Nitrification modeling in pilot-scale chloraminated drinking water distribution systems

Nearly two thirds of drinking water utilities that practice chloramination for secondary disinfection have experienced nitrification in their distribution systems. However, few studies have been conducted to address this problem quantitatively. In this study, pilot-scale chloraminated drinking water distribution systems were set up and a deterministic nitrification model was developed for these systems to predict changes in chloramine, ammonia, nitrite, nitrate, and ammonia oxidizing bacteria concentrations. The model consists of a system of ordinary differential equations (ODE) that were solved by the ODE solvers in Matlab. Parameters were either obtained from the literature or estimated from nonlinear regression analysis of the experimental data. The model provided a mechanistic base to quantify water quality variation during nitrification episodes. The results showed that the developed model was able to predict whether nitrification episodes would occur in the pilot-scale distribution systems.; Parametric sensitivity analysis was conducted to evaluate the significance of processes that contributed to water quality change in the pilot-scale systems. The results showed that the maximum specific growth rate for ammonia oxidizing bacteria (AOB), the inactivation constant for AOB, and the hydraulic detention time were three most sensitive parameters to ammonia consumption and nitrite buildup when nitrification started, which indicated that AOB regrowth, AOB inactivation by chloramines, and hydraulic washout were three most significant processes contributing to nitrification episodes. This result suggests that conditions to be controlled for effective nitrification management could be pH, chlorine residual, and water age, which are directly related to these three kinetic processes, respectively.; Logistic nitrification models were developed to quantify the contribution of pH, chlorine residual, 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 most significant factors that influenced the risk of nitrification occurrence. Free ammonia concentration was not statistically significant contributing to nitrification occurrence in the pilot-scale systems.