Sustainable Solutions for Environmental Pollution, Volume 1 E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
SUSTAINABLE SOLUTIONS FOR ENVIRONMENTAL POLLUTION This first volume in a broad, comprehensive two-volume set, Sustainable Solutions for Environmental Pollution, concentrates on the role of waste management in solving pollution problems and the value-added products that can be created out of waste, turning a negative into an environmental and economic positive. Environmental pollution is one of the biggest problems facing our world today, in every country, region, and even down to local landfills. Not just solving these problems, but turning waste into products, even products that can make money, is a huge game-changer in the world of environmental engineering. Finding ways to make fuel and other products from solid waste, setting a course for the production of future biorefineries, and creating a clean process for generating fuel and other products are just a few of the topics covered in the groundbreaking new first volume in the two-volume set, Sustainable Solutions for Environmental Pollution. The valorization of waste, including the creation of biofuels, turning waste cooking oil into green chemicals, providing sustainable solutions for landfills, and many other topics are also covered in this extensive treatment on the state of the art of this area in environmental engineering. This groundbreaking new volume in this forward-thinking set is the most comprehensive coverage of all of these issues, laying out the latest advances and addressing the most serious current concerns in environmental pollution. Whether for the veteran engineer or the student, this is a must-have for any library. AUDIENCE Petroleum, chemical, process, and environmental engineers, other scientists and engineers working in the area of environmental pollution, and students at the university and graduate level studying these areas
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Table of Contents
Cover
Title page
Copyright
Preface
1 An Overview of Electro-Fermentation as a Platform for Future Biorefineries
1.1 Introduction
1.2 Fundamental Mechanisms
1.3 Value-Added Products from Electro-Fermentation
1.4 Challenges and Future Outlook
1.5 Acknowledgements
References
2 Biodiesel Sustainability: Challenges and Perspectives
Abbreviations
2.1 Introduction
2.2 Biodiesel Production
2.3 Factors Affecting Biodiesel Production Process
2.4 Transesterification Mechanisms
2.5 Production of Biodiesel Using Heterogeneous Catalyst Prepared from Natural Sources
2.6 Challenges and Perspectives
References
3 Multidisciplinary Sides of Environmental Engineering and Sustainability
3.1 Introduction
3.2 System Theory and Integrated System Approach
3.3 Sustainable Development, Sustainable Development Engineering and Environmental Engineering
3.4 Advanced Multi-Disciplinary Sustainable Engineering Education
3.5 Novel Designs for Auto-Thermal Behavior Towards Sustainability
3.6 Conclusions
References
4 Biofuels
4.1 Introduction
4.2 Composition
4.3 Classification of Biofuels
4.4 Examples of Biofuels
4.5 Property Variations with Source
4.6 Properties Compared to Fuels from Crude Oil Tar Sand Bitumen, Coal and Oil Shale
4.7 Fuel Specifications and Performance
4.8 Conclusion
References
5 Sustainable Valorization of Waste Cooking Oil into Biofuels and Green Chemicals: Recent Trends, Opportunities and Challenges
5.1 Introduction
5.2 Waste Cooking Oil (WCO)
5.3 Biofuels from WCO
5.4 Green Chemicals from WCO
5.5 Challenges and Future Work
5.6 Conclusion
References
6 Waste Valorization: Physical, Chemical, and Biological Routes
6.1 Background
6.2 Land Biomass vs. Oceanic Biomass
6.3 Waste Management
6.4 Waste Valorization for Adsorbents Development
6.5 Waste Valorization for Catalysts Preparations
6.6 Bio-Based Waste Valorization for Bio-Fuel and Bio-Fertilizer Production
6.7 Biochemical Mechanism Involved in Anaerobic Digestion System
6.8 Challenges and Recent Advances in Anaerobic Digestion
6.9 Bio-Based Waste and Bioeconomy Perspective
6.10 Conclusion
References
7 Electrocoagulation Process in the Treatment of Landfill Leachate
7.1 Introduction
7.2 Decomposition of Solid Waste
7.3 Landfill Leachate Properties
7.4 Characteristics of Landfill Leachate
7.5 Electrocoagulation Process
7.6 Key Parameters of Electrocoagulation Process
7.7 Operating Mode
7.8 Economic Analysis
7.9 Case Study: Removal of the Organic Pollutant of Colour in Natural Saline Leachate from Pulau Burung Landfill Site
7.10 Gaps in Current Knowledge
7.11 Conclusion and Future Prospect
References
8 Sustainable Solutions for Environmental Pollutants from Solid Waste Landfills
8.1 Introduction
8.2 Domestic Solid Waste and Its Critical Environmental Issues
8.3 Landfill Leachate Characterization and Its Impact on the Environment
8.4 Effect of Landfills on Air Quality
8.5 Effect of Unsuitable Location of Landfill on Environment and Community
8.6 Recent Sustainable Technologies for Leachate Treatment
8.7 Sustainable Solutions for Gas Emission
8.8 Consideration for Selection of Sustainable Locations for Landfills
8.9 Conclusion
References
9 Progress on Ionic Liquid Pre-Treatment for Lignocellulosic Biomass Valorization into Biofuels and Bio-Products
9.1 Introduction
9.2 Lignocellulosic Biomass for Biofuels and Bio-Products
9.3 Pre-Treatment Technologies for Lignocellulosic Biomass
9.4 Ionic Liquids for Lignocellulosic Biomass Pre-Treatment: Characteristics and Properties
9.5 Insights into Pre-Treatment Performance of Ionic Liquids
9.6 Concluding Remarks: Challenges Facing the Development of Ionic Liquids Use at Large Scale and Future Directions
References
10 Septage Characterization and Sustainable Fecal Sludge Management in Rural Nablus – Palestine
List of Abbreviations
10.1 Introduction
10.2 Septage Characteristics
10.3 Study Methodology
10.4 Septage Pre-Treatment Process
10.5 Results and Discussion
10.6 Pre-Treatment of the Fecal Sludge – Results and Discussions
10.7 Treatment Plant Estimated Cost Breakdown
10.8 Conclusion
10.9 Recommendations
References
11 Lipase Catalyzed Reactions: A Promising Approach for Clean Synthesis of Oleochemicals
11.1 Introduction to Oleochemicals Industry
11.2 Sources of Lipases
11.3 Application of Lipases
11.4 Lipase Catalyzed Production of Biodiesel
11.5 Esterification of Fatty Acids with Glycerol
11.6 Interesterification
11.7 Environmental Benefits of Enzymatic Process Against Chemical Process
11.8 Conclusion
References
12 Seaweeds for Sustainable Development
12.1 Introduction
12.2 Types of Seaweeds
12.3 Bioremediation
12.4 Seaweeds in Nutrition
12.5 Seaweeds as a Source of Pharmaceutics
12.6 Seaweeds Hydrocolloids and Biopolymers
12.7 Seaweeds and Bioenergy
12.8 Seaweeds as Biofertilizers
12.9 Seaweeds as Ecological Player in Sulfur Geocycle
12.10 Culturing Seaweeds in the Marine Habitat (Algal Maricultures)
12.11 Conclusion
12.12 Recommendations
References
About the Editor
Index
Also of Interest
End User License Agreement
Guide
Cover
Table of Contents
Title page
Copyright
Preface
Begin Reading
About the Editor
Index
Also of Interest
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Overview of various value-added products produced via electro-ferment...
Figure 1.2 Mechanisms of electro-fermentation: (a) anodic electro-fermentation; ...
Figure 1.3 A conceptual schematic showing combined anodic acidogenic EF coupled ...
Figure 1.4 A conceptual schematic showing anodic ethanol-fermentation using cust...
Figure 1.5 A conceptual schematic showing lipid extraction from microalgae using...
Chapter 2
Scheme 2.1 General transesterification reaction.
Figure 2.1 Simplified flow chart of base-catalyzed biodiesel production.
Scheme 2.2 Saponification reaction.
Scheme 2.3 Hydrolysis reaction.
Scheme 2.4 Acid-catalyzed esterification of FFA.
Scheme 2.5 Poisoning of CaO surface basic sites.
Scheme 2.6 Mechanism of acid-catalyzed transesterification reaction.
Figure 2.2 Surface structure of CaO.
Scheme 2.7 Methoxide formation applying CaO catalyst.
Scheme 2.8 Mechanism of CaO-catalyzed transesterification reaction.
Scheme 2.9 Reaction of calcium oxide with glycerol.
Scheme 2.10 Mechanism of tricalcium phosphate-catalyzed transesterification reac...
Scheme 2.11 Mechanism of hydroxyapatite-catalyzed transesterification reaction.
Chapter 3
Figure 3.1 SD is MD by its very nature. Technology is a subsystem of SD.
Figure 3.2 SDE is a subsystem of technology which is a subsystem of SD. Technolo...
Figure 3.3 Main 10 needs of modern advanced economies and societies and output t...
Figure 3.4 More detailed subsystems of SD.
Figure 3.5 Preliminary bio-refinery structure with the main two sugar and syngas...
Figure 3.6 A sustainable and clean future town.
Figure 3.7 Some routes to bio-diesel and bio-hydrogen. FT= Fischer Tropsch.
Figure 3.8 Routes for cellulosic bioethanol.
Chapter 4
Figure 4.1 Typical sources and uses of biomass for energy purposes.
Figure 4.2 The various representations of the naturally occurring form of the si...
Figure 4.3 Portion of the polysaccharide, starch, which is used as a feedstock t...
Figure 4.4 A typical saturated fatty acid.
Figure 4.5 Bio-butanol is produced through the fermentation of starch and sugars...
Figure 4.6 Examples of typical bioethers.
Chapter 6
Figure 6.1 Interplay between possible adoptable measures for sustainable growth.
Figure 6.2 Raw biomass to value-added products: A perspective to achieve sustain...
Figure 6.3 Intergrated Solid waste management for waste minimization and proper ...
Figure 6.4 Pictorial representation of land-based biomass sources.
Figure 6.5 Global average waste generation (kg/capita/day),
Figure 6.6 Various treatment routes applied for the sake of waste valorization a...
Figure 6.7 Different stages from biomass to briquette production.
Figure 6.8 The Interplay of green economy, circular economy, and bioeconomy.
Chapter 7
Figure 7.1 Schematic diagram of water balance components within a landfill site.
Figure 7.2 Electrocoagulation treatment process: (a) conceptual framework and (b...
Figure 7.3 Basic reaction in the electrocoagulation process.
Figure 7.4 Electrodes arrangement in the electrocoagulation process: (a) connect...
Figure 7.5 Satellite image of PBLS and sampling location.
Figure 7.6 Schematic diagram of the EC experimental set-up. Adopted from Hamid e...
Figure 7.7 The effects of varying (a) current density, (b) contact time, and (c)...
Chapter 8
Figure 8.1 Scheme of leachate generation in a landfill (Eggen
et al
., 2010).
Figure 8.2 An example of birds and scavengers spreading over the landfill.
Figure 8.3 Smoke from landfill due to fire.
Figure 8.4 This landfill is located in Penang city in northwest Malaysia.
Figure 8.5 Deir Al Balah landfill location from Gaza in Palestine.
Figure 8.6 Schematic diagram for stabilized leachate treatment plant by ozone/ p...
Figure 8.7 Various techniques to convert landfill into sustainable biorefinery. ...
Figure 8.8 Cross-sectional diagram for the design of landfill site and the base ...
Figure 8.9 The 3.2 MW gas engine for the generation of electricity.
Figure 8.10 Pilot solar power project on top of the closed landfill cells.
Figure 8.11 The model developed in ArcGIS to select the sustainable location of ...
Figure 8.12 Land suitability evaluation. A case study in Karabuk, Turkey.
Chapter 10
Figure 10.1 Location of the study area (ARIJ, 2018).
Figure 10.2 Average values for septage analyzed at rural Nablus; (a) pH and EC, ...
Figure 10.3 COD/BOD5 ratio for septage analyzed at rural Nablus.
Figure 10.4 Average values for septage analyzed at rural Nablus; (a) TS, TDS, an...
Figure 10.5 Average values for septage analyzed at rural Nablus; (a) Alkalinity,...
Figure 10.6 Designed wastewater treatment plant unit scheme for septage pretreat...
Figure 10.7 Septage treatment plant model scheme designed by GPS-X 7.0 simulator...
Chapter 11
Figure 11.1 Hierarchy of oleochemicals production processes.
Figure 11.2 Triglyceride structure: (a) glycerol structure (b) triglyceride glyc...
Figure 11.3 Lipase catalyzed production of biodiesel using low-quality feedstock...
Figure 11.4 Schematic diagram of a simple esterification reactor with water remo...
List of Tables
Chapter 1
Table 1.1 Summary of electro-fermentation and electro-selective fermentation for...
Chapter 2
Table 2.1 Standard properties of bio- and petro-diesel.
Table 2.2 Empirical formula and structure of popular FAs present in vegetable oi...
Table 2.3 A comparable summary for homogeneous alkaline- and lipase-catalyzed tr...
Chapter 4
Table 4.1 Composition of various oils and fats used for biodiesel production (pe...
Table 4.2 Properties of vegetable oil biodiesel and diesel fuel (Bajpai and Tyag...
Table 4.3 Kinematic viscosity in diesel fuel standards.
Table 4.4 Composition of biogas from different sources (Spiegel and Preston, 200...
Table 4.5 Composition of tall oil extractives from Kraft Black Liquor (Holmlund ...
Table 4.6 Classification of light bio-oil.
Table 4.7 A comparison of the oil content found in green algae.
Table 4.8 Biofuel yields for different feedstocks and countries (Rajagoopal et a...
Table 4.9 Selected properties of certain common bioenergy feedstocks and biofuel...
Table 4.10 Final blend fuel requirements (at point of delivery).
Table 4.11 Climate-related requirements and test methods.
Table 4.12 ASTM D 4806 Standard Specification for denatured fuel ethanol for ble...
Chapter 5
Table 5.1 Summary of the reactions, causes and consequences on the cooking oil s...
Table 5.2 Recently developed methodologies for biodiesel production from WCO.
Table 5.3 Recent reports for catalytic hydro-treatment of WCO into saturated hyd...
Chapter 6
Table 6.1 Different treatment routes for adsorbents preparation from waste mater...
Chapter 7
Table 7.1 The constituents of landfill leachate in different waste biodegradatio...
Table 7.2 Characteristics of landfill leachate in different phases. Adopted from...
Table 7.3 Chemical reactions involved in metal electrodes in the electrocoagulat...
Table 7.4 Advantages and disadvantages of electrocoagulation process.
Chapter 8
Table 8.1 General composition of landfill leachate (Values in mg/l unless otherw...
Table 8.2 Landfill leachate classification (Alvarez-Vazquez
et al
., 2004).
Table 8.3 Details of the possible volatile organic compounds from a landfill sit...
Table 8.4 General summary of volatile organic compounds from landfill during sum...
Table 8.5 Oxidation potential for some common oxidants (Huling
et al
., 2006).
Table 8.6 Summary of advanced oxidation applications for the treatment of stabil...
Table 8.7 Biodegradability improvement after different AOPs.
Table 8.8 Data for treat 100 m
3
of stabilized leachate using ozone/persulfate pr...
Chapter 9
Table 9.1 A comparative summary of the most studied pre-treatment techniques.
Table 9.2 Physico-chemical properties of the most studied imidazolium-based ILs ...
Chapter 10
Table 10.1 Population and no. of houses in 2018 for the villages of the study ar...
Table 10.2 Septage sources, removal pump-out rate, and the characteristics (EPA ...
Table 10.3 Parameters considered for characterizing the collected samples.
Table 10.4 Physical, chemical and bacterial septage characteristics for rural Na...
Table 10.5 Heavy metals septage characteristics for rural Nablus (2018).
Table 10.6 Comparison of domestic septage between rural Nablus (this study) and ...
Table 10.7 Stools characteristics for rural Nablus (2018).
Table 10.8 Urine characteristics for rural Nablus (2018).
Table 10.9 Design values of parameters for septage treatment plant model (Rural ...
Table 10.10 Classification of treated wastewater for agricultural purposes in Pa...
Table 10.11 Effluent characteristics of septage TP using GPS-X 7.0 vs. Pal. Trea...
Chapter 11
Table 11.1 Historical progress of enzymes utilization for oleochemicals producti...
Table 11.2 Lipase-producing microorganisms.
Table 11.3 Lipase catalyzed glycerolysis reactions.
Table 11.4 Enzymatic production of monoglycerides from fatty acid and glycerin.
Table 11.5 Net savings due to implementing enzymatic process (per 1000 kg proces...
Table 11.6 A comparison between chemical methods and enzymatic method from diffe...
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Sustainable Solutions for Environmental Pollution
Volume 1
Waste Management and Value-Added Products
Edited by
Nour Shafik El-Gendy
This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2021 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 9781119785354
Cover image: Stockvault.comCover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
There is a continuous worldwide growth in population which consequently leads to a proliferation of industrial and agricultural activities and a growth in the annual consumption of energy, food, water and different agricultural crops, as well as an upturn of global warming and the produced solid wastes. These come with the depletion of fossil fuel reserves, in addition to the worldwide massive water scarcity problem and issues related to climate change, besides the enormous air, soil and water pollution. Further, solid waste management (SWM) is considered nowadays as a critical public issue because of the negative impact of such wastes on the environment, climate, ecosystem, biodiversity and human health.
The main concern worldwide nowadays is how to achieve the seventeen goals of sustainable development (SDGs) for: (1) attaining water and food security for all, (2) saving life on land and under water, (3) assuring the presence of clean and renewable energy, (4) overcoming poverty, (5) securing decent work as much as it could be for all, (6) diminishing the problem of climate change, (7) guaranteeing growth in innovation, the economy and different industrial sectors, (8) increasing awareness of the concept of a circular economy for ensuring responsible sustainable consumption and production patterns.
This book reports some of the global researchers’ activities on the nexus of biofuels, biorefineries, sustainable energy, environment, climate change, water, health, and the economy. It focuses on the occurred positive impacts on air, water, land use, food, society, and economy via the valorization of different available resources into sustainable biofuels and bio-products.
The chapters are: Chapter 1-An Overview of Electro-Fermentation as a Platform for Future Biorefineries; Chapter 2-Biodiesel Sustainability: Challenges and Perspectives; Chapter 3-Multidisciplinary Sides of Environmental Engineering and Sustainability; Chapter 4-Biofuels; Chapter 5-Sustainable Valorization of Waste Cooking Oil into Biofuels and Green Chemicals: Recent Trends, Opportunities and Challenges; Chapter 6-Waste Valorization: Physical, Chemical, and Biological Routes; Chapter 7-Electrocoagulation Process in the Treatment of Landfill Leachate; Chapter 8-Sustainable Solutions for Environmental Pollutants from Solid Waste Landfills; Chapter 9-Progress on Ionic Liquid Pre-Treatment for Lignocellulosic Biomass Valorization into Biofuels and Bio-Products; Chapter 10-Septage Characterization and Sustainable Fecal Sludge Management in Rural Nablus, Palestine; Chapter 11-Lipase Catalyzed Reactions: A Promising Approach for Clean Synthesis of Oleochemicals; Chapter 12-Seaweeds for Sustainable Development.
Thus, this book addresses the different multidisciplinary sides of environmental and sustainable development engineering which is a subsystem of sustainable development using system theory and its related integrated system approach. It provides a state-of-the-art presentation on how to reach a sustainable and clean environment via waste management and valorization of different readily available resources into biofuels, biorefineries and different value-added products. It discusses the challenges and opportunities for enlarging the world markets of biofuels, biorefineries and bio-products and how countries can set up policies to decrease fossil fuel dependency and petro-based synthesis of commodity chemicals and move steadily towards the bio-based economy. The utilization of renewable resources such as seaweeds and waste biomass is often considered as an attractive candidate for biofuels and different bioproducts. However, limitations regarding the robustness of process and selectivity of target products are often considered bottlenecks to their sustainable commercialization. This book discusses such bottlenecks and suggests many feasible applicable techniques to increase the yield of the targeted bioproducts. For achieving the circular economy with the concept of zero-wastes, this book discusses the production of bioethanol from different lignocellulosic wastes and seaweeds and also the production of biodiesel from waste oils and fats using sustainable heterogeneous catalysts. Further, it explains how to reach for a feasible transesterification process that produces high yield of qualified biodiesel to be successfully used as alternative and/or complementary for the conventional petro-diesel without affecting the engine performance. The book also debates how the applications of heterogeneous catalysts valorized from different readily available natural resources would be very beneficial in the production of biodiesel and bioglycerol. It also states the applicability of lipases in production of biodiesel and oleochemicals. The use of ionic liquids (ILs) in the lignocellulosic wastes pretreatment for further use in bioethanol and other biorefineries production has gained considerable attention in this book.
It also clarifies how solid waste management can in general open a new sustainable revolution in different sectors which would indeed lead to the successful achievement of the three pillars of sustainability, social, environment and economic. Special attention is given to valorization as a technical route adopted for the recycling of waste into desirable products having non-toxic economic worth. Moreover, the valorization of different available bio-wastes into different biofuels and value-added products, for example, glycerol, oleochemicals, bioplastics, biopolymers, biofertilizers, animal feed, etc., with the concept of reaching zero-waste is discussed from the perspective of achieving the seventeen goals of sustainability, and overcoming the problems of water scarcity, waste management and climate change.
One of the most important sectors this book covers is the main environmental problems related to landfills including soil, water and air contamination, and it also highlights and discusses some of the sustainable solutions and management for such problems including landfill design and location, leachate management and treatment, gas and emission control. Moreover, the effect of unsuitable locations of landfills on the environment is discussed and evaluated. This book also goes through optimizing the treatment of landfill leachate for minimizing its adverse impacts on the natural ecosystem as it is an urgent concern with the increased municipal solid wastes. It emphasizes on the promising electrocoagulation process for its outstanding ability in decontaminating leachate pollution in an economic, viabile, and green approach. A case study for septage characterization and sustainable fecal sludge management has been also reported in this book.
The multidisciplinary approach discussed in this book extends the different scientific and engineering disciplines to reach all other disciplines including economics, politics and other social sciences. Thus, it is time for engineers, scientists, medical doctors, economists, politicians, military officers and specialists, etc., to join forces in multidisciplinary research and development projects to achieve sustainability for better, stable, peaceful, non-sectarian, prosperous and more clean and sustainable societies.
Nour Sh. El-Gendy
1An Overview of Electro-Fermentation as a Platform for Future Biorefineries
Tae Hyun Chung and Bipro Ranjan Dhar*
Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB, Canada
Abstract
Many countries have set up policies to decrease fossil fuel dependency and petro-based synthesis of commodity chemicals. Fermentative biofuels and bioresource recovery processes are expected to assist considerably in this context of sustainable transition to a bio-based economy. The utilization of renewable resources such as waste biomass is often considered an attractive feature of fermentative bioprocesses. However, limitations regarding the robustness of process and selectivity of target products are often considered bottlenecks to their sustainable commercialization. Particularly, in conventional fermentation processes, microorganisms produce undesired by-products to attain intracellular redox balance, which leads to a low yield of target products. Recently, electro-fermentation has emerged as an innovative approach for changing metabolic pathways of fermentative microorganisms towards target products with higher yields and productivities by changing intracellular redox potential. Lab-scale EF studies have successfully demonstrated superior performance over conventional fermentation to produce a wide variety of biofuels and commodity chemicals. This book chapter provides an overview of fundamental and applied aspects of various value-added products synthesis with the EF process and identifies research gaps for future development.
Keywords: Electro-fermentation, electro-selective fermentation, fermentation, value-added products, biofuel, biorefinery
1.1 Introduction
Microbial electrochemical cell (MXC) is a unique type of bioreactor, which integrates biological processes (e.g., utilizing electroactive bacteria as biocatalyst) with electrochemistry (e.g., introducing electrodes, potentials) to convert the chemical energy in organic matter into bioenergy. To differentiate the various types of MXCs, different names have been assigned based on the products or their provided services. Over the decades, the MXCs were largely focused to generate bio-electricity in microbial fuel cells (MFCs) (Logan, 2008). More recently, MXCs were engineered to produce various biogas, such as bio-hydrogen in microbial electrolysis cell (MEC) (Logan et al., 2008; Wagner et al., 2009) and bio-methane in microbial electrolysis cell assisted anaerobic digester (MEC-AD) (Huang et al., 2020; Zakaria and Dhar, 2019). Nonetheless, the MXCs were also implemented for many other applications, such as water desalination in microbial desalination cell (MDC) (Al-Mamun et al., 2018; Jafary et al., 2020), nutrient recovery (Barua et al., 2019; Qin et al., 2016; Zou et al., 2017), CO2-reduction-to-value-added-products in microbial electrosynthesis (MES) (Lovley and Nevin, 2013; Rabaey and Rozendal, 2010; Zhang and Angelidaki, 2014) and production of chemicals, such as hydrogen peroxide in microbial peroxide producing cells (MPPCs) (Chung et al., 2020b; Rozendal et al., 2009). Due to the extensive studies since the early 2000s, several studies have been dedicated to scaling-up the MXCs (Dhar et al., 2016; Heidrich et al., 2014; Hiegemann et al., 2016; Liang et al., 2018; Sim et al., 2018). However, the main bottleneck of the aforementioned MXC applications was the requirement of high energy input or output, whether the electrons are the main driving force in MECs, MDCs, MES, and MPPCs, or they are the desired product (e.g., in MFCs). Often, challenges in achieving high current density from MXCs was the main argument against further development and scaling-up (Feng et al., 2014; Heidrich et al., 2014; Sim et al., 2018; Zakaria and Dhar, 2019). On the other hand, the MXCs have gained interest for further development when utilized as a biosensor, where they mainly focus on changes in electrical energy (e.g., signal output), not necessarily required to produce high electrical energy output (Chung et al., 2020a; Do et al., 2020; Jiang et al., 2018). Hence, proposing a new application of MXC focusing on using low energy input can also be a novel means.
Fermentation process is one of the bioprocesses that converts complex organics to various types of soluble molecules, such as alcohols and carboxylates, and sometimes, along with some energy-rich biogases (e.g., hydrogen and methane) (Dhar et al., 2015; Elbeshbishy et al., 2017; Ghimire et al., 2015; Guo et al., 2010; Moscoviz et al., 2016). The process of fermentation is carried out by a large diversity of microorganisms (e.g., pure or mixed cultures), which can utilize different types of substrates and organic wastes (Ghimire et al., 2015; Guo et al., 2010). There are many different environmental parameters, such as the inoculum type, medium composition, pH, temperature, hydraulic retention time, and accumulation of bi- and end-products that can affect alter the fermentative pathways (Moscoviz et al., 2016). Despite the extensive studies conducted for controlling of aforementioned environmental parameters for the fermentation processes, targeting a specific end product is still challenging, especially concerning mixed cultures (Moscoviz et al., 2016). Hence, instead, engineering the oxidation-reduction potential (ORP) of the fermentation medium (e.g., also known as the extracellular ORP) can be an alternative to control the microbial metabolism to generate a target product (Wong et al., 2014; Zhu et al., 2014). The extracellular ORP corresponds to the activity of electrons in the medium (e.g., in this case, substrates or organic wastes during fermentation), where it is mainly influenced by the chemical composition of the medium, the degree of reduction of the metabolites produced by fermentation, and the temperature (Moscoviz et al., 2016). Particularly, the extracellular ORP is critical as it substantially impacts the intracellular ORP via NADH and NAD+ (e.g., reduced/oxidized form of NAD) balance (Liu et al., 2013). The intracellular ORP represents the redox state inside a cell, which can control enzyme synthesis and gene expression; hence, it ultimately affects the entire metabolic process and it can modify the metabolic pathways during fermentation (Liu et al., 2013). Previously, studies have successfully demonstrated the chemical control of using extracellular ORP to enhance the production of succinate (Chen et al., 2012; Li et al., 2010) and 1,3-propanediol (Du et al., 2006) during fermentation, in which, the MXCs can also potentially be implemented to modify the extracellular ORP via supplying or collecting electrical energy (e.g., electrons, current) using electrodes. This process of combining the fermentation process with an MXC was named “electro-fermentation”.
Electro-fermentation (EF) is a unique process, which introduces electrical energy to microbial fermentative metabolism (Moscoviz et al., 2016). The electrical energy in the EF system can control and stabilize the fermentation process, overcoming the metabolic limitations of balanced reactions (Moscoviz et al., 2016). Nonetheless, the EF system requires lower electrical energy compared with other aforementioned applications of MXCs, where high electrical energy (e.g., high current densities) can result in reduced performance or system failure due to the inhibition of fermentative bacteria (Lai and Lan, 2020). More importantly, even small applied current densities can affect both extracellular and intracellular ORP (e.g., overall biological regulation) through the changes in NADH/NAD+ ratio, which can substantially affect the final fermentation products (Moscoviz et al., 2016; Speers et al., 2014; Sturm-Richter et al., 2015; Zhou et al., 2015). In this context, the electrical energy (e.g., current) in the EF system is not the product of interest nor the main driving energy source, but it is a trigger that allows the fermentation to occur under unbalanced conditions (Moscoviz et al., 2016). Furthermore, the features of EF can significantly overcome the problems of the conventional fermentation process. For instance, using electroactive bacteria (e.g., capable of converting volatile fatty acids to electrons, protons, and carbon dioxide) and electrodes in the EF system can alleviate the challenges, such as accumulation of short-chain volatile fatty acids (SCVFAs) (Lai et al., 2016b) and toxicants (e.g., nitrite, if nitrate is used as an electron acceptor) (Takeno et al., 2007), which are experienced by the conventional fermentation. With such advantages, EF systems present an emerging platform for future biorefinery for the synthesis of various value-added products from organic feedstocks. To date, the EF systems have been implemented for enhancing various biofuels and chemical productions, such as carboxylates, alcohols, solvents, lipids, acetoin, biopolymer, and many more (see Figure 1.1) (Bursac et al., 2017; Choi et al., 2014; Jiang et al., 2020; Lai and Lan, 2020; Liu et al., 2019; Mostafazadeh et al., 2016; Vassilev et al., 2018; Villano et al., 2017). This book chapter presents fundamental mechanisms, applied and scientific aspects of EF to produce different value-added products, and finally, perspectives for future development.
Figure 1.1 Overview of various value-added products produced via electro-fermentation.
1.2 Fundamental Mechanisms
A system for EF consists of an anode and a cathode, and the chambers can be separated by an ion-exchange membrane (see Figure 1.2). The use of a membrane is optional; used when preventing product crossover is critical. Briefly describing the entire process, the EF comprises the fermentation of an energy-rich substrate, where the solid electrodes present in the EF system serves as inexhaustible electron donors or acceptors that does not limit the entire fermentation process (Jiang et al., 2019; Moscoviz et al., 2016). The EF system is generally connected with power sources (e.g., power supply, potentiostat, etc.), where the externally poised potential/voltage is utilized to regulate the fermentation pathways for pure and mixed cultures (Jiang et al., 2019; Moscoviz et al., 2016; Schievano et al., 2016). Briefly speaking, the electrons are transferred between the fermentation medium and the bacteria (e.g., fermentative and/or electroactive), and between the bacteria and the electrodes (e.g., anode or cathode). The electron transfer process between bacteria and electrodes are known as extracellular electron transfer (EET). Depending on the type of fermentation, the EET can be outward (during anodic EF) and inward (during cathodic EF) (Gong et al., 2020; Kracke et al., 2018). Although EET mechanisms have not been studied exclusively for various EF processes, literature suggests that bi-directional EET can occur via multiple mechanisms, including direct electron transport via extracellular redox co-factors (e.g., cytochromes, and other redox proteins), nanowires, and mediators (Gong et al., 2020; Kracke et al., 2018).
Depending on the target product (e.g., final product more oxidized vs. reduced form than the initial substrate), the EF can be classified into two main types: (i) anodic electro-fermentation (AEF); and (ii) cathodic electro-fermentation (CEF). When the final product of EF is more oxidized form than the substrate (e.g., ethanol from glycerol), the working electrode acts as an anode and is used to dissipate the excess electrons, known as the AEF. On the other hand, if the final product is in a more reduced form than the substrate (e.g., butanol from glucose), then the working electrode acts as an electron donor, known as the CEF. The electron sinks during AEF are known to synthesize more adenisine triphospates (ATPs) through creating a proton gradient, while the electron sources during CEF have impacts on the generation of more reduced redox cofactors, such as NADH (Kracke and Krömer, 2014). Hence, both AEF and CEF can significantly enhance the entire fermentation performance (e.g., product selectivity, production rate and yield) (Xafenias et al., 2017). Detail descriptions of underlying mechanisms or EF mechanisms can be found elsewhere (Gong et al., 2020; Jiang et al., 2019; Moscoviz et al., 2016).
Figure 1.2 Mechanisms of electro-fermentation: (a) anodic electro-fermentation; (b) cathodic electro-fermentation.
As discussed earlier, in most cases, the reactions and electron transfers associated with EF are usually performed via syntrophic interactions between the fermentative bacteria and electroactive bacteria (Jiang et al., 2019; Moscoviz et al., 2016). However, sometimes, none of the fermentative bacteria are electroactive (e.g., Clostridium species), in which, the redox mediators, such as neutral red (Choi et al., 2012), methyl viologen (Kim and Kim, 1988), or ferricyanide (Xafenias et al., 2017) are required during the fermentation to impact the extracellular ORP (Choi et al., 2012; Kim and Kim, 1988; Sturm-Richter et al., 2015). When the redox mediators are introduced, they can first, be oxidized or reduced by the fermentative bacteria, then they are recycled or recovered electrochemically by the anode or cathode electrodes (Moscoviz et al., 2016). In this context, the redox mediators are used as electron shuttles, and this process is known as the mediated electron transfer (Gong et al., 2020; Rabaey and Rozendal, 2010; Thrash and Coates, 2008). Furthermore, other studies demonstrated another way to add a redox mediator in CEF, such as using produced H2 at the cathode that can be further used as a one-way electron shuttle (Gong et al., 2020; Xafenias et al., 2015; Zhou et al., 2013; Zhou et al., 2015).
On the other hand, metabolically engineered fermentative bacterial strains are another feasible option, for instance, by adding the property of electroactivity (Moscoviz et al., 2016). This approach has been confirmed by adopting the strains (e.g., c-type cytochromes CymA, MtrA, STC) from electroactive bacteria (Shewanella oneidensis) to fermentative bacteria (Escherichia coli), where the electron transfer process can be greatly improved (e.g., by 183%) (Sturm-Richter et al., 2015). Alternatively, electroactive bacterial species (e.g., Shewanella oneidensis) can also be engineered to utilize a variety range of substrates and organic wastes to further aid the whole EF processes (Flynn et al., 2010).
1.3 Value-Added Products from Electro-Fermentation
To date, electro-fermentation has been investigated for a wide variety of value-added products, including carboxylates, alcohols, biopolymers, and other platform chemicals (see Table 1.1). This section reviews the studies related to EF for producing different value-added products.
Table 1.1 Summary of electro-fermentation and electro-selective fermentation for value-added bioproducts.
Product
Feedstock
Inoculum
System configuration
Total working volume (L)
Temperature (°C)/initial pH
Applied voltage/potential
Working electrode
Reference
Butanol
Glucose
C. pasteurianum
Dual chamber
900
37/6.7
0-2.6 V
Cathode
(Mostafazadeh
et al
., 2016)
Butanol
Glucose
Clostridium pasteurianum
DSM 525
Dual chamber
900
37/6.5
+0.045 V vs. SHE
Cathode
(Choi
et al
., 2014)
Ethanol
Glycerol
Clostridium cellobioparum
, +
G. sulfurreducens
Dual chamber
190
30/6
0.24 V vs. Ag/AgCl
Anode
(Speers
et al
., 2014)
Ethanol
Glycerol
Escherichia coli
Dual chamber
50
37/7.4
−44 mV vs. SCE
Anode
(Sturm-Richter
et al
., 2015)
Ethanol
Cellobiose
G. sulfurreducens
+
Cellulomonas uda
Single chamber
1000
30/6.97
0.24 V vs. Ag/AgCl
Anode
(Awate
et al
., 2017)
Ethanol
Food waste
Mixed culture
Single chamber
400
30/6.8
-
-
(Chandrasekhar
et al
., 2015) Acetone-Butanol-
Ethanol (ABE)
Glucose
C. acetobutylicum
Dual chamber
240
37/6.8
-600 mV vs. Ag/AgCl
Anode
(Engel
et al
., 2019)
1,3-propanediol
Glycerol
Mixed-culture +
G. sulfurreducens
pre-colonized cathode.
Dual chamber
900
37/7
-900 mV vs. SCE
Cathode
(Moscoviz
et al
., 2018)
1,3-propanediol
Glycerol
Mixed culture
Dual chamber
520
21/6.9
−0.80 V to −1.10 V vs. SHE
Cathode
(Xafenias
et al
., 2015)
1,3-propanediol
Glycerol
Clostridium pasteurianum
DSM 525
Dual chamber
900
37/6.5
+0.045 V vs. SHE
Cathode
(Choi
et al
., 2014)
Butyric acid
Glucose
Mixed culture
Dual chamber
540
25/5.5
-700 mV vs. SHE
Cathode
(Paiano
et al
., 2019)
3-hydroxypropionic acid
Glycerol
Recombinant
Klebsiella pneumoniae
L17
Dual chamber
620
37/6
+0.5 V vs. Ag/AgCl
Anode
(Kim
et al
., 2017)
Mixed VFAs
Off gases from fermentation (CO
2
+H
2
)
Mixed culture acclimatized homoacetogens
Dual chamber
400
37/6.5
-1.0 V vs. SHE
Cathode
(Zhou
et al
., 2019)
Caproate
Ethanol, acetate
Mixed culture
Dual chamber
240
37/7.2
−0.8 V and −1.1 V vs. Ag/ AgCl
Cathode
(Jiang
et al
., 2020)
Iso-butyrate
Glucose, ethanol, and acetate
Mixed culture
Dual chamber
540
-/5.6
-700 mV vs.SHE
Cathode
(Villano
et al
.,2017)
Lipid
Microalgae biomass
Scenedesmus acutus
Dual chamber
340
30/7.5
-0.3 V vs.Ag/AgCl
Anode
(Liu
et al
., 2019)
Lipid
Microalgae biomass
Scenedesmus acutus
Dual chamber
200
25/7.0
-0.3 V vs.Ag/AgCl
Anode
(Liu
et al
., 2020c)
Lipid
Microalgae biomass
Scenedesmus acutus
Dual chamber
200
-/-
-
Anode
(Liu
et al
., 2020b)
Acetoin
Lactate
Shewanella oneidensis
-
270
-/-
0 mV vs. NHE
Anode
(Bursac
et al
., 2017)
Acetoin
Glucose
Escherichia coli
Dual chamber
23
/-/
0.2 mV vs. NHE
-
(Förster
et al
., 2017)
Polyhydroxybutyrate (PHB)
Glycerol
Ralstonia eutropha H16
Single chamber
500
-/-
10 mA
Anode
(Lai & Lan, 2020)
L-lysine
Glucose
Corynebacterium glutamicum
Dual chamber
360
20/7.0
-1.25 V
Cathode
(Xafenias
et al
., 2017)
L-lysine
Glucose
Corynebacterium glutamicum
Dual chamber
350
30/7.2
0.697 V vs. SHE
Anode
(Vassilev
et al
., 2018)
1.3.1 Carboxylates
Carboxylates (organic acids having a carboxyl group) are primarily categorized as (1) short-chain carboxylates, containing 2–5 carbon atoms, such as acetate, propionate, butyrate, valerate, and (2) medium-chain carboxylates, containing 6–12 carbon atoms, such as caproate, heptanoate, caprylate (Nzeteu et al., 2018). They can be used as valuable platform chemicals and as precursors for the synthesis of a wide variety of marketable products, including fertilizers, pharmaceuticals, polymers, and personal care products (Mohan et al., 2016). The synthesis of these chemicals via microbial fermentation of organic waste and waste biomass provides a biorefinery framework for sustainable waste management for a greener future. However, a high-rate and robust fermentation processes must be achieved to ensure the same and lower prices of these chemicals, as compared to their petroleum-based production pathways. Recent studies have shown that EF could be a potential approach to achieve this goal.
1.3.1.1 Short-Chain Carboxylates
Various short-chain carboxylates, such as acetate, propionate, butyrate, etc., can be produced via microbial fermentation of organic feedstocks, including waste biomass. The acidogenic fermentation of complex organic substrates with mixed consortia can produce a mixture of various short-chain carboxylates (also called volatile fatty acids) in the fermentation broth (Paiano et al., 2019). These short-chain carboxylates can be either extracted or incorporated within other bioprocesses. However, a standalone acidogenic fermentation process is yet to be developed for industrial-scale synthesis of short-chain carboxylates (Paiano et al., 2019). Over the past few decades, significant research efforts have been dedicated towards developing strategies for long-term operation of acidogenic fermentation, optimization of process parameters (e.g., retention time, pH, temperature, substrate concentration, etc.), tuning the product spectrum (i.e., composition of carboxylates) (Arslan et al., 2016; Paiano et al., 2019).
Recently, EF has been investigated for various short-chain carboxylates production (see Table 1.1). These studies showed promising results in enhancing the yield and productivity of short-chain carboxylates with pure and mixed-culture. For example, Villano et al. (2017) studied cathodic EF of single and mixtures of substrates (glucose, ethanol, and acetate) with undefined mixed culture. Their results showed an applied potential of -700 mV vs. SHE could increase iso-butyrate production over the control (open-circuit reactor) by 20-fold (0.43 vs. 0.02 mol/mol glucose) for a mixture of glucose, ethanol, and acetate. Interestingly, iso-butyrate yields were quite comparable for both reactors when operated with glucose as a sole substrate and a combination of glucose plus ethanol or glucose plus ethanol. Thus, their results suggested that in the presence of applied potential and mixture of substrates could stimulate the growth of iso-butyrate producing microbial population. Although no exogenous redox mediator was provided in the fermenter, cyclic voltammetry (CV) of microbial culture showed a reduction peak at -700 mV vs. SHE (same as operating working electrode potential of EF system). Thus, the authors concluded that applied potential altered the redox status of the cells by shifting intracellular NADH/NAD+ ratio, which subsequently redirected their metabolic pathway towards iso-butyrate production. Recently, another study by Paiano et al. (2019) also reported a 4-fold higher n-butyrate yield with unmediated mixed-culture cathodic EF at an applied potential of -700 mV vs. SHE. The authors found that iso-butyrate yield could reach up to 0.6 mol/mol glucose when the system was operated with glucose along with its fermentation products. Furthermore, the authors performed mediated EF tests using Neutral Red and AQDS as exogenous mediators. Only the n-isomer of butyrate was produced in the presence of these mediators, suggesting mediators could introduce a high selectivity towards specific compounds.
Figure 1.3 A conceptual schematic showing combined anodic acidogenic EF coupled with cathodic EF of off-gases produced during anodic EF. The figure drawn with modification after Zhou et al. (2019).
Recently, a study by Zhou et al. (2019) combined anodic acidogenic EF of glucose coupled with cathodic EF of off-gases produced during anodic EF to improve the yield of mixed volatile fatty acids (see Figure 1.3). Their results suggested that the conversion of off-gases (mainly CO2+H2) to mixed volatile fatty acids on the cathode enriched with homoacetogens could contribute to ~14% of the overall VFAs recovery from the integrated process. to improve the product and carbon conversion. Thus, the development of such engineering strategies could further improve product yield from EF.
1.3.1.2 Medium-Chain Carboxylates
Due to low values of short-chain carboxylates, there has been significant interest in upgrading short-chain carboxylates to high-value chemicals, such as medium-chain carboxylates (C6-C12; caproate, heptanoate, caprylate, etc.). Particularly, chain elongation has emerged as an innovative approach for manipulating carbon chain length of the products. The biological upgrading of short-chain carboxylates (e.g., acetate, propionate, butyrate) and alcohols (e.g., ethanol) via chain elongation can be used to synthesize medium-chain carboxylates. Chain elongation can be defined as an anaerobic open-culture secondary fermentation process that converts short-chain volatile fatty acids and an electron donor into medium-chain carboxylates (Angenent et al., 2016). The chain elongating microbes, such as Clostridium kluyveri can use the reverse β-oxidation pathway to convert short-chain volatile fatty acids to medium-chain carboxylates with ethanol as an electron donor (Roghair et al., 2018). For every five chain elongation reactions, one additional mole of ethanol is oxidized into acetate (Roghair et al., 2018; Seedorf et al., 2008).
Among various medium-chain carboxylates, caproate (C6) has received increasing interest due to its application as a precursor for aviation fuels and various commodity chemicals (Roghair et al., 2018). Jiang et al. (2020) studied the impact of cathodic EF on the mixed culture chain elongation for caproate production from acetate+ethanol. Integrating EF with the chain elongation process could increase the caproate specificity by ~28% with a fresh carbon felt cathode compared to a control reactor (open circuit without electrodes). However, EF with an acclimated cathode failed to increase the caproate specificity; caproate specificity was highly varied with the substrate concentrations. Thus, their results suggested a direct interaction between chain elongating microbes and the fresh electrode. Overall, their results suggested that deploying EF can provide an excellent opportunity to tune the chain elongation process for achieving higher efficiency.
1.3.2 Bioethanol
Over the past few decades, there has been a growing interest worldwide to cut transport emissions. Given the rapidly increasing demand for transport fuels, many countries worldwide are now investing in alternative fuels from bio-sources. At present, ethanol (E10, 10% ethanol mixed with 90% gasoline) is the most widely used transport biofuel with potential as a valuable gasoline replacement to reduce transport emissions (Daylan and Ciliz, 2016). Hence, over the last decade, the global bioethanol market has seen rapid growth. Currently, most bioethanol is produced through the fermentation of corn grain using industrial yeast strains (Gomez-Flores et al., 2018). However, pH imbalances introduced by the formation of unwanted metabolites and subsequent inhibition, batch-to-batch inconsistency, decreasing product yield, and product quality have been identified as major bottlenecks in ethanol fermentation complex feedstocks (Awate et al., 2017). Anaerobic fermentation of glucose may produce a wide variety of by-products, including formate, acetoin, glycerol, acetate, and lactate (Awate et al., 2017; Speers et al., 2014). To address these challenges, genetically modified microorganisms have been implemented to redirect the fermentation pathways towards the target product (i.e., ethanol) (Awate et al., 2017). Recently, a few studies investigated EF as a strategy for alleviating these bottlenecks in fermentative ethanol production from different complex feedstocks, including glycerol, food waste, etc. (Table 1.1).
Speers et al. (2014) studied anodic EF of glycerol with a customized co-culture of electro-active Geobacter sulfurreducens and glycerol-fermenting Clostridium cellobioparum. For industrially relevant loadings of glycerol, the system was able to provide ethanol concentration up to 10 g/L with 90% yield (glycerol-to-ethanol). In their dual-chamber system, anodic Geobacter sulfurreducens played a key role in this process by utilizing by-products of glycerol fermentation for the current generation. The electrons transferred to the anode were utilized for electrochemical hydrogen production on the cathode, which could provide an added-value-product from the process.
In another study, Awate et al. (2017) investigated EF of cellobiose with a bioanode comprised of cellulose-degrading fermentative bacteria Cellulomonas uda and a genetically engineered Geobacter sulfurreducens. In their single chamber reactor, Cellulomonas uda efficiently hydrolyzed and fermented cellobiose to ethanol, while Geobacter sulfurreducens rapidly utilized non-ethanol by-products and prevented feedback inhibition of ethanol fermentation. The electron captured by Geobacter sulfurreducens then converted to hydrogen gas on the cathode. Furthermore, genetic modification of Geobacter sulfurreducens provided improve lactate (a fermentation by-product) oxidation, while preventing the oxidation of hydrogen produced on the cathode. Overall, their single-chamber EF demonstrated higher ethanol concentration (0.332 vs. 1.226 g/L), yield (0.103 vs. 0.275 g/g cellobiose), and productivity (0.015 vs. 0.056 g/L/h) over conventional fermentation. Studies by Awate et al. (2017) and Speers et al. (2014) suggested that anodic EF with designer bioanode combing electroactive and fermentative bacteria could efficiently remove fermentation by-products in both single and dual-chamber systems (see Figure 1.4). Thus, EF can potentially reduce downstream purification costs, which can be as high as 80% of the total cost (Schievano et al., 2016).
Figure 1.4 A conceptual schematic showing anodic ethanol-fermentation using customized co-culture of fermentative and electroactive bacteria using (a) dual-chamber, (b) single-chamber electro-fermentation systems. The figures draw on concepts proposed by Awate et al. (2017) and Speers et al. (2014).
Chandrasekhar et al. (2015) demonstrated a solid-state EF for simultaneous biohydrogen and bioethanol generation from food waste. The authors reported a maximum hydrogen production rate of 21.9 ml/h and maximum ethanol production of 4.85% (w/v). Although the authors did not compare the performance with a control reactor, their study demonstrated the possibility of valorization of real organic waste (i.e., food waste) for bioethanol production via EF. As shown in Table 1.1, the majority of EF studies were conducted with synthetic substrates. Therefore, future EF should consider utilizing real waste biomass to demonstrate the feasibility of EF for practical application.
1.3.3 Bio-Butanol
Bio-butanol is another increasingly popular platform chemical that can also be used as liquid biofuel or additive to fossil fuel due to high energy content (27.08 MJ/l) and compatibility with combustion engines (Kumar and Gayen, 2011). The fermentation process driven by solventogenic Clostridia has been widely used for biobutanol production from carbohydrates-rich feedstock due to its low capital cost compared to the petrochemical butanol production process (Elbeshbishy et al., 2015). Before 2005, butanol was mainly used for the synthesis of other industrial chemicals. After being recognized as a biofuel, the demand for butanol has considerably increased in the last 15 years (Kumar and Gayen, 2011). The global market of biobutanol is now expected to reach USD 17.78 billion by 2022 (Grand View Research, 2020).
Low biobutanol yield, as well as production rate, have been identified as major bottlenecks for wide-scale application of fermentative biobutanol production (Elbeshbishy et al., 2015). Over the years, various strategies, including the deployment of genetically modified Clostridia and non-Clostridia organisms as well as designing novel fermentation systems, have been considered for alleviating these bottlenecks. In recent years, a few studies investigated biobutanol production via EF of glucose with pure-culture Clostridium species (Choi et al., 2014; Engel et al., 2019; Mostafazadeh et al., 2016). Choi et al. (2014) studied cathodic EF of glucose with pure culture Clostridium pasteurianum DSM 525 in a dual-chamber EF system. The authors found that C. pasteurianum could produce butanol by utilizing electrons from both cathode and substrate (glucose). Although NADH generation from electricity was trivial as compared to that generated from glucose, EF could increase butanol production by 2.5 times over the control fermenter. Thus, their results suggested a metabolic shift in reduction pathways in C. pasteurianum due to the applied potential.
A study by Mostafazadeh et al. (2016) also reported enhanced biobutanol yield (g butanol/g consumed glucose) and productivity (g butanol/L-h) in cathodic EF using pure-culture C. pasteurianum. Compared to the control (i.e., conventional fermentation), butanol yield and productivity increased by 29% and 30%, respectively. Furthermore, their study highlighted the importance of process optimization of EF for biobutanol production. The authors investigated the effects of various process parameters, including glucose concentration (86.3-153.6 g/L), temperature (27.6-39.4°C), and applied voltage (0-2.6 V) using statistical design of experiment in cathodic EF of glucose to biobutanol using pure-culture C. pasteurianum. The maximum biobutanol concentration of 13.31 g/L butanol was achieved at an applied voltage of 1.32 V under an operating temperature of 33.51°C and at a glucose concentration of 120 g/L. The glucose concentration ≥140 g/L introduced inhibition and led to a decrease in butanol yield. Furthermore, the authors also compared the impact of cathode electrode materials on butanol production. The results showed that up to an applied voltage of 1.5, graphite felt as cathode could provide higher butanol concentration as compared to the stainless-steel electrode. However, with the applied voltage >1.5 V, the performance of both electrodes declined. With an increase in applied voltages, hydrogen production on the cathode linearly increased in both reactors. Moreover, the stainless-steel electrode led to higher hydrogen production as compared to the graphite, which is consistent with some of the reports on superior hydrogen evolution reaction (HER) with the metal electrode (Cheng et al., 2009). Thus, superior HER on the stainless surface could introduce instability of cathode biofilms. Overall, this study demonstrated considerable scope to optimize process parameters and design of EF systems to maximize productivity and yield of target value-added products.
1.3.4 Microalgae Derived Lipids
Up to date, fossil fuels have been widely adopted as energy sources across the world. However, there is a great need to significantly develop the renewable fuels (e.g., non-fossil fuel) as energy sources on a large scale to outcompete the fossil fuels, eliminating the issues of aggravating CO2 concentrations and global warming (IPCC, 2018; Liu et al., 2019; Liu et al., 2020c). Out of many renewable fuels, microalgae-derived biofuels have demonstrated to be highly promising due to their advantages, such as high biomass production per unit area, a high weight ratio of lipids and ability to accumulate lipids in oleosomes, and no competition for arable land usage (Adeniyi et al., 2018; Chisti, 2007; Hallenbeck et al., 2016; Hu et al., 2008; Rittmann, 2008). Despite these attracting features, several drawbacks for the microalgae biofuel generation have been reported, emphasizing their high costs, and environmental concerns associated with algae harvesting and lipids extraction (Markou and Nerantzis, 2013; Pierobon et al., 2018; Rittmann, 2008; Sills et al., 2013). For instance, the requirement of pre-treatment (e.g., acid/alkaline hydrolysis, pulsed electric fields, ultrasound (Sheng et al., 2011; Zbinden et al., 2013)) for microalgae processing are generally highly energy intensive (e.g., in terms of both capital and operational costs), which was one of the arguments against further development and scaling-up. Lipid extraction is a critical step prior to the microalgal biodiesel production process. However, highly toxic chloroform-methanol solvents have been widely adopted for lipid extractions due to their effective performance (e.g., the ability to penetrate cell wall and membrane) (Bligh and Dyer, 1959; Folch et al., 1957). However, they must be replaced by other non-toxic solvents (e.g., hexane-isopropanol) due to the environmental and public health concerns (Lai et al., 2016a; Lai et al., 2014). Hence, a greener approach is required to address the challenges associated with microalgae-derived lipids extraction.
In response, selective fermentation (SF) has been implemented as a new approach for enhancing lipid extraction for microalgae biofuel generation (i.e., biodiesel) (Lai et al., 2016b). SF utilizes the fact that lipids are generally biodegraded more slowly than carbohydrates and proteins under an anaerobic environment (Lai et al., 2016b; Rittmann and McCarty, 2001; Siegert and Banks, 2005), in which, the lipid fermenters, known as the slow-growing microbes (Christ et al., 2000; Rittmann and McCarty, 2001), can be washed out of the system with a relatively short solids retention time (SRT) (Lai et al., 2016b). Hence, the SF solely allows the removal of carbohydrates and proteins in microalgae cells, leaving the lipids intact, providing a state much easier to extract lipids (Lai et al., 2016b). More importantly, the SF process can significantly promote the lipid extraction process (e.g., >5000-fold increase vs. untreated biomass) when using the hexane-isopropanol solvents (Lai et al
