Effects of membrane compositions and operating conditions on the filtration and backwashing performance of the activated carbon polymer composite membranes

Desalination 352 (2014) 181–189

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Effects of membrane compositions and operating conditions on the filtration and backwashing performance of the activated carbon polymer composite membranes Li-Luen Hwang a, Ming-Yen Wey a, Jyh-Cherng Chen b,⁎ a b

Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Department of Safety, Health and Environmental Engineering, Hungkuang University, Taichung 433, Taiwan, ROC

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• We prepared the AC composite membranes with different composition ratios. • Their filtration and backwashing performances were tested and studied. • The AC composite membrane had great PWF, PF, FE, and FRR. • The addition of AC improved the performance of composite membranes.

a r t i c l e

i n f o

Article history: Received 18 July 2013 Received in revised form 26 July 2014 Accepted 26 August 2014 Keywords: Activated carbon Composite membrane Permeation flux Backwashing Flux recovery rate

a b s t r a c t This study prepares different activated carbon (AC) polymer composite membranes and investigates their operation characteristics of filtration and backwashing. Experimental results show that the addition of AC particles and hydrophilic polyethylene glycol (PEG) in the polymer membranes can improve the pure water flux (PWF) and permeation flux (PF) during filtration, as well as increase the flux recovery rate (FRR) after backwashing. The optimum transmembrane flux, filtration pressure and time of AC composite membrane were 45 Lm−2 h−1, 196.8 kPa and 60 min; and the optimum backwashing pressure and time were 393.6 kPa and 10 min, respectively. Under such conditions, the flux recovery rate of the AC composite membranes achieved 87.5%. © 2014 Elsevier B.V. All rights reserved.

1 . Introduction Membrane filtration techniques are widely applied in the treatments of drinking water, waste water, and water recycling nowadays. ⁎ Corresponding author. Tel.: +886 4 26318652x4109; fax: +886 4 26525245. E-mail address: [email protected] (J.-C. Chen).

http://dx.doi.org/10.1016/j.desal.2014.08.022 0011-9164/© 2014 Elsevier B.V. All rights reserved.

Compared with the traditional water treatment processes, membrane technology is more attractive due to its high efficiency, flexibility, and stability in removing various pollutants simultaneously [1–3]. However, membrane fouling is still a major problem in the application of organic polymer membranes. Membrane fouling is caused by the clogging of membrane pores and the accumulation of pollutants on the membrane surfaces, which increases the filtration resistances, declines the

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filtration flux, and reduces the validity period of membranes [4–10]. Therefore, membranes need to be cleaned and backwashed periodically or renewed. To overcome such problems, modification of membranes by various methods is prevailing. The development of a composite membrane has become an attractive issue in membrane technology. With the addition of different materials and compositions, the chemical and physical properties, the antifouling ability, the filtration efficiency, and the backwashing efficiency of composite membranes are improved [11–13]. Table 1 summarizes the results of relevant studies on the preparations, compositions, properties, and operating characteristics of different composite membranes. Many studies indicated that the composite membranes prepared by the phase inversion method can significantly improve the surface morphology and structures of polymer membranes [14–18]. The composite membranes were made by mixing different hydrophilic polymers (such as polyetherimide (PEI), polyvinylpyrrolidone (PVP), PEG, cellulose acetate phthalate (CAP), and polyamideimide (PAI)) and inorganic materials (such as titanium dioxide (TiO2), aluminum oxide (Al2O3), and silica (SiO2)) to modify the hydrophobic polymer membranes (such as polyvinylidene fluoride (PVDF), polysulfone (PSF), polyethylsulfone (PES), and polyethylene (PE)) [19–25]. The results indicated that the modified composite membranes had better hydrophilicity, surface roughness, porosity, permeation flux and separation efficiency. Membrane fouling during filtration processes usually resulted from pollutant precipitations, electrostatic adsorption, and the bio-fouling [4,6,10,26,27]. To remove the reversible and irreversible fouling on the surface and structure of polymer membranes, it is necessary to backwash the membranes with different backwashing pressure, frequency, reagents, and cleaning methods to enhance the permeation flux and flux recovery rates (UF/MF) [1,4,28–30]. The backwashing frequency usually depends on the filtration resistances and pressures. When the filtration pressure increases, the fouling of organic polymer membranes becomes worse and the backwashing efficiency declines. Pure water flux and permeation flux of membranes are thus difficult to recover [31–34]. The membrane structure property, filtration efficiency, and different membrane fouling mechanisms can be characterized through the operation profiles of filtration and backwashing procedures at different pressures and conditions [6,7,35,36]. The preparation and application of activated carbon (AC) composite membranes are seldom investigated in related literatures. The preparation and characteristics of different AC composite membranes have been studied in our previous works [16]. The results illustrated that the addition of activated carbon (AC) particles and hydrophilic polyethylene glycol (PEG) in the composite membranes can improve the porous structures, hydrophilicity, permeation flux, and filtration efficiency. To explore the filtration performance and backwashing characteristics of AC composite membranes, this study prepares different AC composite membranes to perform the filtration and backwashing

experiments for three cycles. The effects of different filtration and backwashing conditions on the pure water flux (PWF), permeation flux (PF), and flux recovery rate (FRR) of the AC composite membranes are also discussed. 2 . Experimental 2.1 . Materials This study prepares five AC composite membranes with different composition ratios and investigates their performance at different filtration and backwashing conditions. The major components of the composite membranes were polyphenylsulfone (PPSU, MW: 53,000– 59,000 g/mol) and polyetherimide (PEI, MW: 529 g/mol). Polyethylene glycol (PEG, MW: 200 g/mol) was used as the pore-formation agent and coconut shell-based activated carbon (AC) (China Activated Carbon Industries Ltd., Taiwan) was the additive particle. The solvent was N-methyl-2-pyrrolidone (NMP). The particle size distributions of AC particles had three peaks of particle sizes at 0.01–0.1 μm, 1–10 μm, and 10–50 μm. The specific surface area, pore volume, micro-, meso-, and macro-pore volumes (percentages), and the average pore size of the AC particle were 524 m2 g−1, 0.431 cm3 g− 1, 0.226 cm3 g− 1 (52%), 0.194 cm3 g− 1 (45%), 0.011 cm3 g−1 (3%), and 3.29 nm, respectively [16]. The prepared AC composite membranes were tested by filtering the simulated water with low concentrations of humic acids (HAs). The simulated water for the composite membrane tests was prepared by dissolving 1.4 g of HA powder (Sigma-Aldrich) into 1 L of deionized water and was pre-filtered using a filter with a mean pore size of 2.5 μm to remove the larger HAs. The pH value of simulated water was controlled at 6.7 ± 0.1 by 0.1 M HCl or NaOH [37–39]. The HA concentration in the feed solutions and resultant permeate solutions were determined by a UV–Vis spectrometer (UV–Vis/DRS; Perkin Elmer Lambda 35) at the wavelength of 254 nm [40–42]. The HA concentration in the pre-filtered solution was 476 mg/L. 2.2 . Membrane preparation All the AC polymer composite membranes were fabricated using the wet phase inversion method. The composition ratios of five different AC composite membranes are shown in Table 2. The major components of the membranes were 35 wt.% PPSU and 5 wt.% PEI polymers dissolved in N-methyl-2-pyrrolidone (NMP) solvent. The contents of the AC particles and pore-formation agent PEG were 0.1, 0.25 wt.% and 6 wt.%, respectively. The casting solutions containing PPSU, PEI, AC particles, PEG, and NMP were stirred at 500 rpm and heated at 60 °C for 2 h. Subsequently, the casting solution was carefully and uniformly spread on a glass plate using a casting knife. The solvent that remained in the casted membrane was evaporated at room temperature overnight. The casted

Table 1 Comparisons of the relevant studies on the compositions, properties, and operating conditions of different composite membranes. aPES: polyethersulfone; bDMMSA: hydrophilic N,N-dimethylN-methacryloxyethyl-N-(3-sulfopropyl); cBMA: n-butyl methacrylate; dMPC: methacryloyloxyethyl phosphorylcholine; eBMA: butyl methacrylate; fPVDF: polyvinylidene fluoride; g SPES: sulfonated polyethersulfone; hPVP: polyvinylpyrrolidone; iTiO2: titanium dioxide; jPSF: polysulfone; kPAI: polyamideimide. Membrane materials

Compositions (%)

Preparation method

Contact angle (°)

Flow rate pressure

Flow rate time (min)

Backwash pressure

Backwash time (min)

Result

Refs.

PSf/MWNTs PES/PVA/PVP/TiO2 PVDF + SPES/PVP/TiO2

15/4 16/1.5/2/0.1 16/4/0.5

Phase inversion Phase inversion Phase inversion

56 43.2 72.2

1–4 bar 5 bar 1 bar

120 30 min 120

– – 1 bar

– –

[19] [20] [21]

PSf/PAI/UV–TiO2

69/30/1

Phase inversion

61.0

1725 kPa

120

1725 kPa

120

PES/PEG/MPC–BMA

5.38/4.49/0.47

Phase inversion

35.0

100 kPa

30

150 kPa

20

PES/PEG /DMMSA–BMA

5.38/4.49/0.47

Phase inversion

49.0

0.1 MPa

60

0.1 MPa

60

PES/PVP/TiO2

15/5/0.3

Phase inversion

72.0

100 kPa

30

100 kPa

1

PVP 55000 rejection = 63% NaCl rejection = 41% BSA rejection = 79% FRR = 76.4% HA rejection = 86% FRR = 79.5% BSA rejection = 59% RFR = 91% BSA rejection = 95% RFR = 47.6% BSA rejection = 71%

10

[22] [23] [24] [25]

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183

Table 2 Compositions and operating conditions of different AC polymer composite membranes. Composite membrane code

PPSU/PEI (wt.%)

PEG 200 (wt.%)

AC (wt.%)

Operating conditions Filtration pressure (kPa)

Filtration time (min)

Backwash pressure (kPa)

Backwash time (min)

PWF measurement time (min)

PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5

35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5 35/5

6 6 6 0 0 6 6 6 0 0 6 6 6 0 0

0 0.1 0.25 0.1 0.25 0 0.1 0.25 0.1 0.25 0 0.1 0.25 0.1 0.25

98.4 98.4 98.4 98.4 98.4 196.8 196.8 196.8 196.8 196.8 393.6 393.6 393.6 393.6 393.6

120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

196.8 196.8 196.8 196.8 196.8 393.6 393.6 393.6 393.6 393.6 393.6 393.6 393.6 393.6 393.6

10 10 10 10 10 10 10 10 10 10 60 60 60 60 60

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

membrane was gently immersed in a water bath for 6 h, and the water temperature was controlled at 28 ± 2 °C to ensure complete precipitation and membrane formation. After immersion for 6 h in water bath, the membranes were peeled off the glass plate and rinsed with deionized water to remove the residual solvent and pore-forming agent (PEG). The finished membranes were stored in the deionized water for the filtration tests [14,15,43].

operating pressures of 98.4, 196.8, and 393.6 kPa for 120 min. The temperature of feeding water was maintained constant at 28 ± 2 °C. The operating conditions (pressure and time) for membrane filtration and backwashing tests were determined from the relevant references [20, 21,45] and our previous work [16]. 2.3.1 . Pure water flux (PWF) The pure water flux (PWF, Jw1) through the membrane filtration system is calculated from the experimental data of 120 min filtration test by Eq.(1).

2.3 . Filtration and backwashing tests of AC composite membranes A laboratory-scale membrane filtration and backwashing system was designed and setup for testing the prepared AC composite membranes. As shown in Fig. 1, the filtration and backwashing system is equipped with the automatic filtration and backwashing units as well as the flow and pressure monitoring/controlling devices. Each membrane was initially compacted with 490.67 kPa pure water for 30 min. Subsequently, the membrane filtration test was conducted at different

Jw1 ¼

Q A  ΔT

ð1Þ

where Jw1 (kg m− 2 h− 1) is the pure water flux (PWF), Q (kg) is the weight of filtrate, A (m2) is the membrane area, and ΔT (h) is the filtration time.

Pressure and flux feedback control

Agitator v

CM

A

F

Concentrated tank

Raw water tank/ Feed tank P P

peristaltic pump

F

Membrane module Permeate tank

Backwash water tank

i Mac

peristaltic pump P:Pressure transducer

F:Flow meter

Electronic Balance

CM:Conductivity Meter

Fig. 1. Schematic diagram of membrane filtration system.

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2.3.2 . Membrane resistance (Rm) The membrane resistance is defined as the pressure drop from the permeation of pure water through the membrane. The membrane resistance is equal to the tolerance of a membrane to the hydraulic pressure during the filtration process [5,31,44]. It can be determined by Eq.(2). Rm ¼

ΔP μ  Jw1

ð2Þ

where ΔP is the transmembrane pressure (98.4, 196.8, and 393.6 kPa) and μ is the viscosity of pure water. 2.3.3 . HA removal efficiency (R) The HA removal efficiency is defined as Eq. (3) R ð%Þ ¼

  Cp  100 1‐ Cf

ð3Þ

where Cp and Cf (mg mL−1) are the HA concentrations in the filtrate and feed, respectively. 2.3.4 . Membrane fouling resistance and FRR of the composite membranes After the AC composite membranes performed the filtration of HA solution for 2 h, the fouled membrane was backwashed with deionized water at different backwashing pressures of 196.8 kPa and 393.6 kPa for 10 and 60 min. The PWF (Jw2) through the membrane was measured again after the backwashing process. The FRR was calculated using the following equation: FRR ¼

Jw2  100 Jw1

ð4Þ

The FRR value can be used to represent the antifouling property of the UF membrane. With the higher FRR values, the membranes have better antifouling ability. On the other hand, the membrane fouling can be indicated by the relative flux reduction (RFR) during the membranes filtration processes. The relative flux reduction can be derived from the equation: RFR ¼ 1−

Jwf  100 JHA

ð5Þ

where Jwf is the final PF after the filtration of HA solution (kg m−2 h−1) and JHA is the initial PF of the membrane (kg m−2 h−1). The fouling mechanism of a membrane can be determined from the reversible (Rc) and irreversible (Rir) fouling resistances. For a simple UF system, Darcy's law is suitable for characterizing the fouling resistance [7,26,43,45]: J¼

ΔP μ  Rt

Rt ¼ Rm þ Rr ¼ Rm þ Rc þ Rir

ð6Þ

ð7Þ

where Rt is the total resistance (m−1) and Rr is the total fouling layer resistance (m− 1) composed of both Rc and Rir. The Rr resistance is calculated using the following equation: Rr ¼

ΔP −Rm μ  JHA

ð8Þ

The membrane fouling that cannot be removed by backwashing is irreversible fouling, which can be calculated using the following equation: Rir ¼

ΔP −Rm μ  Jw2

ð9Þ

The membrane fouling that can be removed by backwashing is defined as reversible fouling: Rc ¼ Rr −Rir

ð10Þ

The definitions and units of all the symbols are summarized in Table 3. 3 . Results and discussion 3.1. The PWF and Rm of different composite membranes The PWF and Rm of five AC composite membranes were measured at different operating pressures (98.4, 196.8, and 393.6 kPa) and shown in Fig. 2(a–c). The results indicated that the PWF of five AC polymer composite membranes were all increased significantly with the rising operating pressures, and the addition of AC particles in the polymer composite membranes can effectively improve the PWF from 42.46, 94.29, and 202.97 L/m2 h to 87.06, 194.57, and 553.94 L/m2 h, respectively. Similar results have been reported in the relevant literatures that additions of inorganic particles in the composite membranes can increase the PWF as well as reduce the Rm. However, overloading of inorganic particles may affect the permeability of composite membranes and decreased the PWF [22,25,44]. The PF and Rt of five AC composite membranes at different operating pressures are shown in Fig. 3(a–c). The results indicate that the composite membrane with AC particles (PA5) had higher PF than the polymer composite membrane (PA1), and the PF increased from 8.92, 17.09, and 26.8 L/m2 h to 45.6,103.09 and 168.8 L/m2 h, respectively. From the results and SEM pictures in our previous study [16], the polymer membrane (PA1) had less porous structures, and the pore number and pore size in the surface and bulk of the AC composite membranes (PA2–PA5) were increased due to the addition of AC particles. The PF decline can be mitigated and the Rt and Rm can be reduced with the addition of AC particles. 3.2 . The PF and HA removal efficiency of AC composite membranes at different operating pressures Fig. 4 summarizes the PF and HA removal efficiency of the AC composite membranes at different operating pressures. The results

Table 3 The definition and unit of symbols used in this study. Symbol

Unit

Definition

Ww Wd Jw1 Q A ΔT Rm ΔP R Cp Cf FRR Jw2 RFR Jwf JHA Rc Rir Rt Rr μ θ

[g] [g] [kg m−2 h−1] [kg] [m2] [h] [m−1] [kPa] [%] [mg mL−1] [mg mL−1] [%] [kg m−2 h−1] [%] [kg m−2 h−1] [kg m−2 h−1] [m−1] [m−1] [m−1] [m−1] Pa s [degree]

Wet membrane weight Dry membrane weight Pure water flux The weight of filtrate Membrane area Filtration time Membrane hydraulic resistance Transmembrane pressure Humic acid removal efficiency Concentration in the filtrate Concentration in feed water The flux recovery ratio Pure water flux The relative flux reduction The final permeate flux The initial permeate flux The reversible fouling resistance The irreversible fouling resistance The total resistance The total fouling layer resistance Viscosity of pure water Contact angle

L.-L. Hwang et al. / Desalination 352 (2014) 181–189

12

100

12

240

(a) 98.4 kPa

210

10

80

185

(b) 196.8 kPa 10

-1

h )

-2

4

-2

-1

PWF (L m h ) 9 -1 Rm (10 , m )

120

6

-1

-1

40

8

150

9

6

PWF (L m

-1 -2

8

9

PWF (L m

-1

R m (10 , m )

-2

PWF (L m h ) 9 -1 Rm (10 , m )

60

R m (10 , m )

h )

180

90

4

60

20

2

2

30

0 PA 2

PA 3

PA 4

0

0

0 PA 1

PA 1

PA 5

PA 2

Composite membranes

PA 4

PA 5

Composite membranes

10

700 600

PA 3

(c) 393.6 kPa 8

-1 -2

-2

-1

4

-1

PWF (L m h ) 9 -1 Rm (10 , m )

300

9

PWF (L m

6

400

R m (10 , m )

h )

500

200 2 100 0 PA 1

PA 2

PA 3

PA 4

PA 5

0

Composite membranes Fig. 2. The pure water flux (PWF) and membrane hydraulic resistance (Rm) of AC composite membranes at different operating pressures.

indicated that the PF of polymer composite membrane PA1 increased but the HA removal efficiency did not as the filtration pressure increased from 98.4 to 393.6 kPa. When adding AC particles and hydrophilic PEG to modify the polymer composite membranes (PA2 and PA3), there were more small pores that formed on the membrane surface and the hydrophilicity of the membrane surface increased [16]. The removal efficiencies of hydrophobic HAs were therefore increased at different filtration pressures. When only adding AC particles to modify the polymer composite membranes (PA4 and PA5), the pore number, pore size, PWF and PF increased significantly but the HA removal efficiency decreased. 3.3 . Membrane resistances of AC polymer composite membranes at different filtration and backwashing pressures Fig. 5(a–c) shows the membrane resistance percentages of AC composite membranes at different filtration and backwashing pressures. At the three filtration and backwashing pressures (Fig. 5(a)–(c)), the polymer composite membrane (PA1) had greater percentage of irreversible resistance Rir than PA2–PA5 because its surface was smooth, hydrophobic, and less porous. From the view point of practical operation, the fouling mechanisms can be divided into reversible and irreversible fouling. The irreversible fouling can't be removed by physically backwashing [7,11,26,29,30].

As shown in Fig. 5(b), the percentages of Rc and Rir were decreased for polymer composite membrane (PA1) and AC composite membranes (PA2–PA5) when the filtration pressure and backwashing pressure were increased to 196.8 kPa and 393.6 kPa, respectively. The increases of filtration and backwashing pressures can effectively prevent the accumulation of fouling on the membrane surface. At the highest filtration pressure and backwashing pressure (393.6 kPa), the results (Fig. 5(c)) indicated that the Rir percentages for the AC composite membranes (PA1–PA5) were greater than those in Fig. 5(b). The possible reason for this result is that the foulants that accumulated on the membrane surface was firm and deep at high pressures and the cross-flow filtration and backwashing were not enough to remove them. Moreover, the Rir percentages of AC composite membranes PA2 and PA3 were greater than those of PA4 and PA5. The addition of hydrophilic PEG in AC composite membranes can change the pore distribution and hydrophilicity of polymer composite membranes [8,10,46], thus increase the irreversible resistance of the membrane. 3.4 . Effects of different backwashing pressures on the flux recovery rates To explore the antifouling ability of the AC composite membranes, the filtration and backwashing cycles were performed for three cycles. The PWF and PF of AC composite membranes during the three cycles of filtration and backwashing are shown in Fig. 6 and Table 4. The results

186

L.-L. Hwang et al. / Desalination 352 (2014) 181–189

60

80 -2

(a) 98.4 kPa

9

-1

-1

Permeate flux (L m h ) 9

120

54

-1

Rt (10 , m )

48

-1

h )

60

-2

42

-1

20

30 24

60

18 12

30

10

6

PA 1

PA 2

PA 3

PA 4

PA 5

0

0

PA 1

PA 2

Composite membranes

-2

PA 5

-1

Permeate flux (L m h )

(c) 393.6 kPa

9

60

-1

Rt (10 , m )

180

50

150 40 120

9

R t (10 , m )

30 90

-1

Permeate flux (L m

PA 4

70 -2

-1

h )

210

PA 3

Composite membranes

240

20

60

10

30 0

0

PA 1

PA 2

PA 3

PA 4

PA 5

Composite membranes Fig. 3. The permeation flux (PF) and membrane total resistance (Rt) of AC polymer composite membranes at different operating pressures.

100

240

90

210 -2 -1

80 180 150 120

98.4 kPa

196.8 kPa

393.6 kPa

-2 -1

Permeate flux (L m h ) HAs removal efficiency (%)

70 60 50 40

90

30 60 20 30

HAs removal efficiency (%)

Permeate flux (L m h )

0

9

-1

20

36

90

R t (10 , m )

30

9

40

Permeate flux (L m

40

R t (10 , m )

Permeate flux (L m

-2

(b) 196.8 kPa

50

-2

-1

h )

Rt (10 , m )

60

150

-1

Permeate flux (L m h )

10 0

0 PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5

Composite membranes Fig. 4. HA removal efficiency and permeation flux (PF) of AC polymer composite membranes at different operating pressures.

0

L.-L. Hwang et al. / Desalination 352 (2014) 181–189

(a) 98.4 kPa

(b) 196.8 kPa 100

90

Rm

90

80

Rir

80

70

Rc

60 50 40 30

Membrane resistances (%)

Membrane resistances (%)

100

Rm Rir

70

Rc

60 50 40 30

20

20

10

10

0

187

0 PA 1

PA 2

PA 3

PA 4

PA 1

PA 5

PA 2

PA 3

PA 4

PA 5

Composite membranes

Composite membranes

(c) 393.6 kPa

Membrane resistances (%)

100 90

Rm

80

Rir Rc

70 60 50 40 30 20 10 0 PA 1

PA 2

PA 3

PA 4

PA 5

Composite membranes Fig. 5. Membrane resistances of AC polymer composite membranes at different operating pressures.

indicate that the PF, FRR1, and FRR2 of AC composite membranes (PA2– PA5) were significantly increased and the RFR1 and RFR2 were decreased as compared with the polymer composite membrane (PA1). With the rise of backwashing pressure, all the FRRs were increased and the RFRs were decreased. Moreover, increasing the filtration and backwashing cycles would decrease the PF and FRR (FRR2 b FRR1) as well as increase the RFR (RFR2 N RFR1). Comparing the results of PA4–5 and PA2–3 in Table 4, FRR1 and FRR2 of PA2–3 were greater than those of PA4–5, the RFR1 and RFR2 of PA2–3 were less than those of PA4–5. The addition of AC particles and pore-formation agent PEG in the composite membranes (PA2 and PA3) was more effective in increasing the antifouling ability than the addition of AC particles only (PA4 and PA5). Because adding hydrophilic PEG can increase the hydrophilicity and change the pore distribution of AC composite membrane [16], the formation of irreversible fouling was effectively reduced in the hydrophilic AC composite membrane. Comparing the filtration and backwashing performance of the AC composite membrane with those of the other composite membranes in the relevant studies (Table 1), we can find that the AC composite

membranes have both better filtration efficiency (i.e. pollutant removal efficiency or rejection rate) and backwashing properties (FRR or RFR). The best HA removal efficiency and FRR of the AC composite membrane (PA2) were 94% and 85.65%, while the best rejection rate and FRR of PSF/PAI/UV–TiO 2 composite membrane were 86% and 79.5% [22]. 4 . Conclusions This study explores the filtration and backwashing characteristics of different AC composite membranes. The addition of AC particles in the polymer composite membrane can improve the pore distribution and porous structures in the membranes, prevent the polymer composite membranes from deforming at high filtration pressures, as well as increase the PF significantly and reduce the formation of irreversible fouling. The optimum transmembrane flux, filtration pressure and time of AC composite membrane were 45 Lm−2 h−1, 196.8 kPa and 60 min; and the optimum backwashing pressure and time were 393.6 kPa and 10 min, respectively. Under such conditions, the flux recovery rate (FRR) of the AC composite membranes achieved 87.5%.

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L.-L. Hwang et al. / Desalination 352 (2014) 181–189

(a) 98.4 kPa

(b) 196.8 kPa

120

300 PWF1

PWF1

PWF2

240

PWF2

210

PF2

PF2

-1

270

PWF3

-2

PF1

Filtration flux (L m h )

80

-2

-1

Filtration flux (L m h )

100

PF3

60

40

20

PF1

PWF3

180

PF3

150 120 90 60 30

0

0

PA1

PA2

PA3

PA4

PA1

PA5

PA2

PA3

PA4

PA5

Composite membranes

Composite membranes

(c) 393.6 kPa 660 PWF1

600

PF1 PWF2

480

PF2

420

PWF3

360

PF3

-2

-1

Filtration flux (L m h )

540

300 240 180 120 60 0 PA1

PA2

PA3

PA4

PA5

Composite membranes Fig. 6. The pure water flux (PWF) and permeation flux (PF) of AC polymer composite membranes during the three cycles of filtration and backwashing at different operating pressures.

References Table 4 The flux recovery rates (FRRs), relative flux reductions (RFRs), and membrane resistances of different AC polymer composite membranes. Composite membrane code

RFR1 (%)

RFR2 (%)

FRR1 (%)

FRR2 (%)

PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5 PA1 PA2 PA3 PA4 PA5

80.48 43.28 47.62 57.28 59.10 81.88 50.25 47.02 66.10 72.25 87.87 71.82 69.53 68.56 67.27

82.13 50.10 53.79 74.89 74.13 83.27 65.03 61.67 75.53 73.85 86.80 71.89 69.43 63.65 69.64

29.63 86.93 86.38 63.15 67.30 36.60 85.65 87.46 71.85 79.91 48.96 86.34 86.31 75.30 77.06

20.80 54.75 68.95 49.82 50.16 29.39 81.75 82.41 66.76 74.94 40.09 79.89 80.55 70.13 71.69

Membrane resistance (m−1) Rm (×109)

Rir (×109)

Rc (×109)

Rt (×109)

8.99 5.06 4.38 6.08 5.53 8.10 4.27 3.92 5.50 4.54 7.52 2.94 2.76 5.99 3.83

21.4 0.76 0.69 3.55 2.69 14.0 0.72 0.56 2.16 1.14 7.84 0.47 0.44 1.96 1.14

15.7 3.10 3.30 4.60 5.31 22.6 3.59 2.92 8.57 10.7 41.6 7.03 5.85 11.1 6.74

46.0 8.92 8.37 14.2 13.5 44.7 8.58 7.41 16.2 16.4 57.0 10.4 9.05 19.0 11.7

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