For removing trace quantities of precipitate from a large volume of water, which filter is most suitable as a polishing step?

Introduction / Context:
In water treatment, polishing steps remove small residual solids at high flow rates and low cost. The selection must balance filtration area, backwashing capability, and operating simplicity.

Given Data / Assumptions:

• Very small amount of precipitate remains.
• Large water throughput is required.
• Economical, continuous operation is preferred.

Concept / Approach:
Rapid sand filters provide deep-bed filtration with periodic backwashing to restore headloss. They are ideal for clarifying large flows with low solids loading. Pressure or vacuum leaf/drum filters are suited for higher solids slurries and cake recovery; plate and frame presses are batch, labour-intensive, and designed for higher solids capture, not trace polishing of municipal-scale flows.

Step-by-Step Solution:
Identify service: polishing of large volumes at low solids.Select deep-bed granular filter: rapid sand filter.Reject batch/cake filters as ill-suited to high-volume low-solid duties.

Verification / Alternative check:
Conventional water treatment trains routinely use rapid sand filters after clarification.

Why Other Options Are Wrong:
Plate and frame / rotary vacuum / shell and leaf: used for slurries with significant solids and cake recovery, not polishing huge clean-water flows.

Common Pitfalls:
Choosing high-solids equipment for low-solids polishing tasks increases cost and complexity unnecessarily.

Final Answer:
Rapid sand (gravity) filter