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A 'Combined sewer' is a type of sewer system which provides partially separated channels for sanitary sewage and stormwater runoff. This allows the sanitary sewer system to provide backup capacity for the runoff sewer when runoff volumes are unusually high, but it is an antiquated system that is vulnerable to sanitary sewer overflow during peak rainfall events.
In many older cities collection of sanitary sewage, the sewage that emanates from homes or businesses, was collected by a single sewage system that also collected the storm water runoff from the streets and roofs. Since this type of combined collection system included both rainwater and sewage from homes and businesses it is referred to as a 'Combined Sewer System' or a CSS. The thought during that time was that it would be cheaper to build just a single system. Since it was expensive to build collection lines, they were only designed to handle certain size storms. These sewers designed relief structures in the sewer system so that when the sewer was overloaded with too much flow the water would exit the sewer system and into a nearby body of water through a relief sewer to prevent back-up into the street or homes.
A ''Combined Sewer Overflow'', or CSO, is an apparatus built into a 'Combined Sewer System'. The arrangement is designed to allow a certain amount of flow to discharge into a water course untreated to keep the system from becoming surcharged in storm conditions. They often contain a screen, which may be either a mechanical or static arrangement, depending on the frequency of spills per year. During heavy rainfall when the storm water exceeds the sanitary flow, the sewage from homes would be diluted.