Mussels and oysters live on the ocean bottom and tend to accumulate less sand than clams. Both hard shell clams and soft shell clams live buried in the substrate, but soft shell clams cannot close their shells tightly, therefore, are more susceptible to sand accumulation. When placed in clean sea water, any of these shellfish will purge themselves of sand.
Place shellfish in a container of cool sea water or a solution of 1/3 cup salt to 1 gallon cool tap water. The water should adequetly cover the shellfish. Leave for 30 minutes and then repeat 2 or 3 times, changing the water each time. Instead, you may use a very large container, such as a wash tub, and leave overnight. Using a large amount of water, or changing a small amount frequently, is necessary to prevent shellfish from using up the oxygen in the water and then suffocating.
Note, by adding cornmeal to the water, shellfish will feed on the cornmeal and replace their stomach contents with the meal. This is desirable for those who do not like the appearance of the normal dark stomach contents of most shellfish.