John E. Tobiason
Aquarion Water Company (AWC) of Connecticut owns and operates the Warner, Stamford and Dean's Mill drinking water treatment plants among several other facilities. Bench-scale experiments were conducted to evaluate the impact of secondary disinfection with chloramines on disinfectant by-products (DBP) for these three supplies. The goal was to determine if a significant decrease in DBP formation could be achieved. The final Stage 2 Disinfectants and Disinfection By-Products (DDBP) Rule has prompted A WC to investigate strategies that can help decrease DBP formation in their finished and distributed water. The Stage 2 DDPB Rule regulates two classes of DBPs, the four trihalomethanes (TTHM) and five of the haloacetic acids (HAA5). Maximum contaminant levels (MCLs) of80 and 60 ~g/L have been set as locational running annual averages (LRAAs) for TTHM and HAA5 respectively. Bench-scale experiments were conducted using both raw water and filtered water from the three Aquarion water supplies. DBP formation potential testing was conducted for the raw waters. Filtered water bench-scale experiments included DBP formation kinetics, and comparisons of free chlorination only versus chloramination after chlorination for disinfection (CT). Results were evaluated by monitoring free and combined chlorine residuals, total organic carbon (TOC), total nitrogen (TN), pH, ultraviolet absorbance at a wavelength of254 nm (UV254), and the formation of THMs and HAAs. Results of the free chlorination only versus chlorarnination after free chlorination experiments showed that depending on the source of water, decreases in THM formation ranged from 20 - 45% for chlorarnination after free chlorination as compared with free chlorination only, while decreases in HAA formation ranged from 30 - 50%, again depending on the source of water. Further decreases in DBP formation were obtained with lower initial free chlorine doses that were converted to chlorarnines. The results of the DBP formation kinetic experiments showed that more than half of the DBP formation occurs within the first four hours of free chlorine contact time. This was consistent for all three supplies. Therefore further decreases in DBP formation may be realized by shorter free chlorine contact time. Chloramination after chlorination did not have a significant impact on increasing nitrosamine formation for the Warner filter effluent.