Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 144

Biochemical oxygen demand (BOD) is a chemical procedure for determining the rate of uptake of dissolved oxygen by the rate biological organisms in a body of water use up oxygen. It is a chemical measure of the power of an effluent to deoxygenate water. The test is widely used as an indication of the quality of water. The biochemical oxygen demand can be used as a gauge of the effectiveness of wastewater treatment plants. There are two recognized methods for the measurement of biochemical oxygen demand which are (i) the dilution method and (ii) the manometric method.

In the dilution method, a small amount of microorganism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionized water. The test is carried out by diluting the sample with oxygen saturated de-ionized water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen, and then sealing the sample to prevent further oxygen dissolving in. The sample is kept at 20°C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final dissolved oxygen and initial dissolved oxygen is the biochemical oxygen demand. The apparent biochemical oxygen demand for the control is subtracted from the control result to provide the corrected value.

In the manometric method, the sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide (typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed, and dioxide is released. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.