Disinfection of drinking water often involves a trade off between inactivating pathogens and creating undesirable disinfection by products in water. Because chlorination is still the most widely used method of disinfection, problems with the formation of chlorinated byproducts due to the interaction of chlorine with organic compounds found in water is a major issue. Two basic classes of compounds are formed: trihalomethanes and haloacetic acids. Both are tightly regulated by the EPA to values under 80 ppb since they are considered suspected carcinogens.
One of the reasons ozone is used as a primary disinfectant in drinking water is that it does not form these compounds. Ozone water treatment can be used as a method of disinfection byproduct control. Chlorine is often used as a secondary disinfectant with ozone to protect the distribution system. In this arrangement, the use of ozone reduces the amount of chlorine required. So, less chlorine results in fewer chlorinated byproducts while maintaining the positive benefits of chlorine.
Ozone is not without the potential to form disinfection byproducts. The most notable is bromate which can be formed if bromide ion is present in the drinking water. Approximately 25% of drinking water sources contain bromide ion, so ozone may not always be indicated for disinfection. In cases where bromide is present, it is still possible to use it by modifying the treatment conditions, a simple method is to limit the amount of ozone used or to adjust the pH to a lower value where possible. More sophisticated methods are also available where the benefits of ozonation are important.
Virtually all disinfection methods have the potential to form disinfection byproducts, but by combining methods and anticipating potential problems, these issues can be minimized.