DISINFECTION PERFORMANCE OF CHLORINE CONTACT TANKS IN NEW YORK CITY WATER POLLUTION CONTROL PLANTS (WPCPs).

New York City water pollution control plants typically provide secondary treatment using the step-feed activated sludge process and disinfection of the plant effluent using sodium hypochlorite as the source of chlorine. The performance of the disinfection facilities were evaluated in terms of chlorine demand, hydraulic efficiency of the chlorine contact tanks, and the disinfection kinetics using both in-situ and bench-scale experimental data. Hypochlorite addition included an initial step of diluting the commercially available concentrated solution using final settling tank effluent. This step initiated breakpoint chlorination which accounted for 80% of the chlorine demand experienced by the plant. Tracer studies showed that the hydraulic regime within the chlorine contact tanks was equivalent to a plug flow reactor as designed. Analysis of the disinfection data collected resulted in a modified form of typical kinetic equations that relate the ratio of survival of the selected biological indicator to the product of retention time and total residual chlorine concentration in the tank effluent. Two sets of equations were formulated, one for fecal coliform bacteria and a second for fecal enterococci.