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
