Urea oxidation and trichloramine reduction in AFM systems
DAISY systems using AFM will reduce the inorganic combined chlorine concentration by as much as 90% as a consequence of a much more efficient water treatment system. AFM filters do not support the growth of bacteria, the removal of dissolved organics and nitrogen must therefore be by chemical and physio-chemical mechanisms as opposed to biological.
AFM does not support an acidic biofilm so the rate constants for dichloramine going to trichloriamine are much slower, this makes a huge difference with regards to the formation of trichloramine. Sand filters are biofilters, autotrophic bacterial activity will actually produce organic matter from inorganic carbon (carbonates). Also it is possible for bacteria to use atmospheric dissolve nitrogen N2. So with sand filters there is going to be a net increase in the nitrogen levels and production of combined chlorine that is not derived from the loading on the system. This may account for over 50% of the demand on the water treatment system.
With AFM systems there may be a higher urea content in the water and / or organochlormaines. The urea is subjected to chlorine oxidation substitution reactions to dichlorurea and monochlorourea and then with further oxidation and hydrolysis to nitrogen trichloride. In the presence of urea as a reduced form of nitrogen, nitrogen trichloride is hydrolysed to diachloramine and monochloramine then eventually nitrogen gas and nitrate. Nitrohen trichloride has a high Henry's constant, so a proportioon of the nitrogen trichloride will be lost to atmosphere.
The urea concentrations are therefore controlled by oxidation to nitrogen trichloride, nitrogen gas and nitrate.
Environ. Sci. Technol., 2010, 44 (22), pp 8529–8534
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society
Experiments were conducted to elucidate the mechanism of the reaction between free chlorine and urea. In combination with findings of previous investigations, the results of these experiments indicate a process by which urea undergoes multiple N-chlorination steps. The first of these steps results in the formation of N-chlorourea; this step appears to require Cl2 to proceed and is the overall rate-limiting step in the reaction for conditions that correspond to most swimming pools. N-Chlorourea then appears to undergo further chlorine substitution; the fully N-chlorinated urea molecule is hypothesized to undergo hydrolysis and additional chlorination to yield NCl3 as an intermediate. NCl3 is hydrolyzed to yield NH2Cl and NHCl2, with subsequent decay to stable end products, including N2 and NO3−. Conversion of urea-N to nitrate is pH-dependent. The pattern of nitrate yield is believed to be attributable to the fact that when urea serves as the source of reduced-N, entry into the reactions that describe chlorination of ammoniacal nitrogen is through NCl3, whereas when NH3 is the source of reduced-N, entry to these reactions is through NH2Cl.