Simultaneous carbon and nitrogen removal accompanied by energy recovery from wastewater in a coupled microbial fuel cells system

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ACCOMPANIED BY ENERGY RECOVERY FROM

WASTEWATER IN A COUPLED MICROBIAL FUEL CELLS SYSTEM

BY

NGUYEN HOANG DUNG

A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY (ENGINEERING AND TECHNOLOGY)

SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY THAMMASAT UNIVERSITY

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BY

NGUYEN HOANG DUNG

ENTITLED

SIMULTANEOUS CARBON AND NITROGEN REMOVAL ACCOMPANIED BY ENERGY RECOVERY FROM WASTEWATER IN A COUPLED MICROBIAL

FUEL CELLS SYSTEM

was approved as partial fulfillment of the requirements for the degree of Doctor of Philosophy (Engineering and Technology)

on June 26, 2023

Chairperson

(Professor Nipon Pisutpaisal, Ph.D.) Member and Advisor

(Professor Sandhya Babel, D.Tech.Sc.) Member

(Associate Professor Rachnarin Nitisoravut, Ph.D.) Member

(Associate Professor Paiboon Sreearunothai, Ph.D.) Member

(Associate Professor Jenyuk Lohwacharin, Ph.D.) Member

(Warunsak Liamlaem, Ph.D.) Director

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Dissertation Title SIMULTANEOUS CARBON AND NITROGEN REMOVAL ACCOMPANIED BY ENERGY RECOVERY FROM WASTEWATER IN A COUPLED MICROBIAL FUEL CELLS SYSTEM

Author Nguyen Hoang Dung

Degree Doctor of Philosophy (Engineering and Technology)

Faculty/University Sirindhorn International Institute of Technology/ Thammasat University

Dissertation Advisor Professor Sandhya Babel, D.Tech.Sc Academic Years 2022

ABSTRACT

For over a decade, microbial fuel cell (MFC) has received much attention as a pioneering technology for wastewater treatment, owing to its ability to generate electricity from organic matter However, most wastewater has the presence of nitrogen and the requirement to remove nitrogen as a contaminant has caused limitations in energy recovery from organic carbon Despite the advantages of integrating biological nitrogen removal (BNR) into MFC and the promising results in organic carbon removal, achieving high nitrogen removal efficiency while optimizing energy recovery from organic matter remains a challenge It is difficult to select an external resistance (ER) value that satisfies both goals The chambers of a stacked MFC system can be sequencing-batch operated to achieve specific goals System can configured accordingly with suitable conditions for each chamber to achieve the goal of energy recovery from organic carbon and nitrogen removal

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was set at a small value (10 Ω) to have a high current density, facilitating nitrogen removal in presence of minimum carbon as required for the denitrification The study evaluated the removal efficiency of carbon and nitrogen accompanied by power generation by applying three different sequencing-batch operation modes to four chambers of a coupled MFC In the first mode, wastewater transferred from the N-MFC anode chamber to the cathode chamber, then to the D-MFC anode chamber, and lastly to the D-MFC cathode chamber In the second and the third modes, the wastewater was fed into anode chamber of the N-MFC The D-MFC received the N-MFC output (in order from anode chamber to anode chamber, from cathode chamber to cathode chamber) The output of the cathode chamber of the D-MFC was the effluent of the coupled MFC system, while the output of the anode chamber of the D-MFC came back to the cathode chamber of the N-MFC The distinction between the second and third modes is the different dissolved oxygen (DO) in the cathode chamber of N-MFC to control the nitrification process The study provided a solution for optimizing electrical energy recovered from organic matter in parallel with efficient nitrogen treatment and better understanding of integrating BNR process into MFC technology Depending on each operation mode, the following specific objectives are proposed: (i) to investigate the effect of DO, and initial ammonium concentration for ammonium oxidizing and power generation from N-MFC; (ii) to assess the ability of ammonium diffusion through cation exchange membrane (CEM) in N-MFC; (iii) to investigate the effect of the COD/N ratio for nitrogen removal in D-MFC; (iv) to evaluate the coulombic efficiency of the system; (v) to evaluate the ratio of autotrophic denitrification in cathodic chamber of D-MFC

The following is a summary of the results and findings of this study

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(ii) The input COD of the anode chamber and the DO concentration in the cathode chamber affect the electricity generation of the N-MFC, directly influencing the diffusion of cations (such as ammonium) from the anode chamber to the cathode chamber in order to balance the charge In addition, the ER of N-MFC in the second operational mode (50 Ω) is lower than that in the third operational mode (100 Ω), resulted in more favorable current production and more ammonium diffusion for charge balance

(iii) The nitrogen removal efficiency at the D-MFC increased when the COD/N ratio of wastewater entering the D-MFC increased The first operational mode, which used both the anode and cathode chambers for denitrification, enhanced nitrogen removal efficiency In the second and third operational modes, the denitrification process only occurred in the cathode chamber of D-MFC The nitrogen removal efficiency at the cathode chamber of the D-MFC was higher in the third operational mode than in the second operational mode for the same COD/N ratio input to the D-MFC, which is the result of a higher reduction rate of nitrite than nitrate

(iv) In the first operational mode, an increase in the COD input of the anode chamber resulted in a decrease in the anodic coulombic efficiency The anodic coulombic efficiencies of N-MFC in the second operational mode was higher than that in the first operational mode The anodic coulombic efficiencies of N-MFC in the third operational mode were relatively low because of low DO in the cathode chamber

(v) In all operational mode, the high ratio of autotrophic denitrification in the cathode chamber of the D-MFC demonstrated that autotrophic denitrification was the primary process assisting in nitrogen removal

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DO concentration in the cathode chamber, and the ER In the second and the third operational mode, the main mechanism for nitrogen removal at the D-MFC was autotrophic denitrification When comparing the second and the third operational modes, the power generation was higher in the second mode, which followed a conventional nitrification/denitrification system However, the third mode with shortcut nitrification-denitrification was more energy-efficient and better in nitrogen removal

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ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my advisor Prof Sandhya Babel, for her patient guidance, continuous support, and valuable encouragement throughout my Ph.D journey Her expertise, kindness, and dedication have been uncountable to me

I am also deeply grateful to my thesis committee members, Prof Nipon Pisutpaisal, Assoc Prof Paiboon Sreearunothai, Assoc Prof Rachnarin Nitisoravut, Dr Warunsak Liamlaem and Assoc Prof Jenyuk Lohwacharin, for their insightful feedback and constructive criticism Their comments have been instrumental in shaping my research

I thank my colleagues and friends, Dr Tuan Anh Ta, Dr Chamath D.Y Yahampath Arachchige Don, and Mr Tan Thong Nguyen, for contributing to the experimental model construction, proofreading, and helpful brainstorming

I am indebted to the staff and faculty of Sirindhorn International Institute of Technology (SIIT), Thammasat University, for providing a stimulating research environment and many personal growth opportunities This work would not have been possible without the full financial support of SIIT through an EFS scholarship in the School of Bio-Chemical Engineering and Technology

Finally, I would like to give my heartfelt appreciation to my family, especially to my father Van Nghia Nguyen, my mother Hong Phuong Ha, my brother Trung Hieu Nguyen, my sister Ngoc Lan Nguyen, my brother Hoang Long Nguyen, for their unwavering love throughout my abroad study Their sacrifices and understanding have been my constant source of strength

Thank you all for your inspiration

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TABLE OF CONTENTS Page ABSTRACT (1) ACKNOWLEDGEMENTS (5) LIST OF TABLES (9) LIST OF FIGURES (10) LIST OF SYMBOLS/ABBREVIATIONS (12) CHAPTER 1 INTRODUCTION 1.1 Problem statement 11.2 Research objectives 71.3 Scope of research 7

CHAPTER 2 LITERATURE REVIEW 8

2.1 Principle of MFCs technology 8

2.2 Design of MFCs 11

2.2.1 Casing 11

2.2.2 Electrode 12

2.2.3 Separator 13

2.2.4 Inoculum and substrate 142.3 Recent MFCs studies for nitrogen and carbon removal, power

generation 15

2.3.1 Nitrification and cathodic denitrification in MFCs 162.3.2 Shortcut nitrification/denitrification process in MFCs 222.3.3 Heterotrophic anodic denitrification in MFCs 23

2.4 Microbial communities 25

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2.6 Challenges in using MFC for carbon and nitrogen removal 30

CHAPTER 3 RESEARCH METHODOLOGY 32

3.1 MFCs configuration 32

3.2 Inoculation and medium 33

3.3 Start-up MFCs 34

3.4 MFCs operation after start-up 353.4.1 First operational mode 373.4.2 Second operational mode 373.4.3 Third operation mode 383.5 Analysis and calculations 403.6 Characterization analysis of electrode surfaces 41

CHAPTER 4 RESULTS AND DISCUSSION 43

4.1 Start-up stage 43

4.2 First operational mode 464.2.1 Electricity generation and COD removal 46

4.2.2 Nitrogen removal 50

4.3 Second operational mode 544.3.1 Electricity generation and COD removal 564.3.2 Nitrogen isolation and removal 604.4 Third operational mode 644.4.1 Power generation from COD removal 64

4.4.2 Nitrogen isolation 72

4.4.3 Shortcut nitrification 754.4.4 Denitrification at the D-MFC 764.5 The advantages and disadvantages of three different operational modes:

A comparison 79

4.6 Characterization of electrode surfaces 81

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 84

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LIST OF TABLES

Tables Page

2.1 Redox reactions with theoretical potentials standard conditions in MFC 82.2 The advantages and disadvantages of some electrode materials 132.3 The performance with pros and cons of various MFC configurations for

simultaneous removal of carbon and nitrogen 192.4 Some microorganisms and their roles in MFC system 274.1 COD output of each chamber of MFC 494.2 Nitrogen parameter outputs of each chamber of MFC 514.3 A comparion of three different operational modes with same influent

wastewater input 80

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LIST OF FIGURES

Figures Page

2.1 Diagram of MFCs technology principles in wastewater treatment 9

2.2 DET mechanisms to anode 11

2.3 Shortcut nitrification-denitrification process in MFCs 222.4 Mechanism of heterotrophic anodic denitrification process in MFCs 243.1 The main components of the coupled MFCs 323.2 The procedure for pretreatment of CEM 333.3 The procedure for pretreatment of electrode 333.4 Schematic diagram of start-up stage 353.5 Summary of operational stage 363.6 Schematic diagram of the first operational mode 373.7 The second mode operating schematic of the coupled MFC system 383.8 The third mode operating schematic of the coupled MFC system for shortcut nitrification-denitrification 404.1 Cell votage progression (Rext = 500 Ω) during 30 days start-up in both MFC

reactors 44

4.2 (A) Polarization curve and power curve of N-MFC (B) Cell votage vs current

of N-MFC 45

4.3 (A) Polarization curve and power curve of D-MFC (B) Cell votage vs current

of D-MFC 46

4.4 Cell voltage progression of the coupled MFC in the first mode with different COD influents (A – 1000 mg L-1, B – 1500 mg L-1, C – 2000 mg L-1) 484.5 Output pH of each chamber in the first mode operation with different COD

influents 54

4.6 (A) Polarization curve and power curve of N-MFC (B) Cell votage vs current

of N-MFC 55

4.7 (A) Polarization curve and power curve of D-MFC (B) Cell votage vs current

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4.8 Cell voltage of N-MFC and D-MFC during the second operational mode

(A, B, C – nitrogen input of 50, 75, 100 mg L-1, respectively) 584.9 Chemical analysis of the effluent from each chamber of the coupled MFC

system in the second operational mode (n.d: not detectable) 614.10 Output pH of chambers in the coupled MFC with different nitrogen influents 644.11 Cell voltage progression of the coupled MFC system during operation

(1, 2, 3, 4 correspond to the first to fourth operating conditions) 664.12 Cell voltage vs current of N-MFC when conducting polarization test

(1, 2, 3, 4 correspond to the first to fourth operating conditions) 714.13 Chemical analysis of the effluent from each chamber of the coupled MFC system in four operating conditions (n.d: not detectable) 734.14 The pH of each chamber’s output under various operating conditions 784.15 SEM images of electrodes (O – before operation; A, B, C, D – anode of

N-MFC, cathode of N-MFC, anode of D-MFC, cathode of D-MFC after

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LIST OF SYMBOLS/ABBREVIATIONS

Symbols/Abbreviations Term

AEM Anion Exchange Membrane AOB Ammonia-Oxidizing Bacteria

AO Anoxic-Oxic

BNR Biological Nitrogen Removal CEM Cation Exchange Membrane DET Direct Electron Transfer DO Dissolved Oxygen ER External Resistance IR Internal Resistance

MET Mediated Electron Transfer MFC Microbial Fuel Cell

NOB Nitrite-Oxidizing Bacteria

SND Simultaneous Nitrification-Denitrification TOC Total Organic Carbon

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CHAPTER 1 INTRODUCTION

1.1 Problem statement

Organic matter contains carbon, hydrogen, oxygen, and other trace elements and is the primary pollutant in wastewater The release of wastewater with a significant quantity of organic matter endanger the water ecosystem because the degradation of these organic compounds utilizes dissolved oxygen (DO) in water, causing a lack of DO for aquatic life (Nora’aini et al., 2005) Besides carbon, nitrogen is also a notable parameter in wastewater High nitrogen concentrations cause eutrophication in ponds, lakes, canals, rivers, and coasts As a result, the planktonic algae overgrowth depletes the DO in water, clogs water intakes, blocks light to deeper waters, and destroys the ecosystem (Howarth and Marino, 2006) An excessive amount of nitrate in groundwater can cause methemoglobinemia, which interferes with the circulation of oxygen in the blood (Feleke and Sakakibara, 2002) Besides, the water bodies where wastewater is received may be the influent of the water treatment plant, supplying water for humans As a result, the cost of water treatment to meet standards for drinking or production Therefore, wastewater treatment is essential before discharging it into the environment Carbon and nitrogen criteria always received special attention from the designers of wastewater treatment plants when choosing technology

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the wastewater Denitrification bacteria use the biodegradable organic carbon as the source of electrons in these processes Many technologies can currently be selected for wastewater treatment, most of which use biotechnology because of its sustainability and low cost Conventional technologies, like the anoxic-oxic (AO) system based on the activated sludge processes are favored for its ability to remove organic substance and nitrogen simultaneously However, these treatment methods consume a significant quantity of energy for aeration, which accounts for nearly half of the total operational energy (Drewnowski et al., 2019) This energy is required to provide oxygen for the oxidation of organic matter and results in the generation of substantial sludge that needs to be properly managed Anaerobic biological technology, such as upflow anaerobic sludge blanket (UASB) reactors, offers advantages including not requiring air supply, low solid yield, methane production as biofuel However, anaerobic treatment has low organic removal efficiency and is very limited in nitrogen removal, resulting in low-quality effluent and needs additional treatment for the remaining pollutant Slow startup and high-temperature requirements are also disadvantages of anaerobic treatment (Chong et al., 2012) In addition, produced biogas needs to be pretreated (separation and purification) before being used to generate electricity In terms of cost-benefit analysis, this is not feasible for small wastewater treatment plants (Gude, 2016) The anammox microorganisms derive their energy from the reaction of ammonium and nitrite to generate nitrogen gas Therefore, anammox processes with the participation of AOB and anammox bacteria have some outstanding benefits such as oxygen-saving for nitrification, eliminating external carbon source requirement for denitrification, and low sludge production (Cho et al., 2010) Nevertheless, its drawback is the complicated operational control The anammox biomass not only grows extremely slowly but is also easily washed out during operation (Huynh et al., 2019)

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aerobic to anoxic for high nitrogen removal efficiencies, resulting in increased pumping cost Also, the aerobic tank's circulating flow with a high DO concentration causes an adverse environment for denitrification at the anoxic tank Another limitation of the DO presence in the anoxic tank is that aerobic digestion of readily biodegradable organic carbon wastes the electron supplier for denitrification (Sander et al., 2017)

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system (Zhang and He, 2012) Wang et al (2013) reported that recovered energy from MFCs helped to save 30–50% in operational costs

Because of these apparent advantages, MFCs have attracted many researchers with a significant quantity of publications in the field of treating wastewater (Santoro et al., 2017) Most of the research aims to improve the power recovered from wastewater and the efficiency of removing carbon and nitrogen Some modified configurations of MFCs have been applied, such as two-chambers MFC (Virdis et al., 2008), chamber MFC with the exposed-air cathode (Hussain et al., 2016), single-chamber MFC with the rotating cathode (Zhang et al., 2013), membrane-less MFC (Zhu et al., 2013), tubular dual-cathode MFC (Zhang and He, 2012), oxic/anoxic-cathode MFC (Xie et al., 2011), stacked five-units MFC (Park et al., 2017) In parallel, many types of ion-exchange membranes and electrode materials are also tested, and certain performance improvements are achieved Although the results showed a good carbon removal efficiency above 80%, the nitrogen removal efficiency fluctuated significantly and ranged from 36% to 97%, depending on experimental conditions The parameters affecting the nitrogen removal in MFC were investigated and evaluated such as carbon sources (Feng et al., 2013), C/N ratio (Huang et al., 2013a), pH (Clauwaert et al., 2009), DO (Virdis et al., 2010), and electrolyte conductivity (Puig et al., 2012) Because successful nitrogen removal in wastewater involves nitrification and denitrification processes, researchers found some methods to integrate these two processes into MFC technology Some studies conducted nitrification and denitrification separately by using an independent nitrification reactor (Virdis et al., 2008), using a rotating cathode or dual cathode (Xie et al., 2011; Zhang and He, 2012; Zhang et al., 2013) Some other research performed simultaneously these two processes by controlling aeration in the cathodic chamber (Virdis et al., 2010; Wu et al., 2017) or using the air cathode (Hussain et al., 2016)

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generation is also decreased by 40% in the nitritation/denitritation process (Beylier et al., 2011) In MFCs, nitrite's electron utilization is better than that of nitrate, resulting in higher coulombic efficiency on cathodic reaction (Virdis et al., 2008) At high temperature (> 30oC), AOB biomass may dominate over NOB biomass, favoring nitrite accumulation for denitritation process (Hellinga et al., 1998) Therefore, industrial wastewater released with high temperature as dyeing wastewater may be suitable for the shortcut nitrification/denitrification process without wastewater heating

Most MFC reactors were designed with a separator between the two chambers to prevent crossover processes that could have adverse effects, drastically reducing MFC performance (Santoro et al., 2017) The fundamental requirement of the separator is to be conductive for ions to balance the overall charge transfer during the operation of the MFC Therefore, ion exchange membranes are a good choice for separators, such as cation exchange membranes (CEMs) However, they are pricey, accounting for the significant investment costs of MFC systems In addition, the poor selectivity of CEMs and the overwhelming concentrations of other cations such as Na+, K+, and NH4+

compared to protons in wastewater resulted in low protons transfer efficiency (Rozendal et al., 2006) This creates a pH imbalance between the two chambers, which leads to a decrease in MFC performance (Nguyen and Babel, 2022) However, from another perspective, this weakness can be exploited to an advantage by using ion exchange membranes to isolate nitrogen from influent wastewater This allows an MFC reactor to be fully configurable to optimize energy recovery from organics while the isolated ammonium oxidation contributes to pH balance Recent research also explored membrane penetration of nitrogen and took advantage of them to aid in nitrogen removal in a dual-chamber MFC (Jin et al., 2022)

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accomplish concurrently in an MFC reactor Firstly, high nitrogen removal occurs at the lowest possible ER of an MFC (Li et al, 2014) However, to obtain maximum power density, ER needs to be set close to internal resistance (IR) of the MFC system (Chen et al., 2019; Li et al., 2016; Park et al., 2017) Basically, utilizing nitrate or nitrite as electron acceptors at the cathode causes the IR to be high compared to utilizing oxygen for the same purpose (Guo et al., 2020; Li et al., 2016; Zhang et al., 2020) As a result, it is difficult to select an ER value that satisfies both goals Secondly, O2/H2O has a higher redox potential (+0.82 V) than that of NO3-/N2 (+0.75 V) and NO2-/N2 (+0.35 V) (Nguyen and Babel, 2022) Therefore, oxygen reduction can produce a higher cell voltage than denitrification at the cathode However, the presence of oxygen at the cathode has a negative impact on the denitrification process, decreasing the nitrogen removal efficiency Finally, conducting simultaneous nitrification-denitrification (SND) in an MFC reactor requires complex control of operating parameters and does not benefit in the power generation from organic material biodegradation (Virdis et al, 2010) A dual-cathode MFC (Li et al, 2016; Zou et al, 2018) or a coupled MFC system (Chen et al, 2019; Xie et al, 2011) has been used to separate the nitrification and denitrification processes It has been reported that a stacked MFC with the effluent of one unit being the influent of the next unit can help improve electricity generation by decreasing IR within the system (Jadhav et al., 2021; Walter et al., 2016) Five stacked MFC units successfully treated low-strength wastewater and reduced the hydraulic retention time (HRT) to 2.5 h (Park et al, 2017)

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second MFC (D-MFC) facilitated nitrogen removal in presence of minimum carbon as required for the denitrification process

1.2 Research objectives

This study evaluates the removal efficiency of carbon and nitrogen accompanied by power generation from synthetic wastewater Different sequencing-batch operation modes are applied to four chambers of a coupled MFC for optimizing recovery of electrical energy from organic matter in parallel with efficient nitrogen treatment Results of study will offer a deeper understanding of integrating the BNR process into MFC technology Batch-sequencing operation is expected to bring a potential technical solution Depending on operation mode, the following specific objectives are proposed

(1) To investigate the effect of DO, and initial ammonium concentration for ammonium oxidation and power generation from N-MFC

(2) To assess the ability of ammonium diffusion through CEM in N-MFC (3) To investigate the effect of the COD/N ratio on nitrogen removal in D-MFC (4) To evaluate the coulombic efficiency of the system

(5) To evaluate the ratio of autotrophic denitrification in cathodic chamber of D-MFC

1.3 Scope of research

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CHAPTER 2

LITERATURE REVIEW

2.1 Principle of MFCs technology

An MFC is a bioelectrochemical system that its electrochemical reactions occur by the interaction between microorganisms and electrodes (Mook et al., 2013) Operation principle of MFCs relied on the electron donation and acceptance of pairs of redox reactions, including the anaerobic oxidation of organic material by microorganisms at the anode and the reduction of higher electrochemical potential substances at the cathode Electrical energy is recovered from the biodegradation of organic matter by connecting the two electrodes with an electric wire to capture electrons' flow Table 2.1 shows the redox reactions in MFCs (Nguyen and Babel, 2022) A typical dual-chamber MFCs with CEM used to treat wastewater is shown in Fig 2.1 Table 2.1 Redox reactions with theoretical potentials standard conditions in MFC

Reductant Anodic reaction Ean (V vs SHE) Acetate CH3COO- + 4H2O  2HCO3- + 9H+ + 8e- -0.279 Glucose C6H12O6 + 12H2O  6HCO3- + 30H+ + 24e- -0.414 Metanol CH3OH + 2H2O  HCO3- + 7H+ + 6e- -0.453 Glycerol C3H5(OH)3 + 6H2O  3HCO3- + 17H+ + 14e- -0.385 Carbon C + 2H2O  CO2 + 4H+ + 4e- -0.207 Oxidant Cathodic reaction Ecat (V vs

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Figure 2.1 Diagram of MFCs technology principles in wastewater treatment

The anodic oxidation reactions and the cathodic reduction reactions are referred to as half-cell reactions Its potentials can be calculated following Logan et al (2006)

0ln redoxMG G RTEnF nF nF M          (2.1)00 0.05916log loglogredredoxoxM MRTE E EnF e M n M             (2.2)

where E (V) is the theoretical electrode potential in a certain condition, E0 (V) is the standard potential (at 298 K, pH2 = 100 kPa, [H+] = 1 M), G is the Gibbs free energy of formation, G0 is the standard Gibbs free energy of formation, R (J/mole/K) = 8.314 is the value of gas constant, T (K) is the thermodynamic temperature on the Kelvin scale, F (Coulombs/mole) = 96500 is the Faraday constant, n is the number of electrons exchanged in the reaction, Mred (mol/L) is the molar concentration of reductants, Mox

(mol/L) is the molar concentration of oxidants For example, if the MFCs reactor is setup following Figure 2.1, we have

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assume that the ideal condition are [NO3-] = [H2O] = [CH3COO-] = [HCO3-] = 1 M, PN2

= 1 atm, pH = 7 ([H+] = 10-7 M) and do not change with time E0 can be calculated from given G0 (Alberty, 2006), we have the same results as shown in table 2.1

 7 90.05916 10.187 log 0.2798 10 ( )An VE       (2.5) 7 120.05916 11.246 log 0.74910 10 ( )CatE V      (2.6)

The ideal electromotive force Eemf (V), also referred to as the ideal cell voltage, is defined as the potential difference between the cathode and the anode

Eemf = ECat – EAn = 0.749 - (-0.279) = 1.028 (V) (2.7) If the ohmic loss and overpotentials of two electrodes are taken into account, the actual cell voltage is considerable lower than the ideal electromotive cell force (Logan et al., 2006) cellemfacOvervoltageE E     IR (2.8)

where a, |c| (V) are the overpotentials of the anode and cathode respectively, I (A) is the produced current, and RΩ is the ohmic resistance of the MFC system Besides, the actual cell voltage also can be calculated as

Ecell = OCV – IRint (2.9)

where OCV (V) is the open-circuit voltage measured immediately after the circuit removes, I (A) is the produced current, Rint is the IR of the MFC system

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Geobacter sulfurreducens, Shewanella oneidensis, Shewanella putrefaciens) were frequently detected in many MFCs studies MET is an indirect electron transfer performed by mediators, additionally provided externally or generated by certain microbes The mediators act as shuttles; they receive electrons from the microbe and transport them to the electrode's surface

Figure 2.2 DET mechanisms to anode

2.2 Design of MFCs

To date, the design of MFCs has reached many advances in the wastewater treatment study Researchers focused on MFCs configuration, electrode, separator, inoculum, and electrolyte solution to improve carbon and nitrogen removal efficiency, increase power productivity, minimize costs, and enhance the MFCs system's stability

2.2.1 Casing

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rectangular Evaluating the impact of various geometries and dimensions on the performance of MFC reactors can be challenging (Krieg et al., 2014)

2.2.2 Electrode

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Table 2.2 The advantages and disadvantages of some electrode materials Material Advantages Disadvantages Carbon cloth High surface area

Relatively high porosity High conductivity Flexibility

Mechanical strength

Relatively high cost

Carbon brush Relatively high surface area High conductivity

High cost due to its titanium core

Carbon rods Affordable cost Low surface area, so primarily utilized as current collectors rather than electrodes

Carbon mesh Relatively low-cost Low conductivity

Low mechanical strength Low porosity

Carbon veil Very cheap

Relatively high conductivity High porosity

Quite fragile

Carbon paper Relatively porous Expensive Fragile Granular activated carbon (GAC) Biocompatibility Low cost Very porous High surface area

Relatively low conductivity

Granular

graphite Similar to GAC, except lower surface area Higher conductivity than GAC Carbonized

cardboard Very low cost High conductivity High porosity

-

Graphite plate Relatively low cost High conductivity

High mechanical strength

Low surface area and surface/volume ratio Reticulated vitreous carbon (RVC) Very conductive Great porosity Quite fragile Very expensive Metallic-based materials (stainless steel, silver, nickel, copper, gold, titanium) Very conductive Robust Cheap

The release of copper and nickel ions from electrodes can be toxic to microbes

2.2.3 Separator

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polarization However, to prevent crossover processes that could have adverse effects, drastically reducing MFC's performance, most MFCs reactors were designed with a separator between the two chambers The separator must be conductive for ions to balance the overall charge transfer Therefore, the ion exchange membranes made from polymer were the good choice for separator Among them, the CEMs were used in most studies about MFCs Two common CEMs are Nafion (DuPont Inc., USA) and Ultrex CMI 7000 (Membranes Inc., USA) because they respectively contain hydrophilic sulfonate groups and sulphonic acid groups, which facilitates to transport of cations and protons (Li et al., 2011) However, the poor selectivity of CEMs and the overwhelming concentrations of other cations such as Na+, NH4+ compared to protons in wastewater resulted in the low protons transfer efficiency (Rozendal et al., 2006) This creates a pH imbalance between the two chambers, which leads to a decrease in MFC performance With the same volume of MFCs reactor, the large designed membrane area minimizes IR, resulting in increased power density (Logan et al., 2006) Anion exchange membranes (AEMs) with ammonium groups in polymer structure help anions travel between two chambers Surprisingly, AEMs have a better proton transfer ability than CEMs if phosphate is added to take the role of proton carrier and pH buffer (Li et al., 2011) However, the practice of AEMs results in the loss of substrates in the anodic chamber by diffusion of anions such as acetate across the membrane to arrive in the cathodic chamber, affecting electron production A bipolar membrane combined from CEM and AEM is suitable for desalination by favoring water splitting, but not proper for MFCs due to the use of two membranes at the same time dramatically increases IR (Li et al., 2011)

2.2.4 Inoculum and substrate

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transfer rate Besides, mixed culture MFCs allows much broader substrate utilization because each microorganism is suitable with different substrates Du et al (2007) compiled a list of microbes that can transfer electrons and their substrate A wide range of biodegradable organic compounds can serve as substrates in the anodic chamber Its composition, characteristics, and concentration in electrolyte affect the efficiency of converting chemical energy into electricity Acetate owned simple structure is the common choice in most design of MFCs because it is less likely to participate in fermentation, methanogenesis, which does not contribute to electricity production and decrease coulombic efficiency Chae et al (2009) reported that MFCs used acetate as a substrate showed the highest power density and coulombic efficiency compared with butyrate, propionate, and glucose In addition, many different substrates in MFCs for electricity production were reviewed carefully (Pandey et al., 2016; Pant et al., 2010) With the goal of nitrogen removal, nitrifying and denitrifying bacteria were often cultured at the cathode Substrates such as ammonium, nitrite, and nitrate are often used for their growth and metabolism In practice, isolating to cultivate a single microbe requires many complicated techniques such as ultrasound, temperature, antibiotics, and fungicides (Santoro et al., 2017) It has also not proven effective in MFCs when compared with using mixed cultures Therefore, most MFCs studies used anaerobic sludge or activated sludge from the wastewater treatment plants as inoculum Another simple way to shorten acclimation time is ultilizing successfully cultured inoculum from a parent MFC Synthetic wastewater consists of substrates, nutrients, and some trace elements needed for microbial growth commonly used as electrolyte solutions in MFCs reactors Some recent studies also applied MFCs to treat actual wastewater treatment (Kim et al., 2008; Park et al., 2017; Ryu et al., 2013; Wen et al., 2010)

2.3 Recent MFCs studies for nitrogen and carbon removal, power generation Clauwaert et al (2007) pioneered the utilization of nitrate as an electron acceptor at the cathode Denitrification was shown to be achievable without H2

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2.3.1 Nitrification and cathodic denitrification in MFCs

Virdis et al (2008) conducted nitrification and denitrification separately, using an independent nitrification reactor combined with a dual-chamber MFC Their study was the first MFC application with complete nitrogen removal in wastewater A synthetic wastewater consisting of acetate and ammonium was continuously supplied to the anode chamber of an MFC It then flowed to an external aerobic reactor where ammonium in the influent underwent oxidation to form nitrate, and finally entered the cathode chamber of the MFC Electrons produced at the anode from acetate oxidation reactions are transferred via an electrical circuit to the cathode where nitrate captures electrons and undergoes reduction to form nitrogen gas The system had positive results with a nitrogen removal rate of 0.41 kg m-3 d-1 (net cathode volume) and a maximum power output of 34.6 W m-3 The COD/N removal ratio of 4.48 was lower than the typical values for the conventional technology and ranged between 10.5 and 12.5 (Beylier et al., 2011) Although a nitrification reactor has high ammonium oxidation efficiency, the cathode chamber effluent still had 26.9 mg L-1 ammonium at an ER of 5 Ohm This is because some of the ammonium ions diffuse from the anode chamber via the CEM

Virdis et al (2010) overcame this weakness by controlling the aeration in the cathodic chamber,enabling SND processes The achieved total nitrogen (TN) removal efficiency of the system was 94.1%, leaving only a small amount of ammonium (1 mg L-1) remaining in the effluent

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To save aeration energy, some MFC configurations with passive nitrification were investigated Zhang and He (2012) constructed a tubular dual-cathode MFC with an outer cathode for nitrification and an inner cathode for denitrification The system used an AEM instead of a CEM to separate the anode and the inner cathode, avoiding ammonium dispersion The wastewater enters the anode and moves to the outer cathode and finally to the inner cathode This system achieved a maximum power density of 6.8 W m-3 The ammonium removal efficiency was 96%; however, the TN removal was only between 66.7 and 89.6%, depending on different nitrogen loading rates corresponding COD/N ratios of 2.81, 4.21, and 8.43 At a high nitrogen loading rate, the effluent contained significant nitrate levels, demonstrating a limitation in the bioelectrochemical denitrification Some of the nitrate ions may diffuse through the AEM from the inner cathode and back to the anode, conducting heterotrophic denitrification at the anode, whereas acetate can diffuse in the opposite direction This resulted in a low Coulombic efficiency and decreased the MFC's performance at a high nitrogen loading rate

A single-chamber MFC with a rotating cathode was another system developed to passively supply oxygen and minimize the pump energy (Zhang et al., 2013) At a COD/N ratio of 5:1, 91.5% and 82.1% of TN were removed under the continuous feeding mode and batch mode, respectively The system achieved a maximum power density of 585 mW m-3 Hussain et al (2016) experimented with a single-chamber MFC with an exposed-air cathode Non-conductive nylon cloth was employed as a separator between the anode and cathode At an influent COD of 2067 mg L-1 and ammonium of 200 mg L-1, their removal efficiency achieved were 84% and 55%, respectively, without output nitrate and nitrite The Coulombic efficiency of the system was about 65%, while the achieved power output was 75 mW L-1

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reported a maximal TN removal efficiency of 80.82% at a low COD/N ratio of 2.8 The Coulombic efficiency also improved, leading to a maximum power density of 4.2 W m-3 Besides single MFC systems, a stacked MFC system, including five air-cathode MFC units connected in series, successfully removed 85% of COD and 94% of TN under a short hydraulic retention time (HRT) of 2.5 h (Park et al., 2017)

Apart from experiments with synthetic wastewater, some studies also applied MFCs to treat actual wastewater For instance, four MFC reactors using different electrode materials and separators were tested to treat piggery wastewater (Ryu et al., 2013) The COD removal rate was 0.523 kg m-3 d-1 (anode compartment volume) and the TN removal rate was 0.194 kg m-3 d-1 (cathode compartment volume) The maximum power density was relatively low at 1415.6 mW m-3 (with a CEM as the separator) because of the poor buffer capacity of real wastewater

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Table 2.3 The performance with pros and cons of various MFC configurations for simultaneous removal of carbon and nitrogen

MFC configuration Results Pros Cons References

Two-chamber MFC and nitrification reactor

Electrode: Granular graphite Separator: CEM

COD removal: 2 kg m-3 d-1

NO3- removal: 0.41 kg m-3 d-1

Power density: 34.6 W m-3

Low COD/N ratio requirement High ammonium concentration in the effluent Virdis et al (2008) Two-chamber MFC

Electrode: Granular graphite Separator: CEM COD removal: 100% TN removal: 94.1% Power density: 0.44-1.58 W m-3Successfully perform SND in the cathode chamber - Virdis et al (2010) Coupled MFC

Electrode: Graphite felt Separator: CEM

COD removal: 98.8% TN removal: 97.3% NH4+ removal: 97.4% Power density: 14 W m-3

High nitrogen removal efficiency

Low Coulombic efficiency for glucose oxidation

High COD/N for denitrification of 15-16

Xie et al (2011)

Tubular dual-cathode MFC Electrode: Carbon cloth (anode, inner cathode), platinum coated carbon cloth (outer cathode)

Separator: CEM, AEM

COD removal: 85-99% TN removal: 66.7-89.6% Power density: 6.8 W m-3

Passive nitrification with no aeration

The energy production higher than energy consumption

Low performance at high nitrogen loading rate Nitrate diffusion through AEM Zhang and He (2012) Single-chamber MFC with a rotating biocathode

Electrode: Carbon felt Separator: None

Total organic carbon (TOC) removal: 85.7%

TN removal: 91.5%

Power density: 585 mW m-3

Passive nitrification High nitrogen removal at COD/N of 5:1

Low Coulombic efficiency because of the open system

Zhang et al (2013)

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Table 2.3 The performance with pros and cons of various MFC configurations for simultaneous removal of carbon and nitrogen (Cont.)

MFC configuration Results Pros Cons References

Single-chamber air cathode MFC

Electrode: Carbon felt anode and gas diffusion cathode Separator: Nylon cloth

COD removal: 5.1 g L-1 d-1

(84%)

NH4+ removal: 0.38 g L-1 d-1

(55%)

Power density: 75 mW L-1

Passive oxygen supply No ion exchange membrane

Low power production and nitrogen removal

Hussain et al (2016)

Two-chamber MFC with a clarifier

Electrode: Carbon paper Air supply through gas-permeable membrane Separator: None COD removal: 97% TN removal: 52% Large-volume reactor of 5 L Inexpensive electrode No ion exchange membrane Nitrogen removal capability at low DO of 0.5 mg/L

Low nitrate reduction efficiency

Yu et al (2011)

Two-chamber MFC Electrode: Carbon felt

(anode), conductivity aerated membrane (cathode) Separator: CEM COD removal: 0.39 kg m-3 d-1(94.74%) TN removal: 0.128 kg m-3 d-1(80.82%) Power density: 4.2 W m-3

High nitrogen removal at

low COD/N ratio of 2.8 Low Coulombic efficiency (9.27%-40.67%)

Wu et al (2017)

Five stacked MFCs with air-cathodes

Electrode: Graphite felt (anode), Pt coated carbon cloth (cathode)

Separator: Non-woven fabric

COD removal: 85%

TN removal: 94% Experiment with actual domestic wastewater The effluent met Korean discharge standard with short HRT of 2.5 h

- Park et al

(2017)

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Table 2.3 The performance with pros and cons of various MFC configurations for simultaneous removal of carbon and nitrogen (Cont.)

MFC configuration Results Pros Cons References

Loop configuration MFC Electrode: Granular graphite (anode), Pt coated graphite felt (cathode)

Separator: Plexiglas-wall, CEM

COD removal: 0.523 kg m-3 d-1

TN removal: 0.194 kg m-3 d-1 Experiment with actual piggery wastewater and large-volume reactor of 5 L

Need external buffer Ryu et al (2013)

MFC system with one anode chamber and two cathode chambers

Electrode: Carbon brushes (anode), carbon cloth (cathode) Separator: CEM TN removal: 99% Power generation: 294.9 mW m-2Successful shortcut nitrification with a nitrite/nitrate ratio of 3:1 High nitrogen removal Produce the net electric power of 0.007 kWh m-3

Li et al (2016)

Two-chamber MFC Electrode: Carbon felt Separator: AEM

TN removal: 91.71%

Power density: 149.76 mW m-2 No need for pH control

at both chambers Zhang et al (2020)

Microscale air-cathode MFC reactor

Electrode: Graphite felt (anode), activated carbon (cathode)

Separator: none

Nitrate reduction efficiency: 96%

Power density: 264.5 mW m-2

Stable pH of 6.5 without buffer

Higher power generation with anodic denitrification Substrate consumption due to denitrification Power generation significantly dropped with nitrate influent of 120 mg L-1

Ren et al (2020)

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2.3.2 Shortcut nitrification/denitrification process in MFCs

Since nitrite can be used to replace nitrate and become the primary electron acceptor of the cathode (Virdis et al., 2008), the shortcut nitrification/denitrification process in an MFC (Fig 2.3) is also of interest

Figure 2.3 Shortcut nitrification-denitrification process in MFCs

In MFC, electron utilization by nitrite is better than that of nitrate, resulting in a higher Coulombic efficiency for the cathodic reaction of up to 97.8% (Virdis et al., 2008) Puig et al (2011) conducted an experiment with nitrite or nitrate medium in the cathode chamber of an MFC reactor Their study indicated that nitrate and nitrite can be interchangeable in the autotrophic denitrification process at the cathode At the same generated current, the nitrogen removal rate with nitrite feeding was higher than with nitrate feeding (257 g m-3 d-1 compared to 203 g m-3 d-1) Similarly, the HRT at the cathode for nitrite feeding was lower than for nitrate feeding (2.76 h compared to 3.10 h), and the nitrogen removal efficiency achieved with nitrite feeding was 37% which was higher than with nitrate feeding (30%) Li et al (2014) employed an MFC reactor with the cathode chamber inoculated with anoxic sludge to perform the shortcut denitrification process However, most of the nitrite removed (over 80%) was converted into nitrate, resulting in a low TN removal rate of only 0.047 kg m-3 d-1 With sodium azide added to inhibit nitrite oxidation, the TN removal rate increased to 0.075 kg m-3

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applied to accumulate nitrite or effectively curb the nitratation step rather than using the sodium azide inhibition Adding phosphate buffer also helped to improve the TN removal rate but did not have much impact on the COD removal A maximum power density of 8.3 W m-3 was attained The power generation was almost the same with and without sodium azide, but it decreased by 50% without the phosphate buffer

Li et al (2016) integrated shortcut nitrification and denitrification into a dual-cathode MFC system The study included an oxygenated dual-cathode for partial nitrification and an oxygen-depleted cathode for shortcut denitrification In the oxygenated cathode chamber, 98% of the ammonium was oxidized to nitrite and nitrate at a DO of 3.5 mg L-1 with a nitrite/nitrate ratio of 3:1 The aerobic cathode chamber's effluent was fed to the anoxic cathode chamber, to perform the autotrophic denitrification process with a COD/N ratio of 9.6:1 (calculated) The TN removal efficiency of their system was 99% Their study also found that the reduction rates of nitrite and nitrate with Pt-coated cathode were higher than those without Pt With partial nitritation, the anammox process can also serve as a shortcut to remove nitrogen in an MFC system It has the potential to be the primary nitrogen removal pathway in MFC (Kim et al., 2016) However, anammox bacteria grow even slower than autotrophic nitrifiers/denitrifiers Thus, a set of environmental conditions to exert some selection pressures for the growth of anammox bacteria in MFC reactor should be investigated further The shortcut nitrification/denitrification process significantly saves the organic carbon used as an electron donor source Consequently, the MFC system can utilize a larger amount of organic matter to produce electricity Further studies should focus on taking advantage of the shortcut nitrification/denitrification process to increase power generation in the MFC system Denitrification at the cathode has achieved remarkable progress in decreasing carbon requirement Theoretically, the anaerobic environment at the anode also facilitates the reduction of nitrate, thus the studies on anodic denitrification process are discussed in next section

2.3.3 Heterotrophic anodic denitrification in MFCs

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low-nitrate influent concentration A dual-chamber MFC was operated with oxygen reduction at the cathode and acetate oxidation combined with heterotrophic denitrification at the anode (Jin et al., 2019) The highest nitrate removal efficiency was 98.9% Results demonstrated that MFC operated based on HAD achieved a higher power density than MFC without HAD (11.9 W m-3 compared to 10.4 W m-3) With nitrate in the medium, the MFC had an IR of 100 Ohm, compared to 150 Ohm in the absence of nitrate A COD/N ratio of 5:1 obtained the optimal efficiency for both HAD and power generation Zhang et al (2020) employed a dual-chamber MFC for nitrogen removal, where the cathode facilitated the nitrification process and the HAD process was implemented Nitrate produced from ammonium oxidation at the cathode passed through the AEM to reach the anode for heterotrophic denitrification The highest nitrogen removal efficiency was 91.71% at a DO of 6.8 mg L-1 without pH control at both the anode and the cathode

Figure 2.4 Mechanism of heterotrophic anodic denitrification process in MFCs

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resulting in better MFC performance Although some of the organic carbon was utilized by denitrification process, the proportion of TOC consumption by electrogenesis was still higher than a nitrate-free MFC (9.6% and 12.9% for 60 or 90 mg L-1 influent nitrate concentration, respectively, compared to 6.3% for without influent nitrate) However, when the nitrate concentration increased to 120 mg L-1, insufficient substrate was observed Denitrifying bacteria strongly competed with exoelectrogens for the substrate, causing only 1.6% of TOC consumption to generate electricity

The HAD process in MFCs offers some benefits in pH balance and decreasing IR With an appropriate nitrate influent concentration and a proper C/N ratio, it can significantly improve the performance of MFCs without buffer In addition to pH balance, other mechanisms that promote the exoelectrogenic bacteria's activity in the HAD-MFC, need to be further investigated The nitrate concentration limit at anode also needs to be overcome for wastewater with high nitrogen influent

2.4 Microbial communities

The bacteria population at both anode and cathode are dominated by Proteobacteria, which accounted for a major proportion (Chae et al., 2009; Jin et al., 2019; Ren et al., 2020; Wang et al., 2018) The β-Proteobacteria were frequently detected in biofilm anode This subclass includes many bacteria capable of biodegrading organic compounds Chae et al (2009) reported Thauera genera accounting for 70.1% of total β-Proteobacteria in the acetate-enriched MFC Geobacter, Desulfovibrio, Pseudomonas, and Shewanella species are well-known anodophilic bacteria with electron transferability to the anode, contributing to electricity generation (Chae et al., 2009; Jin et al., 2019; Ren et al., 2020; Virdis et al., 2011) Geobacter and Desulfovibrio belong to subclass δ-Proteobacteria, while Pseudomonas and Shewanella belong to subclass γ-Proteobacteria Thauera and Pseudomonas genera are common denitrifiers, particularly capable of reducing nitrate under aerobic conditions (Jin et al., 2019; Virdis et al., 2011; Wang et al., 2018)

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Geobacter decreased markedly, while that of Thauera and Pseudomonas increased significantly (Jin et al., 2019) Other genera such as Dysgonomonas, Enterobacter, and Klebsiella were reported as the exoelectrogenic bacteria (Ren et al., 2020) The methanogens were reported to grow in anaerobic conditions at the anode, accounting for 1-6% of the total bacteria (Yu et al., 2011) The methanogenesis does not contribute to electricity production and decrease coulombic efficiency

At the cathode, AOB and NOB are genetically related in the subdivision β-Proteobacteria and α-β-Proteobacteria, respectively Among these AOB, Nitrosomonas genera was frequently detected in the MFC as a main nitrifying bacterium (Park et al., 2017; Wang et al., 2018) Nitrosomonas is also an autotrophic denitrifier that can perform nitrite reduction to nitrogen gas (Ahn, 2006) Besides Nitrosomonas, other genera such as Nitrosococcus, Nitrosopira, Nitrosovibrio, Nitrosolobus, Aquamicrobium, Phycisphaera, and Truepera were also found to be AOB (Ahn, 2006; Wang et al., 2018) Several well-known NOB are Nitrobacter, Nitrospira, Nitrospina, Nitrococcus, and Nitrocystis (Ahn, 2006) The ratio AOB to total bacteria increased with increasing DO concentration maintained in the cathode chamber (Yu et al., 2011)