BOD used as an indicator of water quality
IN sporting circles BOD means Brian O'Driscoll, outstanding rugby player, Irish and Lions captain and one of the best centres to have ever played the game. In scientific circles BOD stands for Biochemical Oxygen Demand and is widely used as an indicator of water quality as far as organic pollution is concerned.
Imagine a crystal-clear stream tumbling under a road bridge. A person leans against the parapet of the bridge eating an apple and admiring the pristine water as it spills along on its journey to the sea. As the apple is eaten and the few moments of admiration come to an end, the core is tossed into the flowing water without much consideration for what is about to unfold.
What unfolds underwater is that bacteria and other microorganisms living in the stream start to feed on the unexpected bounty. In so doing they breed faster and use up some of the stream's reserves of oxygen. The arrival of the apple core puts an additional demand for oxygen on the stream and that additional need is the Biochemical Oxygen Demand or BOD on the water.
Oxygen gas from the air dissolves in the water naturally so the tumbling stream should be well able to cope with a lone apple core. However, a problem would obviously arise in the unlikely event of someone tipping a lorry load of rotting apples into a small stream. A pollution incident would arise and water samples would be collected for analysis.
BOD is measured in laboratories as the number of milligramme of dissolved oxygen consumed by organisms in a litre of water as they break down the organic material present over a period of five days when the water sample is kept in darkness in an incubator at a temperature of 20°C.
A BOD result in the range 1-2mg dissolved oxygen per litre indicates pristine water quality. Results in the range 3-5 indicate doubtful quality; 6-9 suggest pollution. Efficiently treated sewage effluent ranges 10-40 while raw domestic sewage is around 300. Vegetables and washings from food processing are in the high hundreds to low thousands.
Wastes and run-offs from agriculture are particularly polluting: cattle slurry has a BOD of 12,000, pig slurry 30,000 and silage effluent 60,000. Milk is worst of all. All of these alarmingly high figures for agricultural wastes evidence the need for extreme care in dealing with spillages and washings and especially runoff from farmyards following periods of prolonged heavy rain.