Backwash and wash-water distribution are an essential design feature of which filtration unit in water-treatment plants?

Introduction / Context:
Filtration units require periodic cleaning to restore permeability and control headloss. The method of cleaning depends on the filter type. Understanding which unit needs engineered wash-water distribution helps avoid operational issues like media loss or incomplete cleaning.

Given Data / Assumptions:

• Slow sand filters are cleaned by scraping the top schmutzdecke.
• Rapid gravity filters are cleaned by hydraulic backwashing, sometimes with air scour.
• Sedimentation tanks do not use wash-water distribution systems.

Concept / Approach:
Rapid gravity filters employ an underdrain and wash-water distribution system to fluidize the media bed during backwash and to ensure uniform cleaning across the filter area. Designing proper wash-water velocities is key to effective media expansion and scouring.

Step-by-Step Solution:

Verification / Alternative check:
Standard texts and codes specify backwash rates (e.g., 15–20 L/s·m²) and air scour rates for rapid filters, confirming the need for dedicated distribution systems.

Why Other Options Are Wrong:

• Sedimentation tanks: use sludge removal mechanisms, not backwash systems.
• Slow sand filters: cleaned by manual/mechanical scraping, not hydraulic backwash.
• “All of the above” incorrectly generalizes the requirement.

Common Pitfalls:
Confusing rapid gravity filters with pressure filters; both need backwash distribution, but the classic exam context points to rapid gravity filters in open tank configuration.

Final Answer:
rapid gravity filter