Question

The grown algae are useful in the operation of

• A. oxidation pond.
• B. slow sand filter.
• C. cyclone separator.
• D. septic tank.

Hint:

Think about which treatment unit is shallow, sunlit, and depends on a natural relationship between algae and bacteria to stay aerobic.

Answer 1

The Correct Option is A

This question checks if you know where algae is a helpful, working part of the treatment process rather than an unwanted growth.

1. Oxidation pond: Correct. An oxidation pond is a shallow, sunlit basin where algae photosynthesize and release oxygen. Bacteria use that oxygen to break down organic waste, and in turn produce $CO_2$ and nutrients that feed the algae. Algae here is a working partner in the treatment, not a problem.
2. Slow sand filter: Wrong. This unit works through the schmutzdecke, a thin biological film of bacteria and microorganisms on the sand surface, that strains and adsorbs impurities. Excess algae growth on the sand bed is unwanted because it blocks pores and reduces the filter run length.
3. Cyclone separator: Wrong. This is a purely mechanical unit that spins water or air to throw out heavier grit particles by centrifugal force. There is no biological or algal activity involved at all.
4. Septic tank: Wrong. It is a buried, sunlight free tank that decomposes sewage anaerobically, without oxygen. Since no light reaches inside, algae cannot grow there.

The only unit whose working principle actually depends on grown algae producing oxygen for bacteria is the oxidation pond, so option (A) is correct.

Let's summarize:

• Oxidation ponds use natural sunlight and algae-bacteria symbiosis for treatment.
• Algae is unwanted in filters, mechanical separators, and anaerobic tanks.

So the grown algae are useful in the operation of an oxidation pond.