You mentioned you replaced your filters, what filters exactly did you replace them with? What micron ratings, what brand? Did you go with a .5 micron carbon stage? What type of membrane did you install? How many DI stages do you have, one or two? What type of DI are you using? Your TDS after membrane/before DI is high. At 16-18, I'm not surprised that your DI resin did not last as long as it typically would/should have. What is the TDS of your incoming water, before the RO?
RO membranes are typically rated by rejection rate.. Genuine DOW Filmtec 75gpd membranes typically have a 98% rejection rate (keep in mind, 96% for them is still considered within spec). That means the membrane should be dropping 98% of the TDS of your incoming water.. IE: 500 tds coming in, 10 tds before it hits your DI. If you went with a crappy, but higher GPD, membrane, with say a 90% rejection rate, that same example, would be 500 tds coming in, 50 tds before it hits your DI. That means your DI is going to work 5x harder, and last 1/5 as long. The last 75gpd filmtec membrane I purchased happened to have a crappier 96% rejection rate. It went in the trash. I ordered a 98% guaranteed spectrapure tested membrane, and it came in defective, at only about 95% rejection rate. They mentioned they had a bad batch, and sent me a new membrane, rated at 99%. Has been perfect ever since. 400 tds coming in, 4-5 tds before my DI. My DI lasts a very long time, making 1000+ gallons a month. DI does have a shelf life, and depending on how old it is/the quality of it, it may not last as long as you'd expect. Color changing DI doesn't necessary mean it's exhausted when it loses its color, either. If you have a TDS meter to check the TDS before/after DI, you can save your money and not order the color changing kind. A TDS meter is the best way to check. At 1-2 tds, I'd still use it. I have two DI stages, and I monitor the pre/post TDS of each. As soon as the TDS after the first DI stage starts to climb up, I rotate the second DI into it's slot, and replace the second DI stage. This ensures I always have the freshest DI in the very last slot, giving me the cleanest water.
I only use .5 micron carbon filters.. they cost the most typically, but last time I checked, I believe they are rated to last the longest, rated at about 20k gallons of water, filtering out 1ppm of chlorine. I run two. Keep in mind, the 20k includes ALL water that passes through them, water that you keep after RO, and also the rejected water. Your carbon filter will take out the chloramines. You can add an additional GAC carbon stage to deal with chloramines, but after doing research and speaking directly to someone at spectrapure, this is not necessary.. Some people may disagree.. to each his own.
Making only 5-10 gallons at a time is going to put extra stress on your DI as well. You get TDS creep every time you turn your RO system on.. you'll see this when you first turn RO system on, your TDS will be in the two hundred TDS range or more, after the membrane.. and it will slowly drop down to where it needs to be. In the mean time, all that high TDS water passes through your DI stage(s) (unless you're sitting there, and manually diverting it away), and that chews up your DI resin very quickly. I'm lucky enough to have a large water change system, so I only make 50g at a time, sometimes 200g or more. A standard $10 bag of DI resin from BRS will usually last me about 1-2 months, making 1000+ gallons a month. I've used the MaxCAP / Silicabuster DI resin from Spectrapure as well ($$$) and found it works/lasts equally as long as the bulk resin from BRS. As long as your system is intact, I wouldn't purchase a new one, but you may want to do some more research on what filters you choose to use, and add the necessary TDS meters/psi gauges to keep an eye on the health of your filters to ensure everything is working properly. Membranes/pre filters are cheap.. DI can get expensive if you have to keep changing it because the cheap filters aren't doing their job(s).
Anyway, here is what I am doing and so far it works for me.
