For removing very fine suspended solids and colloids from polluted water, which treatment step is most effective as the primary mechanism?

Introduction / Context:
Colloids are particles that remain stably dispersed due to surface charge and Brownian motion, resisting gravity-driven settling. Effective removal requires destabilizing the colloids so that larger flocs form and can be separated by clarification or filtration.

Given Data / Assumptions:

• Target: finely divided suspended solids and colloidal matter.
• Conventional treatment train: coagulation → flocculation → clarification/filtration.
• Mechanical mixing alone does not neutralize charge.

Concept / Approach:
Chemical coagulation (e.g., alum, ferric salts, PAC) neutralizes colloidal charges and promotes microfloc formation. Gentle mechanical flocculation then grows settleable flocs. Sedimentation without coagulation is ineffective for true colloids; clarifiers require prior destabilization to perform efficiently.

Step-by-Step Solution:

Verification / Alternative check:
Jar tests demonstrate dramatic turbidity reduction only when coagulants are dosed at appropriate pH/alkalinity; mechanical stirring without coagulant provides negligible removal.

Why Other Options Are Wrong:

Common Pitfalls:
Underdosing/overdosing coagulant; ignoring pH/alkalinity; insufficient flocculation time or gradients.

Final Answer:
Chemical coagulation (with subsequent flocculation/clarification)