2.1 Case Study

In order to facilitate the choice of a disposable method, CFA was performed on three disposable scenarios: incineration, landfilling, and recycling for 1 million tons of PW. A similar CFA was performed for an upscale version of 100 million tons of PW. Since non-woven polypropylene (PP) is largely used in the preparation of PW, in the current study, PW is assumed to be analogous to PP [8]. Further, in order to compare emissions from both disposal methods, a pragmatic shorter timeframe (30 years) wherein the landfill attains a pseudo-static state (the stage at which chemical modifications are slower than the initial stages), has been considered in the current study [8, 24]. In recycling, wastes are mechanically (granulation) or chemically (monomer blocks) processed with the objective of producing new raw materials. In the present study, due to brevity, only mechanical recycling is considered.

2.2 Scope and Goal of CFA

The disposal of one million tonnes of PW was carefully chosen as the functional unit for CFA. The commonly adopted PW disposal methods of landfilling, incineration, and recycling disposal methods are considered for CFA, and the different sources of PW and stages involved in CFA are presented in Figure 1. The CFA methodology adopted is based on established procedures by ISO 14044. Since the PW considered is the same for landfilling and incineration, it is assumed that emissions due to its embodied carbon are assumed to be zero. Further, to facilitate comparison, the current study considered conventional incineration. However, incineration with energy recovery option has the potential to offset emissions by converting waste into energy (WTE). While WTE is not directly considered in current CFA analysis, its imperative to explore its environmental benefits in future studies using LCA. Further, it is also assumed that emissions from heat and power supply remain the same for all three disposal methods.

2.3 System boundary of CFA

The system boundaries were formulated by adopting a gate-to-gate approach in the PW management chain. The following exclusions were made in the study:

The exclusions made in the current study are in coherence with the guidelines issued by ISO 14044 [7]. The inventory of materials involved in CFA was taken from various databases like [24, 2]. To access CO₂e emissions for three scenarios, a stage-wise calculation approach is adopted and the detailed calculations for disposal and recycling methods are presented in Table 1. Similar calculations for scenario-II (upscale version) are presented in Table 2. Further, PW waste generation and disposal strategies are illustrated in Figure 1. A roundtrip distance of 100 km is assumed as the common haulage distance for all the disposal methods. The type of vehicle/machinery used in the procurement, haulage and operational stage is chosen based on the data from previous studies [14, 19, 28].

3.1 Disposal of PPE

The net emissions (kg CO₂e) for three different scenarios are presented in Figure 2, from which it can be noted that the incineration method of disposal contributed the highest amount of emission with 782280 kg CO₂e and 78774195 kg CO₂e for scenario I and scenario II respectively. The net emission in the landfilling scenario is the least, with a mere 571 and 222756 kg CO₂e for scenario I and scenario II, respectively. The results for emissions related to both disposal methods are presented in Figure 3, and it is observed that the emissions contributed by the operational stage in the incineration method are the highest for both scenarios, with emissions of 0.78 million kg CO₂e/million PPE. The substantially more elevated amount of emissions in the operational stage for the incineration method is primarily contributed by the excessive fuel consumption of combustion chambers.

The lowest amount of emissions was noted in the operational stage of the landfill, which is attributed to the minimal usage of machinery (bulldozer). The emissions from the procurement stage are higher in landfilling and are attributed to variation(s) in vehicle type employed. The emissions in the haulage stage are higher for incineration because of the higher fuel consumption by the same vehicle operated for both the procurement and haulage stages. The emissions in the operational stage for incineration are enormous, with nearly one million kg CO₂e compared to a meagre 46 kg CO₂e emission for landfilling (scenario I). Similar observations were made for scenario II. Further, the results of the CFA on two disposal methods indicate that landfilling is a preferred option over incineration, with substantially lower emissions in both haulage and site operations. The mode of emissions from incineration is immediate, with the release of all the carbon in PPE in the form of methane (CH₄) and carbon dioxide (CO₂), while the emissions from landfills are gradual and time-bound [19]. A primary difference between landfilling and incineration modes of disposal is the timeframe of CFA, which is due to the fact that the emissions from landfills may prevail for extended periods post-closure. Moreover, PPE materials such as polypropylene degrade slowly with time, leading to the formation of microplastics. These microplastics pose environmental risks including groundwater and soil contamination.

Thus, for the surveyable time of 30 years, the emissions from incineration are always higher than from landfills. Considering the total emission balance, landfilling is ranked as the preferable option for PPE [19].

Figure 3 The variation in emissions with stages of CFA for landfilling and Incineration.

The PPE, made of polymers like PE, LDPE/HDPE, fundamentally differs from other industrial wastes like municipal solid waste and coal combustion residues with little or no biodegradation [14]. Finnveden et al. [9] have observed that landfilled PE exhibited 1-3% degradation over 30 years, which is quantified as 9-27 g of CH₄ and 8-24g of CO₂ per kg/PE. However, over hypothetical infinite time periods (beyond 500 years), the emissions substantially increase to more than ten times with 3000g CO₂ per kg/PE. Since the emissions for an infinite time period is considered beyond the purview of the current study, based on the earlier studies studies [8, 9], it can be assumed that the emissions from landfills for a surveyable time period (i.e., 30 years considered in this study) are negligible. On the other hand, PPE incineration coupled with recovery of energy (post-combustion) will offer the benefits over landfilling and are listed below:

Among the disposal strategies considered, in areas with inadequate facilities to perform the disinfection process (autoclaving and microwaving), incineration can be the easiest and preferred method of disinfection or decontamination of PPE. In rural areas, where incineration is also not feasible, the deep burial of PPE can be adopted. However, in resourceful regions, adopting technology-driven disposal strategies such as hydrothermal carbonization and gasification can automate waste valorization with the potential to deliver high-quality by-products.

3.2 Recycling of PPE

From the CFA results presented in Figure 2, it is observed that the emissions from recycling have resulted in positive emissions and is mainly attributed to the embodied carbon of PPE during the production of virgin PPE (6.08 $\times$ 10⁵ kg CO₂e/million PPE) which are substantially higher compared to emissions from the recycling process (449 kg CO₂e/million PPE). The stage-wise emissions in recycling for both scenarios are presented in Figure 4, which reveals that the emissions at the operational stage are substantially higher compared to procurement and haulage owing to multi-stage complex operations notably different from the other disposal methods. It is even observed that the emissions in haulage and procurement stages are comparable to landfilling and Incineration, as shown in Figure 3. From this observation, it can be inferred that the emissions related to the operational stage are the differentiator among the studied disposal methods. Further, one distinct feature of CFA for recycling is the emissions due to the embodied carbon of PPE, which are substantially higher (6.08 $\times$ 10⁵ kg CO₂e/million PPE) compared to collective emissions from the other three stages of recycling (449 and 973 kg CO₂e/million PPE for the scenario I and scenario II respectively).

Figure 4 The variation in emissions with stages of CFA for recycling scenario.

From scenario I to scenario II, the emissions (kg CO₂e/million PPE) related to the recycling process increased by 216%, whereas the emissions (per million PPE) from embodied carbon remained constant. The difference in emissions can be mainly attributed to the variation in machinery/vehicle employed for the upscaled scenario. Another distinct observation was regarding the procurement stage, which decreased by 68% while the emissions for haulage and site operations remained constant. This decrease in emissions for procurement was attained by replacing the pickup van with a medium-duty dumper. The results confirm that PPE recycling results in a carbon-negative/sink situation, which is not achieved for landfilling and incineration modes of disposal. Based on the net emissions for all three scenarios, the order of preference for three disposal methods is recycling $>$ landfilling $>$ Incineration. Similar benefits of recycling plastic waste have been observed by [3, 14]. The industrial sludge or leachate associated with PPE manufacturing units is known to cause the acidification of wetlands, subsequently resulting in eutrophication by photo oxidants [30]. By opting for the recycling option for PPE, apart from the earlier mentioned adverse environmental effect (eutrophication), the demand for energy in the manufacturing of virgin PPE can also be averted [16]. Recycling also avoids emissions associated with the use of plastic as an alternative source of energy (by combustion), which reduces the thrust on non-renewable energy sources like wood and coal [23].

3.3 Comparison of CFA results from both the scenarios

From the CFA results presented in Figures 2, 3 and 4, it is evident that the increase in PPE from 1 million to 100 million has resulted in an exponential increase in the overall emissions. In general, for both scenarios, the emissions in the incineration are much higher than the other two modes of disposal (landfilling and recycling). However, for scenario II, the difference in emission is more pronounced in recycling and landfilling method which exhibited an increment of 383% and 390% respectively compared to an increment of 100% for incineration method. This distinctive spike in emissions for landfilling and recycling method in comparison to the scenario–I are primarily driven by the haulage distance and type of vehicle employed. The vast volume (100 million) of PPE procurement in scenario II necessitates the utilization of vehicles with relatively higher payload capacity and lower fuel efficiency. In the process, the overall fuel consumption is dramatically enhanced, resulting in an exponential rise in emissions. Further, the proportionate increment in emissions in scenario II for the incineration method compared to other disposal methods is primarily contributed by the site operation stage in which the machinery employed remains the same for both the scenarios. However, for landfilling, the highest contributing stage of emissions varied from procurement in the scenario I to haulage in scenario II. Thus, considering the emissions from both the scenarios, it can be inferred that the application of the landfilling method for largescale disposal remains the preferred choice of disposal over incineration in both the scenarios. Another distinct feature observed in scenario II is the increase in the contribution of haulage distance on the overall emissions in the landfilling method is considerably lower (190%) compared to incineration method which exhibited 450% increment. This observation highlights the advantage of landfilling method over ponding method of disposal, particularly for large-scale stabilization.

3.4 Role of haulage distance on the emissions

Figure 5 The variation in emissions with haulage distance.

The variation in emissions associated with the haulage stage with the distance for three vehicles of different payload capacities and mileage is presented in Figure 5. The payload capacity and mileage of the vehicles considered are 0.7 (tons) & 7.5 (km/L), 4 (tons) & 3.36 (km/L) and 24 (tons) & 2.42 (km/L) for pickup van (PV), medium-duty truck (MDT) and heavy-duty truck, respectively. From the results, it is evident that the distance of haulage has a direct influence on the emissions with a linear increase in emissions with the distance for both the vehicles. It is interesting to note that the PV with the best mileage has exhibited the highest emission, which can be attributed to its relatively lower payload capacity. As seen in Figure 5, with the increase in haulage distance from 25 to 100 km, the emissions increased by almost four times for all the vehicles. With the variation in haulage distance from 25 - 100 km, the difference in emissions between the PV and HDT has increased from 195652 to 782606 (kg CO₂e). Due to their relatively lower payload capacity, PV requires more trips to convey the waste, thereby causing substantially higher consumption of fuel. Thus, the higher consumption of fuel is reflected in the form of higher emissions associated with PV. The usage of vehicles like PV is typically seen in the procurement of wastes from households, beaches and street litters (Figure 1). Whereas for mass procurement from hospitals and burial/crematoriums, medium/heavy-duty dumpers of higher payload capacity are generally employed. Further, it can also be inferred that the location of waste procurement indirectly influences the emissions associated with the haulage of waste. It can be noted that the contribution of emissions by haulage to the overall emissions is substantially higher than other stages, with the only exception of emissions due to incineration.

3.5 Sustainable Disposal Approach

Though the COVID-19-induced lockdown has crashed global GHG emissions, the contemporary issue of PPEs is unforeseen and unprecedented, with implications lasting for a substantial period. Albeit the need to address the issue of plastic waste generation and disposal has been highlighted by environmental activists in the past [8, 14, 29], the COVID-19 pandemic makes it a more opportune time to augment a dialogue on the subject of plastic waste. Based on the results from the study, it can be inferred that landfilling is the most preferred disposal option over incineration for the surveyable time of 30 years. This observation is further corroborated by the fact that the landfilling with more than 58% share in total plastic disposal, generates the lowest CO₂e emissions [8, 14, 29]. It is further observed that relying on incineration as the sole method of disposal leads to an increase in global GHG emissions. On the other hand, by recycling PPE, a negative value of GHG emissions will contribute to a net gain in carbon emissions of more than 159000 kg CO₂e/ million PPE. Moreover, due to the energy-intensive production, PPEs recycling also enormously contributes to energy cost savings. Despite efficiencies and net emissions, the major factor that decides the selection of a suitable technique is cost-effectiveness. It is observed that the cost required to dispose off waste products is high compared to the recycling approach, especially waste-to-energy technology. The negative carbon emission signifies the fact that the emissions associated with the embodied carbon of PPE can be averted by choosing recycling as a preferred option over other disposal strategies. As mentioned, mechanical recycling results in a carbon-negative condition, making it an environmentally favourable approach. However, contamination issues can often hinder the feasibility of mechanical recycling. On the other hand, chemical recycling (CR) as an alternative approach to mechanical recycling has excellent potential to recycle contaminated and heterogeneous plastic wastes. The CR is based on the principle of converting complex polymers into a sustainable smaller molecule with relatively lower toxicity [25].

Further, the emerging green technologies should be incentivized to produce biodegradable material. Shifting to sustainable renewable energy sources as fuel for vehicles/machinery will contribute to the overall reduction in emissions. Recently, the authors have suggested the potential usage of molten plastic from PPE as an alternative stabilizer for the treatment of soils [3]. Further, the authors have also highlighted the possibility of fabricating bricks by incorporating molten PPE waste as a binding agent. Utilization of PPE in both applications can avert the substantial carbon emissions associated with the usage of traditional stabilizers like lime and cement [2]. Further, the emission issues related to PPE should be seen as an opportunity to revisit and revamp the fiscal and production policies associated with their manufacturing and disposal methods. With the persistent increase in global temperatures, the occurrence of catastrophic pandemics/disasters are expected to rise shortly [4]. One of the progressive approaches would be to complete adapt bio-based plastics for PPE production, which can have negative carbon footprints. However, disinfection of bio-based PPE post-usage needs comprehensive evaluation before it is promoted as a sustainable alternative to existing non-biodegradable PPE. Post pandemic, there is a need to reflect on the current flawed practice of handling healthcare waste to combat the potential pandemics of unprecedented scale [15]. Lastly, it is prudent to determine sustainable solutions for the limitations identified in the current waste disposal strategies.

3.6 Practical relevance of the research findings

The findings of the study further reinforce the positive benefits of recycling PPE in terms of net savings in emissions. The results of the study highlight the role of haulage distance on the overall emissions. Thus, the estimated emissions in the present study will enable end-users to select appropriate vehicles that are implemented in waste management practices. Though previous researchers have highlighted the role of recycling and haulage distance on emissions from non-biodegradable plastics, the attempts to focus on PPE are sparse. Furthermore, the research findings of the present have greater relevance due to the rapid rise in consumption of PPE (due to the COVID-19 pandemic). The results of the study are an honest attempt to ensure human and environmental safety during the current crisis caused by the pandemic.