Here's how I've been doing it for years:
How to Clean Dry Sand
(without clogging the plumbing)
The method I use is as follows. It is very simple, wastes very little sand, and won’t clog the plumbing.
I clean sand in the utility sink, but any sink large enough to hold a 5g bucket will work. Cut a 6” piece of PVC to fit in the drain to act as a standpipe. You may have to sand a taper into the end of the PVC to get it to fit snugly. Put a 5g bucket in the sink, with standpipe in place. Pour about #10 of sand into the bucket and start filling it up with tap water. The faster, the better. While filling, stir the sand with another piece of PVC (1 ½ “ works well for me). Allow the bucket to overflow into the sink, carrying all the silt and whatnot into the sink, where the heavier particles (sand) will settle out before the wastewater, laden with silt, tops the standpipe and goes down the drain. Continue running the water until the bucket runs clear. . Now drain the bucket of as much water as you can, empty the (now) clean sand into another container (or your tank), and repeat the process until all your sand has been thoroughly washed. When you have finished, there will be several inches of sand in the bottom of the sink. Scoop it up into the bucket, give it another good rinse and you will be just about done. The final thing I do is plug the drain, allow the sink to fill as far as it can, then let it all drain away to give the silt still remaining in the pipes a good chance to be flushed away.