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Old 02-06-2013, 05:39 PM   #1
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328ppm TDS vs. 004ppm

Ok I just got my little handy dandy TDS meter, and got some tap water out of the kitchen sink and it read 328ppm of TDS. I then went out and got one if the gallon jugs that contains my RO/DI water, tested it and its reading 0.004ppm I'm assuming that this is pretty good indication that my RO/DI water filter is working pretty good.

My question is this. Now that I'm armed with this information, at what ppm with TDS should I change out my filters, carbon and resin? I'm currently running a RD-102 6 stage RO+DI unit from WaterGeneral Mfg.
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Old 02-06-2013, 06:57 PM   #2
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Probably a personal preference question.

I normally change the membrane when RO TDS increases into the 7-9ppm range.

I change out the DI resin when the DI TDS reaches 3ppm. I run two DI stages so that the RO/DI output is always 0ppm. When the first DI stage TDS reaches 3ppm, I replace the DI resin, move the remaining DI stage up to the first DI stage spot and intall new DI resin in the 2nd DI stage.

Pre-sediment filters and carbon blocks can be changed out every 6 months (give or take depending on how much water they have filtered.
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Old 02-06-2013, 07:17 PM   #3
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So are you saying its actually the DI that's got my TDS down? I thought it was a combination of all.

Example my TDS are at 004. So would they not drop by changing out the filters instead of the DI?
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Old 02-06-2013, 08:07 PM   #4
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The RO membrane should ideally remove 98%+ of the tap water TDS (RO membrane performance will vary).

The DI removes what the RO membrane cannot.

The pre-sediment and carbon blocks usually have a negligible impact on TDS reduction. If by filters you mean these, then you would not get a reduction in TDS of RO/DI water if you changed out the filters.

You would get a reduction in RO TDS if you are replacing an old RO membrane with a new one.

As an example if the TDS of the tap water is 275ppm, it would be 275ppm or very near it after it passes through the pre-sediment and carbon blocks. After passing through an RO membrane with a 98% rejection rate the TDS of the RO water is reduced to about 5 or 6ppm. The DI stage reduces this to below 1ppm. (registering as a "0ppm" on most TDS meters that cannot detect levels lower than 1ppm but greater than 0ppm).

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Old 02-06-2013, 08:09 PM   #5
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Thanks for the lesson, I appreciate it.
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