I used to get spent hops from my (then) local microbrewery to add to the compost heap. I was initially a bit skeptical, but not after I saw the brewers vegetable garden. It was fantastic. He recommended rinsing them before adding to the compost heap. The sacks they came in were great for storing potatoes, onions etc as well.
What does 15kg of grain look like? A couple of buckets full?