Chlorine decay and bacterial inactivation kinetics in drinking water in the tropics

The decay of free chlorine (Cl₂) and combined chlorine (mostly monochloramine: NH₂Cl) and the inactivation of bacteria was examined in Dar es Salaam, Tanzania. Batch experiments, pilot-scale pipe experiments and full-scale pipe experiments were carried out to establish the kinetics for both decay and inactivation, and to compare the two disinfectants for use under tropical conditions. The decay of both disinfectants closely followed first order kinetics, with respect to the concentration of both disinfectant and disinfectant-consuming substances. Bacterial densities exhibited a kinetic pattern consisting of first order inactivation with respect to the density of the bacteria and the concentration of the disinfectant, and first order growth with respect to the bacterial density. The disinfection kinetic model takes the decaying concentration of the disinfectant into account. The decay rate constant for free chlorine was 114 Ig^-1 h^-1, while the decay rate constant for combined chlorine was 1.84 Ig^-1 h^-1 (1.6% of the decay rate for free chlorine). The average concentration of disinfectant consuming substances in the water phase was 2.6 mg Cl₂/I for free chlorine and 5.6 mg NH₂Cl/I for combined chlorine. The decay rate constant and the concentration of disinfectant consuming substances when water was pumped through pipes, depended on whether or not chlorination was continuous. Combined chlorine especially could clean the pipes of disinfectant consuming substances. The inactivation rate constant λ, was estimated at 3.06 x 10⁴ Ig^-1 h^-1. Based on the inactivation rate constant, and a growth rate constant determined in a previous study, the critical concentration of free chlorine was found to be 0.08 mg Cl₂/l. The critical concentration is a value below which growth rates dominate over inactivation.