A common first step in drinking water treatment is preoxidation. During harmful algal blooms, preoxidation helps to remove algal cells from surface water during physicochemical treatment. However, preoxidation also causes algal cell lysis to release intracellular algal organic matter (AOM), which may react further with subsequently applied oxidants (e.g., chlorine, monochloramine) to increase the formation of disinfection byproducts (DBPs). In this study, we investigated the impact of permanganate at concentrations spanning doses commonly used for preoxidation (i.e., 1-5 mg/L) on DBP generation during chlorination and chloramination of AOM. Our results showed that permanganate preoxidation decreased the generation of trichloromethane but increased that of dichlorobromomethane during both chlorination and chloramination. The favored production of dichlorobromomethane over trichloromethane indicated that permanganate oxidized AOM to precursors that were more reactive toward brominated oxidants (e.g., hypobromous acid). During chlorination of AOM, we also found that permanganate preoxidation decreased dichloroacetonitrile formation (by 26-36%), while increased trichloronitromethane formation (by 2- to 3-fold). The increased formation of trichloronitromethane indicated that permanganate reacted with organic amines from AOM to form organic nitro compounds. During chloramination, permanganate increased the formation of both dichloroacetonitrile (by 30-50%) and trichloronitromethane (up to ~3-fold), indicating that permanganate also degraded AOM to form dichloroacetonitrile precursors that were reactive with monochloramine. Overall, our work indicated that permanganate broke down the higher molecular weight fractions of bulk AOM (e.g., proteins) to form precursors of some DBPs, but may also oxidize some moieties (i.e., amine groups) to prevent the formation of others (i.e., dichloroacetonitrile).

Permanganate preoxidation contributes the formation of disinfection byproducts in drinking water treatment during harmful algal blooms

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