Biological Oxygen Demand

Background Information

Microorganisms such as bacteria are responsible for decomposing organic waste. When organic matter such as dead plants, sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. In this case, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working todecompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) and, subsequently, the dissolved oxygen levels in the water may begin to decline.

Test Procedure

The BOD test takes 5 days to complete and is performed using a dissolved oxygen test kit (from LaMotte or other supplier). Record the dissolved oxygen level (in ppm) on the first day using the method described in the dissolved oxygen test. Then place the water sample in an incubator at 20 C for 5 days. If you don't have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20 C or 68 F). On day 5, take another dissolved oxygen reading (in ppm) using the dissolved oxygen test kit. The BOD level is determined by subtracting the Day 5 reading from the Day 1 reading. Record your final BOD result in ppm.

What to Expect

A BOD level of 1-2 ppm is considered very good. There will not be much organic waste present in the water supply. A water supply with a BOD level of 3-5 ppm is considered moderately clean. In water with a BOD level of 6-9 ppm, the water is considered somewhat polluted because there is usually organic matter present and bacteria are decomposing this waste. At BOD levels of 10 ppm or greater, the water supply is considered very polluted with organic waste.

Generally, as BOD levels increase there is a corresponding decrease in DO levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water. If there is no organic waste present in the water, there won't be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher.

At high BOD levels, organisms that are more tolerant of lower dissolved oxygen (i.e. leeches and sludge worms) may appear and become numerous. Organisms that need higher oxygen levels (i.e. caddisfly larvae and mayfly nymphs) will not survive.

References