The micro filtration is a method for the filtration by membranes with a pore size> 0.1 microns (see Membrane Technology # pore size and selectivity ). In contrast, the pore size in ultrafiltration is less than 0.1 µm. The two processes also differ in their membrane structures and filter materials .
Both are purely physical (mechanical) membrane separation processes. They separate according to the principle of mechanical size exclusion (filtration principle), i.e. H. any particles in the fluids that are larger than the membrane pores are retained by the membrane. The driving force in both separation processes is the differential pressure between the inlet and outlet of the filter surface, which is between 0.1 and 10 bar .
Depending on the application, the filter surface can consist of stainless steel , plastic , ceramic or textile fabric . Microfiltration is preferred for the filtration of beverages and oil as well as for pre-filtration. The different appearances of the filter elements are described below.
In microfiltration, the candle filter consists of a filter housing and one or more candles inserted in it, through which the fluid flows from the outside to the inside.
The candles can be constructed in different ways. A typical design are winding candles that are wound from a thread made of synthetic or natural material. Other candles can consist of ceramic , metal ( sintered metal or multi-layer metal mesh) depending on the application . In addition, there are candles with exchangeable filter media (mostly of textile origin), which are more likely to be used in cake-forming filtration. Exceptions are filter cartridges that can be covered in applications in which precoating is used, whereby the microfiltration takes place in a kind of depth filtration in the pre- washed filter cake of the filter aid .
The advantages of closed candle filters are a closed filter system without any drip losses and good regenerability. A disadvantage is the sometimes significantly higher requirements for the preparation and pre-clarification of the products in order to avoid expensive, premature relocation of the candles.
A service filter for the supply media cold water and hot water, steam and alkali avoids secondary problems caused by particulate contamination of regeneration and sterilization media and ensures the best possible regeneration.
The following have been optimized as cartridge pre-filters for effective protection of a downstream membrane filter:
- a pleated configuration consisting of a glass fiber pre-filter fleece and a membrane with 1.2 µm or 0.8 µm pore size
- Wrapped, asymmetrical polypropylene matrix with a validated separation rate of 1 µm or 0.5 µm (β value 5,000).
- Another type made of coordinated, pleated layers of polypropylene fleece. It is characterized by mechanical and chemical stability as well as excellent regenerability. There are now many areas of application for this candle, e.g. B. in beverage filtration as a validated 1 µm pre-filter cartridge for wines from southern growing areas with low colloid pollution.
Pocket filters are sack-like structures made of textiles or paper. They are usually used as a pre-filter.
Hollow fiber modules
Hollow fibers (also called capillary membranes or hollow fibers) with a length of 2 to more than 200 cm are combined to form hollow fiber modules with a filter area of a few square millimeters up to approx. 100 square meters . The hollow fibers are potted ( potted ) at both ends to prevent hydraulic short-circuiting , so that there is no axial flow through them. Instead, its walls act as a membrane. Their advantage is the accommodation of a large filter surface in a very small space and the generally low manufacturing costs.
Microfiltration modules made of hollow fibers are used in medicine ( plasmapheresis , in-line particle filtration for infusions , sterile filtration) and in water filtration (drinking water: point-of-use, reverse osmosis pre-filtration).