In older cities, the sewers carry both wastewater and rainwater that runs off of the streets. These are called combined sewers. During heavy rain events, the flow in combined sewers can reach capacity and threaten to back up into your house. To prevent this, relief structures called combined sewer overflows (CSOs) are constructed sending the excess water into streams, lakes and harbors. CSOs can grossly pollute rivers and lakes and thus efforts are being made to limit their discharge. Today, separate sewers are installed, with rain runoff discharge to surface waters via storm sewers and wastewater routed to the treatment plant with no discharge en route.

EXPLORE: See how the city of Milwaukee uses deep tunnels to store and later treat rainwater and wastewater from combined sewers.