## Permeate
### Integration of MFCs with Dark Fermentation  
A hybrid procedure of dark fermentation, microbial fuel cells, and microbial electrolysis cell could convert crude glycerol, a waste by-product obtained during biodiesel manufacturing, to bioenergy (MEC). This combined technique addresses the thermodynamic constraint of dark fermentation, where complete breakdown of crude glycerol is challenging [56]. Large percentages of unutilized organic metabolites remain in the dark fermenter's effluent, which can be used to produce power in MFCs or hydrogen in microbial electrolysis cells. The greatest H\({}_{2}\) generation of 332 mL/L (output of 0.55 mol H\({}_{2}\)/mol glycerol) was attained when crude glycerol with an initial carbon oxygen demand concentration of 7610 mg/L was used in dark fermentation. After 50% dilution, the sewage was deteriorated in MFCs to reach a power output of 92 mW/m\({}^{2}\) and a carbon oxygen demand elimination of 49% [57].  
MFCs were also integrated with dark fermentation (DF) for H\({}_{2}\) generation via an anaerobic hydrogen production process. The waste from the hydrogen fermentation is acidic and was reacted further in the MFCs to increase the entire electricity generation and facilitate COD exclusion efficiency [33]. This system was examined to acquire marketable chemicals and fuels, such as biofetrilizers, hydrogen, and methane, for bioelectricity [37].  
An innovative integrated single-stage dark-fermentation-MFC method was designed in an experiment to produce biohydrogen and power from wastewater treatment at the same time. The integrated system's biohydrogen gas was converted into energy using a proton-exchange membrane fuel cell (PEMFC). The impact of hydraulic retention time (HRT) on biohydrogen and electricity production was also investigated. At an 8 day HRT, the greatest volumetric biohydrogen production rate (VHPR) was 0.44 L H\({}_{2}\)/L-d (0.66 L H\({}_{2}\)/g COD removed), with an electricity output of 530 mV (100 mW/m\({}^{2}\)). The bacteria discovered on the anode of the integrated system were _Chryseobacterium_, _Azotobacter_, _Bacillus_, _Enterococcus_, _Citrobacter_, and _Methanobacterium_, per an 16S rRNA gene-based investigation. The PEMFC employed to generate voltage from biohydrogen produced by the integrated system was capable of achieving a maximum voltage of 459 mV (367 mW) and a maximum cell efficacy of 44% (fuel consumption of 1.5 \(\times\) 105 mol/h) [58].  
A hybrid approach of dark fermentation (DF) and MFC known as sDFMC was examined in the study for simultaneous H\({}_{2}\) and electricity production from _Saccharina japonica_ in a single reactor. Due to simultaneous H\({}_{2}\)/carboxylic acid (CA) synthesis by DF and electricity production by MFC, the energy recovery of the sDFMC was significantly improved. A time course of CA concentration in sDFMC validated the coproduction of H2 and power. With an H\({}_{2}\) yield of 110 mL/g-VS and a maximum power density of 1.82 W/m\({}^{2}\), _S. japonica_ provided good energy recovery of 17.3%. The sDFMC demonstrated a diversified microbial population for optimal organic substrate microbial conversion. The findings suggest that the sDFMC could be a promising single reactor process for producing H\({}_{2}\) and energy from a variety of biomass feedstocks while maintaining the individual DF and MFC processes' efficiency [59].