Nanofiltration Membrane Testing

Nanofiltration membrane inspection mainly conducts scientific testing and analysis on flux testing, salt rejection/rejection rate determination, integrity testing, mechanical property testing, chemical stability evaluation and surface characteristic analysis, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Flux testing: The flux is the key indicator for measuring the water production capacity of the nanofiltration membrane, usually referring to the amount of water passing through the unit membrane area per unit time under the specific operating pressure, temperature and recovery rate, with the unit of L/(m²·h) or GFD. Testing the flux can evaluate the initial performance of the membrane element, and provide the basic data for the system design and operation.

Salt rejection/rejection rate determination: This indicator is used to evaluate the separation ability of the nanofiltration membrane for the specific solutes. Usually, the representative standard solutions are selected, such as the sodium chloride (NaCl) or magnesium sulfate (MgSO₄) solutions, the solute concentrations in the feed solution and the product water are determined, and the salt rejection rate is calculated. The difference in the rejection rates of the ions of the different valences is the core manifestation of the separation characteristics of the nanofiltration membrane.

Integrity testing: Aims to confirm whether there are leaks or damages in the membrane elements, membrane housings and connecting pipelines. The commonly used methods include the pressure decay testing and bubble observation method. The pressure decay testing applies a stable air pressure to the feed side of the membrane module, monitors the pressure drop value within the specified time, to judge the intactness of the membrane fibres or sealing parts.

Mechanical property testing: Mainly evaluates the physical strength of the membrane materials and components, including the tensile strength, burst pressure and compaction resistance. These tests are related to the ability of the membrane elements to bear the physical stresses during the long-term operation, cleaning and start-stop processes, directly affecting their service life and operation safety.

Chemical stability evaluation: In the actual applications, the nanofiltration membrane will contact the oxidants (such as the residual chlorine), acid and alkali cleaning agents and the various organic solvents. By immersing the membrane samples in the chemical reagents of the different concentrations and pH values, their flux, rejection rate and appearance changes are regularly detected, to determine the tolerance range, guiding the chemical cleaning and storage.

Surface characteristic analysis: Including the determination of the membrane surface charge (Zeta potential), contact angle (hydrophilicity/hydrophobicity) and roughness. The surface charge affects the rejection behaviour of the membrane for the charged ions and the anti-fouling ability; the contact angle reflects the hydrophilicity of the membrane, related to the flux and the anti-organic fouling performance; and the roughness is related to the risk of the pollutant adhesion.

Pore size and pore size distribution: The pore size of the nanofiltration membrane is usually between 1-2 nanometres. The commonly used characterization methods include the bubble point method, gas adsorption-desorption method, transmission electron microscopy (TEM) and atomic force microscopy (AFM). Accurately understanding the pore size and its distribution is the basis for understanding the rejection mechanism and predicting the separation performance.

Testing Scope

Flat sheet membrane samples: Applicable to the laboratory research and development stage, carrying out the basic performance evaluation of the unformed membrane materials. The testing items usually include the pure water flux, rejection rates of the different standard salts or dye molecules, surface characteristics (such as the contact angle and Zeta potential) and chemical stability testing, providing the data support for the formula optimization.

Single membrane element: This is the most common testing object, referring to the standard products that have completed the industrial packaging such as the spiral wound and hollow fibre. The testing is usually carried out on a standard test bench, comprehensively evaluating its initial performance, including the water production, salt rejection rate and pressure loss. It is the core link of the product ex-factory inspection and user acceptance.

Membrane modules and systems: Referring to the membrane stacks or complete equipment composed of multiple membrane elements in series or parallel. The testing focus shifts from the single element performance to the overall system performance, such as the system recovery rate, inter-stage pressure difference and product water quality uniformity, and also involves the linkage testing of the supporting pipelines, valves, instruments and control logic of the system.

Used membrane performance evaluation: The testing is carried out on the membrane elements that have been in operation for a period of time, aiming to diagnose the causes of the performance decline. By comparing the flux, salt rejection rate and pressure difference data of the new and old membranes, combined with the composition analysis of the pollutants (such as the SEM-EDS), it can be judged whether it is the membrane fouling, scaling or the irreversible deterioration of the membrane material itself.

Special application membrane testing: For the nanofiltration membranes used in the specific fields, the testing scope and standard solutions need to be adjusted accordingly. For example, the membranes used for the sugar liquor decolorization need to test the rejection rate of the pigment molecules; the membranes used for the high-salinity wastewater treatment need to test the anti-fouling and stable operation ability under the high hardness and high organic matter content conditions.

Raw materials and auxiliary materials: The testing scope extends to the membrane production raw materials, such as the purity and molecular weight distribution of the polymer materials such as the polyamide, polysulfone and polyethersulfone; as well as the physical and chemical properties of the auxiliary materials such as the nonwoven support layers, polyester spacer nets, centre tubes, end caps and sealing rings, ensuring their compatibility with the membrane sheets and meeting the long-term operation requirements.

Testing Methods

Standard solution evaluation method: This is the most basic and general method. In accordance with the standards (such as ASTM D4194 and GB/T 32373), the single or mixed salt solutions of the specific concentrations (such as the 2000 mg/L NaCl or MgSO₄) are prepared, operated under the standard test conditions (pressure, temperature and flow rate), and the concentrations of the feed solution and the product water are analysed through the conductivity meter or ion chromatography, calculating the rejection rate.

Cross-flow filtration testing: Simulates the actual industrial operation mode. The feed liquid flows tangentially across the membrane surface at a certain flow rate, and part of the liquid vertically permeates through the membrane driven by the pressure. This method can effectively evaluate the influence of the concentration polarization and membrane fouling, obtaining the stable flux and rejection rate data closer to the engineering applications. It is the key method of the performance evaluation.

Static immersion testing: Mainly used to evaluate the chemical stability of the membrane. The membrane samples are completely immersed in the chemical reagents (such as the NaClO solution, acids and alkalis) of the set concentration and pH value, maintained for a certain time at the constant temperature. After taking out and washing, the flux and rejection rate are retested, and compared with the data before the immersion, quantifying the performance attenuation degree.

Bubble point pressure method: Used to determine the maximum pore size of the membrane. After the membrane sample is wetted and installed in the test cell, the gas (such as the nitrogen) is slowly introduced on one side, and the other side is immersed in the liquid for observation. When the liquid in the membrane pores is pushed out by the gas pressure and the continuous bubbles appear, the recorded gas pressure is the bubble point pressure, and the maximum pore size can be converted through the Washburn equation.

Scanning electron microscopy and energy dispersive spectroscopy analysis: The scanning electron microscope (SEM) is used to observe the micro-morphology, skin layer structure and pollutant morphology of the membrane surface and cross-section. Combined with the energy dispersive spectrometer (EDS), the qualitative and semi-quantitative analysis of the element composition of the membrane surface or the pollution deposits can be carried out. It is an important means for studying the membrane structure, fouling mechanism and cleaning effect.

Infrared spectroscopy and X-ray photoelectron spectroscopy analysis: The Fourier transform infrared spectroscopy (FTIR) is used to analyse the functional groups and chemical structure of the membrane materials. The X-ray photoelectron spectroscopy (XPS) can provide the element composition, chemical states and functional group information within the nanometre depth of the membrane surface, and has the irreplaceable role in studying the membrane surface modification and the chemical properties of the fouling layers.

Long-term operation simulation testing: A small circulation test device is built in the laboratory, and the simulated actual water quality or real feed liquid is used to carry out the continuous or intermittent testing of hundreds or even thousands of hours under the set operation-cleaning cycles. The flux decay rate, rejection rate stability and chemical cleaning recovery rate are monitored, to predict the service life of the membrane.

Testing Standards

Domestic Standards:

1. GB/T 32373, Testing methods for reverse osmosis membrane elements

2. GB/T 1040.1-2018, Plastics — Determination of tensile properties — Part 1: General principles

3. GB/T 9341-2008, Plastics — Determination of flexural properties

International Standards:

1. ASTM D4194, Standard test methods for operating characteristics of reverse osmosis and nanofiltration devices

2. ISO 527, Plastics — Determination of tensile properties

3. ISO 178, Plastics — Determination of flexural properties

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Membrane performance evaluation system: The core equipment, usually composed of a feed tank, high-pressure pump, precision pressure gauges and sensors, flow meters, temperature controller, membrane test cells and data acquisition system. The advanced systems can realize the automatic control of the pressure, flow rate and temperature and the multi-channel parallel testing, meeting the standard testing or customized research needs.

Conductivity meter/ion chromatograph: The conductivity meter is the common tool for the rapid determination of the salt solution concentration and the calculation of the salt rejection rate, applicable to the single-salt systems. For the complex ion systems or multi-component analysis, the ion chromatograph is needed, which can simultaneously and precisely separate and detect the concentrations of the various cations and anions, with more accurate results.

Electronic balance and turbidimeter: The electronic balance is used to precisely weigh the reagents required for preparing the standard solutions. The turbidimeter is used to detect the turbidity of the feed solution and the product water, evaluate the removal effect of the membrane on the suspended particles and colloids, and is also an auxiliary indicator for judging the pre-treatment effect and the membrane fouling degree.

Material mechanical testing machine: Used to execute the mechanical property testing of the membrane. The membrane sample is fixed with the fixtures, and the tensile testing is carried out to obtain the data such as the tensile strength and elongation at break; it can also be used for the burst pressure testing, evaluating the ultimate ability of the membrane to bear the maximum pressure.

Surface analysis instruments: Including the contact angle measuring instrument (used to evaluate the hydrophilicity/hydrophobicity), Zeta potential analyser (used to measure the membrane surface charge) and atomic force microscope (AFM, used for the nanoscale surface morphology and roughness analysis). These instruments provide the quantitative data for the in-depth understanding of the surface properties of the membrane.

Spectroscopy and energy spectroscopy analysis equipment: The Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS) and scanning electron microscope-energy dispersive spectrometer combined system (SEM-EDS) are the essential high-end instruments for the analysis of the chemical structure, surface composition and micro-morphology of the membrane materials, mostly used in the research and development and in-depth failure analysis.

Integrity tester: A dedicated equipment, usually integrating the pressure sensors, precision valves and automatic control unit, used to execute the standardized pressure decay testing or diffusion air testing. The test pressure, pressure stabilization time and allowable pressure drop threshold can be set, automatically judging whether the integrity of the membrane element or module is qualified.