Decay Kinetics of Chlorite under Simulated Distribution System Conditions

Abstract

Nitrification of chloraminated water in distribution systems, particularly in summer time, is a serious problem for some utilities because nitrifying bacteria deplete residual chloramine, allowing the growth of other bacteria. Some water utilities have proposed adding chlorite (ClO2−) to inhibit the nitrifying bacteria responsible for this process. However, chlorite is suspected to degrade due to reaction with chloramine, and the reaction kinetics is poorly understood. In this study, we investigate parameters such as dissolved organic carbon, chloramine, pH, and temperature that might influence the decay of chlorite in synthetic and finished chloraminated water from Louisville Water Company. Our results showed that in the absence of chloramines, chlorite is stable under typical distribution system conditions (buffered water at pH of 7–9; temperature between 15°C and 35°C; and in the presence of natural organic matter, ammonia, nitrite, and nitrate). However, under these same conditions chlorite decays if chloramines are also present; chlorite and chloramines both degrade in the presence of the other, reducing the effective disinfectant residual in the system. An empirical model was developed to show the dependence of chlorite decay on chloramine concentrations and other environmental conditions. Utilities can use the model as a guide for the chlorite feed concentration and estimation of the chlorite decay in the distribution system.