Difference between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)

Difference Between Bod And Cod

Overview of BOD and COD

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are both used to measure how much oxygen is needed to break down organic matter in water. While Biochemical Oxygen Demand depends on the activity of microorganisms under aerobic conditions, Chemical Oxygen Demand relies on a chemical oxidising agent to break down the organic compounds present. Together, these tests are very important for assessing water pollution and ensuring effective wastewater treatment.

What is Biochemical Oxygen Demand (BOD)?

Biochemical Oxygen Demand refers to the amount of dissolved oxygen that microorganisms (such as bacteria) require to decompose biodegradable organic matter in contaminated water under aerobic conditions. It is often measured over five days (BOD₅ test) at a certain temperature (usually 20°C). A higher BOD value suggests a greater concentration of biodegradable matter, which can lead to oxygen depletion in water bodies.

Key Points:

Involves microbial breakdown of organic matter.
Commonly measured in a 5-day test period.
Indicates the level of biodegradable waste in effluent or surface waters.

What is Chemical Oxygen Demand (COD)?

Chemical Oxygen Demand measures the total amount of oxygen required to chemically oxidise both biodegradable and non-biodegradable matter using a strong oxidising agent (often potassium dichromate) under acidic conditions. It is quicker than the BOD test and is often used in wastewater treatment plants to check the overall organic load in a short period.

Key Points:

Involves chemical oxidation, not just microbial activity.
Usually completed within 2–3 hours.
Includes both biodegradable and non-biodegradable organic compounds.

Difference between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)

Parameter	Biochemical Oxygen Demand (BOD)	Chemical Oxygen Demand (COD)
Definition	Oxygen required by microorganisms to break down organic matter under aerobic conditions.	Oxygen required for chemical oxidation of organic (and some inorganic) matter using a strong oxidising agent.
Type of Process	Biological: Involves bacteria and other microbes decomposing biodegradable materials.	Chemical: Uses potent oxidants (e.g., potassium dichromate) to oxidise both biodegradable and non-biodegradable substances.
Compounds Measured	Primarily biodegradable organic pollutants.	Total organic pollutants (including non-biodegradable) and some inorganic reducing agents.
Test Duration	Usually 5 days (BOD₅).	Typically 2–3 hours.
Typical Values	Generally lower than COD.	Often higher than BOD.
Significance in Waste Treatment	Helps assess how much oxygen microbes will consume, indicating the effectiveness of biological treatment processes.	Provides a quick measure of overall organic load, aiding in designing or adjusting treatment systems.
Example of Usage	Evaluating effluent from sewage treatment plants to ensure microbes have sufficient oxygen for breakdown of waste.	Checking industrial discharges to see if they contain large amounts of oxidisable materials that might harm aquatic life or interfere with treatment.
Ease of Analysis	Requires a longer incubation period but reflects actual biological breakdown conditions.	Faster and more encompassing test, but does not distinguish between biodegradable and non-biodegradable substances.

Why are These Tests Important?

Water Pollution Control: Both BOD and COD are crucial in assessing how polluted a water sample is.
Effluent Regulations: Authorities set specific BOD and COD limits for industrial and municipal discharges to protect aquatic life.
Public Health: High organic loads can lead to the growth of harmful microbes and depletion of oxygen in water bodies, affecting both human health and aquatic ecosystems.

Quick Quiz (With Answers)

Which test typically shows a higher value for the same water sample?
A. BOD
B. COD
C. Both are equal
D. Depends on temperature
Answer: B (COD is usually higher than BOD)

Which test directly measures oxygen used by microorganisms?
A. BOD
B. COD
C. Both A and B
D. Neither A nor B
Answer: A (BOD is about microbial consumption)

Which parameter can include non-biodegradable matter?
A. BOD
B. COD
C. Both A and B
D. Neither A nor B
Answer: B (COD includes both biodegradable and non-biodegradable organic matter)

FAQs on Difference between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)

1. Why is COD usually higher than BOD?

COD includes both biodegradable and non-biodegradable substances, while BOD mainly reflects the oxygen required for biological breakdown.

2. Is BOD always measured over five days?

In most standard tests, yes. This duration (BOD₅) offers a good balance between practicality and accuracy.

3. Can BOD and COD tests be used together?

Yes. Many wastewater treatment facilities track both to gain a clearer picture of total organic load and how effectively biological treatments are performing.

4. Does a higher BOD necessarily mean worse water quality?

Generally, yes. It indicates more biodegradable organic pollutants, which can lower dissolved oxygen levels in water bodies.