Water quality changes within a public drinking water distribution system and in private homes: before, during, and after free chlorine conversion

Abstract

Free chlorine conversion (FCC) is a process used by chloramine public water systems (PWSs) to reduce nitrification in distribution networks. FCC temporarily replaces chloramine with free chlorine before reverting to chloramine. This study examined changes in residual disinfectant, microbiological contaminants, disinfection by-products, and inorganic nitrogen in drinking water distribution and household water before, during, and after FCC. The study monitored a chloramine utility for 15 weeks, collecting weekly samples at eight locations (four in the distribution system, four in homes). Analyses included nontuberculous mycobacteria (NTM), heterotrophic bacteria, chlorine species, ammonia, nitrite, nitrate, pH, temperature, and the by-products THM4 and HAA5. Overall, FCC treatment resulted in very few changes in water quality at this PWS. The residual (monochloramine) stability was not improved. At the furthest sampling location, the residual decayed by 80% relative to the concentration measured at the distribution entry point. In distribution and home samples, no change was observed in heterotrophic bacteria or in NTM detection frequencies or concentrations. Many of the NTM isolates identified were species known to cause clinical infections: M. abscessus, M. cosmeticum, M. mucogenicum, and M. phocaicum. Of the thirteen water quality parameters measured, only HAA9, pH, free ammonia, and temperature showed significant changes. In the distribution network, HAA9, pH, and temperature increased significantly from pre-conversion values. In homes, free ammonia decreased, and temperature increased significantly. The limited improvements highlight the need for further research to determine optimal FCC conditions for future water treatment strategies.