Cleaning is done by

i) scraping and removal in slow sand filters

ii) back washing in slow sand filters

iii) scraping and r emoval in rapid sand filters

iv) back washing in rapid sand filters

The correct answer is

- 1
  
  (i) and (ii)
- 2
  
  (ii) and (iii)
- 3
  
  (i) and (iv)
- 4
  
  (ii) and (iv)

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