The Disadvantages of Concentration Polarization Reverse Osmosis

Due to increases in the global demand for water and its dwindling supply, the treatment of existing water sources by reverse osmosis is becoming increasingly popular. Reverse osmosis is an application in water softening, disinfection, desalination and inorganic pollutant removal. The performance of the permeable membrane, during reverse osmosis, is dictated by its ability to reject solutes, otherwise known as concentration polarization.

Reverse Osmosis Models

• Three types of reverse osmosis models currently exist. The irreversible thermodynamics model makes the assumption that the membrane is close to equilibrium and describes fluxes by phenomenological relationships. The homogeneous membrane model assumes that the polymer membrane is nonporous and transport across the membrane occurs via diffusion. In the porous model, transport occurs via both diffusion and convection through the pores that stretch across the entire membrane.

Concentration Polarization

• The buildup of solutes that are unable to cross the membrane surface is referred to as concentration polymerization. As a result, one side of the membrane wall has a higher solute concentration than the other side. Concentration polarization is affected by both membrane and solute properties, as well as transverse and axial flow fields. Concentration polarization has a substantial effect on the overall performance of the reverse osmosis process and is used to predict surface scale formation.

Modeling Difficulties

• Concentration polarization has a negative impact on reverse osmosis modeling as it increases the difficulty in calculating the solute wall concentration, which is required for model design. The increased concentration gradient across the membrane increases the solute flux through the membrane. Concentration polarization also alters the separation properties of the membrane.

Other Disadvantages

• Once the solubility limit is exceeded, concentration polarization causes solute precipitation, which leads to both particle fouling and surface scale formation. Also, the increased osmotic pressure at the membrane wall lowers the water flux. Both fouling and the reducing water flux are enhanced by the accumulation of material in the feed, blocking the surface of the membrane.