all depends on what kinda sand you are using.. most of it can be, stay away from reusing silican sand, as after the first blast on hard surface, it breaks down to a level which is harmful if inhaled..
Make sure to strain and sift reused sand before blasting with it again
i've re-used sand before when blasting in the shop, easier pick-up, strainwhen pouring in to blaster, but only re-use once [maybe] twice. Gets very dusty the more you re-use. Use VERY GOOD mask.