Photodegradation of halogenated organic disinfection by-products: Decomposition and reformation

In this study, the photodegradation of 33 different DBPs (trihalomethanes, haloacetic acids, haloacetaldehydes, and haloacetonitriles) and TOX with low pressure UV light and the subsequent reformation of DBPs with chlorine and monochloramine were investigated. Results indicated that photodegradation followed the order of TOI > TOBr > TOCl, and treated surface water with low SUVA254 background did not impact the photodegradation of highly UV susceptible DBPs such as triiodomethane (TIM), diiodobromomethane (DIBM), tribromomethane (TBM). The mass balance results of chloride, bromide and iodide showed that the main photodegradation mechanism of TOBr and TOI was dehalogenation supported by halide releases (i.e., Cl-, Br- and/or I- ion). In addition, the photodegradation removal effect was higher, when brominated DBPs formation was high. Although low pressure UV light effectively removed halogenated organic DBPs, subsequent use of disinfectants (Cl2 and NH2Cl) reformed photodegraded DBPs, and the overall DBPs concentrations were increased, which suggested that the released Br- and I- ions will reform DBPs in distribution systems, with oxidants present or added (e.g., booster chlorination) in distribution systems. This study showed that although UV photodegradation will reduce halogenated organic DBPs in distribution systems, especially more toxic iodinated and brominated DBPs, it will be a more effective technology towards the end of the distribution system or a point of entry solution rather than in distribution system with post-disinfection and residence time.

Automated summary

This study investigated the photodegradation and subsequent reformation of 33 different DBPs with low pressure UV light. The results showed that the photodegradation order was TOI > TOBr > TOCl, and highly UV susceptible DBPs were not impacted by the low SUVA254 background. The mass balance results indicated that dehalogenation supported by halide releases was the main photodegradation mechanism. Furthermore, the photodegradation removal effect was higher when brominated DBPs formation was high. However, subsequent use of disinfectants reformed photodegraded DBPs and increased the overall DBPs concentrations.