Prevalence of Cryptosporidium oocysts and giardia cysts in the drinking water supply in Japan.

A one-year monitoring of Cryptosporidium oocysts and Giardia cysts was conducted at a water purification plant. A total of 13 samples of 50 L river source water and 26 samples of 2,000 L-filtered water, treated by coagulation flocculation, sedimentation and rapid filtration, were tested. Prior to conducting a survey of a water purification plant, we developed a method for concentrating Cryptosporidium oocysts from a large volume of raw or filtered water using a hollow fiber ultrafiltration (UF) membrane, and this procedure was adapted to survey a water purification plant. Cryptosporidium oocysts were detected in all of the 13 raw water samples. The geometric mean concentration was 40 oocysts 100 L. Giardia cysts were detected in 12 of 13 raw water samples (92%) and the geometric mean concentration was 17 cysts/100 L. Probability distributions of both Cryptosporidium oocyst and Giardia cyst concentration in raw water were nearly lognormal. In filtered water samples, Cryptosporidium oocysts were detected in 9 of the 26 samples (35%) with the geometric mean concentration of 1.2 oocysts /1,000 L and Giardia cysts in 3 samples (12%) with 0.8 cysts/1,000 L. The estimated log10 removal efficiency of Cryptosporidium oocysts and Giardia cysts by rapid-sand filtration was 2.47 and 2.53, respectively. Empty particles were removed at a higher log10 than intact oocysts and cysts. The efficiency of particle removal in the rapid sand filtration process tends to be reduced under cold-water conditions. Close management is necessary in the winter when the water temperature is low.