Use of alum in conventional water treatment Aluminium sulphate, Al₂(SO₄)₃ (alum), is primarily used for which unit process?

Introduction / Context: Clarification of raw water requires aggregation of fine colloids that do not settle on their own. Coagulants like alum are central to producing potable water and protecting downstream filters from fouling.

Given Data / Assumptions:

• Chemical: Aluminium sulphate (alum), Al₂(SO₄)₃.
• Goal: Identify its main purpose in a conventional treatment train.

Concept / Approach: Alum hydrolyzes to form aluminium hydroxide floc and releases acidity, destabilizing negatively charged colloids. This promotes floc formation, which can then be removed by sedimentation and subsequent filtration. Alum is therefore associated with coagulation (and flocculation as a follow-on step).

Step-by-Step Solution:

Verification / Alternative check: Standard treatment trains comprise coagulation–flocculation–sedimentation–filtration–disinfection; alum belongs to the first step.

Why Other Options Are Wrong:

Common Pitfalls: Confusing the chemical addition step (coagulation) with the subsequent physical removal steps.

Final Answer: coagulation