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I have a reverse osmosis and deionization unit that is able to filter my 450 PPM TDS tap water down to 0 PPM TDS. It has been about three months, and my TDS reader now gives 3 PPM TDS.

Do water filters decay at a linear rate for the various compounds it filters out?

For example, if my TDS reader read 10 PPM, and I was to analyze a sample of water at Ward Labs which told me that my water contained 2 PPM of Sodium, 1PPM of Chlorine, and 7 PPM of CaCO3, could I safely assume that at 100 PPM TDS, my water would contain 20 PPM Na, 10 PPM Cl, and 70 PPM CaCO3?

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No

Various particles have different permeability, different solubility. Filter deterioration is not linear one, it will start to leak "most aggressive" ones first. Only real way to tell proportions of what leaked is to test water again... Or obtain deterioration profile from manufacturer, but I doubt they have something they'll be willing to share.

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RO systems are basically a several filter system under pressure that dumps concentrated waste water, producing a couple gallons of low TDS water in 24 hours.

As filters do their job eventually some TDS start comming through. Each RO system has a recommended replacement "time" but that assumes use of all the produced water each day.

My under the counter unit has been on the original filters for over 3 years, manufacturer recommends replacing every three months, but since we only use it for coffee, cooking, Sun tea etc we rarely use it to full capacity. After 3 years my TDS is 6ppm. I won't replace the filters untill there is a noticeable taste resembling my tap water. With all that in mind my tap water is some of the best in the county and very drinkable unfiltered with no noticable chlorine.

I imagine if your water is heavily chlorinated, the oxidizing effects would degrade filter media at a faster rate than I experience.

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