While oxygen is seldom in short supply for terrestrial organisms, the same is not true for aquatic organisms. Living things and the natural processes of decomposition compete for oxygen. Aquatic systems can decompose reasonable amounts of organic wastes without harmful effects. Excessive amounts of sewage wastes, on the other hand, reduce oxygen levels and productivity.
Biochemical oxygen demand (B.O.D.)is a measure of the organic matter in water. The concentration of oxygen in the water sample is tested when the sample is taken and then again after five days of incubation in the dark at 20° C. 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. As your students transfer water samples, recall that mixing increases the D.O. concentration and instead of pouring the sample, siphon the water whenever it is transferred into or out of the sample bottle.
"Part One – Using a Model System to Study the Oxygen Demand of Sewage"employs a simple model using yeast, powdered milk, and methylene blue, instead of actual sewage, to study the effect of sewage on dissolved oxygen concentrations. 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.
The equipment used to determine oxygen concentration in “Measuring Water Quality” is used in this activity as well. Seal the dark sample and cover it with foil or black electrical tape.