Most widely used coagulant for clarifying turbid water: Which chemical coagulant is most commonly applied in conventional water treatment plants to destabilise suspended particles and enhance settling/filtration?

Introduction / Context:
Coagulation–flocculation is a core step for removing turbidity and natural organic matter in surface waters. Selecting the right coagulant affects effluent clarity, sludge production, and chemical costs.

Given Data / Assumptions:

• Treatment objective: reduce turbidity and improve filter performance.
• Common, commercially available inorganic coagulants are considered.

Concept / Approach:
Alum (aluminium sulphate) has long been the default coagulant in conventional plants owing to its effectiveness across a wide pH range, availability, and predictable sludge characteristics. Ferric salts are also effective and sometimes preferred for cold water or high organic loads, but globally alum remains the most widely used. Lime primarily adjusts pH/alkalinity and can aid softening, while sodium aluminate is typically used as an adjunct to boost aluminium content or alkalinity, not as the predominant coagulant by itself.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals commonly present alum dose–response curves and jar-test procedures as the standard baseline, with ferric salts as alternatives.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming one coagulant fits all raw waters; jar testing remains essential to optimise dose and pH.

Final Answer:
Alum (aluminium sulphate)