Towards Anaerobic Biorefineries - Improvement of the anaerobic digestion of the Organic Fraction of Municipal Solid Waste (OFMSW) using substrate pretreatments and two-stage digestion technology

In 2016, the household waste production in EU-28 amounted to about 215 million tons. As for the most industrialized countries, the annual generation overtakes 500 kg per capita with organic residues representing the predominant fraction (European Commission, 2018a). In Italy, in 2016, domestic waste production reached 30 million tons and the content of Organic Fraction of Municipal Solids Waste (OFMSW) accounted for approximately 20% (ISPRA, 2017). According to future predictions, this percentage is estimated to increase due to the improvement of separate collection systems. Achieving high rates of separation requires a strengthening of the recovery of the OFMSW (Ranieri et al., 2018). In response, the recent European environmental policy is twofold: reduction of landfill disposal and valorisation of organic residues. European and national legislations have focused on diverting OFMSW from landfilling due to potential environmental impacts and risks (i.e. odour and greenhouse gas emissions, groundwater contamination by leachate) and promoting of technologies able to transform a waste to be disposed of in a valuable product. These are the milestones of the European Union action plan for the circular economy (European Commission, 2015) aiming at developing a sustainable future built on alternative sources of energy and materials. A world where products at the end of their life are turned into resources for new purposes (Webster, 2013). In this context, the biorefinery concept fits well with this perspective. A biorefinery is a facility where biomass is converted into bioenergy, biofuels or bioproducts for further industrial or commercial applications (Alibardi and Cossu, 2016). Due to the production of energy and bioproducts (digestate), Anaerobic Digestion (AD) is considered one of the first examples of biorefinery (Sawatdeenarunat et al., 2016). AD has been in use for many decades. To date, it has been primarily aimed at stabilising organic waste, ranging from high solid feedstocks (i.e. animal manure, food waste and agro-industrial biomass), as well as municipal wastewaters. Nowadays, a more open mind is required to look beyond these original applications. According to the biorefinery concept, AD is not regarded as a final disposal treatment but is considered the centrepiece of a larger process with multiple functions such as the production of energy, fuels, heat and biobased materials (i.e.: biopolymers and agricultural fertilizers) and the remediation and stabilization of organic waste (Surendra et al., 2015). The aim of the present work is to overcome the traditional AD of the OFMSW through the study of two lines of research: the application of substrate pretreatments and the adoption of a two-stage digestion technology. Pretreatments of OFMSW can be used to solubilize organic matter prior to AD in order to improve the overall process in terms of faster rates and degree of substrate degradation, thus increasing methane production (Cesaro and Belgiorno, 2014). Several methods have been assessed including mechanical, chemical, biological, thermal, hydrothermal and microwave treatments (Ariunbaatar et al., 2014). The present research focuses its attention on these two latter methods. Autoclaving (A) was tested since is able to release the cellulosic materials enmeshed in lignin resulting in an increase of smaller molecules available for further processing (Heerah et al., 2008; Papadimitrou et al., 2010). Similarly, Microwaving (MW) is an optimal method to solubilize organic solids and as such is a suitable candidate to treat OFMSW (Shahriari et al., 2013). With the aim to further improve AD efficiency, the two-stage process has been identified as a promising method because it allows a better reduction of organic load and increases the overall energy conversion efficiency by generating two gases with high combustion power (Liu et al., 2013). The traditional AD is separated into two reactors connected in series. While the first fermentative phase produces a hydrogen rich biogas and releases volatile fatty acids (VFAs) in the liquid solution, the second phase converts VFAs and the residual biodegradable matter into methane and carbon dioxide (De Gioannis et al., 2013). European Union (European Commission, 2003) promotes hydrogen production, as it is a sustainable energy source with no greenhouse gases emissions from its combustion and high-energy yield. Moreover, the significant generation of organic acids during the fermentative stage can be used to produce polyhydroxyalkanoates (PHAs), a class of bio-polyesters completely biodegradable. Their chief property is the mechanical behaviour that make them comparable to common plastics (Colombo et al., 2017). The dissertation consists of four main chapters. Chapter 1 provides a background of the current OFMSW disposal, introduces the concepts of circular economy and anaerobic biorefinery and defines the objectives of the research. Chapter 2 and Chapter 3 are the core elements of the dissertation since they respectively present the study on substrate pretreatments and the study on the two-stage technology. In these chapters, the state of the art of the two lines of research, the materials and methods used, the results and their discussion are provided. Both investigations follow a scale up strategy. Laboratory batch tests play the role of preliminary experiments where process parameters are varied in order to find the optimum condition to be tested on pilot scale semi-continuous trials. Chapter 4 is the final section where conclusions and future developments of the research are stated. The dissertation summarizes the work of three years of research where laboratory equipment changed and evolved over time. The first year (2016) was dedicated to the study of substrate pretreatments. The second and the third year (2017 and 2018) were devoted to the study of the two-stage process and the composition of the doctorate thesis. This line of research was supported by the Bio2Energy project (Pecorini et al., 2017), a project funded by MIUR-Regione Toscana DGRT 1208/2012 and MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013 in sub-programme FAR-FAS 2014 (Linea d’Azione 1.1), which provided new equipment to deeply study the topic.