Pick out the incorrect statement for a rapid sand filter: (i). the period of cleaning is usually 2 to 3 weeks (ii). the effective size of the sand particles is 0.35 to 0.6 mm (iii). the maximum permitted loss of head is 3m
- AOnly (i)Correct
- B(i) and (ii)
- C(ii) and (iii)
- D(i), (ii) and (iii)
Solution
Concept
Rapid sand filter is washed by backwashing every 1 to 3 days, not weeks.
Step-by-step solution
1. (i) Cleaning period is 24 to 72 hours, so (i) is incorrect. 2. (ii) Effective size 0.35 to 0.6 mm is correct. 3. (iii) Maximum loss of head is about 2.5 to 3 m, so it is correct. 4. Only (i) is incorrect.
Explanation
Rapid sand filters are backwashed every 1 to 3 days. Effective size of sand is 0.35 to 0.6 mm and terminal head loss is about 2.5 to 3 m. So only statement (i) is wrong, which is option A.
Why the other options are wrong
B is wrong because (ii) is correct. C is wrong because (ii) and (iii) are correct. D is wrong because (ii) and (iii) are correct.
Shortcut
Rapid filter: wash in days. Slow filter: clean in weeks.
Common mistake
Confusing slow sand filter (weeks) with rapid sand filter (days).
Correct answer: A) Only (i)