Retention rate testing (also known as rejection rate testing) is a core performance evaluation method that quantifies the ability of membranes, filter media, and protective materials to retain particles, dissolved salts, macromolecules, or microorganisms under controlled challenge conditions. This article explains the technical principles, standard test procedures, applicable sample types, and typical industrial applications of retention rate testing, helping engineers and quality professionals select the appropriate verification strategy for their products.
Technical Overview
Retention rate testing measures the percentage of a target substance that is removed, intercepted, or blocked by a porous separation medium when a challenge suspension or solution passes through it. It is one of the most important performance indicators for membrane filtration products, industrial filter media, and respiratory protection materials, because it directly reflects the separation capability of the material in real service conditions.
The retention rate (R) is generally calculated using the following relationship:
R = (1 - Cp / Cf) x 100%, where Cf is the concentration of the target substance in the feed (upstream) stream and Cp is the concentration in the permeate (downstream) stream. A retention rate of 99% means that only 1% of the challenge substance passes through the medium.
Retention is achieved through several mechanisms, including size exclusion (sieving), adsorption, electrostatic attraction, and surface cake formation. Because the measured result depends strongly on challenge particle size, concentration, applied pressure, temperature, flow regime, and solution chemistry, retention rate testing must always be performed under standardized and reproducible conditions. Related performance metrics derived from the same principle include molecular weight cutoff (MWCO), log reduction value (LRV), beta ratio, and overall filtration efficiency.
Testing Samples
Membrane Separation Products
Typical samples include reverse osmosis (RO) membranes, nanofiltration (NF) membranes, ultrafiltration (UF) membranes, and microfiltration (MF) membranes in the form of flat sheets, hollow fibers, tubular modules, spiral-wound elements, and ceramic membranes.
Filter Media and Cartridges
Test items cover nonwoven filter media, melt-blown polypropylene cartridges, pleated filter elements, hydraulic and fuel filter elements, bag filters, and depth filtration materials used in liquid processing systems.
Air Filtration and Respiratory Protection Materials
Samples include HVAC filter media, HEPA and ULPA filter media, electret melt-blown fabrics for respirators and face masks, and microporous laminate films used in protective clothing.
Functional Porous Materials
Lithium-ion battery separator films, sintered metal porous plates, porous ceramics, and other engineered porous structures can also be evaluated for their particle or liquid retention behavior.
Testing Items
- Salt rejection testing of RO and NF membranes using sodium chloride or magnesium sulfate feed solutions
- Molecular weight cutoff verification using polyethylene glycol (PEG) or dextran molecular weight fractions
- Particle retention efficiency testing with monodisperse polystyrene latex (PSL) microspheres of defined diameter
- Bacterial and viral retention testing expressed as log reduction value for sterilizing-grade filter membranes
- Aerosol filtration efficiency testing using sodium chloride and paraffin oil aerosols for respiratory protection materials
- Multi-pass filtration efficiency and beta ratio determination for hydraulic filter elements
- Gravimetric arrestance testing for coarse dust pre-filters and general ventilation media
- Correlation testing with non-destructive integrity methods such as bubble point and pressure hold
Testing Methods
Dead-End Challenge Testing
A challenge solution containing a known concentration of the target substance is pressed through the sample under a defined transmembrane pressure, typically using flat sheet test cells. Feed and filtrate concentrations are measured and the retention rate is calculated directly. This approach is widely used for MWCO verification of ultrafiltration membranes per ASTM E1343 and for latex sphere retention verification of microfiltration membranes, supported by pore structure characterization per ASTM F316.
Cross-Flow Testing
For RO and NF membranes, the sample is installed in a cross-flow test cell and challenged with a saline feed at standardized pressure, temperature, recovery, and pH conditions. Salt concentrations on both sides are quantified by conductivity measurement, and the salt rejection rate is calculated. Standard practices for reverse osmosis performance evaluation, such as ASTM D4195, define the reference operating conditions for these tests.
Microbiological Retention Testing
Sterilizing-grade membranes are challenged with a defined suspension of bacteria or bacteriophages, and the downstream stream is analyzed by culture-based colony counting or microbial enumeration. The result is reported as a log reduction value, following methods such as ASTM F838 for bacterial retention of membrane filters.
Aerosol and Multi-Pass Methods
Air filtration media are challenged with a polydisperse or monodisperse aerosol, and upstream and downstream particle concentrations are measured with laser particle counters to determine fractional efficiency. General ventilation filters are classified according to ISO 16890, EPA, HEPA and ULPA filters are evaluated by the most penetrating particle size (MPPS) method according to EN 1822, and respirator filter materials are tested with sodium chloride and paraffin oil aerosols according to EN 149 or GB 2626. Hydraulic filter elements are assessed with the multi-pass test stand method defined in ISO 16889 to obtain the beta ratio and retention efficiency as a function of particle size.
Analytical Quantification Techniques
Depending on the challenge substance, quantification is performed using UV-Vis spectrophotometry, high performance liquid chromatography (HPLC), gel permeation chromatography (GPC), conductivity meters, total organic carbon (TOC) analyzers, inductively coupled plasma spectroscopy, laser particle counters, gravimetric analysis, or microbiological plate counting. All test programs should be executed in accredited third-party laboratories with calibrated challenge aerosol generators, reference particle standards, and traceable analytical instruments to ensure accuracy and comparability.
Applications
Retention rate testing is applied throughout the development, qualification, and production control of separation and filtration products across many industries:
- Water treatment and desalination: verification of salt rejection for RO and NF membranes and module production acceptance
- Biopharmaceutical manufacturing: qualification of sterilizing-grade filters and validation of virus-retention steps
- Food and beverage processing: confirmation of clarification, sterile filtration, and cold sterilization performance
- HVAC and cleanroom engineering: efficiency classification of air filters for hospitals, laboratories, and semiconductor facilities
- Occupational safety: certification support for respirators, face masks, and protective clothing materials
- Hydraulic and lubrication systems: sizing and qualification of filtration elements for contamination control
- Battery and advanced materials: pore structure and retention performance evaluation of separators and porous substrates
FAQ
What is the difference between retention rate and rejection rate?
The two terms describe the same physical quantity and are calculated with the identical formula. Rejection rate is the preferred term in membrane desalination, while retention rate is more common in filtration, microbiology, and air filtration contexts.
How is retention rate related to pore size and MWCO?
Retention rate is the measured quantity, while pore size rating and molecular weight cutoff are nominal classifications derived from retention data. A membrane is typically assigned a MWCO corresponding to the molecular weight at which it retains at least 90% of the challenge molecule under standard conditions.
Which factors most strongly influence the test result?
Challenge particle size relative to the pore size distribution, applied pressure and flow velocity, feed concentration, temperature, pH and ionic strength, membrane fouling and concentration polarization, as well as the analytical accuracy of the concentration measurement method.
What sample forms and quantities are required?
Flat sheet samples, media coupons, cut discs, finished cartridges, or complete filter elements can be tested. The required quantity depends on the chosen standard; in general, multiple replicate samples are recommended to characterize variability across production batches.
Why should testing be performed by accredited third-party laboratories?
Independent testing with calibrated equipment, reference challenge materials, and qualified personnel according to ISO, ASTM, EN, GB, UL, or JIS methods provides objective evidence for product certification, regulatory submission, customer acceptance, and international market access.
