Parametric study of coagulant recovery from water treatment sludge toward water circular economy

The aim of this study was to recover coagulants from water treatment sludge (WTS) of water treatment plants. The morphology and chemical composition of WTS were investigated using scanning electron microscopy and Fourier-transform infrared spectroscopy analysis, respectively. For coagulant recovery, an acidification method using different normality of sulfuric acid was used to solubilize the alum sludge and polyaluminum chloride (PAC) sludge. The performance of the recovered coagulant was evaluated for the removal of turbidity in surface water treatment at different coagulant dosages and pH values. It was found that the optimum normality of sulfuric acid for coagulant recovery was 1.5 N for alum sludge and 1.0 N for PAC sludge, in which the percentage of turbidity removal achieved more than 70%. In the next stage, the turbidity removal seems to increase with the increase of coagulant dose, which is more than 86% at 30-60 mg/L. The best pH values that promoted high turbidity removal for both recovered coagulants are between pH 5 and 7. Therefore, the overall results show that PAC as the recovered coagulant achieved higher results in the efficiency of turbidity removal than alum and is beneficial for water treatment plants to save costs.