23 Apr


Biochemical Oxygen Demand is used to measure the amount of organic material and nutrients available to microorganisms in water and waste water.  The standard test is the BOD5 which measure the amount of dissolved oxygen used in 5 days at 20°C.  The amount of Oxygen used by the natural organisms in water is directly proportional to the amount of organic matter and nutrients available.  If there are too many of these nutrients available the microorganisms can use all the Oxygen available faster the body of water can take up atmospheric oxygen; fish and other aquatic life can be killed by this lack of Oxygen.  It is important waste water entering lakes streams and rivers do not contain too many of the organic material and nutrients.  The BOD5 test involves determining the dissolved O2 in a water sample and then enclosing the sample in an airtight container.  The container is kept at 20°C and in the dark for 5 days and the amount of dissolved O2 is determined again.  The amount of O2 used is directly proportional to the amount of organic matter available to the natural microorganisms.   The bottles used in BOD5 determination are specially designed to be air tight and to overflow when the cap is applied assuring that no trapped air remains in the bottle.  To keep variables constant the sample is seeded with suitable microorganisms suspended in a buffer which has been saturated with O2.  The dissolved O2 is determined using a DO probe and meter.  Various types of DO electrodes or probes are available for the determination of dissolved Oxygen but probes designed for the BOD5 test are tapered to fit the BOD bottle and may include a stirrer to assure proper sampling of the water in the BOD bottle.  BOD probes can be electrochemical in nature using redox potential and an Oxygen permeable membrane.  Another type of BOD probe is based on quenching by Oxygen of the luminescence emission of a photo active chemical compound.

Robin Prymula