What is a Combined Sewer Overflow (CSO)?

Combined sewers are sewer pipes designed to collect both wastewater and stormwater runoff in the same pipe. A combined sewer overflow (CSO) occurs when a large rainstorm overwhelms the combined sewage system causing stormwater to mix with wastewater (or sewage) and discharge to a nearby river or lake. This relief measure prevents sewage backups into homes, businesses, and streets.

Figure 1. Dry weather conditions in the combined sewer system versus Wet weather conditions in the combined sewer system - high flows can cause a combined sewer overflow (CSO)

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