If they're dried out, and ground up, you won't see rats. I live in the city as well. Rats in my neighbourhood is mostly from people not disposing of garbage (mostly the neighbourhood restaurants) properly. I toss my shells into a container (where they dry out) and every once in a while, I take my frustrations out on them in the pestle and mortar (again, you can use a coffee grinder, just don't use it for coffee after the fact). I've also been planting mint strategically in my back yard (in some small plastic cups... if you plant mint bare, be prepared to do a lot of weeding) on top of making judicious use of my raspberry bush so the rats have been discouraged from feeding in my yard.