Recovery of D-limonene and energy from orange peel waste

Currently, there is not a harmonized definition of food waste. Parfitt et al. (2010) suggested the following definition: “Food losses or waste are the masses of food lost or wasted in the part of food chains leading to edible products going to human consumption”. Therefore, food waste has taken on disquieting proportions worldwide in all steps of the food production and supply chain, and in industrialized countries more than 40 % of the food waste occurs at retail and consumer levels (FAO, 2014). More than one-third of the food produced for human consumption, between 1.2 and 2 billion tonnes per year (FAO, 2014), gets lost or wasted globally, equivalent to 6-10 percent of human-generated greenhouse gas emissions (Gustavsson, 2011; Vermeulen et al., 2012). Food waste is an abundant and renewable biomass for the recovery of high-added value compounds and for the production of bio-fuels, representing a valid alternative for the displacement of polluting and limited fossil fuels. Waste-to-Energy concept is supported by Directive 2008/98/EC (EU, 2008), which is the main effort of European legislation on waste. In this directive, concepts and definitions related to waste management are laid down, as well as waste management principles such as the "polluter pays principle" and the "waste hierarchy". It is of the utmost importance to integrate all of those initiatives/activities aimed at minimizing waste with high recycling targets, in order to respect and protect the environment as well as to promote process sustainability. It is necessary to abandon the idea that an improved quality of life corresponds to immediate economic growth. And, in fact, economic development itself would not be possible without safeguarding the environment and its natural resources. What is the future going to be like if we cannot guarantee the next generations with any resource? The aim of this Thesis is the valorization of by-products generated within a specific sector, namely the citrus processing industry, to help Europe become more resource efficient and sustainable. The principal objective is the evaluation of several processes aiming at the recovery of D-limonene and energy from orange peel waste (OPW), with the task of using collected experimental data for Life Cycle Assessment (LCA) inventory. In order to fulfill this purpose, first, a detailed review was written (and published), with the aim of achieving a deep knowledge on the toxicity of D-limonene, which represents the main challenge when anaerobic digestion is performed. Therefore, experimental works were performed in order to investigate several extraction techniques - including accelerated processes at moderate temperature and pressure-, anaerobic digestion valorization and pyrolysis technology. Afterwards, results were compared through LCA modelling performed by means of EASETECH, a software developed by Technical University of Denmark (DTU) and specifically dedicated to waste management. The most salient results were published on scientific journals, and this Thesis is an assortment of the Author’s literature works. Therefore, this work is structured in the following sections. CHAPTER I reports a deep overview of the most relevant notions concerning citrus waste issue and the main routes implemented for the valorization of this specific waste, both in terms of recovery of high-added value compound and in terms of bio-fuels. To accomplish this, several research works were carefully reviewed in detail. CHAPTER II addresses the effect of several pre-treatment processes on anaerobic digestion, mainly focused on the inhibitory effect of D-limonene. Basic and thermal treatments as well as extractive processes were evaluated in order to select the best strategies for scale-up operation. In CHAPTER III, several anaerobic co-digestion strategies were implemented with the intent of reducing the concentration of toxicants. First, toxicity batch tests were carried out in order to experimentally determine the threshold concentrations for toxic compounds and, afterwards, four Continuous Flow Stirred Tank Reactors (CSTRs) were utilized to investigate the co-digestion process of orange peel waste with manure and/or seaweed. The experimental set-up and work were totally carried out at the Environmental DTU, supervised by Irini Angelidaki and Merlin Alvarado-Morales (from Bio-Energy Group). CHAPTER IV compares the pyrolysis process of orange peel waste with hazelnut shells, a biomass generally accepted as a suitable feedstock for thermochemical processes. After the extraction of D-limonene by Soxhlet extraction with hexane as solvent, the residual waste was subjected to a natural de-hydration at room temperature and then used as a feedstock for pyrolysis process. In CHAPTER V, an accelerated extraction at moderate temperature was suggested as alternative to traditional Soxhlet extraction, with the task of increasing D-limonene recovery yield. The energy expenditure for D-limonene extraction was evaluated in order to select the optimal operating conditions (extraction temperature, extraction time and ratio OPW-solvent). After a natural de-hydration at room temperature, the citrus residue was valorized through pyrolysis process at different heating rates. Furthermore, the Energy Sustainability Index (ESI) was estimated for pyrolysis process, taking into account the total amount of energy available in the form of pyrolytic products and the amount of energy spent to treat the waste. CHAPTER VI presents several waste management scenarios. Environmental savings and burdens were quantified by means of LCA model for mono-treatments (pyrolysis, incineration and anaerobic mono-digestion), co-treatments (four strategies of anaerobic co-digestion) and non-recovery energy processes (landfilling, composting and animal feeding). The whole modelling section was carried out at the Environmental Department of DTU, supervised by Davide Tonini. Finally, the last CHAPTER VII highlights the main results obtained in this work and depicts future perspectives for waste management.