Which of the following statement(s) is / are true for a Slow Sand Filter? i. Water entering the slow sand filter should not have been treated by coagulants ii. Coarsest gravel is used in the bottom most layer and the finest gravel is used in the topmost layer iii. Coarser the sand, purer will be obtained water as more impurities and bacterial will be removed
- AOnly (i)
- B(i) and (ii)Correct
- C(ii) and (iii)
- D(i), (ii) and (iii)
Solution
Concept
Slow sand filter features
Step-by-step solution
1. Coagulants would clog the filter; the water should be plain-sedimented only (i true). 2. The gravel support layer is graded from coarse at the bottom to fine at the top (ii true). 3. Finer sand removes more impurities, so (iii) is false.
Explanation
Only (i) and (ii) are correct.
Why the other options are wrong
Options with (iii) wrongly favour coarser sand.
Shortcut
Finer sand = better removal.
Correct answer: B) (i) and (ii)