What Is the Difference Between BOD and COD?

When it comes to wastewater treatment, there are two important acronyms that you should be familiar with: BOD and COD. Both BOD and COD are measures of the amount of organic pollutants in water, but they differ in how they measure this pollution. Understanding the difference between BOD and COD is important for designing effective wastewater treatment systems.

What is BOD?

BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen that is required by bacteria to break down organic matter in water.

This process takes place over a period of five days at a temperature of 20Β°C, which is why BOD is sometimes referred to as “BOD5”. During this time, microorganisms consume the organic matter in water and use up oxygen as a result.

The level of BOD in wastewater is an indicator of how much organic matter is present. High levels of BOD can indicate that there are high levels of organic pollutants such as sewage or agricultural runoff in the water. This can lead to decreased levels of dissolved oxygen, which can harm aquatic life.

What is COD?

COD stands for Chemical Oxygen Demand. Unlike BOD, COD measures the amount of oxygen required by chemical reactions to break down organic matter in water. These reactions take place over a shorter period than the biological processes measured by BOD – typically just two hours.

COD provides a more accurate measure of total organic pollutants present in wastewater than BOD because it includes both biodegradable and non-biodegradable substances. However, it doesn’t provide information on the specific types or sources of pollution.

Key Differences between BOD and COD

  • BOD measures how much oxygen microorganisms consume while breaking down organic pollutants over five days
  • COD measures how much oxygen chemical reactions require to break down all organic pollutants over two hours
  • BOD only measures biodegradable organic pollutants, while COD measures both biodegradable and non-biodegradable pollutants
  • BOD provides information on the specific types and sources of pollution, while COD doesn’t

When to Use BOD vs. COD

Both BOD and COD are important measures of water quality, but they serve different purposes. BOD is often used to measure the effectiveness of biological treatment methods such as activated sludge or trickling filters. This is because it measures the amount of oxygen that microorganisms consume during wastewater treatment.

On the other hand, COD is useful for measuring the overall level of pollution in water, regardless of whether it’s biodegradable or not. This makes it a good indicator of wastewater strength and can help determine appropriate treatment methods.


In summary, BOD and COD are both important measures of water quality that provide valuable information about organic pollutants in water. While BOD measures the amount of oxygen consumed by microorganisms over five days, COD measures the amount consumed by chemical reactions over two hours. Understanding the differences between these two measurements can help wastewater treatment professionals design effective treatment systems for a variety of applications.

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Emma Gibson