What is BOD? Definition, effect and measurement

To run your system you need oxygen and food. Similarly, micro organism also needs oxygen to consume food? So, the amount of oxygen required by the microorganisms to consume the organic matter of water.

What is BOD?

Most natural water contains some amount of organic content. And in Waste Water more amount of organic content is present. It may be bio-degradable organic content or non-biodegradable organic content.

If an abundance amount of bio-degradable organic content and aerobic micro-organisms are present, then the aerobic micro-organism takes dissolve oxygen from water breakdown or decomposes these molecules. BOD is directly proportional to the amount of oxygen consumed by the micro-organism. BOD, therefore, measures the organic content of the deposit.

What are the sources of BOD?

Biodegradable material consists of organics present in dissolved form. Microorganisms used it as a food resource. And the organic content usually consists of starches, fats, proteins, alcohols, acids, aldehydes, and easter.

Source of biochemical oxygen demands are as follows:

  • Domestic Discharge: Wastewater generated from household activities.
  • Decomposition of plant and animal tissue: The end product of initial microbial decomposition.
  • Industrial waste: Waste generated from different types of Industries.

What are the factors affecting BOD?

BOD of a wastewater body is affected by the following factor

  • Temperature
  • pH value of water
  • The presence of certain kinds of micro-organisms
  • Types of inorganic substances
  • Amount and types of organic substance

What are the stages of BOD?

There are two stages of BOD. Bacteria that oxidize carbon are must stronger than nitrifying bacteria. So in the 1st stage with the help of carbonaceous bacteria oxidized the bio-degradable organic content was almost 68-70% within 5 days. Then 2nd stage of Nitrification demand takes place.

  • Carbonaceous oxygen demand
  • Nitrogenous oxygen demand
This image represents the different stages of biochemical oxygen demand.
Stages of BOD
  1. Nitrogenous oxygen demand

Bacteria that oxidize carbon are must stronger than nitrifying bacteria. So in the 1st stage with the help of carbonaceous bacteria oxidized the bio-degradable organic content was almost 68-70% within 5 days. Then 2nd stage of Nitrification demand takes place.

How to measure BOD?

Bring the waste-water sample to Laboratory. Measure the initial concentration of Dissolve Oxygen( DOi). Filled the bottle with samples without head space to prevent the presence of any unaccounted oxygen. Incubate at 20 degree Celsius and keep it in the dark. Dissolve oxygen is measured after 5 days(DOf). pH of water is maintained before starting the experiment. Make sure that the bacteria are also present in wastewater when bacteria are not present then seeding becomes important.

Dilution: The process of dilution is to supply a sufficient amount of DO to the water sample to carry the reaction smoothly. A BOD experiment is successful if the following condition is present DOf ≥ 1mg/lit, DOi-DOf ≥ 2mg/lit. Condition 1 assume there is sufficient oxygen to complete the reaction and 2nd condition assume that there is sufficient bacteria to hold the reaction. The 5 days BOD is estimated using the following formula.

$BOD_5=DF(DO_i-DO_f)$ when no seeding


Where, B1= Initial DO of seeded dilution water

B2= Final DO of seeded dilution water

f= The fraction of the seeded dilution water volume in a sample to the volume of seeded dilution water in seed control.

Typical values and its indication

The level of BOD represents milligrams per liter. And each value represents various water quality.

  • Below $1$ mg/L: It indicates fresh water
  • $2$-$8$ mg/L: Moderately polluted water
  • Above $8$ mg/L: Severely polluted water

Effect of BOD water

It directly affects the amount of dissolved oxygen in rivers and streams. More oxygen depletes in the river and stream when greater BOD is present. We can understand it better with the help of the oxygen sag curve.

  • Aquatic Life: Reduction in dissolved oxygen level adversely affects aquatic life.
  • Depletion of dissolved oxygen: The higher the BOD more rapidly the oxygen depletes.
  • Aesthetic beauty: The overall aesthetic beauty of the water body is damaged.


With the increase in pollution and urbanization, waste generation also increases. And the waste is directly discharged to the water body without treatment. It significantly reduces the quality of the water bodies.

Testing of BOD is important because it determines the amount of waste in wastewater. According to it, we select the treatment process. After treating the wastewater we discharge it to the water bodies.

In this post you have learned the following key learning points:

  • Determination of waste: BOD test is important because it helps in determining the amount of waste contribution to wastewater.
  • Effect on aquatic life: Due to less dissolve oxygen aquatic organisms become, suffocated, and die.
  • Eutrophication of water bodies: The organic content in the wastewater needs to be reduced before the wastewater is discharged because it can lead to the eutrophication of water bodies.

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    This article was crafted by a group of experts at eigenplus to ensure it adheres to our strict quality standards. The individuals who contributed to this article are:


    Suprabha Panda


    She is an assistant professor with masters in Environmental Engineering.

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