Liquids Prefiltration

Filter action generally becomes more efficient when lesser numbers of particles, whether animate or inanimate, confront the filter. Therefore, filtrations often utilize a combination of two (or more) filters to attain the desired extent of particle removal. The upstream filter is a prefilter. It has larger pores than the downstream or final filter. Depth-type prefilters are traditionally composed of fibrous materials designed as mats of fibers, or fleeces, whose wider pore distributions suit them for the function of prefilters, namely, for the removal of the larger particles of a particulate-load while allowing smaller particles to penetrate. Depth filters are constructed by the progressive deposition of fibers by any of several techniques. This type of construction follows the laws of chance. As a result, the fiber placements take place relatively randomly, regardless of directed attempts to manage their positioning.  In consequence, the depth filters are mats or fleeces of broad pore size distributions and are commonly classified by a nominal retention rating.
Depth filters are thicker than membranes because their fabrication involves a repetitive stacking of fibers into mats or fleeces, until a proper density is reached. The interstices within the fiber matrix invite particle penetrations into their depths, which, in turn, offer the inner space to accommodate them. Hence, their greater dirt loading capacities.
The sacrificial role of the prefilter in accepting part of the total load prolongs the functional life of the final filter which otherwise would require a larger area to accommodate the total particle load. Moreover, the combination of both filters effectively enhances the overall filtrative removal of the particulates (organisms) at a cost savings by substituting less costly prefilters for the greater expanse of final membrane filters that would otherwise have been needed. The rationale of using prefilter / final filter arrangements is that the more open prefilter is in series with the final filter downstream of it. The TSS, total suspended solids, has its larger pieces removed by the more open prefilter; the finer particles pass through to be retained by the less open final filter.  This series arrangement prolongs the service life of the final filter by sparing it a part of the particulate burden. A positive economic benefit results from diminishing the need for more frequent filter change-outs. The cost of prefilter is less than that of membrane.

Glass fibers, with and without binder materials or coatings, have long served as prefilters. They have proved particularly useful in the filtration of sera, blood products in general, and in wine production.  Glass fiber filters coated with cellulose nitrate polymer are especially helpful in removing proteinaceous impurities from various preparations.  Their open structure is enhanced by the strong adsorption of that polymer. The adsorption properties of glass fiber is the desired, as the retention will not only be selective to polar contaminants, but especially separates smaller contaminants than the actual nominal retention. In the past asbestos fiber depth filters, not used anylonger due to the carcinogenic nature, were the optimal protective filter due to the high adsorptivity. The adsorptive properties remove especially lipids from sera and media and colloidals from water and beverages. Glass fiber filters are also often utilized as "polishing" filters, as the adsorptive properties remove haze from media.

Polypropylene fibers are produced by melting, drawing or extruding the bulk polymer into fibers of designed thicknesses and lengths. These are widely used in computer-directed extrusions dedicated to the construction of prefilters of various shapes, sizes, and porosities. Among these are anisotropic mats or fleeces whose graded porosities, from one side of the mat to the other, endows them with some prefilter qualities. The more open (upstream) filter face permits a greater loading of particles within the filter's depths. These particles would otherwise form a more flow-resistant filter cake on the filter's surface. Depending upon the amount and makeup of the suspended solids, the anisotropic structure may yield larger throughputs along with greater load accommodations. In these mat constructions individual fibrous strands are melt (heat) bonded into an integral mass that frustrates individual fiber migration, a bugbear of earlier fibrous compositions.

Diatomeacous earth or cellulosic pads are used in lenticular filter designs and are mainly used as clarifying filters. Highly adsorptive cellulosic or kieselgur containing depth filter pads are bonded together in a plate format. These plate formats commonly have a diameter of 12" or 16" and are welded together in stacks of 4 to 16 to create a depth filter unit. The benefits of these depth filter materials are the tremendous dirt load capacity (total throughput). These filters are commonly used to prefilter solutions, which would blind membrane filters rapidly. The adsorptive depth filter material is ideal to separated colloidal substances and lipids, therefore these filters are very often found in plasma and serum applications. Recently these filters also find their use in the cell harvest step in downstream processing after the fermentation

Porous filters are also prepared from borosilicate, ceramic and metallic compositions. Although possessing their own advantages and limitations, these filters have yet to achieve a significant penetration of the pharmaceutical market.

Microporous membranes of pore size ratings larger than those of the final filter may also be used as prefilters. These filters are commonly of cellulosic material and range from 1.2 µm to 0.65 µm. Such fine prefiltration is required, when the media to be filtered has a wide spectrum of contaminants and size range. To establish which filter combination is the optimal to gain the highest utilization from the entire filter train, filterability trials are required.