Polymeric nanofiltration membranes for textile dye wastewater.pdf

8/19/2019 Polymeric nanofiltration membranes for textile dye wastewater.pdf

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W.J. Lau, A.F. Ismail / Desalination 245 (2009) 321–348 322

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 membrane 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%


13/28


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


14/28


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 compared 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].


15/28


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 membranes 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


16/28


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 membranes 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


17/28


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 proportionally 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


18/28


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.


19/28


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.


20/28


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 membrane 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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