Approximately 100 years ago, Evansville began building a sewer system to carry storm water away from homes, businesses, and streets.
Later, when indoor plumbing arrived, homes and businesses connected their sewage lines into these storm sewers, making them "combined" sewer systems (CSS). Evansville's two wastewater treatment plants were not constructed until the 1950's. Approximately 504 miles of sanitary and 318 miles of combined sewers are under the authority of the Evansville Water and Sewer Utility (EWSU).
During dry weather, the combined sewers transport sewage to the City's two treatment plants. However, after significant rainfall, combined sewers can become overloaded with the incoming storm water. When this happens, overflows are directed into Pigeon Creek and the Ohio River, preventing the sewers from backing up into homes and onto streets. This is referred to as a combined sewer overflow event, or CSO.
Combined sewers serve mainly the older parts of town, while outlying sections of the city are served by separate sanitary and storm sewers. However,sanitary sewage from those outlying areas eventually flows through the combined sewer system before reaching a treatment plant.
Combined sewers are sanitary sewers that convey both sewage and storm runoff. During dry weather, combined sewers carry sewage from domestic, commercial, and industrial sources to the wastewater treatment plant. During wet weather events, the same sewers also convey surface runoff collected from streets, lawns, parking lots, parks, etc. to the wastewater treatment plant (WWTP). When the capacity of the WWTP is exceeded, such as during heavy rainfall events, the excess wastewater is occasionally allowed to bypass directly to surface water bodies, including the Ohio River and Pigeon Creek, through flow diversion structures. This excess wastewater is called combined sewer overflow (CSO).
The combined sewer system in Evansville is located in the south central part of the city, bounded generally by the Ohio River on the south, Pigeon Creek on the north, Carpenter Creek on the west, and Vann Avenue on the east. Figure 1.1 shows the major interceptors and the subsystems of the City's combined sewer area.
The Clean Water Act of 1972 requires CSO communities to restore their waterways. Almost 1,000 other cities in the United States have the same requirements. Pigeon Creek and the Ohio River are valuable natural resources that deserve our protection. Evansville is working closely with state and federal regulatory agencies to remediate its CSOs. In Evansville's combined sewer system, 9 CSOs discharge to Pigeon Creek, and another 13 discharge to the Ohio River. The Utility is currently developing a long-term Integrated Overflow Control Plan to address CSOs. You can see details of this 20-year plan on our Renew Evansville website.