RO Design Flux

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When the average flux for well water RO design is in the range of 27-34 lmh, does this mean that each stage flux will be in this range or the overall flux (for two or three stages) will be in this range?

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4 Answers

  1. Dear Mr. Ali,

    Yes, the flux rate is overall. That mean how much amount of water has pass through the membrane for produce the permeate water. The flux rate is directly proportional to the amount water which pass through the membrane [per unit area]. Unit is lmh or gfd.

  2. Average flux refers to the system's entire permeate being divided by all the elements in the system.

    Systems can be designed with equal flux rates at all stages, but that's not energy efficient and would only be typical for very small systems.  The first element will see water with the lowest osmotic pressure, and will also receive the highest applied pressure. So the first stage would naturally have the highest flux and the last stage will have the lowest.  You would then throttle the permeate at the first stage (and possibly the second in a 3-stage system) to ensure that the last stages receive enough concentrate flow.   The flux ratio between the first and second stage can vary anywhere from 2:1 to 1:1, and is typically around 1.5:1.  

    1 Comment

    1. Thanks for your useful comment.

      In which situation or which water source you will go for permeate throttling or booster pump betweenstages?

      1 Comment reply

      1. It would depend mostly on the osmotic pressure.  If the recovery is high and the feed TDS is high, then the required osmotic pressure to make water in the last element will be high. In that case, the feed pressure needed to satisfy the last element will cause the first element to make too much water.  That is when throttling comes into play.

        Another option would be to have the feed pressure based on the last element of the first stage, and then add a booster pump to provide the additional pressure required for the second stage.

        A membrane projection would help to guide you for the best option.

  3. Average flux is the average flux that the membranes see across the entire system.  Depending on array design, membrane type, recovery, feed water chemistry, system design, etc the flux in the stages can be different than the overall average flux.  In reality each membrane is operating at a different flux with the lead elements being the highest and the tail being the lowest.  Projection software will give you warnings if you exceed recommended stage, lead element, or average flux.

  4. Dear Ali. Each RO membrane module would be in this range.

    Usually the design programs give an warining if a membrane module is working with flux greater  than the recommended one.