Abstract
The engineering of replacements for crude oil is a priority within industrial biotechnology. Biogas, produced by anaerobic digestion (AD) during organic waste degradation, has been used for electricity generation and heating. Microbial electrolysis cells (MECs) are an emerging technology which when combined with AD can produce higher yields of such energy whilst simultaneously treating waste water and sludge. MECs are bioelectrochemical systems which utilize the metabolism of microbes to oxidize organics. The majority of the research has been focused on biohydrogen production, despite associated issues, which has resulted in poor commercialization prospects. Consequently, scientists are now suggesting that methane production should be the focus of MEC technology. This chapter presents lab research on the bioprocessing of biomethane using AD and MECs and addresses important issues, namely the lack of pilot-scale studies. Downstream processing techniques are discussed, as well as a novel suggestion of further utilising MECs in the purification process.
TopIntroduction
The heavy use of fossil fuels has resulted in a global energy crisis which has provided an incentive for researchers to find sustainable alternatives to meet modern-day society’s demands. It has been estimated that current treatment of municipal waste water accounts for approximately 3% of global electricity consumption (Li et al., 2014). Waste water contains significant amounts of energy e.g. domestic waste water contains 7.6 kJ L-1 (Heidrich & Dolfing, 2011). Harnessing the energy contained in these dissolved organics would mitigate the burdens associated with treatment, whilst simultaneously producing renewable fuels. The use of novel bio-technologies is allowing for this energy capture, which has caused a shift in what was traditionally classified as waste. Anaerobic digestion (AD) is a popular technology used to extract bio-energy in the form of biogas during the microbial conversion of organic substrates (Bharathiraja et al., 2016) e.g. high strength waste water and sludges. Unfortunately, there are several bottlenecks to AD technology application such as slow metabolism of methanogens and accumulation of inhibitory volatile fatty acids (VFAs) (Zhang et al., 2009). This combined with the fact that AD can be energy expensive, makes it apparent that more efficient bio-catalytic routes need to be developed which convert organic waste into green fuels.
Over the past decade, bioelectrochemical systems (BSEs) have been gaining speed as a new generation of technologies which show great potential for waste treatment whilst generating hydrogen or methane. A form of these advancements called Microbial Electrolysis Cells (MECs) has been studied as a hopeful candidate to improve gas energy production. The objective of this chapter is to discuss MEC assisted methane production as well as what needs further investigation if MECs are to achieve commercialization.
Background
MECs utilize exoelectrogenic microbes to oxidize organic waste substrates which transfer the electrons to the anode. The electrons are then transferred to the cathode where they can reduce protons for hydrogen production, or produce methane via several pathways (Lu & Ren, 2016). A small external voltage is required between the electrodes to overcome the thermodynamic barrier, which can be supplied by microbial fuel cells. It is apparent that research into hydrogen production using MECs has been prioritised. Hydrogen makes an ideal fuel due to being carbon free and having high energy content (Zhang & Angelidaki, 2014). However, the production of hydrogen presents numerous challenges that prohibit upscaling and commercialization. This was demonstrated in the largest MEC reactor reported to date. After 43 days, methanogenisis led to H2 consumption, and consequently methane became the dominant product (Cusick et al., 2011). Methane is relatively safer than hydrogen for storage, and most importantly, its production has been demonstrated to be more robust and consistent (Clauwaert & Verstraete, 2008). Moreover, the incorporation of MECs into traditional AD systems is proving to be an extremely hopeful technology. All of this suggests that methane production should be at the forefront of MEC research.
Key Terms in this Chapter
Bioelectrochemical Systems (BESs): Systems which harness the metabolisms of microbes to convert chemical energy into electrical energy e.g. Microbial Electrolysis Cell and Microbial Fuel Cell.
Direct Interspecies Electron Transfer (DIET): DIET is a syntrophic interaction that occurs in methanogenic communities. Free electrons are transferred directly from one microbial cell to another rather than being shuttled by reduced molecules such as hydrogen.
Energy Crisis: This is an ongoing global issue that is concerned with the fact that natural resources (fossil fuels), used to power industrial society, are diminishing and the demand for them continues to rise. Suitable renewable replacements must be sought which is the aim of industrial biotechnology.
Methanogen: Microorganisms that produce methane as a metabolic by product under anaerobic conditions.
Downstream Processing: The later stages of bioprocessing namely recovery and purification of biosynthetic products.
Anaerobic Digestion: A sequence of biological processes in which microorganisms break down organic material under hypoxic conditions. One of the metabolic end products is biogas.
Sludge: Sludge is semi-solid slurry and can be produced as part of numerous industrial processes. For example, sewage sludge refers to the residual material that is produced as a by-product during sewage treatment of wastewater. The term activated sludge refers to suspended sludge containing many active bacteria, which remove biodegradable substances from wastewater or waste activated sludge (WAS). WAS refers to excess sludge produced in an activated sludge system which must be processed in a further treatment chain.
Exoelectrogen: Bacteria which can transfer electrons extracellularly e.g. Geobacter species.
Biocathode: In microbial electrolysis cells the cathode is the ‘electron sink’, meaning it accepts electrons and protons produced from the anodic oxidation of the substrate. A bio-cathode utilises microorganisms for this catalytic purpose.