The best way to wash greens (escarole, broccoli rabe, collards, etc.) is to first trim the base and any coarse outer leaves. Cut them to the size that you’ll be using. Then float the cut pieces in a sink full of cold water. Stir it around and any sand will sink to the bottom. Drain them in a colander and they’re good to go.
It may seem like too much when you start to cook the greens but there’s a lot less after they wilt.