Die-off of Human Pathogens in Stored Wastewater Sludge and Sludge Applied to Land
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 92
The beneficial use of municipal wastewater sludge is desirable but is to some extent restricted by the presence of human pathogens. The aim of this study was to provide information about pathogens in sludge in Australia, so that safe management practices could be designed.
In this study pathogen concentrations in the final sludge products from wastewater treatment plants in Perth, Western Australia, were determined. The sludge products had been treated by mesophilic anaerobic digestion and mechanical dewatering. The die-off of pathogens in stored sludge and sludge amended soil was then examined. The pathogens which were monitored in this study were enteroviruses, Salmonella and Giardia. These were chosen on the basis of a qualitative risk assessment. Faecal coliform and faecal streptococci concentrations were also determined.
The concentrations of pathogens in sludge which had been treated by digestion and dewatering were high. The risks associated with Giardia and enterovirus concentrations suggested that this sludge was not suitable for unrestricted marketing and needed further treatment.
It was found that storage of sludge for one year was not suitable as a treatment to further reduce pathogen concentrations. This was because Salmonella and faecal coliforms regrew after one year of storage. Regrowth occurred following rainfall at the end of summer.
Salmonella and faecal coliform regrowth also occurred in sludge amended soil after rainfall at the end of summer. This suggests that sludge which is to be used for amending soil for growing food crops may need further treatment, such as composting, to reduce its regrowth potential prior to soil amendment.
Faecal coliforms and faecal streptococci were not adequate indicators of the die-off of pathogens during anaerobic digestion, sludge storage or soil amendment. These results suggest that studies which examine the effectiveness of treatment processes for removing pathogens should monitor pathogens, and not use faecal indicator bacteria as surrogates.