Abstract
The effect of sludge solids concentration on the process that metals bioleaching and simultaneous microbial production of sulphuric acid by sewage sludge was investigated. In the work five solids concentrations (5, 10, 20, 35, and 55 g/L) were tested. The results showed that the final pH of solids concentrations of 20 g/L was the lowest (pH = 0.91) and the rate of sulfate production decreased in the order: 20 g/L > 35 g/L > 10 g/L > 5 g/L > 55 g/L > Control. The optimum metals (Zn, Cu, Mn, Cd, Mg, Ca, and Cr) solubilization was obtained at a sludge concentration of 10 g/L with the efficiency of 76.36% Cr, 90.8% Ca, and 98–100 % for the other metals. Metals Al and Pb obtained the highest dissolution at a concentration of 20 g/L, with the efficiency of 99.98% and 51.71%, respectively, while the minimum one obtained at sludge concentration of 5 g/L with the solubilization efficiency of 50–90%. In a moderate sludge concentration range, the higher the concentration, the longer time was needed to achieve the favorable metals dissolution. After the solid–liquid separation of the acidified sludge, the concentration of 5 g/L obtained the most quantity of acid liquid but the pH was not the lowest. Sludge of 55 g/L achieved the least acid liquid of higher pH through precipitation.