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76-483: This concept of a membrane has been known since the eighteenth century but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on

152-532: A casein -rich concentrate stream used for cheese making, and a whey/serum protein stream which is further processed (using ultrafiltration ) to make whey protein concentrate. The whey protein stream undergoes further filtration to remove fat in order to achieve higher protein content in the final WPC (Whey Protein Concentrate) and WPI (Whey Protein Isolate) powders. Other common applications utilising microfiltration as

228-436: A bioreactor for biological treatment. Ultrafiltration removes particles higher than 0.005-2 μm and operates within a range of 70-700kPa. Ultrafiltration is used for many of the same applications as microfiltration. Some ultrafiltration membranes have also been used to remove dissolved compounds with high molecular weight, such as proteins and carbohydrates. Also, they can remove viruses and some endotoxins. Nanofiltration

304-435: A general schematic for this process. The major issues that influence the selection of the membrane include A few important design heuristics and their assessment are discussed below: Like any other membranes, microfiltration membranes are prone to fouling. (See Figure 4 below) It is therefore necessary that regular maintenance be carried out to prolong the life of the membrane module. The cost to design and manufacture

380-758: A large scale was with microfiltration and ultrafiltration technologies. Since the 1980s, these separation processes, along with electrodialysis , are employed in large plants and, today, several experienced companies serve the market. The degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive . The latter can be facilitated by pressure , concentration , chemical or electrical gradients of

456-399: A major separation process include Membrane filtration processes can be distinguished by three major characteristics: driving force, retentate stream and permeate streams. The microfiltration process is pressure driven with suspended particles and water as retentate and dissolved solutes plus water as permeate. The use of hydraulic pressure accelerates the separation process by increasing

532-497: A membrane per unit of area are about 20% less compared to the early 1990s and in a general sense are constantly declining. Microfiltration membranes are more advantageous in comparison to conventional systems. Microfiltration systems do not require expensive extraneous equipment such as flocculates, addition of chemicals, flash mixers, settling and filter basins. However the cost of replacement of capital equipment costs (membrane cartridge filters etc.) might still be relatively high as

608-432: A membrane system is given by following equation: Where Qp is the permeate stream flowrate [kg·s], F w is the water flux rate [kg·m·s] and A is the membrane area [m] The permeability (k) [m·s·bar] of a membrane is given by the next equation: The trans-membrane pressure (TMP) is given by the following expression: where P TMP is the trans-membrane pressure [kPa], P f the inlet pressure of feed stream [kPa]; P c

684-685: A physical means of separation (a barrier) as opposed to a chemical alternative. In that sense, both filtration and disinfection take place in a single step, negating the extra cost of chemical dosage and the corresponding equipment (needed for handling and storage). Similarly, the MF membranes are used in secondary wastewater effluents to remove turbidity but also to provide treatment for disinfection. At this stage, coagulants ( iron or aluminum ) may potentially be added to precipitate species such as phosphorus and arsenic which would otherwise have been soluble. Another crucial application of MF membranes lies in

760-613: A polyester layer. An emerging class of membranes rely on nanostructure channels to separate materials at the molecular scale. These include carbon nanotube membranes , graphene membranes, membranes made from polymers of intrinsic microporosity (PIMS), and membranes incorporating metal–organic frameworks (MOFs). These membranes can be used for size selective separations such as nanofiltration and reverse osmosis, but also adsorption selective separations such as olefins from paraffins and alcohols from water that traditionally have required expensive and energy intensive distillation . In

836-400: A portion of the fouling layer can be reversed by cleaning for short periods of time. Microfiltration membranes can generally operate in one of two configurations. Cross-flow filtration : where the fluid is passed through tangentially with respect to the membrane. Part of the feed stream containing the treated liquid is collected below the filter while parts of the water are passed through

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912-419: A procedure termed as 'cold sterilisation', which negate the use of heat. Furthermore, microfiltration membranes are finding increasing use in areas such as petroleum refining, in which the removal of particulates from flue gases is of particular concern. The key challenges/requirements for this technology are the ability of the membrane modules to withstand high temperatures (i.e. maintain stability), but also

988-446: A tube filter housing. Feed water is delivered into the membrane module. It passes through from the outside surface of the hollow fibers and the filtered water exits through the center of the fibers. With the flux rate in excess of 75 gallon per square foot per day, this design can be used for large scale facilities. As separation is achieved by sieving, the principal mechanism of transfer for microfiltration through micro porous membranes

1064-537: Is a primary measurement for fouling. Even though membrane fouling is an inevitable phenomenon during membrane filtration , it can be minimised by strategies such as cleaning, appropriate membrane selection and choice of operating conditions. Membranes can be cleaned physically, biologically or chemically. Physical cleaning includes gas scour, sponges, water jets or backflushing using permeate or pressurized air. Biological cleaning uses biocides to remove all viable microorganisms , whereas chemical cleaning involves

1140-510: Is a process whereby existing imperfections in the membrane (such as microcracks) can grow and propagate due to the complex stress state dynamics. These impacts are not unknown; A 2007 study simulated aging via cyclic backwash pulses, and reported similar embrittlement due to the effects. Additionally, repeated chemical treatment of fouling subjects membranes to excessive amounts of chlorine or other treatment chemicals which can cause degradation. This chemical degradation can lead to delamination of

1216-467: Is also known as "loose" RO and can reject particles smaller than 0,002 μm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening. Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are: (1). minimize particulate and microbial fouling of

1292-399: Is bulk flow. Generally, due to the small diameter of the pores the flow within the process is laminar ( Reynolds Number < 2100) The flow velocity of the fluid moving through the pores can thus be determined (by Hagen-Poiseuille 's equation), the simplest of which assuming a parabolic velocity profile . Transmembrane Pressure (TMP) The transmembrane pressure (TMP) is defined as

1368-454: Is commonly attached to measure the pressure drop between the outlet and inlet streams. See Figure 1 for a general setup. The most abundant use of microfiltration membranes are in the water , beverage and bio-processing industries (see below). The exit process stream after treatment using a micro-filter has a recovery rate which generally ranges to about 90-98 %. Perhaps the most prominent use of microfiltration membranes pertains to

1444-604: Is commonly used in conjunction with various other separation processes such as ultrafiltration and reverse osmosis to provide a product stream which is free of undesired contaminants . Microfiltration usually serves as a pre-treatment for other separation processes such as ultrafiltration , and a post-treatment for granular media filtration . The typical particle size used for microfiltration ranges from about 0.1 to 10 μm . In terms of approximate molecular weight these membranes can separate macromolecules of molecular weights generally less than 100,000 g/mol. The filters used in

1520-614: Is due to concentration polarization . At low feed flow rate or with high feed concentration, the limiting flux situation is observed even at relatively low pressures. Flux, transmembrane pressure (TMP), Permeability, and Resistance are the best indicators of membrane fouling. Under constant flux operation, TMP increases to compensate for the fouling. On the other hand, under constant pressure operation, flux declines due to membrane fouling. In some technologies such as membrane distillation , fouling reduces membrane rejection, and thus permeate quality (e.g. as measured by electrical conductivity)

1596-772: Is fundamentally the same as other filtration techniques utilising a pore size distribution to physically separate particles. It is analogous to other technologies such as ultra/nanofiltration and reverse osmosis, however, the only difference exists in the size of the particles retained, and also the osmotic pressure. The main of which are described in general below: Ultrafiltration membranes have pore sizes ranging from 0.1 μm to 0.01 μm and are able to retain proteins, endotoxins, viruses and silica. UF has diverse applications which span from waste water treatment to pharmaceutical applications. Nanofiltration membranes have pores sized from 0.001 μm to 0.01 μm and filters multivalent ions, synthetic dyes, sugars and specific salts. As

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1672-511: Is given by: Where For micron sized particles the Specific Cake Resistance is roughly. Where Rigorous design equations To give a better indication regarding the exact determination of the extent of the cake formation, one-dimensional quantitative models have been formulated to determine factors such as See External Links for further details Although environmental impacts of membrane filtration processes differ according to

1748-460: Is influenced by numerous factors such as system hydrodynamics, operating conditions, membrane properties, and material properties (solute). At low pressure, low feed concentration, and high feed velocity, concentration polarisation effects are minimal and flux is almost proportional to trans-membrane pressure difference. However, in the high pressure range, flux becomes almost independent of applied pressure. Deviation from linear flux-pressure relation

1824-457: Is often preferred to dead end filtration , because turbulence generated during the filtration entails a thinner deposit layer and therefore minimises fouling (e.g. tubular pinch effect ). In some applications such as in many MBR applications, air scour is used to promote turbulence at the membrane surface. Membrane performance can suffer from fouling-induced mechanical degradation. This may result in unwanted pressure and flux gradients, both of

1900-409: Is proposed to adapt this original concept, by internally reusing older RO membranes within the same pressure vessel. Recycling of materials is a general term that involves physically transforming the material or its components so that they can be regenerated into other useful products. The membrane modules are complex structures, consisting of a number of different polymeric components and, potentially,

1976-450: Is recommended, from two to four times annually. Reuse of RO membranes include the direct reapplication of modules in other separation processes with less stringent specifications. The conversion from the RO TFC membrane to a porous membrane is possible by degrading the dense layer of polyamide. Converting RO membranes by chemical treatment with different oxidizing solutions are aimed at removing

2052-427: Is supported on a thicker layer that has larger pores. These systems are compact and possess a sturdy design, Compared to cross-flow filtration, plate and frame configurations possess a reduced capital expenditure; however the operating costs will be higher. The uses of plate and frame modules are most applicable for smaller and simpler scale applications (laboratory) which filter dilute solutions. This particular design

2128-406: Is that, in some cases, the flux rate and selectivity of the membrane process can be negatively impacted. Once the membrane reaches a significant performance decline it is discarded. Discarded RO membrane modules are currently classified worldwide as inert solid waste and are often disposed of in landfills; although they can also be energetically recovered. However, various efforts have been made over

2204-405: Is the most widely used desalination technology because of its simplicity of use and relatively low energy costs compared with distillation, which uses technology based on thermal processes. Note that RO membranes remove water constituents at the ionic level. To do so, most current RO systems use a thin-film composite (TFC), mainly consisting of three layers: a polyamide layer, a polysulphone layer and

2280-548: Is used for cross-flow filtration. The design involves a pleated membrane which is folded around a perforated permeate core, akin to a spiral, that is usually placed within a pressure vessel. This particular design is preferred when the solutions handled is heavily concentrated and in conditions of high temperatures and extreme pH . This particular configuration is generally used in more large scale industrial applications of microfiltration. This design involves bundling several hundred to several thousand hollow fiber membranes in

2356-562: The RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration. Reverse osmosis is commonly used for desalination. As well, RO is commonly used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO excludes ions but requires high pressures to produce deionized water (850–7000 kPa). RO

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2432-598: The RO system. Four types of fouling are found on RO membranes: (i) Inorganic (salt precipitation), (ii) Organic, (iii) Colloidal (particle deposition in the suspension) (iv) Microbiological (bacteria and fungi). Thereby, an appropriate combination of pre-treatment procedures and chemical dosing, as well as an efficient cleaning plan that tackle these types of fouling, should enable the development of an effective anti-fouling technique. Most plants clean their membranes every week (CEB – Chemically Enhanced Backwash). In addition to this maintenance cleaning, an intensive cleaning (CIP)

2508-547: The active layer of the polyamide membrane, intended for reuse in applications such as MF or UF. This causes an extended life of approximately two years. A very limited number of reports have mentioned the potential of direct RO reuse. Studies shows that hydraulic permeability, salt rejection, morphological and topographical characteristics, and field emission scanning electron and atomic force microscopy were used in an autopsy investigation conducted. The old RO element's performance resembled that of nanofiltration (NF) membranes, thus it

2584-421: The active membrane layer. Microbiological fouling, generally defined as the consequence of irreversible attachment and growth of bacterial cells on the membrane, is also a common reason for discarding old membranes. A variety of oxidative solutions, cleaning and anti-fouling agents is widely used in desalination plants, and their repetitive and incidental exposure can adversely affect the membranes, generally through

2660-653: The application, a generic method of evaluation is the life-cycle assessment (LCA), a tool for the analysis of the environmental burden of membrane filtration processes at all stages and accounts for all types of impacts upon the environment including emission to land, water and air. In regards to microfiltration processes, there are a number of potential environmental impacts to be considered. They include global warming potential , photo-oxidant formation potential, eutrophication potential, human toxicity potential, freshwater ecotoxicity potential, marine ecotoxicity potential and terrestrial ecotoxicity potential. In general,

2736-468: The appropriate membrane for a specific operation. The nature of the feed water must first be known; then a membrane that is less prone to fouling with that solution is chosen. For aqueous filtration , a hydrophilic membrane is preferred. For membrane distillation , a hydrophobic membrane is preferred. Operating conditions during membrane filtration are also vital, as they may affect fouling conditions during filtration. For instance, crossflow filtration

2812-532: The attachment strength of particles to the membrane surface. Reversible fouling can be removed by a strong shear force or backwashing . Formation of a strong matrix of fouling layer with the solute during a continuous filtration process will result in reversible fouling being transformed into an irreversible fouling layer. Irreversible fouling is the strong attachment of particles which cannot be removed by physical cleaning. Factors that affect membrane fouling: Recent fundamental studies indicate that membrane fouling

2888-416: The circular economy principles. Mainly they have a short service life of 5–10 years. Over the past two decades, the number of RO desalination plants has increased by 70%. The size of these RO plants has also increased significantly, with some reaching a production capacity exceeding 600,000 m3 of water per day. This means a generation of 14,000 tonnes of membrane waste that is landfilled every year. To increment

2964-446: The cold sterilisation of beverages and pharmaceuticals . Historically, heat was used to sterilize refreshments such as juice, wine and beer in particular, however a palatable loss in flavour was clearly evident upon heating. Similarly, pharmaceuticals have been shown to lose their effectiveness upon heat addition. MF membranes are employed in these industries as a method to remove bacteria and other undesired suspensions from liquids,

3040-423: The conventional rigid submerged designs. However, their overwhelming success in biological systems is not matched by their application. The main reasons for this are: Microfiltration Microfiltration is a type of physical filtration process where a contaminated fluid is passed through a special pore-sized membrane filter to separate microorganisms and suspended particles from process liquid . It

3116-526: The decrease of their rejection efficiencies. Fouling can take place through several physicochemical and biological mechanisms which are related to the increased deposition of solid material onto the membrane surface. The main mechanisms by which fouling can occur, are: Since fouling is an important consideration in the design and operation of membrane systems, as it affects pre-treatment needs, cleaning requirements, operating conditions, cost and performance, it should prevent, and if necessary, removed. Optimizing

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3192-456: The design must be such to provide a very thin sheeting (thickness < 2000 angstroms ) to facilitate an increase of flux . In addition the modules must have a low fouling profile and most importantly, be available at a low-cost for the system to be financially viable. Aside from the above applications, MF membranes have found dynamic use in major areas within the dairy industry, particularly for milk and whey processing. The MF membranes aid in

3268-449: The driving processes in membrane filtration of solutes and in reverse osmosis . In dialysis and pervaporation the chemical potential along a concentration gradient is the driving force. Also perstraction as a membrane assisted extraction process relies on the gradient in chemical potential. A submerged flexible mound breakwater as a type of using membrane can be employed for wave control in shallow water as an advanced alternative to

3344-594: The equipment may be manufactured specific to the application. Using the design heuristics and general plant design principles (mentioned above), the membrane life-span can be increased to reduce these costs. Through the design of more intelligent process control systems and efficient plant designs some general tips to reduce operating costs are listed below Table 1 (below) expresses an indicative guide of membrane filtration capital and operating costs per unit of flow. Table 1 Approximate Costing of Membrane Filtration per unit of flow Note: The materials which constitute

3420-479: The filter. The suspended liquid is passed through at a relatively high velocity of around 1–3 m/s and at low to moderate pressures (around 100-400 kPa ) parallel or tangential to the semi-permeable membrane in a sheet or tubular form. A pump is commonly fitted onto the processing equipment to allow the liquid to pass through the membrane filter. There are also two pump configurations, either pressure driven or vacuum . A differential or regular pressure gauge

3496-510: The flow goes against the concentration gradient, because those systems use pressure as a means of forcing water to go from low osmotic pressure to high osmotic pressure. Membrane fouling Membrane fouling is a process whereby a solution or a particle is deposited on a membrane surface or in membrane pores in a processes such as in a membrane bioreactor , reverse osmosis , forward osmosis , membrane distillation , ultrafiltration , microfiltration , or nanofiltration so that

3572-432: The flow rate ( flux ) of the liquid stream but does not affect the chemical composition of the species in the retentate and product streams. A major characteristic that limits the performance of microfiltration or any membrane technology is a process known as fouling . Fouling describes the deposition and accumulation of feed components such as suspended particles, impermeable dissolved solutes or even permeable solutes, on

3648-527: The forces of the binding cake to the membrane will be balanced by the forces of the fluid. At this point, cross-flow filtration will reach a steady-state condition [2] , and thus, the flux will remain constant with time. Therefore, this configuration will demand less periodic cleaning. Fouling can be defined as the potential deposition and accumulation of constituents in the feed stream on the membrane. The loss of RO performance can result from irreversible organic and/or inorganic fouling and chemical degradation of

3724-556: The individual components can be recovered for other purposes. Plastic solid waste treatment and recycling can be separated into mechanical recycling, chemical recycling and energy recovery. Mechanical recycling characteristics: Chemical recycling characteristics: Energetic recovery characteristics: Post-treatment Distinct features of membranes are responsible for the interest in using them as additional unit operation for separation processes in fluid processes. Some advantages noted include: Membranes are used with pressure as

3800-420: The lifespan of a membrane, different prevention methods are developed: combining the RO process with the pre-treatment process to improve efficiency; developing anti-fouling techniques; and developing suitable procedures for cleaning the membranes. Pre-treatment processes lower the operating costs because of lesser amounts of chemical additives in the saltwater feed and the lower operational maintenance required for

3876-416: The mean of the applied pressure from the feed to the concentrate side of the membrane subtracted by the pressure of the permeate. This is applied to dead-end filtration mainly and is indicative of whether a system is fouled sufficiently to warrant replacement. Where Permeate Flux The permeate flux in microfiltration is given by the following relation, based on Darcy's Law Where The cake resistance

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3952-428: The membrane field, the term module is used to describe a complete unit composed of the membranes, the pressure support structure, the feed inlet, the outlet permeate and retentate streams, and an overall support structure. The principal types of membrane modules are: The key elements of any membrane process relate to the influence of the following parameters on the overall permeate flux are: The total permeate flow from

4028-562: The membrane material to reduce the likelihood that foulants will adhere to the membrane surface. The exact chemical strategy used is dependent on the chemistry of the solution that is being filtered. For example, membranes used in desalination might be made hydrophobic to resist fouling via accumulation of minerals, while membranes used for biologics might be made hydrophilic to reduce protein/organic accumulation. Modification of surface chemistry via thin film deposition can thereby largely reduce fouling. One drawback to using modification techniques

4104-419: The membrane process. Microfiltration removes particles higher than 0.08-2 μm and operates within a range of 7-100 kPa. Microfiltration is used to remove residual suspended solids (SS), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis. Relatively recent developments are membrane bioreactors (MBR) which combine microfiltration and

4180-565: The membrane structure. The accumulation of foulants can lead to the formation of cracks, surface roughening, and changes in pore size distribution. These physical changes are the result of impacts of hard material with a soft polymer membrane, weakening its structural integrity. Degradation of the mechanical structure makes the membranes more susceptible to mechanical damage, potentially reducing its overall lifespan. A 2006 study observed this degradation by uniaxially straining hollow fibers that were both clean and fouled. The researchers reported

4256-400: The membrane surface and or within the pores of the membrane. Fouling of the membrane during the filtration processes decreases the flux and thus overall efficiency of the operation. This is indicated when the pressure drop increases to a certain point. It occurs even when operating parameters are constant (pressure, flow rate, temperature and concentration) Fouling is mostly irreversible although

4332-406: The membrane surface. Therefore, besides the physical cleaning, chemical cleaning may also be recommended. It includes: Optimizing the operation condition . Several mechanisms can be carried out to optimize the operating conditions of the membrane to prevent fouling, for instance: Membrane alteration . Recent efforts have focused on eliminating membrane fouling by altering the surface chemistry of

4408-504: The membrane untreated. Cross flow filtration is understood to be a unit operation rather than a process. Refer to Figure 2 for a general schematic for the process. Dead-end filtration ; all of the process fluid flows and all particles larger than the pore sizes of the membrane are stopped at its surface. All of the feed water is treated at once subject to cake formation. This process is mostly used for batch or semicontinuous filtration of low concentrated solutions. Refer to Figure 3 for

4484-604: The membrane's performance is degraded. It is a major obstacle to the widespread use of this technology . Membrane fouling can cause severe flux decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant . There are various types of foulants: colloidal (clays, flocs ), biological ( bacteria , fungi ), organic ( oils , polyelectrolytes , humics ) and scaling (mineral precipitates). Fouling can be divided into reversible and irreversible fouling based on

4560-429: The membranes used in microfiltration systems may be either organic or inorganic depending upon the contaminants that are desired to be removed, or the type of application. General Membrane structures for microfiltration include Membrane modules for dead-end flow microfiltration are mainly plate-and-frame configurations. They possess a flat and thin-film composite sheet where the plate is asymmetric. A thin selective skin

4636-426: The microfiltration process are specially designed to prevent particles such as, sediment , algae , protozoa or large bacteria from passing through a specially designed filter. More microscopic, atomic or ionic materials such as water (H 2 O), monovalent species such as Sodium (Na ) or Chloride (Cl ) ions, dissolved or natural organic matter , and small colloids and viruses will still be able to pass through

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4712-407: The operation conditions is important to prevent fouling. However, if fouling has already taken place, it should be removed by using physical or chemical cleaning. Physical cleaning techniques for membrane include membrane relaxation and membrane backwashing . Chemical cleaning . Relaxation and backwashing effectiveness will decrease with operation time as more irreversible fouling accumulates on

4788-469: The past decades to avoid this, such as waste prevention, direct reapplication, and ways of recycling. In this regard, membranes also follows the waste management hierarchy. This means that the most preferable action is to upgrade the design of the membrane which leads to a reduction in use at same application and the least preferred action is a disposal and landfilling RO membranes have some environmental challenges that must be resolved in order to comply with

4864-497: The pore size drops from MF to NF, the osmotic pressure requirement increases. Reverse osmosis (RO) is the finest separation membrane process available, pore sizes range from 0.0001 μm to 0.001 μm. Reverse osmosis is able to retain almost all molecules except for water, and due to the size of the pores, the required osmotic pressure is significantly greater than that for microfiltration. Both reverse osmosis and nanofiltration are fundamentally different from microfiltration since

4940-477: The potential environmental impact of the process is largely dependent on flux and the maximum transmembrane pressure, however other operating parameters remain a factor to be considered. A specific comment on which exact combination of operational condition will yield the lowest burden on the environment cannot be made as each application will require different optimisations. In a general sense, membrane filtration processes are relative "low risk" operations, that is,

5016-528: The potential for dangerous hazards are small. There are, however several aspects to be mindful of. All pressure-driven filtration processes including microfiltration requires a degree of pressure to be applied to the feed liquid stream as well as imposed electrical concerns. Other factors contributing to safety are dependent on parameters of the process. For example, processing dairy product will lead to bacteria formations that must be controlled to comply with safety and regulatory standards. Membrane microfiltration

5092-496: The pressure of concentrate stream [kPa]; P p the pressure if permeate stream [kPa]. The rejection (r) could be defined as the number of particles that have been removed from the feedwater. The corresponding mass balance equations are: To control the operation of a membrane process, two modes, concerning the flux and the TMP, can be used. These modes are (1) constant TMP, and (2) constant flux. The operation modes will be affected when

5168-457: The rejected materials and particles in the retentate tend to accumulate in the membrane. At a given TMP, the flux of water through the membrane will decrease and at a given flux, the TMP will increase, reducing the permeability (k). This phenomenon is known as fouling , and it is the main limitation to membrane process operation. [REDACTED] Two operation modes for membranes can be used. These modes are: Filtration leads to an increase in

5244-404: The relative embrittlement of the fouled fibers. Beyond direct physical damage, fouling can also induce indirect effects on membrane mechanical properties due to the strategies used to combat it. Backwashing subjects not only the particulates, but the membrane to strong shear forces. Greater fouling frequency therefore exposes the membrane to cyclic loading which can lead to fatigue failure . This

5320-456: The removal of bacteria and the associated spores from milk, by rejecting the harmful species from passing through. This is also a precursor for pasteurisation , allowing for an extended shelf-life of the product. However, the most promising technique for MF membranes in this field pertains to the separation of casein from whey proteins (i.e. serum milk proteins). This results in two product streams both of which are highly relied on by consumers;

5396-402: The resistance against the flow. In the case of the dead-end filtration process, the resistance increases according to the thickness of the cake formed on the membrane. As a consequence, the permeability (k) and the flux rapidly decrease, proportionally to the solids concentration [1] and, thus, requiring periodic cleaning. For cross-flow processes, the deposition of material will continue until

5472-651: The solute and the solvent. The mechanism of membrane failure may be the direct consequence of fouling by means of physical alterations to the membrane, or by indirect means, in which the foulant removal processes yield membrane damage. It is important to note that the majority of membranes used commercially are polymers such as polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyethersulfone (PES) and polyamide (PA), which are materials which offer desirable properties (elasticity and strength) to withstand constant osmotic pressures. The accumulation of foulants, however, degrades these properties through physical alterations to

5548-403: The treatment of potable water supplies. The membranes are a key step in the primary disinfection of the uptake water stream. Such a stream might contain pathogens such as the protozoa Cryptosporidium and Giardia lamblia which are responsible for numerous disease outbreaks. Both species show a gradual resistance to traditional disinfectants (i.e. chlorine ). The use of MF membranes presents

5624-444: The upstream segment of the filtration train, followed by high productivity, low energy membranes in the downstream section. There are two ways in which this design can help: either by decreasing energy use due to decreased pressure needs or by increasing output. Since this concept would reduce the number of modules and pressure vessels needed for a given application, it has the potential to significantly reduce initial investment costs. It

5700-529: The use of acids and bases to remove foulants and impurities. Additionally, researchers have investigated the impact different coatings have on resistance to wear. A 2018 study from the Global Aqua Innovation Center in Japan reported improved surface roughness properties of PA membranes by coating them with multi-walled carbon nanotubes. Another strategy to minimise membrane fouling is the use of

5776-535: Was not surprising to see the permeability increase from 1.0 to 2.1 L m-2 h-1 bar-1 and the drop in NaCl rejection from >90% to 35-50%. On the other hand, In order to maximize the overall efficiency of the process, it has lately been common practice to combine RO elements of varying performances within the same pressure vessel, which is called Multi-membrane vessel design. In principle, this innovative hybrid system recommends using high rejection, low productivity membranes in

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