Effectiveness of repeated rechloramination in recovering chloramine from nitrification

Breakpoint chlorination ("burn") is a common practice to recover chloramine from nitrification. It, however, leads to the formation of a high amount of toxic disinfection by-products and is operationally complex. This study evaluated the effectiveness of repeated (daily) rechloramination at a Cl/N mass ratio of 4.5:1 in a continuously flowing reactor system where nitrification (nitrite, 0.187 mg-N L^-1 and chloramine, 0.55 mg-Cl₂ L^-1) occurred. Repeated (daily) rechloramination with 3 mg-Cl₂ L^-1 was more effective than with 2 mg-Cl₂ L^-1 in suppressing nitrification (nitrite 0.016 mg-N L^-1) and bacterial populations (ATP 5pg.μL^-1), thus recovering chloramine residuals (1.5mgL^-1). This effect was noted within three days of rechloramination and remained stable for the next seven days, after which rechloramination was stopped. Chloramine gradually decreased, and in about ten more days, nitrite started to increase. Chloramine drop was triggered by the incoming water quality -chloramine (1.7 mg-Cl₂ L^-1), chloramine decaying proteins and microorganisms contributing to the decay coefficient of 0.0143h^-1. Nitrite in incoming water was < 0.007 mg-N L^-1. Nitrite appeared when the chloramine concentration decreased below the biostable residual concentration (BRC), but it contributed <15% to the decay. Therefore, the necessary conditions for successful recovery by repeated rechloramination are: a) ensuring upstream water quality is suitable to maintain chloramine above the BRC after stopping rechloramination, b) selecting a dose that could consistently keep the chloramine above the BRC, and c) a minimal biofilm-affected tank (e.g., surface to volume ratio <1m^-1). If a tank or pipe is biofilm-affected, chloramine should be kept above 1.7 mg-Cl₂ L¯¹.