Difference Between BOD and COD


Quality of a given water sample depends on a number of variable factors. In addition, it can be categorized in several ways such as biological, physical and chemical. Oxygen demand is one of the most common ways of measuring the quality of water. Both BOD and COD are coming under that phenomenon. The main intention of this article is to give a clear idea about both of the concepts, similarities, differences, and practical usage of them.

What is BOD?

BOD is the abbreviation of biological oxygen demand in water. It is also called as biochemical oxygen demand. Before moving in to a deep discussion about the topic, it is better going through some of the important concepts regarding water quality.

There are some key factors taken in to consideration in order to determine water quality. They are namely pH, turbidity, microorganisms, dissolved-oxygen content, and dissolved-nutrients. The main parameter, which can affect the water quality, is the composition of water. Normally water contains gases, inorganic ions, organic compounds, living organisms and some of the other trace chemical compounds. The composition is varying depending on the different factors such as temperature, source and level of pollution. Particularly the microbial population and the dissolved oxygen concentration are changed with the alterations of the above-mentioned factors.

There is a significant relationship among organic matter, microbial population and finally with the dissolved oxygen content in water. It can be nicely explained like this. Aerobic microbes need oxygen for their metabolism. They use dissolved oxygen and convert organic matter into energy. They utilize the provided energy from organic food, for their further metabolic reactions and especially for their reproduction. The population density is increasing with respect to the gained energy, but it depends on the available food content. This metabolic requirement for the newly created population, again creates a demand for the dissolved oxygen, which is proportional to the available food.

Therefore, the biological oxygen demand can be defined as the amount of dissolved oxygen required by aerobic organisms, to breakdown organic materials, in order to obtain energy for their metabolism. This value should be tested under given temperature for a given period of time, and it will depend on the nutrient concentration and enzymatic reactions too.

BOD value in polluted water is normally higher than the fresh water. Increased BOD can be resulted due to domestic sewage, petroleum residues and wastes of animals and crops.

What is COD?

Chemical oxygen demand, which is commonly abbreviated as COD, is an indirect method of determining organic compounds in water. Not only COD is engaged with decomposition of organic matter, but also, it relates with the oxidation of inorganic chemicals (ammonia and nitrite). It is explained as the capacity of water, to consume dissolved oxygen during both of the cases.

COD and BOD both can be indicated as mg/L or ppm (parts per million)

What is the difference between BOD and COD?

• BOD is only a measurement of consumed oxygen by aquatic microorganisms to decompose or oxidize organic matter. However, COD refers the requirement of dissolved oxygen for the oxidation of organic and inorganic constituents both.

• Although, some of the organic compounds, which can be broken down by microorganisms, are countable for the biological oxygen demand, they may not be encountered in measuring chemical oxygen demand.