Water Treatment MCQ with answer and solution

Concept

Slow sand filters remove bacteria very well through biological action in the schmutzdecke. Rapid sand filters, used with coagulation, are better at removing colour and turbidity.

Step-by-step solution

1. A slow sand filter has a biological layer (schmutzdecke) on top of the sand. 2. This layer removes about 98-99% of bacteria, which is higher than a rapid sand filter. 3. Slow sand filters cannot treat water with high turbidity or colour, and they are used without coagulation. 4. Rapid sand filters, used with coagulation, remove colour and turbidity better. 5. So slow sand filters are better for bacteria but poorer for colour and turbidity. Answer is A.

Explanation

Slow sand filters work mainly by biological action, so they kill and remove bacteria very well. They are less efficient in removing colour and turbidity, while rapid sand filters with coagulation do this better.

Why the other options are wrong

B: Slow sand filters are not better at removing colour and turbidity. C: Slow sand filters are better, not worse, at removing bacteria. D: They are more efficient in bacteria removal, so 'less efficient in both' is wrong.

Shortcut

Slow sand = bacteria. Rapid sand (with coagulation) = colour and turbidity.

Common mistake

Students think a slow, fine filter must be better in every way. It is not good for colour or high turbidity.