Predictive empirical modeling of halogenated by-products during raw water pre-chlorination stage: Toward optimized chlorine dosing

Halogenated disinfection by-products (X-DBPs) formed during chlorination pose significant health risks and remain difficult to control during pre-chlorination of raw surface waters. This study presents an empirical polynomial model for predicting X-DBP formation based on three operational parameters: non-purgeable organic carbon (NPOC), chlorine demand (CD), and pH. Raw water from the Pardo River (Brazil) exhibited marked variability, with NPOC ranging from 2.5 to 9.8 mg L⁻¹, pH 7.1–7.5, and required chlorine doses between 5.4 and 12 mg L⁻¹ to maintain 1–2 mg L⁻¹ free residual chlorine. Chloroform, bromodichloromethane, and chloral hydrate were the only X-DBPs consistently detected, with total concentrations from 52 to 487 μg L⁻¹. The model showed strong performance for total X-DBPs (R² = 0.91, RMSE = 287 μg L⁻¹) and chloral hydrate (R² = 0.70, RMSE = 58.9 μg L⁻¹), comparable or superior to existing empirical models developed mainly for post-treatment conditions. Maximum allowable chlorine doses (D_max) were estimated to ensure compliance with guideline limits (10 μg L⁻¹ for CH, 80 μg L⁻¹ for CHCl₃, 100 μg L⁻¹ for THM). D_max values ranged from 5.2 to 8.5 mg L⁻¹, with doses above 8.5 mg L⁻¹ leading to predicted exceedances. The model provides a practical, field-applicable tool for optimizing chlorine dosing and minimizing toxic by-products during pre-chlorination, particularly in tropical surface waters”.