Ultrafiltration

Ultrafiltration (UF) is basically a pressure-driven separation process, governed by a screening principle and dependent on particle size. Ultrafiltration membranes have a pore size between 1 nm and 100 nm, thus allowing retention of compounds with a molecular weight of 300 to 500 000 Dalton.

Typically, the process is suitable for retaining biomolecules, bacteria, viruses, polymers, colloidal particles and sugar molecules.

 

Ultrafiltration membranes are defined by their nominal molecular weight cut-off (MWCO). The MWCO generally represents the smallest molecular weight for which the membrane has a retention value of more than 90%. In many cases, however, separation efficiency is not only influenced by cut-off but also by interaction between the membrane and the raw solution. The operating pressure for ultrafiltration is usually between 0.1 and 1 MPa.

The following operations are typical for the application of ultrafiltration:

  • Filtration of raw solutions
  • Concentration of substances
  • Fractionating of substances

In the fractionating of dissolved substances or the separation of dissolved substances and solvents, a sufficient degree of separation is reached when the size of the particles differs by the factor 10. Nowadays, different applications for ultrafiltration can be found in nearly all industrial sectors:

  • Sterile filtration of drinking  and beverage water
  • Treatment of surface water
  • Recovery of filter backflushing water
  • Separation of oil/water emulsions
  • Recovery of electrodeposition paint
  • Removal of metal hydroxides in wastewater treatment
  • Separation of biomass in biotechnology
  • Wastewater treatment and re-use
  • Membrane bioreactors

Great significance is given to the use of ultrafiltration for sterile filtration because only a suitable ultrafiltration process can ensure a retention of >log 4 for bacteria, viruses, Legionella and possibly even endotoxins. Ultrafiltration is, therefore, the choice process for point-of-use filtration for the safe production of drinking water.

One of the most sophisticated uses of ultrafiltration lies in the application of membrane bioreactors (MBR) for wastewater. The ultrafiltration can operate in the normal way on a cross-flow by-pass system, or submerged in the bioreactor vessel by means of vacuum suction at low trans-membrane pressures.

The combination of activated sludge with membrane separation in the MBR results in efficiencies of footprint, effluent quality and residuals production that cannot be attained when these same processes are operated in sequence.

 

 

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