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GATE 2026 ES – Question 45

Water and Wastewater Treatment and Management · Water Treatment · 2 marks · Multiple select

Which statements about coagulation–flocculation are true? Select all that apply.

  1. Brownian collisions produce orthokinetic flocculation.
  2. External mixing of the bulk medium produces only perikinetic flocculation.
  3. The Stern layer is more compact than the diffuse layer in an electrical double layer.
  4. Zeta potential is also called Stern potential.

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Correct answer: (C) The Stern layer is more compact than the diffuse layer in an electrical double layer.

Explanation

Brownian-motion collisions are perikinetic; velocity-gradient collisions caused by mixing are orthokinetic. Thus A and B interchange the terms. The Stern layer is the compact near-surface region, so C is true. Zeta potential is evaluated at the slipping plane and is generally different from the Stern-plane potential, so D is false. Select **C**.

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