Q1: For a flow of 5.7 MLD (million litres per day) and a detention time of 2 hours, the surface area of a rectangular sedimentation tank to remove all particles having settling velocity of 0.33 mm/s is

ANS:C - 200 m²

Settling velocity = .00033 m/sec.

Settlement within detention period = .00033 *2 hrs = .00033*2*60*60 = 2.376 m.
Flow = 5.7 MLD = 5.7 * 10^6 LD = 5.7*10^3 M3 per Day = (5.7*10^3)/24 m3 per hour = 237.5 m3 per hour.
Flow within detention period = 237.5 *2 m3 = 475 m3.
Area = Flow/settlement = 475/2.376 = 199.91 m2 = 200 m2.