Combined Sewer Systems and Green Infrastructure

Written by: Sara Dennis

Do you ever wonder where all the rainwater you see in streets goes? Or what’s under a manhole when you open it up? The short answer is pipes. But these aren’t just pipes. They act as an intricate system that collects rainwater runoff, domestic sewage and industrial wastewater, designed to direct each to an appropriate place. Many cities, including Columbus, have a system that collects all of these within the same pipe, called a combined sewer system (CSS). During normal weather conditions the pipe transports all of this wastewater to a sewage treatment plant where it is treated, then discharged into a local body of water. However, when the volume of wastewater exceeds the capacity of the CSS or treatment plant (due to heavy rainfall or snowmelt, for example) the untreated stormwater and wastewater gets discharged into a nearby body of water, which is referred to as combined sewer overflow (CSO).  

Diagrams showing the how the intended operation of a CSS vs. a CSO caused by a wet weather event.  US EPA

Diagrams showing the how the intended operation of a CSS vs. a CSO caused by a wet weather event. US EPA

CSOs can cause various water quality issues because they contain untreated or partially treated human and industrial waste, toxic materials, debris and stormwater. These events can cause drinking water contamination, beach closures, and restrictions on shellfish consumption, as well as ecological problems like harmful algal blooms, fish kills and dead zones in water. 860 municipalities in the US operate using CSSs, so they run the risk of these problems. Most of these municipalities are located in older cities in the Northeast and the Great Lakes Region of the Midwest. The majority of other systems have separate pipes to handle stormwater and sewage, so the two don’t mix, thus preventing CSOs.

Municipalities in the US operating with a CSS.  US EPA

Municipalities in the US operating with a CSS. US EPA

One solution to stop CSOs is to redesign the CSS and separate the stormwater pipes from other wastewater pipes, but this can be expensive and time consuming for a city, and in some cases there are physical barriers preventing this. Effective stormwater management can at least partially, if not fully, solve the problem by decreasing the amount of stormwater going into the CSS. Some common engineering solutions, termed green infrastructure (opposed to gray infrastructure, like pipes), include:

  • Building retention basins, detention basins, bioretention areas and constructed wetlands which can filter and store stormwater as well as provide habitat for wildlife
  • Installing permeable pavements that allow water seep in the cracks, rather than run off it
  • Using green roofs, rain barrels and cisterns to absorb and harvest rainwater from rooftops, which can have a variety of benefits for the building owner and the environment 

By incorporating these stormwater solutions and best management practices we can have cleaner and safer waters and communities!

1. US Environmental Protection Agency. “Combined Sewer Overflows (CSOs).” Washington: US EPA.
2. US Environmental Protection Agency. “What is Green Infrastructure?.” Washington: US EPA.
3. Evans, Mary Anna. "Flushing the Toilet Has Never Been Riskier." The Atlantic. Atlantic Media Company, 17 Sept. 2015. Web.