Analyzing Municipal Solid Waste Treatment Scenarios in Rapidly Urbanizing Cities in Developing Countries: The Case of Dar es Salaam, Tanzania

Currently, large quantities of municipal solid waste (MSW) in many cities of the developing countries are being dumped in informal or formal but unregulated dumpsites that threaten the ecological environment and general public health. The situation in Dar es Salaam, Tanzania is of particular concern and is further challenged by a rapidly growing population and urbanization without adequate waste management systems. Current MSW treatment options have been selected based on the judgment and the experience of individuals with authority while underestimating the role of scientifically derived techniques. This study analyzes the most efficient waste treatment options, particularly scenarios with the lowest economic and environmental costs (EcC and EnC, respectively). It uses 12 years (2006–2017) of MSW management data and compares potential waste treatment options for the identified waste streams. A total of 108 different scenarios were designed, and a multi-criteria analysis method was applied to enable the identification of 11 scenarios with acceptable EcCs and EnCs. These formed an initial decision matrix of aggregation dominance that was then categorized into four groups, each represented by the most ideal point. Finally, the dominant scenario that formed the core for all considered options was found. It costs around $274,100 USD while saving about 1585 metric tons (MT) of CO2 emissions daily. This suggests that after all the MSW generated in the city is collected and segregated, organic waste should be composted whilst plastic, paper, glass, and ferrous metal should be recycled. After treatment, other waste will go to some form of landfill. Sustainable management of MSW in this city and others with similar conditions should consider particular local conditions and could use the methods and the findings of this study as a starting point.


Introduction
Increases in human population, urbanization, economic growth, and associated consumption patterns have significantly added to the volume of materials discarded as municipal solid waste (MSW) [1,2]. From an optimistic perspective, increased volumes of solid waste are not considered a problem; rather, they are considered to provide free resources and employment opportunities, especially for the poor and the marginalized [3][4][5]. Under this perspective, a focus on improving infrastructure, applying modern technologies, and using the best management approaches from a list of scientifically derived options [6] ensures sustainable management of MSW [7]. standards that assist in defining a specific set of possible alternatives. The choice of which approaches to take must be guided by an astute model considering social demand and including, in particular, the environmental and the economic costs [28,37].
Therefore, this study reviews potentially desirable scientific approaches to dealing with the waste problem in the rapidly urbanizing cities in developing countries. It analyzes different waste management scenarios with respect to the actual situation in Dar es Salaam by using the multi-criteria analysis method-the Elimination and Choice Expressing Reality (ELECTRE) method-which uses a process of elimination from a list of developed options until the best choice is reached based on specific local demand and capacities [38,39]. The study provides results that may also be applicable in other cities of similar characteristics and proposes the best scenarios that met the criteria of minimum economic and environmental costs while considering technological ease and level of social acceptance.

Materials and Methods
For ease of analysis, both basic and technical data were sought for the study. The methodology was divided into two major stages; namely, (1) basic data acquisition and (2) scenario development.

Basic Data Acquisition
At this stage, basic data relevant to MSWM in Dar es Salaam was sought. Aspects considered included waste generation level and management status of different waste streams in the period 2006-2017, which was deemed an acceptable "environmental pollution risk assessment period" [40][41][42][43]. Other data were related to risk prevention, protection, and mitigation measures such as waste management systems, population density, settlement patterns, urbanization level and trends, and human factors such as waste management experience and worker skillsets. For population projection, the study used national population data from the National Bureau of Statistics (NBS) [44], as reported in the previous work by Kazuva et al. [11].
To determine per capita generation rates and subsequent projections, we used data obtained from the four sampled wards, Mikocheni, Oyster-bay, Mwananyamala, and Buguruni, which had a total of about 187,000 inhabitants. The selected wards vary in their economic character and their settlement patterns. The former two wards represent the suburbs-areas with properly planned and serviced locations. They include the homes of some major political figures, government officials, and some merchants. The latter two wards are among low-income neighborhoods characterized by poor settlement planning, low-quality housing, and limited social services [45]. Working in these areas, we obtained the average total waste generated per day and then used population data to determine the overall per capita MSW generation rate to be 0.80 kg/capita/day. This was consistent with a previous study by a 2016 Expert Mission on integrated solid waste management (ISWM) from Dar es Salaam, which estimated waste generation to be 0.82 kg/capita/day [18].
Understanding the composition of MSW is an important aspect of developing different treatment and management scenarios. However, this was not an easy task in the city of Dar es Salaam, where waste segregation is very low and recycling systems are poor. The available literature indicates that the composition of MSW varies greatly from one municipality to another and changes significantly over time [22,[46][47][48]. However, we managed to estimate the composition of MSW generated in Dar es Salaam from the surveyed area. In this case, each ton of MSW was composed of 57.21% organic waste, 13.08% plastic waste, 6.12% paper-related waste, 2.32% glass, 1.02% ferrous metal (steel and aluminum), and 20.25% other waste components [18]. This information was particularly important for scenario development.

Scenario Development
This stage was fundamentally important for identification of the ideal conditions for MSWM in Dar es Salaam. In this stage, all possible waste treatment methods were analyzed against each waste type (waste stream), and selections were made based on the qualities of a particular option. The stage involved two steps-scenario design and scenario selection.

Scenario Design
In designing waste treatment scenarios, waste composition was an important consideration, as each stream has its own suitable treatment methods [49,50]. For instance, the methods that are preferable for organic home-based waste are seldom used for metal or electronic waste, and vice versa [51,52]. Six major waste streams-namely, organic, plastic, paper, glass, metal, and others (one group representing all other remaining MSW)-were identified. Each stream was thoroughly examined considering five potential treatment options: composting, recycling, incineration, pyro-gasification, and sanitary and/or bioreactor landfilling. These were numbered 1, 2, 3, 4, and 5, respectively. For the effective organization of all scenarios and ease of analysis and selection, the waste categories/streams were also placed in a precise order, which was used throughout the process [53]. Therefore, the six identified waste streams in the order mentioned above were considered relative to the named treatment options ( Table 1). The symbol "+" stands for a viable treatment option, and "−"represents a non-viable option.  Table 1 shows each stream with its potential MSW treatment options based on expert inputs and comprehensively reviewed literature from reputable sources. All scenarios (see Section 3.2) were identified and given different codes for computational purposes [49,54]. For instance, the codes for the first and the last scenarios (S1 and S108) were 122223 and 555525, respectively. From S1, this means that organic waste must be composted through windrow, vermicomposting, or another suitable composting form, as signified by the first digit of its code (1); plastic and rubber waste, paper waste, glass, and ferrous metal must be recycled, as signified by the second, the third, the fourth, and the fifth digits in the code (2); and other wastes must be incinerated, as shown by the sixth digit of the code (3). The last scenario proposes that all waste streams except metal go to sanitary or bioreactor landfill (5). Metal is suggested for recycling, as signified by the fifth digit of this code (2). All scenarios used this coding system.

Scenario Selection
Selecting the best option for handling MSW from a long list of available alternatives is a complex activity that demands a critical investigation based on the particular advantages and the limitations of each scenario [31]. The complexity is more significant when some of the options are drawn from different localities [55,56]. To make the selected scenarios useful for decision-making and to provide timely demand-specific solutions, it is important that the selection process considers social-cultural, economic, environmental, political, and legal aspects of the specific area in question [57,58]. This study considered these diverse factors, giving special attention to economic cost (EcC) and environmental cost (EnC) for all the scenarios formulated. However, the unavailability of reliable financial and environmental data from the study site was one of the main limitations of this study. Many countries do not make financial and environmental cost figures accessible in the public domain [59]. Both public and private sectors involved in MSWM may lack such important data or, if they have them, may be reluctant to disclose any that could be considered confidential [56]. However, as the provision of a clean environment (including waste management services) is a human right and a proxy of good governance and the sound management of public funds, making such data available for research and other uses should be standard practice [60][61][62].
Despite the lack of complete financial and environmental data, we used basic information available from reliable sources, such as the Vice President's Office (VPO), the National Environmental Management Council (NEMC), and Dar es Salaam Local Authorities (DLAs), to compute economic and environmental costs. The information was obtained through different methods, including an inventory survey whereby information including the cumulative amount of MSW generated and the percentage composition of each waste stream in Dar es Salaam from 2006-2017 was estimated.
For effective analysis of EcC, aspects such as the initial investment in a particular technology and the unit operating costs of treatment facilities need to be assessed [59,63] as shown in Figure 1. However, within EcC, this study considered only operational costs, e.g., technical skills and labor cost, equipment maintenance, environmental charges, and other charges. do not make financial and environmental cost figures accessible in the public domain [59]. Both public and private sectors involved in MSWM may lack such important data or, if they have them, may be reluctant to disclose any that could be considered confidential [56]. However, as the provision of a clean environment (including waste management services) is a human right and a proxy of good governance and the sound management of public funds, making such data available for research and other uses should be standard practice [60][61][62].
Despite the lack of complete financial and environmental data, we used basic information available from reliable sources, such as the Vice President's Office (VPO), the National Environmental Management Council (NEMC), and Dar es Salaam Local Authorities (DLAs), to compute economic and environmental costs. The information was obtained through different methods, including an inventory survey whereby information including the cumulative amount of MSW generated and the percentage composition of each waste stream in Dar es Salaam from 2006-2017 was estimated.
For effective analysis of EcC, aspects such as the initial investment in a particular technology and the unit operating costs of treatment facilities need to be assessed [59,63] as shown in Figure 1. However, within EcC, this study considered only operational costs, e.g., technical skills and labor cost, equipment maintenance, environmental charges, and other charges. Because reports of EcC for different waste treatment options were insufficient, in some cases, researchers used secondary data from in-depth reviews from both developed and developing countries with similar experiences. This helped to generate an average unit cost (expressed in USD) for treating 1 metric ton (MT) of MSW for all treatment options in each waste stream, as shown in Table 2. This made it possible to estimate the entire EcC for the total amount of MSW generated in Dar es Salaam.
The EnC was calculated mainly based on the carbon emissions from MSW treatment. This considered the total waste generated in Dar es Salaam and applied a waste carbon calculator (WCC) to determine net carbon emissions in MT of CO2 equivalent, as adopted from the University of Texas (  Because reports of EcC for different waste treatment options were insufficient, in some cases, researchers used secondary data from in-depth reviews from both developed and developing countries with similar experiences. This helped to generate an average unit cost (expressed in USD) for treating 1 metric ton (MT) of MSW for all treatment options in each waste stream, as shown in Table 2. This made it possible to estimate the entire EcC for the total amount of MSW generated in Dar es Salaam. The EnC was calculated mainly based on the carbon emissions from MSW treatment. This considered the total waste generated in Dar es Salaam and applied a waste carbon calculator (WCC) to determine net carbon emissions in MT of CO 2 equivalent, as adopted from the University of Texas (Table 2). This was supplemented by emissions factors provided by the CA-CP Campus Carbon Calculator, now administered and maintained by the Sustainability Institute at the University of New Hampshire. The estimated EnC for treating 1 MT of each waste stream was obtained from this analysis ( Table 2).
To obtain the total economic and environmental costs, the average cost calculated for treating 1 MT of each waste stream ( Table 2, Sections "A" and "B") was multiplied by a total number of tons of a particular stream produced per day.
Given that waste generation for the entire assessment period (2006-2017) showed a continuous upward trend with no sign of decline, which was also supported by the linear projection (Section 3.1), it was considered appropriate to use data from the last year of assessment (2017) to compute both EcC and EnC. The method is robust; should there be any changes in volumes or trends of generated waste, such changes can easily be accommodated by making adjustments based on the new figures.
Apart from other factors such as the recovery of electricity under some scenarios [31], the criteria for scenario selection was mainly based on the two factors.

Using the Elimination and Choice Expressing Reality (ELECTRE) Method for Scenario Selection
The ELECTRE method is one of the multi-criteria analysis approaches first proposed by Bernard Roy and his colleagues at the consultancy firm SEMA and used to choose a new activity from the list of available options. It ranks relations using value comparisons separately among alternatives under each one of the criteria [87][88][89][90]. This method particularly suits decisions about complex environmental problems such as climate change [91] and urban pollution, including MSWM [38,92].
Analyzing economic and environmental costs for scenario selection was the core objective of this study. A weight vector (w) was subjectively assigned to simplify this analysis and ease the data processing and handling. An ELECTRE algorithmic structure was used to set a matrix and define w for each economic and environmental value. In this case, each factor was given a half weight (50%). Scholars are concerned about the sensitivity of choices to the assigned weights, which normally depend on an expert's point of view [31,93]. In this study, the effect of variations in weight (EcC and EnC) was minimal and was found to have no impact when analyzing indices. Therefore, variations in value were negligible, and the final matrix of aggregate dominance was identified robustly.
Several operational steps in the application of the ELECTRE method have been highlighted by different scholars. These can vary according to the specific purpose. Despite these differences, there are three major common steps; namely, normalizing the decision matrix, weighting the normalized decision matrix, and determining the concordance and the discordance [87,94,95]. However, for ease of analysis, the three steps can be subdivided into several smaller steps [93,96,97]. This study employed eight steps to apply ELECTRE: (1) organizing elements for initial computation, (2) standardizing elements of the initial decision matrix, (3) weighing the standardized decision matrix to construct outranking relationships, (4) constructing matrices of concordance and discordance, (5) constructing a discordance index, (6) setting concordance and discordance thresholds, (7) constructing matrices of concordant and discordant dominance, and (8) constructing a matrix of aggregate dominance. To reach the desired goal when using the ELECTRE method, each step was considered important [93] and was followed systematically using the general ELECTRE equation given below: Since each developed scenario represents two analyzed values, they were presented in a Cartesian coordinate system using the Y and the X axis for economic and environmental values, respectively. From the graphical representation, acceptable scenarios were easily observed based on the criteria set, i.e., EcC ≤ 3 × 10 5 USD and EnC ≤ 360 metric tons of CO 2 emission (see Section 3.2).
The development of the matrix of aggregate dominance provided researchers with the best options from a list of 108 scenarios. This was an important first step in decision-making. Many attempts to reduce environmental pollution from MSW in cities in developing countries fail because of high economic costs [98]. This had also been noted in Dar es Salaam [22,48,99]. It was, therefore, important to make sure that the dominance matrix from which the best scenarios were drawn was appropriate not only in terms of the environmental quality sought, as obligated by the international community, but also in terms of the economic capacity of the country. In this way, the study findings could produce economic benefits and support a sound environment quality for current and future generations in Dar es Salaam and other cities with similar characteristics [59,84,86].
Therefore, scenarios that met the conditions set, i.e., a daily cost of EcC ≤ 3 × 10 5 USD and EnC ≤ 360 metric tons of emitted CO 2 , were selected. The scenarios were thus divided into two sets-acceptable if they met both criteria and unacceptable if they failed to meet one or the other criterion. The sets of acceptable scenarios formed the initial decision matrix. Using the classification guide shown in Table 3, the acceptable scenarios were categorized to support further shortlisting of preferred options (also see Sections 3.3 and 3.4). The acceptable scenarios were then categorized into four major groups based on their similarities and differences, and a scenario with the minimum EnC was selected from each group to be described and used in final decision making.

Generation Trend of MSW in Dar es Salaam
As Table 4 shows, the rate at which MSW was being generated in Dar es Salaam increased significantly for the last 12 years; responses were inadequate. This trend made the environmental risk index (ERI) rise from 0.3-0.5 (risk level) from 2006-2017. More importantly, some indices such as pressure, state, and impact were more worrying than the comprehensive ERI [11].
Based on the actual and the projected situation, it is evident that MSWM in this area is problematic and threatens the ecological environment and human health. As clearly shown in the national population projection report for 2013-2035 [44], the rate of MSW generation has increased over the years. If nothing is done to improve the current management situation, this trend will worsen in the future, and the environmental quality will deteriorate further [11]. To confirm this, Figure 2 shows a fitted linear trend line (R 2 = 0.44) for MSW generation based on the following equation: where x is the year. This means that MSW generation in Dar es Salaam increased by approximately 80 tons every year from 2006-2017. If this trend does not change (and it is likely to increase), it is estimated that by 2031, the amount of MSW generated will be more than 6400 MT per day ( Figure 2).  The results above were consistent with the previous study by Kazuva et al. [11] which showed that, under the impact of urbanization, improving standards of living (tending to change consumption patterns), and other driving forces, MSW generation will rapidly increase and raise the environmental hazard level, with ERI rising to or near the critical point (≥0.8). If there is no prompt and effective response from the current management, MSW will become a major threat to the ecological environment and public health in this city.

Formulated Scenarios for MSW Management in Dar es Salaam Code
As noted, all possible combinations of potential MSW management and treatment options for all waste streams in Dar es Salaam were quantified. As a result, a total of 108 scenarios were obtained, as presented in Table 5. Each scenario represents a complete waste treatment option, considering the differences in the percentage composition of each stream (see Table 4 and the description of the scenario development stage (Section 2.2.1)).
From the set conditions as stated in the Methodology section, all these scenarios were analyzed The results above were consistent with the previous study by Kazuva et al. [11] which showed that, under the impact of urbanization, improving standards of living (tending to change consumption patterns), and other driving forces, MSW generation will rapidly increase and raise the environmental hazard level, with ERI rising to or near the critical point (≥0.8). If there is no prompt and effective response from the current management, MSW will become a major threat to the ecological environment and public health in this city.

Formulated Scenarios for MSW Management in Dar es Salaam Code
As noted, all possible combinations of potential MSW management and treatment options for all waste streams in Dar es Salaam were quantified. As a result, a total of 108 scenarios were obtained, as presented in Table 5. Each scenario represents a complete waste treatment option, considering the differences in the percentage composition of each stream (see Table 4 and the description of the scenario development stage (Section 2.2.1)).
From the set conditions as stated in the Methodology section, all these scenarios were analyzed using economic and environmental variables. The intention was to make sure that any selected scenario is not only economically efficient by considering the local economy but also does not compromise environmental quality for current and future generation. EcC and EnC were computed for all scenarios and, as shown in Table 6, the maximum costs were $512,500 USD (EcC) and 3825 MT of emitted CO 2 (EnC) per day, which occurred in scenarios S70 and S89, respectively. In contrast, the minimum cost was $211,800 USD (EcC) and −3546 MT of CO 2 emissions (EnC) per day for S2 and S55, respectively. Figure 3 is a graphical presentation of the unit costs (EcC and EnC) of all scenarios. Scenarios with the lowest economic and environmental cost fell within the "zone of acceptable scenarios". They were clearly identified and confirmed to have met the set criteria for initial selection.

Selected Scenarios for Initial Decision Matrix
As indicated in Figure 3, scenarios that met the set criteria, i.e., EcC ≤ 3 × 10 5 USD as the daily cost of the comprehensive treatment of all MSW in the city and an EnC ≤ 360 metric tons of emitted CO2, were selected for the next stage of analysis.
However, the action plan toward implementation of goal 13 (climate action) of the 2030 sustainable development goals (SDGs) and the International Organization for Standardization (ISO) standards imply that any development project should include initiatives that reduce greenhouse gas (GHG) emissions to the atmosphere [85,100,101]. The MSWM scenarios should be considered in light of this obligation. In this regard, the most favorable option (the ideal point) would be the one with the lowest environmental cost. From the scenarios generated (Figure 3), this would be S55 (−3546.30 MT CO2 emissions). However, this study also considered the economic costs. S55 had a very high EcC (approximately $470,200 USD per day), which is not favorable given the current economic status of this country and other developing countries [31,[102][103][104]. Therefore, this scenario would not be acceptable in the Dar es Salaam environment. As Figure 3 shows, there were a number of other scenarios with good environmental consequences (S58, S57, S60, and S61) that were outside the acceptable economic cost boundary.

Selected Scenarios for Initial Decision Matrix
As indicated in Figure 3, scenarios that met the set criteria, i.e., EcC ≤ 3 × 10 5 USD as the daily cost of the comprehensive treatment of all MSW in the city and an EnC ≤ 360 metric tons of emitted CO 2 , were selected for the next stage of analysis.
However, the action plan toward implementation of goal 13 (climate action) of the 2030 sustainable development goals (SDGs) and the International Organization for Standardization (ISO) standards imply that any development project should include initiatives that reduce greenhouse gas (GHG) emissions to the atmosphere [85,100,101]. The MSWM scenarios should be considered in light of this obligation. In this regard, the most favorable option (the ideal point) would be the one with the lowest environmental cost. From the scenarios generated (Figure 3), this would be S55 (−3546.30 MT CO 2 emissions). However, this study also considered the economic costs. S55 had a very high EcC (approximately $470,200 USD per day), which is not favorable given the current economic status of this country and other developing countries [31,[102][103][104]. Therefore, this scenario would not be acceptable in the Dar es Salaam environment. As Figure 3 shows, there were a number of other scenarios with good environmental consequences (S58, S57, S60, and S61) that were outside the acceptable economic cost boundary.
On the other hand, if selection were to be based merely on economic efficacy, the ideal scenario would be S2, which costs only $211,800 USD per day. However, from the United Nation (UN) and ISO action plans, this scenario was not acceptable, as its EnC (1189.74 MT CO 2 emission) was high compared to other scenarios. Other scenarios with this behavior (economic efficiency but high environmental cost) included S5, S8, and S11. Choosing one of these to deal with MSW would pose a serious environmental risk by increasing GHG emissions (in this case, CO 2 ), jeopardizing the ecological environment and human health [105]. Indeed, most of them were outside the acceptable scenarios selection zone ( Figure 3). Therefore, after all the analysis, eleven scenarios (S19, S22, S21, S1, S24, S4, S25, S91, S20, S3, and S23) (highlighted in Figure 3) were found to have met the minimum requirements and formed the initial decision matrix. These scenarios and their EcCs and EnCs are listed in Table 7.

Grouping of Acceptable Scenarios
From the initial acceptable points, any scenario could be selected. However, further analysis was needed to reach the most favorable options with both competitive advantages (EcC and EnC). Furthermore, the selected scenarios had some common characteristics, allowing them to be classified into groups based on their similarities and their differences by the use of the acceptable scenarios classification guide (Table 3). Four major groups were identified using the ELECTRE method: (1) the most favorable scenarios, (2) favorable scenarios, (3) less favorable scenarios, and (4) intermediate scenarios ( Table 8). As explained in the Methodology section, each group could be represented by one scenario; that is, the most favored scenario in the group was that with the minimum environmental cost. In each group, this representative scenario was used for further analysis, description, and final decision making. Any scenario at this level was considered economically and environmentally acceptable. In this case, the environmental cost was given the first priority to determine the acceptability level of each point. The ELECTRE aggregate of dominance matrix is graphically presented in Figure 4, indicating four scenarios as the final selected ideal points (S19, S21, S25, and S20). Each point was carrying two analyzed economic and environmental factors. As shown in the figure, the longer the EnC value went far below the "maximum acceptable environmental cost level", the more the scenario became suitable for adoption. Thus, the level of acceptance for each case was identified, whereby S19 had the highest dominance degree (83%) followed by S21 (43%), S25 (35%), and S20 (13%). The scenario suggests that after all the generated MSW in Dar es Salaam is collected and segregated, organic waste would be composted, whilst plastic, paper, glass, and ferrous metal would be recycled. The remaining waste, consisting of 20.25% of the total, would go to sanitary and/or bioreactor landfills. To a great extent, this scenario was consistent with the previous work by the ISWM to Dar es Salaam [18] and other scholars that have worked on MSWM in this city [48,99,106]. The second option (S21) had costs of $261,052.80 USD and −657.24 MT of CO2 emission per day. The scenario was similar to S19 but differed in that plastic waste would go to sanitary and/or bioreactor landfill rather than recycling. This would somewhat increase CO2 emissions (an addition of 1.35 MT of CO2 per ton of MSW treated) but represented a reduction in EcC of approximately $30 USD for every ton of plastic waste compared to the first option (S19). Therefore, despite being more economically competitive than S19, the additional GHG emissions made it not favored for the first position. The level of acceptance for all scenarios must reflect the actual situation with MSW in Dar es Salaam. This includes socio-economic and environmental aspects, population trends, future waste generation projections, available national policies, and international agreements on the environment. Therefore, with the objective of sustainably managing and treating MSW in Dar es Salaam, the four favored scenarios are considered in more detail below.
The first option (S19) was the most favored option with the highest acceptance level (Figure 4), costing $274,065.03 USD and −1585.02 MT of CO 2 emission per day. According to the classification guide, the scenario had a low EnC and a medium EcC. It was a "most favorable option" ( Table 8). The scenario suggests that after all the generated MSW in Dar es Salaam is collected and segregated, organic waste would be composted, whilst plastic, paper, glass, and ferrous metal would be recycled. The remaining waste, consisting of 20.25% of the total, would go to sanitary and/or bioreactor landfills. To a great extent, this scenario was consistent with the previous work by the ISWM to Dar es Salaam [18] and other scholars that have worked on MSWM in this city [48,99,106].
The second option (S21) had costs of $261,052.80 USD and −657.24 MT of CO 2 emission per day. The scenario was similar to S19 but differed in that plastic waste would go to sanitary and/or bioreactor landfill rather than recycling. This would somewhat increase CO 2 emissions (an addition of 1.35 MT of CO 2 per ton of MSW treated) but represented a reduction in EcC of approximately $30 USD for every ton of plastic waste compared to the first option (S19). Therefore, despite being more economically competitive than S19, the additional GHG emissions made it not favored for the first position.
The third option (S25) required that organic waste be composted, while paper waste would be incinerated for energy recovery. The other waste streams (plastic, glass, and ferrous metal) would be recycled, while the remaining waste ''others", as in the previous options, would go to sanitary and/or bioreactor landfills. The costs were $295,045.74 USD/day and −489.97 MT of emitted CO 2 . It was clear that this scenario was significantly less favorable in both economic and environmental cost compared to the previous two options. It had an advantage in the speed of implementation, as it involved the use of more sophisticated technology. Therefore, its adoption could reduce the human health-related impacts from direct contamination. However, it remained the third choice.
The fourth option (S20) had characteristics distinct from the other favored scenarios. Among the four, it had the lowest EcC ($225,908.77 USD/day) but the highest EnC (260.21 MT of CO 2 ). The scenario required that organic waste be composted while plastic waste be incinerated. The other waste streams (paper, glass, and ferrous metal) would be recycled, while ''others" waste would go to landfill. Despite this scenario having the lowest EcC (mainly because the incineration of plastic waste increases the rate of return from energy recovery), this treatment option would not only add to the amount of GHG emitted but would also lead to ashes and dust being deposited in surrounding areas, posing a risk to the ecological environment and general public health [107][108][109]. Because of this, the scenario was the least favored of the acceptable scenarios.
Although each of these four scenarios had independent features, they all included composting of organic waste as the best treatment option due to its economic advantages. However, landfills have been the main option used for the treatment of organic waste in the city of Dar es Salaam. Scholars indicate that the sites used as landfills are usually changed into uncontrolled dumpsites [18,110]. Therefore, this study recommends the adoption of organic waste composting for its competitive advantage rather than relying on landfills, which has thus far caused unsustainable MSWM.
Comparing the amount of CO 2 emitted from composting and pyro-gasification of biodegradable waste, i.e., 0.09 and −0.56 MT of CO 2 /MT of treated waste, respectively, suggests that pyro-gasification is the better option. However, the economic cost for investing in pyro-gasification technology in Dar es Salaam makes it unacceptable. Instead, composting with its economic competitiveness and moderate environmental cost remains the best option for now. As suggested by scholars [111][112][113], composting organic matter will not only ameliorate the MSW problem in the city and other parts of the country but will also improve soil nutrients. This will add value to urban agriculture, which constitutes up to 45% of the economic activity of low-income urban dwellers in the city and its surroundings.

Conclusions
In this study, both economic and environmental costs for comprehensive MSWM were identified and presented. Each designed scenario had distinct features that differentiated it from others. Applying the ELECTRE method was useful for identifying the dominant scenarios in terms of reducing economic and environmental costs. The investigation considered the country's economic, political, sociocultural, and policy aspects and pertinent international treaties on environmental standards. Of all the 108 explored cases, four (S19, S21, S25, and S20) met all the selection criteria. From these, S19 was found the most favorable, forming the core of the four considered. Thus, decision makers should first consider this scenario. The identified economic and environmental costs were estimated on a daily basis. This is seldom the case in current decision-making or in literature. The approach and the methods used are applicable to other cities with similar MSWM issues with or without adjusting the scenarios used in this study. They can also be used to determine the best methods of dealing with urban pollutants other than MSW, leading to sustainable environmental quality in developing countries.
Author Contributions: All authors contributed pointedly to the preparation of this manuscript. E.K. is the principal author, worked on data collection, analysis, interpretation and report writing. J.Z. is the corresponding author, worked on site selection, research methodology, interpretation of the study results and final revision