Filtration processes can be classified according to the pore size of the filter. Depending on this size, we refer to microfiltration (including the specific case of sterilizing filtration) or ultrafiltration. These two techniques are used to remove various particles such as bacteria, viruses, organic residues, and certain colorants.
They are implemented in separation, purification, or concentration steps along production lines. They help improve the sanitary quality, color, taste, turbidity, and odor of many products, particularly in the food and pharmaceutical industries. Our article explains everything you need to know.
Microfiltration and ultrafiltration: how do they work?
Microfiltration and ultrafiltration rely on the physical separation between contaminating particles and the remaining fluid. The quantity and size of particles removed are determined by the pore size of the filter. Substances larger than the filter pores are completely removed, while smaller ones are removed only partially, depending on the nature and thickness of the particle layer deposited on the filter over the course of its use.
Microfiltration
To perform microfiltration, membranes with pore sizes between 0.1 and 10 µm are used. Microfiltration membranes remove all bacteria. They also indirectly retain part of the viral contamination. Although viruses are on average smaller than the pores, they may still be stopped by the membrane by binding to the bacterial biofilm that covers it.
Microfiltration therefore removes microparticles, macromolecules such as polymers, suspended microorganisms, and certain viruses. It is used in the following fields:
• cold sterilization of beverages and pharmaceutical products
• clarification of fruit juices, wines, and beers
• separation of bacteria from water, for example in the treatment of biological wastewater
• effluent treatment
• separation of oil/water emulsions
• water pretreatment
• solid–liquid separation in the food and pharmaceutical industries
Ultrafiltration
PTo eliminate all viruses, ultrafiltration is required. This technique removes dissolved particles from 0.001 to 0.1 µm (i.e., 1 to 100 nm) from the treated fluids. It therefore blocks bacteria, yeasts, and most viruses.
It is used, for example, in:
• the dairy industry
• the food industry
• the metalworking industry, for separating oil/water emulsions
• the textile industry
Ultrafiltration can also be implemented as a pretreatment step before nanofiltration or reverse osmosis. It is also used in fundamental research for the separation and purification of proteins.
When the pore sizes of the filter are even smaller, the process is referred to as nanofiltration, then reverse osmosis.
Ultrafiltration vs. microfiltration: the higher the capability, the broader the application
Ultrafiltration is not necessarily the preferred solution. For example, in the dairy industry, milk is fractionated to isolate and concentrate various components—such as proteins—or to remove others, such as lactose. It may be necessary to eliminate colloids while retaining proteins in the residual fluid. In this case, microfiltration is preferable to ultrafiltration.
In the case of milk, ultrafiltration consists of passing milk through a membrane equipped with micropores, after first skimming it to prevent pore fouling by fat globules. This process yields milk with a satisfactory hygienic quality, without heating, and extends its shelf life.
Focus on sterilizing filtration
Sterilizing filtration refers to the use of microfiltration when the primary objective is the removal of microorganisms. Sterilizing filtration is particularly valuable for products containing thermosensitive molecules that cannot undergo heat-based sterilization.
In practice, this involves using filter pores with a diameter smaller than 0.22 µm (220 nm). To be classified as sterilizing, a filter must meet specific test requirements involving various microbiological or physical methods. Validation of a sterilizing filter notably includes verifying the sterilizing efficiency of the filter under actual operating conditions through a microbial challenge test performed with a reference microorganism, such as Brevundimonas diminuta.
Sterilizing filtration can be carried out, for example, using a pleated PES (polyethersulfone) membrane, which provides excellent bacterial reduction. Additionally, choosing thermally bonded cartridges optimizes integrity and ensures the lowest possible level of extractables in the filtrate.
Expert advice
Beyond the filter media, careful selection of the filter housing used for sterilizing filtration is essential. Using sanitary stainless-steel housings with a mechanically polished finish and a surface roughness below 0.8 µm ensures the absence of product retention and therefore prevents bacterial growth.
To learn more about sterilizing filtration
Discover two examples of sterilizing filtration in operation:
Sterilizing filtration skid for floral waters
Sterilizing filtration solution for a cosmetics wholesaler
Tangential or dead-end filtration?
Filtration techniques can also be classified according to how the fluid passes through the membrane, regardless of pore size. The term dead-end filtration is used when the fluid to be filtered flows perpendicularly across the filter surface, which retains the particles. When the fluid flows tangentially along the filter surface, the process is called crossflow filtration. In this case, it is the fluid pressure that drives permeation through the filter, while the particles remain within the tangential circulation stream.
Expert advice
To perform sterilizing filtration capable of retaining endotoxins, one may use cartridges composed of FiberFlo® hollow-fiber bundles, which offer a large filtration surface within a compact footprint. Used in dead-end filtration, they deliver the full incoming flow, whereas other endotoxin-retention systems require the use of crossflow filtration, which is more costly.
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