x>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA 832-F-99-036
September 1999
Combined Sewer Overflow
Technology Fact Sheet
Maximization of In-line Storage
DESCRIPTION
This fact sheet describes technologies designed to
maximize the in-line storage capability of combined
sewer systems, thus reducing the frequency and/or
severity of Combined Sewer Overflows (CSOs).
Sewer systems that convey both sanitary sewage
and storm water through a single pipe are referred
to as combined sewer systems (CSSs). In dry
weather, the CSS is able to convey all flows to the
wastewater treatment facility. During periods of
heavy rainfall, however, the capacity of the CSS
may be exceeded and an untreated combination of
sewage and storm water may discharged directly
into a nearby receiving water. These overflow
instances are called Combined Sewer Overflows, or
CSOs.
CSOs contain untreated domestic, industrial, and
commercial wastes. Contaminants include
suspended solids, biochemical oxygen demand
(BOD), oils and grease, toxics, nutrients, floatables,
pathogenic microorganisms and other pollutants.
CSOs often contribute to exceedances of water
quality standards and can result in threats to public
health, aquatic species, and aquatic habitat.
Under the U.S. Environmental Protection Agency
(EPA)'s CSO Control Policy, discharge permits
issued to communities with CSSs are expected to
include "nine minimum controls." The nine
minimum controls are measures that can reduce the
magnitude, frequency and impacts of CSOs without
significant construction or expense. Maximization
of storage in the collection system is one of the nine
minimum CSO controls.
In-line storage capacity is typically obtained
through the use of control measures downstream
from a point where excess capacity exists. Typical
control measures include:
• Inspection of the collection system and removal
of obstructions.
• Maintenance, repair and replacement of tide and
control gates.
• Installation and adjustment of regulators.
• Reduction/retardation of inflows.
• Upgrade/adjustment of pumps.
• Real time monitoring.
These control methods are discussed in more detail
below:
Collection system inspection and removal of
obstructions
Collection system inspection involves investigation
of pipes and inflow points to determine where
sediments, debris, and other obstructions are
preventing the system from operating as designed.
Removal of obstructions can reduce CSO volume
by increasing the amount of storage capacity in the
system. Sediments and small obstructions can be
removed by sewer flushing; large obstructions are
often removed manually. Routine inspection
ensures that obstructions are identified and removed
quickly.
Tide and control gate maintenance, repair and
replacement
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Tide gates are designed to reduce or prevent
receiving water from flowing back into the sewer
outfall during high tide. Proper maintenance is
essential for the tide gate to function as designed.
Leaks and cracks that allow flow to pass alter
design performance and reduce available in-line
storage.
Flap style tide gates are commonly used to resist
tidal influences. These gates are typically hinged
stainless steel doors attached to the outfall, opening
and closing dependent on the relative pressure
applied by CSO flows or tidal influence.. While
simple in construction and operation, typical flap
gates are subject to fouling and sticking and require
appreciable hydraulic heads to operate against their
own weight (WEF, 1989). Commonly encountered
problems with typical flap gates include warpage,
corrosion, and the tendency to become stuck in one
position (U. S. EPA, 1993).
Elastomeric tide gates may be considered when
installing or replacing a tide gate. An elastomeric
tide gate is a positive pressure device that forces
CSO flows to open an otherwise collapsed rubber-
based valve. Elastomeric tide gates can overcome
many of the problems encountered by typical flap
gates. They are designed to open with smaller head
requirements and to close over larger debris. A
typical elastomeric check valve appears in Figure 1.
The use of flap gates and elastomeric tide gates
allows greater retention volume in the collection
system by minimizing tidal inflows. The benefits
of these devices are greater at outfalls with
significant tidal influence.
Regulator Installation and Adjustment
Regulators control the amount of flow to a
downstream point and provide an outlet for flows in
excess of the sewer capacity. Adjustment of
regulator settings, proper regulator maintenance and
increasing a regulator outlet to the interceptor are
control measures that can ensure optimal system
performance and maximize in-line storage.
A vortex valve is used to discharge high flows
through a spiral action valve while leaving low
flows untouched. Vortex valves have been used to
Source: WEF, 1989.
FIGURE 1 TYPICAL ELASTOMERIC
CHECK VALVE
divert flows to CSO treatment facilities, control
flow out of storage facilities, and replace failed
mechanical regulators (U.S. EPA, 1993). Since the
flow through the vortex value is constant during
storm events, it serves to dam high flows.
Obstructions to flow such as trash and debris pass
through the vortex, reducing maintenance costs
over traditional valves.
Inflatable dams are popular control measures
whereby a rubberized fabric device is inflated and
deflated to control flows and maximize storage in
designated points in the combined sewer system.
Inflatable dams are usually activated by automatic
sensors that measure flow levels at specified points
in the system. Generally, very little maintenance is
required for inflatable dams. The air or water
supply used to inflate the dam, however, should be
inspected regularly. A typical inflatable dam
appears in Figure 2.
Many other mechanical regulators are designed to
operate under the control of gravity. These devices
are semiautomatic and mechanically variable,
depending on flow. Because these systems are
activated by float mechanisms or pressure plates,
they require little maintenance, and they eliminate
the need for power sources.
As conditions change with system demands over
time, regulator adjustment ensures an optimal
routing of flows in the combined sewer system.
Regulators in areas of growth may need to be
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Underground
Equipment
Vault
River
To Interceptor
Source: WEF, 1989.
FIGURE 2 TYPICAL INFLATABLE DAM
adjusted to allow additional flows beyond the
intended discharge (U. S. EPA, 1995). Increasing
regulator outlet to the interceptor can be allowed if
sufficient capacity exists in the downstream
interceptor or the wastewater treatment plant.
Reduction/Retardation of Inflows and
Infiltration
The reduction or retardation of inflows and
infiltration (I&I) lowers the magnitude of the peak
flow passing through the collection system, helping
to prevent the occurrence of CSO events.
Inflows are wet weather flows resulting from the
entry of storm water directly into the CSS.
Examples of inflows include parking lots and
surface roads containing storm water inlets, and
roof and sump drains connected to storm water
inlets, either directly or from sheet flow over
impervious surfaces. Infiltration is that flow that
enters the CSS through joints, cracks and manholes
as a result of wet weather events. Infiltration can
also enter the system from leaking tide gates.
Inflows can be reduced by detaining or infiltrating
storm water runoff at the point of generation (e.g.
onsite ponds, tanks, infiltration trenches, porous
pavement), in regional detention facilities, and by
disconnecting or blocking roof, sump pump and
perimeter drains. Commercial devices that retard
the rate of inflow can also be installed. They
include storm water inlet flow restriction grates and
piping, and hydrobrakes. Before installing such
devices, care should be taken to ensure that
sufficient drainage is provided to prevent
upgradient flooding.
I&I studies can be performed to determine the
extent and severity of CSS capacity reduction
impacts. Manhole inspection, smoke testing and
television inspection are common methods of
measuring I&I. Infiltration can be corrected
through sewer repair, relining and replacement, and
root control in areas with mature or aggressive
trees.
Upgrade and Adjustment of Pumps
Upgrading/adjusting a lift station pump can result
in additional in-line storage in upgradient portions
of the system, if sufficient hydraulic downgradient
capacity is available. Effects of the additional
pumping rates on the ability of the wastewater
treatment plant to treat increased flows must first
be determined.
Raising Existing Weirs and Installation of New
Weirs
Raising existing weirs and installing new weirs
utilizes in-system storage by controlling discharge
in the overflow conduit. By controlling discharge,
the CSO volume and frequency are typically
reduced. The installation of weirs can be
implemented in stages so as to reduce the risk of
surcharging. Masonry bricks, concrete blocks and
stop logs are commonly used media for weir
construction.
System of Real-time Monitoring/Network
A computerized system to control regulators can be
set up to maximize in-line storage. In 'real-time'
systems, flow and precipitation data are measured
by sensors throughout the system. The collected
data is transferred immediately from the measuring
device to a central control computer through a
communications system. The controlling computer
uses the data as input for system operations. The
control center then automates control of gates,
pumps and other regulators based upon the analysis.
The process allows for the instantaneous change of
control settings in response to changing
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precipitation and flow conditions throughout the
service area.
APPLICABILITY
Implementation of control measures that maximize
in-line storage can result in the lowest cost
additional storage alternative. In-line storage is
appropriate for systems with limited space for other
types of controls. Construction disruption is
minimized, and available storage can include any
part of the system with potential to contain
additional CSO volume. Potential for the greatest
performance improvements will be found in
systems where trunk and interceptor lines can
handle additional flow volumes. Additional
treatment of combined flows can be realized via
settling of sediments within the system. Access to
areas where increased detention and settling is to
occur is important as sediments must periodically
be removed.
Monitoring and modeling systems are frequently
used tools to determine available storage locations.
Hydraulic monitoring of the system provides
necessary information on flow characteristics.
Computer modeling of rainfall and flow data can
simulate system response to event conditions and,
thereby, assist planners in determining the most
appropriate areas for maximizing in-line storage.
ADVANTAGES AND DISADVANTAGES
Limitations of in-line storage include: the potential
for backing flows (surcharging) into basements or
streets; the amount of additional flows the system
can detain; and increased operation and
maintenance requirements.
Before applying an in-line storage control measure,
the hydraulic grade line must be evaluated in order
to determine the effect of the proposed measure on
the system. The hydraulic grade line is an
imaginary line obtained by adding the force of the
flow (pressure head) and the elevation of the flow
relative to the outfall discharge (elevation head) at
a given point in the collection system. A serious
potential for surcharging flows into basements or
streets exists without proper hydraulic evaluation of
proposed in-line storage measures. If a computer
model is used, the maximum height of the hydraulic
grade line should be specified so that control
measures designed to maximize in-line storage do
not cause surcharging of the system.
Combined sewer systems must maintain minimum
flow velocities in order to transport solids and
sediments to the wastewater treatment facility. This
may limit the increase in the volume of flows that
can be detained with additional control measures.
Effects of each in-line storage measure should be
evaluated before implementation to determine the
effect on the conveyance of dry weather flows to
the wastewater treatment facility, and on other
existing and proposed controls.
Most control devices have routine operation and
maintenance requirements. With increased volume,
additional solids and debris will collect in the trunk
and interceptor lines. Therefore, maintenance
requirements for the system usually increase with
control measures designed to maximize in-line
storage.
By reducing the volume of CSOs released to
receiving water, maximization of in-line storage
reduces the adverse impact of BOD, TSS, nutrients,
and other contaminants.
Surcharging of the collection system is a possible
limitation of maximizing in-line storage. The
likelihood of surcharging, and the associated public
health risks, must be evaluated before implementing
control measures to maximize in-line storage.
Flooding problems can be avoided through effective
monitoring efforts, hydraulic modeling, and/or
physical barriers such as disconnection of basement
sump pumps and perimeter drains.
PERFORMANCE
The performance of maximizing in-line storage for
CSO control can be examined through a series of
case study examples outlined below.
Cleveland, Ohio Area
The Northeast Ohio Regional Sewer District began
using an automated control system for its combined
sewer system in 1972 and has expanded the system
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over time. The current system comprises sensing
elements, field control elements, communication
equipment, and a central control center. A network
of 25 rain gauges, 24 remote level monitors, 56
remote flow monitors, and local level monitors at
the regulators comprise the sensing system. Data
collected from these devices are sent via modem or
radio to a central control facility. The entire system
is connected by a central computer which operates
on a local area network of personal computers.
Twenty-nine automated regulators, all which have
distributed control with programmable logic
controllers (PLCs), are used to control the amount
of flow in various portions of the system. This
system was augmented by physical in-line changes,
whereas fixed weirs were replaced by gates and
inflatable dams. The PLCs use the sewer levels,
gate positions, and dam pressures as variables to
compare with fixed points. By comparing the
variables and fixed points, the amount of available
storage and CSO discharge is determined. In-line
storage is maximized by adjusting controlling
regulators based on current system conditions.
Results of the Cleveland area real time monitoring
and control system have proven to be efficient in
achieving significant reductions in overflow.
Interceptor capacities have been maximized by
utilizing in-line storage, and reductions in BOD(5)
and TS S discharges to the environment have proven
to be cost effective through the operation of
automated regulators. Less than one percent of
capital improvement funds since 1972 have been
spent on real time control systems (Hudson, 1994).
On average, the automated system has prevented
about 2 MGD of untreated flow from entering the
environment every day. In 1994 approximately
950,00 Ibs. of BOD(5) and 6.7 million Ibs. of TSS
were captured. The operating cost for storage of
these parameters were $0.15 and $0.16 per pound,
respectively.
Boston, Massachusetts Area
In 1993, the Massachusetts Water Resources
Authority (MWRA) completed a System
Optimization Plan (SOP) for Boston and three other
nearby CSO communities. The SOP identifies
relatively simple structural or operational
modifications that can reduce CSO frequency and
volume. The SOP projects are designed to be low in
cost and easy to implement.
Recommended improvements included raising weir
elevations, repairing regulators, constructing
regulators and weirs, plugging and abandoning
certain overflow pipes, and replacing or repairing
tidegates.
The MWRA recommended SOP measures at 103
locations. As of June 1997, approximately 95% of
SOP projects have been completed (Parker, 1997).
The MWRA SOP analyzed the projected volume
and percent reduction as modeled for each
optimization project. It also analyzed the
cost/benefit ratios for each project. The MWRA
estimates that SOP improvements would reduce the
volume of CSOs (including treated CSOs) by over
25% (MWRA, 1993).
Norwalk, Connecticut
The City of Norwalk, CT, eliminated untreated
CSO discharges in 1980 by separating some outfalls
and bringing all remaining flows to the wastewater
treatment plant (WWTP). Like many old combined
sewer systems, Norwalk has large diameter
connecting and interceptor lines. Utilization of the
maximum capacity in the system allows for the
greatest treatment of CSS flows at the WWTP.
This is accomplished through regulation of the
plant's main gate. Existing in-line storage capacity
is utilized to hold as much flow as possible until the
storm event passes. Operators use rain gauges and
monitoring of a large siphon at the main interceptor
junction to assist in regulating the plant's main gate.
Regular maintenance and cleaning of sewer lines
after storm events ensure that the greatest possible
volume of flow is stored in the collection system.
During certain storm events, excess flow not
handled by the WWTP enters a CSO treatment
facility that provides micro screening and
disinfection. Flows entering the micro screening
units pass through spacings of 33 microns up to 256
microns, depending on volume of flow. All flows
are then routed through the WWTP chorine contact
chamber for disinfection. The CSO treatment
facility averages 30 per cent BOD(5) and TSS
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removal, with some smaller storm event removals
of up to 50 per cent.
The Norwalk WWTP is undergoing an upgrade to
biological nutrient removal (BNR). The plant
treatment capacity will be increased from 20 MOD
to 30 MGD per day. As this facility comes on-line,
maximization of in-line storage will allow
maximum treatment of wet weather events to the
BNR facility. Since the existing CSO treatment
facility provides only the equivalent of primary
treatment, significant water quality improvements
will be obtained by maximizing treatment to the
WWTP.
COSTS
Techniques to maximize in-line storage are most
cost-effective in trunks and interceptors with mild
slopes and large storage capabilities.
Capital and O&M costs vary considerably based the
on characteristics of the system and the type of
control measure implemented. Ancillary costs may
include traffic rerouting and traffic control plans,
proper ventilation requirements, and post
construction monitoring.
Some in-line storage measures do not require
capital improvements. In Norwalk, CT, for
example, no capital costs are involved with
regulation of the main control gate. Expensive
replacement of drum screens and associated O&M
at the CSO treatment facility is reduced by
maximizing in-line storage. O & M for the
collection system does increase as a result of the
operational modification.
A comparison of costs for control measures
implemented by various communities appears in
Table 1.
REFERENCES
1. Association of Metropolitan Sewerage
Agencies (AMSA), 1994. Approaches to
Combined Sewer Overflow Program
Development. Washington, D.C.
2. Hudson, D. M., 1996. Protecting the
Waters of Lake Erie through Real Time
Control of CSO's in the Cleveland Area.
Proceedings, Urban Wet Weather Pollution
Control Sewer Overflow and Storm Water
Runoff, Water Environment Federation,
Quebec City, Quebec.
3. Massachusetts Water Resources Authority,
1997. David Parker, Massachusetts Water
Resources Authority personal communication
with Parsons Engineering Science, Inc.
4. Metcalf & Eddy, 1993. System Optimization
Plans for CSO Control. Prepared for the
Massachusetts Water Resources Authority.
Wakefield, MA.
5. Northeast Ohio Regional Sewer District,
1997. Daniel Hudson, Northeast Ohio
Regional Sewer District, personal
communication with Parsons Engineering
Science, Inc. regarding Construction Cost
for Auto Regulations.
6. City of Norwalk, CT, 1998. Fred Treffeisen,
City of Norwalk, CT, personal
communication with Parsons Engineering
Science, Inc.
7. U. S. EPA. 1993. Manual: Combined
Sewer Overflow Control. Washington D.C.
EPA 625/R-93-007.
8. U.S. EPA, 1995. Combined Sewer
Overflows: Guidance for Nine Minimum
Controls. Washington D.C. EPA 832/B-95-
003.
9. Water Environment Federation, 1989.
Combined Sewer Pollution Control
Abatement. Manual of Practice FD-17.
Alexandria, Virginia.
ADDITIONAL INFORMATION
Massachusetts Water Resources Authority
David Parker
100 First Avenue
Boston, MA 02129
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TABLE 1 COST COMPARISON FOR CONTROL MEASURES
Control
Measure
City
Unit
Capital Cost
Annual O & M
Regulator
Installation
(Inflatable Dams)
Pump Installation
Install Raised
Weir's
Washington D.C. per dam $1,219,0001
San Francisco,
CA
Boston, MA Area
Real Time Cleveland, OH
Monitoring System Area
Collection System
Inspection
Tide Gate
Replacement
Philadelphia, PA
Norwalk, CT
gpd $0.10-$0.11 gpd2
per weir $3,250 brick & mortar3
$12,050 formed concrete3
$18,100 stop logs3
per unit $264,000 auto regulators4
$12,900 remote level
monitor3
system $1,906,0005
None
Boston, MA Area per gate $24,1003
N/A
N/A
N/A
automated regulator
$11,975s
rain gauge $2,8303
flow and level monitor
$3,2703
N/A
reduction in O & M for
microscreening devices;
increase in O & M for
collection system
N/A
ENR = 5750
1 Average cost per dam of 12 dams installed at 8 sites; complete cost includes all monitoring equipment
2 Used to convey flows from CSO treatment facilities to WWTP as conditions allow
3 Typical cost; costs vary by site
4 Average bid cost of automated regulators (usually inflatable dams) in Southerly WWTP district; costs vary by site
5 System = 1 central computer, 8 computer controlled regulators, level measuring devices at 53 regulators, 22 rain gages, flow
metering at 18 stations, several other level devices
Sources: AMSA 1994, Hudson 1996, Metcalf & Eddy 1993, Northeast Ohio Regional Sewer District 1997, Treffeisen 1997.
City of Nashville, Tennessee
Mrs. Lyn Fontana, P.E.
Metropolitan Government
1600 2nd Avenue North 4th Floor
Nashville, TN 37208
Northeast Ohio Regional Sewer District
Daniel Hudson
Environmental and Maintenance Services Center
4747 East 49th Street
Cuyahoga Heights, OH 44125
City of Norwalk, Connecticut
Fred Treffeisen
Malcom Pirnie, Inc.
60 South Smith Street
East Norwalk, CT 06855
City of Philadelphia, Pennsylvania
Gene Foster
Philadelphia Water Department
Fox and Roberts Streets
Philadelphia, PA 19129
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for the use by the U.S.
Environmental Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
MTB
Excellence In compliance through opftnal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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