‌‌Oxygen is equally important as water is for living beings. No, wander oxygen does not contain water but water contains oxygen. Most living things need oxygen to survive, and without oxygen, organisms can not grow, reproduce and turn food into energy. In contrast, water is a basic need for all forms of life to exist on earth. Therefore, we need to conserve the water bodies and not pollute them. Polluting water bodies directly harms every living thing be it mankind or the other form of life. Take a look around you, what do you see? Industrial waste, Marine dumping, Wastewater from homes, oil leaks, and spills all run into the water bodies, rising biological oxygen demand levels, and harming aquatic life which indirectly is harming the ecology of the whole biosphere.


Biological oxygen demand known as BOD is the measure of the level of organic pollution present in water. It is the measure of the rate of oxygen depletion by the presence of microorganisms. How possible can this be?‌‌This is possible because most polluted water contains organic matter and microbes use oxygen in breaking down organic matters present in waters. However, the concentration of dissolved oxygen present in water at a time is an indication of the rate of microbial activity in the decomposition of organic matter. Also, increased organic matter increases microbial decomposition which eventually reduces oxygen concentration in water.‌‌


In the BOD method, the concentration of dissolved oxygen present in a sample of water is determined by titration, the water is then incubated at 20 degrees Celsius for five days during which microbes use some oxygen to break down organic matters. The concentration of oxygen is determined again at the end of the incubation period. The difference in oxygen concentration before and after the incubation period is the concentration of oxygen consumed by microorganisms to break down organic matter.

Now, note that microbial activity might already reduce the oxygen concentration to a very low degree before collecting the sample for the experiment. Or the level of pollution will make all Retrievedtrievedthe water be consumed before the five days. As a result, the sample of water to be tested is diluted with previously parroted distilled water before the experiment.‌‌
Why is it advisable to dilute?‌‌

The amount of dissolved oxygen may not be enough for a microbial activity for the five days and is likely to be zero during the period. Hence, when diluted aerated distilled water, more oxygen will be available initially so that at the end of the five days, all will not be drained.

Another thing to note too is the typical values of biological oxygen demand and its clues.‌‌For instance, below 1 milligram per liter represent pristine water quality, while‌‌2-8  milligrams per liter shows moderately polluted water. And, above 8mg/L indicate severely polluted water.‌‌HIGH


Biological oxygen demand is affected by the following factors:

1. Temperature:‌‌

The amount of oxygen that can dissolve in water depends on temperature. Stormier water will have a higher demand oxygen level than warmer water. A difference in oxygen demand levels at the test site may be detected if tested early in the morning when the water is cool and then later in the afternoon on a sunny day when the water temperature has risen.  Similarly, a difference in oxygen demand levels may be seen at different depths of the water if there is a significant change in water temperature. Also, 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 oxygen demand is high at this location. Therefore, increased water temperatures will speed up bacterial decomposition and result in higher biological oxygen phases.

2. The existence of specific kinds of microorganisms affects the growth of aerobic bacteria.

3. Kinds of inorganic substances in water.

4. Percentage and type of organic substance in water.


Biological Oxygen Demand discloses its importance in numerous fields. These are

  1. To test the level of pollution of water with organic matter.
  2. To measure waste load to be introduced into a waste treatment plant.
  3. To determine waste treatment plant efficiency
  4. Used to determine the effects of waste discharge on receiving water.
  5. BOD is used in the medical and pharmaceutical industries to measure the oxygen consumption of cell cultures.


Sources that improve the Biological Oxygen Demand of water are both natural and man-made. Pollution is a major contributor to increasing the BOD of water bodies. A good lifestyle is linked with a considerable usage of water regularly which results in a lot of wastewater with organic content in it.‌‌With increasing mechanization, pollution is increasing manifold. Factories have tremendous wastewater being produced. Few industries that have large amounts of wastewater are paper mills, food processing plants, jute mills, etc.‌‌The environmental factors contributing to increasing BOD include surface runoff, floating debris, dead animals and plants, soil erosion, etc. Few chemicals affect the BOD of drinking water. One of these is phosphate, which when present in high amounts increases the BOD of water.


‌‌In a nutshell, improving BOD has the same effect as the effects of dissolved depleting oxygen. When the oxygen demand of a water body increases reasonably, aquatic life is adversely affected. The oxygen used by aquatic organisms for respiration and metabolism is quite reduced by the microbes for breaking down organic waste. This impacts the death of fishes and aquatic plants and the complete disruption of the aquatic ecosystem. Oxygen concentration below five parts per million risks even lows oxygen organisms like catfish. Freshwater fishes like Catla do not withstand these concentrations. Hence the overall beauty of the water body is damaged.

Every living organism has a right to a habitat and they are important too. So, we should treat our water bodies with content and save the aquatic organism and ourselves as a whole.


1." Biochemical Oxygen Demand - BOD testing and measurement in water" retrieved from

2. "biochemical oxygen demand | biology | Britannica" Retrieved from



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