Minimization of NDMA Formation during Chlorine Disinfection of Municipal Wastewater by Application of Pre-Formed Chloramines

To evaluate the potential for minimizing the formation of the disinfection byproduct N-nitrosodimethylamine (NDMA) by controlling the conditions under which chlorine is added, a pilot-scale study was conducted under conditions that simulated a microfiltration system at an advanced wastewater treatment facility. NDMA formation was detected consistently during direct addition of sodium hypochlorite to the ammonia-containing wastewater and when chloramines were formed in a dosing tank by the addition of hypochlorite after ammonium chloride in tap water preadjusted to pH 7.0. No NDMA formation was observed when chloramines formed at higher pH or by addition of ammonium chloride prior to sodium hypochlorite. The conditions under which NDMA was formed were accompanied by breakpoint chlorination and enhanced formation of dichloramine in the dosing tank. The formation of NDMA appears to be related to the enhanced reactivity of dichloramine with NDMA precursors. Although direct addition of sodium hypochlorite to ammonia-containing wastewater also has the potential to enhance the formation of dichloramine, this effect was counterbalanced by the reaction of hypochlorous acid with organic-nitrogen containing NDMA precursors, which decreases their reactivity with inorganic chloramines. Although this offset may reduce some of the benefits of using preformed chloramines for disinfection, the application of preformed chloramines may still be a useful way to reduce NDMA formation, provided that the chloramines are produced under the appropriate conditions.