Anaerobic digestion of orange peel in a semi-continuous pilot plant: an environmentally sound way of citrus waste management in agro-ecosystems

The management of residues of citrus processing involves economic and environmental problems. In particular, the uncontrolled disposal of citrus processing waste near production sites can have heavy impacts on air, soil, surface water bodies and groundwater. Anaerobic digestion has been proposed as a viable alternative for citrus waste valorisation, if some problems, linked to the biochemical processes, are overcome. Although many experimental tests have studied the inhibitory effects of the high essential oil content of orange peel on biomethanisation processes, fewer experiences have been carried out in continuous or semi-continuous pilot digesters, more similar to the full-scale biogas plants, using real orange peel. This study has evaluated the methane production through anaerobic digestion of industrial orange peel using a pilot plant (84 L) with semi-continuous feeding at increasing Organic Loading Rates (OLR) and essential oil (EO) supply rates (EOsr) until the complete process inhibition. Under mesophilic conditions, the highest daily specific methane yield was achieved at OLR of 1.0 gTVS L−1 d−1 and EOsr of 47.6 mg L−1 d−1. Partial inhibition of the anaerobic digestion was detected at OLR and EOsr of 1.98 gTVS L−1 d−1 and 88.1 mg L−1 d−1, respectively and the process irreversibly stopped when OLR and EOsr reached 2.5 gTVS L−1 d−1 and 111.2 mg L−1 d−1, respectively. Under thermophilic conditions, the cumulative methane production (0.12 L gTVS−1) was about 25% of that under mesophilic conditions (0.46 L gTVS−1). The thermophilic digestion was completely inhibited at lower OLR (1.98 gTVS L−1 d−1) and EOsr (88.1 mg L−1 d−1) compared to mesophilic conditions. This study confirmed the suitability of anaerobic digestion of orange peel for biomethane production (provided that the right management of the process is set), in view of an environmentally sound way of agricultural residues management in agro-ecosystems.