A town is required to treat $4.2 \: m^3/min$ of raw water for daily domestic supply. Flocculating particles are to be produced by chemical coagulation. A column analysis indicated that an overflow rate of $0.2$ mm/s will produce satisfactory particles removal in a settling basin at a depth of $3.5$ m. The required surface area (in $m^2$) for settling is
- $210$
- $350$
- $1728$
- $21000$