Another question about RO membrane flow rates

Discussion in 'Water Softener Forum, problems, installation and r' started by shopco, Mar 2, 2014.

  1. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    Trying here to substitute logic for knowledge.

    Assume flow meters on concentrate and permeate. If I open the concentrate valve fully then I assume there would be little to no permeate flow. As the concentrate valve is closed permeate flow would begin, so it would seem that the permeate flow is controlled by the concentrate valve.

    If the concentrate valve is closed so as to give permeate flow its calculated volume, i.e. 40 gpd (105ml/min.) is there a way to figure the concentrate flow? Axeon gives a 4:1 ratio @ 50 psi but I assume it would be less (or more efficient) @ 100 psi.

    I am asking this so that I can make sure that the flow meters I buy have the correct flow range. (I realize this is probably overkill and I would probably do ok with a capillary flow restrictor but complex systems amuse me and satisfy my need to quantify stuff.)
  2. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    Your assumptions are missing many important factors.

    Keep permeate flow and concentrate flow as separate for now in your calculations.

    I see your logic, but you are missing the first equation, feed flow

    If you have a pumping system that can pump 5 GPM at 100 PSI then you have to keep the combined permeate and concentrate flows within that parameter or get a bigger pump. Also pre-treatment needs to be able to handle your flow.

    A perfect example is the call that I have had to train my entire customer service staff to handle. We take calls from too many people to count with this problem.

    "The residential under the sink RO is not making any water and it is brand new"

    9 times out of 10 it is missing the flow restrictor on the concentrate waste line. Most companies use a capillary tube insert type (or similar) flow control and it is easily misplaced. Even with the little flags, stickers, notes etc on the drain line and in the instructions. Many of the OEM distributors have this exact same call all the time.

    Why? The pre filtration can only flow .75-1 GPM, and without any restriction, all the available water is going to drain causing no pressure on the membrane. No pressure, no production.

    As to ratios, these are generic and require complex water analysis to determine the proper ratios for longevity of the membrane, water quality, efficiency, etc. Their is not set ratio, just guideline. You can use software to easily determine the optimal ratio, but how large of an RO are you building? Spending a lot of money and tie to protect a $40 membrane may not make a lot of sense. My own system is running at a 3:1 ratio, but the waste is sent to irrigation, so technically, there is no waste.

    Flow meters on a residential system would not make a lot of sense, just set it at a 3 or 4:1 ratio, this will ensure long membrane life in most applications, high quality water, and the system will be simple.

    My own system is nothing more than (2) 150 GPD membranes and (2) permeate pumps.

    Since you want to do it right, are you going to feed a pressurized storage tank? If so, then you really need to consider a permeate pump, but this is another long story. :)
  3. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    No, I’m going with a 20 gallon atmospheric tank with an electric float valve and a Procon pump for distribution pressure.

    I am also using an “Aquatec 6800†before the membrane. I don’t know what my feed pressure will be after the softener and the filters; hence the pump.

    The filters are all 4½ x 20 BB, with 5µ sediment, 10µ Carbon block, and 1µ pleated sediment; located in that order between the softener and the 6800 pump. I will branch off for the house after the carbon block and before the pleated filter.

    I was going to put the sediment before the softener but I became concerned about sufficient flow for the softener’s regeneration cycle so now I am thinking after the softener.

    Your comments are most welcome. I know I need a plan checker.

    P.S. I got it on the flow rates. I don’t know what the flow will be but I get what you are saying.
  4. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    A filter prior to the softener, or after should not have any affect on the backwash flows, softeners have very low flow requirements for proper backwashing. A standard 10" diameter tank maxes out at 2.4-2.7 gpm during the regeneration cycle.

    What is your application? A pressurized storage tank is much simpler and the addition of a permeate pump will eliminate most of the drawbacks of the back pressure caused by the tank.
  5. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    I want to control the type and amount of minerals I put back into the RO water. I plan on doing this by looping flows from the tank through a cartridge(s) and back into the tank until the TDS controller reaches its preset point. At that point a solenoid valve will bypass the mineral cartridge(s). That is one reason. Another reason for using an atmospheric tank is that it seems easier to control the flow from my faucet, which needs to be >4 gpm. I dislike the faucets that come with consumer level under sink units. One of my applications is espresso. Straight RO does not make very good espresso.
  6. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    Is this for a business or a residential application?
  7. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    Residential, although we have a commercial espresso machine. One of the things I am trying to achieve is a Langelier Saturation Index close to 0.0; because of that machine. This whole project is tailored to my desire to make something that does what I want it to, so it is out of the ordinary in that way. It is a commercial level system in a residential size. I am kind of modeling the system after the Watts R-12.
  8. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    A simple 50 GPD RO system with a permeate pump and larger pressurized storage tank and a post membrane calcite /corosex filter 75/25 blend would be perfect for your application. Cheap. simple, reliable.
  9. shopco

    shopco New Member

    Messages:
    45
    Location:
    Riverside, CA
    Can you give me a source(s) for that filter?
  10. ditttohead

    ditttohead Water systems designer, R&D

    Messages:
    1,943
    Location:
    Ontario California
    PM sent. I have several sources and they are a very inexpensive filters.
    Last edited: Mar 3, 2014
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