Hi Martin, I seem to recall this being discussed a while ago and i think the general opinion was that you'd use just as much resin as if you just ran all your water through just a di without a ro. The reason for this is that the tds going into your ro has to go somewhere.
If your tap tds was 200 and your ro gave you an output tds of 5, the waste water from the ro would have a tds of prob about 395, so then if you put that waste through a di vessel the di resin would have to remove all that tds from the waste. It works out that you may as well not bother with the ro, as the di resin is removing the same amount with as without the ro, if you understand what you i'm saying.
I think the short answer is that it doesnt save you anything, in fact it'll cost you a lot in resin.
The best idea is to have a good ro that removes the tds and doesnt have a large waste ratio, if you can get 1 to 1 waste to pure ratio or even better.
Tim
