Biological Oxygen Demand | Activated Sludge Process | Wastewater Treatment Plant

BIOLOGICAL OXYGEN DEMAND

BOD is an abbreviation for Biological Oxygen Demand and, depending on the source, Biochemical Oxygen Demand. Both terms refer to the amount of oxygen required to biologically degrade (oxidize) all organic matter in a wastewater sample. It is the most widely used parameter for determining the relative strength of domestic (and most industrial) wastewater. It is measured in milligrams per liter of DO. These units are used because they are consistent with the measurement of organic content in wastewater, which is also calculated in milligrams per liter.

A BOD test is used to determine BOD. Standard Methods contains a comprehensive description of the standard test procedures. Although this is one of the oldest and most widely used tests in wastewater treatment, it is also one of the most difficult to replicate consistently. In short, BOD bottles, usually 300 mL but sometimes 60 mL, are filled with a mixture of wastewater sample, growth medium, and sterile but not distilled water. The DO concentration is measured and recorded, and the bottles are placed in a 20°C temperature-controlled incubator. After 5 or 7 days, the samples are removed from the incubator and the DO content is measured and recorded again. The difference between the initial and final DO values is the mass of oxygen consumed divided by the volume of actual wastewater in the sample during the incubation period.

Difficulties arise from mixing the growth medium and ensuring sterility in that mix, measuring the DO at the beginning and end of the incubation period with consistent reliability, and minor variations in measuring the actual volumes of wastewater added to each sample bottle. If nothing is done incorrectly and no sample or growth medium contamination occurs, the results should be consistent regardless of the dilution factor used in the various sample bottles. This test is rarely well aligned with the ideal and reality.