Question

A river with an inflow of domestic sewage rich in organic waste may result in
A. Drying of the river very soon due to algal bloom.
B. Increased population of aquatic food web organisms.
C. An increased production of fish due to biodegradable nutrients.
D. Death of fish due to lack of oxygen.

Answer 1

Hint: When sewage enters a lake or stream, microorganisms start to decay the organic materials. Oxygen is consumed as micro-organisms use it in their metabolism.

Complete Answer:
To answer this question, first, we need to know about the domestic sewage. Domestic sewage is the main source of pathogens and putrescible organic substances. Because pathogens are eliminated in feces, all sewage from urban areas and cities is likely to contain various types of pathogens, potentially presenting a direct threat to public health. Putrescible organic matter presents a different sort of threat to water quality. As organics are decomposed naturally in the sewage by bacteria and other microorganisms, the dissolved oxygen content of the water is depleted.

Now, let us find the solution from the option.
- Sewage consists of nitrates and phosphates elements, if an excessive amount of these two elements is present in water, it can encourage the growth of algae, sometimes causing unusually dense and rapid growths known as algal blooms.
- Large algal blooms, however, deplete the solubilized oxygen in the water, leading to dead zones in aquatic systems.
- The solids in sewage cause the water to appear dark and murky, which also affects the ability of fish to breathe and see around them.
- A river consisting of domestic sewage rich in organic waste will decrease the DO (dissolved oxygen). The organic waste will raise biological oxygen demand of the river thus depleting the O2 content and may result in death of fish due to lack of oxygen.

Thus, the correct option is D i.e. Death of fish due to lack of oxygen.

Note: Domestic sewage contains various kinds of solubilized and insoluble impurities. It amounts to a very tiny ratio of the sewage by mass, but it is large by volume and consists of impurities like organic particles and plant nutrients that are likely to decay. The major organic materials are foodstuffs and vegetable wastes. Plant nutrients come from washing powders and chemical soaps etc. There are many substances that we utilize for maintaining our home clean add on to water pollution due to the fact that they consist of harmful materials or elements.