Analysis of Energy losses of Microbial Fuel Cells (MFCs) and Design of an Innovative Constructed Wetlands-MFC THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of the Ohio State University By Ke Li Graduate Program in Food, Agricultural and Biological Engineering The Ohio State University 2017 Thesis Committee: Ann D. Christy, PhD., P.E., Advisor Lingying Zhao, PhD Co-Advisor Olli H. Tuovinen, PhD Copyrighted by Ke Li 2017 Abstract A Microbial Fuel Cell (MFC) is a device used to harvest electrons from living microorganisms to generate electrical power. After decades of development, new architectures have been developed and new materials have been applied to MFC systems. Improvements to this promising technology have been extensively reported. However, scientists and engineers are still facing difficulties on enhancing energy output and increasing the MFC system efficiencies. In addition to physical resistances caused by MFC materials, there are still many unknown factors affecting the electron transfer pathway used by microorganisms in MFC environments. To increase the performance, a series of technologies have been integrated into MFCs. For instance, MFC technology has been combined with other technologies such as algae pounds, anaerobic digesters and constructed wetlands, to increase substrate utilization efficiency. As one example, CW-MFCs have already been studied for wastewater treatment and electricity generation. However, the system efficiencies presented by current models are low, and new designs need be explored to reduce the energy cost during installation and operation of CW-MFC systems. The first objective of this thesis research was to review the development of MFCs and relevant technologies, then to evaluate the energy loses and efficiencies in MFC systems, leading to a comprehensive understanding of MFC system from a thermodynamic viewpoint. The second objective was to design and ii construct a down-flow Constructed Wetland-Microbial Fuel Cell combined system with a semi-air cathode and analyze its performance. This innovative design will be able to enhance the system efficiencies of CW-MFCs by reducing external energy requirements, and this research will provide foundational work for further CW-MFC explorations. Keywords: energy losses, system efficiency, electron transfer, constructed wetland, microbial fuel cell, power generation iii Acknowledgments First, I would like to thank my parents. Their support and encouragement were really important for me. Otherwise, I would not have been able to have such a good opportunity to go abroad and work with these excellent people. I appreciate the effort of my graduation committee: Dr. Ann Christy, Dr. Lingying Zhao and Dr. Olli Tuovinen provided their constant support and insightful suggestions towards my work. Especially Dr. Christy, her patient mentorship helped me a lot during my thesis writing and study. Drs. Beenish Saba and Young Woon Kang also provided their expertise and professional opinions to help me. I would like to thank Ms. Candy McBride, the graduate coordinator in the Department of Food, Agricultural and Biological Engineering. She gave me a lot of help during these two years. And I really appreciate the Ohio State University for providing such a good opportunity to work with these wonderful people. I would like to thank the staff at the Wilma H. Schiermeier Olentangy River Wetland Research Park who allowed me to obtain wetland materials from there for my research iv resources, especially Brent Macolly, who provided really useful background information about wetland plants. Finally, I thank Christopher Gecik, for helping me with fabrication of the electrodes I used during my study. v Vita March 1991 ....................................................Wuhan China 2014................................................................B.S. Jianghan University 2016 to present ..............................................Graduate Teaching Associate, Department of Food, Agricultural and Biological Engineering, The Ohio State University Fields of Study Major Field: Food, Agricultural and Biological Engineering vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. iv Vita ..................................................................................................................................... vi List of Tables ..................................................................................................................... ix List of figures .......................................................................................................................x Chapter 1: Introduction ....................................................................................................1 1.1 Microbial fuel Cells ....................................................................................................2 1.2 Energy recovery technology: Constructed Wetland-Microbial Fuel Cell (CW-MFC) combined system ..................................................................................................................7 1.3 Research objectives .....................................................................................................8 Chapter 2: Energy losses and system efficiencies of Microbial Fuel Cells: An Overview ..........................................................................................................10 2.1 Introduction ...............................................................................................................10 2.2 Energy losses during biotransformation ...................................................................15 2.2.1 Unused organic components .........................................................................16 2.2.2 Alternative metabolic pathway .....................................................................18 2.2.3 Excessive biomass accumulation ..................................................................19 2.3. Energy losses during electron transportation ...........................................................20 2.3.1 The effects of electrodes ...............................................................................20 vii 2.3.2 The effects of membranes .............................................................................21 2.4 Other consideration ...................................................................................................22 2.5 Summary and Conclusions .......................................................................................23 Chapter 3. Down-flow Constructed Wetland-Microbial Fuel Cell system with semi-air cathode .......................................................................................................24 3.1 Abstract .....................................................................................................................24 3.2 Introduction ..................................................................................................................25 3.2.1 Constructed wetlands-MFC ..........................................................................25 3.2.2 Issues in CW-MFC systems ..........................................................................26 3.2.3 Introduction to this research .........................................................................27 3.3 Material and methods ...................................................................................................29 3.3.1 Configuration and setup of CW-MFC ..........................................................29 3.3.2 Assembly of down-flow CW-MFC ..............................................................33 3.3.3 Step tests .......................................................................................................34 3.3.4 Organic Waste Treatment test ......................................................................37 3.3.5 Operation and analysis ..................................................................................37 3.4 Results and Discussion ................................................................................................37 Chapter 4. Conclusions ....................................................................................................43 References .........................................................................................................................46 Appendix A: Experimental data .....................................................................................51 viii List of Tables Table 3.1 Substrate components ........................................................................................32 Table 3.2 Step test external resistance and switch combinations ......................................35 Table 3.3 Voltage and power output of four systems ........................................................40 Table A.1 Resistance decade box data for the step test with variable time intervals ........51 ix List of Figures Figure 1.1 Estimated U.S. Energy Consumption in 2016 (Lawrence Livermore National Laboratory, 2017) ..................................................................................................2 Figure 1.2 A Schematic indicating the operating principle of a Microbial Fuel Cell (Rabaey & Verstraete,