Changes in Contents and Forms of Heavy Mentals in the Aerobic Co-composting Process of Urban Sludge Sewage with Four Crop Straws

Four different crop straws(rape straw,wheat straw,corn straw and rice straw) were selected as the bulking agents for high-temperature aerobic composting of urban sewage sludge.The changes in contents and forms of Cd,Cr and Pb in the sludge sewage compost were investigated.The results showed that composting increased the total contents of the three heavy metals,reduced the content and bioavailability of exchangeable Cr and Pb.Composting increased exchangeable Cd and did not decrease its bioavailability,thus increasing the risk of Cd transfer to the plants when the sewage sludge compost was applied to the soil.Adding corn straw proved to be conducive to reducing carbonate bound and increasing residual fraction with Cd.The most increase of Cr in organic bound was the rape straw treatment,while the most increase in residual fraction was the corn straw treatment.Corn straw was more effective to reduce the organic bound fraction,while rice straw was more effective to increase the residual fraction for Pb than the other three crops.The above results indicated that there is some difference in the risks of heavy metals in the sewage sludge composted with different bulking agents.