Chemical Oxygen Demand (COD) is the amount of oxygen required for the chemical oxidation of the organic matter by the strong oxidizing agent under the acidic condition, it is a measure of oxygen required to oxidize water-soluble and micro-organic materials.
Also, it provides an index to assess the effect that the discharged wastewater will have on the receiving environment.
Higher COD levels mean more significant amounts of oxidable organic materials in the sample, which will reduce dissolved oxygen (DO) levels.
A decrease in DO can lead to anaerobic conditions, which is harmful to higher aquatic life; COD testing is often used as an alternative to BOD due to the shorter length of measurement.
Importance of Chemical Oxygen Demand:
In the COD test, a reliable oxidizing agent is used in acidic medium, and the oxygen equipment of the organic matter that can be oxidized is measured.
For the test, Potassium Dichromate (K2Cr207) is used at a high temperature & for the fast reaction, silver sulfate is added.
Before this test is carried out, the inorganic compounds (which can interfere with the test) are removed from the sample, which is used for the COD test.
On average, the COD of any sewage is always higher than its BOD because more compounds can be chemically oxidized than the compounds which get oxidized biologically.
The limiting value of COD is about 250 ppm or mg / per liter.
Procedure for determination the COD:
The following method is used to determine the chemical oxygen demand (COD) as follows:
- Take 3 COD vials with stopper (two for the samples and one for the blank).
- Add 2.5 ML of the sample to each of the two COD vails, and the remaining blank vail is to be filled with distilled water.
- Also, add 1.5 ML of potassium dichromate reagent (digestion solution, to each of the three COD vials).
- Add 3.5 MI, of sulphuric acid regent (catalyst solution in the same manner).
- Cap the tubes tightly. Switch on the COD Digester and fix the temperature at 150°C and set the time at 2 hrs.
- Place the COD vials into a block digester at 150°C and heat it for 2 hrs.
- After the digester gets switched off, remove jars and allow it to cool down to the room temperature.
- Fill the burette with ferrous ammonium sulfate solution (adjust to zero and fix the burette to the stand).
- Transfer the contents of the empty vial to a conical flask.
- Add fa drops of ferroin indicator (The solution would become Bluish-green).
- Tirate it with ferrous ammonium sulfate (taken in the burette).
- The endpoint of the titration appears to be reddish-brown. Also, read COD conclusion.
- Note down the volume of ferrous ammonium sulfate solution added for the blank (A) is 14.1 ML.
- Transfer the contents of the sample vial to the conical flask.
- Add a few drops of ferrous indicator (The solution becomes green).
- Titrate it with ferrous ammonium sulfate (taken in the burette).
- The endpoint of titration appears to be reddish-brown.
- Note down the volume of ferrous ammonium sulfate solution added for sample (B) is 13.2 ML.
Relationship between BOD and COD:
BOD can be reduced to the permissible level, but COD cannot be reduced to its allowable level so quickly.
COD test is used to measure the organic matter present in industrial waste, which has toxic compounds, which may affect the biological life.
COD can be determined within 3 hours while it requires five days for BOD.
In a domestic waste, the ratio between COD/ BOD.is 1 : (which varies up to 1: 5 in some cases).
When the rate reaches more than three, the sewage is considered as difficult for bio-degradation.
When it is more than 10, it is non-biodegradable sewage.
The above COD/BOD ratio is useful for assessing the amount of sewage available for biological treatment.
COD tests are used very frequently than the BOD tests because of COD tests have high speed to get fast results & used mainly to assess the COD tests.
The COD tests determine the strength of waste material, having toxic and biologically resistant organic matter.
For deciding the industrial waste material, the COD tests are used, it also determines the strength of waste material, which BOD tests cannot identify.
Advantages of Chemical Oxygen Demand:
- When the conditions are not favorable for the growth of microbes due to the presence of toxic matter, BOD cannot be measured accurately.
- COD test is easy to carry out than the BOD test.
- COD tests are not affected by interference as the BOD tests.