AWWA WQTC58808

Ultraviolet (UV) treatment has been shown to produce a nearly undetectable level of disinfection byproducts (DBPs) at the doses commonly used. Most UV treatment scenarios include either chlorination or chloramination for the purpose of maintaining a disinfectant residual during distribution. For this reason, there has also been an interest in impacts UV treatment might have on natural organic matter (NOM) or DBP precursors. Several research groups have examined this question, and results to date have failed to show a substantial effect at typical UV doses. The purpose of this research was to expand upon the prior work to include studies of total organic halide (TOX) and to consider THM and HAA formation under short incubation times. Raw waters from several chemically and geographically distinct sources were collected and treated with UV light (low pressure) and chlorine in the UMass laboratory. Care was taken to use realistic chlorine doses (2-6 mg/L) and controlled pH, along with medium to high UV doses (400-1500 mJ/cm²). Results showed substantial increases in rate of formation of THMs and HAAs when waters were dosed with typical levels of UV radiation. This manifested itself as 8- 23% increases in the two regulated DBP groups at short chlorine contact times. Prior studies have only considered long reaction times, which might explain why these effects have not previously been reported. In addition, there were substantial increases in TOX as a result of UV pretreatment. These increases ranged from 85-270% after about 30 minutes of contact time. What is apparent from this data is that UV treatment has a much greater influence on DPB precursors than previously recognized. It also appears to produce a different distribution of DBPs, favoring the non-THM, non-HAA byproducts. Includes 17 references, tables, figures.