To determine the fate of Cryptosporidium parvum oocysts during wastewater treatment, a model of an activated sludge treatment plant was designed with a flow of 17 ml/min and a detention time of 6 hours. Samples of raw sewage were seeded with oocysts and primary and secondary effluents were analyzed for C. parvum using an immunofluorescent technique. To compare removal efficiencies of oocysts by various wastewater treatment processes, raw sewage, activated sludge, trickling filter and biodisc effluents were seeded with oocysts and settled for 2 hr and for the respective detention times. Sludge produced by a wastewater treatment plant and anaerobically digested at 37° C in a laboratory digester was also seeded with C. parvum oocysts. Oocyst inactivation was measured by excystation and direct counts. Removal of oocysts in primary and secondary sedimentation averaged 83.4% and 90.7% respectively. The total oocyst removal in sewage treatment averaged 98.6%. In comparison with other treatment processes, activated sludge had the maximum oocyst removal efficiency at 92%. The anaerobic digestion process inactivated 90% of the oocysts within four hours of exposure. 99.9% of the oocysts were eliminated by anaerobic digestion after 24 hours. This demonstrates that the activated sludge process and anaerobic digestion can be effective for the removal and inactivation of C. parvum oocysts.
Sludge produced by a model wastewater treatment plant and anaerobically digested at 37°C in a laboratory digester was seeded with G. muris cysts. Samples from the digester were collected at 0:15, 4:00, 8:00, 11:30 and 26:00 hrs. Cyst inactivation was measured by excystation and direct counts of G. muris cysts using a hemacytometer and a phase contrast microscope. The results showed that 99.9% of the cysts were inactivated within apgroximately 18:00 hrs of exposure. This demonstrates that the standard sludge digestion (37°C for 24 hrs) will eliminate most of the cysts (greater than 99.9%).
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