Average flux refers to the ...

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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.  

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Thanks for your useful comment.

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

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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.

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