Biochemical Oxygen Demand

Published: February 27, 2019 | Last updated: July 5, 2023

What Does Biochemical Oxygen Demand Mean?

Biochemical oxygen demand (BOD) is a measure of the amount of oxygen required by aerobic bacteria and microorganisms for decomposing organic matter present in wastewater over a period of five days. BOD gives an estimate of the water quality and is measured in milligrams per liter (mg/l) or parts per million (ppm). BOD also measures the chemical oxidation i.e. oxygen extraction from water by chemical reaction, of inorganic matter present in the water.

When higher BOD levels are present, there's a rapid depletion of oxygen in the wastewater.

Trenchlesspedia Explains Biochemical Oxygen Demand

Dissolved oxygen varies with temperature and altitude. Flowing water has more oxygen content than stagnant water. Decomposition and chemical reactions in the water consume oxygen and if more oxygen is consumed than is produced, dissolved oxygen levels decline. Biochemical oxygen demand can be lowered by primary treatment involving settlement of un-dissolved solids out of suspension forming sludge, in raw sewage. This removes about one third of the BOD but does not remove the dissolved minerals.

Secondary treatment involves bringing the effluent in contact with aerobic bacteria and oxygen that breaks down the organic matter to carbon dioxide. Together the treatment can remove up to 90 percent of the biochemical oxygen demand.

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