Double-Layer vs. Single-Layer Depth Filters
An Automated EZP System can accommodate both double-layer and single-layer depth filters. In total, 19 double-layer Zeta Plus EXT Series grades are available, and each capsule has 1.6 m2 of media. Each Zeta Plus EXT Series grade consists of an open-grade filter media over a tighter grade filter media. The open-grade media functions as a prefilter to extend the throughput of the tighter grade.
In most applications, double-layer depth filters are expected to outperform single-layer depth filters. There are 17 single-layer construction Zeta Plus grades available in the EZP large capsule format. Each capsule has 2.5 m2 of media.
A Zeta Plus filter relies on two major mechanisms to remove contaminants. The first is mechanical sieving, which relies on the pore size, depth, and tortuous path to remove contaminants through mechanical interception of contaminants that are too large to penetrate the physical structure of the media. The second is electro-kinetic adsorption, which is a result of the surface modification of the media.
In electro-kinetic adsorption, a depth filter takes advantage of its positively charged surfaces to remove contaminants having a negative charge. In the applications where electro-kinetic adsorption is desired, such as in the removal of the negatively charged viruses, DNA, endotoxins, and lysed cell debris, the double-layer Zeta Plus EXT grades often outperform the single-layer grades. The added depth in the EXT grades increases particle residence time, thus enhancing the electro-kinetic adsorption effect by allowing more time for mass transfer of contaminants to the internal surfaces of the media.
In applications where mechanical sieving plays a dominant role—such as in the first stage of cell culture clarification, where depth filtration is used primarily to remove whole cells—a single-layer Zeta Plus media may perform better than a double-layer EXT grade due to the higher surface area per capsule.
Factors such as cell density and cell viability must be considered when designing a depth-filtration train in cell culture applications, and often a two-stage system utilizing single-layer, higher area capsules, followed by double-layer finer grade media capsules results in better process economics. It is generally recommended that filtration testing be conducted during process development to optimize the depth-filtration train.