Objective Question & Answer

Statement 1: Correct. In a continuous flow type sedimentation tank, particles are removed

100% if their settling velocity (v_s) is greater than or equal to the surface loading rate (surface

overflow rate, (v_o).

Statement 2: Incorrect. In a continuous flow type sedimentation tank, water moves horizontally while particles settle downward with a resultant velocity (a combination of horizontal flow velocity and vertical settling velocity), meaning they do not move purely vertically down to the bottom.

Statement 3: Correct. When a coagulant (such as alum) is added to water containing natural or added alkalinity, it reacts with the alkalinity (e.g., calcium bicarbonate) to form an insoluble metal hydroxide precipitate (like aluminum hydroxide, Al(OH)₃.

Statement 4: Incorrect. Flocculation is the process of gentle/slow mixing (not vigorous mixing) to encourage destabilized particles to collide and aggregate into larger flocs. Vigorous mixing belongs to the rapid/flash mixing (coagulation) stage