*2.4. Composition of MSW Generated in Harare and Its Dormitory Towns*

The composition of MSW is a vital aspect in MSW management as it is necessary for examining sustainable options for MSW reduction, recovery (reuse and recycle) as well for identifying the most appropriate and sustainable treatment and disposal method [79]. Hoornweg and Bhada-Tata [1] observed that organic waste fraction of MSW in developing countries constitutes a much larger fraction as compared to developed countries. However, like MSW generation data, reliable MSW composition data are absent in the study area. Estimates of averages from the Environmental Management Agency and notable literature studies conducted in Harare, Bulawayo and Chinhoyi were considered for the study as illustrated in Table 3.


**Table 3.** Estimates of percentage composition of MSW generated in Harare.

### *2.5. Integrated MSW Management Options and Treatment Processes*/*Life Cycle Stages*

The integrated MSWM options and their associated processes or life cycle stages are described in the sections below and summarised in Table 4. The transportation system considered is the municipal waste collection service by municipal waste collection trucks [86]. Transport figures for waste collection were derived from the product of annual MSW generation for Harare and the estimated average distance the waste will be transported to the MSW management facility giving 21,028,500 t·km (product of distance to be travelled by the MSW to the treatment facility and the weight of the MSW transported) as waste collection trucks were estimated to travel an average distance of 90 km to and from the MSW management facility. The return trip was modelled only for an empty waste collection truck for the 22,252 trips of 45 km distance each carried out annually.


The recovery of the recoverable materials considered a mixed bag sorting plant equipped with relevant filters to treat waste gases produced during the recovery of the recoverable materials. The materials considered for recovery are metals, paper, plastics and glass at a recovery rate of 20% of their annual estimated generation. The anaerobic digestion plant considered the anaerobic digestion of the estimated biodegradable fraction of MSW amounting to 196,166 metric tons that is generated annually in Harare and its dormitory towns to produce biogas at an estimated average production rate of 115 m3/metric ton [87–90]. The biogas produced will be burnt to produce electrical and heat energy. The digestate or solid residue from the anaerobic digestion process will undergo a compositing process to obtain quality compost for sale as a biological fertilizer or soil enhancer. Gases from the anaerobic digestion process will undergo bio-filtration before being scrubbed or washed with sulphiric acid to produce a generally acceptable leachate that is assumed or considered decontaminated. The mixed bag fraction that reaches the incineration plant will be combusted in a furnace. Combustion engines transform the flue gases from the furnace into electrical energy. Combustion furnace bottom ash will be used in road construction as aggregates prior to its treatment with physical chemical treatment

methods applied to treat the leachate produced during bottom ash recovery. Gaseous emissions from the combustion furnace are treated using appropriate methods such as lime based dry adsorption, bag house filtration, activated carbon-based adsorption and selective noncatalytic reduction. Mixed bag MSW is landfilled with energy recovery. The landfill leachate undergoes nitrification–denitrification process under pressure. Ultrafiltration is used to separate the sludge from the leachate. The treated leachate is sent to a wastewater treatment plant. The transportation of treated leachate from the landfill to the wastewater treatment plant is considered negligible.

### *2.6. Life Cycle Assessment*

LCA was used to estimate and compare the potential acidification, eutrophication, global warming and human health impacts of the various six MSW management scenarios. ISO 14040 standards [91] were the basis for the LCA study. Several studies have been carried out using LCA to assess different MSW management scenarios in a number of countries, namely Spain [57,92,93], Italy [58,94,95], China [96,97], Brazil [59], Australia [98], Indonesia [99], Canada [100], United States of America [101], Lithuania [102] and Nigeria [103,104] to mention just but a few. LCA was therefore applied to assess the human health, acidification, eutrophication and global warming potential of the various MSW management scenarios in Harare and its dormitory towns of Chitungwiza, Epworth, Norton and Ruwa.

### 2.6.1. Goal and Scope

LCA was performed to assess the acidification, eutrophication, global warming and human health impact potentials of the proposed six MSWM scenarios that could be implemented in Harare and its dormitory towns. The LCA results could possibly inform decisions for future MSWM in Harare and its dormitory towns considering the increasing population, lifestyles, global pressure for the need for sustainable cities, the impacts the current MSWM option has on both the environment and human health as well as the imminent closure of Pomona dumpsite whose capacity will be exhausted by 2020 [54].

### 2.6.2. The LCA System Boundaries

The processes that fall under the scope of the study are within the MSWM system boundary as denoted by the dotted line on Figure 2. The entire management processes of all MSW which is not managed by or on behalf of municipalities fall outside the system boundary and study scope. Associated impacts from emissions emanating from the construction of MSWM facilities were assumed negligible compared to those produced from the actual operation of the facilities, hence they were not considered under the study as noted by Mendes et al. [59].

**Figure 2.** Life cycle assessment (LCA) system boundaries.

### 2.6.3. LCA Functional Unit and Software Model

The annual MSW generation for Harare and its dormitory towns of 467,303 tons was considered the functional unit for LCA. Quite a number of studies applied the annual MSW generation as the functional unit [92,95,105]. SimaPro software Version 8.5.2 analyst and its associated database update852 produced by Pre'Sustainability consultants B.V in Amersfoort, Netherlands were used to undertake the LCA. The impacts loads associated with the materials and processes were gathered from the Ecoinvent 3 database (2018) [106]. The detailed input–output pathways for the LCA are as shown in Figure 3.

The anaerobic digestion project database modelled for the rest of the world found on the processing, waste, biowaste and transformation pathway was utilised for the LCA with 2.26 <sup>×</sup> 10<sup>7</sup> m<sup>3</sup> of biogas produced annually being the inputs for MSWM options A3, A4, A5 and A6 where AD was incorporated. Alternatively the AD project database modelled for the rest of the world on processes, waste treatment, waste, transformation and finally biowaste pathway can also be used if the amount of biowaste to be digested is used as input. For waste incineration, the respective individual waste types i.e., metals, glass, paper, biodegradable and plastics that constituted MSW were modelled using their corresponding project databases modelled for the rest of the world on the product selection pathway processes, waste treatment, waste, transformation, incineration then finally municipal incineration with the specific MSW fraction quantities provided in Table 4 under MSWM options A2, A3 and A5 being the inputs. The reason being that Ecoinvent MSW incineration database modelled for the rest of the world is only recommended to be used for MSW with an average of 92.8% burnable fraction which is not a characteristic of the MSW generated in Harare; MSW generated in Harare has a combustible fraction of just over 75%, as reported by Makarichi et al. [81]. The MSW fraction-specific Ecoinvent database modelled for the rest of the world on the processes, waste treatment, waste, transformation, landfilling and then finally sanitary landfilling pathway was used for landfilling with the waste-specific quantities provided in Table 4 for the scenarios that incorporated landfilling being used as model inputs. Waste collection and transportation average distance of 45 km was considered giving a total of 2.10 <sup>×</sup> 10<sup>7</sup> t·km input on the Ecoinvent transport model for the rest of the world on the processes, transport, road and transformation pathway for all the MSWM options.

**Figure 3.** LCA methodological framework.

### 2.6.4. Life Cycle Impact Assessment (LCIA) Method

The LCIA for all the processes under the MSWM scenarios was undertaken using the ReCiPe 2016 v1.02 endpoint method, Hierarchist version, which is the default ReCiPe endpoint method. ReCiPe 2016 method is a new version of ReCiPe 2008 that was created by RIVM, Radboud University, Norwegian University of Science and Technology and PRé Consultants [107,108]. The method has 22 defined endpoint impact categories which are grouped into three damage categories, namely human health, ecosystems and resources. ReCiPe2016 has characterization factors that are globally representative rather than being representative only for Europe while at the same time providing the possible implementation of characterisation factors at national or continental scale for a handful of impact categories. The choices of values used in deriving characterisation factors and the midpoint characterization factors are provided by Huijbregts et al. [107] with the endpoint characterisation factors directly derived from the midpoint characterisation factors according to Equation (1). Therefore, constant global midpoint to endpoint characterisation factors were determined for all the impact categories save for fossil resource scarcity due to limited cause–effect pathway knowledge. The derivation of individual impact category midpoint to endpoint characterisation factors is provided [107,108].

$$\text{CFe}\_{\text{x,c,a}} = \text{CFm}\_{\text{x,c}} \times \text{F}\_{\text{M} \to\_{\text{}} \text{E}\_{\text{,c}} \text{a} \nu} \tag{1}$$

where; CFe and CFm are the end and midpoint characterisation factors respectively, c is the cultural perspective; a is the area of protection, namely human health, freshwater ecosystems, marine ecosystems, terrestrial ecosystems or resource scarcity; x is the stressor of concern; and FM→,E,c,a is the conversion factor from midpoint to endpoint impact for c and a.

### **3. Results**

Figure 3 shows the LCIA results for the acidification, eutrophication, global warming and human health impact potentials for the six MSW management options under consideration. All the MSW management options under consideration lead to a reduction in global warming and human health endpoint impact categories. Detailed results for the endpoint impact categories for acidification, eutrophication, global warming and human health are presented below.

### *3.1. Acidification*

Figures 4–7 show that MSW management options A1, A5 and A6 lead to reduction in acidification while A2, A3 and A4 contribute to increased acidification. The acidification impact potential is measured using the species extinction rates (species-years). A6 contributes the highest reduction in acidification potential of <sup>−</sup>3.9 <sup>×</sup> 10−<sup>2</sup> species-years, followed by A5 with an acidification potential reduction of <sup>−</sup>2.97 <sup>×</sup> <sup>10</sup>−<sup>2</sup> species-years. Results show that A1 contributes the least acidification potential of <sup>−</sup>8.94 <sup>×</sup> <sup>10</sup>−<sup>3</sup> species-years, which is consistent with findings by Mendes el al. that landfilling with gas recovery and leachate treatment leads to reduced acidification impacts. The recovery of metals plays a crucial role in reducing the eutrophication impacts under A5 and A6 as observed by Beigl and Salhofer [105]. A2 leads to the greatest acidification potential of 4.13 <sup>×</sup> <sup>10</sup>−<sup>2</sup> species-years, with A3 giving an acidification increase of 2.48 <sup>×</sup> 10−<sup>2</sup> species-years. A4 leads to the least increase in acidification of 8.57 <sup>×</sup> <sup>10</sup>−<sup>3</sup> species-years. Sensitivity analysis results from Table <sup>5</sup> show that increasing materials recovery levels for A5 and A6 to 28% and 24%, respectively, will result in zero acidification impact potentials.



**Figure 4.** Acidification impact potentials.

### *3.2. Eutrophication*

Figure 5 shows that MSW management options A1, A4, A5 and A6 bring about a reduction in eutrophication, with A2 and A3 leading to increased eutrophication. The eutrophication impact potential is measured using the species extinction rate (species-years). A1 has the highest eutrophication reduction potential of <sup>−</sup>2.16 <sup>×</sup> <sup>10</sup>−<sup>2</sup> species-years followed by A6 with eutrophication potential reduction of <sup>−</sup>6.12 <sup>×</sup> 10−<sup>3</sup> species-years. A4 and A5 have eutrophication reduction potentials of <sup>−</sup>3.77 <sup>×</sup> 10−<sup>3</sup> and <sup>−</sup>2.81 <sup>×</sup> 10−<sup>3</sup> species-years, respectively. A2 and A3 result in eutrophication potential increases of 2.55 <sup>×</sup> 10−<sup>4</sup> and 1.60 <sup>×</sup> 10−<sup>3</sup> species/year, respectively, indicating that the incineration of MSW leads to increased eutrophication, which was also noted by Hong et al. [109]. This confirms that materials recovery contributes to reduced eutrophication potential as it contributes to the reduced eutrophication potential characterizing A5 consisting of incineration, materials recovery and the AD of the biodegradable fraction of MSW. Sensitivity analysis results from Table 5 show that doubling the materials recovery levels under A5 and increasing it to 26% under A6 will result in zero eutrophication impact potentials.

**Figure 5.** Eutrophication impact potentials.

### *3.3. Global Warming*

As shown in Figure 6, all six scenarios lead to reductions in global warming, with A5 having the highest global warming reduction potential estimated at <sup>−</sup>9.05 <sup>×</sup> 10−<sup>1</sup> species-years followed by A3 that has a reduction potential in global warming of <sup>−</sup>8.28 <sup>×</sup> 10−<sup>1</sup> species-years. A2 brings about a global warming reduction potential of <sup>−</sup>7.68 <sup>×</sup> 10−<sup>1</sup> species-years and A1 has a <sup>−</sup>5.04 <sup>×</sup> <sup>10</sup>−<sup>1</sup> species-years reduction potential. A6 has the second from least global warming reduction potential of <sup>−</sup>2.03 <sup>×</sup> <sup>10</sup>−<sup>1</sup> species-years with A4 having the least reduction potential of <sup>−</sup>1.46 <sup>×</sup> <sup>10</sup>−<sup>1</sup> species-years. It is therefore evident that the scenarios that combine other MSW treatment technologies with incineration perform better compared to those combined with landfilling, which is consistent with findings by Wittmaier et al. [110]. The materials recovery also contributed to reduced global warming potential as indicated by the increase in the reductions in global warming potential from A3 to A5 and A4 to A6. Results from Table 5 sensitivity analysis show that no materials recovery effort is necessary under A5 as reduction in global warming impact potential will be realised in its absence. However, under A6, sensitivity analysis indicates that a 6% materials recovery is sufficient to attain zero global warming impact potential.

**Figure 6.** Global warming impact potentials.
