Water treatment: the process used to remove very fine suspended and colloidal impurities (that do not settle by plain sedimentation) is called

Introduction:
Raw water frequently contains colloids and ultra-fine particles that remain stable and do not settle under gravity due to electrostatic repulsion and Brownian motion. This question targets the correct unit process to remove such fines.

Given Data / Assumptions:

• Particles are too small to settle quickly by gravity.
• No membrane filtration is implied.
• Conventional clarification trains are considered.

Concept / Approach:
Coagulation neutralizes surface charges using coagulants (e.g., alum, ferric salts, polymers), destabilizing colloids. Flocculation then gently mixes to form larger, settleable flocs. Disinfection targets pathogens, not turbidity. Softening removes hardness ions rather than colloids. Simple sedimentation alone is ineffective for stable colloids.

Step-by-Step Solution:
Add coagulant to neutralize zeta potential.Induce flocculation to aggregate microflocs into macroflocs.Clarify by settling or dissolved air flotation, followed by filtration.

Verification / Alternative check:
Turbidity and true color reductions after coagulation–flocculation–sedimentation are standard performance indicators in water treatment plants.

Why Other Options Are Wrong:

• Disinfection: microbial inactivation only.
• Softening: targets Ca2+/Mg2+, not colloids.
• Sedimentation only: insufficient for colloid removal without destabilization.

Common Pitfalls:
Overdosing coagulant can restabilize colloids; proper jar testing is essential to optimize dose and pH.

Final Answer:
Coagulation (often followed by flocculation)