Environmental Impacts of Direct Combustion Waste-to-Energy as a Two-Pronged Solution for Waste Management and Power Generation in the Philippines

2.1. System boundary and functional units

This LCA considered the construction of the DC WtE facility, transportation of wastes, DC WtE operations, and landfill operations, which include the residual treatment and disposal (Figure 1(a)) with the following functional units (FU) based on its two (2) functions: MSW management (FU = processing of 1kg of residual MSW, sorted and collected at a materials recovery facility or transfer station) and power generation (FU = generation of 1 kWh energy) comparison. Data for the DC WtE plant was partially based on actual experimental plant built by the Philippines’ Department of Science and Technology (DOST) located at the University of the Philippines – Los Baños (UPLB) (Figure 1(b) ~1(d)).

Fig. 1.

Scope of the study: (a) system boundary [dashed lines indicate material transport], (b~c) model 25-kW DC WtE plant at the UPLB, and (d) process flow diagram of the model DC WtE.

2.2. Waste Data

As of 2020, the Philippines has 54% level of urbanization [16]. To have a good representation on waste profile based on type of settlement in the Philippines, waste characteristics data were obtained from representative urban barangay (Barangay Tanza 1 in Navotas City) and rural sitio (Sitio Masagana in Pulong Yantok, Angat Town in Bulacan Province), which have detailed waste analysis and characterization study (WACS). Waste generation data was sourced from various government reports [e.g., 10-Year Solid Waste Management Plan (SWMP) of LGUs, Regional Department of Energy and Natural Resources (DENR) Environmental Reports, and LGU WACS Reports]. In 2020, WACS data reporting was standardized by the Philippines’ National Solid Waste Management Commission, classifying wastes as biodegradable, recyclable, residual with potential for recycling, residuals for disposal, and special waste (Table 2) [17].

Table 2.

Waste analysis and characterization study (WACS) waste classifications employed in the study based on the Philippines’ National Solid Waste Management Commission [17].

Only residual wastes based from the WACS of the two communities were considered (Table 1), following the DENR Administrative Order No. 2019-21 stipulating that WtE facilities shall only accept source-segregated biodegradables (for biomass WtE) or residual waste (for thermal WtE) [7, 18].

Table 1.

Operation parameters employed in the LCA analysis for evaluation of DC WtE deployment in the Philippines

Lower heating values (LHV) for the two communities were calculated using an artificial neural network (ANN)-generated LHV model (Eq. 1) [15] based on waste composition [i.e., biodegradable or food wastes (F), paper wastes (Pa), and plastic waste (Pl)] and waste fractions (Table 3).

Table 3.

Waste fractions of the different waste compositions.

Meanwhile, from this LHV, energy potential recovery from municipal solid waste (EPmsw) can be estimated (Eq. 2) [14]:

2.3. Life Cycle Inventory and Impact Assessment

DC WtE infrastructure construction data was based on the model DC WtE plant. Waste collection processes were not directly considered in the inventory as only the amount of waste designated to the incinerator was accounted. Data for other parameters were sourced from Ecoinvent v3.8 LCA database [19].

The ReCiPe 1.13(H) method was used to evaluate the impacts, based on the hierarchist approach. Six midpoint indicators in the ReCiPe method were considered and grouped as follows: Global warming/climate change potential (GWP), Ecotoxicity [marine (METP) and terrestrial (TETP)] potentials, and Human health [human toxicity (HTP), particulate matter formation (PMFP), photochemical oxidant formation (POFP)] potentials.

3.1. Contribution analysis

Contribution analysis was conducted to identify the process or life cycle stage of the DC WtE technology that can contribute to a particular emission or impact category, allowing the identification of key problems and consequently, insight improvement or potential preventive actions [20]. Particularly, construction of the DC WtE facility (MSWI facility), transportation of wastes (Transport), DC WtE operations (MSWI operation), and landfill operations, which include the residual treatment and disposal (Landfill), were evaluated for DC WtE technology based on one (1) kg MSW FU and waste profiles generated from urban or rural community (Figure 2(a) ~ 2(f)).

Fig. 2.

Contribution analysis of DC WtE technology life cycle stages on midpoint indicators: a) GWP, b) METP, c) TETP, d) HTP, e) PMFP, and f) POFP [basis: 1 kg MSW].

Albeit almost similar in magnitude for all the overall stacked impacts of all midpoint indicators, impacts based on urban waste profile are relatively smaller than that of rural. However, these differences may not be associated to the waste profiles per type of settlement since impact contribution of MSWI operation, which is the most significant contributor to all midpoint indicators, are equal between urban and rural (Figure 2(a) ~ 2(f)). Construction of MSWI facility and Landfill operations were also noted to have appreciable impacts. However, the difference on the overall stacked impacts between urban and rural may not also be associated to Landfill since these are also almost similar (1~5% difference with respect to rural) to both settlements in all midpoint indicators. The construction of MSWI facility, on the other hand, shows significant difference (~57% with respect to rural), which is the main contributor to the impact difference. Meanwhile, Transport, although showing ~25% difference, can have insignificant contribution to the overall impacts for all midpoint indicators (7.96×10^-4% to 7.70×10^-1%).

GWP (also called climate change impact) was measured in terms of kilograms of equivalent CO₂ emissions (kg CO_2eq) per FU (i.e., 1kg MSW). GWP based on urban (0.5006 kg CO_2eq) and rural (0.5046kg CO_2eq) are relatively similar (Figure 2(a)) and are generally lesser than other GWP ex WtE reported: 0.674 kg CO_2eq to 1.490 kg CO_2eq [21], 0.058 kg CO_2eq to 496 kg CO_2eq [22], and 0.637 kg CO_2eq to 0.736 kg CO_2eq [23]. Landfill use is similar for both types of settlements (urban = 5.43×10^-3 kg CO_2eq; rural = 5.25×10^-3 kg CO_2eq) which contributed to ~1% to their respective total GWPs. Impact of MSWI facility on GWP for urban (3.06×10^-3 kg CO_2eq) is relatively smaller than that in rural (7.20×10^-3 kg CO_2eq) which contributed to 1.43% and 0.61%, respectively, to their overall GWPs. Meanwhile, MSWI operation significantly contributed to the DC WtE GWP both for urban (98.25%) and rural (97.47%). This is expected since DC of MSW involves the generation of climate-relevant emission which are mainly CO₂, N₂O, NO_x, NH₃, and organic C, measured as total carbon [24]. DC of MSW is said to produce 70 to 120% CO₂ [24] although emission profile, as well as efficiency and energy output, can vary on the technology/system specifications [25] and heterogeneity of the feed waste [24]. The MSW composition significantly influences the climate change impact (i.e., GWP) of DC WtE operation, particularly by the proportion of fossil fuel-derived materials such as synthetic plastics. The proportion of waste whose carbon compounds are assumed to be of fossil origin can determine the climate-relevant CO₂ emissions from DC [24]. Thus, the use of Equation 1 to determine the LHV of the wastes is further justified. Generally, urban waste streams commonly from affluent areas have higher concentrations of fossil-derived wastes such as plastic packaging. Similar studies done in Thailand [26] and Vietnam [27] confirm common differences in urban and rural waste profiles. From the waste profiles employed, wastes from urban can have 8.43% to 15.11% plastics while rural can have 6.87% to 10.15% which can be based from fossil fuels thereby, contributing to the GWP of the DC WtE.

DC WtE technology ecotoxicity was evaluated in terms of marine (METP) and terrestrial (TETP) ecotoxicity potentials which can represent the change in potentially disappeared fraction of species due to the environmental concentration of a chemical in marine water and in soils, respectively [28]. Both are measured in terms of 1,4-dichlorobenzene equivalents (kg 1,4-DCB_eq) [29] per kilogram of MSW incinerated. METP of DC WtE based on urban (0.2877 kg 1,4-DCB_eq) and rural (0.2882 kg 1,4-DCB_eq) are similar, with MSWI operation also being the significant contributor (urban = 99.85%; rural = 99.67%) (Figure 2(b)). This can be mainly due to the lack of wastewater treatment facility of the DC WtE design (Figure 1(d)) which can consequently cause possible leaching of heavy metals from residues (e.g., mercury, iron, copper, and zinc) which were also detected in the bottom ash of the DC WtE model plant. Nevertheless, these are lower than reported METPs of WtE in literature ranging from 0.79~4.79kg 1,4-DCB_eq per functional unit [30].

TETP of DC WtE based on urban (8.59×10^-5 kg 1,4-DCB_eq) and rural (8.68×10^-5 kg 1,4-DCB_eq) are also similar, with MSWI operation being the significant contributor (urban = 98.07%; rural=96.99%) (Figure 2(c)). TETP measures potential adverse effects on land-based ecosystems resulting from release of harmful substances (e.g., heavy metals, organic chemicals) during the entire WtE life cycle. However, due to the variable nature of wastes, it is often difficult to pinpoint the exact substances contributing to TETP, which in turn makes it difficult to model their biodegradability, bioaccumulation, and distribution [28]. Since the study includes the construction and installation of the WtE plant (i.e., MSWI facility) as part of the system boundary, capital goods are also expected to contribute to both TETP (Figure 2(c)) and also to the METP (Figure 2(b)). Nevertheless, the evaluated TETP for DC WtE based on urban and rural settings are comparable, if not less than those reported in literature; from a low –7.64×10^-3 kg 1,4-DCB_eq to 5.77×10^-2 kg 1,4-DCB_eq (per functional unit) [21]. Management and treatment of fly ash and bottom ash have a crucial role in affecting the METP and TETP values since these ash components contain heavy metals that could leach out into the surrounding environment after their production [31].

Human health impacts of the DC WtE technology were evaluated based on human toxicity (HTP), particulate matter formation (PMFP), and photochemical oxidant formation (POFP) potentials, which are measured in terms of kg 1,4-DCB_eq, kg PM10_eq, and kg non-methane volatile organic compounds (kg NMVOC_eq), respectively [32, 33]. DC WtE based on urban (0.3855 kg 1,4-DCB_eq) and rural (0.3875 kg 1,4-DCB_eq) have similar HTP of which, MSWI operation is still the significant contributor at 99.35% and 98.85%, respectively (Figure 2(d)). This is expected since the major factor contributing to the HTP of DC WtE is the heterogenous composition of the waste being incinerated, which may also contain hazardous chemicals and pollutants (e.g., acidic gases, NO_x, and dioxins) [34] and thereby pose risk when exposed to human body through ingestion, inhalation, or dermal exposure [35]. Aside from the MSWI operation, construction of MSWI facility (< 1%) and Landfill (<0.5%) contributed to the HTP, albeit almost insignificant.

PMFP of WtE technologies have high ranges due to the large variability brought by the specific technical characteristics of the incinerator [36]. However, the PMFP of MSWI operation of the model DC WtE for both urban and rural settings are equal (6.47×10^-5 kg PM10_eq), which constitutes 73.68% and 66.88%, respectively, to their respective overall stacked PMFP (urban = 8.78×10^-5 kg PM10_eq; rural = 9.67×10^-5 kg PM10_eq) (Figure 2(e)). These significant contributions from MSWI operation is also expected since the combustion nature of the waste incineration can emit precursors (e.g., NH₃, NO_x, and SO₂) [33] and dusts [34] to change ambient concentration of PM [33]. These percent contributions from MSWI operation are the lowest among all the midpoint indicators (Figure 2(e)). Meanwhile, Landfill (urban=17.8%; rural=15.5%) and construction of MSWI facility (urban = 7.90%; rural = 16.9%) contributed appreciably to the DC WtE PMFP.

Lastly, for POFP, MSWI operations still has the highest contribution for both urban (82.76%) and rural (79.80%) settlings although having equal magnitude (2.66×10^-4 kg NMVOC_eq) (Figure 2(f)). Heuristically, this is expected since waste incineration of heterogenous profile of waste feed can lead to the emission of NO_x [34] and NMVOCs, which can consequently change the ambient concentration of ozone [29]. Landfill also contributed (urban = 13.6%; rural = 12.5%) at similar magnitude for both settlings (urban = 4.38×10^-5 kg NMVOC_eq; rural = 4.15×10^-5 kg NMVOC_eq). However, the difference on the overall stacked POFP for urban and rural is mainly due to the contribution of the MSWI facility construction where urban (1.01×10^-5 kg NMVOC_eq) has lower POFP than that at rural (2.37×10^-5 kg NMVOC_eq), each contributing to 3.13% and 7.12%, respectively to their respective POFPs.

Contribution analysis shows that among the DC WtE life cycle stages evaluated, its operation has the most impact in terms of climate change, ecotoxicity, and human health. This necessitates an effective air pollution and wastewater control process to alleviate such impacts, especially from the perspectives of public awareness and acceptance [34]. Preventive actions should also be in place to address the impacts of its other stages such as the construction of the DC WtE facility as well as the handling and treatment of its by-products (e.g., bottom and fly ashes).

3.2. Air pollution and wastewater control process

Air pollutants that should be controlled for DC WtE include dust, acidic gases, NO_x, dioxins, and mercury [34]. In the DC WtE model (Figure 1(d)), bag filters were employed to remove air pollutants from flue gas through filtering. Alkali agent, Ca(OH)₂, and powdered activated carbon (PAC) are injected into flue gas before it passes through the bag filter. This system allows the removal of 1) acidic gases (e.g., HCl and SO₂) through reaction with Ca(OH)₂, 2) dioxins and mercury through adsorption with PAC, and 3) dust though filtering in the dust bag. This is evident through the sample analysis of the particulate matter, sulfur oxides (as SO₂), and nitrogen oxides (as NO2) at the stack of the DC WtE model lower than the DENR Standard DAO 2000-81. Meanwhile, the high dioxins and furans of the sample test result suggest the need for improvement and/or refining on the PAC/adsorption tower operating parameters. Other dioxin control includes stabilization of temperature throughout the operation (i.e., maintaining 3Ts: temperature, (retention) time, and turbulence) and ensure complete combustion, prevention of De Novo synthesis (by not employing electrostatic precipitators that retain flue gas at temperature ~ 300oC), and by finetuning of dust collection via bag filters operation parameters and employment of denitrification catalyst which can decompose dioxins [34]. Removed pollutants discharged from bag filters, together with the injected alkali agent and PAC (viz. fly ash) must then be disposed properly as hazardous waste.

As for the wastewater, the DC WtE model lacks wastewater treatment (Figure 1(d)). Closed systems that do not generate wastewater can be employed where wastewater generated during the treatment process can be sprayed as coolant in furnaces resulting to the evaporation which can then be treated by flue gas treatment systems [34].

3.3. Comparison with other waste disposal method

More than two decades after the enactment of the Ecological Solid Waste Management Act (ESWMA) in 2001 [37], the Philippines is yet to address the growing generation of waste coupled with lack of infrastructure to properly dispose such wastes [7]. Sanitary landfill is currently the only disposal site which will not be enough given the projected annual waste generation of >20Mm³ until 2025 [38]. Hence, the promotion of WtE by the Philippines’ DENR [39] for the integrated management of MSW [18].

Herein, the DC WtE was evaluated for its potential impact as a waste management method in the Philippines based on the aforementioned midpoint indicators. These impacts were compared to those of other waste management methods including the prominent sanitary landfill as well as other methods which include open burning, open dump, and unsanitary landfill (Figure 3(a)~3(f)).

Fig. 3.

Impact assessment comparison of DC WtE as waste management solution to other waste disposal methods: a) GWP, b) METP, c) TETP, d) HTP, e) PMFP, and f) POFP (Basis: 1 kg MSW).

GWP of DC WtE, both at urban (0.5006 kg CO_2eq) and rural (0.5046 kg CO_2eq) settlings are the smallest among all the waste disposal method (Figure 3(a)). Relatively smaller than that of sanitary landfill (0.5079 kg CO_2eq), DC WtE GWP is also smaller than that of open burning (0.6121 kg CO_2eq), open dump (0.6490 kg CO_2eq), and unsanitary landfill (0.9204 kg CO_2eq) which all can have uncontrolled emissions of greenhouse gases (GHGs). In terms of ecotoxicity, DC WtE, both at urban (0.2877 kg 1,4-DCB_eq) and rural (0.2882 kg 1,4-DCB_eq), have the highest METP. This can be mainly due to the possible leaching of the bottom ash which has an appreciable concentrations of heavy metals (Figure 3(b)). Sanitary landfills (0.1418 kg 1,4-DCB_eq) have similar METP with open dump (0.1430 kg 1,4-DCB_eq) and unsanitary landfills (0.1432 kg 1,4-DCB_eq). Meanwhile, open burning (0.0033 kg 1,4-DCB_eq) has the lowest METP which could be mainly due to the elimination of possible source of leachates. However, disposal of wastes via open burning have significantly greater TETP (0.0167×10^-4 kg 1,4-DCB_eq) as compared to DC WtE, for both urban (8.5884×10^-5 kg 1,4-DCB_eq) and rural (8.6834×10^-5 kg 1,4-DCB_eq), sanitary landfill (1.0678×10^-5 kg 1,4-DCB_eq), open dump (9.0001×10^-6 kg 1,4-DCB_eq), and even unsanitary landfill (9.1909×10^-6 kg 1,4-DCB_eq) (Figure 3(c)).

Aside from TETP, open burning also has the highest human health impact as can be seen on its highest HTP (Figure 3(d)), PMFP (Figure 3(e)), and POFP (Figure 3(f)). Open burning of MSW pose serious risks to human health and the ecosystem as toxic compounds are released into the atmosphere and the environment, including dioxins and dioxin-like compounds (e.g., polychlorinated dibenzo-para-dioxins and polychlorinated dibenzofurans) [40] and hazardous heavy metals, such as nickel and cadmium [41, 42].

Meanwhile, comparing the human health impacts of DC WtE to the conventional sanitary landfill in terms of HTP (Figure 3(d)) and POFP (Figure 3(f)) at both urban (HTP = 0.3855kg 1,4-DCB_eq; POFP = 3.21×10^-4 kg NMVOC_eq) and rural (HTP = 0.3875kg 1,4-DCB_eq; POFP = 3.33×10^-4 kg NMVOC_eq), DC WtE impacts are lower than that of the sanitary landfill (HTP = 0.4453 kg 1,4-DCB_eq; POFP = 3.51×10^-4 kg NMVOC_eq). However, in terms of PMFP, the DC WtE both at urban (8.78 ×10^-5 kg PM10_eq) and rural (9.67×10^-5 kg PM10_eq) are relatively higher than that of sanitary landfill (6.29×10^-5 kg PM10_eq) (Figure 3(e)). Results show that DC WtE can be an effective waste disposal method in terms of lower GWP, HTP, and POFP as compared to the sanitary landfills. Meanwhile, more effective water and air pollution control can be designed and integrated to the DC WtE to mitigate its relatively higher METP, TETP, and PMFP than that of sanitary landfills.

3.4. Comparison with other energy sources

While WtE technologies have been promoted by the Philippines’ DENR through its DAO No. 2019-21 [18] to help address the country’s growing waste problem [39], WtE was only mentioned briefly in the ESWMA that the National Solid Waste Management Commission (NSWMC) [43] should look into as part of the National Solid Waste Management Framework [44]. Interestingly, it was the Renewable Energy Act of 2008 (RE Act) which explicitly encouraged the adoption of WtE technologies to which, the Department of Energy remarked that WtE will not only contribute to additional electricity supply, but it will also help increase the RE portfolio in the Philippines [45]. With this, it is just imperative to compare the possible impacts of the WtE, particularly the DC, to the conventional energy sources particularly coal, natural gas, hydro, geothermal, solar PV, wind, and biomass per 1 kWh electrical energy (Figure 4(a) ~ 4(h)).

Fig. 4.

Impact assessment comparison of DC WtE as power generation option to other power generation methods: a) GWP, b) METP, c) TETP, d) HTP, e) PMFP, and f) POFP. Heat map of impact intensity ratio with respect to highest magnitudes per midpoint indicator based on DC WtE at g) urband and h) rural settings (Basis: 1 kWh).

In all the midpoint indicators, it can be seen that the hydropower plant has the least impacts (Figure 4(a) ~ 4(f)) with relative magnitudes <1% with respect to the highest values to each of midpoint indicator: GWP (0.40%), METP (0.03%), TETP (0.26%), HTP (0.18%), PMFP (0.85%), and POFP (0.51%) (Figure 4(g) ~ 4(h)). This shows the overall benignity of hydropower plant in generating electricity. Currently, hydro, which includes impounding hydro, pumped hydro, and run-of-river, contributes 14.1% on the total dependable capacity of the Philippines, ranking second after coal (46%) [46]. Similarly, wind has relative magnitudes <2% with respect to the highest values to each of midpoint indicators: GWP (1.28%), METP (0.89%), TETP (1.64%), HTP (1.32%), PMFP (1.45%), and POFP (1.48%) showing similar benignity of wind as power source (Figure 4(g) ~4(h)). However, wind, mainly onshore, constitutes only 1.7% on the total dependable capacity of the Philippines [46].

Meanwhile, natural gas, which constitutes 13.2% on the country’s dependable capacity [46], can be benign in terms of ecotoxicity relative to the most impactful in each of midpoint indicator: METP (0.07%) and TETP (1.39%). In terms of human health impact, natural gas can still be benign especially in terms of HTP (0.59%), PMFP (3.25%), and POFP (8.81%). The relatively high POFP can be due to the emission of NO_x, which can consequently change the ambient concentration of ozone [29]. Lastly, in terms of GWP, natural gas (0.4266 kg CO_2eq) has high impact which is 42.36% to that of coal (1.0070 kg CO_2eq). This high GWP can be mainly due to the CO₂ emissions, albeit lower than coal and fossil fuel burning, and leaking of natural gas, which is mainly CH4, also a strong GHG. Geothermal, which constitutes 7.1% on the total dependable capacity mix of the Philippines [46], can also have appreciable impacts with respect to those with highest magnitudes to all respective midpoint indicators except METP (0.30%): GWP (6.60%), TETP (19.22%), HTP (4.79%), PMFP (6.32%), and POFP (5.35%).

Solar PV and biomass are also renewable energy contributing 4.7% and 1.6%, respectively, to the dependable capacity of the Philippines [46]. Like all other energy sources except for DC WtE, they can be benign in terms of METP: solar PV (1.74%) and biomass (0.36%) METP with respect to DC WtE. However, they can have appreciable impacts, if not the highest for a particular midpoint indicator. Solar PV has the highest TETP (1.41×10^-4 kg 1,4-DCB_eq) which can be mainly due to the manufacturing of solar PVs which use precious materials (e.g., gold, silver, copper, and platinum) or potentially toxic elements thereby posing threats to soil: both from extraction and release to landfill (if not to be recovered) [47]. Meanwhile, biomass can have appreciable to significant impacts: GWP (6.18%; 0.0622kg CO_2eq), TETP (43.79%; 6.17×10^-5 kg 1,4-DCB_eq), HTP (19.20%; 0.1131kg 1,4-DCB_eq), PMFP (42.85%; 10.83×10^-4 kg PM10_eq), and POFP (77.53%; 30.13 ×10^-4 kg NMVOC_eq). Biomass energy may be perceived to be “carbon neutral” but it can be not “nutrient neutral”, can be intensive in consuming land and water resources, are likely to exacerbate soil erosion problems, and can cause loss of natural biota, habitats, and wildlife for the conversion of natural ecosystems into energy-crop plantations [48]. Pollution problems associated with the production, conversion, and utilization of biomass energy can then be no less significant than the ones vis-à-vis conversion of coal and oil [48, 49].

Coal has been the major fuel type for power generation in the Philippines, constituting 46% of the country’s dependable capacity [46]. Principal emissions resulting from coal burning include CO₂, SO₂, NO_x, particulates, fly and bottom ash, and mercury and other heavy metals [50] which could explain its highest magnitudes in terms of GWP (1.0070 kg CO_2eq), PMFP (25.28×10^-4 kg PM10_eq), and POFP (5.06×10^-4 kg NMVOC_eq). It has relatively the same degree of impacts with DC WtE in terms of HTP (urban = 0.6287kg 1,4-DCB_eq; rural = 0.5889 kg 1,4-DCB_eq; coal = 0.5338kg 1,4-DCB_eq). In terms of ecotoxicity, coal can be benign in terms of METP (2.60 – 2.79%) with respect to DC WtE (urban = 0.4691 kg 1,4-DCB_eq ; rural = 0.4380kg 1,4-DCB_eq) and can have appreciable TETP (17.45%) with respect to solar PV (1.41×10^-4 kg 1,4-DCB_eq).

As subject of interest, DC WtE has the highest METP (urban = 0.4691 kg 1,4-DCB_eq; rural = 0.4380 kg 1,4-DCB_eq) and HTP (urban=0.6287 kg 1,4-DCB_eq; rural=0.5889 kg 1,4-DCB_eq) among the energy sources evaluated. It has significant TETP (urban = 99.40%; rural = 93.67%) with respect to solar PV – significantly higher than coal (17.45%) – and GWP (urban = 81.06%; rural = 76.16%) with respect to coal. The high ecotoxicity (i.e., METP and TETP) could be due to the moisture content in MSW where seeped water can accumulate in the waste pit [34]. Meanwhile, it can have appreciable PMFP (urban = 5.66%; rural = 5.81%) and POFP (urban = 13.48%; rural = 13.03%) with respect to coal. With two midpoint indicators with highest impacts (METP and HTP) and two midpoint indicators with significant impacts (GWP and TETP), DC WtE model (Figure 1(c)) can have more technological improvements, particularly air and water pollution control, to address its potential risks. Nevertheless, it can be a better option than coal in terms of GWP, PMFP, and POFP.

3.5. Comparison with the status quo

WtE’s role in the interplay between waste and energy has consistently been recognized. Global population growth with an accompanied increase in waste generation is also correlated with an increase in energy demand [51]. This energy demand exacerbates the effects on the utilization of fossil fuels which include global climate change, world energy conflicts, and energy source shortages [52]. As a two-pronged technology, WtE can alleviate these concerns as it is not solely about waste management but can also contribute to energy supply security with its enticing potential of harnessing renewable energy resources from waste [53]. With this, concerns associated with fossil fuels can be resolved by green and sustainable alternative in the form of WtE thereby, ceasing waste as a problem and become a valuable renewable energy resource instead [53].

In the Philippines, the RE Act tasked the DOE, in coordination with DENR to spearhead the adoption of WtE technologies [54]. The DOE’s National RE Program (NREP) sets a target of at least a 35% share of RE in power generation and aspires to increase it to at least 50% by 2040 [37]. In this context, it is just imperative to evaluate the combined impact of (DC) WtE as both waste management and energy generation methods based on 1 kg MSW based on urban and rural settings (i.e., waste profiles) versus the status quo, i.e., combined impacts of sanitary landfill and coal (Figure 5(a)~5(f)).

Fig. 5.

Impact assessment comparison of DC WtE (based on urban and rural settling) as both electrical energy generation and waste management options to the combined impacts of conventional methods: coal and sanitary landfill, respectively: a) GWP, b) METP, c) TETP, d) HTP, e) PMFP, and f) POFP (Basis: 1 kg MSW).

To normalize the impacts of coal and sanitary landfill (i.e., FU = 1 kg MSW), thereby allowing the combination of these, conversion factors were calculated by multiplying the impacts of coal-generated electricity to the respective amount of electricity generated of the DC WtE (kWh day^-1) divided by the amount of waste collected for DC (kg MSW day^-1). This yielded conversion factors of 0.1705 and 0.6579 kWh kg^-1 MSW for impacts of coal-fueled power for urban and rural, respectively.

The impacts of sanitary landfill are the same for both urban and rural settings for all midpoint indicators (Figure 5(a)~5(f)) and are discussed in comparing DC WtE to other waste management methods. Similarly, the impacts of DC WtE are similar for all midpoint indicators as also discussed in comparing it with other waste management methods. The differences on the overall stacked impacts of sanitary landfill + coal for all midpoint indicators are mainly due to the difference on the impact magnitude of coal-fueled electricity generation in different settlings which also give better picture on how does the DC WtE, as a two-pronged process, fair against the conventional methods for electricity generation and waste management.

In terms of GWP and human health impacts (i.e., HTP, PMFP, and POFP), DC WtE is significantly more benign than the combined impacts of sanitary landfill and coal (Figure 5(a), 5(d)~5(f)). This lower GWP can be especially significant given the Philippines’ commitment to curb its GHG emissions by 75% for the period of 2020 to 2030. Meanwhile, one of the major oppositions against DC WtE is the health hazards, especially from air pollution [55] which consequently sway policy and public opinion (Figure 1(c)). However, results show that DC WtE, both at urban and rural, has lower human health impacts compared to the status quo. Although, in terms of ecotoxicity (METP and TETP), the DC WtE can have higher impacts (Figure 5(b), 5(c)) where the operation of the MSWI facility is the significant contributor (see Contribution Analysis). Thus, improvements in the DC WtE operation could taper the METP and TETP impacts of WtE. These include securing sufficient volume and quality waste feedstock thereby ensuring the proper utilization of the DC WtE facility technology [7]. Proper waste segregation at the source can drastically reduce waste heterogeneity and could translate to better heating values.

Improvements in energy generation efficiency, wastewater management, and ash management can greatly reduce the impacts of WtE incineration [34]. Selection of WtE technologies with consideration to their recovery efficiencies should be based on geographical, temporal, and technological factors [25].

Separately comparing DC WtE to other waste disposal methods (e.g., landfilling) and power generation methods (e.g., coal-fueled) allow the assessment of DC WtE on each function. However, as a two-pronged process, the comparison would be conclusive if done simultaneously on both functions (i.e., status quo) under the same FU. Doing so, DC WtE model shows favorable advantages in terms of climate change and human toxicity potential and needs refinement/optimization on process operation parameters and/or redesign of pollution control process, particularly integrating wastewater treatment system, to address high ecotoxicity potential.