___ is the recommended rate of flow for proper filter backwash.

• A. 1-2 gpm per square foot
• B. 20-30 inch vertical rise per minute
• C. 50-60 inch per minute
• D. 5-10 inches per minute

Answer: (B)

Backwash flow rate is about how fast wash water moves upward through the filter to lift, expand, and flush out trapped solids. The practical gauge is the vertical rise of wash water in the filter per minute, which reflects the velocity through the media bed. A rate of about 20–30 inches per minute provides enough energy to properly expand and agitate the bed so fines are removed, without washing out the media or causing excessive head loss. If the rate is too low, solids won’t be carried away effectively; if it’s too high, media loss and damage to the bed can occur. Therefore, the recommended rate is 20–30 inches per minute.