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RO / DI Filters - What exactly do they do ?

Are there any experts on water chemistry that can explain what DI filters do? I curse myself for never taking a chemistry class. My question has to do with the difference in DI technology vs. traditional water softening resins. I know that the old water softeners released sodium as they took up calcium and magnesium. Is this a risk with modern RO / DI units, or do the mixed resins in the DI filter remove sodium as well?

I'm not a chemist, but I'll give it a shot.
Most of the dissolved impurities in our tap water are ions, like calcium, sodium, chloride. The de-ionazation process removes these ions by an exchange process. The membrane in an ion filter attracts the positive an negatively charged ions as the water passes through it, and removes it from the water.It may not remove other non-ionic impurites such as organic compounds.
With a water softner all you are doing is replacing the calcium and magnesium ions with sodium ions. Water softners use plastic beads or a membrane made of Zeolite. As water passes over or through the membrane the sodium ions change place with the calcium and magnesium ions. After a while the plastic beads or membrane have absorbed so much calcium and magnesium that they need to bee recharged. A brine solution of sodium is backwashed over the plastic beads or membrane and recharges it with sodium ions that replaces the absorbed calcium and magnesium ions.

Some RO systems offer an additional DI cartridge that you can add.

thanks cindy,

i just recently got the kent ro/di system to replace my old spectrapure ro unit, but had pulled the di cartridge out until i could figure out what it did. a friend told me basically what you just said, so between ya'lls explanations i'm going to go put the di cartridge back in and rest easy.

by the way i really like this new unit! it has a pressure gauge built in which makes it much easier to maximize efficiency, as well as a tds meter to measure intake and output...good stuff!

Thank you Cindy for an explanation that can be understood by people who do not know chemistry. I think in the reverse osmosis scheme, membrane filter work is very important and therefore we have to check it often and replace it when it's time.

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