Treatment
Spent backwash water is either discharged without treatment to a sanitary sewer system or is treated and recycled within the plant. Historically, backwash water was discharged directly to surface water supplies; however, direct discharge is now highly regulated through NPDES discharge permits and is often discouraged. Used backwash water contains high concentrations of particulate material. Typical treatment processes include coagulation, flocculation and sedimentation. High molecular weight synthetic organic polymers are sometimes added to facilitate the formation of settleable floc. Failure of a backwash treatment process and reintroduction of the resulting poor quality water into the main water purification plant flow stream can cause overall process upsets and result in the production of poor quality treated drinking water.
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