Chlorine Concentration Decay in the Water Distribution System of a Town with 50000 Inhabitants

A numerical model of a water distribution network designed for a town with 50,000 inhabitants was implemented in EPANET. The paper presents a methodology for computing the chlorine residual concentration decay in the above urban size water distribution system, over a 3 days period of time. The hydraulic system fits a gravity distribution scheme, consisting of a looped network with a tank, 42 nodes and 78 pipes. Chlorine is injected at the tank, with a constant concentration of 0.45 mg/l. A variable water demand over a 24 hours period is implemented with a one hour flow pattern time step. The hydraulic time step is set to 1 minute, while the water quality time step is set to 0.02 minute, to ensure predicting accurate instantaneous chlorine concentrations. Hydraulic and Water Quality analysis is performed in order to obtain the time dependent flow rate within the network, as well as the time dependent rate of reaction and chlorine residual concentration on pipes. Reactions occurring in the bulk flow, as well as pipe wall reactions are modelled with first-order decay laws, where the instantaneous rates of reaction depend on chlorine concentration. The bulk flow decay coefficient value is set to 0.85 day. The global wall coefficient values computed on each pipe for the average daily water consumption were adopted over the whole daily water consumption pattern; those values range from 0.013 to 0.057 m/day. All reaction coefficients are inserted as negative values within EPANET, to model the disinfectant concentration decay. At peak consumption hours, the chlorine has not enough time to react while transiting the pipes. At off-peak hours, the chlorine concentration decaying process is more pronounced; but the concentration does not decrease below 0.34 mg/l.