In a sand filtration system, the particle size of sand affects the length of time of the filter run. The smaller the size of sand the ___ the run.

• A. longer
• B. no change
• C. shorter
• D. unpredictable

Answer: (C)

Smaller sand grains create a finer filter bed with smaller pore spaces, which increases the flow resistance. With higher resistance, the same amount of solids is trapped more quickly and the head loss across the bed rises faster. When head loss reaches the backwash threshold, the filter run ends. So, finer sand means the filter run is shorter.