The B.O.D. is calculated by subtracting the dissolved oxygen concentration in the sample after incubation from the dissolved oxygen concentration before incubation. In the sealed, darkened sample, organic matter is decomposed by the bacteria normally found in the water. The bacteria require oxygen for decomposition. Thus, the more organic matter present in the water sample, the more dissolved oxygen will be consumed by the bacteria over the five-day test period. The B.O.D. test is easy to conduct and provides your students with a first hand glimpse of the impact of organic pollution.
Yeast represent the decomposers present in sewage and natural waters.
Powdered milk contains sugar represents the nutrient-rich wastes of sewage. Methylene blue is an indicator dye, blue when oxygen is present and colorless when oxygen is absent. Seal the dark sample and cover it with foil or black electrical tape. An increase in any of the sources for dissolved organic matter tends to decrease the amount of oxygen the water can hold. The arrows shown connect the model elements with the components they represent.