How do drains work?

Down your indoor plumbing then out into the street plumbing then to either a water treatment plant then the sea or water treatment plant to a water recycling plant (like in singapore) or from the street straight out to the sea.

The used water goes through your buildings drain pipes into larger collection mains and ultimately to a sewage treatment facility, usually via gravity.

http://www.howstuffworks.com could explain it to ya its a great site on how things work.