of grit and heavy solids,
and are quite effective for control of floatables. For
settleable solids and BOD, the removal capabilities
appear to range from nominal to moderately low, and
performance is influenced to a major degree by the
site-specific paniculate settling characteristics of the
CSOs to be treated. Pollutant removal is provided by a
combination of solids separation from the flows that
discharge and of the underflow that is returned to the
combined sewer system. The latter often accounts for
a significant fraction of the overall pollutant removals.
As a result, appropriate bench testing to define settling
velocities usually is necessary to determine the basic
applicability of this technology at the site and the
appropriate design parameters.
The very short residence time resulting from the typical
high rate design limits the ability to provide disinfection
contact time, and for locations where bacterial control
is required, additional storage/contact chambers are
usually incorporated in the system design.
Disinfection
In all cases where CSO control must include reductions
in bacterial levels to protect a designated use,
disinfection technology must be applied. It essentially
will never be considered independent of any other
control technology, because bacterial contamination is
not the exclusive water quality issue for CSO control
and because pretreatment to reduce solids that
otherwise shield the bacteria normally is necessary for
effective disinfection.
Adequate contact time must be provided, usually in
tanks associated with the control technology applied at
the overflow location. However, in cases where the
outfall lines are sufficiently long to provide necessary
contact, they may be used. For control technologies
involving storage or sedimentation treatment, the
storage volumes usually are sufficient to provide the
required disinfection contact time. For CSO control
technologies such as screens or swirl/vortex units,
which have very short residence times, separate
provisions must be made to provide contact for
disinfection.
Optimizing Storage-Treatment CSO Controls
For CSO control systems that utilize a combination of
storage and treatment methods, a cost-optimization
analysis should be performed to determine the most
cost-effective combination of design sizes. For a given
performance level, the design capacity, and hence cost
of a treatment unit such as a sedimentation device, can
be reduced by using a storage unit that absorbs the
higher flows during parts of the storm, and delivers flow
to the treatment unit at both a lower, averaged rate and
at a more uniform influent rate.
The upper plot in Figure 3-10 illustrates the concept by
indicating the array of combinations of storage capacity
and design capacity for a treatment unit. Large amounts
of equalization storage permit the installation of small
treatment units. Conversely, when little or no storage is
provided to capture the short duration peaks of the
variable CSO flows, a very large treatment capacity
must be provided to avoid either degraded performance
due to excessive applied rates, or the need to bypass
the treatment unit. When this relationship is developed
for a particular set of site conditions (using a SWMM
model analysis or other suitable procedure), and the
relationship between increasing size and increasing
cost is applied for both storage and treatment units, a
summary relationship such as that illustrated by the
lower plot in Figure 3-9 can be developed. This
indicates the relationship between the reduction in the
costs for a treatment unit as smaller design capacities
become applicable and the corresponding use of larger
and more costly storage capacities. Total cost of the
combination system, which is the sum of the individual
unit costs, passes through a minimum, which
represents the most cost-effective design combination.
The concept applies directly to the design of a control
system to be installed at a particular CSO overflow
point, and relates most closely to the off-line near-
surface storage/sedimentation technology discussed in
Chapter 4. It can be applied to the design of
disinfection facilities by associating chemical dosage
and/or type with the "treatment" element, and the
33
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Storage
Capacity
Provided
All combinations provide (he
same operating capacity
(or the treatment unit and
hence the same performance
level
Required Treatment Unit Size
Cost
Optimum
Combinations
Storage
Cost
Treatment
Cost
Physical Size of Storage Device and Treatment Unit
Figure 3-10. Cost optimization of storage-treatment.
contact time required with the "storage" element of the
optimization analysis.
The optimization concept generally is not applicable in
the case of swirl/vortex technologies, because providing
storage upstream of the inlet to these devices usually
is considered inappropriate.
Theoretically, the analysis also could be extended to the
case where the control approach is based on storage
at overflow points, but where treatment at the POTW
follows pump-back or bleed-back of captured overflows.
Direct application of a cost optimization analysis in this
case is complicated by several factors. For example, in
most cases the capacity of the treatment units is already
established by the existing facility design and both the
existing dry weather flows in the system and the
possibility that captured overflows from a number of
overflow points may be involved. In addition, federal or
state regulations may impose requirements for
performance of a storage unit (such as number of
overflows permitted or minimum percent capture) that
may conflict with the strict cost optimization of an overall
system that relies on maximizing the available treatment
capacity of the POTW.
However, in cases where the collection system has the
hydraulic capacity to deliver wet weather flows at rates
greater than a POTW can process, and the design of
additional parallel primary treatment facilities for these
flows is being considered, using an optimization
analysis would increase the cost effectiveness of the
design.
References
When an NTIS number is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
703-487-4650
BGMA and CMT. 1987. Bainbridge, Gee, Milanski &
Associates, Inc., and Crawford, Murphy & Tilly, Inc.
Sanitary district of Decatur combined sewer overflow
operational study.
Dufresne-Henry, Inc. 1992. Report on Rutland CSO:
basis of design. June.
Fair, G.M., and J.C. Geyer. 1954. Water supply and
waste water disposal. John Wiley and Sons.
Henderson, R.J.A., W. McBain, and F. Pettigrew. 1981.
The performance of an off-sewer storm-sewage tank.
Water Poll. Control.
Lygren, Elvind, and T. Damhaug. 1986. The swirl
concentrator as an urban runoff treatment device. In:
Torno, H.C., J. Marsalek, and M. Desbordes, eds.,
NATO ASI series vol. G10, urban runoff pollution.
Mahood, T.W., and G. Zukovs. 1993. A comprehensive
stream study. In: James, William, ed., New
techniques for modeling the management of
stormwater quality impacts. Boca Raton, FL: Lewis
Publishers.
Moffa, RE. 1990. Control treatment of contaminated
sewer overflows. New York, NY: Van Nostrand Reinhold
Environmental Engineering Series.
MWRA. 1993. Massachusetts Water Resources Authority.
Interim report on master planning and CSO facilities
planning. Report prepared by Metcalf & Eddy, Inc.
NYC DEP. 1993. New York City Department of
Environmental Protection. Inner harbor CSO facility
planning project (in progress). Hazen and Sawyer,
and HydroQual, Inc.
U.S. EPA. 1974. U.S. Environmental Protection Agency.
Urban stormwater management and technology: an
assessment. EPA/670/2-74/040.
U.S. EPA. 1977a. U.S. Environmental Protection Agency.
Urban stormwater management and technology:
update and users' guide. EPA/600/ 8-77/014.
September.
34
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U.S. EPA. 1977b. U.S. Environmental Protection
Agency. Cottage Farm combined sewer detention
and chlorination station, Cambridge, Massachusetts.
EPA/600/2-77/046.
U.S. EPA. 1986. U.S. Environmental Protection Agency.
Methodology for analysis of detention basins for
control of urban runoff quality. EPA/440/5-87/001
(NTIS PB 87-116 562). Prepared for Office of
Water, Nonpoint Source Division. September.
U.S. EPA. 1988. U.S. Environmental Protection Agency.
Stormwater management model users' manual
version 4. EPA/600/3-88/001. August.
U.S. EPA. 1989. U.S. Environmental Protection Agency.
Analysis of storm event characteristics for selected
rainfall gages throughout the United States. November.
Washington, DC, Water and Sewer Utility Administration.
1992. CSO abatement program, segment 1: performance
evaluation. April.
WEF. 1992. Water Environment Federation. Design of
municipal wastewater treatment plants volumes I &
II. WEF manual of practice no. 8 and ASCE manual
and report on engineering practice no. 76. 2nd
edition. Alexandria, VA: WEF, and New York, NY: ASCE.
35
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Chapter 4
CSO Control Technologies
In-System Controls/In-Line Storage
Introduction
One of the more readily implementable and cost-
effective approaches to achieving immediate reductions
in CSO volumes is to utilize the available storage and
conveyance capacity of existing collection systems and
the available treatment capacity at the POTW.
Optimizing the use of in-line storage and maximizing
flows to the POTW reduces overflow volumes by
allowing a larger fraction of the total flow from a storm
event to be conveyed to the POTW for treatment. This
control approach is only feasible if sufficient capacity is
available in the collection system and at the treatment
plant.
A number of "in-system" technologies or strategies can
contribute to maximizing in-line storage, maximizing flows
to the POTW, and reducing overflow volumes, including:
• Collection system inspection and maintenance.
• Tidegate maintenance and repair.
• Reduction of surface inflow.
• Adjustment of regulator settings.
• Enlargement of undersized pipes to eliminate flow
restrictions.
• Removal of obstructions to flow, such as sediments.
• Polymer injection to reduce pipe friction.
• In-system flow diversions through existing system
interconnections.
• Adjustment and/or upgrade of pumping station
operations.
• Partial separation of storm drain connections from
combined sewers.
• Infiltration removal.
Prior to implementing these practices, communities
should undertake a program of detailed data collection,
flow monitoring, and modeling to fully characterize the
features and behavior of their combined sewer systems.
The results of these system characterizations can be
used to evaluate the performance of each regulator, and
to identify the potential for utilizing storage in conduits
upstream of the regulator, eliminating flow bottlenecks,
and improving pump station operations. The goal of all
these measures is to optimize the storage and
conveyance capacity of the combined system.
Many types of in-system controls can be implemented
with relatively minimal engineering and cost, although
detailed hydraulic analyses are required. Evaluating
and optimizing system components may serve to
reduce the overall scope of a community's CSO
problem, thereby reducing the size and/or number of
more capital-intensive CSO control facilities required to
meet overall CSO control goals. In-system controls
generally will be most effective where upstream
drainage systems consist of large-diameter pipe laid on
shallow gradients.
Disadvantages to increasing in-line storage through
implementation of in-system controls may include an
increased risk of basement or street flooding, increased
opportunity for sediment deposition, and higher costs
associated with increases in maintenance efforts to
ensure that regulators, tidegates, and other features are
functioning properly and are in good repair.
When evaluating potential in-system controls, criteria
can be set for allowable changes in predicted flow
velocities and peak hydraulic grade line elevation. Using
output from a detailed system model such as SWMM,
time-varying flow velocities in conduits subject to
backwater could be reviewed both for existing
conditions and for conditions once the proposed
in-system controls are implemented. The criteria could
be to ensure that minimum carrying velocities are
restored once the storm ends and the backwater
subsides. Similarly, changes to the peak hydraulic
grade line in conduits upstream of the proposed controls
could be limited to a set maximum above existing
conditions, or to a minimum depth below grade. Setting
criteria for predicted increases to the hydraulic grade
line requires judgment, and the criteria could vary within
a given drainage system, based on knowledge of
conduit depth, previous reports of flooding, tidal impacts,
and other factors.
37
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This manual focuses primarily on regulator
modifications and control, and the use of real-time
control systems, as a means to optimize in-line storage
and conveyance of flows to the POTW. Sewer
separation and infiltration removal are two more
capital-intensive collection system control strategies.
These strategies are discussed only briefly in Chapter
2, but have been well described by others (U.S. EPA,
1974a, 1975a; Metcalf & Eddy, Inc., 1981). Similarly,
actions to increase in-system storage which are related
to maintenance, such as tidegate inspections and sewer
flushing, and actions associated with best management
practices, are not addressed in this manual. Key
references provided at the end of this chapter address
these items (WPCF, 1989; Metcalf & Eddy, Inc., 1991 a;
Field, 1990).
Regulators
Regulators control the amount of flow that enters an
interceptor from an upstream combined system, and
provide an overflow relief point (the CSO) for flows in
excess of the interceptor capacity. Regulators fall into
two broad categories: static and mechanical. Static
regulators feature no moving parts and, once set, are
usually not readily adjustable. Examples of static
regulators include side weirs, transverse weirs, restricted
outlets, swirl concentrators (flow regulators/solids
concentrators), and vortex valves. Mechanical regulators
are more readily adjustable and may respond to
variations in local flow conditions, or be controlled
through a remote telemetry system. Examples of
mechanical regulators include inflatable dams, tilting
plate regulators, reverse-tainter gates, float-controlled
gates, and motor-operated or hydraulic gates.
Many of the older float-operated mechanical regulators
are erratic in operation and require constant
maintenance. In Saginaw, Michigan, many existing
float-operated regulators were replaced by vortex
valves, due to the unreliability and excessive
maintenance associated with the mechanical regulators
(U.S. EPA, 1985b). In Boston, Massachusetts, many
float-operated regulators have been replaced over the
years with static regulators.
Following are descriptions of types of regulators and
gates which have been installed in more recent CSO
control projects, or have been used to replace older,
less reliable types. The reader is referred to other
sources (WPCF, 1989; Metcalf & Eddy, Inc., 1991a; and
Urbonas and Stahre, 1993) for more detailed
descriptions of other regulator types.
Vortex Valves
Vortex valves are static regulators that allow dry
weather flow to pass without restriction, but control
higher flows by a vortex throttling action (Figure 4-1).
Vortex valves have been used to divert flows to CSO
treatment facilities, control flow out of storage facilities,
and replace failed mechanical regulators. The
advantages of vortex valves over standard orifices
include (Urbonas and Stahre, 1993):
• The discharge opening on the vortex valve is larger
than the opening on a standard orifice sized for the
same discharge rate, thereby reducing the risk of
blockage.
• The discharge from the vortex valve is less sensitive
to variations in upstream head than a standard
orifice.
Vortex valves are sized based on design flow and head.
Typically, a vortex valve manufacturer will provide a
table listing the ranges of flow and head appropriate for
the various models and sizes of vortex valves available.
The design flow corresponds to the dry weather flow
capacity of the downstream conveyance capacity. The
Controlled Flow
Low Flow
Free Discharge
(DWF)
Figure 4-1. Example of a vortex valve (Metcalf & Eddy, Inc.,
1991a).
38
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peak head is usually dictated by the maximum upstream
hydraulic grade line allowable without undue risk of
flooding. A high outlet relief should be provided to
ensure that upstream flooding does not occur.
Vortex valves used as CSO regulators are commonly
mounted in reinforced concrete structures. Such
structures should be channelized with benching to
eliminate "dead" flow areas. The valves are typically
constructed of stainless steel and have been reported
to require little maintenance (H.I.L. Technology, Inc.,
1988, 1991).
The European wirbeldrossel and wirbelvalve (Figure
4-2) are similar to the vortex valve, with a tangential inlet
and vertical bottom outlet. The flow chamber for the
wirbeldrossel is horizontal and cylindrical, while the flow
chamber for the wirbelvalve is slanted with a conical
bottom. These devices feature an air supply pipe to
prevent cavitation which might otherwise develop in the
rotational flow. As with the vortex valve, the wirbeldrossel
and wirbelvalve provide greater restriction of flow and
less sensitivity to upstream head than similarly sized
standard orifices. In Europe, these devices have been
used primarily to control the release of flow from storage
facilities (Urbonas and Stahre, 1993).
Inflatable Dams
An inflatable dam is a reinforced rubberized fabric
device which, when fully inflated, forms a broad-crested
transverse weir. When deflated, the dam collapses to
take the form of the conduit in which it is installed. A
dam can be inflated with air, water, or a combination of
both. Water provides the best control of the weir crest,
but air control is usually associated with lower costs for
equipment, particularly if rapid inflation-deflation cycles
are required. Air inflation is also required where the dam
may be exposed to freezing temperatures (APWA,
1970).
Inflatable dams are typically positioned to restrict flow
in an outfall conduit or combined sewer trunk. The
dams, which normally remain fully inflated, can act as
regulators by directing flow into an interceptor, and
preventing the diversion of flow to an outfall until the
depth of flow exceeds the crest of the dam (Figure 4-3).
Inflatable dams are controlled by local or remote flow or
level sensing devices, which regulate the height of the
dam to optimize in-line storage and prevent upstream
flooding. The dam height is controlled by controlling the
air or water pressure in the dam.
Since inflatable dams are typically constructed of rubber
or strong fabric, they are subject to puncture by sharp
objects. These devices generally require relatively little
maintenance, although the associated air supply should
receive periodic inspection to ensure reliable operation
(WPCF, 1989).
Motor- or Hydraulically Operated Sluice Gates
Similar to inflatable dams, motor- or hydraulically
operated gates typically operate in response to local or
remote flow or level sensing devices. Normally closed
gates can be located on overflow pipes to prevent
overflows except under conditions when upstream
flooding is imminent. Normally open gates can be
positioned to throttle flows to the interceptor to prevent
interceptor surcharging. Controls can be configured to
fully open or close gates, or to modulate gate position.
The level of control and general reliability of
motor-operated gates make them well suited for use in
conjunction with real-time control systems (Figure 4-4).
Elastomeric Tide Gates
While not actually regulators, tide gates are intended to
prevent the receiving water from flowing back through
the outfall and regulator and into the conveyance
system. Inflow from leaking tide gates takes up
hydraulic capacity in the downstream interceptors and
increases the hydraulic load on downstream treatment
facilities. Elastomeric tide gates provide an alternative
to the more traditional flap-gate style tide gates, which
are prevalent in many CSO communities. Tide gates
have historically required constant inspection and
maintenance to ensure that the flaps are seated correctly,
and that no objects or debris prevent the gate from
closing. Warpage, corrosion, and a tendency to become
stuck in one position also characterize flap-gate style
tide gates. Elastomeric tide gates are designed to avoid
the maintenance problems associated with the flap
gates (Figure 4-5). In particular, the elastomeric gates
are designed to close tightly around objects that might
otherwise prevent a flap gate from closing (Field, 1982).
Regulator Controls
Static Regulators
Static regulators have no moving parts and thus offer
no opportunity for additional control once the weir
elevations or orifice dimensions are set. Adjustable
weirs may allow some degree of manipulation between
storm events. Modifications to static regulators,
however, generally can be achieved at relatively low
cost. For example, if a collection system hydraulic
model indicates that upstream storage could be
optimized by raising an existing masonry weir in the
overflow pipe at a regulator, the immediate benefits of
such actions may justify the cost of the relatively
minor construction work required to adjust the weir.
Modification of restricted outlets requires more extensive
demolition and excavation work, and would have to
achieve a greater reduction in overflow volume to match
the cost effectiveness of raising weirs.
39
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Aeration Pipe
Inlet
Wirbeldrossel
Wirbeldrossel
Installation
Wirbelvalve
Figure 4-2. Example of a Wirbeldrossel and wlrbelvalve (Urbonas and Stahre, 1993).
40
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Underground
Equipment
Vault
River
To Interceptor
Figure 4-3. Example of an inflatable dam (WPCF, 1989).
Local Control
Local regulator control is most appropriate where a
single regulator is associated with an outfall, or where
the behavior of a regulator would not influence or be
influenced by the behavior of another regulator in the
system. Many of the older style mechanical regulators,
such as reverse tainter gates, respond to local water
level in the sewer through a float mechanism. These
types of regulators are unreliable in many installations,
require constant maintenance, and are slow to react to
rapidly changing flow conditions (U.S. EPA, 1985b).
More advanced local control systems feature electronic
flow or water level monitoring devices, which control
motor-operated gates or inflatable dams. These
systems appear to be more reliable than the old
float-controlled mechanical gates.
System-Wide Real-Time Control
Real-time control (RTC) systems can provide integrated
control of regulators, outfall gates, and pump station
Trunk Water
Level Controls
Outfall Gate
Depth of
Water in Interceptor
Controls Regulator
Gate
May Prevent Gate
Opening
Receiving
Water
Figure 4-4. Example of a motor-operated gate regulator (WPCF, 1989).
41
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Figure 4-5. Example of an elastomeric tide gate (WPCF, 1989).
operations based on anticipated flows from individual
rainfall events. Feedback control adjustments are based
on actual flow conditions within the system. Computer
models associated with the RTC system allow
evaluation of expected system response to control
commands before the commands are executed. As with
any plan for improving in-line storage, to take the
greatest advantage of RTC, a collection system must
have relatively flat upstream slopes and sufficient
upstream storage and downstream interceptor capacity.
An RTC system typically includes the following control
features (James and Stirrup, 1986):
• Sensors to detect flow, water level, rainfall, and/or
pollutant concentration.
• Circuitry and software to transform the signals from
the sensors into numerical quantities.
• Circuitry and software to drive the control
mechanisms (usually gates).
• Rainfall and/or runoff forecasting software running in
real time.
• A computer acting as both data logger and controller.
• For an integrated RTC system, telemetry equipment
for the communication of data among the various
regulators.
A control strategy based on RTC must identify the
control system constraints and evaluate alternatives for
developing the optimum control strategy within those
constraints. Examples of constraints which could define
the limits of an RTC system include (Trotta et al., 1977):
• Capacities of interceptor and trunk sewers, storage
facilities, and POTW.
• Rainfall runoff forecast models.
• Data acquisition system.
• Computer hardware and software.
• Control timestep.
• Human error and equipment malfunction.
Evaluation of the costs and impacts of addressing these
constraints may dictate to what extent an RTC system
will be appropriate for a given community. For example,
if little excess capacity is available in the collection
system, the benefits of RTC may not justify the costs.
Similarly, a community must have sufficient staff
resources to inspect and maintain the remote sensing
equipment to minimize the risk of equipment malfunction.
The configuration of an RTC system depends on a
community's overall CSO control goal. For example,
control of average annual CSO volume may require a
different control strategy than control of mean annual
CSO frequency. For this reason, evaluation of the
feasibility of an RTC system should be conducted as
part of a long-term CSO control plan.
One of the older RTC systems in the United States has
been operated by the Municipality of Metropolitan
Seattle since 1973. Regulators in the Seattle system
feature vertical in-line sluice gates to throttle flow into
the interceptors and store flow in the upstream
combined sewers. Overflow weirs upstream of the gates
prevent upstream flooding during extreme events. The
original computer-augmented treatment and disposal
(CATAD) control system monitored flows and water
surface levels in the combined system, and provided
integrated, remote control of pump station and regulator
gate operation.
A 1990 study evaluated the most cost-effective means
for upgrading the Seattle RTC system and providing
additional storage of CSO flows. The study found that
improvements to the RTC system would be more cost
effective for providing additional in-line storage than
construction of off-line storage facilities. Improvements
to the RTC system included development of a
hydrologic model of the CSO system, instrumentation
upgrades, database development, integration of the
42
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hydrologic model and database into a simulation
program, and control strategy design.
The hydrologic model can estimate flows into the
system for up to 6 hours into the future, based on
predicted rainfall and current conditions within the
system. Information on current conditions is continually
input through a supervisory control and data acquisition
(SCADA) system. Initial simulation runs assume a
relatively simple, local control strategy. Following the
initial simulation, an optimal control strategy is
computed using an optimizing algorithm. A flow-routing
model then checks the optimized strategy for feasibility.
Infeasible strategies are rejected, while optimal
strategies which are modeled to be feasible are sent to
the SCADA system for execution (Vitasovic et al., 1990).
With the continuing improvements to computer and
instrumentation technologies, and the cost-effectiveness
of in-system control vs. new facilities, RTC systems are
gaining in popularity for both CSO and stormwater control.
Off-Line Near-Surface
Storage/Sedimentation
Off-line, near-surface storage/sedimentation facilities
consist of tanks that store and/or treat combined sewer
flows diverted from combined trunk sewers and
interceptors. These facilities provide storage up to the
volume of the tanks, as well as sedimentation treatment
for flows that pass through the facilities in excess of the
tank volume. Coarse screening, floatables control, and
disinfection are commonly provided. The phrase
"near-surface" means that these facilities are constructed
at depths that allow the use of traditional open-cut
excavation techniques, as opposed to the deep tunnel
facilities described in the section of this document on
deep tunnel storage.
Process Theory
The sizing of sedimentation structures at POTWs
historically has been based on discrete particle settling
theory. Type 1 discrete particle settling theory relates
the terminal settling velocity of a particle to an overflow
rate and ratio of depth to detention time, which can then
be used as a basis for sizing a sedimentation basin.
Detailed discussions of discrete particle settling theory
are provided in other texts (Metcalf & Eddy, Inc.,1991 a;
WEF, 1992). For CSO storage/sedimentation facilities,
the basic discrete particle settling model must be
expanded to account for the non-steady-state conditions
that occur as a CSO storage/sedimentation tank fills,
overflows, and is dewatered.
A number of mathematical models have been developed
to predict sedimentation behavior in settling tanks. One
such model, developed for preliminary evaluation of
storage/sedimentation facilities, identifies four distinct
operating phases that may occur in a CSO storage/
sedimentation tank (Lessard and Beck, 1991):
• Fill, when flow is entering the tank but the tank is not
overflowing.
• Quiescent settling, when the tank is partly or
completely filled, with no flow entering or leaving the
tank.
• Dynamic settling, when the tank is full and
overflowing.
• Draw, when the tank is being dewatered to the
sanitary system.
This model can be used to estimate the change in
selected pollutant concentrations and sludge mass over
time, given an array of input parameters such as tank
size, particle settling velocity, and percent settleable
fraction. The model provides an approach for estimating
both the performance of a storage/sedimentation tank,
and the subsequent hydraulic and pollutant load on
the POTW resulting from dewatering of the storage/
sedimentation tank. The following aspects of CSO
storage/sedimentation tank behavior, which were
considered in developing the model, summarize
storage/sedimentation tank dynamics (Lessard and
Beck, 1991).
Fill
During the initial stages of tank filling, turbulence
created by the influent flow prevents the contents of the
tank from settling. As the tank fills, there may be some
fraction of the total tank volume above which the influent
turbulence no longer inhibits sedimentation. For larger
tanks, the fraction of total volume required before
sedimentation occurs is relatively small; for small tanks,
turbulence may influence a greater percent of the total
tank volume.
Quiescent Settling
The percentage of solids that settle during the quiescent
settling phase depends upon the settling velocity profile
and the duration of the settling period. The extent of
quiescent settling that occurs during the lag period
between the end of the storm flows to the facility and
the start of the tank dewatering process may impact the
solids loading rates on the POTW as the tanks are
dewatered.
Dynamic Settling
Under dynamic settling conditions, the behavior of the
tank is essentially the same as a typical primary settling
tank at a POTW. Key parameters for estimating
removals during this phase include an average particle
settling velocity, overflow rate, detention time, and the
43
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fraction of a particular pollutant that could be considered
settleable.
Draw
Two issues are considered when evaluating the draw
phase and the effect of dewatering a CSO storage/
sedimentation facility on the POTW:
• Pollutant concentration
• Timing of dewatering
The pollutant concentration in the dewatering flow
depends on the sludge handling process at the CSO
facility. Some tanks are aerated or provided with
mechanical means for resuspending solids prior to
pumpback. Other facilities may provide high-rate
dewatering of the clarified volume, with separate
pumping of the sludge. Some facilities are dewatered
by gravity, with or without separate disposal of settled
solids. In general, it is assumed that the sludge is
resuspended to a given depth, which corresponds to a
given fraction of the total tank volume. The load on the
POTW can then be estimated from the two volumes, the
concentrations within the two volumes, and the
dewatering rate(s).
Ideally, the dewatering phase should be initiated as
soon as capacity is available at the POTW and in the
conveyance system between the CSO facility and the
POTW. This ensures that the greatest tank capacity is
available for the next storm. Theoretically, a dewatering
system could be activated automatically on a
telemetered signal from the POTW, once flow at the
POTW drops below a specified value. In practice,
however, many CSO facilities are designed with
manually activated dewatering systems. One of the
benefits of manual activation is that POTW operators
can evaluate the impacts of sustained peak storm flows
on POTW performance, before potentially prolonging
the peak flows by dewatering the CSO facility. The
solids loads from a storage/sedimentation facility may
be particularly difficult to handle for a system which
already has been stressed. In addition, some POTWs
that serve combined systems may implement special
operating procedures during wet weather to mitigate the
impact of peak storm flows, particularly on biological
treatment systems. Delaying the dewatering of a CSO
facility may impact the level of control achieved with a
storm that follows immediately. However, if immediate
dewatering of a CSO facility results in the upset or
washout of a secondary treatment system, the overall
impact on receiving waters may be greater.
Process Design
Sizing criteria for a storage/sedimentation tank vary,
depending upon the intended goal of the CSO control
facility. As discussed in Chapter 3, typical CSO control
goals may include:
• Providing a specified minimum treatment level, such
as the equivalent of primary treatment for flows up to
a specified design condition.
• Providing a level of treatment required to meet
receiving water quality standards for storm flows up
to a given recurrence interval.
• Providing full capture of the first flush (highest mass
loadings during the storm event), then a reduced
level of treatment for subsequent flows.
• Reducing the number of annual overflow events
and/or total annual overflow volume to a specified
level.
Selection of the control strategy may depend on local,
state, or federal regulations; a community's long-term
CSO control plan; or requirements of a permitting
authority. All strategies require an estimate of the flows
expected under the given design condition. Computer
models, such as SWMM which is described in
Chapter 3, can be used to simulate the behavior of a
combined sewer system. System behavior can be
simulated for a specific storm of previously defined
characteristics, and on a continuous basis, reflecting the
precipitation history of a selected time period
(commonly one year). The continuous simulation
models the impact of succeeding storms on system
performance, which may impact facility sizing and
dewatering rates. Sizing based on modeled flows is,
therefore, an iterative process.
General concepts related to each of the above CSO
control goals are discussed in Chapter 3. Additional
process design considerations that apply to storage/
sedimentation facilities are described below.
Sizing To Provide a Specified Minimum
Treatment Level
The minimum treatment level most commonly identified
for CSO control facilities is primary treatment, although
facilities also have been sized to provide a specified
minimum detention time for disinfection. Primary settling
tanks at POTWs are sized based on overflow rates and
detention times, which are in turn derived from particle
settling velocities. Tables 4-1 and 4-2 present examples
of design overflow rates and detention times for primary
settling tanks (WEF, 1992). Since combined flows
generally have a higher fraction of heavier solids than
separate sanitary flows, the values presented in
Tables 4-1 and 4-2 may be conservative for CSO
applications. In addition, a study of the relationship
between solids removal performance and overflow rates
suggests that the performance expected from a given
design overflow rate may be affected by other factors
44
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Table 4-1. Typical Overflow Rates for Primary Settling Tanks
(WEF, 1992)
Source
Condition
Overflow Rate
nVWVd gpdrtt2
Metcalf & Eddy, Primary treatment
Inc., 1991 a; U.S. followed by
EPA, 1975b secondary:
WEF, 1992
WEF, 1992
Great
Lakes-Upper
Mississippi River
Board, 1978
Average flow
Peak flow
All units in service:
32-48 800-1,200
80-120 2,000-3,000
Maximum day flow 49 1,200
Peak flow 81 2,000
One unit out of
service:
Peak flow
Larger area of:
Average flow
Peak hour flow
163 4,000
41
61
1,000
1,500
Table 4-2. Typical Detention Times for Primary Settling Tanks
(WEF, 1992)
Source
Condition
Detention Time
Metcalf & Eddy,
Inc., 1991 a
WEF, 1992
Fair et al., 1968
U.S. Army, 1978
Primary treatment
followed by
secondary
Primary treatment
followed by
activated sludge
1.5 to 2.5 hours, 2
hours typical
1 to 2 hours based
on peak flows
Minimum 2 hours in
3m (10 ft) side water
depth
1.5 hours
All other conditions 2.5 hours
such as design details, loading variations, recycle flows,
temperature, and proportions of inert-to-volatile and
soluble-to-insoluble material in the flow (WEF, 1992).
Sizing criteria should consider both overflow rates and
detention times. Determining the actual settling velocity
distribution of the solids in the CSO to be treated, as
described in Chapter 3, will provide a more accurate
assessment of the appropriate design overflow rate.
Since influent hydrographs for CSO control facilities
tend to exhibit more sharply defined peaks, which are
not typical of POTW influent hydrographs, and periods
of no flow, the concept of the "average" flow to a CSO
facility does not have as much physical significance as
at POTWs. Thus, if overflow rates or detention times are
used as a basis for design of a CSO control facility,
considering peak flow conditions may be more
appropriate than attempting to define an "average" flow.
If the shape of the influent hydrograph is known, then
the total volume of flow that has entered the facility at
the timestep when the peak occurs can be determined
and compared against the total tank volume. If the peak
flow occurs before the tank is full, then the actual
maximum overflow rate would occur on the falling leg
of the influent hydrograph.
The actual maximum overflow rate, therefore, would be
less than the calculated overflow rate associated with
the actual peak influent flow. This condition would
provide a degree of conservatism in the sizing of
facilities based on peak overflow rates.
Sizing To Meet Water Quality Standards
Water quality impacts influence the level of treatment
that permitting agencies require CSO control facilities to
achieve. Wasteload allocation studies (which include
water quality monitoring and modeling) can be used to
determine the effluent characteristics and level of
treatment needed to achieve a given water quality goal.
Once the required level of treatment is established, the
design can then evaluate tank sizes and configurations
to meet the required level of control.
Sizing To Capture First Flush
A number of CSO facilities have been designed to
capture the more concentrated combined flows that
may occur during the initial stages of a storm event. The
key to designing such a facility is to define the limits of
the first flush, which will in turn define the volume of
capture required. Chapter 3 defines and discusses the
use of the first flush as a design criterion.
Sizing To Reduce the Number of Overflow Events
Use of this criterion requires continuous simulation to
estimate the flow associated with storms of various
recurrence intervals. As discussed in Chapter 3, the
definition of an "overflow event" also must be clearly
established. An overflow event may be defined as a
treated overflow, an untreated overflow, or an event in
which flow receives less than a defined level of
treatment. The established definition may determine
whether a storage-only or flow-through storage/
sedimentation facility is appropriate for the application.
A related consideration is whether flows in excess of the
design storm are to bypass the facility or flow through
the facility and potentially receive a lower level of
treatment than provided under design conditions.
45
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Process Flow
The arrangement of unit processes at a storage/
sedimentation facility varies depending on the goal of
the CSO control strategy (i.e., primary treatment vs. first
flush capture) and hydraulic considerations. A typical
arrangement includes a regulator, bar screens, settling
tank(s), and an outfall. Disinfection may be required and
can be provided upstream and/or downstream of the
settling tanks. Figure 4-6 presents a schematic for a
typical layout of a storage/sedimentation facility.
Regulators
Typical regulator/influent arrangements include:
• Overflow from remote regulator conveyed to CSO
facility by influent conduit. Depending on the type of
regulator used, the bedload could tend to remain in
the interceptor. The regulator should be designed to
maximize the in-system storage capacity of the
upstream pipe network.
• Overflow from sanitary wetwell. A common arrangement
is to combine a CSO facility with a standard sanitary
sewage pumping station. As flow exceeds the
capacity of the sewage pumps, flow in the pump
station influent channel overtops a weir, and passes
to the CSO facility influent channel. This arrangement
typically occurs downstream of coarse screening
apparatus.
The regulator or influent piping arrangement should
provide some means for a relief overflow to protect the
facility from flooding. In Newport, Rhode Island (Figure
4-6), a high flow relief weir is located in a manhole
adjacent to the side weir regulator controlling flow into the
facility influent conduit. The facility includes provisions
for disinfecting the high level overflow. In addition, a
positive means for isolating the facility should be
provided. The Newport facility and the MWRA Prison
Point facility both feature a modulating influent gate to
throttle flow into the facility when the water surface in
the facility reaches a high level.
Bar Screens
Most storage/sedimentation facilities provide some
means for coarse screening of the influent, such as
catenary-type mechanically cleaned screens. Isolation
gates for the barscreens should be provided, along with
a bypass or a redundant channel. Coarse screening is
described in more detail later in this chapter.
Influent vs. Effluent Pumping
Hydraulic conditions often require influent or effluent
pumping to or from a CSO facility. Operationally, effluent
pumping is preferable, since the pumps run only during
a storm large enough to overflow the tanks. The
disadvantage of effluent pumping is that the elevation
of the entire facility must be set by the hydraulic grade
line of the influent conduit. If the influent conduits are
well below grade, then relying on gravity flow into the
facility could require significant excavation and
construction. Influent pumping allows the elevation of
the facility to be set independently from the influent
conduit elevations. Ideally, the facility elevation is set
such that the facility effluent, and possibly the facility
dewatering piping, flow by gravity. If influent pumping is
selected, wetwell and pump sizing must consider the
range of flows expected.
Disinfection
If required, disinfection is typically provided by liquid
sodium hypochlorite, dosed by metering pumps, and
paced by influent flow. Some facilities include additional
discharge control based on chlorine residual. Dosing
typically is applied at the tank influent channel to
maximize contact time. Some facilities (Newport,
Washington Street; MWRA Prison Point) include a
second dosing location after sedimentation to augment
the original dose. One drawback to dosing upstream of
the settling tanks is that the flow at that location contains
a higher concentration of solids than downstream of the
tanks. Thus, while contact time is gained by dosing
upstream of the tanks, actual "contact" between the
bacteria and the disinfectant may not be as effective.
Auxiliary uses of disinfection may include periodic
disinfection of sludge piping and the spray wash
system. Dechlorination also may be required due to
concerns over the potential toxic impacts of chlorine
residuals on the receiving waters. Disinfection and
dechlorination are discussed in more detail later in this
chapter.
Solids Handling-Screenings
A number of methods have been used to handle
screenings from mechanically cleaned bar screens.
Some methods require separate disposal of the
screenings, while others return the screenings to the
interceptor. Screenings handling options are discussed
in more detail later in this chapter.
Solids Handling-Sludge and Grit
Due to the intermittent operation of CSO facilities,
mechanical sludge collection equipment typically is not
provided. Rather, tank floors are sloped to a center or
side trough and flushing systems help direct solids to
the trough. The trough is sloped back to a dewatering
system drawoff for either pumping or gravity flow back
to the interceptor. Exceptions to this arrangement are
the circular open tanks in the Springfield, Illinois,
46
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Effluent
Flowmeter-
A /-eoa Effluent
/ Conduit (To Outfall)
> (Narranansfitt Bav^
Dewatering
Sluice
Gate
Operations Building
Effluent
Screw
Pumps
10" FM
(Dewatering
and Infiltration
to WPCP Via
36" FM)
(Narragansett Bay)
Flow Direction
Inlet
Sluice
Effluent L Settling Tanks Gate
Collection
Box Influent
Effluent Lift Station
Influent Flowmeter
54"
Parallel
Sewers
48" Settled
Effluent Line
Manhole Station
10 + 64
Dry
Weather
Barrel
Diversion
Manhole
18" Overflow Connection
60" Influent Sewer
LEGEND
Dewatering System
Flushing Water
Chlorine Solution
Flow Direction
Dewatering and
Infiltration Flow
Drain
Sluice gate
Slide Gate
24" Sewer /-w /-Overflow
(To Long Wharf f I Structure
Pump Station) H / No. 2
24" Sewer T
(From Sewer >^
District No. 2)
72" Storm
Drain
(From City)-
FlowmeterJ
-Weir
• 30" Overflow Bypass
-Overflow Structure
No. 1
Figure 4-6. Flow schematic for typical storage/sedimentation facility, Newport, Rhode Island (Metcalf & Eddy, Inc., 1991b).
-------
Sanitary District, Lincoln and Taylorville facilities, which
feature mechanical sludge scrapers that are activated
automatically on tank level. The potential solids loads
on the POTW during pumpback operations should be
considered in evaluating sludge and grit handling
alternatives.
Design Details
Many of the design details for CSO storage/sedimentation
facilities are influenced by variations in treatment goals,
local hydraulic conditions, and other site-specific
considerations. As a result, a variety of methods or
strategies have been developed to address such issues
as facility activation, flow distribution, tank flushing, and
other aspects of facility design. The following examples
of design details have been incorporated into currently
operating CSO storage or storage/sedimentation
facilities.
Facility Influent Gates
Influent gates can prevent dry weather flow from
entering the facility, control the rate of wet weather flow
to the facility, and protect the facility from flooding
or activating during equipment failure or regular
maintenance. Examples of influent gate operations are:
• MWRA Cottage Farm: Flow passes over a weir to an
inlet structure. Three 72-inch sluice gates in the inlet
structure open automatically when the liquid in the
inlet structure reaches a high level. One gate is left
open several inches to prevent splashover from
triggering facility startup (U.S. EPA, 1977a).
• Saginaw, Michigan, Hancock Street: The original
design provided a sonic level sensor in the CSO
facility influent conduit, set to open sluice gates to
the facility on high level, then close the gates on low
level. Once on line, gate operation was erratic and
the controls were changed to require manual closing
of the gates on low level. Non-rising stem sluice gates
were used, but a 1990 study recommended using
rising stem gates to avoid submergence of the stem
threads (U.S. EPA, 1980).
• MWRA Prison Point: A hydraulic sluice gate is
controlled by water level in an inlet structure and in
the stormwater wetwell. The facility gate will modulate
to throttle flow into the facility on a high flow condition
in the inlet chamber. The gate also is provided with
a manual remote control (Maguire, 1981).
• Newport, Rhode Island, Washington Street: The
normally open hydraulic sluice gate throttles flow into
the facility on high level in the effluent screw pump
wetwell by closing and reopening in 2-inch
increments on rising or falling high level. High level
float switches in the influent channel and the effluent
wetwell automatically close the gate completely as a
backup in case the gate-modulating system fails
(Metealf & Eddy, Inc., 1991b).
Flow Distribution
CSO storage/sedimentation facilities commonly are
designed with multiple rectangular tanks. Flow is
distributed such that the tanks fill sequentially to
minimize maintenance and cleanup following small
storms. At some point in the process of filling, flows are
equalized so that all tanks are completely full before
overflow occurs. Examples of flow distribution include
the following:
• MWRA Prison Point: The inlet structure has one dry
weather flow (DWF) channel and three wet weather
channels. On high level in the DWF wetwell, the DWF
gate throttles causing flow to backup in the inlet
structure. On high level in the inlet structure the first
storm gate opens. A bubble tube in the first tank
signals the next gate to open before the first tank
water surface reaches the effluent weir. The third gate
operates in a similar manner. Once all gates are open
and all tanks are at the same level, the water surface
uniformly rises to the effluent weir elevation. This
operating strategy allows the tanks to fill sequentially,
while preventing overflow until all tanks are full.
Available storage capacity in the inlet structure
provides a delay between throttling of the DWF gate
and opening of the first storm gate, preventing
short-cycling of the storm system (Maguire, 1981).
• Newport, Rhode Island, Washington Street: The
elevation of the influent sluice gates for one tank is
lower than the elevation of the gates for the other two
tanks. Flow in the influent channel will pass into tank
1 first, then the second two. All gates are located
below the effluent weir elevation, so all tanks will be
completely full before the facility overflows. There are
three manual sluice gates for each tank. All gates are
normally kept open, although operators can allow
tanks 2 and 3 to fill sequentially by monitoring tank
levels and manually operating the gates (Metealf &
Eddy, Inc., 1991b).
• Saginaw, Michigan, Hancock Street: Influent pumps
initially discharge to one bay. When this bay is 60
percent full, the sluice gate to the second bay is
opened. Two additional bays follow the second bay.
Once all bays are at the 60 percent fill level, they all
will fill evenly, and thus all overflow at once (U.S.
EPA, 1980).
Tank Configuration and Details-Rectangular Tanks
Rectangular tanks are the most common configuration
for CSO storage/sedimentation facilities. A major
advantage of rectangular tanks is that common-wall
construction or installation of interior walls allows the
48
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total volume to be compartmentalized. As noted above,
sequential filling of compartments can reduce cleanup
efforts during smaller storms. Figure 4-7 presents a plan
and profile for a typical rectangular storage/sedimentation
tank. A number of design considerations are related to
tank configuration and appurtenant details:
Surface Geometry
For rectangular primary settling tanks at POTWs,
length-to-width ratios are typically in the range of 3:1 to
5:1. In theory, tank geometry is intended to minimize
flow patterns that might resuspend settled solids or
otherwise disrupt the settling process. The critical scour
velocity is calculated as follows (WEF, 1992):
H = {[8k(s-1)gdyf}
;0.5
where:
VH = critical scour velocity
k = constant for type of scoured particles
s = specific gravity of scoured particles
g = acceleration due to gravity
d = diameter of scoured particles
f = Darcy-Weisbach friction factor
Typical values for k range from 0.04 for unigranular
sand to 0.06 for sticky, interlocking material. The units
in the equation may be either U.S. customary or SI
(Metcalf & Eddy, Inc., 1991a).
In practice, the width of primary settling tanks commonly
is constrained by the available width of mechanical
sludge collection equipment. Since such equipment is
typically not provided at CSO storage/sedimentation
facilities, the constraints on tank width are not as rigid.
In addition, the general reliability of length-to-width ratio
as a design tool has been questioned (WEF, 1992). For
tanks intended for storage only, geometry is of even less
concern. Table 4-3 presents typical dimensions for
-Overflow Conduit
- Collection Trough
[-Influent Channel
nf 1 Sluice Gate and 1
/ Target Bafflen \
^<
*
<
j
/- Influent
S Pressure
Sewer
<- Sluice Gate
L
^~fe
sUp
±
^Coarse Bar
Screens
Effluent Force Main to
Treatment Plant Headworks
(a)
Hnfluent Chamber
Overflow
Conduit —
| ^Walkway With Railway
III 1 1 ^Al 1 1 1
II 1 1*1 1 II
^
£
.r- Contact Chamber Dividing Wall
\- Effluent
Weir
Contact Chamber
Sluice
Trough -v T*
A
r- Influent
/ Pressure
' Sewer
- Coarse
Bar
Screen
- De watering
Pipe
(b)
Figure 4-7. Plan and profile for a typical rectangular storage/sedimentation facility (Metcalf & Eddy, Inc., 1991a).
49
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Table 4-3. Typical Rectangular Tank Dimensions for
Storage/Sedimentation Facilities
Dimension (ft)
Source/Location
Length Width
Side-Water
Depth
Metcalf & Eddy, Inc.,
1991a1
WEF, 19921
Great Lakes-Upper
Mississippi River Board,
19781
80-130
16-32
10-15, 12
typically
10
7 minimum
U.S. EPA, 1975b1
Milwaukee Humboldt Ave. 420 75
(Medina etal, 1981)
Saginaw Hancock St. 206 52
(U.S. EPA, 1980)
Southgate/Wyandotte, 222 362
Michigan2
(Hubbell, 1990)
MWRA Prison Point 94 23.5
(Maguire, 1981)
Newport, Washington St. 130 25
(Metcalf & Eddy, Inc.,
1991b)
10-13
16
10
25
12
12
1 Values for primary settling tanks
2 Large tank is compartmentalized
settling tanks from published texts and examples of
actual dimensions at CSO storage/treatment facilities.
Floors
As noted above, to facilitate solids handling, floors are
typically sloped to a center or side trough, which is in
turn sloped back to the dewatering system drawoff.
Flushing systems generally are required to ensure
complete solids removal. A number of installations have
been developed in Europe with multiple grooves or
channels in the tank floor, in an attempt to maintain
self-cleaning velocities across the entire floor area. This
type of design is more expensive to build, and operating
experience shows that some flushing is still required to
prevent the build-up of solids (Stahre and Urbonas,
1993). Table 4-4 presents examples of floor slopes at
installations in the United States.
Baffles
Most facilities include influent baffles to dissipate energy
in the influent flow and to minimize short circuiting
through the tank. Commonly of timber construction, the
design of the influent baffles for CSO storage/treatment
tanks is similar to influent baffles for rectangular primary
settling tanks. Details of influent baffle designs can be
found in the literature (WEF, 1992; Stahre and Urbonas,
1993). Effluent scum baffles control the discharge of
floatables, a desired goal of CSO abatement strategies.
The effluent baffles for CSO storage/treatment tanks
typically consist of steel plates or concrete beams that
extend below the water surface. Mechanical scum
collection troughs are less common.
Flushing Systems
Flushing systems are provided at storage/sedimentation
facilities to assist in solids handling and general cleanup
following facility activation. Due to the intermittent
operation of CSO control facilities, it is important to keep
tanks clean between storms to minimize odor buildup
and maintain tank capacity. As tanks are dewatered,
heavier solids that have settled out will not necessarily
be resuspended unless some mechanical means such
as aeration or mixing is provided. Without fully
resuspending settled solids, pressurized flushing
systems can be used to direct the settled solids towards
the sludge drawoff hoppers. The two types of flushing
systems commonly found in CSO facilities in the United
States are header-mounted spray nozzles, located
around the perimeter of the tank, and high-pressure,
manually controlled monitor nozzles, mounted on
turrets.
Spray headers are used to wash down walls, but may
not be effective in moving solids along the bottoms of
the tanks. The advantage of spray headers is that little
operator attention is required. Monitor nozzles are
intended primarily for moving solids along tank floors
towards central gutters and sludge drawoffs. They also
are effective for cleaning walls, but are more labor
intensive than the spray headers.
Some facilities feature only one type of system (Oakport,
California), while others (Newport, Washington St.;
Saginaw, Hancock St.) feature both. In some facilities
where monitor nozzles are provided, hose gates and
flexible hose are provided to reach corners or other
Table 4-4. Examples of Floor Slopes for
Storage/Sedimentation Facilities
Slope (ft/ft) Location
Floor slope to
center trough
0.0833
0.0833
MWRA Prison Point
(Maguire, 1981)
Saginaw, Michigan, Hancock
St.
(U.S. EPA, 1980)
0.0870 Newport, Rhode Island,
Washington Street
(Metcalf & Eddy, Inc.,
1991b)
Center trough 0.0167 Saginaw, Michigan, Hancock
slope to drawoff Street
(U.S. EPA, 1980)
0.0115 Newport, Rhode Island,
Washington Street
(Metcalf & Eddy, Inc., 1991 b)
50
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spots out of the range of the monitor nozzles (MWRA
Cottage Farm).
Some European installations feature "tipping flushers,"
which consist of cylindrical containers mounted above
the width of the tank by an off-center swivel mechanism.
Each container has an asymmetrical discharge opening
running the length of the container. At the end of a storm
event, after the storage/sedimentation tank has been
dewatered, a flushing water pump fills the tipping
flusher. When the tipping flusher is nearly full, the center
of gravity shifts, causing the container to rotate and
discharge its contents. The force of the sudden rush of
water moves remaining solids along the floor of the
tank. Tipping flushers may not be effective in tanks
longer than approximately 160 feet (Brombach,
1989).
A number of facilities located near rivers utilize river
water as the source of flushing water. The
Bannockburn, Scotland, facility, which features a
circular tank, experienced silting of the river intake and
clogging of spray nozzles; the spray system was
abandoned. The Saginaw, Hancock Street facility and
the Bay City, Michigan, facility both use river water for
flushing, while the Chapaton, Michigan, Retention Basin
uses lake water. The Chapaton facility can use its
flushing system to resuspend solids for dewatering, but
this option generally is not used.
The MWRA Prison Point facility uses river water from
the Charles River for plant water. The river water is
supplied by gravity through an auto-backflushing
strainer. Astormwater stripping pump provides a backup
source of flushing water. The MWRA Cottage Farm
facility uses an on-site well as a flushing water source.
The Newport Washington Street facility uses city water
as a source for flushing water. The city water is fed
through an air gap into a break tank. On low level in the
break tank, a valve on the city water supply opens. On
high level, the city water valve closes and a "Flushing
Water Ready1 indicator light is activated at the main
control panel.
Tank Configuration and Details-Circular Tanks
Circular tanks typically feature one of two types of inlet
configurations: center column feed with influent baffles,
similar to circular primary settling tanks; and tangential
feed, more common in Europe, which creates a swirling
motion within the tank. Tangential feed tanks are in-line
devices intended primarily for storage of the initial storm
flows, with an overflow located upstream of the tank. A
vortex valve throttles the underflow from a center bottom
drawoff to the downstream interceptor (Brombach, 1989).
Flushing systems for center-feed circular tanks typically
feature header-mounted nozzles and/or manual flushing
with hoses fed from yard hydrants. Tangential feed
tanks are intended to be essentially self-cleaning.
Typical dimensions for circular tanks are presented in
Table 4-5.
Table 4-5. Typical Dimensions of Circular Storage and
Storage/Sedimentation Tanks
Dimension (ft)
Type
Diameter
Side-Water
Depth
Bottom
Slope
Primary
(Metcalf & Eddy, Inc.. 40-150
1991 a)
cso
Bannockburn, Scotland 66
(Henderson et al., 1981)
Decatur, Illinois, 85
McKinley Ave.
(BGMAandCMT, 1977)
Lincoln, Illinois 130
(CMT, undated)
Springfield, Illinois, 180
Spring Creek
(CMT, undated)
Springfield, Illinois, 210
Sugar Creek
(CMT, undated)
10-15
12 typical
9.5
24.8
1:12
1:8.25
1:3*
* 1:3 slope is on sides of tank near bottom
Tank Access
The need for a cover over a tank depends on a number
of factors, including the elevation of the tanks with
respect to grade, proposed development on or adjacent
to the site, public safety and security, the potential for
odors, and general aesthetics. A common practice is to
provide a walkway inside the tank, above the maximum
water surface, to provide access for maintenance and
cleaning. Explosion-proof lighting and electrical fixtures
are required, since the tanks could be exposed to
potentially explosive sewage gases or chemical spills
that could be conveyed through the collection system.
Walkways must be provided with railings and toeplates,
as required by the appropriate building and safety
codes. Covered tanks must be provided with
emergency exits, in accordance with applicable codes.
In some facilities, portions of the roof slab are
removable, allowing a compact front-end loader to be
lowered into the tank to assist in grit handling.
Tank Dewatering Systems
Standard practice is to return the contents of tanks to
the sanitary collection system by gravity, if hydraulically
feasible, or by pumping. Trade-offs between influent
pumping with gravity effluent flow, and gravity influent
flow with effluent pumping should be evaluated during
51
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design. Multiple tanks should have the capability of
sequential dewatering. One common method for
providing sequential dewatering is to pipe the dewatering
drawoff lines to a common header. Motor-operated or
hydraulic valves on the drawoffs are set to open and
close sequentially. The dewatering header could also be
piped to rapid dewatering pumps and solids stripping
pumps. Centrifugal pumps are appropriate for the rapid
dewatering of storage/sedimentation tanks, while
plunger pumps should be considered for solids handling.
Examples of dewatering systems at currently operating
CSO facilities include:
• MWRA Cottage Farm: Tanks are dewatered by
gravity to the interceptor. Approximately 1 hour is
required to drain the full 1.2-million gallon capacity of
the facility (U.S. EPA, 1977a).
• Bannockbum, Scotland: Tank is dewatered by screw
pump to the interceptor (Henderson et al., 1981).
• Oakport, California: Tanks drain by gravity to
interceptor, one tank at a time. Sequential dewatering
minimizes the amount of time that sludge is exposed
to the air, thereby minimizing odors (McCormick et
al., 1990).
• Southgate/Wyandotte, Michigan: Gravity dewatering
to interceptor is controlled by a flow meter and control
valve (Hubbell, 1990).
• MWRA Prison Point: Manually controlled hydraulic
gates drain the tanks to a stormwater wetwell, which
is dewatered by a 500-gpm centrifugal stripping
pump. The stormwater wetwell is sized to hold the
entire contents of one tank (Maguire, 1981).
• Newport, Rhode Island, Washington Street: Two
800-gpm vertical dry-pit non-clog centrifugal pumps
are provided for rapid dewatering, while two 85-gpm
duplex plunger pumps are provided for solids
stripping. One of each type of pump is intended as
a standby, although a "rapid dewatering" mode allows
both centrifugal pumps to run, then both duplex
plunger pumps (Metcalf & Eddy, Inc., 1991b).
Ventilation and Odor Control
Tanks, screenings areas, wetwells, and other areas
within storage/sedimentation facilities that are exposed
to sewage gases must be ventilated to prevent the
accumulation of potentially explosive and/or corrosive
gases and to control the buildup of moisture and
condensation. Evacuation of sewage gases is required
for personnel safety and equipment longevity, while
control of moisture is important since damp surfaces
can provide an environment for bacteria that oxidize
hydrogen sulfide to sulfuric acid, resulting in accelerated
corrosion. Ventilation systems for process areas are
sized based on the volume of the space to be ventilated
and a specified ventilation rate (typically in terms of air
changes per hour). For areas exposed to sewage
gases, ventilation rates of 6-12 air changes per hour
are typical (U.S. EPA, 1985a; Great Lakes-Upper-
Mississippi River Board, 1978). Two-speed fans allow a
lower rate of ventilation when spaces are not occupied
and a higher rate when personnel are present. This
arrangement lowers operating costs for facilities that are
not continuously staffed. Intake louvers are sized based
on a maximum face velocity and commonly are
interlocked with supply and/or exhaust fan operation.
The designer should consult applicable codes,
ordinances, standards, and manufacturer's literature to
establish design criteria for a particular application.
Where storage/sedimentation facilities are located in
the vicinity of residential, commercial, or recreational
areas, or where the exhaust from the facility ventilation
system may otherwise impact a sensitive receptor, a
means for controlling odors in the exhaust gas should
be considered. Two of the more common processes for
removing odors from exhaust gas are wet scrubbing
and activated carbon adsorbtion. Activated carbon
systems generally are applicable for air flows up to
2,000 cfm, while wet scrubbers are more appropriate for
air flows greater than 2,000 cfm (U.S. EPA, 1985a).
Activated carbon may also be applied as a polishing
step, following wet scrubbing.
Discharge permits may be required for ventilation
exhaust from CSO facilities. The designer should
consult current federal, state, and local codes regarding
exhaust air discharge permit requirements.
Wet Scrubbers
Wet scrubbers function by providing contact between
the exhaust air and a scrubbing solution, which removes
the odorous compounds through one or more of the
following processes (U.S. EPA, 1985a):
• Condensation of odorous vapors
• Removal of odorous participates
• Odor adsorbtion into the scrubbing solution
• Odor reaction with an oxidizing scrubbing solution
• Emulsification of odorous gases in a chemical reagent
Wet scrubbers can be arranged in a vertical, counter-
current flow configuration, or in a horizontal, cross-flow
configuration. A typical wet scrubber system includes
the following features:
• Scrubbing tower, housing the following:
- scrubbing solution spray system
- inert packing material, to promote liquid/solid contact
- mist eliminator, to minimize loss of scrubbing fluid
in the exhaust air
52
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- scrubbing solution wet well
- drain
• Exhaust fan with ductwork, including bypass of
scrubbing tower
• Scrubbing solution recycle pumping system
• Chemical storage and feed equipment, including:
- metering pumps
- storage tanks
- piping, valves, and appurtenances
— instrumentation and controls (pH, oxidation-reduction
potential [ORP] probes)
Potassium permanganate and sodium hypochlorite are
two of the more common oxidizing agents used in wet
scrubbers. Scrubbing solutions also may include an
acid or base to keep the pH of the solution in an
optimum range for odor compound removal or
neutralization. Selection of the appropriate scrubbing
solution should be based on the types and
concentrations of the odor-causing constituents to be
removed. Pilot testing should be undertaken to establish
design and performance criteria for a full-scale system.
Some scrubber units are provided with a constant flow
of make-up water and a continuously overflowing
scrubbing solution wetwell. The constant renewal of
scrubbing solution minimizes the build-up of solids in
the scrubbing solution resulting from the reaction
between hydrogen sulfide and sodium hypochlorite. The
overflow from these units is commonly piped to a
sanitary drain.
Activated Carbon Adsorption Unit
Activated carbon adsorbtion units remove odors
through the adsorbtion of odor-causing compounds
onto the surface of the activated carbon media. Atypical
activated carbon system consists of a vessel supporting
a bed of granular activated carbon, an exhaust fan, and
ductwork.
Sodium hydroxide-impregnated carbon commonly is
used in odor control applications, as it has a higher
capacity to remove hydrogen sulfide than non-
impregnated or potassium hydroxide-impregnated
carbon. A further advantage of the impregnated carbon
is that it can be regenerated in-place, using 50-percent
sodium hydroxide solution. Non-impregnated spent
carbon must be removed from the vessel and thermally
regenerated in a multiple hearth furnace. If impregnated
carbon is to be specified, the design must include
appropriate chemical piping, valves, and drains to
permit in-situ sodium hydroxide regeneration.
Design of an activated carbon odor control system
should include the following steps (U.S. EPA, 1985a):
• Characterization of the exhaust air to be treated.
« Identification of effluent criteria.
• Selection of the adsorbent.
• Completion of pilot studies to establish expected
performance, estimated useful life of carbon, and
design criteria.
• Application of pilot data to full-scale design.
The designer is referred to manufacturer's literature and
published design guides (U.S. EPA, 1985a) for additional
information pertaining to odor control system design.
System Controls and Operation
Due to the uncertainty in predicting the nature of
precipitation and storm flows, CSO facilities typically are
designed for automatic activation. Facility activation is
triggered by sensing of flow or water surface elevation,
as a regulator activates and flow is diverted to the
facility. Regulator activation can be passive (i.e., sideweir,
orifice) or mechanical (i.e., inflatable dam). Mechanical
regulator activation can be triggered by a flow signal or
water surface elevation, or through a real time control
system, as described in the first section of this chapter.
In general, the simpler the activation system, the more
reliable the operation. For example, at the Newport,
Rhode Island, Washington Street facility (Figure 4-6),
flow enters the facility influent conduit from a static
sideweir regulator. Once flow reaches the facility,
catenary bar screens and a hypochlorite dosing system
activate from a mercury float switch in the influent
channel. Flow enters the tanks through normally open
manual sluice gates, with the elevations of the gates set
so that one tank fills first. No sophisticated controls are
required until the effluent weir overflows, causing
activation of the effluent screw pumps. Examples of
activation strategies for other facilities are described
above (see "Design Details").
More sophisticated controls are required for influent/
effluent pumping, facility dewatering, and hypochlorite
dosing for disinfection.
Pumping Systems (Influent/Effluent)
Influent and/or effluent pumping commonly is controlled
by wetwell elevation, as in a standard sanitary pumping
station. Wetwell design, pump design, and control
strategy must consider the peaked hydrograph typical
of storm flows and must either attenuate or be able to
respond to relatively rapid changes in flow rate. Some
type of variable speed control is used for pumps.
53
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Screw pumps are well suited for CSO applications since
the pump discharge varies directly with influent flow.
The drawback to screw pumps is the space
requirement, especially for higher lifts. Influent pumps
must be capable of passing solids that fit through coarse
screens.
Overflow relief must be provided to protect the facility
from flooding if the influent or effluent pump capacity is
exceeded. Relief could be in the form of an overflow
weir, which activates when a wetwell backs up, or it
could be a remote regulator, which works in conjunction
with a modulating gate. On high level, the gate would
throttle, causing an upstream regulator to activate. It is
advisable to have simple backup, such as a mercury
float switch, which would close the facility gate
completely on high-high level in the facility, in case the
gate modulating controls fail.
Facility Dewatering
As discussed under Process Theory, initiation of a
facility dewatering sequence typically is a manual
operation. The operators must ensure that the
conveyance system and POTW capacity have been
restored following the storm. Programmable logic
controllers allow for automatic control and switch-over
from rapid dewatering pumps to solids handling pumps
once the dewatering sequence is initiated. Where
gravity dewatering is possible, some facilities feature a
flow control valve for more precise regulation of flow to
the interceptor.
Grit can be troublesome for pumpback operations.
Some facilities have a separate means to handle grit
and heavier solids, such as providing access for
removal via a vacuum truck. Other facilities provide a
separate set of pumps for solids removal. Some
facilities aerate and mix the tank contents to facilitate
solids removal.
Disinfection
A detailed discussion of disinfection control strategies is
provided later in this chapter.
Process Variations
The design of CSO storage/sedimentation facilities is
not standardized. Some early facilities were intended to
be experimental prototypes for evaluating emerging
technologies. Process variations that have been
implemented are described below.
Fine Mesh Screening with Sedimentation
This process variation is intended to augment solids and
floatables removals through storage/sedimentation
tanks. A potential drawback to this variation is that static
fine screens tend to easily clog.
• MWRA Cottage Farm: Horizontal fixed screens with
0.2-inch openings are located between the scum
baffle and effluent weir. The screens are hinged to
allow them to swing open by hydraulic pressure if
clogged. Flow passes under the baffle, up through
screens, then over the weir. The screens are
manually flushed during tank cleanup following a
storm (U.S. EPA, 1977a).
• Atlanta, Georgia, Intrenchment Creek: 50-mesh
(300-u.m) static screens are installed upstream of the
sedimentation tanks. The screens were subject to
blinding during the first flush and when there were
heavy grease loads (West et al., 1990).
Aeration/Mixing of Tank
At some facilities where the tank is not intended to
function as a flow-through treatment facility, aeration
and/or mechanical mixing prevents sludge from turning
septic and generating odors, and facilitates solids
handling. If aeration or mixing is considered, the
potential benefits should be evaluated against the
additional operation and maintenance costs associated
with the aeration or mixing equipment.
• Decatur, Illinois, Seventh Ward and Lincoln Park:
Blowers provide aeration until the first flush tank can
be dewatered. Aerating the tank keeps contents
mixed, minimizes odor problems, and makes the tank
easier to clean after a storm (BGMA and CMT, 1977).
• Milwaukee, Wisconsin, Humboldt Avenue: Seven
mechanical mixers resuspend solids once the storm
is over to facilitate dewatering. Mixing reduces the
cleanup effort after a storm (Medina et al., 1981).
Flow Balance Method
The in-receiving water flow balance method involves
using floating pontoons and flexible curtains to create
an in-receiving water storage facility. CSO flows fill the
facility by displacing the receiving water that normally
occupies the storage facility. The CSO flows are then
pumped to the collection system following a storm. The
technology has been used for CSO control in Brooklyn,
New York, where floating pontoons were permanently
installed in the receiving water near the CSO outlets.
The feasibility of this technology depends in part on
whether the storage facility would significantly impact
the aesthetic value of the surrounding area, and
whether the structure would hinder navigation. Other
site-specific concerns include the availability of volume
due to tidal variations in coastal water, and the need to
protect from damage due to high winds or wave action.
Standard marina technology was used in the Fresh
Creek, Brooklyn demonstration project to prevent
damage.
54
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Deep Tunnel Storage
Introduction
Deep tunnel storage often is considered as an
alternative to near-surface storage/treatment facilities
where space constraints, potential construction
impacts, and other issues challenge the feasibility of
near-surface facilities. The major advantage of deep
tunnel storage is that relatively large volumes can be
stored and conveyed with little disturbance to existing
surface features. In congested urban areas, near-
surface CSO control facilities can be difficult to site, and
the only available open spaces for such facilities often
include recreational areas such as river-front parks or
ball fields. Deep tunnel construction may avoid some of
the issues arising from the use of open spaces and
minimize the disruptions associated with the extensive
open-cut excavation associated with near-surface
facilities.
Deep tunnel construction does not completely avoid the
issues related to siting of open excavations, however,
as excavations at work shafts, access shafts, vent
shafts, and drop shafts are required, along with
excavations for near surface consolidation conduits.
Handling and disposal of excavate can also present
challenges in coordinating truck traffic and identifying
disposal locations. In Rochester, New York, tunnel
excavate was successfully used to fill low areas around
an airport and the POTW, as well as other areas
identified by the owner. Allowable truck routes were
specified to minimize impacts on local neighborhoods.
The feasibility of deep tunneling must be established
through geotechnical investigations, as well as
evaluation of other concerns such as the potential for
encountering hazardous wastes, the impacts on
adjacent structures, and construction logistics.
A typical deep tunnel system includes the following
features:
• Regulators, to divert and control storm flows to the
tunnel system.
• Consolidation conduits, to convey flows from
regulators to the tunnel system.
• Coarse screening, to remove large debris and protect
downstream pumps.
• Vertical dropshafts, to deliver flow to the tunnel and
dissipate energy.
• Air separation chambers, to allow release of air
entrained in the dropshafts.
• Tunnel, sized to store and convey flows from a given
design condition.
• Access shafts, for maintenance personnel and
equipment.
• Vent shafts, for balancing air pressure.
• Dewatering system, to pump volume stored in the
tunnel to the POTW once conveyance system and
treatment capacity is restored.
• Odor control systems at certain venting locations.
Design considerations for each of these components
are described below.
Regulators
Regulators control flow into the interceptor from a
combined trunk sewer and serve as the diversion point
for routing excess flows to the tunnel system. Regulator
types are reviewed earlier in this chapter.
In the Metropolitan Water Reclamation District of
Greater Chicago's Tunnel and Reservoir Project (TARP),
sluice gates in the regulator structures regulate flow into
the tunnel system and can be opened in advance of an
expected storm (Dalton and Goyal, 1989).
Consolidation Conduits
Routing a deep tunnel to provide direct interception of
every CSO outfall in a community's combined sewer
system is not feasible. Rather, construction of near-
surface consolidation conduits from selected outfalls or
regulators to a more centrally located deep tunnel is a
cost-effective alternative. The feasibility and cost of
constructing consolidation conduits may be one of the
factors that influences the configuration, depth, and
route of a deep tunnel system. Conversely, the route of
the tunnel system may dictate the requirements for
consolidation conduits. Aspects to consider in evaluating
the use of near-surface consolidation conduits include:
• Potential disruptions to traffic, utilities, access to
businesses, and other impacts on the community and
the environment during construction of consolidation
conduits.
• Cost of consolidation conduits compared to cost of
multiple drop shafts; minimizing the number of drop
shafts typically will be cost effective.
• Impacts of near-surface soil conditions on consolidation
conduit construction methods (wetlands, unsuitable
soils, bedrock elevation, etc.).
• Impacts of subsurface geology on tunnel construction
methods and tunnel routing.
Consolidation conduits are sized based on the peak
flows from the design condition for which CSO control
is to be provided. For example, suppose a community's
long-term CSO control strategy is to route all flows from
a particular design storm to the deep tunnel system. A
55
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computer model of the community's combined sewer
system, such as SWMM, generates values of the peak
flow from the design storm at each node where the
proposed consolidation conduits intercept flow. These
peak flows are used for preliminary sizing of the
consolidation conduits. The model then is expanded to
include the proposed conduits, and subsequent model
runs are used to refine the peak flows, particularly
where one conduit intercepts the flow from more than
one node. Through this iterative process, the optimum
size of the consolidation conduits can be developed.
Consolidation conduits sized to convey peak storm
flows may be of substantial size, and may present
opportunities for in-line storage. Gates on the
downstream end of consolidation conduits can be
operated in conjunction with a real-time control system
to optimize the use of storage and conveyance capacity
in the tunnel system. Construction methods for
near-surface consolidation conduits can include
open-cut excavation and soft ground tunneling.
Relief points must be provided for flows to the
consolidation conduits in excess of the design storm
peak flow. A convenient location for relief is where the
flow enters the consolidation conduit, as these
structures are commonly located at existing regulators
or on existing outfalls. In some cases, one relief
overflow can serve a number of hydraulically connected
CSOs, thus reducing future monitoring and
maintenance costs by reducing the number of overflow
locations.
Coarse Screening and Grit Removal
At the downstream end of consolidation conduits,
coarse screening equipment can remove large, bulky
solids such as branches and logs before the flow enters
the dropshaft. For a system with a large number of
dropshafts, providing coarse screening at each
dropshaft may not be feasible, both in terms of cost and
operation and maintenance requirements. For this
reason, screening facilities may be located in the tunnel
itself. Coarse screening and grit removal can be
accomplished by screens and sumps located just before
the dewatering pump station, usually a low point in the
tunnel system. Screenings can be removed by a rake
mounted on a bridge crane located at the ground
surface. The rake is lowered to the underground trash
rack to retrieve and raise screenings to the surface for
disposal. A clamshell bucket can remove grit material
from a sump located ahead of the dewatering pumps.
The Chicago TARP system incorporates trash and grit
removal facilities at the intake tunnels of the dewatering
pump station (Variakojis and Quintanilla, 1989). In the
Rochester, New York, deep tunnel system, screening
facilities were provided at the downstream ends of each
of the two main branches of the tunnel system. Coarse
screening equipment is discussed in more detail later in
this chapter.
Vertical Dropshaft
The function of the vertical dropshaft is to deliver flow
from the near-surface conveyance system to the deep
tunnel system, dissipating the energy in the flow to the
extent possible and providing a means to remove air
entrained in the flow as it passes down the shaft.
Dropshaft design considerations and dropshaft types
include (Westfall, 1990):
Dropshaft Design Considerations
Dropshafts have three basic components:
• Inlet structure, to provide the transition between
horizontal and vertical flow.
• Vertical shaft barrel, to convey flow to the lower
elevation and dissipate energy in the flow.
• Bottom chamber, to dissipate energy in the vertical
flow and provide a means to separate and release
air entrained in the dropshaft.
Dropshaft design is influenced by one or more of the
following factors (Westfall, 1990; St. Anthony Falls
Hydraulic Laboratory, 1971).
Variable Discharge
CSO outfall hydrographs tend to show distinct peaks. A
consolidation conduit connecting a number of outfalls or
regulators to a dropshaft may provide some attenuation
of peak flows, but the dropshaft still will be exposed to
a range of flows and must be capable of functioning
within the expected range.
Impact on Dropshaft Floor
The impact of the flow on the dropshaft floor depends
on the depth of the drop. For even relatively shallow
drops, provisions should be made to reduce the
magnitude of the impact. Alternatives for dissipating the
energy in the flow include inducing a hydraulic jump in
the shaft, creating a vortex action in the shaft,
increasing wall friction in the shaft, and providing a
plunge pool in the shaft floor. The plunge pool can be
created by forming a sump in the floor or by providing
a weir downstream of the shaft to back up flow. The
depth of a plunge pool is determined from the Dyas
formula (WPCF/ASCE, 1974):
where:
h = depth of drop (ft)
dc = critical flow depth in shaft inlet (ft)
56
-------
Entrained Air
The air that becomes entrained in the flow as the flow
passes down the dropshaft can be both beneficial and
detrimental to the dropshaft operation. The benefits of
entrained air include:
• Minimizing the potential for subatmospheric
conditions in the shaft, which would cause cavitation.
• Providing a cushion to absorb the impact of the falling
water.
Disadvantages to entrained air include:
• Increasing the volume of the flow, which may
increase the required dropshaft size.
• Requiring a separate means for removing the
entrained air, to avoid a build up of pressure.
Headless in the Dropshaft
If the hydraulic gradeline in the tunnel is close to the
hydraulic gradeline in the near-surface conveyance
system, excessive headless in the dropshaft can cause
flows to back up in the near-surface system.
Surge Relief
Six out of a total of 40 vertical dropshafts in the
Rochester, New York, tunnel system were oversized to
provide surge relief from transient hydraulic pressures
that could develop in the tunnels (Holzbach, 1990).
Types of Dropshafts
Different types of dropshafts have been developed
based on hydraulic studies. Selection of dropshaft type
depends on the relative importance of the design
considerations noted above. Four of the more common
dropshaft types are discussed below (Westfall, 1990).
Drop Manholes
Drop manholes are used in near-surface conveyance
systems to drop flow from a higher sewer into a lower
sewer. The manholes minimize turbulence that would
otherwise promote the release of sewage gas and
erosion of the manhole. A separate access manhole
commonly is provided to the lower sewer, so that
maintenance personnel can avoid climbing down the
wet shaft. Drop manholes are suitable for drops up to
70 feet. For drops of greater than 70 feet, one of the
following dropshaft types should be considered.
Vortex Dropshafts
Flow is introduced into vortex dropshafts tangentially,
causing flow to spiral down the shaft in a vortex flow
pattern. Centrifugal forces keep the flow in contact with
the wall, dissipating energy through friction and creating
an open inner core, through which entrained air may
escape. The vortex flow pattern forces air bubbles
toward the center core, further reducing the amount of
entrained air. The central air core also tends to maintain
atmospheric pressure in the shaft, thereby minimizing
the potential for cavitation. A relatively smooth transition
from horizontal to the vortex vertical flow minimizes
turbulence at the inlet, which also minimizes air
entrainment. Kinetic energy remaining in the flow is
dissipated either through creating a hydraulic jump in
the shaft, or by a plunge pool at the bottom of the shaft.
Five types of tangential inlet configurations have been
developed (Figure 4-8), including (Westfall, 1990):
• Circular: The dropshaft is concentric with the vortex
inlet, which has a horizontal floor.
• Scroll: The sides of the vortex inlet, with a horizontal
floor, curl towards the dropshaft.
• Spiral: The approach channel issues a vortex flow
into the dropshaft by winding downwards.
• Tangential: The approach channel contracts at its
junction with the dropshaft so as to project the flow
tangentially around the walls.
• Siphonic: Generally used for outlets from a reservoir,
a series of siphons are located around the entrance
so as to produce a vortex flow down the dropshaft.
Hydraulic studies have suggested that the spiral and
tangential inlets perform best, while the tangential inlet
is easier to construct (Westfall, 1990). The vortex shaft
results in minimal air entrainment and significant energy
dissipation, but headloss is significant. A vortex type
might not be appropriate where the difference in
hydraulic gradient between the tunnel and the
near-surface conveyance system is minimal.
Morning Glory
These drop structures feature a circular crested weir for
inlet control and commonly are used for reservoir
outlets. Flow characteristics are determined from weir
control, orifice control, and differential head control.
Direct Drop Air Entraining Type
The inlet structure for this dropshaft allows flow to enter
the dropshaft radially, entraining air to the extent that
the shaft flows full. The full-flowing shaft promotes
energy dissipation through wall friction, and the
entrained air provides a cushion for absorbing energy
at the bottom of the shaft. A large air separation
chamber is required at the bottom of the shaft, along
with a separate venting system to evacuate the air
before it enters the tunnel system.
Two variations of the direct drop style were developed
for Chicago's TARP system. Both types are capable of
57
-------
(a) Circular
(b) Scroll
(d) Tangential
(c) Spiral
Siphon
Approach
Channel
Dropshaft
(e) Siphon
Figure 4-8. Examples of tangential inlet configuration (Westfall, 1990).
58
-------
passing a wide range of flows at approximately
20 percent of the headloss of similarly sized vortex-type
dropshafts. The E-15 (Figure 4-9) features an air vent
located inside the drop shaft. Slots in the vertical wall
separating the dropshaft from the air vent allow air to
be re-entrained in the falling water. Recirculating the air
provides a more consistent mix of air and water,
resulting in a more uniform flow in the dropshaft. The
air separation chamber at the bottom of the shaft
features a sloped roof to direct the released air back
towards the vent. The E-15 shaft was used for shaft
diameters up to 9 feet, with a maximum flow of 600 cfs.
The D-4 dropshaft was developed for dropshaft
diameters greater than 9 feet (Figure 4-10). The air vent
for the D-4 shaft is a separate shaft, located downstream
of the dropshaft. The air vent feeds into the dropshaft
above the crown of the incoming sewer, so air is
recycled to the top. The air separation chamber for the
D-4 has a flat roof. The D-4 is suitable for dropshafts up
to 20 feet in diameter, and flows up to 4,500 cfs.
The air separation chambers and shaft structures for
both types of direct-drop dropshafts are large
structures, and must be suitably anchored to withstand
the forces and vibrations generated by the falling flow.
The floors of the air chambers in the TARP system are
lined with a metal coating to minimize erosion from grit
carried in the flow. Drop shafts initially may be oversized
to provide access to the tunnel during construction. The
finished diameter of the dropshaft is sized based on the
following equation (Westfall, 1990):
= 0.2
where:
Q = design flow (cfs)
g = acceleration of gravity (ft/sec2)
D = finished shaft diameter (ft)
Over 200 of the direct-drop style dropshafts, some of
which have been operating for 20 years, have been
installed in Chicago's TARP system.
Tunnels
Sizing and routing of deep storage tunnels, as with any
major CSO control system, is a complex process,
requiring the review of a series of alternatives before
arriving at the optimal configuration. The storage
volume required to meet a given CSO control goal is
developed through the use of a system model such
as SWMM, or a more simplified routing technique.
Alternatives encompassing variations in tunnel diameter,
length, route, depth, and construction methods that
meet the required storage and conveyance needs are
then developed and evaluated. Topics to consider in
Shaft I.D.-r4
r
Exit
Conduit
L>C ELEVATION 30oto75Q
E . K L
W
•—i
Design Discharge
(cfs)
Shaft I.D.
A
B
C
D(Max)
E
F
G
H(Max)
(Min)
W (Min)
HxW(Min)(ft2)
K
L
M
N (Min)
P(Max)
N x P (Min)(ft2)
Q
S (Min)
Head Loss
SECTION C-C
SECTION B-B
600
9'-0"
4'-6"
2' -9"
I'-O"
10' -0"
9'-0"
280
T-2"
3-r
2'-6"
1' - 0"
r-6"
6' -8"
140
5' -8"
2' - 10'
2' -6"
0'-9"
5'-6"
S'-O"
20' -3 1/2" 15' -01/2" 11' -2 1/2"
Variable,
12' - 0"
9'-0"
6'-9"
81
27' - 0"
29' - 0"
56' -0"
4'-0"
Equal to Pipe Dia.
9'-8"
6' -8"
4'-9"
45
20' - 3"
21' -6"
41' -91
3' -6"
(But Not To Exceed I.D. of Shaft)
8'-0"
S'-O"
4'-0"
25
14' -10"
16' - 0"
30' -10"
3'-0"
6
32' - 6"
S'-O"
9.0'
4.4
24' - 0"
4'-6"
5.5'
18'-1"
4'-0"
4.7"
Figure 4-9. The E-15 dropshaft (Westfall, 1990).
59
-------
A
t n
>T-
/-ven
/^~A-
/-T\ Vl
o r:
I, F
ea uover
t
m
4
P ~"
G
TOP VIEW
Inlet Conduit
1
»l
Water
Conduit-
Tl
Air Shaft
o
SECTION A-A
Figure 4-10. The D-4 dropshaft (Westfall, 1990).
/-»
J
Air Separation
Chamber
Exit
Tunnel
Design Discharge
(cfs)
A
B
C
D
E
F
G
H
J
Head Loss
4500
20' - 0"
8'-0"
48' - 0"
40' - 0"
25' - 0"
55' - 0"
90' - 0"
20' - 0"
28' - 0"
20'
2200
15'-0-
6'-0"
36' - 0"
30' - 0"
19' -0"
42' - 0"
67' - 0"
15'- 0"
21'-0"
15'
o
SECTION B-B
1250
12'-0"
5'-0"
29' - 0"
24' - 0"
15'-0"
33' - 0"
53' - 0"
12'-0"
17' - 0"
12'
developing tunnel sizing and route alternatives are
discussed below.
Subsurface Conditions
A substantial subsurface exploration program is
necessary to evaluate the feasibility of deep tunneling
and to identify the most appropriate tunneling
techniques. Depth to bedrock, rock strength,
discontinuities and weaknesses in the rock structure,
and ground-water conditions can impact the selection
of tunnel route and construction method. Softer rocks
allow a faster penetration rate, but also may require
additional temporary and permanent support structures.
Harder rocks, while requiring less support, generally
require a slower rate of excavation. Excessive
ground-water may cause tunnel flooding or local
collapse, and may require special ground treatment
techniques such as dewatering, grouting, or ground
freezing. Discontinuities, fault zones, and other areas of
local weakness or variation may preclude the use of
certain tunneling techniques. The elevation of the top of
the bedrock is an important consideration in setting the
vertical alignment. In some cases, it may also be cost
effective to route the tunnel at a deeper elevation if
doing so would avoid difficult areas or "mixed face"
conditions (Thompson and Dobbels, 1991). In
Rochester, New York, the surface topography would
have allowed the downstream leg of the tunnel system
serving the west side of the city to be set at a constant
grade to the POTW. While allowing gravity flow to the
POTW, the downstream end of this vertical alignment
would have risen above the top of bedrock. The
designer preferred to keep the entire segment in rock
to ensure maximum support against transient pressure
waves when the tunnel was filling or flowing full. The
final vertical alignment, therefore, consisted of an
inverted siphon, which allowed a tunnel boring machine
(TBM) to excavate at a constant but steeper grade
entirely in rock to a downstream control structure, at
which point the rising leg of the siphon was excavated
up to the POTW (Holzbach, 1990).
Encountering hazardous materials can have a severe
effect on project schedule and cost. Delays may occur
to identify the nature and extent of the wastes, develop
and implement disposal and/or treatment options, and
obtain permits. An environmental assessment along the
60
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route of a proposed tunnel and at proposed shaft
locations allows the designer to identify routes that
avoid areas of known or potential contamination. A
limited review for preliminary siting might include review
of available agency files and data on known locations
of oil or hazardous materials spills, and review of past
and present land use along proposed routes
(Thompson and Dobbels, 1991). During design, a more
detailed study, including soil borings, monitoring wells,
and sampling, may be required. The potential
requirement for subsurface easements from property
owners also should be investigated.
Excavation Method
Three methods for deep rock tunnel excavation are
TBMs, road header machines, and drill-and-blast
methods. These methods are described below; the
advantages and disadvantages of each technique are
summarized in Table 4-6.
TBMs
TBMs feature a rotating, cutting head of a size equal to
the full cross-sectional area of the tunnel to be
excavated. Horizontal hydraulic thrust cylinders force
the rotating cutting head forward into the rock. The body
of the TBM is supported by hydraulic legs, which push
against the walls of the tunnel, anchoring the TBM
against the thrust applied to the cutting head. When the
hydraulic thrust cylinders are fully extended, the main
support legs are retracted and the TBM is advanced
forward to start the next cutting cycle. Figure 4-11
shows the cutting cycle for a typical TBM system. The
cutting heads feature a means to remove the excavated
material from the cutting area, commonly by discharging
the excavate to a conveyor system. The type and
configuration of cutters on the cutting head vary,
depending on the anticipated geologic conditions. In
rock, roller or disc cutters are more reliable than drag
cutters, which are more appropriate for soft ground
conditions (Whittaker and Frith, 1990).
Rock Header Machines
These machines feature a relatively small-diameter
boom-mounted rotary cutting head, supported by a
tracked base. The cutting head is worked back and forth
across the rock face by manipulating the boom. The
rock debris is picked up at the base of the rock face by
a conveyor system running through the base of the
machine. The most common boom machines are the
milling-type and ripping-type. The milling-type generally
is best suited for hard rock conditions and features a
conical cutting head that rotates in the same axis as the
boom. The ripping-type cutting head rotates on a
horizontal axis perpendicular to the boom and is best
suited for softer rock conditions (Whittaker and Frith,
1990).
Drill-and-Blast Methods
The steps involved in drill-and-blast methods include
(Whittaker and Frith, 1990):
• Drilling: The number of blast holes required in the
rock face depends on the diameter of the tunnel and
the strength of the rock. In addition to the blast holes,
a number of burncut holes, without explosive
charges, are drilled into the rock face to relieve the
explosive stress. Drilling rigs may be mounted on the
haulage tracks or on separate tracks of wider gauge
than the haulage tracks, or they may be rubber tire-
or tread-mounted.
Table 4-6. Advantages and Disadvantages of Deep Tunnel Excavation Methods
Method Advantages
Disadvantages
TBM
Rockheader machines
Drill and blast methods
Rapid excavation to final tunnel
diameter and grade
Disturbance of surrounding rock
minimized
Well suited for long reaches of constant
cross section
One machine can excavate different
tunnel diameters
Typically lower lead times for delivery
than TBMs
If tunneling conditions change, machine
can be easily withdrawn to allow use of
drill and blast methods
Can be used in most rock conditions
• Cutting face can become jammed where
high rock stresses create "squeezing" condition
• Usually long lead times required to fabricate
new machines (use of reconditioned
machines can reduce lead time)
• Usually not economical if multiple tunnel
diameters are required
• Rate of advancement is lower than for TBMs
• Rate of advance depends more on operator
skill and rock fracture patterns
• Cannot typically apply as much force to
rock as TBMs
Relatively slow rate of advance
Higher potential for damage to surrounding
rock during blasting
61
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Main Legs
Hydraulic
Thrust
Cylinders
Rear
Support
Legs
Cutting
Head
Step 1.
Start of boring cycle. Machine
clamped, rear support legs
retracted.
Step 2.
End of boring cycle. Machine
clamped, head extended, rear
support legs retracted.
Step 3.
Start of reset cycle. Machine
undamped, rear
support legs extended.
Step 4.
End of reset cycle. Machine
undamped, head retracted.
Machine now ready for
clamping and beginning boring
cycle.
Rgure 4-11. Cutting cycle for a typical tunnel boring machine (Whlttaker and Frith, 1990).
62
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• Blasting: The strength of the explosive charge also
depends on the diameter of the tunnel, the local
geology, and the proximity of adjacent structures. The
weight of explosive can be limited by a specified
maximum allowable blasting vibration. Following
detonation, smoke fumes must be exhausted before
personnel return to the blasting areas. Reversible
fans in conjunction with ventilation tubes minimize the
exposure of workers to the blasting fumes.
• Debris Clearance: Once personnel can safely return
to the blasting areas, the roof, face, and sides of the
tunnel are visually inspected and the loose rock is
removed. A variety of methods and machines are
available to handle excavation, including rocker
shovels and scraper action loaders, which discharge
into rail-mounted cars, conveyors, or slurry pipelines.
• Ground Support: Temporary supports protect personnel,
control overbreak, and support loosened rock around
the perimeter of the tunnel. Even tunnels in hard,
competent rock may require temporary supports for
localized weak zones. Common types of temporary
support include arched steel ribs, rock bolting, and
shotcrete. Permanent linings in drill-and-blast tunnels
typically are installed once the excavation is
complete, although in larger tunnels, excavate removal
and concrete placement can be more readily
coordinated.
In general, if geologic conditions are suitable for using
a TBM, then tunnel routing and sizing should be
directed toward longer reaches of constant diameter
tunnel. Straighter tunnels are usually preferable, since
TBMs have a limited turning radius. If subsurface
conditions preclude use of TBMs, then excavation
volume can be optimized by using tunnel segments with
different cross-sectional areas. In any case, work
generally proceeds upgradient to minimize the opportunity
for ground-water infiltration to pond at the work face.
The direction of excavation can influence the location of
work shafts and tunnel layout.
Consolidation Conduit Layout
As noted above, tunnel routing must be developed in
conjunction with near surface consolidation conduit
design. The capacity for storage in consolidation
conduits and upstream collection systems may reduce
the size and/or location of tunnels required.
Potential Operating Strategies
Strategies to control flow into and through a tunnel
should be considered when developing the optimal size
and configuration of the tunnels. For example, certain
diversion structures in the Rochester, New York, tunnel
system are capable of diverting flow to two different
sections of the tunnel system. If one system of the
tunnel fills more rapidly than the other, remotely
controlled gates redivert flow to the tunnel section with
greater remaining capacity (Kent, 1992). Designing
control structures with automatic gates allows such
operating strategies, especially if provided in
conjunction with a real-time control system.
Access, Vent, and Work Shafts
Work shafts and access shafts are required to move
personnel, equipment, and materials in and out of the
tunnel during construction and once the tunnels are
operational. The size of construction work shafts may
be dictated by the size of the excavation machinery
used. The land around a work shaft commonly is used
as a staging area for equipment and supplies, and is
subject to heavy vehicular traffic associated with
excavate removal. One study estimated that 2 to
3 acres are required at each construction access shaft
location (Wheatley, 1991).
Permanent access shafts for tunnel maintenance and
inspection may be developed from construction work
shafts or incorporated into other structures, such as
screening houses or control structures.
Vent shafts provide for the passive movement of air in
and out of the tunnel during filling and dewatering,
responding to differences in pressure between the
tunnel and the above-grade atmosphere. Vent shafts
may be provided with odor control. In Rochester, New
York, intermediate vent shafts are not provided with
odor control, whereas carbon adsorbtion odor control is
provided at a screening facility on the upstream end of
the inverted siphon to the POTW, and also at the
downstream end of the inverted siphon. In addition to
odor-causing compounds, emissions can include
volatile organic compounds (VOCs).
Vent and access shafts typically feature a concrete pad
at grade, with either gratings or an appropriate cover to
secure the opening. The total area required for
permanent access or vent shafts is 1/4 acre or less.
Dewatering Pump Station
Deep tunnel storage and conveyance systems typically
feature a dewatering pumping station at the
downstream end of the tunnel system, although gravity
drainage systems do exist. The pumping station may be
dedicated to the tunnel system or integral to a POTW
sanitary influent pumping station. If coarse screening is
not provided at upstream locations, then screening
facilities should be provided at the pump station.
Figure 4-12 shows a tunnel system dewatering pump
station.
Tunnels are sloped to ensure a sufficient carrying
velocity and to facilitate dewatering. A tunnel slope of
0.1 percent has resulted in minimal problems with grit
63
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Screenings Handling Area
Scrubber Discharge
Fan Building
Clamshell -
Rock Screenings
Shaft
Bndge Crane
Pump Chamber
Equipment
Ingress/
{ Egress Shaft
Figure 4-12. Tunnel system dewatering pump station (MWRA, 1990).
deposition in Rochester, New York. Though much of the
pumping station equipment may be located below
grade, an above-ground superstructure is required to
house screenings handling equipment, odor control
equipment, and/or electrical and operations control
equipment. Dewatering rates are determined by POTW
primary and secondary treatment capacities.
Active tunnel air venting may be provided at pumping
stations for use during dry periods, when anaerobic
conditions in tunnel sediment can produce odors. The
method of venting can be by drawing air into the tunnel
through drop shafts and access shafts, and venting at
the pumping station, or by pushing air through the
tunnel and out the shafts. The selected direction of air
movement may depend on the desired location for odor
control facilities and the location of sensitive receptors.
In Rochester, New York, combined sewage from the
east side of the city feeds a tunnel system that drains
to a 275-mgd pumping station. This pumping station,
which also handles sanitary flows, features nine pumps,
with the largest having a capacity of 38,200 gpm. The
station provides a total lift of approximately 145 feet,
and was designed to allow expansion to a capacity of
400 mgd.
Flows from the west side of Rochester feed a second
tunnel system, which drains by gravity to the POTW.
This tunnel system was originally planned to pass under
the Genessee River, with a gradient which would have
required downstream pumping to the POTW. The local
topography, however, would allow a conduit to be
suspended over the Genesee River, which in turn would
raise the tunnel gradient high enough to allow gravity
flow to the POTW. A second benefit of this design was
that the crossing of the Genesee River gorge was
designed with an integral pedestrian walkway, providing
access between two parks on opposite sides of the
river. At the downstream end of the inverted siphon on
the west side tunnel, gates allowed the siphon to drain
to the main pumping station. This operating strategy
makes the volume of the siphon available for storage at
the start of the storm event (Kent, 1992; Holzbach,
1990).
The pumping station on the Chicago TARP Mainstream
System features pumps housed in two independent
underground chambers, in order to provide backup
capacity. Each housing can accommodate four pumps.
The pumps provide service at two rated heads, 150 feet
and 330 feet; therefore, the required pumping head
range from 60 feet to 330 feet is obtained by throttling
cone valves. Vertical, single-suction volute pumps with
64
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vertical, constant-speed motors are used, with pump
casings encased in concrete to support the motors and
control vibration. Energy for the 85,000 hp of pumping
(57,000 hp of high head and 28,000 hp of low head) is
supplied through two independent substations that tie
into independent transmission lines with switchover
capability (Variakojis and Quintanilla, 1989). Table 4-7
summarizes the capacities of the pumps (Dalton and
Goyal, 1989). Once a storage reservoir is completed,
Table 4-7. Chicago TARP Mainstream Pumping Station
Capacities
No. of
Pumps Rating at Maximum Head Head Range
North Pumphouse
1 330 cfs (213 mgd) at 330 ft
1 220 cfs (142 mgd) at 330 ft
2 330 cfs (213 mgd) at 150 ft
South Pumphouse
1 330 cfs (213 mgd) at 330 ft
1 220 cfs (142 mgd) at 330 ft
1 330 cfs (213 mgd) at 150ft
1 160 cfs (103 mgd) at 150ft
240-330 ft
240-330 ft
60-150 ft (1 future)
240-330 ft
240-330 ft
60-150 ft (future)
60-150 ft
this pumping station will have the capability of operating
in three modes:
• Pumping from the tunnels to the POTW
• Pumping from the tunnels to the storage reservoir
• Pumping from the storage reservoir to the POTW
The pumping rate of storm flows to the POTW is
controlled to maintain a maximum flow to the POTW of
1.5 times DWF. Average DWF to the POTW is 903 mgd,
which leaves 455 mgd of capacity available for treatment
of storm flows (Dalton and Goyal, 1989).
A supervisory control and monitoring system (SCMS)
provides the following remote control functions at the
Mainstream Pumping Station (Dalton and Goyal, 1989):
• Pump control and monitoring
• Power circuit breaker control and monitoring
• Dewatering tunnel gate control and monitoring
• Tunnel discharge valve control and monitoring
• Closed circuit television control
Tunnel System Operation
Deep tunnel storage systems can have various
operating strategies. The simplest operating strategy
allows the system to fill with no restrictions at the
dropshafts until the tunnels, dropshafts, and consolidation
conduits are filled and an overflow occurs. An
alternative operating strategy is to prioritize areas
served by the system. Under this strategy, flows to a
dropshaft of low priority are throttled in order to allow
inflow from a higher priority dropshaft. A higher priority
is assigned to a dropshaft that serves an area with a
more sensitive receiving water. A real time control
system can be used to operate the throttling gates. If
the volume of captured CSO does not exceed the
storage capacity, the stored flow is pumped to the
treatment facility during the dry weather period. The
Chicago TARP system pumps excess volume to large
pit-type storage reservoirs when the tunnel system is
filled to capacity.
Storage of combined flows allows solids deposition in
the tunnel system. In unlined tunnels, a concrete invert
increases velocities and reduces solids deposition.
Solids can be resuspended by flushing the tunnels
during dry weather. Sources of flushing water include
existing interceptor sewers or surface waters. Flushing
water can be removed using high head pumps capable
of handling the high solids concentration expected
during flushing. The required volume of flushing water
is determined by the diameter, length, and grades of the
tunnel sections.
Coarse Screening
Process Description
Coarse screening equipment, consisting of vertical or
inclined steel bars spaced evenly across a channel, with
or without mechanical raking apparatus, traditionally is
located at the headworks of POTWs to remove from the
influent flow large objects that might otherwise damage
downstream equipment or clog downstream pipes.
Depending on the clear spacing between the bars,
coarse screens also can entrain rags and floatables.
Coarse screening equipment is installed at CSO control
facilities, both for the protection of downstream
equipment and to provide floatables removal.
The types of bar screens used at CSO control facilities
include trash racks, manually cleaned screens, and
mechanically cleaned screens. The major features of
each type are (Metcalf & Eddy, 1991 a; WEF, 1992):
• Trash racks
- Typically 1.5- to 3-inch clear spacing between bars
- Intended to remove large objects such as timber
planks and stumps
- Often followed by bar screens with smaller clear
spacing
• Manually cleaned bar screens
- 1- to 2-inch clear spacing between bars
- Bars set 30 to 45 degrees from the vertical
65
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- Screenings are manually raked onto a perforated
plate for drainage prior to disposal
- Commonly used in bypass channels for mechanically
cleaned bar screens
• Mechanically cleaned bar screens
- 0.25- to 1-inch clear spacing between bars
- Bars set 0 to 30 degrees from the vertical
- Electrically driven rake mechanism either continuously
or periodically removes material entrained on the
bar screen
— Common types of mechanically cleaned screens
include chain-driven, with front or back cleaning;
reciprocating rake; catenary; and continuous
Automatically activated, mechanically cleaned screens
are recommended for CSO facilities. Of the
mechanically cleaned bar screens, the catenary type
most commonly is selected for CSO control facilities
(Figure 4-13). Catenary screens are rugged and
reliable. All sprockets, bearings, and shafts are located
above the screenings channel, reducing the potential for
damage and corrosion and facilitating routine
maintenance. The cleaning rake is held against the bars
by the weight of its chains, allowing the rake to be pulled
over large objects that are stuck in the bars and that
might otherwise jam the rake mechanism. The
chain-driven, front or back cleaning types are better
suited for separate sanitary flows, and may be
susceptible to jamming and increased maintenance if
exposed to the range of debris present in combined
flows. Reciprocating rake screens have limited capacity
to handle peak screenings loads. Continuous self-
cleaning screens tend to have a higher capacity for
solids handling than the more traditional design, and
have potential for application at CSO control facilities.
More detailed descriptions of screening devices are in
the literature (Metcalf & Eddy, Inc., 1991a; WEF, 1992;
U.S. EPA, 1977b).
Process Design
The following discussion of design considerations for
coarse screening equipment includes hydraulic
considerations, equipment details, solids handling, and
process flow. Supplemental information is available
from equipment manufacturers and the literature
(Metcalf & Eddy, Inc., 1991 a; WEF, 1992).
Hydraulic Considerations
A straight upstream channel provides the most uniform
flow velocity distribution across a screen, which, in turn,
will result in a more even distribution of solids entrained
on the screen. The headloss across a clean bar screen
can be estimated from the following equation (Metcalf
& Eddy, Inc., 1991 a):
V-V2>|
0.7^ 2g J
where:
hL = headloss, ft (m)
0.7 = an empirical discharge coefficient to account for
turbulence and eddy losses
V = velocity of flow through the openings of the bar
rack, ft/s (m/s)
v = approach velocity in upstream channel, ft/s (m/s)
g = acceleration due to gravity, ft/s2 (m/s2)
For coarse screens, the approach velocity should be at
least 1.25 ft/s to minimize deposition, while the velocity
through the bars should be less than 3 ft/s to prevent
entrained solids from being forced through the bars
(Metcalf & Eddy, Inc., 1991a). Instrumentation provided
with mechanically cleaned bar screens is configured to
trigger cleaning cycles so that headloss across the bar
screen is limited to 6 inches.
At POTWs, solids deposited in the screenings channel
during periods of low flow sometimes can be
resuspended during peak flow periods. At CSO control
facilities, however, peak flows generally occur during
the early part of the storm, after which the flow gradually
tails off. There is typically no equivalent to the diurnal
peak flows which could repeatedly scour the bar screen
channel. In addition, towards the end of a storm, as
flows subside, backwater from a storage/sedimentation
tank effluent weir can create quiescent conditions in the
bar screen channel. A means to flush or otherwise
handle solids deposited in the screenings channel,
therefore, should be provided.
A redundant or standby bar screen should be provided
so that peak flow to the facility can be maintained with
one unit out of service. Screenings channels with sets
of stop log grooves or slide gates will allow each bar
screen to be isolated from the influent flow for
maintenance.
Equipment Details
Data have been published that relate estimated
screenings quantities to bar screen spacing for separate
sanitary sewer systems, but these data do not apply to
combined systems, which may produce higher
screenings loading rates during storm events (WEF,
1992). The 0.5- to 1.0-inch range for bar spacing is
common for mechanically cleaned screens at CSO
control facilities.
Suitable guards, railings, and gratings should be
provided around the screening equipment to ensure
operator safety. Electrical fittings and devices
associated with the screening equipment must conform
to the exposure rating for the space in which the
equipment is located (Great Lakes-Upper Mississippi
River Board, 1978). If screens discharge to screenings
66
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Rgure 4-13. Catenary-type mechanically cleaned bar screen (WEF, 1992).
67
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carts or containers, hoisting and transport equipment is
needed to transfer the screenings from the containers
to a truck for disposal. A monorail-mounted hoist can
provide such capability.
Solids Handling
The quantities of screenings removed at CSO facilities
can be highly variable, depending on the configuration
of the drainage system, the time of year, the interval
between storms, and other factors. Average CSO
screenings loads range from approximately 0.5 to
11 cf/MG, with peaking factors based on hourly flow
ranging from 2:1 to greater than 20:1. The bulk density
of screenings from combined flows range from 40 to
70 Ib/cf (WEF, 1992).
Methods for handling screenings from mechanically
cleaned bar screens vary among existing CSO control
facilities. Examples of handling methods include:
• Newport, Rhode Island, Washington Street: Screenings
from this storage/sedimentation facility are discharged
into 1-cubic yard capacity bins, which are manually
wheeled to a monorail hoist. A lifting/dumping frame
is attached to the bin to keep the bin in an upright
position while being carried on the hoist. The frame
features a release arm for dumping the contents of
the bin into a truck for disposal (Metcalf & Eddy, Inc.,
1991b).
• Atlanta, Georgia, Intrenchment Creek: Screenings
from the headworks of this physical/chemical CSO
treatment facility are discharged into conveyors,
which carry the screenings to collection bins. During
operation material has spilled repeatedly on the floor
and the conveyor belts have jammed. A study of this
facility's operations recommended that future
facilities use an alternative screenings handling
system (West et al., 1990).
• MWRA Cottage Farm: This detention facility features
trash racks upstream of bar racks, both mechanically
cleaned, catenary type. The trash rack screenings
are deposited into dumpsters, while the bar rack
screenings are discharged to a sluiceway and flushed
back to the interceptor (U.S. EPA, 1977a).
• MWRA Prison Point: Screenings from this detention
facility are discharged to a conveyor belt and carried
to a sorting table. From the sorting table, screenings
are fed manually into a hammermill grinder, then
discharged back to the interceptor. Material
unsuitable for grinding, such as bricks or
heavy-gauge scrap metal, is removed at the sorting
table and disposed of separately (Maguire, 1981).
Process Flow
As at POTWs, bar screens at CSO control facilities are
located at the head of the facility, upstream of the tanks
and process equipment, but typically downstream of the
regulator that diverts flow to off-line facilities. Some
facilities, such as the MWRA Cottage Farm and Atlanta
Intrenchment Creek facilities, are equipped with
larger-opening trash racks upstream of smaller-
opening, mechanically cleaned bar screens. As noted
above, at Cottage Farm both the trash racks and bar
screens are catenary-type mechanically cleaned. A
performance study at this facility noted that very little
material was collected on the trash racks, and
suggested that manually cleaned trash racks would
have been suitable (U.S. EPA, 1977a).
At the Decatur, Illinois, McKinley Avenue facility, a
manually cleaned bar screen with 2-inch openings was
provided as a bypass for times when the mechanically
cleaned catenary screen was out of service. During an
operational study, concerns were raised that the manual
screen might become subject to clogging if forced into
operation since personnel may not be available at the
facility (which normally is not staffed) to manually clean
it. The study recommended that if manual bypass screens
were provided, they should be sized to capture only
objects that are large enough to damage downstream
equipment (BGMAand CMT, 1977).
System Controls and Operation
Controls for mechanically cleaned bar screens include
some combination of the following:
• Manual start/stop
• Automatic start/stop on timer
• Automatic start/stop on differential head
At CSO facilities, activation of mechanically cleaned bar
screens is triggered by remote sensing of flow into the
facility, or water level in the screening channel. Timed
cycles, if used, should include a high differential head
override. Near-continuous operation may be required
during the initial phase of the storm when a greater
portion of leaves, litter, and other solids may be carried
in the initial peak flows to the CSO facility. Automatic
start-up of the bar screens should include a time delay
to prevent premature initiation of operation due to
transient surges in flow.
Table 4-8 presents examples of bar screen installations
at CSO control facilities. The table indicates bar screen
type, bar spacing, screenings handling method, and
control sequence.
68
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Table 4-8. Examples of Bar Screen Installations at CSO Control Facilities
Location Facility Type
Bar Screen
Type
Bar Screening Disposal
Spacing (in) Method
Operation/Control
Newport, Rhode Island
Washington St. (Metcalf
& Eddy, Inc., 1991b)
Atlanta, Georgia
Intrenchment Creek
(Westetal., 1990)
MWRA
Cottage Farm (U.S. EPA.
1977a)
Storage/treatment Catenary
Storage/treatment Trash rack
Storage/treatment
MWRA Storage/treatment
Prison Point (Maguire,
1981)
MWRA Coarse screening/
Constitution Beach disinfection
(Hayden-Wegman, 1989)
MWRA
Somerville Marginal
Pretreatment Facility
(Tighe & Bond, Inc.,
1990)
MWRA
Fox Point
(Hayden-Wegman, 1991)
Decatur, Illinois
McKinley Ave. (BGMA
and CMT, 1977)
Washington, DC
NEB Swirl Facility
(O'Brien & Gere, 1992)
Coarse screening/
disinfection
Coarse screening/
disinfection
Storage/swirl
concentrator
Swirl concentrator
Catenary
(trash rack)
Catenary
Catenary
Catenary
Catenary
Catenary
Catenary
Catenary
0.75
3.0
0.75
3.5
0.5
0.5
1.0
0.5
0.75
1.0
1.0
Discharge to 27-cf carts Auto start on high level float
switch; continuous operation
Discharge to conveyor
Discharge to conveyor
Discharge to dumpster Auto start on high level
Discharge to sluiceway Auto start on high level
Discharge to conveyor. Control by timer, differential
to sorting table, then head, or manual
to hammermill grinder
Discharge to carts
Automatic start/stop controlled
by water level
Discharge to conveyor Auto start on water level; stop
to storage container after time delay following gate
closure.
Discharge to container Auto start/stop on water level
Discharge to cart
Discharge to 80-cf
fixed bins; bins
emptied by vacuum
truck
Auto start/stop on differential
head
Control by timer or differential
head
Swirl/Vortex Technologies
Swirl concentrators and vortex separators are compact
flow throttling and solids separation devices that provide
flow regulation and a rough level of solids and floatable
removal in combined flows. The technology originally
was applied in England in the 1960s, and since has
evolved into a number of configurations. Three of the
more common technologies are the EPA swirl
concentrator, the Fluidsep vortex separator, and the
Storm King hydrodynamic separator. The differences
among the three separator types came about through
experimental work aimed at creating flow conditions
within the unit that would optimize the liquid/solid
separation.
Theory
Although each of the three types of separators is
configured differently, the operation of each unit and the
mechanism for solids separation are similar. Flow
entering the unit is directed around the perimeter of a
cylindrical shell, creating a swirling, vortex flow pattern.
The swirling action throttles the influent flow, and
causes solids to be concentrated at the bottom of the
unit. The throttled underflow containing the concentrated
solids passes out through a foul sewer outlet in the
bottom of the unit, while the clarified supernatant
passes out through the top of the unit. The underflow is
typically discharged to the downstream interceptor for
treatment at the POTW. Various baffle arrangements
capture floatables in the supernatant. The floatables are
carried out in the underflow when the unit drains, once
storm flows subside.
The mechanism for solids separation is created by the
flow patterns within the unit. Flow initially follows a path
around the perimeter of the unit. After one revolution,
the flow is deflected into an inner swirl pattern, which
has a lower velocity than the outer swirl. Gravity
separation occurs as particles follow a "long path"
through the outer and inner swirl. The quiescent inner
swirl, as well as tangential breakaway of particles from
the cyclonic flow field and drag forces along the walls,
bottom, and in the shear zone between the inner and
outer swirl, all contribute to solids separation.
Secondary currents direct particles across the floor of
the unit towards the foul sewer outlet. Examples of each
of the three common types of swirl/vortex units are
presented in Figures 4-14 through 4-16.
69
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Foul Sewer
G
Inflow
Overflow
A Inlet Ramp
B Flow Detector
C Scum Ring
D Overflow Weir and Weir Plate
E Spoilers
F Floatables Trap
G Foul Sewer Outlet
H Floor Gutters
I Downshaft
J Secondary Overflow Weir
K Secondary Gutter
Figure 4-14. Example EPA swirl concentrator (U.S. EPA, 1982).
Performance of swirl/vortex devices depends primarily
upon hydraulic throughput and the settling characteristics
of the solids in the combined flow. The settling
characteristics are particularly important. The EPA swirl
concentrator is most effective at removing solids with
characteristics similar to grit (0.2 mm diameter, 2.65
specific gravity), while the particle settling velocity
profile of the flow to be treated is required to predict the
removal efficiency of the Fluidsep and Storm King
devices. Determining the particle settling velocity
distribution in samples of the actual flow to be treated
is strongly recommended in order to better predict
actual solids removal efficiencies.
Opinions differ as to the effect of turbulence and free
vortex formation on performance. The EPA swirl is
designed with vertical baffles to "reduce the rotational
energy of the liquid above the weir plate, and between
the scum ring and the weir" (U.S. EPA, 1982). These
baffles, along with a flow deflector at the inlet, disrupt
the free vortex flow, with the intent of creating a more
"gentle" swirl. Laboratory studies showed that the
baffles improve solids separation performance (U.S.
EPA, 1972). The Fluidsep, by comparison, features no
vertical baffles or flow deflectors, with the specific intent
of encouraging free vortex flow. The researchers who
developed the Fluidsep unit contend that the free vortex
creates less turbulence than the disrupted flow patterns
in the EPA swirl. The differences among the three
common types of swirl devices are outlined in the
section called Design Details.
70
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Effluent Launder
Liquid
Flow Pattern
Effluent
Influent
(enters
tangentially)
Concentrated
Solids -
Figure 4-1S. Example Fluldsep vortex separator (Metcalf & Eddy, Inc., 1991 a).
Process Design
The design of swirl concentrators and vortex separators
is based on the scale-up of empirical data from
experiments on model systems. Researchers used
models to develop optimum configurations and
dimensions for a given set of conditions (flow, solids
settling velocity distribution). Model dimensions were
then scaled-up to match the conditions of the intended
application, using Froude's Law of Similarity.
The methodology for sizing swirl concentrators varies
among the three types of devices. A detailed procedure
for the design of the EPA swirl concentrator has
previously been published (U.S. EPA, 1982), while the
sizing computations for the Fluidsep vortex separator
and the Storm King hydrodynamic separator are mostly
proprietary. A general description of the methodology for
sizing these units is provided below. The reader is
referred to the EPA swirl concentrator design procedures
(U.S. EPA, 1982) and the respective manufacturers of
Fluidsep and Storm King for more specific sizing
information.
EPA Swirl Concentrator
During the 1970s, EPA conducted a series of performance
studies on secondary-flow-motion wastewater control/
treatment devices, which were the prototypes for the
standard EPA swirl concentrator. The purpose of these
studies was to develop the optimum configuration of
swirl chamber elements and dimensions to achieve flow
regulation and maximum solids separation from a
typical CSO flow.
The solids settling velocity distribution in the typical flow
used in the studies was determined by sampling a
number of grit chambers. The actual settling velocity
distribution used in the EPA design is presented in
Figures 4-17 and 4-18. Average settling velocity profiles
from other sampling studies of combined and sanitary
wastewater are presented in Figure 4-19. For the
prototype performance studies, synthetic solids were
created to match the "standard" settling velocity
distribution developed from the grit chamber studies
(U.S. EPA, 1972, 1975c).
71
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Dip Plate and Spillway
Assembly
Support Frame
Top Baffle
Baffle Ptate
Venturi Plate -
Overflow Chamber
Overflow to Outfal
._ Foul Outlet
Pipe to Sanitary Sewer
Tangential Intake Pipe
- Intake Deflector Plate
(Optional)
1 — Sanitary Outlet Pipe Spigot
Concrete Chamber Poured In Place or
Precast Segmental Shaft Rings
(eg. Charcon One Pass)
— Concrete Base (Poured in Place)
Inlet Manhole
Combined Sewer
Storm King
^n7-:-.,v-
Si' Stormwater
Outlet Manhole
Existing
Control Manhole \ \ Stormwater Sewer
S
Max. Permitted Discharge
toWWTP
Reg-u-Flow Dry Weather
Vortex Valve Flow Bypass
Stormwater
Sewer
Figure 4-16. Example Storm King hydrodynamlc separator (H.I.L. Technology, Inc. undated).
72
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400 300 140
U.S. Standard Sieve Numbers
70 40 30 10
433/8
20.0
10.0
8.0
6.0
4.0
8 3-0
1 2-0
I
1.0
0.8
0.6
0.4
0.3
0.2
0.10
0.08
0.06
0.04
0.03
0.02
0.01
f
0.03 0.06 I 0.10
0.04 0.08
i
0.20
0.40 IO.BOI
0.60 1.0
2.0
Particle Diameter, mm
4.06T6T10.0
8.0
1 in = 2.54 cm
Figure 4-17. Particle settling velocities for grit and organic
material in still water (U.S. EPA, 1982).
As a result of the studies with the synthetic waste, solids
removal performance was correlated with flow and the
ratio of swirl chamber diameter to inlet diameter (Da/D,).
A series of curves were produced that related discharge
to the parameter D2/D^, for a range of values for inlet
diameter and percent settleable solids removal. Given
the flow and the desired settleable solids removal, the
curve will yield the value of Dz/D-i corresponding to the
selected inlet diameter, assuming a ratio of unit
diameter to weir height (Da/H) of 0.25. Additional curves
were developed for revising the D2/D^ ratio based on a
Dg/H ratio of other than 0.25, since in some cases the
available head may make H a controlling parameter. All
of these design curves are presented in U.S. EPA,
1982. The remainder of the unit is dimensioned in
proportion to the values of D2, D^ and H determined in
the initial steps.
In practice, the EPA swirl concentrator is intended to be
primarily an in-line flow regulator, and is not intended to
remove the lighter solids that may be found in combined
flows. A suggested method for estimating the actual
solids removal performance of a swirl concentrator
involves comparison of the actual solids settling velocity
profile of the flow to be treated with the theoretical
settling velocity profile for grit developed by EPA. From
3 4
a 10
U.S. Standard Sieve Numbers
16 20 30 40 50
70 100 140
10
1 0.6
Grain Size in mm
0.2
U.S. Sieve Size
4
10
20
40
SO
70
Size
mm in.
5.0 (0.020)
2.0 0.08)
0.84 (0.034)
0.42 (0.017)
0.30 (0.012)
0.20 (0.008)
% Finer by Weight
Grit Organics
100 100
100 53
63 31
31 17
18 14
0 10
Figure 4-18. Typical gradation for grit and organic material
(U.S. EPA, 1982).
Figure 4-17, grit particles of 0.2 mm have a settling
velocity of approximately 2.5 cm/sec. A swirl concentrator
designed in accordance with EPA recommendations
(U.S. EPA, 1982) is intended to remove 90 percent of
grit-sized particles. Given an actual settling velocity
distribution curve, the swirl might be expected to
remove 90 percent of the particles with settling
velocities equal to or greater than 2.5 cm/sec. On this
basis, an overall estimated solids removal rate can be
computed.
Two factors may contribute to the appearance of
lower-than-expected removal efficiencies for the EPA
swirl. Since the expected performance is based on the
removal of particles in the size and weight range of grit,
much of the material the swirl is specifically intended to
remove would be in the bed load carried along the
bottom of the interceptor. If the swirl unit is installed in
an off-line arrangement, where flow is diverted to the
swirl through a side weir or high-outlet regulator, then
the bed load would most likely be carried down the
73
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20.0
10.5
5.0
1.0
0.5
S
in
aj
o
t
co
0.
0.1
.05
.01
U.S. EPA Swirl Concentrator
Solids Basis of Design
Upper Curve = Inorganic Grit
Lower Curve = Organic Settleable Solids
S.G. Upper = 2.65
S.G. Lower = 1.2
Saginaw, Ml
(Weber)
Boston, MA
(heavy commercial)
Philadelphia, PA
Boston, MA
(residential)
San Francisco, CA
10 20 30 40 50 60 70 80 90 100
% of Particles with Settling
Velocity Less Than Stated Value
Rgure 4-19. Settling velocity profiles of combined and sanitary wastewaters (Pisano, 1990).
74
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interceptor and would not be part of the influent to the
swirl. Similarly, a swirl unit located downstream of a
storage tank may not receive the heavier fraction of
solids that it is capable of removing.
In addition, the sampling apparatus used to assess swirl
performance may not take a representative sample of
the influent flow. A single sample draw-off located at
mid-depth in the influent pipe might not pick up heavy
solids passing to the unit in the bed load. The sampler
intake velocity also must be sufficient to carry the
heavier grit particles into the sampler. Since the bed
load solids may constitute a significant fraction of the
total solids mass loading, failure to pick up the bed load
solids in the swirl influent, effluent, and underflow could
skew the overall performance results.
Fluidsep Vortex Separator
As with the EPA swirl, the standard configuration of the
Fluidsep vortex separator evolved through a series of
studies aimed at optimizing the solids removal efficiency
of the unit. However, rather than basing the design on
a "typical" CSO solids settling characteristic, as EPA did,
a generalized design procedure was developed. This
procedure can be used for any given solids settling
distribution, stated performance level for the final swirl
design, and design discharge. Therefore, the user is
required to perform studies of the existing CSO solids
settling characteristics before the Fluidsep vortex
separator can be designed.
The first step in the design procedure for site-specific
applications is to determine the actual solids settling
velocity profile in the CSO to be treated. Froude's Law
of Similarity then is applied to the "real-world" solids
distribution to obtain a model solids distribution curve.
Model tracer removal curves then are applied to the
model solids distribution curve to develop a curve of
predicted removal efficiency versus flow for given
vessel geometries (Pisano, 1990). A nominal D/H ratio
of 2.5 initially is used for the design, although the final
ratio chosen depends on considerations such as the
available head, site area, and construction costs.
Designs are available with D/H ratios varying from 0.5
to 3.0. This information then is used by Fluidsep to
design the final swirl through a proprietary process.
Since the design is based on actual solids settling
characteristics determined through sampling CSO
discharges, removal efficiency predictions for the
Fluidsep vortex separator should be more precise than
for the EPA swirl. However, inflow solids stratification
still can pose problems in accurately characterizing the
influent, thereby making removal efficiency calculations
difficult.
Storm King Hydrodynamic Separator
The design of the Storm King hydrodynamic separator
is based on the solids settling velocity profile of the CSO
to be treated, and the hydraulic loading or overflow rate.
Through studies with its unit, the manufacturer of the
Storm King has developed a series of optimal overflow
rates for particular ranges of solids to be removed.
Thus, if the influent flow rate and the distribution of
solids are known, the manufacturer can provide a unit
of diameter determined from the optimum overflow rate
for the given size of material to be removed. As with the
Fluidsep vortex separator, the specific dimensional
details for the vessel, as well as the inlet, outlet, and
other features of the Storm King, are proprietary. The
Storm King separator differs from the Fluidsep primarily
in the location of the solids outlet and the introduction
of internal components designed to stabilize secondary
flows.
General Hydraulic Considerations
In addition to the specific sizing procedures discussed
above, the configuration of a swirl unit, or even the
feasibility of installing a swirl unit, may depend upon
system hydraulics. One of the main considerations is to
determine to what level the influent sewer can be
surcharged without causing upstream flooding. This
elevation, in conjunction with the elevation head on the
effluent weir, will set the elevation of the unit with
respect to the influent sewer. For example, Figure 4-20
indicates an empirically derived curve of head versus
discharge per linear foot of weir length for a circular weir
(U.S. EPA, 1972). For a maximum flow of 300 cfs and
a circular weir length of 60 feet, the discharge would be
5 cfs/ft, corresponding to a head of approximately 3
feet. If entrance losses were neglected, the weir crest
then would be set 3 feet below the maximum hydraulic
grade line allowed in the influent sewer.
Since overloading the swirl units decreases performance,
the units are provided with some manner of overflow
weir to relieve peak flows. The EPA swirl design
recommends that flows to the unit be limited to twice
the design flow (U.S. EPA, 1982), and both the Fluidsep
and Storm King swirls are designed for the peak flow.
The crest elevation of the side overflow relief weir can
then be determined from the head on the weir at the
maximum flow using Figure 4-20.
If the available head is minimal, the geometry of the unit
may be modified, in certain instances, to reduce the
required head. Pumping the foul sewer discharge also
has been employed at installations where the system
hydraulics prevent gravity flow of the foul sewer
discharge to the interceptor.
75
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a,
I
r 90
-60
0 CFS
l/s
1234
28 56 84 112
Discharge per Linear Foot (30.5 cm)
5
150
Figure 4-20. Head vs. discharge per linear foot of weir length for a circular weir (U.S. EPA, 1982).
Process Flow
Four of the common layouts for swirl concentrator and
vortex separator installations are illustrated in Figure
4-21. Selection of the best arrangement for a particular
application may depend on the overall system-wide
CSO control strategy, treatment goals, cost, and/or site
availability.
Off-Line, Stand Alone
With this arrangement (Figure 4-21 a), a regulator
diverts flows in excess of the interceptor capacity to the
swirl/vortex unit. Diversion weirs and vortex valves are
examples of regulators that have been used for this
purpose. The supernatant or effluent is discharged from
the separator to the receiving water while the concentrated
foul sewer underflow is returned to the interceptor. An
upstream relief overflow, or high level weir in the swirl
unit typically is provided to protect the unit during
extreme storm flows.
Off-Line, With Storage Tank
This arrangement (Figure 4-21 b) is similar to the off-line
stand alone arrangement except that the effluent from
the swirl/vortex unit discharges to a storage tank rather
than directly to the receiving water. The foul sewer
underflow similarly is returned to the interceptor. The
potential benefit of locating a swirl/vortex unit in series
upstream of a storage tank is that the unit can capture
much of the grit and heavy solids that otherwise would
settle out in the storage tank. In addition to making
cleanup easier, removal of a fraction of the solids load
may reduce the required hypochlorite dosage if
disinfection is provided.
In-Line, Stand Alone or With Storage Tank
With this arrangement (Figures 4-21c,d), the swirl/vortex
unit acts as a regulator, throttling flow to allow only the
underflow to continue down the interceptor. The
overflow from an in-line unit passes either to the
receiving water or to a storage tank. The unit is sized
such that peak dry weather flows pass to the interceptor
without being throttled. This arrangement may be the
most appropriate for the EPA swirl concentrator, which
is intended to be primarily a flow-regulating device.
Locating the swirl device in an in-line arrangement also
will provide the best opportunity for the unit to remove
the heavier fraction of solids in the flow. One drawback
to an in-line arrangement is the wear and tear
associated with constant exposure to flow. Should
maintenance or repairs be required, provisions must be
made to handle the dry weather flow until the repairs
are completed.
Other Unit Processes
Many swirl/vortex facilities are designed with upstream
mechanically cleaned bar racks. The bar racks protect
the swirl unit from being clogged or otherwise damaged
by large objects carried in the flow, and may improve
floatables removal. For installations where the
underflow is pumped, the bar racks also help protect the
downstream pumping equipment. Swirl units have been
installed in England without upstream bar racks. At the
Alma Road swirl unit in Bristol, England, no blockages
of the foul sewer outlet were reported during the first 3
years of operation (Smisson, 1968). In-line swirl
concentrator installations that do not require pumping of
the concentrated underflow may not require upstream
76
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OFF - LINE
ON - LINE
Emergency Overflow
1
Swirl/Vortex Unit
Diversion
Weir
9
Storage/Sedimentation
Tank Overflow
Storage/Sedimentation
Tank
9.
Figure 4-21. Common layouts for swirl/vortex Installations (Pisano, 1990).
bar racks, as flow regulators are not provided with
upstream coarse screening.
Performance Considerations
As noted above, the performance of swirl units is
influenced primarily by solids settling characteristics
and flow rate, and the apparent performance also may
be affected by sampling techniques. Performance data
for swirl units is relatively scarce, due in part to the
relatively small number of installations in the United
States, and also to the difficulty in obtaining data from
existing units. Performance evaluations and monitoring
of CSO control facilities present a number of logistical
problems due to the intermittent nature of facility
operation. Sampling and analysis crews must be able
to mobilize on short notice, at odd hours, and for
uncertain durations in order to obtain the types of data
necessary to evaluate performance. Even with
automatic sampling devices, supervision is required to
ensure that the sampling runs smoothly. In addition, as
noted above, sampling systems must be designed and
installed so as to obtain a true representative sample of
the solids distribution in the influent flow. Samples that
do not include an appropriate fraction of the bedload
may result in underestimation of the influent solids load,
which may in turn result in underestimation of the solids
removal performance.
The definition of performance for swirl units is not
uniform. Researchers have distinguished between the
removals attributable to solids separation, and the
removals attributable to the function of the unit as a
regulator. In a performance evaluation of the Washington,
D.C., Northeast Boundary (NEB) swirl facility (O'Brien
& Gere, Inc., 1992), the overall removal efficiency was
determined from the ratio of influent to effluent mass
loadings. The removal efficiency due to flow diversion
was estimated by assuming that the foul sewer solids
concentration equalled the influent solids concentration.
The theoretical effluent load due to flow diversion then
was obtained by subtracting the assumed foul sewer
load from the influent load. The theoretical removal rate
due to flow diversion was then calculated:
removal =
(influent load) - (theoretical effluent load)
influent load
The removal efficiency due to solids separation then
was estimated by the difference between the actual
overall removal efficiency and the theoretical removal
rate due to flow diversion.
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Evaluation of the performance of a Storm King
hydrodynamic separator in Walsall, England (Hedges,
1992), identified a treatment factor e, such that
e =
(foul sewer load/influent load)
(foul sewer flow/influent flow)
If a device provided no treatment, but simply divided the
flow, then e = 1. If some degree of separation and
concentration occurred, then e would be greater than 1.
Design Details
The interior features of swirl units vary among the three
general types: EPA swirl concentrators, Fluidsep vortex
separators, and Storm King hydrodynamic separators.
In addition, since the technology has been evolving,
variation exists in the design details among currently
operating swirl devices of the same general type. For
the EPA swirl concentrator, the dimensions and orientation
of troughs, baffles, and other details are developed from
the basic sizing dimensions "D1" and "D2" (inlet
diameter and vessel diameter) in accordance with
design procedures (U.S. EPA, 1982). For the Fluidsep
and Storm King devices, sizing criteria for weirs, baffles,
and other details are not available in the literature.
Table 4-9 compares the key features of the three
common types of separator units (refer also to
Figures 4-14 through 4-16).
Chamber Construction
Concrete is the most common material of construction
for the swirl chambers, although stainless steel and
epoxy-coated carbon steel also have been used.
Trowelled finishes and/or epoxy wall coatings have
been suggested to minimize clinging of material to the
chamber wall (U.S. EPA, 1984). The performance study
of the Washington, D.C., NEB swirl facility recommended
providing smoother coatings for the floors and walls,
and rounding off the edges of the floor gutters to reduce
turbulence. This study also recommended increasing
the floor slope to minimize the deposit of solids on the
floor (O'Brien & Gere, Inc., 1992). Examples of
chamber construction are presented in Table 4-10.
Interior Baffle/Cone Construction
Interior baffles, flow deflectors, and similar components
typically are constructed of carbon or stainless steel.
Corrosion in the Lancaster, Pennsylvania, EPA swirl,
prompted a recommendation to use stainless steel
(U.S. EPA, 1984). A performance study of the
Washington, D.C., NEB swirl facility presented the
following recommendations regarding interior hardware
for EPA swirls (O'Brien & Gere, Inc., 1992):
• Extend the floatables weir so that the weir skirt is
directly over the entrance flow deflector baffle (as
indicated in U.S. EPA, 1982). This adjustment creates
a more continuous "inner wall" of water, and
therefore, a more quiescent inner settling zone.
• Locate weir plate hangers on the inside face of the
weir to reduce turbulence.
• Provide a continuous, smooth overflow weir plate and
skirt to reduce turbulence.
• For multiple swirl units, set weirs to provide an even
flow split at the most typical flow rates, rather than
the peak design flow.
When a Storm King hydrodynamic separator was
installed for the Walsall Metropolitan Borough in
England, the interior base cone was constructed of light
gauge stainless steel and served as a permanent form
for concrete. The vessel top, including dip plate, top
baffle plate, outlet chamber and channel, and floatables
trap, was fabricated in sections, then bolted together at
the site. A concrete platform ring cast into the vessel
body supported the top assembly (H.I.L. Technology,
Inc., undated).
Piping/Flow Control
The inlet pipe to a swirl unit should be laid at as shallow
a slope as possible to minimize turbulence, yet maintain
self-cleaning velocities. EPA recommends sizing the
foul outlet for three-times DWF, and notes that it is
difficult to size the underflow pipe to act as a throttle.
The use of a sluice gate, motor-operated gate, or vortex
valve on the foul sewer discharge is recommended to
control the underflow (U.S. EPA, 1982). An underflow
pipe sized to throttle flow is of smaller diameter and
more prone to clogging. The level of control required for
the underflow is governed by local hydraulic conditions
and downstream interceptor capacity.
At the Lancaster, Pennsylvania, and West Roxbury,
Massachusetts, EPA swirls, the foul outlets are
controlled by vortex valves, located away from the swirl
chamber. Sediments observed in the lower-velocity flow
upstream of the vortex valves prompted a suggestion
that the vortex valves be cast into the base of the swirl
unit (U.S. EPA, 1984).
At the Decatur, Illinois, McKinley Avenue Swirl Facility
(BGMA and CMT, 1977), a vortex valve on the
interceptor acts as a regulator, allowing dry weather
flows to pass uninhibited, but throttling flows greater
than the interceptor design capacity. The throttled flows
back up and pass over a weir to the McKinley Avenue
facility. During startup, adjustments to the overflow weir
were required to develop the design head on the vortex
valve. Once the proper head was developed, the
discharge from the vortex valve followed its design
curve fairly well. During an 11-month performance
study, the vortex valve did not become plugged.
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Table 4-9. Comparison of Design Details for Swirl/Vortex Devices
EPA Swirl Concentrator Fluldsep Vortex Separator
Storm King Hydrodynamic Separator
Floor
Flat, or less than 2% slope; curved
gutters to direct solids and DWF to
foul sewer outlet
Foul sewer outlet Asymmetric; orifice offset from center
of rotation of swirl
Influent entrance Located at floor of unit; features flow
deflector, to cause first revolution flow
to be deflected inwards, minimizing
turbulence and creating interior
rotating water mass
Treated effluent Past vertical, cylindrical scum baffle,
exit then over vertical, cylindrical weir onto
horizontal clear water plate, leading to
central drop shaft; shaft passes down
through center of unit, discharging
below bottom of vessel
Baffles and flow Series of vertical baffles at surface,
directing features perpendicular to rotational flow,
dampens free vortex formation -
influent flow deflector, effluent weir
scum baffle
Roatables removal Vertical flow deflector extends across
top of unit, directing floating material
to a channel; channel crosses weir
plate to vertical vortex cylinder at wall
of overflow downshaft; vortex draws
material beneath weir plate, where it
Is dispersed and contained under weir
plate and inside weir skirt; when
storm flow subsides, ftoatables are
drawn down and exit through the foul
outlet
Underflow control
Difficult to size underflow pipe to act
as throttle; suggest use of gate or
vortex valve to control underflow
Floor sloped 4-6%;
smooth; no gutters
Conical outlet in exact alignment
with center of rotation
Located at floor; no flow deflector
provided
Flow passes up through annular
opening between vertical, cylindrical
scum baffle, and cone shaped
baffle; flow passes over roof of
vessel to outlet
No baffles to dampen vortex; Inner,
cone-shaped baffle deflects
downward lighter particles carried
up by secondary currents;
cylindrical scum baffle.
Vertical, cylindrical floatables skirt
creates annulus with cone baffle;
floatables retained behind skirt, and
removed out foul sewer as flows
subside
Vortex action at underflow draw off
throttles underflow, or vortex valve
can be used
Roor sloped; no radial gutters
Annular trough along base of interior
cone, halfway between wall and
center of unit
Located at mid-depth of unit; no flow
deflector
Flow passes up through annulus
between horizontal, circular baffle
plate, and cylindrical, vertical dip
plate; flow passes over top of circular
baffle to outlet
Horizontal, circular top baffle;
cylindrical dip plate located at shear
zone; interior cone along centroidal
rotational axis
Floatables captured behind dip plate;
manually removed, or passed to
WWTP when water level subsides
Use vortex valve to control underflow
Table 4-10. Examples of Swirl/Vortex Chamber Construction
Location Swirl Type Diameter (ft) Chamber Construction
Walsall Metropolitan
Borough
Clevedon, Avon
Reference
Lancaster, PA
West Roxbury, MA
Decatur, IL
(McKinley Ave.)
Washington, DC
Presque Isle, ME
Burlington, VT
Decatur, IL
(7th Ward)
EPA Swirl
EPA Swirl
EPA Swirl
EPA Swirl
EPA Swirl
Fluidsep Vortex
Fluidsep Vortex
24
12
25
57
18.5
40
44
Concrete
Carbon steel with epoxy coating
Concrete
Concrete
Stainless steel plate inside concrete shell
Concrete (proposed)
Concrete
U.S. EPA, 1984
U.S. EPA, 1984
BGMA and CMT, 1977
O'Brien & Gere, Inc., 1992
WPCF, 1989
Ganley and McCarthur,
1991
Wilcoxon and Hunsinger,
1991
Storm King
Storm King
17.3 Concrete w/stainless steel base cone
19.7 Concrete
H.I.L. Technology, Inc.,
undated
H.I.L. Technology, Inc.,
1992
79
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From the regulator, flow to the facility passes to a screen
house. A 54-in inlet pipe from the mechanically cleaned
bar screens to the swirl unit was installed at a "minimal"
slope. During operation, solids were deposited in the
bar rack channels and the inlet pipe. During cleanup
following a storm, these solids had to be manually
flushed down through the swirl unit. Increasing the slope
of the inlet pipe would minimize solids deposition in the
pipe, and facilitate cleanup of the screening chamber.
At the Washington, DC, NEB swirl facility (O'Brien &
Gere, Inc., 1992), inflatable dams serve as regulators,
diverting dry and wet weather flows to the facility. In wet
weather, pressure in the dams is increased to divert
more flow to the facility and to optimize in-system
storage. The dams also can be deflated to relieve
upstream flooding and/or excessive flow to the facility.
From the dams, flow passes through mechanically
cleaned bar racks, then to the three 57-foot diameter
EPA swirl concentrators.
During operation of the NEB swirls, air periodically
bubbled up through the effluent downshaft. The release
of this entrained air could possibly reduce the capacity
in the downshaft and cause premature surcharging of
the overflow weir. Vents consisting of four vertical pipes
were installed in the downshaft of one of the units, and
were effective in eliminating this problem. The foul
sewer underflow was pumped to the interceptor using
torque-flow pumps.
For the Storm King hydrodynamic separator installed for
the Walsall Metropolitan Borough, England, the foul
sewer underflow is controlled by a vortex valve (H.I.L.
Technology, Inc., undated).
The Fluidsep vortex separator system in Saginaw,
Michigan, captures "first flush" flows by utilizing
in-system storage and then filling two retention tanks
having vortex throttles to permit continuous drainage.
When the "first flush" storage capacity is filled, pumped
discharge to vortex solids separators commences.
Pumped underflow from the separators is detained by
a third tank, while treated overflow enters a fourth tank.
The continuous drainage from the two retention tanks
permits optimal use of the separators and treatment
tanks, particularly for long intermittent storms (Pisano,
1989).
Spray Wash Systems
Automatic spray wash systems reduce the manual
cleanup effort following activation of a swirl unit. The
potential benefit of an automatic wash system is greater
for multiple-unit installations. EPA suggests installing
two spray headers in each swirl unit: one under the
horizontal circular weir plate, and one along the wall of
the chamber, above the maximum water level. The
hydraulic pressure on the underside of the circular weir
plate may cause material to stick (U.S. EPA, 1982).
Operating experience has yielded mixed results with
respect to spray wash systems. The Lancaster,
Pennsylvania, EPA swirl was designed with two spray
headers, in accordance with EPA recommendations
(U.S. EPA, 1982). During a performance study, material
hung up on the spray header, and a manual wash down
system using hoses was suggested (U.S. EPA, 1984).
The West Roxbury EPA swirl had no spray wash system
or problems regarding clinging material (U.S. EPA,
1984). The Decatur McKinley Avenue EPA swirl does
not have a spray header system. Washdown is
accomplished manually using hoses from adjacent
yard hydrants. A performance study noted that post-
operation cleanup was time consuming, and suggested
installing an automatic spray system in the swirl unit
(BGMA and CMT, 1977). The EPA swirl units at the
Washington, DC, NEB facility were provided with spray
headers around the walls of the chambers. The spray
systems worked well flushing material from the walls,
but were ineffective at moving material across the floor
of the units and into the gutters (O'Brien & Gere, Inc.,
1992). The Fluidsep vortex separators installed at the
Decatur Lincoln Park and 7th Ward Facilities do not
feature automatic spray wash systems. As with the
McKinley Avenue facility, washdown of these units is
accomplished using hoses from adjacent yard hydrants
(Drake and Hunsinger, 1990).
Structural Covering
A roof over a swirl unit is not required for functional
purposes, but can be beneficial for safety and
aesthetics purposes. If a roof is provided, a manhole
should be provided over the floatables vortex cylinder
to allow rodding of the floatables trap and foul sewer
outlet (U.S. EPA, 1982). Walk-in access is most
convenient and safe for removal of large floatables. An
inspection walkway should be provided around the
perimeter of the vessel to allow access to the weir and
scum plate.
A domed cover, grating, and open tank were each
considered in the design of the McKinley Avenue EPA
swirl. The advantage of a dome or grate is that
personnel, especially intruders, are protected from
falling in. The disadvantage of a dome (or other type of
solid roof) is that it creates a confined space, requiring
ventilation, lighting, and appropriate safety procedures
before entering the unit for maintenance. An open-grate
covering eliminates these concerns, but also makes
maintenance more cumbersome by restricting access.
In the end, the McKinley Avenue swirl was constructed
without a roof or grating. These types of considerations,
including safety, maintenance access, and cost of
80
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appurtenances such as lighting and ventilation, should
be evaluated on a site-specific basis.
System Controls and Operation
One of the advantages of swirl/vortex units is that they
have no moving parts, and thus operation of the unit
itself is governed solely by hydraulics. However, upstream
regulators, mechanically cleaned bar screens, upstream
or downstream pumping systems, and disinfection
systems that may be associated with the swirl/vortex
units typically are provided with some degree of
automatic control to allow unattended startup and
operation. Float switches or bubble tubes detect storm
flows through rising water levels. These devices
typically activate the mechanically cleaned bar screens
and the disinfection system, if provided.
Some dewatering and washdown operations are
designed for automatic control, while others are designed
for manual operation. As an example, automatic spray
wash systems could be activated on dropping water
levels in the swirl unit. Refer to the sections on
In-System Controls/ln-System Storage, Coarse Screening,
and Disinfection for additional details on the control
strategies for these operations.
Process Variations
EPA recommends procedures for modifying the basic
swirl design to serve as degritter, primary separator (for
sewage), and for treatment of erosion runoff (U.S. EPA,
1982). In Rochester, New York, a pilot program
evaluated the use of a degritter and primary separator
for treating combined sewage. Flow passed through the
degritter first, then the primary separator. Tests were run
without chemical addition, with polymer, with polymer
and alum, and with polymer, alum, and phosphorus
spikes. Chemical addition did not enhance performance.
The authors believe that the mode of chemical addition
was responsible for the lack of improvement.
Inadequate velocity gradients and/or contact time due
to in-line mixing of chemicals may not have provided
efficient floe development (U.S. EPA, 1979a).
Other studies demonstrate the ability of swirl degritters
designed in accordance with EPA procedures to meet
the projected removal rates for grit-sized particles (U.S.
EPA 1974b, 1977c, 1981). These devices may be
effective as off-line devices at treating swirl concentrator
underflows or CSOs with particularly high fractions of
grit-sized particles. The major difference between a
swirl concentrator and a swirl degritter is that the
degritter features a conical bottom hopper for collecting
the settled grit and delivering it to a separate grit
washing system. The swirl degritter has no capability to
regulate flow, as the bottom outlet is solely intended for
the removal of the grit.
Disinfection
Disinfection of overflows is a common goal of CSO
control strategies. Since liquid sodium hypochlorite is
the most commonly selected means for disinfecting
CSOs, this section focuses on the design of liquid
sodium hypochlorite systems. Alternative technologies
such as ultraviolet radiation, ozone, gaseous chlorine,
and chlorine dioxide are identified in Chapter 2 and
addressed in the literature (White, 1992; WPCF, 1986;
U.S. EPA, 1986; Chlorine Institute), but are not presented
in detail in this manual. Dechlorination is discussed at
the end of this section.
Process Theory
Disinfection effectiveness is measured in terms of
reduction in bacterial concentration. The overall chlorine
dose required to achieve a given bacterial kill is the sum
of the chlorine demand and the chlorine residual. Since
a number of substances found in CSOs can exert a
chlorine demand in addition to the demand created by
bacteria, the actual chlorine demand of the combined
wastewater to be treated is best assessed using actual
laboratory studies or pilot tests.
The expected change in bacterial concentration as a
function of chlorine residual and contact time can be
estimated using the Collins model, which was originally
developed from pilot studies on primary effluent (White,
1992).
y, = y0 (1 + 0.23 Ct)'3
where:
yt = bacterial concentration after time t (MPN/100 ml)
y0 = original bacterial concentration (MPN/100 ml)
C = chlorine residual concentration after time t (mg/l)
t = contact time (min)
This equation, however, does not accurately reflect
bacterial kills at low values of Ct. A modified model
proposed the following relationships, which are
illustrated graphically in Figure 4-22 (White, 1992):
y,/y0 = 1.0 for Ct b
b = X intercept when yt/y0 = 1
n = slope of regression line
This model requires laboratory or pilot data to define the
relationship between y/y0 and Ct. The values of b and
n in the equation are obtained from a regression line
through the data points.
The relationships described by the Collins model
require thorough mixing and ideal plug flow during the
contact time. Since the disinfection capability of chlorine
species depends on physical contact between the
chlorine-containing molecules and the bacteria, the
81
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Log 1=0
Regression
Curve
-Lag Time-*1
Logb->i
-n = Slope
LogCT
CT-"
Arithmetic Plot of Y7VO = —
Rgure 4-22. Graphical representations of log Yt/Y0 versus
log CT (White, 1992).
effectiveness of the dispersion of the chlorine solution
into the flow can have a significant impact on the overall
disinfection efficiency.
Adequate mixing is important to ensure dispersion of
the chlorine solution in the flow. The intensity of mixing
can be defined by the velocity gradient, G, as follows
(White, 1992):
G =
where:
G = mean velocity gradient (sec"1)
P = power requirement (ft x Ib/sec)
H = absolute fluid viscosity (Ib x sec/ft2)
V = mixing chamber volume (ft3)
For mixing by diffuser, the power requirement, P, can be
expressed as:
P = (Q x h)/(3,960 x erf)
where:
Q = flow through diffuser (gpm)
h = headless in diffuser (ft)
eff = efficiency (assume = 1 .0)
An alternative method for computing the velocity
gradient in open channels (U.S. EPA, 1973a) is:
where:
G = velocity gradient (sec"1)
v = velocity (ft/sec)
s = slope of channel (ft/ft)
u. = absolute viscosity (cp)
The parameter "GT, equal to the product of the velocity
gradient and the contact time while under the influence
of mixing, has been identified in pilot studies as a key
to disinfection efficiency at low contact times. It has
been suggested that providing turbulence during the
contact time improves the level of bacterial kill (U.S.
EPA, 1973a, 1979b).
One of the challenges to disinfecting CSOs is the limited
contact time typically available at CSO control facilities.
The limited detention time can be particularly
problematic due to the higher solids concentrations in
CSOs compared with secondary POTW effluent. The
solids can shield the bacteria from exposure to the
disinfecting agent, reducing the efficiency of the
disinfection process. Separate plug-flow contact
chambers generally are not provided at CSO facilities
due to space and/or cost constraints. Contact time for
disinfection commonly is limited to the detention time in
the tanks (for a storage/sedimentation facility) and/or in
the outfall pipe. A number of studies published in the
1970s investigated means for achieving high bacterial
kills at lower contact times, using increased mixing
intensity, increased disinfectant dosage, chemicals
having a higher oxidation rate than chlorine, or a
combination thereof (Crane Co., 1970; Geisser and
Garver, 1977; Tift et al., 1977; U.S. EPA, 1973a,b,
1974c, 1975d, 1976, 1979a,b). These methods are
generally referred to as "high-rate disinfection."
There is no clear definition as to what constitutes a
"high-rate" disinfectant dosage or a "high-rate" value for
GT, other than those values for which acceptable
reduction in bacteria concentrations are achieved at
detention times less than the conventional values of 15
to 30 minutes. For example, one study on high-rate
disinfection produced the relationships between fecal
coliform reduction, GT, and chlorine dosage presented
in Figure 4-23 (U.S. EPA, 1979b). AGT of 10,000 with
a detention time of 1 minute would be considered a
high-rate application. Under these conditions, as
indicated in Figure 4-23, 99.99 percent (4-log) fecal
coliform reductions are achieved with a chlorine dosage
of approximately 5 mg/l. If the velocity gradient G were
reduced, a corresponding increase in chlorine dosage
is required to achieve the same level of bacterial kill at
the same 1-minute detention time. Another study
reported approximately 99.99 percent total coliform
reduction with a 10 mg/l chlorine dosage and a GT of
approximately 5,000 (2-minute detention), with G =
40 sec"1 (U.S. EPA, 1973a).
When using bench-scale or pilot studies to assess the
chlorine demand of the flow to be treated, consideration
should be given to the principles of high rate
disinfection, particularly with regard to the anticipated
available contact time, and the effects of mixing and,
turbulence of projected required dosages.
82
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cc
o
O
O
OJ
U-
LL
2
o
Q
LU
CC
T=1 Win
SS = 200 mg/l
FC = 500,000 counts/100ml
CI2 Dose, as shown, mg/l
pH = 6.8
CI2 Dose
NOTE: All Syracuse Data Included
100,000
Figure 4-23. Relationship between GT and bacterial kill (U.S. EPA, 1979b).
Process Design
Sizing of disinfection equipment is based on the
required dose rate and expected flow. In some states,
the permitting agency may specify a maximum required
dosing capacity, such as 25 mg/l at peak flow. In other
states, the permitting agency may specify the maximum
allowable bacteria concentration, such as 1,000
MPN/100 ml fecal coliform. Where a maximum coliform
bacteria discharge limit is provided, some knowledge of
influent flow bacteria concentration and chlorine
demand is required to estimate the required dosage.
Given an average influent bacteria concentration, y0, a
value for the required effluent bacteria concentration, yt,
and a specified minimum contact time (15 min at peak
flow is typical [Metcalf & Eddy 1991 a]), the required
residual can be estimated from the Collins model. For
example, typical primary effluent might have an initial
bacteria concentration y0 = 38x106 MPN/100 ml (White,
1992). If the discharge requirement is 1,000 MPN/100
ml, then:
1,000 = 38x106 x (1 + 0.23 Ct)'3
where:
Ct= 142
For t = 15 min, C = 9.47 mg/l
Allowing for an immediate (3-5 minutes) demand of
8 mg/l, and a die away demand during contact time
(25+ minutes) of 1 mg/l, then the estimated dosage for
the above example would be about 19 mg/l.
As noted above, final sizing of the disinfectant dosing
system should be based on pilot studies the actual
combined flow to be treated. If a storage/sedimentation
tank is to provide contact time, actual required dosages
may be higher than predicted by models, due to the
concentration of solids in the influent combined flow.
Pilot studies also should allow evaluation of other
high-rate disinfection techniques, such as increasing
mixing intensity and using chemicals with a higher
oxidation rate than hypochlorite. Equations such as the
Collins model estimate the relationship between dosage
and contact time for a given reduction in bacterial
concentration, and therefore provide a basis for
designing pilot tests for assessing actual dosages
required.
Once the required maximum dosage is established, the
metering pump capacity can be computed based on the
estimated strength of the hypochlorite solution. The
strength of hypochlorite solution is presented as a "trade
percent," which is essentially a percent by volume,
computed as:
trade percent = -
available CI2
10
83
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The weight percent of available chlorine is the trade
percent divided by the specific gravity of the solution:
weight percent available chlorine =
trade percent
specific gravity
The specific gravity of a hypochlorite solution of a given
strength is not constant; rather, it depends on the
amount of excess sodium hydroxide the manufacturer
uses to promote stability. For example, to compute the
required metering pump capacity for a CSO facility with
a peak flow of 40 mgd, using 10 percent (trade) sodium
hypochlorite (specific gravity = 1.14) and a 15 mg/l dose
rate, let:
Qp = metering pump capacity (gph)
Cp = concentration of stock sodium hypochlorite
(mg/l)
Q( = peak effluent flow (mgd)
Cf = required sodium hypochlorite dose rate
(mg/l)
then:
Qp x
= Q, x Cf x 41 ,667 gph/mgd
Cp = % available CI2 by weight =
= 8.8% -
Cp = 8.8% x (10,000 mg/l)/1%
= 88,000 mg/l
88,000 mg/l x Qp = 15 mg/l x 40 mgd
x 4 1,667 gph/mgd
Qp = 284 gph
Process Flow
At CSO storage/treatment facilities, liquid sodium
hypochlorite usually is introduced at the upstream end
of the facility to maximize the contact time in the tanks.
Some facilities also provide a second dosing location
downstream of the storage tanks to ensure that a
desired residual is achieved. CSO control facilities may
feature upstream relief overflows to protect the facility
from flooding during extreme events. These remote
overflows also may be suitable locations for
hypochlorite addition. They can be served by piping
from chemical storage and feed equipment located at
the main CSO facility, and controlled by remote
telemetry. Table 4-11 presents examples of hypochlorite
systems currently in use at CSO control facilities. A
schematic of a typical hypochlorite system is presented
in Figure 4-24.
In addition to effluent disinfection, other uses of
hypochlorite at CSO facilities may include:
• Odor control scrubbers
• Sludge pipe disinfection
• Spraywash system
These applications are discussed earlier in this chapter.
Design Details
Components of typical hypochlorite systems include:
• Storage tank
• Metering pumps
• Dilution water supply
• Piping and valves
• Diffuser
• Chlorine residual analyzer
These components are discussed below. Additional
information is available in the literature (White, 1992;
WPCF, 1986; U.S. EPA, 1986).
Storage Tank
Sodium hypochlorite is delivered to a CSO facility in
liquid form by truck, while smaller quantities can be
delivered in 55-gallon drums. Sizing considerations for
bulk storage tanks include projected usage of chemical,
including auxiliary uses such as odor control, delivered
strength, and rate of decomposition of hypochlorite.
Hypochlorite is delivered with solution strengths ranging
from 10 to 15 percent. Higher strength solutions require
less storage volume, but deteriorate more rapidly. A
10 percent solution is most economical (White, 1992).
Chemical storage tanks, whether above or below
ground, must meet the applicable federal, state, and
local regulations and fire codes.
Metering Pumps
Because of the relatively low metering pump capacities
required for hypochlorite dosing, positive-displacement
diaphragm pumps commonly are used for this purpose.
Pumps can be controlled by variable speed drives, by
stroke length positioners, or both. Pacing of the
metering pumps in proportion to flow is a common
control strategy. Dosing locations at the influent end of
the facility should be paced by the influent flow, while
dosing locations at the effluent end of the facility should
be paced by the effluent flow (White, 1992). The need
for additional control of dosage based on effluent
residual may depend on permit requirements and
potential water quality impacts of a high residual.
Dilution Water
Since sodium hypochlorite solution dose rates are low
enough to be expressed in terms of gallons per hour,
dilution water is provided as a carrier fluid to ensure a
reasonable flow velocity. Hypochlorite dilution water
typically is controlled by a solenoid valve and rotameter.
The valve opens when the hypochlorite pumps are
activated. The rotameter is set to provide sufficient flow
to maintain a minimum 2 ft/sec velocity in the piping to
84
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Table 4-11. Examples of Sodium Hypochlorlte Disinfection Systems at CSO Control Facilities
Facility Location Facility Type Dosing Location(s) Startup Control Dose Rate Control Other Details
Newport, Rl
Washington Street
Storage/treatment Influent channel;
tank effluent
collection box
Influent channel
mercury float switch
Pump speed paced by
either influent or
effluent flow; manual
stroke adjustment
Manual dose control
for remote upstream
overflow
Newport, Rl
Wellington Avenue
Saginaw, Ml
Hancock Street
Oakport, CA
MWRA
Prison Point
Microstrainer
Storage/treatment
Storage/treatment
Storage/treatment
MWRA
Cottage Farm
Effluent forcemain
Upstream of tanks
Influent lift station
wetwell, near pump
suction
Influent channel
upstream of tanks
Each of four storm
pump suctions,
downstream of
tanks
Storage/treatment Influent channel
MWRA Coarse screening/
Somerville Marginal disinfection
Pretreatment Facility
MWRA Coarse screening/
Constitution Beach disinfection
MWRA
Fox Point
Coarse screening/
disinfection
Upstream of
influent sluice
gates in inlet
channel
Flow channel
through facility
Flow channel
through facility;
dewatering pump
discharge
Lead stormwater
pump activation
Influent pump
activation
Opening of tank
inlet gate
Storm pump
activation
Influent pump
activation
Capacitance probe
in drainage channel
upstream of facility
opens inlet gates
and starts
equipment on high
level
Start/stop on high/
low water level in
facility
Start/stop controlled
by ultrasonic depth/
velocity flowmeter
in flow channel
Pump stroke controlled
by effluent flow
Pump speed paced on
facility flow
Pump speed controlled
by flow; pump stroke
controlled manually
Pump speed controlled
by flow; pump stroke
controlled by CRA
CRA located halfway
down influent channel
controls dose to
maintain 1 ppm
residual
Initial high first flush
dose provided for first
10 minutes, then
control switched to
pacing on effluent flow
Pump speed controlled
by CRA output;
manual stroke
adjustment
Speed controlled by
facility flow; stroke
controlled by CRA
Progressive cavity
pumps provided for
dosing caused
operational problems
Rotodip feeder
provided for
hypochlorfte dosing
the diffuser. A manual bypass should be provided
around the solenoid and rotameter.
Piping
A typical sodium hypochlorite piping system includes:
• Calibration standpipe, for checking pump capacity.
• Pulsation dampeners, to absorb the impulses from
the diaphragm metering pumps and provide smoother
flow.
• Back pressure valves, to ensure that the check valves
on the metering pump discharges seat properly.
Where chemical storage and feed equipment is located
at a higher elevation than the dosing locations, siphon
conditions may develop that could prematurely drain the
contents of a storage tank. Backpressure valves can
prevent the siphon from developing, depending on the
valve setting and the relative elevations of the fluid
surface in the storage tanks and the valve. Some
installations also feature motor-operated valves on the
drawoff from the tank to prevent the contents of the tank
from siphoning or leaking between storms.
PVC or CPVC piping with solvent-welded joints
commonly is used for hypochlorite systems. Solvent-
welded joints are less susceptible to leakage as compared
with screw-type joints (White, 1992).
Diffuser
The diffuser disperses the sodium hypochlorite solution
into the flow. Proper mixing is critical to disinfection
efficiency, since chlorine species must come in contact
with bacteria to exert the bactericidal effects. A variety
85
-------
Hypochlorite Metering
Pump No.'
2" CLS Line
to Effluent
Collection
Solenoid
Valve -v
1 1/2" SW,
Line J ^
«-. , . *
Rile
Venft,
(Sodium
Stors
Overflow! ^
t >.
^^ Rotameter-j
J*C|I^ \
(0 - 20 GPM)
(0-20GPM)
_. [—Calibration
+ Stand Pipe
J — I . ? r Pulsation
.., •. \ Ed fDampener
Hypochlorite i EJ J=±
ige Tank / ^ j .
-^ t t T
1 Jt. ' '
n ^ r Check
Ball Valve -> \ Valve|
H]^ J(0- 1 0GPM)
prj, _ l—Qiobg valve
L Gate Valve
rL / T. T
T f f ^i
n
oTfloV
In-Line — '
Static Mixer
Hypochlorite Metering
Pump No. 2-7 ^
1 rZ^ ?
_,*I- JH-JSU
Connection
Odor Contrc
System Met
Pumps -v
Back Pressure \
Drain-7 Valve ^H J
1? 1 Tl I
^i. XJlJ
1 — Hvoochlorite M
Hxr
Frc
>l
erir
BtP
BOX
m
ig 1"CL
to 10"
Water
i
:
ri4.HS 2" CLS Line
to Influent
Sewer
,_,H_.,r 2" CLS Line
* Diversion
Manhole
3 Line
Flushing
Line
«
... ,1" niSI inn
Lixtoudol — '•- 1-1 ~>to^Qu
Dewatering
rina Line
Pump No. 3
Figure 4-24. Schematic of typical liquid sodium hypochlorite system (Metcalf & Eddy, Inc., 1991b).
of diffuser types and configurations are commonly used,
including in-channel and in-pipe arrangements. One
common arrangement is to suspend a section of
perforated PVC pipe across a channel or pipe. The exit
velocity of the chlorine solution from across-channel
perforated pipes should be approximately 25-30 ft/sec
(White, 1992). Examples of diffuser types are presented
in Figure 4-25. The reader is referred to the literature
(White, 1992) for additional information on diffusers.
Chlorine Residual Analyzer (CRA)
The CRA is useful as part of a control loop for
hypochlorite feed and for residual monitoring purposes.
Some discharge permits specify a minimum effluent
residual, where residual has been correlated with a
given level of disinfection. Some discharge permits
specify a maximum residual, where the concern is the
toxic effects of chlorine on the receiving waters. For
chlorine dose control based on chlorine residual, the
residual sample point should be located immediately
downstream of the chlorine application point, so that the
sample is taken within 15-30 seconds of application.
The total loop time between application of chlorine and
response of the metering pump controls to the chlorine
residual signal should range from 2 to 5 minutes (White,
1992). If the CRA is located near the sample point, the
sample piping may be coiled or looped to create the
minimum detention time. CRAs should not be located,
however, in areas exposed to explosion hazards.
Although the CRA could be provided with the
appropriately rated enclosure, this enclosure would
have to be opened periodically to replace the stock
chemicals and provide routine service. For chlorine
residual monitoring and reporting purposes, the sample
point should be located as far downstream of the
dosage point as practical.
The design of a CRA installation for a CSO facility must
consider both the higher effluent solids present in CSO
effluent as compared with secondary POTW effluent,
and the intermittent operation of the facility. The higher
solids concentration suggests that the sample may
need to be filtered to minimize the potential for clogging
or other interference with the CRA operation. In
addition, bare electrode cell CRAs may be more
appropriate for CSOs than permeable membrane cells.
Although the bare electrode cells use more chemical
than the permeable membrane cells, the bare electrode
cells may be less sensitive to higher solids concentrations.
During periods between storms, when the CSO facility
is inactive, clean water should be continually flushed
through the CRA to prevent the buildup of slime or algae
and to keep the analyzer cell fresh. A flow of 0.2 to
0.5 gpm should be sufficient for this purpose. The
86
-------
Injector
Chlorine
Solution
Chlorine
Solution
Injector
Chlorine
Solution
(a)
4 in Chlorine
Solution Piping
4 in Perforated PVC
Continuous Weld
Approx. 1/3
Pipe Diameter
Flexible Connection
(Rubber or Teflon)
{fl—Blind Flange
6 in
Steel Sleeve
6 in
Steel Insert Connection
(c)
CI,Gas Jb
~~ U Injector
Injector Water Supply
CI.Gas
Injector If —
Water Supply
Chlorine Solution
3 or 4 in
Chlorine Solution Line
PVC or Other Material
Plan
Water Level
Hose
Clamps
(Typical)
Typical
Diffuser
Section
Flexible Hose
1.5 in
(e)
Minimum Submergence, 1.5 ft
Typical Diffuser
Nozzle for
1.5 in Hose
(0
Figure 4-25. Typical dlffusers used to Inject chlorine solution: (a) single Injector for small pipe, (b) dual Injector for small pipe,
(c) across-the-pipe diffuser for pipes larger than 3 ft In diameter, (d) diffuser system for large conduits, (e) single
across-the-channel diffuser, and (f) typical hanging-nozzle-type chlorine diffuser for open channels (Metcalf &
Eddy, Inc., 1991 a).
87
-------
0.5 gpm should be sufficient for this purpose. The
chemical feed in the analyzer can be deactivated
between storms, but the electronics should remain
powered. When the facility activates during a storm
event, the control system should ignore the signal from
the CRA until the chemical feed system in the CRA is
reactivated, and the CRA starts reading an actual
residual.
System Controls and Operation
A common method for controlling the hypochlorite dose
rate is through pacing the metering pump speed or
stroke in proportion to facility flow. Some facilities
provide compound loops, where pump speed is paced
by flow and stroke is controlled by chlorine residual.
Aspects of dosage control based on plant flow and
chlorine residual were described earlier. Since CSO
control facilities typically are not staffed on a full-time
basis, some means of automatic activation usually is
required. Simpler control systems are considered more
reliable for facilities without a staff. Examples of typical
startup controls include mercury float switches in the
influent channel, and startup on activation of influent
pumps.
One of the disadvantages of hypochlorite disinfection
systems is that sodium hypochlorite solutions are
unstable, and the strength of a hypochlorite solution
decreases over time as the hypochlorite ions break
down. The actual rate of hypochlorite decay depends
on a number of factors. Decay is accelerated by
exposure to heat, light, and certain heavy metal cations,
particularly iron. Sodium hydroxide is added to
hypochlorite solutions by manufacturers to improve
stability. The decomposition rate increases with increasing
solution strength.
Routine testing of the hypochlorite strength should be
part of standard facility maintenance procedures. The
benefits of monitoring hypochlorite strength are:
• Due to the intermittent nature of CSO facility
operations, quantities of hypochlorite may remain in
the storage tank for extended periods. Since the dose
rate depends on the solution strength, overestimation
of the solution strength may result in underdosing the
flow.
• Over a period of time, records on the decay rate and
quantities used of various strengths of hypochlorite
will allow the operator to determine an optimum
strength and delivery schedule.
The strength of a hypochlorite solution in terms of
weight percent can be determined by making two
consecutive 1 MOO dilutions of a sample of hypochlorite,
and then analyzing for chlorine concentration. The
concentration in mg/l of the diluted sample is equal to
the concentration in weight percent of the original
solution.
Process Variations
Dechlorination
A major disadvantage of chlorine-based disinfection
systems is that the residual chlorine concentration may
have a toxic effect on the receiving waters, due either
to the free chlorine residual itself or the reaction of the
chlorine with organic compounds in the effluent. One
report on chlorine toxicity concluded that continuous
effluent concentrations of greater than 0.01 mg/l may
exert toxic effects on resistant organisms, while
concentrations greater than 0.002 mg/l may harm many
aquatic organisms. A variety of stable chloroorganic
compounds may be formed as byproducts of the
chlorination of wastewater, and certain chloroorganics
can exert toxic effects on aquatic species (White, 1992;
Brungs, 1973; Jolley, 1975). With the relatively short
contact times available at many CSO control facilities,
high residuals can be of particular concern and may
require consideration of dechlorination alternatives. Two
common means for dechlorinating treated effluent are
through application of gaseous sulfur dioxide and liquid
sodium bisulfite solution. A sulfur dioxide dosing system
requires apparatus similar to a gaseous chlorine
disinfection system, while sodium bisulfite requires
apparatus similar to a liquid sodium hypochlorite dosing
system. Since the hypochlorite disinfection systems
have been more reliable than gaseous chlorine systems
for intermittent operation in CSO control applications, a
liquid sodium bisulfate system also may be preferable
to a gaseous sulfur dioxide system for dechlorination of
treated CSOs.
Sizing of a sodium bisulfite dechlorination system is
based on the following relationships (White, 1992):
• Sodium bisulfate is usually provided as a 38 percent
solution.
• One gallon of 38 percent sodium bisulfite is
equivalent to 2.17 pounds of sulfur dioxide.
• Stoichiometrically, 0.9 parts of sulfur dioxide are
required to remove 1.0 part of chlorine; actual ratios
can be as high as 1.05 parts sulfur dioxide to 1.0 part
chlorine.
Chemical storage and feed equipment for sodium
bisulfite is identical to the equipment required for
hypochlorite systems. Sodium bisulfite storage tanks
should be located away from and be clearly
distinguished from hypochlorite tanks, since the mixing
of these two chemicals results in a violent temperature
reaction (White, 1992).
88
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National Technical Information Service
5285 Port Royal Road
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703-487-4650
EPA documents with a number that begins with 600 or
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sewer overflows—phase III. EPA/670/2-74/049. NTIS
PB-235771. King of Prussia, PA. August.
U.S. EPA. 1975a. U.S. Environmental Protection
Agency. Handbook for sewer system evaluation and
rehabilitation. EPA/625/6-91/030. Washington, DC.
90
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U.S. EPA. 1975b. U.S. Environmental Protection
Agency. Process design manual for suspended solids
removal. EPA/625/1-75/003a. NTIS PB-259147.
U.S. EPA. 1975c. U.S. Environmental Protection
Agency. Physical and settling characteristics of
particulates in stormwater and sanitary wastewater.
EPA/670/2-75/011. NTIS PB-242001.
U.S. EPA. 1975d. U.S. Environmental Protection
Agency. Bench-scale high-rate disinfection of
combined sewer overflows with chlorine and chlorine
dioxide. EPA/670/2-75/021. NTIS PB-242296.
Syracuse, NY. April.
U.S. EPA. 1976. U.S. Environmental Protection Agency.
Proceedings of workshop on microorganisms in
urban stormwater (March 24,1975). EPA/600/2-76/244.
NTIS PB-263030. Edison, NJ. November.
U.S. EPA. 1977a. U.S. Environmental Protection
Agency. Cottage Farm combined sewer detention
and chlorination station, Cambridge, Massachusetts.
EPA/600/2-77/046. NTIS PB-263292.
U.S. ERA. 1977b. U.S. Environmental Protection
Agency. Urban stormwater management and
technology: update and users' guide. EPA/600/8-
77/014. NTIS PB-275654.
U.S. EPA. 1977c. U.S. Environmental Protection
Agency. Field prototype demonstration of the swirl
degritter. EPA/600/2-77/185. NTIS PB-272668.
U.S. EPA. 1979a. U.S. Environmental Protection
Agency. Combined sewer overflow abatement
program—Rochester, NY: volume II pilot plant
evaluations. EPA/600/2-79/031 b. NTIS PB80-159262.
U.S. EPA. 1979b. U.S. Environmental Protection
Agency. Disinfection/treatment of combined sewer
overflows--Syracuse, NY. EPA/600/2-79/134. NTIS
PB80-113459. Syracuse, NY. August
U.S. EPA. 1980. U.S. Environmental Protection Agency.
Urban stormwater management and technology:
case histories. EPA/600/8-80/035. NTIS PB81-
107153.
U.S. EPA. 1981. U.S. Environmental Protection Agency.
Field evaluation of a swirl degritter at Tamworth, New
South Wales, Australia. EPA/600/2-81/063. NTIS
PB81-187247.
U.S. EPA. 1982. U.S. Environmental Protection Agency.
Swirl and helical bend pollution control devices.
EPA/600/8-82/013. NTIS PB82-266172.
U.S. EPA. 1984. U.S. Environmental Protection Agency.
Swirl and helical bend regulator/concentrator for
storm and combined sewer overflow control. EPA/600/
2-84/151. NTIS PB85-102523.
U.S. EPA. 1985a. U.S. Environmental Protection
Agency. Design manual: odor and corrosion control
in sanitary sewerage systems and treatment plants.
EPA/625/1-85/018. NTIS PB88-184031.
U.S. EPA. 1985b. U.S. Environmental Protection
Agency. In-system storage control for reduction of
combined sewer overflow—Saginaw, Michigan,
executive summary. EPA/905/2-85/001 a. NTIS
PB86-144441.
U.S. EPA. 1986. U.S. Environmental Protection Agency.
Municipal wastewater disinfection design manual.
EPA/625/1-86/021. Washington, DC.
Variakojis, J., and A. Quintanilla. 1989. Facilities
planning study update supplement and summary.
The Metropolitan Water Reclamation District of
Greater Chicago.
Vitasovic, Z., R. Swarner and E. Speer. 1990. Real time
control system for CSO reduction. Water Environ.
and Tech. March.
WEF. 1992. Water Environment Federation. Design of
municipal wastewater treatment plants volumes I &
II. WEF manual of practice no. 8, ASCE manual and
report on engineering practice no. 76. 2nd ed.
Alexandria, VA: WEF, and New York, NY: American
Society of Civil Engineers.
West, M.C., T.L. Moy, and H.A. Elmendorf. 1990. Atlanta
combined sewer overflows present and future. ES
Engineering Science Technical Paper.
Westfall, D.E. 1990. Dropshafts for conveyance of
combined sewer overflows to tunnels. Presented at
the Water Pollution Control Federation Specialty
Conference Series on Control of Combined Sewer
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Wheatley, N.J. 1991. Massachusetts Water Resources
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Mining, Metallurgy, and Exploration.
White, G.C. 1992. Handbook of chlorination and
alternative disinfectants, 3rd ed. New York, NY: Van
Nostrand Reinhold.
Whittaker, B.N., and R.C. Frith. 1990. Tunneling:
design, stability and construction. London: The
Institute of Mining and Metallurgy.
Wilcoxon, N., and C.R. Hunsinger, Jr. 1991. Vortex
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WPCF. 1986. Water Pollution Control Federation.
Wastewater disinfection, manual of practice no.
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91
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WPCF. 1989. Water Pollution Control Federation. WPCF/ASCE. 1974. Joint Committee of Water Pollution
Combined sewer overflow pollution abatement, Control Federation and the American Society of Civil
manual of practice no. FD-17. Alexandria, VA. Engineers. Design and construction of sanitary and
storm sewers. WPCF Manual of Practice No. 9;
ASCE Manual and Report on Engineering Practice
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Chapters
Costs for CSO Control Technologies
This chapter summarizes information on costs associated
with construction and operation of treatment technologies
presented in this report. This information can be used
to develop preliminary budgetary estimates and provides
a basis for comparing technologies. The cost information
also can be useful in characterizing economic sensitivity
in relation to various design alternatives for a proposed
facility. Readers should consult the source documents
cited for additional detail.
The cost relationships reflect a consolidation of cost
data, some of them estimates, compiled from a number
of comprehensive cost assessments that extract data
from facility plans and from a number of individual site
studies not included in the broad-based data sets. As
noted, actual costs for CSO facilities varied considerably
for facilities of similar type and design capacity. This is
attributable to a variety of site-specific factors that
influence project costs.
The primary parameter influencing the estimated cost
for a CSO treatment facility is either design flow rate or
storage volume. For treatment-based control measures
that are designed on the basis of flow rate, costs are
related to flow rate in million gallons per day (MGD). For
storage facilities, the primary design parameter is
storage volume, or the storm size selected as a design
basis, and the cost relationship is based on storage
volume in million gallons (MG).
Sources of Cost Data
Cost estimating procedures for CSO storage and
treatment facilities have been available since the late
1970s. Updates have been developed at regular intervals.
Recent cost data for CSO storage and treatment
facilities have been developed (U.S. EPA, 1992) and
other sources of cost information exist (Field, 1990; U.S.
EPA, 1976, 1977a,b).
The cost curves developed from the literature are based
on a compilation of information extracted from actual
construction costs, estimates developed for CSO facility
plans, consultant-supplied data for specific projects,
and other published sources. The costs provided for
CSO control do not include land acquisition; engineering,
legal, fiscal, or administrative services; contingencies;
or construction loan interest. The exception to this is
screening facilities, where these cost components could
not be isolated and extracted.
Other cost estimating references, such as those
extrapolated from POTW costs (U.S. EPA, 1978,1980),
may be used to refine estimates developed from CSO
costing references, but should be used cautiously. Many
POTW costing procedures are based on average daily
flow, while CSO facilities usually are based on peak flow
or storm volume. Possible differences in the relationship
between peak and average flow and its translation to
the design surface area should be considered.
Construction Costs for CSO Controls
Figure 5-1 summarizes the relationship between
construction cost and design capacity for a range of
CSO control technologies. These cost curves are based
on recent studies that assembled information on CSO
control costs (U.S. EPA, 1992), and reflect the cost of
the basic structure and ancillary equipment (e.g.,
grates, valves, conduits). Associated pumping facilities
are included for some, but not all of the facilities. Land
acquisition and professional service costs are excluded.
The source report provides details on cost elements
included for individual sites. Local cost estimates for
these elements should be developed and added for a
more complete estimate, whenever appropriate.
Most of the cost curves display a line of best fit through
the plotted data, in addition to the plotted points showing
cost versus design size for individual sites. The linear
regression line is described by an equation of the
following form:
COST = aQb or COST = aVb
where Q is the design flow in MGD for treatment units,
and V is the storage volume in MG, and the values a
and b are unique for each line fit.
The construction cost relationships shown by Figure 5-1
are summarized by the equations in Table 5-1.
Inspection of the relative scatter of individual site values
about the regression lines in Figure 5-1 provides a
93
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si 100°
K
S 100
i 10
i
* 10
1 0.1
0.01
100
g 1
I «•'
0.01
Figure 5-1
Table 5-1.
CSO Cont
OFF-LINE STORJ
ju.g.ui }trniM4aooj»i.|j
; : I"1; i \ A
' '*•*' e'o fogf
\"l ipTO?
- A i-^**T < "1 !t
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;<» i!,i!i'n t i li!
LQE: ' '.-' ' '"-.
;.; i i dru »»(*> :ii
It 1 1 1 t t M ' mW I { II
Ij^Sh?: ! ;S
ili ! ]i!l! — c —
~ t ill
Hi ' i \W.\ i ,"l!
,Jf -
• 1 l!:
Jij!
Surface Units 1
Deep Tunnels 1
i 1 II
9.1 1 10 100 1000
Storage Volume MG
i •« i d & \ fcfc
'
l\\\" " i I "iTj 1 '" I"3T$I ii ^,^'1 ! f i-!
;, ,:,:;.:, ^,a^£rf^ : -"-':
Iil| | I | a I kit 11,1 !lj. i I , f. ,1
:|||| 1 ! lli!!! ! ! 1 11 If! 1 ! 1 l.ifE
- .. i ~ t ,.
1 1 i i i i i i i I i i i 1 1 1 1 1 1 1 1 1 ! 1 1 1 [i
1 10 100 1000
• Design Flow MQD
=5 '"I'M S'iS'HI ! i1Jk!ll-!
•;-., SEDIMENTATION t *,!*...
'-" CHEMICAL TREATMENT- ' -'
| EMR-1600 4
!-M ^J*J*^rf^*^*^]"
Y^ lJ«— * '
:ii;i """\"l 'I'1!!1 ' !
1 ! ' 3 1 f "j
* i £ i i £ 1 1 -
! | ill!1!
Sedimentation Basin 1
Wd tor Chem Feed Eqpt|
, |M , ,,,!,,, | , , | , , ,|
1 10 100 1000
Design Row MGD
. Construction costs for CSO controls.
Cost Equations for CSO Control Technology
rol Technology Cost Equation*
Storage basins 3.637 V0'828
Deep tunnels 4.982 V0'795
Swirl concentrators 0.176 Q°'611
Screens 0.072 Q0'843
Sedimentation 0.211 Q0-668
Disinfection 0.121 Q0'464
5 10 S
C 01 t Lit
SCREENS!," , - : , , ,:
^^SBp'i"!1 .'!':=
i i i 1 1 1 > i i i i < 1 1 1 1 i i i 1 1 1 1 1
1 10 100 1000
Design Flow MGD
HP
5 I0 ^J
3 i!:r
ISINFECTION;: " ! ' ' !> 1 : ] - ' }
FNB » tsaa
- ii,i~i- i ,-.,..! ~ ,"i ' =
Ll^r^^^] ' '":
! '!;'n" <-"';': ''>: -
1 10 100 1000
Design Flow MGD
Applicable
Design Range ENR Index
0.15 to 30 MG 4,800
1.8 to 2,000 MG 4,800
3 to 300 MG 4,800
0.8 to 200 MG 4,800
1 to 500 MG 4,500
1 to 200 MG 4,500
* V = volume (MG); Q = flow rate (MGD).
sense of the degree of uncertainty in the above cost
relationships. Reliable cost estimates for a particular
CSO control facility require a conventional material
take-off and application of normal costing procedures,
when facility design is sufficiently advanced to support
such an analysis.
O&M Costs for CSO Controls
Operation and maintenance costs are difficult to predict
because of the intermittent use of CSO treatment
facilities. Actual O&M costs are indicated in the data in
Figure 5-2 to be a function of the number of overflows
experienced by the facility, as well as the design capacity.
94
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The frequency at which a specific CSO facility receives
an overflow event to process depends strongly on the
capacity of the collection system it serves and varies
from one year to another, depending on the amount and
pattern of rainfall experienced.
The significant effect of the frequency of activation is
illustrated by the O&M cost curves for screening facilities,
shown in Figure 5-2. All the major components of O&M
costs, such as energy consumption, labor requirements,
residuals disposal, and equipment maintenance are a
function of the facility use. This, in turn, is a function of
the rainfall quantity and the number of wet weather
events; therefore, O&M costs are highly site specific.
The O&M curves that appear in published reports and
literature should be used cautiously.
References
When an NTIS number is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
703-487-4650
Field, R. 1990. Combined sewer overflows: control and
treatment. In: Moffa, RE., ed., Control and treatment
of combined sewer overflows. New York, NY: Van
Nostrand Reinhold.
U.S. EPA. 1976. U.S. Environmental Protection Agency.
Cost estimating manual—CSO storage-treatment.
EPA/600/2-76/286 (NTIS PB 266 359).
U.S. EPA. 1977a. U.S. Environmental Protection
Agency. Urban stormwater management and
technology—update and user's guide. EPA/600/8-
77/014 (NTIS PB 275 654).
U.S. EPA. 1977b. U.S. Environmental Protection
Agency. Nationwide evaluation of CSOs and urban
stormwater discharges—volume III: cost assessment
and impacts. EPA/600/2-77/064b (NTIS PB 266 055).
U.S. EPA. 1978. U.S. Environmental Protection Agency.
Innovative and alternative technology assessment
manual. EPA/430/09-78/009.
U.S. EPA. 1980. U.S. Environmental Protection Agency.
Treatability manual—volume IV: cost estimating.
EPA/600/8-80/042D (NTIS PB80-223084).
U.S. EPA. 1992. U.S. Environmental Protection Agency.
CSO needs survey, and in a report prepared for ERA
entitled: cost estimates for select combined sewer
overflow control technologies. Office of Water,
Municipal Support Division. Washington, DC.
W 100
3
3
o
10
[SCREENS
ENR = 4500;
10 O/F events /yr
30 O/F events/yr
10 100
Design Flow MGO
10 too
Design Flow MGO
1000
SEDIMENTATION
CHEMICAL PRECIPITATION
o
1
O
100
Design Flow MQD
Figure 5-2. O&M costs for CSO controls.
•U.S. GOVERNMENT PRINTING OFFICE: 1995-650-006/22022
95
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