I just wanna add this
you can do what some lfs stores do. get a water filtration company deliver you large amounts of di water . you just buy or rent a tank and pay for the water . the larger the tank the cheaper the cost. here there are several of them who do it..
you waste nothing ..
Thought about that, but then I have to deal with storage of a larger tank. I might give it a try to buy RODI from a local LFS for awhile. If nothing else it's a stop-gap until I get a new unit rigged up and ready to go.
I would just buy a RODI unit with a pre filter, carbon, two RO membranes and two DI units.
You can also drink your rejection water. It's basically tap that has been through a pre filter and carbon (both are NSF). I do this, also use it for the garden because it's hella better than the chloramine crap from the tap.
You probably will save money on DI alone that way, you might not see it that way making as little as you do, but DI is not cheap, and the 3 filters ahead of it will save some resin.
I was thinking of getting a 4-stage unit with sediment, carbon, RO, and DI, and then replacing the RO with a second DI. No, DI is not cheap, but the cost of running DI myself is much less per gallon than if I buy it.
Drinking the reject water is fine if you are careful about bacterial contamination. I'd be careful about that, personally. The odds of having problems might be low, but getting sick is no fun.
Not gonna happen, probably, since I will have more water than I would use to start with, and then there is the bacterial problem, as you wisely point out.
To the OP, if you are worried about being wasteful, imagine the resources used that go into making your DI resin you are burning through, as well as the resources used to deliver it to you. Unless I missed your point of not wanting to be wasteful, this seems like you are well ahead to dump the RO waste to the ditch. Not trying to lecture you on it, just pointing out the balance.
Point is not about saving money or wasting water. Point is about not plugging up the septic system at my 100+ year old house where there are no plumbing plans on file with the town so I have no idea about the size, location, number, or capacity of the pool(s) in my system. If I suddenly flood it with all this water and have to spend $20k to fix the problem, I'll be a little ****ed.
Thanks to all for replying. I really appreciate it. I was originally thinking of just repacking the API cylinder with fresh carbon and resin, but that seems to be a pain. After reading all your comments and doing a little research, I think I may just get a small unit and doctor the stages to do what I want. I'm hearing to stay away from AquaticLife and to go with BRS for a variety of reasons, mostly to do with the RO piece. I also got several PMs from salespeople pushing other products. Not sure which way I will go, but I'm sure like anything else you get what you pay for.