| Drinking water quality can decay substantially within distribution systems. This decay is exhibited by decreases in disinfectant residuals and increases in bacterial counts, sometimes to levels that affect public heath. The processes that influence water quality decay are complex and depend on many factors, and the raw water properties that influence these factors differ greatly with location. Because of this complexity, no simple rules predict water quality changes in distribution systems.; The purpose of this research is to develop a comprehensive model to track the major processes that affect water quality in distribution systems. This model is called the Comprehensive Disinfection and Water Quality Model (CDWQ). CDWQ tracks all of the major species that affect water quality, including free and combined chlorine, heterotrophic bacteria, nitrifying bacteria, biodegradable organic matter (BOM), soluble microbial products (SMP), dissolved organic carbon (DOC), ammonia, nitrite, and nitrate.; The development of the CDWQ model involved four major steps. First, the model itself was developed, including the mass-balance equations representing the major processes that affect water quality in the distribution system, and the computer code providing the numeric solution of the mass-balance equations. Second, the model was calibrated using extensive measurements taken for a full-scale distribution system. Third, the calibrated model was used to determine the root causes of water quality decay in the distribution system. High levels of bacterial growth was linked to high biodegradable organic matter and ammonia loadings entering the distribution system. Disinfectant loss was linked to several mechanisms, including autocatalytic decay reactions, oxidation of organic matter and corrosion products, and a novel surface catalysis reaction of chloramines on concrete pipe surfaces. The fourth and final step in this study involved developing a water quality improvement plan using the CDWQ model. The plan developed suggested reducing biodegradable organic matter using biofiltration, reducing ammonia using booster chloramination and a higher initial chlorine-to-ammonia ratio, and optimizing the pH. |
