Question

Explain the effect on the characteristics of a river when urban sewage is discharged into it.

Answer 1

A sewage system is a process of treatment, removal, and discharge of sewage and rainwater which contains waterborne pathogens or disease-causing microorganisms like bacteria.

Complete answer:
Sewage treatment involves a process of elimination of impurities from households and industries. This sewage includes house waste liquids such as from toilets, kitchen, etc. which are disposed of via the sewers as well as it includes wastewater from industry discharge. It contains large amounts of organic matter and microbes in which many of them are pathogenic. Therefore, sewage treatment is done to treat the sewage and make it less polluting before discharging off in the river.
Ecology of the river (a natural water body) defines the relationship which living organisms have with each other and with their surrounding environment which coincides with the term i.e. ecosystem. River ecosystem involves flowing water which is unidirectional in movement, having a state of continuous physical changes, many changing microhabitats, and most importantly plants and animals that have adapted to live within the flowing water conditions.
If untreated sewage water is discharged into the river stream, microorganisms present in the sewage water example like human excreta waste containing E.coli contaminated water and cause disease whereas anaerobic bacteria consume the organic waste and produce ${{\text{H}}_{\text{2}}}{\text{S}}$ gas (harmful gas), decompose it which results in pollution and harm to aquatic wildlife, and making the water of the river dark with all the poisonous waste. These microorganisms utilize oxygen from the water for aerobic respiration due to which less oxygen is dissolved in the water body, so aquatic organisms present in the river ecosystem like fish, aquatic plants, or other aquatic animals may be unable to survive.

Note: Partially treated sewage which is disposed of in the river body is the main cause of water pollution and the mass mortality rate of aquatic wildlife. Thus these water bodies become a breeding place for mosquitoes and other disease-causing organisms.