Keywords: 
• sewage sludge /
• compost /
• heavy metals /
• speciation

Abstract: BCR (European Community Bureau of Reference) sequential extraction method was used to investigate the effects of different ratios of maize straw to sewage sludge on speciation of Cu and Zn during composting process. The distribution ratios of different forms of Cu and Zn could reflect the transformation of heavy metals between the mobile fractions (acid-soluble fraction, reducible fraction, oxidizable fraction). The concentrations of Cu and Zn in the composted mixture increased, however, the absolute content of Cu and Zn remained stable after the composting process. It was found that the heavy metals were only transformed between the mobile fractions, the distributions ratio of residual fractions remained stable. The concentration factors of Cu and Zn increased with the ratios of maize straw to sewage sludge. Higher proportion of maize straw might contribute to the transformation of acid-soluble fraction of Cu to oxidizable fraction. Cu was mainly found in reducible fraction and oxidizable fraction. The mobility and bioavailability of Zn increased during the composting process. Zn mainly existed in acid-soluble fraction and reducible fraction. The lower ratio of maize straw, the higher concentrations of Cu and Zn in the compost product, but Cu and Zn didn’t play a decisive role on seed germination.