COD and BOD

COD and BOD are two key parameters used to measure the amount of organic pollutants present in wastewater, providing valuable insights into the level of contamination and the oxygen demand required for their decomposition. While both parameters serve as indicators of water quality, they differ in terms of the testing method, the time frame of analysis, and the types of organic compounds they target.

COD (Chemical Oxygen Demand):

COD is a measure of the oxygen equivalent of the organic matter content in wastewater that can be oxidized by a strong chemical oxidant. It indicates the total quantity of oxygen required for the oxidation of all organic substances present in the water, including both biodegradable and non-biodegradable compounds.

BOD (Biochemical Oxygen Demand):

BOD is a measure of the amount of dissolved oxygen that microorganisms require to decompose the organic matter present in wastewater under aerobic conditions. It primarily assesses the biodegradable organic pollutants in the water that can be broken down by microbial action.

Differences between COD and BOD:

1. Testing Method: COD is determined through a chemical oxidation process, while BOD is measured by monitoring the oxygen consumed during the biological degradation of organic matter.

2. Time Frame: COD provides a rapid assessment of the pollution level, delivering results within a few hours, whereas BOD analysis typically requires 5 days to assess the amount of oxygen required for the complete breakdown of organic matter.

3. Target Compounds: COD measures both biodegradable and non-biodegradable organic compounds, while BOD primarily focuses on the biodegradable organic matter that can be broken down by microorganisms.

Calculation of COD and BOD:

The formulas for calculating COD and BOD are as follows:

1. COD Calculation:

COD (mg/L) = Volume of Oxidizing Solution (in mL) x Normality of Oxidizing Solution x 8000/Volume of Sample (in mL)

Example Calculation:

If 10 mL of a sample is oxidized by 20 mL of 0.25 N solution of potassium dichromate, the COD can be calculated as:

COD= 20 x 0.25 x 8000/10 = 4000 mg/L

2. BOD Calculation:

BOD (mg/L) = (DO₁ – DO₂) x Dilution Factor x 1000/Volume of Sample (in mL)

Example Calculation:

If the initial dissolved oxygen (DO) of a sample is 8 mg/L and the final DO after 5 days is 2 mg/L, and the dilution factor is 5, the BOD can be calculated as:

BOD = (8 - 2) x 5 x 1000/100} = 300 mg/L

Note: Multiplication factor of 8000 in COD Calculation

The multiplication factor of 8000 used in the calculation of Chemical Oxygen Demand (COD) is based on the stoichiometry of the chemical reaction involved in the oxidation process. Specifically, it represents the conversion factor that relates the amount of oxygen consumed during the oxidation of organic compounds to the amount of potassium dichromate (K₂Cr₂O₇) used in the reaction.

In the standard method for COD determination, potassium dichromate (K₂Cr₂O₇) is commonly used as the oxidizing agent. During the oxidation process, the organic compounds present in the sample react with the potassium dichromate, resulting in the reduction of the dichromate ions (Cr₂O₇^2-) to chromium ions (Cr³⁺). The amount of oxygen consumed during this process is proportional to the amount of potassium dichromate used and the stoichiometry of the reaction.

The stoichiometry of the reaction indicates that each mole of potassium dichromate (K₂Cr₂O₇) used corresponds to 8 moles of oxygen (O₂) consumed. Since the molecular weight of oxygen is 32 g/mol, the conversion factor is derived as follows:

Conversion Factor = 8 x Molecular Weight of O₂/Molecular Weight of K₂Cr₂O₇

= 8 x 32/294.18 ≈ 0.8725

To convert the result to milligrams per liter (mg/L) in the final calculation, this conversion factor is often rounded to 8000, making the calculation simpler and ensuring a sufficient margin of safety in the estimation of COD. This multiplication factor of 8000 allows for a convenient conversion from the amount of oxidizing agent used in the reaction to the concentration of the organic compounds in the sample in terms of milligrams per liter.