Rapid sand filtration operations: insufficient washing of sand grains during backwashing most commonly leads to what operational problem inside the filter bed?

Introduction / Context:
Rapid sand filters remove turbidity after coagulation–flocculation. Effective backwashing is essential to detach trapped solids from sand grains and restore permeability. Poor washing causes localized fouling that degrades filter performance.

Given Data / Assumptions:

• Process: rapid sand filtration with periodic backwashing.
• Issue: insufficient washing of sand grains.
• Outcome sought: the most typical operational problem caused.

Concept / Approach:
When backwash intensity/duration is inadequate, sticky floc and fines remain on grains. Over cycles, these residues agglomerate into cohesive spheres called mud balls. They resist normal flow paths, create dead zones, and increase head loss while shortcutting flow around them.

Step-by-Step Solution:

Verification / Alternative check:
Plant inspections of poorly washed filters routinely reveal mud balls; corrective actions include stronger backwash rates, air scouring, surface wash, and proper backwash duration.

Why Other Options Are Wrong:

• Air binding: linked to dissolved gas release or negative pressure zones, not specifically to under-washing.
• Shrinkage of media: sand does not shrink under normal conditions.
• Expansion of filtering media: occurs during backwash, not as a fault caused by insufficient washing.
• Media attrition only: under-washing leaves solids; it does not primarily cause grain size reduction.

Common Pitfalls:
Confusing head loss causes; assuming all filter issues are due to air binding rather than solids accumulation and agglomeration.

Final Answer:
Mud balls