Characteristics of leachate from pyrolysis residue of sewage sludge

The pyrolysis residue (SP) of sewage sludge (SS) produced at 500 °C was subjected to batch and column leaching tests to investigate the release of its organic and inorganic constituents and metals. For comparison, incineration ash (SI) obtained from a SS incinerator was also tested. Pyrolysis and incineration reduced organic matter of SS from 0.78 kg kg−1-dry SS to 0.16 and 0.01 kg kg−1-dry SS, respectively. Heavy metals remained in SP without being volatilized, although Cd and Pb were transferred into the off-gas during incineration.In the batch leaching test with the leaching liquid-to-solid mass ratio (L/S) = 10, the pH of the SS, SP, and SI filtrates was 6.3, 7.9, and 11.0, respectively. The total organic carbon concentrations were in the order SS (877 l mg l−1) ≫ SP (99 mg l−1) > SI (26 mg l−1). The SP and SI filtrates met the landfill standard for the Cd and Pb concentrations (<0.3 mg l−1). In the column tests, although the SP contained more organic matter than that of SI, its carbon discharge into the leachate under aerobic conditions was similar to that of SI under anaerobic conditions. The leaching of heavy metals, such as Cd, Cr, Pb, and Zn, was also suppressed in SP during the active decomposition of organic matter.We demonstrated that pyrolysis reduces the potential release of pollutants from sewage sludge in landfill, making it a promising method of treating sewage sludge before landfilling.