Remember that your schedule for changing prefilters should be completely independent of your DI water TD readings. The concept is better explained in this text from our FAQ's, here: https://www.buckeyehydro.com/faq/
A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the usable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or more of the prefilters (all the filters that touch the water before it reaches the RO membrane) is beginning to clog.
Also be cognizant of the chlorine capacity of the carbon block. A good 0.5 micron carbon block for example will remove much of the chlorine from 20,000 gallons of tap water presented at 1 gpm. Some original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons. Remember that all the water you process, both waste water and purified water, goes through the carbon block.
Regarding your RO membrane and DI resin, use your total dissolved solids (TDS) meter to measure, record, and track the TDS (expressed in parts per million [ppm]) in three places: 1) tap water, 2) after the RO but before the DI, and 3) after the DI.
The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 ppm. Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?
If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO membrane housing and you'll see it is still approximately 400 ppm.
The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 96% (i.e., they reject 96% of the dissolved solids in the feed water). So the purified water coming from your 100 gpd membrane would be about 16 ppm (a 96% reduction). Filmtec 75 gpd (and below) membranes produce purified water (a.k.a. “permeate”) more slowly, but have a higher rejection rate (96 to 99%). The lifespan of an RO membrane is dependent upon how much water you run through it, and how “dirty” the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the TDS in the water coming into the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce purified water more slowly as their function declines.
After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the TDS in the RO water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm, your resin needs to be replaced. Sometimes you'll hear people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin high TDS water. This will exhaust the resin quicker than would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal.
Additionally, don’t forget to sanitize the entire system at least once per year, and wash and lube your housing o-rings with food-grade silicone grease every filter change.
Russ
