As a general rule, membranes with a high water permeability (low feed pressure) also have
a higher salt permeability compared to membranes with lower water permeability. The
permeability of solutes decreases (the rejection increases) with an increase in the:
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degree of dissociation: weak acids, for example lactic acid, are rejected much better
at higher pH when the dissociation is high
ionic charge: e.g. divalent ions are better rejected than monovalent ions
molecular weight: higher molecular weight species are better rejected
nonpolarity: less polar substances are rejected better
degree of hydration: highly hydrated species, e.g. chloride, are better rejected than
less hydrated ones, e.g. nitrate
degree of molecular branching: e.g. iso-propanol is better rejected than n-propanol.