Sequential dark fermentation and microbial electrolysis cells for hydrogen production: Volatile fatty acids influence and energy considerations

Hydrogen production from food waste by coupling dark fermentation (DF) and microbial electrolysis cells (MEC) was studied. Metabolic patterns in DF, their effects on MECs efficiency, and the energy output of the coupling were investigated. Mesophilic temperature and acidic pH 5.5 resulted in 72 +/- 20 mL H2/g CODin and a butyrateenriched profile (C2/C4, 0.5-0.6) contrasting with an acetate-enriched profile (C2/C4, 1.8-1.9) and 36 +/- 10 mL H2/g CODin at pH 7. Assessment in series of the DF effluents in MECs resulted in a higher hydrogen yield (566-733 mL H2/g CODin) and volatile fatty acids (VFAs) removal (84-95%) obtained from pH 7 effluents compared to pH 5.5 effluents (173-186 mL H2/g CODin and 29-59%). Finally, the output energy was lower in DF at pH 7, however, these effluents retrieved the highest energy in the MEC, showing the importance of process pH and VFAs profile to balance the coupling.