Effects of electro-coagulation on fouling mitigation and sludge characteristics in a coagulation-assisted membrane bioreactor

Author’s Accepted Manuscript Effects of electro-coagulation on fouling mitigation and sludge characteristics in a coagulation-assisted membrane bioreactor Lap-Cuong Hua, Chihpin Huang, Yu-Chun Su, Tran-Ngoc-Phu Nguyen, Pei-Chung Chen www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(15)30090-9 http://dx.doi.org/10.1016/j.memsci.2015.07.062 MEMSCI13881

To appear in: Journal of Membrane Science Received date: 16 June 2015 Revised date: 10 July 2015 Accepted date: 29 July 2015 Cite this article as: Lap-Cuong Hua, Chihpin Huang, Yu-Chun Su, Tran-NgocPhu Nguyen and Pei-Chung Chen, Effects of electro-coagulation on fouling mitigation and sludge characteristics in a coagulation-assisted membrane b i o r e a c t o r , Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.07.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Effects of electro-coagulation on fouling mitigation and sludge characteristics in a coagulation-assisted membrane bioreactor

by

Lap-Cuong Hua, Chihpin Huang*, Yu-Chun Su, Tran-Ngoc-Phu Nguyen, Pei-Chung Chen

Institute of Environmental Engineering, National Chiao Tung University Hsinchu, Taiwan, R.O.C.

* Corresponding author. Tel.: +886-3-5712121 ext.55507; fax: +886-3-5725953 E-mail address: [email protected]

Abstract Although much research effort has focused on methods to mitigate fouling of membrane bioreactors (MBRs), fouling is still inevitable. This paper aims to corroborate the roles of electro-coagulation approach within a coagulation-assisted MBR, termed electro-MBR. The effects of electro-coagulation on sludge characteristics and fouling behavior were investigated. 1

A batch-scale experiment was carried out to identify an optimum electric current condition for electro-MBR study, in which the electro-MBR and a traditional MBR were identically set for numerous comparisons. The average fouling rate was substantially reduced 7.8 fold resulting in no chemical cleaning requirements during the entire operation of the electro-MBR. Enhanced sludge compressibility and controlled growth of filamentous bacteria positively affected by electro-coagulation were also obtained in the electro-MBR. This paper suggests that fouling mitigation in electro-MBR can be attributed to two mechanisms: (i) charge neutralization and adsorption for soluble foulants; (ii) electro-chemical oxidation for bound foulants.

Furthermore, we conclude that electro-coagulation can simultaneously alter

morphology and microbial structure of sludge flocs to become more compressible with lower fouling tendency, which would be beneficial for the mitigation of membrane fouling in electroMBR. Keywords: membrane bioreactor; electro-coagulation; membrane fouling; filamentous bulking; fouling mitigation

1. Introduction Membrane bioreactors (MBRs) have increasingly asserted their position for treating municipal and industrial wastewater because they have many advantages over conventional biological processes [1-3]. Nevertheless, the widespread application of MBRs is still hindered by fouling, which substantially reduces flux permeability and thus deteriorates the filtration performance of the membranes [3]. To counteract the negative effects of fouling by, for instance, backwashing or relaxation [4] and chemical cleaning [5] can be variously applied for

2

membrane flux recovery. Fouling can also be prevented before its occurrence by several advanced anti-fouling techniques such as modifying the membrane surface with anti-fouling materials [6], or adding coagulants [7] and absorbents [8] to MBRs. Despite these measures, there still is an urgent need to effectively address membrane fouling. Recently electro-coagulation process combining with MBRs (i.e. electro-MBR) has caught the attention of researchers because of its potential ability to mitigate fouling. Contrary to conventional anti-fouling methods [7] in which chemical coagulants are added directly to MBRs, this promising technique can counter fouling by simultaneously integrating membrane filtration, electro-kinetic phenomena, and biological treatment into a single system [9]. During the electrolytic oxidation of a sacrificial anode, a number of positively-charged coagulants are electrically generated in electro-MBR [10]. Since membrane foulants are negatively-charged [11], it is suggested that such negatively-charged foulants become neutralized by charge neutralization within the electro-MBR, thereby reducing the foulants that accumulate on the membrane surface [12]. Furthermore, the sludge becomes more dewaterable, with less fouling potential resulting from the electro-kinetic processes generated in electro-MBR [12]. The reactions between hydroxide ions and metal ions released during electrolysis facilitate the adsorption of soluble phosphorus into the amorphous hydroxide [13], which markedly improves the phosphorus removal in electro-MBR [14]. Compared with other fouling mitigation techniques, the advantages of electro-MBR include more sustainable permeability, greater flexibility, less chemical usage, better dewaterability, and excellent effluent quality [9, 13-16]. Previous studies have paid much attention to investigating fouling mitigation [9, 12, 15], or to evaluating the performance of electro-MBR [14, 16]. To the best of our knowledge, 3

the principles of electro-kinetics and the changes of mixed liquor properties in electro-MBR, which would have great impact on fouling mitigation, have not yet been investigated. The objectives of this study were therefore to investigate the roles of fouling mitigation and to determine the changes in sludge characteristics within the electro-MBR. A batch-scale experiment was first conducted to determine an appropriate electric current density (ECD) for the continuous operation of the electro-MBR. For comparison, the continuous operation of electro-MBR and traditional MBR (control-MBR) was carried out in parallel for 15 days. Membrane foulants were comprehensively characterized by their relative concentrations of soluble and bound extracellular polymeric substances (EPS). A Portable Series FlowCAM was employed to analyze the changes in sludge morphology and structure in electro-MBR and control-MBR. The treatment efficiency and microbial activity were also investigated. 2. Materials and methods 2.1. Batch-scaled electro-MBR experiment A batch-scale experiment was carried out to determine the optimum ECD for continuously electro-MBR operation by using a 2000 mL glass beaker with 1000 mL of activated sludge and a pair of aluminum (Al) flat-plate electrodes with an effective area of 18 cm2. The ratio between sludge volume and electrode area in the batch-scale experiment was set to correspond with the continuously operating conditions in the electro-MBR. According to the variations in charge surface, microbial activity, and fouling tendency, the optimum ECD value was determined from the ECD, ranging between 10 to 40 A/m2 with 15 min contact time. Instead of using a magnetic stirrer, air was bubbled in the batch-scale tests to simulate heterogeneous aeration conditions. Sludge was sampled after electro-coagulation for further analysis. 4

2.1. Continuous electro-MBR The continuous operation of electro-MBR was carried out according to the outcome from the batch-scale experiment. Fig. 1 is a schematic representation of the control-MBR and electro-MBR, with the working volume of 9 L. The PTFE flat-sheet membranes with nominal pore size of 0.4 μm were vertically immersed at the center of each reactor. Since intermittent operation of electro-MBR was needed to maintain high microbial activity in bioreactors [12, 17], electric currents were applied at 15 min-ON/ 45 min-OFF using a timer. Two Al electrodes, with a fixed distance of 5 cm and an effective area of 200 cm2, connecting to a direct current power supply (Major Science MP–3 AP) were set as shown in Fig. 1b. During operation, water-level sensors were employed to transmit the water level to the computer, which would then adjust pump speed to control the inlet and outlet streams. A diluted hydrochloric acid was automatically added to the bioreactors to control pH at about 6.5–7.5. All pH levels, effluent weights, and transmembrane pressure (TMP) were automatically recorded into the computer.

5

Feed solution

Effluent

Hydrochloric acid Timer

DC power supplied

Peristaltic pump

Pressure meter

pH meter

Level meter Membrane module

Al electrode

(b)

(a)

Air pump

Fig. 1. Schematic of the experimental setup (a) Control-MBR; (b) Electro-MBR. Concentrated synthetic wastewater was prepared according to the composition given by Park [18] (Table 1). This was diluted with tap water to a final concentration with a chemical oxygen demand (COD) of 800 ± 57 mg/L, dissolved organic carbon (DOC) of 318 ± 18 mg/L, and ammonia of 68 ± 5 mg/L. The similar operating conditions for electro-MBR and controlMBR are shown in Table 2. Dissolved oxygen (DO) was maintained higher than 5 mg/L. Whenever TMP exceeded -70 kPa, chemical cleaning was performed by submerging the fouled membranes in 0.05% NaClO solution for 2 h and backwashing with deionized water 6

before reuse. The seeding sludge was collected from the municipal wastewater treatment plant in National Chiao Tung University (Hsinchu, Taiwan), which was inoculated for two weeks prior to use for experiments. Table 1. Compositions of the synthetic wastewater

Composition

Concentrations (mg/L) CH3COONa

1176

NH4Cl

300

KH2PO4

35.2

MgSO4．7H2O

10

Trace inorganic

ZnSO4．7H2O

2.2

compounds

FeSO4．H2O

5

(10 mL of trace solution

CaCl2．2H2O

in 1 L of synthetic

CoCl2．6H2O

wastewater)

MnSO4．H2O

2.15

CuSO4．5H2O

0.2

Concentrated compounds

7.3 0.5 0.5

Table 2. Operation conditions

Parameters

Details

COD (mg/L)

800 ± 57

DOC (mg/L)

318 ± 18

Ammonia (mg/L)

68 ± 5

OLR (kg COD/m3 d)

0.786

HRT (h)

25

SRT (d)

60

MLSS (mg/L)

4440 ± 13

MLVSS (mg/L)

3825 ± 25

7

Flux (L/m2 h)

27

DO (mg/L)

>5

pH

6.5–7.5

2.3. Analytical methods All effluent samples were filtered through 0.45 m membrane filters (Mixed cellulose ester, Adventec) before chemical analysis. The concentrations of DOC and ammonia were measured by using a TOC analyzer (TOC-L, Shimadzu) and a spectrophotometer (Nessler method, HACH), respectively. The mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), specific oxygen uptake rate (SOUR), and sludge volume index (SVI) were measured by following standard methods [19]. DO was monitored by a DO meter (HACH sensION 156). A Portable Series FlowCAM (Fluid Imaging Technologies, US) was used to observe sludge morphology, aspect ratio, and mean particle size (PSM) in the two bioreactors. The FlowCAM has selective capabilities of flow cytometer, microscopy, and fluorescence detection, which could be used to count, image, and analyze particles in a moving fluid. An Auto-Image mode with a 4-x objective that was designed to detect particles ranging from 10 to 500 μm was used. The aspect ratio of a particle is defined here as the ratio between its width per its length as a Legendre ellipse. A spherical particle has the aspect ratio value of 1.0. Particles with aspect values near zero represent thin, elongated particles. For sample preparation, 1 ml of activated sludge was diluted to a total volume of 50 ml with deionized water. The diluted sample was continuously mixed at 60 rpm for FlowCAM measurement. In order to evaluate the tendency of supernatant stability after electro-coagulation, the supernatant was prepared for zeta potential measurement by centrifuging 25 mL of activated 8

sludge samples for 10 min at 4000 rpm and 250C. The zeta potential of the supernatant was subsequently measured by a Zetasizer Nano (Zeta-Meter, US). An attenuated total reflectanceflourier transform infrared spectrometer (ATR-FTIR) (Bruker-FTIR, Tensor 27) was used to characterize the chemical functional groups of the dried sludge samples. Since EPS either in soluble or bound forms are widely accepted as the major components of foulants in MBRs [3], the fouling tendency was therefore investigated by determining the soluble and bound EPS concentrations. Immense methods for EPS extraction are given in the literature [2]. The formaldehyde-NaOH extraction method proposed by Liu and Fang [20] was applied in this study, because the EPS extracted by this method are not contaminated by intracellular substances. Since EPS content can be accounted by their relative components such as polysaccharides and proteins [21], their quantification in both soluble and bound forms was measured to represent the EPS concentration in sludge suspension. The Phenol-sulfuric acid method adopted by Dubois [22] was applied to analyze polysaccharides concentrations, when the Bradford method was used to analyze proteins [23]. A SPSS statistical tool (SPSS Inc, Chicago, IL, USA) was used to analyze the experimental data. All the samples were analyzed in triplicate to minimize errors. 3. Results and Discussions 3.1. Determination of ECD for electro-MBR operation Since zeta potential of the sludge suspension can be used as a precursor to predict the fouling tendency because of its strong correlation with EPS concentrations [11, 24], its variability under various ECDs in the batch-scale experiments of electro-MBR was investigated to ascertain the potential of foulant formation (Fig. 2). The results indicated that

9

the surface charge of the sludge supernatant was neutralized and became less negative after electro-coagulation, particularly at high ECDs (>20 A/m2). This can be ascribed to charge neutralization of the sludge suspension when Al coagulants are added [25, 26]. Jin [27] reported that stable suspensions which hinder close contact of charged particles are caused by the occurrence of repulsive electrostatic interactions. Decreased repulsive forces between colloids resulting from destabilization by electro-coagulants can enhance the aggregation of flocs in suspensions [12], leading to the reduction of charge surface. Therefore, zeta potential decline by the charge neutralization would reflect a lower fouling tendency in mixed liquor after electro-coagulation approach. Fig. 2 also shows the deceleration of microbial activity (SOUR) versus ECDs applied. The diminishing SOUR is possibly due to the sensitivity of microorganisms with electric field [28], or Al coagulants [29], or the toxicity of electrochemical-generated byproducts such as H2O2 and Cl2 [28, 30], which may inhibit and reduce the activity of microorganisms in biomass. Moreover, no remarkable reductions of SOUR were found among ECDs of 10, 20, and 30 A/m2, whereas SOUR was significantly diminished at an ECD as high as 40 A/m2. This result is in agreement with a previous study where electro-coagulation at an ECD of 24.7 A/m2 significantly deactivated microbial activity [28], indicating that a high electric field and a high concentration of Al dissolution are significantly harmful for microorganisms in biomass.

10

-30

8

Zeta potential (mV)

6 -20

-15

4

-10 2

SOUR (mgO2/gVSS/h)

Zeta potential SOUR

-25

-5

0

0 Control

10

20

30

40

Electric current densities (A/m2) Fig. 2. Zeta potential and SOUR of sludge in batch-scale experiment under various ECDs ranging between 0 and 40 A/m2. Furthermore, the quantification of soluble and bound EPS in sludge suspension was carried out to examine the fouling reduction tendency during electro-coagulation. As can be seen in Fig. 3, the results show that after electro-coagulation bound polysaccharides, bound proteins and soluble polysaccharides were substantially reduced at ECDs as low as 10 and 20 A/m2, especially at 20 A/m2, where a maximum reduction rate of total EPS was reached. We assume that charge neutralization between negative-charged sludge flocs and positive electrogenerated coagulants during electro-coagulation would absorb and reduce soluble EPS [25, 26]. On the other hand, we suggest that bound EPS reduction is likely responsible for the coexistence of electro-chemical oxidation, which may convert EPS in solids into more 11

biodegradable compounds. These compounds later may be reduced during aerobic treatment. These mechanisms for fouling mitigation by electro-MBR are discussed below. Fig. 3 also shows that applying ECDs at 30 and 40 A/m2 could suddenly increase the concentrations of polysaccharides and proteins in sludge suspension, which is manifestly a result of the die-off of microorganisms under such high ECDs conditions. During cell lysis, a large amount of polysaccharides and proteins is increasingly produced [17, 24], leading to the reverse increase of both soluble and bound EPS in mixed liquor at high ECDs. These results suggest that operating electro-coagulation at an appropriate ECD can potentially reduce EPS contents in a mixed liquor. According to the evidence from batch-scale experiments mentioned above, 20

30 Bound polysaccharides Soluble polysaccharides Bound Proteins

30

25

25 20 20 15 15 10 10 5

5

0

0 Control

10

20

30

16

40 2

Electric current densities (A/m )

Fig. 3. Changes of soluble polysaccharides, bound polysaccharides, and bound proteins of sludge in batch-scale experiment under various ECDs ranging between 0 and 40 A/m2.

12

15 14 13 12 11 10 9

Bound proteins (mg/gSS) as BSA

35

Soluble polysaccharides (mg/L) as glucose

Bound polysaccharides (mg/gSS) as glucose

A/m2 was selected as the optimum ECD for the continuous operation of electro-MBR.

3.2. Fouling mitigation in electro-MBR 3.2.1. TMP profile For the continuous operation of electro-MBR, TMP is an important index evaluating membrane fouling tendency. The discrepancy of TMP between the electro-MBR and the control-MBR during 15 days of operation is illustrated in Fig. 4. In this study, fouled membranes were removed for chemical cleaning whenever the TMP reached over -70 kPa. As can be seen in Fig. 4, membrane cleaning in the control-MBR was required after only 4 days’ operation, and was carried out 5 times during the entire experiment, while no membrane cleaning was needed in the electro-MBR. Fouling tendency was also reflected by the average fouling rate (kPa/d), which was defined as the average TMP values over the whole period. The data shows that the average fouling rate in the control-MBR was -32.0 kPa/d, which was 7.8 times higher than that of the electro-MBR (-4.1 kPa/d). The significantly low fouling rate in the electro-MBR indicates that integrating an electro-coagulation unit into an MBR system is a promising way for alleviating the negative effects of membrane fouling, thereby prolonging the operation periods for MBRs.

13

Electro-MBR Control-MBR

TMP (kPa)

80

60

40

20

0 0

3

6

9

12

15

Operating time (d)

Fig. 4. Comparison of TMP profiles between the electro-MBR and the control-MBR. Cleaning was conducted when TMP excessed -70 kPa. 3.2.2. Fouling mitigation during electro-coagulation The variations of EPS in the electro-MBR and the control-MBR are shown in Fig. 5. As expected, the amount of EPS in soluble and bound forms in the electro-MBR was rapidly reduced. Compared to the control-MBR, statistical results after SPPS analysis showed that both soluble and bound EPS concentrations in the electro-MBR were significantly lowered with the p <0.05 and p <0.01, respectively. This evidently confirms the better fouling mitigation of the electro-MBR.

14

14

40

12 30

10 8

20 6 4

10

2 0 0

2

4

6

8

10

12

14

0 16

Operating time (d) Bound polysaccharides in control-MBR Bound polysaccharides in electro-MBR Soluble polysaccharides in control-MBR Soluble polysaccharides in electro-MBR Bound proteins in contro-MBR Bound proteins in electro-MBR

Fig. 5. EPS reductions in soluble and bound forms represented by concentrations polysaccharides and proteins. The reduction of soluble EPS in the electro-MBR can possibly be ascribed to the enhanced charge neutralization mechanism through electro-coagulation, in which electrogenerated coagulants can increasingly neutralize the surface charge of sludge, as can be seen in Fig. 6. There is a significantly larger negative surface charge of sludge suspensions in the control-MBR compared to that of the electro-MBR. From an electro-coagulation perspective, the electro-generated Al ions are derived either from the electro-chemical oxidation of the sacrificial anodes or from electrolysis with the Al electrodes [10]. Such positively-charged Al 15

14

12

10

8

6

4

Bound proteins (mg/gSS) as BSA

16

16 Soluble polysaccharides (mg/L) as glucose

Bound polysaccharides (mg/gSS) as glucose

18

50

ions would destabilize the negatively-charged foulants in suspension by neutralization of their charge [25, 26] and thus improve the adsorption of soluble EPS to agglomerates [14], and thereby reducing soluble EPS in the electro-MBR. The results obtained in this study are in strong agreement with that of other works [9, 13]. Furthermore, Gamage [26] concluded that the enmeshment of amorphous Al hydroxide layer formed within electro-coagulation may further contribute to the reduction of EPS in soluble form. Accordingly, soluble EPS becomes substantially reduced in electro-MBR by these electro-kinetics phenomena throughout electrocoagulation process.

-24

Cotrol-MBR Electro-MBR

Zeta pottential (mV)

-21 -18 -15 -12 -9 -6 0

3

6

9

12

15

Operating time (d)

Fig. 6. Surface charge of suspension in the control-MBR and the electro-MBR With respect to bound EPS reduction, the occurrence of electro-chemical oxidation within electro-coagulation appears to cause the degradation of EPS bound in the sludge in the electro16

MBR. Recent reports have stated that an Al electrode can be used as a bipolar electrode in a hybrid electro-coagulation and electro-chemical oxidation process, in which either Al electrocoagulants or oxidizing agents can be simultaneously generated [31-33]. As electro-chemical oxidation occurs, polysaccharides and proteins bounding in sludge are solubilized and broken down into lower molecular-weight and more biodegradable products that can be then degraded during biological processes [34, 35]. As a result, the coexistence of electro-chemical oxidation in electro-MBR is probably able to degrade and reduce the EPS bounding in sludge. In order to corroborate EPS degradation in bound sludge by electro-chemical oxidation, the FTIR analysis was carried out to determine the variations in functional groups of the sludge samples. As shown in Fig. 7, peaks indicating for the functional groups of proteins and polysaccharides were clearly distinguished in the FTIR spectra. The peaks at wave number of 1645 cm-1 (C=O stretching of amide I) and 1642 cm-1 (C=O stretching of amide II) are represented for proteins [36], whilst the peaks at 2928 cm-1 (aliphatic C-H of polysaccharides) and 1046 cm-1 (C-O stretching of polysaccharides or polysaccharide-like substances) are represented for polysaccharides [37]. Obviously, these absorption peaks were decreased with a lower absorbance intensity in the electro-MBR compared to the control-MBR. This result reaffirms that polysaccharides and proteins in bound sludge could be degraded simultaneously by the electro-MBR. Therefore, electro-chemical oxidation appears to enhance the reduction of bound EPS content in the electro-MBR.

17

0.12

Electro-MBR Control-MBR

Absorbance

0.09 2928

1645 1642

1046

0.06

0.03

0.00 4000

3500

3000

2500

2000

1500

1000

Wave number (cm-1)

Fig. 7. FTIR spectra of sludge in the control-MBR and the electro-MBR. Sludge was sampled at day 15. 3.3. Changes of sludge characteristics by electro-coagulation 3.3.1. Sludge floc morphology and compressibility For comparison, sludge flocs in the electro-MBR and the control-MBR were imaged by using the Portable Series FlowCAM. The changes in morphology of the flocs are illustrated in Fig. 8. Bulking sludge aggregated by filamentous bacteria with numerous filaments appeared after several days of operation in the control-MBR (Fig. 8a), while very few filamentous bacteria were observed in the electro-MBR for the entire duration of the experiment (Fig. 8b). 18

Since filamentous sludge under bulking conditions can produce substantially large amount of EPS [24, 37], it is widely accepted that bulking sludge can cause severe cake fouling in MBRs, resulting in considerable deteriorations of membrane filtration performance [24]. Controllable growth of filamentous bacteria by implementing electro-coagulation in our electro-MBR further implies better filtration capability in the electro-MBR.

(a)

(b)

Fig. 8. Morphology of sludge flocs taken by FlowCAM for (a) the control-MBR; (b) the electroMBR. Apart from altering the microbial structure of sludge, electro-coagulation also changed the floc size and shape. Data obtained by FlowCAM also illustrated the differences in sludge 19

properties such as aspect ratio (Fig. 9) and particle size mean (PSM) (Fig. 10) between the two bioreactors. The variations in aspect ratio as shown in Fig. 9 indicate that sludge flocs were of near-spherical shape in the electro-MBR (aspect ratio of near one), whereas they eventually became more elongated in the control-MBR. Results show that electro-generated flocs with an aspect ratio range between 0.7 and 1 accounted for about 50% after electro-coagulation, which was twice as much as in the control sludge without electro-coagulation. Moreover, as can be seen in Fig. 10, PSM variations in the two bioreactors show that the dimensions of electrogenerated flocs rapidly decreased to become more dense flocs, while bulking flocs in the control-MBR became larger, more open, with irregular shapes.

20

33 30 27

Volume (%)

24 21 18 15 12 9 6 3 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Aspect ratio

Control-MBR Electro-MBR

Fig. 9. Aspect ratio distribution of sludge suspension of the control-MBR and the electro-MBR. Sludge was sampled at day 15.

21

300

PSM (m)

250

200

150

Control-MBR Electro-MBR

100

50 0

3

6

9

12

15

Operating time (d)

Fig. 10. PSM variations of sludge flocs taken by FlowCAM in the control-MBR and the electroMBR. Unlike the elongated flocs found after chemical alum coagulation [25], spherical electro-generated flocs in the electro-MBR resulted from the long-time exposure to the electroosmotic force generated during electro-coagulation. This electro-force can extract the tightly bound water retained in flocs and thus decrease their size [12, 16, 38], leading to improve compressibility of the sludge suspension. Furthermore, it was reported that the absence of filamentous bacteria in the electro-MBR would reduce elongation of flocs [24] and increase porosity of the cake layer [27]. As a result, electro-generated sludge flocs in the electro-MBR comprise better compressibility compared to sludge flocs in the control-MBR.

22

As sludge compressibility strongly correlates with SVI values [27], SVI tests were carried out to reaffirm the better compressibility of the electro-generated flocs. Experimental results showed that average SVI values of the electro-MBR and the control-MBR were always in the range of 44–47 mL/g and 143–181 mL/g, respectively. These results further support previous work [14, 38] and corroborate that the compressibility of sludge suspension is markedly improved through electro-MBR, and is of clear benefit to membrane filtration processes and sludge management. 3.3.2. Variations of MLSS, MLVSS, and microbial activity The variations in MLVSS and MLVSS/MLSS ratio were quantified to determine the changes in biomass within the two bioreactors, as illustrated in Fig. 11. Results show that electro-MBR contained a higher biomass concentration of MLVSS (p <0.05) compared to control MBR, while MLVSS/MLSS ratios decreased over the operating period as a result of the Al hydroxide accumulation throughout the electro-coagulation process. This finding accords with those from previous works [14, 16], which consistently have shown the acceleration of inorganic matters in the mixed liquor as well.

23

7000

100

90

MLVSS (mg/L)

80

5000

70

60

MLVSS/MLSS (%)

6000

4000 50

3000

40 0

3

6

9

12

15

Operating time (d) MLVSS in control-MBR MLVSS in electro-MBR MLVSS/MLSS in control-MBR MLVSS/MLSS in electro-MBR

Fig. 11. MLVSS and MLVSS/MLSS ratio in the control-MBR and the electro-MBR. Microbial activity in our study was examined via the SOUR test. As can be seen in Fig.12, the results showed that microorganisms in the control-MBR appeared to be more active than in the electro-MBR. To the best of our knowledge, this phenomenon has not yet been explained. We speculate that the dual effects of electro-coagulation process in which an appropriate electric field could enhance the microbial growth rate [28, 30], together with the high concentration of Al dissolution, might reduce oxygen absorption by microorganisms [29]. Similar results were obtained by [39], where microbial activity rapidly decreased with the

24

addition of polymeric coagulants. On the whole, however, the effective removal of DOC and ammonia was consistently maintained, with an average efficiency rate of more than 98%. It is likely that the effect of relatively low microbial activity was thus not remarkable in terms of the electro-MBR performance.

10 Control-MBR Electro-MBR

SOUR (mgO2/gVSS.h)

8

6

4

2

0 0

3

6

9

12

15

Operating time (d)

Fig. 12. Variations of microbial activity in the control-MBR and the electro-MBR. 4. Conclusions Al electro-coagulants released during electro-coagulation considerably enhances the abilities of electro-MBR to alleviate the fouling tendency and improve sludge properties. The significantly lower fouling rate without requiring any chemical cleaning throughout the entire operating time confirms the excellent performance of electro-MBR in mitigating fouling

25

compared to traditional MBR. The coexistence of charge neutralization, adsorption, and electro-chemical oxidation plays an important role in mitigating membrane fouling in electroMBR. Electro-coagulation also enhances sludge compressibility and demonstrates its ability to control the overgrowth of filamentous bacteria, which markedly contribute to the better filtration performance of electro-MBR. Although microbial activity in electro-MBR was not significant, its effect on electro-MBR performance was negligible. This study indicates that electro-MBR is a promising technique and has the potential to become the next generation of MBR systems because of its potential ability to significantly alleviate membrane fouling Acknowledgements We deeply acknowledge the financial support from the Ministry of Science and Technology, ROC (Grant No. 102-2221-E-009-010-MY3). We also gratefully appreciate the assistances in sludge collection form the municipal wastewater treatment plant in National Chiao Tung University (Hsinchu, Taiwan).

26

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toward

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

Fouling behaviors and sludge characteristics were investigated within electro-MBR.



As fouling was successfully mitigated, no chemical cleaning needed for electro-MBR.



Charge neutralization and adsorption are ascribed to mitigate soluble foulants.



Electro-chemical oxidation can degrade and reduce bound foulants.

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Controlled filamentous growth and better compressibility were archived in electro-MBR.

Roles of electro-coagulation in electro-MBR

DC power supplied

(+)

Severe membrane fouling in traditional MBR

(-)

C o n tro l-M B R E le ctro -M B R 80

Membrane fouling mitigation

Severe membrane fouling

Charge neutralization and adsorption

T M P (k P a )

Timer

60

40

20

0

Electro-oxidation

0

Al electrode

3

6

9

12

15

Membrane module

O p e ra tin g tim e (d )

Uncontrolled filamentous bacteria

Better sludge compressibility

Traditional-MBR Electro-MBR

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