Question

What do you mean by biological oxygen demand (BOD)?

Answer 1

The term biological oxygen demand (BOD) we generally listen whenever there is something related to water pollution so it simply means that there are some organisms in water which demand oxygen.

Complete answer:
BOD refers to the amount of dissolved oxygen required by the aerobic (in the presence of oxygen) biological organisms in order to break down the organic material that is present in a given sample of water at a certain temperature over a specific period of time. Some key factors related to BOD are listed below:

The BOD value is usually expressed in milligrams of oxygen being consumed per litre of the sample during a 5 days period of incubation at a specific temperature of $20^0C$.
More the organic waste present in water more is the biological oxygen demand (BOD). So the biological oxygen demand (BOD) is the measure of organic pollutants in polluted water.
BOD reduction is generally used as an indicator for the effectiveness of WTPs (wastewater treatment plants). BOD of wastewater effluents indicate the short-term influence on the oxygen level of the receiving water.
A low BOD value is an indicator of a good quality water, whereas a high BOD value indicates the polluted water.
The BOD test is a bioassay method which measures the amount of oxygen consumed by the bacteria during decomposition of organic matter. The change in dissolved oxygen (DO) concentration is being measured over a specified period of time in water samples at a particular temperature.

Note: Never get confused between COD and BOD. COD (Chemical oxygen demand) refers to the amount of oxygen being required to breakdown the pollutants using chemicals whereas BOD on the other hand is the amount of oxygen required to breakdown the organic pollutants biologically i.e. through micro-organisms. COD is generally higher than BOD as more organic compounds can easily be chemically oxidized compared to being biologically oxidised.