This paper focused on an exploratory study on reusing waste sludge of a local water treatment works (referred to as “alum sludge”) to enhance the removal of both SS (suspended solids) and COD (chemical oxygen demand) from sewage. Laboratory jar tests were conducted using the raw sewage and the alum sludge taken from a local sewage treatment works with variations in sludge dosages, aging times and pH conditions. It has been found that adding alum sludge at dosage around 20 mg/l as Al to the sewage can increase the SS removal by more than 20% and the COD removal by at least 15%, compared with the control tests without alum sludge addition. The pH of sewage, adjusted from 5 to 9, had little impact on the SS and COD removal induced by the alum sludge. However, the alum sludge aging time substantially affected the SS and COD removal. Experiment results suggested that sweep flocculation and physical adsorption play the key roles in enhancing both SS and COD removal from the raw sewage.

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