I've boiled them in a turkey fryer, it worked nicely but was a pain in the ass and requires propane.
Last time I just took two 5 gallon buckets and drilled holes smaller than the pellets in the bottom. I then put those buckets into another non-drilled bucket, so that way they will still hold water. Fill the buckets about 3 in. from the brim with pellets, give them a quick prerinse without the non-drilled bucket to get some of the solid debris out. Then place preferated bucket in the non-drilled bucket and fill with enough water to cover the pellets then let sit for about 5-10 min. Lift buckets out and let drain, you will see how much nastiness has already been removed in your non-drilled bucket, discard that water. Repeat process, however this time pour small bottle of hydrogen peroxide in with it (about 50 cents from wal mart) and let sit over night.
After you let it drain completely the next day give them another quick rinse without the solid bucket underneath.
It gets them pretty damn clean, and have used them with no problems. It's pretty much impossible to all the old roots out, but I wouldn't worry about it, the h2o2 should have killed any problematic bacteria. The only shitty thing with this method is that if you use a ton of pellets then this might be time consuming, unless you use something bigger than a 5 gallon bucket...Goodluck 😁