I worked in the seafood industry for over 13 years. Farm raised or commercial clams, do not need to be purged, because they have basically gone through a purging process, when they were stored. We sold a ton of clam and never really had an issue with sand. However, if you go clamming and bring back a bunch of clams, they will definitely need to be purged. If you think about it, clams live on the sandy ocean floor(At least the wild ones do). So putting them in sea water, or salty water that is close to the same consistency as the ocean, the clams would naturally eject the sand, as they take in new water. You will want to change the water out, every 20-30 minutes and they should be good to cook, after 2 or 3 soaks.
As far as corn meal goes, I have heard several people tell me that corn meal speeds up the process. We recently went clamming and brought back our limit of cockles. We tried the corn meal, but no one ever really said how much to put in. We put in quite a bit(About a half a cup) into a 5 gallon bucket(Keep in mind the clams came to about 1/3 of the way from the bottom. And we filled the bucket with salt water, and let them sit over night. They still had sand in them. So it looks like, the key is changing the water a few times. I hope this helps