During coagulation–flocculation for drinking water or wastewater clarification, which chemical is commonly added as a coagulant to destabilize colloids and promote floc formation?

Introduction / Context:
Coagulation–flocculation is a crucial step for removing turbidity and natural organic matter. A coagulant neutralizes particle charges so that short-range attractive forces allow aggregation into settleable or filterable flocs. Identifying the typical coagulant helps operators and students understand jar testing and dosage optimization.

Given Data / Assumptions:

• Standard surface water treatment train with rapid mixing and slow flocculation.
• Goal is to reduce turbidity and remove colloids.
• Common coagulants include aluminum and iron salts.

Concept / Approach:
Aluminum sulphate (alum) is one of the world’s most widely used coagulants. In water, alum hydrolyzes to form aluminum hydroxide precipitates that enmesh particles and neutralize negative charges. Alternatives include ferric chloride or polyaluminum chloride; however, aluminum sulphate remains a canonical example, especially in introductory questions.

Step-by-Step Solution:

Verification / Alternative check:
Jar tests demonstrate turbidity removal with alum dose optimization; sludge contains amorphous Al(OH)3 with captured impurities.

Why Other Options Are Wrong:

• Aluminum oxide: Insoluble; not used as a coagulant.
• Calcium chloride: Adjusts ionic strength/hardness but not a standard coagulant.
• None of these: Incorrect because alum is the classic coagulant.

Common Pitfalls:
Confusing alum with polymeric coagulant aids; both can be used, but alum is the primary coagulant in many plants.

Final Answer:
Aluminum sulphate