A change in COD is often used as an objective in water treatment. In laboratory tests the initial amount of COD is noted and ozone is applied to the contaminated solution. A correlation is developed between the ozone applied and the COD level. This is the most direct way to determine the amount of ozone needed. For organic compounds that are treatable with ozone, a rule of thumb can be applied for an initial estimate of ozone demand. It says that you need 2.5 mg ozone/mg of COD where the COD is composed of organic compounds that can be oxidized by ozone.