A Review on Characteristics, Techniques, and Waste-to-Energy Aspects of Municipal Solid Waste Management: Bangladesh Perspective

: Municipal solid waste (MSW) management has become a major concern for developing countries. The physical and chemical aspects of MSW management and infrastructure need to be analyzed critically to solve the existing socio-economic problem. Currently, MSW production is 2.01 billion tonnes/yr. In developing countries, improper management of MSW poses serious environmental and public health risks. Depending on the socio-economic framework of a country, several MSW management procedures have been established, including landﬁlling, thermal treat-ment, and chemical treatment. Most of the MSW produced in underdeveloped and developing countries such as Bangladesh, India, and Pakistan is dumped into open landﬁlls, severely affecting the environment. Waste-to-Energy (WTE) projects based on thermal treatments, e.g., incineration, pyrolysis, and gasiﬁcation, can be feasible alternatives to conventional technologies. This research has explored a comprehensive method to evaluate MSW characteristics and management strategies from a global and Bangladesh perspective. The beneﬁts, challenges, economic analysis, and comparison of MSW-based WTE projects have been analyzed concisely. Implementing the WTE project in developing countries can reduce unsupervised landﬁll and greenhouse gas (GHG) emissions. Alternative solutions and innovations have been discussed to overcome the high capital costs and infrastructural deﬁciencies. By 2050, Bangladesh can establish a total revenue (electricity sales and carbon credit revenue) of USD 751 million per year in Dhaka and Chittagong only. The landﬁll gas (LFG) recovery, waste recycling. and pyrolysis for energy production, syngas generation, and metal recovery are possible future directions of MSW management. The MSW management scenario in developing countries can be upgraded by improving waste treatment policies and working with government, academicians, and environmentalists together.


Introduction
Solid wastes are generally discarded, rejected, abandoned, unwanted, or surplus matter of urban, rural, and industrial streams [1,2].The Resource Conservation and Recovery Act (RCRA) is a federal statute that establishes guidelines for managing solid wastes which are both hazardous and non-hazardous [3].Solid wastes can exist in a variety of physical states, e.g., liquid, semi-solid, and trapped gaseous substances.MSW constitutes product labels, yard waste, furniture, textiles, plastic containers, foodstuffs, papers, electronics, gadgets, cells, etc. [4].The rapid increase of MSW has become a significant obstacle worldwide in keeping the planet pollution-free with the pace of urbanization.2.01 billion metric tonnes of municipal solid garbage are generated yearly across the globe.The global annual MSW generation rate is expected to increase to 2.59 billion tonnes by 2030 and 3.40 billion tonnes by 2050 [5].At the most conservative estimate, 33% of it is not handled in an ecologically sustainable way [6].The percentage is even higher in underdeveloped and developing countries [6].The MSW generation ranges from 0.11 to 4.54 kg/day, which varies with geography and economic development.Waste generation in developed and developing countries is projected to experience an increase of 19% and 40%, respectively [7].South Asia and East Asia (Pacific) will experience the highest growth of MSW in recent years [8].In the South Asia region, the total MSW generation is estimated to increase more than double of current.
The heterogeneity and complexity of MSW composition are causing great difficulty in the sustainable disposal of this enormous amount of waste that also causes many economic losses and poses drastic impacts on the environment and human health [9].The MSW management is maintained very inefficiently in developing and underdeveloped countries, which is linked to the fragmentation of various MSW functions with limited cooperation among stakeholders, weak governance, institutional structures, and management capabilities [10].The prevalent challenges facing these countries reported by several researchers include the use of scattered and low collection coverage, primitive treatment technologies and disposal methods, e.g., crude open dumping and burning without air and water pollution control, lack of legislations and human resources deficiencies.Limited public awareness on proper waste management is also responsible in this case [11].Open landfill dumping, the most common practice in South Asia, has a significant social, environmental, and economic impact, and thus the situation requires urgent action [12].The developed countries dispose of a substantial portion of the MSW in landfills.In 2020, the combined capacity of the two largest landfill corporations in the USA was 9.98 billion cubic yards.However, the environmental impacts of landfill disposal include loss of land area, emissions of GHGs to the atmosphere, and potential leaching of hazardous materials to groundwater, though proper design reduces this possibility.
MSW collection is a crucial step in waste management.While developed countries follow formal waste collection practices, underdeveloped and developing economies mostly rely on informal waste collection [13].High-income countries collect 96% of the MSW, whereas, in low-income countries, it is only 39% [14].In developing countries, waste collection machinery and equipment are outdated, used without proper maintenance, and suffer from breakdowns [15].Alagöz and Kocasoy pointed out that these aspects constitute 80-95% of the total budget of MSW management in developing countries, while in developed countries, the percentage cost is less than 10% because of good financial management and planning [16].
Developed countries have implemented many projects aimed at comprehensively addressing the efficiency of the SW management systems [17].In developing and lesser developed countries, this has not been the case.As a result, there has been challenging to improve the management efficiency and address the related environmental problems [18].Suitable technology for MSW management and processing, e.g., landfill dumping, thermal and chemical process selection, depends on the waste composition, available technologies, and socio-economic structure of the countries [19].In high-income countries, a large portion of MSW is dry waste (plastic, paper, metals, and glasses), which is easily separable and recyclable.But in low-income countries like Bangladesh, India, Pakistan almost 57-60% of MSW is food/agriculture-based waste, which increases the organic portion in MSW.The recyclable materials in the MSW of low-income countries are only 20% of total waste [20].Therefore, selecting suitable technologies for MSW management is crucial for developing counties.However, technology is not the only factor to consider in MSW management [21].In developing countries, the concept of controlled landfills, the proper utilization of waste recycling, and WTE-based project are under development and need to be implemented [22,23].
In Bangladesh, the current MSW management practice relies entirely on informal waste collection, non-regulatory open disposal, and unestablished recycling businesses under insufficient MSW-based legislation.[24].The discrepancies in MSW management incur several limitations, including low resource/budget allocation, little to no practice of technical advancement in waste management, and lack of coordination among nongovernment, local, and central Government [6].If this situation continues, Bangladesh will experience the worst consequence of an insufficient MSW management system in recent decades [22].The potential of MSW to convert it into a valuable resource is still unrevealed in Bangladesh.Since the last decade, the developed and developing countries have been facilitating WTE projects and converting MSW into natural gas (NG), syngas, biofuel, biochar, compost, etc. [25].The global technological era started in Bangladesh.The country has achieved a markable advancement in infrastructure and industrial development [26], through implementing systematic landfill dumping, recycling, and value-added product recovery from MSW, Bangladesh may excel in running WTE projects.
There is a lot of literature available based on MSW generation and management on a global scale; however, minimal resources have been reported for Bangladesh.The lack of accurate data and recommendations is also responsible for Bangladesh's inefficient MSW management.In this paper, the component-based characteristics of MSW were carried out for different cities in Bangladesh.Available MSW management techniques, e.g., landfill, thermal, and chemical treatment processes, are described in general to understand these technologies.A holistic approach has been made to identify the benefits and challenges in WTE project implementation.A feasible recommendation for value-addition to MSW in Bangladesh was provided.LFG recovery, carbonization, pyrolysis, and waste recycling can bring tremendous changes to the environment, ecology, and socio-economic status of the people of Bangladesh.

MSW Characteristics
The sources of MSW, physical and chemical components are the critical parameters for proper MSW management, treatment, and disposal [27].The origins of MSW can be classified into three main categories, e.g., urban, industrial and rural.Subclassification of MSW can be derived and represented in a hierarchy (Figure 1).The urban division consists of residential and non-residential wastes, e.g., commercial wastes, institutional/service wastes, construction wastes, and special types of waste.The wastes related to scientific experiments, medical, manufacturing, automobile maintenance shops, pharmacies, and airports are special waste.Industrial waste is generated in industrial sites.Extraction, utilization, modification, and production of goods and services, as well as the delivery of goods, residues, and effluent, fall into this category.Agriculture items, e.g., fertilizers, animal by-products, animal husbandry, etc., create waste termed a rural division [28] Sustainability 2022, 14, x FOR PEER REVIEW 4 of 26 The organic fraction of MSW consists of putrescible items that deteriorate quickly, such as food and kitchen wastes.Fermentable wastes that decompose quickly produce the foul odors associated with putrefaction.Non-fermentable products tend to resist decom- The organic fraction of MSW consists of putrescible items that deteriorate quickly, such as food and kitchen wastes.Fermentable wastes that decompose quickly produce the foul odors associated with putrefaction.Non-fermentable products tend to resist decomposition and thus usually last a bit longer in fresh conditions (Figure 2).Inorganic MSW consists of metals and non-biodegradable polymers [29].The organic fraction of MSW consists of putrescible items that deteriorate quickly, such as food and kitchen wastes.Fermentable wastes that decompose quickly produce the foul odors associated with putrefaction.Non-fermentable products tend to resist decomposition and thus usually last a bit longer in fresh conditions (Figure 2).Inorganic MSW consists of metals and non-biodegradable polymers [29].The available constituents in MSW make it corrosive, ignitable, reactive, toxic, infectious, and sometimes radioactive (Figure 3).Strong alkaline or acidic wastes are examples of corrosive waste, which pollutes the environment, and damages soil and landfill cells upon dumping.Ignitable wastes tend to burn at lower temperatures and can immediately produce a fire hazard and explosion.Chemically unstable, reactive wastes react aggressively with air or water.Toxic wastes are hazardous and can be detrimental even at trace levels.These may have immediate consequences on the body, perhaps resulting in death or catastrophic diseases.Bandages in hospitals, hypodermic needles used for medical purposes, and other items from hospitals and biological research institutes are infectious waste.Radioactive waste produces radioactive energy, which is hazardous to living things.Before entirely decomposing, certain radioactive elements can remain active for thousands of years and subsequently cause damage to people, animals, and even plants around them [29].

Organic Inorganic
Putrescible Fermentable Non-Fermentable The available constituents in MSW make it corrosive, ignitable, reactive, toxic, infectious, and sometimes radioactive (Figure 3).Strong alkaline or acidic wastes are examples of corrosive waste, which pollutes the environment, and damages soil and landfill cells upon dumping.Ignitable wastes tend to burn at lower temperatures and can immediately produce a fire hazard and explosion.Chemically unstable, reactive wastes react aggressively with air or water.Toxic wastes are hazardous and can be detrimental even at trace levels.These may have immediate consequences on the body, perhaps resulting in death or catastrophic diseases.Bandages in hospitals, hypodermic needles used for medical purposes, and other items from hospitals and biological research institutes are infectious waste.Radioactive waste produces radioactive energy, which is hazardous to living things.Before entirely decomposing, certain radioactive elements can remain active for thousands of years and subsequently cause damage to people, animals, and even plants around them [29].The components and properties of MSW significantly impact MSW management.The properties of solid waste vary according to region and country.The geographic area, income levels of the population, seasons, climate and weather, and a variety of other factors all influence the diversity of solid wastes.In developed countries (the USA, Canada, and Australia), paper and paperboard products account for about 30% of total MSW weight, and food waste contributes less than 20%.Yard scraps, timber industry, glassware, metal, nonbiodegradable polymer, suede, fabric, and other diverse materials make up the rest.Table 1 presents MSW components evaluation in the USA.In the USA, paper-based products and food waste are the major contributors to MSW [30].Several research studies have been conducted to determine the components of MSW in Bangladesh.MSW components in Bangladesh can be classified into physical and chemical categories.In Bangladesh, the MSW is rich in food-based waste.An in-depth analysis of city-based MSW production in Bangladesh gives a clear idea of the component analysis (Table 2).In MSW, the food-based waste varies from 68.3-81.1%.The other fraction contains paper (7.2-10.7%),polythene (2.8-4.3%),textile (1.3-2.2%),rubber (0.1-1.4%), metal (1.1-2.2%), and glass-based (0.5-1.1%) waste and others (4.5-10.4%)[16].
Table 2. Average physical components in weight percentage of MSW in different cities of Bangladesh [24].The chemical characteristic of MSW is vital in deciding its treatment method.MSW with a greater volatile solid content can have a larger calorific value; specifically, the moisture content of MSW in Bangladesh is significantly high.In the six large cities mentioned above, the average nitrogen, phosphorus, and potassium levels were 0.8%, 0.3%, and 0.7%, respectively, in the organic component of MSW.The chemical constituents of MSW recorded by researchers were not precise due to differences in sampling and testing procedures.City-based MSW chemical analysis in Bangladesh is presented in Table 3.The chemical analysis of organic waste shows that it mostly contains carbon, hydrogen, and oxygen.Oxygen is mostly contained in moisture, and other molecules such as N and S are also found in small amounts.Excreta, flushing, and additional grey water or effluents make up wastewater.Sewage systems transport wastewater from residences and workplaces to centralized treatment plants in developed cities.The parameters in measuring wastewater quality, such as pH, TS, BOD, COD, etc., differ from country to country due to the different habits and lifestyles of native people.For example, the average pH of wastewater in Japan stands between 7-9, whereas the value is between 7-8 in Thailand.There is an even larger difference in TS values between them, where the value in Japan ranges from 25,000-32,000 mg/L and 5000-25,400 mg/L in Thailand.The values of COD show an opposite trend where the values are much more prominent in Thailand (5000-32,000 mg/L) than in Japan (8000-15,000 mg/L).

MSW Collection and Transportation
A key component of sustainable MSW management is the collection of solid waste systematically.The existence of an organized collection system is essential for the efficient flow of waste management [32].Several factors affect waste collection in developing and developed countries.Less developed and developing countries lack proper waste collection guidelines, which impacts waste management flow and leads to illegal and unsustainable practices, e.g., open incineration, open dumping at banned places, and backyard burning.In lesser developed and developing countries compared to developed countries, these practices are prevalent and result in significant adverse environmental health impacts due to the emission of toxic gases and the spread of diseases from open dumping.Whilst the infrastructure is well developed in developed countries; this is not the case in lesser developed and developing countries where the roads are in bad conditions or inaccessible [33].In most cases, the waste collection vehicles cannot access areas for waste collection, and wastes remain at their sources of generation for a longer time.Another problem is the lack of financial resources to buy proper equipment and machinery for the generators to store their wastes; these services are expensive for municipalities in lesser developed and developing countries compared to developed countries [34].Accordingly, waste generators resort to indiscriminate dumping in open spaces or water bodies and littering.Moreover, in lesser developed and developing countries, waste collection machinery and equipment are outdated, used without proper maintenance and suffer from breakdowns [35].As a result, these factors affect waste transfer and transportation in less developed and developing countries than in developed countries.
Due to a lack of proper knowledge and inefficient implementation of government policies, approximately 40-60% of the generated waste is not properly collected or disposed of in Bangladesh [20].Residential waste is mostly stored in containers inside or house premises.Designated waste collectors come in a suitable time and collect waste from the containers.They mostly dispose of household waste in their container to carry the waste to the dumping site.Unfortunately, all kinds of waste are dumped together without separation.Some NGOs provide labelled trash bins in urban areas to solve this problem.Waste collection workers are not available in some areas.In that case, residents have to go to the dumping sites.The lack of workforce provided by the authority causes this problem for the residents.Different cities in Bangladesh show a variation in the MSW collection procedure.Authorities of Rajshahi and Chittagong participate in collecting waste from residents.In Khulna, the private sector has handled door-to-door pickups and whole ward management for the past ten years.Secondary disposal site (SDS) is an open area where the side of the road collects a pile of solid waste.They consist of sizable bins, massive steel transport containers, roadside free spaces, etc.According to the current municipal corporation laws, the authority is fully accountable for providing SDS in cities, collecting waste from them, and transportation to the disposal point.Disposal points are mostly based on the population of the area, space available in the place, distance from the city and transportation system.The overall numbers of SDS and community bins in the largest cities of Bangladesh are not satisfactory.Alternative possibilities for SDS should be taken into consideration since effective management with extremely precise time for collection and transfer is essential because SDS is generally located at busy locations.Sylhet City Corporation has already taken steps in this matter and has removed some roadside waste collection points.Dhaka and Khulna City Corporation has already made plans for experimental transfer stations.
Just like the storage and collection of waste, transportation is equally important for fast and safe disposal.The transportation system of waste solely depends on the city authorities as NGOs take part in the collection of waste and storing them in SDS only.But some private companies under the instructions of city corporations have been involved in transportation in Dhaka and Khulna.Several collecting vehicles, mostly trucks, are stationed on the roadside next to SDS.Every vehicle has staff members who are assigned to collect and disposing waste.Waste is often collected throughout the day, impeding traffic and hauled through crowded areas, causing annoyance and pollution.These problems are mostly caused by an insufficient number of vehicles and manpower.

MSW Management Techniques
MSW can be transformed into energy-producing and economic resources by employing appropriate waste management techniques [36].The MSW management techniques depend on multiple factors, e.g., geography, economic development, demography, etc., throughout the world [37].In low-income and middle-income countries, due to a lack of proper waste management systems, most waste disposal is carried out in the form of open dumping [38].Incineration is also widely practiced in developing countries for waste volume reduction and power generation [39].Figure 4 represents different processing techniques of MSW.Landfilling and thermal treatment of MSW are more common in highincome countries [40].Currently, various WTE and waste conversion techniques are being employed worldwide to maximize the utilization of MSW and ensure its proper disposal.As urbanization expands, the environmental impact of waste management is becoming more important [41].
Sustainable alternative waste management techniques are being innovated and modernized to properly utilize MSW as a potential route for resources [42].Recycling and reusing are also becoming a widely followed waste management techniques in developed and developing nations [43].Thermochemical processes for the treatment of waste, such as pyrolysis [44], gasification [45], etc., are being employed for the conversion of MSW to bio-oil, syngas and char products [38].Biochemical conversion techniques such as anaerobic digestion (AD), composting, etc., are promising renewable sources of biogas and fertile compost from municipal solid waste [46].Some of these techniques are presented in Figure 5 and briefly discussed in the following sections.
ume reduction and power generation [39].Figure 4 represents different processing techniques of MSW.Landfilling and thermal treatment of MSW are more common in highincome countries [40].Currently, various WTE and waste conversion techniques are being employed worldwide to maximize the utilization of MSW and ensure its proper disposal.As urbanization expands, the environmental impact of waste management is becoming more important [41].Sustainable alternative waste management techniques are being innovated and modernized to properly utilize MSW as a potential route for resources [42].Recycling and reusing are also becoming a widely followed waste management techniques in developed and developing nations [43].Thermochemical processes for the treatment of waste, such as pyrolysis [44], gasification [45], etc., are being employed for the conversion of MSW to bio-oil, syngas and char products [38].Biochemical conversion techniques such as anaerobic digestion (AD), composting, etc., are promising renewable sources of biogas and fertile compost from municipal solid waste [46].Some of these techniques are presented in Figure 5 and briefly discussed in the following sections.

Landfilling
Landfilling is a common MSW disposal technique in which waste is buried in large empty areas for further treatment and utilization [47].Landfill practices in developed countries are more organized and systematic, whereas, in middle-income countries, there is a lack of organization.Due to a lack of open space for waste burial and regulation landfills, it is a less popular method in developing countries [38].Landfills are categorized into three types, e.g., open dumps, semi-controlled, and sanitary landfills [48].Open landfills are more prevalent in developing countries, where waste is haphazardly piled onto empty spaces.Open landfills can cause environmental pollution and health hazards.Semi-con-

Landfilling
Landfilling is a common MSW disposal technique in which waste is buried in large empty areas for further treatment and utilization [47].Landfill practices in developed countries are more organized and systematic, whereas, in middle-income countries, there is a lack of organization.Due to a lack of open space for waste burial and regulation landfills, it is a less popular method in developing countries [38].Landfills are categorized into three types, e.g., open dumps, semi-controlled, and sanitary landfills [48].Open landfills are more prevalent in developing countries, where waste is haphazardly piled onto empty spaces.Open landfills can cause environmental pollution and health hazards.Semi-controlled landfills have some regulations to ensure proper waste separation from the environment, such as topsoil on the landfill, lining at the bottom of the landfill, and so on [49].However, semi-controlled landfills lack the collection, segregation, and utilization facilities of LFG and leachate [48].In developed countries, sanitary landfills are prevalent in which proper facilities are included for collecting and segregating the energy-generating resources produced by the appropriate treatment of the MSW [50].It is the most costeffective technique because the LFG and leachate can be utilized for energy generation through proper implementation.
Landfills are locations for continuous biochemical reactions and solid waste degradation.The landfill leachate is the liquid that enters the landfill site, such as rainwater, seeps through the wastes, collects various chemicals generated within the landfill, and collects at the bottom lining [51].Sources of the leachate are precipitation infiltration (i.e., rain, snow), surface leachate infiltration, groundwater infiltration, free water in waste, leachate from covering surfaces, and leachate produced from the decomposition of organic matters [52].Leachate composition may vary depending on the location, landfill age, waste quality and quantity, biological and chemical processes occurring during disposal, rainfall density, and water percolation rate through the landfill [53].Leachate contains organic matter, inorganic macro components (calcium, magnesium, sodium, potassium, ammonium ions), heavy metals (cadmium, chromium, lead, nickel), and xenobiotic organic compounds (aromatic hydrocarbons, phenols, chlorinated aliphatics, pesticides, and plasticizers) [54].
The release of the heavy pollutant leachate into the environment without prior treatment poses a high environmental risk [53].Some leachate treatment methods are anaerobic biological treatment [55], aerobic biological treatment [56], reverse osmosis [35], electrocoagulation [57], adsorption [58], physicochemical treatment, e.g., chemical precipitation, pH, oxidation and reduction [59].Leachate leakage into the ground underneath is prevented by using durable landfill liners in properly designed landfills according to existing laws [60].
On the other hand, gases of high energy potential are released due to the chemical, thermal and microbial reactions that occur within the waste in a landfill [48].The evolution of gases from volatile compounds present in MSW is one example of a thermal reaction.Chemical reactions occur due to elevated temperatures and mixing during disposal between different species.Within the diverse chemical environment of the landfill, microbial hydrolysis occurs along with decomposition and fermentation [38].Due to the presence and growth of bacterial communities such as hydrolytic fermentative bacteria, proton-reducing acetogens, hydrogenotrophic methanogens and acetolactic methanogens, AD of organic fraction of the waste occurs and gases are formed [61].The primary components of LFG are methane and carbon dioxide, which are harmful GHGs.The release of LFG into the environment poses environmental pollution risks and the risk of accidental combustion of methane [62].Besides, long-term exposure to LFG emissions poses adverse health impacts to nearby residents of the landfill site [38].In sanitary landfills, these gases are recovered for energy generation.Energy recovery is made by (i) direct combustion in heaters or furnaces, (ii) chemical energy storage, achieved through a conversion process (conversion into biodiesel, methanol, etc. and (iii) as cleanup and relative introduction into the national natural gas grid (4) electric energy generation [62].The gases generated from the proper treatment of landfill leachate are being used for energy generation by gas flaring stations [63].

Thermal Process
The thermal process of MSW management includes incineration, pyrolysis, and gasification.Incineration can be carried out by mass combustion, co-combustion with coal or biomass, refuse-derived fuel (RDF), etc. [64].By mass combustion, the unsorted MSW is usually completely oxidized, generating energy and producing flue gas, ash, and liquid discharge, thereby reducing the waste volume by 90% [65].Although the volume is reduced, the new forms of waste, i.e., bottom ash, fly ash, flue gas, and liquid discharge, are harder to deal with and harmful to the environment.This process is also less feasible for developing countries where the organic fraction of waste is higher [48].
During combustion, the combustible materials in the waste come into contact with oxygen at high temperatures (usually 850 • C to 1450 • C) and go through an oxidation reaction releasing heat energy [66].Incineration plants that generate electricity have efficiencies of up to 20%, while plants that use cogeneration of thermal power and electricity generation have efficiencies of up to 80%.Pre-processing of solid waste prior to incineration through advanced separation, biological and thermal waste treatment, and other methods have been shown to increase the efficiency of the incineration process [67].Screening solid waste allows for the separation of reusable materials (plastics, paper fibers, and metals) as well as the reduction of the presence of hazardous/toxic materials in the waste [67].Treatment processes such as torrefaction, shredding and size reduction of the MSW before incineration increase the heating value and reduce the pollution factor [68].
For MSW treatment, pyrolysis is another technique [69].It is a thermal degradation process in which large chain hydrocarbons (primarily from plastics and polymers in MSW) are cracked in the absence of oxygen at temperatures ranging from 300 • C to 600 • C.This method produces combustible gas (syngas), liquid bio-oil, tar, and char [70] (Figure 6).Factors such as heating temperature, heating rate, and solid residence time, the pyrolysis product, e.g., biochar, bio-oil, or gaseous products (methane, carbon monoxide, hydrogen, and carbon dioxide [71].Pyrolysis involves sorting and pretreating the raw MSW, and the inert materials are segregated, requiring high capital and skilled labor [72].The high heating value of the produced gas and the more recently discovered utilities of biochar and bio-oil make pyrolysis an attractive method [73].Using catalysis in the pyrolysis process may reduce the heating time, enhance the heating value and eliminate the need for rigorous pretreatment of the feed [74].The char produced from pyrolysis has a high calorific value making it using a solid fuel [30], and can be used as feedstock for the gasification process, activated carbon production, and carbon nanofilament production [74].It also has the potential for pollution removal and waste treatment by ion exchange, complication, and precipitation [75].
Sustainability 2022, 14, x FOR PEER REVIEW 11 of 26 (Figure 6).Factors such as heating temperature, heating rate, and solid residence time, the pyrolysis product, e.g., biochar, bio-oil, or gaseous products (methane, carbon monoxide, hydrogen, and carbon dioxide [71].Pyrolysis involves sorting and pretreating the raw MSW, and the inert materials are segregated, requiring high capital and skilled labor [72]. The high heating value of the produced gas and the more recently discovered utilities of biochar and bio-oil make pyrolysis an attractive method [73].Using catalysis in the pyrolysis process may reduce the heating time, enhance the heating value and eliminate the need for rigorous pretreatment of the feed [74].The char produced from pyrolysis has a high calorific value making it using a solid fuel [30], and can be used as feedstock for the gasification process, activated carbon production, and carbon nanofilament production [74].It also has the potential for pollution removal and waste treatment by ion exchange, complication, and precipitation [75].Gasification is another technique for MSW treatment [76].It is a thermochemical process that converts waste (carbon-based material) into a mixture of carbon monoxide, hydrogen, and carbon dioxide byproducts, which are referred to collectively as syngas (synthetic gas or producer gas) with a high heating value [30].It is a partial or incomplete oxidation process at temperatures ranging from 750 to 1100 °C [72].Gasification can be carried out in two methods: direct gasification, in which an oxidizing gasification agent is used, and indirect gasification, where an external energy source is required to carry out gasification [77].The MSW has to undergo a separation process before gasification, where Gasification is another technique for MSW treatment [76].It is a thermochemical process that converts waste (carbon-based material) into a mixture of carbon monoxide, hydrogen, and carbon dioxide byproducts, which are referred to collectively as syngas (synthetic gas or producer gas) with a high heating value [30].It is a partial or incomplete oxidation process at temperatures ranging from 750 to 1100 • C [72].Gasification can be carried out in two methods: direct gasification, in which an oxidizing gasification agent is used, and indirect gasification, where an external energy source is required to carry out gasification [77].The MSW has to undergo a separation process before gasification, where glass, metals, and inert materials are removed.The main components of the syngas produced are carbon monoxide, hydrogen, and methane, and it generally has a calorific value of 4-10 MJ/Nm 3 [77].Utilizing these syngas, energy is recovered from a steam circuit [45].

Biochemical Conversion
Biochemical conversion of MSW is carried out by AD and composting [78].In AD, microorganisms are used to transform biomass (isolated from MSW) into biogas (a mixture of methane and carbon dioxide) [79].It involves the participation of three physiological classes of bacteria, primary fermenting bacteria (hydrolytic-acidogenic), anaerobic oxidizing bacteria (syntrophic-acidogenic), and methanogenic archaea [80].The organic fraction of the waste is segregated, and through a complex process of degradation, biogas is generated that can be utilized for energy production.The digested materials can be used as compost/biofertilizer, which allows recycling of nutrients (nitrogen, phosphorus) [81].The quality of AD has been enhanced by applying enzymatic hydrolysis [82].
The organic fraction in MSW is rich in soil nutrients (nitrogen, phosphorus, etc.By ensuring appropriate segregation of this organic fraction and through vermi-composting [63], mechanical composting [83], and microbial culture [84], the nutrients can be circulated back to the environment [84].This method not only reduces the load on landfills but also proves to be an essential recycling tool for meeting crop nutrient requirements [85].The composting of MSW carries the risk of an increase in metal content of the soil and groundwater contamination, and the mineralization of the organic matter may also release ammonium that can oxidize to nitrate [86], contaminating the surface and groundwater.By carefully manipulating factors (e.g., feedstock selection, aeration, and maturity of the compost), the N content and other contaminants can be maintained to meet the soil requirements [87].By implementing source separation and careful monitoring of MSW compost quality, suitable applications of respective composts have been discovered in many cases [88,89].

Benefits, Challenges, and Cost Analysis of the WTE Projects
For a long time, fossil fuels, e.g., oil, coal, and natural gas, have been utilized as an energy source and account for around 80% of global energy [90].The global energy demand is rising, and by 2035, the market will be approximately 17 billion tons of oil equivalent [91].The modern WTE projects offer an alternative to fossil fuels [92].There are 765 MSWbased WTE plants in operation worldwide, utilizing only 83 million tons of annual capacity, whereas the world produces 2.01 billion tonnes of MSW annually.By 2025, 2.3 billion tonnes and by 2050, 3.4 billion tonnes of MSW are expected to be produced yearly, equivalent to 2.6 × 10 20 GJ of energy [36].As a result, unlike other non-renewable energy sources, MSW is abundant as a raw material to be utilized in WTE projects.If properly utilized, WTE projects can not only mitigate this ongoing global problem of waste management but also shape a new era of renewable energy sources.Hence, it promotes sustainable waste management that ensures proper reuse and recovery of waste materials [42].So, WTE projects utilizing MSW can contribute greatly to energy production which can lower the constant burning of fossil fuel.This automatically offers a promising path to preserve our resources for future generations.
There are several technologies and processes available for WTE projects.A study based in the USA reported that, in a modern WTE power plant, 1 metric tonne of MSW combustion produces 600 kWh of electricity.In contrast, it would require only 0.25 metric tonne of good quality coal to generate a similar amount of electricity.Pandj Prawisudha et.al. implemented Japanese MSW to produce a coal alternative fuel via an innovative hydrothermal treatment with a heating value identical to low-grade sub-bituminous coal [93].
Incineration is the widely used WTE technology.Incineration delivers energy and can significantly reduce the volume of waste by up to 90% [30].Incineration-based WTE projects are economically feasible for developing countries.The fly ash produced in the process can be used as raw material in cement production, road maintenance, and construction [94].The syngas-producing gasification technology is another popular WTE technology.Gasification-based WTE projects are ecofriendly and hold the potential to merge with other thermoelectric plants [77].In recent years, plasma-assisted gasification has offered a sustainable and cleaner technology for WTE-project [30].In several of the neighboring countries of Bangladesh, WTE techniques have been implemented to recover the value of MSW [95].In India, sophisticated methods of WTE techniques are under development, such as bioethanol from waste [96], gasification [97], LFG [72], etc.However, these techniques have not been successful in evolving into long-term solutions due to a lack of consciousness, funding, and different operational and technical insufficiencies [96].The focus on energy production from WTE techniques is increasing daily, and production has been increasing in the last few years [49].Refuse-derived fuel (RDF) is a popular method in India for the production of solid fuel from waste [98] [doi:].In China, due to the huge urban population and insufficient land for continued landfilling, the country has been focusing on research on WTE technologies [99].Incineration and LFG utilization are primary WTE techniques employed in China [100].Although still in the nascent stages of development, biochemical and thermochemical processes of generating power through WTE techniques are occupying a larger margin through the years in the energy sector of Pakistan [101].
On the other hand, compost generated from MSW contributes significantly to enhancing soil properties and plant growth [102].A study showed that the yield becomes maximum if the compost is utilized along with existing fertilizers [103].With increasing population, this holds the potential to aid enhanced food production using not only the available agricultural fields but also urban degraded soil.Another promising alternative to MSW is anaerobic digestion (AD) which produces biogas mainly containing CH 4 around 55-70% and CO 2 around 30-45% with a trace amount of H 2 S. Depending on the composition of the feedstock, around 100 to 200 m 3 of biogas is generated per tonne of MSW, producing 100-150 kWh net electricity [104].The residue left after the digestion is used in a similar manner to compost.The biggest advantage of this process is that it significantly reduces GHG emissions.It also minimizes odors, pathogens, and the risk of ground water pollution as well.For developing countries, AD and composting are the most attractive and economic WTE projects [31].The following sections describe some of the benefits and challenges of WTE projects for developing and underdeveloped countries such as Bangladesh.

Prevention of Unsupervised Landfills
In the world, around 70% of the unused portion of MSW goes into open dumping sites and landfills [40].The dumped waste of landfills produces toxic LFG and landfill leachate that contaminates the ground water resulting in severe health risks and environmental pollution.The contaminated water is highly carcinogenic and can cause cancer in the bronchus, stomach, liver, etc., and congenital disabilities in children [105].The significant components of LFG are CH 4 and CO 2 .The emitted CH 4 is responsible for around 20% of the anthropogenic methane emission worldwide [106].It also contains ammonia, hydrogen sulfide, and VOCs in low concentrations.Even in modern sanitary landfills, the gas collection system is not efficient, and at least 25% of those gases are exposed to the environment [107].Proper utilization of MSW that goes into landfills can save thousands of acres of wasted area and manpower.WTE projects offer an excellent pathway for waste management as it has the potential to significantly reduce landfills and create a cleaner environment worldwide [107].

Reduction in GHG Emissions
Untreated MSW produces a huge amount of GHGs.Integration of proper MSW management and WTE projects can significantly reduce emissions.A study reports that 350 integrated WTE projects around the globe reduced the GHG emission equivalent to 51.3 million metric tonnes of CO 2 in 52 countries [108].In developing countries like Bangladesh, the GHG emission is expected to be 5.4 to 9.6 metric tons of CO 2 equivalent if the ongoing MSW management system is allowed to continue [108].A study in Bangladesh showed that implementing a combination of incineration and LFG recovery systems employed in two major cities of Bangladesh significantly reduced GHGs emissions [36].The prediction showed that the strategy could produce a net negative GHG emission of −0.29 ton of CO 2 equivalent per ton of MSW along with high energy production up to 0.37 MWh per ton of MSW.The WTE plants also produce less GHG compared to other traditional fossil fuel-based plants [107].It is shown in Table 4. Another study showed similar results regarding low CO 2 emissions.It showed that WTE projects could reduce CO 2 emissions from 4 to 48%, depending on the process [109].Thus, WTE projects are cleaner than fossil fuel-based energy production, without any doubt.

Promotes Recycling
The efficiency of the current WTE technologies depends largely on the feedstock composition [104].Hence, many pre-processing and separation schemes are involved in producing the appropriate feedstock [110].This is why recycling and WTE projects complement each other.Some thermal treatment plants, such as incineration, can recover ferrous metals.Other valuable none ferrous metals, glass, ash, etc., can be recovered and recycled as they don't contribute to heat generation.One study showed that more than 80% of the WTE plants in the USA recycle nearly 1.5 million tonnes of materials [107].When burned, plastics that produce harmful toxic gases can also be recycled from the pre-processing units.Recycling is receiving more and more attention in the current world because it contributes highly to a circular economy and sustainable future.With proper planning and implementation, the available WTE projects can make the recycling roadmap simpler and more effective.In order to achieve the benefits that the WTE projects may bring, the several insufficienciesand hurdles faced as a developing nation must first be overcome.

Inefficient Waste Management
For the WTE technologies to perform at their highest efficiency, the feedstock composition plays a huge role [111].For example, thermal treatment requires a dry feedstock, e.g., plastic, paper, rubber [112], and AD or composting requires high moisture content in feedstock, e.g., food waste [113].Hence, pre-processing and effectively sorting the MSW is very important.In many developing countries such as Bangladesh, this is often a challenge.For example, the amount of waste collected is less than 50% in Dhaka city, the capital of Bangladesh [31].For India, that margin lies around 70% [63].The increasing rate of waste generation, lack of government regulation and financial support, inadequate technologies, unsupervised landfills, lack of environmental protection policy, open dumping, and lack of awareness are responsible for such improper management of MSW in developing countries [72].

Unwanted Emissions
Although the WTE projects significantly reduce MSW volume by up to 90% [30], the post-treatment scenario isn't always ecofriendly.For example, incinerators that lack proper planning and appropriate temperature of oxygen composition can produce carbon monoxide, soot due to incomplete combustion, and toxic fumes [29].Another study demonstrated that the incinerators produce a significant amount of toxic dioxin, furan, and mercury [107].The residue after incineration requires safe disposal, which is often tricky.For hydrothermal treatment, organic chlorine elimination can be difficult.These compounds generate hydrochloric acid, which can compromise the furnace structure and generate toxic dioxin [93].Another challenging project is sanitary landfills.Chemical reactions in the landfill can generate tertiary pollutants, e.g., vinyl chloride, methyl-mercaptans, ethyl-mercaptans, hydrogen sulfide, and so on [105].94 additional non-methane organic pollutants are present in the emission of landfills identified by the US EPA.The list includes compounds such as xylene, toluene, benzene, vinyl chloride, chloroform, propyl benzene, hydrogen fluoride, mercuric fluoride, etc. [110].In comparison to WTE, other renewable technologies, e.g., wind and solar, have no such emissions.

Occupational Hazard
In most developing countries, there's an increasing health concern for the workers handling MSW.The workers handling and transporting the MSW are at high health risk but have no or minimal personal protective kits.The workers working directly in landfills are at even higher health risks due to numerous toxic and carcinogenic gases [105].Inefficient incinerators produce toxic NOx gases that can cause countless respiratory diseases among workers and the surrounding neighborhood [29].

High Capital and Operating Costs
Another challenge of most WTE projects is that they are highly cost-intensive.The thermal technologies require around 150,000 to 400,000 US$ per ton per day of MSW [30].Another study showed that non-conventional technologies have high capital and operating cost [110].For example, gasification technology requires a series of syngas cleaning sections, which adds to the cost.The operating cost becomes higher in the case of heterogeneous feedstock composition [30].Incineration often requires air emission control devices to keep up with the environmental protection policies, which adds to the cost [114].It is anticipated that there will always be newer and more rigorous ecological policies in the future regarding the environmental and health risks associated with non-conventional technologies [115].Even though the plasma gasification technology is cleaner, its capital cost is around 100 million US$, making it unattractive to investors.Another problem with non-conventional WTE projects is that all these projects haven't been commercially successful globally due to high capital and operating costs.There's also a safety issue regarding the most recent WTE technologies, e.g., plasma gasification, because it is still under research [110].For developing countries like Bangladesh, some of these technologies, e.g., pyrolysis, gasification, and plasma gasification, seem less attractive because of their high costs [31].

Relative Cost Analysis of Energy Production
The cost of energy production is the crucial parameter for selecting WTE projects.Table 5 compares the capital investment between conventional and WTE technologies.Table 6 represents the relative cost of capital investment among different technologies using fossil fuel, solar, wind, and MSW.It is evident that the average capital investment cost is highest in WTE projects compared to other non-renewable and renewable energy sources, especially for non-conventional technologies.It is because the cost involved in non-conventional technologies is unpredictable.Feedstock processing costs are also high because moisture and materials with low calorific value need to be removed.These reasons make it harder for key stakeholders to estimate and secure a high revenue margin.Several European facilities working on these projects have unfortunately closed down due to financial issues [115].The high cost and limited commercial success of non-conventional technologies make AD, sanitary landfills, and composting more economically friendly because these involve less financial risk.These options require lower capital investment as well as operating costs.The O&M cost associated with WTE technologies is shown in Table 7.Here, operating cost mainly includes labor, overhead, insurance, depreciation, and utility costs.The O&M cost of non-conventional technologies is comparatively higher.However, O&M cost varies slightly from country to country.Usually, highly developed countries with high income have the highest O&M cost in WTE projects.This is illustrated in Table 7.The socioeconomic status, labor cost, maintenance cost of pollution control equipment, e.g., bag filter, scrubber, electrostatic precipitator to keep up with the national environmental policies, high target efficiency, tax, trained staff, and insurance are the parameters that influence the O&M cost in WTE projects [114].In developing countries like Bangladesh, conventional technologies present the highest potential.An estimation study was done in 2016 utilizing LFG recovery and mixed MSW incineration [36].The energy potential of different WTE strategies was assessed using a standard energy conversion model and subsequent GHGs emissions models.The design capacity of the mixed MSW incineration plant to generate electricity in Dhaka and Chittagong city was assumed as 1200 tonne/day.The economic life of the WTE incineration plant was taken as 35 years (comparing the coal plant) On the other hand, a landfill with an LFG recovery system is assumed to have a capacity of greater than 1000 tonnes mixed MSW/day for 35 years.Approximately USD 535 million and USD 251 million of revenue can be generated from electricity sales and claiming of carbon credits from MSW incineration and LFG recovery, respectively, in Dhaka and Chittagong city by 2050.The study suggested incineration requires higher capital and operating costs than the LFG recovery system, and revenue from MSW incineration is much lower.It is illustrated in Table 8.The study only utilized the MSW from two major cities, Dhaka and Chittagong.Hence, implementing MSW in other cities will result in much higher revenue.
In Bangladesh, WTE projects are vital to initiating a nationwide circular economy and industrial ecology.WTE project will ensure the availability of cheaper and greener energy, which will certainly reduce the energy crisis problem to a certain extent and can generate green jobs.The specialized authorities have to set the feasible economic life of the WTE project in comparison with coal/NG-based power plants [36].WTE strategies of MSW to generate electricity and other consumables can deliver socio-economic and environmental benefits to Bangladesh if the challenges can be eradicated.The next section presents some viable strategies for MSW-based valuable fund recovery for Bangladesh.

Possible Strategies for Valuable Fund Recovery from MSW in Bangladesh
Bangladesh has a lot of challenges regarding MSW management, but there is a huge opportunity to use MSW for the greater good.As the amount of waste produced daily is very high, so is the potential to use this waste scientifically and economically.The amount of MSW is expected to rise rapidly in the near future because of the huge population and bad management.This part of the article discusses the potential of these waste materials, how to reuse them, and the hindrances to fulfilling the plans in the perspective of Bangladesh.
However, the conventional techniques, e.g., landfill and incineration, for MSW management in Bangladesh seem ineffective regarding valuable resource recovery.Landfilling is the most common method of waste management in Bangladesh, but it is no longer appropriate due to the deficiency of land in Bangladesh.Incineration is very common in WTE plants in Bangladesh.The main challenges of incineration processes are implementing proper flue gas treatment technology and proper carbon dioxide capture and storage technology.They also have high carbon emissions and high emissions of harmful flue gases [40].Additionally, drying wastes in the rainy season is very difficult, which is a great disadvantage of the incineration process, and a large amount of fuel is required to execute the incineration process [8].
One possible strategy for Bangladesh is the LFG recovery.LFG is produced when the dumped organics in the landfill sites are decomposed.LFG consisting of CH 4 (40-60%), CO 2, and organics are considered landfills' natural byproducts.Sequential aerobic and anaerobic conversion of solid waste yields CO 2 .LFG can be stored and converted into a renewable energy source.LFG utilization as an energy source is beneficial financially and also good for the environment as methane can potentially be a more lethal GHG than CO 2 .The LFG recovery process is illustrated in Figure 7. LFG is collected and compacted to initiate the process.To recover the gases, horizontal trenches, landfill drilling, or a combination of both are used inside the landfill cells.The extracted CH 4 is put through the production process, which removes moisture and produces the pressure necessary to transport the gas to a terminal point or for additional treatment.Knock-out drums, blowers, heat exchangers, control units, etc., are used for this process.
Sustainability 2022, 14, x FOR PEER REVIEW 18 of 26 renewable energy source.LFG utilization as an energy source is beneficial financially and also good for the environment as methane can potentially be a more lethal GHG than CO2.
The LFG recovery process is illustrated in Figure 7. LFG is collected and compacted to initiate the process.To recover the gases, horizontal trenches, landfill drilling, or a combination of both are used inside the landfill cells.The extracted CH4 is put through the production process, which removes moisture and produces the pressure necessary to transport the gas to a terminal point or for additional treatment.Knock-out drums, blowers, heat exchangers, control units, etc., are used for this process.Establishing the LFG process can work as an antidote to the alarming temperature rise in Bangladesh in recent years.Generating power from LFG reduces the quantity of non-renewable materials used to produce energy, such as coal, natural gas, or even oil.Trade of LFG can also lead to creating jobs in the design, implementation, and maintenance of recovery systems.Digging pipeline, building plants, and operations employees account for a large portion of the project expenditures, allowing towns to gain economically from employment creation and local deals.Local companies can save money by switching to LFG instead of relatively expensive energy sources.During the lifetime of the LFG energy projects, several corporations might save vast amounts of money.As a densely populated country, there is always a shortage of adequate electricity in Bangladesh.Landfill energy can solve this problem to an extent if handled properly.The retrieved methane can be transformed into electrical power in various ways, including gas turbines, microturbines, hydrogen fuel cells, reciprocating internal combustion engines, cogeneration, etc.Using reciprocating internal combustion engines should be ideal for Establishing the LFG process can work as an antidote to the alarming temperature rise in Bangladesh in recent years.Generating power from LFG reduces the quantity of non-renewable materials used to produce energy, such as coal, natural gas, or even oil.Trade of LFG can also lead to creating jobs in the design, implementation, and maintenance of recovery systems.Digging pipeline, building plants, and operations employees account for a large portion of the project expenditures, allowing towns to gain economically from employment creation and local deals.Local companies can save money by switching to LFG instead of relatively expensive energy sources.During the lifetime of the LFG energy projects, several corporations might save vast amounts of money.As a densely populated country, there is always a shortage of adequate electricity in Bangladesh.Landfill energy can solve this problem to an extent if handled properly.The retrieved methane can be transformed into electrical power in various ways, including gas turbines, microturbines, hydrogen fuel cells, reciprocating internal combustion engines, cogeneration, etc.Using reciprocating internal combustion engines should be ideal for Bangladesh as it is low-cost and energy-efficient.
By elevating the methane concentration of LFG while reducing the other constituent gases, LFG can be converted into natural gas.This renewable natural gas can be used as an alternative to diesel, octane, etc., to power vehicles.This can be extra beneficial to Bangladesh as there is constant clogging in filling stations due to excessive vehicles.LFGs may be used in various thermodynamic applications in their natural state.Methane from landfills may be used in conservatories, boilers, and dryers.
Bangladesh can also consider the utilization of pyrolysis.Pyrolysis is a fast-evolving method for biomass thermal conversion.The process is very cost-efficient and eco-friendly; valuable fund recovery from MSW using this process is a very popular method around the world.Pyrolysis provides an ample opportunity for Bangladesh to convert MSW into energy sources (Figure 8).It provides an appealing method of transforming waste into products, e.g., biofuel, syngas, and solid residue (char) that may be used to generate heat, chemicals, and power [120].Though pyrolysis is one of the most common methods for WTE conversion, the major disadvantage is the requirement of a very small particle size for fluidized bed reactors [17].Pyrolysis can benefit the environment by lessening greenhouse emissions and water pollution in cities such as Dhaka, Chittagong, Sylhet, Khulna etc. Biofuel can act as a good alternative power source for diesel engines.It can also produce electricity using gas turbines, steam turbines, and boilers.This can work as a blessing for Bangladesh as energy production for its vast population in Bangladesh.Biofuel or bio-oil is relatively cheaper to store and transport than solid fuel.A broad number of organic compounds and specialized chemicals may also be found in bio-oil, which is also beneficiary in the long run.Syngas mostly consists of CO, H2, CH4, and various VOCs.It is mostly used to generate electricity through different turbines (gas, steam, etc.) and boilers.Syngas can also be a handy alternative to produce the heat required in CHP systems.Bangladesh's petrochemical and refinery industries can also use syngas as their basic chemicals.The solid residue of MSW is also known as char [121], which can be used in the water treatment program of WASA.It can also be helpful for rural farmers as char can be used as an agricultural soil amendment.This can help reduce the application of fertilizers and save a huge amount of money used to import these from other countries.However, the setup and operation of the pyrolysis process are quite costly.To overcome this, we have to create a potential business for the pyrolysis products.The syngas has to be circulated to the turbine for power production.Government has to make bio-fuel an available alternative to petroleum products.The use of solid residue for wastewater treatment and soil improvement has to be popularized.
HTC (hydrothermal carbonization) is also an easy yet efficient way.The process of producing solid carbon-rich fuels known as hydrochar is known as HTC.This thermochemical conversion technique might generate a lot of attention due to the possibility of Pyrolysis can benefit the environment by lessening greenhouse emissions and water pollution in cities such as Dhaka, Chittagong, Sylhet, Khulna etc. Biofuel can act as a good alternative power source for diesel engines.It can also produce electricity using gas turbines, steam turbines, and boilers.This can work as a blessing for Bangladesh as energy production for its vast population in Bangladesh.Biofuel or bio-oil is relatively cheaper to store and transport than solid fuel.A broad number of organic compounds and specialized chemicals may also be found in bio-oil, which is also beneficiary in the long run.Syngas mostly consists of CO, H 2 , CH 4, and various VOCs.It is mostly used to generate electricity through different turbines (gas, steam, etc.) and boilers.Syngas can also be a handy alternative to produce the heat required in CHP systems.Bangladesh's petrochemical and refinery industries can also use syngas as their basic chemicals.The solid residue of MSW is also known as char [121], which can be used in the water treatment program of WASA.It can also be helpful for rural farmers as char can be used as an agricultural soil amendment.This can help reduce the application of fertilizers and save a huge amount of money used to import these from other countries.However, the setup and operation of the pyrolysis process are quite costly.To overcome this, we have to create a potential business for the pyrolysis products.The syngas has to be circulated to the turbine for power production.Government has to make bio-fuel an available alternative to petroleum products.The use of solid residue for wastewater treatment and soil improvement has to be popularized.HTC (hydrothermal carbonization) is also an easy yet efficient way.The process of producing solid carbon-rich fuels known as hydrochar is known as HTC.This thermochemical conversion technique might generate a lot of attention due to the possibility of transforming wet organic material into energy.The biomass and water are mixed and heated at temperatures between 180 and 260 • C, with pressures between 2 and 6 MPa.The reactions can take from 5 min to 4 h, depending on the conditions.Produced hydrochar is nearly identical to coal material and can be used as a solid fuel for heat and electricity generation [122].
Waste recycling is a prevalent and efficient method for MSW management [123].Reprocessing of waste into materials and products for further use is known as waste recycling.Inorganic waste in Bangladesh is recycled informally.Even though traditionally, waste recycling is not a regular practice by local authorities in Bangladesh, informal waste recycling (IWR) is a widespread practice among rural people.Mostly paper, glass, plastics, polymers (plastics and PVC), and metals are collected by underprivileged people and sold to vendors (Table 9).They further bring this to recycling sites, and the rest is handled there.
Plastic solid waste (PSW) makes up a considerable number of solid wastes in the urban regions of Bangladesh as well as India [124].They are primarily used for packaging and as shopping bags.However, recycling PSW faces hindrances due to financial and environmental reasons.Recycling plastic materials can benefit both economically and environmentally as recycling would mean less pollution, and the recycled products can be used again.Because of their favorable binding properties, polythene and PVC were utilized in transportation engineering to improve the properties of the asphalt binder.Recently, the government has taken waste recycling seriously.Under government regulations, compost and biogas are produced commercially from organic solid waste (OSW).An estimated 15% of the inorganic waste generated is being recycled on a daily basis in Bangladesh.According to reports, Bangladesh saves up to USD 15.29 million annually.Approximately 0.12 million citizens are involved in Dhaka city in the recycling process and 0.05 million on average per other big cities.In Chittagong, recycling metals from shipbreaking scraps is a widespread practice in Chittagong both informally and under government supervision.In Khulna city, 7.2-8.9% of MSW produced are recycled daily [126].Local agents control the recycling process, and the majority of the recycled products are sent to Dhaka [127].In Dhaka, more than 200 dealers and 4 small businesses recycle almost 19.33 tons of animal bones each day to manufacture items to be used locally and pulverized powder to be transported to Dhaka and even to European regions.140 recycle stores, and around 1906, individuals work in the recycling industry in Rajshahi [126].In Sylhet, 800 traders participate in MSW collection, which has been shown to be a sustainable method of MSW management.They make around BDT 200-400 per day from this [128].
The suitability of the MSW recycling processes in Bangladesh can be analyzed from an environmental, energy, and economic perspective [69].From an environmental point of view, the waste recycling operation had to include dust, acid gas, and NOx abatement section for sustainable operation.MSW recycling should parallel the energy production capacity of other conventional technologies.The cost and valuable fund recovery factors must be considered for MSW recycling.However, the environmental and economic positive impact of waste recycling is the main reason why this process is so popular in Bangladesh.But this process lags in energy revenue drastically.Incineration, pyrolysis, and HTC have higher WTE conversion ratios, ranging from 60-78 percent.Incineration and pyrolysis also decrease the waste volume in significant numbers.Even after all these benefits, incineration is not preferred due to high cost and hazardous environmental impact.Even though the cost for pyrolysis and HTC plant is high, these can be feasible in terms of long term economic and environmental perspectives [122].LFG recovery process can also come into consideration.Bangladesh has to implement a combined practice of LFG recovery process for the replacement of NG/coal reliability, pyrolysis/HTC process for biofuel production, and waste-recycling for overall fund recovery.

Prospectives and Challenges
The production of MSW is ever-increasing in Bangladesh, especially in urban areas.Bangladesh has ample opportunity to generate energy from the MSW.With the implementation of appropriate policies and waste management strategies by the government and local authorities, this supply of MSW could be utilized as a valuable resource for energy production.The energy derived directly from solid waste, and solid-derived fuels may contribute significantly to the overall energy generation and help solve the energy crises.The primary challenges to this are the lack of coordination between city dwellers, local administration, and government authorities.There is also a lack of strict policy for the management of MSW in Bangladesh.To overcome these challenges, solid waste has to be collected, segregated, and treated under proper supervision and control.The huge organic fraction in the waste content contributes to a higher calorific value for thermal processes, thus possessing the quality of a reliable renewable energy source.Once the proper management can be ensured, a WTE plant can be successfully run to generate power for the establishment of a circular economy.

Conclusions
The current global generation of MSW is 2.01 billion tonnes/yr, which will be 3.40 billion tonnes/yr by 2050.The characteristics of MSW vary with the geographic and economic status of a country.The MSW of developing countries is rich in organic fraction.In underdeveloped countries, MSW management is controlled by informal sectors.Insufficient waste collection, improper recycling, and uncontrolled open dumping are the main obstacles in MSW management.The recycled/unrecycled ratio is very narrow.Landfilling is the most popular disposal technique worldwide.However, in developing countries such as Bangladesh, open landfills cause severe environmental problems.Thermal and chemical treatment can solve the problem.Still, these technologies have certain disadvantages, e.g., insufficient resource/budget allocation, little to no practice of technical advancement in waste management, and lack of coordination among non-government, local, and central governments in developing countries.In Bangladesh, feasible WTE projects should be adopted to ensure efficient MSW management.The generation of energy/consumables based on WTE projects will ensure the availability of sustainable energy in Bangladesh.However, high capital costs and inefficient waste management are barriers to properly implementing WTE projects.By 2050, Bangladesh can establish total revenue of USD 791 million per year (Dhaka and Chittagong), including carbon credit revenue.The LFG recovery, waste recycling, and pyrolysis for energy production, syngas generation, and metal recovery are possible approaches Bangladesh can work into.The overall situation can be improved with the collaborative work of the government in terms of MSW policies and environmentalists for suitable WTE technology selection.

Sustainability 2022 , 26 Figure 3 .Figure 3 .
Figure 3. Properties of MSW.The components and properties of MSW significantly impact MSW management.The properties of solid waste vary according to region and country.The geographic area, income levels of the population, seasons, climate and weather, and a variety of other fac-

Figure 6 .
Figure 6.Summary of different MSW management techniques and their ultimate products.

Figure 6 .
Figure 6.Summary of different MSW management techniques and their ultimate products.

Table 4 .
Comparison of GHG emissions from MSW and other fossil fuels.

Table 5 .
Comparison of capital investment between conventional energy production technologies and WTE technologies.

Table 6 .
[30]arison of operating and maintenance costs (O and M) cost for WTE technologies[30].

Table 7 .
[119]rison of O and M cost for conventional WTE technologies among different countries[119].