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

Abstract

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 × 10³ to 7.00 × 10⁵MPN/Kg before anaerobic digestion and from < 6.25 to 2.52 × 10⁵ MPN/Kg after treatment. The concentration of Giardia cysts as determined by immunofluorescence ranged from 7.33 × 10⁴ to 3.30 × 10⁶ cysts/Kg of undigested sludge and 1.00 × 10⁵ to 4.14 × 10⁶ cysts/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.

Key words:

• enteroviruses
• Giardia
• anaerobic digestion
• sewage treatment