Speciation of heavy metals during co-composting of sewage sludge with lime J.W.C. Wong * , A. Selvam Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Received 28 February 2005; received in revised form 24 August 2005; accepted 29 August 2005 Available online 8 November 2005 Abstract During composting the humification of organic matter will have a significant effect on the physicochemical form of existence of heavy metals. Therefore the present study was conducted to investigate the effect of co-composting sewage sludge with lime on heavy metal speciation and the changes in DTPA extractable metals. Metal speciation was con- ducted to evaluate the redistribution of Cu, Mn, Ni, Pb and Zn in sewage sludge composted with lime. Sewage sludge was mixed with sawdust in 2:1 (w/w fresh weight) and then composted with lime at 0%, 0.63%, 1% or 1.63% (dry weight) for 100 days. The lime addition did not cause any changes in the different forms of Cu and Mn, but the com- posting process caused transformations of residual form of Cu and Mn into oxidizable and reducible form, respectively. For Ni, the reducible form was mainly transformed into residual form and lime addition decreased this transformation. Major transformation of different forms of Pb was not found, however the residual form of Pb increased with lime addi- tion. The predominant residual form of Zn was mainly transformed into oxidizable form and the lime addition reduced this transformation. Addition of lime to sewage sludge during composting resulted in lower DTPA extractable metal contents. Therefore, lime is a suitable material to co-compost with sewage sludge to reduce the availability of heavy metals. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Co-composting; DTPA extraction; Heavy metals; Sequential extraction; Sewage sludge; Lime 1. Introduction Composting followed by land application represents one of the most economical ways for the treatment and final disposal of sewage sludge because it combines material recycling and sludge disposal at the same time (Fang et al., 1999). Unfortunately, the presence of high levels of heavy metals often hinders agricultural land application of the composted sludge. The total heavy metal content in sewage sludge is about 0.5–2% on a dry weight basis (Wong and Henry, 1988), and in some cases, about 4% (Tyagi et al., 1991; Jain and Tyagi, 1992). Uptake of heavy metals by plants and subsequent accumulation along the food chain is a potential threat to animal and human health. Consequently, focus on the improvement of the composting process to minimize the mobility of heavy metals using various additives is receiving more attention (Wong et al., 1997; Ho and Qiao, 1998; Fang et al., 1999; Zorpas et al., 2000; Chiang et al., 2001). Lime is considered as one of the common amendment materials to co-compost with 0045-6535/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.08.045 * Corresponding author. Tel.: +852 3411 7056; fax: +852 3411 5995. E-mail address: jwcwong@hkbu.edu.hk (J.W.C. Wong). Chemosphere 63 (2006) 980–986 www.elsevier.com/locate/chemosphere