A new chloramine recovery method in nitrifying water without "chlorine burn"

A better alternative to breakpoint chlorination (BPC) or "chlorine burn" to recover from nitrification is proposed. The BPC involves adding chlorine to achieve Cl/N mass ratios more than 7.5 g-Cl2/g-NH4+-N to convert to free chlorinated system. Thus, it is operationally complex and increases the chance of the formation of regulated disinfection by-products (DBPs). Reverting the system back to (mono)chloraminated system is also operationally complex. All these processes require informing public and disruption to service. We tested three Cl/N mass ratios (5.5:1, 7.5:1, 10:1) and three predetermined reaction times (breakpoint reaction time). After the breakpoint reaction time, the samples were rechloraminated (2.5 mg/L at a Cl/N mass ratio of 4.5:1) twice. The second rechloramination was carried out when chloramine had reached 1.0 mg/L. The improvement was evaluated based on the ability of the method to improve chloramine stability, suppress nitrification, and reduce active bacterial cells. Results showed a Cl/N ratio of 5.5:1 (a new method) followed by two rechloramination doses achieved the same improvement as the traditional Cl/N ratio of 10:1 and one rechloramination. Major factor controlling the effectiveness of the new method relies on the ability of the disinfectant to deactivate chloramine-decaying proteins present in nitrified water. The new method does not need the conversion to a free chlorinated system and thus significantly minimises the operational complexity, disruption to service and potential to form DBPs. It offers a potentially novel solution but needs optimisation and testing in a continuous flow system.