AWWA WQTC71612

Chloramines are unstable at neutral pH, even without the presence of reactive inorganic or organic substances, and auto-decompose by complex sets of reactions that ultimately result in the oxidation of ammonia and the reduction of active chlorine. Granular activated carbon (GAC) filters become biologically activated carbon (BAC) filters due to microbiological growth which occurs after the ozonation process. To compare filter efficiency, chloramine decay tests were carried out to observe the chloramine decay patterns of filter effluents during the dry and wet seasons. The auto-decomposition rate increased with decreasing pH. Chloramine disinfects and reacts with microorganisms. The residual chloramines of Log 9.0 CFU HPC/mL sample decreased by 20% within 1 hour of incubation time. 90% of the chloramine was consumed within 96 hours of incubation time. Chloramine was initially consumed to kill bacteria, and then reacted with dead bacterial matter. Monochloramine decay also occurred through reactions with inorganic ions such as nitrite, bromide, and ferrous iron, and with inorganic carbon from GAC particles. Oxidation of bromide by monochloramine is rapid under the conditions of chloramination. The oxidation of ferrous iron by monochloramine has apparently never been investigated at the quantitative level. This may be because of the belief that the reactions are too slow to be of economic relevance. Monochloramine might also react with GAC fines, but 0.1 mg/L GAC fine did not affect the monochloramine loss within 140 hours. BAC filtration can remove NOM and particle matter through microbiological processes. Particle matter and the growth of microorganisms from the filter media contribute to pressure head loss. Heterotrophic Plate Count (HPC) from the initial samples taken after filter backwashing showed a decrease until 3 hours after backwashing, then increased until 24 hours as the BAC filter matured. The residual chlorine increased after 1 hour of backwashing. The sample taken 1 hour before backwashing showed a decrease in residual chlorine because matter might have come out of the mature filter because of the high loss of head. Over 80% of the residual chlorine was consumed within 96 hours. The lignite GAC and bituminous GAC did not show a difference in chloramine consumption. The chloramine demand was greater during the wet season than the dry season. Samples were collected immediately after backwashing during the wet and dry seasons, and incubated for 96 hours. Results showed a chloramine demand of 9.4 mg/L during the wet season, while the dry season demand was 7.7 mg/L. Includes 8 references, table, figures.