Coagulants in water treatment: “Alum” used for coagulation during sedimentation and filtration is chemically which compound?

Introduction / Context:
Coagulation is a key step in conventional water treatment. The commonly used coagulant, known as “alum,” destabilizes colloids so they can aggregate into flocs and be removed by sedimentation and filtration.

Given Data / Assumptions:

• Alum refers to the water treatment coagulant used widely in municipal plants.
• No modifiers (e.g., polyaluminium chloride) are implied.

Concept / Approach:

In water treatment, “alum” is the commercial name for aluminium sulphate, generally Al2(SO4)3·nH2O. When added to water with alkalinity, it produces aluminium hydroxide floc and reduces turbidity and natural organic matter through adsorption and sweep flocculation.

Step-by-Step Solution:

Verification / Alternative check:

Standard water treatment texts consistently define alum as aluminium sulphate, not ferric/ferrous salts or copper sulphate.

Why Other Options Are Wrong:

• Copper sulphate: An algicide, not the primary coagulant “alum.”
• Ferrous/ferric sulphate: Iron-based coagulants (effective alternatives) but not alum.
• Magnesium sulphate: Not used as a primary coagulant.

Common Pitfalls:

• Confusing “alum” with “alumina” or with iron coagulants.
• Ignoring alkalinity needs for effective coagulation with alum.

Final Answer:

Aluminium sulphate.