The driving force for solvent flux in RO is the difference between the pressure drop across the membrane and the osmotic pressure difference across the membrane (Δp − Δπ). Then the mass flux of solvent is
In these equations K′solv is the permeability of the solvent (usually water) through the membrane with an effective membrane skin thickness of tms. The pressure drop across the membrane, Δp = pr − pp. Δπ is the difference in the osmotic pressure across the membrane (see description below). The mass solute flux across the membrane J′A can be written as
where K′A is the solute permeability ...