I have 4 and rotate them. The dirty one goes in a thermos filled with hot water and about 1/4 cup of bleach. I leave it there for a few days (until the one that replaced it is dirty) and then pull it out and spray it down with hot water. It comes out white as can be every time.
i set up a 5 gallon bucket with ro/di water and 1/2 gallon bleach it always works for me. soak it over night at least. then blast it off with a high pressure hose. i do this for my lifeguard micron filters.