The global water shortage has increased the importance of seawater desalination technologies, with reverse osmosis (RO) 

emerging as the most widely used process. However, the performance of RO membrane modules is signifcantly infuenced 

by the trade-of between concentration polarisation and pressure drop, both of which are strongly afected by the geometry 

and arrangement of spacers. In this study, a computational fuid dynamics (CFD)-based surrogate model was developed to 

quantitatively capture the infuence of spacer geometry on RO performance and subsequently integrated into module- and 

process-level simulations. Process-level economic optimisation was performed as a demonstration of the proposed framework.

Water and salt fuxes at the membrane surface, as well as the pressure drop, were obtained from three-dimensional 

CFD simulations at the unit-cell level. These results were generalised into regression-based surrogate models and integrated

with process models at the module and stage levels. Subsequently, optimal operating conditions were determined 

to minimise freshwater cost. The analysis showed that freshwater cost converged within a narrow range of approximately 

0.62–0.63 $/m3

, depending on the spacer geometry, while the resulting process confgurations varied signifcantly (e.g., 

total module counts ranging from 1,981 to 2,193). Through this framework, spacer efects can be explicitly incorporated 

into process-level models, where they are typically treated as fxed constants (e.g., mass transfer coefcients and friction 

factors) in simplifed mathematical models