Process placement in conventional treatment: A rapid sand filter should be placed after which pretreatment steps in a conventional water-treatment train?

Introduction / Context:
Conventional treatment for surface waters typically follows the sequence: coagulation, flocculation, sedimentation, filtration, and disinfection. Rapid sand filters are designed to remove the fine turbidity remaining after clarification.

Given Data / Assumptions:

• Turbid surface water requiring full conventional treatment.
• Granular media filtration stage is rapid sand filtration.

Concept / Approach:
Coagulation and flocculation convert colloids into settleable flocs. Sedimentation removes most suspended solids. The rapid sand filter polishes the clarified water by removing remaining fine particles. Disinfection is a separate, final step; it is not “combined” inside the rapid sand filter unit.

Step-by-Step Solution:
Establish train: coagulation → flocculation → sedimentation → rapid sand filtration → disinfection.Therefore, rapid sand filtration should be preceded by coagulation and sedimentation.

Verification / Alternative check:
Design manuals specify maximum allowable turbidity to filters to achieve target effluent turbidities and acceptable filter runs, confirming the need for prior clarification.

Why Other Options Are Wrong:

• “Uses rapid sand as filter media”: wording is imprecise; media is graded silica sand (often with anthracite/GS support), not “rapid sand.”
• “Used after slow sand filtering”: slow sand is an alternative technology, not a pretreatment to rapid sand.
• “Can combine disinfection also”: disinfection is a separate process.

Common Pitfalls:

• Feeding high turbidity directly to filters causes short runs and breakthrough.

Final Answer:
should be preceeded by coagulation and sedimentation