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Biochemical Oxygen Demand is the amount of oxygen needed or demanded by aerobic microorganisms. It is also known as Biological Oxygen Demand (BOD).
- BOD specifies the amount of oxygen needed to break down organic matter in water.
- Aerobic microorganisms use BOD to break down the organic matter present in water at a specific temperature, at a given time.
- BOD is a metric to measure water purity and quality.
- The more the BOD of a water body or water sample, the more it is polluted.
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Key Terms: Biochemical Oxygen Demand, Biological Oxygen Demand, BOD, Water Quality, Aquatic System, Water, Oxygen, Aerobic degradation, Pollution, Fish
What is Biochemical Oxygen Demand?
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Biochemical Oxygen Demand is the biological approach of determining how much Dissolved Oxygen (DO) was consumed overall by bacteria during the biological process of metabolizing organic compounds found in water. Biochemical Oxygen Demand or BOD is used to assess the susceptibility of an organic substance to aerobic degradation.
- Dissolved oxygen (DO) refers to the entire amount of oxygen gas contained in the water.
- Water may include either dissolved oxygen gas or non-compound oxygen, which is a byproduct of the photosynthesis of aquatic plants.
- In some water bodies, organic matter is a great source of BOD.
- These organic matters include sewage and other pollutants present in the water bodies.
- The greater the BOD, the lower the dissolved oxygen available for aerobic animals such as fish and other aquatic organisms.
- BOD is a reliable measure of the organic pollution of water bodies.
- The main reason for treating wastewater before its discharge into a water resource is to reduce its BOD level.
Biochemical Oxygen Demand
How to Measure BOD?
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BOD is measured using a bioassay approach.
- BOD measurement test was given by Sawyer and McCarty in 1978.
- The test analyses the oxygen required by the bacteria from the decomposition of the organic matter for five days at an incubation temperature of 20°C.
- The demand for a water sample is also determined through the same.
- BOD is measured in milligrams per liter of the sample water collected.
- Although it is not an exact quantitative test, this method is frequently used to determine how contaminated a water body is.
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Values of BOD and Its Indication
Given below are the values of BOD that helps to determine the pollution level in a water body:
| BOD Value | Water Quality |
|---|---|
| Below 1 mg/L | Fresh Water |
| Between 2-8 mg/L | Moderately Polluted Water |
| Above 8 mg/L | Highly Polluted Water |
Factors Affecting BOD
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BOD of a Water Body is affected by the given factors:
- Water Temperature
- Water's pH Level
- Growth of Aerobic Bacteria is affected by Specific Types of Microorganisms
- Inorganic Materials found in Water
- Type and Amount of Organic Content in Water
Why is BOD Important?
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Biological Oxygen Demand is significant due to the given reasons:
- It helps to analyze the quantity of oxygen that microorganisms in water bodies use to break down organic waste.
- It identifies the organic pollution levels present in an aquatic ecosystem.
- BOD calculation is helpful in sewage treatment or wastewater treatment.
- It helps to find how much organic matter is present in soils, garbage, etc.
- BOD values help calculate the rate of respiration in living beings.
- Biochemical Oxygen Demand is studied by pharmaceutical industries to understand the consumption level of oxygen in cell cultures.
Sources of BOD
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Sources of Biochemical Oxygen Demand for water includes both natural and man-made sources.
- Pollution is majorly responsible for increased BOD levels of the water body.
- The synthetic factors that increase BOD include sewage residue, factory wastes, etc.
- Industry wastes and toxic chemicals like Phosphate hampers the water quality and increases the BOD of water.
- The environmental factors that increase BOD include dead animals and plants, soil erosion, etc.
Effects of High BOD on the Aquatic Ecosystem
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Effects of High BOD are similar to those of dissolved oxygen depletion.
- Aquatic life suffers when the BOD of a body of water considerably grows.
- The bacteria that break down organic waste reduce the oxygen ratio needed by water species need for respiration and metabolism.
- Fish and aquatic vegetation die as a result, completely upsetting the aquatic ecology.
- Even low-oxygen organisms like catfish and carp are at risk when the oxygen concentration falls below 5 ppm (parts per million).
- At this quantity, freshwater fish species like Catla and rohu cannot survive.
- Damage is done to the overall appeal and attractiveness of the water body.
Effects of Pollution on BOD
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Pollution and Urbanization lead to a reduction in the water quality of the water bodies. Management of water quality is crucial for proper ecological processes.
- Urbanization causes sewage production to increase significantly.
- For handling this massive amount of sewage, there aren't adequate sewage treatment facilities.
- Water bodies get polluted due to the direct discharge of untreated waste, due to which BOD increases.
- The pollution and high BOD water give birth to water-borne illnesses like cholera, dysentery, jaundice, etc.
- Ganga and Yamuna are severely polluted as a result of the rising BOD and pollution levels.
To protect these significant national rivers and create awareness, the Ministry of Environment and Forests took several initiatives and actions. The Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993 were part of this initiative. To make it easier to release solely treated sewage into rivers, these plans led to the construction of numerous sewage treatment facilities.
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How to Reduce BOD in Water?
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BOD is an indicator of the pollution level of a water body. The higher the BOD, the more water is polluted. Thus, it is necessary to lower BOD in the water body to retain its quality. The rise in biochemical oxygen demand levels in water harms the aquatic ecosystem and further disturbs the natural ecological balance.
Here are some scientific methods to reduce BOD in water:
- Advanced Oxidation Processes (AOP)
- Coagulation
- Flocculation and Sedimentation
- Adsorption through Charcoal
- Electro-flocculation
- Using the Up-flow Anaerobic Sludge Blanket Reactor (UASB)
- Reverse Osmosis
- Dissolved Air Floatation Technique
Things to Remember
- Biochemical Oxygen Demand (BOD) is the amount of oxygen utilized by aerobic microbes to degrade the organic waste present in water.
- BOD is calculated over five days at a temperature of 20°C.
- It is important for understanding the polluting potential of the water.
- The more the BOD level in a water body, the more polluted the water is.
- BOD is used as an index in wastewater or sewage treatment plants.
- BOD levels of wastewater need to be lowered essentially before discharging it into water bodies.
Sample Questions
Ques. Describe various sources of BOD. (3 Marks)
Ans. The biological oxygen demand (BOD) rises through multiple natural and human sources. Natural sources like soil erosion and floating debris play a key role in increasing BOD. The waste generated through paper mills, jute mills, and food processing plants results in increased BOD.
Ques. What are the factors that affect BOD levels? (3 Marks)
Ans. BOD levels of a water body are affected by multiple factors like water temperature, water pH level, growth of aerobic bacteria in the water, inorganic materials, and organic content found in water. All of these factors contribute to determining the BOD of a water body.
Ques. Define BOD and briefly explain its importance. (3 Marks)
Ans. The biochemical oxygen demand or BOD refers to the amount of oxygen needed by microorganisms to break down organic matter in water. BOD identifies the organic pollution levels present in an aquatic ecosystem. It helps in treating wastewater discharged from sewage and factories. One can also calculate the rate of respiration in living beings through BOD.
Ques. How to measure BOD? (3 Marks)
Ans. A test was designed by Sawyer and McCarty in 1978 to measure the BOD of the water bodies. The test administers the oxygen used by the bacteria from the decomposition of the organic matter for a period of five days at 20°C to calculate the biochemical oxygen demand of the water. It is measured and written in milligrams per liter units of the water collected water sample.
Ques. What pollutants contribute to increasing in BOD? (3 Marks)
Ans. Pollution is majorly responsible to increase BOD levels in the water. The wastes are directly discharged from industries, factories, and sewages which also give rise to toxicity and BOD in the water. Toxic chemicals like phosphates and nitrates contribute to higher BOD levels. The increased BOD provides nutrients for algae and aquatic plants to grow quickly leading to rising in organic waste.
Ques. What does a BOD value indicate? (5 Marks)
Ans. The BOD value indicates water purity levels. The BOD values determine the purity or toxicity of the collected water. There are several indications of each BOD value. If a water sample has a BOD value below one mg/L it means that the particular water sample is fresh, clean, and fit for consumption. As the BOD unit increases, the water becomes contaminated. Therefore, the BOD levels between 2-8 mg/L suggest that the water sample is moderately polluted. Any water sample that scores above eight mg/L BOD is highly polluted and unfit for consumption.
Ques. How to reduce Biochemical Oxygen Demand? (5 Marks)
Ans. More water is contaminated when the BOD is higher. In order to maintain the water body's purity, BOD must be reduced. The aquatic ecology suffers as a result of the increase in biochemical oxygen demand levels in the water, which also upsets the natural ecological equilibrium. Here are some proven techniques for lowering BOD in water:
- Advanced Oxidation Processes (AOP)
- Coagulation
- Flocculation and sedimentation
- Adsorption through charcoal
- Electro-flocculation
- Using the up-flow anaerobic sludge blanket reactor (UASB)
- Reverse osmosis
- Dissolved air floatation technique
Ques. How does pollution effects Biochemical Oxygen Demand? (3 Marks)
Ans. BOD determines a water body's amount of pollution. It serves as a metric for assessing the ability of water bodies to pollute. The BOD of a body of water increases as pollution levels rise. When wastewater from homes, businesses, power plants, and other companies reaches water bodies, it raises the water's organic content, causing anaerobic bacteria to proliferate and use the oxygen there to break down organic waste. As a result, fish and aquatic plants perish. Phosphate contamination has become one of the inorganic chemicals that significantly raises BOD in water bodies.
Ques. Explain the effects of high BOD on the aquatic ecosystem in detail. (3 Marks)
Ans. The amount of BOD value in a water body is dangerous for aquatic life. Due to the increase in BOD, fish and aquatic vegetation die. It also hampers the bacteria that break down the organic waste resulting in a completely imbalanced aquatic ecology. Severely high BOD levels make it difficult for even low-oxygen organisms like catfish and carp to survive as the oxygen concentration falls below 5ppm (parts per million). It is also dangerous for freshwater fish species like Catla and rohu cannot to survive. The rise in Biological Oxygene Demand for a water body reduces the water body's overall appeal and attractiveness.
Ques. What actions are taken by the Government to reduce BOD? (5 Marks)
Ans. It is highly essential to keep BOD levels for the sake of aquatic life and ecology. The toxic chemicals discharged from factories and sewages are major contributors to increasing BOD. In fact, high BOD value and pollution hampered the two primary river systems of India; The Ganga and Yamuna rivers.
Therefore, the Government had to step in and create awareness regarding the issue. The Ministry of Environment and Forests launched The Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993 to deal with this issue. As a result of these plans, numerous sewage treatment facilities were built and it was ordered that only treated waste shall be discharged in the water bodies.
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