Biological oxygen demand or also called biochemical oxygen demand, refers to the amount of oxygen in the water, which is required or consumed by organic material for oxidation. This indicator is used to evaluate the quality of water available to consumers for basic or commercial use.
A good water quality is very important, because water is needed in variety of uses from domestic to industrial supplies. The water quality is controlled by the authorities to protect users form health and other adverse effects of poor water quality. A high BOD level can indicate the existence of faecal contamination or particulate and dissolved organic carbon from various sources, which are neither from human nor from animal. This kind of contamination can seriously affect human health and cause problems to the industry.
So it is very important for every government to ensure a good water quality with low BOD level. For this purpose international conventions and agreements have been made such as The Resolution II and the Plan of the United Nations Water Conference.
There are two methods to measure the BOD level. Both methods are empirical tests.
- Method I: This is the most common method. A specified sized bottle is filled to the brim and the sample is left over 5 days at a constant temperature of 20° in the dark.
- Method II: This method is similar to the Method I. Here again a specified sized bottle is filled with a water sample and then it is diluted with de-ionized water. The amount of dissolved oxygen in the de-ionized water is known. The bottle is also left over 5 days at a constant temperature of 20° in the dark.
A drinking water should have after the 5 days well below 1 mg/l. A sewage plant will have around 20 mg/l.
As those methods are empirical the BOD indicator gives no absolute results. The indicator provides a good comparison among samples but it gives no exact measure of the contamination in the water. An alternative indicator to BOD is COD – Chemical Oxygen Demand.