Minimization of NDMA formation

As the predominant mechanism responsible for NDMA formation during disinfection involves a reaction between dichloramine and unchlorinated organic nitrogen precursors, removal or deactivation of organic nitrogen precursors, or minimization of dichloramine, would reduce NDMA formation. Chloraminating utilities employing cationic coagulation polymers, such as polyDADMAC, are attempting to reduce NDMA formation by reducing their polymer dosage [78]; however, these attempts must be balanced by the need to ensure proper coagulation.

In the case of wastewater-impacted source waters, the organic nitrogen precursors may be deactivated by a period of free chlorination [79] or ozonation [80]. As the dichloramine-associated formation mechanism requires an initial nucleophilic attack by the lone electron pair of organic amines on dichloramine, pretreatment with a strong oxidant, such as free chlorine or ozone, would oxidize the lone pair, preventing the formation. Interestingly, ozone was not effective at destroying NDMA itself.

Lastly, previous research indicated that dichloramine formation during chloramination, and the associated NDMA formation, could be reduced by altering the method by which the chlorine and ammonia reagents are added [79]. Chloramine formation reactions are fast compared to the timescale of reagent mixing. When chlorine is added downstream of ammonia, the chlorine to ammonia molar ratio can exceed 1 at the point of chlorine addition prior to complete mixing of chlorine into the flow stream. These conditions promote dichloramine formation at the point of chlorine addition. When chlorine is added upstream of ammonia, the opposite conditions prevail, and monochloramine formation is favored.

Moreover, as noted above, the short contact time with free chlorine before ammonia addition would aid in deactivating organic nitrogen precursors.

In certain waters, such as nonnitrified secondary municipal wastewaters in some wastewater recycling operations, ammonia is present in the influent water. In these situations, chloramines could be preformed under conditions promoting monochloramine formation (i.e., high pH with chlorine added prior to ammonia), and then applied to the process stream. This process was successfully pilot-tested at a wastewater recycling facility [81].

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