AWWA WQTC65976

Systems utilizing chloramines for residual disinfection often are required to stop ammonia addition for at least one month per year. This time is utilized to "burn" or "flush" the system with a high strength disinfectant and is typically accompanied by a program of hydrant flushing to ensure the chlorinated water travels completely through the distribution system. A previous study indicated that switching from chloramines to free chlorine residual disinfectant reduced bacteria levels as measured by heterotrophic plate count (HPC); the effect was temporary, however, because the levels increased again upon return to chloramination. To build upon this work, one of the project goals was to examine the effect of switching from combined to free chlorine and then back to combined chlorine on bacteria levels in two neighboring distribution systems (DS-1 and DS-2) with similar raw water quality, and to link these effects to water age and water quality parameters including disinfectant residual, assimilable organic carbon, and nitrogen species (nitrate, nitrite, ammonia). A second project goal involved examining a "dead-end" site in the distribution system for bacterial growth after periods of unsustained chlorine residuals. Additionally, nitrogen species were analyzed to examine the possibility of nitrification at the site. Includes 15 references, table, figures.