Biological Oxygen Demand

Biological Oxygen Demand (BOD) is a measure of the rate of use of oxygen by water borne microorganisms in removing organic compounds dissolved in the water.

The BOD in England is carried out by taking two freshwater samples, testing for dissolved oxygen in one sample, then sealing the other sample (to prevent further oxygen dissolving in),and placing it in the dark (to prevent photosynthesis adding oxygen). This second sample is kept at room temperature and tested for dissolved oxygen after five days. The ratio of the dissolved oxygen in both samples is the BOD(five). Five days is chosen because this is the longest time that river water takes to travel from source to estuary in the UK.