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 al so reports the derivation of empirical formulae which describe these effect s. 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 con centration of about 0.5 mg/1. The decay constants were found to be inversel y 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 d ecay is influenced by its initial concentration and explains why chlorine d ecays more rapidly in the initial stages. It also explains why the decay co nstant in the first-order model varies with the initial chlorine concentrat ion. (C) 1999 Elsevier Science Ltd. All rights reserved.