Biomethane and acidogenic potential of agri-food waste streams : towards the production of volatile fatty acids (VFAs)

Abstract

Dark fermentation results in an appropriate alternative in the valorization of residues derived from agri-food industry, through the transformation of simple sugars into value-added products. CLAMBER R&D Biorefinery is a public Spanish research center that is currently working on the assessment of agri-food residues to produce volatile fatty acids (VFAs), which then might be used in the production of bioplastics such as poly3-hydroxybutyrate-co-valerate (PHBV). This study investigates the acidogenic potential of agri-food residues as precursors in the production of VFAs. Four agri-food wastes (AFW) (Dairy Industry Residues (AFW1), Winery Industry Residues (AFW2), and two fruits and vegetable residues (AFW3 & AFW4) ) were initially characterized to determine physico-chemical and biological properties. Centrifugation and thermal hydrolysis were applied to liquid and solid residues, respectively, as pretreatment prior to the dark fermentation experiments, which were carried out under mesophilic conditions (35°C). Experiments were run for residues with and without pretreatment, as well as for operating conditions with and without pH control. Quantification of VFAs was performed by using HPLC Chromatography, and kinetic parameters of the study were estimated using a second-order model. Results showed an important improvement on the final VFA yield for the four residues when pH control was implemented, with a percentage increase of 423% for AFW1, 367% for AFW2, 103% for AFW3 and 33% for AFW4 in comparison with experiments without pH control. Centrifugation resulted in a proper option for pretreatment of liquid residues, with improvement of 24% for AFW1 and 53% for AFW2 in VFA yield. Thermal hydrolysis showed poor potential as a pretreatment of solid residues, with a decrease of 4% for AFW3 and 25% for AFW4 in VFA yield. The maximum VFAs yield was determined for each residue, resulting in values of 0.927 gVFAs/gVS for AFW1, 0.668 gVFAs/gVS, for AFW2, 0.936 gVFAs/gVS for AFW3 and 0.841 gVFAs/gVS for AFW4. The results of this work provide useful information about the potential of the four tested residues, becoming products of interest for implementation in further analysis to produce PHBV.