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Understanding Impacts and Meaning of Maintaining Detectable Residuals in Distribution Systems
Citation:
Mistry, J., G. Abulikemu, D. Wahman, M. Alexander, S. Pfaller, J. Lu, AND J. Pressman. Understanding Impacts and Meaning of Maintaining Detectable Residuals in Distribution Systems. AWWA Water Quality Technology Conference, Dallas, TX, November 03 - 07, 2019.
Impact/Purpose:
This study evaluated the residual disinfectant and resulting water quality in four drinking water distribution systems over the course of one year. Sampling events were conducting in four representative locations in each system and important pathogens, regulated DBPs, and significance of nitrification and organic chloramines were assessed. The results demonstrated that Legionella and Mycobacterium can exist in water distribution systems even when disinfectant is present. The results also showed organic chloramines are more persistent than monochloramine. This has important implications on the management of chloramines and the operation of water distribution systems. This research will be of high interest to water utilities and practitioners in the field of drinking water.
Description:
EPA’s Region 6, Office of Research and Development, and Office of Water conducted a drinking water research project to understand how maintaining various drinking water distribution system disinfectant residual concentrations (due to water age or operational practice) corresponded to the pathogenic microbial communities (Mycobacteria and Legionella), disinfection byproducts (trihalomethanes (THMs) and haloacetic acids (HAAs)), organic chloramine formation, and extent of nitrification found in the distribution system. Sampling was conducted at four utilities at the following four sites: (1) system entry point, (2) near a storage tank, (3) state residual monitoring site, and (4) maximum residence time (MRT). All field samples were analyzed for pathogens, disinfectant residual, THM4, and HAA9. In addition, entry point and MRT samples were used to conduct hold studies to evaluate residual stability, nitrification, and organic chloramine formation. Drinking water culture analyses were conducted for M. avium, M. intracellulare, and heterotrophs (HPC). In addition, pathogen qPCR analyses for Legionella and other pathogenic species (Legionella spp., L. pneumophila serogroup 1, 2 virulence factors, Mycobacterium spp., Pseudomonas, Vermamoeba vermfomis, Acanthamoeba, Naegleria spp.) were also conducted. Legionella and other opportunistic pathogens were detected in the four water utility distribution systems during the four sampling events. Legionella detections correlated with the detections of other opportunistic pathogens and total bacteria (i.e., lowest in the entry point and becoming higher with increasing distance within the distribution system). However, Legionella detections increased more than other pathogens with increasing water age. Disinfectant residuals were not sufficient to control for the occurrence of Mycobacterium intracellulare/chimaera, which was detected by qPCR in > 50% of samples, or other clinically-relevant mycobacteria like M. mucogenicum, which was frequently isolated by culture. Disinfectant concentrations may correlate with heterotroph concentrations but were not significantly associated with Mycobacterium concentrations in these distribution systems. Water hold studies demonstrated that organic chloramines were more persistent than monochloramine and resulted in a larger fraction of total chlorine as residual increased. An inorganic chloramine model closely simulated chloramine decay in hold studies for entry point samples but overestimated chloramine stability in MRT samples, indicating production of chloramine demanding materials within the distribution system. Overall, the project (1) provides occurrence data that correlates drinking water pathogens and DBPs with various chloramine residual concentrations, (2) evaluates organic chloramine formation and the resulting positive interferences on measuring disinfectant residuals, and (3) evaluates nitrification impacts on organic chloramine formation.