Mud balls observed after backwashing in a sand filtration system indicate which condition? (Variant wording)

• A. Filter media fouling
• B. Filter backwash rate is too low
• C. Inadequate chemical coagulation
• D. Sediment bypass

Answer: (B)

Backwashing is meant to lift and suspend solids and re-expand the bed so that accumulated fines are flushed to waste. Mud balls after backwashing show the wash energy isn’t high enough to fully mobilize and remove the settled fines. When the bed isn’t adequately scour ed, fine particles such as silt and clay can clump together into ball-like masses that resist removal and fall back into the bed. So the issue is insufficient backwash rate to properly fluidize and cleanse the filter. This differs from media fouling (which is about clogging and higher headloss over time), inadequate coagulation (which affects how well particles coagulate before filtration), or sediment bypass (which would let untreated sediment flow around the filter, not form mud balls during backwash).