Effects of mixing on chloramination process

Abstract

According to a recent study more than 90 percent of water treatment plants utilizing chloramination for distribution system residuals indicate a certain level of dissatisfaction toward the process performance. One factor that may lead to such dissatisfaction is the inadequacy of mixing when ammonia is added to chlorinated water. If mixing is not instantaneous and uniform, the actual chlorine to ammonia nitrogen molar ratio will become variable at a micro-level, even though the overall ratio at the macro-level is close to the desired 1:1 ratio. Because of the non-uniform mixing, certain portions of the mixture might have a molar ratio exceeding the stoichiometric ratio of 1:1. In such instances, certain unintended reactions (e.g. breakpoint type of chlorine chemistry) can occur. This will lead to the resultant monochloramine concentration being significantly less than the stoichiometric concentration, based upon the calculation using the overall molar ratio. Other factors, such as pH variation in the micro environment, could also affect the final chemical composition of the chloramination process. In this study, the effect of mixing was studied by conducting breakpoint chlorination experiments under different levels of mixing, represented by the average velocity gradient, G in s−1. A unique way of plotting breakpoint chlorination curve was utilized to analyze the data, which allowed a clear delineation if the monochloramine formation was according to the stoichiometry. A quantitative comparison between experimental data and stoichiometry can clearly indicate the impact of non-uniform mixing. The experimental data showed that as the G value increased from 35 to 500 s−1, the monochloramine formation increased from 75 to 87 percent of the stoichiometric value. The location of the breakpoint, correspondingly, increased from a molar ratio of 1.25 to 1.75. Comparison of 40 s−1 (50 rpm) and 300 s−1 (200 rpm) experimental data was conducted and a breakpoint curve was plotted imposing one over the other. It has been observed from previous literature that in ideal conditions, breakpoint occurs at chlorine to ammonia nitrogen molar ratio of 1.5:1, and the peak of monochloramine is expected at a molar ratio of 1:1. Hence, breakpoint curve was plotted at mixing speed of 50 and 200 rpm, indicating free chlorine, monochloramine, dichloramine, trichloramine, and total chlorine concentration at contact time of 45 minutes. Few studies were found in literature on mixing effects in chloramination. Data from a previous study was re-analyzed and compared with the current study, and a similar trend was observed. In another case study, the design G value for a modern water treatment plant in metropolitan Boston was found to be 800 s−1, which was higher than the maximum G value used in this study (500 s−1), and is likely to be more than sufficient. In conclusion, when chlorine and ammonia are combined to produce monochloramine, the degree of mixing indeed has significant impact on the performance of the chloramination process, and therefore must be a critical consideration in its design and operation.