where ΔΠ is the system's osmotic pressure difference, ΔP is the hydrostatic pressure applied to the compartment of larger solute concentration, and 𝐹 = exp(-𝐽 ! (54S) R ). 𝐴 and 𝐵 are the solvent and solute permeability, respectively, which are generally regarded as constant for a membrane-solvent/solute pair, particularly when membrane sorption is low, and thus characteristic of a membrane separation system. The reflection coefficient 𝜎 is a third modelling parameter in the Spiegler-Kedem equations which acts as a measure of membrane selectivity: 𝜎 = 1 signifies solute flow completely uncoupled from solvent flow (perfect membrane permselectivity), while 𝜎 < 1 indicates the extent to which the solute permeation is coupled to that of the solvent.