Statement (i) is TRUE & Statement (ii) is FALSE
According to Hazen’s classic ideal sedimentation theory, the removal efficiency (
) of a
discrete particle is given by the ratio of its settling velocity (Vs) to the Surface Overflow Rate
(SOR) Vo:
The Surface Overflow Rate is calculated as the discharge (Q) divided by the surface area (As
= LB) of the tank:
Substituting (Vo) into the efficiency equation shows that for a given discharge, settling
efficiency depends strictly on the plan/surface area (L B) and is independent of the depth (D)
of the tank. While increasing the depth changes the detention time, it simultaneously
increases the vertical distance the particle must fall to hit the floor, meaning the two depth
effects mathematically cancel each other out.
Statement (iii) is FALSE:
Increasing the surface overflow rate Vo increases the upward velocity of the water leaving the basin. Since a higher Vo makes it harder for slow-settling particles to reach the floor before escaping, it decreases the settling efficiency rather than increasing it. To maximize efficiency, the surface overflow rate needs to be reduced.
Substituting (Vo) into the efficiency equation shows that for a given discharge, settling efficiency depends strictly on the plan/surface area (L B) and is independent of the depth (D) of the tank. While increasing the depth changes the detention time, it simultaneously increases the vertical distance the particle must fall to hit the floor, meaning the two depth effects mathematically cancel each other out.
Statement (iii) is FALSE:
Increasing the surface overflow rate Vo increases the upward velocity of the water leaving the basin. Since a higher Vo makes it harder for slow-settling particles to reach the floor before escaping, it decreases the settling efficiency rather than increasing it. To maximize efficiency, the surface overflow rate needs to be reduced.
