Unlike filtration membranes, filter sheets provide depth filtration throughout the entire media, allowing for a high contaminant retention capacity. However, this is not their only advantage in solid/liquid separation. Filtration principle, filter media specificities, applications… our article covers everything you need to know about filter sheets.
A triple action against impurities
Filter sheets offer a high contaminant retention capacity. They combine surface filtration, depth filtration, and adsorption. During the filtration process, the liquid to be purified flows, under the effect of a differential pressure, through the fibres of the sheet.
The largest particles to be removed are stopped at the inlet surface by a sieving effect. Intermediate-size particles are mechanically retained, trapped within the asymmetric porous structures of the media, as the liquid passes through progressively smaller pores as it moves through the sheet. Finally, residual particles—the finest ones, whose size is smaller than that of the smallest pores—are retained by adsorption on the outlet face of the sheet.
The main advantage of depth filters is therefore their high pollutant retention capacity, as opposed to surface filtration, where contaminants are retained solely by a mechanical surface screening action.
Which media for depth filtration?
Filter sheets are composed of cellulosic fibres, to which inorganic filtration aids are added in varying proportions: natural diatomaceous earth and perlite. Moisture-resistance agents, such as polyamidoamine, are also incorporated.
The cellulose fibres, finely fibrillated and refined, form a three-dimensional sieve featuring a large number of small, fine, and tortuous channels. The manufacturing method used makes it possible to obtain an asymmetric structure: pore size gradually decreases between the inlet face and the outlet face of the sheet.
Filtration aids are then incorporated into this cellulosic matrix in order to increase the internal surface area and ensure the desired porosity. Diatomaceous earth, or kieselguhr, is a powder obtained from the crushing of diatomite, a silica-based rock resulting from the sedimentation and fossilisation of microscopic algae, the diatoms. It makes it possible to form a natural sieve between the cellulose fibres and preferentially retains polysaccharides and proteins.
Perlite is used to fill the voids left by the cellulose and diatomaceous earth. It is a volcanic rock, composed mainly of silica. Once extracted, it is thermally expanded to form a very fine cellular structure, which is then crushed.
Finally, thermosetting resins, added in small quantities as wet-strength agents, prevent water-saturated sheets from disintegrating. These resins also facilitate the adsorption of negatively charged particles or micro-organisms whose size is smaller than that of the channels, through electrostatic interactions.
Expert advice
Caution: the amount of resin present in the filter media must be perfectly controlled by the manufacturer to avoid undesirable retention of active ingredients and premature filter clogging.
Formats and characteristics tailored to your filtration needs
Filter sheets generally have a thickness between 2.5 and 4.5 mm. They are available in several grades suitable for microbial reduction and for applications requiring fine filtration, clarifying filtration (pore diameter between 10 and 450 µm), or coarse filtration. When the pore diameter is below 0.22 µm (220 nm), filter sheets allow the retention of micro-organisms.
Filter sheets are available in various formats (20 × 20, 40 × 40, 60 × 60 cm) to match the standard dimensions of square filter presses, perforated or non-perforated, or in disc format. They can also be custom-sized. They must be handled with care to avoid damage; in particular, impacts, twisting, and abrasion should be avoided.
The manufacturer provides a nominal retention rating for the filter sheet, for indicative purposes. However, the actual retention characteristics of a filter sheet also depend on operating conditions and the properties of the liquid to be filtered (viscosity, pH, concentration, and polarity of contaminants). It is therefore always necessary to carry out on-site trials to validate the choice of a filter sheet.
For optimal filtration, each filter sheet model must be used under specific conditions of differential pressure (pressure difference between the inlet and outlet of the filter), flow rate, and filtration velocity, particularly to avoid any risk of premature clogging or contaminant release. Some models are suitable for the filtration of highly viscous fluids (liquids with a high content of colloids or particles) or are approved for the production of products intended for direct human consumption.
When its internal matrix is filled with particles, the depth filter is clogged and must be replaced. If the differential pressure is abnormally high and the outlet liquid flow rate is increasingly low, this is probably the case. Filter sheets offer a longer service life compared with surface filters.
Expert advice
During installation, care must be taken to position the filter sheets in the correct orientation, taking into account the direction of filtration. Generally, a filter sheet has a rough side and a smooth side. The liquid must always enter on the rough side and exit on the smooth side.
A wide range of applications
Filter sheets are used in the food industry, chemical and pharmaceutical industries, and in cosmetics and perfumery. In the food industry, they are notably used for the treatment of oils and beverages, such as spirits, wine, or beer. For beer, for example, they make it possible to retain ultra-fine colloidal hazes.
Some models incorporate activated carbon. The adsorption properties of the carbon retain fatty acids, decolourise, and deodorise the product. Filter sheets are also used to round out flavour and reduce astringency in certain beverages.
In oenology, clarifying filter sheets are used to achieve a filtration compromise: they make it possible to preserve the organoleptic qualities of the filtered wine while ensuring acceptable visual and microbiological stability. Filter sheets are used for coarse filtration of musts, clarification, and sterilising filtration of wines, up to deodorisation.
In the production of olive oils, depth filtration makes it possible to stabilise the product and obtain an organoleptically superior extra virgin olive oil. It enhances the quality of the product presented to the consumer by removing solids initially present in the olive oil.
This prevents their combustion during frying and therefore the formation of smoke and unpleasant odours. Filtering oil also improves its shelf life.
Expert advice
For olive oil filtration, it is recommended to use filter sheets manufactured with high-quality α-cellulose. Highly crystalline, they have a high degree of polymerisation and are the most stable and pure of the three types of cellulose.
The sediment present at the bottom of bottles is indeed rich in carbohydrates and protein substances, which undergo anaerobic fermentation, and in lipolytic enzymes that break down fat molecules. If they are not removed by filtration, prolonged storage of the oil may lead to the appearance of fermentation odours and an increase in acidity due to the acids released.
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