WASTE BIOREFINERIES: OPPORTUNITIES AND PERSPECTIVES

The progressive implementation of the circular economy concepts, the population growth and associated concerns in terms of availability of non-renewable resources, the will of many countries to diversify their strategic sources and to free themselves from the supply of materials and energy resources from areas politically and socially unstable, the fight against climate change, the need to favor delocalization of production systems and promote regional and rural development, the improvement of the knowledge of the factors that govern biological processes, entail that the context of biowaste management currently looks at more ambitious and articulated targets which find the most appropriate and complete synthesis in the concept of waste biorefinery. The potential inherent in biorefineries is huge. The global industrial production of organic chemicals accounts for a major share of the overall global chemicals industry and is estimated to amount, excluding fuels, to more than 300 Mt/year; the associated market was worth over 6 billion $ in 2014 and grew at an average of 8% per year from 2009 to 2014. The primary outputs of the chemical industrial activity are represented by a relatively limited number of building blocks used to produce a plethora of end products such as food and beverages, pharmaceuticals, pesticides, agrochemicals, water treatment, crop protection, personal care products and cosmetics, fertilizers, automotive industry, gasoline additives, polymers and chemicals, etc. Each building block can also be obtained from biomass, enabling the supply of raw materials at the local level, releasing the industrial activity from expensive and risky supplies, and opening the door to economic sustainability even in disadvantaged contexts such as, for instance, the insular ones. The demand for bioproducts from renewable sources is estimated to reach, depending on more or less favorable market conditions, 26-113 Mt/year in 2050, which would correspond to 38 and 17% of the total organic chemicals production, respectively; the associated market should account for some 7-8 billion $, with a growth rate of 15%/year which could further benefit from the increasing demand for biopolymers (IEA Bioenergy - Task 42 Biorefinery, 2012). The concept of biorefinery is not new in its more traditional meaning, and has evolved over time driven by three pivotal aspects (Akhlaghi et al., 2016): - cascade approach; - environmental sustainability; - economic sustainability. The cascade approach involves the flexible integration of different processes aimed at producing a mix of biofuels and bioproducts. The integration of processes and products according to the traditional or inverse cascade, is basically linked to economic sustainability, which requires an appropriate mix of products characterized either by significant market sizes - typical of biofuels - or high added values, but also to environmental aspects. In fact, as the number of usable and marketable outputs increases, this would logically correspond to less waste production, thus approaching the zero waste concept. The improvement in environmental sustainability is the main element underlying the hypothesis of transition towards a new generation of biorefineries: waste biorefineries. The environmental sustainability of the first and current biorefinery generations was, and still is, linked mainly to benefits related to the reduction of the consumption of non-renewable resources and CO2 emissions. To this respect, it is estimated that the production of a large share of synthetic organic compounds from renewable resources could lead to a global reduction of CO2 emissions ranging between 400 and 1000 Mt/year. The use of residual biomass would bring further environmental benefits: - first, the environmentally sound management of residues through their valorisation; - waste biomass should not be grown/bred, leading to a reduction in production costs; no biomass for food use would be treated nor areas that could be dedicated to other uses would be occupied; - the economic budget would benefit also from the waste treatment fees and short supply chain, besides the sale of the obtained bioproducts; - the different environmental and economic dynamics that would characterize waste biorefineries could make sustainable process schemes characterized by greater simplicity and smaller plant size as compared to traditional biorefineries. However, despite the potential advantages highlighted above, it is not conceivable that all the technological and economic perspectives associable to traditional biorefineries could be fully extended to waste biorefineries, if only due to the nature of the residual biomass, which would be, in most cases, qualitatively more heterogeneous and quantitatively less controllable. Therefore, the following challenge awaits environmental researchers and technicians: is the biorefinery concept feasible for waste management? and to what extent? A univocal answer probably does not exist. In contexts where large traditional biorefineries were available, the search for synergies could probably lead to consider the waste biomass as a secondary inflow that may contribute mainly to the recovery of products characterised by lower added value (energy, energy carriers, biofuels). Conversely, where the organic waste is the main biomass to be treated, the question would be whether and under what circumstances (availability and type of residues, market conditions, etc.) process schemes were applicable which, though lacking the complexity and the articulation of treatment steps/final products achievable in a traditional biorefinery, move away from current relatively simple valorization options, such as the recovery of biogas/biomethane and composting, to approach the concept of industrial biorefinery. Indeed, though some types of organic waste contain appreciable quantities of substances whose value may reach even 15,000 EUR/g - or are suitable to be converted in valuable building blocks (e.g. lactate) - thus being worth exploiting in relatively small dedicated plants where extraction of compounds and energy recovery from the resulting residues are performed, the minimum size that is considered sustainable from an economic point of view, the qualitative/quantitative characteristics of the waste biomass to be treated, the applicable processes, and the recoverable products, are still subjects of debate. Traditional biorefineries are meant to require rather large plants; it is well aknowledged that the minimum size to ensure economic sustainability is about 500,000-700,000 t/year (Kuchta, 2016). The use of waste biomass may lead to lowering this threshold, provided that the benefits in terms of simpler process schemes and revenues from waste treatment fees exceed the drawbacks related to the anticipated poorer quality of the final outputs and less ambitious market targets. However, under most of the circumstances, the less stringent requirements in terms of minimum plant size would not be such as to allow for a process based on the treatment of a single type of residue. Therefore, in the current state of affairs waste biorefineries should probably co-treat different types of residues, either of municipal origin or deriving from production activities, with all the advantages and problems which are associated to co-treatment, as pointed out by the experience gained with reference to anaerobic digestion. As for the processes to be applied, it is important to emphasize the pivotal role that fermentation would play in a waste biorefinery scheme, due to its ability to hydrolyze and simplify the organic substance and convert it to marketable products or building blocks. Indeed, the number of building blocks attainable through fermentation is remakable and current global production of fermentation products accounts to more than 8 Mt/year with an associated market of more than 20 billions $ (IEA Bioenergy - Task 42 Biorefinery, 2012). However, fermentation is a complex process, in particular when applied to substrates which are heterogenous and contain indigeneous microorganisms, and strongly depends on numerous and interconnected factors such as substrate chemical composition, concentration and pretretament methods, presence/type of inoculum and eventual pretreatment, inoculum- to substrate- ratio, reactor type and operation regime, applied operating conditions (e.g., pH, hydraulic and cell residence time, temperature, organic loading rate, etc.) (Alibardi and Cossu, 2015; De Gioannis et al., 2013; Dionisi and Silva, 2016; Ghimire et al., 2016). Therefore, there is still a strong claim for better understanding the complex interrelations among the relevant factors and, in turn, predicting the evolution of the process and optimizing its performance when has to be applied to residual biomass. As far as the obtainable products are concerned, if one looks at the current conditions in several European countries, characterized by strong incentives to the production of biomethane, a possible simplified and readily applicable waste biorefinery scheme could consist of an anaerobic process performed according to two stages, with the first one properly managed in order to recover H2 + CO2 from fermentation, besides the CH4 + CO2 mixture produced in the second stage. Both mixtures should be refined to recover biohydrogen and biomethane which could be then used individually or as a mixture (hythane). The separated CO2 could be reused or marketed or, alternatively, if the H2 use were not well established yet, both could be fed to reactors where carbon dioxide is biologically reduced in order to increase the overall biomethane production. Possible alternatives could include the commercialization of the organic acids-rich solution produced in the fermentative hydrogenogenic stage, or its further and innovative valorization as readily biodegradable substrate for biopolymers production or to be fed to microbial electrochemical systems (MES), known also as bioelectrochemical systems (BES), with production of electric energy (microbial fuel cells - MFC), or further biohydrogen or hydrogen peroxide or caustic solutions (microbial electrolysis cells - MEC), or even synthesis of organic compounds (microbial electrosynthesis - MES).