Chemical Oxygen Demand (COD)

What is COD?

Chemical Oxygen Demand or COD is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water.

How is COD measured?

A common method for Chemical Oxygen Demand analysis is Method 410.4. The method involves using a strong oxidizing chemical, potassium dichromate Cr2O72-, to oxidize the organic matter in solution to carbon dioxide and water under acidic conditions. Often, the test also involves a silver compound to encourage oxidation of certain organic compounds and mercury to reduce the interference from oxidation of chloride ions. The sample is then digested for approximately 2 hours at 150°C. The amount of oxygen required is calculated from the quantity of chemical oxidant consumed.

Why is COD important?

Chemical Oxygen Demand is an important water quality parameter because, similar to BOD, it provides an index to assess the effect discharged wastewater will have on the receiving environment. Higher COD levels mean a greater amount of oxidizable organic material in the sample, which will reduce dissolved oxygen (DO) levels. A reduction in DO can lead to anaerobic conditions, which is deleterious to higher aquatic life forms. The COD test is often used as an alternate to BOD due to shorter length of testing time.

Measuring COD with UV-VIS Absorbance technology

Chemical Oxygen Demand is a valuable test for assessing organic pollution levels; however, like BOD, the test requires time for the chemical oxidant to oxidize the substances in the sample. In addition, interference from inorganic ions may cause erroneously high COD results.

Using UV-VIS absorbance technology, COD can be measured in real-time. A number of organic compounds, including humic substances, lignin, tannin, and various aromatic compounds strongly absorb UV radiation between 200-400 nm. Measuring absorbance at several wavelengths, excellent calibrations can be made between multi-dimensional absorbance data and laboratory COD data.

Where is COD measured?

  • Influent wastewater streams for process control
  • Effluent wastewater streams to sewer or environment for regulatory compliance