Drinking-water clarification: which filter type is commonly used for the clarification and polishing of potable water in municipal treatment?

Introduction / Context:
Municipal water treatment relies on robust, low-maintenance unit operations for large throughputs. After coagulation, flocculation, and sedimentation, filtration is used to remove remaining turbidity and pathogens. The bed-type filters used here are different from pressure-driven cake filters used in the chemical industry.

Given Data / Assumptions:

• Application: potable (drinking) water.
• Goal: clarification/polishing, not cake formation.
• Steady large flow, continuous service, backwashable media.

Concept / Approach:
Gravity sand (or dual-media sand-anthracite) filters pass water downward through a granular bed under near-atmospheric head. Particles are captured within the depth of the bed (depth filtration). The units are periodically backwashed to restore headloss. Plate-and-frame, vacuum leaf, and rotary vacuum filters are cake filters intended for slurries, not for bulk water treatment.

Step-by-Step Solution:
Identify process need: municipal clarification, continuous, sanitary.Match to technology: gravity sand filters or rapid gravity filters.Exclude industrial cake filters not suited to potable water polishing.

Verification / Alternative check:
Standard water works designs specify rapid gravity sand filters following sedimentation; media are selected for effective size and uniformity coefficient to meet turbidity targets.

Why Other Options Are Wrong:
Plate and frame / vacuum leaf / rotary vacuum: designed for slurries, generate cakes, and are impractical for ultra-large municipal flows.

Common Pitfalls:
Confusing “filtration” generically with cake filtration; potable water plants mostly use granular media filters operating by gravity or pressure, not vacuum cakes.

Final Answer:
gravity sand