BOD stands for “biochemical oxygen demand.” It is used as a measure of pollution in water. COD stands for “chemical oxygen demand.” This is also a measure of pollution in water, but the indicators that detect the state of the water under study include different factors for each of these two measures.
Biochemical oxygen demand
BOD measures the amount of oxygen that organisms within water need to digest the organic matter present in the water in which they live. The measure is also known as “biological oxygen demand” and is primarily a measure of the levels of sewage in a body of water.
Chemical oxygen demand
COD measures the amount of water-dissolved oxygen that has been destroyed by chemical pollution present in that sample of water. The pollutants may be organic or inorganic. Like BOD, COD is an indirect measure of examining the presence of pollutants in water.
Both BOD and COD are expressed as parts per million or milligrams per litre of water. In both cases, testing requires a sample of the contaminated water. Rather than testing directly on the body of water in the field, tests are carried out in lab conditions.
A BOD test requires that the water sample be kept in a container in a dark place for five days. The temperature of the test location should be kept at a regular ambient level of 20 degrees centigrade. Changes in the oxygen content of the water indicate BOD levels. Higher consumption of oxygen indicates higher levels of contamination. BOD tests do not measure non-organic pollutants and so a COD test is usually performed on a different sample sourced from the same body of water, taken at the same time as the BOD sample. The COD test lasts only two hours and involves adding a solution of boiling potassium dichromate to the sample.
Both measurements measure pollution in water and both are expressed in the same unit of measure. However, the BOD test relies on biological processes to consume oxygen in the water sample and the COD test uses the chemical, potassium dichromate to decompose pollutants in the water. The BOD test is a much longer process than the COD test. The COD test is able to measure the rates of destruction of both organic and inorganic matter in the water, but the BOD test can only measure the destruction of organic pollutants in the water sample.