What is concentration polarisation in relation to Reverse Osmosis
Concentration polarisation (CP) is the term that describes the tendency of the solute to accumulate at membrane solution interference within a concentration boundary layer or stringent liquid film. This layer contains near stringent liquid, since at the membrane surface itself the liquid velocity must be nil. This implies that the only mode of transport within this layer is diffusion, which is around two orders of magnitude slower than convective transport in the bulk liquid region. Rejected materials thus build up in the region adjacent to membrane, increasing their concentration over the bulk value. This build up occurs exponentially with increasing flux. This thickness of the boundary layer, on the other hand, is determined entirely by the system hydrodynamics, decreasing in thickness when turbulence is promoted.
For pressure driven processes, the greater the flux, the greater the build up of solute at interface. The greater the solute build up, the higher the concentration gradient. The steeper the concentration gradient, the faster the diffusion. These mass transfers are all in dynamic equilibrium with one another. CP increases the propensity for sparingly soluble solutes to precipitate out into the membrane, forming a gel layer, as well as generally increasing the concentration of colloidal or suspended material at the membrane surface. Furthermore, CP increases the permeation of the rejected materials through the membrane because of the increase in the trans-membrane concentration gradient generated. For RO, CP raises the effective osmotic pressure at the membrane surface interface, increasing the required trans-membrane pressure for operation. This is thus always desirable to suppress CP by promoting turbulence.