What are the applications of ultrafiltration?

Technical Applications of Ultrafiltration UF

The working principle of ultrafiltration (UF):

Ultrafiltration (UF) technology is an advanced membrane separation technology developed in recent years relying on materials science, and has been widely used in various fields of industrial and municipal construction.

Ultrafiltration (UF) is a pressure-driven membrane separation process that utilizes the interception capacity of porous materials to separate particulate matter from fluids and dissolved components. Typical pore sizes of ultrafiltration membranes are in the range of 0.01-0.1 micrometers, and offer extremely high removal rates for bacteria and most viruses, colloids, sludge, etc. The smaller the nominal pore size of the membrane, the higher the removal rate. Ultrafiltration membranes are usually made of polymers whose basic properties are mainly hydrophobic. Hydrophilic modifications such as blending are possible. The process is operated at room temperature with no phase change and no secondary contamination.

Ultrafiltration using polyvinylidene fluoride (PVDF) material, the Department of double skin layer hollow fiber structure. Among the PVDF ultrafiltration membranes used in industrial applications, ultrafiltration has the smallest nominal pore size and is capable of removing almost all particles, bacteria (4-log removal rate), most viruses, and colloids. Despite its small pore size, its extremely high porosity allows UF to achieve fluxes comparable to microfiltration, making it a better choice than microfiltration in most cases.

Ultrafiltration utilizes an external pressure structure that is not easily clogged, providing a higher dirt-holding capacity, a larger filtration area, and easier and more thorough cleaning. The flow design is based on full-flow filtration, but the elements can be easily converted to a staggered-flow filtration mode. Compared to staggered flow, full-flow filtration consumes less energy and operates under lower pressure, resulting in lower operating costs. Staggered flow filtration, on the other hand, can handle fluids with higher suspended solids. The specific form of operation is therefore determined by the suspended solids content of the feed water.

Ultrafiltration is usually operated at constant flow, and the differential transmembrane pressure (TMP) will gradually increase over the operating time, at which time the contamination layer can be removed by regular backwashing or gas scrubbing, while the use of biocides or other cleaning agents can more thoroughly control microbial reproduction and remove contaminants.

In the field of water treatment, ultrafiltration can be used to purify water by removing particles, colloids, bacteria, viruses, heat sources, proteins and macromolecular organics.

Advantages and characteristics of ultrafiltration

Long service life: ultrafiltration adopts PVDF material with special performance and hydrophilic modification, which has excellent oxidation resistance and fatigue strength, anti-pollution and clear resistance, greatly extending the service life of the membrane filament.

High quality of water production: the average filtration precision of ultrafiltration reaches 0.03µm, the bubble point pressure is higher, and the removal rate of bacteria reaches 6-log, which makes it obtain better quality of water production.

Wide range of application: the external pressure structure of ultrafiltration and the patented water distribution method, allowing a wider range of inlet suspended solids content, more suitable for poor water quality application conditions, and at the same time to ensure a high water recovery rate.

Low operating cost: The external pressure type of ultrafiltration can adopt low-cost air-water mixing cleaning method, which can efficiently maintain the long-term stability of the flux and save the consumption of chemical cleaning agent.

Main Applications of Ultrafiltration

The applications of ultrafiltration mainly involve three aspects: industrial applications, food biochemistry, and drinking water, which are described below.

The industrial applications of ultrafiltration can be divided into three types: (1) concentration (2) separation of small molecule solutes (3) classification of large molecule solutes.

The vast majority of industrial applications belong to this aspect of concentration. Separation of small molecule solutes can be achieved by combining or complexing with macromolecules. The separation of free calcium and protein-bound calcium is an example. Separation of small solutes, such as desalting and salt exchange, can be accomplished by ultrafiltration or by combining ultrafiltration with dialysis. Classification of macromolecular solutes can be performed using membranes with different molecular weight cut values, or by using a combined system where several ultrafiltration cells are combined so that the liquid from one cell can go to the next, with the membranes in each cell cutting a progressively decreasing molecular weight value.

Industrial Applications

-Electrophoretic paint recovery using ultrafiltration

-Recovery of oily wastewater by ultrafiltration

-Treatment of heavy metal-containing wastewater by ultrafiltration

-Other industrial applications

1 One of the industrial applications of ultrafiltration —— Electrophoretic Paint Recovery by Ultrafiltration

In the process of metal electrophoretic painting, an electrically charged metal object is immersed in a pool of oppositely charged paint. Due to the oppositely charged attraction, the paint forms a uniform coating on the metal surface, and the metal object is fished out of the pool and washed to remove the paint that was carried out with it. In order to protect the environment and save energy in a closed loop, an ultrafiltration process can be used to retain the polymer resin and pigment particles while allowing the inorganic salts, water and solvents to pass out through the ultrafiltration membrane. The blocked components are then returned to the electrophoretic paint storage tank. The filtrate is used to rinse newly applied parts freshly removed from the electrodeposition paint to recover excess paint trapped in the part.

As early as 1968, the U.S. PPG company’s patent proposed the use of ultrafiltration and reverse osmosis combination of technology to treat electrophoretic paint wastewater. At present, the technology has been widely used in automated assembly lines, there have been hundreds of membrane area greater than 100m2 membrane module put into operation, which is mainly for the tube type. As the solution in the pool is electrically charged, a membrane with the same charge on the surface has been developed, which is not easily contaminated due to homogeneous repulsion. Membrane permeate flow rate for several months to stay above 1m / d without cleaning, membrane life is generally more than 2 years.

2 ultrafiltration industrial applications of the second – the use of ultrafiltration for the recovery of oily wastewater

Oil-water emulsion in the metal machining process is widely used as tools and workpieces repeatedly cold-drawing operations and metal roll-forming, lubrication and cooling of cutting operations, but because of the use of the process is easy to be mixed with metal debris, bacteria, and cleaning of metalworking surfaces of the rinse water, resulting in its service life is very short. Individual molecules on its molecular weight is small enough to pass through the ultrafiltration membrane, and these oily wastewater ultrafiltration can be successfully separated from its oil phase, this is because the oil-water interfacial tension is sufficient so that the oil droplets can not pass through the membrane has been wetted by water, after ultrafiltration of permeate oil concentration is usually less than 10g/m3, has reached the discharge standard can be discharged to the gutter, and the concentrate of the final oil content of up to 30% to 60% can be used to combustion or It can be used for combustion or other purposes. The operation process is shown in the figure below. In addition, alkaline cleaning solution baths are often used to clean oily or dirty metal parts. Ultrafiltration can also be used to process this cleaning solution to remove grease, oil and dirty particles and recover most of the cleaning agent in the form of filtrate.

3 Ultrafiltration industrial applications of the third —- use of ultrafiltration to treat heavy metal-containing wastewater

Micelle-enhanced ultrafiltration (MEIJF) is a recently developed ultrafiltration method combined with surfactant technology. Its principle is shown in the figure below. In industrial wastewater injected concentration higher than the critical concentration of micelles of surfactants, the hydrophobic end of the inward entanglement, while the negatively charged hydrophilic end of the arrangement of the surface, thus making the surface of the micelles with a negative charge. The metal cations in the wastewater are adsorbed on it due to the electrostatic effect, and the ultrafiltration membrane with the molecular weight cut-off less than the molecular weight of micelles can make the metal ions to be retained.

The surfactant sodium dodecyl sulfate (SDS) was added to the simulated wastewater to separate four ions, Cd2+, Zn2+, Cd+ and Ca2+, respectively, and the retention rate was above 96%, and the membrane permeate flow rate was the same as that of the pure water, which indicates that the industrial application of MEUF is possible. Some researchers also used natural deoxycholic acid and lecithin as surfactants, and the results show that its separation effect on Ca2+, Pb2+, Cu2+, Ni2+, Zn2+ is better than SDS, with retention rate above 99.9%. Since the surfactant concentration in MEUF must be higher than the critical micelle concentration in order to form micelles, MEUF cannot be used for the separation of metal ions in low concentrations.

Even if a small amount will not be depolymerized polyelectrolyte completed methyl cellulose (CMC) and polystyrene sulfonic acid (PSS) instead of surfactant added to the wastewater, the polyelectrolyte that is dissociated, the counter-ion (Na +) into the water, the polymer is negatively charged, the wastewater of heavy metal ions, such as Cu2 + and polymer combination with a molecular weight cut-off of the polymer can be used to retain the wastewater Cu2 + in the ultrafiltration membrane. Cu2+ in the wastewater can be retained by the ultrafiltration membrane with MWCO less than the polymer. This improved MEUF is called polyelectrolyte-enhanced ultrafiltration (PEIJF), and the concentration of Cu2+ in wastewater can be reduced from 100 × 10-6 to 1 × 10-6 using this technique.

There is an ultrafiltration process called IEIJF (Ion-Expulsion Ultrafiltration) that utilizes the repulsive effect of ions for separation. It is well known that micelles bind less than the stoichiometric number of counter ions in water and thus they have a charge which is close to a constant over a wide range of ionic concentrations. IEUF utilizes this property of micelles to separate ions with the same charge as it, and the following figure shows the schematic diagram of the IEUF process. According to the Donnan equilibrium principle, when the dissociated anions and cations in solution are in equilibrium in the permeate and concentrate on both sides of the membrane, their ionic activity products are equal, and thus the concentration of each ion can be calculated. Calculations show that Cr042- can be concentrated 21.5 times in the permeate when equilibrium is reached.

Other industrial applications of ultrafiltration:

(1) Ultrafiltration application in the preparation of high-purity water

Many industrial water requirements are very strict, especially in the electronics industry, many places have to use high-purity water, which plays an important role in ensuring product quality. For example, in the integrated circuit semiconductor device slicing, grinding, epitaxial, diffusion and evaporation processes, to be repeatedly cleaned with high purity water, integrated circuits in a very small area, there are many circuits between neighboring components is only 0.002mm or so, so the cleaning of water requirements are very strict. General requirements for ion-free, non-soluble organic matter, no bacterial body and no particles larger than 0.5μm. Each IC plant, has a central system for manufacturing high purity water, which is then delivered to the point of use through a distribution system. The purification process is as follows:

Tap water → pre-filtration → ultrafiltration (or microfiltration) → reverse osmosis → anion and cation exchange resin mixed bed → ultrafiltration → distribution system microfiltration → point of use microfiltration → use

Ultrafiltration is mainly used to remove colloids, particles, and bacteria in the process of high-purity water preparation. Ultrafiltration components used for high-purity water preparation are mostly hollow fiber type, with membrane permeate flow rate as high as 2~4m/d.

(2) Containing starch and enzyme wastewater treatment.

In some food processing industries, such as potato processing, its wastewater contains low concentration of starch, brewing industry discharges contain enzymes and so on. Ultrafiltration can be used to recover starch and enzymes, and derive wastewater that can be allowed to discharge.

(3) Treatment of desizing water from the textile industry.

Sizing materials such as starch and water-soluble polymers (polyvinyl alcohol) are often used in textile processes to facilitate the process. The woven cloth is washed to remove the sizing material, resulting in a dilute solution containing the sizing material. Ultrafiltration can be used to recover this sizing material for reuse and results in a good quality aqueous filtrate that can be discharged or reused.

(4) Emulsion concentration.

In the manufacture and application of synthetic rubber, the washing water of containers, reactors, etc. contains a dilute emulsion solution, which is successfully concentrated using the ultrafiltration process.

(5) Processing of the discharge solution of the rinsed wool.

This discharge contains emulsified lanolin-type oils and fats emulsified by detergents, which can be dewatered by ultrafiltration (often combined with centrifugal operation).

(6) Treatment of pulp mill discharge.

This discharge contains high molecular weight lignosulfonates, which can be separated and concentrated by ultrafiltration.

(7) Application in the process of Chinese medicine preparation.

At present, China’s Ministry of Health has included membrane separation technology as one of the separation and refining methods for traditional Chinese medicine. Ultrafiltration technology is mainly used in the preparation of traditional Chinese medicine injection (such as compound danshen injection, Yinhuo Huang injection, Wumai disinfectant drink injection, etc.), the extraction of active ingredients (such as the extraction of huangling test from huangling) and the preparation of medicinal extracts and so on. Ultrafiltration alone or with activated carbon, reverse osmosis is also used to effectively remove bacteria and pyrogens; the preparation of Chinese medicine oral solution (ginseng essence oral solution, Hailong Haji four essence oral solution), the preparation of pure water for pharmaceutical use, health care beverages.

Food Biochemical Applications of Ultrafiltration

Food biochemical applications of ultrafiltration ——- Dairy treatment by ultrafiltration

Dairy industry milk vinegar production process will produce a large amount of whey, according to statistics, only the United States every year there are 25 million m3 whey produced, so the field has become the largest field of ultrafiltration applications. Through ultrafiltration, the concentrate containing 10% protein can be obtained, and if it is spray dried, whey powder containing 65% protein can be obtained, which can be used as a substitute for skimmed milk powder in bakery products. If it is further desalted, the product with protein content higher than 80% can be obtained, which can be used in baby food. And the permeate containing lactose can be used as animal feed after concentration and drying.

Treatment of whey by ultrafiltration process

In ultrafiltration of whey various forms of modules are used, the largest of which contains a membrane area of 1800 m2 and a daily capacity of 1000 m3 of whey. it is usually operated at 50°C. The membrane permeate flow rate is initially greater than 1 m3 . The membrane permeate flow rate is initially greater than 1m/d, but when the whey is concentrated by a factor of 10 the viscosity is greater than 0.002Pas (0.02P), the membrane permeate flow rate drops to 0.5m/d, and the concentration limit is therefore largely determined by the membrane contamination and the increase in viscosity of the whey concentrate. Ultrafiltration applied to the food industry is the most important issue is the daily cleaning and sterilization. Generally first alkaline wash, then acid wash, and finally sterilized with sodium hypochlorite solution. Membrane life can be more than 1 year.

A new cheese production process is to first concentrate skimmed milk by ultrafiltration 3~4 times, and then use its concentrate for fermentation to produce milk vinegar, which is being gradually popularized with its great superiority. Because the treated concentrate is used to produce milk enzyme, its yield can be increased by more than 20%, according to the conservative estimate can save 6% of the milk; In addition, because of the lactose is removed from the milk to make the milk vinegar taste more delicious, and finally also reduces the amount of whey processing.

Food Biochemical Applications of Ultrafiltration —— Juice Clarification by Ultrafiltration

Fresh juice extracted from apples is cloudy due to the presence of compounds such as pectin. Conventional methods use enzymes, saponins and gelatin to precipitate them, and then the supernatant is taken and filtered to obtain clarified juice. Through ultrafiltration or microfiltration to clarify the juice, only partially remove pectin first, can reduce the amount of enzyme, eliminating saponin and gelatin, saving raw materials, and save labor and time. At the same time, the juice recovery rate has been improved, up to 98% ~ 99%. In addition, the ultrafiltration treatment of juice quality has also been improved, turbidity only 0.4 ~ 0.6 NTU (traditional process for 1.5 ~ 3.0 NTU). And because ultrafiltration can be meta-thermal removal of bacteria in the juice, thus extending the shelf life of the juice.

Food biochemical applications of ultrafiltration —- serum albumin extraction by ultrafiltration

The isolation of serum albumin from plasma involves a complex series of processes. A fraction that has been processed containing 3% albumin, 20% ethanol, and other small molecules is separated from ethanol in a three-step process using an ultrafiltration membrane with a MWCO of 30,000. During the first and second steps, the membrane permeate flow rate was 0.5-0.7 m/d, and during the last step, it decreased to below 0.1 m/d. According to the latest report [called, the use of zirconia metal membrane with a cut-off molecular weight of 10,000, which has a longer membrane life and more energy. Coupled with the use of backflush and pulse feed can reduce membrane contamination, the permeation rate of the first and second step can be increased nearly 1 times, and the concentration of albumin in the permeate is much lower, usually less than 0.4 g/L.