____ are an aggregate of solids accumulated in sand filter beds.

• A. Sludge
• B. Mud balls
• C. Biofilm
• D. Fines

Answer: (B)

In sand filtration, solids that are trapped in the bed can clump together to form cohesive lumps called mud balls. These aggregates—typically made from clay, silt, and organic matter bound by moisture and biofilm—collect within the sand layer as filtration proceeds. They reduce pore space and can hinder flow, contributing to clogging unless removed by backwashing or bed cleaning. The other terms don’t describe these lump-like accumulations: sludge is settled solids from clarifiers, biofilm is the microbial film on surfaces, and fines are the very small particles themselves rather than the bonded aggregates they can form.