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Biochemical oxygen demand Measurement Research Paper

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Updated: Sep 29th, 2019


Biochemical oxygen demand (BOD) denotes the amount of dissolved oxygen in a water body required by aerobic microbes to oxidize organic matter in a water sample at a specific temperature over a fixed time. The term, BOD, may also be taken to signify the chemical process used to measure this amount. A more refined definition of BOD terms it as the amount of oxygen (in milligrams) taken up by a one liter sample of water incubated in the absence of light for a period of 5 days (Hocking 2007, 127).

BOD is the total amount of dissolved oxygen required by microbes to metabolize organic compounds in water. Most of the world’s natural waters have some amount of organic compounds. Aquatic microbes utilize these compounds to produce energy. They oxidize organic compounds in the presence of dissolved oxygen yielding carbon dioxide, water, and energy.

The microbes use this energy for their growth and reproduction processes. The Population of the aquatic microbes is directly correlated to the amount dissolved organic compound in water; that is to say, the higher the amount of organic compounds the larger the microbe population (Hocking 2007, 127).

The oxygen demand of a water system at any particular time is dependent on factors such as nutrient concentration, temperature, and availability of enzymes to the microbes. Total BOD is the sum of oxygen required to “oxidize the organic compounds to carbon dioxide and water through generations of microbial growth, death, and decay” (Hocking 2007, 127). In some cases, the rate of microbial metabolism may surpass that of oxygen dissolution in water leading to the depletion of oxygen in water.

This situation will lead to loss of aquatic life forms that are dependent on the dissolved oxygen, chief of them being fish and aquatic insects. The dissolved oxygen is introduced into the water system by aeration, photosynthesis, and stream flow. The rate at which oxygen dissolves in water bodies is influenced by the prevailing temperature of the water body; more oxygen dissolves in cold water than warm water (Hocking 2007, 127).

The development of the BOD test

This test was first conducted in England where it was assumed that waste discharge to the English rivers took an average of 5 days to be deposited to the sea, thus the use of the 5 days incubation period for the samples. The subscript 5 is often used with the BOD label to denote the 5-day period of the test.

In the event that the test period differs from this, then the subscript used with the label must indicate the number of days that the test took. The 25-day test is the lengthiest and yields results that are complex to interpret. Then again, the lengthy test is important as it yields results that are closest to imitating the likely oxidation and recovery conditions in the water bodies (Hocking 2007, 127).

The Actual test

The BOD test is conducted by incubating microbes in oxygenated water at a temperature of 20oC. The BOD level is measured in mg/L units. The test sample is placed in an airtight jar and left to stand in darkness for a period of five days. This test measures the level of organic pollution in a given water sample.

The results obtained from the sample can then be extrapolated to determine the level of pollution in the wider water body where the sample was obtained. Biochemical oxygen demand measurement can be conducted using one of three possible approaches. The first approach is the direct technique where the oxygen required for oxidation comes solely from the oxygen dissolved in the sample water.

The second approach, also known as the dilution technique uses aerated dilution water as the primary supply of the oxygen needed. The third and final approach is the manometric technique whose source of oxygen is the closed space above the analyzed water sample. The third technique is tricky to employ with precision and is thus reserved for use with systems that have high oxygen demands (Hocking 2007, 127).

BOD for clean river waters is best determined using the direct technique. This entails the complete filling of two 300mL bottles with sample water from the river. The oxygen content of the first river is the tested and recorded as DOinitial. The second bottle is incubated in the dark for 5 days at a temperature of 20oC. The oxygen content of the second bottle is then obtained after the 5 days elapse and recorded as DOfinal. (Hocking 2007, 128).The difference between the two oxygen measurements is the direct measure of river’s BOD, which is given by:

BOD=DOfinal –DOinitial

The dilution technique is ideal for measuring BODs of waste streams polluted with sewerage and industrial effluents. The sample is first diluted with distilled aerated dilution water laden with nutrient salt. Then two 300mL bottles are filled with the water sample. The first bottle’s dissolved oxygen measurement is taken immediately and recorded as DOinitial. The second bottle is incubated in the dark for 5 days at a temperature of 20oC.

The oxygen measurement of the second bottle is then taken after 5 days and recorded as DOfinal. The difference between the two oxygen measurements is the dilution measure of waste stream’s BOD. The BOD loading gives the extent of how polluted a water source is. A BOD load of 1 or less indicates that the water source is exceptionally clean with no pollutants, while a BOD load of 20 or more indicates that the test water body is severely polluted (Hocking 2007, 129).

The Effects of High BOD

The primary focus of wastewater treatment is to reduce the BOD of the effluent released in water. The treatment plants utilize aerobic bacteria, which oxidizes the organic waste found in the wastewater. Excess bacterial growth along with other solid waste is removed as sludge at the treatment plant. Organic pollution disturbs the oxygen balance in the water ecosystem and may result in lethal pathogenic contamination.

Phosphorous pollution of water bodies has been credited with the emergence of the alga bloom; a condition where algal population outbursts occur in waters that has been polluted by the phosphorus. The algal bloom turns water green, rendering it unsuitable for human consumption.

Water pollution can generally be classified under four broad categories. These include pathogenic, biological, toxic, and physicochemical pollution (Goel 2006, 61). Physicochemical pollution occurs when the pollutants impart color, odors, and disagreeable taste to the receiving waters, making them unfit for domestic use. Heavy metals, cyanide and biocides convey toxicity to the receiving waters making them unfit for aquatic life and human use (Goel 2006, 63).

Aside from bearing chemical elements, some wastes like sewerage, also have pathogenic microbes, which leads to pathogenic pollution of the receiving waters. Consumption of pathogenically polluted waters has resulted in waterborne diseases like colitis, typhoid, and cholera. Chemicals contained in the effluents directed to the receiving water bodies may also lead to the death of aquatic flora and fauna.

Algae can be affected by pollution in several ways such as reduction of algal growth due to coloration of the waters; heavy metals and pesticides may directly harm the algae; the pollutants may alter the environment preventing or impeding algal growth. Zooplanktons are also affected by aquatic water pollution. High pollution by heavy metals kills zooplanktons while thermal pollution is lethal to this group of aquatic life forms (Goel 2006, 69).

Water Quality monitoring and management

Water quality describes the characteristics of water that makes it suitable for a particular purpose. The suitability of water at any point is highly influenced by the substances that are deposited or suspended in it. The quality of water varies according to its intended use.

Human activities and natural processes affect the quality of water at the receiving water bodies. Monitoring programs are noteworthy as they help in ensuring that the water ecosystems are protected from preventable pollution. Monitoring programs have helped in a big way to narrow down uncertainties.

These programs help in explaining how changes have occurred in the water ecosystem. The results from monitoring are crucial in comparing and evaluating the benefits of management options (National Research Council 1990, 21). Successful monitoring is one, which results in the formulation of sound management decisions. The initial water quality modeling that focused on BOD has in the recent decades proved sophisticated.

Efforts are now diverted to monitoring of transportation and accumulation of toxic waste substances in aquatic food webs (National Research Council 1990, 23). Extensive monitoring has helped in promotion of effective pollution management. Industries conduct monitoring programs ensuring that effluents released to water bodies have the least possible BOD level.

Regulated river and water dischargers principally conduct monitoring programs either to comply with the law or to source for information for decision and policy makers about the effect of their activities.

Scientists also conduct monitoring programs. This helps them to comprehend the processes of nature and further hone their technical capabilities (National Research Council 1990, 27). Elected public officials also participate in the monitoring programs. They develop and modify legislations touching on marine monitoring programs. These officials are instrumental in echoing public concerns on environmental matters thereby ensuring that policies made are those that favor the people they represent.


Processing and chemical industries are the greatest pollutants of the water bodies. Ironically, they are also the greatest consumers of water from these sources. Industrial effluents weighed down with chemical reducing agents are notorious in that they rapidly take up oxygen exerting an immediate demand for oxygen in the receiving water body. Untreated effluents from industrial processes pollute the river and seas where they are discharged.

This then leads to severe levels of pollution to the waters. An ideal wastewater treatment is one that occurs in an environment the survival of the decomposing microbes and at the same time keeps their population under control. BOD levels can be reduced by ensuring that the waste waters discharged contains minimal or no pollutants (Jørgensen and Johnsen 1987, 287). Reduction of the high BOD levels will be possible only if concerted monitoring and management efforts are employed by the relevant stakeholders.


Goel, P. K. 2006. Water pollution: causes, effects and control. New Delhi: New Age International.

Hocking, Martin B. 2005. Handbook of chemical technology and pollution control. San Diego: Academic.

Jørgensen, Erik, and Ib Johnsen. 1989. Principles of environmental science and technology. Amsterdam: Elsevier Science Publishers.

National Research Council (U.S.). 1990. Managing troubled waters: the role of marine environmental monitoring. Washington, D.C.: National Academy Press.

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