What is the difference between COD and BOD in Sewage Treatment?

What is the difference between COD and BOD in Sewage Treatment?

Both BOD and COD measure the amount of oxygen required to degrade organic pollutants in water, but they do so in different ways and reflect different aspects of pollution.

➤ Biochemical Oxygen Demand (BOD)

~Definition:

BOD is the amount of dissolved oxygen needed by aerobic biological organisms to biologically degrade biodegradable organic matter in water over a specified period—typically 5 days at 20°C, hence denoted as BOD₅.

~ Characteristics:

→ Biological test: Uses microorganisms to oxidize biodegradable organics.

→ Reflects biodegradable organic pollution: It does not capture non-biodegradable compounds.

→ Time-consuming: Standard BOD test takes 5 days.

→ Units: mg/L (milligrams of O₂ per liter of sample).

➤ Importance in Sewage Treatment:

~ Indicates how much oxygen the natural environment or treatment plant will need.

~ High BOD = high organic pollution = potential oxygen depletion in receiving water bodies, which harms aquatic life.

➤ Chemical Oxygen Demand (COD)

~ Definition:

COD is the total amount of oxygen required to chemically oxidize both biodegradable and non-biodegradable organic matter (and some easily oxidizable inorganic substances, like ammonia) in water using a strong chemical oxidant like potassium dichromate in acidic conditions.

~ Characteristics:

→ Chemical test: No microbes involved; uses a strong oxidizer.

→ Captures total organic content: Including non-biodegradable pollutants.

→ Faster test: Typically 2–3 hours.

→ Units: mg/L (like BOD).

➤Importance in Sewage Treatment:

~ Gives a faster estimate of the organic load.

~ Useful for real-time monitoring or industrial wastewater with toxic substances not captured by BOD.

➤ Typical Values in Sewage

~ Raw Domestic Sewage:

√ BOD₅: 200–400 mg/L

√ COD: 400–800 mg/L

~ COD is typically 2 to 2.5 times higher than BOD in raw domestic sewage.

➤ BOD/COD Ratio

√ BOD/COD > 0.5: Wastewater is readily biodegradable; good for biological treatment.

√ BOD/COD < 0.3: Wastewater has poor biodegradability; may need advanced treatment (e.g., chemical oxidation, membrane processes).

✪ In simple terms:

→ BOD tells us how much oxygen nature will need to clean the water.

→ COD tells us how much oxygen is required overall, whether nature can do it or not.

In wastewater engineering, we often measure both. COD provides a rapid and comprehensive measure of organic load, while BOD offers a better reflection of the natural biodegradation potential and environmental impact.

P.S. Biological Treatment Plants are typically designed based on BOD, but COD is used for screening, monitoring, and regulatory compliance, especially for industrial effluents