Recovery and reuse of coagulants from treatment of water and wastewater

Coagulants used in many water treatment processes are increasing in cost and supply problems. For this reason, finding more efficient means for reclaiming and reusing these chemicals would offer substantial economic benefits in the operation of water and wastewater treatment systems. This was the objecti ve of the research project reported herein. In laboratory experiments, it tested procedures for economical recovery of hydrolyzed ·metallic coagulants used in treatment of polluted surface waters and selected wastewater discharges . Investigations focused on treatment systems using aluminum and iron salts as coagulants and precipitants. The experiments sought answers to three principal questions: ( 1) How much coagulant can be recovered? (2) What is the best means of separating and dewatering the remaining sol ids? (3) How effective is the recovered coagulant during reuse? The investigation included an economic analysis to estimate the capital cost of implementing the process, and to determine what savings may be expected in chemical costs. In addition, a computer model was developed for determining, under selected conditions, the economic break-even point with reference to treatment plant capacity. Research findings indicate that recovery of both aluminum and iron salts is technically feasible, though the potential for aluminum recovery appears greater . Among addjtional benefits from the process described are large reductions in sludge volume and improved settleability and filterability of the remaining sludge. Laboratory tests also indicate that the efficiency of recovered metal in coagulating suspended solids is not materially reduced from that of fresh coagulant.