Biological oxygen demand
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Description
The biological oxygen demand is a measure of the quantity of oxygen required to aerobically decompose an organic pollutant in waste water, converting it into carbon dioxide and water. Conventionally, it is measured over five days and is abbreviated to BOD5.
Detailed description
The following information is an edited excerpt from Wikipedia
Typical BOD values
Most pristine rivers will have a 5-day BOD below 1 mg/l. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/l. Municipal sewage that is efficiently treated by a three stage process would have a value of about 20 mg/l. Untreated sewage varies, but averages around 600 mg/l in Europe and as low as 200 mg/l in the U.S., or where there is severe groundwater or surface water infiltration. (The generally lower values in the U.S. derive from the much greater water use per capita than in other parts of the world.)
The BOD5 test
BOD measures the rate of oxygen uptake by micro-organisms in a sample of water at a temperature of 20°C and over an elapsed period of five days in the dark.
There are two recognized methods for the measurement of BOD.
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 activated sludge with de-ionized water. The BOD test is carried out by diluting the sample with de-ionized water with added nutrients, saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and 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 The apparent BOD for the control is subtracted from the control result to provide the corrected value.
The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called the BOD5. In most European countries, allylthiourea is also added at the start of the test to prevent oxidation of ammonia. Results from such tests are represented as BOT5(ATU) and referred to as Carbonaceous BOD (CBOD) in the United States. Less frequently used is the Ultimate BOD (UBOD) test, in which DO is repeatedly measured by DO meter in the same specialised bottles until it has reached equilibrium.
BOD can be calculated by:
- Undiluted: Initial DO - Final DO = BOD
- Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor
BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidised, rather than just levels of biologically active organic matter.
Manometric method
This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxydation is inhibited.
The sample is kept in a sealed container fitted with a pressure sensor. A substance absorbing carbon dioxide (typically LiOH) 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, thus the pressure, decreases because carbon dioxide is absorbed. Form the drop of pressure,the electronics computes and displays the consumed quantity of oxygen.
The main advantages of this method compared to the dilution method are:
- its 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
Furthermore, as the BOD measurement can be monitored continuously, a graph of its evolution can be plotted. Interpolation of several graphs on a similar water may build an experience of its usual evolution, and allow an estimation of the five days BOD after as early as the first two days of incubation. [1]
