Water Treatment – Process placement of coagulation–flocculation using alum In a conventional surface water treatment train, coagulation–flocculation with alum is carried out:

Introduction / Context:
Coagulation–flocculation is the essential pretreatment step to destabilize colloids and form settleable/ filterable flocs. Correct placement in the treatment train ensures that sedimentation and filtration units operate efficiently and that downstream disinfection is effective and economical.

Given Data / Assumptions:

• Surface water with turbidity and natural organic matter.
• Coagulant: alum (aluminum sulfate) with pH/alkalinity control.
• Conventional sequence: mixing → flocculation → sedimentation → filtration → disinfection.

Concept / Approach:

Alum is added prior to flocculation and sedimentation to remove turbidity and a portion of dissolved organics. Rapid sand filters should receive water with low residual turbidity to prevent rapid head loss and breakthrough. Disinfection (chlorination) is typically placed after filtration (post-chlorination) to maximize pathogen inactivation and minimize disinfectant demand.

Step-by-Step Solution:

Verification / Alternative check:

Plant P&IDs and textbooks consistently place coagulation–flocculation before filtration. Pre-chlorination may be used in special cases but is not paired “immediately with” coagulation–flocculation as an after-step.

Why Other Options Are Wrong:

(a) Misstates sequence; chlorination is typically after filtration. (b) Pre-chlorination has specific objectives but coagulation–flocculation does not follow it immediately as a rule. (c) Performing coagulation–flocculation after filtration defeats its purpose.

Common Pitfalls:

Confusing pre-chlorination practices with standard post-chlorination, or overlooking alkalinity needs for alum dosing which affects coagulation efficiency.

Final Answer:

Before rapid sand filtration