Ozone is often used to reduce the amount of organic compounds in water treatment applications via a process called oxidation. Commonly used measures of organic levels are COD (chemical oxygen demand) and BOD (biological oxygen demand). The first measure uses high temperature and aggressive chemical oxidants to completely oxygenate organic compounds in a test sample and measure the equivalent amount of oxygen required for this to take place, thus the chemical oxygen demand.
The BOD test uses bacterial to oxygenate the sample, the bacteria use oxygen to digest the organic compound and produce carbon dioxide as a waste product. Thus, it indicates the biological oxygen demand of the water over the time it takes to run the test, typically 1-5 days. Because some compounds are not readily oxidized by bacteria, the COD value can exceed the BOD value. The larger the COD:BOD ratio the less biodegradable the organic compounds are.
Ozone tends to work better on compounds that are not biodegradable. In fact, as the ozone oxidation breaks down the organic compound it tends to to form more biodegradable compounds, typically aldehydes, ketones and carboxylic acids. Ozone is considered a specific oxidant, targeting certain molecules and in some cases specific parts of a molecule. In terms of specific parts of molecules, this applies to compounds that create color, odor or taste in water.
The advantage of a specific oxidant is that less oxidant is required since only the target molecules or portions of the target molecules are effected. This characteristic can be used in water treatment to employ ozone as a pretreatment for a biological process, the recalcitrant compounds are made biodegradable and the biological process can deal with them.
When ozone is combined with peroxide or UV light, the ozone is converted to hydroxyl radical, this is an indiscriminate oxidant, it will attack virtually all organic compounds. Processes that make hydroxyl radicals are typically referred to as advanced oxidation processes.