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
- $19$
- $20$
- $21$
- $22$