1. Introduction
The annual global municipal solid waste (MSW) generation rate is projected to reach 2.2 billion metric tons per annum by 2025 from 1.3 billion metric tons per annum in 2012 [
1]. Member countries of the Organization for Economic Co-operation and Development (OECD) however, are reporting a reduction in MSW generation [
2]. Dramatic population increase in urban areas within Africa and Asia was singled out by the United Nations [
3] as a typical phenomenon that leads to the astronomical increase in MSW generation. Standards of living, rapid urbanization, ever increasing population and obtaining economic environments in a given locality were cited as some of the factors that influence MSW generation [
4,
5,
6,
7]. Dongquing et al. [
8] also cited the type and abundance of a region’s natural resources apart from the above mentioned factors as a factor that influences MSW generation.
The best way to identify and manage solid waste streams is the fundamental environmental issue globally, both in industrialised and developing nations [
9]. Global initiatives are supporting the prioritization of solid waste management (SWM) because it is viewed an important facet for the sustainable development of any country [
10]. Sustainable development is the reduction of ecological footprints while improving quality of life for current and future generations within the earth’s capacity limit [
11]. UNDESA [
12] Agenda 21 of the Rio Declaration on Environment and Development affirmed the need for environmentally friendly waste management since it is an environmental issue of major concern in maintaining the quality of the earth’s environment.
1.1. Solid Waste Management Dynamics in Developed Nations
Solid waste (SW) mass production characterised human life since the formation of non-nomadic communities around 10,000 BC [
13]. Seadon [
14] argued that small communities could bury the SW they generated in environments surrounding their settlements or dispose it in rivers, which could not prevent the wide spread of diseases or foul odours from accumulated SW and filth emanating from increased population densities that characterised the formation of non-nomadic communities. Exceptional cases on waste management existed Worrell and Vesilind [
13] reported that by 200 BC, organised (SWM) systems had been under implementation in Mohenjo–Daro, an ancient Indus Valley metropolis, and the Chinese had established disposal police to enforce waste disposal laws. Melosi [
15] also reported that by 500 BC, the Greeks had issued a decree that banned the disposal of waste in streets and organised first accepted MSW dumps in the Western world.
Middle Ages’ city streets were characterised by odorous mud with stagnant water, soil, household waste and excreta from both humans and animals creating favorable conditions for disease vectors [
16]. Therefore, the disposal of biodegradable or organic waste in streets is argued to have partly contributed to the Black Death of the 1300s that occurred in Europe [
13,
16,
17]. Developments in SWM in developed nations were and are initiated to address environmental, land use, natural resources depletion, human health, climate change, waste value, aesthetic, economic, public information and participation issues associated with improper waste disposal [
13,
16,
18,
19,
20]. SWM has evolved in developed nations driven by historical forces and mechanisms which can possibly inform the development of SWM strategies in developing nations [
20]. Marshall and Farahbakhsh [
21] noted five drivers for integrated SWM paradigm in developed nations, namely the environment, climate change, resource scarcity, public health and public awareness and participation.
Public health concerns remain a driver of SWM transformation in the developed world characterised with continued review of public health legislation. The need to reduce land, air and water contamination [
20,
22] was a primary driver of policy changes in SWM development in the 1970s and beyond [
20]. Waste control characterised the SWM policy framework between the 1970s and mid-1980s focusing on daily landfill compacting and covering and incinerator retrofitting for dust control. The SWM policies enacted from the 1980s to date focus on increasing technical standards, starting with control of landfill leachate and gas, reduction of incinerator flue gas and dioxin and the current span covering control of odour at composting and anaerobic digestion (AD) facilities [
20]. The last decade of the 20th century saw the increased focus and attention towards the adoption of integrative policy due to the inadequacies of advocating for continued increase in environmental protection only from both the technical (engineering and scientific) and environmental perspective without considering the political, economic, social, cultural and institutional dimensions of SWM [
20,
23,
24]. The waste hierarchy upon which the European Union (EU) current policy on waste is based reignited materials recycling and reuse of the 19th century in the 1970s [
20,
25] in light of the increasing scarcity of resources. The EU’s Second Environment Action Programme of 1977 introduced the waste hierarchy model for SWM priorities derived from the “Ladder of Lansik” [
26].
Climate change has also driven the development of SWM from the early 1990s to address greenhouse gas (GHG) emissions from biodegradable waste landfilling, a major contributor of methane gas emissions, complimented with a strong focus on the recovery of energy from SW [
20,
22]. The concerns by the public on poor SWM practices with their increased awareness have also contributed in driving the developments in SWM [
20]. The public became concerned with the location of SWM facilities in the vicinity of their households, ‘not in my backyard’ (NIMBY), though they appreciate the need of SWM facilities. Therefore, effective communication, wide public knowledge of SWM needs, the active engagement of all stakeholders during the entire SWM cycle have been successful in overcoming NIMBY public behavior and opposition to numerous developmental projects [
27] thereby acting as drivers for developments in SWM [
21].
1.2. Solid Waste Management Dynamics in Developing Nations
Despite the increase in waste generation, global call and acceptance that waste management must take an integrated approach to derive economic benefits while reducing environmental burdens, Africa is still lagging in this regard. This lag is also being witnessed despite the reported increased globalization as poor SWM challenges and their associated public health impacts are affecting urban environments in many developing nations [
22,
28,
29] one and a half centuries after the sanitary revolution in the EU [
30]. Unlike developed nations that are concerned with diseases associated with affluence (cancer, cardiovascular disease, alcohol and drug abuse), poor SWM derived public health impacts in developing nations are evidently manifesting in the form of communicable diseases giving the double headache of dealing with both communicable diseases and emerging diseases of affluence [
30]. Public health mostly drives SWM development in developing nations, though other factors as in developed nations are considered because the key priority is waste collection and removal from population centres as it was in European and American cities before the 1960s [
20,
31,
32,
33]. Wilson [
20] noted that environmental protection remained relatively low on the SWM priorities despite the presence of legislation prohibiting unregulated waste disposal with minor changes towards its prioritization taking place. The value of waste as a resource is also another vital driver within developing nations currently providing livelihoods to the urban poor through informal recycling [
20,
22]. Climate change is a significant driver globally with a number of nations having incorporated the municipal solid waste (MSW) sector amongst the sectors considered for low-emission development strategies (LEDs) on the national emission reduction commitments or targets within the nationally determined contributions (NDCs) framework of the Paris agreement under the United Nations Framework on Climate Change Convention (UNFCCC).
A number of similarities do exist between the current conditions characterizing many cities in developing nations and those experienced in European and American cities during the 19th century with regards to increased urbanisation levels, degraded sanitary environment emanating from lack of adequate sanitation and environmental services, inequalities and social exclusions in SWM systems, unprecedented mortality and morbidity levels due to inadequate sanitation, potable water supply and waste disposal services [
30]. Thus, developing nations are likely to go through almost similar SWM development pathways as those developed nations went through. However, Marshall and Farahbakhsh [
21] argued that despite these similarities, complex local-level-specific technical, political, social, economic and environmental challenges in developing nations have been created from rapid urbanization, increasing population, the fight for economic growth, institutional, governance and authority issues, international influences, along with their interaction with diverse economic, cultural, political and social dynamics which are bringing associated SWM complexities in developing nations.
In developing countries therefore, SWM is complicated by levels of urbanization, economic growth and inequality as well as socio-economic dimensions, governance, policy and institutional issues coupled with international interferences [
21] which limit the application of SWM approaches that succeeded in SWM development pathways for developed nations. The understanding of the origins and critical drivers in the past developments in SWM in developed nations provides contextual knowledge on the current changes occurring in developing nations. Simelane and Mohee [
34] identified African social norms with their associated concerns including economic and environmental issues, national and regional legislative deficiencies, technological and human resources developments and historical influences among other factors necessitating this lag. Iriruaga [
35], on another note, cited low private investment in infrastructure, industry linkages and academic research as the drivers of Africa’s inability to effectively derive benefits from the waste it generates. Muzenda et al. [
36] identified the increased demand for SWM provision, MSW minimization, and recovery of materials for reuse and recycle, constraining factors including physical, land use and environmental constraints, as well as demographic and socio-economic factors as the core drivers for the need of integrated waste management (IWM) techniques.
MSW generation and its disposal are causing enormous environmental and human health challenges in urban environments of developing countries [
37,
38,
39]. It is considered hazardous and to have toxic impacts on the biological environment, thereby affecting lifestyles and economic activities [
40]. This, therefore, calls for the need to sustainably manage waste to reduce its impact in the ecosystem and human health [
41]. The need to design and develop integrated waste management (IWM) options that seeks to meet the economic, technical, environmental and social constraints of products or production processes has become paramount and urgent. McDougall et al. [
42] defined IWM as a combination of technically sound, economically feasible, environmentally sustainable and socially acceptable collection and treatment processes that handle materials constituting MSW.
1.3. Municipal Solid Waste Management in Zimbabwe
Like many developing countries facing enormous MSW generation and disposal associated environmental and human health challenges in urban environments, the Government of Zimbabwe acknowledged that its urban local authorities (city municipalities, town councils, district councils and local boards) are experiencing major challenges in managing MSW due to rapid population growth. Most of Zimbabwe’s local authorities fail to cope with the ever increasing volumes of waste being generated by the public [
43]. Several studies have also affirmed that municipal solid waste management (MSWM) is one of the greatest challenges facing urban environments in Zimbabwe [
41,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53]. In Zimbabwe, about 60% of the MSW generated in urban environments is disposed at official dumpsites with the remaining waste being dumped illegally in undesignated areas namely storm water drains, open spaces, alleys and road verges [
45]. The dumping of waste in open and illegal dumpsites is not only an eyesore but creates an environment where disease causing vectors can thrive, contribute to air, soil and water pollution and emit greenhouse gases that cause global warming [
43].
MSW problems in Harare specifically are evidently manifesting in the form of both surface and groundwater pollution due to the dumping of MSW in waterways and untreated leachate from dumpsites. The storage capacity of the sole official MSW dumpsite in Harare is expected to reach its limit in the next five years [
54]. This calls for the need to redefine future MSWM options as well as redefining the models of operating the MSWM facilities considering biogas recovery for electricity generation as well as the production of saleable products from MSW. To date no or few studies have been carried out focusing on determining the most probable integrated MSWM option with the least environmental impacts for Harare. Such study results could possibly inform future decisions and policies on MSWM considering the increasing population, changing lifestyles, global pressure for the need for sustainable cities, the impacts the current MSWM practices have on both the environment and human health as well as the imminent closure of the existing dumpsite whose service life is anticipated to come to an end in 2020. This study, therefore, is a life cycle-based comparative assessment of the various probable MSWM scenarios to be implemented in Harare. The study seeks to identify the scenario with the least burden with regards to human health, acidification, eutrophication and global warming impact categories.
1.4. Life Cycle Assessment
Life cycle assessment (LCA) is a tool that could be used in the design and development of IWM options. LCA holistically quantifies the environmental burdens and impacts for entire products’ or processes’ life cycles [
55]. Winkler and Bilitewski [
56] described LCA as a science-based impact assessment methodology for the impacts of a product or system on the environment, which is not purely a scientific tool. LCA application in sustainable MSWM started over two decades ago, as argued by Güereca et al. [
57] that it has been applied for MSWM since 1995. The use of LCA for decision making and strategy development in MSWM systems has expanded rapidly over the recent past years as a tool with the capacity to capture and address complexities and interdependencies characterizing modern IWM systems [
58]. Mendes et al. [
59] noted the appropriateness of LCA application as a tool for decision making and strategy development in MSWM because of the associated wide differences in spatial locations, waste composition and characteristics, sources of energy, waste disposal options available as well as available nature and size of products from various waste treatment methods. Therefore LCA has emerged as an appropriate holistic method increasingly being applied in MSWM decision making and strategy development processes [
60].
LCA has been previously applied to assess the associated impacts of MSWM systems thereby assisting in comparing alternative MSWM systems and/or identifying areas of major concerns that need potential improvements [
61]. It has been applied to identify and probe likely negative impacts of various MSWM practices [
62] because it is capable of calculating and comparing impacts of different MSWM scenarios [
63]. It incorporates environmental impact weighing or valuation to estimate the performance of a specific MSWM scenario [
62]. The intensification of MSWM policies in Europe and global call for the implementation of LCA methodology ISO 14044: 2006 standards have resulted in a positive trend towards the adoption of life cycle studies on MSWM [
64]. To date, numerous studies have been undertaken worldwide applying LCA to the different MSW life cycle stages that cover the entire life cycle of MSW [
60,
62,
63,
64,
65,
66]. Khandelwal et al. [
64] reviewed 153 studies that applied LCA on MSWM, undertaken globally and published between 2013 and 2018. The distribution of the selected LCA studies reviewed by continents showed that 72 were in Asia, 53 in Europe, 10 in North America, 9 in South America, 3 in America, 2 in Africa, 2 addressed generic cities assuming MSW generation, characteristics and associated environmental emissions together with other remaining studies that focused on at least one country. Very few life cycle studies on MSWM were found in Africa and poor LCA methodology penetration in Africa was cited as the cause of the limited LCA studies on MSW. The only two LCA studies found for Africa were done in Nigeria.
4. Discussion
LCIA results show that scenario A6 is the best option with regards to acidification while scenario A2 is the worst option. MSWM option A1 is the best scenario considering eutrophication potential and A3 is the worst. In terms of global warming and human health impact potential, A5 is the best option and A4 is the worst MSWM option. Overall, MSWM option A5 emerges as the best option for managing MSW in Harare as shown in
Table 6. This is confirmed from findings by Sharma and Chandel [
111] that MSWM systems that combines incineration, anaerobic digestion, composting and materials recovery have the least environmental impacts.
The recovery of landfill gas for combined heat and power (CHP) generation under the current study is attributed to the reduction of impact potentials across all the impact categories under consideration, except for A4 under acidification, in the MSWM options that incorporated landfilling because energy recovery from waste bring significant environmental benefits [
95,
110,
112,
113,
114,
115,
116]. Khandelwal et al. [
64], in their review of 153 LCA based MSWM studies published between 2013 and 2018, had 9 studies concluding the appropriateness of AD compared to biodegradable waste landfilling. The same review noted the conclusions from 11 studies regarding the appropriateness of landfilling with landfill gas recovery for CHP generation. This was also noted by Yadav and Samadder [
62] in their review analysis of 91 LCA studies on MSWM undertaken from 2006 to 2017 in Asian countries with 5% of the reviewed studies reporting the relative environmental friendliness and sustainability of landfilling with landfill gas recovery—an observation that was also observed by Menikpura et al. [
117].
Yadav and Samadder [
62] further observed that incineration was reported as a better option than landfilling by 9% of the reviewed studies largely due to the reduced methane emissions associated with incineration. This observation is in agreement with this study’s conclusions with regards to human health and global warming impact categories since MSWM options A2, A3 and A5 that incorporated incineration bring more global warming and human health impact potential reductions than A1, A4 and A6 which incorporated landfilling. Cleary [
65], like Yadav and Samadder [
62], also noted the better performance of thermal treatment with regards to global warming, which is consistent with this study’s findings. Thermal treatment was also reported to perform better than landfilling in a critical review of 222 published LCA studies on SWM systems in general, accessed from 216 peer reviewed articles and 15 public reports undertaken by Laurent et al. [
63] and Laurent et al. [
66].
Overall review results by Yadav and Samadder [
62] show that 71% of the reviewed LCAs found landfilling to be the worst or least preferred MSWM treatment option with 8% of the studies concluding incineration to be the worst or least preferred MSW treatment option among other treatment options due to its associated harmful emissions in the form of dioxins and furans as well as human toxicity. Cleary [
65], in their review of 20 LCA-based MSWM assessments undertaken and published in peer-reviewed journals between 2002 and 2008, observed that 19 studies confirmed the low environmental performance of landfilling. A review by Abeliotis [
60] of 21 LCA studies further observed that landfilling was reported as the worst option for managing and treating MSW, as was observed by Mendes et al. [
118], Hong et al. [
109], Wanichpongpan and Gheewala [
116], Cherubini et al. [
95] and Miliūtė and Staniškis [
102]. However, despite these reported low environmental performances of landfilling, it performed better than incineration with regards to acidification and eutrophication impact potentials under this study. This is also contrary to observations made by Cleary [
65], who noted the better performance of thermal treatment compared to landfilling with regards to eutrophication and acidification impact categories.
The better environmental performance of recycling and thermal treatment of plastics and paper compared to landfilling, as shown by the best performance of A5 which combined incineration and recovery of materials together with AD, was observed by Laurent et al. [
63] and Laurent et al. [
66] in their reviews consistent with findings by Michaud et al. [
119], Lazarevic et al. [
120] and Tyskeng and Finnveden [
121]. Materials recovery and recycling are environmentally appropriate and sustainable as they lead to reduced environmental impacts potentials [
60,
62,
63,
64,
65,
66,
102]. This is confirmed by the better performances of A5 compared to A3 and of A6 compared A4 under this study; sensitivity analysis results that reveal an inverse relation between materials recovery levels and the magnitudes of environmental impact potentials.
Differences in results from LCA studies were observed by Laurent et al. [
66] who noted little agreements with regards to the conclusions and no definite agreement except for landfilling with regards to which amongst thermal treatment, anaerobic digestion and recycling is most preferable for managing or treating plastic, paper, organics and metals. De Feo and Malvano [
122] observed that the best IMSWM option is subject to the examined impact categories, hence the differences amongst impact categories considered render other MSWM or treatment methods environmentally sustainable while simultaneously rendering others as unsustainable. Khandelwal et al. [
64] singled out the heterogeneous nature of MSW as a factor that makes no single MSW treatment method capable to be applied to all the MSW fractions, inevitably resulting in different LCA results from region to region due to differences in MSW generation and composition, MSWM structures, system boundaries, MSWM practices and the choice of impact categories.