Ultrafiltration

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The ultrafiltration is a filtration process in the field of membrane technology , with the macromolecular substances and small particles separated and a medium concentrate blank.

It was invented in 1907 by Heinrich Jakob Bechhold .

classification

A distinction is made between microfiltration , ultrafiltration and nanofiltration based on the degree of separation. If the exclusion limit (or " cut-off ") is 100  nm or more, it is called microfiltration. If the exclusion limit is in the range between 2 and 100 nm, this is known as ultrafiltration. In the case of nanofiltration, the exclusion limit is below 2 nm.

The exclusion limits of ultrafiltration membranes are also given in the form of the NMWC (Nominal Molecular Weight Cut-Off, also MWCO , Molecular Weight Cut Off, unit: Dalton ). It is defined as the minimum molecular mass of globular molecules that are 90% retained by the membrane. In practice, the NMWC should be at least 20% lower than the molar mass of the molecule to be separated.

Further qualitative statements about the filtration can be made using the flux ( water value ) (transmembrane flow or penetration rate). Ideally, this is proportional to the transmembrane pressure and reciprocally to the membrane resistance. These variables are determined both by the properties of the membrane used and by concentration polarization and any fouling that may occur . The penetration rate is based on 1 m 2 of membrane area. Its unit is l / (m 2 h).

Applications

Diafiltration

One application of ultrafiltration is diafiltration . This procedure can be used to replace the buffer or change the salt concentration in a suspension . It combines the properties of dialysis with those of ultrafiltration. During the process, solvent is continuously added to the retentate until the old solvent has been completely exchanged across the membrane. If the inflow and outflow are adjusted, the suspension volume is not changed. This process is used in industry, but also in medicine (hemodiafiltration).

Drinking water treatment

Since the introduction of the new drinking water ordinance and the associated limit values ​​for the parameter turbidity, ultrafiltration has been used more and more in municipal drinking water treatment. This method is operated as a dead-end technique, whereby the membrane converts the entire amount of raw water into filtrate. Another advantage of this method is that the filtrate produced is sterile, which means that even contaminated groundwater can be safely treated after the effects of heavy rain. In 2005 the WAG Nordeifel waterworks in Roetgen put the world's largest system with a capacity of 6000 m³ / hour into operation.

Another area of ​​application for ultrafiltration is the treatment of circulating water in swimming pools in dead-end operation. At the end of 2002, the first system with 40 m³ / h was put into operation in Bad Steben, meanwhile more than 200 (status 12/2012) circuits have been equipped with the new process. The currently largest systems are in the Bad Aibling thermal baths with five circuits with a total of 800 m³ / h circulation capacity.

Wastewater treatment

Among other things, ultrafiltration is being used more and more often in wastewater treatment . This can be done in a variety of ways. It can be used as a so-called "polishing step" (1), i.e. after the conventional wastewater treatment process. However, ultrafiltration can also be used directly in the aeration tank (2), where it can replace other downstream process steps.

  1. This process is usually used when older systems are to be upgraded to further treat the wastewater.
  2. Membrane activation reactors are usually used where there is little space available or where the concentration of the biomass in the reactor sludge enables additional degradation rates to be achieved. This is especially true for substances that are difficult to biodegrade, such as B. Drug residues . Another advantage is the almost sterile filtration due to the selected pore size and the resulting separation of pathogenic germs .

Industrial parts cleaning

Ultrafiltration systems are used to reduce surfactant consumption in industrial parts cleaning . The filters and the pressures are selected so that the unused surfactant molecules (diameter <5 nm) can pass through the filters, but the enclosed colloids of dirt cannot. This means that the utilization rate of the surfactants can be increased to 95%. One application example is the production of sheet metal for car bodies .

Other uses

  • Separation of proteins (e.g. from milk)
  • cold sterilization in pharmacy (antibiotic production)
  • Metal recovery and wastewater treatment in metallurgy
  • Food treatment
  • Dialysis (filtering off plasma water without substitution) and other blood treatments
  • Concentration of milk before making cheese
  • Fractionation of proteins
  • Clarification of fruit juices
  • Separation of vaccines and antibiotics from fermentation broths

literature

  • Munir Cheryan: Handbook of Ultrafiltration . B. Behr's Verlag GmbH & Co, 1990, ISBN 3-925673-87-3 .
  • Rautenbach, Robert: Membrane process basics of module and system design . Springer-Verlag, 1997, ISBN 3-540-61573-3 .
  • Staude, Eberhard: Membranes and membrane processes . VCH Verlagsgesellschaft mbH, 1992, ISBN 3-527-28041-3 .

Individual evidence

  1. ↑ Terminology for membranes and membrane processes. In: Journal of Membrane Science , 120, 149-159, 1996.
  2. ↑ enwor supplies drinking water from one of the world's largest ultrafiltration systems
  3. ↑ W. Koller, TH Luger, Ch. Putensen, G. Putz: Blood purifying procedures in intensive care medicine. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid. 1994, ISBN 3-540-57904-4 , pp. 404-419; here: p. 408.

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