RO unit TDS ---- and yes the question is tied to a SOFTENER

Discussion in 'Water Softener Forum, problems, installation and r' started by ByteMe, Nov 16, 2012.

  1. ByteMe

    ByteMe New Member

    Messages:
    61
    Location:
    Midland Texas
    The TDS of my water ranges between 950-990 at my kitchen sink. After the RO (5 stage with permeate pump) unit under the sink I get a range of 50-55 TDS. I figure I average a rejection of 94.5%.

    What is the cost cost effective way to reduce this even more. Should I run the permeate water through a second RO membrane?

    How it ties in to the softener. Are my TDS level this high because my city water comes in with a hardness that ranges from 30gpg to 50 gpg and the softener replaces with the salt?

    But then even 50gpg * 17.1 = 855. With this type of water it seems easy to justify a whole house RO with whatever pretreatment needed.
  2. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    Whole house RO may be an answer, but it depends on the results you are currently getting with your existing water treatment and are you happy with the results? 50 ppm for an RO is fine. Lowering the TDS by doing a double pass RO design would work but is not feasible for a small application like yours. This is typically done in commercial applications only and requires considerable design considerations. You could try a higher rejection membrane and remember to not use the RO for small amounts of water. A permeate pump is one of my favorite items for RO systems, but if you use small amounts of water regularly, it will cause a slight raise in the product tds. To get an accurate test of your RO membrane rejection, turn off the tank, open the faucet, and let it run for 30 minutes. Test the TDS out of the faucet, this will give you an accurate test of the membranes performance. What membrane do you have? Different manufacturers have different rejection ratings for their membranes. Why do you want the TDS lower?
  3. ByteMe

    ByteMe New Member

    Messages:
    61
    Location:
    Midland Texas
    FilmTec 50 gpd membrane TW30-1812-50.

    With my excellent water pressure, I expect the permeate pump is not as needed as my last place.

    I would be willing to lose much of my RO gpd if I could get lower than a 20 TDS.

    Is there that much better of a membrane available?


    *edit* as to why i want a lower TDS, because I can.
  4. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    The permeate pump will typically improve rejection rates, it eliminates the back pressure on the membrane, almost simulating atmospheric storage. It also increases the systems efficiency by removing the back pressure caused by a bladder style storage tank. The only time a PP causes higher TDS is in extremely low use applications. The Filmtec is a 96-98% rejection membrane, but that is under ideal conditions. All things considered, 94% is acceptable, and actually just fine. A simple DI cartridge could be used to reduce the TDS, but the water will taste like a Sunday newspaper, or maybe even fish, but your tds meter will read 0.

    As to the Filmtec membrane, it is one of the higher rejection membranes. Pentek and CSM also make very high quality membranes, but they will all have similar performance. Did you test the actual membranes performance potential as I suggested earlier? Turn off the tank, open the faucet for 30 minutes, take a reading. Let me know what it is then. When was the last time you replaced the membrane?

    Regardless, 94% is noramal, if not very good performance for a residential style system.
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