Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane. This separation process is used in industry and research for purifying and concentrating macromolecular (10^3 - 10^6 Da) solutions, especially protein solutions. Ultrafiltration is not fundamentally different from reverse osmosis, microfiltration or nanofiltration, except in terms of the size of the molecules it retains.

Industries such as chemical and pharmaceutical processing, food and beverage processing, and waste water treatment, employ UF in order to recycle flow or add value to later products. UF's main attraction is its ability to purify, separate, and concentrate target macromolecules in continuous systems. UF does this by pressurizing the solution flow, which is tangential to the surface of the supported membrane (cross-flow filtration). The solvent and other dissolved components that pass through the membrane are known as permeate. The components that do not pass through are known as retentate. Depending on the Molecular Weight Cut Off (MWCO) of the membrane used, macromolecules may be purified, separated, or concentrated in either fraction. Currently, the study of UF processing occurs mainly in laboratory setups because it is very prone to membrane fouling caused by increased solute concentration at the membrane surface (either by macromolecular adsorption to internal pore structure of membrane, or aggregation of protein deposit on surface of membrane), which leads to concentration polarization (CP). CP is the major culprit in decreasing permeate flux. Ultrafiltration is used in reverse osmosis processes in many Middle Eastern countries to produce fresh water as there is little fresh water available in those areas.

(References 1-4 used in above summary.)

Biology

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It is also used in hemodialysis to clean whole blood while keeping its composition intact. In biological terms this occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent arteriole and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:

1. The renal artery contains blood at very high pressure which enters the glomerulus via the short afferent arteriole.
2. The efferent arteriole has a smaller diameter than the afferent arteriole.

The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate. Large molecules such as plasma proteins and blood cells do not pass through the filter because they are too big. Glomerular pressure is about 75 millimeters of mercury (10 kPa). It is opposed by osmotic pressure(30 mmHg, 4.0 kPa) and hydrostatic pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This difference in pressure is called effective pressure(25 mmHg)(3.3 kPa).

References

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1. Atra R., Vatai G., Bekassy-Molnar E., Balint A. (2005). Investigation of ultra- and nanofiltration for utilization of whey protein and lactose. Journal of Food Engineering, 67, 325-332pp
2. Li Y., Shahbazi A., Kadzere C. (2005). Separation of cells and proteins from fermentation broth using ultrafiltration. Journal of Food Engineering, Accepted 26 April 2005, Article In Press
3. Lo M.Y., Cao D., Argin-Soysal S., Wang J., Hahm T. (2005). Recovery of protein from poultry processing wastewater using membrane ultrafiltration. Bioresource Technology, 96, 687-698pp
4. Magueijo V., Semiao V., Norberta de Pinho M. (2005). Fluid flow and mass transfer modelling in lysozyme ultrafiltration. International Journal of Heat and Mass Transfer, 48, 1716-1726pp

Kidney

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