What is Biochemical Oxygen Demand ???

Biochemical Oxygen Demand (BOD) : BOD is an important measure of water quality. It is a measure of the amount of oxygen needed by microorganisms to oxidize the organic matter in water over a period of 5 days. The BOD of drinking water should be less than 1. Raw sewage will produce a BOD value of several hundered parts per million.

Primary Treatment of Water :
The simplest method of treatment is to allow the solids in raw sewage to settle out. THis forms a sludge. Primary treatment removes only a small portion of BOD and virtually none of the dissolved minerals and chemicals.

Secondary Treatment :
Many treatment plants then pass the wastewater from primary treatment to secondary treatment. Here the wastewater is brought in contact with oxygen and microorganisms. They break down much of the organic matter.

Primary and secondary treatment together can remove up to 90% of the BOD. Chlorination is then used to remove the content of bacteria, then the sample from secondary treatment is returned to the local surface water.

Advanced Waste Treatment:
The combination of primary and secondary treatment removes most of the organic matter in sewage and lowers the BOD. But even after that, most of the nitrogen and phosphorus in sewage remains in the wastewater from secondary treatment. These inorganic nutrients can cause algae. A third stage of treatment called tertiary or advanced waste treatment is used to dissolve slats and these chemicals from the sample.