Long-Term Preservation of Orange Peel Waste for the Production of Acids and Biogas

Orange peel waste (OPW) has a remarkable biorefinery potential. In this paper, a biorefinery strategy is proposed at a laboratory scale in order to overcome the issue of the OPW seasonality. OPW was preliminary subjected to a long-term ensiling (up to 12 weeks) with the twofold purpose of preserving the OPW potential for methane production through anaerobic digestion (AD) and stimulating the production of value-added compounds by means of biological (leachate of a previous ensiling process) and chemical (MnCl2) supplements and/or their combination. On the liquid fraction of the ensiled OPW, lactic acid (LA), acetic acid (AA), and ethanol concentrations were detected. Instead, solid fractions were used as substrates for AD both in batch and semi-continuous modes. Specifically, the combined stimulation led to LA and AA yields of 54.5 and 16.6 g kgTS-1, respectively, after 8 weeks, whereas the largest yield of ethanol (i.e., 70.4 g kgTS-1) was achieved after 8 weeks of ensiling without any stimulation. Chemical and combined stimulation allowed us to preserve in the solid fraction, separated by centrifugation after long-term (8-12 weeks) ensiling, about 50% of the methane potential of the fresh OPW. Moreover, semi-continuous AD resulted in semi-stable processes for all the solid fractions (methane yields ranging from 0.23 to 0.28 NL gVSloaded-1) even though nutrients supplementation was necessary.