Which of the following is a characteristic feature of a Rapid Sand Gravity Filter?
- AIt uses sand with an effective size of 0.2 to 0.3 mm
- BCleaning is done by backwashing with water and compressed airCorrect
- CIt requires a very large land area compared to a slow sand filter
- DIt is primarily used for removing bacteria through a 'Schmutzdecke' layer
Solution
Concept
Rapid sand filter operation and backwash
Formula
NONE
Step-by-step solution
1. Rapid sand gravity filters use coarse granular media (effective size 0.45 to 0.70 mm). 2. They operate at high filtration rates where impurities penetrate deep into the bed. 3. Washing is carried out by agitating the bed with compressed air and reverse water backwashing. 4. Hence, option B correctly states the characteristic operational feature. Answer: B
Explanation
Rapid sand gravity filters operate with coarse sand media (effective size 0.45 to 0.70 mm) at high filtration rates, forcing particulate accumulation throughout the bed depth. Cleaning is accomplished by scouring the sand with compressed air and backwashing with treated water under pressure to expand and rinse the media.
Why the other options are wrong
A) Effective size of 0.2 to 0.3 mm applies to slow sand filters. C) Rapid sand filters require roughly 1/20 to 1/30 the footprint of slow sand filters. D) Biological Schmutzdecke skin is the primary mechanism of slow sand filters.
Shortcut
Rapid sand filter = High rate + Backwash with water and air.
Common mistake
Attributing slow sand filter features (Schmutzdecke, fine sand, surface scraping) to rapid sand filters.
Correct answer: B) Cleaning is done by backwashing with water and compressed air