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Simulations and Experiments Reveal the Relative Significance of the Free Chlorine/Nitrite Reaction in Chloraminated Systems - slides
Citation:
WAHMAN, D. AND G. E. Speitel Jr. Simulations and Experiments Reveal the Relative Significance of the Free Chlorine/Nitrite Reaction in Chloraminated Systems - slides. Presented at 2011 AWWA Water Quality and Technology Conference, Phoenix, AZ, November 13 - 17, 2011.
Impact/Purpose:
To inform the public.
Description:
Nitrification can be a problem in distribution systems where chloramines are used as secondary disinfectants. A very rapid monochloramine residual loss is often associated with the onset of nitrification. During nitrification, ammonia-oxidizing bacteria biologically oxidize free ammonia to nitrite. The nitrite produced can subsequently react with monochloramine, and this mechanism often is cited as a possible cause for rapid monochloramine loss. Another possible pathway for the nitrite is a direct reaction with the extremely low free chlorine concentrations (e.g., 10-3 to 10-6 mg Cl2/L) present in chloraminated systems because free chlorine is approximately 1.8x105 more reactive than monochloramine with nitrite. Previous research has either dismissed this reaction pathway or operated under conditions where it would be minimally important. Therefore, an evaluation is necessary to understand the relative importance of this pathway under the range of typical chloramination conditions to determine the relative importance of this reaction pathway and whether it should be included in water quality models. The Unified Model was implemented into AquaSIM along with the free chlorine/nitrite and monochloramine/nitrite reactions. Using this model, an evaluation of the relative significance of the free chlorine/nitrite and monochloramine/nitrite reactions was conducted under conditions representative of chloraminated drinking water systems (i.e., pHs of 7, 8, and 9; chlorine to nitrogen mass ratios (Cl2:N) of 3:1, 4:1, and 5:1; monochloramine concentrations of 1, 2, and 4 mg Cl2/L; nitrite concentrations of 0.5 and 2 mg N/L, and a 4 mM carbonate buffer). These simulations showed that even though free chlorine is present at extremely low concentrations, its reaction with nitrite is important under certain conditions. Typically, the reaction of free chlorine with nitrite has been dismissed as an important pathway of nitrite loss because it would be limited by the release of free chlorine from monochloramine hydrolysis, but based on simulations, this is only the case at pHs of approximately 7.5 and lower. As pH increases above 7.5, monochloramine hydrolysis is no longer rate limiting and free chlorine becomes the dominant reacting species with nitrite. Subsequent to the modeling effort, targeted batch experiments were conducted to evaluate the model’s ability to predict experimental results and to determine if the inclusion of the free chlorine/nitrite reaction is required to accurately simulate the experimental data. These experiments and related simulations confirmed that under certain conditions, the experimental data was best represented with the inclusion of the free chlorine/nitrite reaction pathway. Model simulations and experimental results will be presented that highlight the conditions when the free chlorine/nitrite reaction is important.
