Die-Offof Human Pathogens in Stored Wastewater Sludge and Sludge Applied to Land

ReportNo WSAA 92

March 1995

SYNOPSIS

Thebeneficial use of municipal wastewater sludge is desirable but is to someextent restricted by the presence of human pathogens. The aim of this study wasto provide information about pathogens in sludge in Australia, so that safemanagement practices could be designed.

Inthis study pathogen concentrations in the final sludge products from wastewatertreatment plants in Perth, Western Australia, were determined. The sludgeproducts had been treated by mesophilic anaerobic digestion and mechanicaldewatering. The die-off of pathogens in stored sludge and sludge amended soilwas then examined. The pathogens which were monitored in this study wereenteroviruses, Salmonella and Giardia. These were chosen on the basisof a qualitative risk assessment. Faecal coliform and faecal streptococciconcentrations were also determined.

Theconcentrations of pathogens in sludge which had been treated by digestion anddewatering were high. The risks associated with Giardia and enterovirus concentrations suggested that this sludgewas not suitable for unrestricted marketing and needed further treatment.

Itwas found that storage of sludge for one year was not suitable as a treatmentto further reduce pathogen concentrations. This was because Salmonella and faecal coliforms regrewafter one year of storage. Regrowth occurred following rainfall at the end ofsummer.

Salmonella and faecal coliform regrowthalso occurred in sludge amended soil after rainfall at the end of summer. Thissuggests that sludge which is to be used for amending soil for growing foodcrops may need further treatment, such as composting, to reduce its regrowthpotential prior to soil amendment.

Faecalcoliforms and faecal streptococci were not adequate indicators of the die-offof pathogens during anaerobic digestion, sludge storage or soil amendment.These results suggest that studies which examine the effectiveness of treatmentprocesses for removing pathogens should monitor pathogens, and not use faecalindicator bacteria as surrogates.

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