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Performance study of ZrO2 ceramic micro-filtration membranes used in pretreatment of DMF wastewater
Desalination 346 (2014) 1–8
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Performance study of ZrO2 ceramic micro-filtration membranes used in pretreatment of DMF wastewater Qi Zhang a, Rong Xu a, Pengwei Xu a, Ruoyu Chen a, Qiang He a, Jing Zhong a,⁎, Xuehong Gu b a b
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, Jiangsu, China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
H I G H L I G H T S • 19-Channel ZrO2 microfiltration membrane was used in pretreatment of DMF wastewater. • 0.2 μm ZrO2 membrane can effectively remove fine particles in DMF wastewater. • Combined flushing, ultrasonic and chemical cleaning was effective for flux recovery.
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 20 April 2014 Accepted 5 May 2014 Available online 22 May 2014 Keywords: DMF wastewater ZrO2 micro-filtration membrane Pretreatment Backflushing process Membrane fouling
a b s t r a c t Three kinds of 19-channel ZrO2 micro-filtration membranes with different pore size were used for pretreatment of dimethylformamide (DMF) wastewater from the polyurethane (PU) synthetic leather factories, which would enter the distillation column for further purification. The effects of membrane pore size, cross flow velocity (CFV) and transmembrane pressure (TMP) on ceramic membrane filtration performances were investigated. Experimental results indicated that the optimum membrane pore size, CFV and TMP were 0.2 μm, 3 m·s−1 and 0.2 MPa, respectively, with the corresponding liquid turbidity removal rate of 99.62% and suspended solid content retention rate of 99.99%. The membrane fouling mechanism was analyzed by resistance in series model. The main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of particle adsorption and sedimentation resistance to total resistance was above 70% for the 0.2 μm membrane. The backflushing technology was applied and the effect of backflushing on the permeate flux was studied. The permeate flux eventually increased by 50% with the optimum backflushing pressure of 0.6 MPa, backflushing time of 5 s and backflushing interval of 20 min. A combination cleaning method was used to regenerate the ZrO2 micro-filtration membrane. The pure water flux recovered to 608.2 L·m−2·h−1 and flux recovery rate was 96.8% which indicated that the combination cleaning method was effective. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dimethylformamide is an important solvent which is primarily used in the production of polyurethane synthetic leather [1]. In China alone, the emission amount of DMF wastewater from leather factories is up to 1 billion tons a year, which seriously harm the environment because of its high toxicity and poor biodegradation. Therefore, the recovery of DMF is very essential. At present, the main method for DMF recovery from wastewater is two-tower distillation including atmospheric distillation and vacuum distillation. However, the suspended solid content and turbidity of DMF wastewater from the PU synthetic leather factories are very high. Besides, DMF wastewater contains fine particles (soft salt, fibers, dusts, etc.) [2], which would lead to the blockage of equipment pipes and distillation columns and thus increase the cleaning times for ⁎ Corresponding author. Tel.: +86 519 86330330. E-mail address: [email protected] (J. Zhong).
http://dx.doi.org/10.1016/j.desal.2014.05.006 0011-9164/© 2014 Elsevier B.V. All rights reserved.
distillation equipment. Therefore, the pretreatment of DMF wastewater is very necessary to provide a high-quality feed for further purification in distillation columns. The conventional pretreatment system relies on sedimentation, flocculation or sand filtration processes [3]. However, these processes show a limited separation effect because the size of fine particles in DMF wastewater is less than 2 μm. Abundant literature has reported that the fine particles could be effectively removed from the wastewater by membrane filtration technology. Ceramic membrane filtration based on the screening mechanism can effectively reject micron- and nano-sized particles. Cui et al. [4] reported a pre-treatment process using 50 nm ZrO2/Al2O3 ceramic membranes for the desalination of seawater. The ceramic membrane system maintained a stable permeability throughout the long-term experiment. The permeate turbidity and removal ratio produced by the multi-channel ceramic membrane were 0.076 NTU and 99.2% respectively. Agana et al. [5] applied a 50 nm ZrO2 ceramic ultrafiltration membrane to wastewater containing
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cathodic electrodeposition paint particles. The crossflow velocity of 3.2 m·s− 1 and transmembrane pressure of 100 kPa produced the highest steady-state permeate flux and the optimum energy consumption. Turbidity rejection rates of all experiments were larger than 99.95%. It was demonstrated that a stable, high-quality feed was obtained by ceramic membrane pre-treatment process, which can provide an economical and reliable separation for DMF wastewater due to its excellent selectivity, permeability and thermal and chemical stability [6]. However, due to concentration polarization and membrane fouling, the permeate flux of the micro-filtration (MF) process decreases with time [7,8], which will significantly diminish the filtration performance during the pretreatment of DMF wastewater. Fine particles in DMF wastewater would accumulate on the membrane surface and form a cake layer. At the same time, some particles may adsorb on or block the membrane pores. Now, the dominant fouling cause and the physical or chemical interactions between these foulants are not clear, which are critical for membrane permeability and cleaning strategies [9]. The decline of membrane flux can be converted into the membrane resistance [10]. Therefore in order to investigate the membrane fouling mechanism, membrane resistances were analyzed in terms of resistance-inseries model which would guide the choice of methods for membrane regeneration [11,12]. In order to maintain high efficiency for the filtration of DMF wastewater, membrane regeneration is an integral part of the microfiltration systems and effective membrane cleaning is necessary [13]. Numerous attempts have been made to reduce concentration polarization and control membrane fouling by mechanical (backflushing) and chemical cleaning [14]. Among them, backflushing technology is the most effective for ceramic membranes. Unlike polymeric membranes, ceramic membranes are able to withstand the high pressures associated with backflushing. However, it should be noted that although the membrane fouling that caused by caking is reduced, backflushing could not remove severe pore blocking. Gabrus et al. [15] investigated the application of backflushing to microfiltration of yeast suspension with inorganic TiO2/Al2O3 MF membranes. They found that backflushing could efficiently remove surface deposits but less effective in removal of internal pore fouling. This type of irreversible fouling causing by particles inside the membrane pores may only be eliminated by chemical cleaning. This work was done to explore the pretreatment processes of DMF wastewater from PU synthetic leather factories using ZrO2 microfiltration membranes. The objective of this work was to investigate the effects of membrane pore size, cross flow velocity and transmembrane pressure on ceramic membrane filtration performances. The focus is on optimizing DMF wastewater pretreatment process conditions and combining with resistance in series model to explore the membrane fouling mechanism, and on determining effective membrane regeneration cleaning methods. This study can provide an insight into the applicability of ZrO2 microfiltration membrane for pretreatment of DMF wastewater.
Fig. 1. The particle size distribution of the DMF wastewater.
quantified with a particle size analyzer (ZEN3600, Malvern, UK). The average particle size of DMF wastewater was 1.38 μm. 2.2. Ceramic membranes Three kinds of 19-channel ZrO2/Al2O3 composite micro-filtration membranes with different pore sizes (50 nm, 0.2 μm and 0.5 μm) were used in this study supplied by Jiangsu Jiuwu High-Tech Co., Ltd. (Nanjing, China). The dimensions of these membranes are presented as follows: channel diameter, 4 mm; length, 200 mm; porosity, 35%; effective membrane area, 477 cm2. The isoelectric point of the ZrO2 active layer is approximately 7 and therefore the membrane surface charge is positive or negative at pH b7 or N7, respectively. 2.3. Ceramic MF system The schematic diagram of ceramic membrane cross-flow microfiltration for pretreatment of DMF wastewater and backflushing process is shown in Fig. 2. The volume of the feed tank jacketed for temperature control was 50 L. The DMF wastewater was continuously pumped into the ceramic membrane module with a centrifugal pump. The operating transmembrane pressure and crossflow velocity were adjusted by valves before and after the membrane module. The permeate and retentate were recirculated continuously into the feed tank. The backflushing arrangement consisted of a gas buffer reservoir and an air compressor. The temperature of the feed solution was controlled at 298.15 K by a thermostatic bath. The effects of cross flow velocity (0.5 m·s−1, 0.8 m·s−1, 1.0 m·s−1, 2.0 m·s−1, 3.0 m·s−1) and transmembrane pressure (0.08 MPa, 0.12 MPa, 0.16 MPa, 0.20 MPa, 0.24 MPa) on ceramic membrane filtration performance were investigated. The permeate flux was determined by collecting the permeate sample in a graduated cylinder at specific time intervals.
2. Materials and methods
2.4. Analysis of membrane fouling
2.1. DMF wastewater
In order to investigate the membrane fouling, the permeate flux of DMF wastewater micro-filtration process was used to analyze membrane resistances using resistance-in-series model. Four membrane resistances commonly exist according to the characteristics of the micro-sized particle system, which are: Rm (the inherent resistance of the clean membrane which depends on its pore size and material, Eq. (1-1)), Rc (the resistance resulting from the concentration polarization, Eq. (1-2)), Ri (the resistance resulting from the membrane pores blocking, Eq. (1-3)) and Rg (the resistance resulting from the particle adsorption and sedimentation on membrane surface, Eq. (1-4)).
The DMF wastewater (~20 wt.% of DMF) is from a synthetic leather company in Jiangsu including some fine particle and fibers. The characteristics of the DMF wastewater are given in Table 1. The particle size distribution of the DMF wastewater is shown in Fig. 1, which was Table 1 Characteristics of the DMF wastewater. Parameters
pH
Conductivity (μS·cm−1)
Suspended solid content (mg·L−1)
Turbidity (NTU)
Average size (μm)
Value
7.68
698
408
452.50
1.38
Rm ¼
ΔP μ w J0
ð1 1Þ
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3
Fig. 2. Schematic diagram of cross-flow microfiltration apparatus. (1) feed tank; (2) centrifugal pump; (3) flow meter; (4) membrane module; (5) buffer tank; (6) air compressor; V1–V7: valves.
Rc ¼
ΔP ΔP − μ J1 μ w J2
ð1 2Þ
Ri ¼
ΔP ΔP − μ w J3 μ w J0
ð1 3Þ
Rg ¼
ΔP ΔP − : μ w J2 μ w J3
ð1 4Þ
In these equations, J0 is the initial pure water flux; J1 is the permeate flux after DMF wastewater filtration; J2 is the pure water flux after the membrane fouling; J3 is the pure water flux after physical cleaning; μw and μ are the viscosity of pure water and DMF wastewater and their values at 25 °C are 0.960 mPa·s and 1.779 mPa·s respectively. 2.5. Backflushing process Backflushing is an in situ cleaning method involving reverse flow of permeate by applying higher pressure on the permeate side [10]. The permeate backflushing in pretreatment of DMF wastewater was applied
by air pressure, as shown schematically in Fig. 2. During backflushing, the reverse flow through the membrane removed the concentration polarization and cake layers from the membrane surface. In the present study, the effects of three important factors in the backflushing process were investigated: (1) backflushing pressure (from 0.3 to 0.6 MPa); (2) backflushing time (from 3 to 9 s); and (3) backflushing interval (from 10 to 40 min).
2.6. Membrane cleaning The membrane cleaning methods are classified into physical method and chemical method. Both types of membrane cleaning method are aimed at recovering membrane permeability. In order to obtain the agreement of cleaning conditions, the ZrO2 microfiltration membrane was used for pretreatment of DMF wastewater after 2 h MF process with TMP, CFV and temperature at 0.12 MPa, 3 m·s−1 and 25 °C. The physical cleaning consisted of deionized water flushing, ultrasonic and tap water cleaning. Ultrasonic cleaning is an effective physical cleaning method. The membrane module was submerged in the ultrasonic bath (KQ2200DB, Kunshan Shumei Ultrasonic Instruments Co., Ltd, China) equipped with a powerful generator with 100 W output and 40 kHz frequency. Systematic investigation was performed on the influence of ultrasonic power, ultrasonic temperature and ultrasonic time. The composition of the chemical cleaning solution was 0.5 wt.% HCl 0.5 wt.% NaOH and 0.02 wt.% NaClO. Chemical cleaning parameters were set at pressure of 0.08 MPa, CFV of 3.0 m·s− 1, temperature range of 25– 28 °C and cleaning time of 30 min. After membrane cleaning, the pure water flux was measured and compared with the initial value.
Table 2 Effect of pore size on the permeate qualities.
Fig. 3. Effect of pore size on permeate flux (TMP: 0.12 MPa, CFV: 3 m·s−1, T: 25 °C).
Pore size (μm)
Turbidity (NTU)
Turbidity removal rate (%)
Suspended solid content (mg·L−1)
0.5 0.2 0.05
12.8 1.7 1.4
97.17 99.62 99.69
0.30 0.01 b0.01
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Fig. 4. Effect of pore size on the particle size distribution of the permeate.
3. Results and discussion 3.1. Effect of membrane pore size Membrane pore size is one of the most important parameters in the MF process. The initial permeabilities of ZrO2 microfiltration membranes with different pore sizes (50 nm, 0.2 μm and 0.5 μm) were determined. The initial pure water flux for 0.5 μm ZrO2 membrane is 1047 L·m−2·h− 1, 0.2 μm ZrO2 membrane is 628 L·m−2·h− 1 and 50 nm ZrO2 membrane is 363 L·m− 2 · h− 1 under the conditions of TMP 0.12 MPa, CFV 3 m·s−1 and temperature 25 °C. The effects of membrane pore size (50 nm, 0.2 and 0.5 μm) on the permeate flux and the permeate water qualities are shown in Fig. 3 and Table 2. The particle size distribution of the permeate is also shown in Fig. 4. It is interesting that, contrary to our expectations, the bigger pore size did not always provide the higher permeate flux. As shown in Fig. 1, it appears that a fraction of the particles in DMF wastewater are between 0.2 and 0.5 μm, this fraction could cause severe pore blocking and thus a higher permeate flux decrease for membrane with pore size of 0.5 μm in comparison with other membranes. The 0.2 μm membrane showed better effects in permeate flux, turbidity removal rate and suspended solid content. The liquid turbidity of the DMF
wastewater reduced from 452.5 to 1.7 NTU and the suspended solids content varied from 408 to 0.01 mg·L− 1. The permeate flux of the 0.2 μm membrane was very close to that of the 0.5 μm membrane. Meanwhile, the turbidity removal rate was 99.62% of the 0.2 μm membrane which nearly achieved that of the 50 nm membrane. Table 3 shows the effect of pore size on membrane resistances and relative percentages in pretreatment of DMF wastewater. It can be seen that the 50 nm membrane shows the highest total resistance (Rt = 14.82 × 1012 m− 1), and the resistances for the membranes of 0.2 and 0.5 μm are very close (Rt = 7.16 × 1012 m−1 and Rt = 7.98 × 1012 m−1). The concentration polarization resistance Rc increased with decreasing membrane pore size. The fouling resistance Rg of the 0.2 μm membrane accounted for 75.84% of the total resistances. It is concluded that the main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of pore blocking resistance to total resistance was 19.03% for the 50 nm membrane, which was greater than that of the 0.2 μm membrane, but less than that of the 0.5 μm membrane. As shown in Fig. 1, there is a fraction of the particles lower than 0.2 μm. These particles in DMF wastewater would penetrate into the membrane pores of 0.2 μm and 50 nm, resulting in internal pore blockage. Moreover the smaller the membrane pores, the more difficult for the particles to penetrate them, resulting in more severe pore blocking. Therefore, the ratio of pore blocking resistance to total resistance for the 50 nm membrane was greater than that of the 0.2 μm membrane. Since most particles in the DMF wastewater were in the range of 0.20–2.48 μm in size, the fine particles easily penetrated the 0.5 μm membrane pore. So the percentage of total resistance for Ri was the highest for the 0.5 μm membrane. The 0.2 μm membrane was less prone to internal blocking and had higher permeate flux. Therefore, the most appropriate membrane pore size is 0.2 μm in pretreatment of DMF wastewater. 3.2. Effect of TMP The effect of TMP on the permeate flux and turbidity removal rate during microfiltration of the DMF wastewater is illustrated in Fig. 5, using the 0.2 μm ZrO2 membrane. The permeate flux decreased rapidly with time for the first 20 min which is mainly attributed to the deposition of fine particles on the membrane surface and the subsequent
Table 3 Effect of pore size on membrane resistances and relative percentages. Size (μm)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.5 0.2 0.05
0.43 0.72 1.24
5.39 10.06 8.37
0.44 0.58 1.98
5.51 8.10 13.36
1.88 0.43 2.82
23.56 6.01 19.03
5.23 5.43 8.78
65.54 75.84 59.24
7.98 7.16 14.82
Fig. 5. Effect of TMP on the permeate flux and turbidity removal rate (CFV: 3 m·s−1, T: 25 °C, membrane pore size: 0.2 μm.).
Q. Zhang et al. / Desalination 346 (2014) 1–8
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Table 4 Effect of TMP on membrane resistances R and relative percentages. TMP (MPa)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.08 0.12 0.16 0.20 0.24
0.72 0.72 0.72 0.72 0.72
10.23 10.06 8.64 9.78 7.11
0.82 0.58 0.76 0.64 0.62
11.65 8.10 9.12 8.70 6.13
0.38 0.43 0.62 0.67 1.24
5.40 6.01 7.44 9.10 12.25
5.12 5.43 6.23 5.33 7.54
72.73 75.84 74.79 72.42 74.51
7.04 7.16 8.33 7.36 10.12
Fig. 6. Effect of CFV on the permeate flux and turbidity removal rate (TMP: 0.20 MPa, T: 25 °C).
formation of membrane fouling. The steady-state flux increased with increasing TMP. The highest steady permeate flux of 47.64 L·m−2·h−1 was obtained at a TMP of 0.24 MPa. However, there is a threshold pressure of 0.20 MPa. When the TMP is lower than 0.20 MPa, the steadystate flux is directly proportional to the TMP. Beyond the threshold pressure, an obvious deviation from the linear progression is observed. The turbidity removal rates are higher than 99.38% in the range of 0.08– 0.20 MPa, while at a TMP of 0.24 MPa the turbidity removal rate decreased significantly from 99.49% to 97.44%. The effect of TMP on membrane resistances and relative percentages is presented in Table 4. When the TMP was 0.24 MPa, the membrane total resistance Rt rose by 41% compared to TMP of 0.20 MPa. It can be seen from Table 4 that the concentration polarization decreases when the transmembrane pressure increases. In this study, the concentration polarization has little effect on membrane fouling because the suspended solid content in DMF wastewater was only about 408 mg·L−1, and the values of concentration polarization resistance Rc shown in Table 4 are relatively small. When the pressure increased, the concentration polarization resistance transformed into the particle adsorption and sedimentation resistance, and the pore blocking resistance. As shown in Table 4, the percentage of total resistance for Ri increased from 5.40% to 12.25% with increasing TMP. These resistances cause membrane fouling and consequently permeate flux and turbidity removal rate decline. To achieve an optimum operating pressure, obtaining the maximum permeate flux and the best effluent quality are needed [16]. In our comprehensive consideration, 0.20 MPa is the optimum TMP.
3.3. Effect of CFV The effects of CFV on permeate flux and turbidity removal rate in a range of 0.5–3 m·s− 1 are presented in Fig. 6. It shows that steadystate permeate flux increases with increasing CFV and the turbidity removal rates measured for all experiments are higher than 99.6%. This is mainly due to increasing CFV can reduce aggregation of DMF wastewater components in the filter cake layer. High shear stress in the cross-flow filtration prevented the fine particle adsorption and sedimentation on the membrane surface making the fouling layer thinner. Therefore, this weakens the effects of concentration polarization and particle layer formation resulting in the increase in the permeate flux and the slight decrease in turbidity removal rates. Table 5 shows the effect of CFV on membrane resistances and relative percentages. It can be seen that the lowest total resistance (Rt = 7.36 × 1012 m−1) can be obtained at 3 m·s−1. It is clear that the ratio of concentration polarization resistance to total resistance decreased from 16.76% to 8.70% with increasing CFV which caused the permeate flux increasing directly consistent with the previous analysis. Therefore, in this research scope, highest CFV at 3 m·s−1 was chosen as the relatively appropriate value. 3.4. Backflushing process In the backflushing cycle, the permeate flow was applied through the ZrO2 membrane in the reverse direction to the filtration for a few seconds once every several minutes. Under the conditions of
Table 5 Effect of CFV on membrane resistances R and relative percentages. CFV (m·s−1)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.5 0.8 1.0 2.0 3.0
0.72 0.72 0.72 0.72 0.72
7.06 8.07 8.04 8.58 9.78
1.71 1.23 1.02 0.78 0.64
16.76 13.79 11.38 9.30 8.70
0.54 0.43 0.49 0.64 0.67
5.29 4.82 5.47 7.63 9.10
7.23 6.54 6.43 6.25 5.33
70.88 73.32 71.76 74.49 72.42
10.20 8.92 8.96 8.39 7.36
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Fig. 7. Effect of backflushing interval on permeate flux (backflushing pressure: 0.60 MPa, backflushing time: 5 s).
Fig. 9. Effect of backflushing pressure on permeate flux (backflushing time: 5 s, backflushing interval: 20 min).
transmembrane pressure 0.2 MPa, systematic investigation was performed on the backflushing parameters in Figs. 7, 8, 9, 10. The effect of backflushing interval (10 min, 20 min, 30 min, 40 min) on permeate flux is illustrated in Fig. 7. The higher permeate flux was observed with the shorter backflushing interval. When the backflushing interval was 10 and 20 min, the maximum recovery fluxes were 72.42 L·m −2 ·h− 1 and 71.24 L·m− 2·h−1 respectively. The flux recovery rate was above 90%. This is mainly because during shorter backflushing interval, a thinner sedimentation layer was formed on the membrane surface and it was more easily removed. Furthermore, in actual production process, more frequent backflushing needs more energy. Considering the permeate flux recovery and energy consumption, 20 min was chosen as the relatively appropriate backflushing interval in the present study. Fig. 8 illustrates the effect of backflushing duration ranging from 3 to 9 s on permeate flux. It was found that the highest permeate flux recovery increased firstly and then decreased with increasing backflushing duration. When backflushing duration was 5 s, the permeate flux recovery reached the maximum. Although the increase of backflushing duration will temporarily reduce membrane fouling, with the extension of backflushing duration, bubbles produced by backflushing might enter the membrane pores, and block the wastewater permeating through the membrane. Thus, the optimum backflushing duration was concluded to be to 5 s in this system. The effect of backflushing pressure on permeate flux is presented in Fig. 9, where the higher permeate flux was observed at higher backflushing pressure. This can be explained by the fact that the impact of backflushing works at the base of the particles and this impact loosens and detaches the particles from the membrane surface. Moreover, higher backflushing pressure can remove the particle adsorption
and sedimentation on the membrane surface more quickly and powerfully. In this study, optimum backflushing pressure was 0.6 MPa. Fig. 10 shows the time course of the permeate flux with and without backflushing for pretreatment of DMF wastewater. It is clear that backflushing is an effective method to keep the permeate flux of DMF wastewater at higher values. The initial permeate flux slightly decreased with regular backflushing, whereas a rapid decline in the flux was observed without backflushing. After prolonged operation, the steady permeate flux with backflushing increased approximately by 50%, compared with the flux without backflushing.
According to the investigation of membrane fouling mechanism, the predominant membrane fouling cause was the formation of cake layer in the pretreatment of DMF wastewater. In order to remove the cake layer, several physical cleaning methods such as ultrasonic, deionized water flushing and tap water cleaning were adopted. Table 6 shows the effect of ultrasonic conditions on membrane cleaning. It was observed that the pure water flux after ultrasonic cleaning increased with increasing ultrasonic power from 35 to 45 W, ultrasonic temperature from 40 to 60 °C and ultrasonic time from 10 to 30 min. However, the pure water flux had no obvious variation when ultrasonic power is 50 W, ultrasonic temperature 70 °C and ultrasonic time 40 min. The highest pure water flux of 346.8 L·m−2·h−1 was obtained at ultrasonic power of 45 W, ultrasonic temperature 60 °C and ultrasonic time 30 min. Judging from these the optimal experimental parameters within the scope of this study are ultrasonic power of 45 W, ultrasonic temperature 60 °C and ultrasonic time 30 min.
Fig. 8. Effect of backflushing time on permeate flux (backflushing pressure: 0.60 MPa, backflushing interval: 20 min).
Fig. 10. Permeate flux versus time with/without backflushing (backflushing time: 5 s, backflushing interval: 20 min, backflushing pressure: 0.60 MPa).
3.5. Membrane cleaning
Q. Zhang et al. / Desalination 346 (2014) 1–8
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Table 6 Effect of ultrasonic conditions on pure water flux after cleaning. Ultrasonic power (W)a
Pure water flux (L·m−2·h−1)
Ultrasonic temperature (°C)b
Pure water flux (L·m−2·h−1)
Ultrasonic time (min)c
Pure water flux (L·m−2·h−1)
35 40 45 50
282.6 292.4 346.8 345.2
40 50 60 70
282.3 312.3 346.8 344.4
10 20 30 40
119.2 218.2 346.8 342.3
a b c
Ultrasonic temperature 60 °C, ultrasonic time 30 min. Ultrasonic power 45 W, ultrasonic time 30 min. Ultrasonic power 45 W, ultrasonic temperature 60 °C.
Table 7 shows the recovery of pure water flux after different physical cleaning procedures. The initial pure water flux for 0.2 μm ZrO2 membrane is 628 L·m−2 · h−1 at 0.12 MPa. Judging from the cleaning effect, deionized water flushing can break the layer of particle sedimentation. The percentage flux recovery is 39.1%, which proves that deionized water flushing has a certain cleaning effect. However ultrasonic cleaning has a remarkable effect, the percentage flux recovery is 55.2%. The main reason is that the strong ultrasonic cavitation can form a large number of cavities in the layer of particle sedimentation. When the cavities collapse, the deposited layer on the membrane surface could be destroyed at the same moment. In addition, the ultrasonic impact may loosen the particles in the membrane pores, which contributes to some particles flowing out of the membrane pores to reduce the pore blocking resistance. When deionized water flushing and ultrasonic cleaning were used at the same time, the percentage flux recovery was 61.9%. DMF wastewater from PU synthetic leather factories has high turbidity and suspended solids with fine particles including some kinds of soft salts and fibers. These particles could dissolve well in alkaline or acid solutions. So three types of chemical cleaning agents: 0.5 wt.% HCl, 0.5 wt.% NaOH and 0.02 wt.% NaClO were chosen. Table 8 shows the effects of different chemical cleaning methods on pure water flux in the absence of physical cleaning. It can be seen that some pollutants existing on the membrane surface or in the membrane pores could dissolve in chemical reagents. However, after the chemical cleaning the maximum flux recovery rate is 48.1% of the initial pure water flux. Comparing Tables 7 and 8 with Table 9, we can see that a combination of the physical and chemical cleaning methods has better effects on pure water flux recovery, but the cleaning effects are not a simple superposition of physical and chemical cleaning. In this research, the best cleaning method is the combination of flushing, ultrasonic, 0.5 wt.% NaOH and 0.02 wt.% NaClO. Firstly, some of the particles deposited on the membrane surface or in the membrane pores could be easily broken up by flushing and ultrasonic cleaning. The membrane surface sediment layer also becomes loose or even falls off. And then 0.5 wt.% NaOH and 0.02 wt.% NaClO was applied to dissolve the fine particles such as soft salts. This method made the recovery of pure water flux reach 608.2 L·m−2·h−1 and recovery rate of 96.8%. The cleaning method selected should not only have better recovery of membrane flux but also more important have good repeatability. Table 10 shows the repeatability investigation of the above optimal cleaning method. The pure water flux remained between 565.7 and 608.2 L·m− 2·h−1 after 10 times
Table 7 Effect of physical cleaning method on pure water flux. Physical methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
Ultrasonic Deionized water flushing Tap water cleaning Flushing + ultrasonic
346.8 245.7 109.2 389.2
55.2 39.1 17.4 61.9
Ultrasonic power of 45 W, ultrasonic time of 30 min and ultrasonic temperature of 60 °C.
cleaning. It indicated that the combination cleaning method was effective for regeneration of the ZrO2 micro-filtration membrane. 4. Conclusions ZrO2 micro-filtration membranes with different pore sizes were applied to pretreatment of DMF wastewater from the polyurethane synthetic leather factories. The optimum membrane pore size, crossflow velocity and transmembrane pressure were 0.2 μm, 3 m·s− 1 and 0.2 MPa, respectively. The liquid turbidity of the wastewater reduced from 452.5 to 1.7 NTU and the suspended solid content varied from 408 to less than 0.01 mg·L− 1. The liquid turbidity removal rate was 99.62% and the suspended solid content retention rate was 99.99%. The membrane fouling mechanism was analyzed by resistance in series model. The main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of particle adsorption and sedimentation resistance to total resistance was above 70% for the 0.2 μm membrane. Thus backflushing method was adopted to recover the permeate flux. The optimum backflushing conditions were confirmed: 0.60 MPa backflushing pressure, 5 s backflushing time, and 20 min backflushing interval. The permeate flux could be raised approximately 50% after backflushing. A combination of flushing, ultrasonic, 0.5 wt.% NaOH and 0.02 wt.% NaClO cleaning methods showed better effect on membrane flux recovery. By this method, the pure water flux recovered reached 608.2 L·m−2·h−1 and the flux recovery rate was 96.8%. The combination cleaning method was effective for regeneration of the ZrO2 micro-filtration membrane. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21276029, 21306012), the Natural Science Foundation of Jiangsu (BK20131142), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents, and the State Key Laboratory of Materials-Oriented Chemical Engineering. Table 8 Effect of chemical cleaning on pure waste flux. Chemical methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
0.5 wt.% HCl 0.5 wt.% NaOH 0.02 wt.% NaClO 0.5 wt.% HCl + 0.02 wt.% NaClO 0.5 wt.% NaOH + 0.02 wt.% NaClO
102.3 109.9 182.3 298.4 302.2
16.3 17.5 29.0 47.5 48.1
TMP of 0.08 MPa, CFV of 3.0 m·s−1, temperature range of 25–28 °C and cleaning time of 30 min.
8
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Table 9 Effect of combination cleaning method on pure water flux. Methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
Flushing + 0.5 wt.% NaOH +0.02 wt.% NaClO Ultrasonic + 0.5 wt.% NaOH + 0.02 wt.% NaClO Flushing + ultrasonic + 0.5 wt.% NaOH + 0.02 wt.% NaClO
389.4 586.4 608.2
62.0 93.4 96.8
Table 10 Repeatability investigation of the optimal cleaning method. Cleaning times
Pure water flux (L·m−2·h−1)
Recovery rate (%)
1 2 3 4 5 6 7 8 9 10
608.2 592.5 589.2 592.1 570.4 573.2 595.5 578.8 572.1 565.7
96.8 94.3 93.8 94.3 90.8 91.3 94.8 92.2 91.1 90.1
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Performance study of ZrO2 ceramic micro-filtration membranes used in pretreatment of DMF wastewater Qi Zhang a, Rong Xu a, Pengwei Xu a, Ruoyu Chen a, Qiang He a, Jing Zhong a,⁎, Xuehong Gu b a b
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, Jiangsu, China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
H I G H L I G H T S • 19-Channel ZrO2 microfiltration membrane was used in pretreatment of DMF wastewater. • 0.2 μm ZrO2 membrane can effectively remove fine particles in DMF wastewater. • Combined flushing, ultrasonic and chemical cleaning was effective for flux recovery.
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 20 April 2014 Accepted 5 May 2014 Available online 22 May 2014 Keywords: DMF wastewater ZrO2 micro-filtration membrane Pretreatment Backflushing process Membrane fouling
a b s t r a c t Three kinds of 19-channel ZrO2 micro-filtration membranes with different pore size were used for pretreatment of dimethylformamide (DMF) wastewater from the polyurethane (PU) synthetic leather factories, which would enter the distillation column for further purification. The effects of membrane pore size, cross flow velocity (CFV) and transmembrane pressure (TMP) on ceramic membrane filtration performances were investigated. Experimental results indicated that the optimum membrane pore size, CFV and TMP were 0.2 μm, 3 m·s−1 and 0.2 MPa, respectively, with the corresponding liquid turbidity removal rate of 99.62% and suspended solid content retention rate of 99.99%. The membrane fouling mechanism was analyzed by resistance in series model. The main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of particle adsorption and sedimentation resistance to total resistance was above 70% for the 0.2 μm membrane. The backflushing technology was applied and the effect of backflushing on the permeate flux was studied. The permeate flux eventually increased by 50% with the optimum backflushing pressure of 0.6 MPa, backflushing time of 5 s and backflushing interval of 20 min. A combination cleaning method was used to regenerate the ZrO2 micro-filtration membrane. The pure water flux recovered to 608.2 L·m−2·h−1 and flux recovery rate was 96.8% which indicated that the combination cleaning method was effective. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dimethylformamide is an important solvent which is primarily used in the production of polyurethane synthetic leather [1]. In China alone, the emission amount of DMF wastewater from leather factories is up to 1 billion tons a year, which seriously harm the environment because of its high toxicity and poor biodegradation. Therefore, the recovery of DMF is very essential. At present, the main method for DMF recovery from wastewater is two-tower distillation including atmospheric distillation and vacuum distillation. However, the suspended solid content and turbidity of DMF wastewater from the PU synthetic leather factories are very high. Besides, DMF wastewater contains fine particles (soft salt, fibers, dusts, etc.) [2], which would lead to the blockage of equipment pipes and distillation columns and thus increase the cleaning times for ⁎ Corresponding author. Tel.: +86 519 86330330. E-mail address: [email protected] (J. Zhong).
http://dx.doi.org/10.1016/j.desal.2014.05.006 0011-9164/© 2014 Elsevier B.V. All rights reserved.
distillation equipment. Therefore, the pretreatment of DMF wastewater is very necessary to provide a high-quality feed for further purification in distillation columns. The conventional pretreatment system relies on sedimentation, flocculation or sand filtration processes [3]. However, these processes show a limited separation effect because the size of fine particles in DMF wastewater is less than 2 μm. Abundant literature has reported that the fine particles could be effectively removed from the wastewater by membrane filtration technology. Ceramic membrane filtration based on the screening mechanism can effectively reject micron- and nano-sized particles. Cui et al. [4] reported a pre-treatment process using 50 nm ZrO2/Al2O3 ceramic membranes for the desalination of seawater. The ceramic membrane system maintained a stable permeability throughout the long-term experiment. The permeate turbidity and removal ratio produced by the multi-channel ceramic membrane were 0.076 NTU and 99.2% respectively. Agana et al. [5] applied a 50 nm ZrO2 ceramic ultrafiltration membrane to wastewater containing
2
Q. Zhang et al. / Desalination 346 (2014) 1–8
cathodic electrodeposition paint particles. The crossflow velocity of 3.2 m·s− 1 and transmembrane pressure of 100 kPa produced the highest steady-state permeate flux and the optimum energy consumption. Turbidity rejection rates of all experiments were larger than 99.95%. It was demonstrated that a stable, high-quality feed was obtained by ceramic membrane pre-treatment process, which can provide an economical and reliable separation for DMF wastewater due to its excellent selectivity, permeability and thermal and chemical stability [6]. However, due to concentration polarization and membrane fouling, the permeate flux of the micro-filtration (MF) process decreases with time [7,8], which will significantly diminish the filtration performance during the pretreatment of DMF wastewater. Fine particles in DMF wastewater would accumulate on the membrane surface and form a cake layer. At the same time, some particles may adsorb on or block the membrane pores. Now, the dominant fouling cause and the physical or chemical interactions between these foulants are not clear, which are critical for membrane permeability and cleaning strategies [9]. The decline of membrane flux can be converted into the membrane resistance [10]. Therefore in order to investigate the membrane fouling mechanism, membrane resistances were analyzed in terms of resistance-inseries model which would guide the choice of methods for membrane regeneration [11,12]. In order to maintain high efficiency for the filtration of DMF wastewater, membrane regeneration is an integral part of the microfiltration systems and effective membrane cleaning is necessary [13]. Numerous attempts have been made to reduce concentration polarization and control membrane fouling by mechanical (backflushing) and chemical cleaning [14]. Among them, backflushing technology is the most effective for ceramic membranes. Unlike polymeric membranes, ceramic membranes are able to withstand the high pressures associated with backflushing. However, it should be noted that although the membrane fouling that caused by caking is reduced, backflushing could not remove severe pore blocking. Gabrus et al. [15] investigated the application of backflushing to microfiltration of yeast suspension with inorganic TiO2/Al2O3 MF membranes. They found that backflushing could efficiently remove surface deposits but less effective in removal of internal pore fouling. This type of irreversible fouling causing by particles inside the membrane pores may only be eliminated by chemical cleaning. This work was done to explore the pretreatment processes of DMF wastewater from PU synthetic leather factories using ZrO2 microfiltration membranes. The objective of this work was to investigate the effects of membrane pore size, cross flow velocity and transmembrane pressure on ceramic membrane filtration performances. The focus is on optimizing DMF wastewater pretreatment process conditions and combining with resistance in series model to explore the membrane fouling mechanism, and on determining effective membrane regeneration cleaning methods. This study can provide an insight into the applicability of ZrO2 microfiltration membrane for pretreatment of DMF wastewater.
Fig. 1. The particle size distribution of the DMF wastewater.
quantified with a particle size analyzer (ZEN3600, Malvern, UK). The average particle size of DMF wastewater was 1.38 μm. 2.2. Ceramic membranes Three kinds of 19-channel ZrO2/Al2O3 composite micro-filtration membranes with different pore sizes (50 nm, 0.2 μm and 0.5 μm) were used in this study supplied by Jiangsu Jiuwu High-Tech Co., Ltd. (Nanjing, China). The dimensions of these membranes are presented as follows: channel diameter, 4 mm; length, 200 mm; porosity, 35%; effective membrane area, 477 cm2. The isoelectric point of the ZrO2 active layer is approximately 7 and therefore the membrane surface charge is positive or negative at pH b7 or N7, respectively. 2.3. Ceramic MF system The schematic diagram of ceramic membrane cross-flow microfiltration for pretreatment of DMF wastewater and backflushing process is shown in Fig. 2. The volume of the feed tank jacketed for temperature control was 50 L. The DMF wastewater was continuously pumped into the ceramic membrane module with a centrifugal pump. The operating transmembrane pressure and crossflow velocity were adjusted by valves before and after the membrane module. The permeate and retentate were recirculated continuously into the feed tank. The backflushing arrangement consisted of a gas buffer reservoir and an air compressor. The temperature of the feed solution was controlled at 298.15 K by a thermostatic bath. The effects of cross flow velocity (0.5 m·s−1, 0.8 m·s−1, 1.0 m·s−1, 2.0 m·s−1, 3.0 m·s−1) and transmembrane pressure (0.08 MPa, 0.12 MPa, 0.16 MPa, 0.20 MPa, 0.24 MPa) on ceramic membrane filtration performance were investigated. The permeate flux was determined by collecting the permeate sample in a graduated cylinder at specific time intervals.
2. Materials and methods
2.4. Analysis of membrane fouling
2.1. DMF wastewater
In order to investigate the membrane fouling, the permeate flux of DMF wastewater micro-filtration process was used to analyze membrane resistances using resistance-in-series model. Four membrane resistances commonly exist according to the characteristics of the micro-sized particle system, which are: Rm (the inherent resistance of the clean membrane which depends on its pore size and material, Eq. (1-1)), Rc (the resistance resulting from the concentration polarization, Eq. (1-2)), Ri (the resistance resulting from the membrane pores blocking, Eq. (1-3)) and Rg (the resistance resulting from the particle adsorption and sedimentation on membrane surface, Eq. (1-4)).
The DMF wastewater (~20 wt.% of DMF) is from a synthetic leather company in Jiangsu including some fine particle and fibers. The characteristics of the DMF wastewater are given in Table 1. The particle size distribution of the DMF wastewater is shown in Fig. 1, which was Table 1 Characteristics of the DMF wastewater. Parameters
pH
Conductivity (μS·cm−1)
Suspended solid content (mg·L−1)
Turbidity (NTU)
Average size (μm)
Value
7.68
698
408
452.50
1.38
Rm ¼
ΔP μ w J0
ð1 1Þ
Q. Zhang et al. / Desalination 346 (2014) 1–8
3
Fig. 2. Schematic diagram of cross-flow microfiltration apparatus. (1) feed tank; (2) centrifugal pump; (3) flow meter; (4) membrane module; (5) buffer tank; (6) air compressor; V1–V7: valves.
Rc ¼
ΔP ΔP − μ J1 μ w J2
ð1 2Þ
Ri ¼
ΔP ΔP − μ w J3 μ w J0
ð1 3Þ
Rg ¼
ΔP ΔP − : μ w J2 μ w J3
ð1 4Þ
In these equations, J0 is the initial pure water flux; J1 is the permeate flux after DMF wastewater filtration; J2 is the pure water flux after the membrane fouling; J3 is the pure water flux after physical cleaning; μw and μ are the viscosity of pure water and DMF wastewater and their values at 25 °C are 0.960 mPa·s and 1.779 mPa·s respectively. 2.5. Backflushing process Backflushing is an in situ cleaning method involving reverse flow of permeate by applying higher pressure on the permeate side [10]. The permeate backflushing in pretreatment of DMF wastewater was applied
by air pressure, as shown schematically in Fig. 2. During backflushing, the reverse flow through the membrane removed the concentration polarization and cake layers from the membrane surface. In the present study, the effects of three important factors in the backflushing process were investigated: (1) backflushing pressure (from 0.3 to 0.6 MPa); (2) backflushing time (from 3 to 9 s); and (3) backflushing interval (from 10 to 40 min).
2.6. Membrane cleaning The membrane cleaning methods are classified into physical method and chemical method. Both types of membrane cleaning method are aimed at recovering membrane permeability. In order to obtain the agreement of cleaning conditions, the ZrO2 microfiltration membrane was used for pretreatment of DMF wastewater after 2 h MF process with TMP, CFV and temperature at 0.12 MPa, 3 m·s−1 and 25 °C. The physical cleaning consisted of deionized water flushing, ultrasonic and tap water cleaning. Ultrasonic cleaning is an effective physical cleaning method. The membrane module was submerged in the ultrasonic bath (KQ2200DB, Kunshan Shumei Ultrasonic Instruments Co., Ltd, China) equipped with a powerful generator with 100 W output and 40 kHz frequency. Systematic investigation was performed on the influence of ultrasonic power, ultrasonic temperature and ultrasonic time. The composition of the chemical cleaning solution was 0.5 wt.% HCl 0.5 wt.% NaOH and 0.02 wt.% NaClO. Chemical cleaning parameters were set at pressure of 0.08 MPa, CFV of 3.0 m·s− 1, temperature range of 25– 28 °C and cleaning time of 30 min. After membrane cleaning, the pure water flux was measured and compared with the initial value.
Table 2 Effect of pore size on the permeate qualities.
Fig. 3. Effect of pore size on permeate flux (TMP: 0.12 MPa, CFV: 3 m·s−1, T: 25 °C).
Pore size (μm)
Turbidity (NTU)
Turbidity removal rate (%)
Suspended solid content (mg·L−1)
0.5 0.2 0.05
12.8 1.7 1.4
97.17 99.62 99.69
0.30 0.01 b0.01
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Fig. 4. Effect of pore size on the particle size distribution of the permeate.
3. Results and discussion 3.1. Effect of membrane pore size Membrane pore size is one of the most important parameters in the MF process. The initial permeabilities of ZrO2 microfiltration membranes with different pore sizes (50 nm, 0.2 μm and 0.5 μm) were determined. The initial pure water flux for 0.5 μm ZrO2 membrane is 1047 L·m−2·h− 1, 0.2 μm ZrO2 membrane is 628 L·m−2·h− 1 and 50 nm ZrO2 membrane is 363 L·m− 2 · h− 1 under the conditions of TMP 0.12 MPa, CFV 3 m·s−1 and temperature 25 °C. The effects of membrane pore size (50 nm, 0.2 and 0.5 μm) on the permeate flux and the permeate water qualities are shown in Fig. 3 and Table 2. The particle size distribution of the permeate is also shown in Fig. 4. It is interesting that, contrary to our expectations, the bigger pore size did not always provide the higher permeate flux. As shown in Fig. 1, it appears that a fraction of the particles in DMF wastewater are between 0.2 and 0.5 μm, this fraction could cause severe pore blocking and thus a higher permeate flux decrease for membrane with pore size of 0.5 μm in comparison with other membranes. The 0.2 μm membrane showed better effects in permeate flux, turbidity removal rate and suspended solid content. The liquid turbidity of the DMF
wastewater reduced from 452.5 to 1.7 NTU and the suspended solids content varied from 408 to 0.01 mg·L− 1. The permeate flux of the 0.2 μm membrane was very close to that of the 0.5 μm membrane. Meanwhile, the turbidity removal rate was 99.62% of the 0.2 μm membrane which nearly achieved that of the 50 nm membrane. Table 3 shows the effect of pore size on membrane resistances and relative percentages in pretreatment of DMF wastewater. It can be seen that the 50 nm membrane shows the highest total resistance (Rt = 14.82 × 1012 m− 1), and the resistances for the membranes of 0.2 and 0.5 μm are very close (Rt = 7.16 × 1012 m−1 and Rt = 7.98 × 1012 m−1). The concentration polarization resistance Rc increased with decreasing membrane pore size. The fouling resistance Rg of the 0.2 μm membrane accounted for 75.84% of the total resistances. It is concluded that the main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of pore blocking resistance to total resistance was 19.03% for the 50 nm membrane, which was greater than that of the 0.2 μm membrane, but less than that of the 0.5 μm membrane. As shown in Fig. 1, there is a fraction of the particles lower than 0.2 μm. These particles in DMF wastewater would penetrate into the membrane pores of 0.2 μm and 50 nm, resulting in internal pore blockage. Moreover the smaller the membrane pores, the more difficult for the particles to penetrate them, resulting in more severe pore blocking. Therefore, the ratio of pore blocking resistance to total resistance for the 50 nm membrane was greater than that of the 0.2 μm membrane. Since most particles in the DMF wastewater were in the range of 0.20–2.48 μm in size, the fine particles easily penetrated the 0.5 μm membrane pore. So the percentage of total resistance for Ri was the highest for the 0.5 μm membrane. The 0.2 μm membrane was less prone to internal blocking and had higher permeate flux. Therefore, the most appropriate membrane pore size is 0.2 μm in pretreatment of DMF wastewater. 3.2. Effect of TMP The effect of TMP on the permeate flux and turbidity removal rate during microfiltration of the DMF wastewater is illustrated in Fig. 5, using the 0.2 μm ZrO2 membrane. The permeate flux decreased rapidly with time for the first 20 min which is mainly attributed to the deposition of fine particles on the membrane surface and the subsequent
Table 3 Effect of pore size on membrane resistances and relative percentages. Size (μm)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.5 0.2 0.05
0.43 0.72 1.24
5.39 10.06 8.37
0.44 0.58 1.98
5.51 8.10 13.36
1.88 0.43 2.82
23.56 6.01 19.03
5.23 5.43 8.78
65.54 75.84 59.24
7.98 7.16 14.82
Fig. 5. Effect of TMP on the permeate flux and turbidity removal rate (CFV: 3 m·s−1, T: 25 °C, membrane pore size: 0.2 μm.).
Q. Zhang et al. / Desalination 346 (2014) 1–8
5
Table 4 Effect of TMP on membrane resistances R and relative percentages. TMP (MPa)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.08 0.12 0.16 0.20 0.24
0.72 0.72 0.72 0.72 0.72
10.23 10.06 8.64 9.78 7.11
0.82 0.58 0.76 0.64 0.62
11.65 8.10 9.12 8.70 6.13
0.38 0.43 0.62 0.67 1.24
5.40 6.01 7.44 9.10 12.25
5.12 5.43 6.23 5.33 7.54
72.73 75.84 74.79 72.42 74.51
7.04 7.16 8.33 7.36 10.12
Fig. 6. Effect of CFV on the permeate flux and turbidity removal rate (TMP: 0.20 MPa, T: 25 °C).
formation of membrane fouling. The steady-state flux increased with increasing TMP. The highest steady permeate flux of 47.64 L·m−2·h−1 was obtained at a TMP of 0.24 MPa. However, there is a threshold pressure of 0.20 MPa. When the TMP is lower than 0.20 MPa, the steadystate flux is directly proportional to the TMP. Beyond the threshold pressure, an obvious deviation from the linear progression is observed. The turbidity removal rates are higher than 99.38% in the range of 0.08– 0.20 MPa, while at a TMP of 0.24 MPa the turbidity removal rate decreased significantly from 99.49% to 97.44%. The effect of TMP on membrane resistances and relative percentages is presented in Table 4. When the TMP was 0.24 MPa, the membrane total resistance Rt rose by 41% compared to TMP of 0.20 MPa. It can be seen from Table 4 that the concentration polarization decreases when the transmembrane pressure increases. In this study, the concentration polarization has little effect on membrane fouling because the suspended solid content in DMF wastewater was only about 408 mg·L−1, and the values of concentration polarization resistance Rc shown in Table 4 are relatively small. When the pressure increased, the concentration polarization resistance transformed into the particle adsorption and sedimentation resistance, and the pore blocking resistance. As shown in Table 4, the percentage of total resistance for Ri increased from 5.40% to 12.25% with increasing TMP. These resistances cause membrane fouling and consequently permeate flux and turbidity removal rate decline. To achieve an optimum operating pressure, obtaining the maximum permeate flux and the best effluent quality are needed [16]. In our comprehensive consideration, 0.20 MPa is the optimum TMP.
3.3. Effect of CFV The effects of CFV on permeate flux and turbidity removal rate in a range of 0.5–3 m·s− 1 are presented in Fig. 6. It shows that steadystate permeate flux increases with increasing CFV and the turbidity removal rates measured for all experiments are higher than 99.6%. This is mainly due to increasing CFV can reduce aggregation of DMF wastewater components in the filter cake layer. High shear stress in the cross-flow filtration prevented the fine particle adsorption and sedimentation on the membrane surface making the fouling layer thinner. Therefore, this weakens the effects of concentration polarization and particle layer formation resulting in the increase in the permeate flux and the slight decrease in turbidity removal rates. Table 5 shows the effect of CFV on membrane resistances and relative percentages. It can be seen that the lowest total resistance (Rt = 7.36 × 1012 m−1) can be obtained at 3 m·s−1. It is clear that the ratio of concentration polarization resistance to total resistance decreased from 16.76% to 8.70% with increasing CFV which caused the permeate flux increasing directly consistent with the previous analysis. Therefore, in this research scope, highest CFV at 3 m·s−1 was chosen as the relatively appropriate value. 3.4. Backflushing process In the backflushing cycle, the permeate flow was applied through the ZrO2 membrane in the reverse direction to the filtration for a few seconds once every several minutes. Under the conditions of
Table 5 Effect of CFV on membrane resistances R and relative percentages. CFV (m·s−1)
Rm (1012 m−1)
Rm/Rt (%)
Rc (1012 m−1)
Rc/Rt (%)
Ri (1012 m−1)
Ri/Rt (%)
Rg (1012 m−1)
Rg/Rt (%)
Rt (1012 m−1)
0.5 0.8 1.0 2.0 3.0
0.72 0.72 0.72 0.72 0.72
7.06 8.07 8.04 8.58 9.78
1.71 1.23 1.02 0.78 0.64
16.76 13.79 11.38 9.30 8.70
0.54 0.43 0.49 0.64 0.67
5.29 4.82 5.47 7.63 9.10
7.23 6.54 6.43 6.25 5.33
70.88 73.32 71.76 74.49 72.42
10.20 8.92 8.96 8.39 7.36
6
Q. Zhang et al. / Desalination 346 (2014) 1–8
Fig. 7. Effect of backflushing interval on permeate flux (backflushing pressure: 0.60 MPa, backflushing time: 5 s).
Fig. 9. Effect of backflushing pressure on permeate flux (backflushing time: 5 s, backflushing interval: 20 min).
transmembrane pressure 0.2 MPa, systematic investigation was performed on the backflushing parameters in Figs. 7, 8, 9, 10. The effect of backflushing interval (10 min, 20 min, 30 min, 40 min) on permeate flux is illustrated in Fig. 7. The higher permeate flux was observed with the shorter backflushing interval. When the backflushing interval was 10 and 20 min, the maximum recovery fluxes were 72.42 L·m −2 ·h− 1 and 71.24 L·m− 2·h−1 respectively. The flux recovery rate was above 90%. This is mainly because during shorter backflushing interval, a thinner sedimentation layer was formed on the membrane surface and it was more easily removed. Furthermore, in actual production process, more frequent backflushing needs more energy. Considering the permeate flux recovery and energy consumption, 20 min was chosen as the relatively appropriate backflushing interval in the present study. Fig. 8 illustrates the effect of backflushing duration ranging from 3 to 9 s on permeate flux. It was found that the highest permeate flux recovery increased firstly and then decreased with increasing backflushing duration. When backflushing duration was 5 s, the permeate flux recovery reached the maximum. Although the increase of backflushing duration will temporarily reduce membrane fouling, with the extension of backflushing duration, bubbles produced by backflushing might enter the membrane pores, and block the wastewater permeating through the membrane. Thus, the optimum backflushing duration was concluded to be to 5 s in this system. The effect of backflushing pressure on permeate flux is presented in Fig. 9, where the higher permeate flux was observed at higher backflushing pressure. This can be explained by the fact that the impact of backflushing works at the base of the particles and this impact loosens and detaches the particles from the membrane surface. Moreover, higher backflushing pressure can remove the particle adsorption
and sedimentation on the membrane surface more quickly and powerfully. In this study, optimum backflushing pressure was 0.6 MPa. Fig. 10 shows the time course of the permeate flux with and without backflushing for pretreatment of DMF wastewater. It is clear that backflushing is an effective method to keep the permeate flux of DMF wastewater at higher values. The initial permeate flux slightly decreased with regular backflushing, whereas a rapid decline in the flux was observed without backflushing. After prolonged operation, the steady permeate flux with backflushing increased approximately by 50%, compared with the flux without backflushing.
According to the investigation of membrane fouling mechanism, the predominant membrane fouling cause was the formation of cake layer in the pretreatment of DMF wastewater. In order to remove the cake layer, several physical cleaning methods such as ultrasonic, deionized water flushing and tap water cleaning were adopted. Table 6 shows the effect of ultrasonic conditions on membrane cleaning. It was observed that the pure water flux after ultrasonic cleaning increased with increasing ultrasonic power from 35 to 45 W, ultrasonic temperature from 40 to 60 °C and ultrasonic time from 10 to 30 min. However, the pure water flux had no obvious variation when ultrasonic power is 50 W, ultrasonic temperature 70 °C and ultrasonic time 40 min. The highest pure water flux of 346.8 L·m−2·h−1 was obtained at ultrasonic power of 45 W, ultrasonic temperature 60 °C and ultrasonic time 30 min. Judging from these the optimal experimental parameters within the scope of this study are ultrasonic power of 45 W, ultrasonic temperature 60 °C and ultrasonic time 30 min.
Fig. 8. Effect of backflushing time on permeate flux (backflushing pressure: 0.60 MPa, backflushing interval: 20 min).
Fig. 10. Permeate flux versus time with/without backflushing (backflushing time: 5 s, backflushing interval: 20 min, backflushing pressure: 0.60 MPa).
3.5. Membrane cleaning
Q. Zhang et al. / Desalination 346 (2014) 1–8
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Table 6 Effect of ultrasonic conditions on pure water flux after cleaning. Ultrasonic power (W)a
Pure water flux (L·m−2·h−1)
Ultrasonic temperature (°C)b
Pure water flux (L·m−2·h−1)
Ultrasonic time (min)c
Pure water flux (L·m−2·h−1)
35 40 45 50
282.6 292.4 346.8 345.2
40 50 60 70
282.3 312.3 346.8 344.4
10 20 30 40
119.2 218.2 346.8 342.3
a b c
Ultrasonic temperature 60 °C, ultrasonic time 30 min. Ultrasonic power 45 W, ultrasonic time 30 min. Ultrasonic power 45 W, ultrasonic temperature 60 °C.
Table 7 shows the recovery of pure water flux after different physical cleaning procedures. The initial pure water flux for 0.2 μm ZrO2 membrane is 628 L·m−2 · h−1 at 0.12 MPa. Judging from the cleaning effect, deionized water flushing can break the layer of particle sedimentation. The percentage flux recovery is 39.1%, which proves that deionized water flushing has a certain cleaning effect. However ultrasonic cleaning has a remarkable effect, the percentage flux recovery is 55.2%. The main reason is that the strong ultrasonic cavitation can form a large number of cavities in the layer of particle sedimentation. When the cavities collapse, the deposited layer on the membrane surface could be destroyed at the same moment. In addition, the ultrasonic impact may loosen the particles in the membrane pores, which contributes to some particles flowing out of the membrane pores to reduce the pore blocking resistance. When deionized water flushing and ultrasonic cleaning were used at the same time, the percentage flux recovery was 61.9%. DMF wastewater from PU synthetic leather factories has high turbidity and suspended solids with fine particles including some kinds of soft salts and fibers. These particles could dissolve well in alkaline or acid solutions. So three types of chemical cleaning agents: 0.5 wt.% HCl, 0.5 wt.% NaOH and 0.02 wt.% NaClO were chosen. Table 8 shows the effects of different chemical cleaning methods on pure water flux in the absence of physical cleaning. It can be seen that some pollutants existing on the membrane surface or in the membrane pores could dissolve in chemical reagents. However, after the chemical cleaning the maximum flux recovery rate is 48.1% of the initial pure water flux. Comparing Tables 7 and 8 with Table 9, we can see that a combination of the physical and chemical cleaning methods has better effects on pure water flux recovery, but the cleaning effects are not a simple superposition of physical and chemical cleaning. In this research, the best cleaning method is the combination of flushing, ultrasonic, 0.5 wt.% NaOH and 0.02 wt.% NaClO. Firstly, some of the particles deposited on the membrane surface or in the membrane pores could be easily broken up by flushing and ultrasonic cleaning. The membrane surface sediment layer also becomes loose or even falls off. And then 0.5 wt.% NaOH and 0.02 wt.% NaClO was applied to dissolve the fine particles such as soft salts. This method made the recovery of pure water flux reach 608.2 L·m−2·h−1 and recovery rate of 96.8%. The cleaning method selected should not only have better recovery of membrane flux but also more important have good repeatability. Table 10 shows the repeatability investigation of the above optimal cleaning method. The pure water flux remained between 565.7 and 608.2 L·m− 2·h−1 after 10 times
Table 7 Effect of physical cleaning method on pure water flux. Physical methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
Ultrasonic Deionized water flushing Tap water cleaning Flushing + ultrasonic
346.8 245.7 109.2 389.2
55.2 39.1 17.4 61.9
Ultrasonic power of 45 W, ultrasonic time of 30 min and ultrasonic temperature of 60 °C.
cleaning. It indicated that the combination cleaning method was effective for regeneration of the ZrO2 micro-filtration membrane. 4. Conclusions ZrO2 micro-filtration membranes with different pore sizes were applied to pretreatment of DMF wastewater from the polyurethane synthetic leather factories. The optimum membrane pore size, crossflow velocity and transmembrane pressure were 0.2 μm, 3 m·s− 1 and 0.2 MPa, respectively. The liquid turbidity of the wastewater reduced from 452.5 to 1.7 NTU and the suspended solid content varied from 408 to less than 0.01 mg·L− 1. The liquid turbidity removal rate was 99.62% and the suspended solid content retention rate was 99.99%. The membrane fouling mechanism was analyzed by resistance in series model. The main resistance derived from the particle adsorption and sedimentation on the membrane surface. The ratio of particle adsorption and sedimentation resistance to total resistance was above 70% for the 0.2 μm membrane. Thus backflushing method was adopted to recover the permeate flux. The optimum backflushing conditions were confirmed: 0.60 MPa backflushing pressure, 5 s backflushing time, and 20 min backflushing interval. The permeate flux could be raised approximately 50% after backflushing. A combination of flushing, ultrasonic, 0.5 wt.% NaOH and 0.02 wt.% NaClO cleaning methods showed better effect on membrane flux recovery. By this method, the pure water flux recovered reached 608.2 L·m−2·h−1 and the flux recovery rate was 96.8%. The combination cleaning method was effective for regeneration of the ZrO2 micro-filtration membrane. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21276029, 21306012), the Natural Science Foundation of Jiangsu (BK20131142), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents, and the State Key Laboratory of Materials-Oriented Chemical Engineering. Table 8 Effect of chemical cleaning on pure waste flux. Chemical methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
0.5 wt.% HCl 0.5 wt.% NaOH 0.02 wt.% NaClO 0.5 wt.% HCl + 0.02 wt.% NaClO 0.5 wt.% NaOH + 0.02 wt.% NaClO
102.3 109.9 182.3 298.4 302.2
16.3 17.5 29.0 47.5 48.1
TMP of 0.08 MPa, CFV of 3.0 m·s−1, temperature range of 25–28 °C and cleaning time of 30 min.
8
Q. Zhang et al. / Desalination 346 (2014) 1–8
Table 9 Effect of combination cleaning method on pure water flux. Methods
Pure water flux (L·m−2·h−1)
Recovery rate (%)
Flushing + 0.5 wt.% NaOH +0.02 wt.% NaClO Ultrasonic + 0.5 wt.% NaOH + 0.02 wt.% NaClO Flushing + ultrasonic + 0.5 wt.% NaOH + 0.02 wt.% NaClO
389.4 586.4 608.2
62.0 93.4 96.8
Table 10 Repeatability investigation of the optimal cleaning method. Cleaning times
Pure water flux (L·m−2·h−1)
Recovery rate (%)
1 2 3 4 5 6 7 8 9 10
608.2 592.5 589.2 592.1 570.4 573.2 595.5 578.8 572.1 565.7
96.8 94.3 93.8 94.3 90.8 91.3 94.8 92.2 91.1 90.1
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