In EU Countries of the Mediterranean basin the solid wastes generated from the olive oil extraction process (2- or 3-phases or intermediate processes) pose a number of environmental, economic and social concern. The main management options for wet husks are (a) electric power generation, (b) domestic heating and (c) return to agriculture via composting. 82.5% of the world olive oil production takes place in the EU27, i.e. 2.34 vs. 2.84 million tonnes, respectively, the majority of which occurs in Spain (46%), Italy (16%), Greece (12%), Portugal (2%), France, Cyprus, Slovenia and Malta (≤ 1% each). The percentage rises to 94.1% of the world olive oil production, i.e. 2.63 million tonnes, if the non-EU Countries of the Mediterranean basin are included. In Italy olive trees are cultivated in 18 Regioni over 20, but 88.0% of the olive production, ca. 600.000 tonnes/year (2/3 extra-virgin and 37 DOP recognized in EU) is obtained in the Southern ones (Puglia, Calabria, Sicilia, Basilicata and Sardegna). Puglia has 267.203 olive farms, Sicily 196.352, Calabria 136.016, Campania 112.093, less in the others. Tuscany is contributing with 4%, i.e. 0.2 million tonnes/year to the national olive oil production. In the six major olive oil producing EU countries the wastewater and solid waste produced are 6.01 million m3 and 8.06 million tonnes per year, respectively. The estimate is based on the relative distribution of the olive oil extraction technology and on the amount of olives processed during five years (2009-2013). However, the increasingly adopted 2-phase decanter centrifugation process (due to its higher effectiveness in olive oil yields) could lead in the near future to a further increase in the amount of solid (i.e. wet husks) residues vs. wastewaters. When considering the Mediterranean basin, annually up to 30 million m3 of olive mill wastewater and 20 million tonnes of solid waste are produced. In Tuscany, 296 olive mills produce an average of 66.800 tonnes/year of wet husks, at ca. 54% humidity (equivalent to a water content of 36.000 m3), ranging between 44.350 tonnes in the lower production years and 99.250 tonnes in the higher production years. By using the life cycle assessment (LCA) and life cycle impact assessment (LCIA) tools, it has been shown that domestic heating and power generation with wet husks are the most important impact factors in damage to human health, ecosystems, and natural resources depletion. Composting wet husks is 2-4 orders of magnitude less impacting than domestic heat and power generation. Considering human health, the impact of climate change, human toxicity and particulate matter formation represent the main impacted categories. Considering ecosystems, climate change and natural land transformation are the main impacted categories. Within natural resources, fossil fuel depletion is impacted three orders more than metal depletion. Within domestic heating and power generation scenarios, storage of wet husk along with the extraction by organic solvent and the waste treatment are the most impacting phases for global warming potential, ozone layer depletion, acidification and non-renewable fossil resources depletion. The results obtained for the waste disposal have been comparatively assessed with respect to the environmental impact of the olive oil production chain. When considering the entire chain of the olive oil production (i.e. from olive tree cultivation to waste disposal) the disposal options “power production” and “domestic heating” affect significantly the eutrophication potential with respect to the disposal option “composting”. To achieve an appropriate management of solid and liquid waste (LCM) in Tuscany, as well as for Mediterranean basin, other factors have to be taken into account, namely agricultural soils profile (i.e. low content of soil organic matter, ca. 2% in agricultural soils), social and economic impacts (employment opportunities). Waste-to-energy disposal looks appropriate when centralized plants are industrially feasible in large olive tree cultivation areas where soils are characterized by a content of organic matter >3.5%, or where 2-phase decanter centrifugation process is the sole extraction process available, and no alternative sources of energy are available or are available at higher prices with respect to electricity obtainable from wet husk. The above scenario seems to be fitting more for Central Spain and much less for Tuscany. Waste-to-agriculture disposal strategies are more appropriate where sustainable agriculture management (low chemical fertilizers and chemical pesticides, increased adoption of biofertilizers and bio-based control agents, or integrated pest management regimes) is widespread or to be promoted, or where small to medium size olive farms are fragmented and the transportation costs of the wastes would affect the economic return. The latter scenario seems to be fitting more for Tuscany than Central Spain. Furthermore, for waste-to-agriculture disposal strategy in the areas where 3-phase decanter centrifugation process is dominant, liquid waste could be used partly for biogas generation while composting would be retained for solid waste disposal at both small-size olive mills and at farm level, using the liquid waste to maintain appropriate process conditions. The liquid waste generated from the 3-phase-decanter extraction process (still largely present in Tuscany) to be used for biogas production offers other advantages, in particular it allows to upgrade the digestate from biogas generation by flowing it into the composting process.
Marco Nuti - University of Pisa - firstname.lastname@example.org