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Application Overview

The determination of the Biochemical Oxygen Demand or Biological Oxygen Demand (BOD) evaluates the amount of biodegradable organic material present in wastewater, effluent and polluted waters. The BOD test reflects the amount of dissolved oxygen (DO) consumed by bacteria while oxidizing these materials. Dissolved oxygen is essential for the life of aquatic fauna and flora, and the BOD test is a measure of the ecological impact that effluent water may have on the receiving body of water (river, lake, etc.). This test is often required in discharge permits, as it is a means of assessing the degree of water pollution.

Two methods are widely used for BOD measurement: the dilution method and the manometric method:
  • Dilution method
    This is a standard method. As an example, the APHA standard methods 5210B from the American Public Health Association describes in detail this protocol. Dilution water is prepared by adding inorganic nutrients and buffer salts to purified water. If the sample does not contain sufficient amounts of bacteria, or contains compounds toxic to bacteria (e.g. chlorinated effluent), it may be necessary to add microbial seed as well. Various dilution levels of the sample water are then prepared using the dilution water. The BOD bottles are filled to the top, capped and sealed. They are incubated in the dark at 20°C for 5 days. The levels of dissolved oxygen are measured prior to and after the 5-day incubation period. The difference between these two values, corrected for the dilution and the blank, is the BOD5 value. BOD tests results are expressed in mg/L of dissolved oxygen.

  • Manometric method
    In this test, a manometer is fitted into a bottle containing the undiluted sample. It continuously measures the drop in air pressure in the bottle, which reflects the amount of oxygen uptake by the sample. This method is easier than the dilution method because no dilution is necessary, and continuous measurements are obtained.
The presence of toxicants or poor seeding material in water samples may lead to falsely low BOD results. Therefore, it is recommended to regularly use a glucose - glutamic acid (GGA) solution as a standard check solution. The oxygen uptake of this solution should be 198 +/- 30.5 mg/L.

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