Water Treatment MCQ with answer and solution

Concept

Surface overflow rate in sedimentation

Formula

SOR = Q / A = H / t_d = v_s

Step-by-step solution

1. Surface overflow rate (SOR = Q/A) is the hydraulic settling threshold of an ideal clarifier. 2. Any particle with settling velocity vs ≥ SOR is 100% captured, making (i) true. 3. Dividing plan area by volume gives A/V = A/(A·H) = 1/H (inverse depth), not overflow rate, making (ii) false. 4. Therefore, only statement (i) is true. Answer: A

Explanation

In sedimentation basin design, the surface overflow rate (SOR = Q/A) represents the hydraulic loading per unit plan area and corresponds to the settling velocity of the smallest particle intended to achieve 100% removal. Statement (ii) is false because dividing plan area by volume yields the reciprocal of water depth (1/H), not the overflow rate.

Why the other options are wrong

B) Statement (ii) is false as plan area divided by volume equals 1/depth. C) Statement (ii) is false, so both cannot be true. D) Statement (i) is a fundamental axiom in sedimentation theory.

Shortcut

SOR = Flow / Plan Area = Critical settling velocity for 100% removal.

Common mistake

Thinking plan area divided by volume equals overflow rate instead of reciprocal of depth.