BOD vs COD || Biochemical Oxygen Demand and Chemical Oxygen Demand

 Biochemical oxygen demand (BOD or Biological oxygen demand) is an analytical parameter representing the amount of dissolved oxygen (DO) consumed by aerobic bacteria growing on the organic material present in a water sample at a specific temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20 °C and is often used as a surrogate of the degree of oragnaic water pollution. 

The chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution, which in SI units is milligrams per liter (mg/L). A COD test can be used to quickly quantify the amount of organics in water.

Biological Oxygen Demand (BOD) vs Chemical Oxygen Demand (COD)

Definition

• BOD: The amount of dissolved oxygen required by aerobic microorganisms to break down biodegradable organic matter in a water sample.
• COD: The total amount of oxygen required to chemically oxidize all organic and inorganic substances in a water sample using a strong chemical oxidant.

Process

• BOD: Biological oxidation by microorganisms.
• COD: Chemical oxidation using strong oxidizing agents (e.g., potassium dichromate and sulfuric acid).

Time taken

• BOD: Typically 5 days (BOD5 is the most common measurement).
• COD: A few hours.

Pollutants measured

• BOD: Primarily biodegradable organic matter.
• COD: Both biodegradable and non-biodegradable organic matter, as well as some inorganic compounds.

Value

• BOD: Generally lower than COD.
• COD: Always higher than BOD (as it accounts for more oxidizable substances).

Application

• BOD: Commonly used for municipal wastewater treatment to assess the efficiency of biological treatment processes and the impact on receiving water bodies.
• COD: Widely used for industrial wastewater treatment due to its speed and ability to measure a broader range of pollutants, including those toxic to microorganisms.

Nature of test

• BOD: Biological process, influenced by microbial activity, temperature, and pH.
• COD: Chemical process, less affected by microbial inhibitors or specific environmental conditions.

Reagents

• BOD: No specific chemical reagents are added for oxidation (relies on natural microorganisms).
• COD: Strong chemical oxidants like potassium dichromate, sulfuric acid, etc.

Toxicity sensitivity

• BOD: Sensitive to toxic substances that inhibit microbial growth.
• COD: Less sensitive to toxic substances as it's a chemical process.