Due to the retention effect of the semi-permeable membrane on solutes, a concentration gradient of solutes is formed from the membrane surface to the main body of the stock solution, that is, the phenomenon of concentration polarization is formed.

Under the cross-flow operation mode, the flow state of the raw liquid in the flow channel is divided into three types. The one near the membrane surface is the laminar flow layer, the one in the middle of the flow channel is the turbulent flow layer, and the one between the remaining laminar flow layer and the turbulent flow layer is the transition layer.

Driven by the pressure difference on both sides of the membrane, some of the solvent passes through the membrane to form an permeable flow, while the solute is retained and accumulates on the membrane surface. Solute concentration throughout the entire flow channelcIt varies in the direction of the vertical line on the membrane surface. Due to the fact that turbulence can effectively achieve the reverse diffusion of solutes from the membrane surface to the mainstream, the concentration of solutes in the turbulent layercfIt can be regarded as uniform. If the solute concentration on the membrane permeate side is setcpIf it is constant, then the solute concentration in the laminar flow layercThere exists a gradient and it reaches a high value on the membrane surfacecm. The so-called concentration polarization degreeβIt is the solute concentration on the surface of the laminar flow filmcmAnd the solute concentration in the turbulent layercfThe ratioβ=cm/cf.

"Influence"

Concentration polarization only temporarily increases the concentration of retained substances on the membrane surface, which is a recoverable process and does not lead to membrane fouling. However, it is precisely due to the existence of concentration polarization phenomena in the membrane process that the relative increase in the concentration of substances retained on the membrane surface either accelerates the formation of gel layers on the surface of porous membranes such as microfiltration and ultrafiltration, or speeds up the saturated precipitation of insoluble salts on the surface of dense membranes such as reverse osmosis and nanofiltration, thereby exacerbating the contamination of various types of membranes.

For porous membranes such as microfiltration and ultrafiltration, the concentration polarization layer formed by the retained substances on the membrane surface also has certain filtration and retention effects, which can improve the retention effect of the separation membrane on solutes to a certain extent and correspondingly increase the resistance of solvent permeation. For dense membranes such as nanofiltration and reverse osmosis, the phenomenon of concentration polarization will increase the salt concentration on the retention side, raise the osmotic pressure on the retention side, that is, increase the water production resistance and reduce the water production flow rate. Since the permeability of inorganic salts is directly proportional to the salt concentration difference on both sides of the membrane, the concentration polarization phenomenon also increases the permeability of inorganic salts and reduces the desalination rate of the membrane process.

The main methods to avoid concentration polarization are: reducing the pressure difference on both sides of the membrane can alleviate the concentration polarization that has already occurred; Increase the turbulence on the membrane surface to reduce the solute concentration on the membrane surface.