Units of BOD and COD

 The units of BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) are typically expressed as a concentration of oxygen in water, measured in:

Common Units

1. Milligrams per Liter (mg/L):

   • This is the most commonly used unit in wastewater analysis and environmental standards.
   • Indicates the mass of oxygen (in milligrams) required per liter of water.

2. Parts Per Million (ppm):

   • Equivalent to mg/L in dilute aqueous solutions.
   • Often used interchangeably with mg/L in wastewater treatment contexts.

Example

• A BOD of 200 mg/L means 200 milligrams of oxygen are required to biologically degrade the organic matter present in one liter of water.
• A COD of 500 mg/L means 500 milligrams of oxygen are chemically required to oxidize all organic and inorganic matter in one liter of water.

Why These Units Are Used

• Convenience: mg/L directly correlates with pollution concentration in water.
• Regulatory Standards: Effluent discharge limits are usually specified in mg/L (e.g., BOD ≤ 30 mg/L, COD ≤ 250 mg/L).

Understanding these units is essential for monitoring and comparing the performance of sewage treatment plants and ensuring compliance with environmental regulations.