I have a 8x12 run with 4" of sand and gravel. The bulk of sand was purchased from our local sand and gravel lot, which wasn't a very homogeneous. It's more like a sand/gravel mix, so sifting it really isn't an option since I would end up with more rocks than poo. I've added bagged sand and pea gravel along the way as well.
The run is covered, and I rake occasionally, and a thorough hose down if needed. Water and liquid droppings go away.
The problem, which I've been contemplating for a while now, is that while what will liquefy drains away nicely, the bio-solids from whatever didn't digest will remain. Hosing and agitating will float some of the solids to the surface. The sand is noticeably firm compared to last year.
What I plan on doing is to either do a partial replace (but what to do with the sand?) or create some sort of a sand treatment device designed to clean and separate debris. I've been thinking of using some deep trays with a slot cut out a couple inches from the base on one end, then agitate with a hose to raise and pour out debris. Maybe a series of trays would work better, cascading into each other to reduce sand loss. *shrugs*