The biochemical oxygen demand (BOD) test is an indirect measure of the biodegradable organic matter in an aqueous sample. The test is indirect because oxygen used by microbes as they degrade organic matter is measured, rather than the depletion of the organic materials themselves. Generally, the test is performed over a five-day period (BOD₅) at 68°F (20°C) in 10 fl oz (300 ml) bottles incubated in the dark to prevent interference from algal growth. Dissolved oxygen (DO) levels (in mg/l) are measured on day zero and at the end of the test. The following equation relates how the BOD₅ of a sample is calculated:

Note that in the above equation "ml sample" refers to the amount of sample that is placed in a 300 ml BOD bottle. This is critical in doing a test because if too much sample is added to a BOD bottle, microbes...