I agree with Michael ...

I agree with Michael Chipps, Amirtharajah's does conclude that a sub fluidized backwash with air is the best way to backwash a filter. The difficulty is that the media will be highly fluidized and may backwash out of a pressure vessel (not sure if this is gravity filter or pressure vessel). Air scour prior to backwash will help, if you don't have it you may need to install it. Simply changing the media will not solve the problem Media that is backwashed without an air scour cycle does not change size or change shape, be definition "fluidization"  in water only means the filter grains don't touch during backwash. You need to make sure you getting full fluidization (use ~30% expansion as a guideline) and make sure your backwash is long enough. Pull grabs from the backwash waste and backwash until the waste effluent is  < 50 ntu (closer to 25 would be better) and this will tell you if your backwash is long enough.