Surface washers should be turned off ____ min before the end of the main backwash.

Surface washers should be turned off ____ min before the end of the main backwash.

• A. 3-5
• B. 7-10
• C. 4-7
• D. 1-2

Answer: (D)

During backwashing, surface washers help clean the top of the filter bed, but they should be turned off a short time before the end of the main backwash. Stopping them about one to two minutes before finish allows the bed to settle and recompact as the final portion of backwash removes the loosened material. If the surface wash stays on too long, it can keep the surface agitated, re-suspend fines, or erode the top layer, hindering proper bed consolidation before filtration resumes. Stopping too early would leave the surface inadequately cleaned, while stopping later would unnecessarily disrupt the final rinse.

During backwashing, surface washers help clean the top of the filter bed, but they should be turned off a short time before the end of the main backwash. Stopping them about one to two minutes before finish allows the bed to settle and recompact as the final portion of backwash removes the loosened material. If the surface wash stays on too long, it can keep the surface agitated, re-suspend fines, or erode the top layer, hindering proper bed consolidation before filtration resumes. Stopping too early would leave the surface inadequately cleaned, while stopping later would unnecessarily disrupt the final rinse.