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Biological Oxygen Demand (BOD) – Definition, Importance, Measurement

Biological oxygen demand refers to the amount of oxygen required by bacteria and other microorganisms to decompose organic matter under aerobic (oxygen present) conditions at a certain temperature (BOD). The greater the concentration of organic contaminants in the water, the greater the oxygen demand of the bacteria. Consequently, the level of contamination in a body of water is proportional to its BOD.

What is Biological Oxygen Demand (BOD)?

  • Biochemical oxygen demand refers to the amount of oxygen needed for the biological oxidation of organic materials in water (BOD).
  • Aquatic life depends on a precise concentration of dissolved oxygen for its respiratory demands.
  • When organic matter is present in a body of water, aerobic microbes use dissolved oxygen to decompose it, hence reducing the amount of oxygen available to aquatic life.
  • Water includes molecular oxygen, which is either produced by photosynthesis in aquatic plants or dissolved air oxygen.
  • The greater the BOD of a water body or sample, the worse its pollution.
  • Organic matter concentrations are increasing due to a variety of factors, the most of which are of human origin, such as pollution.
  • BOD is also used to evaluate the quality of water.
  • Biochemical Oxygen Demand of a water sample is determined by a Bioassay process that analyses the oxygen consumed by the bacteria from the decomposition of organic materials over a period of five days at 20°C.
  • BOD is measured in milligrammes per litre of water sample. Although this is not a quantitatively accurate test, it is often used as an indicator of the water’s contaminating potential. This test was administered in 1978 by Sawyer and McCarty.

Typical Values of BOD and its Indication:

  • Below 1 mg/L- Pristine water quality.
  • 2-8 mg/L- Moderately polluted water.
  • Above 8mg/L- Severely polluted water.

How does water pollution increase the Biological Oxygen Demand (BOD)?

  • Oxygen will be depleted if too much organic waste (water pollution) is introduced to water.
  • This results in the demise of aquatic organisms depending on oxygen.
  • Consequently, anaerobic bacteria (those that do not require oxygen) begin to degrade the organic waste and produce chemicals with a foul odour that are hazardous to human health.
  • Aerobic (oxygen-requiring) bacteria will consume more oxygen and degrade these organic wastes, reducing the water’s dissolved oxygen.
  • Therefore, the amount of BOD in the water indicates how much oxygen is necessary to biologically decompose organic matter.
  • Therefore, when sewage and river water are combined, the BOD levels in the water increase.

Factors Affecting BOD

The BOD of a body of water is affected by the following variables: 

  • Temperature.
  • The pH of the water.
  • Certain microorganisms inhibit the growth of aerobic bacteria when present.
  • Type of inorganic compounds found in water.
  • Quantity and type of organic compounds in water.

Sources of BOD

  • Sources that boost the level of Biological Oxygen Demand for water is both natural and anthropogenic. Pollution is a major factor in the rise of BOD in water bodies.
  • A healthy lifestyle is related with the regular consumption of large quantities of water, which generates large quantities of organic-rich wastewater. As industrialization increases, pollution multiplies exponentially.
  • Massive quantities of wastewater are created by factories. Paper mills, food processing plants, jute mills, etc., are among the few businesses that generate enormous amounts of wastewater.
  • Environmental elements that contribute to an increase in BOD include surface runoff, floating trash, dead animals and plants, soil erosion, and others.
  • Few substances have an effect on the BOD of drinking water. One of these is phosphate, which increases the BOD of water when present in high concentrations.

Usage of BOD in Sewage Treatment Plants

  • The Biochemical Oxygen Demand is utilised in secondary or biological sewage treatment.
  • After the initial treatment, in which floating material is removed by sequential filtration and sedimentation, the primary effluent is transferred to aeration tanks, where it is continuously agitated and air is injected into it.
  • There is a strong development of heterotrophic microorganisms forming flocs in aeration tanks. Flocs are bacterial clusters linked with filamentous fungi.
  • These microorganisms devour the primary effluent’s organic materials. The water is treated till its BOD concentration is decreased. This is currently known as the activated sludge.
  • This effluent from the aeration tanks is then treated using anaerobic microorganisms and physicochemical processes before being discharged into bodies of water.

Effect of High BOD on the Aquatic Ecosystem

  • Increasing BOD has the same impact as oxygen depletion due to dissolution. When the BOD of a body of water considerably rises, aquatic life is negatively impacted.
  • The oxygen required by aquatic species for respiration and metabolism is drastically reduced by bacteria responsible for decomposing organic waste. This causes the extinction of fishes and aquatic plants and a total disturbance of the aquatic ecology.
  • Under 5 ppm (parts per million) of oxygen, even low oxygen creatures such as catfish and carps are at risk.
  • At high concentrations, freshwater fishes like Catla and rohu cannot survive.
  • The entire appearance and elegance of the body of water has been compromised.

Pollution and Its Effects on Biochemical Oxygen Demand

  • With increased pollution and urbanisation, the water quality of the bodies of water is deteriorating substantially. Water quality management is vital for the proper functioning of ecosystems.
  • Urbanization results in the generation of significantly more sewage. The number of sewage treatment plants was insufficient to treat these massive sewage volumes.
  • Often, untreated sewage was released directly into water bodies, resulting in huge contamination and an increase in BOD levels. This also contributed to a rise in water-borne ailments such as cholera, dysentery, jaundice, etc.
  • This growing BOD and pollution led to the extreme pollution of India’s two major rivers, Ganga and Yamuna.
  • The Ministry of Environment and Forests launched the Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993 in an effort to preserve these main rivers of the country.
  • These plans launched the construction of a large number of sewage treatment plants so that only treated sewage could be discharged into waterways.

Methods to Reduce BOD in Water

The subsequent techniques can aid in decreasing the biological oxygen requirement of water.

  • To reduce the biological oxygen demand, the first and most important step is to reduce pollution-causing sources.
  • Other advanced oxidation processes include H2O2/UV, O3/UV, Fenton’s reagent (H2O2+FeSO4), and Fenton’s reagent (H2O2+FeSO4).
  • For coagulation, alum or cationic polymers are utilised.
  • sedimentation and flocculation (e.g., chitosan, isinglass, polyelectrolyte).
  • Utilizing activated charcoal for absorption.
  • Flocculation using electrical means.
  • Utilizing an up-flow anaerobic sludge blanket reactor (UASB).
  • Reverse osmosis.
  • Flotation method using dissolved air.


  • The following uses of biological oxygen demand have been mentioned:
  • In research, biological oxygen demand is used to measure the self-purification capacity of streams.
  • It is an essential technique in sanitary analysis for determining industrial waste, sewage concentration, and polluted water.
  • In addition, it functions as a checkpoint for the quality of effluents discharged into the stream water.

Significance of BOD

Biochemical Oxygen Demand is essential in numerous fields. Listed below are –

  • BOD is most important in wastewater treatment systems. It provides the respiration rate for sewage, sludge, soil, and trash.
  • It determines the respiration rate of living organisms.
  • BOD measurement yields the Chemical Oxygen Demand (COD) of inorganic compounds.
  • It signifies the potential for water pollution.
  • In the medical and pharmaceutical industries, BOD is utilised to determine the oxygen consumption in cell cultures.

What is Dissolved Oxygen?

  • Similar to terrestrial creatures and people, aquatic animals require oxygen to survive.
  • Oxygen from the atmosphere dissolves in river and lake water, and fish and other aquatic organisms breathe this oxygen.
  • When water flows over rocks in creeks and rivers, oxygen can enter the water. The image below depicts rapids in an Ellesmere Island glacier stream.
  • Oxygen levels depend on whether or not water is moving, if there are rocks or other impediments for the water to flow over, how many plants are growing in the water, and the water’s temperature.
  • Cold, flowing water with several impediments and a moderate number of plants contains more oxygen. Plants absorb carbon dioxide and release oxygen, but if there are too many plants, the bacteria that decompose them when they die will use all of the oxygen.
  • Extremely cold water contains more oxygen than extremely warm water. This may lead us to believe that winter water contains a great deal of oxygen, but this is not the case.
  • During the winter, ice covers lakes and rivers and very little oxygen from the atmosphere reaches the water, effectively sealing the lake.
  • Oxygen levels in lakes vary with depth. In deep lakes that do not get very much breeze, oxygen levels fall lower as we travel deeper.
  • Oxygen levels are often low at the bottom of all lakes, where the water hits the sediment or muck. This is because the silt contains numerous germs and organisms that live and breathe.
  • These bacteria and animals consume oxygen while decomposing dead matter that sinks to the ocean floor. In some lakes and ponds with extremely low oxygen levels, we install aerators to maintain high oxygen levels.
  • This occurs frequently in lakes that have been stocked with fish and lakes that receive sewage inputs.

Why does dissolved oxygen matter?

  • Fish and other aquatic organisms require oxygen to survive. Typically, oxygen levels in natural lakes and rivers cannot be excessive.
  • Conversely, if oxygen levels in the water are too low, fish and other animals may suffocate and die. For instance, oxygen cannot enter the water when it is frozen, therefore fish frequently suffocate towards the end of winter.
  • The likelihood of winter fish kills increases if the fish inhabit a system that is contaminated or overgrown (overproductive).
  • Overgrowth of animals, plants, and germs in polluted systems depletes oxygen quickly, occasionally causing fish to choke.
  • The majority of Arctic lakes and rivers lack an overgrowth of animals, plants, and germs, so “fish kills” at the end of winter or during the hottest days of summer are uncommon.
  • Large-scale industrialization, intensive use of fertilisers, and dumping of human waste can rapidly pollute water and contribute to oxygen depletion.
  • Each kind of aquatic fish requires a particular amount of dissolved oxygen to survive. For instance, Northern Pike cannot thrive in water containing less than around 6 milligrammes of dissolved oxygen per litre (6 mg/L). If dissolved oxygen levels fall to approximately 3 to 4 mg/L, even the most robust fish could suffocate.

How do we measure dissolved oxygen?

  • Direct measurement of dissolved oxygen in water using a calibrated dissolved oxygen sensor is optimal.
  • This sensor can directly measure the amount of oxygen dissolved in water as mg/L or as a percentage of oxygen dissolved (%DO).
  • Water at lower temperatures should have a higher concentration of dissolved oxygen and a higher %DO, but warmer, more polluted water will have a lower concentration and %DO.
  • In general, healthy water should have dissolved oxygen concentrations between 6.5-8 mg/L and 80-120%.


  • Li, D., & Liu, S. (2019). Water Quality Monitoring in Aquaculture. Water Quality Monitoring and Management, 303–328. doi:10.1016/b978-0-12-811330-1.00012-0 

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