Biosolids from two-stage bioleaching could produce compost for unrestricted use.

A biological process, called BIOSOL, developed in our laboratory, efficiently reduces metals and reduces pathogenic indicator bacteria in sewage sludges in a single-stage system to levels meeting Ontario Ministry of the Environment (OME) requirements for land application of biosolids. Since the requirements for unrestricted use of compost are 10 to 50 times (depending on the metal involved) more restrictive than those for land application, this study was carried out to determine whether the efficiency of the BIOSOL system could be increased to meet the more stringent limitations required for compost. A two-stage modified BIOSOL system was successfully operated for a period of 4% months, at 30 degrees C, treating anaerobically digested sludge from the Guelph Wastewater Treatment Plant. Elemental sulphur (4g l(-1)) was the energy source for the autotrophic bacteria (thiobacilli). The pH, metals (Cd, Cr, Cu, Pb and Zn), and sulphate were used to evaluate system effectiveness. At an HRT of 8 days in each solubilization tank, and a S*-concentration of 4g l(-1), sulphur oxidation efficiency was about 70%, while the metal removals were: Cd 90%, Cr 93%, Cu 96%, Pb 67% and Zn 98%. The finished product (biosolids) met the OME requirements for metal concentrations for producing compost or fertilizer for unrestricted use. The reduction of pathogenic indicator organisms was demonstrated in earlier studies.