United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-99-041

September 1999

Combined Sewer Overflow

Management Fact Sheet

Sewer Separation

DESCRIPTION

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 and during light to moderate rainfall, 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,

often causing untreated combined sewage and storm

water to back up into basements and to overflow

from manholes onto surface streets. Traditionally,

CSS outfalls were designed to discharge directly

into receiving waters during combined sewer

overflows (CSOs). This was done to prevent the

excessive combined flows from directly impacting

public health via basement and street flooding.

In addition to flooding problems, CSOs can cause

problems in receiving water bodies. CSOs can

contain untreated domestic, industrial, and

commercial wastes, as well as storm water runoff.

Contaminants contributed by these sources include

potentially high concentrations of suspended solids,

biochemical oxygen demand (BOD), oils and grease,

toxics, nutrients, floatables, pathogenic

microorganisms, and other pollutants. CSO

pollution has caused many receiving waters to

exceed water quality standards, resulting in threats

to public health, aquatic species, or aquatic habitat.

CSO pollutants have impaired receiving water body

uses and have contributed to restrictions on shellfish

harvesting, occasional fish kills, and numerous

beach closures. Potential odors and solids deposits

in the receiving water body can also compromise

aesthetics and limit recreational uses of the water

body.

Many communities have studied and evaluated CSO

control strategies that would effectively reduce, if

not necessarily eliminate, CSOs and their associated

health and ecological risks. One of the strategies

often considered is sewer separation.

Sewer separation is the practice of separating the

combined, single pipe system into separate sewers

for sanitary and storm water flows. In a separate

system, storm water is conveyed to a storm water

outfall for discharge directly into the receiving

water. Based on a comprehensive review of a

community's sewer system, separating part or all of

its combined systems into distinct storm and sanitary

sewer systems may be feasible. Communities that

elect for partial separation typically use other CSO

controls in the areas that are not separated.

APPLICABILITY

Sewer separation can be considered wherever there

is a CSS. However, an evaluation of the most

appropriate CSO control should be performed prior

to selecting sewer separation or any other measure.

Sewer separation has often been the appropriate

technology in areas where one or more of the

following conditions exist: 

• Most sewers are already separated; 

• Siting constraints and costs prohibit the use

of other structural measures; 

• The uses and the assimilative capacities of

receiving waters prohibit the use of other

CSO controls; 
• Other CSO strategies are not publicly

acceptable;

• Additional infrastructure improvements,

such as road repaving, are also required;

• The combined system is undersized;

• Elimination of CSOs is desired; and/or

• Other CSO measures are not able to achieve

the community's goals.

Sewer separation has been used effectively in many

communities. Most of the approximately 1,000

communities that are served by CSSs are located in

the Northeast and the Great Lakes region.

Complete or partial separation of CSSs has

occurred in many of these areas, as well as in several

locations in the West. Cities that have completely

or partially separated CSSs include: Minneapolis,

St. Paul, and South St. Paul, MN; the metro

Detroit, MI, area; the metro Boston, MA, area;

Salem and Portland, OR; the metro Seattle, WA,

area; Lynchburg, VA; Bangor, ME; Hartford and

Norwich, CT; and Remington, IN. Columbus, OH,

has recently elected to separate its CSS as well. 

One of the largest sewer separation projects

occurred in Minneapolis, St. Paul, and South St.

Paul, MN. The project involved pipe separation in

more than 21,000 acres of drainage area. By

December 1996, 189 miles of storm sewers and

11.9 miles of sanitary sewers had been installed.

This program was needed to reduce the number of

overflows that were estimated to occur an average

of once every three days (American City and

County, 1996). Overflows have been significantly

reduced by this separation project.

ADVANTAGES AND DISADVANTAGES

Positive impacts resulting from sewer separation

include: reduction or elimination of basement and

street flooding; reduction or elimination of sanitary

discharges to receiving waters; decreased impacts to

aquatic species and habitat; decreased contact risk

with pathogens and bacteria from domestic sewage

in the receiving water; and relief from CSO

regulations. In addition, incidental infrastructure

work (e.g., road repaving and the repair or

replacement of miscellaneous utilities, such as water

and cable lines) could be conducted more cost

effectively if it were to coincide with sewer

separation. For example, as a result of the CSO

program in the City of St. Paul, MN, streets were

paved and handicap ramps were added to sidewalks,

gas and water mains were installed, gas services

were renewed or replaced, lead water service

connections were replaced, and street lights were

installed.

Separating CSSs may contribute to improvements to

water quality due to the reduction or elimination of

sanitary discharges to receiving water bodies.

However, the increased storm water discharges

resulting from sewer separation could decrease the

positive impacts of the separation unless storm

water discharges are mitigated. Without mitigation,

increased loads of storm water pollutants, including

heavy metals, sediments, and nutrients, may run off

into local water bodies. For example, in Atlanta,

GA, sewer separation was predicted to increase

pollution to local creeks (AMSA, 1994) as polluted

storm water previously reaching the treatment plants

now is discharged directly into receiving waters.

However, in many cases, separating sewers reduces

pollution to receiving waters, as described above for

St. Paul, MN. A second example of successfully

reducing pollution to receiving water bodies has

occurred in Juneau, AK. It has been reported that

in Juneau, where there is in excess of 70 inches of

precipitation a year, the storm water concentrations

conveyed through the recently separated storm

water sewers are rather dilute. This has also been

attributed to large quantities of clean groundwater

that infiltrate into the storm sewer, relatively clean

activities within the watershed, and several

non-point source pollution control programs within

the City (City of Juneau, 1997). Existing and future

storm water impacts to the receiving water body

should be evaluated prior to implementing sewer

separation.

Negative impacts associated with sewer separation

include extensive construction and construction

related impacts (e.g., noise, dust, erosion),

disruption to residents and businesses, possible

disruptions in sewer service, and the need for storm

water controls or best management practices.
In addition, complete separation of sanitary and

storm water flows can be hard to accomplish

whether the combined sewer is converted to a storm

sewer or to a sanitary sewer. Complete separation

of a CSS would involve disconnection of all storm

water drainage structures, sump pumps, and roof

and footer drains. Disconnection of footer drains is

often not cost effective. Some communities have

offered financial incentives to homeowners and

businesses for voluntarily disconnecting some of

these storm water sources from sanitary sewers.

Many communities have also passed ordinances

requiring the disconnection. Despite these

provisions, there is still potential for some storm

water flows to remain connected to sanitary sewers.

Likewise, complete disconnection of sanitary flows

from a converted storm water sewer may be difficult

to accomplish, but is usually more successful than

eliminating all storm flow connections from

connected sanitary sewers.

KEY PROGRAM COMPONENTS

Decisions for a CSO control strategy should be

made on a site-by-site basis utilizing drainage area

data, receiving water use and water quality data, and

sewer system data. Sewer system information can

be obtained from review of sewer plans, television

inspection, and flooding records. Communities may

consider performing house to house inventories of

house connections to the combined system (i.e.,

sanitary and roof drains). This was successfully done

in parts of the metropolitan Boston area. Modeling

and Geographical Information Systems (GIS) may

be useful data analysis and prediction tools. 

Using these data, communities should determine

what CSO controls, or combination of controls, will

meet performance goals established by the

community. Other factors, such as cost

effectiveness, natural and urban topography and soil

types, siting constraints, location of current and

future utilities, land use and cover, existing sewer

capacity, layout, and condition, pump and treatment

plant capacities, and requirement for other

infrastructure work in the same area, should be

considered before finalizing project plans. For

example, sewer separation was selected in

Minneapolis, South St. Paul, and St. Paul, MN, due

to local needs for eliminating sewage backups into

basements, reducing street flooding, and

reconstructing aging portions of the sewer system

(MWCC, 1984).

Sewer separation can be accomplished through

installing new storm or sanitary sewers to be used in

conjunction with the existing sewer. Economics,

capacity, condition, and layout of the combined

sewer are the typical factors used in deciding the

existing line’s post-separation use.

An advantage of converting the combined sewer to

a sanitary sewer (referred to as a converted sanitary

sewer in this document) is that all sanitary flows are

already connected to the converted sanitary sewer.

Using the existing combined sewer as the sanitary

sewer and installing a new storm sewer would likely

require that any overflow weirs, gates, or other

regulating devices remaining in the converted

sanitary system be bulkheaded or otherwise disabled

to eliminate the potential for sewage to overflow.

In addition, storm water drainage structures, sump

pumps, and roof drains must be disconnected from

the converted sanitary system and connected to the

new storm water sewer. This will provide more

capacity in the converted sanitary sewer and will

reduce the possibility of overflows. Building footer

drains, however, are often left connected to the

existing combined system and do consume some of

the converted sanitary sewer capacity.

Rehabilitation or relining of the converted sanitary

system, storage tanks, and/or equalization basins

may be required if infiltration is a significant

problem due to cracks or inadequate construction

materials (e.g., brick sewers). In some cases, such

as in Juneau, AK, the existing combined sewer may

be in such poor condition that new sanitary sewers,

as well as new storm sewers, are constructed.

There are some circumstances that may make the

conversion of the combined sewer to a storm sewer

(referred to as a converted storm sewer in this

document) more appropriate. For instance,

combined sewers that have a large diameter and

have little slope (less than 3 percent) would not

have the flushing velocity required of a sanitary

sewer. In cases such as this, the existing CSS may

be more appropriately converted to a storm sewer,

provided that the sewer has sufficient capacity for

safe conveyance of the local design storm. A
smaller sewer should be appropriately designed,

sized, and constructed to convey the sanitary flows.

Storm, roof, and footer drains, as well as

catchbasins could remain connected to the

converted storm sewer. Sanitary connections,

however, would need to be disconnected and

conveyed to the new sanitary line. Any remaining

sanitary lines connected to the converted storm

sewer will cause direct discharges of sanitary flows

to the receiving water body. Post-separation

sampling and monitoring of the converted storm

sewer is typically required to confirm that all

sanitary flows have been removed from the

converted storm sewer and redirected into the

sanitary sewer. Conversion of the combined sewer

to a storm sewer would also require that the

interceptor connection at the regulating device (e.g.,

weir or gate) be plugged, and may potentially

require modifications to prevent water from

stagnating upstream of the regulator.

Consideration should be given to coordinating

sewer separation with improvements to other

utilities, as this enhances the cost-effectiveness of

both/all projects and minimizes disruption to the

public.

IMPLEMENTATION

Sewer separation reduces and often eliminates

untreated sanitary discharges from discharging into

receiving water bodies, and therefore positively

impacts receiving water quality. Sewer separation,

however, greatly increases untreated storm water

discharges to the receiving water body. In a CSS,

at least some of the storm water flows are treated at

the treatment plant. The performance achieved with

sewer separation will vary depending on the existing

storm water pollutant loading and the existing

sanitary pollutant loading. For example, a study

performed for North Dorchester Bay, MA,

estimated that the overall fecal coliform removal

potentially achieved by sewer separation was only

45 percent (Metcalf & Eddy, 1994). This was

attributed to the increase in storm water discharges

to the receiving water body, and the corresponding

increase in non-point runoff pollutants.

Actual fecal coliform removal rates have been

determined for several sites where sewer separation

has been implemented. Water quality monitoring

data collected in St. Paul and Minneapolis from

1976 to 1996 indicated a fecal coliform

concentration reduction of 70 percent. One of the

four sites where data was collected reduced fecal

coliform concentrations from an average of 500

organisms per 100 mL to 150 organisms per 100

mL. At another site, fecal coliforms were reduced

from 489 organisms per 100 mL to 143 organisms

per 100 mL (Richman, 1996). This reduction has

been attributed to sewer separation and to the

reduction in the number of overflows occurring

every year. 

Sewer separation may also result in other related

improvements to water quality. In stretches of the

Mississippi, water quality improvements attributed

to sewer separation projects have resulted in the

return of the pollution-sensitive Hexagenia mayfly

after a 30 year absence; the return of Bald Eagles to

the area; and the recovery of fish populations and

diversity from 3 species to over 25 species (Cities of

Minneapolis, et. al., 1996).

Monitoring the performance of CSO control

strategies at the Rouge River Demonstration

Program has been underway since the summer of

1997. Part of the monitoring program will identify

the effectiveness of sewer separation in terms of

improvements to water quality. Instream

monitoring is also occurring in Portland, OR. The

supplemental data will add to the performance data

collected in Minnesota (70 percent fecal coliform

reduction) and estimated for Massachusetts (45

percent fecal coliform reduction).

OPERATIONS AND MAINTENANCE

The Operations and Maintenance (O&M

requirements of separated sewers are generally the

same as those of other sewer systems. Maintenance

must be conducted on pump stations (including

routinely cleaning wet wells, testing for adequate

pumping capacity, and exercising pumps and

stand-by generators), sewer lines, and catchbasins

and grit chambers. Catchbasins and grit chambers

located in the sanitary or storm sewer system will

require routine cleaning to prevent accumulation of

sediment. Jet spray cleaning, pumping, and
vacuuming are common methods for cleaning

catchbasins and grit chambers. 

In addition, all sewer lines, and in particular sewers

that were previously combined, need to be

monitored to verify hydraulic capacity and to

identify infiltration and inflow. Basement or street

flooding is a likely indication of hydraulic capacity

or gradient problems in the sewer and may require

major repairs. Excessive infiltration into a converted

sanitary sewer may require rehabilitation of the

sewer system. Methods for assessing the condition

of the sewers include modeling, smoke testing, and

television inspection. Monitoring will identify

cracked and collapsed sewers that will need to be

repaired. In addition, monitoring can identify the

location and cause of sewer blockages. To prevent

blockages, lodged debris, sediment, and grit must be

removed on a regular basis.

Post-separation monitoring and sampling may be

required to ensure that no sanitary flows are still

connected to the storm sewer and being directly

discharged to the receiving water body.

Alternatively, simple dye studies can be employed to

verify separation.

COSTS

Separation costs vary considerably due to the

location and layout of existing sewers; the location

of other utilities that will have to be avoided during

construction; other infrastructure work that may be

required; land uses and costs; and the construction

method used (e.g., open cut verses microtunneling).

Communities that have other infrastructure

requirements (such as road repairs) in addition to

sewer separation may find that upgrading the

facilities simultaneously can result in a much lower

cost relative to upgrading them independently.

Construction occurring in existing right-of-ways

would probably not require land acquisition, and

thus would not add to the project cost. Project

costs could increase depending on the land use. For

example, project construction occurring in an

industrial area that contained hazardous materials or

wastes would likely increase the project cost.

Methods of construction, such as the need to tunnel

or bore versus open cutting, can also add to the

cost. Project costs could also increase if sanitary

equalization basins are required as part of the

separation project or if storm water best

management practices are required to control the

increased storm water discharges to the receiving

water body.

Actual construction costs are available from the St.

Paul sewer separation project. For that project,

sewer separation costs ranged from $8,350/acre

to$40,060/acre, with an average cost of

$15,400/acre (all costs are in 1984 dollars).

Estimates from the City of Portland and Detroit are

$18,000/acre and $67,800/acre, respectively.

The Rouge River project has also generated good

cost data for sewer separation. Costs were

approximately $377,000 for separating

approximately 600 meters of pipe on a small

residential street (CSO Area 42, Windsor Avenue),

which included costs for removing existing

pavement, laying a new sewer line, and re-paving

and re-sodding. A second project (CSO Area uing

cost $1.3 million to separate approximately 2,600

meters of pipe. All costs are presented in 1995

dollars. 

The cost of operation and maintenance (O&M) of

the separated sewer system is difficult to predict. 

Factors contributing to the O&M costs include the

age and the condition of the previously combined

sewer, the length and diameter of the sewers, the

frequency and the amount of sand and grit removed,

and the size of drainage areas.

Sewer separation can reduce treatment and O&M

costs at the receiving treatment plant by potentially

eliminating storm water flows to the plant. Energy

costs for transporting flows to the treatment plant

could also be reduced due to the reduced flow

volume.

REFERENCES

1. American City and County, 1996. "CSO

Control Revitalizes Stretch of the

Mississippi," pp. 31-32.
2. Association of Metropolitan Sewerage

Agencies (AMSA), 1994. Approaches to

Combined Sewer Overflow Program

Development. Washington, D.C.

3. City and Borough of Juneau, Alaska, 1997.

Personal Communication with Ernie Mueller

with the City and Borough of Juneau, Public

Works.

4. Metcalf & Eddy, 1994. Final CSO

Conceptual Plan and System Master Plan:

Part II CSO Strategies. Prepared for the

Massachusetts Water Resources Authority.

Wakefield, MA.

5. Metropolitan Waste Control Commission

(MWCC), 1984. MWCC Metro 201

Facilities Plan. St. Paul, MN.

6. Cities of Minneapolis, Saint Paul, South St.

Paul, and Metropolitan Council

Environmental Services, 1996. Separating

Combined Sewers to Improve and Protect 

Mississippi River Water Quality: A

Ten-Year Commitment. Annual Progress

Report to the Public - Year Ten.

Minneapolis, St. Paul, South St. Paul, MN.

7. City of Portland, Oregon, 1997. Personal

communication with Lester Lee, City of

Portland.

8. Richman, M., 1996. "Sewer Separation

Lowers Fecal Coliform Levels in the

Mississippi River". Water Environment &

Technology, Vol. 8, No. 11, pp 20-22.

9. Wade-Trim, 1997. Personal communication

with Laura Crane, Wade-Trim.

ADDITIONAL INFORMATION

City of Columbus, Ohio

Laurie Mehl

Public Utilities, Division of Sewerage and Drainage

910 Dublin Road, Room 32

Columbus, OH 43215

Borough and City of Juneau, Alaska

Ernie Mueller

Department of Public Works

5433 Shaune Dr. 

Juneau, AK 99801

Michigan City, Indiana, Sanitary District

Tim Haus

532 Franklin Street

Michigan City, IN 46361

City of New Haven, Connecticut

Raymond Smedberg

Water Pollution Control Authority

345 East Shore Parkway

New Haven, CT 06510

City of Portland, Oregon

Lester Lee

City of Portland Bureau of Environmental Services

1211 Southwest 5th Avenue, Suite 800

Portland, OR 97204

Rouge River Demonstration Project

Vyto Kaunelis

Wayne County Department of Environment

415 Clifford Street, 7th Floor

Detroit, MI 48226

City of Saco, Maine

Larry Nadeau

Department of Public Works

300 Main Street

Saco, ME 04072

City of St. Paul, Minnesota

Mike Kassan

Sewer Utility, Department of Public Works

1000 City Hall Annex

St. Paul, MN 55102
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

The mention of trade names or commercial products

does not constitute endorsement or recommendation

for the use by the U.S. Environmental Protection

Agency.