BOD can be measured by three different techniques – the Winkler method, by using a dissolved oxygen probe or manometrically.
The Winkler method is a titrimetric wet chemistry test that measures the amount of oxygen present based on conversion of oxygen to iodine. Although considered by some to be it the “Gold Standard” in BOD determination, it requires skilled and time-consuming sample preparation; is laborious to perform and so is relatively expensive.
The dissolved oxygen (DO) method of BOD determination involves the electrochemical measurement of DO in the sample.
The manometric method. The sample is placed in an airtight bottle, with a pressure sensor mounted on the bottle. The carbon dioxide evolved during aerobic respiration is absorbed using an alkali (KOH). As oxygen is used up during aerobic respiration and the carbon dioxide is absorbed there will be a pressure drop in the bottle proportional to the amount of oxygen used. This pressure drop is measured and the oxygen consumption (BOD) is calculated from the pressure drop. The manometric method requires less sample preparation and less reagents than the other 2 methods. Manometric measuring instruments are also able to take readings automatically and store them for retrieval at a later time.
BOD levels are expressed in terms of mg/l O2 and are measured over a 5-day period. The samples must be maintained at 20°C ±1°C for the duration of the test.
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