Nanofiltration was defined as a procedure intermediate between rearward osmosis and Ultrafiltration that rejects molecules which have a size in the order of one nanometre ” [ 1 ] . The history of NF dates back to the 1970s when RO membranes with a sensible H2O flux runing at comparatively low force per unit areas were developed. Hence, the high force per unit areas traditionally used in RO resulted in a considerable energy cost. Therefore, membranes with lower rejections of dissolved constituents, but with higher H2O permeableness, would be a great betterment for separation engineering. Such low-pressure RO membranes became known as NF membranes [ 2 ] . By the 2nd half of the 1980s, NF had become established, and the first applications were reported [ 3 ] , [ 4 ] .
Chiefly, in membrane procedure, membrane offprints two stages. The membrane allows conveyance of one or few constituents more readily than that of other constituents. The driving force for conveyance can be a force per unit area gradient, a temperature gradient, a concentration gradient or an electrical possible gradient. A conventional representation of a membrane procedure is given in Fig.1 provender watercourse is divided into two watercourses, the retentate or concentrate watercourse and the permeate watercourse. Either retentate or permeate can incorporate or be the coveted merchandise depending on the application [ 1 ] .
Fig.1. Simplified concept conventional of a membrane separation operation
The aims of a separation can be classified approximately as follows [ 5 ] :
Concentration: the coveted constituent is present at a low concentration and dissolver has to be removed.
Purification: unwanted constituents have to be removed.
Fractionation: a mixture must be separated into two or more coveted constituents.
Chemical reaction mediation: combination of chemical or biochemical reaction with a membrane separation procedure to increase the reaction rate.
Normally, specific application nonsubjective determine the proper pick of a membrane processes. Such aim could be: particulate or dissolved solids remotion, hardness decrease or extremist pure H2O production, remotion of specific gases/chemicals etc. The end-use may besides act upon choice of membranes for industries such as drinkable H2O, outflowing intervention, desalinization or H2O supply for electronics or pharmaceutical fabrication.
NF membranes categorization
There are several facets involve in NF membranes categorization, taking into history the nature of membrane behaviour every bit good as filtration considerations. Applications, procedures, and stuffs are the chief classs of sorting NF membranes every bit good as other types of membranes. In add-on, membranes constellation and faculties can besides sort NF membranes from the fabrication and market handiness points of position. The undermentioned subdivisions illustrate briefly the three classs.
NF membranes use and application
Despite the consideration of the nano filtration engineering as new assuring one, it has been widely applied in many industries, such as H2O intervention and desalinization [ 1-42 ] , assorted types of effluent wastewater intervention [ 11, 12, 21, 23, 43-67 ] , chemical processing [ 68-72 ] , pharmaceutical and endocrinal disrupters [ 46, 49, 52, 73-75 ] , auto rinsing industry [ 47 ] , and others. However, H2O industry ( intervention and desalinization ) is of the major application of nanofiltration. Therefore, use and application tend to go a tool that can find and sort the type of membrane needed in footings of stuff, substances removal, faculty, and even maker ( for commercial membranes ) . The undermentioned subdivisions illustrate the categorization of NF membrane based on pertinence in land H2O and surface H2O industries.
Groundwater and surface H2O intervention and softening are the major Fieldss of application for the NF membranes. The natural and/or contaminated substances that lead to unwanted or deteriorated quality in footings of imbibing and usage every bit good as meeting H2O criterions require using several types of purification. NF membranes have been applied to take these substances significantly. However, taking of a substance and/or group of substances require using certain or different types of NF membranes. Many research workers have applied NF membranes to take pesticide [ 76-86 ] , arsenic [ 84, 87-96 ] , hardness [ 4, 97, 98 ] , nitrates [ 2, 80, 82, 99-104 ] , fluoride [ 87, 88, 102, 105-109 ] , organic affairs and disinfection by merchandises DBP [ 52, 110-131 ] , viruses [ 2, 132-134 ] , pollutants [ 2, 12, 135, 136 ] , mircopollutants [ 2, 81, 137-139 ] , and endocrines [ 140, 141 ] .
Kiso et al [ 76 ] examined and reported on the rejection belongingss of 11 sorts of aromatic pesticides by four polymeric commercial NF membranes in a level sheet form. The highest desalting membranes found to hold an effectual rejection rate of about all pesticides. Van der Bruggen and Everaert [ 80 ] applied four commercial NF membranes denoted: NF70, NF45, UTC-20 and UTC-60 to take four pesticides ( atrazine, simazine, diuron and isoproturon ) , hardness, and nitrates from land H2O. This led to accomplishing satisfactory rejection of pesticides. A sensible high rejection of pesticides was besides achieved by Kosutic et al [ 84 ] who applied two polymeric commercial NF membranes to take pesticides from natural land H2O.
Eric et al [ 91 ] applied polymeric NF membrane to take arsenic and succeeded to make 90 % removal per centum. Harisha et al [ 94 ] investigated arsenic remotion utilizing thin movie complex ( TFC ) NF membrane, denoted NF300. The remotion of arsenate ions from H2O by this NF membrane reached a high rejection rate of 99.80 % along with entire dissolved solids ( TDS ) and other contaminates achieved.
Removing of hardness was studied by Gorenflo et al [ 97 ] . He used polymeric commercialised NF200B that rejected Ca+A? and Mg+A? with high per centums ( & gt ; 74 % and & gt ; 86 % , severally ) compared to a pure CaCl solution ( & gt ; 45 % ) . For Nitrate remotion, an effort was made by Jolanta et al [ 101 ] to take nitrates.. He attempted to take nitrate ions from tap H2O utilizing combined RO/NF procedures. The survey achieved removing of nitrate ions with 83 % and a H2O recovery rate of about 68 % . Garcia et al [ 104 ] attempted to take NF membrane suitable to reject adequate nitrate in H2O intervention through proving four level sheet polymeric commercial NF membrane. The effort reported success in rejecting nitrates with about 88 % but non for all tested membranes.
Kang Hu and James [ 108 ] reported fluoride remotion utilizing negatively charged polymeric commercial thin-film composite ( TFC ) membranes and achieved different fluoride rejection rates. Padilla and Situa [ 87 ] used TFC coiling lesion polymeric commercial NF membrane to take fluoride ion and achieve rejection rate between 73 % at low force per unit area ( 2 saloon ) to 89 % at higher force per unit area ( 7 saloon ) .
Park and Kwon [ 127 ] performed bench and pilot-scale membrane trials to take natural organic affair NOM arising from Paldang Lake in Korea by using assorted membrane processes including RO, NF, and UF, describing an effectual remotion of NOM from coagulation/sedimentation treated H2O. Alborzfar et al [ 112 ] applied coiling lesion NF45 membrane to expeditiously take NOM from two humic land Waterss in Denmark. Lee and Cho [ 129 ] compared ceramic cannular UF to polymeric level sheet NF membranes with regard to NOM remotion. They reported similar behaviour for both types of membranes in footings of NOM remotion. Taro and Yamamoto [ 134 ] used assorted types of commercialized MF and NF membranes for virus keeping. Retention recorded was enormously high ( 99-99.9999 % ) . Organic micropollutants rejection such as: disinfection byproduct ( DBP ) , endocrinal disrupting chemicals ( EDCs ) , and pharmaceutically active compounds ( PhACs ) have been investigated by Kimura et al [ 137 ] utilizing two polymeric ( Polyamide ) thin movie complex ( TFC ) commercial NF membranes. Therefore, an effectual rejection ( & gt ; 90 % ) for negatively charged compounds was reported. While lower rejection ( & lt ; 90 % ) was reported for the non-charged compounds. Removal of hexavalent Cr, which is considered as an organic pollutant, from H2O by NF membrane was reported by Hafiana and Dhabi et al [ 135 ] . The riddance of endocrines from H2O was investigated by Dudziak and Bodzek [ 140 ] by using two commercial NF membranes that differ in polymer ( An integrally asymmetric membrane prepared from cellulose ethanoate and a composite with a surface bed made of Polyamide ) . Both membranes reported endocrines keeping per centum between 60.3 % – 100 % .
The abilities of NF membranes to selectively reject different dissolved salts, and have high rejection of low molecular weight, dissolved constituents highlighted the singularity of these membranes [ 4 ] . Such rejection capacity makes NF membranes applicable in H2O desalinization, chiefly as pretreatment of the provender. Several types of NF membranes have been applied to desalt brackish and sea Waterss associated with different types of engineerings applied, such as NF – SWRO [ 28 ] and NF – MSF [ 9, 28 ] . Polymeric NF membrane, commercially denoted as NF90, was tested by AL -Zoubi et Al [ 41 ] for pretreatment and partial demineralisation procedures, demoing an ability of rejecting monovalent and bivalent ions of sea H2O every bit good as decrease of salt. Al – Ahmad et al [ 13 ] developed a extremely hydrophilic NF membrane by modifying commercial polymeric polysulfone NF membrane. The modified membrane has high salt rejection rate of about 95 % . Hilal et al [ 6 ] tested to three commercial NF membranes, two made of Polyamide ( PA ) and one made of Polyethersulfone for high rejection of salts up to sea H2O salt as pretreatment option. Salinity degree ranged from 5000 to 25000 ppm. Achieved rejection rates by using each NF membrane varies depending on provender concentration every bit good as force per unit area.
Similar to sea H2O desalinization, some polymeric NF membranes have late applied to desalt land H2O. Haddada et al [ 15 ] investigated the usage of polymeric cellulose ethanoate NF membrane for desalinization of two Tunisian brackish Waterss, with a salt content between 3500 mg/L to 4000mg/L. Inorganic cannular NF membrane for brackish land H2O desalinization have been investigated by Walha et al [ 42 ] and win to cut down the concentrations of Ca2+ , Mg2+ and SO4 2- every bit good as bring forthing H2O with TDS reduced to 1890 mg.L-1.
Membrane processes classify types of membranes basically. Membrane procedures can be divided harmonizing to their drive forces. As drive forces, gradients in force per unit area, concentration, temperature and electrical potency are used ( see Table 1 ) . The electrically impulsive procedures are electrodialysis and membrane electrolysis. The driving force for ( ionic ) conveyance in these procedures is supplied by an electrical potency difference. Electrically goaded procedures can be employed merely when charged molecules are present, utilizing ionic or charged membranes [ 142 ] .
Concentration driven membrane procedures are dialysis and osmosis. In dialysis procedure, the transportation of the solute across the membrane occurs by diffusion and separation is obtained through differences in diffusion rates because of differences in molecular weight [ 143 ] .
Table.1: Categorization of membrane procedures via inactive conveyance [ 142 ] .
Concentration ( I”C )
Electrical potency ( I”E )
Pressure ( I”P )
Micro, Ultra, NF, RO
Temperature ( I”T )
Osmosis is the conveyance of H2O across a selectively permeable membrane from a compartment of higher H2O chemical potency to a compartment of lower H2O chemical potency until the osmotic force per unit areas of both compartments are equal. It is driven by a difference in solute concentrations across the membrane that allows transition of H2O, but rejects most solute molecules or ions. Osmotic force per unit area ( Iˆ ) is the force per unit area which, if applied to the more concentrated solution, would forestall conveyance of H2O across the membrane [ 50 ] .
In heat goaded procedure, membrane distillment is a separation procedure for aqueous solutions, based on the usage of hydrophobic microporous membranes. The membranes are non wetted by the aqueous stage, until the operating force per unit area remains lower than the minimal incursion force per unit area of the membrane, so that the entryway of the pores acts as the physical support for a liquid vapor interface which can arise the separation of constituents of different volatility. The driving force for mass transportation across the membrane is a difference in the partial force per unit area between the two terminals of the membrane pores. That can be maintained by moving on the temperature difference across the membrane, as in direct contact MD, by utilizing a sweeping gas on the permeate side, by presenting an air spread or by using vacuity in the permeate side ( Cabassud et al. 2003 ) .
Pressure driven membrane procedures use the force per unit area difference between the provender and permeate side as the driving force to transport the dissolver through the membrane. Atoms and dissolved constituents are ( partly ) retained based on belongingss such as size, form and charge. Four membrane procedures can be distinguished when the drive force is a force per unit area difference across the membrane, dividing two liquid solutions. These procedures are Microfiltration ( MF ) , Ultrafiltration ( UF ) , nanofiltration ( NF ) and rearward osmosis. Traveling from MF through UF and NF to RO, the hydrodynamic opposition additions and accordingly higher drive forces are needed. On the other manus the merchandise flux through the membrane and the size of the molecules being retained lessenings. The merchandise flux obtained is determined by the applied force per unit area and the membrane opposition [ 89 ] . Typical values for applied force per unit areas and fluxes are given in Table 2.
Table 2: Molecular weight cut off, flux scope and used force per unit area scope in MF, UF, NF and RO membrane procedures. [ 70 ] :
Pressure scope ( saloon )
permeableness scope ( L.hE‰A?.mE‰A?.barE‰A? )
MWCO ( Da )
0.1 – 2
& gt ; 1000
& gt ; 1000000
1 – 5
10 – 1000
1000 – 300000
3 – 20
1.5 – 30
200 – 1000
5 – 120
0.05 – 1.5
& lt ; 200
Membrane morphology, construction, & A ; separation mechanism
Another facet to sort NF membranes is the membrane morphology. E. Curcio and E. Drioli [ 35 ] have considered that membranes are classified harmonizing to their morphology ( Fig. 1.1 ) : dense homogenous polymer membranes, porous membranes and ( thin movie ) complex membranes, dwelling of a dense top bed on a porous construction of a different stuff.
However, Cot et al 1998 stated that categorization occurs harmonizing to the homogeneousness of the pore construction along the membrane cross subdivision into symmetric, asymmetric, and composite membranes.
Porous membranes can be besides symmetric or asymmetric. Symmetric membranes may hold straight or sponge-like pores. Asymmetric membranes display a thin dense tegument bed, with or without pores, on top of a deep porous bomber bed: the thickness of the selective tegument offers the advantage of low opposition to transport through the membrane [ 35 ] .
Figure.2 Membrane categorization based on morphology ( E. Curcio and E. Drioli, 2009 )
Membrane separation mechanism is another attack that structurally classifies membranes. Ulbricht et al 2006 classified membranes as porous ( NF as micro porous, UF as meso porous, and MF as macro porous ) , dense, and ion-exchange ( negatively charged, positively charged, and bipolar ) .
From the filtration point of position, NF membranes are considered as cross-linked filtration membranes that are chiefly porous. ( Colophon ) . In such instance, asymmetric membranes, besides called skinned membranes can be classified as integrally or non-integrally skinned. On the other manus, microporous or symmetric membranes are classified as isotropic ( with pores of unvarying size throughout the organic structure of the membrane ) or anisotropic ( where the pores change in size from one surface of the membrane to the other ) ( munir, MF & A ; UF book )
Based on type of membrane stuff, membranes can be classified into organic, inorganic and loanblends of organic/inorganic stuffs. Organic membranes are made of different types of polymers and considered as biggest proportion of membrane installed and presently in usage. Organic membranes are normally made of natural or man-made polymer. The common stuffs include ; cellulose ethanoate, polysulfone, aromatic polymeric amides, polyacrylonitrile ( Suen et al 2003 and Ulbricht 2006 ) .
Inorganic NF membranes are made of different types of inorganic stuff such as ceramics, metals and glass. Two chief categories of membranes can be distinguished: dense ( they are made of metals, intercrossed organic-inorganic or assorted conductive oxides ) and porous ( ceramic ) membranes. Sol-gel processing, plasma-enhanced chemical vapour deposition and hydrothermal synthesis are methods that can be used for inorganic membrane readying. Inorganic membranes compete with organic membranes for specific applications in drastic conditions. They can run at elevated temperatures, with metal membranes stable at temperatures runing from 500 – 800A°C and with many ceramic membranes useable at over 1000A°C. They are much more immune to chemical onslaught and have long life rhythm ( Caroa et al 2006 ) . But on the other manus, their pore belongingss, cost, capableness for surface alteration may non be competitory. Consequently, inorganic stuffs are infrequently adopted as the affinity membrane supports ( Suen et al 2003 ) .
Organic-inorganic intercrossed stuffs offer specific advantages for the readying of unreal membranes exhibiting high selectivity and flux, every bit good as a good thermal and chemical opposition ( Sforca et al 1999 ) . Hybrid organic/inorganic stuffs are normally classified in two classs ( Cot et al 2000 ) :
Type I in which lone interactions like new wave der Waals forces or H bonds exist between organic and inorganic parts. Hybrid stuffs can be described here as micro or nano complexs in which one portion ( organic or inorganic ) is dispersed in the other portion moving as the host matrix.
Type II in which covalent adhering exists between organic and inorganic parts, ensuing either in homogenous intercrossed stuff at the molecular degree or in high surface country inorganic stuffs modified through surface grafting of organic groups.
Membrane constellation and faculties
Membrane constellation and faculties classify NF membranes in a kind of a contrary order, as the faculty is the form on which the membrane available for utilizations depending on type of operated system that membrane will be installed in or applied for.
Normally, when employed for practical applications, membranes are normally housed in a faculty. The design of membrane faculty depends on the membrane form. Various membrane forms and faculty designs have been adopted in different membrane procedures. The techno-economic factors for the choice, design and operation of membrane faculties include cost of back uping stuffs and enclosure ( force per unit area vass ) , power ingestion in pumping and easiness of substitutability ( Aptel et al.1996 ) .
Membrane faculties manufactured in different forms to fulfill assortment of applications and systems using membranes. However, typical membrane faculties are: level sheet, coiling, and hollow fibre for organic membranes and tubular for inorganic membranes. The rule advantages and disadvantages of different faculties are given in table 3.
Table 7: Chief advantages and disadvantages of different faculties ( morocco thesis )
Shape of faculty
Packing denseness ( m2/m3 )
( 10 – 300 )
( 9000 – 30000 )
( 10 – 400 )
( 300 – 1000 )
Hydraulic diameter ( millimeter )
5 – 15
0.1 – 1
1 – 5
0.8 – 1.2
Replacement of membranes
Hazard of choke offing
Flat sheet faculty
The simplest device for packing level sheet membranes is a plate-and-frame faculty. Plate-and frame faculties can be constructed in different sizes and forms runing from lab-scale devices that hold individual, small-size membrane vouchers to all-out systems that hold more than 1700 membranes. Two of the chief restrictions of plate-and-frame elements for membrane applications are deficiency of equal membrane support and low wadding denseness. Lack of equal membrane support bounds operation to moo hydraulic force per unit area and/or operation at similar force per unit areas on both sides of the membrane ( necessitating comparatively high procedure control ) . Low packing denseness leads to a larger system footmark, higher capital costs, and higher operating costs ( labour for membrane replacing ) . Other restrictions of the plate-and-frame constellation include jobs with internal and external waterproofing, trouble in supervising membrane unity, and a limited scope of operating conditions ( e.g. , flow speeds and force per unit areas ) ( Cath et al 2005 ) .
Flat sheet faculties are widely applied for research and synthesis membranes tested in laboratory graduated table experiments.
Spiral lesion faculty
A coiling lesion faculty contains from one to more than 30 membranes leafs, depending on the component diameter and component type. Each foliage is made of two membrane sheets glued together back-to-back with a permeate spacer in between them ( Aptel et al 1996 ) . Glue lines seal the inner ( permeate ) side of the foliage against the outer ( feed/concentrate ) side. The unfastened side of the foliage is connected to and sealed against the pierced cardinal portion of merchandise H2O tubing, which collect permeate from all foliages. The foliages are rolled up with a sheet of provender spacer between each of them, which provides the channel for the provender and dressed ore flow. In operation, the provender H2O enters the face of the component through the provender spacer channels and issues on the opposite terminal as dressed ore. Spiral lesion designs offers many advantages compared to other faculty designs. Typically, a coiling lesion constellation offers significantly lower replacing costs, easier care, high wadding denseness and higher force per unit area application. As to the possible jobs for the spiral-wound design, hard faculty cleansing, and the flow complexness caused by the diminution in transmembrane force per unit area bead along the radial way ( Cath et al 2006 ) .
Spiral lesion constellation is the industry criterion for rearward osmosis and nanofiltration membranes in H2O intervention and desalinization.
Tubular membranes are non self-supporting membranes. They are located on the interior of a tubing which is the back uping bed for the membrane. Because the location of cannular membranes is inside a tubing, the flow in a cannular membrane is normally indoors out. The chief job for this is that the fond regard of the membrane to the back uping bed is really weak. Tubular membranes have a diameter of approximately 5 to 15 millimeter. The tubings are encased in strengthened fibre glass or enclosed inside a stiff PVC or unstained steel shell. As the provender solution flows through the membrane nucleus, the permeate passes through the membrane and is collected in the cannular lodging. Because of the size of the membrane diameter, plugging of cannular membranes is non likely to happen. This type of faculty can be easy cleaned. A drawback of cannular membranes is that the wadding denseness is low, which consequences in high monetary values per faculty ( Bouchard et al 2000 ) .
Hollow fibre faculty
Hollow fibre membranes are little cannular membranes with a diameter of below 2mm. Hollow fibre membranes are self back uping membranes. The selective barrier is sufficiently strong to defy filtration force per unit areas. Because of this, the flow through these membranes can be either inside out or outside in ( Maurel, 1993 ) . The opportunities of plugging of a hollow fibre membrane are really high. The membranes can merely be used for the intervention of H2O with low suspended solids content. The packing denseness of a hollow fibre membrane is really high. The cartridges contain several hundred of fibres. The cardinal belongingss of efficient membrane faculties are ( Starthmann, 1999 ) :
High wadding denseness.
Good control of concentration polarisation and membrane fouling.
Low operating and care costs ; and
Cost efficient production.
Figure 2 shows the four membrane faculties forms.
Figure.3 Membrane faculties ( A. P. Sincero and G. A. Sincero, 2003 )
NF membranes choice
The assortment of NF membranes available and the large-scale of application influence NF membrane choice from the user point of position. In add-on, several parametric quantities are involved in NF membrane choice and chosen of proper NF membrane for certain application. These parametric quantities are cost, system designed, and R & A ; D issues. Aptel et al 1996 stated that techno – economic parametric quantities that are chiefly design and operation facets including cost, enclosure ( force per unit area vass ) , power ingestion in pumping, and easiness of substitutability influence NF membrane choice. However, the coveted rejection and substances removal play a major function in taking a membrane.