Disinfection by-products in drinking water: a comparison between chlorine and chlorine dioxide

To evaluate Disinfection By Products (DBPs) and relative amounts in drinking water following chlorine and/or chlorine dioxide treatments in comparison. In July 2005 to February 2006 12 waterworks (managed by Agenzia d'Ambito per i Servizi Pubblici di Modena-ATO) in Northern Italy were sampled twice, in summer and winter. In each sampling session 4 samples were collected: just before and immediately after the treatment, at the tap close to the disinfection plant and far from the plant. Main DBPs (trihalomethanes, chlorite, chlorate, bromate, and haloacetic acids) were investigated by headspace gas chromatography and ionic chromatography/mass spectrometry techniques.Nine water networks were supplied by ground water and 3 by mixed spring and surface water: chlorine was used for disinfection in 4 water systems and chlorine dioxide in 8. Bromate and haloacetic acids were never evidenced, whereas trihalomethanes showed very low values, increasing according to the distance from the disinfection plant and higher in summer. In chlorine dioxide-treated samples THMs ranged from <0.01 to 1 μg/L, whereas in chlorine-treated they ranged from 0.3 to 25 μg/L. Brominated THMs were measured at detectable levels in chlorine-treated samples only (range: 0.1-1 μg/L). Chlorite was evidenced in chlorine dioxide-treated samples only (range: 21-290 μg/L). Chlorate was measured in samples collected from all water networks (range: 2-276 μg/L).Neither chlorine nor chlorine dioxide form THMs at high levels in our samples; this is due to source water, which contains very low levels of organic substance even in case of surface water. Chlorite, produced by chlorine dioxide only, deserves attention as little information is available on its chemical reactions and fate in drinking water within water systems. More important to be investigated is the presence of chlorate, unexpectedly present in several water samples whatever disinfection treatment is used.