Sterile air and gas filters

A compressed air filter is a pressure vessel. While in service, filters must be depressurized before any maintenance is attempted. The filter should be depressurized by slowly opening the drain or trap provided for condensate removal. Failure to depressurize slowly can result in damage to equipment and injury to personnel. 

The manufacturer s warnings should be observed at all times with regard to cleaning the filter, to avoid danger to filter parts. Since some cleaning solvents attack seals, or transparent bowls or housings, these should be cleaned only with soap and water. 

Any product that can be contaminated by airborne bacteria must be protected against them. In the case of foods, beverages and chemicals produced by fermentation, ingress of bacteria would cause serious defects in the products, if not their complete rejection. 

Air for process input will include aeration of liquids, seed fermentation and laboratory applications. Air for mixing into the preparatory chemicals or the ultimate product requires to be as clean and sterile as the material with which it mixes, so there can be no question of fouling by solids, liquids or micro-organisms. 

Filter medium material for the removal of viable organisms from compressed air tends to be either of borosilicate glass microfibres or meltspun fleece or PTFE membranes, supported on 100% glass fibre woven fabric or polysulphone or polypropylene spunbonded textiles. 

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Infection of the wort damages beer quality, and a major sonrce of contamination is improperly filtered aeration of the wort. Aeration requires a clean, well-filtered sterile air source for successful beer production. Hydrophobic (non-wettable) membrane filter cartridges are ideal for sterile air and gas filtration. 

Because of the much closer pore distribution than with flat manbrane media, the number of pores per unit filter area is far greater, thereby extending the service life of the element. Equally a closer pore distribution means that the largest pore is considerably reduced a conventional membrane has a maximum pore size at least 0.3 pm larger than that of hollow fibre material. This development of membrane filter elements with a rated pore size of 0.1 pm, a longer service life and a reduced pore size distribution is a considerable aid to safer compressed air and gas sterilization, as this means the ability to retain viruses as well as bacteria. 

Drug manufacturers go to great lengths to avoid contamination of product in gas, air, and water systems. It is the authors opinion that stopper manufacturers should identify and quantify the volatile components in their products to help the users determine which stoppers are most suitable for their product. Similar information is routinely available for such product contact items as tubing and filters used for filling and sterilization operations. 

In the diffusive-flow test, also called the forward flow test, the filter membrane or candle in its tightly closed case is set under continuous test gas pressure. The amount of air that diffuses through the filter membrane per time unit is measured downstream, on the sterile side of the filter. The pressure drop over the membrane should be constant during the test to prevent variations in diffusion rate. Collection and measurement of the air on the sterile side often require actions that may lead to contamination of the setup, and thus these actions should be performed aseptically. Furthermore, for small filter surface areas the volume of air that diffuses through the filter is small and therefore no accurate measurements are possible for these small filters. 

The importance of clean cuvettes is self-evident. Routinely, all non-disposable cuvettes should be emptied immediately after use, rinsed repeatedly in the solvent (e.g. water), then with clean ethanol or acetone and dried with low pressure air or nitrogen from a cylinder. It is prudent to install a filter (such as those with pore sizes of 0.45 pm used in filter sterilization) in the gas line. Cuvette washers (e.g. Aldrich) wash, rinse, and dry cuvettes. Cotton wool buds can also be usefiil for dislodging interior, stubborn marks and for drying. The outside optical surfoces should be polished with clean lens tissue. Note that plastic squeety bottles generally used for solvents contain plasticizers such as butyl phthalate, which can interfere with critical UV spectra. 

Sterile Filtration of Gases. Primary appHcations for sterile gas filtration are the sterilization of fermentor inlet air, fermentor vent gas, vents on water for injection tanks, and vacuum break filters during lyophilization. Operational and process considerations apply. Typically, the membrane in gas... 

Filters for use in sterile gas filtration must conform to standards similar to those mandated for sterile hquid filtration. Nondestmctive integrity tests may be apphed. The tests are performed by wetting the filter with an appropriate solvent, commonly 60/40 isopropyl alcohol/water for hydrophobic membranes, and applying air or nitrogen gas at a preset pressure. 

A known amount of steam is then introduced and the sterilizer is left to soak" for a short period. This is followed by injection of the sterilizing gas to its specified pressure. The sterilizer and its contents are then held under these conditions for the specified time of exposure. At the end of this period the gas is removed by evacuation and replaced by air that has passed through a bacteria-retentive filter. 

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