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March 18, 2014
The green approach to combined sewer overflows

The EPA has issued a new document to assist cities in developing green infrastructure solutions to combined sewer overflows (CSOs).  Greening CSO Plans: Planning and Modeling Green Infrastructure for Combined Sewer Overflow (CSO) Control is highlighted by a description of EPA’s Storm Water Management Model (SWMM), a tool that can be used to optimize different combinations of gray and green infrastructure.

9,000 outfalls

Today, about 700 U.S. cities, concentrated in the Northeast, the Great Lakes region, and the Pacific Northwest, have combined sewer systems (CSSs).  CSSs link sanitary sewers and stormwater conveyances to outfalls to surface water bodies.  When the weather is dry or moderately wet, the CSS carries both urban street runoff and sanitary sewage to waste water treatment plants.  During extreme wet weather, the capacities of the plant are overwhelmed and the combined and untreated flows are diverted to surface waters.  The EPA has estimated that inventoried CSSs have about 9,000 outfalls into surface waters. 

Raw sewage in CSOs has contributed to beach closures, contamination of drinking water supplies, and other environmental and public health concerns. In a 2004 report to Congress, the EPA estimated that each year about 850 billion gallons of untreated wastewater and stormwater are released as CSO.  

Long-term control plans

In 1994, the EPA issued a CSO control policy that directed federal, state, and local governments to control CSOs through the use of the National Pollutant Discharge Elimination System (NPDES) permitting system.  In 2000, Congress amended Section 402 of the Clean Water Act to require both NPDES permits and enforcement orders for CSOs to conform to the CSO control policy.  Under their NPDES permits, communities are required to implement nine minimum controls and to develop and implement long-term control plans (LTCPs).  But the EPA emphasizes that many communities are still searching for cost-effective ways to implement their LTCPs.

Gray and green infrastructure

Many communities have attempted to address CSOs through “gray” infrastructure.  Gray infrastructure refers to traditional stormwater engineering systems such as pipes, sewers and other structures involving concrete and steel.  Off-line storage tanks, basins, or deep tunnels, which are adjacent to the sewer system and hold CSOs until the plant has the capacity to treat the overflow, are among the most commonly implemented types of gray infrastructure. 

But gray infrastructure is capital-intensive, highlighting the advantages of green CSO planning.  Green infrastructure practices mimic natural hydrologic processes to reduce the quantity and/or rate of stormwater flows into the CSS.  By controlling stormwater runoff through processes such as infiltration, evapotranspiration, and capture and use (rainwater harvesting), green infrastructure can help keep stormwater out of the CSS.

Green infrastructure also supports the principles of low-impact development—an approach to land development (or redevelopment) that works with nature to manage stormwater as close to its source as possible.


Governments can use an SWMM to determine how much CSO can be reduced by green projects.  For example, the EPA reports that Toledo, Ohio, used the model before and after installing bioswales on a residential street.   Long-term simulations indicated an annual average reduction of runoff volume from the bioswales of approximately 64 percent.  Using these data, the city was able to evaluate the cost effectiveness of implementing bioswales as an element of its CSO control program.

The EPA also discusses the use of Green LTCP-EZ, a simplified tool small communities can use to develop CSO LTCPs, using, at least in part, green infrastructure.

EPA’s green infrastructure CSO guidance

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