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Disinfection By-Products in Potable Reuse Water and Interpretation of Their Relative Toxicities
Civil and Environmental Engineering
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To preserve water resources, many arid regions have started recycling water via direct or indirect potable reuse treatment systems. One of the pitfalls of reused water is the potential for disinfection by-products (DBPs) to form and persist in the systems–many of which are carcinogens. DBPs form from reactions between disinfectants and organic or inorganic matter.Many methods have emerged to analyze and interpret DBP data. One such method, called “predicted toxicity,” has been used to compare the efficacy of different water treatments. Dividing each measured DBP concentration by an experimentally determined toxicological index, results in the relative contribution to the overall DBP associated toxicity of a sample, and summing the products results in a total “predicted toxicity” of a water sample, which is compared to other samples or water treatment methods. This thesis shows that this method of data use may result in biased conclusions due to the inability of any study to measure all DBPs. I removed or added actual or simulated DBP measurements to those of published studies which evaluated granular activated carbon as a treatment to reduce the relative toxicity of the effluent. In one example, removing measured haloacetaldehydes from the data caused the predicted cytotoxicity of a treated sample to decrease by up to 47%, reversing the initial conclusion that activated carbon increased the toxicity of the water. Though this additive approach should be carefully used when evaluating water treatments, it is an appropriate tool to demonstrate which species dominate toxicity. Understanding which compounds are more toxic than others can help inform future water regulations.Nitrosamines, especially N-nitrosodimethylamine (NDMA), are potential human carcinogens that form from reactions of chloramines (a common disinfectant) with nitrogenous organic matter. These are persistent potential carcinogens and are best prevented from forming by removing their precursors. I attempted to identify precursors at a potable reuse facility in California. The reuse facility’s chemical additions, cleaning agents, UV/AOP process, and oxidant conditions were evaluated for NDMA precursor contribution by performing NDMA formation potential tests. Chemical additions and cleaning agents formed some nitrosamines but incorporating a conservative dilution factor to simulate the facility’s conditions resulted in negligible contribution to the total NDMA formation observed at the plant. Precursor loading was higher in HOCl treatment train than in the H2O2 treatment train, but precursors were well removed across the treatment process for both oxidant trains. Finally, many nitrogen-containing anthropogenic compounds were subjected to simulated UV/AOP conditions to determine if they transformed into reactive nitrosamine precursors. Yields were generally less than 0.2%, except for diphenhydramine whose yield was 1.4%. Diphenhydramine (and similar compounds containing a tertiary amine outside of a ring structure) requires further study to determine the precursor load it contributes to the water reuse facility.