The BOD test is an indirect measure of the biodegradable organic matter in an aqueous sample. The test is indirect because oxygen used by microbes as they degrade organic matter is measured. Generally the test is performed over a 5 day period (BOD5) of 20°C in 300 ml bottles incubated in the dark to prevent interference from algal growth. Dissolved oxygen (DO) levels (in mg/L) are measured in day 0 and at the end of the test.
When biodegradable organic matter is released into a body of water, micro-organisms, especially bacteria, feed on the wastes, breaking it down into simple organic and inorganic substances.
The aerobic decomposition of wastes present in water by micro-organisms produces non objectionable, stable and products such as CO2, sulphate (SO4), orthophosphate and nitrate.
BOD is most often expressed in milligrams of oxygen required per liter of wastes (mg/L) or the equivalent gm/m3.
BOD values of various effluents are:
Characteristics BOD (mg/L)
Domestic sewage 165
Industrial waste water 200
Paper industry effluent 372
Food industry effluent 747
Metal industry effluent 13
The effects of oxygen demanding wastes on rivers
The concentration of oxygen demanding wastes greatly influence the quality of water in the river which may be described as follows:
1. As dissolved oxygen drops below 4 to 5 mg/L, the forms of life that can survive in rivers begin to be reduced.
2. In the extreme case, when anaerobic condition exist, most higher form of life are killed in rivers driven off.
3. Noxious conditions, including floating sludges, bubbling, odorous gases and slimy fungal growth take place in water body.