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I have my own personal data. I keep logbooks on every gallon of RO/DI water I produce. From my experience I can tell you what kills you is DI resin life if you make much water at all. What happens is lesser quality prefilters and carbons due a poor job of protecting the RO membrane from both particulates and chlorine thus shortening the life of the membrane and causing it to work harder ending up with lower quality RO product water. Higher RO TDS shortens the life of the DI dramatically. A very accurate rule of thumb tells us for every 2% you increase the efficiency of the RO membrane you effectively DOUBLE the life of the DI resin.
In my own case, I used a modified drinking water unit for about 10 years. It started out as a Watts Premier unit from Costco but being as I live almost across the street from Premier, I carried the new unit in still in the box and they swapped out the 25 GPD membrane for a 75 GPD membrane for $35. I added a vertical refillable DI and always used high quality fresh sealed DI resin from trusted sources and averaged 150 gallons per DI refill before I started to see a RO/DI TDS of 2-3 when I refilled the cartridge. My tap water TDS is 750-850 and I do have a water softener. After RO I was seeing a TDS of 13-15 which is pretty good.
I now own a reef quality RO/DI unit. (Spectrapure MaxCap). The tap water TDS is still the same but I now average a RO only TDS of 6.2 to 6.4 max. My DI now lasts in excess of 630 gallons per cartridge and thas changing it out when I first see a reading over 1.0 instead of 2-3. Recently Randy Holmes Farley posted his Spectrapure membrane lasted between 9 and 10 years before switching it out. I can say with confidence that is a direct result of using high quality replacement filters and maintaining the system as directed. My old membranes lasted 15 to 24 months using Premiers replacements and not much longer using slightly better filters. With the MaxCap and using their replacements I fully expect to see 5+ years at the very minimum before I need to replace the membrane and hopefully 8 to 10 years.
As I said I keep log books of every gallon showing TDS at tap, post carbon/pre RO, RO only, post MaxCap and final effluent. I also monitor incoming pressure and RO pressure, temperature and total gallonage so I have pretty good records. Time will tell.
I know Spectrapure has some spreadsheets showing operating costs of various units on the market but I don't know if they are available online or not.
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