Membrane filters microfilters

Efforts to overcome the limitations of the fragile membranes (as delicate as soap bubbles) have evolved with the use of membrane supports, such as polycarbonate filters (straight-through pores) [543] or other more porous microfilters (sponge-like pore structure) [545-548]. 

A microfilter for this industry is considered sterile if it achieves a log reduction factor of better than 7. This means that if 107 bacteria/cm2 are placed on the filter, none appears in the filtrate. A direct relationship exists between the log reduction factor and the bubble point of a membrane. 

Filters are available in several constructions, effective filtration areas, and configurations. Depending on the individual process, the filter construction and setup will be chosen to fit its purpose best. Most commonly used for RO filters are tubular devices, so-called spiral wound modules due to the spiral configuration of the membrane within the support construction of such device. UF systems can be found as a spiral wound module, a hollow fiber, or a cassette device. The choice of the individual construction depends on the requirements and purposes towards the UF device. Similar to the different membrane materials, UF device construction has to be evaluated in the specific applications to reach an optimal functioning of the unit. Microfilters and depth filters can be lenticular modules or sheets but are mainly cylindrical filter elements of various sizes and filtration areas, from very small scale of 300 cm to large scale devices of 36 m. A 10-inch high cylindrical filter element can be seen in Fig. 6. 

Microfilters use membranes with pores in the 0.1-1 pm range. They can filter out particles of dust, activated carbon, and ion exchange resin fines, and most microorganisms. Microfilters require low differential pressures (5-20 psi) and are available both as normal flow ( dead end ) and crossflow configurations. In pharmaceutical water purification systems, they are often used as disposable cartridge filters after activated carbon filters, softeners, and ion exchange beds. 

Table 12 shows the typical LRV values obtained using a polymeric and ceramic microfilter. Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts of bacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use of ceramic filters may allow the user to combine the sterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life (5 years or longer). 

Microfilters, Fig. 1 Left. (a) schematic and (b) picture of a microfiuidic blood filtration device [32]. The top and bottom pieces are made up of PDMS, and the membrane is a commercially available hydrophilic polypropylene filter. Right, schematic of another device fabricated similarly but with multiple layers of nanoporous membranes [33]... 

Further distinction has to be made between conventional filtration of fine particle less than 10 pm in diameter, and microfiltration. It would be unusual for the filtration of such particles on a conventional fiher cloth to be described as ndcrofiltratian. Thus microfihration is constituted by the filtration of small particles and by the medimn which is used for the filtration. Conventional fihration is undertaken on filter cloths with a very open structure, see Chapter 4, whereas membrane fihration is usua% concerned with fihration enq>loying membrane media where the equivalent pore size is of the order of 10 pm, or less. These definitions are, however, becoming less distinct as it is now possible to obtain conventional fihration equ ment employing membrane-type fiher media, as discussed in Chapter 4, and crossflow microfilters enqploying conventional filter cloth. 

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