Rex,
The five moving bed filters I've built out of blue plastic drums using mb3 media from WMT in Baton Rouge are self cleaning as the plastic media just like your sand is in constant motion. However as with any biofilter except for polygeysers, it's important to get rid of as much solids as possible first isn't it? That way you don't have heterotrophs out competing the autotrophs and creating anoxic conditions in you biofilter.
I built an upflow sand filter 6 inches in diameter identical to one sold by AES / Pentair complete with a clear pvc top section to keep tabs on the sand, but as if yet haven't used it. I was told about a few disasters that occurred with them which scared me off. I was also told they need constant attention and are really touchy.
You had no issues?
Here's the plastic media:
http://www.w-m-t.com/Products/WaterTek_MB3_Moving_Bed_Media.php
