Difference Between BOD and COD

The key difference between BOD and COD is that the BOD is the oxygen demand of microorganisms to oxidize organic matter in the water under aerobic conditions while the COD is the oxygen demand to oxidize all the pollutants in the water chemically.

Quality of a given water sample depends on some variable factors. Also, it can be categorized in several ways such as biological, physical and chemical. 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 varies depending on the different factors such as temperature, source and level of pollution, etc. Oxygen demand is one of the most common ways of measuring the quality of water. Both biological oxygen demand (BOD) and chemical oxygen demand (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.

1. Overview and Key Difference
2. What is BOD
3. What is COD
4. Similarities Between BOD and COD
5. Side by Side Comparison – BOD vs COD in Tabular Form
6. Summary

What is BOD?

BOD is the abbreviation of biological oxygen demand in water. It is also called as biochemical oxygen demand. There is a significant relationship among organic matter, microbial population and dissolved oxygen content in water. Aerobic microorganisms need oxygen for their metabolism. Hence, they use dissolved oxygen and convert organic matter into energy. Moreover, they utilize the provided energy from organic food, for their further metabolic reactions and especially for their reproduction. Thus, the population density is increasing concerning the gained energy, but it depends on the available food content. The metabolic requirement for the newly created population again creates a demand for the dissolved oxygen, which is proportional to the available food.

Figure 01: BOD Test

Therefore, the biological oxygen demand is the amount of dissolved oxygen required by aerobic organisms to breakdown organic materials, to obtain energy for their metabolism. This value should be tested under a given temperature for a given period, and it will depend on the nutrient concentration and enzymatic reactions too. Besides, BOD value in polluted water is normally higher than the fresh water. High BOD can be a result due to domestic sewage, petroleum residues and wastes of animals and crops.

What is COD?

Chemical oxygen demand (COD) is an indirect method of determining total organic compounds in water. Hence, COD refers to the amount of oxygen necessary to oxidize all of the organic carbon completely to CO₂ and H₂O. Not only COD engages with the decomposition of organic matter, but also, it relates to 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.

Figure 02: Organic Pollution

Moreover, COD testing uses chemical reagent to oxidize the pollutants. Hence, it does not rely on the use of microorganisms to break down the organic material in the sample by aerobic respiration. Not only that, COD always high in comparison to BOD since COD is a measurement of a complete breakdown of pollutants, unlike BOD in which biochemical breakdown is often not complete as a chemical breakdown.

What are the Similarities Between BOD and COD?

COD and BOD can be indicated as mg/L or ppm (parts per million).
Both parameters measure the amount of oxygen needed to oxidize pollutants in water.
Also, both measurements indicate the intensity of water pollution.
Furthermore, both BOD and COD are critical in wastewater for determining the amount of waste in the water.

What is the Difference Between BOD and COD?

BOD refers to the amount of oxygen consumed by microorganisms to decompose organic matter in water under aerobic conditions. On the other hand, COD refers to the amount of oxygen needed to oxidize all the organic and inorganic pollutants in water chemically. Therefore, this is the key difference between BOD and COD.

Furthermore, COD test utilizes a strong oxidizing reagent to oxidize materials in water while BOD test does not use a chemical reagent instead it allows microorganisms to act on the organic matter. Thus, it is also a difference between BOD and COD. Another difference between BOD and COD is that it is possible to complete the COD test within a few hours in the lab. But, BOD test takes five days. Moreover, since COD measures the amount of oxygen necessary to oxidize total pollutants, COD value is always greater than the BOD value. Also, in COD, a complete breakdown of pollutants takes place while it does not occur in BOD.

The below infographic on the difference between BOD and COD describes these differences comparatively.

Summary – BOD vs COD

BOD and COD are two parameters to measure water quality and the amount of organic pollution of water. BOD measures the amount of oxygen utilizing by aerobic microorganisms to breakdown organic matter in water. On the other hand, COD measures the amount of oxygen needing to oxidize total organic and inorganic pollutants in water chemically without the involvement of microbes. So, this is the key difference between BOD and COD. Moreover, COD is always higher than the BOD. Since COD uses a strong oxidizing agent, it oxidizes pollutants completely into CO₂ and H₂O unlike in BOD. BOD is a time-consuming test while COD can finish within a few hours. Thus, this summarizes the difference between BOD and COD.

Reference:

1. “Chemical Oxygen Demand.” NeuroImage, Academic Press. Available here
2. “Biochemical Oxygen Demand.” Wikipedia, Wikimedia Foundation, 11 Dec. 2018. Available here