Membrane Filtration: Efficient Particle Retention and Bioburden Reduction in Bioprocessing Fluid Streams
Membrane prefilters are used extensively in biopharma manufacturing to separate unwanted particulate impurities from process streams. Prefiltration removes plugging impurities, reduces the variability of complex process streams, and improves throughput capacity of filtration operations, improving overall process efficiency.
Some membrane prefilters, such as Milligard® PES filters, have bioburden reduction claims and can be used standalone as a cost-effective alternative to sterilizing filters for bioburden control in applications such as such as buffer filtration or filtration of process intermediates. For process intermediates that will be sterilized by terminal sterile filtration, bioburden monitoring and control should be considered as part of a larger contamination control strategy that relies on the principles of Quality Risk Management (QRM) to ensure that microbial, particulate and endotoxin/pyrogen contamination is prevented in the final product1.
This technical article discusses:
- Materials and Methods
- Throughput Performance of Milligard PES® Filters
- Bioburden Reduction with Milligard PES® Filters
- Case Study Showing Throughput Performance of Milligard® PES Filters as Compared to a Sterilizing Filter
- Economic Benefits of Milligard® PES Bioburden Reduction Filters for Aseptic Filtration
Figure 1.Particle size distributions of the challenge streams. Particle sizing was performed with Malvern MasterSizer and PMS Liquilaz.
Throughput Performance
Water permeability of Milligard® PES filters was measured at 10 psi and 21-25oC. Filters were challenged with model streams until permeability was reduced by 90% relative to water permeability (90% flow decay). Tests assessed throughput performance of the filter in the absence of a downstream sterilizing filter.
Retention Performance
Bacterial retention by Milligard® PES filters in OptiScale® 25 formats was assessed with model streams containing Brevundimonas diminuta or Serratia marcescens at concentrations exceeding 2x107 cfu/mL. Samples were collected at different points of filter plugging to assess retention performance.
For more details, read our Tech Note.
Throughput Performance of Milligard® PES Filters
Standalone throughput performance of Milligard® PES filters of each membrane pore size in OptiScale® 25 formats was benchmarked against competitive filters with similar bioburden retention specifications. Throughput differences less than 20% are not considered statistically significant.
Figure 2 shows that in most streams, normalized throughput of Milligard® PES 1.2/0.2 µm nominal filters was at least 20% higher than the competitive filter. For the Soy T stream, with larger particles, Milligard® PES 1.2/0.2 µm nominal filters would not be the preferred choice.
Figure 2.Throughput performance of Milligard® PES 1.2/0.2 µm nominal filters compared to competitive filter A in four model streams. Each bar represents the average results of two OptiScale® 25 filters.
Milligard® PES 1.2/0.45 µm filters showed the most notable differentiated performance from the competitive filters when challenged with the Soy Peptone stream, Figure 3. For streams containing larger particle sizes, performance was equivalent to competitive filters, but for the CHO harvest stream, alternative filters could be considered.
Figure 3.Throughput performance of Milligard® PES 1.2/0.45 µm filters compared to competitive filter B. Each bar represents the average results of two OptiScale® 25 filters.
Throughput performance of Milligard® PES 1.2/0.8 µm filters was benchmarked against Polysep™ II filters, which are high-capacity filters containing borosilicate glass and mixed esters of cellulose media. These filters are typically used in highly plugging streams.
Figure 4 shows equivalent throughput performance in streams containing both the smallest and largest particle sizes, but for the CHO and Whey streams containing mid-sized particles, throughput of Polysep™ II filters exceeded that of Milligard® PES 1.2/0.8 µm filters. This is most likely due to high levels of particle adsorption on the Polysep™ II filter media.
Figure 4.Throughput performance of Milligard® PES 1.2/0.8 µm filters as compared to Polysep™ II filters. Each bar represents the average results of two OptiScale® 25 filters.
Each application and process fluid will have a different particle composition and impact on filter throughput and capacity. Overall, Milligard® PES filters throughput performance compared favorably with that of commercially available filters in different challenge streams. Importantly, Milligard® PES filters deliver both excellent throughput performance and gamma compatibility, a desired combination for many process steps.
In practice, we recommend evaluating your process streams using different pore sizes of Milligard® PES filters to identify the preferred filter for maximizing throughput capacity.
Bioburden Reduction with Milligard® PES Filters
The two smaller pore sizes of Milligard® PES filters were designed for aseptic processing applications where high throughput capacity and bioburden or contamination control, rather than sterile operations are performed. Applications could include buffer filtration or intermediate processing, among others.
Milligard® PES filters were challenged with Soy Peptone and Whey model streams containing the relevant test bacteria out to 90% flow decay. No bacteria were detected in any filtrate grab samples; a representative subset of results is shown in Figure 5.
Figure 5.Retention performance of triplicate Milligard® PES filters with Soy Peptone challenge. A: Milligard® PES 1.2/0.2 µm nominal filters challenged with Brevundimonas diminuta, B: Milligard® PES 1.2/0.45 µm filters challenged with Serratia marcescens. Arrows indicate no bacteria were detected in the filtrate.
Filtrate pools, collected at 90% flow decay, were analyzed for the presence of bacteria. In one test, a low level of bacterial passage was detected, and in all cases, log reduction values (LRVs) confirmed robust bioburden reduction, even when the membrane pores were highly fouled, and permeability was 90% lower than water permeability.
Milligard® PES filters are an attractive option for contamination control and bioburden risk reduction in moderately critical process steps. However, even with their excellent retention performance, they should not be considered as an alternative to sterilizing filters in critical final sterile filtration operations.
Case Study
Clarification removes cellular debris from the cell culture harvest. The clarification approach is highly dependent on the individual process conditions and processing volume. Following clarification, harvest material is filtered through a sterilizing-grade membrane filter to protect downstream unit operations from particulate impurities and bioburden contaminants.
In this study, several approaches were used to clarify a monoclonal antibody (mAb) harvest. Following clarification, the fluid streams were processed with either Milligard® PES 1.2/0.2 µm nominal bioburden reduction filters or Millipore Express® SHC (0.5/0.2 µm) sterilizing filters and the filter throughput at 90% flow decay was determined, Figure 6.
Figure 6.Throughput of clarified harvest streams with Millipore Express® SHC sterilizing filters normalized to Milligard® PES 1.2/0.2 µm nominal filters.
Results demonstrated that irrespective of the clarification method, Milligard® PES 1.2/0.2 µm nominal filters had higher throughput than Millipore Express® SHC filters with capacity improvements ranging from 1.3-2.7 times. These results demonstrate that Milligard® PES filters deliver the high throughput and filter capacity needed for efficient processing while providing excellent bioburden control.
Economic Benefits of Milligard® PES Filters for Aseptic Filtration
When used as standalone bioburden reduction filters, Milligard® PES 1.2/0.2 µm nominal filters deliver higher throughput and increased filter capacity as compared to market-leading Millipore Express® SHC sterilizing grade filters. This increased capacity results translates into meaningful economic benefits through the reduced filtration area requirements:
- Reduces warehousing costs
- Reduced integrity testing
- Shorter installation times
- Minimize hold-up volume
- Reduced plastic waste
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References
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