Your waste water will have far higer TDS readings, every succesive litre of re-purified waste water will become further contanimated. And you would need to power a pump to push it through your membranes. Think of all the water Butts you will need.
Money for a pump + running costs.
Money for water butt or butts rather, as you will need somewhere to store the waste water from the waste water you are trying to purify
What effect is this going to have on the life span of your membranes?
An awful lot of faffing around to try and cut possibly £20 or £30 quid of your water bill.
I don't think it is worth doing.
Ian