Measurement of BOD5
The traditional BOD test is conducted in the standard 300-mL glass BOD bottles. The test for 5-d BOD of water sample involves taking two DO measurements: an initial measurement when the test begins, at time t = 0, and a second measurement, at t = 5, after the sample has been incubated in the dark for 5 d at 20oC. The BOD5 is simply the difference between the two measurements.
For example consider that a sample of water from a stream is found to have an initial DO of 8.0 mg/L. It is placed directly into a BOD bottle and incubated for 5 d at 20oC. After the 5 d, the DO is determined to be 4.5mg/L.The BOD is the amount of oxygen consumed, or the difference between the two DO readings. That is, BOD5 = 8.0 – 4.5 = 3.5mg/L.
Very clean bodies of surface water usually have a BOD5 of about 1 mg/L due to the presence of naturally occurring from decaying leaves and animal wastes. BOD5 values in excess of 10 mg/L, however usually indicate the presence of sewage pollution.