polymeric nanofiltration membranes for textile dye wastewater.pdf
TRANSCRIPT
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1. Introduction
Textile industries traditionally use a huge
amount of water, which is normally discharged
after the wastewater treatment system to decrease
the pollution load in order to meet the legislativerequirement for the discharge. With increasing
regulatory pressures and demand for cost reduc-
tion of water and chemicals, such treatment
systems have been enhanced to address these
challenges. Textile manufacturers have therefore
converted the traditional “money-wasting” pro-
cess of pollution control to a profitable operation
through recycling the waste effluent [1]. This
operation allows for the recovery of the valuable
chemical components and water from a number
of different textile process streams. Due to ineffi-
ciency of conventional treatment systems, nano-
filtration (NF) frequently becomes the chosen
treatment process.
NF has been recognized having the properties
in between ultrafiltration (UF) and reverse
osmosis (RO) and thus offers significant advan-
tages, e.g. lower osmotic pressure difference,
higher permeate flux, higher retention of multi-
valent salts and molecular weight compounds
(>300), relatively low investment and low opera-tion and maintenance costs [2]. Many researchers
have evaluated the performance of NF mem-
branes in terms dye retention, salt rejection, per-
meate flux and COD retention. The effects of
different operating conditions of wastewater and
membrane properties have been systematically
studied. The results have proven that NF mem-
branes are the suitable separation process to be
employed for the treatment of textile wastewater
and generally showed an acceptable rejection [3].However, to maintain the efficiency of NF mem-
branes at a reasonable operating cost, it is neces-
sary to use a suitable pre-treatment in order to
prevent fouling and severe module damage [4].
In view of integrating the various aspects of
NF membranes for the treatment of textile
wastewater, the aim of this paper is to review and
critically discuss performance evaluation of
various technologies for the treatment of textile
effluents, at either the laboratory-scale or pilot-
plant-scale level, specifically discharged from the
dyeing process, in comparison with NF mem-
branes.
The preparation of one of the well-known
thin-film composite NF membranes (TFC–NF) as
well as the performance evaluation of commer-
cially available NF membranes in terms of dye
rejection, salt rejection, flux and COD retention
under various operating conditions is the main
focus. To gain further understanding on the trans-
port properties of dyes and salts in NF, a
discussion of the currently available transport
models is also carried out. Furthermore, a funda-mental knowledge of fouling mechanisms and
suggested methods for fouling control are also
discussed in order to provide the most efficient
solutions to minimize fouling. In addition, the
future direction of NF in textile industries is also
provided in view of developing a more compe-
titive NF membrane, particularly for textile
wastewater treatment.
2. Nature of textile effluent
In textile refining processes, substantial
amounts of water, mineral salts and reactive dyes
are used on average for every kilogram of cotton
processed. As a consequence, they generate a
large amount of wastewater which contains com-
plex contaminants from its daily operation. The
textile effluents typically contain many types of
dyes, detergents, solvents and salts depending on
the particular textile process such as scouring, bleaching, dyeing, printing, finishing, etc. [5].
Table 1 shows the typical characteristics of
wastewater from the effluents of the dyeing and
finishing processes that contain a variety of com-
ponents of varying concentrations [6]. For each of
the parameters involved, a range of values is
given, confirming the large variability of the
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Table 1
Typical characteristics of wastewater from a textile
dyeing process [6]
Aspect/component Value
pHTemperature, ECCOD, mg/LBOD, mg/LTSS, mg/LOrganic nitrogen, mg/LTotal phosphorus, mg/LTotal chromium, mg/LColor, mg/L
2–1030–8050–5000200–30050–50018–390.3–150.2–0.5>300
wastewater from the dyeing process [7]. Besides
these components, surfactants are also used to
reduce surface tension of water during proces-
sing; however, it only contributes to a small
amount of the wastewater. Non-ionic surfactants
(alkyl phenol ethoxylates) in wastewater should
be properly treated as it can be biodegraded to
alkyl phenols which are much more toxic than the
ethoxylated [8].
There are many classes of dyes used during
the dyeing process. The method of dye applica-
tion and estimated degree of fixation for differentdye-fibre combinations are described in Table 2.
Further details regarding this information are
available elsewhere [3,9,10]. Nowadays, reactive
dyes are the most widely used dyes due to the
rapid growth in the use of this kind of dyes for
cellulosic fiber and the technical and economic
limitation of the other dyes [11]. Generally
speaking, reactive dyes which have the reactive
groups enable them to react chemically with the
fiber substrate to form a covalent bond [12]. Incomparison to other classes of dyes, the degree of
fixation of reactive dyes on the fabric is still very
low where about 5–50% of dyes remain in textile
wastewater due to their incomplete exhaustion
and dye hydrolysis in the alkaline dye bath during
the dyeing processes (Table 2). The hydrolyzed
dyes are derived when the reactive dyes react
with water instead of reacting with the functional
group of textile fabrics. The loss of dyes to the
effluent, however, is dependent on the degree of
fixation of the combination of different dye and
fiber [13]. Generally, all of the dye classes pre-
sent the same problem in terms of not being
environmentally friendly. Hence, it is important
to decolorize the effluents properly before dis-
charging into the environment in order to mini-
mize the water pollution.
On the other hand, in the dyeing process, the
inorganic salt is added in order to enhance the
dye uptake by the fabric. Monovalent salt-sodium
chloride (NaCl) is the most common inorganic
salt that has been widely used in dyeing process.
Besides NaCl, divalent salts, e.g. sodium sulphate(Na2SO4), are the alternative salts being used
during the process. A higher concentration of salt
in the waste stream may be a main environmental
problem in some areas due to the salination of the
soil. Therefore, it must be emphasised that a
wastewater treatment system is not just a process
to cope with the environmental problem but also
a step to recover valuable rinsed water as well as
to minimize the waste volume discharged.
3. Performance evaluation of various tech-
nologies for the treatment of textile effluents
Many researchers had investigated the per-
formance of textile effluent treatment using var-
ious technologies [14,15]. Textile wastewater
typically consists of different types of dyes, deter-
gents, grease and oil, heavy metal, inorganic salts
and fibers in amounts depending on the proces-
sing regime [16]. The effluents are generallycharacterized using parameters such as biological
oxygen demand (BOD), chemical oxygen
demand (COD), total organic carbon (TOC), pH,
color and suspended solids (SS). Nowadays,
many of the world’s textile manufacturers are
equipped with their own wastewater treatment
plant, which usually combines an aerobic
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Table 2
Method of dye application and estimated degree of fixation for different dye fiber combinations [3,9,10]
Class Characteristics Substrates Method of application Fiber Degre of fixation(loss to effluent), %
Acid Anionic, water soluble
Nylon, wool,silk
Usually from neutral to acidicdyebaths
Polyamide 89–95 (5–20)
Basic Cationic, water soluble
PAN, modifiednylon, inks,
polyester
Applied from acidic dyebaths Acrylic 95–100 (0–5)
Direct Anionic, water soluble
Cotton, rayona,leather, nylon
Applied from neutral or slightlyalkaline baths containingadditional electrolytes
Cellulose 70–95 (5–30)
Disperse Very low water solubility
Polyester, poly-amide, acetate,
plastic, acrylic
Fine aqueous dispersions oftenapplied by high temperature
pressure or lower temperaturecarrier methods
Polyester 90–100 (0–10)
Reactive Anionic, water soluble
Cotton, wool,silk, nylon
Reactive site on dye reacts withfunctional group on fiber to bindeye covalently under influence of heat and pH (alkaline)
Cellulose 50–90 (10–50)
Sulfur Colloidal,insoluble
Cotton, rayona Aromatic substrate vatted withsodium sulfide and re-oxidized toinsoluble sulfur-containing
products on fiber
Cellulose 60–90 (10–40)
Vat Colloidal,
insoluble
Cotton, rayona Water-insoluble dyes solubilized
by reducing with sodiumhydrosulfite, then exhausted onfiber and re-oxidized
Cellulose 80–95 (5–20)
aAlso known as viscose.
biological process and a physicochemical pro-
cess. However, most of these traditional methods
were found inadequate due to the large variability
of composition of textile wastewater.
Table 3 illustrates the efficiencies of various
treatment systems on decolorization and CODremoval which have been employed on textile
reactive dyeing effluent. According to Marmagne
and Coste [15], the coagulation and flocculation
process is not an excellent one for reactive dye
removal. The poor quality of floc resulted in
uneven settlement even after introduction of a
flocculant. This treatment method, however, was
suitable to be used in sulphur and dispensed dye
removal due to the good quality of floc forma-
tion. They also revealed their studies on color
removal of different types of dyes using an
activated carbon treatment. Results indicated that
high removal rates (>90%) could only beachieved for acid and cationic dyes. For reactive
dyes, a moderate removal (>50%) is considered
good. As shown in Table 3, the ozonation process
shows a higher reactive dye removal compared to
the other treatments, regardless of types of reac-
tive dyes used. It is very effective towards oxida-
tion of dyes and removing color, which is the
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Table 4
Summary of applications of combined treatment systems on textile effluent
Treatment processes Firststage
Secondstage
Remarks
Physical/membranetreatment (2007)
Coagulation UF Achieved substantial colloidal particle removal (>97%) of turbidity removal) regardless of type and dosage of coagulantsused, but degree of membrane fouling was highly dependent ontype of coagulants used. Study has proven that inorganiccoagulants were more efficient to reduce fouling compared to
polymeric coagulants [22]
Membranetreatment (2006)
UF NF Authors claimed that UF was an appropriate pre-treatment of a NR/RO process for textile wastewater reuse. To deal with thewastewater with high variability values of COD and conductivity,they observed flux decline was significant at the lowest crossflow velocity studied due to the solid deposition onto the
membrane surface [5]
Physical/membranetreatment (2005)
Coagulation/flocculation
NF Study reported that the quality of permeate after coagulation/flocculation did not match the requirement of reuse on the site.However, this method could act as pretreatment of NF to limitmembrane fouling. By using this integrated approach, high-quality permeate could be obtained [7].
Chemical/membranetreatment (2005)
Electro-chemicaloxidation
Membr. Study indicated the feasibility of combined processes for treatment of textile wastewater. Membrane prior toelectrochemical oxidation process showed promising results interms of COD, turbidity and color removal ( RCOD = 89.2%,
Rturbidity = 98.3%; Rcolor = 91.1%;) compared to electrochemical
oxidation prior to membrane process ( RCOD = 86.2%, Rturbidity =95.1%, Rcolor = 85.2%). This is due to lower color concentrationremaining in wastewater after the electrochemical oxidation
process [23]
Chemical/biologicaltreatment (2003)
Ozonation Aerobic Use of ozonation as pretreatment was able to increase the bioavailability of the dye before it was treated with the aerobic process. To achieve higher color (99.8%) and DOC (85%)removal, higher doses of ozone were required. This would make itless economically favorable [24]
Physical/membranetreatment
Sandfiltration
and MF
NF Sand filtration and MF in a pilot plant were fundamental inreduction of suspended solids (100%) and turbidity (78%). To
completely remove COD, conductivity and color, NF wasresponsible for removal [4]
Physical/chemicaltreatment (1997)
Coagulationand electro-chemicaloxidation
Ionexchange
Water produced from this integrated treatment was reported goodto reduce color, turbidity and COD; however, efficiency of treatments was significantly different with varying reaction timesof H2O2 and current of electrochemical treatment [25]
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Treatment processes Firststage
Secondstage
Remarks
Physical/chemical/ biological treatment
(1996)
Coagulationand electro-
chemicaloxidation
Activatedsludge
Continuous treatment showed that promising quality of permeatewater could be achieved as well as 24% cost savings over
conventional methods. However, oeprating variables (e.g.,wastewater flow rate, applied current, aeration time in activatedsludge, egc.) should be taken into account in order to optimizetreatment performance [26]
Physical/chemicaltreatment (1994)
Coagulation Ozonation Ozonation of wastewater after coagulation treatment exhibitedmore efficient color and COD reduction compared to coagulationof wastewater after ozonation treatment under the sameconditions of wastewater. It was due to further 90% and 20–25%of reduction of color and COD, respectively, could be achieved
by using ozonation after the coagulation process [18]
very limited, mainly because research on MMH processes has only recently started. However, as
the amount of research and industrial applications
increases, MMH is expected to attract more atten-
tion for treating the textile wastewater in the near
future. Processes that combine membrane with
conventional treatment process/membrane pro-
cess have been widely applied to achieve lower
capital cost and higher productivity. However,
fouling is often the main problem of membrane
system for complex textile manufacturing waste-
water. Dyes are the components which mainlycontribute to colloidal fouling layer onto the
membrane surface. To investigate the fouling
mechanism and fouling control techniques on NF,
Section 7 of this review discusses the state-of-the-
art on the topic in details.
For economic and environmental reasons, it is
necessary that as much of this waste as possible
is recycled instead of being disposed of in landfill
sites. Due to the recent technological innovations
in membrane technology, the cost of membranesystems has decreased and has led to an increase
in the use of membrane systems for wastewater
treatment processes. Though cost analysis is a
paramount exercise to undertake for textile indus-
try process, estimating the cost of installing a
treatment system is very difficult. This is because
of variation in the raw water characteristics; the
efficiency of the process; the technologicalinnovations; the system’s capacity; the permeate
characteristics and etc., which make the system
cost evaluation vary significantly [27]. Apart
from this, the price of purchasing various com-
ponents as well as capital and operational costs
are different in each country. Further, the reports
on full-scale application of membrane systems in
the textile industry are apparently lacking. There
are only a limited number of cost analysis studies
looking into the reuse and recovery of textile
effluent using integrated membrane systems on pilot-plant scale. This, however, could be an indi-
cation of the economical feasibility of the imple-
mentation on full industrial scale in the following
days.
In a study by Ciardelli et al. [28], it was
reported that about US $1.27/m3 treated waste-
water would be a reasonable cost for the imple-
mentation of membrane techniques for treatment
of dyehouse effluents for reuse in Italy. But it is
expected to increase in the future. In India, thetotal expenses incurred for water treatment and
recovery using RO/NF membrane are about
US $1.80/m3 of the effluent [29]. The cost of
recovery may be too high in some countries.
They, however, reported that the cost was still
lower than the cost of water purchased due to
non-availability of good quality water for dyeing
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processes in Tirupur, India. Babursah and co-
workers [27] reported that the cost of recovering
wastewater within the textile industry using a
membrane recovery system was US $0.55/m3
based on the current market conditions in Istan-
bul, Turkey. Details of economic analysis based
on the membrane technologies can be found
elsewhere [30,31]. With this limited information,
there is a need for comprehensive studies to
assess the economic feasibility of using NF
membrane technology for producing purified
water from wastewater, particularly in the textile
industry.
4. Preparation and characterization of poly-meric nanofiltration composite membranes
There are a number of commercially available
NF membranes in the current market, which are
mostly monopolized by the niche international
companies. Among the most widely used is the
TFC–NF membrane. Its excellent permeability
and selectivity over asymmetric NF membranes
offered competitive improvement of this kind of
membrane [32]. The currently available TFC–NF
membranes are mainly prepared by forming avery thin polyamide (PA) active layer on the
porous support layer which is mainly prepared
from polysulfone (PSf) or polyethersulfone (PES)
[33–35]. The substrate membrane is commonly
prepared through a dry–wet phase inversion tech-
nique while the top active layer is formed via the
interfacial polymerization (IP) technique.
The combined techniques offer significant
advantages as either thin skin layer or porous
substrate layer can be optimized independently.The support layer membrane can be optimized to
enhance strength and compression resistance
while the top skin layer can be optimized to
enhance desired solvent flux and solute rejection.
Through such optimization process, TFC–NF
membranes generally exhibit higher salt rejection
over asymmetric NF membrane due to the ultra-
thin skin layer (300–400 nm) formed onto the
porous support membrane [33]. Although TFC–
NF is manufactured based on the IP method, its
performance in terms of rejection and flux profile
is different. The performance very much depends
on the support membrane employed, concen-
tration of reactant, reaction time, organic solution
used and others, which are not fully explored.
At present, a number of studies on the pre-
paration and characterization of TFC–NF mem-
branes via the IP technique have been reported.
Interfacial polymerization reaction occurs be-
tween two very reactive monomer (or one pre-
polymer) e.g., amine-type and acid chloride, at
the interface of two immiscible solvents [36]. In
this section, studies on the effects of manufac-turing conditions on the TFC–NF membrane have
been reviewed. In addition, it is very important to
provide valuable information for those who are
going to choose NF membranes in textile waste-
water treatment.
Song et al. [32] introduced a thin active layer of PA on polysulfone (PSf)/sulfonated poly-sulfone (SPSf) alloy substrates via IP using three
different types of PA: p-phenylenediamine(PPD), m-phenylenediamine (MPD) and pipera-
zine (PIP). The PSf/SPSf substrates prepared bythe dry–wet phase inversion method wereimmersed into an aqueous solution before dipping
into an organic solution. Polyamide, surfactantand phase transfer catalysts were dissolved into
distilled water to form the first solution while acertain amount of trimesoyl chloride (TMC) wasdissolved into hexane to form the organic solu-
tion. The results indicated interactions of ionic bonds of an interpenetrating layer between the
PAs active layer and the substrates using Fourier transfer infrared (FTIR) and attenuated totalreflection infrared (ATIR). The thermal proper-
ties of different membranes were also investi-
gated using differential scanning calorimetry
(DSC) and thermalgravimetry (TGA) under
nitrogen flow at a heating ramp of 10 K/min. The
thermal analysis results confirmed that an inter-
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penetrating layer was formed between the active
layer and the support membrane based on the
investigation of the chemical composition and the
thermal property of these membranes. The strong
interaction will not cause the active layer to be
detached from support layer under harsh con-
ditions of wastewater from the dyeing process. It
is because the chemicals in the wastewater are
able to swell the support layer and then cause the
TFC–NF membrane in an undesirable condition.
In this case, they proposed the use of SPSf into
PSf porous substrates where SPSf could also act
to further improve the hydrophilicity of the
membrane and provide higher permeability in-
stead of using PSf as the substrate.
Oh et al. [37] used the interfacial polymeri-zation of PIP with TMC on the surfaces of microporous polyacrylonitrile (PAN) supports toform a strong interaction between the active and
support layers. Interestingly, they observed thatthe functional groups of –CN in PAN could be
modified to be –COOH groups through a simpletreatment with NaOH solution at ambient tem-
perature. The ionic bond between these two
layers is shown in Fig. 1. In addition, they alsostudied the influence of modified PAN concen-
tration on the membrane surface roughness usingatomic force microscopy (AFM). The surfaceroughness increased significantly with increasing
the modified PAN concentration from 10 wt% to20 wt%. Therefore, to remove dyes effectively
from water solutions, membrane surface rough-
ness is an important factor to be considered.
Membrane fouling by dyes would reduce the
water flux, thus resulting in the membrane being
less economical.
In a study on the influence of monomer compositions and organic solutions, Jegal et al.
[38] found that PA composite membranes pre-
pared by IP of piperazine/m-phenylene diamine
(8/2 w/w) and TMC on the microporous PSf
membrane, with hexane as organic solution, were
able to increase the flux to 2.5 m3/ (m2.day) at
200 psi as the composition ratio was changed
Fig. 1. Ionic bond formation between the PIP of PA
active layer and –COOH on the PAN support [37].
from 7/3 w/w. In addition, with changing hexane
solution into benzene/hexane mixture solutions,
interesting results were achieved. They attributed
this to benzene that was a good cosolvent to con-
trol the permeation properties of the membrane.With the addition of 40 vol% of benzene into
hexane, a PA composite membrane could provide
promising results both in flux and solute
rejection.
Mohammad et al. [39] used Bisphenol A
(BPA) as the top active layer material on the
porous substrate made from a mixture of PSf and
polyvinylpyrrolidone (PVP) using the IP tech-
nique. They observed that an increase in the
reaction or concentration of BPA resulted in
decreasing of water permeability. This conditionwas attributed to the increase of active layer
thickness. Though the thickness was increased
with increasing the reaction time, it was found
that the membrane pore size was independent of
the reaction time. However, based on AFM
results, the pore size differs considerably with
increasing the monomer concentration.
Detailed studies on the characteristics of top
active layer have been conducted by Ooi [40] and
Ahmad and Ooi [41]. Ahmad and Ooi [41]observed that by immersing the PSf support
membrane in two different solutions containing
(a) PIP, 3,5-diaminebenzoic acid and distilled
water and (b) TMC and n-hexane, respectively, a
strong interaction of these materials would occur
where the structure of organic chemistry for the
reaction is shown in Fig. 2. They also indicated
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Fig. 2. Reaction scheme of trimesoyl chloride with piperazine and 3,5 diaminobenzoic acid [41].
that strong interactions of these materials would
improve the efficiency of separation process. In
addition to the effects of top layer materials on
the TFC–NF performance, different kinds of PAsused as the top skin layer membrane would also
have the influence on separation performance.
Verisimmo et al. [42] used PAs such as PIP, N,N-
diaminopiperazine (DAP), 1,4-bis(3-amino-
propyl)-piperazine (DAPP) and N-(2-amino-
ethyl)-piperazine (EAP) to react with TMC sepa-
rately during composite membrane manufacture.
Among these membranes, it was reported that
PIP–TMC exhibited higher water permeability
and rejection of monovalent and divalent salts
than those of other membranes. This may be thereason for an increase in the use of PIP for
commercial products.
To further improve the separation performance
of TFC–NF membranes via the IP technique,
Chen et al. [43] proposed the use of dimethyl
formamide (DMF) as a swelling agent in the
aqueous solution. It was found that 20 vol% of
DMF in solution was the optimum composition
since higher concentrations of swelling agents
added would not improve the thin layer forma-tion. They observed that permeation rate of
3 L/(m2.h) and NaCl rejection rate of 94% were
achieved at optimum composition when a salt
solution of 2000 ppm was fed at 13.6 bar. This
represents an interesting finding as the rejection
of monovalent salt could be achieved as high as
divalent salt.
To date, membrane processes such as MF, UF
and RO have often been used as the wastewater
treatment. Due to certain technical reasons, NF
membranes have grown significantly as the mem- brane separation process during the last decade.
However, if NF can be applied successfully in the
textile industry, the TFC–NF membrane is the
way to meet the requirements for the applications
with achieving both high permeability and high
rejection of inorganic salt, e.g., NaCl. The perfor-
mance of TFC–NF membranes has been widely
studied on textile wastewater removal; however,
in most cases, it was just carried out at laboratory
or pilot-plant scale. Therefore, to be more practi-
cal, extensive efforts are still needed to further enhance TFC–NF membrane performance.
5. Performance evaluation of NF membranes
for specific textile effluent: Effects of process
conditions
In the literature, there are a number of studies
reported on the effects of different operating con-
ditions of textile effluents on NF performance.
The laboratory or pilot studies indicated the high potential of using NF for reuse of water and
chemicals from textile effluents. The following is
a summary of studies based on the performance
of commercial NF under different operating con-
ditions. Table 5 summarizes the application of the
commercially available polymeric NF membranes
used for textile effluent treatment. It was found
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that some of the commercial membranes achieved
maximum separation of dye and salts while others
achieved higher flux.
5.1. Dye retention of nanofiltrationIn a study of color and COD retention by NF
at pilot-plant scale, Lopes et al. [44] reported that
NF membranes such as NF 45 and DK 1073
exhibited good performance in terms of dye
retention. The maximum dye rejection was up to
99.2% and 99.8% respectively, with an initial dye
concentration of 450–500 mg/L. Meanwhile, the
performance of MPS 31 was also investigated and
gave results of dye retention which varied from
90.1–97.3%. However, on average, the percen-tage of color rejection of MPS 31 was slightly
higher than NF 45 and DK 1073 (Table 5). This
may be due to its smaller molecular weight cut
off (MWCO). On the other hand, Sungpet and co-
workers [52] attributed the dye rejection to the
secondary layer formed by retained dye on the
membrane surface. It was because the MPF 36
(MWCO 1000), having larger MWCO than MPF
34 (MWCO 200), showed higher dye removal in
the presence of a reactive dye and sodium
chloride. Thus, they found that secondary layersformed by dye and the Donnan effect may be
responsible for dye removal instead of membrane
MWCO. Fouling layer occurred resulting from
the absorption of dye onto the membrane, result-
ing in an increase of dye rejection.
Tang and Chen [46] studied dye retention
using the TFC-SR2 membrane. They found that
with increasing dye concentration of Reactive
Black 5 gradually from 92 ppm to 1583 ppm, the
dye rejection remained constant at a feed pressureof 5 bar. This indicates that dye rejection is inde-
pendent of dye concentration. The results were
also supported by Akbari et al. [3] and Van der
Bruggen et al. [48]. Akbari et al. [3] reported that
dye rejection only slightly decreased with an
increasing concentration from 2000 ppm to
6000 ppm at feed pressure of 10 bar. In this case,
they concluded that dye molecules could perform
a good mass transfer throughout the membrane
and avoid build-up of dye concentration polariza-
tion on the membrane surface. Nevertheless, dye
molecules were able to induce color on the
membrane surface, which resulted in a fouling
problem [49].
Apart from the effect of concentration of dye
itself on dye removal, Koyuncu [53] conducted a
study to investigate the effect of salt concen-
tration on dye rejection using the DS5 DK mem-
brane. They reported that lower color removal
was observed with increasing NaCl concentra-
tion. Similar results were also reported elsewhere
[46]. By increasing salt concentration, the Don-
nan effect becomes less effective on the nega-tively charged membrane. This would promote
the penetration of dye molecules through the
membrane and further decrease dye retention.
However, in the work of Jiraratananon et al. [54],
the unchanged dye rejection in three different NF
membranes (ES20, NTR-729HF and LES90) in
the presence of NaCl salt indicated that retention
of a reactive red dye (Benefix) was mainly domi-
nated by the steric effect rather than the Donnan
effect. This is reasonable as the pore radius of
these NF membranes is typically smaller than the
effective hydrodynamic radius of the dye.
The efficiency of color removal was also
dependent of cross flow velocity. The DS5 DK
membrane was further tested with different cross
flow velocities (1.11 m/s, 0.41 m/s and 0.11 m/s)
to investigate its influence on the efficiency of
color removal [53]. Color removal for cross flow
velocities of 1.11 m/s and 0.41 m/s was better
than that of 0.11 m/s due to the decrease in con-
centration polarization on the membrane surface.However, no significant color rejection was
observed when increasing salt concentration to
the feed sample. Cross flow velocity, therefore,
has not played an important role in color removal
due to the presence of dye agglomeration at high
NaCl concentrations.
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Table 5
Summary of the applications of commercially available polymeric NF membranes on textile effluent
Membrane(manufacturer)
Configuration(polymer material)
MWCO(Da)
Process conditions Evaluation
MPS 31(Weizmann)
Spiral wound(NA)a
NAa Experiments conducted at dyeconcentrations varying between 400– 500 mg/L at 60EC and operating pressure of 25 bars. NaCl (10 g/L), CaCl2 (10 g/L) and
Na2SO4 (15 g/L) were added to the solution[44]
Pave = 66.25 L/(m2.h)
Rdye, ave = 94.9%
NF45(Dow/Film Tec)
Spiral wound(PA) b
200 Same as above. Pave = 39.2 L/(m2.h)
Rdye, ave = 92%
DK 1073(Osmonics)
Spiral wound(PA) b
300 Same as above. Pave = 60.25 L/(m2.h)
Rdye, ave = 94.5%
ATF 50(Adv. Membr.Tech.)
Spiral wound(TFCc of PIPd onPSf e)
340 Two kinds of industrial wastewater wereanalyzed with (a) COD = 14,200 mg/L at pH 10.2; (b) COD = 5430 mg/L at pH 5.5.Experiments were carried out attransmembrane pressure of 0.2–1.1 MPaand temperatures of 25–40EC [45]
RCOD = 95% for pH 10.2wastewater
RCOD = 80.9% for pH 5.5 wastewater
TFC–SR2(Fluid System)
Flat sheet(TFCc of PSf e)
200– 400
92–1583 ppm of Reactive Black 5 and10–80 g/L of NaCl were used to synthesizewastewater. Solutions were filtered under cross flow velocity of 3–5 L/min andoperating pressures of 100–500 KPa [46]
Pave = 45.05 L/(m2.h)
Rdye, ave = 97.71%
DK 2540
(Osmonics)
Spiral wound
(NA)a NAa Industrial wastewater with COD 1576 g/L,
color >500 Hz and conductivity of 3.5 µS/cm were used during experiments attransmembrane pressure of 20 bars and crossflow velocity of 1.66 m/s [47]
Pave = ~60 L/(m2.h)
Rsalt = 60–80%
NTR 7450(Nitto–Denko)
Flat sheet(sPES)f
600–800 Synthetic dyebath solution containingReactive Orange 16 (RO16) or ReactiveBlue 2 (RB2) (15 g/l), Na2SO4 (56 g/L),surfactant-EDTA (0.2 g/L), Na2SO3 (1 g/L)and NaOH (2.5 g/L) were used in experi-ments at operating pressures of 0–60 bar and cross flow velocity of 0–0.75 m/s [48]
P = 64 L/(m2.h) Rdye = 92.1% Rsalt = 87.3% at operating pressure of 20 bar
UTC 20(Toray Ind.)
Flat sheet(PA) b
180 Dye retentions for RO16 and RB2 werestudied with cumulative addition componentof 1 g/L dye, followed by 2.5 g/L of NaOH,1 g/L of Na2SO3, 0.2 g/L of EDTA, 11 g/Lof Na2SO4 and 19 g/L of Na2SO4 [48]
Rdye, RO16 = ~99% Rdye, RB2 = >99.3%
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Membrane(manufacturer)
Configuration(polymer material)
MWCO(Da)
Process conditions Evaluation
NF 70(Dow/Film Tec)
Flat sheet(PA) b
250 Experiments were carried out using twotypes of exhausted dye baths from the wool
dyeing process: (1) acid dye bath (SA), (2)metal complete dye bath (MC).Transmembrane pressure in experiments was10 bar [49]
PSA = 33 L/(m2.h)
PMC = 32 L/(m2.h)
DL 4040F(Osmonics)
Spiral wound(NA)a
150–300 Wastewater from a dyeing process andfinishing plant was treated using a three-stage treatment system: sand filtration, UFand NF. Mean value of the parameters was
pH 7.8, COD 142 mg/L, TSS 12 mg/L andconductivity 3950 µS/cm. UF and NFmodules worked at 0.4 and 9 bar,respectively [50]
RCOD = >93% RTSS = >60% Rconductivity = 40.5%
Desal 5 DK (Osmonics)
Flat sheet(TFCc –PA b)
150–300 Synthetic dye solution was prepared at a dyeconcentration of 1 g/L without addingauxiliary compounds. Experiments werecarried out at pressure of 10 bar, temperature25EC and pH 6 with and without stirring[51]
Pave = 41.1 L/(m2.h) Rdye, ave = 100% for Direct Red 80 at Reynolds no. of 4100
a Not available. bPolyamide. cThin-film composite. dPiperazineamide. ePolysulfone.f Sulfonated polyethersulfone. Pave = average permeability. R = rejection.
Different observations were reported by
Chakraborty et al. [55] on dye retention using an
organic NF membrane with MWCO 400. Theyattributed the decrease in dye retention after a
certain period of study to the build-up of con-
centration polarization of solute particles over the
membrane surface, thus enhancing the solute per-
meation by convection through the membrane.
Besides these effects, more studies on color
removal have been conducted using NF [51,56–
58]. The research reports that the efficiency of
color removal also depended on a number of
other factors such as wastewater characteristics,molecular weights of the dyes used, hydraulic
conditions, volume reduction factor, temperature,
pH, pressure, etc.
5.2. Salt rejection of nanofiltration
Practically, sodium chloride and sodium sul-
phate are used as the exhausting and retarding
agents during the dyeing processes. The amount
of salts required depends on production require-ments. To determine the salt transport across a
membrane, Eq. (1) is normally used.
(1)( ) /s sQ K C A
where Qs is salt flow through membrane, K s the
membrane permeability coefficient for salt, ΔC
the salt concentration difference across the mem-
brane, A the membrane surface area and τ the
membrane thickness. Salt transport through themembrane is proportional to the salt concen-
tration difference but independent of the applied
pressure [59]. Referring to Eq. (1), increase in
applied pressure will not affect the value of the
salt flow.
Tang and Chen [46] reported that a decrease
of salt rejection occurred with increasing salt
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concentration. Inorganic salt (NaCl) can be ion-
ized completely into Na+ and Cl! in acid, alkali
or pure water. When the salt concentration
increases, so does the concentration of Na+. Based
on the principle of the Donnan equilibrium,
repulsive force from the negatively charged
membrane decreases with increasing electrolyte
concentration. Lower repulsive force means that
more Cl! anions are allowed to pass through the
membrane and thus salt rejection is reduced.
Moreover, higher salt concentration could lead to
a build-up of concentration polarization on the
membrane surface resulting in lower flux and
separation. Other researchers attributed the
decrease in NaCl rejection with increasing NaCl
concentration to the shield effect [60]. The unde-sirable effect of concentration polarization,
however, can be minimized by maintaining a high
flow rate of the liquid phase along the membrane
surface and by applying turbulence promoters
(spacers) between the membranes [61].
Previous studies demonstrated that adoption of a NF hollow fiber membrane (HA 3110, Toyobo,Japan) in a submerged MBR was feasible because
it could provide extra-clean permeate for reuse[62]. The rejection rates of monovalent and
divalent ion by this NF membrane varying from40% to 60% and from 70% to 90%, respectively,during the initial 80 days of filtration process.
The lower rejection rates of monovalent salt com- pared to divalent salts were also reported pre-
viously [51,60]. Choi and co-workers [62] foundthat salt rejections tended to decrease graduallyafter 80 days, which was probably due to the
increase in pore size and decrease in the surfacecharge of membranes, which deteriorated the
membrane properties.A neutral surface membrane typically shows
a lower salt rejection as compared to a charged
membrane for a given pore size. The mechanism
of salt rejection is primarily based on the steric
effect in neutral surface membrane. The Donnan
exclusion, however, plays an important role in
retaining salt in negatively charged membranes.
The Donnan effect becomes less effective with
increasing salt concentration in the feed due to
the lower repulsive force. Jiraratananon et al. [54]
elucidated that a higher concentration of Cl! ions
would contribute to an increase in the Donnan
equilibrium of Cl! ions in the membrane,
resulting in higher ionic flux through the mem-
brane. Consequently, a lower salt rejection is
obtained.
Vrijenhoek and Waypa [63] used the NF-45
membrane to investigate the performance of NF
membranes under single and multiple salt
solutions. In the case of uncharged membranes,
the observed order of single salt rejection is CaCl2> NaSO4 > NaCl, which generally is dominated
by steric exclusion. Rejection of divalent ionsincreased with increasing feed concentration and
flux, while rejection of monovalent ions
decreased with increasing feed concentration.
Based on molecular weight of the ions, though
the size of Ca2+ is smaller than SO42!, the rejection
of Ca2+ is slightly higher than SO42!
. This is due to
cations that are able to accumulate more water
molecules around the ions, thus resulting in a
larger hydrated radius than anions. For the sepa-
ration of multiple salt solutions (NaCl/Na2SO
4)
by NF-45, Vrijenhoek and Waypa [63] found that
size exclusion is the dominant mechanism for ion
retention where the observed order of rejection is
SO42! > Na+ > Cl!, which is in reverse order of the
ionic diffusion coefficients (Table 6). Theoreti-
cally, the diffusion of ion in the liquid can be
determined based on Eq. (2) [64]:
4 # n # 6 (2)
i
kT D
n a
where k is the Boltzmann’s constant, a is the
radius of solute, η is the solution viscosity and n
is the Stokes–Einstein coefficient. However, for
a NaCl/CaCl2 mixture, they reported that ionic
diffusivity is responsible for the ion retention
instead of the size exclusion effect based on the
observed rejection of Ca2+ > Cl! > Na+ [63].
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Table 6
Ions, ion diffusivities, ion atomic or molecular weights
and hydrated radii for salt solutions [63]
Order of rejection
(highest to lowest)
D bulk, I
(10!9
m2
/s)
AW or
MW
r H2O,
nm
NaCl/Na2SO4 saltmixture: SO4
2!
Na+
Cl!
(lowest tohighest)1.061.662.03
96.0622.9935.45
NA0.360.33
NaCl/CaCl2 saltmixture: Ca2+
Cl!
Na+
(highest tolowest)0.922.031.33
40.0835.4522.00
0.410.330.36
On the contrary, the results obtained byPeeters et al. [65] illustrated that salt rejection inneutral NF membrane were affected both by size
exclu-sion and Donnan exclusion effects. Theorder of rejection for various salts in neutral NF
is NaSO4 > CaCl2 > NaCl. Anionic NFmembranes which have positive groups attachedto a polymer back-bone are able to repel cations,
particularly divalent cations such as Ca2+, andattract anions, particularly divalent anions such as
SO42!. The result is an order in salt rejection such
as CaCl2 > NaCl > Na2SO4. The cationic NFmembrane with fixed negative charges prefer-
entially rejects SO42! but permeates Ca2+ and
results in an order of salt rejection NaSO4 > NaCl
> CaCl2. Du and Zhao [34] also observed that saltrejection in a positively charged NF membrane isdependent not only on pore size of the membrane
but also on the static electric action between theion in solution and membrane surface. This
observation of salt rejection is similar to those byPeeter et al. [65], i.e. the order of the rejectionwas MgCl2 > MgSO4 > NaCl > Na2SO4.
5.3. Permeate flux of nanofiltration
Water reclamation is a key subject in the
textile industry. When the level of solute
retention is met, the permeate flux becomes a
fundamental factor in the process optimization. A
study by Akbari et al. [66] showed that
wastewater pH variation from 6 to 10.3 did not
affect dye retention significantly. However, the
permeate flux was affected by the type of
membrane used. Sungpet et al. [58] proved that
lower acid concentration in the industrial effluent
could lead to higher flux. To increase permeate
flux, Bowen and Mohammad [67] claimed that
the characteristics of a membrane play a main
role for the improvement. A significant improve-
ment in flux could be obtained using looser NF
membranes with higher effective charge density
compared to the tight membranes with typical
charge density, X d .Another aspect that significantly influences
the permeate flux is operating temperature. High
temperature water requires lower operating pres-
sure to achieve a desired flux when compared to
operating at low temperature water. Lower solute
rejection occurs at higher temperatures due to a
greater solute penetration through the membrane.
For a NF membrane, an approximation of per-
meate flux can be made by:
(3)o( 25)
( 25 C)1.03 T p pQ Q
where Q p is the permeate flow at temperature T ,
Q p(25EC) the permeate flow at 25oC and T is the
water temperature, oC. To obtain a desired quan-
tity of permeate flux, controlling the temperature
of the feed solution is one of the critical para-
meters which should be considered. All poly-
meric membranes have their own maximum
operating temperature. In general, these mem- branes are considered sustainable in most of the
separation processes which require not very high
operating temperature [6,68]. However, good
control of feed temperature is still required in
order to minimize the change in the physical and
chemical properties of the membrane.
Studies by Chen et al. [45] clearly concluded
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that permeate flux increases proportionally with
an increase in the transmembrane pressure drop
and with an increase in the operating temperature
due to the decrease in water viscosity. Viscosity
usually decreases significantly when temperature
is increased whereas viscosity increases with
increasing pressure. However, the effect is gene-
rally insignificant at a pressure less than 4 MPa
(~39.5 bar) [69]. Though the influence of feed
pressure on viscosity is insignificant, it has great
influence on permeate flux. Bes-Pia et al. [70]
studied the relationship between the permeate
flux and feed pressure of NF-90 and DK-5 and
found that by increasing the feed pressure from
10 to 20 bar, the permeate flux changed
significantly. Nevertheless, no influence of feedflow rate (in the studied range of 200–400 L/h)
on permeate flux was noticed.
On the other hand, with permeate recycled tothe system, the NF 70 membrane could provide a
stable flux even after 6–10 h of filtration [49].However, when no permeate was recycled, no
stable value was achieved. This is due to theosmotic pressure and adsorption of organic com-
pounds on the membrane material. Thus, in order
to maintain stable permeate flux over the periodof study, a pre-treatment system was applied.
Marcucci et al. [4] proposed the use of MF mem- branes as the pre-treatment. The DL 4040Fmembrane resulted in a very constant permeate
flux even after 530 h of operation. In addition,Van der Bruggen et al. [71] proposed an alter-
native option by using biological treatment as a pre-treatment before NF to lower the flux declineas well as to provide higher permeate quality in
long-term operation. Without the pre-treatment,
significant flux decline was reported for a single-stage treatment of either the NF 70 or UTC-20.
Flux decline is suspected to be stronger over long
operation hours than that indicated in the study.
However, more intensive studies are needed to
confirm the applicability of the combined treat-
ment processes as NF of the biological effluent
on a real scale might suffer from biofouling.
As mentioned before, the permeate flux may
be influenced by the feed solution velocity. In
such a case, the Reynolds number is widely used
to express solution turbulence:
(4)Re Dv
where D, v̄ , ρ and μ are the diameter of tube,
average velocity of liquid, density of liquid and
viscosity of liquid, respectively. The transition of
the fluid from laminar to turbulent flow was
found to be dependent on the Reynolds number
[72]. Tang and Chen [46] observed that in the NF
experiments, an increasing flow rate at theapplied pressure of 200 kPa did not increase the
flux since the Reynolds numbers for the cross
flow velocity of 3 L/min and 5 L/min were 3000
and 6000, respectively in the turbulent region.
However, for the Reynolds number which falls in
either the transition region or laminar region, it
may have an effect on the permeate flux since the
solid deposition happens onto the membrane
surface at low flow velocity [59]. To further
promote the turbulence flow, Auddy et al. [73]
employed thin wires as turbulent promoters in thecross flow NF system. The flux was enhanced in
the range of 40% to 100% due to the decrease in
concentration polarization, resulting in less solute
deposition over the membrane surface.
The influence of inorganic salts on permeate
flux was also studied by several researchers. Van
der Bruggen et al. [49] have evaluated the influ-
ences of monovalent salts (NaCl) and bivalent
salts (Na2SO4, Na2CO3) on the permeate flux of
NF. It was observed that the presence of inorganic salt in the feed solution increased the
osmotic pressure due to the increase of ionic
concentration in the solution. Thus, a higher feed
operating pressure is required to achieve the same
permeate flux. To optimize the separation per-
formance, there are two models which are widely
employed to estimate osmotic pressure before the
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filtration process. The osmotic pressure based on
the Van’t Hoff model is defined as:
(5) j RT
vc
M
where v is the number of ions, c j is concentration
of component j (kg/m3), R is 8.314 (J/mol.K), T is
absolute temperature (K), and M is molecular
weight (g/mol). This model, however, resulted in
an overestimation of osmotic pressure at high
concentrations of ions. Therefore, for better pre-
diction of osmotic pressure for higher concen-
tration solutions, the Pitzer model is recom-
mended. Van der Bruggen et al. [71] employed
this model to evaluate the flux decline in threedifferent membranes at 10 bar where the osmotic
pressure can be expressed as follows:
(6) 1000
sm
s
RTM v
v
where M s is the molecular weight of the solvent
(g/mol), vs the molar volume of solvent (m3/mol),
vm the number of positive ions and N is the osmo-
tic pressure coefficient. By using the Pitzer model, they estimated the osmotic pressure was
about 2.7 bar when the salt (Na2SO4) concen-
tration was 7.8 g/L. Therefore, they reported that
flux decline was 27% at an operating pressure of
10 bar.
Apart from salt osmotic pressure, a study was
carried out by Gomes et al. [74] to investigate the
influence of dye osmotic pressure on permeate
flux in the NF 45. For a very low dye concen-
tration, the osmotic pressure is increased pro- portionally with the concentration. However, dye
osmotic pressure was found to be concentration
independent, especially for concentrations higher
than 3 g/L. In this case, dye osmotic pressure was
not the dominant factor to affect the permeate
flux as dye would aggregate with increasing dye
concentration.
5.4. COD retention of nanofiltration
The COD test is normally used to measure theoxygen equivalent of the organic material inwastewater that can be oxidized chemically using
dichromate in an acid solution. Severalresearchers reported NF membranes had merit to
minimize the COD values from the textileeffluent. Bes-Pia et al. [70] observed that themeasured COD values in permeate were lower
than 50 mg/L (initial values varying between200–400 mg/L) in all experiments using both NF-
90 and DK-5 membranes. With such percentageof removal (76–83%), they considered that thereduction in textile industry wastewater was
satisfactory. According to Sojka-Ledakowicz et
al. [75], a very high reduction of COD (up to99%) could only be achieved using RO mem-
branes. However, with the combined treatmentsystem of NF and physical/chemical treatment, it
was reported that nearly 100% of COD removalwas achieved [76].
On the contrary, combined treatment with the NF DL4040F as the final membrane process in a pilot-plant scale testing showed that the quality of
the effluent did not match the requirement of
water to be reused since the percentage removalof COD, conductivity and total hardness were93%, 40% and 75%, respectively [50]. It wasrecommended that the integrated approach (sand
filtration, UF and NF) should be used for minor processes in the textile industry such as washing
and dyeing, since they required a relatively lowquality of water.
Studies by Chen et al. [45] on desizing waste-
water for the bleaching and dyeing industry inHong Kong showed that only a minor increase in
COD retention was achieved for an increase intransmembrane pressure drop as well as operatingtemperature. The minor increase, however, is
expected from the analysis of COD transportmodel as follows:
(7)COD
COD COD COD
1 1 1.s
s s v
B
R R R J
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where RCOD, RsCOD, BsCOD and J v are the overall
retention of COD, overall retention parameter for
COD, overall mass transfer parameter for COD
and permeate flux, respectively. Referring to
Eq. (7), with an increase in transmembrane pres-
sure, permeate flux would be increased since
1/ RCOD decreases and RCOD increases. It was a
very interesting case where higher rejection of
COD was achieved with increasing permeate
flux. However, Chakraborty et al. [55] observed
that the COD removal decreased with increasing
operating pressure since more solutes were able
to permeate through the NF membrane at a higher
pressure. Therefore, more studies are needed to
verify the relationship between the COD removal
and operating pressure.The influence of pH on COD removal was
also investigated by Chen et al. [45]. They
reported that a higher retention of COD was
achieved at a higher pH value. It might be the
acidic environment for lower pH value that made
the hydrolyzation of starch more significant.
Thus, the COD retention (95%) at pH 10.2 waste-
water was higher than at pH 5.5 wastewater
(80–85%) using the ATF 50 membrane. Fur-
thermore, NF membranes, e.g., MPS 31, NF 45
and DK 1073, have been proven to reduce the
COD values ranging from 73% to 87% with an
initial COD value of 700 mg/L [44]. The remain-
ing COD in filtrate was probably produced by the
solutes and other oxidizable materials.
6. Transport modelling in nanofiltration
membranes
Nowadays membrane filtration processes are
widely used for industrial separations. In thiscircumstance, there is an increasing need for a
model-based tool to design new membrane sys-
tems for a variety of product separations or to
optimize existing membrane installations. Al-
though the applicability of NF membranes in the
textile industry was increased significantly, their
transport mechanism is not yet well understood
due to their unknown structure and the com-
plexity in composition of textile effluent. To date,
there are numerous studies reported in the litera-
ture regarding various salt removals using NF.
Generally, two main approaches have been used
to model the transport of inorganic salts through
NF. The first was based on the extended Nernst–
Plank model (ENP) [41,77,78] and the second
was based on the Spiegler and Kedem model (SP)
[79–81]. Other than these models, the Teorem–
Meyer–Siever (TMS) and the Donnan–Steric
pore model (DSPM) also were used to determine
salt rejection [79,80,82–84].
To model dye mixture separation, anunsteady-state mass transfer model was
developed and successfully tested for predictionof permeate flux and permeate concentration
polarization of dye components without con-
sidering the presence of salt in an unstirred NF batch [85]. The research was continuously con-ducted to investigate the effects of different
operating parameters on per-eate flux and per-meate concentration of dye in an unstirred batch
and a cross-flow cell [86]. Nevertheless, in theliterature there are no studies covering thetransport model that specifically is versatile for
textile colored wastewater. The mechanism of salttransport in NF is complicated in the presence of
organic dye and many other components, thusmaking the analysis of transport much moredifficult.
Generally, the fouling layer or gel polarization
layer occurs from the absorption of dye onto the
membrane. The situation could be worsened in
the presence of other components, e.g., wax,
fibers, oil, etc. in the textile effluent. Therefore,
the currently available transport models areinsufficient to predict the performance of NF in
textile wastewater. More intensive studies are
therefore desired. However, a brief review of
transport models of NF membranes relevant to
textile dyeing effluent is made below to provide
the basic knowledge for those who are interested
in further improving transport models.
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According to Koyuncu and Topacik [87],
apart from the two layers which were considered
in the previous studies—a concentration
boundary layer on the high pressure side of the
membrane and membrane itself—they also took
into consideration the effects of gel polarization
or fouling on salt and dye removal. Therefore, the
average mass transport of salt, k avg, was defined
as:
(8)avg
1 1 1
s sd k k k
where k s is the mass transport coefficient of salt in
front of the gel layer of organic ion (dye) and k sd the mass transport coefficient of salt inside the
gel layer of organic ion. They observed that
accumulation of dye molecules on the membrane
surface resulted in a decrease of permeate flux
due to the friction loss of the gel layer. Based on
the Spiegler–Kedem and Perry–Linder models
and film theory equations, they described that dye
concentration had a significant effect on flux
values for a fixed NaCl concentration of
26 mol/m3, 340 mol/m3, 600 mol/m3 and
1145 mol/m3. However, interesting results wereobtained with increasing salt concentration higher
than 340 mol/m3. This is because high salt con-
centration increases the degree of aggregation and
subsequently has a positive effect on membrane
fouling, which in turn results in lower flux.
Comparison of the model with the experiments
showed that the model is able to predict rejection
as well as flux reduction behavior for systems
containing NaCl, dye and water.
Al-Bastaki [88] investigated the efficiency of the membrane process in removing color and salt
from a synthetic colored wastewater using a
theoretical model which is based on the solution
diffusion (SD) mass transport theory, where both
salt and dye concentration polarization effects
were included as well as the possibility of
dynamic membrane formation. Similar to
Koyuncu and Topacik [87], they found that
dynamic membranes formed in the presence of
dye could reduce water permeability due to an
increase in membrane resistance. In this case,
water permeability, K w, was proposed in terms of
the combined resistance of membrane and
dynamic membrane:
(9)1
w M DM K
R R
where R M is the resistance of membrane and R DM
is the resistance of a dynamic membrane in the
presence of dye. Al-Bastaki [88] described the
transport of ions with different feed solutions
containing different concentrations of dye andsalt at different operating pressures based on the
SD theory. The theoretical results generally
showed good agreement with experimental results
in terms of salt rejection, color removal as well as
permeate flux, and made it possible to determine
the permeability of dyeing wastewater.
Because the Donnan effect leads to a dif-
ference in the rejection and permeability of NF, it
has been widely utilized in transport models to
improve the dye–salt separation [89,90].
Although salt passage in NF is expected to occur both by diffusion and convection, Levenstein et
al. [90] proposed a simple two-parameter model:
the diffusivity parameter, B0, and the power
exponent in salt permeability equations, n, by
neglecting the convection term. This model
enables characterization of salt rejection in NF of
multi-component aqueous solutions, e.g., NaCl–
H2O, NaCl–H2O–dye and NaCl–H2O–dye–PNa.
With increasing dye concentrations in solutions,
lower permeate fluxes as well as lower chloriderejections were obtained since the Donnan effect
is expected to occur, which could enhance
chloride passage through membranes. Generally,
the model proposed in this work was found to be
applicable for predicting salt removal due to very
good agreement with the experimental results.
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7. Fouling control in polymeric nanofiltration
membranes
Fouling is often a weakness of NF for
complex textile manufacturing. Dyes can produce
a colloidal fouling layer, which further introducesan undesirable flux decline in the operation [91].
Heavy membrane fouling is expected since dyes
can be accumulated on the active layer of poly-
amide NF membranes by chemical bonds either
in the ionic or covalent bond, depending on pH or
the class of dye [51].
Generally, the phenomenon of membrane
fouling is inevitable, but it is reversible by using
feed pre-treatment, modifying the membrane
surface or by controlling membrane cleaning
procedures. To evaluate the fouling potential in
an early stage, a membrane fouling simulator
(MFS) was developed for evaluation of fouling
control by using different chemicals in order to
make NF membrane systems less susceptible to
fouling [92]. The evaluation was also done in
combination with liquid chromatography–organic
carbon detection (LC–OCD) and other analytical
methods to characterize fouling material in
membranes and identify species responsible for
fouling [93].The conventional prevention of fouling by
applying a pre-treatment can still be coped with
by using appropriate cleaning procedures. The
cleaning procedures are typically conducted using
physical and chemical methods [94]. Physical
methods can be intermittent back-washing,
application of critical flux, critical TMP, inter-
mittent suction operation, low TMP, high cross
flow velocity and hydrodynamic shear stress
scouring. On the other hand, chemical cleaningagents can be acids (strong or weak), alkalis
(NaOH), detergents, enzymes, complexing agents
(EDTA) and disinfectants. Furthermore, chemical
cleaning agents are recommended by membrane
manufacturers since they have the ability to
recover completely the initial membrane perme-
ability and require less energy consumption
compared to physical methods [95]. Nevertheless,
chemical treatments are relatively expensive and
may cause severe membrane damage and produce
toxic by-product waste [96].
Some researchers reported that each class of
dyes could cause membrane fouling. The differ-
ence between these dyes was the thickness or
hardness of dye cake layer accumulated on the
membrane surface [56]. Fouling makes the NF
membrane separation process less economically
favorable. Thus, they suggested that a pretreat-
ment with chemical coagulant-alum was required
to decrease the extent of membrane fouling. In
comparison, the degree of flux decline of pre-
treated wastewater over the operation time
became smaller than when original wastewater was used (Fig. 3). The results showed that
approximately 20% of improvement of flux was
observed using the pre-treatment system prior to
NF. Furthermore, the use of an ozonation process,
MF as well as UF as pre-treatment for the NF
membrane in textile effluent has also been inves-
tigated in order to minimize membrane fouling
and deterioration to meet the objective of pro-
longing the NF membrane life span [4,5,70].
In terms of membrane modification, Mulder
[36] reported that negatively charged membranes
Fig. 3. Effect of pre-treatment with alum on the fluxdecline along the operation time when artificial dyeingwastewater was treated with the NF PA composite
membrane [97].
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have the potential of reducing fouling, especially
in the presence of negatively charged colloids in
the feed. On the contrary, Kim and Lee [97]
modified the NF membrane surface by coating
with a neutral polyvinyl alcohol (PVA) to reduce
surface charge and membrane surface roughness
so that the ionic bridge of the cations between the
membrane and dye could be reduced. They exper-
ienced fouling could easily have resulted from the
divalent cations in the effluent using a typically
higher surface charge of NF. The results con-
firmed that the PVA-coated NF membranes were
successful in increasing fouling resistance and
subsequently reducing membrane fouling.
Furthermore, the antifouling NF membrane
was introduced in the work of Asatekin et al. [98] by coating the PVDF support membrane with
an amphiphilic graft copolymer, poly(vinylidenefluoride)-graft-poly(oxyethylene) methacrylate,PVDF-g-POEM. The higher fouling resistance
and higher water produced can be attributed to both the nanoscale dimensions of the hydrophilic
channels through the coating and to the neutralcharge of POE which creates the barrier to theadsorption. Recently, the amphiphilic comb co-
polymer additive, polyacrylonitrile-graft-poly-ethylene oxide (PAN-g-PEO) was also found to
have excellent antifouling characteristics by coat-ing it on a PAN UF membrane [99]. The authorsattributed this to the surface segregation and local
orientation of PAN-g-PEO molecules at the mem- brane surface and pore walls, forming a dense
brush layer as the barrier to the adsorption.
Instead of membrane surface coating, hydro-
philicity or hydrophobicity of membrane is the
factor that should be taken into account in reduc-
ing the extent of fouling. Less fouling is observedfor aqueous solutions or suspensions when the
membranes are strongly hydrophilic due to the
preferential wetting of such material by water. In
the work conducted by Boussu et al. [100], they
reported that NF 270 (27E) was less fouled com-
pared to the large contact angle membrane,
namely NF 90 (54E) and BW30XLE (51E) since
a small contact angle which is corresponding to
the hydrophilic surface could reduce the tendency
of membrane fouling.
Sungpet et al. [52] elucidated that the use of
NaCl in the dyeing process could enhance the
penetration of reactive dye into membranes,
which results in NF membranes heavily colored
after the experiments. This, however, subse-
quently led to a flux decline in the membrane
process. Interestingly, the use of chemical clean-
ing with 0.2 wt% HNO3 followed by 0.5 wt%
NaOH could recover 80–100% of the flux where
the chemical cleaning procedures were periodi-
cally carried out. On the other hand, Marcucci et
al. [4] used alkaline detergent (l–2%) for removal
of organic material and acid detergent (1–2%) for removal of inorganic material from a textile
effluent using a combined membrane treatment
system. This method can be used immediately if
the hydraulic performance is worsened. The
results showed that after the chemical washing,
the initial permeate flux of MF and NF membrane
was re-established even after 300 h of operation.
However, a delay in chemical cleaning of NF
membranes led to irreversible changes in mem-
brane structure and eventually deteriorated the
membrane performance [101].
In the work conducted by Shu et al. [102], itwas concluded that membrane fouling caused by
dye absorption was reversible, but was highlydependent on membrane cleaning. Similar obser-
vations were also obtained by Lopes et al. [44]with applying chemical cleaning on commercial
NF membranes. However, no details of the
chemical solution used as well as the frequencyof chemical cleaning procedure were given.
On the other hand, Van der Bruggen et al. [49]claimed that membrane fouling caused by theadsorption or pore blocking of organic com-
pounds on the membranes had a large influenceon the permeate flux due to the high concen-
tration of organic compounds used in textiledyeing. In the application of NF membranes for the treatment of exhausted dye baths, 26–46% of
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the irreversible flux declines were reported as therelative difference between the initial pure water
flux and pure water flux measured after theexperiment. Although absorbed organic com-
pounds might be desorbed by rinsing water, it
was still difficult for those dyes which weresupposed to attach strongly to the membranes.
The absorption of dyes onto the membrane sur-face is due to the reaction between the polyamide
fibers with dye either in ionic bonding, covalent bond or Van der Waals.
Moreover, the fouling phenomenon is also
found to be linked with the hydrodynamic condi-tions of the filtration system. The hydrodynamic
of cross-flow velocities (CFV) plays an important
role in influencing membrane fouling to controlthe build-up of solute in NF membrane surfaces.
A study was conducted by Petrinic et al. [103] to
evaluate the membrane fouling caused by the dye
bath wastewater at variable CFV. In comparison,
the higher CFV of 0.8m/s was sufficient to keep
the concentration polarization layer small enough
compared to the CFV of 0.4 m/s and 0.6 m/s and
provided promising results in terms of the per-
meate flux and color removal. This is supported
by Koyuncu [53] where he experienced that flux
was increased with increasing CFV from 0.11 m/sto 1.11 m/s, regardless of the concentration of salt
in wastewater. However, the effects of CFV were
not significant for the high NaCl concentrations
(80 g/L) due to the aggregation of dye molecules
at high NaCl concentrations. Instead of control-
ling operating parameters, an alternative approach
for modeling flux decline during membrane
separation processes is based on the filtration
theory [104, 105]. Elimelech and Bhattacharje
[105] developed a theoretical model which is based on the principles of thermodynamics and
hydrodynamics for prediction of permeate flux
during steady-state cross flow membrane filtra-
tion. The results showed that the predictions of
permeate flux compare remarkably well with a
detailed numerical solution of the convective
diffusion equation coupled with the osmotic
pressure model. Moreover, the model is also
capable of predicting the point where cake
formation is initiated. The prediction is useful
since cake formation on membranes is an inevi-
table phenomenon in the textile industry.
8. Future direction of research and develop-
ment of nanofiltration membranes for textile
wastewater treatment
NF membranes have been proven to be one of
the most important separation processes for
textile dyeing effluents treatment based on
numerous research carried out so far. However,
improvements on these membranes are stillneeded in order to further enhance its perfor-
mance before NF becomes a dominant commer-
cialized wastewater treatment system in a large-
scale industrial plant. At present, many published
papers are still at laboratory- or pilot-scale level
and, consequently, further works would be
required in the near future.
Continuous operation of pilot plants should be
optimized and intensive investigations on the
long-term performance of NF membranes should
be carried out to provide a good indication of how a specific component in textile wastewater
leads to membrane fouling. This could assist in
the development of suitable pretreatment systems
prior to NF in order to achieve higher permea-
bility and minimize membrane fouling on long-
term operation.
With respect to the manufacture of TFC–NF
membranes using the IP technique, development
of NF membranes should be further expanded
both in the range of chemical compatibilities and physical operating conditions (including pressure,
CFV, temperature and pH) of membrane systems
due to the high variability of textile effluent
values in order to offer a greater potential over
other membranes. In terms of energy consump-
tion, extensive development effort would help to
find the hidden energy sources in membrane
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processes since the textile wastewater having
operating temperatures in a range between 30–
80EC can be reused in their daily operation. A
detailed economic evaluation of this NF tech-
nology on textile wastewater treat-ment plants,
including maintenance and operation costs,
should also be conducted in order to gain
confidence in NF in the competitive market. With
all of these improvements, a major breakthrough
in NF membrane technologies research will
definitely overcome the limitations and weak-
nesses of current technologies and contribute
greatly to the textile industry worldwide in the
near future.
9. Conclusions
It is difficult to draw a general conclusion on
the feasibility and the efficiency of NF for dyeing
effluent treatment in the textile industry due to
the large variability of textile wastewater
parameters and the quality of the permeate
required. However, based on the numerous
studies conducted so far, NF membranes have
proved applicable in dealing with textile
wastewater which is highly colored as well as
highly loaded with monovalent and/or divalentsalts.
Generally, it can be concluded that NF offers
many more advantages compared to conventional
treatment methods and the other categories of
membrane technologies. To commission the full
scale of NF membrane treatment plants in the
textile industry, a long-term performance of the
system should be carried out along with the
installation of a pretreatment system prior to NF
for the purpose of minimizing membrane foulingand prolonging the membrane life span as well as
increasing the efficiency of the overall treatment
system. Besides, with the understanding of the
transport mechanisms in NF membrane, it will
lead toward better prediction and optimization of
separation processes in the textile industry.
Finally, research and development in this field are
a must in order to gain confidence in NF for
textile wastewater treatment system.
10. Symbols
a — Stokes–Einstein radius, m
A — Membrane surface area, m2
c j — Concentration of component j,
kg/m3
ΔC — Salt concentration difference across
the membrane, kg/m3
D — Diameter of tube, m
Di — Diffusivity, m2/s
J v — Permeate flux, m3/m2.h
k — Boltzmann’s constant
k avg — Average mass transport coefficientof salt
k s — Mass transport coefficient of salt
k sd — Mass transport coefficient of salt
inside the gel layer of organic ion
K s — Membrane permeability coefficient
K w — Water permeability of combined dy-
namic membrane, m2.s/kg
M — Molecular weight, g/mol
M s — Molecular weight of the solvent,
g/mol
n — Stokes–Einstein coefficient
R — Gas constant, J/mol.K
R M — Resistance of membrane, kg/m2.s
R DM — Resistance of dynamic membrane in
the presence of dye, kg/m2.s
Re — Reynolds number
Q p — Permeate flow, kg/m2
Qs — Salt flow through membranes, kg/m2
T — Water temperature, EC
v̄ — Average velocity of liquid, m/s
v — Number of ionsvm — Number of positive ions
vs — Molar volume of solvent, m3/mol
Greek
η — Solution viscosity, N.s/m2
μ — Viscosity of liquid, N.s/m2
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π — Osmotic pressure, bar
ρ — Density of liquid, kg/m3
τ — Membrane thickness, m
N — Osmotic pressure coefficient
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