The presence of free oxygen is necessary for the livelihood of organisms present in the sewage water The aerobic action continuous till the free oxygen is present. As the oxygen exhausts, the anaerobic action or putrification starts, when complex organic compounds split up. setting free gases like CO2, H2S NH3,CH4 etc.and foul smell starts coming. The length of aerobic action can be increased, if the percentage of free oxygen in the sewage is increased in the beginning.
Biological oxygen demand (BOD) of sewage is define some amount of free oxygen required for the biological oxidation of the organic matter under aerobic at 20°C and for period of 5 days. The unit of Biological Oxygen Demand is mg/liter or ppm. An average sewage a BOD of 100 to 150 mg/liter
The test is based upon determination of dissolved oxygen prior to and following a 5 day period at 20°C. A known volume of sample of sewage is diluted with a known volume of dilution water (water containing nutrients for bacterial growth), whose dissolved oxygen content is predetermined. The whole solution is incubated in a closed bottle at 20° C for period of 5 days. After this unused oxygen is determined.The difference between the original oxygen content in the ‘dilution water and unused oxygen of solution after 5 days gives Biological Oxygen Demand.
Biological Oxygen Demand test is the most important in sewage treatments as it is indicates the amount of decomposable organic matter in the sewage. Larger the concentration of decomposable organic matter. greater the BOD and consequently more the strength or nuisance potential. This test has a special significance in pollution control as it enables us to determine the degree of pollution at any titan in the sewage stream.