Correct Answer - Option 1 : (i) and (iv)
Explanation:
(i) The cleaning of slow sand filters is not done by back washing as is done for rapid gravity filters, but is done by scrapping and removing the top 1.5 to 3 cm of top sand layer. The top surface is finally raked, roughened, cleaned and washed with good water.
(ii) The amount of wash water required is generally of the order of 0.2 to 0.6 percent of the total water filtered. Cleaning is repeated as often as necessary until the sand depth is reduced to about 40 cm or so.
(iii) After each cleaning, the filter is again used and raw water is admitted into it. But the effluents that will be obtained in the beginning will not be pure and are not used for about 24 to 36 hrs until formation of a film of arrested impurities around the sand grains (i.e. the formation of schmutzdecke) has taken place. Since the filtering action of slow sand filters depends largely upon the formation of this film, the effluents obtained in the beginning, when such a film is absents, shall not be pure.
(iv) The interval between the two successive cleanings depends mainly on the filtering sand used in the filter. This interval however, normally ranges between one to three months.
Backwashing Of rapid gravity filter:
(i) In rapid gravity filter, when sand becomes dirty, as is indicated by the excessive loss of head, the filter must be cleaned and washed. For cleaning, the raw supplies as well as the filtered supplies have to the cut off, bed drained down, and wash water sent back upward through the filter beds. This forced upward movement of wash water and compressed air will agitate the sand particles and thus removing the suspended impurities from it.
(ii) The rapid gravity filters get clogged very frequently and have to be washed every 24 to 48 hours.
