Biomethanization of citrus waste: Effect of waste characteristics and of storage on treatability and evaluation of limonene degradation

This study proposes the evaluation of the suitability of mesophilic anaerobic digestion as a simple technology for the treatment of the citrus waste produced by small -medium agro-industrial enterprises involved in the transformation of Citrus fruits. Two different stocks of citrus peel waste were used (i.e., fresh and stored citrus peel waste), to evaluate the influence of waste composition (variability in the type of processed Citrus fruits) and of storage (potentially necessary to operate the anaerobic digester continuously over the whole year due to the seasonality of the production) on anaerobic degradation treatability. A thorough characterization of the two waste types has been performed, showing that the fresh one has a higher solid and organic content, and that, in spite of the similar values of oil fraction amounts, the two stocks are significantly different in the composition of essential oils (43% of limonene and 34% of linalyl acetate in the fresh citrus waste and 20% of limonene and 74% of linalyl acetate in the stored citrus waste). Contrarily to what observed in previous studies, anaerobic digestion was successful and no reactor acidification occurred. No inhibition by limonene and linalyl acetate even at the maximum applied organic load value (i.e., 2.72 gCOD(waste)/gVS(inoculum)) was observed in the treatment of the stored waste, with limonene and linalyl acetate concentrations of 104 mg/1 and 385 mg/1, respectively. On the contrary, some inhibition was detected with fresh citrus peel waste when the organic load increased from 2.21 to 2.88 gCOD(waste)/gVS(inocuium), ascribable to limonene at initial concentration higher than 150 mg/I. A good conversion into methane was observed with fresh peel waste, up to 0.33lCH(4)/gCOD(removed) at the highest organic load, very close to the maximum theoretical value of 0.35 lCH(4)/gCOD(removed), while a lower efficiency was achieved with stored peel waste, with a reduction down to 0.24 lCH(4)/gCOD(removed) at the highest organic load. (C) 2018 Elsevier Ltd. All rights reserved.