The inactivation of Cryptosporidium parvum oocysts and the formation of bromate were assessed simultaneously by performing experiments with a full-scale ozone bubble-diffuser contactor used for drinking water disinfection. Fluorescence-dyed polystyrene microspheres were used as surrogates for C. parvum oocysts. Semi-batch ozonation experiments were performed to determine the fluorescence-intensity decay of individual microspheres, which was measured by flow cytometry. The results obtained with the microspheres were correlated to the inactivation kinetics of C. parvum oocysts by choosing an appropriate threshold fluorescence intensity below which microspheres were considered to be equivalent to nonviable oocysts. A mathematical model was then used to predict the inactivation efficiency and bromate formation. The contactor hydrodynamics were characterized by running tracer tests, and the kinetic parameters for ozone decomposition and bromate formation were obtained by performing batch experiments. Model predictions were in good agreement with full-scale experimental results. Additional model simulations revealed that ozone contactors should be designed with the lowest possible backmixing so that the target inactivation efficiency can be achieved with the lowest possible formation of bromate.
Fluorescent‐dyed polystyrene microspheres can be used to assess disinfection efficiency.
The authors investigated the use of fluorescent‐dyed polystyrene microspheres to assess drinking water disinfection efficiency in full‐scale ozone contactors. A demonstration was conducted with one of two parallel full‐scale ozone bubble‐diffuser contactors at the Alameda County Water District Water Treatment Plant in Fremont, Calif. Microsphere suspensions were analyzed by flow cytometry to determine changes in fluorescence distribution. Selection of threshold fluorescence intensities allowed the determination of microsphere “survival” ratios that were correlated with Cryptosporidium parvum inactivation curves. Compared with the concentration times contact time (C X T) concept, use of the surrogate indicators offered a potentially simpler and more accurate approach for assessing and optimizing ozone disinfection efficiency achieved in full‐scale contactors and for demonstrating compliance with disinfection requirements.
The Surface Water Treatment Rule (SWTR) contains tables of disinfectant CT(concentration × time) values that have been demonstrated to achieve the required degree of inactivation of viruses and Giardia cysts. If the CT product for a water treatment plant meets or exceeds those in the SWTR tables, it is assumed this inactivation has been achieved. The objective of the study was to compare two methods for calculating C and T. Tracer tests were conducted to characterize the behavior of flow through the process units of water treatment plants. Effective CT values, which were calculated based on the residence time distribution and the kinetics of chlorine decay, were compared with the most conservative CT values recommended in the US Environmental Protection Agency guidance manual.
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