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 6.0 m/h and length to width ratio of filter bed as $1.35$. The width of each filter bed is to be kept equal to $5.2$ m. 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

1. $19$
2. $20$
3. $21$
4. $22$
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