I agree; although, if you can find jetties and secluded areas where there is abundant aquatic life it would be better than beach sand. Stay away from nearbye streams and other areas with run-off. Fact of the matter is, the safest bet is purchasing dry sand from a reputable source.
**Ha, forgot to answer the question. To sterilize I would bleach heavily (Clorox) and dry it out completely, letting it sit for days in the sun. I'm not sure it's worth your time flaming as well when you could just purchase quality sand