Pretreatment, membrane technologies are relatively simple to install, and the systems require little more than a feed pump, a cleaning pump, the membrane modules, and some holding tanks. According to a 1997 report by the National Research Council, most experts foresee that membrane filtration will be used with greater frequency in small systems. Note on the Membrane Filtration Guidance Manual Purpose The purpose of this guidance manual is to provide technical information on the use of membrane filtration and application of the technology for compliance with the Long Term 2 Enhanced Surface Water Treatment Rule, which would require certain systems to provide additional treatment for Cryptosporidium.
Ultrafiltration (UF) is a variety of membrane filtration in which forces like pressure or concentration gradients lead to a separation through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is used in industry and research for purifying and concentrating macromolecular (103 - 106Da) solutions, especially protein solutions.
Ultrafiltration is not fundamentally different from microfiltration. Both of these separate based on size exclusion or particle capture. It is fundamentally different from membrane gas separation, which separate based on different amounts of absorption and different rates of diffusion. Ultrafiltration membranes are defined by the molecular weight cut-off (MWCO) of the membrane used. Ultrafiltration is applied in cross-flow or dead-end mode.
- 1Applications
- 3Membrane fouling
- 3.2Types of fouling
- 4Membrane arrangements
- 6Process design considerations
- 6.2Membrane specifications
- 6.3Operation strategy
Applications[edit]
Industries such as chemical and pharmaceutical manufacturing, food and beverage processing, and waste water treatment, employ ultrafiltration in order to recycle flow or add value to later products. Blood dialysis also utilizes ultrafiltration.
Drinking water[edit]
Drinking water treatment 300 m3/h using ultrafiltration in Grundmühle waterworks (Germany)
Ultrafiltration can be used for the removal of particulates and macromolecules from raw water to produce potable water. It has been used to either replace existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in water treatment plants or as standalone systems in isolated regions with growing populations.[1] When treating water with high suspended solids, UF is often integrated into the process, utilising primary (screening, flotation, filtration) and some secondary treatments as pre-treatment stages.[2] UF processes are currently preferred over traditional treatment methods for the following reasons:
- No chemicals required (aside from cleaning)
- Constant product quality regardless of feed quality
- Compact plant size
- Capable of exceeding regulatory standards of water quality, achieving 90–100% pathogen removal [3]
UF processes are currently limited by the high cost incurred due to membrane fouling and replacement.[4] Additional pretreatment of feed water is required to prevent excessive damage to the membrane units.
In many cases UF is used for pre filtration in reverse osmosis (RO) plants to protect the RO membranes.
Protein concentration[edit]
UF is used extensively in the dairy industry; particularly in the processing of cheese whey to obtain whey protein concentrate (WPC) and lactose-rich permeate.[5][6] In a single stage, a UF process is able to concentrate the whey 10–30 times the feed.[7]
The original alternative to membrane filtration of whey was using steam heating followed by drum drying or spray drying. The product of these methods had limited applications due to its granulated texture and insolubility. Existing methods also had inconsistent product composition, high capital and operating costs and due to the excessive heat used in drying would often denature some of the proteins.[5]
Compared to traditional methods, UF processes used for this application:[5][7]
The original alternative to membrane filtration of whey was using steam heating followed by drum drying or spray drying. The product of these methods had limited applications due to its granulated texture and insolubility. Existing methods also had inconsistent product composition, high capital and operating costs and due to the excessive heat used in drying would often denature some of the proteins.[5]
Compared to traditional methods, UF processes used for this application:[5][7]
- Are more energy efficient
- Have consistent product quality, 35–80% protein product depending on operating conditions
- Do not denature proteins as they use moderate operating conditions
The potential for fouling is widely discussed, being identified as a significant contributor to decline in productivity.[5][6][7] Cheese whey contains high concentrations of calcium phosphate which can potentially lead to scale deposits on the membrane surface. As a result, substantial pretreatment must be implemented to balance pH and temperature of the feed to maintain solubility of calcium salts.[7]
A selectively permeable membrane can be mounted in a centrifuge tube. The buffer is forced through the membrane by centrifugation, leaving the protein in the upper chamber.
Other applications[edit]
- Filtration of effluent from paper pulp mill
- Cheese manufacture, see ultrafiltered milk
- Removal of some bacterias from milk
- Process and waste water treatment
- Enzyme recovery
- Fruit juice concentration and clarification
- Dialysis and other blood treatments
- Desalting and solvent-exchange of proteins (via diafiltration)
- Laboratory grade manufacturing
- Radiocarbon dating of bone collagen
Principles[edit]
The basic operating principle of ultrafiltration uses a pressure induced separation of solutes from a solvent through a semi permeable membrane. The relationship between the applied pressure on the solution to be separated and the flux through the membrane is most commonly described by the Darcy equation:
where J is the flux (flow rate per membrane area),TMP is the transmembrane pressure (pressure difference between feed and permeate stream), μ is solvent viscosity, Rt is the total resistance (sum of membrane and fouling resistance).
Membrane fouling[edit]
Concentration polarization[edit]
When filtration occurs the local concentration of rejected material at the membrane surface increases and can become saturated. In UF, increased ion concentration can develop an osmotic pressure on the feed side of the membrane. This reduces the effective TMP of the system, therefore reducing permeation rate. The increase in concentrated layer at the membrane wall decreases the permeate flux, due to increase in resistance which reduces the driving force for solvent to transport through membrane surface. CP affects almost all the available membrane separation processes. In RO, the solutes retained at the membrane layer results in higher osmotic pressure in comparison to the bulk stream concentration. So the higher pressures are required to overcome this osmotic pressure. Concentration polarisation plays a dominant role in ultrafiltration as compared to microfiltration because of the small pore size membrane.[8] It must be noted that concentration polarization differs from fouling as it has no lasting effects on the membrane itself and can be reversed by relieving the TMP. It does however have a significant effect on many types of fouling.[9]
Types of fouling[edit]
Particulate deposition[edit]
The following models describe the mechanisms of particulate deposition on the membrane surface and in the pores:
![Qualification of membrane filtration pdf Qualification of membrane filtration pdf](/uploads/1/2/4/8/124870837/349189665.jpg)
- Standard blocking: macromolecules are uniformly deposited on pore walls
- Complete blocking: membrane pore is completely sealed by a macromolecule
- Cake formation: accumulated particles or macromolecules form a fouling layer on the membrane surface, in UF this is also known as a gel layer
- Intermediate blocking: when macromolecules deposit into pores or onto already blocked pores, contributing to cake formation [10]
Scaling[edit]
As a result of concentration polarization at the membrane surface, increased ion concentrations may exceed solubility thresholds and precipitate on the membrane surface. These inorganic salt deposits can block pores causing flux decline, membrane degradation and loss of production. The formation of scale is highly dependent on factors affecting both solubility and concentration polarization including pH, temperature, flow velocity and permeation rate.[11]
Biofouling[edit]
Microorganisms will adhere to the membrane surface forming a gel layer – known as biofilm.[12] The film increases the resistance to flow, acting as an additional barrier to permeation. In spiral-wound modules, blockages formed by biofilm can lead to uneven flow distribution and thus increase the effects of concentration polarization.[13]
Membrane arrangements[edit]
Hollow fibre module
Depending on the shape and material of the membrane, different modules can be used for ultrafiltration process.[14] Commercially available designs in ultrafiltration modules vary according to the required hydrodynamic and economic constraints as well as the mechanical stability of the system under particular operating pressures.[15] The main modules used in industry include:
Membrane Filtration Pdf Free
Tubular modules[edit]
The tubular module design uses polymeric membranes cast on the inside of plastic or porous paper components with diameters typically in the range of 5 – 25 mm with lengths from 0.6 - 6.4 m.[5] Multiple tubes are housed in a PVC or steel shell. The feed of the module is passed through the tubes, accommodating radial transfer of permeate to the shell side. This design allows for easy cleaning however the main drawback is its low permeability, high volume hold-up within the membrane and low packing density.[5][15]
Hollow fibre[edit]
This design is conceptually similar to the tubular module with a shell and tube arrangement. A single module can consist of 50 to thousands of hollow fibres and therefore are self-supporting unlike the tubular design. The diameter of each fibre ranges from 0.2 – 3 mm with the feed flowing in the tube and the product permeate collected radially on the outside. The advantage of having self-supporting membranes as is the ease at which it can be cleaned due to its ability to be backflushed. Replacement costs however are high, as one faulty fibre will require the whole bundle to be replaced. Considering the tubes are of small diameter, using this design also makes the system prone to blockage.[7]
Spiral-wound modules[edit]
Spiral-wound membrane module
Are composed of a combination of flat membrane sheets separated by a thin meshed spacer material which serves as a porous plastic screen support. These sheets are rolled around a central perforated tube and fitted into a tubular steel pressure vessel casing. The feed solution passes over the membrane surface and the permeate spirals into the central collection tube. Spiral-wound modules are a compact and cheap alternative in ultrafiltration design, offer a high volumetric throughput and can also be easily cleaned.[15] However it is limited by the thin channels where feed solutions with suspended solids can result in partial blockage of the membrane pores.[7]
Plate and frame[edit]
This uses a membrane placed on a flat plate separated by a mesh like material. The feed is passed through the system from which permeate is separated and collected from the edge of the plate. Channel length can range from 10 – 60 cm and channel heights from 0.5 – 1 mm.[7] This module provides low volume hold-up, relatively easy replacement of the membrane and the ability to feed viscous solutions because of the low channel height, unique to this particular design.[15]
Process characteristics[edit]
The process characteristics of a UF system are highly dependent on the type of membrane used and its application. Manufacturers' specifications of the membrane tend to limit the process to the following typical specifications:[16][17][18][19]
Hollow Fibre | Spiral-wound | Ceramic Tubular | Plate and Frame |
---|---|---|---|
pH | 2–13 | 2–11 | 3–7 |
Feed Pressure (psi) | 9–15 | <30–120 | 60–100 |
Backwash Pressure (psi) | 9–15 | 20–40 | 10–30 |
Temperature (°C) | 5–30 | 5–45 | 5–400 |
Total Dissolved Solids (mg/L) | <1000 | <600 | <500 |
Total Suspended Solids (mg/L) | <500 | <450 | <300 |
Turbidity (NTU) | <15 | <1 | <10 |
Iron (mg/L) | <5 | <5 | <5 |
Oils and Greases (mg/L) | <0.1 | <0.1 | <0.1 |
Solvents, phenols (mg/L) | <0.1 | <0.1 | <0.1 |
Process design considerations[edit]
When designing a new membrane separation facility or considering its integration into an existing plant, there are many factors which must be considered. For most applications a heuristic approach can be applied to determine many of these characteristics to simplify the design process. Some design areas include:
Pre-treatment[edit]
Treatment of feed prior to the membrane is essential to prevent damage to the membrane and minimize the effects of fouling which greatly reduce the efficiency of the separation. Types of pre-treatment are often dependent on the type of feed and its quality. For example, in wastewater treatment, household waste and other particulates are screened. Other types of pre-treatment common to many UF processes include pH balancing and coagulation.[20][21] Appropriate sequencing of each pre-treatment phase is crucial in preventing damage to subsequent stages. Pre-treatment can even be employed simply using dosing points.
Membrane specifications[edit]
Material[edit]
Most UF membranes use polymer materials (polysulfone, polypropylene, cellulose acetate, polylactic acid) however ceramic membranes are used for high temperature applications.
Pore size[edit]
Membrane Filtration Pdf Download
A general rule for choice of pore size in a UF system is to use a membrane with a pore size one tenth that of the particle size to be separated. This limits the number of smaller particles entering the pores and adsorbing to the pore surface. Instead they block the entrance to the pores allowing simple adjustments of cross-flow velocity to dislodge them.[7]
Operation strategy[edit]
Schematic of cross flow operation.
Schematic of dead-end operation
Flowtype[edit]
UF systems can either operate with cross-flow or dead-end flow. In dead-end filtration the flow of the feed solution is perpendicular to the membrane surface. On the other hand, in cross flow systems the flow passes parallel to the membrane surface.[22] Dead-end configurations are more suited to batch processes with low suspended solids as solids accumulate at the membrane surface therefore requiring frequent backflushes and cleaning to maintain high flux. Cross-flow configurations are preferred in continuous operations as solids are continuously flushed from the membrane surface resulting in a thinner cake layer and lower resistance to permeation.
Flow velocity[edit]
Flow velocity is especially critical for hard water or liquids containing suspensions in preventing excessive fouling. Higher cross-flow velocities can be used to enhance the sweeping effect across the membrane surface therefore preventing deposition of macromolecules and colloidal material and reducing the effects of concentration polarization. Expensive pumps are however required to achieve these conditions.
Flow temperature[edit]
To avoid excessive damage to the membrane, it is recommended to operate a plant at the temperature specified by the membrane manufacturer. In some instances however temperatures beyond the recommended region are required to minimise the effects of fouling.[21] Economic analysis of the process is required to find a compromise between the increased cost of membrane replacement and productivity of the separation.
Pressure[edit]
Typical two stage membrane process with recycle stream
Pressure drops over multi-stage separation can result in a drastic decline in flux performance in the latter stages of the process. This can be improved using booster pumps to increase the TMP in the final stages. This will incur a greater capital and energy cost which will be offset by the improved productivity of the process.[21] With a multi-stage operation, retentate streams from each stage are recycled through the previous stage to improve their separation efficiency.
Multi-stage, multi-module[edit]
Multiple stages in series can be applied to achieve higher purity permeate streams. Due to the modular nature of membrane processes, multiple modules can be arranged in parallel to treat greater volumes.[23]
Post-treatment[edit]
Post-treatment of the product streams is dependent on the composition of the permeate and retentate and its end-use or government regulation. In cases such as milk separation both streams (milk and whey) can be collected and made into useful products. Additional drying of the retentate will produce whey powder. In the paper mill industry, the retentate (non-biodegradable organic material) is incinerated to recover energy and permeate (purified water) is discharged into waterways. It is essential for the permeate water to be pH balanced and cooled to avoid thermal pollution of waterways and altering its pH.
Cleaning[edit]
Cleaning of the membrane is done regularly to prevent the accumulation of foulants and reverse the degrading effects of fouling on permeability and selectivity.
Regular backwashing is often conducted every 10 min for some processes to remove cake layers formed on the membrane surface.[7] By pressurising the permeate stream and forcing it back through the membrane, accumulated particles can be dislodged, improving the flux of the process. Backwashing is limited in its ability to remove more complex forms of fouling such as biofouling, scaling or adsorption to pore walls.[24]
These types of foulants require chemical cleaning to be removed. The common types of chemicals used for cleaning are:[24][25]
Regular backwashing is often conducted every 10 min for some processes to remove cake layers formed on the membrane surface.[7] By pressurising the permeate stream and forcing it back through the membrane, accumulated particles can be dislodged, improving the flux of the process. Backwashing is limited in its ability to remove more complex forms of fouling such as biofouling, scaling or adsorption to pore walls.[24]
These types of foulants require chemical cleaning to be removed. The common types of chemicals used for cleaning are:[24][25]
- Acidic solutions for the control of inorganic scale deposits
- Alkali solutions for removal of organic compounds
- Biocides or disinfection such as Chlorine or peroxide when bio-fouling is evident
When designing a cleaning protocol it is essential to consider:
Cleaning time – Adequate time must be allowed for chemicals to interact with foulants and permeate into the membrane pores. However, if the process is extended beyond its optimum duration it can lead to denaturation of the membrane and deposition of removed foulants.[24] The complete cleaning cycle including rinses between stages may take as long as 2 hours to complete.[26]
Aggressiveness of chemical treatment – With a high degree of fouling it may be necessary to employ aggressive cleaning solutions to remove fouling material. However, in some applications this may not be suitable if the membrane material is sensitive, leading to enhanced membrane ageing.
Disposal of cleaning effluent – The release of some chemicals into wastewater systems may be prohibited or regulated therefore this must be considered. For example, the use of phosphoric acid may result in high levels of phosphates entering water ways and must be monitored and controlled to prevent eutrophication.
Summary of common types of fouling and their respective chemical treatments [7]
Cleaning time – Adequate time must be allowed for chemicals to interact with foulants and permeate into the membrane pores. However, if the process is extended beyond its optimum duration it can lead to denaturation of the membrane and deposition of removed foulants.[24] The complete cleaning cycle including rinses between stages may take as long as 2 hours to complete.[26]
Aggressiveness of chemical treatment – With a high degree of fouling it may be necessary to employ aggressive cleaning solutions to remove fouling material. However, in some applications this may not be suitable if the membrane material is sensitive, leading to enhanced membrane ageing.
Disposal of cleaning effluent – The release of some chemicals into wastewater systems may be prohibited or regulated therefore this must be considered. For example, the use of phosphoric acid may result in high levels of phosphates entering water ways and must be monitored and controlled to prevent eutrophication.
Summary of common types of fouling and their respective chemical treatments [7]
Foulant | Reagent | Time and Temperature | Mode of Action |
---|---|---|---|
Fats and oils, proteins, polysaccharides, bacteria | 0.5M NaOH with 200 ppm Cl2 | 30-60 min 25-55 °C | Hydrolysis and oxidation |
DNA, mineral salts | 0.1M – 0.5M acid (acetic, citric, nitric) | 30-60 min 25-35 °C | Solubilization |
Fats, oils, biopolymers, proteins | 0.1% SDS, 0.1% Triton X-100 | 30 min – overnight 25-55 °C | Wetting, emulsifying, suspending, dispersing |
Cell fragments, fats, oils, proteins | Enzyme detergents | 30 min – overnight 30 – 40 °C | Catalytic breakdown |
DNA | 0.5% DNAase | 30 min – overnight 20 – 40 °C | Enzyme hydrolysis |
New developments[edit]
In order to increase the life-cycle of membrane filtration systems, energy efficient membranes are being developed in membrane bioreactor systems. Technology has been introduced which allows the power required to aerate the membrane for cleaning to be reduced whilst still maintaining a high flux level. Mechanical cleaning processes have also been adopted using granulates as an alternative to conventional forms of cleaning; this reduces energy consumption and also reduces the area required for filtration tanks.[27]
Membrane properties have also been enhanced to reduce fouling tendencies by modifying surface properties. This can be noted in the biotechnology industry where membrane surfaces have been altered in order to reduce the amount of protein binding.[28] Ultrafiltration modules have also been improved to allow for more membrane for a given area without increasing its risk of fouling by designing more efficient module internals.
The current pre-treatment of seawater desulphonation uses ultrafiltration modules that have been designed to withstand high temperatures and pressures whilst occupying a smaller footprint. Each module vessel is self supported and resistant to corrosion and accommodates easy removal and replacement of the module without the cost of replacing the vessel itself.[27]
References[edit]
- ^Clever, M.; Jordt, F.; Knauf, R.; Räbiger, N.; Rüdebusch, M.; Hilker-Scheibel, R. (1 December 2000). 'Process water production from river water by ultrafiltration and reverse osmosis'. Desalination. 131 (1–3): 325–336. doi:10.1016/S0011-9164(00)90031-6.
- ^Laîné, J.-M.; Vial, D.; Moulart, Pierre (1 December 2000). 'Status after 10 years of operation — overview of UF technology today'. Desalination. 131 (1–3): 17–25. doi:10.1016/S0011-9164(00)90002-X.
- ^American Water Works Association Research Foundation .. Ed. group Joël Mallevialle (1996). Water treatment membrane processes. New York [u.a.]: McGraw Hill. ISBN9780070015593.
- ^Edwards, David; Donn, Alasdair; Meadowcroft, Charlotte (1 May 2001). 'Membrane solution to a 'significant risk' Cryptosporidium groundwater source'. Desalination. 137 (1–3): 193–198. doi:10.1016/S0011-9164(01)00218-1.
- ^ abcdefTamime, A. Y. Membrane Processing Dairy and Beverage Applications. Chicester: Wiley. ISBN1118457021.
- ^ abNigam, Mayank Omprakash; Bansal, Bipan; Chen, Xiao Dong (1 January 2008). 'Fouling and cleaning of whey protein concentrate fouled ultrafiltration membranes'. Desalination. 218 (1–3): 313–322. doi:10.1016/j.desal.2007.02.027.
- ^ abcdefghijCheryan, Munir (1998). Ultrafiltration and Microfiltration Handbook. CRC Press. ISBN1420069020.
- ^Brian, P.L., 1965, Concentration polarization in reverse osmosis desalination with variable flux and incomplete salt rejection, Ind. Eng. Chem. Fund. 4: 439−445.
- ^Rizvi, Anil Kumar; Pabby, Ana Maria; Sastre, Syed S.H., eds. (2007). Handbook of membrane separations : chemical, pharmaceutical, and biotechnological applications. Boca Raton, Fla.: CRC Press. ISBN978-0-8493-9549-9.
- ^Bruijn, J P F; Salazar, F N; Borquez, R (September 2005). 'Membrane blocking in ultrafiltration: A new approach to fouling'. Food and Bioproducts Processing. 83 (3): 211–219.
- ^Antony, Alice; Low, Jor How; Gray, Stephen; Childress, Amy E.; Le-Clech, Pierre; Leslie, Greg (1 November 2011). 'Scale formation and control in high pressure membrane water treatment systems: A review'. Journal of Membrane Science. 383 (1–2): 1–16. doi:10.1016/j.memsci.2011.08.054.
- ^Flemming, H.-C.; Schaule, G.; Griebe, T.; Schmitt, J.; Tamachkiarowa, A. (1 November 1997). 'Biofouling—the Achilles heel of membrane processes'. Desalination. 113 (2–3): 215–225. doi:10.1016/S0011-9164(97)00132-X.
- ^Baker, J.S.; Dudley, L.Y. (1 September 1998). 'Biofouling in membrane systems — A review'. Desalination. 118 (1–3): 81–89. doi:10.1016/S0011-9164(98)00091-5.
- ^Futselaar, Harry; Weijenberg, Dick C. (1 September 1998). 'System design for large-scale ultrafiltration applications'. Desalination. 119 (1–3): 217–224. doi:10.1016/S0011-9164(98)00159-3.
- ^ abcdBelfort, Georges (1 February 1988). 'Membrane modules: comparison of different configurations using fluid mechanics'. Journal of Membrane Science. 35 (3): 245–270. doi:10.1016/S0376-7388(00)80299-9.
- ^Koch Membrane Systems. 'Membrane Products'. Koch Membrane Systems. Retrieved 9 October 2013.
- ^US Department of the Interior Bureau of Reclamation. 'Water Treatment Primer for Communities in Need'(PDF). US Department of the Interior Bureau of Reclamation. Retrieved 11 October 2013.
- ^Con-Serv Manufacturing. 'Operation and Maintenance Manual - UF-6-HF Ultrafiltration System'(PDF). Con-Serv Manufacturing. Retrieved 10 October 2013.
- ^Laîné; prepared by Joseph G. Jacangelo, Samer Adham, Jean-Michel (1997). Membrane filtration for microbial removal. Denver, CO: AWWA Research Foundation and American Water Works Association. ISBN0898678943.CS1 maint: multiple names: authors list (link)
- ^Water, Sydney. 'Rosehill Recycled Water Scheme - Fairfield Recycled Water Plant'(PDF). Sydney Water.
- ^ abcNordin, Anna-Karin; Jönsson, Ann-Sofi (1 November 2006). 'Case study of an ultrafiltration plant treating bleach plant effluent from a pulp and paper mill'. Desalination. 201 (1–3): 277–289. doi:10.1016/j.desal.2006.06.004.
- ^Farahbakhsh, Khosrow; Adham, Samer S.; Smith, Daniel W. (June 2003). 'Monitoring the Integrity of Low-Pressure Membranes'. Journal AWWA: 95–107.
- ^American Water Works Association Research Foundation .. Ed. group Joël Mallevialle (1996). Water treatment membrane processes. New York [u.a.]: McGraw Hill. ISBN0070015597.
- ^ abcCui, edited by Z.F.; Muralidhara, H.S. (2010). Membrane technology : a practical guide to membrane technology and applications in food and bioprocessing (1st ed.). Amsterdam: Butterworth-Heinemann. pp. 213*254. ISBN978-1-85617-632-3.CS1 maint: extra text: authors list (link)
- ^Gao, Wei; Liang, Heng; Ma, Jun; Han, Mei; Chen, Zhong-lin; Han, Zheng-shuang; Li, Gui-bai (1 May 2011). 'Membrane fouling control in ultrafiltration technology for drinking water production: A review'. Desalination. 272 (1–3): 1–8. doi:10.1016/j.desal.2011.01.051.
- ^Wallberg, Ola; Jönsson, Ann-Sofi; Wickström, Peter (1 December 2001). 'Membrane cleaning — a case study in a sulphite pulp mill bleach plant'. Desalination. 141 (3): 259–268. doi:10.1016/S0011-9164(01)85004-9.
- ^ abBennett, Anthony (1 November 2012). 'Membrane technology: Developments in ultrafiltration technologies'. Filtration + Separation. 49 (6): 28–33. doi:10.1016/S0015-1882(12)70287-2.
- ^Ag, S (1 September 2012). 'Energy-efficient membrane is designed for MBR systems'. Membrane Technology. 2012 (9): 4. doi:10.1016/S0958-2118(12)70178-7.
External links[edit]
- Media related to Ultrafiltration at Wikimedia Commons
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Ultrafiltration&oldid=914727442'
Membrane technology covers all engineering approaches for the transport of substances between two fractions with the help of permeablemembranes. In general, mechanical separation processes for separating gaseous or liquid streams use membrane technology.
- 2Mass transfer
Applications[edit]
Ultrafiltration for a swimming pool
Venous-arterial extracorporeal membrane oxygenation scheme
Membrane separation processes operate without heating and therefore use less energy than conventional thermal separation processes such as distillation, sublimation or crystallization. The separation process is purely physical and both fractions (permeate and retentate) can be used. Cold separation using membrane technology is widely used in the food technology, biotechnology and pharmaceutical industries. Furthermore, using membranes enables separations to take place that would be impossible using thermal separation methods. For example, it is impossible to separate the constituents of azeotropic liquids or solutes which form isomorphic crystals by distillation or recrystallization but such separations can be achieved using membrane technology. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include the production of drinking water by reverse osmosis (worldwide approximately 7 million cubic metres annually), filtrations in the food industry, the recovery of organic vapours such as petro-chemical vapour recovery and the electrolysis for chlorine production.
In waste water treatment, membrane technology is becoming increasingly important. With the help of ultra/microfiltration it is possible to remove particles, colloids and macromolecules, so that waste-water can be disinfected in this way. This is needed if waste-water is discharged into sensitive waters especially those designated for contact water-sports and recreation.
About half of the market is in medical applications such as use in artificial kidneys to remove toxic substances by hemodialysis and as artificial lung for bubble-free supply of oxygen in the blood.
The importance of membrane technology is growing in the field of environmental protection (NanoMemPro IPPC Database). Even in modern energy recovery techniques membranes are increasingly used, for example in fuel cells and in osmotic power plants.
Mass transfer[edit]
Two basic models can be distinguished for mass transfer through the membrane:
- the solution-diffusion model and
- the hydrodynamic model.
In real membranes, these two transport mechanisms certainly occur side by side, especially during ultra-filtration.
Solution-diffusion model[edit]
In the solution-diffusion model, transport occurs only by diffusion. The component that needs to be transported must first be dissolved in the membrane. The general approach of the solution-diffusion model is to assume that the chemical potential of the feed and permeate fluids are in equilibrium with the adjacent membrane surfaces such that appropriate expressions for the chemical potential in the fluid and membrane phases can be equated at the solution-membrane interface. This principle is more important for dense membranes without natural pores such as those used for reverse osmosis and in fuel cells. During the filtration process a boundary layer forms on the membrane. This concentration gradient is created by molecules which cannot pass through the membrane. The effect is referred as concentration polarization and, occurring during the filtration, leads to a reduced trans-membrane flow (flux). Concentration polarization is, in principle, reversible by cleaning the membrane which results in the initial flux being almost totally restored. Using a tangential flow to the membrane (cross-flow filtration) can also minimize concentration polarization.
Hydrodynamic model[edit]
Transport through pores – in the simplest case – is done convectively. This requires the size of the pores to be smaller than the diameter of the two separate components. Membranes which function according to this principle are used mainly in micro- and ultrafiltration. They are used to separate macromolecules from solutions, colloids from a dispersion or remove bacteria. During this process the retained particles or molecules form a pulpy mass (filter cake) on the membrane, and this blockage of the membrane hampers the filtration. This blockage can be reduced by the use of the cross-flow method (cross-flow filtration). Here, the liquid to be filtered flows along the front of the membrane and is separated by the pressure difference between the front and back of the membrane into retentate (the flowing concentrate) on the front and permeate (filtrate) on the back. The tangential flow on the front creates a shear stress that cracks the filter cake and reduces the fouling.
Membrane operations[edit]
According to the driving force of the operation it is possible to distinguish:
- Pressure driven operations
- Concentration driven operations
- Operations in an electric potential gradient
- membrane electrolysis e.g. chloralkali process
- Operations in a temperature gradient
Membrane shapes and flow geometries[edit]
Cross-flow geometry
Dead-end geometry
There are two main flow configurations of membrane processes: cross-flow (or) tangential flow and dead-end filtrations. In cross-flow filtration the feed flow is tangential to the surface of membrane, retentate is removed from the same side further downstream, whereas the permeate flow is tracked on the other side. In dead-end filtration the direction of the fluid flow is normal to the membrane surface. Both flow geometries offer some advantages and disadvantages. Generally, dead-end filtration is used for feasibility studies on a laboratory scale. The dead-end membranes are relatively easy to fabricate which reduces the cost of the separation process. The dead-end membrane separation process is easy to implement and the process is usually cheaper than cross-flow membrane filtration. The dead-end filtration process is usually a batch-type process, where the filtering solution is loaded (or slowly fed) into the membrane device, which then allows passage of some particles subject to the driving force. The main disadvantage of a dead end filtration is the extensive membrane fouling and concentration polarization. The fouling is usually induced faster at higher driving forces. Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle back flow (concentration polarization). The tangential flow devices are more cost and labor-intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow. The most commonly used synthetic membrane devices (modules) are flat sheets/plates, spiral wounds, and hollow fibers.
Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules. Spiral wounds are constructed from similar flat membranes but in the form of a 'pocket' containing two membrane sheets separated by a highly porous support plate.[1] Several such pockets are then wound around a tube to create a tangential flow geometry and to reduce membrane fouling. hollow fiber modules consist of an assembly of self-supporting fibers with dense skin separation layers, and a more open matrix helping to withstand pressure gradients and maintain structural integrity.[1] The hollow fiber modules can contain up to 10,000 fibers ranging from 200 to 2500 μm in diameter; The main advantage of hollow fiber modules is very large surface area within an enclosed volume, increasing the efficiency of the separation process.
Spiral wound membrane module
- Hollow fiber membrane module
- Separation of air into oxygen and nitrogen through a membrane
Disc tube module is using a cross-flow geometry, and consists of a pressure tube and hydraulic discs, which are held by a central tension rod, and membrane cushions that lie between two discs.[2]
Membrane performance and governing equations[edit]
The selection of synthetic membranes for a targeted separation process is usually based on few requirements. Membranes have to provide enough mass transfer area to process large amounts of feed stream. The selected membrane has to have high selectivity (rejection) properties for certain particles; it has to resist fouling and to have high mechanical stability. It also needs to be reproducible and to have low manufacturing costs. The main modeling equation for the dead-end filtration at constant pressure drop is represented by Darcy's law:[1]
where Vp and Q are the volume of the permeate and its volumetric flow rate respectively (proportional to same characteristics of the feed flow), μ is dynamic viscosity of permeating fluid, A is membrane area, Rm and R are the respective resistances of membrane and growing deposit of the foulants. Rm can be interpreted as a membrane resistance to the solvent (water) permeation. This resistance is a membrane intrinsic property and is expected to be fairly constant and independent of the driving force, Δp. R is related to the type of membrane foulant, its concentration in the filtering solution, and the nature of foulant-membrane interactions. Darcy's law allows for calculation of the membrane area for a targeted separation at given conditions. The solutesieving coefficient is defined by the equation:[1]
where Cf and Cp are the solute concentrations in feed and permeate respectively. Hydraulic permeability is defined as the inverse of resistance and is represented by the equation:[1]
where J is the permeate flux which is the volumetric flow rate per unit of membrane area. The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance.
Membrane separation processes[edit]
Membrane separation processes have a very important role in the separation industry. Nevertheless, they were not considered technically important until the mid-1970s. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrolysis, dialysis, electrodialysis, gas separation, vapor permeation, pervaporation, membrane distillation, and membrane contactors.[3] All processes except for pervaporation involve no phase change. All processes except (electro)dialysis are pressure driven. Microfiltration and ultrafiltration is widely used in food and beverage processing (beer microfiltration, apple juice ultrafiltration), biotechnological applications and pharmaceutical industry (antibiotic production, protein purification), water purification and wastewater treatment, the microelectronics industry, and others. Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations (removal of CO2 from natural gas, separating N2 from air, organic vapor removal from air or a nitrogen stream) and sometimes in membrane distillation. The later process helps in the separation of azeotropic compositions reducing the costs of distillation processes.
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Ranges of membrane based separations
Pore size and selectivity[edit]
The pore distribution of a fictitious ultrafiltration membrane with the nominal pore size and the D90
The pore sizes of technical membranes are specified differently depending on the manufacturer. One common distinction is by nominal pore size. It describes the maximum pore size distribution[4] and gives only vague information about the retention capacity of a membrane.The exclusion limit or 'cut-off' of the membrane is usually specified in the form of NMWC (nominal molecular weight cut-off, or MWCO, molecular weight cut off, with units in Dalton). It is defined as the minimum molecular weight of a globular molecule that is retained to 90% by the membrane. The cut-off, depending on the method, can by converted to so-called D90, which is then expressed in a metric unit. In practice the MWCO of the membrane should be at least 20% lower than the molecular weight of the molecule that is to be separated.
Filter membranes are divided into four classes according to pore size:
Pore size | Molecular mass | Process | Filtration | Removal of |
---|---|---|---|---|
> 10 | 'Classic' filter | |||
> 0.1 μm | > 5000 kDa | microfiltration | < 2 bar | larger bacteria, yeast, particles |
100-2 nm | 5-5000 kDa | ultrafiltration | 1-10 bar | bacteria, macromolecules, proteins, larger viruses |
2-1 nm | 0.1-5 kDa | nanofiltration | 3-20 bar | viruses, 2- valent ions[5] |
< 1 nm | < 100 Da | reverse osmosis | 10-80 bar | salts, small organic molecules |
The form and shape of the membrane pores are highly dependent on the manufacturing process and are often difficult to specify. Therefore, for characterization, test filtrations are carried out and the pore diameter refers to the diameter of the smallest particles which could not pass through the membrane.
The rejection can be determined in various ways and provides an indirect measurement of the pore size. One possibility is the filtration of macromolecules (often dextran, polyethylene glycol or albumin), another is measurement of the cut-off by gel permeation chromatography. These methods are used mainly to measure membranes for ultrafiltration applications. Another testing method is the filtration of particles with defined size and their measurement with a particle sizer or by laser induced breakdown spectroscopy (LIBS). A vivid characterization is to measure the rejection of dextran blue or other colored molecules. The retention of bacteriophage and bacteria, the so-called 'bacteriachallenge test', can also provide information about the pore size.
Nominal pore size | micro-organism | ATCC root number |
---|---|---|
0.1 μm | Acholeplasma laidlawii | 23206 |
0.3 μm | Bacillus subtilis spores | 82 |
0.5 μm | Pseudomonas diminuta | 19146 |
0.45 μm | Serratia marcescens | 14756 |
0.65 μm | Lactobacillus brevis |
To determine the pore diameter, physical methods such as porosimetry (mercury, liquid-liquid porosimetry and Bubble Point Test) are also used, but a certain form of the pores (such as cylindrical or concatenated spherical holes) is assumed. Such methods are used for membranes whose pore geometry does not match the ideal, and we get 'nominal' pore diameter, which characterizes the membrane, but does not necessarily reflect its actual filtration behavior and selectivity.
The selectivity is highly dependent on the separation process, the composition of the membrane and its electrochemical properties in addition to the pore size. With high selectivity, isotopes can be enriched (uranium enrichment) in nuclear engineering or industrial gases like nitrogen can be recovered (gas separation). Ideally, even racemics can be enriched with a suitable membrane.
When choosing membranes selectivity has priority over a high permeability, as low flows can easily be offset by increasing the filter surface with a modular structure. In gas phase filtration different deposition mechanisms are operative, so that particles having sizes below the pore size of the membrane can be retained as well.
See also[edit]
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Notes[edit]
- ^ abcdeOsada, Y., Nakagawa, T., Membrane Science and Technology, New York: Marcel Dekker, Inc,1992.
- ^'RCDT Module - Radial Channel Disc Tube (RCDT) Module'. Radial Channel Disc Tube (RCDT) Module. Retrieved 2016-05-11.
- ^Pinnau, I., Freeman, B.D., Membrane Formation and Modification, ACS, 1999.
- ^TU Berlin script - 2 Principles of Membrane Processes ('Archived copy'(PDF). Archived from the original(PDF) on 2014-04-16. Retrieved 2013-09-06.CS1 maint: archived copy as title (link); PDF-Datei; 6,85 MB) Page 6 (German)
- ^Experience and potential application of nanofiltration - University of Linz (German) (PDFArchived 2013-04-05 at the Wayback Machine)
References[edit]
- Osada, Y., Nakagawa, T., Membrane Science and Technology, New York: Marcel Dekker, Inc,1992.
- Zeman, Leos J., Zydney, Andrew L., Microfiltration and Ultrafitration, Principles and Applications., New York: Marcel Dekker, Inc,1996.
- Mulder M., Basic Principles of Membrane Technology, Kluwer Academic Publishers, Netherlands, 1996.
- Jornitz, Maik W., Sterile Filtration, Springer, Germany, 2006
- Van Reis R., Zydney A. Bioprocess membrane technology. J Mem Sci. 297(2007): 16-50.
- Templin T., Johnston D., Singh V., Tumbleson M.E., Belyea R.L. Rausch K.D. Membrane separation of solids from corn processing streams. Biores Tech. 97(2006): 1536-1545.
- Ripperger S., Schulz G. Microporous membranes in biotechnical applications. Bioprocess Eng. 1(1986): 43-49.
- Thomas Melin, Robert Rautenbach, Membranverfahren, Springer, Germany, 2007, ISBN3-540-00071-2.
- Munir Cheryan, Handbuch Ultrafiltration, Behr, 1990, ISBN3-925673-87-3.
- Eberhard Staude, Membranen und Membranprozesse, VCH, 1992, ISBN3-527-28041-3.
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