I have a semi-automated graywater system at my house.
Used laundry water goes through a sandfilter/marshplants, gravity fed into a big rubbermaid trough, and then gets pumped to the toilet with a well pump on a pressure switch.
When you flush the toilet, pressurized water starts pumping into it from the graywater tank.
It works great.
Read more at:
Ben's DIY Graywater System - EcoRenovator