I've never seen any sand in asparagus, so I've always just washed it under running water. Maybe
you should buy it elsewhere?
If that's not an option, here's what I do when I need to wash the sand off cilantro, parsley, etc. I place the cilantro in a bowl of warm water, and just soak it, dipping it down to the bottom once. The trick is NOT to wash it, just to soak it, and the sand falls to the bottom. Then, I remove the cilantro, wash out the bowl, and repeat. If there's still sand, I repeat it once more, then wash it under cold, running water. Maybe this will work with your asparagus.