Biochemical Oxygen Demand

The purpose of this exercise is to investigate the relationship of water quality and the consumption of oxygen by aerobic bacteria. Biological Oxygen Demand (BOD) is a measure of how polluted a particular aquatic system is with nutrients which increase the activity of aerobic bacteria. Researchers measure BOD as an index of water pollution or general water quality in streams, rivers, and lakes.

Biochemical Oxygen Demand is a common, environmental procedure for determining the extent to which oxygen within a sample can support microbial life. The following tutorial explores the theory and basics of performing this test when one has little or no prior experience. This method is popular in many environmental laboratories analyzing waste water, compost, sludge, and soil samples. Although methods for each matrix are similar, this tutorial focuses on the method associated with only waste water effluents.

The test for Biochemical Oxygen Demand is especially important in waste water treatment, food manufacturing, and filtration facilities where the concentration of oxygen is crucial to the overall process and end products. High concentrations of dissolved oxygen (DO) predict that oxygen uptake by microorganisms is low along with the required break down of nutrient sources in the medium (sample). On the other hand, low DO readings signify high oxygen demand from microorganisms, and can lead to possible sources of contamination depending on the process.

Performing the test for Biochemical Oxygen Demand requires a significant time commitment for preparation and analysis. The entire process requires five days, and it is not until the last day where data is collected and evaluated. During this time, samples are initially seeded with microorganisms and supplied with a carbon nutrient source of glucose-glutamic acid. The sample is then introduced to an environment suitable for bacterial growth at reproducible temperatures, nutrient sources, and light within a 20 degree Celsius incubator such that oxygen will be consumed. Quality controls, standards and dilutions are also run to test for accuracy and precision. Determination of the dissolved oxygen within the sample can be determined through Winkler titration methods. The difference in initial DO readings (prior to incubation) and final DO readings (after 5 days of incubation) predicts the BOD of the sample. A suitable detection limit as per environmental QC is 1 mg/L.

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