Two-stage anaerobic digestion of orange peel without pre-treatment: Experimental evaluation and application to Sao Paulo state

Orange juice production is an important industry worldwide, specifically in Brazil, which is the leading producer. Nevertheless, approximately 50 % of the feed used for industrial production of orange juice is wasted, primarily as peels, a promising raw material for being used in the different biotechnological processes. This study evaluated the potential of two-stage anaerobic digestion (reactor of 4.3 L total volume) to valorize orange juice residues. The system replaced the use of other pretreatments and increased methane concentration (approximately 60 % compared with about 50 %) and volume (by 13 %) relative to one-stage anaerobic digestion. The accumulated biogas yield was 0.79 L/g SVT and 0.49 L/g SVT for the methanogenic and control reactors, respectively. Also, the anaerobic reactors were able to start-up with high solids feed, which could be applied to the Upflow Anaerobic Sludge Blanket (UASB) reactors currently in operation in the citrus juice industry. The potential electric power generated was estimated as 97.5 x 10(3) MWh/year, which might be used on-site or sold back to the energy grid where it represents 0.2 % of the total energy used by Sao Paulo State. The potential greenhouse gas mitigation from the substitution of electric energy from the Brazilian energy grid to a more renewable one from biogas burning was estimated to be 9.05 x 10(3) tCO(2)eq/year. The results demonstrated that two-stage anaerobic digestion is a promising technology for the disposal of orange peel waste while producing a useful bioenergy product.