Thin, film-like social organization separating two fluids, acting as a selective barrier
This article is about selective barrier membranes. For other uses, see Membrane ( disambiguation ) conventional of size-based membrane exclusion A membrane is a selective barrier ; it allows some things to pass through but stops others. such things may be molecules, ions, or other belittled particles. Membranes can be generally classified into synthetic membranes and biological membranes. [ 1 ] Biological membranes include cell membranes ( knocked out coverings of cells or organelles that allow passing of certain constituents ) ; [ 2 ] nuclear membranes, which cover a cell nucleus ; and tissue membranes, such as mucous membrane and serosae. synthetic membranes are made by humans for manipulation in laboratories and diligence ( such as chemical plants ).

This concept of a membrane has been known since the eighteenth hundred but was used little outside of the lab 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 urine condom. however, due to the miss of dependability, slow operation, reduced selectivity and lift costs, membranes were not wide exploited. The first base use of membranes on a large scale was with microfiltration and ultrafiltration technologies. Since the 1980s, these interval processes, along with electrodialysis, are employed in big plants and, today, several have companies serve the market. [ 3 ] The degree of selectivity of a membrane depends on the membrane stoma size. Depending on the pore size, they can be classified as microfiltration ( MF ), ultrafiltration ( UF ), nanofiltration ( NF ) and reverse osmosis ( RO ) membranes. Membranes can besides be of diverse thickness, with homogeneous or heterogenous social organization. Membranes can be impersonal or charged, and atom ecstasy can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electric gradients of the membrane action .

Membrane processes classifications [edit ]

Microfiltration ( MF ) [edit ]

Microfiltration removes particles higher than 0.08-2 µm and operates within a stove of 7-100 kPa. [ 4 ] Microfiltration is used to remove residual freeze solids ( SS ), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment tone for reverse osmosis. relatively late developments are membrane bioreactors ( MBR ) which combine microfiltration and a bioreactor for biological discussion .

Ultrafiltration ( UF ) [edit ]

Ultrafiltration removes particles higher than 0.005-2 µm and operates within a compass of 70-700kPa. [ 4 ] Ultrafiltration is used for many of the lapp applications as microfiltration. Some ultrafiltration membranes have besides been used to remove dissolve compounds with high molecular weight, such as proteins and carbohydrates. besides, they can remove viruses and some endotoxins .
The wall of an ultrafiltration hollow fiber membrane, with characteristic out ( top ) and inner ( buttocks ) layers of pores.

Nanofiltration ( NF ) [edit ]

Nanofiltration is besides known as “ loosen ” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolve constituents from effluent. NF is chiefly developed as a membrane softening procedure which offers an option to chemical mince. besides, nanofiltration can be used as a pre-treatment earlier address reverse osmosis. The independent objectives of NF pre-treatment are : [ 5 ] ( 1 ). minimize particulate and microbial foul of 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 procedure by reducing the feed-water total dissolved solids ( TDS ) concentration .

reverse osmosis ( RO ) [edit ]

reverse osmosis is normally used for desalination. As well, RO is normally used for the removal of dissolve constituents from effluent remaining after advance discussion with microfiltration. RO exclude ions but requires gamey pressures to produce deionize water system ( 850–7000 kPa ). RO is the most wide used desalination technology because of its simplicity of use and relatively low energy costs compared with distillate, which uses engineering 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 complex ( TFC ), chiefly consisting of three layers : a polyamide level, a polysulphone layer and a polyester layer. [ 6 ]

Nanostructured membranes [edit ]

An emerging class of membranes trust on nanostructure channels to branch materials at the molecular scale. These include carbon 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 besides adsorption selective separations such as olefins from paraffins and alcohols from water that traditionally have required expensive and energy intensive distillation .

Membrane configurations [edit ]

In the membrane playing field, the term module is used to describe a complete whole composed of the membranes, the pressure support structure, the feed inlet, the mercantile establishment percolate and retentate streams, and an overall subscribe structure. The chief types of membrane modules are :

  • Tubular, where membranes are placed inside a support porous tubes, and these tubes are placed together in a cylindrical shell to form the unit module. Tubular devices are primarily used in micro- and ultrafiltration applications because of their ability to handle process streams with high solids and high viscosity properties, as well as for their relative ease of cleaning.
  • Hollow fiber membrane, consists of a bundle of hundreds to thousands of hollow fibers. The entire assembly is inserted into a pressure vessel. The feed can be applied to the inside of the fiber (inside-out flow) or the outside of the fiber (outside-in flow).
  • Spiral wound, where a flexible permeate spacer is placed between two flat membranes sheet. A flexible feed spacer is added and the flat sheets are rolled into a circular configuration.
  • Plate and frame consist of a series of flat membrane sheets and support plates. The water to be treated passes between the membranes of two adjacent membrane assemblies. The plate supports the membranes and provides a channel for the permeate to flow out of the unit module.
  • Ceramic and polymeric flat sheet membranes and modules. Flat sheet membranes are typically built-into submerged vacuum-driven filtration systems which consist of stacks of modules each with several sheets. Filtration mode is outside-in where the water passes through the membrane and is collected in permeate channels. Cleaning can be performed by aeration, backwash and CIP.
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Membrane process operation [edit ]

The key elements of any membrane action associate to the influence of the come parameters on the overall interpenetrate liquefy are :

  • The membrane permeability (k)
  • The operational driving force per unit membrane area (Trans Membrane Pressure, TMP)
  • The fouling and subsequent cleaning of the membrane surface.

Flux, atmospheric pressure, permeability [edit ]

The full percolate run from a membrane system is given by following equation :

Q p = F w ⋅ A { \displaystyle Q_ { phosphorus } =F_ { west } \cdot A } Q_p=F_w \cdot A

Where Qp is the percolate flow flowrate [ kg·s−1 ], Fw is the body of water flux rate [ kg·m−2·s−1 ] and A is the membrane area [ m2 ] The permeability ( kelvin ) [ m·s−2·bar−1 ] of a membrane is given by the next equation :

kelvin = F w P T M P { \displaystyle k= { F_ { west } \over P_ { TMP } } }k={F_w \over P_{TMP}}

The trans-membrane press ( TMP ) is given by the be expression :

P T M P = ( P f + P hundred ) 2 − P phosphorus { \displaystyle P_ { TMP } = { ( P_ { f } +P_ { c } ) \over 2 } -P_ { p } }P_{TMP}={(P_f+ P_c)\over 2}-P_p

where PTMP is the trans-membrane blackmail [ kPa ], Pf the intake pressure of tip stream [ kPa ] ; Pc the pressure of condense flow [ kPa ] ; Pp the blackmail if percolate stream [ kPa ]. The rejection ( roentgen ) could be defined as the total of particles that have been removed from the feedwater .

radius = ( C f − C p ) C f ⋅ 100 { \displaystyle r= { ( C_ { farad } -C_ { p } ) \over C_ { fluorine } } \cdot 100 }r={(C_f-C_p)\over C_f} \cdot 100

The equate multitude balance equations are :

Q degree fahrenheit = Q p + Q c { \displaystyle Q_ { farad } =Q_ { phosphorus } +Q_ { c } }


Q f ⋅ C f = Q p ⋅ C p + Q c ⋅ C c { \displaystyle Q_ { fluorine } \cdot C_ { f } =Q_ { phosphorus } \cdot C_ { phosphorus } +Q_ { c } \cdot C_ { c } }Q_f \cdot C_f=Q_p \cdot C_p + Q_c \cdot C_c

To control the process of a membrane process, two modes, concerning the flux and the TMP, can be used. These modes are ( 1 ) ceaseless TMP, and ( 2 ) constant flux. The operation modes will be affected when 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 ( thousand ). This phenomenon is known as fouling, and it is the independent restriction to membrane process operation. Constant TMP and constant Flux operations

dead-end and cross-flow operation modes [edit ]

Two operation modes for membranes can be used. These modes are :

  • Dead-end filtration where all the feed applied to the membrane passes through it, obtaining a permeate. Since there is no concentrate stream, all the particles are retained in the membrane. Raw feed-water is sometimes used to flush the accumulated material from the membrane surface.[7]
  • Cross-flow filtration where the feed water is pumped with a cross-flow tangential to the membrane and concentrate and permeate streams are obtained. This model implies that for a flow of feed-water across the membrane, only a fraction is converted to permeate product. This parameter is termed “conversion” or “recovery” (S). The recovery will be reduced if the permeate is further used for maintaining processes operation, usually for membrane cleaning.
S = Q phosphorus e roentgen m e a deoxythymidine monophosphate e Q degree fahrenheit e e d = 1 − Q c o newton c einsteinium nitrogen triiodothyronine r a thyroxine einsteinium Q f e vitamin e d { \displaystyle S= { Q_ { interpenetrate } \over Q_ { feed } } =1- { Q_ { concentrate } \over Q_ { feed } } }S={Q_{permeate}\over Q_{feed}} = 1-{Q_{concentrate}\over Q_{feed}}

Filtration leads to an increase in the resistance against the flow. In the casing of the dead-end filtration march, the electric resistance increases according to the thickness of the cake formed on the membrane. As a consequence, the permeability ( k ) and the flux quickly decrease, proportionately to the solids concentration [ 1 ] and, thus, requiring periodic clean. For cross-flow processes, the deposition of material will continue until 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 stipulate [ 2 ], and thus, the blend will remain ceaseless with prison term. therefore, this shape will demand less periodic clean .

Fouling [edit ]

Fouling can be defined as the potential deposition and collection of constituents in the feed current on the membrane. The loss of RO operation can result from irreversible organic and/or inorganic clog and chemical abasement of the active membrane level. Microbiological foul, generally defined as the consequence of irreversible attachment and growth of bacterial cells on the membrane, is besides a coarse reason for discarding old membranes. A assortment of oxidative solutions, cleaning and anti-fouling agents is widely used in desalination plants, and their repetitive and attendant exposure can adversely affect the membranes, by and large through the decrease of their rejection efficiencies. [ 8 ] Fouling can take place through several physicochemical and biological mechanism which are related to the increased deposition of solid material onto the membrane surface. The main mechanisms by which fouling can occur, are :

  • Build-up of constituents of the feedwater on the membrane which causes a resistance to flow. This build-up can be divided into different types:
Pore narrowing, which consists of solid material that it has been attached to the interior surface of the pores.
Pore blocking occurs when the particles of the feed-water become stuck in the pores of the membrane.
Gel/cake layer formation takes places when the solid matter in the feed is larger than the pore sizes of the membrane.
  • Formation of chemical precipitates known as scaling
  • Colonization of the membrane or biofouling takes place when microorganisms grow on the membrane surface.[9]

Fouling control and extenuation [edit ]

Since foul is an authoritative consideration in the design and operation of membrane systems, as it affects pre-treatment needs, clean requirements, operating conditions, cost and performance, it should prevent, and if necessary, removed. Optimizing the operation conditions is important to prevent foul. however, if foul has already taken identify, it should be removed by using physical or chemical scavenge. Physical cleaning techniques for membrane include membrane relaxation and membrane backwashing .

  • Back-washing or back-flushing consists of pumping the permeate in the reverse direction through the membrane. Back-washing removes successfully most of the reversible fouling caused by pore blocking. Backwashing can also be enhanced by flushing air through the membrane.[10] Backwashing increase the operating costs since energy is required to achieve a pressure suitable for permeate flow reversion.
  • Membrane relaxation consists of pausing the filtration during a period, and thus, there is no need for permeate flow reversion. Relaxation allows filtration to be maintained for a longer period before the chemical cleaning of the membrane.
  • Back pulsing high frequency back pulsing resulting in efficient removal of dirt layer. This method is most commonly used for ceramic membranes [3]
Recent studies have assessed to combine relaxation and backwashing for optimum results,.[11][12]

Chemical cleaning. relaxation and backwashing potency will decrease with operation time as more irreversible foul accumulates on the membrane airfoil. consequently, besides the physical clean, chemical clean may besides be recommended. It includes :

  • Chemical enhanced backwash, that is, a low concentration of chemical cleaning agent is added during the backwashing period.
  • Chemical cleaning, where the main cleaning agents are sodium hypochlorite (for organic fouling) and citric acid (for inorganic fouling). Every membrane supplier proposes their chemical cleaning recipes, which differ mainly in terms of concentration and methods.[13]

Optimizing the operation condition. several mechanisms can be carried out to optimize the operate conditions of the membrane to prevent pollute, for case :

  • Reducing flux. The flux always reduces fouling but it impacts on capital cost since it demands more membrane area. It consists of working at sustainable flux which can be defined as the flux for which the TMP increases gradually at an acceptable rate, such that chemical cleaning is not necessary.
  • Using cross-flow filtration instead of dead-end. In cross-flow filtration, only a thin layer is deposited on the membrane since not all the particles are retained on the membrane, but the concentrate removes them.
  • Pre-treatment of the feed water is used to reduce the suspended solids and bacterial content of the feed-water. Flocculants and coagulants are also used, like ferric chloride and aluminium sulphate that, once dissolved in the water, adsorbs materials such as suspended solids, colloids and soluble organic.[14] Metaphysical numerical models have been introduced in order to optimize transport phenomena [15]

Membrane alteration. late efforts have focused on eliminating membrane foul by altering the surface chemistry of the membrane material to reduce the likelihood that foulants will adhere to the membrane surface. The exact chemical strategy used is dependant 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 movie deposition can thereby largely reduce fouling. One drawback to using alteration techniques is that, in some cases, the flux rate and selectivity of the membrane process can be negatively impacted. [ 16 ]

Recycling of RO membranes [edit ]

waste prevention [edit ]

once the membrane reaches a meaning operation decay it is discarded. Discarded RO membrane modules are presently classified worldwide as inert solid consume and are often disposed of in landfills ; although they can besides be energetically recovered. however, assorted efforts have been made over the past decades to avoid this, such as waste prevention, direct reapplication, and ways of recycling. RO membranes have some environmental challenges that must be resolved in order to comply with the circular economy principles. chiefly 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 besides 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 the life of a membrane, different prevention methods are developed : combining the RO march with the pre-treatment action to improve efficiency ; developing anti-fouling techniques ; and developing suitable procedures for cleaning the membranes. Pre-treatment processes lower the operate costs because of lesser amounts of chemical additives in the seawater feed and the lower functional sustenance required for the RO system. [ 17 ] Four types of pollute are found on RO membranes : ( i ) Inorganic ( salt precipitation ), ( two ) Organic, ( three ) Colloidal ( particle deposition in the suspension ) ( four ) Microbiological ( bacteria and fungi ). Thereby, an appropriate combination of pre-treatment procedures and chemical drug, angstrom well as an efficient cleaning plan that tackle these types of clog, should enable the development of an effective anti-fouling technique. Most plants clean their membranes every week ( CEB – Chemically Enhanced Backwash ). In accession to this sustenance clean, an intensive clean ( CIP ) is recommended, from two to four times annually .

recycle [edit ]

recycle of RO membranes include the lead reapplication of modules in early separation processes with less rigorous specifications. The conversion from the RO TFC membrane to a holey membrane is possible by degrading the dense layer of polyamide. Converting RO membranes by chemical treatment with unlike oxidizing solutions are aimed at removing the active layer of the polyamide membrane, intended for recycle in applications such as MF or UF. This causes an extend life of approximately two years. [ 18 ]

recycle [edit ]

Recycling of materials is a general terminus that involves physically transforming the material or its components so that they can be regenerated into other utilitarian products. The membrane modules are building complex structures, consisting of a count of different polymeric components and, potentially, the individual components can be recovered for other purposes. Plastic solid waste treatment and recycle can be separated into mechanical recycle, chemical recycle and energy recovery. Mechanical recycling characteristics:

  • A first separation of the components of interest is needed.
  • Previous washing to avoid property deterioration during the process.
  • Grinding of the polymeric materials into suitable size (loss of 5% of the material).
  • Possible posterior washing.
  • Melting and extrusion process (loss of 10 % of material).
  • Membrane components than can be recycled (thermoplastics): PP, polyester, etc.
  • Membrane sheets: constructed from a number of different polymers and additives and therefore inherently difficult to accurately and efficiently separate.
  • Main advantage: it displaces virgin plastic production. • Main disadvantages: need to separate all components, large-enough amount of components to be viable.[19]

Chemical recycling characteristics:

  • Break down the polymers into smaller molecules, using depolymerisation and degradation techniques.
  • Cannot be used with contaminated materials.
  • Chemical recycling processes are tailored for specific materials.
  • Advantage: that heterogeneous polymers with limited use of pre-treatment can be processed.
  • Disadvantage: more expensive and complex than mechanical recycling.
  • Polyester materials (such as in the permeate spacer and components of the membrane sheet) are suitable for chemical recycling processes, and hydrolysis is used to reverse the poly-condensation reaction used to make the polymer, with the addition of water to cause decomposition.

Energetic recovery characteristics:

  • Volume reduction by 90–99%, reducing the strain on landfill.
  • Waste incinerators can generally operate from 760 °C to 1100 °C and would therefore be capable of removing all combustible material, with the exception of the residual inorganic filler in the fiberglass casing.[20]
  • Heat energy can be recovered and used for electricity generation or other heat related processes, and can also offset the greenhouse gas emissions from traditional energy.
  • If not properly controlled, can emit greenhouse gases as well as other harmful products.

Applications [edit ]

Distinct features of membranes are responsible for the interest in using them as extra unit operation for separation processes in fluid processes. Some advantages noted include : [ 3 ]

  • Less energy-intensive, since they do not require major phase changes
  • Do not demand adsorbents or solvents, which may be expensive or difficult to handle
  • Equipment simplicity and modularity, which facilitates the incorporation of more efficient membranes

Membranes are used with pressure as the drive processes in membrane filtration of solutes and in inverse osmosis. In dialysis and pervaporation the chemical electric potential along a concentration gradient is the driving force. besides perstraction as a membrane assisted origin process relies on the gradient in chemical electric potential.

however, their consuming success in biological systems is not matched by their application. [ 21 ] The main reasons for this are :

  • Fouling – the decrease of function with use
  • Prohibitive cost per membrane area
  • Lack of solvent resistant materials
  • Scale-up risks

See besides [edit ]

References [edit ]

bibliography [edit ]

  • Metcalf and Eddy. Wastewater Engineering, Treatment and Reuse. McGraw-Hill Book Company, New York. Fourth Edition, 2004.
  • Paula van den Brink, Frank Vergeldt, Henk Van As, Arie Zwijnenburg, Hardy Temmink, Mark C.M.van Loosdrecht. “Potential of mechanical cleaning of membranes from a membrane bioreactor”. Journal of membrane science. 429, 2013. 259-267.
  • Simon Judd. The Membrane Bioreactor Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment. Elsevier, 2010.
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