Trichloramine in swimming pools – Formation and mass transfer

Trichloramine is a volatile, irritant compound of penetrating odor, which is found as a disinfection by-product in the air of chlorinated indoor swimming pools from reactions of nitrogenous compounds with chlorine. Acid amides, especially urea, ammonium ions and α-amino acids have been found as most efficient trichloramine precursors at acidic and neutral pH. For urea a relative NCl3 formation of 96% at pH 2.5 and 76% at pH 7.1 was determined. Even under sub-stoichiometric molar ratios of Cl/N the formation of NCl3 is favored over mono and dichlorinated products. However, the reaction kinetics of urea with chlorine is slow under conditions relevant for swimming pools. Also the mass transfer of NCl3 from water to the gas phase which was calculated by the Deacon’s boundary layer model could be shown as a relatively slow process. Mass transfer would take 20 h or 5.8 d for a rough or a quiescent surface of the water, respectively. This is much more than a typical turnover rate of 6–8 h of a treatment cycle of a 25 m swimming pool. Therefore processes to remove NCl3 and its precursors can help to minimize the exposure of bathers.

► Trichloramine precursors.
► Reaction parameters of trichloramine formation.
► Mass transfer of trichloramine from the water to the gas phase.
► Indoor swimming pools.