how to calculate Flux in RO?

Avinash Patil
Started by Avinash Patil on
28 Aug 2014 at 08:52
Know-how: Water Supply, Treatment

May be its a simple question for expert, but please guide me on how to calculate Flux in RO?


Industry Tags: 
  • WaterDesalinationReverse Osmosis


Engr. Mansoor Ahmed PhD. Scholar
Dear Yahia Albasha,

You might calculate flux / permeate of each Vessel but not each element. As a Vessel contains different configuration elements say up to 7 or so on. So all the elements are connected to each other to give you resultant permeate. You can put sampling point or flow meter at each vessel to understand how much feed you give in and how much you are getting as permeate and how many elements are there in the vessel (so you could have total area of membranes per vessel).

This is how it works. By the way why are you willing to know what each element is giving you the flux as normally the membranes manufacturer while selecting the membranes for your specific needs, they give you what could be the estimated flux / permeate from their system. In case if you are not getting the desired, then you can investigate the reasons which could be many and most of the time these are learned by experience while you are running a plant specially at the initial stages to stabilize the system.

In other words, sometimes theoretical things becomes difficult while you go practical.
Yahia Elbasha
Please how to calculate the flux of each element in the vessel, i.e how to calculate the permeate flow of each element in the vessel.
Dear Avinash:
There are two types of flux:
Average flux is the result of dividing the total flow of water produced by the whole skid or individual vessel and the total membrane area of skid or individual vessel.
This number although very used, does not give too much information, because it's an average.
To have a real knowledge of what's happening, you should pay attention to the flux of each element in the vessel. (Each element product flow divided by element area)
Several vessels with same average flux, can have very different patern of individual flux, depending on the type of membranes or their disposition ( Hybrid staging for example, gives lower maximum individual flux than standard staging, helping to reduce fouling)
area of membranes from membrane manufacturer!!!
You will get the details of the area from the manufacturer. Total numbers of membranes multiply by membrane area will give you total membrane area ( I prefer m2 unit) Convert product (permeate) in to ltres per hour (LPH). DIvide LPH by M2 to get flux in LMH (liter per meter square per hour)
Engr. Mansoor Ahmed PhD. Scholar
Dear Avinash,
Here is the Simple Flux Calculation you can do easily.

Flux = Permeate Flow Rate/Unit Membrane Area/Unit Time
i.e. Flux = liter/m2/hr (or Gallons/Ft2/Day)

Flux = Qp/A

Qp = Permeate Flow in liters or gallons
A = No. of Membranes x Membrane Area

In typcial SWRO Membrane Flux is in the range between 12 - 18 lit/m2/hr

Membrane area is given by every supplier easily in the documents and number of membranes are already mentioned in each vessel by your RO Supplier.

Hope this clears, should you need any further clarifications, please write to me on
Pavel Makhnev
Avinish, the best is to use RO systems designing software from corresponding RO membrane manufacturers. For example, for Russia's RM Nanotech RO membrane you can download software at
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