Green Infrastructure vs. Gray Infrastructure

A combined sewer system (CSS) is one in which both storm water and sanitary sewage are collected and conveyed in the same system. Some 700 United States cities rely on such combined systems to handle waste water issues.

It doesn’t take much imagination to envision potential problems for such systems, especially in light of the hurricanes and coastal flooding of recent years. When excess rain water hits the area, the system can overflow its pipes and tanks (known as “combined sewer overflow,” or CSO), resulting in harmful contamination of drinking water and of surface water systems like streams and ponds. This contamination puts humans and wildlife at risk for disease and death, as well as threatening the local economy by shutting down beaches, fisheries and other recreational and business activities.

How can this kind of threat be addressed, in an age when science tells us that storms are likely to get more severe and flooding is more likely to become an issue in coastal areas? Rather than adding more pipes and tanks of steel and concrete, could there be a greener solution?

The United States Department of Environmental Protection recently issued a paper describing ways to design green infrastructure adaptations to existing CSSs that helps control CSO overflow. (Planning and Modeling Green Infrastructure Scenarios, (March 2014).

“Green infrastructure” generally relies upon three processes to keep rainwater out of the CSS. Through bioinfiltration, vegetation assists runoff water to saturate or infiltrate the subsoil. Evapotranspiration is simply the process of water being absorbed by a plant’s roots and released (or transpired) through its leaves. Finally, by capture and use or harvesting, the system prevents rainwater from ever entering the CSS by diverting it to a useful purpose: watering plants.

Water channel funneling into rain garden, St. Paul, MN. Image (c) Alisha Goldstein
Water channel funneling into rain garden, St. Paul, MN. Image (c) Alisha Goldstein

A green system, unlike a traditional “gray infrastructure” of tanks and pipes, can enhance public life by creating new park-like areas instead of burying miles of pipes underground at taxpayer expense (and disrupting their lives in the process with road closures and muddy mess). In addition, green infrastructure can be expanded fairly rapidly as needed, typically more quickly than traditional CSSs, and with less disruption and expense to the public. These swathes of green throughout a city increase air quality, improve community morale, reduce urban heat effects, and can bring down energy use. Further, because a green system uses rainwater to feed the plants that are part of the system itself, the system is self-sustaining. At the same time, the amount of water that enters the CSS is reduced and therefore the costs of improving or maintaining the CSS are similarly lessened. Because green systems are typically passive in nature, they require little to no investment of money or energy to run and maintain, unlike their “gray infrastructure” counterparts. Local wildlife benefit from improved habitat and biodiversity. Even when initiated on a small scale, such as a roof-top garden, these same benefits obtain.

Rooftop garden, Chicago, IL. Image (c) Abby Hall, U.S.E.P.A.
Rooftop garden, Chicago, IL. Image (c) Abby Hall, U.S.E.P.A.

Have you taken a look at this report? Do you see any potential problems with these green systems? How do they compare cost-wise? Do you think city governments will be open to these kinds of alternatives?


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