If you “backwash” a slow sand filter, you will destroy it. Backwashing is for rapid sand filters. I posted a bit of info on this about a year ago. I did not mention why backwashing will ruin a slow sand filter. Here is why:
A slow sand filter works by allowing layers of living microscopic organisms to actually grow in the sand. As water flows down through the sand, these organisms form layers, the most dense being on the top. There are layers further down in the sand that never need to be disturbed when the filter is “cleaned”. Also, most slow sand filters use a layer of gravel at the bottom to prevent the sand from clogging up the drainage pipes. Backwashing (forcing water back up through the bottom of the filter) will mix the sand and gravel, and mix the biological layers. This will destroy the filters effectiveness and ruin it. Don’t backwash a slow sand filter.