Laboratory studies on the effect of ozonation on THM formation in swimming pool water

Water samples from indoor swimming pool were ozonated at different pH values to evaluate the effect of pH on decomposition of ozone in swimming pool water. Furthermore, drinking and pool water were repeatedly ozonated followed by chlorination to evaluate THM formation. Decomposition of ozone was not affected by pH in the range relevant to swimming pools (pH 6.8 – 7.8) and a half-life time at 10-12 min was obtained. Repeating the ozonation, the decomposition of ozone increased at the second dose of ozone added (t_½,2=8 min) and then decreased again at the third and fourth dose of ozone (t_½,3=17 min; t_½,4=18 min). For drinking water, ozone decomposition was fast for the first dose of ozone (t_½,1=4 min) and then decreased for the second and third dose of ozone (t_½,2=19 min; t_½,3=17 min). Chlorination after ozonation revealed that ozone removed reactivity of the dissolved organic carbon toward chlorine for drinking water as lower TTHM formation occurred than in non-ozonated samples. For pool water, a higher TTHM formation was observed in ozonated than non-ozonated pool water. Thus, it was observed that ozone reacts markedly different in swimming pool water from the known pattern in drinking water production.