Below is very similar to my own process. Hope this helps a bit:
from
https://www.foodrepublic.com/2017/04/11/purge-clams/
The first thing to know is that most commercially available clams and mussels have already been filtered. If you’ve bought your clams in a supermarket, they are likely to be ready to go – just follow step 1 below for weeding out any bad ones and step 5 in case there are any closed dead ones hoarding a shellfull of sand. Trust me on step 5 – it sounds tedious but this is the most important step! If you have even one of these dead ones open in your pan while tossing, your entire dish will be ruined. If you’re sourcing them from a fishmonger and you’re not sure, just ask if they have already been purged.
If you need to purge the clams yourself, the best procedure is this:
1. Rinse clams quickly under water, weed out any with crushed shells or that are open and don’t move when touched or squeezed. Put the clams in a large non-reactive bowl (such as glass or ceramic).
2. Cover the clams in salt water by 3/4–1 1/4 inch. Actual seawater (filtered to remove any sand) is best, of course, but otherwise use sea salt (not regular table salt) and water to a salinity of about 3.5 percent – or 35 g (1 1/4 ounces) to every 1 liter (34 fluid ounces or 4 cups) of water. Fresh water will kill the clams. Try not to shock them to death by changing their temperature too rapidly so keep them somewhere relatively close to their current temperature. If they have been stored chilled (for example, at the fishmongers), then you can use cool water and keep them chilled in the fridge. Otherwise, set them somewhere like in a cool corner of the room.
3. Purge for at least 1 hour. I find this time sufficient for clams bought from the fishmonger. If you leave them for significantly longer than that, check on them from time to time and change the water so they don’t die from loss of oxygen. When you tap or agitate them, they should close (perhaps slowly, but they should eventually completely close). The last thing you want is to forget about them and come back to a bowl of dead clams.
4. Transfer the clams to a colander using your hands or a perforated spoon (don’t tip the water out directly into the colander as you’ll end up pouring any purged sand back over them).
5. You’ll see Italian fishmongers tapping or bouncing their clams on the counter to weed out any dead ones that look like they are closed. It’s incredibly important to do this (if you’ve got little ones running around, they might like to help). With a plastic chopping board underneath, tap or bounce the clams one by one. Live ones will stay tightly shut. If there is a dead one in there, it will open when you do this – and will likely be full of sand that you’ve just saved from getting into your sauce. Now they’re ready to cook.
