The greenhouse gas performance of selected biodegradable and recalcitrant plastics in U.S. landfills

Biodegradable plastics are often considered to exhibit superior environmental performance compared to conventional recalcitrant plastics. Here, we assess the greenhouse gas (GHG) emissions of selected biodegradable and recalcitrant plastics made from both fossil and biogenic carbon (C) as disposed in a national average U.S. landfill. This average landfill incorporates consideration of size, precipitation, landfill gas management, and gas collection installation schedule. The GHG emissions of an 80% biodegradable polycaprolactone (PCLf) made from fossil C and a 2% biodegradable poly(butylene succinate) (PBSb) made from biogenic C were evaluated to represent the range of anaerobic biodegradabilities. The 2% biodegradable PBSb has lower GHG emissions than the 80% biodegradable PCLf in the national average landfill. In the best case, which includes aggressive gas collection, conversion of gas to energy, and disposal in a large landfill, the PCLf results in 2423 kg CO2e/mt, which is well above PBSb (−1956 kg CO2e/mt), a hypothetical biogenic and 80% biodegradable PCLb (4739 kg CO2e/mt), and recalcitrant fossil plastic (0 kg CO2e/mt). From a disposal perspective, a recalcitrant biogenic plastic is optimal given the long-term storage of carbon. This study informs the direction of materials research to develop materials that minimize their overall environmental footprint at end-of-life.


Introduction
The annual global demand for plastics was 367 million metric tons in 2020, and an increase of more than 60% is projected by 2050 [1].Despite the recycling of some commonly used plastics, ∼75% of plastics are disposed in landfills at the end of their useful life in the U.S. [2].Growth in the production of plastics from biogenic carbon (C) (e.g.corn, potato, or sugarcane) as well as plastics that are biodegradable is projected to quadruple by 2026 from 2020 levels, as such materials are often considered to be more environmentally beneficial [3,4].There are many plastic formulations with differing raw material sources and biodegradabilities.Not all biogenic plastics are biodegradable, and not all biodegradable plastics are biobased.For instance, polylactic acid (PLA) is a biogenic plastic used in straws and shopping bags.PLA exhibits limited anaerobic biodegradability under mesophilic conditions and higher biodegradability in a thermophilic environment [5,6].Polyethylene terephthalate, used to make water bottles, can be made from fossil or biogenic C but does not biodegrade [7,8].No report of a traditional (fossil C based) plastic that biodegrades under anaerobic conditions without applying enzymes has been identified [9].
Whether a biodegradable or biogenic plastic is more desirable than a conventional recalcitrant fossil-based plastic depends in part on the problem being addressed.If the objective is to reduce marine debris, then a plastic that biodegrades under marine conditions is desirable.Similarly, if the objective is to save landfill space, then a plastic that biodegrades under landfill conditions would meet the objective.Most plastics are disposed in landfills at the end of their useful life [10,11].Greenhouse gas (GHG) emissions are an important consideration for both new materials and as existing materials are deployed in new applications.Therefore, we focus on the GHG emissions of plastics disposal in U.S. landfills.Combustion is not considered in this study as only ∼16% of plastics are combusted with energy recovery in the U.S. [2].Furthermore, the GHG performance of different plastics when combusted for energy will only vary based on their relative energy content and their biogenic and fossil C content.
In earlier work, we showed that biodegradable plastics that are disposed in landfills result in increased GHG emissions relative to conventional (recalcitrant) plastics, even at landfills where the recovered methane is utilized as an energy source [12].Since the earlier work, the variety of alternative plastics has increased, we have shown that landfill size has an impact on GHG emissions as larger landfills are required to collect gas for a longer period, and New Source Performance Standards (NSPS) regulating landfill gas (LFG) collection have been strengthened [13].In addition, the fraction of waste disposed in landfills that convert LFG to energy increased from 35% in 2008%-55% in 2020 [14,15].It is thus important to evaluate the GHG impact of landfill disposal of a range of plastics based on actual landfill operation.
Our overarching objective is to understand and compare the GHG emissions of selected traditional and biogenic plastics as disposed in a national average U.S. landfill in consideration of governing regulations that control how landfills are operated in practice.We focus on a national average landfill because estimating the GHG emissions of a material during landfill disposal must include the range of disposal and gas treatment scenarios that occur in practice.This study should inform the direction and focus of materials research and the adoption of new plastics in consumer products.Specifically, manufacturers that want to deploy materials that represent a reduced environmental footprint require information on the end-of-life management of their materials.
The overall study workflow is presented in figure 1.The modeling approach, including a description of the U.S. average landfill is described in section 2-Materials and Methods.Section 2 also describes the rationale for selection of a set of polymers for study and their relevant properties.The results of this study, including sensitivity analysis are presented in section 3-Results and Discussion.Finally, we discuss the implications of the results for material selection and optimal end-of-life management.

Materials and methods
We first define the modeled national average and regulatory compliant landfill where a given mass of plastics is disposed.We then describe the plastics made from biogenic or fossil carbon selected for analysis.Finally, two alternate carbon accounting methods are presented to evaluate the GHG emissions from landfilling the studied plastics.a The relative shares for small, medium, and large landfills were estimated based on the range of <25th percentile, 25-75th percentiles, and >75th percentile in size, which are summarized in figure 2. b The flare start and end years were calculated based on the size of the landfill (waste acceptance and operating years), modeled waste generation using equation HH-1 in the GHGRP, and compliance with NSPS regulations that require gas collection and control when NMOC generation exceeds 34 Mg year −1 .The engine start and end year was based on the availability of 10 m 3 min −1 of LFG.c '-' indicates that Small landfills in Arid and Moderate regions as well as Wet regions under NSPS minimum collection are sufficiently small that collected LFG is less than the minimum allowable gas flow rate of 10 m 3 min −1 .These Small landfills are only required to operate a flare system.

National average U.S. landfill
We recently described a model of a national average U.S. landfill (Wang et al) [13] In this study, the model is used to evaluate the GHG performance of the disposal of polymers with a range of biodegradabilities.The national average landfill is comprised of landfills with no gas collection and control system (GCCS) in place, landfills that collect and flare gas, and landfills that convert the collected gas to electricity.In addition, the national average landfill considers disposal at different (1) precipitation levels which impacts the rate of waste decomposition, (2) landfill sizes which informs gas collection requirements, and (3) GCCS practices.The landfill model considers LFG generation, collection, CH 4 oxidation of emissions passing through the landfill cover, and combustion in a flare or engine to produce electricity, based on Levis and Barlaz [16].

LFG generation
Methane generation from a national average landfill is estimated as the weighted average of methane generation at each precipitation level, landfill size, and gas treatment scenario using the fraction of annual waste disposal (%AWD precip,GCCS,size ) (equation ( 1)).The %AWD precip,GCCS,size values to parameterize equation ( 1) are described in table 1 The mass of municipal solid waste (MSW) disposed in each landfill category was estimated using the U.S. GHG Inventory of landfills [15] (as reported in the Greenhouse Gas Reporting Rule (GHGRP) [17]) and data from the Landfill Methane Outreach Program database (LMOP) [3] (figure 2).We classified landfill size as small, medium, and large at each precipitation level based on the USGHGI [15] and the LMOP database [18], because LFG management varies as a function of landfill size.For example, a small landfill may not be required to collect LFG, while a relatively large landfill may require LFG collection after 5 yr [19].Facility-reported annual waste disposal and gas treatment practice were used to calculate the fraction of waste disposed in small, medium and large landfills in each climate and for each gas management alternative (vent, flare, convert to energy) (table 1).
Three decay rate constants were analyzed to reflect the mix of landfills in the U.S., across three gas treatment scenarios: passive vent, flare, and conversion to electricity (figure 2 and table 2).The U.S. GHG Inventory categorizes landfills based on annual precipitation (wet > 102 cm, moderate 51-102 cm, dry < 51 cm), as the MSW decay rate constant increases with precipitation (0.06 yr −1 , 0.04 yr −1 , and 0.02 yr −1 , respectively) [20].To categorize landfills into arid, moderate, and wet regions, 30 year annual precipitation averages from the National Oceanic and Atmospheric Administration [21] were mapped by facility location information reported by GHGRP and LMOP.In addition, landfills were classified as small, medium, and large based on previous work that showed that larger landfills will operate their GCCS for a longer time before they are allowed to terminate gas collection [13] (tables 1 and 2).

LFG collection
Two gas collection scenarios were analyzed including a regulatory minimum (NSPS minimum) scenario and an aggressive collection scenario as defined in table 2. These scenarios cover the range of schedules for the initiation of gas collection in the U.S. A given mass of waste will experience different gas collection efficiencies depending on when the waste is buried relative to the landfill's age, the gas collection well installation schedule, and the number of years that the LFG collection system operates.To simulate the time-varying LFG collection efficiency, a temporally averaged gas collection efficiency was applied to the 'average' mass of MSW buried in a landfill each year [16].These temporally averaged efficiencies were then used to estimate the volume of CH 4 collected, flared, combusted for energy, oxidized, and emitted.The longer GCCS operating time originates from the requirement that landfills install a GCCS once they produce more than 34 metric tons (mt)/yr of nonmethane organic carbon (NMOC), and that once a GCCS has been installed, it must be operated for at least 15 years and until NMOC collection is below 34 mt yr −1 [22].Finally, the GHG impact for the national average landfill was calculated for both an aggressive and regulatory (NSPS) minimum gas collection scenario to consider both the time-varying gas collection and methane oxidation efficiencies given the schedule of cover placement and GCCS installation (table 2).
Gas collection system turn-on and turn-off times were calculated based on the landfill size and its operating lifetime, the MSW decay rate constant, the gas collection system installation schedule and assumed collection efficiency, and when the landfill is projected to reach the 34 mt yr −1 NMOC threshold [22] (table 1).LFG control includes, at a minimum, an enclosed flare with a CH 4 destruction efficiency of 99.9% and may include beneficial use of the CH 4 (i.e.energy recovery through combustion in an internal combustion engine [ICE] with an electricity conversion efficiency of 35% [23] and a CH 4 destruction efficiency of 99.9% [24]).The energy system will typically be turned off when less than 10 m 3 LFG/min is collected, and the turn-off time depends on the MSW decay rate, the collection efficiency, and the mass of waste disposed.The cut-off of 10 m 3 was developed from discussions with landfill operators.When energy is recovered, the electricity is offset based on the U.S. national average grid [25].

Methane oxidation
A fraction of the uncollected CH 4 is oxidized to CO 2 by microorganisms in the landfill cover soil, and the remaining fraction is described as a fugitive emission (table 2).The fraction of CH 4 oxidized was varied from 10% to 35%, based on U.S. EPA guidance [26].When no gas collection is in place, the landfill has a relatively high CH 4 flux and 10% oxidation efficiency is assumed based on the U.S. EPA's Mandatory Reporting of Greenhouse Gases [27].The model assumes 20% oxidation for landfills with gas collection prior to landfill closure, and 35% for landfills that have installed gas collection and the final cover.

Properties of selected biodegradable and recalcitrant plastics that are made from biogenic and fossil carbon
End-of-life disposal is evaluated for a metric ton of plastics that were selected to represent the range of actual and potential plastics made from biogenic or fossil carbon that are biodegradable or recalcitrant (table 3).This includes a range of biodegradation extents (0%-80% C conversion) and methane potentials.A survey of published literature on the rate and extent of anaerobic biodegradation of a wide variety of plastics indicates that PBS and PCL represent biodegradable plastics with the lowest and highest biodegradability, i.e. 2% and 80% C conversion, f Based on the cumulative CH4 yield curve in Yagi et al [34].
respectively (table S1).Industry surveys [28][29][30] on plastics classification show that PCL is a fossil C-based degradable plastic and PBS is bio-based degradable plastic.Therefore, PBS from biogenic C (PBS b ) and PCL from fossil C (PCL f ) were analyzed to include the plausible range of plastic biodegradabilities in every landfill disposal scenario.To comprehensively compare the GHG performance from fossil and biogenic plastics with a range of biodegradabilities, we studied a hypothetical PCL made from biogenic carbon (PCL b ).All three biodegradable plastics (PCL f , PCL b and PBS b ) are compared to a traditional recalcitrant fossil plastic which represents the business-as-usual case given the dominance of such plastics in the market.This analysis is focused on LFG as the fossil fuel driven processes at a landfill in the other processes (construction, operation, and closure) are similar for both fossil-based and biogenic plastics.
Because plastics are a small fraction of the waste buried in landfills, the bulk MSW will control methane generation.Thus, precipitation specific MSW decay rate constants were used to estimate the GCCS operation period (table 1).The rate of plastic decomposition is important, as the decomposition rate interacts with the GCCS installation schedule to determine the fraction of generated gas that is collected (table 1) [31].

Emissions calculation and global warming potential (GWP) accounting
Landfill emissions are calculated as in equation ( 2) and this value is used to calculate the GWP for each scenario.The calculation scheme for a national estimate of methane emissions from landfills in the U.S. is illustrated in SI figure S1 CH 4 emissions = CH 4 generation ( To capture the effect of decay rate and gas collection schedules on the turnon and turnoff times of GCCS systems, we estimated LFG generation for 1000 years following waste burial.This time is long enough to include 99.99% of LFG generation at a minimum decay rate constant of 0.02 yr −1 , which applies to MSW in an arid region. We estimated the net GWP for the disposal of each polymer in the national average U.S. landfill as the mass of carbon dioxide equivalents (CO 2 e).Differences in the origin of the C used to manufacture a plastic were considered by using the two common biogenic C (CO 2 b) accounting methods [35].In the Neutral CO 2 b method (equation ( 3)), biogenic CO 2 emissions have a GWP of zero because the CO 2 is considered part of the short-term carbon cycle.In the Positive CO 2 b method (equation ( 4)), the biogenic C is assumed to have been stored in the waste before landfill disposal.The coefficients in equations ( 3) and ( 4) are the 100 yr GWP values as adopted from the International Panel of Climate Change [36] (table S3).An electricity offset of 0.38 kg CO 2 e kWh −1 is used based on the U.S. mix grid to account for collected CH 4 that is converted to energy [37].(4)

Sensitivity analysis
We conducted a breakeven analysis to evaluate whether a biodegradable PCL b or PCL f could result in lower GHG emissions than a recalcitrant fossil plastic as LFG collection increases.To this end, we modeled a 'best-case' scenario to ensure that if the 'breakeven' gas collection efficiency is not practically achievable, then it is likely not possible for a biodegradable PCL to have lower GHG emissions than recalcitrant fossilbased plastics when disposed in a landfill.Table S4 shows the inputs for the best-case landfill model.Critical assumptions include the aggressive collection of LFG and that all collected LFG is used for electricity production in an internal combustion engine.In reality, the fraction of collected gas that is used for energy recovery is uncertain but below 50% when averaged across all U.S. landfills.In addition, the sensitivity of the GWP to climate (correlated to decay rate), landfill size, and the rate and extent of plastics biodegradation was investigated to illustrate how results vary with polymer properties in different landfill disposal scenarios.

GHG emissions of biodegradable and recalcitrant plastics in a landfill
Figure 3 compares the GWP of biogenic plastics with varying biodegradabilities to fossil-based plastics for a national average landfill (equation ( 1)).The biogenic plastic with low C conversion has lower GHG emissions than the biogenic plastic with high C conversion as less CH 4 is emitted and, in the case of the Neutral CO 2 b method, there is a credit for biogenic C storage.The fossil carbon-based recalcitrant plastic has the lowest GWP in the Positive CO 2 b case, while the low C conversion biogenic plastic is lowest in the Neutral CO 2 b case.The difference is due to the C storage credit for the biogenic plastic in the neutral CO 2 b method (equation ( 3)).When only the biodegradation of biogenic waste is considered, the rankings of the plastics should not change as long as the climate system boundaries and C balance are defined clearly and consistently.However, the accounting method will change the numerical results.In cases like this study, where both fossil and biogenic C are compared, the rankings of alternatives may not hold and are sensitive to the choice of biogenic C accounting method (equations ( 3) and ( 4)).
The results are consistent with our earlier study in showing that less biodegradability is desirable from a GWP perspective [12].This is because keeping C in the ground leads to less GHG emissions compared to biodegradation with resulting CH 4 and CO 2 emissions.As expected, the GWP is higher for the NSPS minimum gas collection scenario, in which gas collection wells are not installed until 5 years after waste burial, relative to the aggressive gas collection scenario in which gas collection begins in under a year (figure 3 and table 1).The range of scenarios and carbon accounting schemes evaluated here suggests that the results are robust to a range of landfill scenarios and accounting schemes aligned with current U.S. practices.
The GHG emissions from landfilling plastic waste are dominated by landfill CH 4 emissions (figure 3), which contribute 70%-91% to 100 yr GWP estimates in all cases except for biogenic plastics with low C conversion using neutral CO 2 b accounting.For PBS b , landfilling leads to the storage of 98% of the biogenic C (table 3).This results in net negative GHG emissions as compared to landfilling a recalcitrant fossil plastic because in the neutral CO 2 b method (equation ( 3)), biogenic C is considered to be removed from immediate atmospheric circulation for storage in the landfill.

Breakeven gas collection efficiency for biodegradation to be desirable in climate mitigation
SI figure S2 shows the GHG impacts associated with the best case, which includes aggressive gas collection, conversion of CH 4 to energy and disposal in a large landfill (table S4).The 80% biodegradable PCL f exhibits a GWP (Neutral CO 2 b method) of 4739 kg CO 2 e/mt plastic, which is well above the hypothetical 80% biodegradable PCL b (2423), 2% biodegradable PBS b (−1956 kg CO 2 e/mt plastic), and the recalcitrant fossil plastic (0 kg CO 2 e/mt plastic).
The breakeven analysis evaluates whether a biodegradable PCL b or PCL f could result in lower GHG emissions than a recalcitrant fossil plastic as LFG collection increases.Recalcitrant fossil plastic has no GHG emissions (0 kg CO 2 e/mt) associated with LFG or C storage.For the biodegradable PCL b and PCL f to hypothetically have lower GHG emissions than a recalcitrant fossil plastic, the cumulative LFG collection efficiency must be high enough for the electricity offsets to outweigh the fugitive CH 4 and CO 2 emissions from uncollected LFG.Even at a landfill in which all collected LFG is used to offset electricity generation from coal (1.1 kg CO 2 e kWh −1 vs the U.S. mix grid of 0.38 kg CO 2 e kWh −1 ), a cumulative LFG collection efficiency of >92% is required for a biodegradable PCL f to have lower GHG emissions than a recalcitrant fossil plastic (figure 4).However, a 92% collection efficiency is not realistic over the life of a landfill.In previous research, cumulative LFG collection efficiencies were estimated to range from 31%-86% for various materials and bulk decay rates under aggressive (not typical) gas collection schedules [13].The upper end of this range is for wood, which decays considerably slower than a biodegradable plastic.In contrast, PCL b will result in lower GHG emissions than a recalcitrant fossil plastic at a cumulative LFG collection efficiency of >72% and >88%, when electricity is offset based on 100% coal and the U.S. mix grid, respectively.While a 72% collection efficiency is attainable at a specific landfill, it is not attainable for the national average landfill in the absence of regulations to require LFG collection at more landfills.

Impact of decay rate, landfill size, and extent and rate of plastic biodegradation on GWP
Figure 5 shows the fate of landfill CH 4 for the national average landfill and illustrates the effect of climate (correlated to decay rate) and landfill size.The high biodegradability PCL exhibits a higher methane yield and more rapid decay than the low biodegradability PBS.PCL thus releases more CH 4 than PBS when landfilled.For the national average case, the Aggressive gas collection scenario can reduce CH 4 emissions by 25%-37% compared to the NSPS minimum scenario due to the earlier and longer gas collection and control period (figure 5(a)).
In the case of the rapidly degrading PCL, large landfills have a similar gas collection efficiency to medium landfills because the PCL decays relatively rapidly (∼7 times faster than MSW) and yields 80% of its CH 4 potential in the first five years.During the first five years, gas collection efficiencies are 30%-75% (figure 5(b)).The medium and large landfills that collect and control gas are important, as they accept 82% of U.S. waste that is landfilled and contribute 65% to the national average GWP impact (figures 2 and 3).The medium and large landfills have a higher gas collection efficiency than small landfills because NSPS regulations require the collection of gas until NMOC generation is less than 34 mt yr −1 .This threshold will result in a longer period of gas collection and control at larger landfills.
Figure 6 illustrates that the GHG impact of polymers in a national average landfill increases linearly with increasing extent of biodegradation.GWP initially decreases until a decay rate of 0.01 yr −1 , as more LFG is generated before the gas system is shut off based on regulatory requirements to collect gas.Above 0.01 yr −1 , GWP increases sub-linearly as more gas is produced before high gas collection efficiencies are achieved.The improvement in GWP performance of the aggressive collection scenario relative to the NSPS minimum scenario becomes larger as biodegradability rate and extent increase because more CH 4 is being generated as the extent of biodegradability increases.In the NSPS case, more CH 4 is generated prior to the beginning of gas collection, resulting in higher CH 4 emissions.We emphasize that GHG emissions of polymers increase with increasing biodegradation rate and extent regardless of the difference between fossil and biogenic PCL.

Summary and implications
While anaerobic biodegradability is often considered to be a desirable attribute for a plastic, this is not the case for a plastic that is disposed in a U.S. landfill at the end of its useful life when considering GWP.Lower biodegradability leads to lower GWP given the longterm storage of carbon and lower methane emissions.This overall conclusion proved robust to a range of landfill practices and regulations.Furthermore, the GWP increases as the decay rate constant for a plastic increases, which is not consistent with the desire to design materials for rapid biodegradation.The LFG collection efficiencies required for an 80% biodegradable PCL (f, b) to have the same GWP as a recalcitrant fossil C-based plastic are not realistic for the national average U.S. landfill.In solid waste management systems that include anaerobic digestion, where methane collection efficiencies are above 90%, a biodegradable plastic is beneficial from a GWP perspective and desirable as it reduces contamination of the resultant digested streams.Finally, the selection and design of new materials should also include the energy and emissions associated with their raw materials and manufacture.It is estimated that 50%-61% of the GHG emissions associated with manufacture conversion that is flared, combusted for energy production, oxidized, and emitted in a national average landfill across three GCCS scenarios, three landfill sizes and three precipitation levels, using Aggressive and NSPS Minimum gas collection scenarios.Plastics with 80% conversion refer to PCL f with 421 kg CH4 generated at a field decay rate of 0.27 yr −1 while a 2% C conversion plastic refers to PBS b with 8.39 kg CH4 generated at a field decay rate of 0.19 yr −1 for a bulk MSW k of 0.04 yr −1 .(b) Effect of precipitation and landfill size on the fraction of methane collected for PCL with 80% C conversion, under the energy recovery and aggressive collection scenario.The schedules of gas collection and control system installation for different landfill sizes are presented in table 2. Small landfills in arid and moderate regions are sufficiently small that collected LFG is less than the required gas flow rate of 10 m 3 min −1 needed to run an engine for electricity conversion and thus only a flare system is utilized.and disposal are attributable to fossil fuel extraction and resin production for both biogenic and fossilbased plastics [38,39].Thus, comprehensive materials assessment must consider the entire life cycle.This research shows that emissions associated with disposal are an important consideration.

Figure 1 .
Figure 1.Workflow describing the inputs required to evaluate the greenhouse gas emissions from polymer disposal in a U.S. national average landfill.

Figure 2 .
Figure 2.Relative shares of annual waste disposal in arid, moderate, and wet regions with three gas treatment scenarios in small, medium, and large landfills.The shares for small, medium, and large landfills were assigned based on the range of <25th percentile, 25-75th percentiles, and >75th percentile in annual waste acceptance.The values supporting this figure are presented in table 1. Gas generated from the landfills that have no GCCS in place is either oxidized or emitted, regardless of landfill size.

Figure 3 .
Figure 3. GWP estimates (left) and percent absolute contribution to the net GWP (right) from disposal of biogenic-and fossil-based plastics with different extents of biodegradability in a national average landfill for two gas collection schedules and two biogenic C accounting methods.

Figure 4 .
Figure 4. Net greenhouse gas emissions versus gas collection efficiency for PCL b and PCL f using either coal or the U.S. mix grid average for the electricity offsets associated with LFG recovery from the PCL.

Figure 5 .
Figure 5. (a) Percent of methane generated (absolute mass values in parenthesis) from plastics with 80% (PCL f ) and 2% (PBS b ) Cconversion that is flared, combusted for energy production, oxidized, and emitted in a national average landfill across three GCCS scenarios, three landfill sizes and three precipitation levels, using Aggressive and NSPS Minimum gas collection scenarios.Plastics with 80% conversion refer to PCL f with 421 kg CH4 generated at a field decay rate of 0.27 yr −1 while a 2% C conversion plastic refers to PBS b with 8.39 kg CH4 generated at a field decay rate of 0.19 yr −1 for a bulk MSW k of 0.04 yr −1 .(b) Effect of precipitation and landfill size on the fraction of methane collected for PCL with 80% C conversion, under the energy recovery and aggressive collection scenario.The schedules of gas collection and control system installation for different landfill sizes are presented in table 2. Small landfills in arid and moderate regions are sufficiently small that collected LFG is less than the required gas flow rate of 10 m 3 min −1 needed to run an engine for electricity conversion and thus only a flare system is utilized.

Figure 6 .
Figure 6.The effect of biodegradation rate (a) and extent (b) on the GWP (neutral CO2b) associated with PCL b with 80% C conversion based on aggressive and NSPS minimum gas collection scenarios as defined in table 2.
Figure 6.The effect of biodegradation rate (a) and extent (b) on the GWP (neutral CO2b) associated with PCL b with 80% C conversion based on aggressive and NSPS minimum gas collection scenarios as defined in table 2.

Table 1 .
Waste acceptance rate, operating years, and GCCS turnon and turnoff timings of small, medium, and large landfills across three precipitation levels using NSPS minimum and aggressive collection schedules (described in table 2).

Table 2 .
[20]dules and values of landfill gas oxidation and collection, based on the underlying assumptions from EPA 2020[20].NSPS = New Source Performance Standards.Final cover is placed one year after final waste placement based on the landfill's operating years (table1).
a b The aggressive gas collection schedule represents the best possible scenario based on judgement.

Table 3 .
[32]tic materials selected for study and their biodegradation characteristics.Relevant properties of PCL and PBS are presented in tableS2.Stoichiometric methane yield is calculated using the Buswell equation[32]and adjusted by the % biodegradability (% C conversion).bCarbonstorage factor was calculated from the stoichiometries and methane yields of each polymer.cThevalues reflect the biodegradability of plastics in simulated mesophilic landfill reactors at temperatures of 35-38 • C.
[33]A hypothetical PCL made from biogenic C. e Based on the cumulative biodegradation curve in Cho et al[33].