Figure 1. Trends of the Scientific Production (Zheng et al., 2014)
This article provides a summary of the strategies currently in use to extract energy from municipal solid waste. Special trash is ignored since its characteristics greatly vary from type to type and from region to region. However, municipal solid waste has properties that make it amenable to a more standardized investigation of its energy potential. The initial part examines the methods for extracting usable energy from garbage. The second part discusses energy availability in the context of the changing qualitative and quantitative characteristics of Municipal Solid Waste (MSW). The third section takes a look at selective collection and how it serves the larger goal of energy recovery. In the fourth part, the significance of Directive 1999/31/CE and its effect on energy recovery strategies are discussed in the context of the current trend in the use of residual municipal solid waste. A mandatory pre-treatment makes less interesting the option of landfilling, moving the energy exploitation of residual municipal solid waste towards thermal treatments. Also, research shows how thermal treatments and anaerobic digestion could work together to provide for a local resident's energy needs.
Production, treatment, management, and disposal of MSW have all been the topic of substantial research and discussion from a wide range of perspectives and locations around the world Tomic et al., 2017). When a country decides to optimize its environmental management, it attracts more attention. Recently, the importance of the link between Municipal Solid Waste (MSW) and energy has increased greatly for sustainable development, as indicated in the Rio Declaration from 1992 and updated in 2012 through the "Rio+ii20" Earth Summit. To get there, people need to reduce their use of primary resources generally, especially non-renewable ones, and increase their use of secondary resources (materials from recycling processes and leftover garbage).
To meet the 2020 EU goals, 50% of a country's energy needs must come from renewable sources, with biomass making up the other 50%. According to Directive 2009/28/CE, the organic fraction of MSW and the biogas produced and consumed from it are now considered biomass and energy from biomass, respectively (Murphy & McKeogh, 2004).
From extraction (mine waste) through production and distribution (industrial, hazardous, and packaging waste), consumption (MSW, waste from electric and electronic equipment), and treatment, waste is always created as the cheapest resource (slag and ash).
There is a shift from a landfill-based concept in which the sanitary landfill is viewed as a bioreactor to a reactorbased scenario in which the anaerobic digester permits the collection of 100% of the biogas generated and selective collection of food waste can alter the strategy for utilizing biogas.
Special trash is not taken into account because its characteristics vary greatly depending on location and type. The energy potential of MSW, on the other hand, may be studied more uniformly on a worldwide scale. Municipal Solid Waste (MSW) can be used in a variety of facilities for the generation of thermal and electric energy as well as biofuels after pretreatment. Research into the potential energy in trash has become increasingly important in recent decades. Scopus gives the total number of publications in the last 30 years, including those on MSW and energy (Tsai & Kuo, 2010; Zheng et al., 2014). The graphs depicting the trends in scientific production are shown in Figure 1.
Figure 1. Trends of the Scientific Production (Zheng et al., 2014)
For a long-time, municipal solid waste was simply "stuff to be disposed of," but after the European Union passed laws like Directives 74/224, 75/442, and 94/62 CE, MSW was finally recognized as a valuable resource. As a result of these laws, MSW must now be minimized, reused, recycled, and recovered. This research explains how the last two ideas have direct applications to the energy producing industry (Beylot & Villeneuve, 2013).
Considering the evolution of waste-to-energy solutions over the past decades, this paper aspires to contribute to the spread of accurate information about MSW valorization from an energy perspective.
Tomic et al. (2017) depict gate-fee changes of the wasteto- energy plants being investigated in the conditions set by European Union legislation and by the introduction of the new heat market. Waste management and sustainable energy supply are core issues in the sustainable development of regions, especially urban areas. These two energy flows logically come together in the combined heat and power facility through waste incineration.
Murphy and McKeogh (2004) four technologies are investigated that produce energy from Municipal Solid Waste (MSW) namely incineration, gasification, generation of biogas, and utilization in a Combined Heat and Power (CHP) plant, followed by conversion to transport fuel. Typically, the residual component of MSW (non-recyclable, non-organic) is incinerated, producing electricity at an efficiency of about 20% and a thermal product at an efficiency of about 55%.
Tsai and Kuo (2010) gave a concise summary of the current status of domestic energy consumption and power generation, MSW generation and MSW incineration treatment, and electricity generation from MSW incineration plants since 2000. Based on the electricity generation in 2008 (i.e., 2967 GWh), the environmental benefit of mitigating CO2 emissions and the economic benefit of selling electricity were preliminary calculated.
Zheng et al. (2014) illustrate the current status of MSW management in China. The application of landfill gasfired and MSW incineration power generation is then analyzed in detail. Meanwhile, the present situation of Clean Development Mechanism (CDM) projects converting MSW to energy is presented. In addition, a series of preferential policies and regulations are offered to encourage the expansion of the use of MSW for energy, especially MSW incineration.
Beylot and Villeneuve (2013) compare the environmental performances of 110 French incinerators (i.e., 85% of the total number of plants currently in operation in France) in a life cycle assessment perspective, considering five nontoxic impact categories namely, climate change, photochemical oxidant formation, particulate matter formation, terrestrial acidification, and marine eutrophication. The mean, median, and lower and upper impact potentials are determined considering the incineration of 1 ton of residual French municipal solid waste.
First and foremost, the energy content of each Municipal Solid Waste (MSW) fraction needs to be considered in the MSW waste-to-energy schemes. Each of these components' Lower Heating Value (LHV) is listed. These numbers are helpful for setting up multiple-scenario balances to prevent issues with waste that should be disposed of in a landfill. LVH should be less than 13 MJ/kg, and the Respirometric Index (RI) should be less than 1,300 MgO2 kg-1TS/h. The LHV of Municipal Solid Waste (MSW) is about 6-7 MJ/kg in regions with a low to medium standard of living. But when economic growth has led to a rise in the amount of lightweight packaging found in MSW, the resulting LHV can be as high as 13 MJ/kg. South Carolina (SC) needs to be considered as well for the whole scenario. Quantitative analysis shows that switching from a SC scenario with an efficiency of 35% to one with an efficiency of 65% results in a nearly 50% reduction in the flow of Residual Municipal Solid Waste (RMSW). When this occurs, it is necessary to create a thorough evaluation of the many MSW treatment alternatives (López-González et al., 2014). Figure 2 shows the bar diagram of the LHV of each MSW fraction.
Figure 2. LHV of Each MSW Fraction (López-González et al., 2014
It is important to remember that efficient MSW management depends on measures that go against the prevailing pattern of increasing MSW output for per-capita production. Unlike how a worldwide economic catastrophe would impair people's ability to spend money and the consequent waste generation, this is not likely to happen. The details of the qualitative features must stand out (Lin et al., 2016).
The qualitative features has to do with a new subset of fractions with diapers. Their prevalence in Residual Municipal Solid Waste (RMSW), where their percentage may exceed even 10%, is a reflection of their increasing popularity in certain regions of the EU, where their usage is facilitated by the lack of SC for this proportion. Since the energy exploitability ratio decreases as SC increases, it is evident that the separation of waste fractions with a positive energy content reduces the energy exploitability ratio, which includes plastics, paper, and cardboard in particular. The data come from a real-life research conducted in Italy's northeast (Moon et al., 2015). Thanks to the implementation of European Union (EU) directives on waste management, SC efficiency in this area has increased considerably in the past 20 years. Figure 3 shows the bar diagram of the comparative analysis of energetic efficiency and SC efficiency between 1994 and 2012.
Figure 3. Comparative Analysis of Energy Efficiency and SC Efficiency Between 1994 to 2012 (Moon et al., 2015)
The proportion of organic matter in Residual Municipal Solid Waste (RMSW) is decreasing. This is because the SC of the organic fraction is activated, reducing its presence in the RMSW, and the prevalence of packaging is rising without necessarily leading to an increase in the SC of this fraction. In the most severe cases, where people are extraordinarily resourceful, the percentage of organic matter in RMSW can drop below 10% (Meng et al., 2015; Rajasekhar et al., 2015). It is important to note that numerous European countries, including the recently admitted EU members like Romania and Bulgaria, already have a considerable share of organic components in their RMSW.
The world generates about 2.01 billion tons of Municipal Solid Waste (MSW) annually, and at least 33 percent of that is not handled in an environmentally responsible manner. For example, India's first attempts at managing solid waste lacked an appreciation for the need for hierarchy. Waste production in India increased from 52,971,720 Million Tons (MT) in 2017–18 to 53,175,755 MT in 2018–19. However, solid waste management has seen consistent transformation over the past few years, and displays the annual waste management volume in India. However, more work remains before effective solid waste management practices are implemented (Rezaei et al., 2018; Appleby & Foulkes, 1993).
The situation has worsened because of the rise in garbage produced by a larger urban population. Mismanaged municipal solid waste is becoming a major issue. Garbage collection, storage, and transportation are the three main facets of solid waste management that need to be addressed. Waste segregation is becoming increasingly important since different waste components call for different management methods. India's major cities have sorted, collected, and processed their garbage since February 2020 (Tilahun et al., 2015).
Different states and cities in India experience different difficulties with the Municipal Solid Waste Management (MSWM) plan. To give only a few examples, recycling and sorting efforts are inadequate, there is no scheduled street cleaning, and primary collection is not performed at the front door. Nothing is recycled or separated, streets aren't swept regularly, trash is hauled by open tractors and trucks, very little is processed, and Municipal Solid Waste (MSW) is deposited in landfills without any prior investigation into its effects. The increasing amounts of MSW in India cannot be processed or disposed of due to a lack of appropriate infrastructure (Subramani & Murugan, 2014; Okafor, 2011). The Bar Graph represents the waste processing facilities for the various states across India and is shown in Figure 4.
Figure 4. Bar Graph Represents the Waste Processing Facilities Across India (Okafor, 2011)
This has led to the unrestrained disposal of trash on land resources, which has not only resulted in the creation of large rubbish mounds but also poisoned the air and tainted the groundwater, posing a threat to public health.
Over the last century, energy valorization in MSW management has seen a significant shift, with landfills, once considered a reactor for biogas production, now giving way to environmentally friendly thermal treatment as the primary waste-to-energy option. Enhancement of South Carolina (SC) efficiency served as a driving force in this progression. The importance of SC is discussed in the context of the energy value of MSW. The material flows can be taken into consideration. An in-depth examination of biogas's function is provided. Biogas is produced by anaerobic digesters that capitalize on the high quality of source separation that can be achieved through efficient citizen collaboration. The amount of biogas can reach 150 m3/t of food waste with about 60% methane content (Hoornweg & Bhada-Tata, 2012).
Residues from paper and cardboard recycling have a high energy content and can be exploited in authorized waste-to-energy plants, or they can be treated alone or with Residual Municipal Solid Waste (RMSW) to create Refuse Derived Fuel (RDF), which can then be valorized as industrial fuel. Residues from plastic recycling, on the other hand, produce a sizable secondary stream that cannot be easily valorized as material (Yong et al., 2019).
The paper provides a European Union perspective on the evolution of Organic Fraction of Municipal Solid Waste (OFMSW) exploitation over the last few decades. OFMSW greatly contributed to the biogas generation from a landfill when landfilling without pre-treatment was prominent in the industry. Only around 50% of its amount could be used due to fugitive emissions. The initiatives of OFMSW the SC were bolstered by the introduction of pretreatment as a mandatory option before landfilling through the aforementioned directive. The subsequent introduction of economic incentives for the generation of electricity from OFMSW food waste shifted the sector from direct composting to anaerobic digestion in dedicated reactors. It can ensure 100% biogas collection (Subramani & Murugan, 2014).
The same method was used for biogas after it has been cleaned for energy production using an engine connected to an alternator to produce both electricity and heat remains in use. In the next scenario, in which biogas treatment aims to produce methane to be injected into the natural gas grid or to be used as fuel for methane cars, is predicted to arise in the near future (Alzate et al., 2019; Agarwal & Kudapa, 2022).
The amount of energy available from Municipal Solid Waste (MSW) is contingent on numerous variables, including the energy content of MSW. The amount of energy diverted by SC (plastics, paper, cardboard, etc.), the amount of OFMSW sent to energy recovery through SC and the amount of energy content left in the RMSW. The energy efficiency of the process adopted for the valorization of RMSW, is basically the co-generation efficiency. There is a function for both OFMSW anaerobic digestion and RMSW thermochemical treatment, but it varies with factors such as regional energy needs. Reports suggest that a simplified review of the situation in a region of Northern Italy, home to roughly 500,000 people can shed light on this supposedly pivotal function (Kudapa, 2022; Mohan & Kudapa, 2021).
Further changes in MSW composition toward greater lightpackaging content occurred in the late 1990s; indeed, despite a higher SC that diverted some MSW energy, the total amount of RMSW energy increased due to increases in both MSW generation and Lower Heating Value (LHV).
While it may appear that SC has negative effects on energy recovery, this is actually an effect that has to do with the possibility of generating energy in plants specifically designed to process waste, and it is important to remember that SC can help conserve energy during the manufacturing process.
With an efficiency of 25% for RMSW combustion and 40% for OFMSW anaerobic digestion, estimates of their respective electrical generating potentials can be made. By refining the computation, it is possible to show that the MSW sector plays a substantial role, even if it is unable to meet the full demand, as has been shown in the most recent scholarly works. Heat availability from cogeneration is anticipated to be around 60% for RMSW combustion and its use in heating the reactor from anaerobic digestion, would be barely enough to meet the minimum requirements (Alzate et al., 2019).
This article examines on energy recovery demonstrating MSW's shift toward multi-stream administration, with the Organic Fraction of Municipal Solid Waste (OFMSW) and Residual Municipal Solid Waste (RMSW) offering the most concerning out-of-the-ordinary possibilities. In order to promote a clean SC stream into the anaerobic digesters, OFMSW relies on the active participation of its citizens. Composting digestate after post-treatment provides an opportunity to add carbon and other components to the soil, though the amount of vitality produced is lower than that accessible by RMSW. RMSW for energy is making strides, from coordinated combustion to alternative processes. This progress is associated with the evolution of the Solid Recovered Fuel (SRF) concept, which could supply a consistent and uniform input. Inadequate management of municipal solid waste (MSW) cannot meet the entire population's energy needs, but its contribution should not be overlooked either.