Effect of anaerobic digestion on the occurrence of enteroviruses and giardia cysts in sewage sludge

The disposal of anaerobically digested sewage sludge onto farmland has created the necessity to evaluate the fate of pathogens which are often present. The occurrence of enteroviruses and Giardia cysts in mesophilic anaerobically digested sludge before and after treatment, was monitored for a period of 14 months. The concentration of enteroviruses in sludge determined by Most Probable Number (MPN) ranged from 4.36 x 103to 7.00 x 105MPN/Kg before anaerobic digestion and from < 6.25 to 2.52 x 105MPN/Kg after treatment. The concentration of Giardia cysts as determined by immunofluorescence ranged from 7.33 x 104to 3.30 x 106cysts/Kg of undigested sludge and 1.00 x 105to 4.14 x 106cysts/Kg of anaerobically digested sludge. The percentage virus removal after anaerobic digestion varied from 68 % to > 99.94%. The levels of intact Giardia cysts did not show any significant removal after sludge treatment. However, cyst viability could not be assessed by the method used for detection. Results of this study suggest that significant concentrations of both groups of pathogens can be present in anaerobically digested sludge.