Three statements are provided here which define the term – sewage.
Pick out the correct definition for these terms.
A. The settled solids that are removed in wastewater treatment with a scraper.
B. Water from the kitchen is used for washing dishes.
C. Wastewater released from homes, industries, hospitals and other public buildings.
D. None of these.

Hint: The sewage contains many harmful substances. The sewage needs to be treated efficiently before discharging into the river or other water bodies. Otherwise, it will cause water pollution and many water-borne diseases such as diarrhoea, typhoid etc. It could also lead to eutrophication of water bodies.

Complete answer
With increasing population, large quantities of waste water are generated. This wastewater is conveyed to the sewers and hence, called sewage. Municipal wastewater or domestic wastewater is also called sewage. The wastewater from the cities and towns contains many pathogenic microorganisms. This is because
The sewage contains wastewater from various household activities such as washing clothes, utensils, bathing etc. It has detergents and soaps in it.
Large part of the sewage contains human excreta.
It contains a large amount of organic matter.
It also contains wastewater produced from various industries.
The wastewater from hospitals is also conveyed to the sewer. It may contain various harmful microbes in it.
The sewage is treated in sewage treatment plants. It involves the two stages of treatment- primary and secondary treatment. In primary treatment, the suspended particles present in sewage are removed by sedimentation and physical process. There is no involvement of microbes during primary treatment. In secondary treatment, microbes are used to remove the organic matter present in sewage. Once the BOD of water is reduced, it is discharged into the water bodies.

Thus the answer of the above question is option (C).

Note: BOD or biological oxygen demand means the amount of oxygen that the bacteria will consume while decomposing all the organic matter in the water. The more BOD indicates the more polluted water while less BOD signifies that the water is less polluted. Hence, by measuring the BOD of water, we can determine the pollution level of water.