Sustainable Solutions for Environmental Pollution. Группа авторов

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