Primer

Combined Sewer Overflow

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.

Published January 1, 2010