The reuse of water treatment sludge as a coagulant for post-treatment of UASB reactor treating urban wastewater

The feasibility of reusing polyaluminium chloride-based water treatment sludge as a coagulant for the post-treatment of upflow anaerobic sludge blanket (UASB) reactor treating urban wastewater was assessed. Response surface methodology with Box-Behnken design was employed to optimise three variables affecting coagulation/flocculation process namely sludge dose, initial pH and fresh coagulant dose. Results of the modelling study gave the following optimum conditions: sludge dose 15 g/L, initial pH 9 and fresh coagulant dose 4.2 mg Al/L; and predicted 72% COD removal and 88% turbidity removal. Confirmative experiments at the optimum conditions gave COD and turbidity removals of 74% and 89%, respectively, which were in close agreement with the predicted values. Further, at optimum conditions high removals of phosphate (79%), suspended solids (84%), BOD (78%) and total coliforms (99.7%) were obtained. The experiments also showed that a combination of water treatment sludge with fresh coagulant would give a substantially higher contaminant removal compared to the removals obtained by fresh coagulant alone. The study presents a novel reuse alternative for water treatment sludge and the results suggest that reuse of water treatment sludge as a coagulant for the post-treatment of UASB reactor effluent would be an attractive option.