Water treatment sequence – where does sedimentation occur? In a conventional water-treatment train (without direct filtration), sedimentation is carried out at which stage relative to filtration?

Introduction / Context:
Conventional surface-water treatment typically includes coagulation, flocculation, sedimentation, filtration, and disinfection. Each stage has a defined purpose in removing specific classes of impurities and reducing load on subsequent units.

Given Data / Assumptions:

• Conventional plant layout (not direct filtration or package units).
• Coagulant aids forming settleable floc before filtration.
• Gravity settling basins used.

Concept / Approach:
Sedimentation removes most of the flocculated suspended solids, thereby protecting filters from excessive head loss and breakthrough. This step should therefore precede filtration to ensure filters polish the clarified water, removing fine particulates that escaped settling.

Step-by-Step Solution:
Sequence: Rapid mix → Flocculation → Sedimentation → Filtration → Disinfection.Thus, sedimentation occurs before filtration.

Verification / Alternative check:
Process flow diagrams in standard texts confirm that clarifiers are upstream of filters in conventional treatment.

Why Other Options Are Wrong:

• After filtration or simultaneously is characteristic of other designs (e.g., direct filtration or solids-contact units), not the conventional sequence asked here.
• Chlorination is a disinfection step and not paired-inseparably with sedimentation.

Common Pitfalls:
Confusing conventional plants with direct filtration systems that omit sedimentation when raw turbidity is low.

Final Answer:
Before filtration (after coagulation–flocculation)