Case studies on nitrosamine formation and control at full-scale drinking water treatment plants

As part of the case studies, detailed information on water quality and operations were obtained, which provided valuable information on NDMA formation at these plants. A combination of FP and SDS tests were conducted, in addition to measuring actual full-scale formation. The highest formation was at plants that were more wastewater impacted and/or used polyDADMAC. These plants also tended to have short free chlorine contact times (typically no more than a few minutes) before ammonia addition. The lowest formation (or FP) was at two plants that used GAC for TOC removal and at one plant that used RBF, both of which can remove NDMA precursors, and at another plant that appeared to have low (if any) wastewater impact, used polyacrylamide as the coagulant aid, and used ozonation. Other plants that used ozone substantially destroyed NDMA precursors with this oxidant. Moreover, free chlorine (with sufficient contact time) significantly lowered NDMA formation, especially at pH levels at which deprotonated amines are present. In addition, warmer water increased the efficacy of chlorine in destroying NDMA precursors. However, halogenated DBP formation needs to be balanced with NDMA control when free chlorine is used to destroy NDMA precursors. Another important observation was that NDMA formation in the plant effluents was relatively high (compared to what was detected in the distribution systems) when chloramine contact times were high in the plant (e.g., 12-18 hr), whereas NDMA occurrence was low in the plant effluents and there was more formation in the distribution systems when chloramine contact times were low in the plant (e.g., 0.5-2 hr). These case studies are providing information to help explain the relative formation of nitrosamines at the plants under study.