Combined sewer overflow

A combined sewer overflow is a discharge of untreated sewage mixed with stormwater when a combined sewer system exceeds its capacity, often during heavy rain.

Combined sewer: A sewer system that carries both sanitary wastewater and stormwater in the same pipes. Its shared pipes create the conditions that produce overflow during intense rainfall.
Fecal indicator bacteria: Microorganisms used to estimate contamination by fecal waste in water. Their elevated levels can signal sewage contamination after an overflow.
Sewer separation: The conversion of a combined sewer system into separate sanitary and stormwater networks. Separating flows prevents stormwater from consuming capacity needed for sewage.
Chicago Tunnel and Reservoir Plan: A large Chicago-area system of deep tunnels and reservoirs built to manage stormwater and sewage. Its storage network was designed in part to reduce combined sewer overflows.
Urban flooding: Flooding in built-up areas caused by intense rainfall, overflowing waterways, or drainage failures. The same storms that trigger overflows can flood streets and overwhelm drainage networks.
Sanitary sewer: A sewer system that carries wastewater from buildings separately from stormwater. Separate wastewater pipes avoid the rainfall-driven capacity conflicts of combined systems.
Waterborne disease: Illness caused by pathogens transmitted through contaminated water. Untreated sewage can expose people to pathogens through recreation or contaminated water supplies.
Deep tunnel: A large underground tunnel designed to store or convey stormwater and sewage. Deep tunnels can temporarily hold peak flows that would otherwise overflow.
Milwaukee Metropolitan Sewerage District: The regional agency responsible for wastewater and stormwater management in the Milwaukee area. Its infrastructure and planning illustrate regional responses to overflow pollution.
Climate change and water cycle: The ways climate change alters evaporation, precipitation, and water movement. Shifts in heavy-rainfall patterns challenge infrastructure designed around historical conditions.

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