Author(s): S. N. Chan; Q. S. Qiao; J. H. W. Lee
Linked Author(s): Joseph Hun-Wei Lee
Keywords: Chlorine disinfection; Chlorine demand; Breakpoint chlorination; Ammonia
Abstract: Chlorine is extensively used in disinfection processes in water and wastewater treatment. In the chemically- enhanced primary treatment (CEPT) plant of the Hong Kong Harbour Area Treatment Scheme, high concentration (10 percent) chlorine solution is dosed into the treated sewage flow (2 × 106 m3/d) by jet mixing. Due to the fast reaction of chlorine with organic and inorganic nitrogen compounds, field observations have demonstrated significant loss of chlorine within a short distance or travel time from the dosing point (in the order of 0.1 m or 1 second or less). It is essential to understand the mixing and reaction of chlorine jet with ammonia nitrogen for disinfection dosage optimization. In this paper, an integral reacting chlorine jet model for predicting the changes in free chlorine, chloro-amine and ammonia nitrogen is developed for the first time. The model is validated against measurements in a bench- scale “toy” model experiment of chlorine jet discharging in coflowing ammonia nitrogen solution and CEPT effluent. The results suggest that in ammonia solution, the dosed chlorine predominantly reacts with ammonia to form combined chlorine with negligible chlorine demand. In CEPT effluent, the chlorine demand is mainly due to the preferential oxidation of organic debris by free chlorine. Combined chlorine is then formed due to the reaction of remaining free chlorine with ammonia nitrogen under jet mixing.