The performance characterization of depth filtration focuses on the physical and thermal properties of depth filters and should be assessed based on potential interactions with specific process fluids. Key performance indicators include filter service life, adsorption capacity, and filtrate turbidity.
Filter Service Life
When determining the service life of a depth filter, the following factors should be considered:
• A rise in pressure differential or a decline in flow rate (hydrodynamic limitation) usually indicates filter clogging. In many applications, optimal filtration performance is defined by establishing a maximum differential pressure.
• A decline in filtrate quality (breakthrough of impurities). Acceptable turbidity limits should be established based on the specific product, process, and operational unit. Evaluation criteria may also need to account for downstream steps such as final filtration, endotoxin removal, or chromatography.
Adsorption
The adsorption performance of depth filters is typically influenced by:
• Filter media composition
• Properties of feed stream (e.g., drug product or buffer solutions)
• Process operating parameters
At lower (optimal) flow rates, residence time of the drug product within the depth filter reaches its maximum, thereby enabling sufficient contact between contaminants and depth filter media. Higher flow rates generally reduce contact time, potentially compromising clarification performance. These factors significantly affect impurity removal and protein binding. Establishing these performance parameters enables maximal impurity adsorption while minimizing the adsorption of the target protein.
Turbidity Testing
When determining the filter area for a specific application, it is critical to identify whether the dominant removal mechanism is sieving or adsorption. Adsorption can help capture particles that may otherwise penetrate the filter. Once the filter reaches its adsorption capacity, turbidity in filtrate may increase.
When adsorption is the dominant impurity removal mechanism, the rise in differential pressure is typically slower than turbidity. This condition is referred to as ‘breakthrough point’. An increase in filtrate turbidity may negatively impact downstream processes, such as column chromatography clogging or sterilizing-grade filter blockage. Turbidity can be monitored either online or offline, based on the capabilities of turbidity measurement system.