Nonbiological surrogate indicators for assessing ozone disinfection: The authors investigate the use of fluorescent-dyed microspheres for assessing disinfection efficiency in ozone contactors

Fluorescent-dyed polystyrene microspheres were used as nonbiological surrogate indicators for assessing ozone disinfection efficiency. Microspheres were exposed to various constant dissolved ozone concentrations ranging from 0.1 to 1.5 mg/L in a semibatch reactor. Experimental results showed that fluorescence decay rates were first-order with respect to dissolved ozone concentration and independent from solution pH in the range of 6.2 to 8.5. Fluorescence intensities from individual microspheres were measured by flow cytometry. Resulting microsphere fluorescence intensity distributions at various ozonation contact times were analyzed by defining a fluorescence intensity threshold. Microspheres with fluorescence intensities above and below the threshold were considered equivalent to viable and nonviable cysts, respectively. Corresponding microsphere survivals correlated well with ozone disinfection data reported for Giardia cysts. The authors propose an experimental protocol for assessing full-scale ozone contactor disinfection efficiency with fluorescent-dyed microspheres.