Evolution of heavy metal speciation during the aerobic composting process of sewage sludge

The contents of heavy metals and their bioavailability to the soil-plant system were the major limitation to the application of sewage sludge compost in soil. This study was conducted to determine the evolution of heavy metal speciation in the course of an aerobic composting, and investigate the influence of changes of composting process parameters including pH, temperature and organic matter (OM) content on distribution of heavy metal speciation in composted sludge. The sequential extraction procedure developed by Tessier et al. was used in sludge compost to determine the heavy metal speciation. Results showed that, during composting, (1) the contents of the residue fraction for Pb, Zn and Cd were decreased but those for Ni and Cr were increased; the Cu residue fraction was almost constant; (2) the contents of the total mobile fractions (including fractions 1-4) for Zn and Pb were significantly increased, but the increase of those for Cu and Ni were not so remarkable; (3) there were significant degrees of correlation between heavy metal fractions and changes of some selected parameters (for example, pH, composting temperature, and OM content). Only the content of the total mobile fractions for Cu could be predictable from its total content. For the prediction of the total mobile fractions of Zn, Ni, Cd and Cr, the R(2) value was significantly increased by the inclusion of other variables such as pH, temperature and OM content.