The pollution present in wastewater cannot be measured directly. The biochemical oxygen demand (BOD) serves as a means to measure the amount of oxygen that is needed to oxidize the substances present in one liter of wastewater. The BOD imitates the oxidative processes taking place as part of the natural self-purification of bodies of water but in a laboratory setting.
The BOD of water or wastewater is defined as the amount of oxygen that must be consumed by microorganisms in order to bring about oxidative degradation of the organic substances at 20 °C in one liter of the water sample. The incubation period is generally five days (BOD5) and in special cases, it can be set to span different time periods, e.g., two days for BOD2 or 20 days for BOD20.
Under normal conditions only approx. 9 mg/l oxygen dissolves in water; however, the oxygen demand of untreated wastewater is normally higher by 10 to 100 times. Therefore, the oxygen needs to be introduced into the sample artificially. This can be achieved by blending a small quantity of wastewater with pure water containing oxygen (the dilution method).
Wastewater generated in the industrial production of beer and food
The water sample is diluted with oxygen-rich water “free of oxidizable organic substances” so that after the incubation period there is still at least 2 mg oxygen per liter left in the water. Afterward, the dissolved oxygen in the diluted sample should be determined immediately and then again after five days using an iodometric method or with oxygen electrodes. The difference between the two values yields the BOD5 (taking into account the dilution factor).