COD Test in Wastewater: Overview

 COD test

Test method- APHA 5220 B

COD Test in Wastewater: Overview

The Chemical Oxygen Demand (COD) test is a key method used to measure the amount of organic pollutants in wastewater. It indicates how much oxygen would be required to chemically oxidize organic and inorganic matter in the water. COD is expressed in mg/L (milligrams of oxygen per liter of sample).

Purpose of COD Test

• To assess the organic pollution level in wastewater.

• To help monitor and control treatment processes in wastewater treatment plants.

• To comply with environmental discharge regulations.

Principle of COD Test

The COD test is based on the chemical oxidation of organic matter using a strong oxidizing agent (usually potassium dichromate, K₂Cr₂O₇) in acidic conditions.

1. The sample is digested with potassium dichromate (K₂Cr₂O₇) in the presence of sulfuric acid (H₂SO₄).

2. Silver sulfate (Ag₂SO₄) acts as a catalyst, and mercuric sulfate (HgSO₄) is added to remove interference from chlorides.

3. The mixture is heated (typically at 150°C for 2 hours).

4. The amount of dichromate consumed is measured to determine the COD.

Steps in Performing COD Test (Closed Reflux Method - Standard)

1. Sample Preparation:

• Take a known volume of the wastewater sample (usually 2 mL).

• Add digestion solution (potassium dichromate and sulfuric acid) and catalyst.

2. Digestion:

• Place the mixture in a sealed reflux apparatus or COD digestion vial.

• Heat at 150°C for 2 hours.

3. Cooling and Titration/Measurement:

• After digestion, cool the vials.

• Measure the amount of oxidant remaining (via titration) back titration method with FAS.

Typical COD values

Type of Watst. COD Range (mg/L)

Domestic sewage 250 – 1000

Treated effluent. < 100

Industrial wastewater. Up to 10,000