| Research Abstract |
The effects of free chlorine and ozone on infectivity of Cryptosporidium parvum oocysts were evaluated. Free chlorine CT (the product of concentration and contact time) requirement for reduction in infectivity was highly dependent on exposed chlorine concentration at the range of 1 to 100 mg/L.Obtained relationship between CT values for n log reduction in infectivity (CTn mg min/L) and exposed mean free chlorine concentration (C mg/L) was CTn = n・(21.3C+793) at 20 C, pH 7.0. As for ozone, CT requirement did not show any dependency on ozone concentration at the tested range of 0.1 to 1.0 mg/L and obtained ozone CT values for 2 log reduction in infectivity were in narrow range of 2 to 3 mg min/L.
The efficacy of combined disinfection with ozone and chlorine was tested. Oocysts were exposed to ozone with the CT of 0.75, 2.0 and 3.0 mg min/L and then exposed to free chlorine of 1,400 mg min/L.Achieved reductions in infectivity by combined disinfection with chlorine following ozone were 2.17, 1.68, 1.43 log for 0.75, 2.0, 3.0 mg min/L of ozone, respectively. As reduction in infectivity by chlorine of 1,400 mg min/L was 1.74 log, excess reductions by combined disinfection were as small as 0.97 log, -0.06 log, -0.31 log for 0.75, 2.0, 3.0 mg min/L, respectively. Thus, synergistic increase in reduction in infectivity was observed in case of 0.75 mg min/L of ozone, but additional reduction by synergistic effect was limited only 1 log. In addition, any significant synergistic effect was not observed in other two cases. From these results, combined disinfection by sequential mode of chlorination following ozonation is not a practical method for expecting excess reduction in infectivity of oocysts by synergistic effect.
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