What Physical Characteristics Should Be Considered When Using Depth Filters? – Part II

The flow characteristics of depth filters are determined by filter media (permeability and surface properties) and feed stream parameters (viscosity). Permeability is typically characterized by water, with results typically expressed as the permeate volume per unit area, per unit time, and per unit pressure at a given temperature, such as LMH/psi.

Pressure Limits of Depth Filters

Depth filters must withstand typical operating conditions in pharmaceutical manufacturing processes. As pressure directly impacts filtration performance, parameters should be made to the specifications provided by the supplier.

Excessive Differential Pressure

During filtration, as impurities are retained within the filter media, the differential pressure increases. Once the threshold is reached, the filter element may deform, leading to breakthrough of impurities.

• Hydraulics: The pressure generated by forcing liquid through an orifice, pipe, or other passage.
• Pressure surge: Also known as hydraulic shock, it refers to a sudden and rapid increase in fluid flow, resulting in an instantaneous high-pressure peak within the system. This can include both forward pressure surges and reverse pressure surges.

Forward pressure surges：

During the initiation of filtration or occurrence of process events, sudden pressure changes—such as hydraulic transients (water hammer)—can generate positive pressure surges that can compromise the filter's ability to capture impurities and potentially cause the release of retained contaminants. This phenomenon should be evaluated during the qualification and validation phases of the filtration process.

Reverse pressure surges：

Depth filters are typically designed to withstand the differential pressure encountered during forward flow filtration, but most are not resistant to reverse flow, which may lead to filter damage. Reverse flow may occur if upstream pressure drops below downstream pressure, typically due to following situations:

• Vacuum created during the cooling phase after steam sterilization
• The feed stream source is positioned below the depth filter shell, and the pump is turned off

Depth Filter Operation Should Consider:

A rise in pressure differential or a decline in flow rate usually indicates filter clogging. In many applications, optimal filtration performance is defined by establishing a maximum differential pressure.

Reduced filtrate quality (e.g., breakthrough or turbidity). Turbidity limits should be established based on the specific product, process, and operating conditions. In some cases, the criteria may also consider requirements from downstream unit operations, such as filtration, endotoxin removal, or chromatography.