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Question

# 8 rapid sand filters are used to treat 35 MLD of water with rate of filtration 4000 lit/hr/m2. If back washing rate is 4 times rate of filtration and duration of back wash is 15 minutes operated once in every 24 hrs, then the percentage of filtered water required to back wash the filter is

A
3.9 %
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B
4.21 %
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C
3.46 %
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D
4.566 %
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Solution

## The correct option is B 4.21 %The volume of water used in back wash = ROB × DOB × Area of each filter The volume of water filtered = ROF × DOF × Area of each filter % of water used in back wash = Volume of water in back washVolume of water filtered×100 = ROB × DOB × Area of each filter ROF × DOF × Area of each filter×100 =4×4000×15604000×(24−1560)×100=4.210%

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