Water Treatment MCQ with answer and solution

Concept

Plain sedimentation removes discrete particles, so it is Type-1 settling. Type-2 settling is flocculent settling.

Formula

v_s = g(G - 1)d² / (18ν)

Step-by-step solution

1. Screening removes big floating matter. It comes first, before sedimentation, so (A) is correct. 2. Plain sedimentation settles discrete particles without chemicals. This is Type-1 settling, not Type-2. So (B) is wrong. 3. Coagulation forms flocs that settle faster and remove fine and colloidal particles. So (C) is correct. 4. Type-1 settling of discrete particles follows Stokes' law. So (D) is correct. 5. The incorrect statement is (B).

Explanation

In plain sedimentation, particles settle alone, without changing size or shape. This is discrete or Type-1 settling, which follows Stokes' law. Type-2 settling is flocculent settling, as in sedimentation aided with coagulation.

Why the other options are wrong

A: True, screening is the first unit in water treatment. C: True, coagulation removes fine and colloidal particles that plain sedimentation cannot remove. D: True, Stokes' law gives the settling velocity of discrete particles (Type-1).

Shortcut

Type-1 = discrete (plain sedimentation), Type-2 = flocculent (coagulation), Type-3 = zone/hindered, Type-4 = compression.

Common mistake

Mixing up Type-1 (discrete) and Type-2 (flocculent) settling.