7.1.3. Fermentation Effluents

The fermentation effluents comprise various byproducts such as acetate, butyrate, ethanol, lactate, and formate. These byproducts can still undergo further reactions, resulting in the production of more hydrogen. The fermentation of sugars by bacteria is a common source of H2 production, although transformation of carbohydrates to hydrogen is insufficient. Additional hydrogen is frequently generated from the effluent of an ethanol dark fermentation reactor, according to the results of a single-chamber MEC test. At an applied voltage of E(ap) = 0.6 V, overall H2 recovery of 83% ± 4% was achieved using a pH-controlled effluent (pH = 6.7–7.0), with a hydrogen generation rate of 1.41 ± 0.08 m<sup>3</sup> H2·m<sup>−</sup>3·day−1; furthermore, after combining the MEC and fermentation system, the overall H2 recovery increased to 96%, and the system was able to produce an average of 2.11 m<sup>3</sup> H2·m<sup>−</sup>3·day−1, which corresponds to a voltage efficiency of 287%. At applied voltages ranging from 0.5 to 0.8 V, high cathodic hydrogen recoveries (ranging from 70% ± 5% to 94% ± 4%) were obtained [96].

When the MEC was integrated with the other fermentation system, 96% of the H2 was recovered at a production rate of 2.11 m<sup>3</sup> H2·m<sup>−</sup>3·day−1, resulting in electrical energy productivity of 287%. Sosa-Hernández et al. investigated the potential of spent yeas<sup>t</sup> (SY) for energy recovery in MEC. Tests were conducted on the concentrations of SY produced by bench alcoholic fermentation and ethanol, which ranged between 750 and 1500 mg COD·L−<sup>1</sup> and between 0 and 2400 mg COD·L−1, respectively. The removal efficiency (RE), Coulombic recovery (CR), Coulombic efficiency (CE), H2 production, and current density of the COD removal system were all measured and analyzed. The combination of 1500 mg COD·L−<sup>1</sup> SY + 1200 mg COD·L−<sup>1</sup> ethanol produced an appealing current density (222.0 ± 31.3 <sup>A</sup>·m<sup>−</sup>3) and H2 generation (2.18 ± 0.66 LH2·d−1·LReactor−1) [97]. Cai et al. developed a bioelectrochemically assisted anaerobic reactor and compared it to an anaerobic digestion (AD) control reactor in order to generate methane. They achieved an average methane production rate of 0.070 mL CH4·mL−<sup>1</sup> reactor·day−1, which was 2.59 times greater than the AD control reactor (0.027 m<sup>3</sup> CH4·m<sup>−</sup>3·day−1), as well as an increase in COD removal of approximately 15% above the AD control. When the fermentation liquid is changed to sludge fermentation liquid, the rate of methane generation was raised even further, reaching 0.247 mL CH4·mL−<sup>1</sup> reactor·day−<sup>1</sup> [98].
