Editing Ultrafiltration (section)

=== Concentration polarization ===
When filtration occurs the local concentration of rejected material at the membrane surface increases and can become saturated. In UF, increased ion concentration can develop an [[osmotic pressure]] on the feed side of the membrane. This reduces the effective TMP of the system, therefore reducing permeation rate. The increase in concentrated layer at the membrane wall decreases the permeate flux, due to increase in resistance which reduces the driving force for solvent to transport through membrane surface. CP affects almost all the available membrane separation processes. In RO, the solutes retained at the membrane layer results in higher osmotic pressure in comparison to the bulk stream concentration. So the higher pressures are required to overcome this osmotic pressure. Concentration polarisation plays a dominant role in ultrafiltration as compared to microfiltration because of the small pore size membrane.<ref>Brian, P.L., 1965, Concentration polarization in reverse osmosis desalination with variable flux and incomplete salt rejection, Ind. Eng. Chem. Fund. 4: 439−445.</ref> [[Concentration polarization]] differs from fouling as it has no lasting effects on the membrane itself and can be reversed by relieving the TMP. It does however have a significant effect on many types of fouling.<ref name=Sablani>{{cite book|editor-last1=Rizvi|editor-first1=Anil Kumar |editor-last2=Pabby |editor-first2=Ana Maria |editor-last3=Sastre |editor-first3=Syed S.H. |title=Handbook of membrane separations : chemical, pharmaceutical, and biotechnological applications|year=2007|publisher=CRC Press|location=Boca Raton, Fla.|isbn=978-0-8493-9549-9}}</ref>

==== Types of fouling ====

==== Types of Foulants ====
<ref>Rouzan Shoshaa, Mohammad Y. Ashfaq, Mohammad A. Al-Ghouti,
Recent developments in ultrafiltration membrane technology for the removal of potentially toxic elements, and enhanced antifouling performance: A review,
Environmental Technology & Innovation, Volume 31, 2023, 103162, ISSN 2352-1864,
https://doi.org/10.1016/j.eti.2023.103162</ref> 
The following are the four categories by which foulants of UF membranes can be defined in:
:*''biological substances''
:*''macromolecules''
:*''particulates''
:*''ions''

==== Particulate deposition ====
The following models describe the mechanisms of particulate deposition on the membrane surface and in the pores:<br />
:*''Standard blocking'': macromolecules are uniformly deposited on pore walls
:*''Complete blocking'': membrane pore is completely sealed by a macromolecule
:*''Cake formation'': accumulated particles or macromolecules form a fouling layer on the membrane surface, in UF this is also known as a gel layer
:*''Intermediate blocking'':  when macromolecules deposit into pores or onto already blocked pores, contributing to cake formation <ref name=Bruijn>{{cite journal|last1=Bruijn|first1=J P F|author2=Salazar, F N |author3=Borquez, R |title=Membrane blocking in ultrafiltration: A new approach to fouling|journal=Food and Bioproducts Processing|date=September 2005|volume=83|issue=3|pages=211–219|doi=10.1205/fbp.04012}}</ref>

==== Scaling ====
As a result of concentration polarization at the membrane surface, increased ion concentrations may exceed solubility thresholds and precipitate on the membrane surface.  These inorganic salt deposits can block pores causing flux decline, membrane degradation and loss of production. The formation of scale is highly dependent on factors affecting both solubility and concentration polarization including pH, temperature, flow velocity and permeation rate.<ref name=Antony>{{cite journal|last1=Antony|first1=Alice|author2=Low, Jor How |author3=Gray, Stephen |author4=Childress, Amy E. |author5=Le-Clech, Pierre |author6= Leslie, Greg |title=Scale formation and control in high pressure membrane water treatment systems: A review|journal=Journal of Membrane Science|date=1 November 2011|volume=383|issue=1–2|pages=1–16|doi=10.1016/j.memsci.2011.08.054}}</ref>

==== Biofouling ====
{{main|biofouling}}
Microorganisms will adhere to the membrane surface forming a gel layer – known as [[biofilm]].<ref name=Flemming>{{cite journal|last1=Flemming|first1=H.-C.|author2=Schaule, G. |author3=Griebe, T. |author4=Schmitt, J. |author5= Tamachkiarowa, A. |title=Biofouling—the Achilles heel of membrane processes|journal=Desalination|date=1 November 1997|volume=113|issue=2–3|pages=215–225|doi=10.1016/S0011-9164(97)00132-X|bibcode=1997Desal.113..215F }}</ref> The film increases the resistance to flow, acting as an additional barrier to permeation. In spiral-wound modules, blockages formed by biofilm can lead to uneven flow distribution and thus increase the effects of concentration polarization.<ref name=Baker>{{cite journal|last1=Baker|first1=J.S.|author2=Dudley, L.Y. |title=Biofouling in membrane systems — A review|journal=Desalination|date=1 September 1998|volume=118|issue=1–3|pages=81–89|doi=10.1016/S0011-9164(98)00091-5|bibcode=1998Desal.118...81B }}</ref>