MEMBRANE FILTRATION

 A liquid consists of a number of components with different molecular or particle sizes. By using membranes with pores of different sizes, it is possible to separate exactly the components you wish to concentrate or remove from the liquid stream.Membrane filtration is a straightforward technology that separates a liquid into two streams using a semi-permeable membrane

 A difference in pressure forces the components that are smaller than the membrane pores through the membrane as “permeate”. The remaining components are retained as “retentate”. A substantial flow moving parallel to the membrane prevents the membrane surface from getting blocked during the process. This is known as cross-flow filtration.

  SPIRAL-WOUND MEMBRANES

 Figure illustrates the basic structure of a spiral-wound membrane module. A permeate spacer is placed between two membranes. The two membranes and permeate spacer are glued together along three edges to form what is called a leaf. The unglued edge is connected to a perforated central collection tube. The membrane leaf along with a mesh feed spacer is spirally wound around the central collection tube.

FOULING AND CONCENTRATION POLARIZATION

Fouling refers to any phenomena that result in reduced transmembrane flux over time. Fouling is primarily caused by concentration polarization. Concentration polarization is the accumulation of molecules at the membrane surface. Three types of fouling are cake layer formation, pore blockage, and internal pore fouling. Cake layer formation and pore blockage refer to fouling on the surface of the membrane.

Deposited molecules build up on the membrane surface during cake layer formation while, rejected molecules block pore openings during pore blockage. Internal pore fouling refers to fouling within the pores of the membrane. Internal pore fouling occurs when molecules are deposited within the pores of the membrane, which reduces the average membrane pore size. All three forms of fouling resist the transport of solvent and small solute molecules through the membrane pores

In general, it is observed that high cross flow velocity and low operating pressure provide the most economical solute-solvent separations Higher crossflow velocities can reduce concentration polarization and fouling At low operating pressures, the permeate flux increases almost linearly with the applied pressure.. the higher the crossflow velocity, the higher the limiting flux. A balance of crossflow velocity and pressure must be achieved for optimal separation. The balance varies by membrane type and by feed solution characteristics.

Depending on the pore size of the membrane, the process is called: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) or microfiltration (MF)