Slow sand filters are cleaned by scraping the top ____ off the top of the sand filter.

• A. 1 In
• B. 2 In
• C. 3 In
• D. 4 In

Answer: (A)

In slow sand filtration, the surface layer becomes clogged with organic matter and a biological film, so cleaning is done by removing a thin portion of that top layer to restore flow and performance. Scraping off about 1 inch of the top sand takes away the accumulated solids and biofilm—the part most responsible for headloss—while leaving the deeper, still-filtering sand in place. This balance keeps filtration effective without wasting media. Removing more than 1 inch would waste media and reduce overall capacity, while removing less wouldn’t adequately reduce clogging. This method is specific to slow sand filters and differs from the backwashing used in rapid filters.