Chemical Oxygen Demand (COD)
Updated: Sep 20
What is Chemical Oxygen Demand?
Chemical Oxygen Demand (COD) is the quantity of specified oxidant that reacts with the sample under controlled conditions.
The amount of oxidant consumed is expressed in terms of its oxygen equivalence.
Both organic and inorganic components of a sample are subject to oxidation, but in most cases the organic component predominates and is of the greater interest.
In other words, COD measures the amount of oxygen consumed for the oxidation of total organic matter (biodegradable and non-biodegradable)
Therefore, COD is an indirect representation of the amount organic matter in a sample.
Why is Chemical Oxygen Demand important?
COD is one of the most important parameters in water quality analysis of wastewater. COD is used to:
Determine the pollution level and the acceptance criteria for discharge of treated wastewater.
Determine the biodegradable fraction of wastewater effluent through the ratio between Biological Oxygen Demand (BOD) and COD
COD together with Biological Oxygen Demand (BOD) is an important indicator in designing wastewater treatment plants.
How is Chemical Oxygen Demand test conducted?
The basic idea for COD test is that almost all organic compounds can be oxidized into carbon dioxide in acidic conditions at high temperatures.
In COD test, wastewater sample is digested using Potassium Dichromate and Sulphuric Acid that is heated to 150 Celsius for 2 hours.
In such acidic conditions, Potassium Dichromate will act as a strong oxidant that will oxidise organic matter in the sample into carbon dioxide and water. In this process, dichromate ions, Cr2O2-7 , in potassium dichromate is reduced to chromate ions, Cr2O2-4 .
In the digestion step, the oxygen transaction from the dichromate ion to carbon dioxide reduces the dichromate ion to the chromium ion.
After the digestion step, the concentration of organic substances in the sample is calculated using a titrimetric or spectrophotometric determination of the remaining oxidant.
Meaning in COD test, the amount of organic materials is determined by measuring the amount of oxygen consumed during the oxidation of organic materials. This can be done in the presence of excess dichromate or chromate ions.
At SaniChem, we perform COD tests using the spectrophotometric method.
The basis for the spectrophotometric method is that both dichromate ions and chromate ions are coloured and absorb light at different wavelengths.
The dichromate ion is detected at 420nm and is used in low range COD measurement while the chromic ion is measured at around 600 nm to 620 nm and is used for high range COD.
Thus, COD is quantified by colometric determination using a spectrophotometer at 600 nm which measures the absorbance of light and correlates it to a COD concentration value based on the Beer-Lambert Law.
What does high Chemical Oxygen Demand mean?
Since COD measures the amount of oxidizable organic matter in a water sample, high COD value means that the sample contains high amounts of organic matter.
High organic matter will reduce the amount of dissolved oxygen (DO) levels.
Low DO value can lead to anaerobic conditions that can be disastrous to aquatic life.
What is the difference between chemical oxygen demand and biological oxygen demand?
BOD measures the amount of oxygen required by microorganism to breakdown organic material, whereas COD is the amount of oxygen required to breakdown organic material via oxidation.
BOD is an indirect representation of the amount of biodegradable organic matter while COD is and indirect representation of the amount of biodegradable and non-biodegradable organic matter.
COD does not differentiate between biologically available and inert organic matter. Because of this, COD value is always higher than BOD value (about 1.3 to 1.5 times higher)
BOD test requires a minimum of 5 day incubation period whereas COD test is a rapid test which only requires 2 hours digestion.
Contact us for more information about COD tests in Malaysia.