Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:35 PM IST

BOD is the amount of oxygen required by microorganisms to decompose organic matter dissolved in water. It becomes an important parameter of environmental science in defining the quality of the water and the health condition of aquatic ecosystems. In a progressively polluted world, the BOD will act as an indicator of the extent of organic Pollution within the water and has serious consequences for aquatic life.

This Story also Contains

Understanding Biochemical Oxygen Demand
Parameters and Forms of Biochemical Oxygen Demand
How Biochemical Oxygen Demand is useful and its Applications in Real Life
Some Solved Examples
Summary

Biochemical Oxygen Demand

Understanding Biochemical Oxygen Demand

• The milligrams of dissolved oxygen used by microorganisms in breaking down the organic matter present in a water sample in a five-day period usually at 20°C.
• BOD is the most important and essential parameter to determine the organic load of pollution on receiving waterbodies. Higher values indicate a large amount of organic matter; this may signify severe water pollution.
- Measurement of BOD involves incubating a water sample in the dark and measuring the initial and final DO levels before finding the difference, which will yield the oxygen consumed by the micro-organisms.
- The parameters that influence the value of BOD are temperature, pH, type of microorganisms, and kind of organic and inorganic materials in water.

Dissolved oxygen(DO) is essential for sustaining animal and plant life in any aquatic system. If the DO level in water decreases, the aquatic organisms may not survive. The wastes such as domestic, industrial and biodegradable organic compounds are oxygen-demanding wastes. These are decomposed by the bacterial population which in turn decreases the oxygen from water.
BOD is a measure of the oxygen utilized by microorganisms during the oxidation of organic materials. It is the most widely known measure for assessing the water pollution of a given organic waste. BOD is directly proportional to the amount of organic waste that is to be broken down.
The determination of the BOD of a sample of water requires 20-30 days for the complete decomposition of organic waste present in the sample. This is too long a time for obtaining data. Therefore, the time has been fixed as five days, i.e., BOD is the amount of oxygen consumed in five days. The measurement is done at 20^oC. The sample of water is saturated with oxygen. It is then incubated for five days at 20^oC. Micro-organisms utilize oxygen to oxidize waste during this period. The remaining oxygen is measured and subtracted from the amount of oxygen originally present to get the BOD value. It is expressed in ppm.
BOD is a real measure of water quality. Clean water has a BOD value of less than 5 ppm. The polluted river water could have a BOD value of 17ppm or more. The untreated municipal sewage has a BOD value of 100-400 ppm.

Parameters and Forms of Biochemical Oxygen Demand

There are a few types of BOD, all of which have different purposes in the area of environmental science.
Five-Day BOD: This is the standard measurement taken for five days, and this one is widely adopted to classify or measure the degree of water pollution.
Ultimate BOD: It shows the total oxygen demand by organic material in a sample for a long period and is hence applied for long-term assessment purposes.
• Carbonaceous BOD: The amount of oxygen required for the decomposition of only carbon-containing organic matter by microorganisms, i.e., excluding nitrogenous contributions.
• Nitrogenous BOD: That contribution to the oxygen demand made by nitrogenous compounds, mainly because of ammonia, which can equally contribute towards degradation in water quality.

NEET Highest Scoring Chapters & Topics

This ebook serves as a valuable study guide for NEET exams, specifically designed to assist students in light of recent changes and the removal of certain topics from the NEET exam.

Download EBook

How Biochemical Oxygen Demand is useful and its Applications in Real Life

BOD has relevant implications for environment management and public health with regard to wastewater treatment and regulatory compliance.
- In that respect, BOD measurements become very important in wastewater treatment since they express the amount of organic wastes that shall be broken down during such processes, and whether the treated water is safe for release into natural water bodies.
- It provides the regulatory agencies' basis of allowable values of BOD with respect to the industrial discharges for the protection of aquatic ecosystems; hence industries must treat their wastewaters prior to discharge to a water body to ensure low discharged BOD values.
• Long-term monitoring for BOD can identify sources of impairment, focus restoration efforts, and further inform policy strategies likely to improve the quality of the water.
• Academic researchers use BOD data for understanding the self-purification capacity of rivers and streams and modeling water quality as a function of the suite of different pollutant types impacting on aquatic ecosystems.

Some Solved Examples

Example 1 Biochemical Oxygen Demand(BOD) is the amount of oxygen required (in ppm):

1)For the photochemical breakdown of waste present in 1 m³ volume of a water body

2)by anaerobic bacteria to break down inorganic waste present in a water body.

3) (correct)by bacteria to break down organic waste in a certain volume of a water sample

4)for sustaining life in a water body

Solution

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand (BOD).

Hence, the answer is the option (3).

Example 2 Given below are two statements :

Statement I: The value of the parameter " Biochemical Oxygen Demand (BOD)" is important for the survival of aquatic life.

Statement II: The optimum value of BOD is 6.5 ppm.

In the light of the above statements, choose the most appropriate answer from the options given below:

1) (correct)Statement I is true but statement II is false

2)Both statement I and statement II are false

3)Statement I is false but statement II is true

4)Both Statement I and Statement II are true

Solution

Statement I is correct.

Statement II is incorrect.

If too much organic matter is added to water, all the available oxygen is used up. This causes oxygen-dependent aquatic life to die. The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand (BOD). The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically. Clean water would have a BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more.

Hence, the answer is the option (1).

Question:

Example 3 The measured BOD values for four different water samples (A-D) are as follows: A= 3 ppm; B=18 ppm; C=21 ppm D=4 ppm. The water samples which can be called as highly polluted with organic wastes, are

1)A and B

2)A and D

3) (correct)B and C

4)B and D

Solution

As we have learned,

Highly polluted water has a BOD of more than 17 ppm.

Thus, water samples B (18 ppm) and C (21 ppm) are highly polluted.

Hence, the answer is the option (3).

Example 4 Given below are two statements :
Statement I: In polluted water values of both dissolved oxygen and BOD are very low.

Statement II: Eutrophication results in a decrease in the amount of dissolved oxygen.
In the light of the above statements, choose the most appropriate answer from the options given below :

1)Both Statement I and Statement II are true

2)Both Statement I and Statement II are false

3)Statement I is true but Statement II is false

4) (correct)Statement I is false but Statement II is true

Solution

Eutrophication is a process due to which excessive growth of weeds in water bodies takes place leading to an increase in pollution levels. Eutrophication leads to a decrease in the amount of dissolved oxygen in water bodies. As the amount of dissolved oxygen decreases, the value of BOD [Biological oxygen demand] increases.
So statement I is false and statement II is true.
Hence, the answer is the option (4).

Example 5 The water having more dissolved O₂ is :

1)Boiling water

2)Water at $80^{\circ} C$

3)Polluted water

4) (correct)Water at $4^{\circ} C$

Solution

The solubility of gases in water decreases with the increase in temperature.

Thus, more dissolved O₂ will be present in water at $4^{\circ} C$

Hence, the answer is the option (4).

Summary

Lastly, BOD is the basic concept meaning biochemical oxygen demand, which is the amount of oxygen needed for microorganisms to break down organic matter in the water. The paper tried to define, explain, and expound on the meaning, types, and factors affecting BOD in a way that would underline its role as a pointer or indicator of water quality and pollution. We shared with them practical applications of BOD determination in the realm of wastewater treatment, regulatory compliance, and academic research.

Frequently Asked Questions (FAQs)

1. Why is BOD important in environmental chemistry?

BOD is crucial in environmental chemistry because it helps assess water quality and pollution levels. High BOD levels indicate high amounts of organic matter in water, which can lead to oxygen depletion and harm aquatic life. It's used to monitor wastewater treatment efficiency and evaluate the impact of pollutants on water bodies.

2. What's the difference between BOD and COD?

BOD (Biochemical Oxygen Demand) measures the amount of oxygen consumed by microorganisms in breaking down organic matter, while COD (Chemical Oxygen Demand) measures the amount of oxygen required to chemically oxidize organic compounds in water. COD is typically higher than BOD as it includes both biodegradable and non-biodegradable organic matter.

3. What causes high BOD levels in water?

High BOD levels are typically caused by the presence of large amounts of organic matter in water. Common sources include sewage, agricultural runoff, food processing waste, and industrial effluents. When these organic materials decompose, they consume oxygen, leading to elevated BOD levels.

4. What is the typical BOD range for unpolluted water?

Unpolluted, natural water bodies typically have BOD levels between 1 and 3 mg/L. Levels above 5 mg/L are generally considered indicative of pollution, with higher values suggesting more severe contamination.

5. How does BOD relate to dissolved oxygen levels in water?

BOD and dissolved oxygen levels are inversely related. As BOD increases, the amount of dissolved oxygen in water decreases. This is because the microorganisms consuming the organic matter use up the available oxygen. If BOD becomes too high, it can lead to hypoxic (low oxygen) or anoxic (no oxygen) conditions, which are harmful to aquatic life.

6. How is BOD measured?

BOD is typically measured by taking a water sample, measuring its initial dissolved oxygen content, incubating it in darkness at 20°C for 5 days, and then measuring the dissolved oxygen again. The difference between the initial and final oxygen levels represents the BOD, usually expressed in milligrams of oxygen consumed per liter of sample.

7. How does nitrogen affect BOD measurements?

Nitrogen-containing compounds can interfere with BOD measurements. Nitrifying bacteria can oxidize ammonia to nitrites and nitrates, consuming oxygen in the process. This nitrification demand is not typically considered part of the carbonaceous BOD and can lead to overestimation of BOD if not accounted for or suppressed during testing.

8. How does temperature affect BOD?

Temperature significantly affects BOD because it influences microbial activity. Higher temperatures generally increase microbial metabolism, leading to faster decomposition of organic matter and higher oxygen consumption. This is why BOD tests are standardized at 20°C to ensure consistent results across different samples and locations.

9. What is the significance of the 5-day BOD test?

The 5-day BOD test (BOD5) is the standard method for measuring BOD. This duration was chosen because it typically takes 5 days for microorganisms to oxidize about 70-80% of the organic matter in a sample. It provides a consistent timeframe for comparing different water samples and assessing pollution levels.

10. How do algal blooms affect BOD?

Algal blooms can significantly increase BOD. While living algae produce oxygen through photosynthesis, when they die, their decomposition consumes large amounts of oxygen. This sudden increase in organic matter leads to a spike in BOD, potentially causing oxygen depletion and fish kills in severe cases.

11. What is Biochemical Oxygen Demand (BOD)?

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. It's a key indicator of water quality and pollution levels in aquatic environments.

12. What is the difference between carbonaceous BOD and nitrogenous BOD?

Carbonaceous BOD (CBOD) refers to the oxygen demand from the breakdown of carbon-containing organic compounds, while nitrogenous BOD (NBOD) is the oxygen demand from the oxidation of nitrogen compounds, primarily through nitrification. CBOD is typically measured in the first 5-7 days of a BOD test, while NBOD becomes more significant in longer-term tests.

13. How does BOD relate to the carbon cycle in aquatic ecosystems?

BOD is closely linked to the carbon cycle in aquatic ecosystems. It represents the rate at which organic carbon is being oxidized by microorganisms, releasing CO2 and consuming oxygen. High BOD levels indicate rapid carbon cycling and potential imbalances in the ecosystem's carbon dynamics.

14. What are some limitations of the BOD test?

Limitations of the BOD test include: 1) It's time-consuming, taking 5 days for standard results. 2) It doesn't account for non-biodegradable pollutants. 3) Toxic substances can inhibit microbial activity, leading to underestimation. 4) It requires a seed population of microorganisms, which may not always be representative. 5) Results can be affected by factors like pH, temperature, and the presence of certain chemicals.

15. How does pH affect BOD measurements?

pH can significantly influence BOD measurements. Most aquatic microorganisms prefer a pH range of 6.5-8.5. Extreme pH values can inhibit microbial activity, leading to lower BOD readings that don't accurately reflect the organic load. Additionally, pH can affect the solubility of oxygen in water, further impacting BOD results.

16. How does seasonal variation affect BOD?

BOD can vary seasonally due to several factors. In summer, warmer temperatures increase microbial activity, potentially raising BOD. Algal blooms in warmer months can also increase BOD. In contrast, colder winter temperatures may lower BOD due to reduced microbial activity. Seasonal changes in organic matter inputs, such as leaf fall in autumn, can also impact BOD levels.

17. How does BOD relate to the concept of ecological succession in aquatic ecosystems?

BOD can influence and reflect ecological succession in aquatic ecosystems. High BOD levels, often associated with pollution or eutrophication, can drive succession towards communities dominated by pollution-tolerant species. As BOD levels change over time (e.g., due to natural recovery or restoration efforts), the ecosystem may undergo further succession, potentially returning to a less polluted state with a more diverse community.

18. What is the relationship between BOD and eutrophication?

BOD and eutrophication are closely related. High BOD levels often indicate the presence of excess nutrients like nitrogen and phosphorus, which can lead to eutrophication. As algae and other aquatic plants grow rapidly due to these nutrients, their eventual death and decomposition further increase BOD, creating a feedback loop that can severely impact water quality.

19. What is the role of microorganisms in BOD?

Microorganisms play a crucial role in BOD. They are responsible for breaking down organic matter in the water, consuming oxygen in the process. The BOD test essentially measures the metabolic activity of these microorganisms. Different types of microorganisms may be involved, including bacteria, fungi, and protozoa, each contributing to the overall oxygen demand.

20. How does BOD relate to water treatment processes?

BOD is a key parameter in water treatment processes. It's used to assess the efficiency of treatment plants in removing organic matter. The goal of many treatment steps is to reduce BOD to acceptable levels before discharging treated water. Monitoring BOD helps operators adjust treatment processes and ensure compliance with environmental regulations.

21. What is the significance of BOD in aquaculture?

In aquaculture, BOD is crucial for maintaining water quality and fish health. High BOD levels can lead to oxygen depletion, stressing or killing fish. Aquaculturists must carefully manage organic inputs (like feed and waste) to keep BOD levels low. Regular BOD monitoring helps prevent oxygen-related problems and optimize production.

22. How does BOD change along the course of a river?

BOD typically varies along a river's course. It may be low near the source, increase as the river passes through urban or agricultural areas due to organic inputs, then decrease downstream as natural purification processes occur. This pattern, known as the river continuum concept, reflects changes in organic matter sources and processing along the river.

23. What is the relationship between BOD and dissolved oxygen sag curve?

The dissolved oxygen sag curve illustrates the relationship between BOD and dissolved oxygen levels in a stream after receiving organic waste. As BOD increases due to pollution, dissolved oxygen levels decrease, creating a "sag" in the oxygen profile. The curve typically shows oxygen levels recovering downstream as natural purification processes occur and re-aeration takes place.

24. What is the impact of industrial effluents on BOD?

Industrial effluents can significantly impact BOD levels. Many industries, such as food processing, paper mills, and textile manufacturing, produce wastewater high in organic content. When discharged into water bodies, these effluents can dramatically increase BOD, potentially leading to oxygen depletion and harm to aquatic ecosystems. This is why industrial wastewater treatment is crucial before discharge.

25. How does BOD relate to fish kills in aquatic ecosystems?

High BOD levels can lead to fish kills in aquatic ecosystems. As microorganisms break down excess organic matter, they consume large amounts of dissolved oxygen. If BOD becomes too high, oxygen levels can drop below the minimum required for fish survival, resulting in mass mortality events. This is particularly problematic in slow-moving or stagnant water bodies.

26. What is the role of BOD in assessing ecosystem health?

BOD is a key indicator of ecosystem health in aquatic environments. It provides insights into the balance between organic matter inputs and the ecosystem's capacity to process them. High BOD levels suggest an imbalance that can stress or alter ecosystem functions. By monitoring BOD, scientists and environmental managers can assess overall water quality and ecosystem integrity.

27. How does BOD relate to the concept of self-purification in rivers?

BOD is closely tied to the concept of self-purification in rivers. As organic pollutants enter a river, BOD initially increases. However, natural processes like microbial decomposition, dilution, and re-aeration gradually reduce BOD levels downstream. This self-purification capacity has limits, and excessive pollution can overwhelm it, leading to long-term water quality issues.

28. What is the significance of BOD:COD ratio?

The BOD:COD ratio provides information about the biodegradability of organic matter in water. A high BOD:COD ratio (e.g., >0.5) suggests that most of the organic matter is easily biodegradable. A low ratio indicates the presence of non-biodegradable or slowly biodegradable substances. This ratio is useful in assessing the treatability of wastewater and the potential effectiveness of biological treatment processes.

29. How does BOD relate to the nitrogen cycle in aquatic ecosystems?

BOD is linked to the nitrogen cycle through nitrogenous BOD (NBOD). As organic nitrogen compounds break down, they release ammonia. Nitrifying bacteria then oxidize this ammonia to nitrites and nitrates, consuming oxygen in the process. This nitrification contributes to the overall BOD and can significantly impact oxygen levels, especially in systems with high nitrogen inputs.

30. What is the impact of BOD on aquatic biodiversity?

High BOD levels can negatively impact aquatic biodiversity. As oxygen levels decrease due to high organic loads, species that require well-oxygenated water (like many fish and invertebrates) may die off or migrate. This can lead to a shift in community composition, often favoring more pollution-tolerant species. In severe cases, high BOD can create "dead zones" with drastically reduced biodiversity.

31. How does aeration affect BOD in water treatment?

Aeration plays a crucial role in managing BOD during water treatment. By introducing oxygen into the water, aeration supports the growth and activity of aerobic microorganisms that break down organic matter. This accelerates the reduction of BOD. In activated sludge systems, for example, aeration is key to maintaining the microbial population that processes organic waste, effectively lowering BOD levels.

32. What is the relationship between BOD and biological indicators of water quality?

BOD is often used alongside biological indicators to assess water quality. Certain organisms, like macroinvertebrates, are sensitive to changes in oxygen levels and organic pollution. The presence or absence of these indicator species can corroborate BOD measurements. For instance, high BOD levels often correlate with a decrease in sensitive species and an increase in pollution-tolerant organisms.

33. How does BOD relate to the concept of trophic state in lakes?

BOD is related to the trophic state of lakes, which describes their nutrient and productivity levels. Oligotrophic lakes with low nutrient levels typically have low BOD. As lakes become more eutrophic due to increased nutrient inputs, BOD levels tend to rise. In highly eutrophic systems, BOD can become extremely high, especially near the bottom where dead organic matter accumulates.

34. What is the impact of storm water runoff on BOD in urban areas?

Storm water runoff in urban areas can significantly increase BOD in receiving water bodies. Runoff can carry organic matter, nutrients, and pollutants from streets, lawns, and other surfaces. This sudden influx of organic material and nutrients can cause spikes in BOD, potentially leading to temporary oxygen depletion in the receiving waters, especially after heavy rainfall events.

35. How does BOD relate to the concept of carrying capacity in aquatic ecosystems?

BOD is closely tied to the carrying capacity of aquatic ecosystems. The carrying capacity represents the maximum organic load an ecosystem can process without significant degradation. If BOD consistently exceeds this capacity, it can lead to long-term changes in ecosystem structure and function. Understanding BOD helps in managing organic inputs to stay within an ecosystem's carrying capacity.

36. What is the role of BOD in setting water quality standards?

BOD is a key parameter in setting water quality standards. Regulatory agencies often establish maximum allowable BOD levels for different types of water bodies and effluents. These standards aim to protect aquatic life and maintain ecosystem health. BOD limits may vary depending on the water body's intended use (e.g., drinking water, recreation, aquatic life support) and its natural characteristics.

37. How does BOD relate to the decomposition of different types of organic matter?

Different types of organic matter decompose at varying rates, affecting BOD patterns. Easily biodegradable substances like sugars and proteins contribute to rapid initial oxygen demand. More complex molecules like cellulose decompose more slowly, contributing to long-term BOD. Understanding these decomposition rates is crucial for accurately interpreting BOD test results and managing organic inputs in aquatic systems.

38. What is the significance of BOD in anaerobic environments?

While BOD is typically associated with aerobic decomposition, it's also relevant in anaerobic environments. In the absence of oxygen, anaerobic microorganisms break down organic matter through fermentation and other processes. This doesn't contribute to oxygen demand directly but can produce reduced compounds (like hydrogen sulfide) that exert oxygen demand when they reach aerobic zones, indirectly affecting BOD.

39. What is the relationship between BOD and sediment oxygen demand?

BOD is closely related to sediment oxygen demand (SOD). While BOD primarily measures oxygen consumption in the water column, SOD represents the oxygen consumed by biological and chemical processes in bottom sediments. High BOD can lead to increased organic matter deposition, enhancing SOD. Together, BOD and SOD provide a more complete picture of oxygen dynamics in aquatic systems, especially in shallow or slow-moving waters.

40. How does BOD testing differ for fresh and salt water?

BOD testing procedures differ slightly for fresh and salt water due to their different chemical compositions. Saltwater typically has lower dissolved oxygen levels and different microbial communities. When testing saltwater, dilution with specially prepared seawater or artificial sea salts is often necessary. Additionally, nitrification inhibitors may be more commonly used in saltwater BOD tests to prevent interference from marine nitrifying bacteria.

41. What is the impact of BOD on the carbon footprint of wastewater treatment?

BOD levels directly impact the carbon footprint of wastewater treatment. Higher BOD requires more energy-intensive treatment processes, including increased aeration and chemical treatments. This leads to higher energy consumption and, consequently, a larger carbon footprint. Reducing BOD through source control or more efficient treatment technologies can significantly decrease the overall environmental impact of wastewater management.

42. How does BOD relate to the concept of resilience in aquatic ecosystems?

BOD is an important factor in ecosystem resilience. Ecosystems with naturally low BOD and high dissolved oxygen levels are often more resilient to organic pollution. They can better withstand temporary increases in BOD without significant long-term impacts. However, chronic high BOD can reduce resilience by stressing organisms and altering community structure, making the ecosystem more vulnerable to other disturbances.