Impact of Temporary Switches from Chloramines to Free Chlorine on Distribution System Water Quality
Creators: name: Ferguson, Bree A.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2005

description: Nitrification in chloraminated distribution systems (DS) can reduce water quality by causing reduced chioramine residuals and pH, and increased heterotrophic plate count bacteria, nitrate and nitrite. To understand how temporarily switching from chloramines to free chlorine affects nitrification, bacterial regrowth and overall distribution system water quality, a study was performed to examine the impact of disinfectant switching in three distribution systems of varying sizes and water quality. All three utilities annually switch from chloramines to free chlorine for the month of March. Grab samples were collected from these systems before and during the switch to free chlorine, soon after the return to chloramines and up to three months later. Samples were analyzed for total and free chlorine, pH, trihalomethanes (THMs), ammonia (total and free), nitrite, nitrate and nitrifier enumeration by the most probable number (MPN) method. Additionally, a device was installed in one of the distribution systems to obtain biofilm samples from polyvinylchloride (PVC), cast iron and ductile iron coupons for microbial analyses. Biofilm samples were taken before, during and after the switch to free chlorine and analyzed for total cell concentrations by the acridine orange direct count (AODC) method, viable bacteria concentrations by HPC with R2a agar, and fluorescent in situ hybridization (FISH) was used to detect the presence of active ammonia oxidizing bacteria (AOB) in the biofilms. Total chlorine residuals were temporarily reduced throughout each DS as a result of the breakpoint reaction generated by switching disinfectants. Extended periods of low residuals (~1-2 weeks) were measured in water storage tanks due to this breakpoint reaction. Trihalomethane concentrations greatly increased during the free chlorination period in all systems, where relative concentrations were dependent upon water residence time and finished water total organic carbon (TOC)concentrations. Chloramine, nitrate, nitrite and pH results indicated that a switch to free chlorine terminated a nitrification episode for at least three months in a small DS. However, evidence of nitrification was found in another DS only four months after the free chlorination period. FISH results revealed AOB surviving in biofilms on PVC, cast iron and ductile iron coupons before and during the free chlorination period in one DS; however, there was no chemical evidence of nitrification occurring in this system throughout this study. Total cell concentrations in the biofilms were unchanged by the free chlorination period and viable cell concentrations were temporarily reduced on the PVC coupons, but not cast iron or ductile iron. These findings indicate that switching to free chlorine is effective at terminating nitrification episodes, but not effective at long-term prevention. The disadvantages associated with this practice, namely increased THMs and temporarily reduced chlorine residuals, must be weighed against the benefits of temporary nitrification control, where the degree of success of this practice for maintaining water quality will depend on the characteristics of a specific utility.