Process sequence for a rapid gravity filter in water treatment Which type of influent is appropriate for a rapid gravity filter in a conventional treatment train?

Introduction / Context:Rapid gravity filters are the polishing step in conventional water treatment. They remove remaining fine flocs and reduce turbidity to potable levels after prior clarification steps.

Given Data / Assumptions:

• The plant follows a conventional train: screening → coagulation–flocculation → sedimentation → rapid gravity filtration → disinfection.
• Filter media typically includes sand (and sometimes anthracite) with graded support gravel.

Concept / Approach:Filters are not designed to handle high raw-water turbidity without upstream clarification. Coagulants destabilize colloids; flocculated particles settle in sedimentation basins. Filters then remove residual fines, achieving low turbidity and pathogen reduction (as a physical barrier) before final disinfection.

Step-by-Step Solution:

Verification / Alternative check:Headloss profiles and filter run times deteriorate sharply if raw water is fed directly to rapid filters, underscoring the need for coagulation/sedimentation first.

Why Other Options Are Wrong:

• (a) Raw water overwhelms filters and reduces efficiency.
• (b) Disinfection before filtration is atypical and ineffective for turbidity control.
• (e) Backwash water must be clarified (or wasted) before any reuse; direct feed is improper.

Common Pitfalls:Assuming filters can replace clarification; in conventional systems they are complementary, not substitutes.

Final Answer:Water that has passed through coagulation–flocculation and sedimentation (clarified water)