Trihalomethane formation during chemical oxidation with chlorine, chlorine dioxide and ozone of ten Italian natural waters

The chlorination of water containing natural organic matter (NOM) and bromide leads to the formation of total trihalomethanes (TTHMs), for which the Italian new regulation on drinking water (Legislative Decree 31/01) indicates a restrictive maximum concentration level of 30 µg/l. The extent of total THMs (TTHMs) and brominated THMs (THM-Br) depends on many factors: bromide, organic matter, chlorine dose, pH, alkalinity and ammonia concentration. In this study the formation of TTHMs and their speciation was investigated. Different oxidation batch tests with chlorine, chlorine dioxide and ozone were performed on natural surface waters coming from ten different sources (seven artificial lakes and three rivers) used for drinking water production in real potabilisation plants. The results suggest that in TTHMs formation hypochlorite ion reacts effectively with organic matter, and bromide plays an important role in these reactions for THM speciation.

The experimental data of batch tests performed on ten different surface waters show that: C TTHMs increase with chlorine dose, applied during water treatments, organic matter and bromide initial concentration in raw water; C THM-Br formation depend on bromide concentration in raw water: they are about 40% of TTHMs for low bromide and more than 80% for high bromide; C the use of alternative oxidants, like chlorine dioxide and ozone, reduces TTHM formation of about 97% with respect to chlorine treatments; nevertheless, bromate can be generated during ozonation of bromide-containing waters. The maximum concentration of 10 µg/l for bromate can be fulfilled only for water with low bromide concentration (0– 30 µg/l).