Chloramines are an attractive option for secondary disinfection - they're chemically stable, persistent and produce much lower levels of disinfection by-products (DBPs). However, because chloramination involves mutiple chemical reactions between ammonia and chlorine, managing the chemistry of chloraminated water distribution systems is tricky. Chlorine is consumed as water travels through the distribution system and reacts with organic matter, and free ammonia can be left behind. For this reason, nitrification becomes a principal water quality management issue for chloraminated water systems. We have received many inquiries from operators about how to manage and avoid nitrification in water storage tanks, below we answered three common questions.
Q: What are the recommended parameters to monitor nitrification at the water tank?
At a bare minimum, utilities with a nitrification monitoring plan should take residual measurements once a week at their water storage tank. The key detail is to take these measurements from both the bottom and the top of the tank. If you grab a sample from the top and it is different from the sample at the bottom, this is a strong indication of stratification and must be addressed with active mixing. If active mixing doesn't improve residual levels, this suggests that there is low residual water entering the tank, which may require some additional boosting at the tank site.
Q: What should I do if my storage tank has high free ammonia but low nitrate?
Having a high level of free ammonia in your tank puts you at a greater risk of nitrification. It is important to have enough chlorine to cancel out the free ammonia, so you should try to maintain a weight ratio of 5:1 (chlorine to ammonia).
Q: Can a baffled water tank improve chloramine levels and reduce nitrification?
No! Baffling is the procedure of putting walls or partitions in a tank to encourage plug flow. Plug flow prevents water from mixing: instead, water travels as a single slug in a serpentine path from one end of the tank to the other. Because water is traveling in a single slug, baffling maximizes contact time. Baffling can be highly effective in clearwells, but in finished water tanks, baffling is a bad idea. Baffles prevent lateral mixing and greatly increase the risk of thermal stratification - this maximizes water age in the tank and increases residual loss. Thermal stratification allows water in the top of the tank to get warm - also accelerating the process of nitrification.Interested in learning more about managing and avoiding nitrification in the distribution system? Read our Opflow article, "Nitrification Control Strategies."