Elutriation and chemical addition are the two sludge conditioning alternatives. Conditioning is an intermediate process between primary and secondary anaerobic digesters. It also improves dewatering of digested sludge.
Iron, aluminum salts, and lime are the most common chemicals for conditioning.
Elutriation is the washing of suspended sludge held in suspension by air or stirring. It reduces alkalinity and makes the sludge more filterable. Single and multitank (countercurrent) elutriation are both used. The single-tank system uses repeated sludge washing, whereas in the coun-tercurrent system, fresh wash water is added to the last sludge tank. The wash water overflow from the last-stage tank furnishes the wash for the previous tank. Although the countercurrent system requires additional tanks and piping, it uses less makeup wash water.
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