Water bodies also contain living life in the form of microorganisms. These microorganisms feed on the organic matter present in the water and respire through the oxygen present in the water.
Biological oxygen demand (BOD) is the amount of oxygen required by bacteria to consume all the organic matter in aerobic conditions at a specific temperature.
Depending on the type of aquatic life a certain amount of oxygen should be present in water which can support aquatic life in water. This is the measure of quality of water as well.
Increase in the organic matter in water will certainly increase its oxygen demand. If sufficient oxygen is not available because of any reason, microorganisms would not be able to decompose all organic matter and this will affect the quality of water.
The decay of organic matter in water is measured as biochemical or chemical oxygen demand.
BOD value is expressed in milligrams of oxygen consumed per litre in 5 days of incubation at 20 C.
Total biochemical oxygen demand
The amount of oxygen required to completely oxidize the organic compounds to carbon dioxide and water through generation of microbial growth, decay and cannibalism is total biochemical oxygen demand of water.
- Most pristine rivers will have a 5 day carbonated BOD below 1 mg/L.
- Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L.
- Severely polluted river BOD may jump to 8 mg/L.
- Municipal sewage that is efficiently treated by a three stage process would have a value of 20 mg/L.
- Untreated sewage may have a BOD value of 600 mg/L in Europe and 200 mg/L in US.
Use of BOD
- BOD is used in measuring waste loadings to treatment plants and in evaluating the BOD removal efficiency of such treatment systems.
- It also tells about the quality of water and about the aquatic life present in particular ecosystems.