NDMA FORMATION IN CHLORAMINATED DISTRIBUTION SYSTEM.IS THERE AN EFFECT OF NITRIFICATION? – EBMUD EXPERIENCE

摘要

N-Nitrosodimethylamine (NDMA) is one of the disinfection by-products of concern forrnsystems that chloraminate. NDMA has been shown to form preferentially with chloramine than with free chlorine when diallyldimethylammonium chloride (DADMAC)rncationic polymer was used as a treatment coagulant aid. DADMAC is used nationwide for coagulation. East Bay Municipal Utility District (EBMUD) has extensively tested the conditions under which NDMA was formed at the treatment plants and minimized its formation (Wilczak et al, 2003a). Literature references report that under acidicrnconditions NDMA may form upon the contact of dimethylamine (DMA) or certain tertiaryrnamines with nitrite, which is of concern because nitrite is often present in chloraminatedrndistribution systems that nitrify and amine precursors may occur in DADMAC cationicrnpolymer. However, this reaction was deemed unlikely under typical water treatment pHrnconditions. Mechanistic studies reported in the literature suggest that the mainrnpathways of NDMA formation under conditions of water treatment and distribution wouldrnbe a reaction between amine precursors and monochloramine, as well as a reactionrnbetween DMA, nitrite and the hypochlorous acid. Bench tests indicated that NDMA was not a DADMAC polymer contaminant and did not form in the presence of polymer and ammonia, nitrite and nitrate. It was, therefore, unlikely that nitrification would contribute to NDMA formation when DADMAC was the only significant NDMA precursor (Wilczak et al, 2003a). A nitrification monitoring program was conducted in the distribution system to confirm the effect of nitrification on NDMA levels.