In municipal water treatment, for which filter configuration is a high backwash (wash-water) velocity required to effectively expand and scour the media bed?

Introduction:
Backwashing is essential for restoring headloss and removing trapped solids from granular media filters. The hydraulics of backwash depend on the underdrain system. When standard strainers/nozzles are absent, higher wash-water velocities are generally needed to achieve uniform expansion and adequate scouring of the bed.

Given Data / Assumptions:

• Compare rapid gravity filters with and without strainers.
• Slow sand filters are normally cleaned by scraping, not by backwashing.
• Objective: identify the case requiring high backwash velocity.

Concept / Approach:
In a rapid gravity filter, the underdrain distributes wash water. With nozzles/strainers, distribution is uniform, so design backwash rates (e.g., 40–60 cm/min at 20 °C, adjusted for temperature and media) suffice to fluidize the bed. Without dedicated strainers, distribution becomes less uniform; to achieve the same degree of bed expansion and lift heavier agglomerates, the gross velocity must be increased to compensate for maldistribution and local short-circuiting.

Step-by-Step Solution:
Identify which systems are routinely backwashed: rapid gravity filters yes; slow sand filters no (scraping is used). Recognize the underdrain role: strainers help uniform flow and lower the required gross velocity. Conclude that absence of strainers requires higher wash-water velocity to achieve target expansion.

Verification / Alternative check:
Design texts recommend adjusting backwash rates for distribution quality; poorer distribution (no nozzles) implies increasing the applied rate to reach similar bed expansion percentage.

Why Other Options Are Wrong:

• Rapid gravity filter with strainers: standard, requires lower design velocities due to uniform distribution.
• Slow sand with/without strainers: slow sand filters are not backwashed; they are refurbished by scraping the schmutzdecke and top sand.
• None of these: incorrect because the no-strainer rapid filter clearly demands higher velocity.

Common Pitfalls:

• Assuming all filters are cleaned identically; slow sand and rapid sand have different O&M methods.
• Ignoring distribution uniformity when setting backwash rates.

Final Answer:
Rapid gravity filter without strainers in the underdrain.