Recently, increasing attention has been paid to heavy metals in sludge. However, limited literature could be found on the distribution characteristics of heavy metals in sludge and their potential risks. In this study, sludges from wastewater plants in Jiangsu Province (China) were selected for the investigation of heavy metal loadings, showing that typical heavy metal levels were in the order of Zn > Cu > Cr > Ni > Pb > As > Hg > Cd, ranging from 154 to 2970 mg/kg, 28 to 1150 mg/kg, 10 to 136 mg/kg, 9 to 262 mg/kg, 0 to 79 mg/kg, 12.1 to 41.6 mg/kg, 0.67 to 19.50 mg/kg and 0.21 to 2.77 mg/kg, respectively. Analysis of the typical heavy metal distribution in sludge indicated that Hg, Zn and Cu were obviously influenced by the degree of industrial intensity and exploitation of human activities, while Ni, Cd, Pb, As and Cr were more evenly distributed. Effects of sewage sources and wastewater-treatment processes on heavy metal levels implied that different industrial wastewaters resulted in different metal contents, but the distribution of Ni, Cd, Pb, As and Cr in different treatment processes was similar. Furthermore, Hg and Cd had the strongest ecological risk, with their levels reaching severe, suggesting that sludge was not recommended for agricultural reuse in this study.

Recently, due to the abundance of carbon, nitrogen and phosphorus in municipal sewage sludge (MSS), the energy potential of MSS has become increasingly prominent. Economically developed regions possess more financial and policy support advantages for the development of MSS energy recovery technology; hence, the selection of the appropriate sludge treatment and disposal technologies to maximize the energy potential of MSS is of great importance. In this study, the energy recovery potential of MSS was evaluated on the basis of regional economies, sludge analysis, a main sludge energy reuse technology review and legislative profile analysis. As the most commonly adopted technology, incineration had a lower energy potential, which may be ascribed to the high moisture content, high disposal costs and difficulties in in situ energy recovery. In contrast, the energy potential for anaerobic digestion, pyrolysis and gasification was relatively high. By conducting a SWOT-FAHP analysis, management evaluation of these four technologies was carried out from the following four perspectives: problem solving, technology development, ecological environment and laws and regulations. Pyrolysis was evaluated to be the most suitable technology from the technical and environmental perspectives because its products had high energy potential and avoided heavy metal problems. The obstacles and problems that pyrolysis technology might encounter in commercial applications in the future are discussed. With this energy-generating, low energy consumption, low-pollution sludge energy reuse technology, the potential of sludge pyrolysis would be high in the long run. These results revealed the factors affecting the energy recovery potential of sludge, and comprehensively evaluated the technologies from the aspects of problem solving, environmental impact, technology development and law, the optimal solution obtained could provide reference on the management decision of sludge disposal technology for economically developed areas in the future.