CHEMICAL OXYGEN DEMAND (COD): DEFINITION AND APPLICATION

Chemical oxygen demand (COD) is defined as a measure of the amount of chemicals (usually organic compounds) that consume dissolved oxygen in a body of water. It is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water bodies. COD is often used to measure the level of pollution in wastewaters and natural sources of water such as rivers, lakes and streams. Other related analytical values or techniques that give us the true state of pollution of a given body of water include the BOD, total oxygen demand (TOD), and total organic carbon (TOC). While TOD is a measure of the amount of dissolved oxygen consumed by all the chemical elements in a given water sample when complete (total) oxidation is achieved; the TOC is a measure of the dissolved oxygen consumed by the microorganisms under specific environmental conditions. COD measurement is an important determinant of water quality; and like the BOD measurement, COD is expressed in milligrams per liter (mg/L). This unit for COD measurement (mg/L) indicates the mass of oxygen that is consumed per liter of the test solution.

The chemical oxygen demand (COD) represents the amount of oxygen necessary for the aerobic biological oxidation (breakdown) of the organic substances in a given water body or sample to carbondioxide (CO2) and water (H2O) if the organics are actually biodegradable. COD is different from BOD – which is a measure of the amount of oxygen consumed by the bacteria that are decomposing organic matter in a given body of water. Oxygen is critical to the sustenance of life in aquatic environment, and the amount of dissolved oxygen in such environment must be sufficient enough to support organisms whose habitat is in the water. However, some human and environmental factors that affect the aquatic environment may deny organisms that live in such habitats the oxygen they require to continue to exist. Several factors including agricultural practices and natural factors like storms and heavy rainfall can affect the COD of a particular water body – since these phenomenon or occurrences could move organic and inorganic materials or compounds into water bodies, thereby consuming the available dissolved oxygen (DO) required to sustain aquatic life.

COD measurements are important in accessing the quality of a given water body as aforementioned; and such values helps us to determine the extent of pollution that the water has undergone. COD tests also measure the amount of inorganic contaminants that is dissolved or suspended in a water body. The higher the COD, the higher the amount of pollution in the test water sample. The COD test usually involves using a strong oxidizing chemical agent such as potassium dichromate (Cr2O72-); and the basis for the COD test is that nearly all organic compounds can be fully oxidized to carbondioxide with a strong oxidizing agent (e.g. Cr2O72-) under acidic conditions. The potassium dichromate is used to oxidize the organic matter in the test water sample to carbon dioxide and water under acidic conditions. Silver compounds can also be included in the test to encourage the oxidation of certain organic compounds and mercury to reduce the interference from oxidation of chloride ions.

Any chemical substance that reduces Cr2O72- will interfere with the COD test; and chloride ions (Cl) which is quantitatively oxidized to Cl2 by dichromate is one of the most common interferences in the COD experiment. The test water sample is then digested for approximately 2 hours at 150°C; and the amount of oxygen required is calculated from the quantity of chemical oxidant consumed. There are automated COD analyzers that can rapidly and accurately give the COD levels of a given water sample at any time; and these analyzers are used for the detection of chemical oxygen demand of both natural waters and wastewaters (Figure 1). The analysis for organic matter in natural water bodies or wastewaters can be based on: (1) those methods that quantify an aggregate (total) amount of the organic matter comprising organic constituents with common characteristics or (2) those methods that quantify the individual organic compounds present in the test water sample.

Figure . A chemical oxygen demand (COD) analyzer. This apparatus is a product of JM Science Inc, USA; and it is a Quick COD Analyzer (Model HC-607) that provides fast, accurate analyses and is easy to use with coulometric titration.

The COD methods are used to assess the total amount of organic compounds present in the test water sample. COD is an important water quality parameter that is similar to BOD. It provides an index to assess the effect discharged wastewater will have on the receiving aquatic environment or water bodies. Higher COD levels in a water sample mean a greater amount of oxidizable organic material in the sample, and this will reduce the amount of dissolved oxygen (DO) required to sustain aquatic life. A reduction in DO can lead to anaerobic conditions, which is deleterious to higher aquatic life forms and can cause the death of the organisms in the water. The COD of a water body is usually measured at the influent wastewaterstreams for process control and at the effluent wastewater streams to sewer or environment for regulatory compliances as it pertains to water bodies for domestic or industrial uses. The analysis of organic compounds present in a body of water are usually carried out to assess the concentration and general composition of organic matter present in raw water supplies, treated effluents and receiving waters and wastewaters; and these values also helps us to determine the efficiency of the water treatment processes carried out. COD is one type of analytical parameter that can help us determine the quality of water bodies and the actual efficiency of the water treatment processes.     

Further reading

Jee C and Shagufta (2007). Environmental Biotechnology. APH Publishing Corporation, Darya Ganj, New Delhi, India.

Latha C.D.S and Rao D.B (2007). Microbial Biotechnology. First edition. Discovery Publishing House (DPH), Darya Ganj, New Delhi, India.

Maier R.M, Pepper I.L. and Gerba C.P (2000). Environmental Microbiology. Academic Press, San Diego.

Mishra B.B, Nanda D.R and Dave S.R (2009). Environmental Microbiology. First edition. APH Publishing Corporation, Ansari Road, Darya Ganj, New Delhi, India.

Paul E.A (2007). Soil Microbiology, ecology and biochemistry. 3rd edition. Oxford: Elsevier Publications, New York.

Pelczar M.J., Chan E.C.S. and Krieg N.R. (2003). Microbiology of Soil.  Microbiology, 5th Edition. Tata McGraw-Hill Publishing Company Limited, New Delhi, India.

Pepper I.L and Gerba C.P (2005). Environmental Microbiology: A Laboratory Manual. Second Edition. Elsevier Academic Press, New York, USA. 

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