I started rainwater harvesting because I hated the water waste plus were on a meter as well . I have a 1000l tank in back garden to store rainwater then pump to water butt in garage ,comes into water butt at 35 tds ish . I have been pumping through RO with booster pump and let waste back into water butt so the water is going round and round effectivley. The product is o.ootds and waste comes out at 35 .My theory is this: If my tapwater that I did use goes into RO at 160 ish and down drain then if my rainwater is going in at 35 ish i.e at least 4 times less TDS than tap then sending back round system via water butt isnt going to affect my membranes very much , its quite early days so far but I will keep you posted. I have noticed sediment pre-filter getting dirty quicker so have ordered a re-usable washable sediment pre-filter to get round that.
Others seem to be able to get rainwater at 00 tds but here in muck leeds it just isnt happening.