Closed water system

In the past, I've done some rambling on an off-grid house and today I was wondering if you could build a closed water system.  Most people pull city water and output waste to the sewer so you get charged twice for water used.  I think it would be pretty easy to build a filter for the dooker and the stuff in piss, but you would want to sterilize the water or you'd have cholera constantly.  Rather than use chemicals maybe you could use UV.  However, UV requires a lot of juice so you'd need to be able to generate a hefty amount of electricity from solar, wind and hydrogen cells.

The good news is you could hookup your gutters to the system to collect rain water.  Unless you live in the desert the rain water would compensate for watering the garden and other losses.

An interesting project would be to design a completely self-sufficient house without a septic system.