Agro-industrial waste valorization by mixed microbial consortia: production of hexanoic acid and biomethane-rich biogas through anaerobic fermentation processes

The present work aims at the evaluation of the possibility to use different organic wastes, as substrates for the production of hexanoic acid and methane, through anaerobic fermentation process. The substrates that will be examined are red and white grape pomaces, for the production of volatile fatty acids, and different agro-industrial waste, coming from insects breeding, for the production of biomethane. The first goal is the establishment of an appropriate culture medium, for the acidogenesis and chain elongation processes, in order to achieve a high concentration of hexanoic acid in the fermentation broth. The culture medium is mostly composed by ethanol, acetic acid and sodium hydrogen carbonate: ethanol is used as the electron donor of the chain elongation process; acetic acid is the source of organic carbon; sodium hydrogen carbonate is the alkalinity regulator. The essay’s results show that each of them is mandatory inside the fermentation broth, with a specific concentration and molar ratio. The study of the production of hexanoic acid was conducted in batch reactors, using red or white grape pomaces. The concentrations of VFAs achieved through the batch culture were high, allowing an easy recovery of them from the fermentation broth. The yield of the process, using red grape pomace, was higher than the yield of white grape pomace. A pseudo-continuous process was set up, in order to monitor its performance with respect to the batch process. The results showed lower concentrations achieved with the last process, with respect to the batch one. The biochemical methane potential study of different agro-industrial residues was carried out: each substrate produced a certain amount of methane, demonstrating the possibility to use them as feeding for an industrial anaerobic digestor. The methane yields found were lower than the average of methane yield of fresh organic waste.