The purpose of biological treatment of waste-water is to
A. Reduce BOD
B. Increase BOD
C. Reduce sedimentation
D. Increase sedimentation

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Hint: The BOD I.e. Biological Oxygen Demand is used for measuring the level of purity of water. In other words, it acts as a measure of pollution present in the wastewater. The sewage treatment plants employ this method for determining the pollution level.

Complete answer: Water has a certain ratio of dissolved oxygen. It may also come from the atmosphere or may be added as the byproduct of the photosynthesis performed by aquatic plants. The aquatic organisms are able to survive using this dissolved oxygen. The amount of dissolved oxygen required by aquatic creatures is measured as BOD and it is an important indicator of water pollution.
The more the water is polluted, the more will be the BOD, thus indicating more pollution in the water. This property is extensively used in sewage treatment plants; aerobic bacteria are introduced in the raw sewage water and these bacteria break down the organic pollutants present in it thus decomposing the raw sewage.
This decreases the pollution level hence, decreasing the BOD of raw sewage.

So, the correct answer is option A. Reduce BOD.

Note: In the biological treatment of wastewater the decomposed remnants of the raw sewage are collected and disposed off into the oceans. Is one of the most commonly used methods of sewage treatment as it is very economic as well as effective compared to other mechanical or chemical processes.