Guillaume Parent
email: guillaume.parent@univ-artois.fr

Rémi Penin

Jean-Philippe Lecointe

Jean-Franc ¸ois Brudny

Thierry Belgrand

A new approach to the critical induction in transformer cores

Keywords: Finite element method, grain oriented electrical steel, magnetic cores, transformers

The most critical areas in power transformers are their corner joints where the magnetic flux distribution presents a very high heterogeneity. This heterogeneity results from several parameters such as the geometry of the magnetic core, the anisotropy level of the electrical steel laminations and a critical induction. This paper is focused on the magnetic steel grades influence on the critical induction, adding a new parameter to those classically considered, such as the number of steps in a multiple step lap stack. Due to the difficulty to perform local measurements inside a real transformer corner, the study is performed both experimentally and by Finite Element analyses on a simplified magnetic core structure. The paper highlights that, in a corner of a core, it exists a critical induction level which depends on the magnetic steel grades and that, in a lamination, the magnetic flux density value along the Transverse Direction can be of the same order that it has along the Rolling Direction.

Introduction

Knowing the influence of Grain Oriented Electrical Steel (GOES) grade on the magnetic flux distribution inside transformer cores is a key point for their optimization in terms of core losses and acoustic noise. It is now admitted that this noise founds its origin into two magnetic phenomena: the Maxwell's forces and the magnetostriction [START_REF] Hasenzagl | Magnetostriction of 3% SiFe for 2-D magnetization patterns[END_REF][START_REF] Weiser | Mechanisms of noise generation of model transformer cores[END_REF][START_REF] Krell | Relevance of multidirectional magnetostriction for the noise generation of transformer cores[END_REF][START_REF] Weiser | Relevance of magnetostriction and forces for the generation of audible noise of transformer cores[END_REF][START_REF] Loizos | Core vibration analysis in Si-Fe distributed gap wound cores[END_REF]. In both cases, the most critical areas are the corner joints where the magnetic flux distribution presents a high degree of heterogeneity [START_REF] Mechler | Magnetic flux distributions in transformer core joints[END_REF][START_REF] Penin | Impact of mechanical deformations of transformer corners on core losses[END_REF]. It is the consequence of various parameters such as the geometry of the magnetic core [START_REF] Ilo | Influence of geometric parameters on the magnetic properties of model transformer cores[END_REF], the anisotropy level of the electrical steel [START_REF] Pfützner | Concept for more correct iron loss measurements considering path length dynamics[END_REF] and a critical induction B C [START_REF] Weiser | Relevance of magnetostriction and forces for the generation of audible noise of transformer cores[END_REF][START_REF] Löffler | Relevance of step-lap joints for magnetic characteristics of transformer cores[END_REF][START_REF] Ilo | Critical induction-a key quantity for the optimisation of transformer core operation[END_REF]. In particular, the latter parameter, which is the average induction level for which there is enough energy in the core to generate domains in the Transverse Direction (TD), significantly modifies the ratio between the in-plane and out of plane magnetic flux density. Now, according to the literature [START_REF] Weiser | Relevance of magnetostriction and forces for the generation of audible noise of transformer cores[END_REF][START_REF] Löffler | Relevance of step-lap joints for magnetic characteristics of transformer cores[END_REF][START_REF] Ilo | Critical induction-a key quantity for the optimisation of transformer core operation[END_REF], B C only depends on the geometry in a first hand, ie the number of steps in a Multiple Step Lap (MSL) configuration, and on the saturation polarization in a second hand.

In this paper, the influence of the magnetic steel grade on B C inside the corners is investigated. But due to the difficulty to perform local measurements inside a real transformer corner, the study is performed both experimentaly and by Finite Element (FE) analysis on a simplified structure enabling to reproduce the same phenomena. The experimental device, as well as its numerical model, are presented in the first part of the paper. In the second part, the experimental and numerical results are compared in order to validate the FE model. The third part is devoted to the study and the analysis of the influence of the electrical steel grade on B C , and thus on the magnetic flux distribution in transformer corners. In particular, it will be shown that in a corner:

• B C does depend on the magnetic steel grade,

• in a lamination, the magnetic flux density value along the TD can be of the same order of magnitude than in the Rolling Direction (RD)

1 Studied Structure 1.1 GOES stack in frame shape Studying especially the magnetic flux distribution in a transformer corner joint (Fig. 1a) is a difficult task, particularly in the in-plane air gaps. Due to their very small size, it is not obvious to insert measurement devices without modifying the local configuration [START_REF] Hihat | Equivalent permeability of step-lap joints of transformer cores: Computational and experimental considerations[END_REF][START_REF] Jones | Flux distribution and power loss in the mitered overlap joint in power transformer cores[END_REF]. Numerical models would give a possibility to avoid the instrumentation of a studied device, but modeling a whole transformer corner, including every laminations and air gaps would lead to extremely high or even unacceptable computation time. Hence, one of the best way to study the magnetic flux distribution in such areas consists in developing simplified structures as easy to equip with non invasive sensors as it is to model.

The structure reported in this paper is presented in Fig. 1b. The test assembly is made of a stack of hollow GOES square plates shifted one another from 90 • . There is no other air gap but the ones between each lamination out of the plane as shown in Figs. 1c and1d. By contrast, each core section presents a succession of RD and TD. Fig. 2 shows the first magnetization curves along the RD and TD for three different GOES grades: HGO 0.30 mm, CGO 0.30 mm and HGO 0.23 mm [START_REF]Grain oriented electrical steel PowerCore R -Our products[END_REF]. Due to the low value of the magnetic permeability along the TD, the reluctance of the parts of the legs oriented into that direction corresponds to the reluctance of a less than 1 mm thickness air gap. Then, it is assumed that the studied structure has a magnetic permeability arrangement comparable to the one locally encountered in the corners of a single-phase transformer, as well as in the external corners of a three-phase transformer built with Butt Lap or Single Step Lap joints.

Experimental setup

The experimental setup of the simplified structure is presented in Fig. 3. The stack is composed of 35 HGO 0.30 mm sheets. The dimension of a frame is 0.5 m×0.5 m and the width of a leg is 0.1 m. Primary and secondary windings are placed along each leg (coils A, B, C and D in Fig. 3). All the coils have to same number of turns and coils B and D are split in order to be able to insert sensors, as indicated in the following.

The excitation is made through the primary winding with a sinusoidal voltage at 50 Hz whereas the secondary winding is used to measure the magnetic flux density value in a whole leg, noted B global . Flux sensor coils disposed inside the legs (Fig. 3) give access to the local magnetic flux density along the RD and the TD, noted B RD and B T D respectively. Their thickness is of 100 µm which is bigger than to coating thickness (a few microns) and in order not to influence the behavior of the magnetic circuit those coils are located in the middle of a leg (Fig. 3), which is far enough from the corners.

Note that, in the following, B global , B RD and B T D will refer to peak values.

Numerical model

An FE model of the simplified structure, taking into account the laminations as well as the interlaminar air gaps, has been developped using the software GETDP [START_REF] Dular | A general environment for the treatment of discrete problems and its application to the finite element method[END_REF] in order to interpret the magnetic flux distribution in addition to the measured results. As previously stated (Fig. 1b), the structure is composed of a stack of GOES laminations whose RD are shifted one another from 90 • . Then, the periodic shape of the structure have been taken as advantage for simplification and model reduction, making it possible to model only two sheets, with the interlaminar air gaps. As an illustration, a plane cut of the model in the same axis system as the one used in Fig. 1 is presented in Fig. 4. This strategy has already successfuly been used in [START_REF] Parent | Analysis of the magnetic flux distribution in a new shifted non-segmented grain oriented AC motor magnetic circuit[END_REF]. As regards the mesh, it is constituted of prisms obtained by extrusion along the z axis (Fig. 1b). The use of an extruded mesh allows to easily control its density as well in the thickness of the laminations and as in that of the interlaminar air gaps. In this study, it is consituted of 8 and 4 prismatic layers in a lamination and in an air-gap respectively. Moreover, in order to get results as valid as possible, every simulations are performed with both scalar and vector magnetic potentials formulations [START_REF] Bossavit | Whitney forms: a class of finite elements for three-dimensional computations in electromagnetism[END_REF]. Note that the characteristics of the aforementioned mesh, especially its thinness, have been chosen so that both formulations give the same results. Nevertheless, in order to improve the readability of the figures, only the results obtained by the hφ formulation are shown.

Several methods exist to take into account the anisotropy of the GOES [START_REF] Nakata | Numerical analysis of flux distributions in cores made of anisotropic materials[END_REF][START_REF] Shirkoohi | A finite element method for modelling of anisotropic grain-oriented steels[END_REF][START_REF] Th | Study of anisotropic B-H models for transformer cores[END_REF][START_REF] Elleuch | Anisotropy in three-phase transformer circuit model[END_REF][START_REF] Lee | Nonlinear analysis of the three-phase transformer considering the anisotropy with voltage source[END_REF][START_REF] Tamaki | Comparison of magnetic field analysis methods considering magnetic anisotropy[END_REF].

Usually, the choice of a method with respect to another is made by considering the accuracy level, the implementation complexity, the computation time and the number of experimental data required. In this work, the method named Newton-Raphson in [START_REF] Lee | Nonlinear analysis of the three-phase transformer considering the anisotropy with voltage source[END_REF] is used. The mains rationales on the use of this method are its simplicity of implementation and its accuracy given the structure studied in this paper, as it will be shown in the following section. It requires the definition of the magnetic permeability along three directions: the RD, the TD and the Normal Direction (ND). In each direction, the non linearity of the sheets is taken into account by using first magnetization curves (Fig. 2). The ones related to the RD and the TD were obtained via standardized single sheet tester method [24,[START_REF] Pfützner | Concept for more correct magnetic power loss measurements considering path length dynamics[END_REF], whereas the one related to the ND is obtained by using a specific test bench [START_REF] Hihat | Experimental and numerical characterization of magnetically anisotropic laminations in the direction normal to their surface[END_REF].

Numerical Model Validation

In order to validate the numerical model, FE simulations have been performed using the steel grade used in the experimental device presented in section 1.2. That way, the experimental and numerical results can be compared. To do so, the frames have been magnetized as stated in sections 1.2 and 1.3 respectively, so that B global varies in a range from 0 T and 1.35 T. In Fig. 5 it appears clearly that, for B global < B C , the whole magnetic flux takes place along the RD, and there is no flux along the TD. But as soon as B C is reached, B RD does not evolve anymore, and B T D increases.

As an illustration of this phenomenon, Figs. 6 and7 show the magnetic flux distribution in two contiguous laminations in the case of B global = 0.48 T (induction for which there is a real possibility for both directions to be magnetized) and B global = 1.22 T (the value of the magnetic flux density just at the end of the knee of the TD magnetization curve) respectively. This distribution is as expected:

• in the case of the low B global value, the magnetic flux establishes along the RD only and then passes from a lamination to another one in the corner areas in order to fulfill the principle of energy minimization (Fig. 6),

• in the case of the high B global value, it still tends to establish along the RD, but also has to spread out along the TD (Fig. 7). As an example, in this figure, B T D = 0.578 T. Fig. 8, which presents the temporal evolution of measured and numerical values of B RD and B T D , also shows that the experimental and numerical results are in very good accordance since the maximum error value is 4 %. Hence, in the next section, the FE model will be applied to various steel grades.

Influence of the Electrical Steel Grade

As previously stated, the FE model can be used to compare and interpret the influence of the eletrical steel grades (HGO 0.30 mm, CGO 0.30 mm and HGO 0.23 mm) on the value of B C , and thus, on the magnetic flux distribution inside the structure. • the maximum flux density which can be established in the RD of a lamination cross section differs for the three different grades: it is equal to 1.86 T, 1.72 T and 1.82 T in the cases of HGO 0.30 mm, CGO 0.30 mm and HGO 0.23 mm respectively. That was expected given their first magnetization curves (Fig. 2).

• Fig. 9 also shows that B C depends on the electrical steel grade. Indeed, it is equal to 0.90 T, 0.72 T and 0.82 T in the cases of HGO 0.30 mm, CGO 0.30 mm and HGO 0.23 mm respectively This relation between the electrical steel grade and B C has never been mentioned in the literature. Actually, in the literature, it is stated that B C depends on the number of steps in the corner, on one hand, and on the saturation polarization [START_REF] Weiser | Relevance of magnetostriction and forces for the generation of audible noise of transformer cores[END_REF][START_REF] Löffler | Relevance of step-lap joints for magnetic characteristics of transformer cores[END_REF][START_REF] Ilo | Critical induction-a key quantity for the optimisation of transformer core operation[END_REF], on the other hand. However, in our case, all the tested grades have the same saturation polarization since they all are FeSi 3 %. Then, those results show that B C does not depend on the saturation polarization but depends on the approach to saturation of the magnetization curve, which is different from a grade to another one; the approach to saturation of CGO 0.30 mm is slower than the one of HGO 0.30 mm, and at the same time B C of CGO 0.30 mm lower than B C of HGO 0.30 mm. This may have a great influence on the efficiency of the tranformer in term of losses, on one hand, since the presence of magnetic flux in the TD of electrical steel sheets increases the core losses [START_REF] Jones | Flux distribution and power loss in the mitered overlap joint in power transformer cores[END_REF], and it may also have influence on the core vibration and noise, on the other hand. Indeed, the differences noted on B RD (t) and B T D (t) (Fig. 10) turn into a clipping on B RD (t) and thus to distortion. The lower the B C value, the higher the distortion, and thus, the more important the related harmonics. Now, exciting a tranformer so that B global is higher than B C results in an increase of the acoustic noise harmonics [START_REF] Valkovic | Effect of electrical steel grade on transformer core audible noise[END_REF][START_REF] Penin | Evaluation a priori des performances environnementales d'un noyau magnétique de transformateur triphasé sur la base de tests simplifiés[END_REF]. This B C is not a one for all deterministic value for noise and it has also to be associated to the geometry of the core joints, but it can be used as an indicator for noise increase.

As previously stated, the noise emission results from vibrations, which have two different origins: the Maxwell's forces and the magnetostriction. As both phenomena are related to the square of the local magnetic flux density, Figs. 11a and11b show the spectra of the square of the local magnetic flux densities along the RD and TD respectively. It appears that for B RD , the fundamental (100 Hz) and the first harmonic (200 Hz), which are the most significant spectral lines in this case, are the highest in the case of HGOES. This is in good accordance with the fact that B C HGO > B C CGO as previously stated, in one hand, and the fact that CGO sheets are noisier than HGO ones [START_REF] Taguchi | New grain-oriented silicon steel with high permeability "ORIENTCORE HI-B[END_REF][START_REF] Snell | Noise generated by model step lap core configurations of grain oriented electrical steel[END_REF][START_REF] Varga | Challenges in optimizing the magnetic properties of bulk soft magnetic materials[END_REF][START_REF] Penin | Grain-Oriented steel rings for an experimental comparison of relative magnetostriction and Maxwell's forces effects[END_REF], on the other.

Conclusion

In this paper, a simplified magnetic core enabling to analyze the distribution of the magnetic flux density in transformer corners has been presented. An FE model of this structure has been made, and the numerical results lead to a good accordance with the measurement results. Additional simulations, performed using various electrical steel grades, have shown the influence of the latters on the critical induction B C . The analysis of the results, such as the variation of the local magnetic flux density inside the structure as well as its harmonic spectrum, showed that the critical induction B C seems to be a key parameter to describe the occurrence of losses or noise but this quantity can not be identified without a suitable setup. Complementary investigations are needed to enable a simple access to this quantity.

Fig. 9

 9 Fig. 9 shows the variation of B RD and B T D with respect to B global for the three electrical steel grades previously mentioned. The influence of the latters appears on two points:

Figure 1 :

 1 Figure 1: Equivalence between a real corner joint and the simplified structure

Figure 2 :Figure 3 :Figure 5 :Figure 8 :B

 2358 Figure 2: First magnetization curves for HGO 0.30 mm, CGO 0.30 mm and HGO 0.23 mm steel sheets

Figure 9 :Figure 10 :Figure 11 :

 91011 Figure 9: Variation of B RD and B T D with B global inside a lamination for various electrical steel grades

(a) Odd layer (b) Even layerFigure 6: Magnetic flux distribution in the structure for B global = 0.48 T

(a) Odd layer (b) Even layerFigure 7: Magnetic flux distribution in the structure for B global = 1.22 T

Acknowledgment

This work is supported by the French national technological research cluster on electrical machine efficiency increase. This program, including ThyssenKrupp Electrical Steel UGO, is sponsored by the Region Nord Pas-de-Calais (France), the French ministry and the European founds (FEDER).

List of Figures


Zhenxing Ren

Guixia Ji

Hongbo Liu

Ming Yang

Suyun Xu

Mengting Ye

Eric Lichtfouse

Accelerated start-up and improved performance of wastewater microbial fuel cells in four circuit modes: Role of anodic potential

Keywords:

Negative potential accelerates the startup speed and enhances the performance of MFCs.

• The higher effluent pH of the MFCs corresponds to a higher COD removal. • The OC-MFC mode produces the slightest membrane fouling during start-up. • The abundance of exoelectrogens under the CI-MFC mode is as high as 54.7%.

Introduction

Dwindling fossil fuel supplies and increasing energy demand could lead to a global energy crisis [START_REF] Chen | Electrochemical technologies in wastewater treatment[END_REF]. Microbial fuel cells (MFCs) have gained great interest, because of the conversion system utilize chemical energy from wastewater into alternative bioenergy [START_REF] Gurreri | Electrodialysis applications in wastewater treatment for environmental protection and resources recovery: a systematic review on progress and perspectives[END_REF]. The conversion is achieved by coupling oxidation reactions that provide electrons at the anode with reduction reactions using electrons at the cathode [START_REF] Gajda | Water formation at the cathode and sodium recovery using Microbial Fuel Cells (MFCs)[END_REF]. These reactions are separated electronically inside the system, forcing electrons to flow through an external circuit, while the movement of ions inside the system maintains charge balance and completes the circuit. As a hybrid bioelectrochemical, different approaches of MFCs have been explored, including electricity generation mechanism [START_REF] Mandal | Chlorine mediated indirect electrooxidation of ammonia using non-active PbO2 anode: influencing parameters and mechanism identification[END_REF][START_REF] Schrott | Electrochemical insight into the mechanism of electron transport in biofilms of Geobacter sulfurreducens[END_REF], wastewater treatment [START_REF] Chen | Electrochemical technologies in wastewater treatment[END_REF][START_REF] Gurreri | Electrodialysis applications in wastewater treatment for environmental protection and resources recovery: a systematic review on progress and perspectives[END_REF], electrode materials [START_REF] Hindatu | Mini-review: anode modification for improved performance of microbial fuel cell[END_REF], reactor geometry and structure [START_REF] Zhang | Performance improvement of air-cathode singlechamber microbial fuel cell using a mesoporous carbon modified anode[END_REF], membranes [START_REF] Kim | The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR)[END_REF], substrate types [START_REF] Song | Effects of initial sediment properties on start-up times for sediment microbial fuel cells[END_REF], environmental monitoring [START_REF] Adekunle | A comparison of microbial fuel cell and microbial electrolysis cell biosensors for real-time environmental monitoring[END_REF] and bioremediation [START_REF] Gajda | Water formation at the cathode and sodium recovery using Microbial Fuel Cells (MFCs)[END_REF]. However, biofilm formation is a slow and complex process involving bacterial adhesion to the electrode surface, followed by cell-cell adhesion to form a multilayer biofilm [START_REF] Yuan | Microorganism-immobilized carbon nanoparticle anode for microbial fuel cells based on direct electron transfer[END_REF]. Consequently, the start-up time of MFC usually takes days to months, limiting its scale application.

Table 1 demonstrates that the start-up period of various wastewater MFCs ranges from 1 to 40 days. Mainly three strategies are used to decrease the start-up period: anodic material modification, cell architecture optimization and application of external potential. Anode is the main location for microbial adhesion and electron transfer from the substrate, whose material composition plays an important role in the performance (e.g., current, power output) and cost of MFCs [START_REF] Hindatu | Mini-review: anode modification for improved performance of microbial fuel cell[END_REF]. Anodic materials modification can improve the biocompatibility and electron mobility to promote the growth of microorganisms. Configurations of MFC vary widely, ranging from single-chamber reactor, to plate and frame (equally sized electrodes) and tubular configurations [START_REF] Rossi | Unraveling the contributions of internal resistance components in two-chamber microbial fuel cells using the electrode potential slope analysis[END_REF]. Differences in configuration impact the internal resistance and mass transfer efficiency, therefore even when experimental conditions and reactor setups are identical, the biofilm development, MFC performance and start-up time cannot be directly compared [START_REF] Huang | Preparation of conductive microfiltration membrane and its performance in a coupled configuration of membrane bioreactor with microbial fuel cell[END_REF]. The electrochemical limitations which result from ohmic, kinetic, and transport resistance can be reduced through cell architecture optimization. Anodic potential is a key factor affecting the start-up process, and is directly related to the microbial catalytic activity [START_REF] Aelterman | The anode potential regulates the bacterial activity in microbial fuel cells[END_REF], community composition [START_REF] Almatouq | Microbial community structure of anode electrodes in microbial fuel cells and microbial electrolysis cells[END_REF] and metabolic pattern [START_REF] Kumar | Catalytic response of microbial biofilms grown under fixed anode potentials depends on electrochemical cell configuration[END_REF]. Growth of microbial biofilms can be induced by precise control of anode potential vs. a reference electrode using potentiostat or DC power supply to apply external power, but the connection to the external power changes the MFC into MEC. Nevertheless, applying a positive or negative anode potential is still under debate [START_REF] Rossi | Unraveling the contributions of internal resistance components in two-chamber microbial fuel cells using the electrode potential slope analysis[END_REF]. Based on the Gibbs free energy ΔG = -nΔEF, where ΔE is the potential difference of electron donor and electron acceptor, F is the Faraday constant, n is the mole of electron transferred. A lower (negative) anodic potential naturally indicates more energy for microbes when the potential is near to NADH reaction potential (-520 mV vs. Ag/AgCl reference electrode) [START_REF] Wang | Key factors affecting microbial anode potential in a microbial electrolysis cell for H-2 production[END_REF]. Another point of view is that positive potential could increase the energy yield per equivalent of substrate oxidation and could accelerate the microbial acclimation [START_REF] Finkelsein | Effect of electrode potential on electrode-reducing microbiota[END_REF]; the key exoelectrogens are negatively charged Gram-positive bacteria easier attracted by the positive anodic potential [START_REF] Wang | Accelerated start-up of two-chambered microbial fuel cells: effect of anodic positive poised potential[END_REF].

The separation membranes (e.g., the proton exchange membrane, the cation exchange membrane and the anion exchange membrane) play a crucial role in the performance of wastewater MFCs. The function of a membrane is to transfer protons from the anode to the cathode while preventing the transfer of harmful substances such as oxygen and toxic substrates. Membrane fouling is a major issue that induces physical blockage of charge transfer channels [START_REF] Venkatesan | Characterization and performance study on chitosan-functionalized multi walled carbon nano tube as separator in microbial fuel cell[END_REF]. Chemical fouling is caused by chemicals (e.g., nutrients), present in the liquid media. Chemical fouling occurs mainly on the anodic side of the membrane. Biological fouling refers to membrane pollution by biological processes, e.g., by formation of a biofilm that thickens the membrane and, in turn, increases its resistance to mass transfer and ion transportation [START_REF] Yang | Assessment of different ceramic filtration membranes as a separator in microbial fuel cells[END_REF]. Although researches have recently focused on developing new membrane materials and modifying existing materials to reduce membrane fouling [START_REF] Huang | Preparation of conductive microfiltration membrane and its performance in a coupled configuration of membrane bioreactor with microbial fuel cell[END_REF][START_REF] Zinadini | High power generation and COD removal in a microbial fuel cell operated by a novel sulfonated PES/PES blend proton exchange membrane[END_REF][START_REF] Kondaveeti | The performance and long-term stability of low-cost separators in single-chamber bottle-type microbial fuel cells[END_REF], little is known on the effect of circuit modes on membrane fouling during the start-up of the process.

Therefore, here we studied the effect of four circuit modes, including the closed-circuit constant potential mode (CC-MFC, control), the closed-circuit intermittent potential mode (CI-MFC), the closed-circuit step potential mode (CS-MFC) and the open-circuit constant potential mode (OC-MFC), on the start-up period and cell performance of a wastewater MFC system. Meanwhile, the membrane fouling produced during the start-up stage were measured to reveal the influencing mechanism of circuit modes on membrane fouling. Furthermore, anodic microbial community composition was analyzed to investigate the effect of the circuit modes on the microbial communities. Based on these investigations, we attempt to provide a faster and more effective start-up strategy for the wastewater MFCs.

Materials and methods

Construction of wastewater microbial fuel cells

Four typical two-chamber wastewater MFCs with the same structure were constructed. The reactors were rectangular of 9 × 9 × 12 cm, and the two internal chambers were cylinders of 5 cm in length and 4 cm in diameter, whose working volume of a single chamber is 50 mL. Reactors were screwed in place, whose interface was sealed by a rubber ring and two chambers are separated by a cation exchange membrane (CMI-7000, Membranes International Inc, USA). The anode and cathode are 2 × 3 × 0.2 cm graphite felt and 3 × 3 × 8 cm carbon fiber brush, respectively, both of which are fixed on the top of the reactor, keeping /: not explicitly stated.

Table 1

Comparison of various start-up strategies for microbial fuel cells (MFCs).

-300mV (vs Ag/AgCl).

Electrochemical analysis and calculations

This investigation was conducted for 20 days, and voltage-recovery tests were done on day 5, 12, and 20 compared to the performance between individual MFCs. During the test, MFCs were disconnected from DC power supply, and the voltage between anode and cathode was recorded every 1 min using a data acquisition device (Picolog1216, Pico Technology Inc, UK).

The Coulombic efficiency C E , an important parameter related to electron transfer efficiency in the whole MFC reaction system, is calculated according to Eq [START_REF] Chen | Electrochemical technologies in wastewater treatment[END_REF].

C E = M S ∫ t 0 Idt Fb es V An ΔCOD ( 1
)
Where ΔCOD is the substrate concentration variation in one cycle (1 day); t is the cycle time; M S is the molar mass of the substrate; F is the Faraday constant; V An is the volume of liquid in anode chamber; b es is the amount of transferred electron matter. Electrochemical tests were measured using electrochemical workstations (CHI 660E, CHI Co. Ltd, Shanghai, China). During tests, the anode of MFC connected to the working electrode while the reference electrode and the cathode connect to the reference electrode and the counter electrode respectively. Polarization and power density curve were obtained by linear sweep voltammetry (LSV) with a scan rate of 1mv/s, using open-circuit potential as the initial and short-circuit potential as the final potential, then obtaining results from the V-I curve. Cyclic voltammetry (CV) can characterize the redox behavior of bioanode. The experiment was operated in three-electrode mode, setting the scan potential range from -0.8 -0 V with a scan rate of 0.01 mV/s.

Before the test of electrochemical impedance spectroscopy (EIS), the external circuit was disconnected for 1 h, setting the open circuit potential as the starting potential, with frequency range of 10 5 -10 -2 Hz, amplitude of 5 mV/s and stabilization time of 100s. The results obtained from the test were fitted in Zview software for analysis.

Characterization of cation exchange membranes

After the start-up investigations, cation exchange membranes (CEM) were washed with deionized water, to determine the transmittance of four CEM of MFCs in wavenumber range of 600-4000 cm -1 using FTIR (Thermo Scientific Nicolet iS5, USA), which was used to identify the variation of functional groups in the CEM under different circuit modes.

The specific proton conductivity (K M ) of CEM was measured to character membrane pollution; two stainless steel electrodes (2 cm apart) were used to provide a constant current and two platinum-wire electrodes (1 cm apart) to record the voltage drop in response to the current supply [START_REF] Fu | Sulfonated polystyrene/polyvinyl chloride composite membranes for PEMFC applications[END_REF]. The specific proton conductivity (K M ) is calculated by Eq (2), setting potentiostat (CHI 660E, CHI Co. Ltd, Shanghai, China) in the frequency range of 10 5 -50 Hz to measure the impedance.

K M = d R M × A (2)
Where R M is the resistance of the membrane; d is the thickness of the membrane; and A is the area of the membrane.

The ion transport number (t + ) is the fraction of the total current carried by a given ion in the electrolyte. We measured t + of four cation exchange membranes (single-sided area 12.6 cm 2 ) in a new reactor equipped with two Luggin capillary reference electrodes (Ag/AgCl) that were placed near the membrane-solution interface to monitor the potential variations. Different concentrations of sodium ions were added to two chambers to drive the concentration gradient [START_REF] Choi | Effects of biofouling on ion transport through cation exchange membranes and microbial fuel cell performance[END_REF]; measured electrode potential between the two reference electrodes after 30 min is calculated with Eq (3).

E M = RT F (2t + -1)ln( C 1 C 2 ) (3)
Where E M is the cell potential (mV); F is the Faraday constant (9.64853 × 104 C); R is the molar gas constant (8.31447 J/mol⋅K); T is the temperature (K) and t + is the transport number of the cation; C 1 and C 2 are the concentrations of electrolytes in different chambers (C 1 = 0.05 mol/ L NaCl, C 2 = 0.01 mol/L NaCl).

Determination of water quality indicators

The chemical oxygen demand (COD) and pH of the influent and effluent were measured daily. COD was measured by a rapid digestion spectrophotometry. 2 mL samples were mixed with digestion solution, then were heated at 150 • C for 2 h in a thermal reactor (DRB200-USA). After samples were cooled, they were measured by spectrophotometry (DR 2800, HACH company, Germany) and recorded averagely of three times. The pH of influent and effluent was measured by a pH meter (FG20, Mettler-Toledo, Switzerland) whose electrode was calibrated using buffers with pH = 4.0 and pH = 6.9 before testing.

Anode microbial community analysis

For comparative analysis of the bacterial community on anode under different circuit modes, electrodes enriched in microorganisms were collected and immediately stored at -80 • C. According to the manufacturer's protocol, microbial DNA was extracted from the inoculum and electrode samples were prepared using the E.Z.N.A. ® Soil DNA Kit (Omega Bio-tek, Norcross, GA, USA). The V4-V5 regions of the bacterial 16S rRNA gene were amplified by PCR using primers 338F (ACTCC-TACGGGAGGCAGCA) and 806R (GGACTACHVGGGTWTCTAAT). PCR the distance between the anode and the cathode at 5 cm. The Ag/AgCl reference electrode(SCE,+197 mV vs SHE; CHI Co. Ltd, Shanghai, China) was inserted near the anode.

Inoculation and start-up operations

Anaerobic sludge from the YE Wastewater Treatment Plant, Shanghai, China was used as inoculum, mixed with a 2:3 vol ratio with anode electrolyte. Chemical oxygen demand (COD) of 740 ± 50 mg/L and pH of 6.7 ± 0.1 were measured after centrifugation of the mixture. The anode electrolyte contains 1.64 g NaCH 3 COOH, 0.31 g NH 4 Cl, 4.40 g KH 2 PO 4 , 3.40 g K 2 HPO 4 , 0.10 g CaCl 2 ⋅H 2 O, 0.10 g MgCl 2 ⋅6H 2 O, 12.5 mL trace mineral solution and 5 mL vitamin solution per liter [START_REF] Wang | Concentration responses of toxicity sensor with Shewanella oneidensis MR-1 growing in bioelectrochemical systems[END_REF]. The anode mixture was aired with nitrogen for 20 min to remove dissolved oxygen before transferred into the anode chamber. The cathode electrolyte was prepared by dissolving 16.64 g K 3 Fe (CN) 6 , 4.40 g KH 2 ⋅PO 4 and 3.40 g K 2 HPO 4 ⋅3H 2 O in 1 L deionized water. Solution of two chambers should be changed daily, during which time the poorly adherent bacteria were washed off with 50 mM PBS buffer solution. During the investigation, temperature of all wastewater MFCs was controlled at 25 ± 2 • C, and reactors were homogenized by a magnetic stirrer to eliminate concentration gradients.

The conventional mode of CC-MFC was served as the control group. The MFC was connected to the DC power supply (SPD3303X-E, Siglent Technologies Co. Ltd, America), and kept the anodic potential at -300 mV vs Ag/AgCl in closed circuit by using a wire to connect anode and cathode. The CI-MFC and the CS-MFC modes were connected in the same way as CC-MFC. The CI-MFC was set to apply -300 mV (vs Ag/ AgCl) potential to the anode every 1 h. The starting potential of anode in CS-MFC was set to -150mV (vs Ag/AgCl), followed by the applied voltage V d = -[150 + (D-1) × 10] mV, where D is the number of experimental days, until V d = -300 mV, then kept the potential constantly. In the OC-MFC mode, the external circuit of MFC was disconnected and the DC power supply kept the anode potential at was conducted in ABI GeneAmp® 9700 (Applied Biosystems, USA) with the following steps: initial denaturation at 95 • C for 3 min, then denaturing at 95 • C for 30 s, annealing at 55 • C for 30 s, extension at 72 • C for 45 s, and finally extension at 72 • C for 10 min with a total of 27 cycles. After purification and quantification of the PCR products, the microbial communities of anodic biofilms were analyzed using Illumina MiSeq sequencing conducted by Majorbio (Shanghai, China).

Results and discussion

pH balance and substrate degradation

In this study the anodic reaction refers to the oxidation of sodium acetate to CO 2 (Eq (4)) and the cathodic reaction refers to the reduction of ferricyanide (Eq (5)). Typically, in the dual-chamber MFC, due to the generation rate of H + exceeds the transfer rate of H + with the growth of biofilm, H + was accumulated and resulted in the declination of pH at anode, which would affect the microbial activity of subsequent chemical reactions.

CH 3 COO -+ 2H 2 O→2CO 2 + 8e -+ 7H + (4)
Fe(CN) 3- 6 + e -→Fe(CN) 4- 6 (5)

However, compared with the influent pH, the effluent pH of CC-MFC, CS-MFC and OC-MFC increased during the start-up period (Fig. 1a). The effluent pH of OC-MFC and CS-MFC provided the most noticeable rising.

Only the effluent pH of CI-MFC decreased obviously after Day 11. These results suggest that some mechanism may exist to maintain anode pH balance. Fig. 1b demonstrated variations of COD removal rate during 20-day start-up under four circuit modes. All MFCs obtained significantly COD removal rates, with 20-day average COD removal rates of 70.5%, 54.2%, 78.3%, and 81.7% for CC-MFC, CI-MFC, CS-MFC, and OC-MFC, respectively. Interestingly, the COD removal rate showed similar distribution profiles as the effluent pH under four circuit modes, and the COD removal rate of CI-MFC also decreased after Day 11. The higher effluent pH in MFC corresponded to a higher COD removal.

We believe that this pH shift originates from the application of the anodic negative potential. For the closed-circuit mode, an anode potential (E) of -300mV was applied and the energy output of ΔE 3 can reach 927 mV (Fig. 2a), which forces the anodic electrons to reach the cathode quickly and bind to electron acceptors. Simultaneously, more protons were driven through the CEM to participate in the reduction reaction and accelerate the pH balance. The neutral pH is preferred by anodic microorganisms for substrate degradation, and the microbial activity declines at higher or lower pH [START_REF] Mohan | Biohydrogen production from chemical wastewater treatment in biofilm configured reactor operated in periodic discontinuous batch mode by selectively enriched anaerobic mixed consortia[END_REF]. Therefore, the pH shift improves the COD removal rate and plays a positive in the closed-circuit mode.

For the open-circuit mode, external negative potential cannot drive protons to the cathode. Another mechanism of pH shift may be occurring (Fig. 2b). Acetate acts as the sole electron donator in anode and is metabolized by microorganisms to generate energy and CO 2 (Eq (4)). CO 2 could be captured by anaerobic bacteria to produce CH 4 through directly electron transfer (Eq ( 6)) [START_REF] Villano | Bioelectrochemical reduction of CO2 to CH4 via direct and indirect extracellular electron transfer by a hydrogenophilic methanogenic culture[END_REF][START_REF] Rabaey | Microbial fuel cells: novel biotechnology for energy generation[END_REF], which reduces the acidity in the anode and thus favors substrate consumption. Besides, the low anodic potential favors biomass production accelerating the growth of microorganisms [START_REF] Aelterman | The anode potential regulates the bacterial activity in microbial fuel cells[END_REF]. In conclusion, the application of negative anode potential is the main factor to promote pH to neutral shift, regardless of closed-circuit or open-circuit mode, thereby enhancing the COD removal rate.

CO 2 + 8e -+ 8H + →CH 4 + 2H 2 O (6)

Electrochemical performance during start-up

Start-up time and coulombic efficiency

Voltage-recovery tests revealed the difference in electrochemical activity of MFCs in different circuit modes during the start-up stage. As shown in Fig. 3a, although voltage output was generated in both four MFCs on day 5, there was significant difference between them. Compared with the other three MFCs, the OC-MFC generated the highest voltage output and plateaued at 810 mV for about 20h. Furthermore, the OC-MFC reached the maximum voltage level at the beginning, while the CS-MFC and the CI-MFC took longer time (550 and 980 min respectively) to reach the maximum voltage output of 600 mV and 585 mV respectively, Then, their voltage decreased as the substrate concentration decreased, showing a typical bell-shaped curve. These results indicate that the started up of MFC in the open-circuit mode collects exoelectrogens with higher electrochemical activity effectively, with higher electron transfer rate. It can be seen from the voltage-recovery tests on day 12 and day 20 (Fig. 3b andc) that the voltage output of the O-MFC was still 810 mV without a significant increase, but the voltage stabilization time was longer. This is because the bacterial community stabilized partially when the voltage output was stable, while other genus still developed or declined after the voltage stabilization [START_REF] Paitier | Microbial fuel cell anodic microbial population dynamics during MFC start-up[END_REF]. With the gradual formation of the biofilm, the substrate utilization of the CI-MFC and the CS-MFC also increased continuously. It was noteworthy that although the CC-MFC can generate a high voltage output (about 650 mV) in a short time (5 days), its voltage level could not remain stable after 20 days, indicating that the competition between anode microorganisms is more complex in the closed-circuit constant potential mode, which is not conducive to the exoelectrogens forming predominant genus. According to the results in Fig. 3d, the Coulombic efficiency in CI-MFC remained high in all investigations. Since the Coulombic efficiency was influenced by the competition between exelectrogens and non-exelectrogens on the anode [START_REF] Juang | Comparison of electrogenic capabilities of microbial fuel cell with different light power on algae grown cathode[END_REF], we speculate that CI-MFC favors the growth of exelectrogens.

Polarization and power density

Fig. S1 depicted the polarization and power density curve of MFCs in different circuit modes at day 5, 12, and 20. The open circuit voltage (OCV)of OC-MFC stabilized at 500 mV in all polarization tests, which is higher than that of other MFCs. The OCV of CC-MFC, CI-MFC and CS-MFC increased with time, and the slope of their polarization curves decreased gradually, implying the decreasing of polarization resistance and increasing of microbial catalytic activity. On day 20, their OCV were 423 mV, 438 mV and 463 mV respectively. The maximum power density of OC-MFC increased from 15.6 mW/m 2 (day 5) to 25.94 mW/m 2 (day 20). This showed that electrochemical properties of the biofilm are still evolving even after day 5 when the voltage reached a plateau. But the power density of OC-MFC were also higher than other modes at day 5, 12 and 20, presenting the best electrochemical performance. The power density of CI-MFC was increased most significantly, which might be attributed to its highest Coulombic efficiency, with the maximum power density increasing from 0.9 m W/m 2 to 19.1 mW/m 2 in 15 days. In contrast, the power density of CC-MFC increased slowly by 8.1 mW/m 2 in 15 days, which was related to its internal colony competition.

Electrochemical activity and impedance analysis

Fig. 4a demonstrated CV curve of the four MFCs before and after start-up in different circuit modes. No visible current generation was observed before the start-up completed. After 20 days of start-up operation, clearly redox peaks were observed. The peaks as current response was considered proportional to the growth of the film. The closed area of CV curve of CC-MFC, CI-MFC, CS-MFC and OC-MFC were 7.6, 8.2, 7.8 and 8.3 times greater after start-up. The oxidation peak potential of the four MFCs were more positive than the applied potential of -300 mV, in agreement with previous studies speculating a self-regulatory strategy of the microbe to obtain higher energy [START_REF] Finkelstein | Effect of electrode potential on electrode-reducing microbiota[END_REF]. All four MFCs exhibited oxidation peaks at approximately -0.27 V (vs Ag/AgCl), indicating the existence of the same redox species. OC-MFC produced the highest peak current of 5.54 mA, which had high level of redox species. Another oxidation peak of CC-MFC at -0.38 V (vs Ag/AgCl) was found, which might be relative to cytochrome Omc B [START_REF] Rousseau | A theoretical model of transient cyclic voltammetry for electroactive biofilms[END_REF]. The reduction peak at -0.26 V (vs Ag/AgCl) of CC-MFC was much higher than other peaks, which might be resulted from difference of biofilm performance, allowing more electron-mediated or electroactive enzymes to be produced.

EIS presents the charge transfer impedance on electrode interface, which exhibits a semicircular part in the high frequency region and a linear part in the low frequency region. Fig. 4b showed the Nyquist plots corresponding to the impedance spectra after the completion of the start-up under different circuit modes. The equivalent circuit of Wang et al. was chosen for simulation, where R 0 is the ohmic resistance, R 1 is the anode charge transfer resistance, and R 2 is the cathode charge transfer resistance [START_REF] Wang | Accelerated start-up of two-chambered microbial fuel cells: effect of anodic positive poised potential[END_REF]. The simulation results in Table 2 indicated that the R tans was one order of magnitude higher than the R ohm for each circuit mode, implying that R tans was the main component of R toal . R tans of OC-MFC (13.33 Ω) was smaller than that of CI-MFC, which was equivalent to 3 R ohm of CC-MFC, indicating that the circuit mode had a significant influence on the internal resistance of wastewater MFCs. Although the internal resistance and power density was closely related, there was no absolute linear relationship found. For example, OC-MFC obtained the maximum power density with minimum internal resistance, but the maximum power density of CI-MFC with the biggest internal resistance was slightly lower than that of OC-MFC. Overall, OC-MFC is an effective start-up strategy that not only improves the electrochemical activity of wastewater MFCs, but also reduces the internal resistance.

Effect of circuit modes on cation exchange membrane fouling

After start-up investigations, the fouling of MFC membranes in four circuit modes were evaluated (Fig. 5a). Results showed that the front sides near the anode were more contaminated than the back sides near the cathode. The black fouling appeared higher for the CC-MFC, whereas CI-MFC and OC-MFC modes induced less contamination. We further analyzed residues by FTIR, specific proton conductivity (K M ) and ion transport number (t + ). Fig. 5b contained information on the CEM and the fouling layer. It was observed that characteristic peaks of CEM of all four MFCs appeared below 1500 cm -1 with almost identical characteristic peaks obtained, indicating that there was no significant effect of different circuit modes on the functional groups in CEM. The stretching vibration of water (above 1500 cm -1 ) in CEM of CC-MFC was weaker than in others, due to the difference in membrane performance. The distinctive peaks of sulfonate groups, which are the main functional groups of the CMI-7000 CEM, were observed at 1190 cm -1 and 1037 cm -1 . The characteristic peak at 1401 cm -1 was in the V C=N band associated with the amide group (stretching vibration of the C-N bond), indicating the presence of proteins, and implying the occurrence of microbial fouling of the membrane.

K M represents the lateral conductivity of membrane, which reflects the condition of membrane fouling. t + represents the longitudinal conductivity of the membrane, including the biological fouling covering the membrane surface and chemical fouling. In Fig. 5c, CC-MFC produced the lowest K M and t + , implying the heaviest membrane fouling, which is consistent with the observation in Fig. 5a. The OC-MFC performed with K M and t + 6.5 and 2.7 times higher than those of CC-MFC respectively. The difference between CC-MFC and OC-MFC is the circuit mode. Since strong electric field force was applied in CC-MFC, electrons were constantly transferred to cathode to introduce reduction reaction, which allowed cation supporting bacterial transportation across the CEM to maintain charge balance, resulting in blocked ion channel. Some divalent cations (e.g., Mg 2+ , Ca 2+ ) form precipitates with naturally occurring organic matter in water could create a dense fouling layer on the membrane [START_REF] Yang | Critical review of bioelectrochemical systems integrated with membrane-based technologies for desalination, energy selfsufficiency, and high-efficiency water and wastewater treatment[END_REF][START_REF] Vermaas | Fouling in reverse electrodialysis under natural conditions[END_REF], causing chemical contamination. Moreover, multivalent ions can bridge bacterial extracellular polymers, leading to the aggregation and stabilization of biopolymers and microorganisms, resulting in biological fouling [START_REF] Kim | The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR)[END_REF]. The anode ions in OC-MFC would not be driven by the electric field force to cathode, which greatly reduced the possibility of membrane fouling. Because the potential applied to the anode was stepped up in CS-MFC, the electric field force applied to CS-MFC was smaller than that of CC-MFC, thus the membrane was not contaminated as severely as in CC-MFC. Therefore, it can be concluded that the magnitude and duration of the electric field force during start-up is another important factor affecting the degree of membrane fouling.

Anode biofilm microbial community analysis

The composition of anode microbial community under different circuit modes were analyzed. Table S1 showed the results of Alpha diversity analysis. The Good's coverage of MFCs is generally greater than 0.99, indicating that the sequencing results are reliable. The control group (CC-MFC) presented low abundance but high diversity, showing that the circuit mode made the competitive pressure in anode colony greater than the synergistic effect, which was unsuitable for the development of the dominant colony. The high abundance and diversity of both CS-MFC and OC-MFC implied that the two modes were more In total, 13 bacterial phyla were identified at the phylum level. As shown in Fig. 6b, Firmicutes (58.32%), Actinobacteriota (16.63%), Chloroflexi (6.59%), and Proteobacteria (5.97%) were the dominant bacteria of CC-MFC, and the relative abundance of the minor bacteria Desulfobacterota was only 2.15%. Whereas, Desulfobacterota was the dominant bacteria of CI-MFC, CS-MFC, and OC-MFC with the relative abundance of 56.31%, 31.44%, and 35.79%, respectively, indicating that different circuit modes caused significant selective variation in the development of anodic microbial community. The relative abundance of Desulfobacterota in different modes revealed that it could not be adapted to the strong negative potential condition. Desulfobacteria have been reported to be involved in power generation and organic matter degradation and are the main exoelectrogens found in anode biofilm [START_REF] Liu | Characterization of microbial communities during anode biofilm reformation in a two-chambered microbial electrolysis cell (MEC)[END_REF]. Synergistetes are usually found in acetate-fed MFC, which is used to introduce fermenting bacteria in the anaerobic environment of anode, but with low abundance [START_REF] Lee | Characterization of the COD removal, electricity generation, and bacterial communities in microbial fuel cells treating molasses wastewater[END_REF]. Firmicutes, Chloroflexi and Proteobacteria were the dominant bacteria of CC-MFC, which are also essential for bioelectricity production, such as Geobacter sulfurreducens, Shewanella oneidensis and Clostridiumbutyricum EG3 are genera belong to these phyla [START_REF] Rabaey | Microbial ecology meets electrochemistry: electricity-driven and driving communities[END_REF][START_REF] Suzuki | Bacterial communities adapted to higher external resistance can reduce the onset potential of anode in microbial fuel cells[END_REF]. Therefore, it showed that all four MFC anodes had been adapted to different operating modes and possessed corresponding level of power generation capacity.

We further analyzed anodic biofilm at the genus level, with the 50 dominant genera shown in Fig. 6c. Geobacter was the main exoelectrogens of CI-MFC, CS-MFC and OC-MFC. Notably, the highest abundance of Geobacter in CI-MFC was 51.35%, indicating that the CI-MFC mode was favorable for the growth and colonization of Geobacter. Geobacter can provide critical indirect electron transfer without any medium and is one of the most important genera for electron transfer between bacteria and electrodes. A study also indicated that different Geobacter evolutionary branches are associated with specific potential [START_REF] Ishii | Microbial population and functional dynamics associated with surface potential and carbon metabolism[END_REF]. In Table 3, the relative abundance of Geobacter in CC-MFC was only 0.11%, together with other known exoelectrogens of Rhodopseudomonas (0.38%) and Desulforhabdus (0.10%); the total abundance was differed by two orders of magnitude from the CI-MFC (54.70%). However, the electrochemical performance of the CI-MFC mode was not optimal, suggesting that the microbial structural components did not influence the MFC performance directly. Dominant bacteria of Clos-tridium_sensu_stricto_1 (21.79%), Romboutsia (9.14%), and Acidaminococcaceae (5.01%) in CC-MFC all belong to Firmicutes phylum, which may be due to the inhibition of the growth of exoelectrogens by the high current of CC-MFC [START_REF] Almatouq | Microbial community structure of anode electrodes in microbial fuel cells and microbial electrolysis cells[END_REF]. The abundance of exoelectrogens is grossly inconsistent with electrochemical performance; we speculate that unknown exoelectrogens may be present in CC-MFC. The abundance of Pseudomonas in CS-MFC (1.31%) and OC-MFC (1.74%) were similar. Pseudomonas are regarded as exoelectrogens responsible for the conversion of VFA into electricity and intermediate compounds, such as phenazine chlorophyll acting as electron shuttles [START_REF] Srinivasan | Ecological and transcriptional responses of anoderespiring communities to nitrate in a microbial fuel cell[END_REF].

Conclusion

The effect of circuit modes on start-up performance and cation exchange membrane fouling of wastewater MFCs were evaluated in this study. The OC-MFC presented the shortest start-up time of 5 days and the highest voltage output of 810 mV. The degree of membrane fouling was significantly affected by circuit modes and the electric field force; the OC-MFC produced the slightest fouling with specific proton conductivity (K M ) of 0.013 ± 0.003 S/cm and ion transport number (t + ) of 0.954 ± 0.06. Besides, the hypothesis involved in pH balance to enhance substrate (acetate) degradation in anode chamber were proposed. For closed-circuit modes, the application of negative potential in anode produced a high electric field force that accelerates the pH shift to 7.0. For open-circuit modes, anodic pH balance was maintained by side reactions generated by applying negative anode potential, which consumes H + to generate biomass and CH 4 . The analysis of anode microorganisms demonstrated that microbial community structure varied significantly among four circuit modes, and the total abundance of exoelectrogens in CI-MFC mode was the highest.

Fig. 1 .

 1 Fig. 1. Variations of effluent pH (a) and COD (b) during 20 days of start-up in MFCs under four circuit modes. The dashed line represents the influent pH.

Fig. 2 .

 2 Fig. 2. Tentative mechanism of pH shift of microbial fuel cells (MFC) in closed-circuit (a) and open-circuit (b) conditions. ΔE 1 indicates the potential driving acetate oxidation in Fig. 2a; ΔE 2 represents the potential energy of electron transfer from bacteria to anode, which depended on the energy-saving strategy of the bacteria; ΔE 3 indicates the energy output of MFCs.

Fig. 3 .

 3 Fig. 3. Voltage-recovery tests on day 5 (a), 12 (b) and 20 (c), and the corresponding coulombic efficiency (d) in different circuit modes.

Fig. 4 .

 4 Fig. 4. (a) Cyclic voltammetry (CV) before and after start-up in different circuit modes; (b) Nyquist plots corresponding to the impedance spectra in different circuit modes on day 20.

Fig. 5 .

 5 Fig. 5. Analysis of membrane fouling. (a) membrane photos, (b) FTIR spectra of the membranes and (c) characterization of membrane fouling in different circuit modes after 20 days of operation.

Fig. 6 .

 6 Fig. 6. (a) Principal component analysis on anodic biofilm under four circuit modes; (b) composition and relative abundances in bacterial communities at the phylum level based on 16S rRNA sequences; (c) heatmap of the samples tested for classified bacterial genera with the relative abundance of top 50 illumina sequencing.

Table 2

 2 Simulation results of internal resistance in MFC under different circuit modes.

	Modes	R total (Ω)	R ohm (Ω) (R 0 )	R trans (Ω) (R 1 +R 2 )
	CC	51.44	3.64	47.80
	CI	58.05	3.12	54.93
	CS	46.46	2.60	43.86
	OC	45.62	2.52	43.10

Table 3

 3 Relative abundances of known exoelectrogens in anodic microflora at the genus level based 16S rRNA sequences under four circuit modes (relative abundance ≥ 0.10%).

	Samples	Relative abundance(%)

/: not available.

Acknowledgements

The authors have stated that there was no conflict of interest. We gratefully acknowledged the co-funding of this work by the National Natural Science Foundation of China (No.52070130) and the Shuguang Project of Shanghai (Education and Scientific Research Project of Shanghai,18SG45).

CRediT authorship contribution statement

Zhenxing Ren: Investigation, mechanism modelling. Guixia Ji: Investigation, Data curation, Supervision. Hongbo Liu: Funding acquisition, Conceptualization, Project administration, Supervision, Writingoriginal draft. Ming Yang: Methodology, Data curation. Suyun Xu: Writingreview & editing. Mengting Ye: Investigation. Eric Lichtfouse: Writingreview & editing.

Declaration of competing 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.

Appendix A. Supplementary data

Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2022.231403 and below.