When I went to a Chicago public school in the mid '80's, they used to 'recycle' food. And not in a good way, either. If someone dumped an uneaten portion of food in the garbage can, there was a lady there that scooped it out and put it in a plastic bag.
She told anyone who asked that she was taking the food home for her dog. But interestingly enough, the same entree would be served as a side dish the next day.
Ammonia is probably one of the 'good' things they let slip through. Ugh, I'm getting the shivers just thinking about this.