Effect of co-pyrolysis of sewage sludge with different plastics on the nitrogen, sulfur, and chlorine releasing characteristics and the heavy metals ecological risk of biochar

The high calorific value, low ash content and high volatility of the plastic can compensate for the deficiencies of the single pyrolysis of sewage sludge (SS), and these properties can improve the quality of the pyrolysis product. However, secondary contaminants may be generated during the co-pyrolysis of sludge and plastics. Plastics contain polluting elements that may also enter sludge, so different plastics may affect the release of N, S, and Cl as well as the ecological risk of heavy metals. In this study, polypropylene (PP), polyamide 6 (PA6) and polyvinyl chloride (PVC) were added to sludge to analyze the release of N, S and Cl from sludge pyrolysis products by different plastics. An environmental risk assessment was also conducted to evaluate the various forms of heavy metals. The results showed that PP, PA6, and PVC increased the total mass losses of sludge by 16.88%, 6.82% and 9.46%, respectively, inhibiting the release of NH3, CH3Cl, H2S and SO2 harmful gases from sludge. PP and PA6 increased the calorific value of pyrolysis oil and released more aliphatic hydrocarbons and alcohols. However, PA6 and PVC will increase the release of HCN and HCl, respectively. The presence of plastics enhanced the carbon structure of biochar, with PP and PA6 particularly enriching the pore structure and surface properties of biochar. PA6 and PVC reduced the potential ecological risk of biochar. These results will provide new insights into the co-pyrolysis of sludge and plastics.