Risk analysis of nitrification occurrence in pilot-scale chloraminated distribution systems

The implementation of chloramine disinfection introduces free ammonia into drinking water distribution systems, and free ammonia serves as an energy source for ammonia oxidizing bacteria that cause nitrification. The occurrence of nitrification may be affected by many factors including temperature, pH, water age, chloramine residual, and chlorine-to-ammonia application ratio. No studies have been conducted to quantitatively evaluate the individual and total contributions of these factors to the risk of nitrification occurrence. This study evaluated the individual and total contributions of pH and other factors on nitrification occurrence using factorial-designed pilot-scale chloraminated distribution system experiments. The pilot-scale systems were operated at pH values of approximately 7.0, 8.0, and 9.0. Logistic models were developed to quantify the contribution of pH and other factors to the risk of nitrification occurrence in the pilot-scale systems. Results showed that pH, total chlorine residual, hydraulic detention time, and temperature were the significant factors that influenced the risk of nitrification occurrence. However, none of these factors appeared to be dominant in predicting the risk of nitrification occurrence under the studied conditions. A simplified model was able to predict the risk of nitrification occurrence in the pilot-scale systems based on a combination of pH, temperature, and total chlorine residual. The developed logistic models provide an easy-to-use tool for drinking water utilities to manage operation strategies to minimize the risk of nitrification occurrence based on ambient conditions (temperature and pH) and operating parameters (water age and chloramine residual).