Depth filter media selection for bioprocessing

Introduction to depth filtration

Depth filtration is a type of direct-flow filtration used in downstream processing to remove unwanted particles from a process liquid. It is most commonly applied during the clarification step immediately after harvesting material from a bioreactor. At this stage, the goal is to remove whole cells, cell debris, cell organelles, and impurities such as host cell proteins and DNA. Depth filtration can also be used later in the process if aggregates form and need to be removed.  

Here, we’ll look at key principles for selecting a depth filter for your clarification process.

Filter selection based on particle size distribution

Choosing the right depth filter begins with understanding the particle size distribution in the process fluid. Different upstream processes generate particles of different sizes. For example, mammalian cell cultures such as HEK293 typically produce particles up to around 15 µm. In contrast, bacterial systems like E. coli generate much smaller particles, often no larger than 2 µm. To accommodate these differences, depth filter media are available in a range of retention ratings (Fig 1). Because depth filters capture particles within the filter matrix rather than only on the surface, it is important to select a retention rating that matches the particle size distribution of the process.

• If the retention rating is too small, larger particles will accumulate on the surface instead of entering the filter material. This leads to premature clogging and reduced filter capacity.
• If the retention rating is too large, particles may pass through the filter and remain in the process liquid, reducing clarification efficiency.

Selecting the appropriate retention rating ensures that particles are effectively captured throughout the depth of the filter, maximizing both performance and capacity.

Fig 1. Nominal retention rating of Seitz P-series (green), HP-series (orange), and Bio-series (blue) depth filter sheet media.

Product recovery and nonspecific binding

Depth filters retain particles through two main mechanisms. The first is mechanical retention, which is based on pore size—a particle larger than an opening simply cannot pass through. The second mechanism is adsorptive retention, where particles bind to the filter material through charge‑based or hydrophobic interactions. Unlike mechanical retention, this mechanism does not depend on particle size. Adsorptive interactions are beneficial because they help remove impurities such as host cell DNA and endotoxins. Depth filters have a large internal surface area, meaning there are many more potential sites for these interactions than there are in membrane filters.

However, adsorptive retention can also lead to undesired product loss. In some cases, the target molecule may interact with the material’s binding sites and become retained within the filter material. How common is this problem? For most mAb processes, adsorptive losses are minimal. The issue is more likely to arise with:

• Molecules with physicochemical properties (such as low isoelectic point or high surface hydrophobicity) that favor binding. Examples include peptides, enzymes, or other small biologics.
• Low-concentration or low-yield products, where a small absolute loss represents a large percentage of total yield. Examples may include viral vectors such as AAV or lentivirus. Depth filter media that are better suited to these products are listed in Table 1.

Recommended starting points by application

Table 1 provides filter recommendations as starting points for common applications. These are based on field experience across a range of processes, but they are not a substitute for process-specific trials.

Table 1. Recommended starting depth filters for common applications. 

*Requires addition of 1.0 M CaCl₂ to precipitate residual RNA and 2.0–2.5 w/% filter aid such as Celpure C300 (diatomaceous earth). †PKA = prekallikrein activator.

Because every molecule–filter interaction is different, the most reliable way to assess product recovery is to measure it directly during small-scale filtration trials. Learn more about how to run a filtration trial here.

What to do if you observe product loss

If you encounter unacceptable product loss during depth filtration, there are two main strategies to address it.

1. Adjust the filter’s surface chemistry. Some depth filters are treated with specialized surface chemistries that adjust the filter’s charge characteristics (zeta potential) across different pH ranges. These modified materials shift the binding sites from predominantly positive to predominantly negative, depending on pH conditions. Cytiva depth filters with this treatment carry the prefix “Supra (S)” in their product names.
2. Change the filter material. If product loss persists even after adjusting the zeta potential, it may be necessary to change the filter material entirely. Cellulose‑based depth filters contain filter aids such as diatomaceous earth and perlite, which can sometimes interact nonspecifically with certain molecules. In such cases, switching to a polypropylene‑based filter (such as our Profile filters) can help minimize unwanted binding.

When depth filtration isn't enough: alternative clarification technologies

Because depth filters capture particles throughout the filter matrix rather than only at the surface as a membrane filter would, they can hold a large amount of contaminants before becoming clogged. This gives depth filters significantly higher capacity than membrane filters when removing particles from solution.

When the particle load of a fluid becomes too high, however, depth filters can no longer operate efficiently. This situation is common in certain bacterial processes, where cell densities and the resulting cell‑debris levels after lysis can exceed what depth filters are designed to handle. For high particle loads, methods such as cake filtration (also called surface filtration), tangential flow filtration (TFF), or centrifugation may be more suitable. Table 2 provides a general guideline for which clarification technology is best suited for different levels of particle loads.

Table 2. The cell debris content (CDC) of a process fluid—measured as grams of dry cell debris per gram of process solution—helps determine which clarification technology is appropriate.

Depth filtration is typically suitable for culture harvests with particle loads up to 5% CDC. At this level, the filter can capture particles throughout its matrix without becoming overloaded.

Above 5% and up to about 9%, cake filtration (also called surface filtration) becomes a viable alternative. In this method, filter aids such as diatomaceous earth are added to the process solution at concentrations of 0.5–2%. As filtration proceeds, particles accumulate on the filter surface, forming a cake. The filter aid keeps this cake porous, allowing filtration to continue until the cake reaches its maximum allowable thickness, usually limited by the filter housing.

TFF using hollow‑fiber modules is often the preferred option for CDCs between 10% and 29%. In this setup, cell debris is retained on the membrane surface while the crossflow continuously sweeps particles away, preventing rapid clogging. The target molecule passes through the membrane and is collected in the filtrate.

Once the CDC reaches 30% or higher, centrifugation is generally the only practical solution. However, centrifugation becomes increasingly expensive at larger scales and is therefore used only when necessary or when process economics justify it.

Summary

Depth filtration can be highly effective for clarification, provided you’ve selected suitable retention ratings and filter materials for your specific application. Filterability trials are the best way to test depth filter performance. We also offer support (remote or on-site) for small-scale process development. If you have any questions or would like to get support for your process development, don’t hesitate to contact us.