Modelling of chlorine decay in municipal water supplies

This paper describes the effects which temperature and the initial chlorine concentration have on the chlorine decay in different water samples. It also reports the derivation of empirical formulae which describe these effects. The data show that chlorine decays more rapidly in fresh samples than in those which had been re-chlorinated with sodium hypochlorite to give a concentration of about 0.5 mg/l. The decay constants were found to be inversely proportional to the initial chlorine concentration in the former case and to the concentration added in the latter case. A combined first and second-order model is described which describes the decay more precisely than the conventional first-order model. Specifically, it shows that the chlorine decay is influenced by its initial concentration and explains why chlorine decays more rapidly in the initial stages. It also explains why the decay constant in the first-order model varies with the initial chlorine concentration.