There are so many ways and myths floating about regarding purging clams, mussels, etc. Some swear by cornmeal, others use oatmeal and some use nothing. Common sense will tell you what to do.
Does cornmeal or oatmeal grow in the ocean? Of course not.
A simple soaking in very salty water for about 30 minutes is fine. Clams live burrowed in the sand. Mussels cling (hence the beard) to rocks and eat what is floating by. They don't get as sandy as clams. But if you want, a short soaking in salty water for a short time is fine. It certainly won't hurt them.
You never want to soak any seafood in fresh water. Crazy as it may sound, they will drown and die on you. They cannot breath in fresh water. And your fresh water from the tap has chlorine in it. A chemical that is not conducive to happy living for small live sea animals.
If you are going to use wine or some other alcohol product in you final dish, make sure you cook off the alcohol before you add the seafood to the dish. Alcohol often has a way of making the little critters become tough.
When you are removing the meat from the shells, do so over a clean bowl so you can catch every drop of liquor. Before you add that delicious juice to your final dish, strain it through a really fine mesh strainer with a cheesecloth placed inside. It will catch the last bit of sand. Place the meat from the shells in that liquor also so you can give them a final rinse.
The water that you steamed the clams or mussels in also has great flavor. Do no waste it. That too should be strained before adding it to your final recipe.
When you buy seashells by the bag, there should be a tag on it stating where and when they were dug. Save this tag in the event all of them were dead by the time you got them home or you became sick. The information will be very helpful to the authorities.
Make sure you only make your purchases from a reputable vendor. If you see someone selling the at the side of the road from the back of his truck, I can almost guarantee that person is a poacher and does not have a license for clam digging. And more than likely, the clams are not sitting on a bed of ice.
Here hoping you enjoy your meal of seafood and are happy as a clam.