L19 BOD and COD values

Aim: To show what these terms mean and how the values are measured.

The BOD (biological oxygen demand) value is the amount of oxygen required, with the assistance of bacteria, to oxidise the oxidisable organic materials present in 1litre of water to carbon dioxide and water. This is therefore a measure of the degree of pollution of the water and is expressed in mg of oxygen /L.
To perform the test a small amount of bacteria is added to a measured quantity of dirty water. The initial oxygen content is measured and then the water is kept in the dark for 5 days at a temperature of 20°C. No photosynthesis can occur under these conditions, so no oxygen is produced. The oxygen content is then re-measured and the decrease in oxygen content is called the BOD value.



   Heavily polluted water has to be diluted first. The decomposition has to occur with the oxygen dissolved in the water.

A major disadvantage with this method is the time involved, the result only being available after 5 days. Another disadvantage is that toxic substances present in the water can adversely affect the bacteria making the measured BOD value too low.

If the BOD value is less than the solubility of oxygen in water (1,38.10-3 mol/L; 44 mg/L), then the bacteria can oxidise the polluting organic materials present in the water. If the BOD value is higher than the solubility of oxygen then this cannot occur and the water remains polluted.

The COD value (chemical oxygen demand) indicates how much oxygen, measured in mg/L is required to oxidise most of the organic material including cellulose in 1litre of dirty water.

To perform this test a measured volume of acidified potassium dichromate solution is added to the dirty water and the mixture is refluxed for 2hours at a temperature of 148°C.The solution is then cooled and the quantity of remaining potassium dichromate is determined by back titration using a solution of Fe2+ ions. From the amount of dichromate that has been consumed the equivalent amount of oxygen in mg/L can be calculated.

This test method oxidises more organic material than the BOD test and therefore the COD value is always higher than the BOD value. Despite this, measurement of the COD value is often preferred because the results can be obtained more quickly.