Fluid Purification Using Charge- Modified Depth Filtration Media

NOVEMBER 2005 P ositioned at various stages during bioprocessing, depth filters are used as prefilters that clarify fluid streams to protect, prolong the life, and increase the efficiency of final (sterilizing) filters, chromatography columns, and other separation technologies. Depth filters are particularly well-suited for use in prefiltration during bioprocessing, removing large contaminants and microorganisms, cellular debris, nucleic acids, host-cell–derived aggregates, and other contaminants that foul downstream processing steps and lead to reduce product recoveries. Examples of applications where depth filtration is used this way include clarification of cultures following fermentation and following centrifugation or tangential flow filtration of biomass. Charge-modified depth filters use two distinct mechanisms to remove contaminants. The first is physical capture within tortuous paths (Figure 1), where particles become entrapped when they enter the depth filter matrix. The second mechanism, electrokinetic adsorption, removes bacteria, fungi, viruses, and other negatively charged contaminants that are smaller than the pore size rating of the media. Filters are designed to exploit electrokinetic adsorption by using one of several amine chemistries that impart to the media a net positive charge that is maintained under typical process pH ranges and ionic strengths.