Coagulation in water treatment:\r Which chemical is most commonly used as a primary coagulant in conventional surface water treatment plants?

Introduction / Context:
Coagulation is the first key step in conventional treatment to destabilize colloids and form settleable flocs. Selecting an appropriate coagulant is crucial for turbidity removal, natural organic matter reduction, and optimal filter performance.

Given Data / Assumptions:

• We are dealing with standard municipal plants using coagulation–flocculation–sedimentation–filtration.
• Common coagulants include alum, ferric salts, and polyaluminium chloride; chlorine and bleaching powder are disinfectants, not coagulants.

Concept / Approach:
Alum (aluminium sulphate) has historically been the most widely used primary coagulant due to availability, effectiveness over a broad pH range with alkalinity adjustment, and predictable sludge characteristics. Alternatives are used based on raw water chemistry and cost considerations.

Step-by-Step Reasoning:
Identify chemical function: coagulant vs. disinfectant vs. pH modifier.Chlorine/bleaching powder are oxidants/disinfectants; lime is primarily for softening/pH/alkalinity adjustment.Select alum as the standard coagulant.

Verification / Alternative check:
Design manuals list alum as the default choice, with ferric salts and PACl as principal alternatives. Jar tests verify dose and performance.

Why Other Options Are Wrong:

• Chlorine/bleaching powder: not coagulants; used for disinfection.
• Lime: coagulant aid/softening agent, not the primary coagulant for colloid destabilization in typical surface water treatment.
• Ferrous sulphate alone is less commonly the primary coagulant in modern municipal practice compared to alum or ferric chloride.

Common Pitfalls:

• Confusing coagulation (particle destabilization) with disinfection.
• Overlooking the need for alkalinity when dosing alum to maintain target pH.

Final Answer:
Alum (aluminium sulphate)