Membrane filters applications

Hayashi, K. et al., Membrane filter applications for cmd separation from radioactive waste water generated in LWR power plants. In proceedings of Symposium on On-site Management of Power Plant Wastes, Zurich, 1979, OECD/NEA, Paris, 229, 1979. 

Ultrafiltration utilizes membrane filters with small pore sizes ranging from O.OlS t to in order to collect small particles, to separate small particle sizes, or to obtain particle-free solutions for a variety of applications. Membrane filters are characterized by a smallness and uniformity of pore size difficult to achieve with cellulosic filters. They are further characterized by thinness, strength, flexibility, low absorption and adsorption, and a flat surface texture. These properties are useful for a variety of analytical procedures. In the analytical laboratory, ultrafiltration is especially useful for gravimetric analysis, optical microscopy, and X-ray fluorescence studies. 

Dead-end filtration through membrane filters is common in some industries where high purity is imperative. When clogged, the membrane has to be replaced. The water is first purified, and the filters serve as a final polisher. They are unsuitable for applications where they have to remove any significant concentration of particulate matter, as the cost of membrane replacement can become very high. 

They may also be required in industrial applications where they become part of venting systems on fermenters, centrifuges, autoclaves and freeze-dryers. Certain types of filter (membrane filters) also have an important role in sterility testing, where they can be employed to trap and concentrate contaminating organisms from solutions under... 

Other applications of filters include sterilization of venting or displacement air in tissue and microbiological culture (carbon filters and hydrophobic membrane filters) decontamination of air in mechanical ventilators (glass fibre filters) treatment of exhausted air ftom microbiological safety cabinets (HEPA filters) and the clarification and sterilization of medical gases (glass wool depth filters and hydrophobic membrane filters). 

Airborne particles were collected outside the laboratory by drawing air at the rate of 30 cu. meters/day through a 4.7 cm. diameter, 0.8-p pore size, membrane filter (I). The air intake was 5 meters above ground and was shielded from precipitation by a glass tube. Air was drawn through the filter for 24-hour periods. The filter was collected at 8 30 a.m. (either EST or EDT, as applicable), Tuesdays through Fridays, and the a-activity was measured with a scintillation detector within 2 minutes of collection. 

The earliest commercially available filters were manufactured in two pore sizes 0.45 and 0.8 pm. The 0.45 pm-rated membranes were considered to be sterilizing-grade filters and were successfully used in the sterile filtration of pharmaceuticals and parenterals. The membrane filters were qualified using Serratia marcescens, a standard bacterium, having dimensions of 0.6 x 1 pm. However, in the late 1960s it became apparent that the matrix of the 0.45 pm-rated filters could be penetrated by some pseudomonad-like organisms (1). For sterile filtration applications in the 1990s, 0.2 pm-rated membranes are the industry standard in the manufacture of sterile parenterals and pharmaceuticals. 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Depth membrane filters are usually preferred for in-line filtration. As particles are trapped within the membrane, the permeability falls, and the pressure required to maintain a useful filtrate flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particle loading of the feed solution. A typical application of in-line depth microfiltration membranes is final polishing of ultrapure water just prior to use. Screen membrane filters are preferred for the cross-flow microfiltration systems shown in Figure 7.1(b). Because screen filters collect the retained particles on the surface of the membrane, the recirculating fluid helps to keep the filter clean. 

The May Pack has been used world-wide to characterise radioiodine. In some applications, glass fibre-filters have replaced membrane filters and copper or silver gauzes have been used to trap elemental iodine. The separation of iodine species in the May Pack is at best qualitative. Some inorganic iodine vapour may be adsorbed on the particulate filter. Conversely, iodine adsorbed on particles, and trapped on the particulate filter, may be desorbed during extended periods of sampling. 

Filtration is a unit operation commonly employed nowadays in biotechnological processes. In this unit operation, a filter medium acts as a physical barrier to particles larger than its pores. Traditional filtration devices such as filter presses and rotary vacuum drum filters have so far found no application for the separation of animal cells. Nevertheless, membrane filters are commonly employed, as well as some alternative filter designs such as spin-filters. In the next sections, the most common types of filters used for animal cell separation will be discussed. 

Membrane polymeric materials for separation applications are made of polyamide, polypropylene, polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, cellulose diacetate, polystyrene resins cross-linked with divinylbenzene, and others (see Section 2.9) [59-61], The use of polyamide membrane filters is suggested for particle-removing filtration of water, aqueous solutions and solvents, as well as for the sterile filtration of liquids. The polysulfone and polyethersulfone membranes are widely applied in the biotechnological and pharmaceutical industries for the purification of enzymes and peptides. Cellulose acetate membrane filters are hydrophilic, and consequently, are suitable as a filtering membrane for aqueous and alcoholic media. 

Eumstead ( ) chose native fluorite as an internal standard for the analysis of quartz in coal dust His procedure consisted of mixing 0 20 mg fluorite into each water suspension of standard and sample and depositing on silver membrane filters for a calibration curve (fluorite/quartz intensity ratio i[ . mg quartz) The application of this method to coal dust samples containing less than 1% quartz produced acceptable results relative standard deviation (RSD) was 18 2 ... 

Various formulations of nylon membrane filters have appeared on the market in recent years.32 The filter material is less brittle than nitrocellulose, and the stable linkages formed with DNA allow stripping off probes by heat while still retaining the target DNA on the filter, thus allowing reuse. The membranes have proved particularly effective for Southern blot hybridizations and are quite amenable to use in slot-blot and dot-blot devices, which allow application of up to 96 DNAs on a single filter. The membranes can be incubated in thermostable plastic pouches that require only a small volume of buffer. However, quantitation of DNA hybridizations on these filters has been no better than for nitrocellulose (C. P. Kurtzman, unpublished, 1987). 

Cell harvesters were developed to capture multiple samples of cells on membrane filters, wash away unincorporated isotopes, and prepare samples for liquid scintillation counting on special equipment developed to process and count multiple samples. Despite miniaturization and improvements in efficiency of this technique, the disadvantages of multiple liquid handling steps and increasing costs for disposal of radioactive waste materials severely limit its usefulness. Although specific applications require measuring DNA synthesis as a marker for cell proliferation, much better choices are available for detecting viable cell number for HTS. 

The maximum contaminant levels for coliform bacteria are applicable to community and noncommunity water systems (Shelton, 1989). Two methods are used to measure coliform bacteria in water. One is the membrane filter technique and the other is the fermentation tube method. For details, see Shelton (1989). 

In phase contrast microscopy particles are sampled on to a membrane filter that is treated with acetone and glycerin triacetate to make the filter transparent. Problems associated with the preparation step, and low depth of focus limit thus the applicability of the method [153]. 

---