Enhanced alleviation of ultrafiltration membrane fouling by regulating cake layer thickness with pre-coagulation during drinking water treatment

Journal Pre-proof Enhanced alleviation of ultrafiltration membrane fouling by regulating cake layer thickness with pre-coagulation during drinking water treatment Baiwen Ma, Wenjing Xue, Yaohui Bai, Ruiping Liu, Wei Chen, Huijuan Liu, Jiuhui Qu PII:

S0376-7388(19)33035-2

DOI:

https://doi.org/10.1016/j.memsci.2019.117732

Reference:

MEMSCI 117732

To appear in:

Journal of Membrane Science

Received Date: 29 September 2019 Revised Date:

30 November 2019

Accepted Date: 7 December 2019

Please cite this article as: B. Ma, W. Xue, Y. Bai, R. Liu, W. Chen, H. Liu, J. Qu, Enhanced alleviation of ultrafiltration membrane fouling by regulating cake layer thickness with pre-coagulation during drinking water treatment, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2019.117732. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Author Statement

Baiwen Ma: Data curation, Validation, Writing- Original draft preparation Wenjing Xue: Investigation, Software, Visualization Yaohui Bai: Conceptualization, Project administration, Supervision, Writing- Review & Editing Ruiping Liu: Methodology, Resources Wei Chen: Formal analysis Huijuan Liu: Resources Jiuhui Qu: Conceptualization

1

Enhanced alleviation of ultrafiltration membrane fouling by

2

regulating cake layer thickness with pre-coagulation during drinking

3

water treatment

4 5

Baiwen Maa, Wenjing Xuea,b, Yaohui Baia*, Ruiping Liua,c, Wei Chend,e, Huijuan Liuf,g, Jiuhui

6

Qua,c

7 8

a

9

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Key Laboratory of Drinking Water Science and Technology, Research Center for

10 11

b

12

Qingdao 266042, China

College of Environment and Safety Engineering, Qingdao University of Science and Technology,

13 14

c

University of Chinese Academy of Sciences, Beijing 100049, China

15 16

d

17

Shallow Lakes, Hohai University, Nanjing 210098, China

Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on

18 19

e

College of Environment, Hohai University, Nanjing 210098, China

20 21

f

22

Control, Tsinghua University, Beijing 100084, China

School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution

23 24

g

Research Center for Water Quality and Ecology, Tsinghua University, Beijing 100084, China

25 26

* Corresponding author. E-mail address: [email protected]

27 28 29 30 31 32 33 34 1

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Abstract: Ultrafiltration (UF) membrane modules are static in membrane tanks; thus,

36

result in the continuous development of a cake layer and serious membrane fouling.

37

Thickness regulation is the most convenient method to solve this owing to the

38

looseness induced by flocs. Recently, integrated membrane technology is increasingly

39

being applied due to its high pollutant removal efficiency and low space requirements.

40

Herein, with the injection of Fe-based flocs, the UF membrane performance was

41

investigated with module rotation in the presence of humic acid (HA) and

42

south-to-north water in China. The obtained results showed that the outer cake layer

43

was easily shed away owing to the strong flow shear force. The thickness of the cake

44

layer decreased and membrane fouling was significantly alleviated. The faster the

45

rotation speed, the thinner the cake layer and the lower the membrane fouling was.

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However, the reduction rate of the cake layer thickness decreased as the rotation

47

speed increased owing to the high density of the inner cake layer. Although both the

48

rotation speed and rotation time played an important role in reducing the membrane

49

fouling, the removal efficiency of HA remained constant, and the cake layer tended to

50

be the main fouling mechanism; thus, indicating that module rotation is beneficial to

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the integrated technology. Owing to the smaller particle size and higher positive

52

charge of flocs formed under an acidic condition, more negatively charged HA

53

molecules were adsorbed, and the UF membrane performance was superior to that in

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an alkaline condition. Raw water experiments further confirmed the excellent UF

55

membrane performance. Based on this, the proposed technology has great potential

56

for wide applications in rural areas, particularly with the rapid development of clean

57

energy (e.g., solar energy) and intelligent water services.

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Key words: Coagulation; Ultrafiltration; Membrane module rotation; Thickness

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regulation of cake layer; Fouling alleviation. 2

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1. Introduction

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Ultrafiltration (UF) membrane is a promising technology that is gradually

62

becoming primarily used in drinking water treatment owing to the excellent water

63

quality it provides and its low space usage [1-4]. However, membrane fouling is

64

inevitable over time owing to pollutants coming into contact with the membrane

65

surface, which is the key problem in the development of membrane technology.

66

Previous studies have demonstrated that pore adsorption, pore blocking, and cake

67

layer formation are the main UF fouling mechanisms that occur during filtration [5,6].

68

Traditional coagulation plays an important role in effectively alleviating

69

membrane fouling because of the excellent removal efficiency of pollutants and loose

70

flocs that form. To date, at least three pretreatment technologies that combine

71

coagulation and membrane filtration have been investigated: (1) pre-adsorption,

72

which involves a sedimentation tank after the coagulation and the flocs with/without

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pollutants are pre-deposited before membrane filtration [7]; (2) direct-filtration, in

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which raw water is applied directly to a subsequent membrane system after the

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coagulation [8]; (3) integrated filtration, in which flocs are pre-deposited/suspended

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on a membrane surface before filtration [9,10]. Owing to the involved low space

77

usage and excellent membrane performance, the majority of studies in recent years

78

have focused on integrated filtration [11-13]. For this, the formation of a cake layer

79

from flocs (micrometer scale) is the main UF membrane fouling mechanism owing to

80

the small membrane pore size (nanometer scale). However, the chance of subsequent

81

pollutants coming into direct contact with the membrane surface becomes extremely

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low owing to the adsorption/interception of the cake layer, which is beneficial for

83

further alleviating membrane fouling.

84

It has been noted that the UF membranes in membrane modules comprising 3

85

existing drinking water treatment technologies are static in the membrane tank,

86

resulting in the continuous development of a cake layer and severe membrane fouling

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over time, despite periodic backwashing. It has been demonstrated that the

88

contribution of membrane fouling induced by a cake layer is greater than 90% after

89

long-term operation with reservoir water [13]. Furthermore, microorganisms are

90

easily nourished in a thick cake layer; thus, further aggravating the membrane fouling.

91

Fortunately, the cake layer formed by flocs is loose and can be easily destroyed by

92

application of a small force. As a result, a simple and effective method to alleviate UF

93

membrane fouling is to regulate the cake layer using membrane module rotation. In

94

previous researches, the majority of such investigations have focused on wastewater

95

treatment [14-16] with inorganic ceramic membranes in particular [17-20], while little

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attention has been paid to drinking water treatment.

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Fe- and Al-based salts are commonly used as coagulants in water treatment

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because of their high floc-adsorption ability [21-24]. In comparison with Al-based

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flocs, Fe-based flocs settle to a greater extent owing to their higher density [25]; thus,

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they have a significant advantage as they fall off in response to the scouring effect.

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For this reason, Fe-based coagulants were investigated in actual operation in this

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study to obtain a suitable cake layer thickness. It has been demonstrated that severe

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UF membrane fouling is easily induced by humic substances (HS) [23, 26-28], which

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comprise important natural organic matter and commonly exist in natural water

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bodies [29]. Previous studies have reported that the concentration of HS in natural

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waters ranges from a few mg/L to several hundred mg/L C; and have a large

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molecular weight (MW) distribution that ranges from a few thousand Daltons to a few

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hundred thousand Daltons [26,27]. HS can also cause environmental problems, for

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instance, they can serve as a food source for bacteria in water [30], enhance the 4

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transport characteristics of heavy metals by complexation [31], and even form

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disinfection byproducts with chlorine during water treatment [32].

112

Herein, Fe-based flocs were directly injected into a membrane tank in the

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presence of HS. The UF membrane module was rotated in the membrane tank, which

114

was driven by an electrical machine. The flow regime in the membrane tank was

115

simulated using a computational fluid dynamics (CFD) model. To further test the

116

rotation membrane process, raw water collected from south-to-north water in the

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Beijing area was also investigated. The objectives of this study are as follows: (1) to

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understand the UF membrane performance with the use of membrane module rotation;

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(2) to explore the factors influencing membrane behavior, particularly the rotation

120

speed and rotation time; and (3) to examine the feasibility of the rotation membrane

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process in the presence of raw water.

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2. Materials and methods

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2.1 Materials

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All chemical reagents used in this work were of analytical grade, except when

125

specified. FeCl3·6H2O, HCl, NaOH, and kaolin were obtained from Sinopharm

126

Chemical Regent Co., Ltd (China). The humic acid sodium salt (HA, Sigma-Aldrich,

127

USA) and kaolin were dissolved in tap water (Beijing, China). Raw water, collected

128

from south-to-north water in the Beijing area (China), was used immediately.

129

Furthermore, the specific characteristics of the feed water are shown in Table 1.

5

Table 1 Characteristics of feed water

130 Items

With 20 mg/L HA

Raw water

Water temperature ( C) pH Turbidity (NTU) Conductivity (µS/cm) Dissolved organic carbon (DOC, mg/L)

22.7±3.6 7.2±0.4 10±0.3 93.4±5.2 6.8±0.4

19.3±2.7 7.7±0.4 12.7±2.2 341.6±21.3 3.2±0.6

Suspended solids (mg/L) Residual chlorine (mg/L)

42.7±5.9 0.4±0.1

21.3±4.9 -

o

131

2.2 Filtration process

132

To explore the effects of membrane module reciprocating rotation, an electrical

133

machine set above the membrane module was used for performing the integrated

134

filtration (Fig. S1), which involved one and a half turns forward, then one and a half

135

turns backward. To prevent the membrane module touching the wall of the membrane

136

tank during rotation, a 25-g weight was tied to the bottom of the membrane module

137

(Fig. S2).

138

During the filtration, feed water was continuously supplied to the membrane tank,

139

controlled by a peristaltic pump. For the membrane filtration, a polyvinylidene

140

fluoride hollow fiber membrane (100 kDa, Motimo, China) module was immersed in

141

the membrane tank, which had an inner diameter and height of 64 mm and 800 mm,

142

respectively. The operation flowrate during the filtration was 1 L/h, and the filtration

143

cycle was set to 30 min, including 1 min backwashing (2 L/h). Fresh Fe-based flocs

144

were continuously injected into the membrane tank via a periodic pump, and a

145

ceramic aeration device (0.1 L/min) was placed in the bottom of the membrane tank

146

to ensure the flocs remained well suspended. The final solution pH was adjusted to

147

the desired value (e.g., pH 6, pH 9) using 0.1 M HCl or NaOH via a periodic pump. 6

148

To reduce the influence of microorganisms, the system was only operated for 12 d

149

each time.

150

The transmembrane pressure (TMP) was used to represent the UF membrane

151

fouling [13], which was recorded after operation for 25 min before backwashing (8

152

a.m. to 9 a.m. each day). Meanwhile, the rotation speed was set as 18 rpm (G: 4.1 S-1),

153

24 rpm (G: 6.3 S-1), and 30 rpm (G: 8.9 S-1), respectively, to investigate its influence.

154

Additionally, to analyze the effect of the rotation time, the continuous rotation time

155

was set as 2 h, 6 h, 12 h, and 24 h in one day, which was precisely controlled by a

156

relay. Furthermore, the membrane module was removed from the membrane tank

157

after water sampling (at 20 min before backwashing) and the TMP value was recorded

158

on day 12, which was at the end of the rotation. Then, 2 cm of the membrane was

159

carefully cut away from the middle of the membrane module for measurements of the

160

morphology of the fouled membrane surface, while the ends were tied in knots to

161

ensure normal operation in the filtration afterward. Finally, tap water was used to

162

wash away the cake layer from the UF membrane surface with the aim of exploring

163

the specific proportion of membrane fouling resistance.

164

2.3 Floc preparation and characteristics during filtration

165

Flocs formed by FeCl3·6H2O were directly injected into the membrane tank,

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which significantly affected the membrane performance. For the floc preparation,

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FeCl3·6H2O was dissolved in 0.5 L tap water (Beijing) each time, with its pH adjusted

168

to 7.5 using 1 M NaOH (Table 1). It has been demonstrated that Fe hydrolytic flocs

7

169

are the dominant iron species around neutral pH conditions [33]; therefore, the

170

concentration of the Fe-based flocs was almost equal to that of the Fe-based

171

coagulants used.

172

To understand the specific characteristics of the Fe-based flocs under various

173

rotation conditions, floc samples were obtained from 2 cm below the water surface on

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day 1 to minimize the influence of impurities. Images of the flocs were captured using

175

a microscope with a charge coupled device camera (GE-5, Aigo, China). A

176

nano-particle-size and zeta potential analyzer (BECKMAN COULTER Ltd., USA)

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was used to measure the surface charge of the flocs.

178

2.4 Computational fluid dynamics model

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To investigate the flow regime in the membrane tank with the membrane module

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rotation, a CFD model was used. In this work, tetrahedrons and boundary layer hybrid

181

grids were employed, and the first layer grids’ height of the boundary layer was

182

calculated using Y+ [34]. ANSYS FLUENT software was employed to simulate and

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post-process the internal flow process in the computational domain. The

184

pressure-velocity coupling algorithm was solved using a pressure implicit split

185

operator. The discretization of the momentum, turbulent kinetic energy, and turbulent

186

energy dissipation rate was performed using the second order upwind scheme. To

187

improve the calculation stability, the pressure and momentum relaxation factors were

188

set as 0.3 and 0.5, respectively, the time step size as 0.01 s, and the calculation

189

residual as 1e-4 [35].

8

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2.5 Other analytical measurements

191

The solution pH was measured using pH meter (Orion, USA). The HA

192

concentration and the peak value of the HA MW distributions were determined using

193

gel permeation chromatography (GPC, Agilent Technologies, USA; Detector: UV254;

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Column: TSK). The UF fraction method was used to investigate the removal

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efficiencies realized with various HA MW distributions [36]. Furthermore, scanning

196

electron microscopy (SEM, JSM-7401F, JEOL Ltd., Japan) was used to obtain images

197

of the membrane surface and cross-section before and after the fouling.

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3. Results and discussion

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3.1 UF membrane performance with/without module rotation

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Owing to the small UF membrane pore size (average pore size: 30 nm,

201

manufacture provided), the HA molecules were rejected during filtration (Fig. S3) and

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a relatively dense and thick cake layer formed (Figs. 1a and 1b). When the Fe-based

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flocs were injected, continuous low aeration was used to maintain the suspension of

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the flocs in the membrane tank. As a result, a relatively loose cake layer formed

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gradually on the membrane surface without continuous membrane module rotation

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and a few flocs (yellow area, same below) could also be observed (Fig. 1c). Owing to

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the looseness of the cake layer, its thickness increased from 17.2 µm (without flocs) to

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24.6 µm with 13 mM Fe-based flocs (Fig. 1d). With the continuous membrane

209

module rotation, however, part of the cake layer was shed, particularly the loose outer

210

layer, because of the induced scouring effect. Therefore, a smoother and thinner cake 9

211

layer formed gradually on the membrane surface (Figs. 1e and 1f). The thickness of

212

the cake layer clearly reduced from 24.6 µm to 2.4 µm with a rotation of 24 rpm. (a)

(b)

(c)

(d)

(e)

(f)

213

214

215 216 217 218 219 220

Fig. 1. (a) Morphology of membrane surface and (b) cross section without flocs; (c) morphology of membrane surface and (d) cross section in the presence of flocs without rotation; (e) morphology of membrane surface and (f) cross section in the presence of flocs with continuous rotation at 24 rpm. Other experimental conditions: 20 mg/L HA, 13 mM Fe-based flocs, and pH 7.5.

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Owing to the particle size distribution of the HA molecules (Fig. S3a), the HA

222

removal efficiency was 26.1% in the absence of flocs with a 24-rpm continuous

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rotation at day 12. With the injected flocs, however, the removal efficiency gradually

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increased to 43.2%, 65.7%, and 75.9% in the presence of 6.5 mM, 13 mM, and 26

225

mM flocs, respectively (Fig. 2a), owing to the strong adsorption ability of the flocs.

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With the removal of the HA, the peak value of the HA MW distribution reduced from 10

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12,071 Da to 9,284 Da for 26 mM flocs. It is interesting to note that the HA removal

228

efficiency was slightly influenced by the membrane module rotation, indicating the

229

stability and validity of the appropriate membrane module rotation (Fig. S4). However,

230

the smaller the HA molecules, the more difficult they were to remove (Fig. 2b). The

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removal efficiency of a large MW (> 30 kDa) of HA was 81.2±3.3%, while that of a

232

small MW (< 3 kDa) of HA was 54.3±5.2% in the presence of 26 mM flocs with a

233

continuous rotation at 24 rpm.

234

Owing to the module rotation, different UF membrane fouling behaviors were

235

clearly observed (Figs. 2c and 2d). As shown in Figs. 1a and 1b, a dense and thick

236

cake layer formed on the membrane surface for HA alone, and the TMP significantly

237

increased to 63.8 kPa after 12 days of operation. The greater the amount of flocs

238

injected, the higher the removal efficiency of the HA molecules and the lower the

239

membrane fouling was. It was observed that the TMP reduced gradually to 57.5 kPa,

240

30.4 kPa, and 11.5 kPa in the presence of 6.5 mM, 13 mM, and 26 mM Fe-based flocs,

241

respectively. With UF membrane module rotation, however, the outer cake layer was

242

shed because of the induced scouring effect (Figs. 1e and 1f), even in the absence of

243

flocs. As a result, the membrane fouling reduced from 63.8 kPa (without flocs and

244

rotation) to 49.7 kPa (without flocs at a continuous rotation of 24 rpm). With the

245

injected flocs, the TMP further reduced to 33.1 kPa, 10.6 kPa, and 7.7 kPa in the

246

presence of 6.5 mM, 13 mM, and 26 mM flocs, respectively, at a continuous rotation

247

of 24 rpm.

11

(b)100

15

20 mg/L HA No flocs + 24 rpm 6.5 mM + 24 rpm 13 mM + 24 rpm 26 mM + 24 rpm

12,071 Da

Removal efficiency (%)

Response (mV)

(a) 20

10

5

850

900 Time (s)

248

>30

<3 3-30 Molecular weight (kDa)

(d) 80 No flocs + No rotation 6.5 mM flocs 13 mM flocs 26 mM flocs

40

40

20

20

0

0 0

2

4

6

8 10 Time (d)

No flocs + No rotation No flocs + 24 rpm 6.5 mM + 24 rpm 13 mM + 24 rpm 26 mM + 24 rpm

60

TMP (kPa)

60

TMP (kPa)

40

950

(c) 80

254

60

20

800

250 251 252 253

80

9,284 Da

0

249

0 mM + 24 rpm 6.5 mM + 24 rpm 13 mM + 24 rpm 26 mM + 24 rpm

12

14

0

16

2

4

6

8

10

12

14

16

Time (d)

Fig. 2. (a) Variation of HA concentrations and peak value of HA MW distribution at day 12; (b) Removal efficiency of various MWs HA at day 12; (c) TMP development without membrane module rotation; (d) TMP development with continuous membrane module rotation of 24 rpm. Other experimental condition: pH 7.5.

3.2 Effect of rotation speed and rotation time on UF membrane performance

255

Owing to the excellent UF membrane performance with module rotation (Figs. 1

256

and 2), further experiments were conducted in the presence of 13 mM Fe-based flocs

257

(same below). With respect to the loose cake layer, the membrane module rotation

258

speed plays an important role in regulating the thickness of the cake layer. As can be

259

observed from Figs. 3a, 3b, and 3c, the higher the rotation speed, the faster the

260

velocity of the water passing through the membrane surface and the stronger the

261

turbulence degree simulated in the CFD model. As a result, a larger shear force was 12

262

applied to the cake layer. The average shear forces on the membrane surface were

263

0.0039 Pa, 0.0061 Pa, and 0.0084 Pa with a continuous rotation speed of 18 rpm, 24

264

rpm, and 30 rpm, respectively (Figs. 3d, 3e, and 3f, respectively). (a)

(c)

(b)

265

(d)

266 267 268 269

(e)

(f)

Fig. 3. CFD simulation of velocity magnitude distribution on membrane surface (m/s) for continuous rotation of: (a) 18 rpm, (b) 24 rpm, and (c) 30 rpm; simulation of shear force on membrane surface (Pa) for continuous rotation: (d) 18 rpm, (e) 24 rpm, and (f) 30 rpm.

270

The greater the shear force, the smoother the cake layer surface (Figs. 4a and 4b).

271

The flocs could also be observed on the cake layer surface even with a rotation speed

272

of 30 rpm. In addition, with the shear force induced during the membrane module

273

rotation, the thickness of the cake layer further reduced from 4.9 µm to 2.4 µm and

274

2.1 µm with the continuous rotation speeds of 18 rpm, 24 rpm, and 30 rpm,

275

respectively, on day 12, (Figs. 1f, 4c, and 4d, respectively). It seemed that the rate of

276

the reduction in the cake layer thickness decreased with further increasing of the

277

rotation speed, which was mainly ascribed to the high density of the inner cake layer 13

278

[37]. In addition, the longer the rotation time, the thinner the cake layer that formed.

279

As can be observed in Figs. 1f, 4e, and 4f, the thickness of the cake layer was 9.7 µm,

280

3.6 µm, and 2.4 µm with continuous rotation for 6 h, 12 h, and 24 h in a day,

281

respectively. (a)

(b)

(c)

(d)

(e)

(f)

282

283

284 285 286 287 288

Fig. 4. Morphology of membrane surface with continuous rotation of (a) 18 rpm and (b) 30 rpm; morphology of membrane cross section with continuous rotation of (c)18 rpm and (d) 30 rpm; morphology of membrane cross section with continuous rotation time of (e) 6 h and (f) 12 h in a day at 24 rpm. Other experimental conditions: 13 mM Fe-based flocs, and pH 7.5.

289

With the reduction in the cake layer thickness, however, the membrane fouling

290

was further alleviated and the TMP reduced from 30.4 kPa without rotation to 14.8

291

kPa, 10.6 kPa, and 5.9 kPa at a continuous rotation of 18 rpm, 24 rpm, and 30 rpm,

292

respectively (Fig. 5a). Owing to the difficulty in further reducing the thickness of the 14

293

cake layer (Figs. 1f, 4c, and 4d), the mitigation trend of the membrane fouling

294

decreased with the increase in the rotation speed. One possible reason for this was

295

how easy it was for the outer layer to fall off, which could be ascribed to the

296

following reasons. Firstly, the adsorption force between the membrane surface and

297

inner layer was greater than that between the membrane surface and outer layer owing

298

to the effect of the intramembrane vacuum. Secondly, the outer cake layer was always

299

much looser than the inner cake layer [37].

300

To further investigate the effect of the rotation time during filtration, the

301

membrane module rotation time was set as 2 h/day, 6 h/day, and 12 h/day with

302

continuous rotation at 24 rpm (Fig. 5b). It was found that the longer the membrane

303

module rotation time, the lower the membrane fouling was owing to the thinner the

304

cake layer that formed (Figs. 1f, 4e, and 4f). The corresponding specific TMPs

305

reduced from 30.4 kPa (no continuous rotation) to 21.9 kPa (2 h/day), 16.8 kPa (6

306

h/day), 12.6 kPa (12 h/day), and 10.6 kPa (continuous rotation), respectively.

307

Moreover, owing to the slight influence of the membrane module rotation on the floc

308

characteristics, the corresponding HA removal efficiency was almost the same for the

309

various rotation speeds and rotation times under consideration (Fig. S5).

15

(b) 40

(a) 40 0 rmp 18 rmp 24 rmp 30 rmp

20

313

20

10

10

0

0 0

310 311 312

30

TMP (kPa)

TMP (kPa)

30

2

4

No rotation 2 h/day + 24 rpm 6 h/day + 24 rpm 12 h/day + 24 rpm 24 h/day + 24 rpm

6

8 10 Time (d)

12

14

0

16

2

4

6

8 10 Time (d)

12

14

16

Fig. 5. TMP development for various (a) membrane module continuous rotation speeds and (b) rotation times. Other experimental conditions: 20 mg/L HA, 13 mM Fe-based flocs, and pH 7.5.

3.3 Effect of solution pH on UF membrane performance with module rotation

314

Previous studies have demonstrated that the solution pH significantly affects the

315

floc characteristics [38,39]. To further understand the effect of pH on the UF

316

membrane performance with module rotation, the continuous rotation speed of 24 rpm

317

was selected. As can be observed from Figs. 6a and 6b, the particle size of the

318

Fe-based flocs in the membrane tank at pH 6 was much smaller than that at pH 9. The

319

specific particle size of the flocs were 48.7±6.1 µm, 81.6±5.3 µm, and 132.7±15.1 µm

320

at pH 6, 7.5, and 9, respectively. The smaller the floc size, the smoother and thinner

321

the cake layer was during the filtration (Figs. 6c-6f). The thickness of the cake layer

322

was 1.7 µm at pH 6, and increased to 2.4 µm and 5.1 µm at pH 7.5 and 9, respectively.

323

Furthermore, the zeta potential of the HA maintained a negative charge [40], while

324

that of Fe-based flocs was 2.13±0.37 mV, 0.28±0.11 mV, and -4.37±0.86 mV at pH 6,

325

7.5, and 9, respectively.

16

(a)

(b)

(c)

(d)

(e)

(f)

326

327

328 329 330 331 332

Fig. 6. Floc morphology in membrane tank at (a) pH 6 and (b) pH 9; (c) morphology of membrane surface and (d) cross section at pH 6; (e) morphology of membrane surface and (f) cross section at pH 9. Other experimental conditions: 20 mg/L HA, 13 mM Fe-based flocs, and continuous rotation of 24 rpm.

333

Thus, more HA molecules were adsorbed/rejected at pH 6 than at pH 9. The

334

lower the solution pH, the higher the removal efficiency of HA was (Fig. 7a). The

335

corresponding HA removal efficiencies on day 12 were 46.7%, 65.7%, and 88.9% at

336

pH 9, 7.5, and 6, respectively. With the removal of HA, the variation of the peak value

337

of HA also varied, decreasing from 12,071 Da (HA alone) to 11,052 Da, 9,487 Da,

338

and 8,077 Da at pH 9, 7.5, and 6, respectively, with 13 mM flocs. Furthermore, the

339

lower the MW of the HA, the lower the removal efficiency was (Fig. 7b). However,

340

the removal efficiency of the small MW HA (< 3 kDa) was much higher (63.2±7.3%) 17

341

at pH 6 than (33.8±2.4%) at pH 9. As a result, the UF membrane performed better at

342

pH 6 (16.9 kPa on day 12) than (51.2 kPa on day 12) at pH 9 with the membrane

343

module rotation owing to the higher HA removal efficiency (Fig. 7c). In comparison

344

with the performance at pH 7.5 (10.6 kPa on day 12), although the removal efficiency

345

was higher at pH 6, the membrane fouling was more severe because of the relatively

346

dense cake layer that formed (Figs. 1e and 6c). In addition, Fig. S6 shows the iron

347

concentration in the effluent as a function of time; the iron concentration remained

348

lower than 0.1 mg/L, even at pH 6.

15

(b)

20 mg/L HA pH 6 pH 7.5 pH 9

12,071 Da

pH 6 pH 7.5 pH 9

100

Removal efficiency (%)

Response (mV)

(a) 20

11,052 Da

10 9,487 Da

5

80

60

40 0 800

349

8,077 Da

850

900 Time (s)

(c)

<3

Molecular weight (Da)

80 pH 6 pH 7.5 pH 9

60

TMP (kPa)

3-30

>30

950

40

20

0 0

350 351 352 353 354

2

4

6

8

10

12

14

16

Time (d)

Fig. 7. Effect of solution pH on UF membrane performance with membrane module rotation: (a) HA concentration and peak value variation of HA on day 12; (b) removal efficiency for various MWs of HA on day 12; (c) TMP development as a function of time. Other experimental conditions: 20 mg/L HA, 13 mM Fe-based flocs, and continuous rotation of 24 rpm.

18

355

3.4 UF membrane performance induced by raw water with module rotation

356

The membrane fouling induced by the HA at pH 6 was almost the same as that at

357

pH 7.5. However, the HA removal efficiency was much higher (Fig. 7). Thus, the

358

membrane performance induced by the raw water was further investigated at pH 6

359

with a rotation speed of 24 rpm and rotation time of 6 h/day. Similar to the case of

360

only HA, a dense and thick cake layer was also observed in the absence of flocs (Figs.

361

8a and 8b) because of the particle size distribution of raw water (Fig. S7). With

362

membrane module rotation, a smoother and thinner cake layer formed on the

363

membrane surface with 13 mM Fe-based flocs injected (Figs. 8c and 8d). The

364

thickness of the cake layer induced by raw water was 4.1 µm without injected flocs,

365

while the thickness became 4.8 µm without rotation and was further reduced to 1.9

366

µm with membrane module rotation (6 h/day at 24 rpm) in the presence of 13 mM

367

Fe-based flocs (Figs. 8e and 8f). (a)

(b)

(c)

(d)

368

369

19

(e)

(f)

370 371 372 373 374

Fig. 8. (a) Morphology of membrane surface and (b) cross section with raw water; (c) morphology of membrane surface and (d) cross section at pH 6 without rotation in the presence of 13 mM flocs; (e) morphology of membrane surface and (f) cross section at pH 6 with rotation speed of 24 rpm and rotation time of 6 h/day in the presence of 13 mM Fe-based flocs.

375

With the injected Fe-based flocs, a high pollutant removal efficiency was

376

observed, and the corresponding peak value of the MW of raw water also decreased.

377

As can be observed from Fig. 9a, the removal efficiency of raw water with a relatively

378

high MW (> 10,000 Da) was 64.3%, and the corresponding peak value was reduced

379

from 12,481 Da to 10,236 Da, while the removal efficiency of raw water having a

380

relatively low MW (MW < 10000 Da) was 40.1%, and the corresponding peak value

381

was reduced from 8,252 Da to 7,824 Da at 24 rpm with a rotation time of 6 h/day.

382

Although the pollutant removal efficiency was slightly influenced by the membrane

383

module rotation (similar to HA), the membrane fouling with rotation was alleviated to

384

a greater extent than that without rotation. The TMP significantly increased to 87.1

385

kPa after operation for 9 days, while the membrane fouling reduced to 55.4 kPa

386

without rotation and further reduced to 34.2 kPa with rotation at day 12 (Fig. 9b).

387

After washing with tap water, the membrane fouling was significantly reduced with/

388

without rotation; thus, indicating that the cake layer formation was still the main

389

fouling mechanism.

20

8

(b) 150

Raw water pH 6 pH 6 + 24 rpm + 6 h/day

6 12,481 Da

Raw water pH 6 pH 6 + 24 rpm + 6 h/day

120

TMP (kPa)

Response (mV)

(a)

10,236 Da

4 8,252 Da

90 60

2

30 7,824 Da

0 800

390 391 392 393

394

850

900 Time (s)

0 0

950

2

4

6 8 10 Time (d)

12

14

16

Fig. 9. (a) Variation of pollutant concentration and peak value for raw water on day 12; (b) TMP development with raw water as a function of time. Other experimental conditions: 13 mM Fe-based flocs.

3.5 Mechanism of membrane module rotation and application in environment

395

For traditional UF membrane drinking water treatment, the membrane modules

396

are static in the membrane tank. With the accumulation of pollutants, a thick cake

397

layer gradually forms on the membrane surface, which becomes the main fouling

398

mechanism, despite periodic physical cleaning. As a result, serious membrane fouling

399

is caused over time. However, the farther the cake layer from the membrane, the

400

looser that layer structure is due to the smaller attractive force that the cake layer

401

experiences owing to the negative pressure induced by the peristaltic pump in the

402

membrane cavity. With the rotation of the membrane module, intense turbulence is

403

required to be induced in the membrane tank and particularly on the membrane

404

surface. As a result, the faster the membrane module rotation, the greater the shear

405

force induced by scouring effect on the cake layer and the easier it is to shed the outer

406

cake layer. Thus, the membrane fouling was alleviated to a large extent because of the

407

shedding of the outer cake layer, including the flocs and pollutants adsorbed by the 21

408

flocs. The higher the rotation speed, the thinner the cake layer and the lower the

409

membrane fouling was. However, the membrane fouling was not significantly

410

alleviated by a low rotation speed, while the membrane fouling alleviation rate was

411

reduced by a high rotation speed owing to the dense inner cake layer. In addition, the

412

cake layer structure can be destroyed with a very high rotation speed and increase the

413

risk of severe UF membrane fouling. Furthermore, the longer the rotation time, the

414

thinner the cake layer and the lower the membrane fouling was. In comparison with

415

the alkaline condition, the floc particles were smaller and had a higher positive charge

416

under acidic conditions. Thus, more negatively charged HA molecules were

417

adsorbed/rejected by the flocs during filtration, and a lower membrane fouling was

418

induced. A schematic of the membrane fouling alleviation mechanism with module

419

rotation is shown in Fig. 10.

420 421

Fig. 10. Schematic of UF membrane fouling alleviation mechanism with module rotation.

422

To date, three key trends of drinking water treatment technologies have gradually

423

emerged with the rapid development of technology (particularly in rural areas): low

424

energy consumption [1], injection of few chemicals [41], and integration [42] and

425

intelligence [43]. Membrane technology, as an advanced separation technology, 22

426

facilitates the effective removal of pollutants without the addition of chemicals, which

427

plays an important role in achieving these goals. With the development of

428

gravity-driven membrane filtration technology [44], the energy consumption has

429

reduced, and it has gradually become used more in rural areas. To further reduce the

430

utilization of chemicals, much attention has been focused on the utilization of clean

431

energy (e.g., solar energy) in water treatment [45,46]. With the gradual application of

432

intelligent water services, it will become possible to precisely regulate the cake layer

433

structure, which is necessary. Although the development of microorganisms is

434

inevitable and plays an important role in membrane fouling, the extracellular

435

polymeric substances will be adsorbed/rejected by flocs, and the cake layer can also

436

be regulated by rotation during filtration. This work is beneficial for realizing a green

437

and intelligent membrane drinking water treatment process, particularly in rural areas

438

with small scale.

439

4 Conclusions

440

Owing to the static nature of UF membrane modules in membrane tanks, severe

441

membrane fouling is induced due to the continuous development of a cake layer. Here,

442

the effect of the use of the reciprocating rotation of a membrane module was

443

investigated to regulate the cake layer thickness in the presence of HS and raw water.

444

For the organic UF membrane, excellent membrane performance was also

445

obtained with the appropriate module rotation in drinking water treatment. The outer

446

cake layer was easily shed and the thickness was dramatically reduced owing to the

23

447

looseness of the cake layer. The higher the module rotation speed, the thinner the cake

448

layer and the lower the membrane fouling was. However, the reduction rate of the

449

cake layer thickness decreased as the rotation speed increased owing to the dense

450

inner cake layer. In addition, the longer the rotation time, the thinner the cake layer

451

and the better the UF membrane performance was. Although both the rotation speed

452

and rotation time played an important role in alleviating UF membrane fouling, the

453

cake layer formation was the main fouling mechanism. It should be noted, however,

454

that the floc characteristics were not influenced by the module rotation. Thus, the HA

455

removal efficiency remained constant during the filtration. Owing to the smaller size

456

of flocs with higher positive charge under acidic conditions, more negatively charged

457

HA molecules were adsorbed/removed, and the UF membrane performed better

458

compared to that in alkaline conditions.

459

According to the excellent UF membrane performance, the UF membrane

460

module rotation shows considerable potential for application in UF membrane

461

drinking water treatment. With the synergistic application of solar and wind energy, a

462

green and low-energy-consumption water treatment technology can be proposed.

463

Acknowledgements

464

This study was supported by the National Natural Science Foundation for Young

465

Scientists of China (51608514), Funds for International Cooperation and Exchange of

466

the National Natural Science Foundation of China (51820105011), and National Key

467

R&D Program of China (2016YFC0400802).

24

468

469

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30

Highlights

Module rotation in membrane tank was used to alleviate UF membrane fouling. Strong flow shear force was induced by the scouring effect during rotation. UF membrane fouling was significantly alleviated with module rotation. Rotation speed and rotation time significantly affected the membrane performance. UF membrane performed well under acidic conditions.

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.