Bread and stale bread: starch is the delicate part, because in water it swells and turns sticky. One grinding stage is enough, and nothing is left behind in the trap. Fresh bread turns doughy with water, stale bread crumbles: either way, better in small pieces, not a whole loaf. With that one precaution it becomes an ordinary scrap. If you produce a lot of it, it is worth checking your water utility’s rules.