[mike31154]

Depending on the quality (or lack thereof) of your source/tap water, you may be able to get by with a 4 stage RODI unit. I ordered a 4 stage from BWI in Ontario but they sent me a 5 stage unit designed to handle Chloramines. Mistake, who knows but I don't mind the extra stage even though it's not necessary since my water supplier uses chlorine, not chloramine so a single carbon block would have been sufficient. At some point I may replace the first carbon stage with another poly pre filter or just leave the housing empty for better flow to the membrane.

If you can get your hands on a TDS meter (handhelds are available cheap at WalMart or Can Tire these days) you can check your tap water TDS and get an idea of how elaborate to go with your RODI system. Might save a few dollars that way before ordering a 5 stage when a 4 stage may do the trick. My unit is a 75 gpd and I understand that the 75 gpd membranes provide a better rejection rate than the 100 gpd membranes. This translates to less load on your DI media and resulting savings in replacing that. DI media and RO membranes are the more costly replacement items in these systems, so any savings there are a good thing. I collect my waste water for use in the garden or around the house, so don't see the need for a water saving add on. I think that's simply a second 75 gpd membrane in series with the first. You'll need to have very good water pressure to run two membranes efficiently, so checking your water pressure with a guage is also a good move before making a purchase. You may need a booster pump to run a dual membrane system, which means additional cost and complexity.