Coagulation chemistry in potable water treatment: Among common chemicals used at the rapid-mix stage, which is the most widely applied coagulant for turbidity and color removal?

Introduction / Context:
Coagulation–flocculation is central to conventional water treatment, destabilizing colloids and promoting aggregation so that particles can be settled and filtered. Selecting an appropriate coagulant affects removal efficiency, sludge production, and cost. This question identifies the most widely used coagulant worldwide for municipal treatment plants.

Given Data / Assumptions:

• Goal: Remove turbidity, natural organic matter, and color.
• Coagulants considered: aluminium and iron salts, and alkalinity control agents.
• Operational context: rapid-mix followed by flocculation and settling/filtration.

Concept / Approach:
Alum (aluminium sulphate) hydrolyzes to form positively charged aluminum hydroxide species that neutralize negatively charged colloids and adsorb dissolved organics. It is economical, readily available, and effective across a wide pH range with proper alkalinity. Ferrous sulphate can be used but requires oxidation and pH control. Lime is primarily for softening and pH adjustment, not as a primary coagulant in conventional turbidity removal. Hydrazine is a reducing agent used in boilers, not a coagulant.

Step-by-Step Solution:
Identify chemicals that act as primary coagulants: alum and iron salts.Evaluate global usage and practicality: alum is most common.Select alum (aluminium sulphate).

Verification / Alternative check:
Industry surveys, procurement data, and design manuals routinely list alum as the default coagulant, with ferric chloride/sulphate as alternatives depending on source water characteristics.

Why Other Options Are Wrong:
Ferrous sulphate: Less common; process complexity is higher.Lime: Used for softening/alkalinity control; not a primary coagulant for turbidity.Hydrazine: Not a coagulant; safety concerns in potable treatment.

Common Pitfalls:

• Confusing coagulants (charge neutralization) with floc aids (polymer bridging) or conditioners.
• Overdosing alum without alkalinity, leading to low pH and carryover.

Final Answer:
alum (aluminium sulphate)