Combined use of inorganic coagulants and cationic polyacrylamide for enhancing dewaterability of sewage sludge

Coagulation or flocculation of waste sludge by coagulants or flocculants is often used alone or in hybrid conditioning with other preconditioning for sludge dewatering. However, combined conditioning with coagulants and flocculants for sludge dewatering is rarely reported. This work explored the combined use of inorganic coagulant (Fe3+ or polyaluminum chloride) and cationic polyacrylamide for sludge dewatering. The results showed the combined use of coagulant and flocculant could achieve the same or similar filterability of that with individual use of coagulant or flocculant based on capillary suction time value, while the dosage decreased by 56.55% for Fe3+ and 44.49% for cationic polyacrylamide, or 49.94% for polyaluminum chloride and 29.12% for cationic polyacrylamide. Moreover, a higher efficiency of water content reduction was obtained for sludge with the combined use of coagulant and flocculant. By studying the basic physicochemical properties and rheological profiles of the sludge system, the results indicated that the mechanism was a synergistic effect of coagulation and flocculation, including disintegration of microbial cells and agglomeration of sludge flocs, which efficiently reduced bound water, leading to the lowest apparent viscosity in the Non-linear regime. Accordingly, the lowest bound water content and best flowability were obtained for sludge with the combined use of coagulant and flocculant. These results suggested that the combined use of inorganic coagulants and cationic polyacrylamide is more favorable for improving sludge dewatering performance than with individual use of coagulant or flocculant. (C) 2018 Elsevier Ltd. All rights reserved.