BOD Test Explained | Principle, Procedure, Calculation & Environmental Importance

Biochemical Oxygen Demand (BOD) Explained | Definition, Principle, Procedure, Calculation & Importance

Introduction

Biochemical Oxygen Demand (BOD) is one of the most important parameters used to assess the level of biodegradable organic pollution in water and wastewater. It indicates the amount of dissolved oxygen required by microorganisms to biologically decompose organic matter under aerobic conditions.

BOD is widely used in environmental science, wastewater treatment, water-quality monitoring, and pollution assessment.

What is BOD?

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen consumed by microorganisms while decomposing biologically degradable organic matter present in a water sample under aerobic conditions.

The standard BOD₅ test measures the oxygen consumed during a 5-day incubation period at 20°C.

Definition

BOD₅ is the amount of dissolved oxygen consumed by microorganisms during the biological decomposition of biodegradable organic matter in a water sample after incubation for 5 days at 20°C.

Unit: mg/L (milligrams per litre)

Principle of BOD Test

The BOD test is based on the measurement of the difference in dissolved oxygen (DO) concentration before and after incubation.

A water sample is initially tested for its dissolved oxygen concentration. It is then incubated for 5 days at 20°C in the dark. After incubation, the DO is measured again.

The reduction in DO represents the oxygen consumed by microorganisms.

Basic Principle

Initial DO − Final DO = Oxygen consumed

Therefore:

BOD₅ ≈ Initial DO − Final DO

For diluted samples, the dilution factor must also be considered.

Procedure for BOD₅ Test

The general procedure includes:

Collect the water or wastewater sample properly.

Measure the initial dissolved oxygen (DO).

Fill a BOD bottle carefully without introducing air bubbles.

Incubate the sealed bottle at 20°C for 5 days in darkness.

Measure the final DO after incubation.

Calculate the BOD value from the decrease in DO.

BOD Flow

Water Sample → Initial DO Measurement → 5 Days Incubation at 20°C → Final DO Measurement → BOD Calculation

BOD Calculation

For an undiluted sample:

Where:

D₁ = Initial DO (mg/L)

D₂ = Final DO after 5 days (mg/L)

For a diluted sample, the calculation is adjusted according to the dilution used and any oxygen demand from the dilution water.

Example

Suppose:

Initial DO = 8.0 mg/L

Final DO = 3.5 mg/L

Then:

Importance of BOD

BOD is important because it helps determine the organic pollution load of water.

Major applications

Assessment of water pollution

Monitoring wastewater treatment plants

Evaluating the quality of rivers, lakes and other water bodies

Determining the efficiency of wastewater treatment

Estimating the oxygen-depleting potential of wastewater

Environmental monitoring and regulatory compliance

High BOD vs Low BOD

BOD Level

General Indication

Low BOD

Lower biodegradable organic pollution

Moderate BOD

Moderate organic pollution

High BOD

High biodegradable organic pollution

A high BOD means microorganisms are consuming more oxygen to break down organic matter. This can reduce the dissolved oxygen available for aquatic organisms.

Relationship Between BOD and DO

BOD and dissolved oxygen are closely related.

High biodegradable organic matter → High microbial activity → High oxygen consumption → Lower DO

If DO becomes too low, aquatic organisms such as fish and other aerobic organisms may be adversely affected.

BOD vs COD

BOD and Chemical Oxygen Demand (COD) are both used to assess oxygen demand, but they are different tests.

BOD

COD

Biological process

Chemical oxidation

Uses microorganisms

Uses chemical oxidizing agents

Standard BOD₅ takes 5 days

Usually much faster

Measures biodegradable organic matter

Measures chemically oxidizable matter

Useful for biological wastewater treatment assessment

Useful for rapid pollution assessment

Generally, COD is higher than BOD because COD can oxidize substances that may not be readily biodegradable.

Factors Affecting BOD

Several factors can influence BOD results:

Temperature

Microbial population

Amount and type of organic matter

pH

Availability of nutrients

Dissolved oxygen concentration

Toxic substances

Incubation conditions

Conclusion

Biochemical Oxygen Demand (BOD) is a key parameter for evaluating biodegradable organic pollution in water and wastewater. The standard BOD₅ test measures the decrease in dissolved oxygen after 5 days of incubation at 20°C.

Understanding BOD is essential for environmental studies, water-quality analysis, wastewater treatment, and pollution control.

Quick Revision

BOD = Oxygen required by microorganisms to biologically decompose biodegradable organic matter under aerobic conditions.

Standard BOD₅ = 5 days at 20°C

Unit = mg/L

Basic measurement = Initial DO − Final DO