Purpose of chemical coagulation in drinking-water treatment What is the primary reason for adding coagulants (e.g., alum, PAC) in conventional water treatment plants?

Introduction / Context:
Chemical coagulation–flocculation is a core step of conventional water treatment. Coagulants destabilize colloids and fine suspended solids so that they can be agglomerated into larger flocs that settle or are filtered out.

Given Data / Assumptions:

• Standard potable-water plant with rapid mixing, flocculation, sedimentation, and filtration.
• Coagulants such as alum, ferric salts, or polyaluminium chloride are used.

Concept / Approach:
Colloidal particles carry surface charge and remain suspended. Coagulant species neutralize charges and promote bridging, creating larger flocs. This significantly increases the settling velocity and removability in clarifiers and filters.

Step-by-Step Solution:
Identify the functional outcome of coagulation → enhanced aggregation into floc.Link to process performance → increased settling rate and improved turbidity removal.Select the option that explicitly states this purpose.

Verification / Alternative check:
Jar tests demonstrate steep reductions in turbidity when coagulant dose and pH are optimized, confirming faster settling and better filterability.

Why Other Options Are Wrong:

• (a) Omits the mechanism of destabilization and floc formation.
• (c) Coagulation is not a disinfectant step; chlorination/ozonation handles pathogens.
• (e) Hardness removal requires softening (lime-soda, ion exchange), not simple coagulation.
• (d) Inapplicable since (b) is correct.

Common Pitfalls:
Expecting coagulation to remove dissolved species; overdosing coagulant causing residual aluminium or excessive sludge.

Final Answer:
To increase the rate of settlement by forming larger, settleable floc