What Is Ultrafiltration?

Ultrafiltration is one of the membrane separation technologies driven by pressure. For the purpose of separating large and small molecules, the pore size of the membrane is between 20-1000A °. Hollow fiber ultrafilter (membrane) has the advantages of high filling density in a unit container, small footprint and so on.

Basic information of ultrafiltration

Ultrafiltration Overview

During the ultrafiltration process, the aqueous solution flows through the membrane surface under pressure, and the solvent (water) smaller than the membrane pores and the small-molecule solute permeable membrane become the purification solution (filtration solution). It is entrapped and discharged with the water flow to become a concentrated liquid. The ultrafiltration process is dynamic filtration, and the separation is done in a flowing state. The solute is only deposited on the membrane surface to a limited extent, the ultrafiltration rate is attenuated to a certain extent and tends to be balanced, and can be recovered by cleaning.

Ultrafiltration originated in 1748. Schmidt used a cotton rubber membrane or Lu membrane to filter the solution. When a certain pressure was applied, the solution (water) passed through the membrane, and proteins, colloids and other substances were trapped. Its filtration accuracy far exceeded Filter paper, so he proposed the term ultrafiltration. In 1896, Martin produced the first artificial ultrafiltration membrane. In the 1960s, the concept of molecular weight was proposed, which was the beginning of modern ultrafiltration. In the 1970s and 1980s, During the period of rapid development, it began to mature after the 1990s. China's research on this technology is relatively late. It is still in the research period in the 1970s, and only entered the stage of industrialized production and application at the end of the 1980s.

Ultrafiltration devices are like reverse osmosis devices, which are plate type, tube type (internal pressure tube type and external pressure tube bundle type), roll type, medium

Ultrafiltration membrane module

Empty fiber type and other forms. Concentration polarization is a natural phenomenon in the membrane separation process. How to reduce this phenomenon to a minimum is one of the important topics of ultrafiltration technology. The measures adopted are: increase the water flow speed on the membrane surface to reduce the thickness of the boundary layer and allow the trapped solute to be taken away by water in time; take physical or chemical washing measures.

Ultrafiltration definition

Ultrafiltration is a new technology of hollow fiber filtration, combined with three-stage pretreatment filtration to remove impurities in tap water; ultrafiltration micropores are less than 0.01 microns, which can completely remove harmful substances such as bacteria, rust, colloids in the water, and retain the original traces in the water. Elements and minerals.

Ultrafiltration principle

Ultrafiltration is a pressurized membrane separation technology, that is, under a certain pressure, small molecule solutes and solvents are passed through a special membrane with a certain pore size, so that macromolecular solutes cannot pass through and remain on one side of the membrane, so that The macromolecular material was partially purified. The principle of ultrafiltration is also a membrane separation process principle. Ultrafiltration uses a pressure-active membrane to trap colloids, particles, and relatively high molecular weight substances in water under the influence of external driving force (pressure), and water and small solute particles penetrate The separation process through a membrane. Materials with molecular weights ranging from 3x10000 to 1x10000 can be retained through micropore screening on the membrane surface. When the treated water passes through the membrane surface with a certain flow rate by the effect of external pressure, water molecules and solutes with a molecular weight of less than 300-500 pass through the membrane, and particles and macromolecules larger than the membrane pores are trapped due to sieving. Thereby the water is purified. That is, after water passes through the ultrafiltration membrane, most of the colloidal silicon contained in the water can be removed, and at the same time, a large amount of organic matter can be removed.

The principle of ultrafiltration is not complicated. During the ultrafiltration process, as the trapped impurities continue to accumulate on the membrane surface, a concentration polarization phenomenon will occur. When the concentration of the solute on the membrane surface reaches a certain limit, a gel layer is formed, which drastically reduces the water permeability of the membrane. This makes the application of ultrafiltration limited to some extent. For this reason, research is needed through experiments to determine the optimal process and operating conditions, to minimize the effects of concentration polarization, and to make ultrafiltration a reliable pretreatment method for reverse osmosis.

Ultrafiltration is a membrane separation technology (UItrafil-tration for short). Capable of purifying, separating or concentrating solutions. Ultrafiltration is between microfiltration and nanofiltration, and there is no obvious dividing line between the three. Generally, the pore size of the ultrafiltration membrane is between 0.05 um1 nm and the operating pressure is 0.10.5 Mpa. It is mainly used to trap and remove large-molecule substances such as suspended solids, colloids, particles, bacteria and viruses in water. Ultrafiltration membrane can be divided into organic membrane and inorganic membrane according to the membrane material. According to the appearance of the membrane, it can be divided into: flat type, tube type, capillary type, hollow fiber and porous type. At present, domestic ultrafiltration water purifiers are mostly hollow membranes.

The work of the ultrafiltration membrane is mainly based on the sieving mechanism, and the water is purified by the working pressure and the pore size of the membrane. Taking hollow fiber as an example, the water inlet method can be divided into external pressure type: raw water enters from the outside of the membrane filament, and purified water is prepared from the inside of the membrane filament. Otherwise it is the internal pressure type. The working pressure of the internal pressure type is lower than that of the external pressure type. Ultrafiltration membranes have been widely used in many fields such as deep drinking water treatment, industrial ultrapure water and solution concentration and separation.

Advantages and disadvantages of ultrafiltration

Ultrafiltration membrane elements use products from world-renowned membrane companies to ensure that customers get the highest quality organic membrane elements in the world, thereby ensuring retention performance and membrane flux. The ultrafiltration equipment control system can be personalized according to user specific requirements. Combined with advanced control software, on-site centralized monitoring of important process operating parameters, to avoid manual misoperation, and to ensure long-term stable operation of the system from multiple aspects.

Since each ultrafiltration module is added with a protective solution before it leaves the factory, the protective solution in the module must be thoroughly rinsed before use. It should be rinsed with low pressure (0.1MPa) water for 1 hour, and then washed with high pressure (0.2MPa) water for 1 hour. When flushing at low or high pressure, the system's water discharge valve should all be opened. When using product water, check and confirm that the product water does not contain any fungicides.

The ultrafiltration equipment system has a high recovery rate, and the quality of the obtained product is excellent, which can realize efficient separation, purification and high-fold concentration of materials. The system is made of sanitary pipe valves, which are clean and hygienic on site, and meet GMP or FDA production specifications. The system has advanced process design, high integration, compact structure, small footprint, easy operation and maintenance, and low labor intensity for workers.

There is no phase change in the process, and it has no adverse effect on the composition of the material. It is always at normal temperature during the separation, purification, and concentration. It is especially suitable for the treatment of heat-sensitive substances and completely avoids the disadvantage of high temperature destroying biologically active substances. Effectively retain the biologically active substances and nutrients in the original material system.

Ultrafiltration components should be handled with care, and care should be taken. Because ultrafiltration components are precision equipment, be careful when using and installing them. Handle them with care, and do not throw them away. If the component is not used, it should be rinsed with water and then sterilized by adding 0.5% formaldehyde solution, and sealed. In winter, the components must be antifreeze, otherwise the components may be scrapped.

Ultrafiltration equipment system has low energy consumption and short production cycle. Compared with traditional process equipment, the equipment operation cost is low, which can effectively reduce production costs and improve economic efficiency of enterprises.

The advantages of ultrafiltration technology are simple operation, low cost, and no need to add any chemical reagents, especially the experimental conditions of ultrafiltration technology are mild, compared with evaporation and freeze drying, there is no phase change, and it does not cause changes in temperature and pH. It can thus prevent the denaturation, inactivation and autolysis of biological macromolecules. In the preparation of biological macromolecules, ultrafiltration is mainly used for desalination, dehydration and concentration of biological macromolecules. The ultrafiltration method also has certain limitations. It cannot directly obtain dry powder preparations. For protein solutions, generally only a concentration of 10-50% can be obtained.

Ultrafiltration classification

Filter membranes can be divided into three types: microfiltration, ultrafiltration, and reverse osmosis, depending on the operating pressure applied and the average pore diameter of the membrane used. The operating pressure for micropore filtration is usually less than 2 × 10 ^ 5 Pa, and the average pore diameter of the membrane is 500 Angstroms to 14 micrometers, which is used to separate larger particles, bacteria and pollutants. The operating pressure used for ultrafiltration is 1 × 10 ^ 5 Pa to 6 × 10 ^ 5 Pa, and the average pore diameter of the membrane is 10-100 Angstroms, which is used to separate macromolecular solutes. The operating pressure used for reverse osmosis is greater than ultrafiltration, often reaching 20 × 10 ^ 5 Pa to 70 × 10 ^ 5 Pa. The average pore diameter of the membrane is the smallest, generally less than 10 angstroms, and is used to separate small molecule solutes, such as seawater desalination , Making high-purity water, etc.

Ultrafiltration applications

Ultrafiltration membranes have a minimum cut-off molecular weight of 500 Daltons and can be used to separate proteins, enzymes, nucleic acids, polysaccharides, peptides, antibiotics, viruses, etc. in biopharmaceuticals. The advantage of ultrafiltration is that there is no phase transfer, no need to add any strong chemicals, it can be operated at low temperature, the filtration rate is fast, and it is convenient for aseptic processing. All these can simplify the separation operation and avoid the loss of vitality and denaturation of biologically active substances.

Because of the many advantages of ultrafiltration technology, it is often used as:

(1) Desalting and concentration of macromolecular substances, and exchange equilibrium of solvent systems for macromolecular substances.

(2) Fractionation of macromolecular substances.

(3) Depyrogenation of biochemical or other preparations.

Ultrafiltration technology has become an indispensable and powerful tool in the pharmaceutical industry, food industry, electronics industry, and environmental protection ^[2] .

Ultrafiltration membrane

The key to ultrafiltration technology is the membrane. There are various types and specifications of the membrane, which can be selected according to the needs of the work. Early films were isotropic, uniform films, commonly used microporous films, and their pore diameters were usually 0.05 mm and 0.025 mm. In recent years, some anisotropic asymmetric ultrafiltration membranes have been produced. One type of anisotropic diffusion membrane is composed of a very thin, porous "skin layer" with a certain pore size (about 0.1mm ~ 1.0mm). And a relatively thick (about 1mm) layer that is more permeable and used as a "sponge layer" for support. The skin layer determines the selectivity of the membrane, while the sponge layer adds mechanical strength. Because the skin layer is very thin, it is highly efficient, has good permeability, large flow rate, and is not easily blocked by solutes, resulting in a decrease in flow rate. Commonly used membranes are generally made of acetate or nitrocellulose or a mixture of both. Recently, in order to meet the needs of sterilization in the pharmaceutical and food industries, non-fibrous isotropic membranes have been developed, such as polysulfone membranes, polysulfoneamide membranes, and polyacrylonitrile membranes. This membrane is stable at pH 1 to 14 and can work normally at 90 ° C. Ultrafiltration membranes are usually relatively stable and can be used continuously for 1 to 2 years if used properly. Not used for the time being, it can be immersed in 1% formaldehyde solution or 0.2% NaN3 for storage. The basic performance indicators of ultrafiltration membranes are: water flux [cm3 / (cm2? H)]; rejection rate (expressed as a percentage); chemical and physical stability (including mechanical strength).

Ultrafiltration membrane

Ultrafiltration device

Ultrafiltration devices are generally composed of several ultrafiltration components. Generally, it can be divided into four main types: plate and frame type, tube type, spiral roll type and hollow fiber type. Most of the liquids treated by ultrafiltration method contain water-soluble biological macromolecules, organic colloids, polysaccharides and microorganisms. These substances are easily adhered and deposited on the surface of the membrane, causing severe concentration polarization and blockage. This is the most critical problem of ultrafiltration. To overcome concentration polarization, it is usually possible to increase the liquid flow, strengthen turbulence and Increase stirring.

Ultrafiltration device pictures (2 photos)

Ultrafiltration wastewater treatment

The application of ultrafiltration in biological products has high economic benefits. For example, 25% of human placental albumin (ie, fetal white) for intravenous injection is usually prepared by ammonium sulfate salting-out method, dialysis desalination, and vacuum concentration. This process has large ammonium sulfate consumption, high energy consumption, long operation time, and easy to produce pollution during dialysis. After switching to the ultrafiltration process, the average recovery rate can reach 97.18%; the adsorption loss is 1.69%; the transmission loss is 1.23%; and the retention rate is 98.77%. The production and quality of albumin has been greatly improved, and 6.2 tons of ammonium sulfate and 16,000 tons of tap water can be saved each year. At present, the most famous manufacturers of ultrafiltration membranes and ultrafiltration devices in foreign countries are Milipore of the United States and Sartorius of Germany. Well-known domestic manufacturers have Lisheng.

Application of ultrafiltration in wastewater treatment

(1) Treatment of reducing dye wastewater;

(2) Treatment of electrophoresis paint wastewater;

(3) Treatment of wastewater containing emulsified oil;

(4) Domestic sewage treatment

Ultrafiltration water purifier

A microporous filter membrane with consistent pore size specifications and a nominal pore size range of 0.001-0.02 microns. Membrane filtration using ultrafiltration membrane driven by pressure difference

The filtration method is ultrafiltration membrane filtration. Ultrafiltration membranes are mostly made from acetate fibers or polymer materials with similar properties. It is most suitable for the separation and enrichment of solutes in processing solutions. It is also commonly used for the separation of colloidal suspensions that are difficult to complete by other separation technologies. Its application field is constantly expanding. Membrane filtration using pressure difference as the driving force can be divided into three categories: ultrafiltration membrane filtration, microporous membrane filtration and reverse osmosis membrane filtration. They are distinguished by the smallest particle size or molecular weight that can be retained by the film. When the rated pore size range of the membrane is used as a criterion, the rated pore size range of the microporous membrane (MF) is 0.02 to 10 m; the ultrafiltration membrane (UF) is 0.001 to 0.02 m; and the reverse osmosis membrane (RO) is 0.0001 to 0.001 m. It can be seen that the ultrafiltration membrane is most suitable for the separation and enrichment of solutes in a processing solution, or the separation of colloidal suspensions that are difficult to accomplish by other separation technologies. The ultrafiltration membrane production technology, that is, the technology to obtain the desired size and narrow distribution of micropores is extremely important. There are many factors controlling pores. For example, ultrafiltration membranes with different pore sizes and pore size distributions can be obtained according to the type and concentration of the solution, evaporation and aggregation conditions during film formation. Ultrafiltration membranes are generally polymer separation membranes. The polymer materials used as ultrafiltration membranes are mainly cellulose derivatives, polysulfone, polyacrylonitrile, polyamide, and polycarbonate. Ultrafiltration membranes can be made into flat membranes, roll membranes, tubular membranes or hollow fiber membranes. They are widely used in pharmaceutical industry, food industry, environmental engineering, etc. We all know that a sieve is used to sieve things. It can release small objects and retain larger ones. But have you heard of sieves that can sieve molecules? Ultra-membrane-this super sieve can separate molecular sieves of different sizes! So, what exactly is an ultrafiltration membrane? Ultrafiltration membrane is a kind of porous membrane with super "sieving" separation function. Its pore diameter is only a few nanometers to tens of nanometers, which means that it is only 1 of a hair strand! With appropriate pressure on one side of the membrane, solute molecules larger than the pore size can be sieved to separate particles with a molecular weight greater than 500 Daltons and a particle size greater than 2-20 nm. The structure of the ultrafiltration membrane is divided into symmetrical and asymmetrical. The former is isotropic, there is no cortex, and the pores in all directions are the same, which belongs to deep filtration; the latter has a dense surface layer and a bottom layer mainly composed of finger-like structures. The thickness of the surface layer is 0.1 microns or less. It has ordered micropores, and the thickness of the bottom layer is 200-250 microns, which belongs to surface filtration. Ultrafiltration membranes used in industry are generally asymmetric membranes. The membrane materials of ultrafiltration membranes are mainly cellulose and its derivatives, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, polysulfone amide, sulfonated polysulfone, and cross-linked Polyvinyl alcohol, modified acrylic polymers, etc.

Ultrafiltration kitchen drinking dual-use machine: PP cotton filter element, activated carbon, nano-membrane surface ultrafiltration membrane filter element, composite filter element, five-stage filter equipment adds an additional activated carbon, and six-stage adds a mineralized filter element Established direct drinking fountains seen on the market. More grades add more targeted filter elements.

Ancillary equipment

(1) Booster pump ultrafiltration membrane uses the force difference as the driving force to filter. When the water pressure of the raw water cannot meet the filtering requirements, the system needs to increase the pump pressure to achieve the ultrafiltration membrane separation effect. The pressure is low, generally less than O · 7MPa. Therefore, in the system design, centrifugal pumps are generally selected. The main basis for selecting a centrifugal pump is head, flow, and pump body material, followed by the volume, appearance and price of the pump.

Choice of head and flow rate Select the pump head and flow rate according to the working water inlet pressure, transmembrane pressure difference and water flow rate required in the design of the ultrafiltration system. Generally, the head and flow of the selected pump should be equal to or slightly larger than the designed water supply and working pressure to meet the normal operation of the ultrafiltration system.

Selection of the material of the pump body According to the quality of the raw water, the appropriate material of the pump body is selected to reduce the investment cost. The material of the pump body must not have any reaction with the components in the raw water, and there must be no dissolution. When the pH of the raw water is 6.5 ~ 8.5, cast iron pump body can be used; when the raw water is seawater, the plastic pump body that is resistant to seawater corrosion should be selected; the pharmaceutical and food industry water treatment generally chooses stainless steel pump body.

Chemical cleaning pumps generally choose a pump body that is resistant to chemicals.

(2) Pressure reducing valve When the water pressure of the raw water is greater than the system design water pressure, the raw water should be decompressed. Generally, a pressure reducing valve capable of reducing static pressure is used to achieve the pressure reducing accuracy of the pressure reducing valve depends on the ultrafiltration system. In addition, according to the quality of the raw water, a pressure reducing valve suitable for the material is selected. Generally, the materials that can be selected are copper, stainless steel, iron, and plastic.

(3) Physical cleaning and chemical cleaning system The cleaning system is mainly composed of a dispensing tank, a water purification tank, and a circulating pump. The air-water mixed cleaning also includes an air compressor. General physical cleaning is divided into isostatic flushing and back flushing. During isostatic flushing, the water production valve is closed and the concentrated water valve is fully opened, so that the raw water flushes the membrane surface at a faster flow rate than in normal working conditions to remove dirt. Backwashing is to close the raw water valve and use a circulating pump to drive the water in the water purification tank into the membrane module from the water production port. Make the purified water pass through the membrane in the opposite direction of normal filtration, flush out the pollutants on the surface of the membrane, and let it drain from the concentrated water port. After backwashing, immediately perform isostatic flushing. It can discharge the trapped pollutants more effectively. In order to enhance the cleaning effect, when flushing, you can use a gas-water mixture to rinse.

The chemical cleaning system uses a circulating pump to send the cleaning liquid in the dispensing tank to the ultrafiltration system for circulating cleaning and soaking, and the dirt on the membrane surface is removed by the action of chemicals to restore the water production capacity of the membrane and maintain the design flow rate requirements.

(4) Disinfection and sterilization system The equipment and operating procedures of the ultrafiltration sterilization and sterilization system are the same as those of the chemical cleaning system, and only the cleaning solution needs to be replaced with a sterilizing solution. The commonly used sterilants are sodium hypochlorite and hydrogen peroxide In the selection of the sterilant, the material of the agent film and the concentration of the sterilant should be considered. For example, Ps membranes cannot be sterilized with anionic surfactants, or they will cause irreversible flux loss to the membranes.

(5) Automatic measurement, monitoring and instrumentation

The flow meter is used to measure the water flow. There are rotor flowmeters, float flowmeters, electromagnetic flowmeters, and earning-pin flowmeters. Most glass float (rotor) flowmeters are used in ultrafiltration systems, which are mainly intuitive and inexpensive. An ultrafiltration system requires at least two flowmeters for observation, one is a water production flowmeter and the other is a concentrated water flow. Meter or raw water inlet flow meter. The selection of the flowmeter specifications is based on the flow rate of the system. The selection of the float flowmeter is usually 1.5 to 2 times the actual maximum measurement flow rate.

The monitoring system and the instrument ultrafiltration system must be operated in strict accordance with the design parameters during operation. This requires the relevant parameters of the system to be monitored. The main monitoring items are water quality, flow and pressure, which can be manually operated, and instruments and The programmable controller controls the system automatically.

The water quality can be monitored using a water quality monitor, the water pressure can be monitored using a pressure switch and a pressure gauge, and the flow rate can be monitored using an electronic flow meter. The monitoring signal is fed back to the PLC and then the pump is controlled. , Valve and cleaning system, thus realizing the automation of the system.

Pressure is an important parameter of the ultrafiltration system, so when selecting a pressure gauge, pay attention to its accuracy and durability. The selection of the pressure gauge range is appropriate to use the pressure to make the pointer at 1/2 to 2/3 of the scale, and the impact of the water hammer on the pressure gauge should be considered.