Backwashing should expand the filter bed by ________%.

Backwashing should expand the filter bed by ________%.

• A. 30 to 50
• B. 5 to 15
• C. 60 to 70
• D. 15 to 25

Answer: (A)

Backwashing works by reversing the flow to lift and loosen the media and the accumulated cake so solids can be carried away. The bed should expand enough to break up the settled layer and reopen voids for effective cleaning, but not so much that media is washed out. A target expansion of about 30 to 50 percent provides that balance: it’s large enough to detach trapped material and reconfigure the media, yet moderate enough to avoid excessive media loss or instability. Expanding less than this (such as 5 to 15 percent) won’t fully disrupt the cake, leaving more solids behind; expanding far beyond (60 to 70 percent) increases the risk of media washout and inefficient backwashing.

Backwashing works by reversing the flow to lift and loosen the media and the accumulated cake so solids can be carried away. The bed should expand enough to break up the settled layer and reopen voids for effective cleaning, but not so much that media is washed out. A target expansion of about 30 to 50 percent provides that balance: it’s large enough to detach trapped material and reconfigure the media, yet moderate enough to avoid excessive media loss or instability. Expanding less than this (such as 5 to 15 percent) won’t fully disrupt the cake, leaving more solids behind; expanding far beyond (60 to 70 percent) increases the risk of media washout and inefficient backwashing.