Combined Sewer Overflows
A combined sewer overflow (CSO) is the discharge of wastewater and stormwater from a combined sewer system directly into a river, stream, lake, or ocean. Overflow frequency and duration varies both from system to system, and from outfall to outfall, within a single combined sewer system. Some CSO outfalls discharge infrequently, while others activate every time it rains. During heavy rainfall when the stormwater exceeds the sanitary flow, the CSO is diluted.
The storm water component contributes a significant amount of pollutants to CSO. Each storm is different in the quantity and type of pollutants it contributes. For example, storms that occur in late summer, when it has not rained for a while, have the most pollutants. Pollutants like oil, grease, fecal coliform from pet and wildlife waste, and pesticides get flushed into the sewer system. In cold weather areas, pollutants from cars, people and animals also accumulate on hard surfaces and grass during the winter and then are flushed into the sewer systems during heavy spring rains.
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