The Structure of Depth Filters

Depth filters are playing an increasingly vital role in the pharmaceutical industry, especially in the biopharmaceutical sector, and come in various forms and materials to meet different process requirements. In this Alioth “Deep In, Clear Out” Knowledge Series , we will provide a detailed introduction to the structure and product types of depth filters.

Structure of depth Filters

The earliest development of the filter media began in the early 1890s, typically using materials such as cellulose and asbestos fibers. Both materials offered excellent heat resistance, chemical stability, and resistance to hydraulic stress, making them ideal for creating various high-efficiency filtration models. However, with the prohibition of asbestos fibers in pharmaceutical processes, inorganic filter aids and charge-modified filtration media emerged, bringing new vitality to the advancement of filtration technology.

Since then, depth filters have undergone a series of technological innovations, including chemical modification to enhance surface charge, submicron particle media, and more efficient structural designs. Today, most depth filtration media (i.e., filter plates) are composed of cellulose, diatomaceous earth, and high-performance resins. Key parameters such as thickness, surface area, and porosity play crucial roles in particle removal.

Cellulose

• Provides a three-dimensional framework
• Offers large internal space
• Exhibits strong hydrophilicity

Depth filters primarily use cellulose as the filtration medium. The cellulose forms a reticular structure within the filter, with gaps between the fibers capturing particles from the process fluid. As filtration progresses, particles accumulate on the surface to form a filter cake, which initially enhances filtration efficiency. However, as the filter cake thickens, it also leads to a decrease in filtration speed.

Filter aids

• Provides space for contaminant retention
• Adsorbs impurities, Delays fouling, and increases capacity

To increase the flow rate and loading capacity of filters, filter aids are often incorporated into cellulose media. Common filter aids include diatomaceous earth, perlite, activated carbon, and other silica-based materials. Other filter components typically consist of various elastomeric and polymeric materials.

Resin

• Wet-strength agent
• Binder
• Positive charge modification

The third component of depth filtration media is resin. A certain proportion of resin, acting as a binder for the media, can enhance the wet tensile strength. Some special resins can also provide a positive surface charge, whicheffectively adsorbs impurities such as DNA, host cell proteins (HCP), and endotoxins.

Product formats of Depth Filters

Depth filters typically come in various structural formats, including membrane disc-type, stack-(lenticular) type, and plate-and-frame configurations.

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1.Membrane Disc-Type

The membrane disc-type configuration typically consists of flat filter sheets enclosed within a supporting frame and is one of the more traditional designs, with a long history of use in pharmaceutical processes. Its advantages include high loading capacity and low operating costs. Additionally, the filter surface area can be adjusted by adding or removing flat sheets to accommodate different filtration volume requirements. However, this design is gradually being replaced by other configurations due to complexity in assembly, disassembly, and cleaning. Moreover, older designs are prone to leakage, though newer designs are addressing these shortcomings.

2.Stack- (Lenticular) Type

The stack-type (Lenticular) configuration typically consists of multiple disc-shaped filter modules installed vertically or horizontally within a reusable filter housing (e.g., stainless steel). Its main advantages over the disc-type design are easier assembly and disassembly, as well as easier cleaning of the reusable housing.

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3.Plate-and-Frame Filter

The plate-and-frame filter is typically designed for single-use applications and is available in various filtration areas to accommodate different processing volumes required for small scale R&D, pilot-scale, and production-scale operations. The filtration medium is usually sealed within a polymeric material housing, eliminating the need for additional equipment cleaning, and allowing direct connection to process piping.