There's probably tiny ( pea size and under) snails in your municipality's gray water output. Tie a chunk of raw meat to a rock, put the rock in a coffee can with a 'coin slot ' cut into the plastic lid. Tie a 10 foot string to the can, toss it in the water and check on it in a day or two. I get bloodworms, snails, all sorts of stuff.
Be aware though that any wild-caught lifeforms may introduce various nasties into your system...parasites etc. I've never had any trouble, but conventional wisdom does say that WC stuff is always risky