What is a sewer overflow and why does untreated sewage spill into our rivers, streams and creeks?

Our sewer system carries human and industrial waste and rainwater runoff away from our homes, businesses and public facilities in order to protect the public’s health and safety. When the weather is dry, sewage flows from where we live, work and play to wastewater treatment plants where it is cleaned before being returned to the environment. However, when the weather is wet and lots of rain or snow falls in short time periods, the system is unable to transport and ultimately, treat all of the combined rainwater and sewage due to its limited capacity. The additional rainwater overwhelms our aging system during these moderate to heavy storm events. To avoid flooding people’s streets and homes, the system discharges excess rainwater and wastewater into local waterways. These discharges are commonly referred to as sewer overflows.

Two types of sewer overflows can occur – combined and separated. The Clean Rivers Healthy Communities program addresses overflows that occur from combined sewers. A combined sewer is a channel or pipe which carries both sanitary sewage and stormwater runoff. When stormwater is high, due to excessive rain, the sanitary sewer provides backup capacity for stormwater runoff. A separate sewer consists of two separate pipes, one for commercial and residential wastewater or sewage; and the other for stormwater runoff.