Due to the relatively late development of China's membrane separation industry, there are few national and industry standards. There are 21 standards for membranes and membrane modules, and 24 standards related to membrane products. All are recommended standards; 5 are national standards, while the rest are industry standards, mainly marine industry standards, totaling 27.

Standards, as an important step in industry development, play a significant role in affirming existing achievements and guiding the direction of industry development. They are highly instructive. Therefore, the formulation of relevant standards plays an important technical support role in promoting the scientific and standardized management of the membrane industry, guiding its healthy and orderly development, and promoting the widespread application of membrane technology.

Membrane Product Technical Indicators

Membrane products have numerous performance indicators, mainly divided into three categories: separation and permeation performance, physical properties, and chemical properties. Separation and permeation performance includes: water production, water flux, pure water permeability, molecular weight cut-off (MWCO), rejection rate, desalination rate, recovery rate, maximum pore size, average pore size, pore size distribution, porosity, airtightness and integrity, etc.; physical properties include: structural properties (appearance, membrane area, membrane thickness, inner and outer diameter of membrane fibers), mechanical properties (tensile strength, burst strength, bending strength, flexibility index, elongation at break), electrical properties (charge, Zeta potential), hydrophilicity (contact angle), and heat resistance (maximum operating temperature), etc.; chemical properties include: chemical stability (chemical compatibility), oxidation resistance (short-term free chlorine tolerance, hydrogen peroxide tolerance), acid and alkali resistance (operating and cleaning pH range), and fouling resistance, etc.

Membrane separation and permeation performance reflects the applicable range of the filter membrane, while physical and chemical properties reflect the operating conditions. Membrane separation and permeation performance is the most important technical indicator of membrane products. There is a lot of research and testing methods, and it is also the main technical content of existing membrane product standards. Except for structural properties, relevant standards for membrane physical and chemical properties are still blank.

Analysis of Membrane and Membrane Module Standards

Membrane products are classified by membrane separation process into microfiltration (MF) membranes, ultrafiltration (UF) membranes, nanofiltration (NF) membranes, reverse osmosis (RO) membranes, and ion exchange membranes, etc.; by membrane module type, they can be classified into flat membranes, spiral-wound membranes, hollow fiber membranes (columnar, curtain), tubular membranes, and disc-tube membranes, etc.

General Standards

General standards include "Terminology of Membrane Separation Technology" (GB/T20103—2006) and "Naming of Membrane Modules and Devices" (GB/T20502—2006). GB/T20103—2006 defines common terms in the membrane separation field, including electrodialysis, reverse osmosis, nanofiltration, ultrafiltration, microfiltration, gas separation membranes, and ion exchange membranes. It defines 221 terms in the membrane separation technology field and is applicable to membranes and membrane materials, membrane modules, liquid separation, gas separation, and other membrane separation processes. GB/T20502—2006 specifies the naming rules for membrane modules and devices and is applicable to reverse osmosis, nanofiltration, ultrafiltration, microfiltration, gas separation membranes, electrodialysis, and electrodialysis devices.

Reverse Osmosis (RO) Membrane Standards

There are 4 reverse osmosis membrane standards:

《Testing Method for Hollow Fiber Reverse Osmosis Membrane》(HY/T049—1999),

《Hollow Fiber Reverse Osmosis Component in Hollow Fiber Reverse Osmosis Technology》(HY/T054.1—2001),

《Testing Method for Hollow Fiber Reverse Osmosis Component in Hollow Fiber Reverse Osmosis Technology》(HY/T054.2—2001),

《Testing Method for Spiral Wound Reverse Osmosis Membrane Component》(HY/T107—2008).

HY/T049—1999 specifies the testing conditions and methods for the desalination rate and water flux of hollow fiber reverse osmosis membranes. It is applicable to the testing of hollow fiber reverse osmosis membranes with tap water, brackish water, and seawater as feed water. Before testing, sample preparation is required. The membrane fibers are bonded to nylon tubes or rubber stoppers with epoxy resin, and then the cured membrane sample is placed in a test tube for testing on a testing device. However, the standard does not provide specific and detailed regulations or explanations for sample preparation and test tube requirements. Water flux is measured using a graduated cylinder and a stopwatch, without requiring constant temperature. The measured data is converted to the value at 25℃ using a table.

HY/T107—2008 specifies the testing methods for six parameters of spiral-wound reverse osmosis membrane elements: desalination rate, recovery rate, water production, water flux, seal leakage points, and airtightness. The desalination rate can be calculated by the sodium chloride concentration method or the conductivity method.

Currently, hollow fiber reverse osmosis membranes are rarely seen, and almost all engineering applications use spiral-wound reverse osmosis membranes. However, there is a lack of standards that specify the technical requirements for the performance of spiral-wound reverse osmosis membrane elements. Quality inspection institutions can only conduct quality inspections based on the manufacturer's nominal values, resulting in inconsistent evaluation criteria.

Nanofiltration (NF) Membrane Standards

There is one nanofiltration membrane standard: 《Nanofiltration Membrane and its Elements》(HY/T113—2008).

This standard specifies the classification and model, requirements, testing, and inspection rules for nanofiltration membranes and their elements. The technical requirements specify the water flux and removal rate of monovalent and divalent ions for nanofiltration membranes under standard conditions. The desalination rate can be measured by the gravimetric method or the conductivity method. For monovalent ions, 500mg/L NaCl is used as the test solution, and the removal rate should be no less than 30%; for divalent ions, 2000mg/L MgSO4 is used as the test solution, and the removal rate should be no less than 90%.

Ultrafiltration (UF) Membrane Standards

There are 3 ultrafiltration membrane standards:

《Testing Method for Hollow Fiber Ultrafiltration Membrane》(HY/T050—1999),

《Hollow Fiber Ultrafiltration Membrane Component》(HY/T062—2002)

《Ultrafiltration Membrane and its Components》(HY/T112—2008).

HY/T050—1999 specifies the determination methods for pure water permeability, molecular weight cut-off, and rejection rate. The concentration of the polyethylene glycol test solution is set at 5000mg/L, which is difficult to achieve in operation and should provide an allowable error range; the concentration range of the bovine serum albumin test solution is 1000-3000mg/L, which is too wide and unscientific. Regarding the testing conditions for pure water permeability, the test solution temperature and recovery rate, which have a significant impact on the test results, are not specified in the standard, lacking rigor.

HY/T062-2002 specifies the classification and model, requirements, and test methods for hollow fiber ultrafiltration membrane modules. All test methods require operation according to the provisions of HY/T050-1999, but there are inconsistencies between these two standards. For example, the "appearance inspection" and "leakage detection" regulations cannot be found in HY/T050-1999; in addition, the terminology used is inconsistent. For example, the "pure water flux" indicator in HY/T062-2002 is expressed as "pure water permeability" in HY/T050-1999.

HY/T112-2008 specifies the technical requirements for ultrafiltration membranes, specifically the technical requirements for ultrafiltration membrane components used in seawater pretreatment. The test methods for pure water flux and retention performance are required to be implemented according to the provisions of HY/T050-1999. In addition, it specifies the methods for integrity testing, leakage testing, and pressure resistance performance testing, but the latter two are tested according to the manufacturer's rated maximum working pressure, without a uniformly determined test pressure specification. The pressure resistance performance test requires operation under the rated maximum transmembrane pressure difference for 12-24 hours, which is too long a test cycle.

Microfiltration (MF) membrane standards

There are 6 microfiltration membrane standards:

《Determination Method of Microporous Membrane Pore Performance》(HY/T039-1995)

《Test Method for Hollow Fiber Microporous Membrane》(HY/T051-1999)

《Microporous Membrane》(HY/T053-2001)

《Hollow Fiber Microfiltration Membrane Module》(HY/T061-2017)

《Polyvinylidene Fluoride Microporous Membrane》(HY/T065-2002)

《Polypropylene Hollow Fiber Microporous Membrane》(HY/T110-2008)

The HY/T039-1995 standard specifies performance indicators such as maximum pore size, average pore size, pore size distribution, and porosity, and their detection methods. This standard is applicable to the determination of the maximum pore size (which can also be expressed by bubble point pressure), average pore size, pore size distribution, and porosity of multi-porous membranes with an average pore size of 0.1-20 μm. The test object is a membrane sheet. The testing device uses the CTL-D type membrane pore size tester specified in HY/T038-1995, which is self-made by Hangzhou Water Treatment Center and does not have universality.

The HY/T051-1999 standard specifies the method for testing the maximum pore size of hollow fiber microporous membranes using the bubble point pressure method and the water flux (also known as pure water permeability) test method.

The HY/T053-2001 standard specifies the product marking, requirements, and test methods for microporous membranes. It specifies the thickness range of the membrane, the hydrophilicity and hydrophobicity of the microporous membrane, the bubble point pressure, and the flux value. The bubble point pressure is determined according to HY/T039-1999, and the flux is measured by placing a wet membrane sheet with a diameter of 47 mm on a filter under a negative pressure of 0.09 MPa. Constant temperature is not required. The flux value at the measured water temperature is converted to the flux at 25 ℃ using a temperature correction factor.

The HY/T061-2017 standard specifies the requirements, testing methods, inspection rules, marking, packaging, transportation, and storage of shell-type and shell-less hollow fiber microfiltration membrane modules. This standard is applicable to the research, production, use, and management of shell-type and shell-less hollow fiber microfiltration membrane modules.

The HY/T065-2002 standard is applicable to polyvinylidene fluoride microporous membranes used for gas purification. The filtration accuracy is divided into two types: 0.01 μm and 0.3 μm. The performance requirements specify the air flux, porosity, and retention rate. The filtration accuracy and retention rate are determined using aerosol and integrity test methods, and the porosity is determined using a mercury pressure meter.

The HY/T110-2008 standard specifies the classification and model, requirements, and test methods for polypropylene hollow fiber microporous membranes. It specifies the maximum pore size, porosity, and gas flux for hydrophobic and hydrophilic membranes, respectively. Corresponding regulations are made for the average diameter and wall thickness of the membrane fibers. The maximum pore size and pure water flux are implemented according to HY/T051-1999, the porosity is implemented according to HY/T065-2002, and the standard also specifies the measurement methods for the average diameter, wall thickness, and gas flow rate of the membrane fibers.

Curtain membrane standards

There is one curtain membrane standard: 《Hollow Fiber Curtain Membrane Module》(GB/T25279-2010).

This standard is applicable to two types of membrane modules: hollow fiber curtain microporous membrane modules and hollow fiber curtain ultrafiltration membrane modules. It specifies the classification and model, requirements, testing methods, and inspection rules of the membrane modules. Specific requirements are made for appearance, external dimensions, non-leakage, and water production. Water production test conditions: tap water, 25℃, -0.02MPa.

Ceramic microporous membrane standards

There are 3 ceramic microporous membrane standards:

《Tubular Ceramic Microporous Membrane Element》(HY/T063-2002)

《Test Method for Tubular Ceramic Microporous Membrane》(HY/T064-2002)

《Ceramic Microporous Membrane Module》(HY/T104-2008)

The HY/T063-2002 standard specifies the appearance, dimensional tolerances, performance requirements, test methods, and inspection rules for tubular ceramic microporous membrane elements. Performance requirements include: maximum pore size, average pore size, porosity, pure water flux, bending strength, and acid and alkali resistance. Performance testing methods are carried out according to the HY/T064-2002 standard.

The HY/T064-2002 standard is compatible with HY/T063-2002. The HY/T104-2008 standard specifies the classification and model, requirements (including main dimensions, operating conditions, etc.), test methods, and inspection rules for ceramic microporous membrane modules.

Ion exchange membrane standards

There is one ion exchange membrane standard: 《Hetero-phase Ion Exchange Membrane for Electrodialysis Technology》(HY/T034.2-1994)

This standard specifies the technical requirements, test methods, and inspection rules for electrodialysis hetero-phase ion exchange membranes. Indicators include appearance, water content, exchange capacity, surface resistance, and selective permeability.

Membrane product device standards

Membrane products are assembled into devices or equipment and applied to many industries. The establishment of device standards has promoted the widespread application of membrane technology. Reverse osmosis membrane product device standards include:

《Reverse Osmosis Water Treatment Equipment》(GB/T19249-2003)

《Technical Specification for Cleaning of Reverse Osmosis System Membrane Elements》(GB/T23954-2009)

Pure Water Machine (CJ/T168—2002), Sewage Permeate Disc Tube Reverse Osmosis Treatment Equipment (CJ/T279—2008),

Reverse Osmosis Seawater Desalination Device (CB/T3753—1995),

Acceptance Guide for Reverse Osmosis Water Treatment Devices in Thermal Power Plants (DL/T951—2005),

Reverse Osmosis Device (HCRJ065—1999),

Technical Requirements for Environmental Protection Products Reverse Osmosis Water Treatment Devices (HJ/T270—2006),

Integrity Requirements and Inspection and Evaluation Methods for Reverse Osmosis Devices (SJ/T31455—1994),

Drinking Purified Water Preparation System SRO Series Reverse Osmosis Equipment (HY/T068—2002), Membrane Water Treatment Reverse Osmosis Seawater Desalination Engineering Design Specifications (HY/T074—2003); Nanofiltration membrane product device standards include: Nanofiltration device (HY/T114—2008); Ultrafiltration membrane product device standards include: Ultrafiltration water treatment equipment (CJ/T170—2002),

Hollow Fiber Ultrafiltration Device (HY/T060—2002),

Technical Requirements for Environmental Protection Products Ultrafiltration Devices (HJ/T271—2006),

Acceptance Guide for Ultrafiltration Water Treatment Devices in Thermal Power Plants (DL/Z952—2005),

Household and Similar Use Ultrafiltration Water Purifiers (QB/T4143—2010); Microfiltration membrane product device standards include: Microfiltration water treatment equipment (CJ/T169—2002),

GTL——D Type Membrane Pore Size Tester (HY/T038—1995),

Polyvinylidene Fluoride Microporous Filter Membrane Pleated Filter (HY/T066—2002),

Hollow Fiber Microporous Filter Membrane Device (HY/T103—2008); Ion exchange membrane product device standards include: Electrodialysis technology electrodialyzer (HY/T034.3—1994),

Electrodeionization Membrane Stack (Component) (HY/T120—2008),

Technical Requirements for Environmental Protection Products Electrodialysis Devices (HJ/T334—2006).

Analysis of Key Content of Membrane Technology Standards

Ultrafiltration Membrane Retention Performance

The retention performance of ultrafiltration membranes includes two indicators: molecular weight cutoff and retention rate. These are the most important separation performance indicators of ultrafiltration membranes. The retention rate refers to the ratio of the specific solute retained by the ultrafiltration membrane to the total specific solute in the solution. The molecular weight cutoff is the minimum molecular weight at which the membrane retains more than 90% of a specific solute.

They are alternative parameters reflecting the size and distribution of membrane pores, determining the separation performance of the ultrafiltration membrane.

In the three Chinese standards for ultrafiltration membranes, the test for retention performance uses the method specified in standard HY/T050—1999. This standard specifies the following reference substances for measuring the retention performance of ultrafiltration membranes: polyethylene glycol (molecular weight 6000, 10000, 20000u); cytochrome C (molecular weight 13000u); ovalbumin (molecular weight 45000u); bovine serum albumin (molecular weight 67000u).

Although many reference substances are currently used in the testing of ultrafiltration membrane retention performance, these compounds do not fully possess the characteristics of reference substances for ultrafiltration membrane retention performance testing and cannot well meet the requirements for accurately determining the retention performance of ultrafiltration membranes. The main problems are:

① The molecular weight distribution of a single compound is not concentrated, and the weight average/number average molecular weight ratio (Mw/Mn) measured by gel permeation chromatography is >1.1, which is not suitable for use as a reference substance;

② Although polyethylene glycol, polyethylene oxide, and dextran have advantages such as high purity, stable properties, and narrow molecular weight distribution, the molecular configuration of these compounds is linear and the concentration changes with environmental changes, which cannot accurately characterize the retention performance of ultrafiltration membranes, and the measured retention rate is unreliable.

③ Although proteins such as bovine serum albumin and ovalbumin have advantages such as a very small molecular weight distribution range and a basically spherical molecular shape, the spherical shape of proteins is non-rigid and is easily affected by testing conditions (such as pressure and temperature changes). In addition, only a very small number of protein molecular weights are available, and proteins are expensive. More seriously, protein substances such as bovine serum albumin have strong adsorption to membrane materials, which easily causes large errors in the test results, especially for polysulfone ultrafiltration membranes, and this adsorption is even irreversible.

④ Currently, ultrafiltration membranes with a molecular weight cutoff of more than 100,000u are widely used in engineering, but the highest molecular weight of the reference substances in the test standard is only 67,000u, which cannot meet the testing needs. In addition, with the development of technology, the surface modification of ultrafiltration membranes may undergo significant changes, such as the adsorption of functional substances or the presence of charges. In this case, the use of traditional reference substances can no longer accurately measure the retention performance of ultrafiltration membranes.

Based on the shortcomings of existing reference substances and the industry's needs for ultrafiltration membrane retention performance testing, it is urgent to develop a series of high-performance reference substances.

Microfiltration Membrane Pore Size

The pore size of a microfiltration membrane is closely related to the membrane's separation performance and flux and is the most important characteristic parameter of a microfiltration membrane. Standard HY/T051—1999 determines the maximum pore size of the filter membrane based on the bubble point method, but this method is based on the assumption that the membrane pores are uniform straight circular holes, and the results obtained are often different from the actual results. Because microfiltration membrane pores are extremely complex and irregular, for microfiltration membranes, the pore size refers to the radius of the narrowest part of the pore channel that penetrates the two surfaces of the membrane, that is, the radius of the neck of the through-hole. In addition, the measurement process is easily affected by factors such as wetting effect, pressure increase rate, pore length, and the affinity between the wetting liquid and the membrane material, resulting in poor reproducibility.

To accurately determine the pore size of microfiltration membranes and components, the method for determining the retention performance of ultrafiltration membranes can be used for reference. A standard particle solution with a certain particle size range is prepared as raw water, and after filtration through a microfiltration membrane, a particle counter is used to determine the number of standard particles of different particle sizes in the raw water and the permeate. Based on the retention of standard particles by the microfiltration membrane, the pore size and pore size distribution data of the microfiltration membrane component can be obtained.

Summary

First, although China's membrane technology standards have made considerable progress, the standards still lag behind industry development, and the quantity and quality of standards are still insufficient to meet the needs.

Second, the performance parameters involved in existing membrane technology standards are mainly separation and permeation performance indicators, and relevant standards for membrane physical and chemical properties need to be established.

Third, some early standards need to be revised as soon as possible.

Fourth, existing reference materials cannot meet the industry's needs for ultrafiltration membrane retention performance testing, and the development of a series of high-performance reference materials is urgently needed.

Fifth, the standard particle method can be used to accurately determine the pore size and pore size distribution of microfiltration membranes and components.

Sixth, standards are the quality criteria for membrane products and the basis for conducting quality supervision and inspection of membrane products. The following suggestions are made for the development of membrane technology standards in China:

First, improve the standard system construction. Construct a separation membrane standard system framework with a reasonable structure, complete content, prominent focus, and clear levels to guide the scientific development of separation membrane standardization work;

Second, conduct in-depth research on standard fundamentals and improve the technical level of separation membrane standards;

Third, formulate mandatory standards to ensure the health and safety of separation membranes used in drinking water and other aspects;

Fourth, strengthen the quality supervision of separation membrane products and conduct regular national quality supervision and spot checks of membrane products;

Fifth, promote membrane product quality certification and implement market access;

Sixth, strengthen the dissemination of separation membrane standards and promote the popularization of basic knowledge of standardization in the separation membrane field and the regulatory role of standards in the industry.