Microfiltration membrane pore size

The most popular nominal pore size in microflltration is 0.2 pm, with many polymer membranes and some ceramic types available at this size. These fibers are believed to provide a sterile liquid because bacteria are retained in the retentate and the permeate is bacteria flee. In order to ensure sterile conditions a 0.1 pm membrane is sometimes used. This is also, therefore, a popular commercial microfiltration membrane pore size. For details of the standard biological test see Section 6.6.1. Other cellular material of a biological origin, such as beer and wine yeasts, can be filtered by similar pore dze membranes. Coarser pore sizes are available 0.45, 0.8,1.0, 1.2,2.0, 3.0, 5.0 and 10 pm. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

The principle of microfiltration is the application of hydrostatic pressure on a microporous filter membrane, so that the pressure difference forces solutes, water molecules, and particles smaller than the membrane pore size to flow across the pores, retaining and concentrating the larger particles in the suspension. 

MWCO), usually defined as the molar mass at which the membrane rejects 90% of solute molecules. However, as in microfiltration, the molecular shape can affect permeability through the membrane pores. For example, a membrane with a nominal cut-off of 100 kDa, which does not allow globular molecules with a molar mass of 100 kDa to flow through, may allow fibrous molecules with higher molar masses to flow across the pores. As in microfiltration, the membrane pore size is not uniform, with a normal distribution around an average value. 

According to Castelas and Serrano [53] microfiltration with pore sizes over 0.4 pm does not influence the wine, whereas pore sizes of 0.25 pm and lower disturb the organoleptic characteristics of the wine. However the complete removal of bacteria can only be achieved by 0.2 pm. Fouling of the membranes (Membralox) with coarser pore sizes, limits fluxes to 40-601/m h, 0.2 pm is less affected and retains a flux of 851/m h. 

Once the cells have been removed, the enzyme broth is concentrated by evaporation or ultrafiltration. In ultrafiltration, membrane pore sizes are much smaller than in microfiltration, allowing only water and small molecules to travel through the membrane. Thus, enzymes can be concentrated under mild process conditions. Typical molecular weight cut off pore sizes for ultrafiltration membranes are 5000, 10,000, or 30,000 Daltons. 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The membrane pore size, shape, and electrostatic surface charge are fundamental to particle removal. Reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) relate to separation of ions, macromolecules and particles in the 0.001-10 pm range. 

In cross-flow flltration, the wastewater flows under pressure at a fairly high velocity tangentially or across the filter medium. A thin layer of solids form on the surface of the medium, but the high liquid velocity keeps the layer from building up. At the same time, the liquid permeates the membrane producing a clear filtrate. Filter media may be ceramic, metal (e.g., sintered stainless steel or porous alumina), or a polymer membrane (cellulose acetate, polyamide, and polyacrylonitrile) with pores small enough to exclude most suspended particles. Examples of cross filtration are microfiltration with pore sizes ranging from 0.1 to 5 pm and ultrafiltration with pore sizes from 1 pm down to about 0,001 pm. 

One process sometimes used for low flow rates (<50 m /h) is lime or caustic soda softening followed by cross-flow microfiltration (XMF) and RO poHshing [21]. The MF membrane (pore size <0.2 pm) is tubular with a diameter of 1.27 or 2.54 cm. Due to the large diameter of the tubes, the membranes can handle feeds with sohd levels of up to 5% at a very high membrane flux (375—500 huh). The XMF filtrate is of high quality with turbidity <0.1 NTU and SDI < 3.0. 

Separation processes such as ultrafiltration and microfiltration use porous membranes which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration membranes have pore sizes from 0.001 to 0.1 pm while microfiltration membranes have pore sizes in the range of 0.02 to 10 pm. The production of these membranes is almost exclusively based on non-solvent inversion method which has two essential steps the polymer is dissolved in a solvent, cast to form a fUm then the film is exposed to a non-solvent. Two factors determine the quality of the membrane pore size and selectivity. Selectivity is determined by how narrow the distribution of pore size is. In order to obtain membranes with good selectivity, one must control the non-solvent inversion process so that it inverts slowly. If it occurs too fast, it causes the formation of pores of different sizes which will be non-uniformly distributed. This can be prevented either by an introduction of a large mun-ber of nuclei, which are uniformly distributed in the polymer membrane or by the use of a solvent combination which regulates the rate of solvent replacement. 

Ultrafiltration is a membrane process whose nature lies between nanofiltration and microfiltration. The pore sizes of the membranes used range from 0.05 um (on the microfiltration side) to 1 am (on the nanofiltration side). Ultrafiltration is typically used to retain macromolecules and colloids from a solution, the lower limit being solutes with molecular weights of a few thousand Daltons. Ultrafiltration and microfiltration membranes can both be considered as porous membranes where rejection is determined mainly by the size and shape of the solutes relative to the pore size in the membrane and where the transport of solvent is directly proportional to the applied pressure. Such convective solvent flow through a porous membrane can be described by the Kozeny-Carman equation (see eq. VI - 27) for example. In fact both microfiltration and ultrafiltration involve similar membrane processes based on the same separation principle. However, an important difference is that ultrafiltration membranes have an asymmetric structure with a much denser toplayer (smaller pore size and lower surface porosity) and consequently a much higher hydrodynamic resistance. 

Microfiltration (MF), like ultrafiltration, is a pressure-driven membrane process. In the case of microfiltration, the pore size is typically in the 0.03- to O.l-pm range. In this range, individual bacteria and viruses will pass through the membrane. However, colloidal suspended sohds, floe particles, and parasite cysts are prevented from passing. No removal of dissolved solids is accomplished by microfiltration membranes. 

Separation processes such as ultrafiltration and microfiltration use porous membranes which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration membranes have pore sizes from 0.001 to 0.1 pm while microfiltration membranes have pore sizes in the range of 0.02 to 10 pm. The production of these membranes is almost exclusively based on non-solvent inversion method which has two essen-... 

Membrane Cliaraeterization MF membranes are rated bvtliix and pore size. Microfiltration membranes are imiqiielv testable bv direct examination, but since the number of pores that rnav be obsen ed directlv bv microscope is so small, microscopic pore size determination is rnainlv useful for membrane research and verification of other pore-size-determining methods. Furthermore, the most critical dimension rnav not be obseiA able from the surface. Few MF membranes have neat, cvlindrical pores. Indirect means of measurement are generallv superior. Accurate characterization of MF membranes is a continuing research topic for which interested parties should consult the current literature. 

Fig. 16.6. Atomic force microscope image of a polycarbonate microfiltration membrane (cyclopore), 0.2 p,m pore size.

Fig. 16.8. Atomic force microscope image of anodise microfiltration membrane, 0.2 xm pore size.

The physical characterisation of membrane structure is important if the correct membrane is to be selected for a given application. The pore structure of microfiltration membranes is relatively easy to characterise, SEM and AFM being the most convenient method and allowing three-dimensional structure of the membrane to be determined. Other techniques such as the bubble point, mercury intrusion or permeability methods use measurements of the permeability of membranes to fluids. Both the maximum pore size and the pore size distribution may be determined.1315 A parameter often quoted in manufacturer s literature is the nominal... 

S-layer ultrafiltration membranes (SUMs) are isoporous structures with very sharp molecular exclusion limits (see Section III.B). SUMs were manufactured by depositing S-layer-carrying cell wall fragments of B. sphaericus CCM 2120 on commercial microfiltration membranes with a pore size up to 1 pm in a pressure-dependent process [73]. Mechanical and chemical resistance of these composite structures could be improved by introducing inter- and intramolecular covalent linkages between the individual S-layer subunits. The uni-... 

Membrane Morphology—Pores, Symmetric, Composite Only nucleopore and anodyne membranes have relatively uniform pores. Reverse osmosis, gas permeation, and pervaporation membranes have nonuniform angstrom-sized pores corresponding to spaces in between the rigid or agamic membrane molecules. Solute-membrane molecular interactions are very high. Ultrafiltration membranes have nonuniform nanometer sized pores with some solute-membrane interactions. For other microfiltration membranes with nonuniform pores on the submicrometer to micrometer range, solute-membrane interactions are small. 

Membrane Characterization MF membranes are rated by flux and pore size. Microfiltration membranes are uniquely testable by... 

---