Role of alum in water treatment In conventional water-treatment trains, alum (aluminium sulphate) is primarily used as a:

Introduction / Context:
Alum is one of the most widely used chemicals in water treatment. Understanding its function ensures proper jar testing, dosing control, and integration with subsequent sedimentation and filtration steps.

Given Data / Assumptions:

• Chemical is aluminium sulphate (“alum”).
• Applied at rapid mix stage before flocculation.

Concept / Approach:
As a coagulant, alum destabilizes colloids by neutralizing their surface charges, enabling particle agglomeration. Coagulation is followed by flocculation (gentle mixing) to grow settleable flocs. Alum is not a disinfectant or catalyst; and while it aids floc formation, the distinct term “flocculant” often refers to polymer aids.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals and jar test protocols classify alum under primary coagulants, often aided by polymers (flocculants) for improved settling.

Why Other Options Are Wrong:

• Flocculant: usually refers to polymer aids, not alum’s primary classification.
• Catalyst: not applicable.
• Disinfectant: chlorination/ozonation are used instead.
• Scale inhibitor: unrelated to coagulation.

Common Pitfalls:
Using the terms coagulant and flocculant interchangeably; in practice they are distinct steps/chemicals.

Final Answer:
Coagulant