Activated sludge is a process dealing with the treatment of
sewage and
industrial wastewaters. Basically atmospheric air or pure
oxygen is forced into raw sewage (or industrial wastewater) combined with organisms to develop a biological
floc which reduces the
organic content of the
sewage. The combination of raw sewage (or industrial wastewater) and biological mass is commonly known as Mixed Liquor. In all activated sludge plants, once the sewage (or industrial wastewater) has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated
supernatant is run off to undergo further treatment before discharge. Part of the settled material, the
sludge, is returned to the head of the
aeration system to re-seed the new sewage (or industrial wastewater) entering the tank. This fraction of the floc is called Return Activated Sludge (R.A.S.). Excess sludge which eventually accumilates beyond what is returned is called Waste Activated Sludge (W.A.S.). W.A.S is removed from the treatment process to keep the ratio of biomass to food supplied (sewage or wastewater) in balance. This is called the F:M ratio. W.A.S is stored away from the main treatment process in storage tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal. Sometimes another term for W.A.S is S.A.S (Surplus Activated Sludge), both terms have the same meaning.
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Product that results when primary effluent is mixed with bacteria-laden sludge and then agitated and aerated to promote biological treatment, speeding the breakdown of organic matter in raw sewage undergoing secondary waste treatment.
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