In order to investigate the occurrence of Giardia cysts and Cryptosporidium oocysts in raw sewage and the removal in sewage treatment processes, we conducted a field survey at 9 sewage treatment plants. The results can be summarized as follows : 1) In all plants, Giardia cysts were detected in raw sewage and effluent of primary sedimentation. Cyst densities in raw sewage ranged from 130 to 5,500 cysts·l-1 (geometric mean 1,350 cysts·l-1). Cyst densities in effluent of primary sedimentation ranged from 150 to 5,200 cysts·l-1 (geometric mean 800cyst·l-1). 2) Cryptosporidium oocysts were detected from only one raw sewage sample with the density of 28 oocysts·l-1. 3) Indicator bacteria counts tested, C.perfringens spores, coliforms and E.coli, were statistically correlated with Giardia cyst densities and the most significant relationship was observed between C.perfringens spore counts and Giardia cyst densities. 4) No correlation was observed between Giardia cysts removal and indicator bacteria removal. However, significant relationship was observed between average removal of C.perfringens spores and Giardia cysts. These results indicated that C.perfringens spores can be useful indicator for the evaluation of protozoan densities and removal. 5) Turbidity appeared to be the most appropriate indicator for Giardia cysts removal in activated sludge treatment process.
