Because dichloramine formation from monochloramine takes time, applying pre-formed monochloramine minimizes the exposure to dichloramine. Monochloramine can be formed by injection into high pH tapwater of first free chlorine and then ammonia, because these conditions favor monochloramine formation.
Schreiber, I.M.; Mitch, W.A. Nitrosamine
formation pathway revisited: the importance of dichloramine and
dissolved oxygen. Environ. Sci.
Technol., 2006, 40 (19), 6007-6014.
This technique was pilot tested at the West Basin Municipal Wastewater Recycling Plant. The application of pre-formed monochloramine significantly decreased NDMA formation compared to direct injection of free chlorine into ammonia-containing wastewater at pH 7.
Mitch,
W.A.; Oelker, G.L.; Hawley, E.L.; Deeb, R.A.; Sedlak, D.L. Minimization of
NDMA formation during chlorine disinfection of municipal wastewater by
application of pre-formed chloramines. Environ. Eng.
Sci., 2005, 22 (6): 882-890.
It also helps minimize the formation of dichloroacetonitrile, a toxic N-DBP:
It also helps minimize the formation of dichloroacetonitrile, a toxic N-DBP:
Hayes-Larson, E.L.; Mitch, W.A. Influence of the method of reagent addition on dichloroacetonitrile formation during chloramination. Environ. Sci. Technol. 2010, 44 (2), 700-706. (Full Text)