Factors which control bulk chlorine decay rates

Several computer programs for modelling water distribution networks have been developed which incorporate a facility for modelling chlorine decay. Problems have been experienced with the calibration and durability of these models due to both temporal and spatial variability in the decay constants. Chlorine will decay either due to reactions at the pipe wall or due to reactions in the bulk water. The aim of the work presented in this paper is to investigate the factors which influence bulk decay. Over 200 determinations of bulk chlorine decay against time were performed on waters taken from 32 sampling locations within the Severn Trent region, U.K. The bulk decay constant was observed to show significant variation with temperature, the initial chlorine concentration and the organic content of the water. An equation was derived relating these parameters which could be used to update the decay constants in network models and improve their durability.