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Hello guys! I have a 2yr. old RO/DI that haven't been used for awhile. And its only been used for 6mos. since then. Now my brother is getting into the hobby and wants to use it. We tested the water today with a TDS meter and it show 0ppm. Considering the unit haven't been used for 2 years, should we still use it? Or should we replace all the membrane, resin and carbon?

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I would replace the membranes and cartridges, It is possible for them to dry out if the unit was emptied before not being used for so long, Or if it was kept wet there is the possibility of bacterial growth inside the unit

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Test the TDS from the tap water, RO only and RO/DI. If the membrane is still functioning at close to 96-98% removal then it is probably OK to use.


I would replace the sediment and carbon block filters, disinfect the system using the method I have described here many times and replace the DI cartridge though.


Since it has been a couple years I would remove the RO membrane from its housing and disinfect the entire system.

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Thanks for the reply. Yeah we tested the tap water and its around 300ppm and the output of my ro/di is 0ppm. How to disinfect the enitire system? So i could still use the RO membrane? I'm confused..

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What is the TDS of the RO only water, not the RO/DI water?

You need to know the RO only before the DI to help determine the membrane condition. The final TDS or 0 may be a result of the DI resin doing all the work in which case the resin would only last a matter or hours per cartridge. Since the RO membrane does 90-98% of the work its important to know its condition.

Disconnect or remove the DI and test the RO only.


Do a search with my username and RO disinfection and many many threads will pop up on the subject with edetailed directions.


You only know if the membrane can be used or not after you test the rejection rate which is calculated using the tap water TDS and RO only TDS readings. Tap TDS-RO TDS/tap TDSx100 will give you the rejection rate or removal efficiency. So for example your tap TDS is 300 and you find the RO only TDS to be say 50. 300-50=250, 250/300=.833, .833x100= 83.3% rejection or removal so that membrane would require replacement. You want the number to be in the 95-98% range for best water quality and DI resin life.

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