From a study on river X, this finding is obtained: Biochemical oxygen demand (B.O.D) value is high: Which statement is correct about the river:
1. The river is POLLUTED.
2.The content of dissolved OXYGEN is LOW
3.The river contains LESS/NO living aquatic organisms.
4.The DECOMPOSITION activity by microorganisms is HIGH.
Many students get confused over the B.O.D stuff which is related with the BOD experiment to test the pollution in different water samples..
Biochemical oxygen demand or BOD is a chemical procedure for determining the uptake rate of dissolved oxygen by the biological organisms in a body of water.
It is not a precise quantitative test, although it is widely used as an indication of the quality of water.
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 or less. Untreated sewage varies, but averages around 600 mg/L in Europe.
The following notes will help you to understand more abot BOD:
Microorganisms such as bacteria are responsible for decomposing organic waste. When organic matter such as dead plants, leaves, manure, sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live.
Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline.
Nitrates and phosphates in a body of water can contribute to high BOD levels. Nitrates and phosphates are plant nutrients (maybe from organic fertilisers) and can cause plant life and algae to grow quickly. When plants grow quickly, they also die quickly. This contributes to the organic waste in the water, which is then decomposed by bacteria. This results in a high BOD level.
The temperature of the water can also contribute to high BOD levels. For example, warmer water usually will have a higher BOD level than colder water. As water temperature increases, the rate of photosynthesis by algae and other plant life in the water also increases. When this happens, plants grow faster and also die faster. When the plants die, they fall to the bottom where they are decomposed by bacteria. The bacteria require oxygen for this process so the BOD is high at this location. Therefore, increased water temperatures will speed up bacterial decomposition and result in higher BOD levels.
When BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive.
Test Procedure:
The BOD test takes 5 days to complete and is performed using a dissolved oxygen test kit.
The BOD level is determined by comparing the DO level of a water sample taken immediately with the DO level of a water sample that has been incubated in a dark location for 5 days.
The difference between the two DO levels represents the amount of oxygen required for the decomposition of any organic material in the sample and is a good approximation of the BOD level.
1.Take 2 samples of water
2.Record the DO level (ppm) of one immediately using the method described in the dissolved oxygen test.
3.Place the second water sample in an incubator in complete darkness at 20 °C for 5 days. If you don't have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20 °C or 68 °F).
4. After 5 days, take another dissolved oxygen reading (ppm) using the dissolved oxygen test kit.
5. Subtract the Day 5 reading from the Day 1 reading to determine the BOD level.
2.Record the DO level (ppm) of one immediately using the method described in the dissolved oxygen test.
3.Place the second water sample in an incubator in complete darkness at 20 °C for 5 days. If you don't have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20 °C or 68 °F).
4. After 5 days, take another dissolved oxygen reading (ppm) using the dissolved oxygen test kit.
5. Subtract the Day 5 reading from the Day 1 reading to determine the BOD level.
6.Record your final BOD result in ppm.
RESULT:
BOD Level(in ppm) Water Quality
1 - 2 Very Good:There will not be much organic waste present in the water supply. 3 - 5 Fair: Moderately Clean
6 - 9 Poor: Somewhat PollutedUsually indicates organic matter is present and
bacteria are decomposing this waste.
100 or greater Very Poor: Very PollutedContains organic waste.
NOTE: Generally, when BOD levels are high, there is a decline in DO levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water. If there is no organic waste present in the water, there won't be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher.
..As for our experiment in our Bio Lab (Malaysian version), we are using methylene blue solution (which is blue in colour ) and will DECOLORIZE faster if there is less dissolved oxygen in the water sample…Vice versa……The more polluted the water sample is, the faster the methylene blue solution to decolorise ..
Notes are extracted..TQ
http://www.ciese.org/curriculum/dipproj2/en/fieldbook/bod.shtml
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