Ozonation By-Products

Ozone is a highly reactive gas that can be used for both the destruction of taste- and odor-causing compounds and for the disinfection of drinking water. Ozone forms a variety of organic and inorganic by-products. Many of the by-products caused by ozone are less harmful than the disinfection by-products formed by chlorine. Careful application of ozone can make it a powerful tool for reducing the total amount of harmful by-products produced when treating drinking water.

Water may be safe to drink but still contain natural organic matter (NOM) that may cause undesirable taste, odor, or color. In the industrialized world, consumers may object to water with discernable tastes and odors, or even mistake it as unsafe for consumption. As NOM is primarily found in surface water sources, utilities that obtain their water from these sources often use some means to destroy taste and odor compounds. Ozone readily reacts with these chemicals in water to oxidize them, causing the chemicals to break down.

This same reactivity also causes ozone to act as a disinfectant by attacking and inactivating pathogens within water. Although ozone can cause complete disinfection, it evaporates quickly from water, leaving behind no residual disinfection ability. Within the United States, water distribution systems are required to have a disinfection residual, referred to as a secondary disinfectant. Residual disinfectant will inactivate any pathogens that water may encounter on its way from a treatment facility to the tap. Some form of chlorine is generally used to provide this residual. But regulation of the disinfection by-products formed by chlorine has caused increased use of ozone as a primary disinfectant for drinking water, used earlier in the treatment process.

The disinfection by-products formed by ozone occur in two main forms: nonhalogenated and halogenated. Of these two by-products, the halogenated by-products form the greatest health risk. Halogens refer to the nonmetal elements of fluorine, chlorine, bromine, iodine, astatine, and ununseptium. Because they are highly reactive, halogens exist in the environment only as compounds or within ions.

When ozone reacts with natural organic matter, it does not form halogenated by-products. In this case, it typically forms aldehydes and acids. Nonhalogenated by-products include formaldehyde, acetaldehyde, glyoxal, methyl glyoxal, oxalic acid, succinic acid, formic acid, acetic acid, pyruvic acid, and hydrogen peroxide. None of these are regulated contaminants for which maximum contaminant levels exist for drinking water within the United States.

Although the nonhalogenated by-products form some health concern, of greater concern are halogenated by-products, which form when ozone is used to treat water that has the bromide ion present. The halogenated by-products formed include bromate, bromo-form, brominated acetic acids, bromopicrin, and brominated acetonitriles. Because of the [Page 339]formation of these more hazardous substances, ozone may not be suitable to treat water that contains bromide. Many of the halogenated disinfection by-products are regulated within drinking water within the United States due to known cancer risks.

Within the United States, the Environmental Protection Agency's Stage I Disinfection and Disinfection Byproducts Rule (Stage I DBPR) was promulgated to regulate the allowable concentrations, known as maximum contaminant levels (MCLs), of various disinfection by-products within drinking water, including bromate, halogenated acetic acids, and trihalomethanes. The MCL for bromate is 0.010 mg/L due to increased cancer risks.

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