hiya chez. yes I had same idea previously, but if you measure the waste it is far higher than your input is to start with, as got higher concentration of dirt in.
as a thought tho, if you can be bothered to go to hassle, and depending if you have tank in house roof, it could be pumped into storage tank and possibly reused for tap/flushing toilet/bath water if you don't want to waste it?!
that or have lovely green grass Ilib the summer!