Field-based estimation of bulk and wall chlorine decay in continuous and intermittent water distribution systems: a study from Coimbatore, India

Maintaining appropriate residual chlorine in drinking water distribution systems is crucial for ensuring disinfection protection. This study uses field data from two urban distribution zones in Coimbatore, India—Jayaram Nagar, operating under continuous water supply, and Gandhi Park, operating under intermittent water supply to develop a deterministic first-order kinetic-based web framework for estimating bulk and wall chlorine decay coefficients. During the summer of 2024, 96 locations were sampled, comprising 37 in Gandhi Park and 59 in Jayaram Nagar. Bulk ( \({k}_{b}\) ) and wall ( \({k}_{w}\) ) decay coefficients were estimated using first-order kinetic equations, with hydraulic contact time approximated from pipe length and flow velocity and empirical adjustments applied for selected water-quality parameters. Across both zones, \({k}_{b}\) ranged from 0.059 to 0.092 h⁻ 1 and \({k}_{w}\) from 0.015 to 0.034 h⁻ 1 . Gandhi Park exhibited mean \({k}_{b}\) and \({k}_{w}\) values of 0.078 and 0.026 h⁻ 1 , respectively, compared with 0.076 and 0.025 h⁻ 1 in Jayaram Nagar. The mean numerical ratio of the estimated bulk to wall decay coefficients was approximately 3:1 in both zones; however, this ratio is reported only as a comparison of coefficient magnitudes and does not represent the actual relative mass contribution of bulk and wall reactions to chlorine loss. However, the differences between the two zones were only 0.002 h⁻ 1 for \({k}_{b}\) and 0.001 h⁻ 1 for \({k}_{w}\) and were smaller than the corresponding within-zone variability; therefore, the observed differences were interpreted descriptively rather than as statistically established effects of the supply regime. The developed application supports CSV-based and manual data entry and provides transparent sample-level estimates for preliminary engineering assessment.