Fate of Cryptosporidium oocysts, Giardia cysts, and microbial indicators during wastewater treatment and anaerobic sludge digestion

The extent of reduction in selected microorganisms was tested during both a erobic wastewater treatment and anaerobic digestion of sludge at the wastew ater treatment plant in Ottawa to compare the removal of two encysted patho genic protozoa with that of microbial indicators. Samples collected include d the raw wastewater, the primary effluent, the treated wastewater, the mix ed sludge, the decanted liquor, and the cake. All of the raw sewage samples were positive for Cryptosporidium oocysts and Giardia cysts, as well as fo r the other microorganisms tested. During aerobic wastewater treatment (exc luding the anaerobic sludge digestion), Cryptosporidium and Giardia were re duced by 2.96 log(10) and 1.40 log(10), respectively. Clostridium perfringe ns spores, Clostridium perfringens total counts, somatic coliphages, and he terotrophic bacteria were reduced by approximately 0.89 log(10), 0.96 log(1 0), 1.58 log(10), and 2.02 log(10), respectively. All of the other microorg anisms were reduced by at least 3.53 log(10). Sludge samples from the plant were found to contain variable densities of microorganisms. Variability in microbial concentrations was sometimes great between samples, stressing th e importance of collecting a large number of samples over a long period of time. In all cases, the bacterial concentrations in the cake (dewatered bio solids) samples were high even if reductions in numbers were observed with some bacteria. During anaerobic sludge digestion, no statistically signific ant reduction was observed for Clostridium perfringens, Enterococcus sp., C ryptosporidium oocysts, and Giardia cysts. A 1-2 log(10) reduction was obse rved with fecal coliforms and heterotrophic bacteria. However, the method u tilized to detect the protozoan parasites does not differentiate between vi able and nonviable organisms. On the other hand, total coliforms and somati c coliphages were reduced by 0.35 log(10) and 0.09 log(10), respectively. T hese results demonstrate the relative persistence of the protozoa in sewage sludge during wastewater treatment.