I am presuming that you ...

I am presuming that you don't mean to actually backwash the filter but how to clean the sand removed from  the filter for reuse.

Slow sand filters are generally initially charged with sand o around 600 mm or more deep and after initial ripening a clogging layer develops which is biologically active to clean the water. As clogging develops it is necessary to remove this and this is done by scraping off the upper layer say 25 to 50 mm. Depth removed depends on the technology employed. After a short run to waste period the filter then goes back in with less sand.

After several cycles there is not enough sand left and The filter needs to be reconstructed with clean sand.

The removed sand can be cleaned up and that is what I think you mean. This is done generally in a washing plant that is basically what the sand would have been processed in when quarried with washer and classifier screens to get the right size cut and conveyors to return sand to  stockpile.