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Old 11-19-2005, 11:19 AM   #1
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Understanding RO/DI systems

This got started on another thread but needed a new subject to attract more input...

I'm just turning around to getting a RO/DI unit and don't know an awful lot about them, which makes selecting one rather difficult. What do I look for? I don't know anything about necessary add ons, PSI readings, TDS readings, resin, membranes...you get the idea. Can someone point me in the right direction for information I can sift through? Any or all advice is greatly appreciated.

If I unit states it is 110GPD, does that mean it's "gallons per day"?

I have a 60 gallon that I was going to set up with tap water this weekend, but will put off a few more weeks until I get the RO/DI system all sorted out. It's been a major let down to say the least, but will be worth it to get it right.

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Old 11-19-2005, 01:07 PM   #2
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Yes, GPD means gallons per day. The type of unit you need will have alot to do with the quality of your source water and indeed it's pressure through the "house". Higher TDS from the tap will mean a lower GPD rated RO membrane could be more advantageous as it will have a higher rejection rate and cleaner output before heading to the DI stage. Low TDS can allow you to get away with the higher (sometimes cheaper) GPD units.

See if this article helps with some of your questions...

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Old 11-19-2005, 06:16 PM   #3
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Yes it does, thank you very much.
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Old 11-20-2005, 04:13 AM   #4
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TDS is your total disolved solids (in parts-per-million). After a trip through your RO+DI system, you should be at 0. Tap water is around 150-200 (or if you're me, 450-570 ). You have to use a TDS meter to get a reading. It may be a good investment to see that the system is operating.

Normal RO+DI systems have several stages:
1. a simple fibrous sediment filter
2. a carbon filter to remove chlorine
3. another carbon filter to remove more chlorine (chlorine destroys organic (the better kind) of RO membrane, so needs to be removed).
4. The RO membrane. This is the slowest part of the system. It takes in water and slowly pushes it through tiny holes to pass through only water. It also generates reject water (at a higher rate than clean water). Either pipe it to the drain, or some systems can feed it back into your hot-water line (more exotic, saves water).
5. The DI resin. Its a special ion-exchange resin that captures hard ions (calcium, magnesium, etc) in your water which may make it through the RO membrane. This is the part that generaly gets eaten up the fastest if you have hard water. While carbon filters can last 6-months to a year, the resin is usualy have that.

Higher input pressure leads to better filtered water at a higher rate.
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Old 11-20-2005, 06:02 AM   #5
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too high water preasure (around 80psi+) results in more rejection water/per filtered water and that is depenant on the flow rating of the membrane from the charts Ive seen..
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ro/di, stem, tan

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