Chemical treatment of sludge: in-depth study on toxic metal removal efficiency, dewatering ability and fertilizing property preservation.

The presence of toxic metals in municipal sewage sludge restricts the application of this biomass in agricultural area. A chemical leaching process using a combination of inorganic acid and two oxidants has been developed for sludge decontamination. The present study investigated the effects of the concentrations of sulphuric acid (H2SO4), hydrogen peroxide and ferric chloride on metal solubilization from sewage sludge, as well as preservation of fertilizing properties (nutrient content) and dewatering ability of the treated sludge. The analysis of the results from batch leaching tests has allowed to define the optimal conditions for the reagent concentrations, which are 56 kg Fe3+ tonne(-1) of dry sludge solids (tds), 8 kg H2O2 tds(-1), and enough H2SO4 to reach a pH between 2.0 and 2.5 but less than 142 kg H2SO4 tds(-1). Finally, under these conditions, oxidoreduction potential values are found to be between 450 and 475 mV.

Electrolytic recovery of lead in used lime leachate from municipal waste incinerator.

The purification of municipal incinerator gases generates huge quantities of both fly ashes and used lime, containing high amount of heavy metals (Pb, Cd, Zn and Hg). The used lime is mainly contaminated by Pb and often considered as hazardous waste. This paper is related to a laboratory study on the performance of electrochemical techniques used to recover Pb from leachate produced during used lime decontamination. Two types of electrolytic units (monopolar and bipolar electrode cells) using different electrode materials were studied. Effectiveness was measured in terms of energy consumed, weight of residue sludge produced and reduction in Pb concentration. Results showed that the best performances for Pb removal (in terms of effectiveness and cost) were obtained by initially adjusting the pH of UL leachate to pH 7.5-8.5 with sulphuric acid followed by electrochemical treatment using bipolar electrode (mild steel) system operated at current intensity of 1.5A through only 5 min. The yield of Pb removal was 98.8+/-1.3% and an amount of 4.9+/-1.3 kg trt-1 of metallic sludge was produced. The optimal conditions determined for Pb recovery involved a total cost of CAN$ 1.66 t-1 of dry residue treated, including acid consumption, energy consumption and metallic sludge disposal.