Valorization of Residual Orange Peels: Limonene Recovery, Volatile Fatty Acids, and Biogas Production

Abstract

The new European Union directives on waste management encourage the adoption of the "pyramidal hierarchy" for the extraction or synthesis of high-added-value compounds rather than the conventional mere biogas production. This approach was applied to orange peels (OPs), rich in limonene, a molecule used as an additive in the cosmetics market. Soxhlet extraction was optimized, investigating the effects of different solvents, operative temperatures, cycle number, and substrate-solvent ratios on limonene recovery. It was demonstrated that n-hexane, used at a substrate-solvent ratio of 1:2 at 70 °C, gave the best performance in limonene extraction yield (about 1.20% w/w of OPs). The residual OPs after limonene extraction were fermented in batch conditions for the production of volatile fatty acids (VFAs) at different total solids (TS) concentrations. The best yield of 0.34 gVFA per gram of TS treated was obtained with a TS concentration of 15% w/w and a fermentation duration of 5 days. Finally, the methane potential of extracted and unextracted OPs was tested through anaerobic digestion. Both the tests achieved the same level of methane productivity of 355-365 NLCH4/kgTVS, but the OPs without a previous limonene extraction needed double the time to reach the final methane production.