Combined sewer

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.

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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 stormwater 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.^[1] 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 stormwater exceeds the sanitary flow, the sewage from homes would be diluted.

Recently, municipalities have began to look for ways to mitigate the environmental effects of such overflow locations. One solution is to build a CSO facility, which consists of some low level treatment, storage, and return of the sewage to the normal system. As described above, in the initial setup, water would flow through the relief structure out into a body of water. In this new arrangement, the water would be diverted through a channel into a treatment building. Typically, only mechanical treatment (screening of solids) would be completed. The sewage which previously flowed into the water, would flow into a large storage tank - typically underground. That tank would have the capacity to hold runnoff from all but the largest storms which occur once every 100 years or less. Once the storm passes, the facility's pumps would send the retained water back into the system to be treated under the normal dry-weather process. The result of this effort is the near elimination of raw sewage flowing into the body of water. An example of this type of system is currently being constructed for the New York City Department of Environmental Protection (NYCDEP) in Brooklyn at Paerdegat Basin

References

See also

• Sewage treatment
• Storm drain
• Stormwater
• Water pollution