A few notes here about backwashing a small slow sand filter. This mainly applies to DIY (Do It Yourself) filters, with one exception noted below in the next paragraph. Do not forcefully backwash your DIY slow sand filter. You may very well destroy it.
Now, that said; here is another note that I must include: There is a type of slow sand filter (biological sand filter) designed to be cleaned by a process very similar to “backwashing”, in fact, the designer refers to the method as backwashing. Know that the process is intended to be used on a filter with a carefully regulated water flow which is in no way similar to the “backwashing” process used in rapid sand filters. Do not confuse the issues here. This person has spent time designing a filter that is very different than a Do-it-yourself small slow sand filter. If you just hook up a hose to the output of your home built slow sand filter and force water through it to clean it, you will likely destroy its effectiveness.