Environmental Engineering
As compared to rapid sand filters, slow sand filters give (i) Slower filtration rate (ii) Higher filtration rate (iii) Lesser efficiency in removal of bacteria (iv) Higher efficiency in removal of bacteria The correct answer is

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

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Environmental Engineering
Scour valves are provided

At every summit of rising mains
At every depression and dead ends to drain out the waste water that may collect there
At street corners to control the flow of water
At the foot of rising main along the slope to prevent back running of water

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