A kinetic study of nitrification inhibition in water distribution systems using low levels of chlorite
Water utilities use operational strategies such as increasing pH, chlorine-ammonia ratio, and/or chloramine residual, distribution main flushing, and periodic break-point chlorination to control nitrification of chloraminated water in distribution systems. Although these methods are proven to be useful in controlling nitrification, the results are utility dependent and sometimes not effective in controlling the loss of chloramines. In various pilot studies, the direct application of chlorite at 0.1 to 0.8 mg/L is shown to be an effective alternative to control and prevent nitrification. Chlorite inactivates ammonia-oxidizing bacteria (AOB), the root cause of nitrification. In this study, we developed a kinetic model for nitrification inhibition through the addition of chlorite and the observation of residual chloramines and AOB. The important water quality variables examined were: chlorite concentration ranging from 0.02 to 0.4 mg/L, pH ranging from 7 to 9, ammonia concentrations ranging from 0.5 to 2.0 mg/L-N, and temperature ranging from 15 to 35 °C. Instead of measuring the viability of the AOB cells, the production of nitrite and the consumption of ammonia are used as surrogates for cell activity. This information was used to develop a kinetic model (modified Intrinsic Quenching model) that was able to fit the experimental data.