All membranes say 1-2 years on the package. All filters say between 3-9 months. I from experience know for a fact that unless water is flowing through the membrane constantly, bacteria and other undesirables build up in both filters and membranes. Most ppl look at TDS. TDS has nothing to do with reefing whatsoever. I can use 112 TDS and I can use 8 TDS water side by side and neither tank would have a difference. Now what that TDS is comprised of means something.
There's zero reason to replace a reef use RO membrane before the flow drops off or the rejection drops too much. It may never happen, and didn't in my case twice for 10 years each.
Certainly bacteria may grow between the RO membrane and the DI. That's no concern for reef use. Drinking by people? Sure, they need to be sanitized periodically for that reason.
Of course TDS is not the actual thing we care about. TDS is always used as a surrogate for RO/DI functionality. If the TDS is 0 ppm, the RO/DI is functioning and we can be assured that most things we care about (e.g., copper) are being removed in the expected fashion.
IMO, the only filter that might usefully be removed before there's an indication of an issue is the carbon block. All of the others can be replace only when needed based on pressure, flow, and TDS.
I discuss these issues here:
Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com
These are my suggestions from it that are somewhat contrary to your suggestions:
Tips on Using RO/DI
If you have a pressure gauge on the RO membrane, change the sediment filter when the pressure drops to about 40 PSI. If the pressure does not rise adequately after replacing the sediment filter, change the carbon filter.
If you do not have a pressure gauge, change the sediment and carbon filters when the output of purified water becomes significantly reduced.
Avoid running the RO/DI if your town’s water system is flushing fire hydrants in your area, as the sediment and carbon filters will clog very fast.
If the sediment and carbon filters do not seem to get clogged, at least replace the carbon filter periodically to ensure that chlorine and chloramine are being broken down appropriately. Both of these can be measured with an inexpensive chlorine test kit if you want to be sure they are being removed.
If you have especially hard water, you might consider running it through a water softener before the RO/DI filter. That process removes calcium and magnesium by swapping sodium for them. While aquarists are not generally worried about calcium and magnesium in their source water, they can foul RO membranes by forming calcium and magnesium carbonate precipitates inside it. That precipitation will reduce flow and eventually make the membrane unusable.
Make sure that your waterline pressure is appropriately high (at least 40-60 PSI). In some situations where the pressure is low, an inline pump may be desirable before the RO/DI system to boost the pressure.
If there is excessive carbon dioxide in the source water (such as well water), you might consider degassing the water first to remove some of the carbon dioxide, and thereby reduce the rate of depletion of the DI resins. Note, however, that this option is not inexpensive, and typically involves repressurizing the water with a pump.
If you are evaluating an existing RO membrane and can collect water from the tap and after the RO membrane, the conductivity (in mS/cm or ppm TDS) should drop by a factor of more than 10 across it (to as much as 100), relative to the tap’s water. If the drop is less than a factor of 10, it is not working properly, and may have holes in it.
Monitor the DI resins by measuring the effluent’s conductivity, either with an inline meter (set to its most sensitive level), or by measuring the effluent manually. If you are using a TDS or conductivity meter, then the measured value should drop to near zero, or maybe 0-1 ppm TDS or 0-1 mS/cm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. That does not necessarily mean, however, that 2 ppm TDS water is not OK to use. But beware that the flow of impurities and the conductivity may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm versus zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity; about 0.7 mS/cm for saturation with normal levels of CO2, possibly higher indoors) and the conductivity/TDS meter itself may yield results of 1 or 2 ppm even from totally pure water by not being exactly zeroed properly. Also note that the first impurities to leave the DI resin as it becomes saturated may be things that you are particularly concerned with (such as ammonia if your water supply uses chloramine or silica if there is a lot in the source water).
If you recharge your DI resins yourself, be very careful with the acid and base used, as they can be dangerous.
