Sludge management paradigms: impact of priority substances and priority hazardous substances

As a by-product of treatment processes, municipal wastewater treatment plants (WWTP) generate large quantities of sludge, with sludge treatment focused on sterilisation, volume reduction and biogas production. Whilst the EU Sewage Sludge Directive sets limits on the concentrations of selected metals in sludge applied to agricultural land, the potential impact of many EU Water Framework Directive priority and priority hazardous substances (PS/PHS) on human or environmental health has yet to be fully addressed. Research presented here shows that treated sludge from five urban WWTPs experiencing differing local conditions contain a range of PS/PHS including substances whose use has been banned or heavily restricted. Concentrations reported in this study do not exceed the limit values set for the four PS/PHS currently included in the EU Sewage Sludge Directive. However, more stringent national limits are exceeded. The basis for developing and applying Predicted No Effect Concentration (PNEC) values for the application of sludge to agricultural land is still unclear. However, comparison between PS/PHS sludge concentrations and available PNEC soil values clearly indicate the need for further research. Implications and research priorities arising from these findings in terms of achieving compliance with EU Sewage Sludge and Water Framework Directives are discussed.