Evaluation of the dewaterability, heavy metal toxicity and phytotoxicity of sewage sludge in different advanced oxidation processes

This study thoroughly evaluated six types of sludge dewatering processes in terms of sludge dewaterability, environmental risk of heavy metal release and phytotoxicity. The assessed sludge dewatering processes included two conventional coagulants (polyacrylamide (PAM); FeCl3/CaO) and four advanced oxidation processes (AOPs) (Fenton's reagent/CaO; Fe2+/Na2S2O8/CaO; Ca(ClO)(2)/FeCl3/walnut shell (WS); thermal and acid-washed zero-valent iron (TA-ZVI)/H2O2/CaO). Compared to the conventional coagulants, the use of AOPs as conditioners presents greater potential for sludge dewatering, as AOPs have dual roles in alleviating the environmental risk of heavy metals and the phytotoxicity of dewatered sludge. Among the four tested AOPs, Fenton's reagent/CaO treatment achieved the highest sludge dewaterability improvement, with a higher water removal rate, lower specific resistance of filtration (SRF) and lower water content of dewatered sludge (Wc). This treatment was also the most cost-effective among the four AOPs. However, this method could not substantially mitigate the environmental risk due to the leaching toxicity of heavy metals and the phytotoxicity of dewatered sludge. Conversely, when considering the reduction of environmental risk, Ca(ClO)(2)/FeCl3/WS could be a better option, although its operational cost was slightly higher than that Fentons reagent/CaO. Collectively, this study provides novel insights for the selection of sludge conditioning treatments.