You should be replacing the sediment filter and carbon block every 6 months and disinfect the system at that time to protect the membrane.
Sediment and carbon filters have very little to absolutely nothing to do with TDS, they remove suspended solids, things in the 1 micron range and larger depending on the quality of your replacement filters, and of course chlorine. The RO membrane is the workhorse of the system and removes dissolved solids, TDS, things in the 0.0001 micron range, many times smaller.
Always use high quality replacement sediment and carbon filters, not the ebay specials or those found at HD or Lowes. The better the filter the more stuff they trap so the membrane never sees it and does not have to deal with it, thus lasting longer and working better. I suggest no larger than 1 micron sediment filter and carbon block and 0.5 micron in both will work even better plus the 0.5 micron carbon block will last up to 20,000 gallons of chlorinated water versus 9 to 12,000 gallons for the 1 micron and as little as 1,500 gallons for a 10 micron or even only 300 gallons for granular activated carbon filters. Thats total gallons including waste so with a normal 4:1 waste ratio the 0.5 will make 4,000 gallons of treated and 16,000 waste, the 1 micron may make 2400 gallons of treated and 9600 waste bu the 10 micron would only make 300 gallons of treated water and 1200 gallons of waste. You could be ruining your membrane with chlorine if you are using low end replacements.
What are your tap water, RO only and RO/DI TDS readings? What is your water pressure and water temperature? What is your exact waste ratio as measured with a measuring cup and clock or watch? All these things have a bearing on how long a RO membrane and DI resin will last.
Are you using bulk DI resin or new fresh cartridges? DI resin has a shelf life of about 6 months when stored properly. Properly means vacuum sealed and damp like in a seal a meal bag or mylar foil bag and stored in a cool place out of light like the back of the refrigerator. Otherwise resin exhausts quickly. Never let the resin dry out, always make sure there is water in the DI housing and membrane housing when stored. Always use the RO/DI system at least every 10-14 days at a minimum. If its going to be much over two weeks the membrane and DI shuld be removed and stored wet in the refrigerator to stay fresh.
You can just about figure a fresh DI cartridge will remove 3000 to 6000 total TDS per pound of resin. A properly packed refillable 10" vertical cartridge holds 20 oz of resin so if you know your RO only TDS going in you can figure how long DI should last. Color changing resins are a poor indicator of resin condition since they can change in streaks, in layers, top to bottom, bottom to top, not at all or all at once. Not very reliable and yyou will find 90% of the time you are passing TDS before the resin has changed colors. The problem is it starts to release weakly ionized substances even BEFORE it is exhausted and these include nitrates, silicates and phosphates all or which do not register ell on a hobbyist grade TDS meter. You may be putting these in your tank and not even realize it. A TDS meter is the only economical way to test RO/DI water with a conductivity meter being a more expensive way.
In short, to protect the membrane you replace the sediment and carbon block every 6 months with high quality low micron eplacements and disinfect the system to reduce the likelyhood of virus or bacteria growth in the housings. You monitor the TDS on a regular basis with a handheld TDS meter to keep track of your rejection rate or removal efficinecy and you check and adjust your waste ratio to4:1 if its not there presently. Other things are keep the system in a climate controlled place out of bright lighting and temperature extremes, a membrane melts at 113 degrees. Never run hot or warm water through the membrane for that reason, cold water only.
