Why Biochemical Oxygen Demand Matters in Environmental Science
Biochemical Oxygen Demand or alternatively termed as Biological Oxygen Demand (BOD) is the amount of oxygen needed or demanded by aerobic microorganisms to break down the organic matter present in a certain sample of water at a specific temperature and over a given time period.
Water bodies have a certain amount of oxygen dissolved in it on which the aquatic life is dependent for its respiratory needs. When there is organic matter present in the water body, aerobic microbes use the dissolved oxygen in the water to break down the organic substance thus reducing the oxygen available for aquatic life. The molecular oxygen present in water is either a byproduct of photosynthesis occurring in aquatic plants or is atmospheric oxygen in the dissolved state. The more the BOD of a water body or water sample, the more it is polluted. The increase in organic matter can be due to environmental factors but are mostly due to anthropogenic causes like pollution. BOD is used as an index for measuring water quality. Determining organic matter present in a water body and its effect on the ecosystem and aesthetics of the water body is an integral part of water quality management.
Measuring Biochemical Oxygen Demand
Biochemical Oxygen Demand of a water sample is measured by a Bioassay procedure which measures the oxygen consumed by the bacteria from the decomposition of the organic matter over a period of five days at an incubation temperature of 20°C. BOD is expressed in milligrams per litre of sample water. Although this is not a precise quantitative test, it is widely used as an indication of the polluting potential of water. This test was given by Sawyer and McCarty in 1978.
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.
Factors Affecting BOD
BOD of a Water Body is Affected by the Following Factors -
Temperature
pH value of the water
The presence of certain kinds of microorganisms affects the growth of aerobic bacterias.
Type of inorganic substances in water.
Amount and type of organic substance in water.
Significance of BOD
Biochemical Oxygen Demand finds its importance in various fields. These are -
BOD finds its primary importance in sewage treatment plants. It gives the respiration rate of sewage, sludge, soil, and garbage.
It determines the rate of respiration in living beings.
Measuring BOD gives the COD or Chemical Oxygen Demand of inorganic substances.
It indicates the polluting potential of water.
BOD is used in the medical and pharmaceutical industries to measure the oxygen consumption of cell cultures.
Sources of BOD
Sources that increase the Biological Oxygen Demand of water are both natural and man-made. Pollution is a major contributor to increasing the BOD of water bodies. A good lifestyle is associated with an ample usage of water on a regular basis which results in a lot of wastewater with organic content in it. With increasing industrialization, pollution is increasing manifold. Factories have enormous wastewater being generated. Few industries that have huge quantities of wastewater are paper mills, food processing plants, jute mills, etc. The environmental factors contributing to increasing BOD include surface runoff, floating debris, dead animals and plants, soil erosion, etc. There are few chemicals that affect the BOD of drinking water. One of these is phosphate, which when present in high amounts increases the BOD of water.
Usage of BOD in Sewage Treatment Plants
Biochemical Oxygen Demand is used in secondary sewage treatment or biological sewage treatment. After the primary treatment in which the floating debris is removed by sequential filtration and sedimentation, the primary effluent is passed to aeration tanks where it is constantly agitated and the air is pumped into it. In aeration tanks, there are vigorous growth heterotrophic microbes into flocs. Flocs are masses of bacteria associated with fungal filaments.
These microbes consume the organic matter present in the primary effluent. The water is treated until the BOD of the water is reduced. This is now called the activated sludge. This effluent from aeration tanks goes into settling tanks for bacterial flocs to settle down and after which they are treated with anaerobic microbes and physicochemical processes before discharge in water bodies.
Effect of High BOD on the Aquatic Ecosystem
Increasing BOD has the same effect as the effects of dissolved depleting oxygen. When the BOD of a water body increases significantly, aquatic life is adversely affected. The oxygen used by aquatic organisms for respiration and metabolism is significantly reduced by the microbes for breaking down of organic waste. This results in the death of fishes and aquatic plants and complete disruption in the aquatic ecosystem. Oxygen concentration below 5ppm (parts per million) risks even low oxygen organisms like catfish and carps. The freshwater fishes like Catla and rohu do not survive at these concentrations. The overall aesthetic and beauty of the water body are damaged.
Pollution and Its Effects on Biochemical Oxygen Demand
With increasing pollution and urbanization, the water quality of the water bodies is significantly reducing. Water quality management is essential for the correct ecological functions. Urbanization leads to the production of much larger quantities of sewage. The number of sewage treatment plants was not enough for treating these large quantities of sewage. Untreated sewage was often discharged directly into water bodies which caused massive pollution and an increase in BOD of the water bodies. This also led to an increase in water-borne diseases like cholera, dysentery, jaundice, etc.
This increasing amount of BOD and pollution led to extreme pollution of the two main river bodies in India, Ganga, and Yamuna. The Ministry of Environment and Forests initiated the Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993, to save these major rivers of the country. These plans initiated the building of a large number of sewage treatment plants to facilitate the discharge of only treated sewage in rivers.
Methods to Reduce BOD in Water
The biological oxygen demand of the water can be reduced by the following methods
Advanced Oxidation Processes (AOP) by using H2O2/ UV, O3/UV, Fenton's reagent ( H2O2+FeSO4), etc.
Coagulation using alum or cationic polymers
Flocculation (e.g. chitosan, isinglass, polyelectrolyte) and sedimentation.
Adsorption using activated charcoal.
Electro flocculation.
Using the up-flow anaerobic sludge blanket reactor(UASB).
Reverse osmosis.
Dissolved air floatation technique.
Water is a basic need for all life forms to exist on earth. Therefore it is extremely important for us to conserve water and not pollute it. Polluting water directly harms every living creature be it mankind or the other forms of life. Rising biochemical oxygen demand levels in water are harming the ecosystem of the water bodies which indirectly is harming the whole ecology of the whole biosphere as a whole. It is our duty to keep these ecosystems alive. The other forms of life have the same amount of rights on environmental resources as human beings. We must keep our greed away and strive towards conservation of water bodies and reduction of BOD levels in them.
The measurement of the amount of dissolved oxygen consumed by aerobic microorganisms while decomposing organic matter in stream water is known as Biochemical Oxygen Demand.
Affecting Factors
Mentioned below are the affecting factors of biological chemical demand:-
Biological oxygen demand affects the amount of dissolved oxygen in streams and rivers. The rate of oxygen consumption is affected by: pH, temperature, various kinds of microorganisms, and organic and inorganic materials.
Less oxygen is consumed by higher forms of aquatic life. The consequences of the high BOD are the same as those for low dissolved oxygen.
Uses
Various uses of biological oxygen demand have been mentioned as follows :
Biological oxygen demand is used in studies for measuring self purification capacity of the streams.
It is an important method in sanitary analysis in determining industrial waste, the strength of sewage, and polluted water.
It also serves as a source through which the quality of effluents discharged into the stream water can be checked.
Importance
Let's discuss about the environmental significance of biological oxygen demand:
Biological oxygen demand is important for water quality because it provides an index to assess the effect discharged wastewater will have on our environment.
BOD is used extensively for treating wastewater, as decomposition of organic waste by microorganisms is commonly used for treatment.
Data obtained from the BOD test is used for designing the wastewater treatment plants.
FAQs on Biochemical Oxygen Demand Explained: Factors, Importance & Uses
1. What is Biochemical Oxygen Demand (BOD) and what does it indicate?
Biochemical Oxygen Demand, or BOD, is the amount of dissolved oxygen needed by aerobic microorganisms to break down the organic material present in a given water sample at a specific temperature over a period of time. It is a key indicator of water pollution; a higher BOD value signifies a higher degree of organic pollution, making the water less suitable for aquatic life.
2. Is a high BOD level in water considered good or bad?
A high BOD level is considered bad for a water body. It indicates that the water is heavily polluted with organic waste. This waste serves as food for decomposer bacteria, which consume large amounts of dissolved oxygen, depleting the supply available for fish and other aquatic organisms, potentially leading to their death.
3. What are the main sources that contribute to high BOD in water bodies?
The main sources of high BOD are primarily organic pollutants from various human and natural activities. Key examples include:
- Sewage: Untreated or partially treated domestic wastewater.
- Industrial Effluents: Waste from food processing plants, paper mills, and tanneries.
- Agricultural Runoff: Fertilizers and animal manure washed into rivers and lakes.
- Decomposing Algae: From algal blooms caused by nutrient pollution (eutrophication).
- Natural Sources: Decomposing leaves, dead plants, and animals.
4. What is the fundamental principle behind the BOD test?
The principle of the BOD test is based on a bioassay procedure. It measures the oxygen uptake by a microbial population over a set period. The core idea is that the amount of oxygen consumed by aerobic bacteria to decompose organic pollutants in a water sample is directly proportional to the concentration of biodegradable organic matter. It essentially simulates the natural self-purification process of water in a controlled lab environment.
5. How is the BOD of a water sample typically measured?
BOD is typically measured using the BOD₅ test. A water sample is taken, and its initial dissolved oxygen (DO) level is measured. The sample is then sealed and incubated in the dark at 20°C for five days. This prevents photosynthesis from adding oxygen. After five days, the final DO level is measured. The BOD is calculated as the difference between the initial and final DO levels, expressed in milligrams of oxygen per litre (mg/L).
6. How are Biochemical Oxygen Demand (BOD) and Dissolved Oxygen (DO) related?
BOD and Dissolved Oxygen (DO) have an inverse relationship in a polluted water body. As the amount of organic waste increases, so does the BOD, because more oxygen is required by microbes for decomposition. This increased consumption of oxygen by bacteria leads to a sharp decrease in the available DO. Low DO levels stress or kill aquatic life like fish and insects.
7. What is the difference between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)?
The key difference lies in what they measure. BOD specifically measures the amount of oxygen required by microorganisms to break down only the biodegradable organic matter. In contrast, COD measures the oxygen required to break down nearly all organic matter—both biodegradable and non-biodegradable—through strong chemical oxidation. Therefore, the COD value for a water sample is always higher than its BOD value.
8. Why is it crucial to reduce the BOD of wastewater in treatment plants?
Reducing the BOD of wastewater before discharging it is crucial to protect aquatic ecosystems. Wastewater with high BOD, if released directly into a river or lake, would cause a rapid depletion of the water body's dissolved oxygen. This process, known as deoxygenation, creates hypoxic (low oxygen) or anoxic (no oxygen) conditions, leading to the mass death of fish and other aerobic organisms and severely degrading the water quality.
9. How does the secondary treatment stage in a sewage treatment plant (STP) reduce BOD?
Secondary treatment significantly reduces BOD through biological processes. The primary effluent is passed into large aeration tanks where it is constantly agitated and air is pumped into it. This promotes the vigorous growth of aerobic microbes (bacteria and fungi) which form masses called flocs. These flocs consume the majority of the organic matter in the effluent, thereby reducing its BOD by about 90-95% as per the NCERT curriculum for the 2025-26 session.
