Influence of bulking agents on speciation of heavy metals during sewage sludge composting

The major limitation of soil application of sewage sludge compost is the total heavy metal contents and their bioavailability and mobility. This study was conducted to evaluate the effect of mature compost as a bulking agent on reduction the mobility and bioavailability of Zn, Cu, Pb and Cr during the sewage sludge composting process. The three composting processes: sewage sludge(SS), sewage sludge + wood chips(SS+WC) and sewage sludge + mature compost(SS+MC) in our study showed that stable fractions of Zn, Cu, Pb and Cr could be increased over time by composting. While the different composting processes, exhibited different effects on the speciation of heavy metal. All the metals showed the relatively lower mobility and bioavailability in the SS+MC treatments. The GPC spectra shown that the concentrate of humci substances decreased in the order of SS+MC>SS+WC>SS. Furthermore, the FTIR spectral studies confirmed that humic substances ( drived from SS+MC) has more content of reactive functional groups such as C=O and carboxylic groups. Investigation of the correlation matrices of metal fractions with pH, composting temperature, humic substances and organic matter content indicated that the humic substances heavily influenced the changes of heavy metal speciation during SS+MC composting. Accordingly, the higher reduction of mobility and bioavailability of heavy metal in the SS+MC treatments mainly attributed to the enhancement of humification process by adding mature compost as bulking agent.