Caproic Acid Formation by Carbon Chain Elongation During Fermentative Hydrogen Production of Cassava Wastewater

In this work, cassava wastewater was used to evaluate hydrogen production and the subsequent production of caproic acid via acetate carbon chain elongation, adding ethanol as electron donor. Batch reactors were operated with an initial chemical oxygen demand of 10 and 20 g O 2 /L (A10 and A20, respectively), under self-fermentative operation at 40 ± 1 °C. The initial pH was set to 5.5 to inhibit methanogens growth. Hydrogen production yields were 1.01 and 1.98 mol H 2 /mol glucose for A10 and A20, respectively. In both cases, the main metabolites were ethanol and acetic, butyric, propionic, and caproic acids during the fermentative step; when adding ethanol for the chain elongation the pH was adjusted to 7.0 using a 2-M NaOH solution. The maximum total caproic acid concentration obtained after ethanol addition was 4.2 and 7.2 g/L for A10 and A20, respectively; of which 84% and 90% resulted from the carbon chain elongation step, respectively. Graphic abstract