Advanced treatment of a complex pharmaceutical wastewater by nanofiltration: Membrane foulant identification and cleaning

Desalination 251 (2010) 167–175

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Advanced treatment of a complex pharmaceutical wastewater by nanofiltration: Membrane foulant identification and cleaning Xinyu Wei a,b,c, Zhi Wang a,b,c,⁎, Fanghui Fan a,b,c, Jixiao Wang a,b,c, Shichang Wang a,c a b c

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China

a r t i c l e

i n f o

Article history: Received 18 August 2009 Accepted 28 August 2009 Available online 17 October 2009 Keywords: Membrane fouling Membrane cleaning Nanofiltration Pharmaceutical wastewater

a b s t r a c t Advanced treatment of a complex pharmaceutical wastewater by Desal-5 DK nanofiltration (NF) membrane was carried out. The membrane fouling and chemical cleaning in this application were investigated. It was found that at the initial stage of NF process, the deposition of sulfate and carbonate of calcium was the main cause of membrane fouling. At the later fouling stage, besides calcium salts, complex organic foulants containing carboxyl acid, amide, and alkyl halide functional groups also deposited onto membrane surface and gradually formed a densely packed fouling layer. Accordingly, in cleaning process, the efficiency of a cleaning agent depended on its ability to break down the integrity of the compact fouling layer by reacting with the foulants. The results showed that the cleaning efficiencies of the agents increased in the sequence of NaOH (pH 11) < HCl (pH 2) < citric acid (pH 2) < EDTA (10 mM). After cleaning with EDTA for 60 min, the membrane flux recovery ratio reached 99.0%. The SEM image and element content of the membrane cleaned by EDTA were quite similar to those of the virgin membrane. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Wastewater of pharmaceutical industry is characterized by high organic matter contents, toxicity, deep color, and high salt contents [1]. The traditional biological treatment methods are not effective in removing the pollutants from the wastewater, because the toxic compounds in the wastewater may greatly inhibit the activities of microorganisms [2,3]. Therefore, the treatment of pharmaceutical wastewater requires some complementary techniques that could efficiently remove pollutants and enable the wastewater to be discharged into receiving water or be reused for industrial purposes. Membrane processes nanofiltration (NF) and reverse osmosis (RO) with high efficiencies in removing organic and inorganic pollutants can overcome the shortcomings of the traditional methods. They can replace or may operate together with the traditional methods for advanced treatment of pharmaceutical wastewater [3–5]. Membrane fouling is a main obstacle to the application of membrane techniques in pharmaceutical wastewater treatment. The rate and extent of membrane fouling are influenced by feed stream quality, membrane surface properties, and operating conditions [6–12]. The feed stream quality plays a significant role in determining foulant–membrane and foulant–foulant interactions, and hence membrane performance [9]. ⁎ Corresponding author. Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel.: +86 22 27404533; fax: +86 22 27890515. E-mail address: [email protected] (Z. Wang). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.08.005

Membrane surface foulants are identified to confirm the above interactions as well as to provide a good evidence for the choice of cleaning agent in membrane regeneration process. Numerous studies have investigated membrane fouling by studying simulated wastewater which contains one or more of identified colloids, dissolved solids, natural organic matters (NOM), and synthetic organic matters [10,11,13,14]. It is relatively easy to identify membrane foulants in simulated wastewater treatment process. However, a real wastewater always contains complex matters which are difficult to be separated. Studies on membrane foulant identification in real wastewater treatment process are rather scarce [15–17]. Almost none of them related to the membrane foulant identification in pharmaceutical wastewater treatment process. Therefore, the previous studies on membrane fouling are not able to provide systematic information to account for membrane fouling in pharmaceutical wastewater treatment process. Despite many preventive methods, membrane fouling is inevitable in wastewater treatment process. Chemical cleaning is an effective method to remove the foulants deposited on membrane surface [18]. When there is a significant drop in permeate flux or salt rejection, or when there is a need to increase the trans-membrane pressure (TMP) significantly to maintain the desired permeate flux, chemical cleaning is performed [18,19]. The factors that need to be considered in chemical cleaning include cleaning agent type, cleaning agent concentration, temperature, pH, pressure, flow rate, and cleaning time. It is critical to select appropriate cleaning agents for a particular cleaning situation, because the use of non-optimal cleaning agents will incur unnecessary costs through chemical over-use [20]. Moreover, some cleaning agent–

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membrane combinations are incompatible and will result in irreversible loss of membrane performance [20,21]. Thus, the optimal cleaning agent should be chosen in consideration of both foulants and membrane material, in a way that it is not only effective in removing foulants, but also in restoring membrane performance. In pharmaceutical wastewater treatment process, NF/RO membrane had good separation performance as discussed in previous studies [3–5]. However, no information on chemical cleaning has been reported to restore NF/RO membrane performance after fouling. Hence, there is an urgent need to investigate chemical cleaning for the NF/RO membranes fouled by pharmaceutical wastewater, in order to obtain more feasible, cost-saving membrane performance restoration method. In this work, experimental study on NF for advanced treatment of a real complex pharmaceutical wastewater was carried out. There were two major objectives. One was to determine membrane foulants and understand membrane fouling process in this application. The other was to develop an effective and economical chemical cleaning based on previous membrane foulant identification. Both of them will contribute to the full-scale application of membrane technique in pharmaceutical wastewater treatment process. 2. Experimental 2.1. Wastewater and membrane The raw wastewater was the biotreatment effluent of sequencing batch reactor (SBR) from a pharmaceutical company in Shijiazhuang (China). It was pretreated with hollow fiber polysulfone ultrafiltration (UF) membrane module (UEOS810, Motianmo Corp., China) before NF. The water quality parameters were measured according to the Standard Method for the Examination of Water and Wastewater [22] and shown in Table 1. It is seen that the NF feed is characterized by high concentrations of several parameters, such as chemical oxygen demand (COD), color, total hardness, total alkalinity, total dissolved

Table 1 The water qualities of NF feed and permeate and the national standard of China for the reclaimed water quality used in the industry (GB/T 19923-2005). Itema

pH COD (mg/L) BOD5 (mg/L) Color (Pt–Co) Turbidity (NTU) Total hardness (as CaCO3) (mg/L) Total alkalinity (as CaCO3) (mg/L) TDS (mg/L) P (mg/L) NH3-N (mg/L) SiO2 (mg/L) (mg/L) SO2− 4 HCO− 3 (mg/L) Cl− (mg/L) Br− (mg/L) Fe2+ and Fe3+ (mg/L) Na+ (mg/L) K+ (mg/L) Ca2+ (mg/L) Mg2+ (mg/L) Ba2+ (mg/L) Conductivity(μs cm− 1) a b c d

NF feedb

NF permeate mean value ± standard deviation (mean removal percent/%)c

Standard

7.8 240 23.4 682 0.0 498.9

7.4 ± 0.1 20.0 ± 8.0 (91.7) <2.0 (91.5) 4 ± 1 (99.4) 0.0 15.4 ± 0.2 (96.9)

6.5–8.5 ≤60 ≤10 ≤30 ≤5 ≤450

835.6

296.6 ± 2.4 (64.5)

≤350

384 ± 4 (84.4) <0.01 (>99.9) 2.48 ± 0.2 (81.6) 1.66 ± 0.04 (76.9) 29.9 ± 1.2 (97.0) 180.9 ± 2.4 (64.3) 49.76 ± 1.02 (70.3) 7.9 ± 0.2 (71.1) 0.020 ± 0.002 (98.7) 92.52 ± 2.0 (80.3) 20.3 ± 0.4 (81.7) 5.28 ± 0.10 (96.9) 0.53 ± 0.01 (96.9) 0.000 (100) 558 ± 12 (85.2)

≤1000 ≤1 ≤10 ≤30 ≤250 /d ≤250 / ≤0.3 / / / / / /

2460 13.8 13.5 7.18 998.4 506.5 167.5 27.3 1.56 469.1 110.8 170.9 17.2 0.009 3760

The water quality parameters were tested at 25 °C. The NF feed was the same batch wastewater which had been pretreated by UF. NF permeate quality was measured ever 30 h during 180 h filtration process. The corresponding water quality parameter is not required in the standard.

− solids (TDS), and inorganic ions (such as Na+, Ca2+, SO2− 4 , and HCO3 ). The values of these parameters for the wastewater were much higher than the national standard of China for the reclaimed water quality used in the industry (GB/T 19923-2005) [23]. In our experimental work, Desal-5 DK and DL NF membranes (Osmonics, USA) were used for pharmaceutical wastewater treatment. Table 2 shows the characteristics for the two membranes [12,24]. It was found that compared with the DL membrane, the DK membrane had higher rejections of organic and inorganic matters in the wastewater treatment process due to its lower pore radius. Moreover, the DK membrane had lower flux decline, since it has smooth surface that may reduce the deposition of foulants onto membrane surface [11,25]. Thus, considering the better performance, DK membrane was chosen to deal with the wastewater in this work.

2.2. Chemicals All chemicals used were of analytical grade. Sodium hydroxide (NaOH), hydrochloric acid (HCl), tetra-sodium salt of ethylene diamine tetraacetic acid (EDTA), and citric acid (CA) were used as cleaning agents in membrane regeneration process. They were purchased from Tianjin Kewei Company (China) and used with no further purification. In addition, pure water with conductivity less than 10 μs/cm was produced by a two stage reverse osmosis system. 2.3. Membrane fouling and foulant identification 2.3.1. Membrane fouling experiments The membrane fouling experiments were carried out with a flatsheet laboratory-scale NF equipment described in detail elsewhere [13]. A membrane piece with effective filtration area of 66.47 cm2 (4.6 cm radial) was loaded into a radial cross-flow cell, after being thoroughly rinsed and soaked in pure water for 24 h. At the beginning of all the experiments, the membrane pieces were precompacted with pure water under TMP of 1.8 MPa until permeate flux stabilized. Full circulation mode was used during the experiments where the retentate and permeate were returned to the feed tank in order to maintain constant concentration of NF feed. As some feed solutes deposited onto membrane surface or pipeline wall, there was a slight decrease in feed solute concentration during the process. Adding extra feed into the feed tank to minimize the change was performed. Two groups of fouling experiments were performed for 3 and 180 h, respectively. NF of the wastewater was carried out under TMP of 1.4 MPa, cross-flow velocity of 0.11 m s− 1, and temperature of 25.0 ±0.3 °C. The permeate flux was measured by weighing permeate on an electronic digital weighing scale. The rejections of organic/inorganic matters were calculated after measurements of the water quality parameters of NF

Table 2 The characteristics of Desal-5 DK and DL NF membranes. Itema

DK membrane

DL membrane

pH tolerance Temperature (max) (°C) MWCO/Da MgSO4 retention (%)b NaCl retention (%)b Membrane pore radius (nm)c Isoelectric point (pH)c Mean roughness (nm)c Permeability (ms− 1 Pa− 1)d

2–11 50 200 98.6 52.8 0.48 <3.0 9.5 1.4 × 10− 11–2.2 × 10− 11

2–11 50 400 91.9 51.0 0.52 3.0 12.0 1.6 × 10− 11–2.1 × 10− 11

a

Values without special illustration are according to the manufacturers. The test solutions are MgSO4 and NaCl aqueous solutions with concentration of 2000 mg/L, and the test conditions applied are 0.69 MPa and 25 °C. c Morão et al. [12]. d Bargeman et al. [24]. b

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feed and permeate using available standard methods [22]. For 3 h fouling experiments, after filtration of the wastewater, hydrodynamic cleaning was carried out for 30 min under TMP of 0.1 MPa, cross-flow velocity of 0.24 m s− 1, and temperature of 30.0 ±0.5 °C. For 180 h fouling experiments, hydrodynamic cleaning was carried out every 3 h. The membrane samples of 3 h and 180 h fouling after hydrodynamic cleaning were removed from the testing cell, and dried at 25 °C in vacuum for the following analyses. 2.3.2. ICP-AES/IC analysis A membrane sample with definite area of 66.47 cm2 was cut and weighed. Then, the sample was immersed in HCl (pH 2) solution and extracted by an ultrasonic cleaner for 24 h, to make sure that most of the inorganic foulants on membrane surface were dissolved in the solution. The HCl solution was titrated with 0.01 M NaOH to pH of 7.6, before being transferred into a 250 mL calibrated flask and diluted with deionized water to the volume. The concentrations of inorganic cations as well as total phosphorus and silicon in the solution were analyzed by ICP-AES (9000(N+M), Thermo Jarrell-Ash) at 25 °C. The concentrations of inorganic anions were analyzed by IC (DX-120, Dionex) at 25 °C using carbonate eluents. At least three membrane samples of the same fouling stage were prepared for the ICP-AES/IC analysis. 2.3.3. ATR–FTIR analysis In order to investigate the changes in chemical bonds on membrane surfaces, Fourier transform infrared spectroscopy (FTIR, MAGNA-560, Thermo Nicolet Corp., USA) with an attenuated total reflectance spectra (ATR, ZnSe crystal, 45°) was employed. FTIR spectra were recorded in the wave number range of 600–4000 cm− 1.

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3. Results and discussion 3.1. Membrane fouling by pharmaceutical wastewater The experiments for NF of pharmaceutical wastewater were performed with the same batch of wastewater under identical operating conditions (TMP of 1.4 MPa, cross-flow velocity of 0.11 m s− 1, and temperature of 25.0 ± 0.3 °C). Almost the same flux decline behaviors were observed with the above conditions. After the first 3 h operation, there was about 12.3% decrease in membrane permeate flux, and the flux can be restored by hydrodynamic cleaning. However, after 180 h operation, the membrane permeate flux decreased obviously from 64.12 to 17.63 L/(m2h) (about 72.5% decrease in permeate flux), which can hardly be restored by hydrodynamic cleaning. This indicates that the membranes suffered from severe fouling after a relatively long time operation. The membrane foulant identification for the above two different fouling stages (i.e., after 3 h and 180 h operations) and the chemical cleaning to remove the foulants from membrane surface after 180 h operation will be discussed in Sections 3.2 and 3.3, respectively. Table 1 shows the average NF permeate quality and its standard deviation calculated from 6 times of measurements during 180 h NF process. It is observed that although DK NF membrane suffered severe fouling in the process, the membrane was effective in removing organic and inorganic matters with acceptable rejections. The DK NF permeate quality always reached the national standard of China (GB/T 19923-2005) in the filtration process. Thus, it is feasible to treat the pharmaceutical wastewater using the DK NF membrane, providing that membrane foulants are identified, and an effective method is developed to control fouling. 3.2. Membrane foulant identification

2.3.4. SEM–EDX analysis The membrane samples were cut into 5 mm × 5 mm size and coated with gold powder on the surfaces by a sputter coating machine before the observation of SEM–EDX (XL30, Philips). The images of membrane surfaces were taken by SEM, and the surface element contents were analyzed by EDX. To get the average values, 4 different sites on each of two membrane samples prepared under the same fouling conditions were tested for the element contents. 2.4. Membrane chemical cleaning In order to investigate the cleaning efficiencies of different cleaning agents, chemical cleanings for the membranes of 180 h fouling were carried out according to the following protocol: (1) before and after fouling, the pure water fluxes were measured; (2) the membrane samples were cleaned with a cleaning agent under TMP of 0.1 MPa, cross-flow velocity of 0.24 m s− 1, and temperature of 30.0 ± 0.5 °C; and (3) the pure water fluxes were measured again after the residual of cleaning agent was removed. During the entire test run, the pure water fluxes were measured under TMP of 1.4 MPa, cross-flow velocity of 0.11 m s− 1, and temperature of 25.0 ± 0.3 °C. For each cleaning experiment with a cleaning agent, at least 3 repetitions were performed. Flux recovery ratio (FRR) was calculated from Eq. (1) to quantify the cleaning efficiency [26]. J −J FRR ð%Þ = w fw × 100 Jiw −Jfw

In order to understand the membrane fouling process during NF of pharmaceutical wastewater, the membrane foulants after 3 h and 180 h operations were identified in detail by ICP-AES/IC, ATR–FTIR, and SEM–EDX as follows. 3.2.1. Analysis by ICP-AES/IC The surface inorganic compositions of virgin membrane, membrane of 3 h fouling and membrane of 180 h fouling were analyzed by ICP-AES and IC, The results were shown in Table 3. It is seen that after 3 h operation, some sulfate and carbonate of calcium appear on membrane surface, and their contents increased after 180 h operation. This could be confirmed by the following analysis. As shown in Table 1, there are high concentrations of Ca2+, SO2− 4 , and −3 , 10.4 × 10− 3, and HCO− 3 ions with molar concentrations of 4.27 × 10 8.30× 10− 3 mol/L, respectively, in NF feed. Moreover, the concentration of CO2− 3 can be calculated by Eq. (2) [27].

2−

½CO3  =

½HCO− 3  −pH

1 + 10K

a2

+

Table 3 ICP-AES/IC analytical results for NF membrane surface foulants at different fouling stages. Item

ð1Þ

where Jiw is the pure water flux before fouling; Jfw is the pure water flux after fouling; and Jw is the pure water flux after cleaning. After chemical cleaning, SEM–EDX was applied to evaluate the effectiveness of cleaning and the presence of foulants on the membrane surfaces.

ð2Þ

10−2pH Ka1 ⋅Ka2

Calcium ion (Ca2+) Iron ion (Fe2+ and Fe3+) Silicon (Si) Phosphorus (P) Sulfate ion (SO2− 4 ) Carbonate ion (CO2− 3 ) Total a

Surface foulant content (µg/cm2 membrane area) Virgin

3 h fouling

180 h fouling

−a 0.43 ± 0.04 0.10 ± 0.00 – – – 0.53

11.2 ± 0.7 1.76 ± 0.14 0.24 ± 0.04 – 16.9 ± 0.6 8.80 ± 0.03 38.90

20.4 ± 1.2 7.62 ± 1.22 0.64 ± 0.02 3.26 ± 0.15 40.4 ± 1.6 16.1 ± 2.5 88.42

The matter is undetected by the analytical method. It is the same for Tables 4 and 5.

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− in which, [CO2− 3 ] and [HCO3 ] refer to the bulk ion concentrations, and carbonic acid dissociation constants Ka1 and Ka2 are equal to 4.3× 10− 7 and 5.61 × 10− 11, respectively, at 25 °C [28]. Thus, at pH 7.8, the molar −5 mol/L. concentration of CO2− 3 was 2.82 × 10 It should be noted that the intensity of the scale formation on the membrane is governed by Sm, the supersaturation ratio at the membrane surface, defined by:

Sm ðCaSO4 Þ =

½Ca2 + m ½SO2− 4 m Ksp ðCaSO4 Þ

Sm ðCaCO3 Þ =

½Ca2 + m ½CO3 m Ksp ðCaCO3 Þ

ð3Þ

2−

ð4Þ

where Ksp is the thermodynamic solubility product. At 25 °C, Ksp for CaSO4 and CaCO3 is 7.1×10− 5 and 5.0×10− 9 mol2/L2, respectively. 2− [Ca2+]m, [SO2− 4 ]m, and [CO3 ]m refer to membrane surface ion concentrations. They are related to the bulk ion concentrations [Ca2+], [SO2− 4 ], and [CO2− 3 ] according to the concentration polarization degree CP, given by [29]: CP =

  ½Ca2 + m ½SO2− ½CO2− J 4 m 3 m = = = ð1−R0 Þ + R0 exp v ð5Þ 2 + 2− 2− k ½Ca  ½SO4  ½CO3 

where Jv is the permeate flux. R0 is the observed (nominal) salt rejection. k is the mass transfer coefficient. The value of k is estimated based on the Lévêque equation using the average Sherwood number (Sh) for a radial cross-flow membrane cell [30]: Sh =

  k⋅h h 1=3 = 1:47 Re⋅Sc⋅ D r

ð6Þ

in which, Re=u0h/ν, Sc =ν/D, h is the channel height, r is the channel radial, u0 is the average cross-flow velocity, ν is the kinematic viscosity of the feed solution, and D is the solute diffusion coefficient. D is about 0.90×10− 9 m2/s at 25 °C for the calcium salt system at infinite dilution [31,32]. In the present study, with u0 = 0.11 m/s, h = 0.0040 m, r=0.046 m, and v=1.0×10− 6 m2/s, it was obtained that Re=440, Sc ≈1110, and k≈1.2×10− 5 m/s. The average permeate flux and salt rejection of the NF membrane were about 8.5×10− 6 m/s and 85%, respectively. Thus, the estimated degree of concentration polarization in the present work was 1.9. From Eqs. (3), (4), and (5), it was calculated that the supersaturation ratios of CaSO4 and CaCO3 at the membrane surface were 2.3 and 87, respectively. In the previous studies [31–33], it was reported that the scale formations of CaSO4 and CaCO3 occurred at the supersaturation ratios at the membrane surface higher than 1 and 4, respectively. The much higher supersaturation ratios observed in the present work could result in severe depositions of CaSO4 and CaCO3 onto the membrane surfaces. The total value of the tested scales on the membrane surface (shown in Table 3) reaches as high as 88.42 µg/cm2 after 180 h operation, which confirms that NF membrane suffered from severe scaling [16]. The severe scaling on membrane surface can be removed by effective chemical cleaning after fouling as discussed in Section 3.3. Another option is to prevent or minimize scaling by pretreatment of NF feed (such as pH adjustment and anti-scalant dosing), which will be studied in our future work. From Table 3, it is also observed that small amounts of iron and silicon are detected on the virgin membrane surface. No iron and silicon in the composition of DK membrane have been reported by manufacturer or other researchers. Thus, we consider that the appearances of iron and silicon in the virgin membrane surface could be from instruments and/or vessels used in the sample preparation process before ICP-AES analysis. In addition, the amount of iron increased on membrane surface especially after a relatively long time operation. This could be partly caused by the deposition of iron salts from the NF feed,

and partly caused by the corrosion of steel pipelines used in NF equipment during the relatively long time operation. 3.2.2. Analysis by ATR–FTIR In order to identify the compositions of organic and inorganic deposits on the membrane surfaces at different fouling stages, the FTIR spectra of membrane samples including virgin membrane, membrane of 3 h fouling and membrane of 180 h fouling were compared and shown in Fig. 1. The FTIR spectrum for the virgin DK membrane, presented in Fig. 1, shows characteristics of both polyamide barrier layer and polysulfone support layer [34]. The broad band centered at 3372 cm− 1 is attributed to the overlapping of bands from the stretching vibrations of N–H and O–H. The bands at the ranges of 2850–3000 cm− 1 and 670–1050 cm− 1 are attributed to C–H stretching and blending vibrations, respectively. The bands at 1584 and 1480 cm− 1 are assigned to C C stretching in aromatic ring. The bands at 1652 and 1540 cm− 1 corresponding to C O stretching (amide I) and N–H in-plane bending (amide II), respectively are characteristics for polyamide in the barrier layer [34]. The band at 1245 cm− 1 corresponding to Ar–O–Ar stretching (Ar denotes an aromatic ring) is characteristic for polysulfone in the support layer [35]. From the FTIR spectrum for the membrane of 3 h fouling, it can be seen that the bands in the virgin membrane spectrum are either eliminated or severely attenuated due to coating by the foulants. There is no obvious band in the wave number range of 2500–4000 cm− 1, suggesting that the fouling layer on membrane surface of 3 h fouling could consist a large amount of inorganic matters, as organic matters always have apparent bands in this wave number range [36]. A new intense band at 1040 cm− 1 appears for the membrane of 3 h fouling, as compared with the virgin membrane. This band is in the wave number range of 1040–1100 cm− 1 2− which is characteristic for SO2− 4 and/or CO3 ions [36]. Combined with the analysis results of ICP-AES/IC, it is concluded that sulfate and carbonate of calcium appeared on the membrane surface after 3 h operation. Since no other intensified bands or new bands besides

Fig. 1. Comparison of the FTIR spectra of NF membranes: virgin membrane, membrane of 3 h fouling, and membrane of 180 h fouling.

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1040 cm− 1 were observed in the FTIR spectrum for the membrane of 3 h fouling, the calcium salts could be the main foulants on the membrane surface after 3 h operation. The deposition of calcium salts on the membrane surface can be seen from SEM images shown in the next section. The FTIR spectrum for the membrane of 180 h fouling is also shown in Fig. 1. It is seen that compared with the bands in the FTIR spectrum for the membrane of 3 h fouling, some bands are obviously intensified, and some new bands appear for the membrane of 180 h fouling, due to the severe fouling and formation of thicker foulant layer after a relatively long time operation. The much more intensified band at 1040 cm− 1 indicates more severe deposition of calcium salts on the membrane surface after 180 h operation. Moreover, the new broad bands at 3287 cm− 1 (O–H and/or N–H stretching) and 2850–3000 cm− 1 (C–H stretching) suggest an obvious increase in the amount of organic matters in the fouling layer. In addition to the broad band at 3287 cm− 1, new bands at 1730 cm− 1 (C O stretching) and 1230 cm− 1 (C–O stretching) appear, suggesting the presence of carboxylic acid functional groups in the foulants. The obviously intensified bands at 1652 cm− 1 (amide C O stretching) and 1540 cm− 1 (amide N–H bending) confirm that there are some other sources of amide functional groups (such as protein), besides polyamide in membrane barrier layer. The new bands that appear in the range of 600–800 cm− 1 are probable for the stretching vibration of alkyl halide (such as C–Cl and C–Br), as confirmed by EDX analysis in the next section. All these results indicate that after 180 h operation, besides calcium salts, complex organic foulants containing carboxyl acid, amide, and alkyl halide functional groups also deposited onto membrane surface, causing severe fouling. As discussed above, the concentration polarization caused the supersaturation of CaSO4 and CaCO3 at the membrane surface and their severe deposition on the membrane surface at the initial fouling stage. At this stage, the electrostatic adsorption of inorganic cations onto the negative membrane surface could cause a decrease in surface charge, as confirmed by Hong [9] and Mo [37] through zeta-potential measurements. On the other hand, the calcium ions with high concentration of 4.27× 10− 3 mol/L in NF feed could react with acidic functional groups (predominantly carboxyl acid) of organic matters to form complexes [9], which were accelerated at membrane surface due to the higher concentrations of both calcium ions and organic matters that resulted from concentration polarization. This led to a decline in the negative charges of organic matters especially those at the membrane surface [9,13,31,32]. The decreases in the negative charges of both membrane surface and organic matters could cause a decrease in electrostatic repulsion between membrane surface and organic matters, and thus cause an increase in deposition rate of organic matters onto membrane surface, which led to gradual formation of a densely packed fouling layer. Moreover, bridging between organic macromolecules mediated by calcium complexation may also contribute to formation of compact fouling layer [9]. The above discussions could be the reasons for the severe fouling after 180 h operation. 3.2.3. Analysis by SEM–EDX In order to characterize the changes in morphology at different fouling stages, membrane samples including virgin membrane, membrane of 3 h fouling, and membrane of 180 h fouling were analyzed by SEM. From Fig. 2, it is observed that after 3 h operation, a large amount of white crystal substances appear in the fouling layer, which should be sulfate and carbonate of calcium as confirmed by ICPAES/IC and ATR–FTIR analyses. After 180 h operation, a densely packed fouling layer which could consist of both organic matters and inorganic salts was observed on the membrane surface. Combined with SEM, EDX was used to analyze the element contents in fouling layers [16]. The results are shown in Table 4. It is observed that the element calcium appears on the membrane surface after 3 h operation, and its weight percent increases from 3.54 to

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4.84% after 180 h operation. This should be caused by the deposition of calcium salts during NF of pharmaceutical wastewater as discussed above. In addition, chlorine and bromine were also found on the fouled membrane surfaces (especially for the membrane of 180 h fouling). These were probably from alkyl halides, as consisted with ATR–FTIR analysis. The alkyl halides with hydrophobic properties could deposit onto membrane surface via hydrophobic interactions [8]. 3.3. Membrane chemical cleaning and regeneration For the membranes of 180 h fouling, chemical cleanings with citric acid (CA), hydrochloric acid (HCl), sodium hydroxide (NaOH), and

Fig. 2. Comparison of SEM images of NF membrane samples at different fouling stages: (a) virgin membrane, (b) membrane of 3 h fouling, and (c) membrane of 180 h fouling.

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Table 4 EDX analytical results for NF membrane surfaces at different fouling stages. Element

C O S P Si Ca Fe Cl Br Total

Weight percent (%) Virgin

3 h fouling

180 h fouling

69.30 27.52 3.18 – – – – – – 100

61.32 28.38 5.02 – – 3.54 0.34 1.07 0.33 100

57.23 25.22 6.12 0.54 0.08 4.84 0.96 3.41 1.60 100

tetra-sodium salt of ethylene diamine tetraacetic acid (EDTA) aqueous solutions were performed to remove foulants from membrane surfaces. The chemical cleaning was carried out for a time within the range of 10 to 100 min under TMP of 0.1 MPa, cross-flow velocity of 0.24 m s− 1, and temperature of 30.0 ± 0.5 °C. The pH values of acid (CA or HCl) and alkaline (NaOH) used in cleaning process had been optimized by comparisons of the cleaning efficiencies of 60 min at different pH values from 2 to 5 and from 9 to 11, respectively. The concentration of EDTA had been optimized by comparisons of the cleaning efficiencies of 60 min at different concentrations from 5 to 15 mM. The optimization results showed that the best cleaning efficiency for each cleaning agent was obtained when the cleanings were carried out with CA solution of pH 2, HCl solution of pH 2, NaOH solution of pH 11, and EDTA solution of 10 mM, respectively. 3.3.1. Characterization by membrane flux and rejection The chemical cleaning efficiencies of CA (pH 2), HCl (pH 2), NaOH (pH 11), and EDTA (10 mM) with respect to membrane flux recovery ratios (FRR) are shown in Fig. 3. The cleaning efficiency of pure water is used to serve as a baseline for the comparison. It is observed that with increasing cleaning time, FRR increase more obviously for the membranes cleaned by metal chelating agent (EDTA) or acids (CA and HCl) than that for the membranes cleaned by alkaline (NaOH). After 60 min cleanings, FRR increased in the sequence of NaOH (pH 11) < HCl (pH 2) < CA (pH 2) < EDTA (10 mM), which are 60.4%, 83.3%, 93.6%, and 99.0%, respectively. The reasons are shown below. As discussed in Section 3.2, at the initial fouling stage, the main foulants were sulfate and carbonate of calcium, and at the later fouling stage, complex organic foulants also deposited onto membrane surface. In another word, the inorganic foulants were the main composition in the fouling layer closest to membrane surface, and the organic foulants were mostly in the outer surface of fouled membrane. Accordingly, in cleaning process, a cleaning agent should react with these foulants to lessen foulant–membrane and foulant–foulant interactions, and finally formed a lessoned fouling layer that can be removed from membrane surface via mass transfer [18]. Metal chelating agent or acids could react with the inorganic foulants closest to membrane surface to lessen foulant–membrane interaction, and made the whole fouling layer become looser and easier to be removed via mass transfer. Alkaline could react with the outer surface organic foulants to lessen foulant– foulant interaction to a certain extent. But it had difficulty in removing the foulants closest to membrane surface and the whole fouling layer. As a consequence, higher cleaning efficiency was obtained after cleaning with metal chelating agent or acids than with alkaline. A schematic representation for membrane fouling and effective chemical cleaning for NF membrane in pharmaceutical wastewater treatment process was described and shown in Fig. 4. In the cases of cleaning with CA and HCl, FRR increased obviously with increasing cleaning time (see Fig. 3), due to their efficiencies in dissolving the inorganic salts especially those that closest to membrane

Fig. 3. Variation of membrane FRR with respect to cleaning time.

surface to lessen foulant–membrane interaction as discussed above. In addition, CA is a metal chelating agent. It may have some reactions with calcium ions in organic matter–calcium complexes to form soluble complexes. Thus, the cleaning efficiency of CA was higher than that of HCl. However, since CA and HCl are not efficient in removing the organic foulants and calcium sulfate scale, they are not the best cleaning agents for the membrane fouled by the pharmaceutical wastewater. EDTA was the most favorable cleaning agent for the membrane fouled by the pharmaceutical wastewater with highest FRR during the cleaning process (see Fig. 3), as compared with other investigated cleaning agents. EDTA is a strong metal chelating agent. It can react with calcium ions in sulfate and carbonate of calcium to form soluble complexes. It can also react through ligand-exchange with calcium ions in organic matter– calcium complexes. These two kinds of reactions between EDTA and the foulants across the whole fouling layer to form soluble complexes cause the total breakdown of the densely packed fouling layer, which could be the reason for the best efficiency of the EDTA cleaning. Moreover, it was found that after relatively long time cleaning, cleaning efficiencies of EDTA and CA exceeded 100%. That is, the FRR was 102% after cleaning with EDTA for 80 min, and it was 101% after cleaning with CA for 100 min. These results suggest that not only cleaning agents recover membrane permeability but also increase it. The phenomena are attributed to the interactions between cleaning agents and membrane surfaces [18,35]. Since it is inevitable that cleaning agents contact membrane surfaces directly, the adsorption of cleaning agents which contain polar covalent bonds (e.g., CA contains –OH and –COOH bonds in molecular structure) onto membrane surfaces may improve surface hydrophilicity, causing higher permeability for the cleaned membranes [35]. Furthermore, an impact of cleaning on the membrane rejection has been studied by measurements of conductivity and COD in NF feed and permeate. There were about 0.7% and 0.8% increases in the rejections of conductivity for the membranes cleaned by CA and HCl, respectively. However, there were about 1.2% and 1.0% decreases in the rejections of conductivity for the membranes cleaned by NaOH and EDTA, respectively. Consistent with Refs. [19,21], the acidic cleaning plays a role in preserving the membrane ion retention capability, probably by making the membranes tighter by charge neutralization, which is just opposite for the alkaline cleaning. Moreover, there were no obvious changes in the rejections of COD after cleanings with the agents. The membrane permeate quality

X. Wei et al. / Desalination 251 (2010) 167–175

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Fig. 4. Schematic representation for NF membrane fouling process and effective chemical cleaning in pharmaceutical wastewater treatment process: (a–b) At the initial fouling stage, the foulants closest to membrane surface are formed by deposition of inorganic matters. (b–c) At the later fouling stage, the organic foulants bound by calcium ions in the form of complexes gradually deposit onto membrane to form densely packed fouling layer. (c–d) During cleaning process, the cleaning agent contacts membrane surface foulants via mass transfer. (d–e) The agent reacts with the foulants especially those that closest to membrane surface, yielding loosened fouling layer. (e–f) These reaction products are removed from membrane surface via mass transfer. (f–g) A cleaned membrane surface is obtained after an effective chemical cleaning.

always reaches the national standard (GB/T 19923-2005) after each chemical cleaning, in the scope of the presented work. 3.3.2. Characterization by SEM–EDX The 180 h fouled membranes after being cleaned with chemicals for 60 min were also characterized by SEM–EDX (see Fig. 5 and Table 5). As shown in SEM images, the clean degrees of membrane surface after cleaning with the agents increased in the sequence of NaOH (pH 11) < HCl (pH 2) < CA (pH 2) < EDTA (10 mM). The SEM image of the membrane cleaned with EDTA was quite similar to that of the virgin membrane (see Fig. 2). This confirms that EDTA has the best cleaning efficiency for the membrane fouled by the pharmaceutical wastewater. Moreover, EDX analytical results (Table 5) show that after cleaning with the agents, the weight percents of foulant elements decrease in the sequence of NaOH (pH 11) > HCl (pH 2) > CA (pH 2) > EDTA (10 mM). For example, the weight percents of element calcium are 5.12, 2.16, 0.44, and 0%, respectively. The element contents of the membrane cleaned by EDTA are quite similar to that of the virgin membrane (see Table 4). The SEM–EDX analysis results further confirm the effectiveness of the chemical cleanings characterized by FRR. 4. Conclusions The foulant identification and chemical cleaning for Desal-5 DK NF membrane were studied during advanced treatment of a complex pharmaceutical wastewater. The membrane foulants were identified by

ICP-AES/IC, ATR–FTIR, and SEM–EDX. It was found that the fouling layer was a mixture of organic and inorganic matters. There were two distinct fouling stages. At the initial fouling stage, membrane fouling was mostly caused by the deposition of sulfate and carbonate of calcium. This was due to high contents of inorganic ions in NF feed and concentration polarization. At the later fouling stage, besides calcium salts, complex organic foulants containing carboxyl acid, amide, and alkyl halide functional groups also deposited onto membrane surface. This was attributed partly to the adsorption of cations to membrane surface at the initial fouling stage which reduced the negative charge of membrane surface, and partly to the formation of complexes between organic matters and calcium ions in the bulk which reduced the negative charge of feed organic matters. The decreases in the negative changes of both organic matters and membrane surface could reduce the electrostatic repulsion between organic matters and membrane surface and accelerate the deposition of organic matters onto membrane surface, gradually forming a densely packed fouling layer. Accordingly, in cleaning process, the efficiency of a cleaning agent depended on its ability to break down the integrity of the compact fouling layer by reacting with the foulants. The results showed that the cleaning efficiencies of the agents increased in the sequence of NaOH (pH 11) < HCl (pH 2) < CA (pH 2) < EDTA (10 mM). EDTA was the most favorable cleaning agent in the process, because it could react through ligandexchange with calcium ions in both inorganic foulants and organic matter–calcium complexes to produce soluble complexes, which finally resulted in the total breakdown of the densely packed fouling layer.

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Fig. 5. Comparison of the surface SEM images of 180 h fouled membranes after 60 min cleaning with different agents: (a) NaOH (pH 11), (b) HCl (pH 2), (c) CA (pH 2), and (d) EDTA (10 mM).

Symbols CP [Ca] [Ca]m [CO2− 3 ] [CO2− 3 ]m [SO2− 4 ] [SO2− 4 ]m [HCO− 3 ] D h FRR Jv Ksp Jfw Jiw

concentration polarization degree, dimensionless concentration of Ca ion in the bulk of the solution, mol/L concentration of Ca ion at the membrane surface, mol/L concentration of CO2− 3 ion in the bulk of the solution, mol/L concentration of CO2− 3 ion at the membrane surface, mol/L concentration of SO2− 4 ion in the bulk of the solution, mol/L concentration of SO2− 4 ion at the membrane surface, mol/L concentration of HCO− 3 ion in the bulk of the solution, mol/L solute diffusion coefficient, m2/s channel height, m flux recovery ratio, % Permeate flux, m/s thermodynamic solubility product, mol2/L2 pure water flux after fouling, L/(m2h) pure water flux before fouling, L/(m2h)

Table 5 Comparison of the EDX analytical results of 180 h fouled membranes after 60 min cleaning with different agents. Element

C O S Ca Fe Cl Br Total

Jw k Ka1 Ka2 r R0 Re Sc Sh Sm u0 ν

pure water flux after cleaning, L/(m2h) mass transfer coefficient, m/s the first dissociation constant of carbonic acid, dimensionless the second dissociation constant of carbonic acid, dimensionless channel radial, m observed (nominal) salt rejection, % Reynolds number, dimensionless Schmidt number, dimensionless Sherwood number for feed side, dimensionless supersaturation ratio at the membrane surface, dimensionless average cross-flow velocity, m/s kinematic viscosity of the feed solution, m2/s

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20676095), the Program for New Century Excellent Talents in University, the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education (No. IRT0641) and the Program of Introducing Talents of Discipline to Universities (No. B06006).

Weight percent (%) after cleaning with NaOH (pH 11)

HCl (pH 2)

CA (pH 2)

EDTA (10 mM)

57.23 29.25 7.28 5.12 1.12 – – 100

63.75 28.01 4.41 2.16 – 1.23 0.44 100

67.72 27.92 3.53 0.44 – 0.39 – 100

68.81 27.65 3.54 – – – – 100

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