Environmental health: coagulation: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Environmental health: coagulation

Sample item

After monsoon floods a town's river water becomes highly turbid. The plant uses rapid gravity filters, and turbidity in the filtered water is high. Which step must be added or optimised before filtration?

AAlum coagulation and flocculation followed by sedimentationBAeration of the raw waterCBoiling the raw water at the plantDA high dose of chlorine directly into the raw water

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Alum coagulation and flocculation followed by sedimentation

Rapid sand filters need the raw water to be coagulated first, usually with alum, so that fine colloidal particles form a floc that settles in sedimentation tanks. The filter then removes the remaining floc. Chlorine is less effective in turbid water because particles shield organisms and consume chlorine.

Option by option

A. Alum coagulation and flocculation followed by sedimentation (correct)
Correct. Coagulation and flocculation remove colloidal turbidity before rapid filtration.
B. Aeration of the raw water
Aeration removes gases and iron, not suspended turbidity.
C. Boiling the raw water at the plant
Boiling is impractical at municipal scale and does not remove particles.
D. A high dose of chlorine directly into the raw water
Chlorine alone cannot remove turbidity and is wasted on suspended matter.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is A, Alum coagulation and flocculation followed by sedimentation. Rapid sand filters need the raw water to be coagulated first, usually with alum, so that fine colloidal particles form a floc that settles in sedimentation tanks.

Why is "Aeration of the raw water" not the answer?

Aeration removes gases and iron, not suspended turbidity.

Why is "Boiling the raw water at the plant" not the answer?

Boiling is impractical at municipal scale and does not remove particles.

Why is "A high dose of chlorine directly into the raw water" not the answer?

Chlorine alone cannot remove turbidity and is wasted on suspended matter.

What concept does this question test?

It tests Environmental health: coagulation in PSM. The filter then removes the remaining floc. Chlorine is less effective in turbid water because particles shield organisms and consume chlorine.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Environmental health: coagulation?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.