What Physical Characteristics Should Be Considered When Using Depth Filters? – Part I

Porosity

Also known as void volume, refers to the percentage of the filter’s total volume occupied by air within its pores. It is defined as the ratio of non-solid volume to total volume of material.

Depth filters can have porosities (void volumes) as high as 90%, which typically affect the following characteristics:

• Filter permeability

• Loading capacity per unit filter medium volume

Under identical conditions (e.g. fiber shape, diameter, thickness, and binder content), at a given flux, deep filters with high void volume typically exhibit higher contaminant loading capacity and lower pressure drop. However, their retention efficiency for fine particles might be inferior to filters with a denser structure and lower void volume.

The thickness of depth filters also impacts filtration removal efficiency. Typically, thicker filters possess greater internal surface area, which enhances filtration removal efficiency while increasing filter resistance.

Retention Efficiency in Depth Filtration

Depth filters are typically named their nominal pore sizes based on their media structure. However, due to the absence of standardized method in the industry for determining retention efficiency, these nominal pore sizes are intended only as a reference for filter selection.

Common methods for determining retention efficiency in depth filtration include:
・ Inorganic particle testing
・ Latex bead testing

Inorganic Particle Testing

Particle challenge systems for filter characterization can test particles ranging in size from submicron levels to those larger than 50 μm. While AC Coarse Test Dust (ACCTD) and AC Fine Test Dust (ACFTD) were extensively used, ISO Medium Test Dust (ISO MTD) is predominantly employed.

Advantages:

• More extended particle size distribution provides retention efficiency data across various particle sizes
• Superior reproducibility. Standardized particulate systems result in minimal variation in particle size distribution between batches.

Limitations:

• 100% inorganic systems may inadequately represent various types of processes, particularly for deformable biologic product impurities
• Large pores are prone to clogging by large particles, so the retention rate is partly determined by the particle size distribution.
• The results are not directly comparable to retention rate from membrane filtration.

Latex Testing

Latex particles are mainly used as a challenge system due to their very narrow particle size distribution.

Advantages:

• The test system consists of a large number of particles of a given size, and is well-defined

Limitations:

• Latex particles are the least comparable of actual process conditions.

• A given test provides information for only one particle size (and one pore size distribution) under a single test condition.