Case studies were conducted at 18 drinking water treatment plants in the U.S. and at one in Canada on the occurrence and control of the chloramine by-product N-nitrosodimethylamine (NDMA). Plants were studied that represented a range of (1) precursor issues (treated wastewater discharges in the watersheds were a source of NDMA precursors; the polymers polyDADMAC and polyamine, but not polyacrylamide, were a source of precursors); (2) pre-oxidation scenarios (e.g., use of free chlorine or ozone, which can destroy/transform NDMA precursors; and (3) precursor removal unit processes (i.e., river bank filtration [RBF], powdered or granular activated carbon [PAC or GAC] absorption, where AC removed wastewater-derived precursors, but not polyDADMAC-derived precursors). As part of the case study approach, a source-to-tap analysis was conducted, where (1) precursor loadings (using formation potential [FP] tests) were determined in the plant influent and after polymer usage or after precursor removal with RBF, pre-oxidation, PAC, and GAC; and (2) nitrosamine formation in the plant effluent and distribution system were measured. Moreover, simulated distribution system tests were conducted to evaluate the impact of different free chlorine contact times, pH levels, and temperatures. Plants that used polyDADMAC increased the precursor loading by 6 ng/L (median) to 12 ng/L (maximum). With 6 mg/L of PAC, ~40-50% of wastewater-derived precursors were removed, where higher PAC doses did not substantially impact precursor removal. GAC was still removing wastewater-derived precursors (e.g., ~60%) after TOC breakthrough occurred. In general, ozonation was effective at destroying NDMA FP (median and maximum = 37 and 89%, respectively). When there was short free chlorine contact time (0-3 min), there was a range of NDMA values, whereas NDMA formation was relatively low for moderate and high free chlorine contact times. These case studies present insights into how best to control NDMA formation at drinking water treatment plants.

Case studies on NDMA occurrence and control

248th National Meeting (2014)

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