Biology,  Earth & Environmental Sciences

Modeling Biological Oxygen Demand

March 25, 2015

Biological oxygen demand (BOD) relates to the change in oxygen that occurs in bodies of water as a result of microorganisms taking in and using dissolved oxygen as they decompose organic matter. These decomposers utilize oxygen in the same way that other organisms do—for the process of cellular respiration. A body of water with a large amount of organic waste will provide plenty of material for decomposers to feed upon, but the high rate of oxygen use by decomposers can severely limit the oxygen available to other aquatic organisms such as fish. In this situation the water is said to have a high BOD level. You can accurately measure these changes with either of these PASCO sensors: the Optical Dissolved Oxygen Sensor or the Advanced Water Quality Sensor. This lab uses readily available consumables and PASCO's Optical Dissolved Oxygen Sensor to teach your students about BOD, which prepares them for independent inquiry.)

Note: You can download a FREE copy of this lab activity at the bottom of the page.

Typical BOD studies take numerous samples from the field and monitor dissolved oxygen levels for five days at a constant temperature. (More information on this method is provided in the extension to this lab activity.) The lab procedure is a model for what occurs in aquatic ecosystems. Milk is used to model the presence of an organic pollutant and yeast is used to model the action of decomposers.


The sample data show what was collected with 50%, 25%, 12.5% and 6.25% whole milk solutions. The higher concentrations of organic waste (milk in the model) clearly correlated to a higher oxygen demand by the yeast.

After completing this investigation, try extending the inquiry by exploring one or more of the following:

  • Does an increase in temperature affect dissolved oxygen in aquatic ecosystems? Design and carry out an experiment to test this question.
  • How are BOD levels measured in the field for water quality studies? Use these methods to test the BOD levels of some of the bodies of water in your area.
  • Research water treatment facilities and the methods used within these facilities. How does your local facility treat wastewater that contains sewage?

Lab Activity Download

Related Products:

  • PASPORT Optical Dissolved Oxygen Sensor (PS-2196)
  • Advanced Water Quality Sensor (PS-2230)