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Performance evaluation of nanofiltration membranes for treatment of effluents containing reactive dye and salt
DESALINATION Desalination 130 (,2000) 177-183
ELSEVIER
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Performance evaluation of nanofiltration membranes for treatment of effluents containing reactive dye and salt Ratana Jiraratananon*,Anawat Sungpet, Piyanoot Luangsowan Department of Chemical Engineering, King Mongkut 's University of Technology, Thonburi, Bangkok 10140, Thailand Tel. +66 (2) 470-9221; Fax +66 (2) 428-3534; email: [email protected] Received 12 May 2000; accepted 12 June 2000
Abstract
The main purpose of this work is to investigate the preliminary performance of nanofiltration membranes for treatment of effluents containing salt and reactive dye. The membranes, designated as ES20, NTR-729HF, and LES90, were kindly supplied by the Nitto Denko Corporation. ES20 and LES90 are negatively charged membranes. NTR-729HF, a substituted poly(vinyl alcohol) membrane, is neutral. Regardless of the feeds, LES90 provided the highest permeate fluxes, while ES20 gave the lowest flux. ES20 and LES90 membranes achieved the higher salt rejection than NTR-729HF. LES90, however, suffered a great loss in the salt rejection as the concentration of NaC1 increased. The dye rejections of ES20 and LES90 are sufficient for the reuse of the permeate. The neutral NTR-729HF membrane exhibited lower dye retention than the charged membranes. For the feed containing salt and dye, the superposition of trans-membrane osmotic pressure and concentration polarization resulted in substantial flux decline. Nevertheless, with the presence of dye in the feed, the charged membranes were able to retain more salt. It was likely that a highly concentrated layer formed by the accumulation of dye near the membrane surface attributed to the higher salt rejection. An increase in the feed temperature contributed to a slight increase in the LES90 flux while the salt and dye rejections remained almost unchanged.
Keywords: Nanofiltration; Textile effluents; Dyeing; Reactive dyes; Salt
1. Introduction
Textile dyeing is a chemically intensive process and consumes large quantities of water. The wastewater generated usually contains *Corresponding author.
substances that are severely harmful to the environment. Dyes from dyeing operations are the major sources of color in textile effluent. In typical dyeing processes, 50-100% of the dye is fixed on the fiber, and the unfixed dyes are discharged in spent dye-baths or in wastewater from subsequent textile-washing operations.
0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S001 I-9164(00)00085-0
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Chemical auxiliaries such as salt and surfactants also contribute to aquatic toxicity. Reactive dyes are widely used to color cellulosic fibers such as cotton and rayon. Dyeing with reactive dyes, however, provokes serious environmental concerns. The process requires large amounts of salt to assist the dye exhaustion. Nevertheless, reactive dye classes typically exhibit poor fixation characteristics [1], leaving substantial amounts of dye in the wastewater. Some reactive dyes, particularly vinyl sulfone types, react with water. The hydrolyzed dyes are unreactive and quite adsorptive to the cellulose. As a consequence, large volumes of water, preferably at high temperature, are required in the post-washing step. Difficulties in the effluent treatment arise from the low level of aerobic biodegradation and/or adsorption of dye color onto activated sludge during treatment. Furthermore, removing large amounts of salt in the wastewater to regulation level is also unattainable by conventional treatment systems. Reactive dyeing consequently represents perhaps the greatest challenge to the textile industry in terms of minimizing color and salt discharges. Considerable research and development have been done to resolve the long-standing problem of reactive dyeing. One of the promising solutions is integration of membrane filtration into the dyeing process. With some modifications made to the dyeing process, a charged membrane was used to separate reactive dye residuals from the effluents while allowing sodium chloride to pass through the membrane [2]. Results obtained from a small membrane test cell indicated that the reuse of reactive dye liquors was technically feasible using ultrafiltration (UF). Experiences with water reclamation and reuse in the reactive dyeing of cotton by several techniques including membrane filtration were presented [3]. It was concluded that the membrane technology-based solution made possible the hot water reuse in rinsing and the reuse of dye bath water and salts.
Recently, nanofiltration (NF) and reverse osmosis (RO) systems were tested for the reuse of rinsing water from reactive dyeing of cotton [4]. The experiments, carried out in laboratory and pilot scale, suggested that NF permeate could be used for rinsing. The RO system was able to produce a higher quality permeate which could be used along with process water. The solute rejection of a NF membrane is principally based on steric and Donnan effects. The membrane, having a molecular weight cutoff in the range of 200--400 Dalton, can be classified as being intermediate between UF and RO membranes. Compared with RO membranes, NF membranes possess a looser structure. Because of this, NF membranes can be operated at a lower pressure while providing a high flux. The membranes are preferred for the separation of effluent containing microsolutes, including dyes, with molecular weights ranging approximately from 500 to 1000 Dalton. For an ionic dye class such as reactive dyes, a charged NF membrane also retains the dye through the Donnan effect. This allows the membrane to have a more open structure with a consequentially higher permeation flux while maintaining the dye retention. In the present work, commercially available NF membranes were tested with the effluents containing reactive dyes and salt. The experiments were carried out with the main purpose to investigate the preliminary performance of the membranes.
2. Experimental 2.1. Materials
Three types of flat-sheet membranes were kindly given by the Nitto Denko Corporation. The membranes are designated as ES20, LES90, and NTR-729HF. ES20, having a corrugated skin layer and large surface area, is designed to be
179
R. Jiraratananon et al. / Desalination 130 (2000) 177-183
operated at low pressure while giving flux comparable to earlier generation RO membranes. ES20 and LES90, composite membranes, are negatively charged. NTR-729HF is a substituted poly(vinyi alcohol) membrane with a neutral surface charge. Reactive red dye, commercially known as Benefix, was used. Laboratory-grade sodium chloride was used to study the effects of salt. RO-deionized water was used in all experiments. 2.2. Methods
The membrane testing system is shown in Fig. 1. The made-up feed entered a booster pump, which supplied the required head for the highpressure pump. Particles of larger than 5 lam in size were removed by the particulate filter, preventing damage to the high-pressure pump. Before entering the membrane module, the effluents were pressurized to the desired pressures, ranging from 5-16 bar. The membrane module was a thin-channel circular type,
providing a radial crossflow over the membrane surface. The membrane, with an area of 12.57 cm 2, rested on a metal-porous support. The permeate was periodically collected until the constant flux was obtained. The retentate was cooled down by a heat exchanger before circulating back to the feed tank. The experiments were performed with ROdeionized water and with 1 g/l and 10 g/l NaC1 aqueous feeds. The effect of the reactive dye on membrane performance was studied with feeds containing 180 and 300 ppm dye. The membranes were also tested with feeds consisting of both dye and sodium chloride. The composition of feed in the 70-1 feed tank essentially remained constant because the amount of permeate collected was relatively small. All experiments were done at a constant feed temperature of 35°C unless otherwise specified. Dye concentration was determined by UV-absorption at 571 nm using a Hach spectrophotometer (Model DR/3000). Sodium chloride concentration was obtained by measuring conductivity of the aqueous solution.
kd Heat exchanger
l
Valve Valve
r:%oro
Retentate
Feed tmak I Membrane module
Valve Heater
Booster Pump
Particulate Filter
Fig. 1. Schematic diagram of the nanofiltration system.
High Pressure Pump
Pemmate
180
R. Jiraratananon et al. / Desalination 130 (2000) 177-183
A small amount of dye in the sodium chloride solutions was found to insignificantly affect the measurement of salt concentration.
3. Results and discussion 3.1. RO-deionized water flux
The membranes were tested with ROdeionized water at various pressures. The results are shown in Table 1. The specific fluxes of ES20 and NTR-729HF (flux per net driving pressure) are approximately 5.1 and 7.21/m2.h. bar at 35°C, respectively. LES90 has a remarkably high specific flux of 10.81/m2.h.bar. The number becomes 12.61/mZ.h.bar, a 16.7% increase, as the feed temperature is raised from 35°C to 45°C. Note that both NTR-729HF and LES90 give higher fluxes than ES20, the low operating pressure membrane. This may be indicative of a more compact morphology of ES20. 3.2. Sodium chloride aqueous solution
The membranes were tested with the feeds containing 1 g/l and 10g/1 of sodium chloride at a constant pressure of 10 bar. The results are shown in Tables 2 and 3. The presence of sodium chloride in the feed leads to the reduction in permeate flux. The transmembrane salt concentration gives rise to transmembrane osmotic pressure, Ax, lowering the driving pressure. By taking account of the net driving pressure in the flux calculation, it can be shown that Ax is mainly responsible for the flux decline. The specific fluxes, calculated by noting that the net driving pressure is the applied pressure less Ax, are listed in Tables 2 and 3. For the most part, the specific fluxes are comparable to the pure water specific fluxes. It is therefore evident that the reduction of the net driving force is the underlying cause of the flux decline. However, with the 10 g/i NaC1 feed, the specific flux of LES90 is notably less than the pure water
specific flux. This indicates that LES90 significantly suffers from the concentration polarization. Because of the inherently high flux, LES90 still provides the largest specific flux. As the operating temperature increases from 35°C to 45°C, LES90 shows a slight increase in flux at the expense of salt rejection. NTR-729HF shows the lowest salt rejection (Tables 2 and 3). This comes at no surprise because the membrane has the neutral surface charge. The rejection is primarily based on size of the solute and morphology of the membrane. As the NaC1 concentration increases from 1 g/l to 10 g/l, the salt rejection becomes lower because of increasing sorption of salt into the membrane. The ability to reject salt of the negatively charged ES20 and LES90 membranes also becomes lower as the salt concentration increases. Unlike NTR-729HF, the Donnan effect most likely plays an important role in retaining salt in these membranes. The diffusion of salt through the charged membranes involves the electrochemical potentials in the membrane and solution phases. The C1- ions are retained by the negative charges of ES20 and LES90, while the Na ÷ ions can diffuse through the membranes only in conjunction with the CI- ions. Given a fixed charge density of the membranes, the Donnan exclusion, however, becomes less effective with increasing salt concentration in the feed. The higher concentration of the CI- ions in the feed solution contributes to an increase in the ionic or Donnan equilibrium of the CI- ions in the membranes. This results in the higher ionic flux through the membranes and, as a consequence, the lower salt rejection. LES90 suffers a greater loss in salt rejection compared with ES20, suggesting that ES20 possesses a higher fixed charge density. 3.3. Dye aqueous solution
The dye content in the dyeing wastewater varies greatly depending on the dyestuffs, the
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Table 1 Fluxes (l/m2.h) of pure water through the membranes at various pressures (bar) Membrane
Pressure, bar
ES20 NTR-729HF LES90 LES90 (45°C)
Specific flux, l/mZ.h.bar
5
8
10
13
16
24.8 ----
41.1 ----
52.5 71.6 113.4 126.5
-90.7 133.6 --
-119.3 171.8 --
5.1 7.2 10.8 12.6
Table 2 Performance of the membranes with the feed containing I g/l NaCI
Table 3 Performance of the membranes with the feed containing 10 g/l NaCI
Membrane
Flux, I/m2.h
Ag, bar
Specific Percent flux, rejection l/mZ.h.bar
Membrane
Flux, l/mZ.h
Ag, bar
Specific Percent flux, rejection l/m2.h.bar
ES20 NTR-729HF LES90 LES90 (45°C)
48.9 66.3 104.8 112.2
0.98 0.55 0.89 0.76
5.4 7.0 11.5 12.1
ES20 NTR-729HF LES90 LES90 (45°C)
16.9 36.8 40.1 41.1
6.78 4.60 5.80 5.58
5.2 6.8 9.5 9.3
90.1 61.2 89.3 80.9
fiber substrate, and the dyeing procedures. In the present work, the membranes were tested with the feeds consisting o f 180ppm and 300 ppm o f the reactive dye. The feed pressure was held constant at lObar. The performance o f the membranes is shown in Table 4. C o m p a r e d with the pure water fluxes, the permeate fluxes o f ES20 and LES90 decreased to some degree. The highly probable cause is the concentration polarization at the f e e d - m e m b r a n e interface. On the other hand, the permeate flux o f N T R - 7 2 9 H F , the neutral membrane, is unaffected by the presence o f dye in the feed stream. Much the same flux characteristic was observed in a test with effluent from a paper regeneration wastewater treatment plant [5]. There was also a report on the property o f neutral membrane
85.2 55.0 72.8 63.2
Table 4 Flux and rejection of the membranes as a function of dye concentration Membrane
Flux, l/m2.h
Percent rejection
180 ppm 300 ppm
180 ppm 300 ppm
ES20 40.6 NTR-729HF 71.4 LES90 108.2 LES90 120.6 (45oc)
40.3 71.3 107.4 118.2
96.2 81.3 95.9 94.7
97.5 84.9 95.8 97.1
against the absorption o f charged colloidal particles or charged organic molecules. The membrane, as a consequence, possessed a low-
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R. Jiraratananon et al. / Desalination 130 (2000) 177-183
fouling feature [6]. Then the neutral surface charge of NTR-729HF may contribute to the constant flux. Note that the permeate fluxes of all the membranes remain unchanged upon an increase in the dye concentration from 180 ppm to 300 ppm. The negatively charged membranes, ES20 and LES90, exhibit the dye rejection at roughly the same level, 95-97%, irrespective of the color concentration in the feed. The electrostatic interaction between the membranes and dye may promote the dye retention efficiency of these charged membranes. By contrast, the rejection of lower than 85°/'o is obtained from the neutral NTR-729HF membrane. 3.4. Sodium chloride and dye aqueous solution
The preliminary studies were done to determine the performance of the membranes in the decolorization of the effluents composed of salt and dye. The dye concentration in the feed was 300 ppm and the sodium chloride concentration was 10 g/l. The applied pressure was maintained at 10 bar for all experiments. The results are presented in Table 5. The permeate fluxes are lower than those obtained from the feeds containing only salt or dye. It is likely that the ES20 and LES90 membranes experience the superimposition effects of transmembrane osmotic pressure and dye concentration polarization. The presence of dye has an interesting impact on the salt rejection of ES20 and LES90. These membranes exhibit the higher salt rejection compared with those observed with the dye-free feeds. As discussed in the previous section, ES20 and LES90 undergo dye concentration polarization. A boundary layer formed by the retained dye near the membrane surface serves as a resistance to the permeation of salt. For a charged membrane, a solute is retained through a combination of steric and Donnan effects. The membrane properties taking part in
Table 5 Flux and rejection of the membranes with the feed containing 10 g/1NaCI and 300 ppm dye Membrane
Flux, 1/m:.h
Percent Percent dye NaCI rejection rejection
ES20 NTR-729HF LES90 LES90 (45°C)
11.0 26.5 37.2 30.2
94.8 54.1 86.3 91.7
97.8 82.5 96.0 97.4
the dye rejection therefore include the effective pore radius, the effective charge density, etc. [7]. These properties, the effective charge density in particular, responds to characteristics of the effluent being treated. For a salt-containing effluent, the cations can be adsorbed onto the membrane surface, shieldingthe negative groups. As a consequence, the Donnan effect is reduced. For a charged UF membrane, the decrease of dye rejection due to an increase in salt concentration was a case in point [8]. In the present work, the unchanging dye rejection in the presence of salt indicates that the retention of dye is mainly controlled by the steric effect. This is quite reasonable providing that the pore radius o f a NF membrane is typically smaller than the effective hydrodynamic radius of the dye. The membrane of choice for the water reclamation should effectively remove dyes to avoid possible staining of fabric. The color rejection of NTR-729HF is obviously much lower than an acceptable level for water reuse. Although ES20 and LES90 produce a permeate of comparable quality, the LES90 membrane provides a higher production rate.
4. Conclusions
Textile dyeing processes generate a large amount of water-based effluent containing
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pollutants such as dyes, electrolytes, and metals. Recent environmental attention and effluent discharge regulations have prompted increasing demands on the recovery of water, chemicals, and energy. NF membranes have promisingly shown the application in treating textile dyeing effluent where the high retention of dye is required. The present work intends to investigate the initial performance of commercially available NF membranes in treating the effluents consisting of reactive dye and salt. ES20 and LES90 are the negatively charged membranes and show higher NaCI rejection than the neutral NTR-729HF membrane. An increase in the salt concentration, however, reduces the retention ability of the membranes due to the decrease in the Donnan effect. The membranes also suffer from flux decline, primarily caused by the transmembrane osmotic pressure. The ES20 and LES90 membranes exhibit the decrement in volume flux which can be attributed to the concentration polarization of dye on the membrane surface, while the NTR-729HF membrane is unaffected by the presence of dye. However, the color rejection of NTR-729HF is lower than that of the negatively charged membranes. For the feed containing salt and dye, the effects of transmembrane osmotic pressure and concentration polarization are superimposed, resulting in the substantial flux decline. The presence of dye in the effluent significantly affects the salt removal effectiveness of ES20 and LES90. It is probable that the concentration build-up of dye near the membrane surface involves increasing salt retention of the mem-
183
branes. The ES20 and LES90 membranes can effectively retain color, producing the permeate with reuse possibility. At the same time the LES90 membrane achieves a significantly higher flux which makes it a more attractive alternative. Nevertheless, in-depth evaluation of this membrane is required prior to the implementation in industry.
Acknowledgements The authors gratefully acknowledge the kindness of the Nitto Denko Corporation for providing us with the membranes. We also would like to offer special thanks to Mr. Chaiwichit Hansuri and Mr. Chanyut Maneemat for their assistance and participation in this work.
References [1] S. Wagner, Improvementsin products and processing to diminish environmental impact, COTTECH Conference, Raleigh, NC, 1993. [2] A. Erswell, C.J. Brouckaert and C.A. Buckley, Desalination, 70 (1988) 157. [3] H. Wenzel, H.H. Knudsen, G.H. Kristensen and J. Hansen, Desalination, 106 (1996) 195. [4] J. Sojka-Ledakowicz, T. Koprowski, W. Machnowski and H.H. Knudsen, Desalination, 119 (1998) 1. [5] K.H. Ahn, H.Y. Cha, I.T. Yeom and K.G. Song, Desalination, 119 (1998) 169. [6] R. Gerard, H. Hachisuka and M. Hirose,Desalination, 119 (1998) 47. [7] W.R. Bowen and A.W. Mohammad, Desalination, 117 (1998) 257. [8] K. Majewska-Nowak, M. Kabsch-Korbutowicz and T. Winnicki, Desalination, 108 (1996) 221.
ELSEVIER
ww~.elsevier.com/Iocate/desal
Performance evaluation of nanofiltration membranes for treatment of effluents containing reactive dye and salt Ratana Jiraratananon*,Anawat Sungpet, Piyanoot Luangsowan Department of Chemical Engineering, King Mongkut 's University of Technology, Thonburi, Bangkok 10140, Thailand Tel. +66 (2) 470-9221; Fax +66 (2) 428-3534; email: [email protected] Received 12 May 2000; accepted 12 June 2000
Abstract
The main purpose of this work is to investigate the preliminary performance of nanofiltration membranes for treatment of effluents containing salt and reactive dye. The membranes, designated as ES20, NTR-729HF, and LES90, were kindly supplied by the Nitto Denko Corporation. ES20 and LES90 are negatively charged membranes. NTR-729HF, a substituted poly(vinyl alcohol) membrane, is neutral. Regardless of the feeds, LES90 provided the highest permeate fluxes, while ES20 gave the lowest flux. ES20 and LES90 membranes achieved the higher salt rejection than NTR-729HF. LES90, however, suffered a great loss in the salt rejection as the concentration of NaC1 increased. The dye rejections of ES20 and LES90 are sufficient for the reuse of the permeate. The neutral NTR-729HF membrane exhibited lower dye retention than the charged membranes. For the feed containing salt and dye, the superposition of trans-membrane osmotic pressure and concentration polarization resulted in substantial flux decline. Nevertheless, with the presence of dye in the feed, the charged membranes were able to retain more salt. It was likely that a highly concentrated layer formed by the accumulation of dye near the membrane surface attributed to the higher salt rejection. An increase in the feed temperature contributed to a slight increase in the LES90 flux while the salt and dye rejections remained almost unchanged.
Keywords: Nanofiltration; Textile effluents; Dyeing; Reactive dyes; Salt
1. Introduction
Textile dyeing is a chemically intensive process and consumes large quantities of water. The wastewater generated usually contains *Corresponding author.
substances that are severely harmful to the environment. Dyes from dyeing operations are the major sources of color in textile effluent. In typical dyeing processes, 50-100% of the dye is fixed on the fiber, and the unfixed dyes are discharged in spent dye-baths or in wastewater from subsequent textile-washing operations.
0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S001 I-9164(00)00085-0
178
R. .liraratananon et al. / Desalination 130 (2000) 177-183
Chemical auxiliaries such as salt and surfactants also contribute to aquatic toxicity. Reactive dyes are widely used to color cellulosic fibers such as cotton and rayon. Dyeing with reactive dyes, however, provokes serious environmental concerns. The process requires large amounts of salt to assist the dye exhaustion. Nevertheless, reactive dye classes typically exhibit poor fixation characteristics [1], leaving substantial amounts of dye in the wastewater. Some reactive dyes, particularly vinyl sulfone types, react with water. The hydrolyzed dyes are unreactive and quite adsorptive to the cellulose. As a consequence, large volumes of water, preferably at high temperature, are required in the post-washing step. Difficulties in the effluent treatment arise from the low level of aerobic biodegradation and/or adsorption of dye color onto activated sludge during treatment. Furthermore, removing large amounts of salt in the wastewater to regulation level is also unattainable by conventional treatment systems. Reactive dyeing consequently represents perhaps the greatest challenge to the textile industry in terms of minimizing color and salt discharges. Considerable research and development have been done to resolve the long-standing problem of reactive dyeing. One of the promising solutions is integration of membrane filtration into the dyeing process. With some modifications made to the dyeing process, a charged membrane was used to separate reactive dye residuals from the effluents while allowing sodium chloride to pass through the membrane [2]. Results obtained from a small membrane test cell indicated that the reuse of reactive dye liquors was technically feasible using ultrafiltration (UF). Experiences with water reclamation and reuse in the reactive dyeing of cotton by several techniques including membrane filtration were presented [3]. It was concluded that the membrane technology-based solution made possible the hot water reuse in rinsing and the reuse of dye bath water and salts.
Recently, nanofiltration (NF) and reverse osmosis (RO) systems were tested for the reuse of rinsing water from reactive dyeing of cotton [4]. The experiments, carried out in laboratory and pilot scale, suggested that NF permeate could be used for rinsing. The RO system was able to produce a higher quality permeate which could be used along with process water. The solute rejection of a NF membrane is principally based on steric and Donnan effects. The membrane, having a molecular weight cutoff in the range of 200--400 Dalton, can be classified as being intermediate between UF and RO membranes. Compared with RO membranes, NF membranes possess a looser structure. Because of this, NF membranes can be operated at a lower pressure while providing a high flux. The membranes are preferred for the separation of effluent containing microsolutes, including dyes, with molecular weights ranging approximately from 500 to 1000 Dalton. For an ionic dye class such as reactive dyes, a charged NF membrane also retains the dye through the Donnan effect. This allows the membrane to have a more open structure with a consequentially higher permeation flux while maintaining the dye retention. In the present work, commercially available NF membranes were tested with the effluents containing reactive dyes and salt. The experiments were carried out with the main purpose to investigate the preliminary performance of the membranes.
2. Experimental 2.1. Materials
Three types of flat-sheet membranes were kindly given by the Nitto Denko Corporation. The membranes are designated as ES20, LES90, and NTR-729HF. ES20, having a corrugated skin layer and large surface area, is designed to be
179
R. Jiraratananon et al. / Desalination 130 (2000) 177-183
operated at low pressure while giving flux comparable to earlier generation RO membranes. ES20 and LES90, composite membranes, are negatively charged. NTR-729HF is a substituted poly(vinyi alcohol) membrane with a neutral surface charge. Reactive red dye, commercially known as Benefix, was used. Laboratory-grade sodium chloride was used to study the effects of salt. RO-deionized water was used in all experiments. 2.2. Methods
The membrane testing system is shown in Fig. 1. The made-up feed entered a booster pump, which supplied the required head for the highpressure pump. Particles of larger than 5 lam in size were removed by the particulate filter, preventing damage to the high-pressure pump. Before entering the membrane module, the effluents were pressurized to the desired pressures, ranging from 5-16 bar. The membrane module was a thin-channel circular type,
providing a radial crossflow over the membrane surface. The membrane, with an area of 12.57 cm 2, rested on a metal-porous support. The permeate was periodically collected until the constant flux was obtained. The retentate was cooled down by a heat exchanger before circulating back to the feed tank. The experiments were performed with ROdeionized water and with 1 g/l and 10 g/l NaC1 aqueous feeds. The effect of the reactive dye on membrane performance was studied with feeds containing 180 and 300 ppm dye. The membranes were also tested with feeds consisting of both dye and sodium chloride. The composition of feed in the 70-1 feed tank essentially remained constant because the amount of permeate collected was relatively small. All experiments were done at a constant feed temperature of 35°C unless otherwise specified. Dye concentration was determined by UV-absorption at 571 nm using a Hach spectrophotometer (Model DR/3000). Sodium chloride concentration was obtained by measuring conductivity of the aqueous solution.
kd Heat exchanger
l
Valve Valve
r:%oro
Retentate
Feed tmak I Membrane module
Valve Heater
Booster Pump
Particulate Filter
Fig. 1. Schematic diagram of the nanofiltration system.
High Pressure Pump
Pemmate
180
R. Jiraratananon et al. / Desalination 130 (2000) 177-183
A small amount of dye in the sodium chloride solutions was found to insignificantly affect the measurement of salt concentration.
3. Results and discussion 3.1. RO-deionized water flux
The membranes were tested with ROdeionized water at various pressures. The results are shown in Table 1. The specific fluxes of ES20 and NTR-729HF (flux per net driving pressure) are approximately 5.1 and 7.21/m2.h. bar at 35°C, respectively. LES90 has a remarkably high specific flux of 10.81/m2.h.bar. The number becomes 12.61/mZ.h.bar, a 16.7% increase, as the feed temperature is raised from 35°C to 45°C. Note that both NTR-729HF and LES90 give higher fluxes than ES20, the low operating pressure membrane. This may be indicative of a more compact morphology of ES20. 3.2. Sodium chloride aqueous solution
The membranes were tested with the feeds containing 1 g/l and 10g/1 of sodium chloride at a constant pressure of 10 bar. The results are shown in Tables 2 and 3. The presence of sodium chloride in the feed leads to the reduction in permeate flux. The transmembrane salt concentration gives rise to transmembrane osmotic pressure, Ax, lowering the driving pressure. By taking account of the net driving pressure in the flux calculation, it can be shown that Ax is mainly responsible for the flux decline. The specific fluxes, calculated by noting that the net driving pressure is the applied pressure less Ax, are listed in Tables 2 and 3. For the most part, the specific fluxes are comparable to the pure water specific fluxes. It is therefore evident that the reduction of the net driving force is the underlying cause of the flux decline. However, with the 10 g/i NaC1 feed, the specific flux of LES90 is notably less than the pure water
specific flux. This indicates that LES90 significantly suffers from the concentration polarization. Because of the inherently high flux, LES90 still provides the largest specific flux. As the operating temperature increases from 35°C to 45°C, LES90 shows a slight increase in flux at the expense of salt rejection. NTR-729HF shows the lowest salt rejection (Tables 2 and 3). This comes at no surprise because the membrane has the neutral surface charge. The rejection is primarily based on size of the solute and morphology of the membrane. As the NaC1 concentration increases from 1 g/l to 10 g/l, the salt rejection becomes lower because of increasing sorption of salt into the membrane. The ability to reject salt of the negatively charged ES20 and LES90 membranes also becomes lower as the salt concentration increases. Unlike NTR-729HF, the Donnan effect most likely plays an important role in retaining salt in these membranes. The diffusion of salt through the charged membranes involves the electrochemical potentials in the membrane and solution phases. The C1- ions are retained by the negative charges of ES20 and LES90, while the Na ÷ ions can diffuse through the membranes only in conjunction with the CI- ions. Given a fixed charge density of the membranes, the Donnan exclusion, however, becomes less effective with increasing salt concentration in the feed. The higher concentration of the CI- ions in the feed solution contributes to an increase in the ionic or Donnan equilibrium of the CI- ions in the membranes. This results in the higher ionic flux through the membranes and, as a consequence, the lower salt rejection. LES90 suffers a greater loss in salt rejection compared with ES20, suggesting that ES20 possesses a higher fixed charge density. 3.3. Dye aqueous solution
The dye content in the dyeing wastewater varies greatly depending on the dyestuffs, the
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R. Jiraratananon et al. /Desalination 130 (2000) 177-183
Table 1 Fluxes (l/m2.h) of pure water through the membranes at various pressures (bar) Membrane
Pressure, bar
ES20 NTR-729HF LES90 LES90 (45°C)
Specific flux, l/mZ.h.bar
5
8
10
13
16
24.8 ----
41.1 ----
52.5 71.6 113.4 126.5
-90.7 133.6 --
-119.3 171.8 --
5.1 7.2 10.8 12.6
Table 2 Performance of the membranes with the feed containing I g/l NaCI
Table 3 Performance of the membranes with the feed containing 10 g/l NaCI
Membrane
Flux, I/m2.h
Ag, bar
Specific Percent flux, rejection l/mZ.h.bar
Membrane
Flux, l/mZ.h
Ag, bar
Specific Percent flux, rejection l/m2.h.bar
ES20 NTR-729HF LES90 LES90 (45°C)
48.9 66.3 104.8 112.2
0.98 0.55 0.89 0.76
5.4 7.0 11.5 12.1
ES20 NTR-729HF LES90 LES90 (45°C)
16.9 36.8 40.1 41.1
6.78 4.60 5.80 5.58
5.2 6.8 9.5 9.3
90.1 61.2 89.3 80.9
fiber substrate, and the dyeing procedures. In the present work, the membranes were tested with the feeds consisting o f 180ppm and 300 ppm o f the reactive dye. The feed pressure was held constant at lObar. The performance o f the membranes is shown in Table 4. C o m p a r e d with the pure water fluxes, the permeate fluxes o f ES20 and LES90 decreased to some degree. The highly probable cause is the concentration polarization at the f e e d - m e m b r a n e interface. On the other hand, the permeate flux o f N T R - 7 2 9 H F , the neutral membrane, is unaffected by the presence o f dye in the feed stream. Much the same flux characteristic was observed in a test with effluent from a paper regeneration wastewater treatment plant [5]. There was also a report on the property o f neutral membrane
85.2 55.0 72.8 63.2
Table 4 Flux and rejection of the membranes as a function of dye concentration Membrane
Flux, l/m2.h
Percent rejection
180 ppm 300 ppm
180 ppm 300 ppm
ES20 40.6 NTR-729HF 71.4 LES90 108.2 LES90 120.6 (45oc)
40.3 71.3 107.4 118.2
96.2 81.3 95.9 94.7
97.5 84.9 95.8 97.1
against the absorption o f charged colloidal particles or charged organic molecules. The membrane, as a consequence, possessed a low-
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fouling feature [6]. Then the neutral surface charge of NTR-729HF may contribute to the constant flux. Note that the permeate fluxes of all the membranes remain unchanged upon an increase in the dye concentration from 180 ppm to 300 ppm. The negatively charged membranes, ES20 and LES90, exhibit the dye rejection at roughly the same level, 95-97%, irrespective of the color concentration in the feed. The electrostatic interaction between the membranes and dye may promote the dye retention efficiency of these charged membranes. By contrast, the rejection of lower than 85°/'o is obtained from the neutral NTR-729HF membrane. 3.4. Sodium chloride and dye aqueous solution
The preliminary studies were done to determine the performance of the membranes in the decolorization of the effluents composed of salt and dye. The dye concentration in the feed was 300 ppm and the sodium chloride concentration was 10 g/l. The applied pressure was maintained at 10 bar for all experiments. The results are presented in Table 5. The permeate fluxes are lower than those obtained from the feeds containing only salt or dye. It is likely that the ES20 and LES90 membranes experience the superimposition effects of transmembrane osmotic pressure and dye concentration polarization. The presence of dye has an interesting impact on the salt rejection of ES20 and LES90. These membranes exhibit the higher salt rejection compared with those observed with the dye-free feeds. As discussed in the previous section, ES20 and LES90 undergo dye concentration polarization. A boundary layer formed by the retained dye near the membrane surface serves as a resistance to the permeation of salt. For a charged membrane, a solute is retained through a combination of steric and Donnan effects. The membrane properties taking part in
Table 5 Flux and rejection of the membranes with the feed containing 10 g/1NaCI and 300 ppm dye Membrane
Flux, 1/m:.h
Percent Percent dye NaCI rejection rejection
ES20 NTR-729HF LES90 LES90 (45°C)
11.0 26.5 37.2 30.2
94.8 54.1 86.3 91.7
97.8 82.5 96.0 97.4
the dye rejection therefore include the effective pore radius, the effective charge density, etc. [7]. These properties, the effective charge density in particular, responds to characteristics of the effluent being treated. For a salt-containing effluent, the cations can be adsorbed onto the membrane surface, shieldingthe negative groups. As a consequence, the Donnan effect is reduced. For a charged UF membrane, the decrease of dye rejection due to an increase in salt concentration was a case in point [8]. In the present work, the unchanging dye rejection in the presence of salt indicates that the retention of dye is mainly controlled by the steric effect. This is quite reasonable providing that the pore radius o f a NF membrane is typically smaller than the effective hydrodynamic radius of the dye. The membrane of choice for the water reclamation should effectively remove dyes to avoid possible staining of fabric. The color rejection of NTR-729HF is obviously much lower than an acceptable level for water reuse. Although ES20 and LES90 produce a permeate of comparable quality, the LES90 membrane provides a higher production rate.
4. Conclusions
Textile dyeing processes generate a large amount of water-based effluent containing
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pollutants such as dyes, electrolytes, and metals. Recent environmental attention and effluent discharge regulations have prompted increasing demands on the recovery of water, chemicals, and energy. NF membranes have promisingly shown the application in treating textile dyeing effluent where the high retention of dye is required. The present work intends to investigate the initial performance of commercially available NF membranes in treating the effluents consisting of reactive dye and salt. ES20 and LES90 are the negatively charged membranes and show higher NaCI rejection than the neutral NTR-729HF membrane. An increase in the salt concentration, however, reduces the retention ability of the membranes due to the decrease in the Donnan effect. The membranes also suffer from flux decline, primarily caused by the transmembrane osmotic pressure. The ES20 and LES90 membranes exhibit the decrement in volume flux which can be attributed to the concentration polarization of dye on the membrane surface, while the NTR-729HF membrane is unaffected by the presence of dye. However, the color rejection of NTR-729HF is lower than that of the negatively charged membranes. For the feed containing salt and dye, the effects of transmembrane osmotic pressure and concentration polarization are superimposed, resulting in the substantial flux decline. The presence of dye in the effluent significantly affects the salt removal effectiveness of ES20 and LES90. It is probable that the concentration build-up of dye near the membrane surface involves increasing salt retention of the mem-
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branes. The ES20 and LES90 membranes can effectively retain color, producing the permeate with reuse possibility. At the same time the LES90 membrane achieves a significantly higher flux which makes it a more attractive alternative. Nevertheless, in-depth evaluation of this membrane is required prior to the implementation in industry.
Acknowledgements The authors gratefully acknowledge the kindness of the Nitto Denko Corporation for providing us with the membranes. We also would like to offer special thanks to Mr. Chaiwichit Hansuri and Mr. Chanyut Maneemat for their assistance and participation in this work.
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