Question

A rapid sand filter comprising a number of filter beds is required to produce 99 MLD of potable water. Consider water loss during backwashing as 5%, rate of filtration as $6m/h$, and length to width ratio of filter bed as $1.35$. The width of each filter bed is to be kept equal to $5.2m$. One additional filter bed is to be provided to take care of break-down, repair, and maintenance. The total number of filter beds required will be

• A. 19
• B. 22
• C. 21
• D. 20

Answer 1

The correct option is C 21

Flow rate = 99 MLD
Rate of filtration = 6m/h
Water lost in backwashing = 5%

Total water to be produced per day
$=99\times {10}^3{m}^3$

Hence, water to be filtered per day
$=\frac{99\times {10}^3}{0.95}$

$=104210.526$

Water to be filled per hour

$=\frac{104210.526}{24}$

$=\frac{4342.105}{6}=4342.105$

$=723.684m^2$

L:B = 1.35 (given) and B = 5.2

$\therefore$ Surface area of one filter bed

= $1.35B^2$
$=1.35\times (5.2{)}^2=36.504m^2$

$\therefore$No. of filter bed required

=$\frac{723.684}{30.504}+1$

$=20.825\simeq 21$