AWWA ACE91022

The authors conducted a comprehensive evaluation of disinfection at three water treatment plants of the South Central Connecticut Regional Water Authority. Disinfection at all three plants was inadequate to comply with the Surface Water Treatment Rule. Tracer studies indicated that significant hydraulic short-circuiting occurred in the filtered water reservoirs (FWRs) at all three plants. At one plant, FWR uniformity of flow increased with increasing flow rate; this contradicts information presented by the USEPA. This article discusses contact time and CT values in theory and as prescribed by the Surface Water Treatment Rule. CT values can be increased in three ways: increase time, increase disinfectant residual, or use alternative disinfectants or modify water quality parameters. Each of these problems and its solution are discussed: the use of baffles to prevent short circuiting, thereby increasing contact time; construction of additional FWRs for use as chlorine contact basins; use of alternative disinfectants; and modifications to the treatment processes and changes in plant operation/control, such as prechlorination, increasing free chlorine residual in the FWRs, or decreasing the pH of the filtered water. Detailed results and recommendations are given for each filter plant.