A technical review of bioenergy and resource recovery from municipal solid waste
Graphical Abstract
Introduction
Rapid industrialization and energy consumption go hand-in-hand with globalization and urbanization. The prosperity of a nation for economic development and urbanization often leads to compromising the natural environment. It is projected that the world population will increase from the current 7.7 billion to 8 billion by 2030 (United States Census Bureau USCB, 2020). With the rise in the global population, the basic needs of survival are also indispensable, which includes food, water, air, shelter, clothing, energy source and lifestyle requirements such as fuel and electricity. There is an upsurge in the per capita energy consumption with a human birth eight seconds (Nanda et al., 2015). Fossil fuels (e.g. crude oil, gasoline, diesel, coal and natural gas) have dominated the global energy market since the industrial revolution. Fossil fuels have augmented the worldwide industrialization, but have also compromised environmental quality. The exploiting use of fossil fuels has led to an increase in greenhouse gas (GHG) emissions, air pollution, acid rain, smog, global warming and climate change.
Fawzy et al. (2020) have systematically reviewed some key approaches to abate climate change, which include conventional mitigation, negative emissions and radiative forcing geoengineering. Conventional mitigation technologies aim to mitigate CO2 emissions from fossil fuels by making a paradigm shift and adapting to nuclear power and carbon-neutral renewable energy sources (e.g. solar, tidal, wind and biofuels). On the other hand, negative emissions technologies intend to capture and sequester CO2 through the application of biochar, afforestation, reforestation, ocean fertilization, terrestrial weathering, ocean alkalinity enhancement, wetland restoration as well as direct air carbon capture and storage strategies. On the contrary, radiative forcing geoengineering strategies to curb climate change by regulating the earth’s radiative energy budget and stabilizing global temperatures. This approach includes some notable techniques such as stratospheric aerosol injection, marine sky brightening, space‑based mirroring, surface‑based brightening and Cirrus cloud thinning, to name a few. A detailed description of these aforementioned topics can be found in the review by Fawzy et al. (2020).
Every year, the energy consumption and urban population augment by 1.1% and 1.5%, respectively ([International Energy Agency IEA, 2007], [Kumar and Samadder, 2017]). Population growth, rapid urbanization and economic development have significantly increased the generation of municipal solid waste (MSW). Municipal solid waste is also known as garbage, thrash and refuse refers to the solid wastes discarded by the public daily. Currently, about 1.9 billion tonnes of MSW are produced every year on a global scale, out of which nearly 30% remain uncollected by municipalities' waste management systems (Waste Atlas, 2018). However, the generation of MSW is expected to escalate to 3.4 billion tonnes by 2050 (The World Bank, 2020). According to The World Bank (2020), an average individual generates nearly 0.74 kg of solid waste footprint every day. After the collection of MSW, approximately 70% ends up in landfills, 19% is recycled and 11% is used for energy recovery. The effective management of MSW is categorized under two of the United Nations Sustainable Development Goals (SDGs), namely Goal 12 (Responsible Consumption and Production) and Goal 11 (Sustainable Cities and Communities) (United Nations, 2020). However, an additional goal (i.e. Goal 7: Affordable and Clean Energy) can also be achieved if the MSW is diverted for waste-to-energy conversion.
The composition of MSW varies greatly among different municipalities worldwide, but it typically comprises of both biodegradable and non-biodegradable materials from organic and inorganic sources. MSW is collected from households, offices, institutions and commercial enterprises, which typically include organic wastes (e.g. kitchen waste and yard waste), paper, plastics, glass, metal and miscellaneous garbage (e.g. electronic waste, inert materials, pharmaceuticals as well as construction, demolition and renovation wastes) as summarized in Table 1. The management of MSW varies within municipalities, cities, states and countries. The basic platforms in MSW management are: (i) generation of wastes at the source; (ii) collection and transfer of waste; and (iii) disposal, processing and treatment of wastes (Fig. 1).
MSW is one of such waste resources that require immediate attention to effective management techniques. Moreover, MSW is a valuable, renewable and economical waste resource that can recover usable solid, liquid and gaseous fuels to supplement the amplifying energy demands. The composition and management of MSW vary largely from municipalities to countries. Fig. 2 illustrates the composition of MSW on a global scale. Waste-to-energy is an economically viable and environmentally sustainable solution to recover energy from waste resources in the form of fuel, heat and electricity. It is indispensable for realizing the true potential of WtE conversion pathways to address the concerns of solid waste management and alternative energy generation. Typically, the WtE technologies are selected based on MSW composition, seasonality, communal socio-economic levels, local municipality policies, economic assessment and environmental impacts (Moya et al., 2017).
The available WtE technologies for MSW can be chiefly classified into thermochemical and biological processes (Fig. 3). Depending on the thermochemical conversion technologies, the major fuel products can be bio-oil, producer gas, synthesis gas and char. On the contrary, biological conversion processes include anaerobic digestion and composting that produce biogas (i.e. biomethane) and compost (i.e. decomposed organic matter). Some next-generation biological conversion processes that could be applied to the solid organic fraction and leachate of MSW to produce biohydrogen are dark and photo-fermentation, direct and indirect bio-photolysis, microbial electrolysis cells as well as microbial electro-hydrogenesis cells ([Zhen et al, 2016], [Allegue et al, 2020], Kamaraj et al., 2020, [Osman et al, 2020], [Sarangi and Nanda, 2020]). The chief WtE technologies applied for MSW are conventional incineration and hydrothermal incineration and oxidation. Opposed to landfill disposal, these technologies have the candidacy to significantly reduce GHG emissions and transform MSW to usable fuels to energy products.
There is a lack of standard methods to assess the effective energy recovery and environmental impacts of different waste-to-energy (WtE) conversion technologies. Certain properties of MSW can aid in determining its candidacy for either thermochemical or biological WtE conversion technology. For example, a higher proportion of moisture-containing organic fraction in MSW (i.e. kitchen waste, food waste and yard waste) is found to be suitable for biological conversion technologies (e.g. anaerobic digestion and composting) as well as hydrothermal technologies (i.e. subcritical/supercritical water-assisted liquefaction, gasification and incineration). The biological conversion pathways involve the use of microorganisms and their enzymes to decompose the organic fraction of MSW for conversion to biomethane, biohydrogen, compost and digestate. The feedstock of biological conversion must be prepared considering the organic content, dry solids, volatile solids, carbon/nitrogen (C/N) ratio, moisture, micro-nutrients and macro-nutrients for microorganisms without any contaminants (e.g. pesticides, insecticides, disinfectants, antibiotics, pharmaceuticals, inert materials such as glasses, plastics and metals). Based on the metabolism of the responsible microorganisms (i.e. aerobic, anaerobic, photosynthetic or dark), the appropriate biological conversion pathways such as composting, anaerobic digestion, bio-photolysis, dark fermentation and photo-fermentation are determined.
On the contrary, MSW containing recalcitrant organic components (e.g. paper waste, packaging boxes and cardboards) and non-biodegradable organics (e.g. plastics, rubber, polymers and tires) are more suitable for thermochemical conversion technologies (e.g. pyrolysis, liquefaction and gasification). Some physicochemical properties of broadly heterogeneous MSW, which can help access their WtE valorization are proximate composition (i.e. moisture, volatile matter, fixed carbon and ash), ultimate composition (i.e. carbon, hydrogen, nitrogen, sulfur and oxygen), thermal stability (i.e. devolatilization pattern), elemental composition (micro- and macro-elements), particle size, porosity, bulk density and biopolymeric composition (i.e. cellulose, hemicellulose, lignin, fatty acids, proteins, lipids, amino acids and salts), to name a few.
The WtE technologies applied to MSW management can contribute to the circular economy for offsetting the environmental risks and impacts and GHG emissions, while at the same time, increasing waste recycling as well as resource and energy recovery rates. Farrell et al. (2020) reported a critical analysis of closed-loop recycling and open-loop cascading options of the circular economy, which are primarily used to evaluate the environmental impacts in waste management. Through the closed-loop recycling of MSW, the main products and by-products can enter its supply chain with wide-ranging applications and marketability. In contrast, open-loop recycling of MSW can be performed when the main products do not meet the quality requirement for direct commercial applications but can find utility in alternative industries. The cascade option in open-loop recycling refers to the post-processing of the product before it can enter into other industrial processes as a precursor to undergo re-designing and engineering for opportunistic recovery.
There is a growing interest in effectively and safely managing municipal solid waste in nexus with its efficient valorization. According to the Scopus database, in the last ten years (2010–2020), more than 13,000 technical documents (e.g. articles and reviews) were published in indexed journals on “municipal solid waste” mentioned in the article title, abstract and keywords (Scopus, 2020). However, only 240 articles were indicated during the same decade with “municipal solid waste to energy” as the search option. Although tremendous amounts of MSW are generated in both developing and developed countries, there is a lack of technology transfer between the nations. Based on this notion, the current paper intends to comprehensively review MSW as a potential resource for alternative energy generation across the world. This paper attempts to systematically review the incineration as a popular WtE technology as well as different thermochemical and biological conversion technologies. These MSW treatment technologies have been technically evaluated to identify the opportunities, challenges and barriers to effective implementation and sustainable waste management.
Section snippets
Conventional incineration
Incineration is the most common waste treatment technology dealing with the combustion or burning of organic waste materials. Incineration is referred to as a traditional and widely used WtE technology that recovers high-temperature heat from the combustion of waste materials for combined heat and power plants. The heat released through the incineration of wastes can also be used to generate electricity. Incineration of MSW has been a preferred alternative to landfills that are malodorous,
Anaerobic digestion or biomethanation
Biogas or biomethane is produced by the natural or induced decomposition of organic matter under anaerobic conditions. The anaerobic digestion or biomethanation of biodegradable materials takes place in the absence of oxygen with the aid of anaerobic microorganisms (Prajapati et al., 2018). The process produces combustible biogas and stabilized biosludge. Wide varieties of biodegradable feedstocks that can be used for anaerobic digestion are MSW, cattle manure, sewage sludge and agricultural
Conclusions and perspectives
The adoption of MSW as a candidate feedstock for alternative fuel production can not only reduce the dependency on fossil fuels but also provide new ways for their eco-friendly remediation. Table 3 summarizes the advantages and disadvantages of different WtE technologies potentially applied to MSW. Pyrolysis and liquefaction of MSW can produce energy-dense bio-oil, whereas gasification results in H2-rich syngas. Char, a solid residue from pyrolysis and gasification, is rich in stable aromatic
CRediT authorship contribution statement
Sonil Nanda: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Roles/Writing - original draft, Writing - review & editing. Franco Berruti: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), Mitacs and the City of London, Ontario for funding this research.
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