Deciphering chlorine decay influenced by corrosion scale and biofilm from pipe walls in water distribution system: A variable rate exponential model

The chlorine decay in the bulk water of water distribution systems has been thoroughly described, while the description of wall decay often presents challenges. An important challenge lies in the difficulty of isolating the influence of pipe wall components from the system. Herein, we focused on local key factors corrosion scales and biofilms at the wall, establishing a mathematical description to elucidate their role in the chlorine decay process. This study developed a variable rate exponential (VRE) model with minimal parameters to predict chlorine decay kinetics. The model has a variable reaction rate coefficient that decreases as the reaction progresses. The study evaluated and validated the model's performance in fitting chlorine decay influenced by corrosion scales and biofilms, demonstrating superiority compared to traditional first-order models. The VRE model accurately fits the chlorine decay process under varying initial chlorine concentrations, temperatures, corrosion scale concentrations, and biofilm biomass (R² = 0.95–1.00). The temperature dependence of chlorine decay influenced by corrosion scales was explained using the Arrhenius formula. Furthermore, a linear positive correlation was found between corrosion scale concentration/biofilm biomass and the initial reaction rate coefficient. The parameter results of the VRE model confirmed its suitability for different initial chlorine concentrations without requiring any further parameter calibration. This study offers novel modeling perspectives for analyzing chlorine decay in water distribution systems.