The presence of solutes in a feed solution will increase the osmotic pressure difference across the membrane **(∆π_{f} = π_{f} – π_{p})**, reducing the net effective driving force. To account for this when calculating , the following equation needs to be used:

Note: is generally reported between 0 and 100%, but it should be a value between 0 and 1 for the calculation of (step 4)

Note: taken from step 2

Note: , , , and taken from steps 2–3. can be considered equal to the feed-side mass transfer coefficient (**k**_{f}) for single-solute solutions. Details for calculating **k**_{f} can be found __here__. If unavailable, **k**_{f} and **k**_{sol} can be roughly estimated as 100 L m^{-2} h^{-1}. For more information on the estimation of and , please see the page concentration polarization __here__

Note: *∆π*_{f} can be calculated __here__. Details for calculating can be found __here__. If unavailable, can be roughly estimated as 100 L m^{-2} h^{-1}. For more information on the estimation of and , please see the page concentration polarization __here__. R_{g}, T, and V_{W} are the ideal gas constant (cm^{3} bar K^{-1} mol^{-1}), temperature (K), and molar volume of water (cm^{3} mol^{-1}), respectively. All other values taken from steps 1 and 4.

Note: and taken from steps 4–5. The factor of 2.778×10^{-12} is for unit conversion to cm^{2} s^{-1} when using units of nm for **δ** and L m^{-2} h^{-1} for **B**.

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- [3] D.R. Paul, Relation between hydraulic permeability and diffusion in homogeneous swollen membranes, J. Polym. Sci. B Polym. Phys. 11 (1973) 289–296.