Membrane Testing Glossary: 25+ Lab & Analytical Terms

This glossary covers key terms related to membrane testing and characterisation, membrane performance parameters. Each term is defined with technical accuracy for membrane researchers, engineers, and water treatment professionals. For the complete glossary covering all 80+ membrane technology terms, see our comprehensive membrane technology glossary.

Membrane Testing and Characterisation

Crossflow Test Cell

A laboratory device for evaluating flat sheet membrane performance under controlled crossflow conditions, simulating the hydrodynamic environment of commercial spiral wound modules. Crossflow test cells allow researchers to measure flux, rejection, and fouling behaviour at various operating pressures, temperatures, and crossflow velocities. Tech Inc. crossflow test cells are precision-engineered with stainless steel construction, high-pressure capability, and interchangeable membrane coupons.

Membrane Compaction

The densification of a membrane structure under applied pressure over time, resulting in reduced water permeability. Compaction occurs as the porous support layer and selective layer compress, reducing pore volume and free volume. New membranes are typically pre-compacted at operating pressure before performance measurements to establish stable baseline flux values.

Coupon Testing

The evaluation of small, flat sheet membrane samples (coupons) in laboratory test cells to determine membrane performance characteristics before committing to full-scale element production. Coupon testing allows rapid screening of membrane formulations, surface modifications, and operating conditions. Tech Inc. test cells accept standard coupon sizes for consistent and reproducible membrane evaluation.

Membrane Characterisation

The comprehensive analysis of membrane physical, chemical, and transport properties using techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurement, zeta potential analysis, and porometry. Characterisation data informs membrane development, quality control, and failure analysis.

Accelerated Fouling Test

A laboratory test protocol that exposes membranes to concentrated or aggressive feed solutions to rapidly simulate long-term fouling behaviour in a short time frame. Accelerated tests help researchers compare the fouling resistance of different membranes and evaluate cleaning protocols. Tech Inc. test systems support accelerated fouling studies with precise flow and pressure control.

Water Permeability Test

A standardised procedure to measure the pure water flux of a membrane at a given pressure and temperature, providing the membrane's intrinsic permeability coefficient (A value). Pure water testing establishes a baseline against which fouling-induced flux decline can be measured. It is typically the first performance test conducted on a new membrane sample.

Salt Rejection Test

A standardised procedure to measure the percentage of dissolved salt rejected by a membrane under defined operating conditions. Standard test conditions for RO membranes typically use 2000 ppm NaCl at 15.5 bar and 25°C, or 32,000 ppm NaCl at 55 bar for seawater membranes. Results enable comparison between membrane products from different manufacturers.

Membrane Performance Parameters

Flux

The volumetric flow rate of permeate through a membrane per unit area, typically expressed in litres per square metre per hour (LMH) or gallons per square foot per day (GFD). Flux is a primary performance indicator for membrane systems, influenced by transmembrane pressure, feed concentration, temperature, and membrane properties. Higher flux indicates greater productivity but may accelerate fouling.

Rejection

The fraction of a solute that is retained by the membrane, expressed as a percentage. Rejection is calculated as (1 minus the ratio of permeate concentration to feed concentration) multiplied by 100. High rejection rates are critical for applications such as desalination, where salt passage must be minimised to produce potable water.

Recovery

The percentage of feed water that is converted to permeate in a membrane system. Higher recovery reduces the volume of concentrate requiring disposal but increases the concentration of rejected species, which can lead to scaling and fouling. Optimal recovery balances water production efficiency with membrane longevity and concentrate management costs.

Molecular Weight Cut Off (MWCO)

The molecular weight at which a membrane rejects 90% of a solute, commonly used to characterise ultrafiltration and nanofiltration membranes. MWCO is typically determined using a series of polyethylene glycol (PEG) or dextran solutions of known molecular weights. A lower MWCO indicates tighter pore size and the ability to retain smaller molecules.

Salt Rejection

The percentage of dissolved salts removed from the feed water by a membrane, a critical specification for RO and NF membranes. Salt rejection is typically measured using standardised sodium chloride or magnesium sulphate solutions under defined pressure and temperature conditions. Commercial RO membranes typically achieve salt rejection rates above 99%.

Permeability (A Value)

The intrinsic water permeability coefficient of a membrane, representing the water flux normalised by the transmembrane pressure difference and corrected for osmotic pressure. The A value is a fundamental membrane property that allows comparison between membranes tested under different conditions. It is expressed in units such as LMH/bar.

Selectivity

A measure of a membrane's ability to preferentially transport one component over another, often expressed as the ratio of permeabilities or the separation factor between two species. High selectivity is crucial in gas separation, pervaporation, and nanofiltration applications where the goal is to separate components with similar sizes or properties.

Specific Energy Consumption (SEC)

The energy required per unit volume of permeate produced, typically expressed in kilowatt-hours per cubic metre (kWh/m³). SEC is a key economic metric for membrane desalination systems, influenced by operating pressure, recovery rate, and pump efficiency. Modern seawater RO plants achieve SEC values of 3–4 kWh/m³.

Transmembrane Pressure (TMP)

The pressure difference between the feed side and the permeate side of a membrane, representing the net driving force for permeation. TMP is a critical operating parameter that determines flux and rejection in pressure-driven membrane processes. Monitoring TMP over time provides an indication of membrane fouling. Tech Inc. test cells feature precision pressure measurement for accurate TMP determination.

Silt Density Index (SDI)

A standardised empirical measurement used to quantify the fouling potential of feed water for RO and NF systems. SDI is determined by measuring the time required to filter a specified volume of water through a 0.45 micrometre membrane filter at a constant pressure of 2.07 bar (30 psi). An SDI value below 3 is generally considered acceptable for RO feed water. Tech Inc. offers SDI testing equipment and sampling stations for accurate water quality assessment.

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Frequently Asked Questions

What is Crossflow Test Cell?

A laboratory device for evaluating flat sheet membrane performance under controlled crossflow conditions, simulating the hydrodynamic environment of commercial spiral wound modules. Crossflow test cells allow researchers to measure flux, rejection, and fouling behaviour at various operating pressures, temperatures, and crossflow velocities. Tech Inc. crossflow test cells are precision-engineered with stainless steel construction, high-pressure capability, and interchangeable membrane coupons.

What is Membrane Compaction?

The densification of a membrane structure under applied pressure over time, resulting in reduced water permeability. Compaction occurs as the porous support layer and selective layer compress, reducing pore volume and free volume. New membranes are typically pre-compacted at operating pressure before performance measurements to establish stable baseline flux values.

What is Coupon Testing?

The evaluation of small, flat sheet membrane samples (coupons) in laboratory test cells to determine membrane performance characteristics before committing to full-scale element production. Coupon testing allows rapid screening of membrane formulations, surface modifications, and operating conditions. Tech Inc. test cells accept standard coupon sizes for consistent and reproducible membrane evaluation.

What is Membrane Characterisation?

The comprehensive analysis of membrane physical, chemical, and transport properties using techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurement, zeta potential analysis, and porometry. Characterisation data informs membrane development, quality control, and failure analysis.

What is Accelerated Fouling Test?

A laboratory test protocol that exposes membranes to concentrated or aggressive feed solutions to rapidly simulate long-term fouling behaviour in a short time frame. Accelerated tests help researchers compare the fouling resistance of different membranes and evaluate cleaning protocols. Tech Inc. test systems support accelerated fouling studies with precise flow and pressure control.

What is Water Permeability Test?

A standardised procedure to measure the pure water flux of a membrane at a given pressure and temperature, providing the membrane's intrinsic permeability coefficient (A value). Pure water testing establishes a baseline against which fouling-induced flux decline can be measured. It is typically the first performance test conducted on a new membrane sample.

What is Salt Rejection Test?

A standardised procedure to measure the percentage of dissolved salt rejected by a membrane under defined operating conditions. Standard test conditions for RO membranes typically use 2000 ppm NaCl at 15.5 bar and 25°C, or 32,000 ppm NaCl at 55 bar for seawater membranes. Results enable comparison between membrane products from different manufacturers.

What is Flux?

The volumetric flow rate of permeate through a membrane per unit area, typically expressed in litres per square metre per hour (LMH) or gallons per square foot per day (GFD). Flux is a primary performance indicator for membrane systems, influenced by transmembrane pressure, feed concentration, temperature, and membrane properties. Higher flux indicates greater productivity but may accelerate fouling.

What is Rejection?

The fraction of a solute that is retained by the membrane, expressed as a percentage. Rejection is calculated as (1 minus the ratio of permeate concentration to feed concentration) multiplied by 100. High rejection rates are critical for applications such as desalination, where salt passage must be minimised to produce potable water.

What is Recovery?

The percentage of feed water that is converted to permeate in a membrane system. Higher recovery reduces the volume of concentrate requiring disposal but increases the concentration of rejected species, which can lead to scaling and fouling. Optimal recovery balances water production efficiency with membrane longevity and concentrate management costs.