Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting

A batch composting study was performed to evaluate the feasibility of co-co mposting sewage sludge with lime, aiming at reducing the availability of he avy metals in the sludge compost. Sewage sludge with sawdust as bulking age nt was amended with lime at 0, 0.63, 1.0, and 1.63% w/w, and composted for 100 days in laboratory batch reactors. The changes in temperature, pH, elec trical conductivity (EC), and extractable heavy metal contents were measure d while compost maturity was determined by C/N-organic and cress seed germi nation during the composting period. Liming raised pH of compost effectivel y at the initial stage of composting and caused a decrease in EC through pr ecipitation of soluble ions. Lime amendment also significantly reduced wate r-soluble and Diethylene triamine pentracetic acid (DTPA)-extractable metal contents. The maximum reductions were 60 and 40% for Cu, 80 and 40% for Mn , 55 and 10% for Zn, and 20 and 25% for Ni at the end of the composting per iod for the lime-amended sludge as compared to the control. In spite of the inhibitory effect of lime amendment on the decomposition activity of sewag e sludge, all treatments reached maturation after 63 days of composting as indicated by the results of C/N-organic ratio and cress seed germination te st results. A lime amendment of less than or equal to 1.0% is recommended t o co-compost with sewage sludge. (C) 1999 Elsevier Science Ltd. All rights reserved.