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.