What is the critical parameter to monitor and control in a chloramination system to minimize the formation of undesirable disinfection byproducts?

The critical parameter to monitor and control in a chloramination system to minimize the formation of undesirable disinfection byproducts (DBPs) is the chlorine-to-ammonia ratio. Chloramination involves adding ammonia to chlorine to form chloramines, which are used as a secondary disinfectant to maintain a residual in the distribution system. The goal is to form monochloramine, which is the most effective disinfectant and produces fewer DBPs than free chlorine. However, if the chlorine-to-ammonia ratio is not properly controlled, other chloramine species, such as dichloramine and trichloramine (nitrogen trichloride), can form. Dichloramine and trichloramine have several undesirable properties. They have lower disinfection effectiveness than monochloramine, can cause taste and odor problems, and trichloramine can be corrosive and pose health risks at high concentrations. The ideal chlorine-to-ammonia weight ratio is typically in the range of 3:1 to 5:1. Monitoring this ratio and adjusting the chlorine and ammonia feed rates accordingly is essential to ensure that monochloramine is the dominant species. Regular monitoring of chloramine species, pH, temperature, and chlorine demand is also necessary to optimize the chloramination process and minimize DBP formation. Careful control of the chlorine-to-ammonia ratio is more effective than simply adding ammonia without close monitoring, which can actually increase the formation of undesirable DBPs if the ratio is incorrect. For example, if too much chlorine is added relative to ammonia, free chlorine will persist, leading to the formation of trihalomethanes (THMs) and haloacetic acids (HAAs), which are regulated DBPs. If too much ammonia is added, nitrification can occur in the distribution system, leading to loss of disinfectant residual and potential microbial growth.