Introduction
Retention rating is one of the most important indicators in membrane filtration. It defines how effectively a membrane can remove particles, bacteria or other contaminants. In critical applications such as pharmaceuticals, food and high-purity chemicals, understanding retention and LRV (Log Reduction Value) is essential to selecting the right filtration solution.
1. What Is Retention Rating?
Retention rating describes the ability of a membrane to block particles of a specific size. It is closely associated with membrane pore size and structure. A higher retention rating indicates stronger ability to remove contaminants.
- Indicates which particle sizes are effectively removed
- Helps determine compatibility with critical applications
- Directly linked to membrane performance and safety
2. Nominal vs. Absolute Retention
Retention values can be described using nominal or absolute ratings.
| Type | Description |
|---|---|
| Nominal Rating | Indicates approximate removal efficiency; not guaranteed for all particles of a given size |
| Absolute Rating | Defines the largest particle that will definitely be retained under tested conditions |
3. What Is LRV (Log Reduction Value)?
LRV quantifies how effectively a membrane removes microorganisms or particles using a logarithmic scale. It is commonly used in sterile filtration, water treatment and high-purity chemical processes.
- LRV 1 = 90% reduction
- LRV 3 = 99.9% reduction
- LRV 6 = 99.9999% reduction
Higher LRV means better microbial retention. Sterile-grade filters typically require LRV ≥ 7 for bacteria such as Brevundimonas diminuta.
4. Factors Influencing Retention Performance
Retention is affected by both membrane structure and process conditions.
- Pore Size Distribution: Narrower distributions provide more reliable retention.
- Membrane Material: Hydrophilicity, surface charge and morphology can influence performance.
- Operating Pressure: Excessive pressure may deform pores in some membranes.
- Viscosity and Fluid Composition: Proteins or particles may cause partial pore blockage.
5. Common Retention Ratings by Application
Different industries require different retention levels.
- Biopharmaceuticals: 0.22 μm sterile filtration; LRV ≥ 7 required.
- Food & Beverage: 0.45 μm for microbial reduction; tighter grades for yeast removal.
- Electronics: Submicron retention for particle-sensitive chemical processes.
- Water Treatment: UF and MF membranes with specific LRV ratings for pathogens.
6. How Retention Is Tested
Retention testing ensures that a membrane meets performance requirements before use.
- Microbial Challenge Tests: Common for sterile-grade filters using standardized organisms.
- Particle Challenge Tests: Use latex beads or polystyrene spheres.
- Integrity Tests: Bubble point and diffusion tests confirm membrane integrity without destroying the filter.
Conclusion
Retention rating and LRV provide essential information about membrane filtration capability. Understanding these metrics helps users select the correct pore size, ensure filtration safety and achieve consistent process performance across different industries.
Purchase Suggestion
We offer membranes with precise retention ratings, including 0.22 μm sterile-grade filters, 0.45 μm clarification filters and high-LRV options for critical applications. Contact our team for technical guidance and product recommendations.