Depth filtration

Depth filtration employs the thickness, or depth, of the filter media to capture insoluble suspended particles, separating them from the process fluid that flows through the media. Particles are trapped by various mechanisms along the tortuous or complex pathway of the media, such as direct interception, inertia, impaction, and electrostatic adsorption.

Depth filters differ from membrane filters in that they do not have a defined, or absolute, pore size. Membrane filters use mechanical sieving alone, having holes or pores in the surface of the filter that trap objects larger than the pores while letting smaller objects flow through. Depth filters, on the other hand, rely on a combination of materials and thickness to create a maze or tortuous pathway to trap the suspended particles.

Depth filters are typically composed of three components: cellulose, a filter aid, and a polymer binder resin. Functional chemistry can also be applied to the material for enhanced performance.

Depending on the process, a mAb purification train will traditionally use a two-stage depth filter process. The filter(s) used in the first stage have a wider nominal pore size to remove larger cells and insoluble fragments. The fluid is then fed into a filter with narrower nominal pore size to remove finer debris. Depth filters with a certain level of positive charge may also remove some negatively charged DNA and host cell proteins.

Single-layer depth filters contain only a single layer of filtration media. 3M™ Zeta Plus™ EXT Series Filter Capsules and Cartridges have a dual-layer configuration with two distinct layers or zones of filter media. This construction increases the holding capacity of the filter, extending its service life and providing enhanced clarification for biological fluids. In some processes, using two or more single-layer depth filters in a chain may be advantageous, as each filter can serve a specific purpose that might not be possible with a dual-layer filter.