In many older coastal cities, especially in the northeastern United States, storm sewers in the street that collect stormwater runoff from rainfall are connected to municipal sewage treatment plants that process household sewage and industrial wastewater. Under normal, relatively dry conditions runoff and municipal waste go to a sewage treatment plant where they are treated. However, when it rains, in some cases less than an inch, the capacity of a sewage treatment plant can be exceeded; the system is overloaded. The mixed urban stormwater runoff and raw municipal sewage is released to nearby creeks, rivers, bays, estuaries or other coastal waters, completely untreated. This is a combined sewer overflow (CSO) event.

Combined sewer overflow events are not rare. In Boston Harbor, for example, there are 88 pipes or outfalls that discharge combined stormwater runoff and sewage. It has been estimated that CSO events occur approximately...