I saw recipies here where people would mix a tsp into a quart or half gallon of water and then shake it up and drink it through out the day.. or even a pinch into a cup. Super tiny doses spread out.
No clue about kidneys... i can't see either being bad for kidneys, but i really don't know. DE dehydrates some people, so you'd want to start small and increase fluids which should be good for kidneys.