A
combined sewer is a
sewage collection system of pipes and tunnels designed to also collect
surface runoff. Combined sewers can cause serious
water pollution problems during
combined sewer overflow (CSO) events when wet weather flows exceed the
sewage treatment plant capacity. This type of sewer design is no longer used in building new communities (because current design separates
sanitary sewers from runoff), but many older cities continue to operate combined sewers.