Ultrafiltration Technique

Membrane separation processes are the simplest among all non-chromatographic techniques currently used and are based on the molecular weight and hydrodynamic radius of the proteins. PEGylated species can be efficiently fractionated and recovered using ultrafiltration. Even though ultrafiltration is not able to separate PEGylated conjugates according to their positional isomerism, it is still a powerful alternative to size exclusion chromatography (SEC) for the purification due to its certain advantages, such as cost and throughput.

Fig. 1 Schematic of ultrafiltration stirred cell apparatus. (Ruanjaikaen K. 2013)

How Does Ultrafiltration Work?

Fig. 2 Schematic illustration of dynamic ultrafiltration process. (Baker, R. W. 2000)

Ultrafiltration is an intermediate between microfiltration and reverse osmosis. It is one of the membrane separation technologies driven by pressure. In the ultrafiltration process, the aqueous solution flows through the membrane surface. The solvent and small molecular solutes smaller than the membrane pores can permeate the membrane to become purification fluid (filtrate), whereas the solutes larger than the membrane pores will be trapped and discharged with the water flow to become concentrated liquid.

The ultrafiltration process is dynamic, and the separation is completed in a flowing state. Solutes unable to pass through the membrane are accumulated on the membrane surface, forming a solid gel-like film that acts as a barrier to the flow of permeate through the membrane. The thickness of the fouling film is controlled by the sweeping action of the feed solution past the membrane surface. This circulating flow of solution hydrodynamically scrubs the membrane surface, continuously removing the surface film. Thus, a balance is achieved between circulation of solution past the membrane surface, which removes the gelled material, and the flux of permeate through the membrane, which brings fresh material to the membrane surface.

General Considerations

In the ultrafiltration process, the influencing factors that affect the efficiency of ultrafiltration mainly include the following aspects:

• Operating pressure

Since a gel-like layer will be formed at the interface of the membrane during the ultrafiltration process, when the operating pressure increases to a certain value, the permeable flux will no longer increase. Therefore, the pressure should be appropriate. Excessive permeable flux will compact or damage the membrane. The practical operating pressure during ultrafiltration is about 0.5 to 0.6 MPa.

• Flow rate of the feed

Increasing the flow rate of the feed liquid can aggravate the turbulence of the liquid flow on the membrane surface, which is beneficial to slow down the concentration polarization and improve the processing capacity of the equipment. However, if the flow rate is too high, the energy consumption and the operating cost will correspondingly increase. Therefore, it is necessary to control the flow rate of the feed liquid in an appropriate range. Under normal circumstances, the flow rate is around 1~ 3 m/s.

• Pretreatment of feed liquid

In order to ensure the normal and stable operation of the ultrafiltration system and improve its efficiency, the feed liquid should be pretreated. There are various types of pretreatment methods including filtration, chemical coagulation, pH adjustment, disinfection, and activated carbon adsorption, etc., and can be selected according to the property of the feed and the unique needs of the research.

• Operating temperature

The operating temperature of the system mainly affects the viscosity of the feed liquid. Under certain conditions, increasing the operating temperature will help to increase the mass transfer efficiency and the permeable flux.

• Maintenance of membrane

It is recommended to pay attention to the maintenance of the membrane during daily use. If not, the membrane's permeable flux will gradually decay and the separation ability will be influenced. Membrane cleaning methods include hydraulic cleaning, chemical cleaning and mechanical cleaning, etc., which usually be determined according to the property of the membrane and the feed liquid.

What Can Ultrafiltration Be Used For?

• Separation of proteins, enzymes, nucleic acids, polysaccharides, peptides, antibiotics, viruses, etc. in biopharmaceuticals.
• Separation of mono-, multi- PEGylated species from unreacted proteins.
• Desalination, concentration and fractionation of macromolecular substances.
• It could also be used as the depyrogenation treatment of biochemical preparations.

Strengths & Weaknesses of Ultrafiltration

Summary of Developed Membrane Separation Techniques

In addition to ultrafiltration, there are several commonly used commercial membrane separation processes. They all well-established and are briefly listed as follows:

References

1. Baker, R. W. Membrane separation. Membrane Technology & Research Inc. (MTR) 2000.
2. Ruanjaikaen K. Purification and production of pegylated proteins using membrane processes. 2013.