COD, or chemical oxygen demand, is a testing tool to determine water quality after wastewater has been treated. The test is based on how much oxygen is required to break down organic compounds and is expressed in parts per million.
In a chemical oxygen demand test, a high COD value indicates a high amount of pollution in the water sample. A COD test can identify chemically digestible material that cannot be determined through a biological oxygen demand test. The principle is that the organic matter is oxidized with an oxidizing agent under acidic conditions to determine the total water toxicity.