BackPage Primer from Environmental Building News
January 1, 2010
Combined Sewer Overflow
Combined sewer overflow (CSO) occurs when heavy rains overload antiquated sewer systems that combine wastewater and stormwater, then dump this polluted water into rivers or estuaries without treatment. These CSO events can dramatically increase bacteria levels in estuaries and beaches, often leading to shellfishing restrictions and swimming bans.
Most modern municipal wastewater systems collect sewage (“sanitary” wastewater from domestic, commercial, and industrial sources) and stormwater separately. Sewage is piped to a treatment plant (sometimes referred to as a publicly owned treatment works or POTW), while stormwater flows directly into nearby surface waters. Since the mid-20th century, all new sewer systems have separated wastewater and stormwater.
Many older cities, especially in the Northeast, Upper Midwest, and Pacific Northwest (see map), still have combined sewer systems in which a single pipe carries both wastewater and stormwater to the sewage treatment plant. Depending on the system, a moderate to large storm will cause a CSO.
Nationwide, according to the U.S. Environmental Protection Agency (EPA), combined sewer systems exist in 772 communities with a total of 40 million people. In New York City, according to Riverkeeper, an organization devoted to protecting the Hudson River and its tributaries, there are 460 CSO outflow locations emptying into New York Harbor that dump an average of 27 billion gallons (100 billion liters) of untreated sewage into the Harbor each year. At some of these locations, as little as 1/20 of an inch (1.3 mm) of rain can trigger a CSO event, though at most locations a significantly larger rainstorm is required.
Eliminating CSOs requires a two-part strategy. In the long term, sanitary sewers must be separated from storm sewers and used to pipe only the sanitary wastewater to sewage treatment plants. According to EPA, eliminating combined sewer systems nationwide would cost about $50 billion, and that scale of funding is not available in most areas.
In the short term, and in municipalities with constrained budgets, the priority should be to reduce stormwater flows so that only the largest rainstorms cause CSOs. This can be done by reducing impervious area and allowing rainwater to infiltrate onsite instead of entering storm sewers, and by slowing down the flow of stormwater by retaining some of it in landscaping features. This is an important function of green roofs.
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Illustration: Julia Jandrisits
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Source: U.S. EPA
