CFD modelling of electro-osmotic permeate flux enhancement on the feed side of a membrane module

• Positive slip velocity and high gradient drive flux enhancement.
• Non-uniform slip velocity is more effective than uniform slip velocity.
• Electro-osmosis is more effective for systems with high concentration polarisation.
• Better enhancement at larger Schmidt and lower Reynolds number and rejection.
• Effectiveness reaches a peak in the permeability range of BWRO.

Electro-osmosis has the potential to enhance mass transfer at the membrane surface, thereby minimising concentration polarisation, particularly for nanofiltration and reverse osmosis processes. Electro-osmotic flow can result in a disruption of the concentration boundary layer because of the movement of a thin layer of fluid in the vicinity of the membrane surface. A Computational Fluid Dynamics (CFD) model is used to simulate steady electro-osmotic flow with permeation inside a 2D unobstructed empty membrane channel. It is found that a slip velocity in the direction of bulk flow results in permeate flux enhancement, while a slip velocity in the opposite direction results in flux decline. A non-uniform slip velocity shows greater permeate flux enhancement than a uniform slip velocity. In addition, the results show that electro-osmosis is more effective in enhancing flux for systems with a higher level of concentration polarisation (low Reynolds number and/or high Schmidt number). The data suggests that for seawater RO, electro-osmosis is more effective as the permeability of the membrane is increased, and reaches a peak in the permeability range of brackish water RO membranes. The data also reveals better electro-osmotic enhancement for membranes with lower intrinsic rejection, which might be particularly suited for ultra-osmosis.