Variations in nitrogen trichloride concentration over time in swimming pool water: Experimental and modeling study

The anthropogenic substances e.g. urea, ammonia, amino acids and creatinine, found in large quantities in swimming pools, are involved in a very complex series of reactions with chlorine. This leads to the formation of by-products such as trihalomethanes, halogenoacetic acids, halogenoketones and chloramines of which the most halogenated form, nitrogen trichloride (trichloramine, NCl3), is very volatile. NCl3 is an irritant for the respiratory tract and the eyes, when released into the atmosphere of aquatic centers. The work presented here proposes a model to predict the NCl3 concentration over time in a traditional indoor swimming pool, based on its operating parameters and attendance. The approach used relies on the chemical kinetics of NCl3 formation, mainly due to free chlorine reacting with ammonia and urea in the water, and its transfer at the air/water interface. The model was validated in an aquatic center with a single swimming pool. In this swimming pool, the working parameters and concentrations of the main compounds were followed over several days. The model satisfactorily predicted variations over time in the concentrations of chemical species of interest in water, such as nitrogen trichloride or urea.