Evaluation of conventional and alternative anaerobic digestion technologies for applications to small and rural communities

Highlights

• Exploiting biogas in small communities is economically and technically challenging.

• Evaluating the spatial scale for waste collection and treatment is crucial.

• Food waste anaerobic digestion and post-composting were studied at different scales.

• Energy savings and carbon balance were analyzed considering two digestion processes.

• Conventional wet processes are preferable if the input waste flow is > 3500 t/y.

Abstract

The management of food waste has been considered an extremely important issue since the 1990s but finding efficient solutions for small and rural communities is still challenging. Anaerobic digestion (AD) may provide interesting opportunities in terms of carbon emissions and economic payback in the long term, but the choice of the correct technology and the spatial scale requires attention. The focus of this study is on a small rural municipality, which is selected as a case study to assess the environmental and economic sustainability of the application of two options for AD (a conventional and an alternative wet process) and two spatial scales (municipality and a consortium of municipalities). Both the AD configurations are examined in terms of biogas exploitation, through a combined heat and power generator, and in combination with a post-composting stage of the digestate. From economic and environmental perspectives, the consortium-scale application of the conventional wet process is expected to generate greater benefits in the long term, as it enables 80% more electric energy production and economic revenues/savings, and avoids carbon emissions. However, before selecting the technology, decision makers should consider the public acceptance of local communities (e.g., the susceptibility to the “not-in-my-backyard” syndrome), as the best technical-economical solution may not be the most appropriate to specific communities. The methodology developed in this paper and the discussion of the results will inform decision makers about how to identify the most appropriate alternative for their purposes.

Introduction

The Intergovernmental Panel on Climate Change (IPCC) reported in 2007 that the transition towards renewable sources of energy was unavoidable due to climate change and was a priority (IPCC, 2007, IPCC, 2013). The IPCC was ratified in December 2015 by the Paris Agreement. In the European Union (EU), climate change mitigation and the development of an energy union are stated to be priorities in the EU’s long-term strategy on the reduction of greenhouse gases (European Commission, 2020), which aims at limiting the mean temperature increase from that of the pre-industrialized era to 1.5 °C. To achieve this goal, the European Union has set various short-term targets. The European Directive 2009/28/EC (European Union, 2009) established that by 2020 at least 20% of energy consumption must come from renewable energy sources. In 2018, a new target was set by Directive 2018/2001/EC: renewable energy sources must constitute a minimum of 32% of all energy consumption by 2030 (European Union, 2018). The 2017 dataset (the most recent available) reveals that 17.5% of the energy demand of the 28 EU Member States (EU-28) is covered by renewable energy sources, rather than the target of 20%. Estimates for 2020 show that 34.3% of the electric energy demand in the EU-28 can be covered by renewable sources, although this percentage decreases when considering thermal energy (21.3%) and the demand of fuels in the transportation sector (10.3%) (European Renewable Energy Council, 2011). In 2017 renewable energy sources in Italy met 34.1%, 20.1% and 6.5% of the electric energy, thermal energy and fuels demands, respectively, (Gestore Servizi Energetici, 2017).

As an emerging renewable energy source, biogas valorization has increased over the last decade in Europe, and the number of plants increased from 6,227 in 2009 to 17,432 in 2017, while the installed electric energy capacity increased from 4,128 MW to 9,985 MW during the same period, contributing to 6.5% of the electric energy produced from renewable sources (European Biogas Association, 2019). In Italy, 2,116 biogas plants were operative in 2017, which treated agricultural and forest residues (49%), zootechnical waste (28%), municipal solid waste (19%) and sewage sludge (4%) (Gestore Servizi Energetici, 2017). The biogas sector accounts for 7.8% of the national electric energy production from renewable sources and 43% of that from bioenergy sources, i.e., biogas, solid biomass, bioliquids, biomethane and biodegradable waste in landfills (Gestore Servizi Energetici, 2017).

Public incentives in Italy were introduced in 2018 through a decree (Italian Republic, 2018) focusing on the development of biomethane production, aimed at achieving specific European targets for the transport sector, including a minimum proportion of 10% to be derived from biofuels by 2020 (European Union, 2009). This target was modified to 14% to be achieved by 2030 by a recent Directive (European Union, 2018). The anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) plays a fundamental role in the achievement of these targets.

AD is a biodegradation process carried out by microorganisms in the absence of oxygen. AD is a growing sector with a total biogas capacity of 15 GW worldwide, of which 10 GW in Europe (Scarlat et al., 2018). AD is highly sensitive to temperature, as it controls the microbial growth rate, the kinetics of the process and biogas production (Panigrahi and Dubey, 2019). Thermophilic conditions enable a higher biogas yield, a lower hydraulic retention time (HRT) and thus a smaller plant size (Schiavon et al., 2018), while mesophilic conditions allow for more stability in the process, a lower energy demand and result in a digestate of higher quality (Panigrahi and Dubey, 2019). The AD can be classified as a dry (60–75% water) or a wet (85–90% water) process, depending on the moisture content. The dry process has clear advantages in terms of digester volume, water consumption and the production of wastewater. However, wet processes result in higher methane productivity, lower mixing and pumping costs, and are able to dilute peak concentrations of substrate and inhibitors (Angelonidi and Smith, 2015, Rocamora et al., 2020), which makes them advantageous in small-scale applications. The AD process can also be affected by the organic loading rate (ranging from 1.2 to 12.0 kgVS/m³/d), the pH in the digester (recommended to be in the range 6–8), the carbon-to-nitrogen ratio (generally between 20 and 30) and the HRT (15–20 d) (Zhang et al., 2019).

The level of biomethane production in Europe in 2017 was estimated to be 1.2 billion m³ from 414 biomethane plants, from a total biogas production level of 18.2 billion m³ (Scarlat et al., 2018). However, biomethane production requires major investments in biogas upgrading equipment, and thus it is only feasible for medium-large installations. In a recent study, it was estimated that a plant with biogas production of 500 Nm³/h would have an investment payback time of about six years, when also accounting for public incentives (Barbera et al., 2019). Biogas upgrading in smaller plants would therefore be economically disadvantageous.

Thus, the technological limitations of small-scale waste production for use in biogas plants must be considered in the energy management conducted by local municipalities when attempting to achieve the targets of the Paris agreement. The Covenant of Mayors (CoM) is an EU initiative focusing on the actions local communities can take to achieve the goals of this agreement, and represents the largest organization involved in local climate and energy activities (Pablo-Romero et al., 2015, Adami et al., 2019a). CoM is part of the Global Covenant of Mayors (GCoM) for Climate and Energy, which is aimed at bringing together local municipalities that voluntarily sign a Sustainable Energy and Climate Action Plan (SECAP) to implement short-term (2030) EU climate and energy objectives and middle-term (2040 and 2050) targets (Abarca-Alvarez et al., 2019).

Although primary energy production in the EU has increased since 1991, with a total production that surpassed 600 PJ in 2014 (Scarlat et al., 2018), only 23 biogas plants are regarded as good practice by the CoM, from a total of 6,800 + good practices that cover nearly 6,400 Action Plans. Of these 23 plants, 14 (over 60%) use crops or sewage, 3 use wood, seaweed and waste from the food industry, respectively, and only 6 plants transform waste or water waste into biogas. All of the plants are located in cities or territories with >100,000 inhabitants, but the potential of biogas plants as local renewable sources of energy has only been partially analysed (Adami et al., 2019b).

Food waste management is considered an extremely important social and environmental issue (Loizia et al., 2019) and has presented challenges since the ‘90s (Lettinga et al., 1993), but identifying efficient solutions for small and rural communities in modern and developing countries (Regattieri et al., 2018, Thiriet et al., 2020) has proven difficult. Rajendran et al. (2012) reviewed various digester configurations, construction materials, and parameters such as pH, temperature, substrate, and loading rate, and biogas production for digesters ranging from 1 to 150 m³. Regattieri et al. (2018) focused on simple and feasible biogas micro-production solutions for developing countries and humanitarian camps, and Thiriet et al. (2020) proposed an optimization method for locating micro-AD in the Grand Lyon Metropole region of France. However, few studies investigate the susceptibility of the local community to the so-called “not-in-my-backyard” (NIMBY) syndrome, which is another important criterion in the selection of the best AD configuration, and the NIMBY phenomenon has been examined more thoroughly in terms of waste incineration than it has for AD (Cossu, 2009, Baxter et al., 2016).

Thus, this study aims to provide a comprehensive overview of the advantages of selecting biogas production from waste as a strategy for complying with the sustainability targets of the CoM initiative. The environmental and economic feasibility of two AD options for a small municipality that participates to the CoM initiative are assessed, with the aim of identifying the best solution for local-scale waste management. A conventional wet AD process and an alternative wet AD process that uses the liquid proportion of waste extracted through a specific pre-treatment (Sibisi, 2006) are compared in terms of balancing energy, environmental and economic factors. The alternative configuration was selected as it has a lower surface occupation and digester volume than the conventional wet process. This may prove to offer a level of social validity, as a smaller digester could be considered as the local community’s “own” digester. The negative connotation of the NIMBY syndrome could then be transformed into the “beauty-in-my-backyard” (BIMBY) concept (Lu et al., 2019). However, the new concept of “just-in-my-backyard” (JIMBY) (i.e., a plant representing a source of pride for the local community) is also positive and may be more appropriate in this context.

Both the alternatives will be combined with a post-composting process, which enhances the environmental sustainability of food-waste management (Rincón et al., 2019, Al-Rumaihi et al., 2020) and increases the economic efficiency of the whole process (Babalola, 2020). The two alternatives will be applied to the locally produced OFMSW and analyzed to estimate the energy production from local biogas combustion, quantify the avoided and generated emissions of greenhouse gases (GHGs), and to evaluate the investment and operating costs involved and the economic profit/savings. Two spatial scales will be considered for each alternative to assess whether the alternatives proposed are more suitable to a municipality or a larger scale (e.g., a consortium of municipalities). In addition to providing a more complete picture of the opportunities AD can provide in the CoM initiative, the potential GHG emissions/savings from the selected options are analysed in detail, which have not previously been fully examined in the literature. The effects of indirect emissions related to the management of the OFMSW, i.e., GHG emissions from road transport, are also analysed.

In this research a specific case study is considered as an example of the application of different AD strategies, but the developed methodology can also be applied to other contexts and can help decision makers evaluate the most appropriate opportunity for their specific cases. To generalize the results and make them applicable to other contexts, the calculations were applied to an extended range of input OFMSW, from 500 t/y to 50,000 t/y.