Effects of pressure and water age on residual chlorine decay in a water distribution subsystem: a case study of Kolfe, Addis Ababa, Ethiopia

ABSTRACT Maintaining adequate residual chlorine in water distribution systems is essential for maintaining water quality, especially in rapidly growing cities of developing countries. This study evaluated the combined influence of hydraulic pressure conditions and water age on residual chlorine concentration in the Gefersa bypass water supply subsystem serving Kolfe Keraniyo sub-city, Addis Ababa, Ethiopia. Field measurements of pressure and free residual chlorine were conducted at selected nodes across the network and complemented by laboratory bottle tests to determine chlorine decay kinetics. A hydraulic and water quality model of the system was developed using Bentley WaterGEMS and calibrated using observed pressure and residual chlorine data. Chlorine decay was represented using first-order bulk and wall reaction coefficients of −0.75 and −0.55 day−1, respectively. Simulation results indicated that residual chlorine concentrations generally decreased with the increasing water age, particularly in peripheral zones characterized by low hydraulic circulation and extended residence times. Most monitoring locations maintained residual chlorine concentrations above the World Health Organization recommended minimum value of 0.2 mg/L; however, isolated nodes exhibited values below this threshold. Although localized reductions in residual chlorine may indicate areas requiring operational attention, microbiological contamination and associated water quality outcomes were not directly evaluated in this study. Other environmental and sanitation-related factors may also influence waterborne disease occurrence within the study area. The findings demonstrate the importance of integrating hydraulic and water quality modeling for identifying vulnerable zones in intermittent urban water distribution systems and support targeted operational interventions aimed at improving disinfectant persistence within the network. These findings may also be relevant to other intermittent and hydraulically variable urban water distribution systems in sub-Saharan Africa that experience similar operational constraints.