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Role of Manganese Oxide in the Formation of Disinfection Byproducts in Drinking Water Treatment

This work examined the role of manganese oxide (MnOx) in the formation of disinfection byproducts (DBPs) in drinking water treatment. DBPs are of increasing concern as more is being learned about their carcinogenicity and genotoxicity. Studies were performed to determine the impact of MnOx and free chlorine (Cl2), used for dissolved manganese (Mn(II) removal, on the formation of these undesirable byproducts. Batch experiment results showed that the presence of MnOx did not significantly increase the haloacetic acid or trihalomethane concentrations. Rates of DBP formation were also similar in the absence and presence of Mn(II) at the beginning of the reaction. The purported reason for similar DBP concentrations is that aquatic natural organic matter (NOM) does not readily adsorb to MnOx under the conditions (pH, time, etc.) typical of drinking water treatment plants (WTPs). Results of column studies were consistent with batch results; exposure to MnOx coated granular filter media did not increase DBP formation. Measured instantaneous DBP concentrations were higher when Cl2 was applied ahead of the column as compared to after the column. However, measured DBP concentrations when all samples were held for 24 hours were similar. The practice of only post-filter chlorination is not feasible for WTPs with the treatment goal of removing Mn(II) across a MnOx-coated granular media filter. An alternative method which allows the post-filtration application of Cl2 while removing Mn(II) by sorption and catalytic oxidation by Cl2 is employing second stage contactors (SSCs) solely for Mn(II) removal. This decoupling of particle (and NOM) and Mn(II) removal was documented at the full-scale at a newly reconstructed direct filtration WTP. The SSCs successfully removed Mn(II), and a substantial decrease in DBP formation was observed. There was no measurable increase in DBP formation across the SCCs.