Removal of Trace Heavy Metals from Sewage Using the Lime-Magnesium Process

Abstract

Abstract The research described in this paper is directed toward optimizing conventional lime precipitation of heavy metals by the addition of small quantities of magnesium. Specifically, jar tests were performed to evaluate the effectiveness of adding various concentrations of Mg2+ to both raw sewage and primary effluent to improve the precipitation of metal ions at various reaction pH values. The desired pH levels (10.0, 10.6 and 11.4) were obtained by the addition of lime. General results of the study, using Cr3+, Cu2+, Pb2+, Ni2+ and Zn2+ in concentrations of 0.5, 2.5 and 5.0 mg/l, indicated the following: 1. as expected, the higher the reaction pH, the greater the removal efficiency; 2. for a given reaction pH, the presence of some added Mg++ increased the removal efficiency for all metals tested except Ni2+; 3. on the average, a lime dosage of 220 mg/l (reaction pH=10.6) coupled with an Mg2+ concentration of 30 mg/l, give equivalent removals to high lime treatment at 400 mg/l (reaction pH=11.4); 4. the removal efficiencies varied between 10% and 35% for Ni2+, while the other metals gave results of 35% to 95%, depending upon treatment conditions. It can be concluded that the addition of Mg2+ does indeed enhance the removal of many heavy metals although more work is needed to optimize the combinations for a given type of sewage and to subsequently investigate relative economics.