Retention rate analysis (also known as rejection rate analysis) is a fundamental performance evaluation for filtration membranes and filter media, quantifying the percentage of dissolved salts, particles, macromolecules, or microorganisms that a material prevents from passing through its structure. Accredited third-party laboratories perform retention rate testing in accordance with ASTM, ISO, EN, and GB standards, delivering critical data for product development, quality control, and regulatory compliance in water treatment, biopharmaceutical, air purification, and protective equipment industries.
Technical Overview
Retention rate, also referred to as rejection rate, is the primary performance parameter used to characterize the separation capability of a membrane or filter medium. It expresses the fraction of a target substance that is retained by the barrier relative to the amount present in the feed stream. The retention rate is calculated using the following standard formula:
R = (1 − Cp / Cf) × 100%, where Cf is the concentration of the target species in the feed solution and Cp is its concentration in the permeate (filtrate). A retention rate of 99.5% indicates that only 0.5% of the target substance passes through the membrane into the permeate.
Retention rate analysis distinguishes between observed retention, which is calculated from bulk feed and permeate concentrations, and intrinsic retention, which corrects for concentration polarization at the membrane surface. This distinction is important because fouling, transmembrane pressure, cross-flow velocity, feed concentration, temperature, and pH can all significantly influence the measured value.
- Membrane pore size distribution and molecular weight cut-off (MWCO)
- Operating pressure, cross-flow velocity, and recovery rate
- Feed solute concentration, ionic strength, and pH
- Temperature and solution viscosity
- Membrane fouling, compaction, and aging effects
- Electrostatic interactions between solutes and membrane surface charge
Accurate retention rate measurement requires controlled test conditions, calibrated instrumentation, and standardized challenge materials. Deviations in feed concentration or hydrodynamic conditions can produce misleading results, which is why testing is typically performed in accredited third-party laboratories following internationally recognized standards.
Testing Samples
Membrane Materials
Flat sheet membranes made from polymeric materials such as polyamide, polyethersulfone (PES), polyvinylidene fluoride (PVDF), and cellulose acetate, as well as ceramic membranes and hollow fiber or tubular membrane modules used in reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) applications.
Filter Elements and Cartridges
Pleated filter cartridges, melt-blown depth filters, string-wound filters, bag filters, and sterilizing-grade liquid filter elements used in process water, chemical, food, and pharmaceutical filtration systems.
Air Filter Media and Finished Filters
HVAC panel and bag filters, HEPA and ULPA filter units, cleanroom filter modules, engine intake filter media, and respirator materials evaluated for particulate interception efficiency.
Protective Textiles and Barrier Materials
Medical protective clothing fabrics, surgical masks, and respiratory protective materials assessed for particle retention and filtration efficiency in accordance with applicable EN and GB standards.
Testing Items
- Salt rejection rate for RO and NF membranes (NaCl, MgSO4, CaCl2, Na2SO4)
- Molecular weight cut-off (MWCO) determination using polyethylene glycol (PEG), dextran, or protein markers
- Protein rejection performance (bovine serum albumin, BSA, and similar model proteins)
- Particle retention efficiency and filtration ratio for liquid filter media
- Bacterial retention and log reduction value (LRV) for sterilizing-grade filters
- Particulate filtration efficiency for air filter media and respiratory protective equipment
- Retention profile curves showing rejection as a function of molecular weight or particle size
- Pressure- and time-dependent retention stability testing
Testing Methods
Salt Rejection Testing by Conductivity
RO and NF membranes are challenged with standard saline solutions under defined pressure, temperature, and recovery conditions following ASTM D4194. Conductivity meters or ion chromatography quantify the feed and permeate salt concentrations, from which the salt rejection rate is calculated. Performance normalization may be conducted in accordance with ASTM D4516 to allow comparison between different test conditions.
MWCO Determination by Challenge Solutes
Flat sheet and hollow fiber membranes are tested in dead-end stirred cells or cross-flow rigs following ASTM E1343. A series of PEG or dextran standards of known molecular weight is filtered, and the permeate and feed concentrations are measured by total organic carbon (TOC) analysis, gel permeation chromatography (GPC), or refractive index detection. The MWCO is reported as the molecular weight at which the membrane retains at least 90% of the challenge solute.
Particle Retention and Multi-Pass Testing
Liquid filter media are evaluated using multi-pass test procedures in accordance with ISO 16889, in which standardized test dust is continuously injected and online particle counters measure upstream and downstream particle counts. The filtration ratio (beta value) is reported at specified particle sizes. Air filter media are tested under ISO 16890 for ePM fractional efficiency against fine dust fractions, while EPA, HEPA, and ULPA filters are characterized at their most penetrating particle size (MPPS) in accordance with EN 1822 and ISO 29463.
Bacterial Retention Challenge Testing
Sterilizing-grade membranes are challenged with a defined bacterial suspension of Brevundimonas diminuta at a minimum challenge level of 10⁷ colony-forming units per square centimeter, following ASTM F838. The downstream filtrate is assayed for penetration, and results are expressed as a log reduction value, with values of 7 or greater typically required for liquid sterilizing-grade filter claims.
Respirator and Protective Material Efficiency Testing
Particulate filtration efficiency of respiratory protective equipment is measured using sodium chloride or paraffin oil aerosol challenge methods in accordance with EN 149 and GB 2626, with total inward leakage and filtered particle counts determining the assigned protection class.
Applications
Retention rate analysis supports a wide range of industrial and regulatory objectives across multiple sectors:
- Water and wastewater treatment: verification of RO and NF membrane desalination performance, UF selectivity, and process qualification for potable and reuse water projects
- Biopharmaceutical manufacturing: validation of virus retention, sterile filtration, and tangential-flow filtration steps required by regulatory submissions
- Food and beverage processing: confirmation of dairy concentration, protein fractionation, cold sterilization, and clarification membrane performance
- Electronics and ultrapure water: qualification of high-retention polishing membranes for semiconductor fabrication water systems
- Air purification and cleanrooms: certification of HEPA and ULPA installations, HVAC filter classification, and indoor air quality compliance
- Personal protective equipment: conformity assessment of masks and protective clothing against particulate retention requirements
- Research and development: screening of new membrane chemistries, pore size engineering, antifouling coatings, and competitive benchmarking
FAQ
What is the difference between retention rate and rejection rate?
The two terms are used interchangeably in membrane science. Both describe the percentage of a target solute or particle blocked by the membrane, calculated as R = (1 − Cp/Cf) × 100%. Some industries prefer one term over the other, but the underlying measurement is identical.
How is molecular weight cut-off (MWCO) determined?
MWCO is determined by filtering a series of challenge solutes, typically PEG or dextran standards of increasing molecular weight, and plotting retention rate against molecular weight. The MWCO is defined as the molecular weight at which the membrane achieves 90% retention, following the methodology of ASTM E1343.
What sample size is required for flat sheet membrane testing?
Most dead-end stirred cell and cross-flow test rigs accept circular membrane coupons with effective diameters between 25 and 90 millimeters. Laboratories typically request several replicate coupons to account for material variability and to enable repeat measurements.
What is considered a good salt rejection rate for RO membranes?
Commercial seawater RO membranes generally achieve NaCl rejection above 99%, while brackish water RO membranes typically achieve 97% to 99%. Nanofiltration membranes show selective rejection, often exceeding 95% for divalent ions such as MgSO4 while allowing a larger fraction of monovalent salts to pass.
Which standards govern retention rate testing?
Commonly applied standards include ASTM E1343 for MWCO evaluation, ASTM D4194 and ASTM D4516 for RO and NF device performance, ASTM F838 for bacterial retention, ISO 16889 for hydraulic filter multi-pass testing, ISO 16890 for general ventilation air filters, EN 1822 and ISO 29463 for EPA, HEPA, and ULPA filters, and EN 149 and GB 2626 for respiratory protective equipment.
How long does retention rate testing take?
Standard salt rejection and particle retention tests are typically completed within five to ten working days. MWCO profiling with multiple challenge solutes, bacterial challenge testing, and full filter element qualification can require two to four weeks due to conditioning, replicate measurements, and incubation requirements.
Can retention rate testing be combined with other membrane characterizations?
Yes. Retention rate analysis is frequently combined with pure water flux measurement, pore size characterization by bubble point methods, contact angle and zeta potential testing, and fouling resistance evaluation to provide a complete membrane performance profile for product development or quality assurance purposes.