Production, purification and recovery of caproic acid, Volatile fatty acids and methane from Opuntia ficus indica

Opuntia ficus-indica can grow in arid and semi-arid environments characterized by low water and nutrients availability. These features make it a more sustainable alternative to the common energy crops for biorefinery purposes. This work focused on the potential benefits of anaerobic processes applied to this plant. Specifically, it considered i) the substrate preparation, demonstrating the effect of the apparent viscosity on the process; ii) the evaluation of biomethane, Volatile Fatty Acids (VFAs), and caproic acid production in semi-continuous mode at different hydraulic retention times; iii) the purification of the Fatty Acids-rich output through pressure-driven membrane filtration. The rheology analysis found that a 5 %w/w water dilution of the substrate is needed to lower the apparent viscosity to 173 cP, which is below the acceptable apparent viscosity level of 200 cP for a good bioreactor mixing. Keeping this condition, the semi-continuous trials with the best biomethane performance was at HRT of 20 days, with 210 mLCH4/gVS and 232 mLCH4/gCODin of production and specific yield, respectively. The VFAs and caproic acid production reached their best at Hydraulic Retention Time 5, with 26 and 7.9 gCOD/L of VFAs and caproic acid, corresponding to specific yields of 79 and 30 % respectively. Pressure-driven filtration at 330 kDa allowed to obtain a permeate with a VFAs and caproic acid content of 96.72%w/w. Finally, the adsorption and desorption tests allowed to separate caproic acid from the permeate and to concentrate it from about 7.5 gCOD/L to about 26 gCOD/L.