Biochemical Oxygen Demand: Overview, Sources, Principle, Procedure, Measurement Methods

What is Biochemical Oxygen Demand?

The biochemical oxygen demand (BOD), also known as biological oxygen demand, is the quantity of oxygen required or demanded by aerobic microorganisms to decompose the organic matter present in a specific water sample at a particular temperature and over a specific period. Some of the basic points about biochemical oxygen demand are discussed below:

• Biochemical oxygen demand is one of the important factors which decides water quality and pollution levels in the aquatic environment.

• BOD is measured in milligrams of oxygen consumed per liter of water by the standard incubation period which is usually 5 days at 20°C.

• High BOD can also lead to hypoxia where there are low oxygen levels or anoxia where there is no oxygen present. Due to this, there is death of the fish and other aquatic organisms.

• BOD is also used as a wastewater treatment process as it helps in assessing the effectiveness of the treatment methods that are being used to reduce the organic pollutants such as discharge from hospitals and households.

Sources of BOD

Sources that raise biological oxygen levels are both natural and man-made demands for water. The BOD of water bodies is largely influenced by pollution. Some of the major sources are discussed below:

How is BOD calculated?

After five days of incubation at a temperature of 20 degrees Celsius, BOD is calculated and expressed as milligrams of oxygen consumed per litre of sample. BOD is calculated using the formula,

BOD = [D_i - D_f]/P

Where:

D_i = Initial concentration of dissolved oxygen

D_f = Dissolved oxygen concentration for the last five days

P = Wastewater Volumetric Fraction

BOD Levels in Drinking Water

Biochemical oxygen demand is one of the important factors in deciding the quality of the water. It helps in configuring organic pollution levels in the drinking water. It helps to decide the safety and suitability of the water for consumption and survival of the aquatic organisms.

• The BOD levels of drinking water can be normally from 1 to 2 ppm. This is considered the safe quality of water for consumption.

• If the value of water ranges from 3 to 5 ppm then it is considered moderately clean water but not very good for consumption.

• While the BOD from 6 to 9 ppm indicates the contaminated water due to organic waste. Water is not suitable for drinking.

• The water which exceeds the value of 100 ppm is considered highly contaminated water with high health risks.

• Although apart from these some organisms like leeches and sludge worms can thrive in low BOD environments due to ecological adaptability.

Significance of BOD

The importance of biochemical oxygen demand is found in many different fields.

• These are sewage treatment plants are where BOD is most important. It provides information on how quickly soil, sewage, sludge, and garbage breathe.

• The Chemical Oxygen Demand (COD) of inorganic substances is determined by measuring BOD. It shows that water has the potential to pollute.

• BOD is used in the healthcare and pharmaceutical sectors to gauge how much oxygen cell cultures are consumed.

• BOD is a measurement of how much oxygen is used by microorganisms in the decomposition of organic matter in water bodies.

• It shows how much organic pollution there is in an aquatic ecosystem.

• To determine how effectively aerobic microbes are destroying organic wastes, BOD is calculated during sewage treatment or wastewater treatment.

Factors Influencing BOD

Several factors influence BOD because it reduces water's dissolved oxygen content and speeds up metabolism. Some of the factors are mentioned below:

1. pH: An irregular pH value causes a noticeable decrease in the rate at which organic compounds are removed, which has a big impact on BOD measurements.

2. Organic substances in water: Water's biochemical oxygen demand is influenced by the types and concentrations of organic substances, which determine the rate of dissolved oxygen consumption.

3. Microorganism: Some microorganisms promote the growth of aerobic bacteria, which in turn affect BOD.

BOD vs COD

The major difference between BODand COD is, BOD measures the oxygen consumed by microorganisms to decompose organic matter, while COD measures the oxygen required to chemically oxidize organic compounds. Other differences are:

Exam Relevance

During exam preparation, different types of questions about the Biochemical Oxygen Demand (BOD) are asked. The table below contains the different patterns of questions asked in different exams.

Biochemical Oxygen Demand NEET MCQs

Q1. A higher biochemical oxygen demand in a particular segment of a river indicates that

1. the segment is free from pollution

2. the segment is highly polluted

3. aquatic life has started flourishing

4. the river has high number of aquatic animals

Correct answer: 2) the segment is highly polluted

Explanation:

A higher BOD in a river segment signifies organic pollution. BOD quantifies oxygen utilised by microbes to decompose organic substances like sewage and decaying plants. An elevated BOD implies increased organic content, necessitating more oxygen. This excessive demand can deplete water oxygen, adversely impacting aquatic life by decreasing available oxygen for fish and similar organisms.

Hence, the correct answer is option 2). The segment is highly polluted.

Q2. Biochemical Oxygen Demand (BOD) may not be a good index for pollution for water bodies receiving effluents from

1. domestic sewage

2. dairy industry

3. petroleum industry

4. sugar industry

Correct answer: 3) petroleum industry

Explanation:

Causes of water pollution -

Oil pollution is a source of pollution in seawater. Oil pollution is due to ship accidents, loading and discharging of oil at the harbour, oil refineries and off-shore oil production. The degree of impurity due to organic matter is measured in terms of BOD (Biochemical Oxygen Demand). It is the demand for O2 to decompose organic wastes in a litre of water. Petroleum industry waste is rich in radium-based toxic substances that do not favour algal and plankton growth and have no effect on BOD.

Hence, the correct answer is 3) the petroleum industry.

Q3. The amount of biodegradable organic matter in sewage water can be estimated by measuring.

1. Biochemical oxygen demand

2. The growth of anaerobic bacteria in water

3. Biogeological oxygen demand

4. The growth of aerobic bacteria in water.

Correct answer: 1) Biochemical oxygen demand

Explanation:

The Biochemical Oxygen Demand (BOD) can be used to assess the amount of biodegradable organic matter in sewage water.

BOD is the quantity of oxygen needed by microbes to decompose the organic matter in water during a predetermined time frame often five days at 20°C. Because microorganisms need more oxygen to break down organic material, a greater BOD denotes a larger degree of organic pollution. This measurement is crucial for evaluating the quality of water and figuring out how polluted sewage or wastewater is.

Hence, the correct answer is option 1) Biochemical oxygen demand.

Also Read:

• MCQs on Algal Bloom

• Eutrophication

• Biomagnification

FAQs on Biochemical Oxygen Demand

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 ecosystems. BOD is crucial in environmental studies because it helps assess water quality and pollution levels. High BOD indicates high levels of organic matter, which can lead to oxygen depletion in water bodies, harming aquatic life and ecosystem balance.

How is BOD calculated?

It takes two steps to measure BOD in a water sample. Take two water samples, test one of them right away, and incubate the other one for five days in the dark at 20°C. The incubated sample should then be tested 5 days later. The quantity of BOD is equal to the difference in milligrams of oxygen per litre between two samples.

BOD = [D_i - D_f]/P.

Where:

D_i = Initial concentration of dissolved oxygen

D_f = Dissolved oxygen concentration for the last five days

P = Wastewater Volumetric Fraction

What are safe BOD levels in drinking water?

BOD levels vary widely in different water bodies. Pristine rivers and lakes typically have BOD levels below 1 mg/L, while moderately polluted rivers may have BOD levels between 2-8 mg/L. Untreated sewage can have BOD levels exceeding 200 mg/L.

What are the main sources of BOD?

The main sources of BOD include domestic sewage, agricultural runoff, industrial effluents, and natural organic matter from decaying plants and animals. These sources introduce organic materials that consume oxygen as they decompose.