1) collect driftwood
2) pressure wash (if you don't have pressure washer, scrub with stiff bristled brush, then hose off, then repeat)
3) boil, if too large to boil - take racks out of dishwasher and sanitize it (no soap)
4) now it's safe and clean. Just submerge in large tub. If water darkens, change every couple days. When it doesn't float any more - put in tank.
This always works for me.