Ideas needed for "outdoor" kitty litter

We've just completed our wonderful new outdoor kitty enclosure, a 10' x 20' fully enclosed "cage" to keep my 3 little darlings in, and the coyotes out. They love it so!! There is not enough space for them to do their poopin just anywhere, so we put in a 3'x 5' sand-box, about 15" deep. Even though I clean the poops out, I'm worried that since the sand gets wet, and urine and a little poop remain, it might become a damp breeding ground for unsavory microorganisms. What could I add to the sand to keep it sanitary without harming the kitties or environment? Any ideas?

Thanks! -Miakoda and Dusty, Cairo and Tundra, the 3 happiest cats on Earth.