Environmental Engineering
Cleaning is done by (i) Scraping and removal in filters slow sand (ii) Back washing in slow sand filters (iii) Scraping and removal in filters rapid sand (iv) Back washing in rapid sand filters The correct answer is

(ii) and (iii)
(i) and (ii)
(ii) and (iv)
(i) and (iv)

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Environmental Engineering
Sludge volume index is defined as the ratio of

Percentage of sludge by volume to percentage of suspended solids by weight
Percentage of suspended solids by weight to percentage of sludge by volume
Percentage of sludge by volume to percentage of total solids by weight
Percentage of total solids by weight to percentage of sludge by volume

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Environmental Engineering
The major disadvantage of lime soda process of water softening is that

The effluent cannot be reduced to zero hardness
It is unsuitable for turbid and acidic water
Huge amount of precipitate is formed which creates a disposal problem
It is unsuitable for softening the water of excessive hardness

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