Inactivation of Giardia lamblia and Giardia muris cysts was compared by using an ozone demand-free 0.05 M phosphate buffer in bench-scale batch reactors at 22°C. Ozone was added to each trial from a concentrated stock solution for contact times of 2 and 5 min. The viability of the control and treated cysts was evaluated by using the C3H/HeN mouse and Mongolian gerbil models for G. muris and G. lamblia, respectively. The resistance of G. lamblia to ozone was not significantly different from that of G. muris under the study conditions, contrary to previously reported data that suggested G. lamblia was significantly more sensitive to ozone than G. mudis was. The simple Ct value for 3 log unit inactivation of G. lamblia was 2.4 times higher than the Ct value recommended by the Surface Water Treatment Rule.
Giardia muris cyst viability after ozonation was compared by using fluorescein diacetate-ethidium bromide staining, the C3H/HeN mouse-G. muris model, and in vitro excystation. Bench-scale batch experiments were conducted under laboratory conditions (pH 6.7, 22°C) in ozone-demand-free phosphate buffer. There was a significant difference between fluorogenic staining and infectivity (P c 0.05), with fluorogenic staining overestimating viability compared with infectivity estimates of viability. This suggests that viable cysts as indicated by fluorogenic dyes may not be able to complete the life cycle and produce an infection. No significant differences between infectivity and excystation and between fluorogenic staining and excystation (P c 0.05) were detected for inactivations up to 99.9%. Only animal infectivity had the sensitivity to detect inactivations greater than 99.9%. Therefore, the animal model is the best method currently available for detecting high levels of G. muris cyst inactivation.
The disinfection effects of the ozone molecule alone and that of ozone decomposition products when inactivating Giardia muris cysts were investigated at bench-scale using two different ozone demand-free laboratory buffer systems. The first water was a 0.05 M phosphate buffer with hydrogen peroxide added at a 10:1 weight ratio. The second water was a 0.05 M phosphate -0.01 M bicarbonate buffer which quickly scavenged radical species from ozone decomposition. The C3H/HeN mouse model was used to assess the infectivity of ozone treated cysts.The phosphate-bicarbonate buffer system had significantly greater (P < 0.05) inactivation of G. muris cysts than that observed in the phosphate bufferperoxide system where ozone was completely decomposed in less than 120 s. Consequently, the design of ozone disinfection processes should maintain ozone residual for disinfection prior to the addition of hydrogen peroxide for the oxidation of other compounds.
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