Filter Sizing Examples

Naturally, the size ofthe membrane pores should depend on the size of microbes to be filtered. For example, membranes with 0.2 pm pores are often used to sterilize culture media, while membranes with 0.45 pm pores are often used for the removal of microbes from culture products. It should be noted here that the so-called pore size of a membrane is the size of the largest pores on the membrane surface. The sizes of the pores on the surface are not uniform moreover, the pore sizes usually decrease with increasing depth from the membrane surface. Such a pore size gradient will increase the total filter capacity, as smaller microbes and particles are captured within the inner pores of the membrane. 

Although it is beyond the scope of this book to provide equipment design information, the example below is given for a first look filter-sizing exercise. 

Collection on porous filter media is perhaps the most efficient means of particle removal. Aerosol filtration is an effective means of air purification, while at the same time it has been widely used for sampling airborne material for mass and chemical composition determination. A wide variety of filter media is available, ranging from fibrous mats of relatively inert material to porous membranes. Fibrous mats and model filter arrays appear microscopically as stacks of overlaid cylinders, where the cylinders may be smooth or rough. In contrast, the membrane media are plastic films with microscopic holes of nearly uniform size nuclepore filters, for example, are produced of sheets of polyester, and the holes are introduced by neutron bombardment. 

Example 1 Substance of known triple-point in a triple-point cell Example 2 Glass of known optical density in a transmission filter holder Example 3 Spheres of uniform size mounted on a microscope slide. 

An example of a solid-liquid phase separation - often referred to as a mechanical separation - is filtration. Filters are also used in gas-sohd separation. Filtration may be used to recover liquid or sohd or both. Also, it can be used in waste-treatment processes. Walas [6] describes many solid-hquid separators, but we will only consider the rotary-drum filter. Reliable sizing of rotary-drum filters requires bench and pilot-scale testing with the slurry. Nevertheless, a model of the filtering process will show some of the physical factors that influence filtration and will give a preliminary estimate of the filter size in those cases where data are available. 

Proper selection of the filter medium is more of an art than a science. The filter cutoff must be chosen to capture the smallest particles of interest. Other factors that must be considered are the type of filter (bulk or surface), the required flow rate, and the size of the membrane. These parameters are not independent and the best choice will usually involve trade-offs. Finally, the material from which the filter is made must be considered. It must be selected for compatibility with the intended postfiltration processing. Glass-fiber filters, for example, often have very high blanks for common ions such as chloride and sodium. 

Various types of filtration equipment are available commercially and can be operated in batch, semicon-tinuous, or continuous modes. Among the commonly used types are the plate and frame filter, rotary drum filter, leaf filter, plate filter, and tray filter. Apart from the plate and tray filters, all other are enclosed and therefore are easy to work with when sterility of the solids is an important issue. Moreover, all these filters are examples of dead-end filters. Cross-flow filtration is mostly used in the purification stage through membranes with very low pore sizes and is discussed later. 

Filter size selection depends on the MW and solution concentration. Use an appropriate size of filter to prepare polymer solutions, so the large molecules will not be excluded by the filter. If there is no information about the MW of the polymer, a large size filter of 5, 8, or 10 micrometers is recommended. Examples are shown below. 

An example for a filter design is given in Reference 58. Because of the considerably low working frequency of the UMTS (Universal Mobile Telecommunication System)-filter (about 2 GHz), a combline topology was chosen to achieve a small filter size. Figure 9.72 shows a drawing of the filter witiiout external grotmd planes. 

Particulate interferents can be separated from dissolved analytes by filtration, using a filter whose pore size retains the interferent. This separation technique is important in the analysis of many natural waters, for which the presence of suspended solids may interfere in the analysis. Filtration also can be used to isolate analytes present as solid particulates from dissolved ions in the sample matrix. For example, this is a necessary step in gravimetry, in which the analyte is isolated as a precipitate. A more detailed description of the types of available filters is found in the discussion of precipitation gravimetry and particulate gravimetry in Chapter 8. 

Although Brevundimonas (Pseudomonas) diminuta (ATCC 19146) is most commonly used for steriliziug-grade filter vaUdation, iu certain appHcations other bacteria are used. For example, when it is necessary to demonstrate removal of mycoplasma in appHcations involving sera and tissue culture media, membranes having a smaller pore size rating, eg, 0.1 p.m, are frequentiy used. For these membranes,laidlawii may be employed for vaHdation purposes (9). 

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