Treatmentofhighlyconcentratedwastewatercontainingmultiple.pdf

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Treatment of highly concentrated wastewater containing multiplesynthetic dyes by a combined process of coagulation/occulationand nanoltration

Can-Zeng Liang, Shi-Peng Sun n, Fu-Yun Li, Yee-Kang Ong, Tai-Shung Chung n

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

a r t i c l e i n f o

Article history:Received 11 April 2014

Received in revised form

26 June 2014

Accepted 29 June 2014Available online 8 July 2014

Keywords:

Multiple dye wastewater

Coagulation/occulation

Nanoltration

Hollow ber membranes

a b s t r a c t

The treatments of dyes (acid, basic and reactive dyes) wastewater were studied by applying individualcoagulation/occulation (CF) and nanoltration (NF) processes as well as their combination (referred as

CF–NF). For the treatment of highly concentrated multiple dyes wastewater (MDW, 1000 ppm),

polyaluminum chloride (PAC) and polydiallyldimethyl ammonium chloride (PDDA) were found to be

the most effective coagulant and occulant, respectively. The CF process can achieve about 90% of dye

removal at the optimal dosage of PAC/PDDA¼400/200 ppm, and the MDW with pH43 is favorable for

the CF treatment. A positively charged NF hollow ber membrane was fabricated and used for NF

treatment. It is able to remove almost 100% dyes with a permeate ux of about 1.0 L m2 h1 under an

operating pressure of 1 bar. The combination of CF and NF can complement each other's strengths and

overcome their individual limitations. The NF treatment can completely remove the strong color left in

CF treated dye solutions, while the ef ciency of coagulant/occulant is improved by treating NF

concentrated streams and subsequently results in much less sludge. In addition, membrane fouling is

abated and NF permeate ux is increased by applying the CF process as a pretreatment. Thus, the

combination of CF–NF improves the overall performance for the dyes wastewater treatment.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

A dye molecule consists of two components; namely, the dye

chromophore and the dye auxochrome. When the dye molecule is

exposed to light, the chromophore structure which includes

double bonds (CQC) oscillates to absorb light and generates

visible color [1,2]. By estimation, more than 100,000 synthetic

dyes and over 700,000 t of dyestuff are produced annually [3,4].

The color that appears in industrial wastewater ef uents caused

by residual dyes is esthetically undesirable and harmful to the

environments and the ecosystems. Even a very low concentration

of dyes can generate strong color [5]. The dye wastewater is

generated during dye-related activities such as dye production,textile dyeing, leather tanning and paper production, etc [6].

Particularly in the textile industry, about 10–15% dyes are lost in

dyeing processes, and typically 200–350 m3 wastewater are gen-

erated to produce one ton nished product [7]. It is becoming

more dif cult to directly discharge this kind of wastewater

because the legislations related to environments are becoming

more stringent in many countries [8].

Generally, there are two decolorization methods [9]; namely,

destruction of dye molecules and separation of dyes from water.

To destruct or transform the dyes, conventional processes are

usually applied, such as chemical oxidation, photo-catalysis and

biodegradation [10]. However, the destruction methods is found to

be inadequate and require extensive energy to break down the dye

molecules, most of which are stable to light, oxidizing agents and

microbiological degradation [1,6,11]. The separation methods

include adsorption, coagulation/occulation (CF) and membrane

separation [9]. Adsorption of dyes on powder activated carbon is

popular and effective. However, the activated carbon is not cheap,and the adsorption performance is reduced sharply after regen-

eration or reactivation, which also results in a 10–15% loss of the

sorbent [3,7,12]. Coagulation/occulation is widely used for dyes

removal due to its low capital cost and simple operation [12–14].

Coagulation of dye solutions is a process of destabilizing the dye

solution systems to form agglomerates or ocs. Flocculation is the

process of destabilizing suspended particle systems and bridging

the aggregated ocs to form larger agglomerates that settle down

under gravity [15,16]. Charge neutralization is regarded as a

prerequisite condition in most of coagulation processes [17].

The CF process generates sludge and but sometimes it is ineffective

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/memsci

Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.06.057

0376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding authors. Tel.: þ65 6516 6645; fax: þ65 6779 1936.

E-mail addresses: [email protected] (S.-P. Sun),

[email protected] (T.-S. Chung).

Journal o f Membrane Science 469 (2014) 306–315

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for some soluble dyes. In fact, it is always challenging in selecting

appropriate coagulant/occulant due to the large number of exist-

ing and emerging dyes that have complex structures [7]. Another

important separation method is the pressure-driven membrane

technology including ultraltration (UF), nanoltration (NF) and

reverse osmosis (RO) [1]. Ultraltration has been successfully used

for separating high molecular weight and insoluble dyes from

water. However, UF is not able to remove those water-soluble dyes

with low molecular weights [1,18]. Although ef cient dye removalcan be achieved by RO, the high pressure required in the RO process

hinders its wide applications to treat dye wastewater. NF is

positioned in between UF and RO [9], the nominal molecular weight

cutoff (MWCO) of NF membranes is in the range of 100 to 1000 Da

and with the pore size of about 0.5–2.0 nm [19,20]. NF was

introduced in the early 1980s and has become popular due to its

low operating pressures and relatively low capital and operating

costs [20].

Currently most NF membranes are negatively charged at-

sheet composite membranes, which may have high fouling ten-

dency because most foulants are positive charged. In recent years,

there is a growing interest to fabricate hydrophilic and positively

charged NF membranes which are less prone to organic fouling

[19–30]. Compared to the sophisticated module fabrication pro-

cess for at-sheet or spiral-wound membranes, it is convenient to

fabricate hollow ber membrane modules, which also provide a

high surface area per unit volume and thus to reduce manufactur-

ing costs [31]. NF hollow ber membranes have attracted much

attention due to its superiorities to the conventional at-sheet NF

membranes. Recently, several efforts have been made on the

development and application of NF hollow ber membranes for

removing dyes from water. For instance, Sun et al. employed both

crosslinking and interfacial polymerization to coat a positively

charged polyethyleneimine (PEI) layer on the outer surface of a

negatively charged polyamideimide (PAI) hollow ber substrate to

produce a double-repulsive NF membrane that can remove 99.8%

of the positively charged Safranin O and 98.8% of the negatively

charged Orange II sodium salt [19,21]. Shao et al. developed a thin-

lm composite NF hollow ber membrane by interfacial polymer-ization on the inner surface of polyetherimide (PEI) hollow ber

supports for removal of Safranin O and Aniline blue and achieved

higher than 90% dye rejections [25]. Wei et al. fabricated a NF

hollow ber membrane through interfacial polymerization on a

polysulfone/polyethersulfone supporting membrane for the treat-

ment of reactive brilliant blue X-BR and acid red B dye solutions

and obtained above 99.9% dye rejections [26]. Zheng et al. devel-

oped positively charged TFC hollow ber NF membranes via the

dip-coating method on polypropylene hollow ber microltration

membranes. The resultant NF membrane can achieve dye rejections

of 99.8%, 99.8% and 99.2% for Brilliant green, Victoria blue B and

Crystal violet, respectively [27].

However, the drawbacks of membrane technologies including

NF are the ux decline caused by membrane fouling and the

generation of concentrated streams [32]. To minimize the ux

reduction, one approach is to implement a right pretreatment

process [33], the other approach is to produce fouling-resistant

membranes [34]. The generation of concentrated streams is an

intrinsic issue for membrane separation processes since mem-branes only achieve separation rather than destruction or trans-

formation. For the dye-containing wastewater treatment, the

concentrated stream is usually an unwanted by-product. It must

be further treated before discharge [35,36]. To overcome the

aforementioned problems, a combination of various separation

methods is necessary to achieve a high dye removal and high

separation ef ciency.

To our best knowledge, there is no study on treatment of

synthetic dye wastewater containing multiple dyes of different

classes by a combined process of CF and NF with a positively

charged NF hollow ber membrane. In this work, reactive, acid

and basic dyes were chosen because of their popularity in textile

and dye industries as well as environmental concerns [6,7,12,37].

The representative dye concentrations in dye wastewater streams

are in the range of 0.05–0.1 g/L [38]. For this study, aqueous dye

solutions with a dye concentration of 1000 ppm (1.0 g/L) were

prepared to simulate the highly concentrated dye house waste

ef uent and the concentrated stream after membrane ltration.

In order to maximize the ef cacy of the CF process and

minimize the generation of sludge, proper CF formulations were

carefully evaluated, screened and selected. To reduce membrane

fouling, hydrophilic and positively charged NF hollow ber mem-

branes were made in our laboratory for this study. The CF, NF and

the combination of CF–NF were studied to remove dye(s) from

synthetic dye wastewaters, which consist of either single dye or

multiple dyes. The objectives of this work are to demonstrate that

both CF and NF techniques can remove dyes from wastewater

effectively and the combination of CF–NF is able to improve the

overall performance. This could lead to a new approach for the dyewastewater treatment by a hybrid system.

2. Experimental

2.1. Chemicals and materials

7 inorganic and 4 organic chemicals, listed in Table 1, were

chosen and used as coagulants and occulants in this study. 5 dyes

(2 acid dyes, 2 reactive dyes and 1 basic dye, purchased from

Table 1

Characteristics of coagulants and occulants used in this work.

Name of coagulant/occulant Code name Molecular formula Molecular weight

(g/mol)

Purity

Polyaluminum chloride PAC Aln(OH)mCl(3nm) (0omo3n) Z115b

Z30% (as Al2O3)

Aluminum sulfate-octadecahydrate AS Al2(SO4)3 18H2O 666 Extra pure

Aluminum potassium sulfate-dodecahydrate APS AlK(SO4)2 12H2O 474 99.5%

Iron(III) chloride-anhydrous IC FeCl3 162 98%

Iron(III) sulfate-pentahydrate IS Fe2(SO4)3 5H2O 490 97%

Calcium oxide CO CaO 56 Z98 %

Magnesium chloride-anhydrous MC MgCl2 95 pure

Cationic polyacrylamide CPAM a(CH2–CH–CONH2)m 800–1000 million Z90%

Anionic polyacrylamide APAM a(CH2–CH–CONH2)m 800–2500 million Z90%

Polydiallyldimethyl ammonium chloride (cationic, dissolved in water) PDDA (C8H16ClN)n 200,000–350,0 00 20 wt%

Cyanoguanidine CYGU NH2C(¼NH)NHCN 84 Z99 %

a Obtained from the specication of the products.b

Calculated by assuming m¼n¼1 for PAC.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306 – 315 307


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a ow rate of 0.15 L/min and a pressure of 1.0 bar. The permeate

solution was collected from the lumen side of the membrane. The

retentate stream was circulated back to the feed solution. Each NF

experiment included 3 steps: (1) the initial pure water permeability

(PWP, L m2 bar1 h1) was measured using DI water at 1 bar; (2) a

1.0 L dye solution was used as the feed solution and circulated at

1 bar. Then permeate samples were taken at each consecutive time

interval of 1 h. Finally, DI water was used to ush the system and the

module thoroughly until the circulating water was clean, then thenal PWP was measured.

Pure water permeability (PWP, L m2 bar1 h1) was calcu-

lated according to the following equation:

PWP¼ Q

ΔPAt ð1Þ

where Q is the volume (L) of permeate collected during a specic

period of sampling time t (h), A is the effective membrane area

(m2), and ΔP is the transmembrane pressure (bar).

The permeate ux (J, L m2 h1) can be determined by the

following equation:

J ¼Q

At ð2Þ

where Q is the volume (L) of permeate collected during a specicperiod of sampling time t (h), and A is the effective membrane

area (m2).

2.5. Physicochemical analyses

Two methods were chosen to determine the dye concentration;

namely, total organic carbon (TOC) method and UV –vis integral

method. The implementation of these two methods depends on

specic situations as discussed later.

The TOC method was based on the TOC concentration deter-

mined by a TOC analyzer (TOC, ASI 5000A, Shimazu). Appendix

Fig. 1 shows the calibration curves of TOC values as a function of

dye concentration for various dyes. The TOC value has an almost

linear relationship with dye concentration in the range of 0 to1000 ppm. The dye removal (Rd, %) was calculated by the following

equation:

Rd ¼ðC bC aÞ

C b 100% ð3Þ

where C b and Ca are the TOC concentrations of the dye solution

before and after the CF treatment, respectively.

The UV –vis integral method was based on the UV –vis integral

(integrated range: 350–650 nm) obtained through scanning the

sample by a UV –vis spectrophotometer (Pharo 300, Merck).

Appendix Fig. 2 shows the calibration curves of UV –vis integral

values as a function of dye concentration for various dyes. The UV –

vis integral has a linear relationship with dye concentration when

the dye concentration is below 100 ppm. However, the relation-

ship becomes non-linear when the dye concentration is above

100 ppm. Based on UV –vis integral, the dye removal (Rd, %) by CF

was calculated by the following equation:

Rd ¼ðI bI aÞ

I b 100% ð4Þ

where I b and I a are the UV –vis integral values of the dye solution

before and after the CF treatment, respectively.The dye removal (Rd, %) by the NF membrane was calculated by

the following equation:

Rd ¼ðI f I pÞ

I f 100% ð5Þ

where I f and I p are the UV –vis integral values of the feed and the

permeate solutions, respectively.

It is worthy to point out that the concentrated dye solutions

had to be diluted to around 100 ppm when the UV –vis integral

method was used in order to achieve reasonable accuracy.

The normalized ef ciency of coagulant/occulant (E c ) is dened

as

E c ¼mdmc

ð6Þ

where md is the weight of the dye removed by CF, while mc is the

total weight of coagulant/occulant added for the CF treatment.

The particle size of the settlement after CF was analyzed by

using a laser diffraction particle size analyzer (Beckman Coulter

LS230, analysis range: 0.04 mm to 2000 mm). The solution pH was

determined by using a pH meter (pH/ion S220, Mettler Toledo).

The aluminum concentration was measured using Inductively

Table 4

Preliminary performance evaluation of coagulants on dye removal.

Coagulant Dye removala (%)

INCA RBBR BB-R RB-5 AB-8 MDW

PAC 95.6 90.0 85.5 87.6 0 90.2

AS 64.2 42.6 81.0 44.3 0 75.7

APS 51.1 42.7 84.3 25.6 0 72.1

IC 72.8 59.2 84.6 22.3 0 69.5

IS 27.4 44.9 84.1 21.3 0 65.3

CO 0 0 43.1 0 75.2 5 9.9

MC 29.2 12.6 24.5 0 0 34.4

a Dye removal was based on the TOC method. The coagulant dosage was

1000 ppm, the original dye concentration was 1000 ppm.

0.0

0.5

1.0

1.5

2.0

2.53.0

3.5

4.0

50

60

70

80

90

100

0 400 800 1200 1600 2000

E c

R d

( % )

PAC dosage (ppm)

Rd

Ec

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

50

60

70

80

90

100

0 40 80 120 160 200

E c

R d

( % )

PAC dosage (ppm)

Rd

Ec

MDW=1000 ppm

MDW=100 ppm

Fig. 1. Coagulation performance of PAC for MDW treatment at different dye

concentrations: (a) the dye concentration in MDW was 1000 ppm; (b) the dye

concentration in MDW is 100 ppm. Dye removal is based on the UV –vis integral

method.



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Coupled Plasma-Optical Emission Spectrometry (ICP-OES, iCAP

6000 Series, Thermal Scientic).

3. Results and discussions

3.1. Coagulation and occulation

Both single dye and multiple dye wastewaters (referred to asSDW and MDW, respectively, hereafter) were studied in order to

evaluate the effectiveness of coagulants and occulants in remov-

ing various types of dyes, and to select the most proper coagulant

and occulant. The following procedures were carried out: rstly

to screen and select a proper coagulant; secondly to optimize the

coagulant dosage; thirdly to identify the suitable occulant (or

coagulant aid); and nally optimize the occulant dosage.

3.1.1. Selection of the proper coagulant

Table 4 summarizes the dye removal for both SDW and MDW

based on the TOC method. The reasons of choosing the TOC

method are: (1) to exclude the color interference introduced by

ironic ion (Fe3þ); (2) To avoid a large pH change that may change

the intensity of UV –

vis absorbance caused by the coagulants. Thisis particularly true for calcium oxide (CO) because 1000 ppm CO

can dramatically change the pH value of dye solutions; (3) there is

an almost linear relationship between TOC and dye concentration

in the range of 0–1000 ppm, which makes it simple and reliable

for analyses.

It can be observed from Table 4 that an inorganic coagulant can

be very effective in treating certain dyes, but totally ineffective for

the others. In the case of MDW, every coagulant appears to be

more or less effective, this may suggest that interactions between

dyes take place and subsequently change the MDW's charge

characteristics. As a result, when a coagulant is added into the

dyes solution, charge neutralization initially occurs and promotes

to formation of particles/ocs; then dyes may be also involved in

the process of co-precipitation and sorption onto ocs; nally the

coagulation happens and dyes are removed [7,15,17,40]. The

ability of removing dyes from the MDW sample by the 7 coagulants

is in the order of PAC4AS4APS4IC4IS4CO4MC. Therefore,

PAC was identied as the most proper coagulant for further

studies.

3.1.2. Optimization of the coagulant dosage

The dye removal is also studied by the UV –vis integral method

to identify the optimal coagulant dosage because of the followingreasons: (1) the PAC solution induces no visible color change;

(2) the organic occulant may contribute to the TOC value. Fig. 1

shows examples of multiple dyes removal from MDW at different

dye concentrations. For the 1000 ppm MDW treated by PAC, the dye

removal increases sharply with an increase in PAC dosage initially,

but nally approaches a plateau, which indicates that the maximum

dye removal by coagulation is around 97%. Mathematically, the

0

20

40

60

80

100

CPAM PAC PAC -CPAM

R d

( % )

0

20

40

60

80

100

APAM PAC PAC -APAM

0

20

40

60

80

100

PDDA PAC PAC -PDDA

0

20

40

60

80

100

CYGU PAC PAC -CYGU

R d

( % )

R d

( % )

R d

( % )

Fig. 2. Performance of PAC and different organic occulants to treat 1000 ppm MDW. The dosage of each PAC, APAM, CPAM, CYGU and PDDA was 400 ppm; For the PAC-

occulant combination, the dosage of each component was 400 ppm. Dye removal was based on the UV –vis integral method.

0.0

0.5

1.0

1.5

2.0

2.5

50

60

70

80

90

100

0 100 200 300 400

E c

R d ( % )

Dosage of PDDA (ppm)

Rd

Ec

Fig. 3. Effect of different dosages of PDDA combined with 400 ppm PAC to treat

1000 ppm MDW. E c refers to the overall normalized ef ciency of PAC and PDDA.

The dye removal was based on the UV –vis integral method.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306 – 315310


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removal ef ciency per gram of the coagulant (E c ) declines accord-

ingly. For the 100 ppm MDW treated by PAC, Fig. 1(b) shows that

the dye removal reaches a maximum when about 100–150 ppm

PAC is added and then decreases with a further increment in PAC

dosage. The exceeded amount of PAC might lead to the reverse of

surface charge of the coagulated particles/ocs, thus resulting in

particles re-stabilization and lowering the treatment ef ciency [41].

Before the dye removal reaches the maximum, the E c value of Fig. 1

(a) is always greater than that of Fig.1(b), which implies coagulationtaking place more favorably in a high dye concentration than a low

dye concentration. In order to achieve a high dye removal without

much compromising the coagulant ef ciency, we chose a PAC

dosage of 400 ppm because it can remove about 85% dyes with

an E c value of about 2.1 for further studies to identify a proper

occulant.

3.1.3. Identi cation of the suitable occulant

A typical coagulation/occulation process involves two stages of

mixing process; namely, rapid stirring (75–700 rpm) for 0.5–3 min in

order to achieve a good dispersion of the coagulant/occulant, and

slow stirring (30–150 rpm) for 5–30 min in order to propagate the

growth of ocs and minimize the breakdown of formed aggregates

[16,42]. In this work, the experiments associated with both coagulantand occulant, the rapid and slow stirring processes were combined

into one single stirring scheme; namely, stirring at 200 rpm for

1 min. Adopting the single stirring scheme is due to the following

reasons: (1) to simplify the process; (2) to allow the coagulation and

occulation take place together. Some organic polyelectrolytes, such

as CPAM and PDDA, exhibit the ability of functioning as both

coagulant and occulant, which is demonstrated in Fig. 2 with

following discussions. In terms of dyes removal by coagulation/

occulation, as depicted in Fig. 2(a) and (c), neither APAM nor CYGU

is effective in dye removal. The combination of PAC-APAM does not

make much improvement compared with PAC alone; while the

combination of PAC-CYGU produces antagonistic effect, which

reduces the ef cacy of PAC. As shown in Fig. 2(b) and (d), both

CPAM and PDDA alone are effective but with less ef ciency than PAC.The combinations of PAC-CPAM and PAC-PDDA generate synergetic

effects, which improve the total dye removal compared to that of PAC

alone. Both CPAM and PDDA are positive charged, but PDDA is

superior to CPAM according to its individual or combined perfor-

mance. Additionally, the suspension produced by PAC-PDDA was

easier to lter out than that by CPAM-PDDA. This is due to the fact

that CPAM possesses a huge molecular weight (800–1000 million)

which is prone to entangling with the lter paper and forming a gel-

like sticky lm that blocks the lter pores. In summary, PDDA was

identied as the most proper occulant among the 4 examined

occulants.

3.1.4. Optimization of the occulant dosage

The effects of different PDDA dosages on dye removal werestudied by xing the PAC dosage at 400 ppm. Fig. 3 displays that

the dye removal increases with an increase in PDDA dosage. This

might be attributed to: (1) PDDA possesses a high positive charge

density (as shown in Table 3) and water solubility. It can function

as an effective coagulant in the PAC-dye solution system; (2) PDDA

serves as a occulant that enhances the bridging mechanism for

coagulation and improves the aggregating capacity [16], thus

increasing the coagulation ability of PAC. However, the E c value

(i.e., the ef ciency per gram of the total coagulants (400 ppm

PACþPDDA)) decreases when the PDDA dosage increases. There-

fore, The CF settlement's particle size was investigated to optimize

the PDDA dosage.

Fig. 4 displays the particle size (or diameter) distributions of CF

settlements when 1000 ppm MDW was treated by 400 ppm PAC

without and with PDDA. The particle sizes were analyzed after CF

experiments and 1 h settling. As observed at the bottom of Fig. 4,

trace amounts of large-size particles in the range of 50–100 mm are

formed with the addition of PDDA. In contrast, there are no

particles with a size larger than 30 mm for the CF settlement

without adding PDDA. This is probably owing to the fact that the

ne precipitates formed by PAC are not able to grow bigger

without the aid of occulant (PDDA). As shown in the enlarged

picture of Fig. 4, the particle size distribution curve shifts towardthe right side when increasing the PDDA dosage. This suggests that

the mean particle size becomes larger with an increase in PDDA

dosage. Without PDDA, a mean particle size of 7.4 mm is produced

by PAC, the mean particle size increases to about 7.8, 9.2, 9.3, or

9.7 mm when adding 100, 200, 300 or 400 ppm PDDA, respectively.

The bigger the particle size, the easier the ltration is. It is found

that the combination of 400 ppm PAC and 200 ppm PDDA pro-

duces a settlement with a moderate size of 9.3 mm and an

acceptable dye removal of 90% with a relatively high normalized

ef ciency (E c ) of 1.5. Therefore, the (400/200 ppm) PAC/PDDA

dosage was chosen to study the effects of pH.

0

2

4

6

8

0 20 40 60 80 100

V o l u m e ( % )

Particle size (µm)

0

2

4

6

8

3 6 9 12 15

V o l u m e ( % )

Particle size (µm)

PDDA=0 ppm

PDDA=100 ppm

PDDA=200 ppm

PDDA=300 ppm

PDDA=400 ppm

Fig. 4. Particle size (or diameter) distributions of CF settlements when 1000 ppm

MDW was treated by 400 ppm PAC without and with PDDA.

0

2

4

6

8

10

12

0

20

40

60

80

100

0 2 4 6 8 10 12

F i n a l p H ( - )

R d ( % )

Initial pH (-)

Rd: PAC

Rd: PAC-PDDA

Final pH: PAC

Final pH: PAC-PDDA

Fig. 5. Effects of the initial pH on the dye removal for 1000 ppm MDW when using

PAC (400 ppm) and PAC-PDDA (400/200 ppm). Dye removal was based on the UV –vis

integral method.



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a result, the NF membrane shows excellent rejections for both

positively and negatively charged dye molecules.

Fig. 6 displays the permeate ux and dye removal as a function

of time for MDW. Two MDW concentrations were employed;

namely, 100 and 1000 ppm. The 1000 ppm MDW was treated

with and without the CF treatment. For all cases, the permeate ux

declines initially but becomes stable after 7 h, while the dye

removal remains above 99%. For the 100 ppm and 1000 ppm

MDW, they have almost the same initial permeate ux and declinein the same speed until the fourth hour. After the fourth hour, the

permeate ux of the 100 ppm sample tends to drop slightly,

whereas that of the 1000 ppm sample drops dramatically. After

7 h, both permeate uxes become stable, and the permeate ux of

the 100 ppm sample is approximately 20% higher than that of the

1000 ppm sample. These phenomena may be explainable as

follows: (1) the dyes adhered to the membrane surface and

partially blocked the surface pores in the beginning; (2) membrane

fouling due to the concentration polarization of the dyes became

serious for the 1000 ppm sample after 4 h. Since the higher the

dye concentration, the heavier the concentration polarization is

[44], the ux decline of the 100 ppm sample is mainly controlled

by membrane fouling, while that of the 1000 ppm sample is

affected by both membrane fouling and concentration polarization

fouling.

Since about 90% dyes were removed after the CF treatment, the

dye concentration in the CF treated sample is around 100 ppm. As

a result, the relationship between permeate ux vs. time of the CF

treated sample is nearly parallel with that of the 100 ppm MDW

sample, but the former has a lower ux (i.e., about 10%) than the

latter. The similar relationship between permeate ux vs. time

indicate that membrane fouling for both cases are similar due to

their similar dye concentrations. The lower ux of the CF treated

sample may arise from the osmotic pressure increase in the CF

treated sample. As shown in Fig. 7, about 24 ppm aluminum ion is

found in the CF treated solution. As a consequence, the effective

driving force across the NF membrane is reduced even though

they have the same transmembrane pressure [45,46]. Since the CF

treated sample has less fouling propensity, it has a higher ux thanthat of the 1000 ppm sample after the 7-h test (i.e., about 10%

higher). Clearly, applying the CF process as a pretreatment, not

only can it lower membrane fouling but also sustain the

permeate ux.

In terms of rejections, nearly 100% dyes were removed by the

NF for all the 3 cases over 8-h tests. The fate of added aluminum

was also studied for the CF–NF process. As shown in Fig. 7, about

58% of the total added aluminum (i.e., 57 ppm) reacted and

remained in the settlement as the solid stream, while around

41% dissolved and left in the treated solution as the feed stream for

the NF process, and about 0.5% nally went to the permeate ux as

the ef uent stream. Ideally, there should be no aluminum in the

NF feed stream after the CF process since the aluminum ion would

lead to an increase in osmotic pressure and ultimately a decrease

in effective driving force across the membrane. Additionally, the

less aluminum in the ef uent stream, the better it is since the

ef uent would eventually be discharged or reused.

The UV –vis was also used to conrm the above conclusion.

Fig. 8 displays the UV –vis spectra in a wavelength range between

200 and 800 nm for the original 1000 ppm MDW, the CF treated

MDW, and the NF permeate of the CF treated sample. The

absorbance decreases signicantly after the CF treatment because

of about 90% dyes removal at this stage. The UV absorbance drops

to almost zero after the NF process. This is consistent with theaforementioned results of almost 100% dye removal. The pictures

at the up right side of Fig. 8 show the color changes during the

processes. The original 1000 ppm MDW is deeply dark blue. After

the CF treatment, it becomes light blue, and then the color

completely disappears after the CF-NF treatment.

4. Conclusions

The treatments of synthetic dye wastewaters were studied by

applying coagulation/occulation (CF), nanoltration (NF) and the

combination of CF–NF. It is found that the removal of dye through

CF from single dye wastewater (SDW) depends on both dye and

coagulant. Some coagulants are very effective in treating certainSDW, but totally ineffective for the others. The ef ciency of

removing dyes from the highly concentrated multiple dyes waste-

water (MDW, 1000 ppm) by the 7 coagulants follows the order of

PAC4AS4APS4IC4IS4CO4MC. For the treatment of MDW,

polyaluminum chloride (PAC) is the best coagulant and polydial-

lyldimethyl ammonium chloride (PDDA) is the best occulant.

A dosage of 400/200 ppm PAC/PDDA is the optimal CF formulation,

which is able to remove about 90% of dyes from 1000 ppm MDW.

With the optimal dosage, the CF process is more favorable for dye

solutions with a higher concentration and pH.

The positively charged hollow ber NF membrane shows dye

removal of almost 100% for both anionic and cationic dyes. The NF

performance is also nearly independent of dye concentration with

a permeate ux of about 1.0 L m2

h1

under an operating

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Total Solid Feed Effluent

D i s t r i b u t i o n o f A l ( % )

C o n c e n t r a t i o n o f A l ( p p m )

C F s e t t l e m e n t s t r e a m

C F t r e a t e d M D W

N F p e r m e a t e

T o t a l a d d e d A l f o r C F

Fig. 7. The fate and distribution of aluminum during the CF–NF treatment process.

0

10

20

30

40

50

200 300 400 500 600 700 800

A

b s o r b a n c e

Wave length (nm)

Original

After CF

After CF-NF

a

bc

Fig. 8. The UV –vis absorbance spectra of (a) the original MDW (1000 ppm),

(b) solution after the CF treatment and (c) solution after the NF –NF treatment.

The UV –vis spectrum of the original MDW was obtained via diluting the original

MDW by 20 times, then the diluted sample was measured, the resultant absor-

bance was multiplied by a factor of 20.



9/10

pressure of 1 bar. Additionally, this NF membrane shows an

excellent rejection of trivalent ions (e.g., Al3þ).

The generation of concentrated stream and membrane fouling

is inevitable in the NF process, while the production of sludge and

the remaining strongly visible color are encountered in the CF

process. The combination of CF and NF can complement each

other's strengths and overcome their individual limitations. The

NF treatment can completely remove the color left in the CF

treated MDW, meanwhile the ef ciency of coagulant/occulant issignicantly enhanced by treating the concentrated streams of the

NF process. As a result, much less sludge is generated. In turn, by

applying CF process as a pretreatment method, the membrane

fouling is reduced and the NF permeate ux is increased. Those

benets and strengths lead to better performance for the treat-

ment of synthetic dye wastewater. Future works will aim at

developing a proper CF formulation and a higher ux positively

charged NF membrane for the treatment of real dye wastewater.

Acknowledgments

The authors would like to acknowledge the nancial supports

provided by National Research Foundation (NRF) of Singaporeunder its NRF Proof-of-Concept 8th Grant Call (NRF2012NRF-

POC001-059) for the project entitled ‘Development of advanced

nanoltration membranes for high removing rate of dyes in textile

wastewater’ (NUS grant number: R-279-000-389-281) as well as

the Singapore-MITAlliance for Research and Technology (SMART)

Centre under its Innovation Grant (ING12045-ENG) for the project

entitled ‘Development of robust high-performance nanoltration

membranes for textile wastewater treatment’ (NUS grant number:

R-279-000-377-592). Special thanks are given to Mr. Bai-Wang

Zhao for his supports during the preparation of this work.

Appendix. Supporting information

Supplementary data associated with this article can be found in

the online version at http://dx.doi.org/10.1016/j.memsci.2014.06.057.

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