Potential of exausted pomace as a matrix for biogas production after polyphenol recovery

Italy ranks second in Europe as for production and export of olive oil. Oil production means proportional volumes of organic waste that need to be disposed of. Assuming that to be sustainable in the long term a production process must have a circular flow of energy and raw material, a system has been studied for oil waste valorization so to create a technical cycle where waste of olive production is used to : a) extract polyphenols, b) produce energy through Anaerobic Digestion (AD), c) use organic residual from AD as a fertilizer for olive grove. The study was based on the assumption that the vegetation waters and the exhausted pomace have a high content of potentially useful molecules, among which polyphenols are the largest part. However the polyphenols, if left in the exhausted pomace, give it bactericidal capacity which prevents extensive use as soil fertilizer as it interferes with soil microbiome. For the same bactericidal activity the exhausted pomace shows difficulty in AD for the production of biogas. For these reasons, at present, in Italy, the most common destination for depleted pomace is incineration with consequent loss of organic matter and energy. Both for economic and environmental reasons the study was developed to manage the exhausted pomace in a circular economy perspective: extraction of polyphenols with a multi-membrane system and subsequent AD of residual pomace with biogas, energy and digestate production. Data used in the study refer to an olive treatment plant of average dimensions. Results demonstrate the possibility to put in place an economically sustainable production cycle.