I've just been looking at grey water stuff because about to start a bathroom reno that will include moving waste pipes under the house. Timber floor and a subfloor space, so simple stuff. Thinking of all of the relatively clean water going down the drain, I'm thinking maybe a diverter valve that sends the water to a small box with a pump and float switch. When the box fills up, the pump switches on and puts the water into the back lawn. Unfortunately we slope down from the back, otherwise I wouldn't even need a pump.