Methods of Biological Oxygen Demand (BOD) Measurement : Week 5/10/2012
In the last article, we discussed about basics of BOD, i.e. the Biological Oxygen Demand. In this article, let us discuss about the available methods of measuring BOD & their limitations.
What are current methods of measuring BOD ?
There are two commonly recognized methods for the measurement of BOD.
(A) Dilution method
To ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each sample being tested. This seed is typically generated by diluting organisms with buffered dilution water. The BOD test is carried out by diluting the sample with oxygen saturated dilution water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen (DO) 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 DO and initial DO is the BOD.
(B) Manometric method
This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxidation is inhibited.
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, as ammonia oxidation is inhibited, carbon 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. The main advantages of this method compared to the dilution method are:
- Simplicity: no dilution of sample required, no seeding, no blank sample.
- Direct reading of BOD value.
- Continuous display of BOD value at the current incubation time.
Method used worldwide today takes 3 or 5 days to monitor BOD value of any effluent. This is a major limitation in taking any corrective action if required, to keep discharge from plant within required limits. One has to control effluent treatment process based on indirect parameters like Dissolved Oxygen or COD(Chemical Oxygen Demand).
What if we get some other quicker method of BOD measurement ?
Efforts are on to develop a new method of Monitoring BOD in 30 minutes. With this, the user need not wait for 3 or 5 days to know BOD of the effluent. One can take quick corrective action in effluent treatment or any unrest in actual process. Furthermore, you need not monitor COD to get indirect measurement of BOD which will save lot of Chemicals used in daily COD monitoring and can save on manpower required for same.
Most importantly, one can have better control over water recycling processes and can increase efficiency of same. The user can go away with large holding areas. He can discharge effluent as soon as it gets treated. It will be then possible to keep daily records of BOD levels of the effluent plant and will help statutory bodies to work better.
In our next article, we will discuss more about BOD & see if there are any alternate methods to measure it quickly