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 main details of this method are taken specifically from Standard Methods for the Examination of Water and Wastewater (Method 507: 1985, and Method 218B: 1971) and the United States Environmental Protection Agency of 1979 (Method 405.1). Slight variations and additional insight are added from my experience as an analyst, and modifications will be noted. Other methods may exist amongst laboratories performing this test, so it must be stressed that this method, although approved, is not all inclusive. In addition, this procedure is only suitable for samples void of serious matrix interferences. To gain a broader appreciation of oxygen demand, additional avenues of interest may be explored including CBOD (carbonaceous oxygen demand), COD (chemical oxygen demand), and TOC (total organic carbon).
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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Last revision dated December 9, 1994
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