In a field study conducted previously in Pinellas County, Florida, implementing UV treatment following chemical disinfection hindered control of HPC bacteria and resulted in higher counts in most samples. It was believed that free chlorine and monochloramine were either decaying in the presence of UV light or that they were absorbing UV irradiation. These occurrences would both lead to a decrease in available disinfection and in turn lead to increased bacteria counts. A bench-scale study was conducted to further investigate interactions between UV light and chemical residuals in drinking water. Three separate water sources (surface water, blended water and deionized water) were treated with chlorine, monochloramine or chlorine dioxide at different concentrations, then exposed to low-pressure UV light. It was confirmed that chemical disinfectants decay when samples are exposed to UV light. Specifically, for deionized water free chlorine was lowered to approximately 95% of initial concentration following UV treatment. In contrast, free chlorine was decreased to 89% and 84%, on average, in the tested groundwater and surface water, respectively. A similar range in decreased disinfectant concentration was also observed for chlorine dioxide and chloramines. However, no significant absorption of UV irradiation by chemical disinfectants was observed. These results provide greater insight into the confounding effects between chlorine-based disinfectants and UV light, which may be important for utilities or institutions that are considering UV treatment of previously disinfected water.

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