Global biomethane and carbon dioxide removal potential through anaerobic digestion of waste biomass

Anaerobic digestion is a bioenergy technology that can play a vital role in achieving net-zero emissions by converting organic matter into biomethane and biogenic carbon dioxide. By implementing bioenergy with carbon capture and storage (BECCS), carbon dioxide can be separated from biomethane, captured, and permanently stored, thus generating carbon dioxide removal (CDR) to offset hard-to-abate emissions. Here, we quantify the global availability of waste biomass for BECCS and their CDR and biomethane technical potentials. These biomass feedstocks do not create additional impacts on land, water, and biodiversity and can allow a more sustainable development of BECCS while still preserving soil fertility. We find that up to 1.5 Gt CO2 per year, or 3% of global GHG emissions, are available to be deployed for CDR worldwide. The conversion of waste biomass can generate up to 10 700 TWh of bioenergy per year, equivalent to 10% of global final energy consumption and 27% of global natural gas supply. Our assessment quantifies the climate mitigation potential of waste biomass and its capacity to contribute to negative emissions without relying on extensive biomass plantations.


Introduction
Reaching net-zero greenhouse gas (GHG) emissions by 2050 requires a rapid decarbonization as well as the deployment of carbon dioxide removal (CDR) solutions to offset hard-to-abate emissions [1].Bioenergy with carbon capture and storage (BECCS) is a key CDR technology that features prominently in most net-zero emissions by 2050 pathways [1][2][3][4].Scenarios compiled by the Intergovernmental Panel on Climate Change suggest that to limit warming below 2 • C, 170-650 billion metric tonnes of CO 2 must be sequestered via BECCS by 2100 [5].BECCS involves the capture and permanent sequestration of biogenic CO 2 produced during energy production from biomass feedstock and is considered promising because of its commercial maturity compared to other CDR technologies [6][7][8][9][10][11][12].While previous studies estimated the potential bioenergy generation from biomass resources worldwide [13][14][15][16], the global CDR potential from waste biomass resources has not been estimated yet.
Given its high technology readiness level, a prominent near-term opportunity for BECCS is biogas production through anaerobic digestion (figure S1).Anaerobic digestion is a promising bioenergy conversion process that converts biomass feedstocks to biogas (i.e.biomethane and biogenic CO 2 ) and digestate [17][18][19].The biomethane generated from this process can serve as an alternative to fossil fuel to decarbonize the energy system.The pure CO 2 generated during biogas upgrading can be captured and permanently sequestered for CDR [20].The digestate can also be used as a valuable fertilizer product [21,22] to preserve soil fertility [23].Biogenic CO 2 needs to be captured, transported, and durably stored [11] via conventional carbon capture and storage (CCS), which involves a complex supply chain [24][25][26], or carbon dioxide mineralization, which relies on reacting CO 2 with alkaline rich oxides to form stable carbonate minerals [27][28][29].
There are several concerns on the biophysical and economic feasibility of large-scale BECCS deployment [8,30,31].Biomass feedstocks often originate from dedicated purpose-grown biomass plantations, which have major negative socioenvironmental externalities, including increase in food prices [32,33], exacerbation of water scarcity [34][35][36][37], increase in fertilizers usage [38], competition for agricultural and natural land [39][40][41], and biodiversity loss [42][43][44].A global expansion of BECCS would thus create additional conflicts between climate mitigation and socioenvironmental impacts of biomass feedstock production.Considering these negative environmental impacts associated with purpose-grown plantation, waste biomass from agriculture residues and food waste could provide promising potential considering that global agricultural demand is expected to increase by 50% by 2050 [45,46].
Food systems account for about 30% of the global annual GHG emissions [47][48][49].Within food systems, waste biomass, including manure, crop residue, and food waste, contribute to 16% of GHG emissions the food sector emits, predominantly releasing nitrous oxide and methane [49].Utilizing this waste biomass and producing biogas during anaerobic digestion leads to a reduction in nitrous oxide and methane emission [49], which will significantly enhance agricultural sustainability and contribute to a circular economy towards net-zero agriculture.In addition, waste biomass is a limited resource [20] and there is growing consensus that the social value of deploying biomass for CDR via BECCS may exceed its value for bioenergy production only [50][51][52].Recent work has quantified BECCS potential in Europe, China, and the United States considering biomass feedstocks that do not create additional impacts on natural resources, namely crop residues, organic food waste, forestry residues, livestock manure, and wastewater [20,[53][54][55][56][57][58].These studies focused on regional-scale analyses of BECCS potential and used different methods and sources of biomass feedstock, making results difficult to adopt in future climate mitigation policies.In addition, the global bioenergy and CDR potential from waste biomass remain unknown.Therefore, an assessment of the global availability of agricultural waste that can serve as feedstocks for various biochemical processes, including BECCS and decarbonization of other sectors, such as the chemical industry and aviation, is needed [59].
Here, we quantify the global technical potential of four different waste biomass sources, including livestock manure, crop residues, organic municipal solid waste, and wastewater, on a resolved scale of 10 km.These waste biomass sources are then analyzed for their technical potential to produce biogenic CDR and bioenergy from biomethane via anaerobic digestion.Waste biomass is defined as biomass produced, processed, and used in a sustainable way, which does not have additional impacts on natural resources, such as land and water use, and biodiversity loss [20].The technical potential of utilizing waste biomass takes into account the proportion of agriculture residues allocated to improve soil fertility and prevent soil erosion [60], and the available livestock manure can be sustainably collected [61,62].Biogenic CDR is defined as the amount of biogenic CO 2 captured and sequestered during biomass conversion into bioenergy.First, we assess the global availability of waste biomass feedstocks after accounting for the quantity that must be left in the soil to preserve soil organic carbon, soil fertility, and biodiversity [63][64][65].Second, we quantify the amount of CDR and biomethane produced after anaerobic digestion and biogas upgrading of available waste biomass.Third, we quantify the extent to which domestic BECCS potential would enable CDR to mitigate hardto-decarbonize emissions and the extent to which biomethane from waste biomass would meet final energy consumption and energy supply in each country.Finally, we perform a techno-economic analysis of biomethane from waste biomass and compare its production cost with fossil natural gas.By considering these techno-economic factors, we aim to provide a comprehensive understanding of the feasibility and viability of utilizing waste biomass for biomethane production as an alternative to fossil fuels.Our results identify regions and countries with the highest potential for biogenic CDR and low carbon biomethane from waste biomass, and thus a vision on the future role of BECCS in global climate policies.

BECCS technology chain
We designed a BECCS technology chain using waste biomass feedstocks with suitable bioenergy technologies (figure S1).The BECCS technology chain converts wet biomass feedstocks into biogas through anaerobic digestion.Biogas, a mixture of biogenic CO 2 and biomethane, is subsequently upgraded into pure CO 2 and biomethane through a biogas upgrading process.Depending on the biomass type, different biogenic CO 2 and biomethane quantities are generated.CO 2 can be used for CDR via permanent sequestration, while biomethane can be used in energy systems.

Biomass feedstocks availability
We consider four different waste biomass feedstocks, including crop residues, livestock manure, household organic municipal solid waste, and wastewater.For each biomass feedstock, we assess spatially explicit biomass availability at 10 km resolution worldwide.
For livestock manure, we assess the quantity of manure produced, M (metric tons, t), from 8 livestock species (i) by multiplying the number of livestock of each species, N i (head), by the quantity of manure produced, Q i (t/head/year), and the availability factor A i (%), which represents the share of manure that can be technically collected from the ground: ( The number of livestock per pixel (livestock density distribution) is from a global gridded dataset at 10 km resolution, that considers eight major livestock species, namely, cattle, buffaloes, horses, sheep, goats, pigs, chickens, and ducks [66].The quantity of manure varies by different livestock species, and the availability factor depends on species, farming system, and disposal practices [67].Values of the quantity of manure produced and the availability factor are reported in supplementary table 5 and obtained from a literature review of previous studies covering main livestock husbandry areas, such as Europe [67,68], North America [69,70], South America [71].
For crop residues, we assess spatially explicit biomass availability at 10 km resolution worldwide for 42 crop types (j).A total of 42 crops in nine groups, including cereals, root, bean, fruit trees, vegetables, oil crops, sugar, fiber, coffee, are considered in this study which account for nearly 100% of food crops reported by the United Nations Food and Agriculture Organization [72].For each crop type, the weight of available crop residues, R j (t), is assessed by multiplying crop production, C j (t), times the straw to grain ratio of each crop, S j (−), and the sustainable removal rate, R j (%), which represents the share of crop residues that can be collected from the ground 42 ( Crop production (C j ) is derived from a global gridded agricultural-production datasets at 10 km resolution [72].The straw to grain ratio (S j ) varies by each crop and represents the ratio of straw weight to grain weight.To promote soil fertility and prevent erosion, it is recommended to allocate a portion of crop residues for direct agricultural use [63].This can include leaving the residues on the ground, utilizing them as animal bedding, employing them for frost prevention in horticulture and vegetable cultivation, among other competitive agricultural applications [64,73,74].Therefore, we applied a sustainable removal rate (R j ), which is the fraction of crop residues that can be extracted without significant impacts on soil and other direct agricultural uses.Depending on the variety of crops, the values of sustainable removal rate range from 30% to 70% [65].The straw to grain ratio and the sustainable removal rate are obtained from a literature review of previous studies covering main crop production regions in Europe [64,75], the United States [63], China [76], and other countries [71,74] and available in table S6.
For organic municipal solid waste, we assess spatially explicit biomass availability at 10 km resolution worldwide by multiplying spatially explicit information of population density, P (number of people per area), with per capita production of organic municipal solid waste, O (kg/capita/year) For population density, P, we use year 2020 gridded world population density at 1 km resolution [77].The capita production of organic municipal solid waste, O, accounts for the weight of the organic fraction of municipal solid waste produced per capita per year and it varies by country [78,79].It is worth noting that current methodology with country-specific ratio might overlook the possible rural-urban difference in municipal solid waste generation.
For wastewater, we consider wastewater facilities [80], where wastewater is treated through anaerobic digestion to produce biogas.We consider a total of 58 500 wastewater treatment facilities and their characteristics, including population served, and wastewater discharges.From the population served by a wastewater treatment plant we then quantify biogas production potential.

Assessment of biogas from anaerobic digestion
Biomass feedstocks are converted into biogas through anaerobic digestion, which decompose organic matter in an oxygen-free environment by microbial communities [81].This process produces biogas (a mixture of biomethane and carbon dioxide) and digestate, which contains nitrogen and phosphorus and can be returned to the soil to add nutrients [22].
Biomethane yields depend on the biodegradability of the biomass, which change with time and location [82].In this global study, we estimate the average biomethane yields from multiple types of waste biomass and agricultural residues.Depending on the biomass feedstock, different biomethane production rates are achievable [82].For livestock manure, biomethane yields from anaerobic digestion vary by species, breed, body weight, and age [67].Biomethane yields from manure of different types of livestock are derived from previous studies and are listed in table S7, with values ranging from 6 to 34 kg biomethane per tonnes manure [67,83].For crop residues, biomethane yields are estimated to be 140-160 kg biomethane per tonnes crop residues from previous studies in China [82,84], Europe [85], and India [86].For organic municipal solid waste, biomethane yields are assumed to be between 200 to 310 kg biomethane per tonnes solid waste from studies in China [82] and India [86].Lower biogas yields and methane contents usually indicate irregularities in the anaerobic digestion process.
For wastewater treatment facilities, we assume that sludge in wastewater is treated through anaerobic digestion.We quantify CO 2 emissions using the population served by each wastewater treatment facility.We consider that 18-26 l of biogas are produced per person equivalent per day [87][88][89].Previous studies have shown that up to 25% of the organic carbon in wastewater is of fossil origin, most likely from cleaning products, pharmaceuticals, and other fossilderived materials [90,91].Therefore, to calculate the biogenic CO 2 content in wastewater, we make a conservative assumption that 75% of carbon generated from biogas production is biogenic [20].
The composition of methane and CO 2 in biogas is dependent on the feedstock composition and pretreatment steps [88].Here we used different ranges of methane and CO 2 composition for different feedstock.For livestock manure, we assume that biogas has 35%-45% CO 2 content and 55%-65% of biomethane content by volume (table S7).For crop residues and organic municipal solid waste, we assume that biogas has a 40% CO 2 content and a 60% biomethane by volume [54] (table S7).For wastewater, we assume that biogas has a biomethane content by volume of 63% to 67%, and the rest, 33%-37%, is considered CO 2 [88] (table S7).We then derived CO 2 production rates from biomethane yields based on their composition.
The four types of biomass feedstocks are considered as wet biomass.Therefore, they are not suitable for combustion in a waste to energy plants.Waste to energy plants require dry biomass (which has less than 10%-15% moisture content) [92][93][94].An additional amount of energy is needed to dry this biomass to the appropriate moisture level making the overall process unfavorable.On the other hand, a high moisture level is suitable for microorganisms to break down the biomass [95].For example, livestock manure has water content ranging from 60% to 90%, depending on the livestock type, animal diet, and storage methods [96][97][98].The moisture content of crop residues varies largely by residue types and it can range from 10% to 15% in wheat straw [99] to 80%-90% in sugarcane tops [100].For organic municipal solid waste, the moisture content ranges from 50% to 90% [101,102].The moisture content of wastewater slurry is >80% [103].

Biogas upgrading
Biogas produced from anaerobic digestion is upgraded to remove CO 2 and other contaminants and produce pure biomethane which can be used as renewable energy source.The most used biogas upgrading technology includes absorption (such as water scrubbing, amine scrubbing, physical scrubbing), adsorption (pressure swing adsorption), membrane separation, and cryogenic separation.These biogas upgrading technologies are currently developed and available at both small (e.g.farms, landfills) [104,105] and large (industry) scales [106].A detailed breakdown of energy requirements of multiple biogas upgrading technologies is included in table S8.While different upgrading technologies show a large range of energy use between 0.1 and 0.8 kWh m −3 biogas, water scrubbing and physical scrubbing are cheaper, with widespread deployment, and have a more consistent energy use between 0.2 and 0.5 kWh m −3 biogas [106].In this study, we assume that an energy use between 0.2 and 0.5 kWh m −3 biogas for upgrading biogas is supplied by an on-site generator powered by biomethane.Therefore, 3.6%-10.8% of the total biomethane producible from waste biomass is used in the biogas upgrading process.Because high purity biomethane is required for subsequent uses, we assume that biomethane is upgraded with a 99.5% purity [107].Biomethane can be used as a fuel for vehicles, heating, engines, and gas turbines, and can also be injected into the natural gas grid.We use a lower heating value of 13.9 kWh kg −1 biomethane to calculate the energy content of biomethane [108].

CO 2 and biomethane leaks
The transport of CO 2 and methane generates leaks along the whole supply chains, including gathering, transport, and production [109].Biogenic CO 2 generated from anaerobic digestion can be captured and transported to suitable sites for permanent sequestration.As for current natural gas networks, CO 2 transport, and storage will have some leaks.Current natural gas networks have a leakage rate ranging from 1.8% to 6% [109][110][111].Here we assume CO 2 leaks of 1.8% in the optimistic scenario and 6% in the conservative scenario [20] along the whole supply chains.While biogas production using waste biomass can potentially reduce GHG emissions, it can also increase emissions due to methane leakage from biogas digesters, piping, and appliances [111].It is important to consider leaks in biomethane supply chains in future planning, as current measurements show leakage rates of ∼6% [111], which is much higher compared to current natural gas leaks.In this study we assume biomethane leaks of 1.8% and 6% in the optimistic and conservative estimates, respectively [109,110].

Sobol sensitivity analysis
In our analysis, we performed Sobol's sensitivity analysis, a global sensitivity method measuring the influence of variables on the output variance [112].The implementation of Sobol's analysis in this study quantifies the contribution of each input variable to the variance of the output variable independently (first-order analysis).It is achieved through Monte Carlo simulations with Saltelli's sequence sampling technique [113].The input variables in the sensitivity analysis include manure production from different livestock (summarized from table S5), availability factor of manure (summarized from table S5), straw to grain ratio of different categories of crops (summarized from table S6 by crop categories), sustainable removal rate of crop residues (summarized from table S6), the range of organic municipal solid waste fractions from the top ten countries (derived from Piercy et al [78]), biogas yield rates from livestock manure, crop residues, organic municipal solid waste, and wastewater (table S7), and the range of biomethane leakage (1.8%-6%).The output variable is the total biomethane production.After finding the factors that have significant impacts on biomethane production, we further evaluate the effect of increasing the top two variables on biomethane production.

Sensitivity analysis results
We conducted a sensitivity analysis on the variables used to produce results.Crop removal rate and manure availability are the two most important variables that affect the biomethane production potential, accounting for 88% of the variance in the technical potential of biomethane (figure S6(a)).Therefore, it is crucial to increase the crop sustainable removal rate and manure availability for an improved biomethane production.For example, by increasing the availability of manure by 10%-50%, there can be a corresponding increase in total biomethane production ranging from 2%-11%.Similarly, if the sustainable removal rate is increased by 10%-50%, the total biomethane production can increase by 6%-27% (figure S6(b)).Future work should focus on detailed quantification of crop removal rates and manure availability to accurately quantify biomethane potential from agricultural waste.

Techno-economic analysis
In this study, we performed a technical and economic analysis.The technical analysis contains an evaluation of the technical feasibility and capacity of the technology in the supply chain.Data for the technical readiness level was collected from previous literature [114][115][116][117][118] (table S1).The economic analysis of biomethane in different regions covers the costs of multiple processes in the biogas production, including the costs of feedstock, anaerobic digestion technology, and biogas upgrading (table S2).The economic analysis also considers the proportion of detailed categories in the investment (capital) cost, annual cost, and operational and maintenance cost to the total cost (table S5 and S6).Data for the economic analysis was collected from the International Energy Agency [19] and previous literature [12,119,120].

Energy costs of biogas upgrading
The energy costs of biogas upgrading are included in the study (table S8).The costs was collected from multiple previous literature [106,119,[121][122][123][124][125][126].The electricity used for upgrading biogas is assumed to be produced by onsite generators powered by the biomethane produced.In this study, we considered water scrubbing and organic physical scrubbing as the two most common and cost-effective biogas separation technologies.Using an energy requirement of 0.2-0.5 kWh per m 3 biogas input, we calculated that 3.6%-10.8% of biomethane will be needed for biogas upgrading, assuming a volume distribution of 40% CO 2 and 60% biomethane.

Life-cycle assessment
We use the GHGs, Regulated Emissions, and Energy Use in Technologies (GREET) model, developed by Argonne National Laboratory and sponsored by the U.S. Department of Energy, to perform well-to-wheel analyses [127].The first part, well-to-pump, typically represents the stages from exploration and recovery from the well, but in this study, it starts from waste since we use waste biomass as the input.The second part, pump-to-wheel, corresponds to combustion in an internal combustion engine.GREET model allows us to assess the GHG emissions associated with each step in the complete cycle, within the system boundary shown in figure S7, accounting for three GHGs, namely CO 2 , CH 4 , N 2 O [127].The model assumes that the anaerobic digestion facilities are located where the waste biomass is currently treated, so no additional transportation of feedstock is needed [128].Biomethane generated from waste biomass involves the recycling of carbon and emissions in both a reference case and a modeled pathway.In this study, traditional manure management is considered as the reference case.Emissions from the reference case are subtracted from the emissions occurred in the 'new' pathway in the model [128,129].The model considers not only GHG emissions along the each step in the supply chain, but also GHG emissions associated with the digestate produced from anaerobic digestion and GHG emissions vented or leaked during the anaerobic digestion and biogas upgrading process (assuming a leakage rate at 2%) [129].It is worth noting that methane leakage during the storage and transport of waste to the digester and during digester maintenance is not considered in this model, which is estimated at 5%-10% [130].To account for these leakages, we have considered an additional methane leakage in the estimation of biomethane production at 1.8%-6%.
The GHG emissions from waste-to-pump are −158 ± 18 g carbon dioxide equivalent (CO 2eq ) kWh −1 (figure S8).Waste-to-pump GHG emissions are largely negative because carbon dioxide is stored in biomethane and digestate.When the output fuel is combusted in light duty vehicle, carbon dioxide is released and pump-to-wheel emissions become positive.In total, waste-to-wheel emissions from livestock manure are 50 ± 18 g CO 2 e kWh −1 .All production pathways have significant GHG emission reduction benefit, as they have lower GHG emissions compared to GHG emission from North American natural gas production estimated to be 302 g CO 2 eq kWh −1 (figure S8).
Net-zero carbon dioxide emissions can be achieved by implementing carbon capture and storage (CCS) during biogas upgrading.For example, studies show that GHG emission of producing ethanol from corn stover for use in light duty vehicle with CCS result in −378 ± 22 g CO 2 e kWh −1 .If we assume all the biomethane produced from waste biomass via anaerobic digestion in this study follows the pathway in figure S7 to be used as compressed fuel for light duty vehicle operation, 0.57 ± 0.28 Gt of CO 2 per year will be generated from the full life cycle process.If the technical potential of CDR from biogas upgrading estimated in this study can be all achieved, net GHG emissions will be generated from during the life cycle of BECCS supply chains in this study are −0.97 ± 0.52 Gt CO 2 per year.This number may be overestimated since anaerobic digestion from manure takes up more CO 2 than organic municipal solid waste and wastewater due to that manure has higher emission .In addition, CDR can be achieved by capturing and permanently sequestering the CO 2 emissions generated during the combustion of biomethane.If biomethane were burned at large pointsources, such as natural gas-fired power plants, it would be possible to perform BECCS with the adoption of pre-combustion or post-combustion CCS technologies and enable additional CDR development from the capture of biogenic CO 2 from biomethane combustion.Alternatively, additional CDR can be obtained by capturing the CO 2 produced during the conversion of biomethane into hydrogen through steam methane reforming [55].

Spatial distribution of global waste biomass
Spatially explicit waste biomass has been estimated at pixel level (figure 1).The regions with highest potential in manure, crop residues and organic municipal solid waste are shown in figure 1 in red colors (over 10 000 t per year per pixel).The regions with highest potential in manure are the Midwest in United States, Mexico, the Caribbean region, Brazil, Europe, sub-humid zones in Africa, India, and southeastern China.The regions with highest potential in crop residues are southeastern China, India, Europe, coastal countries in West Africa, Midwest United States, and the Pampas.The regions with highest potential in organic municipal solid waste are south Asia, east Asia, central Europe, Eastern United States, and Caribbean regions.The largest wastewater facilities that served more than 5000 000 people are in Peru, Brazil, Greece, Pakistan, and India (figure S2).

Carbon dioxide removal potential from waste biomass via anaerobic digestion
Our study focuses on estimating the potential of biogenic CDR through the anaerobic digestion of waste biomass.To determine this potential, we consider various factors such as biomass availability, biogas yields, and the carbon dioxide content in the biogas produced.Our assessment is conducted at a highresolution scale of 10 km, allowing us to analyze the global distribution of biogenic CDR potentials.
Figure 2 illustrates the geospatial distribution of biogenic CDR potential resulting from the anaerobic digestion of waste biomass across the world.We observe significant potential for biogenic CDR in regions characterized by high population density and extensive agricultural production.Notable areas exhibiting substantial potential include the Midwestern United States, southern Brazil, western Europe, west Africa, India, Pakistan, and eastern China (figure 2).These findings highlight the geographic locations where the anaerobic digestion of waste biomass can be harnessed to achieve significant biogenic CDR contributions.
Spatially explicit CDR potentials have been aggregated to determine country-scale and globalscale CDR potentials (figure 3 and table 1).We find that a total of 1545 million tonnes (Mt) of CO 2 per year can be captured by treating waste biomass with anaerobic digestion (table 1).Of the total biogenic CDR potential via anaerobic digestion, 54% (826 Mt of CO 2 per year) is from crop residues, 31% (486 Mt of CO 2 per year) is from organic municipal solid waste, and 14% (224 Mt of CO 2 per year) is from livestock manure.Although wastewater treatment is an important solution to improve water quality, in terms of CDR it accounts for only 1% (or 9 Mt CO 2 per year) of global biogenic CDR potential via anaerobic digestion (table 1).Because our results are affected by a variability in the parameters used to quantify biogenic CO 2 production from anaerobic digestion and potential CO 2 losses during transport, table 1 shows uncertainty ranges in biogenic CDR potentials from different waste biomass sources.Results are presented considering a central scenario, while conservative and optimistic scenarios are depicted with uncertainty bars whenever necessary.
Figure 3 shows country-and biomass feedstockspecific biogenic CDR potentials from anaerobic digestion.China has the highest potential in the world, with 266 Mt CO 2 per year, accounting for 2% of its annual GHG emission, followed by India with 170 Mt CO 2 per year (5% of its annual GHG emissions), the United States with 126 Mt CO 2 per year (2% of its annual GHG emissions), Brazil with 124 Mt CO 2 per year (12% of its annual GHG emissions), and Indonesia with 54 Mt CO 2 per year (5% of its annual GHG emissions) (figure 3).For other countries, biogenic CDR potentials are below 50 Mt CO 2 per year, although biogenic CDR can offset more than 10% of annual GHG emissions in 27 countries (Supplementary Datasets).Crop residues are the most important biogenic CDR biomass feedstock and account for more than half of the share of biogenic CDR potential in 73 countries, including China,  The figure shows at 10 km resolution the distribution of biogenic CDR that can be produced from anaerobic digestion of livestock manure, crop residues, organic municipal solid waste, and wastewater sludge.
India, United States, Brazil, Indonesia, Thailand, Russia, Nigeria, Argentina, France, Vietnam, Turkiye, Malaysia, Philippines, and Bangladesh (figure 3).In China, crop residues have a biogenic CDR potential of 147 Mt CO 2 per year (or 55% of CDR potential from anaerobic digestion in China) (figure 3).Livestock manure accounts for more than 15% of biogenic CDR potential in Argentina, Brazil, China, Table 1.Global carbon dioxide removal and biomethane potential from waste biomass via BECCS.The table shows the CDR and biomethane attainable from anaerobic digestion of waste biomass.For CDR, uncertainty ranges are shown in parentheses and determined by considering variability in biogenic CO2 production during bioenergy conversion, and CO2 leaks during transport.For biomethane, uncertainty ranges are shown in parentheses and determined by considering variability in biogas yields, biogas upgrading, and biomethane leaks during transport.Note: 1 kg of biomethane is equal to 13.9 kWh [108].Mexico, Bangladesh, and Pakistan (figure 3).Organic municipal solid waste accounts for more than 40% of biogenic CDR potential in Japan, Mexico, Pakistan, Germany, Italy, Vietnam, and Turkiye (figure 2).

Potential to offset total GHG emissions
We assess the extent to which biogenic CDR could meet annual GHG emission to enable a transition to net-zero emission (figure 3).We show that the overall biogenic CDR potential from anaerobic digestion process assessed in this study can meet 3.3% (or 1545 Mt CO 2 per year) of annual global GHG emissions in 2019 [131].Annual global GHG emissions are estimated to be ∼48 Gt CO 2 equivalent per year and include energy (electricity, heat and transport), direct industrial processes, waste, and agriculture, but do not consider emissions from land-use change, and forestry [131].The year 2019 is used as the most up-to-dated data before the COVID-19 pandemic, which has affected GHG emissions.As illustrated in figure 3, by deploying sustainable BECCS, China can offset ∼2% of its annual GHG emissions with anaerobic digestion, the United States ∼2%, and India 5%.In the top 20 countries with highest biogenic CDR potential from anaerobic digestion, Brazil, Nigeria, Philippines, Bangladesh, Argentina, Pakistan, Thailand, Malaysia, France, Indonesia, and India can offset more than 5% of their annual GHG emissions with domestic biogenic CDR potentials (figure 3).

Biomethane potential from anaerobic digestion
By accounting for biomass feedstock-specific biomethane yields, the energy needed to upgrade biogas into pure biomethane, and biomethane leakages, we estimate bioenergy from biomethane that could be produced from anaerobic digestion at 10 km resolution (figure S3 for geospatial distribution).We then aggregated spatially explicit biomethane production potentials at the country-and global-scale (figure 4; table 1).We estimate that the global bioenergy production potential from biomethane is 10 781 TWh (table 1).Crop residues have the greatest biomethane production potential of 5799 TWh per year (or 54% of global biomethane potential from waste biomass), followed by organic municipal waste (3372 TWh per year or 31% of global biomethane potential), manure (1539 TWh per year or 14% of global biomethane potential), and wastewater (70 TWh per year or 1% of global biomethane potential) (table 1). Figure 4 shows country-level and feedstockspecific bioenergy production potentials from biomethane produced during anaerobic digestion of waste biomass.China, with 1862 TWh from biomethane per year, has the greatest biomethane potential production among all the countries, followed by India (1186 TWh), the United States (881 TWh), Brazil (863 TWh), and Indonesia (377 TWh) (figure 4).
We assess the extent to which biomethane could meet final energy consumption to enable a transition to net-zero emission (figure 4).We show that the overall biomethane production potential assessed in this study can meet 10% (or 10 781 TWh per year) of year 2019 final energy consumption.Final energy consumption considers the energy used by end users, including households, industry, and agriculture [132].Final energy consumption in 2019 is used as the most updated energy consumption before COVID-19 pandemic, which impacted the final energy consumption.As illustrated in figure 4, in top 20 countries with highest biomethane production potential from anaerobic digestion process, China can meet 8% of final energy consumption with domestic biomethane production from waste biomass, the United States can meet 5%, and India 16% (figure 4).Brazil, Philippines, Bangladesh, Argentina, Indonesia can meet over 20% of final energy demand with domestic biomethane potentials (figure 4).The share of biomethane produced from anaerobic digestion compared with natural gas supply in each country.Total energy supply from natural gas data is taken from International Energy Agency for year 2019 [132].Countries with no data on natural gas supply are depicted in grey.

Potential of biomethane to offset current natural gas supply
Biomethane can be adopted as a fuel to replace fossil natural gas [133].We assess the extent to which biomethane could offset current total energy supply from fossil natural gas in each country, which is defined as the total fossil natural gas including the use of natural gas in the power production sector [132] (figure 5).We find that the total amount of 10 781 TWh of biomethane produced from anaerobic digestion of waste biomass can offset 27% of global annual natural gas supply.Biomethane can be directly adopted as a fuel to replace 64% of domestic annual natural gas supply in China, 10% in United States, 83% in Indonesia, 26% in Mexico, 58% in Pakistan, and 45% in Thailand (figure 5).Because of low natural gas consumption in India, biomethane production can offset nearly two times natural gas supply in India (figure 5).Biomethane can offset 25%-50% of natural gas demand in countries in south America and Europe and can offset over 100% of natural gas in countries in Africa, Brazil, Sweden, and India.

Techno-economic analysis of biomethane
We assessed the techno-economic potential of biomethane from waste biomass.Anerobic digestion and biogas upgrading technologies have reached full commercial maturity [114][115][116], while carbon dioxide capture, transport, and permanent storage span the full range of technological readiness, with high readiness in conventional onshore CO 2 injection and CO 2 storage in saline formations and through enhanced oil recovery [117,118] (table S1).The global average biomethane production cost is $66 MWh −1 ($ represents US dollar) [19].Over 60% of the total cost is from investment in biogas plant, digestate storage, upgrading plant, filling stations [19,119] (table S2, S3).However, biomethane cost varies by region mainly due to cost difference in feedstock types, anerobic digestion systems and storage, and upgrading plants (figure S4, table S4).We collected data from previous work [19,119,120,134] that assessed cost of biomethane production by region (figure S5) to assess the cost of biomethane production from waste biomass including manure, crop residues, municipal solid waste, and wastewater (figure 6).Eurasia and North America have the lowest average biomethane production costs under $5 MWh −1 , while average biomethane cost in Africa is the highest at $83 MWh −1 (figure 6).Depending on the biodigester sizes, biomethane cost at household level in North America can be as low as $10-17 MWh −1 [19], which is competitive compared to average natural gas prices at $11.5 MWh −1 in the United States in 2014-2022.The lowest biomethane costs in Eurasia ($14 MWh −1 ) and Europe ($17 MWh −1 ) are also competitive compared to average natural gas prices in Europe ($21 MWh −1 ).For multiple feedstocks, biomethane produced from landfill gas recovery containing organic municipal solid waste at economic scale is less than $16 MWh −1 [19].Compared to the peak natural gas price in Europe in 2022 ($138 MWh −1 ), biomethane from waste biomass can be a good alternative at lower production costs.

Discussion
We quantify the technical potential of BECCS via anaerobic digestion from waste biomass worldwide.However, social, economic, and political factors will likely reduce the actual quantity of CDR and bioenergy achievable via BECCS.In fact, BECCS supply chains have a high design complexity and degree of customization influenced by local social, economic, environmental, and political factors [135].BECCS costs are estimated to be US$100-200 per t of CO 2 sequestered [5].A successful BECCS development will therefore require institutional support, including  S4).The bar width represents the total bioenergy potential of the region.The average and 1 standard deviation of the United States (2014-2022) and European (2014-2020) natural gas prices are displayed as solid and dashed lines.The price of natural gas in Europe for the year 2022 increased sharply from previous years due to Russia's invasion of Ukraine, so it is displayed separately.In addition, natural gas prices in Europe for 2021-2022 are excluded from calculating the average European natural gas prices.The regions in the figure are defined by the International Energy Agency [19] (figure S5).
accounting and rewarding for negative emissions [136].Site-specific life cycle assessment studies will be needed to determine the actual net-negative emissions generated from the CDR potential quantified in this study [137].

Carbon dioxide removal potential
This study quantifies the global technical potential for BECCS by leveraging the use of waste biomass, namely livestock manure, crop residues, organic municipal solid waste, and wastewater.The waste biomass dataset we created can be used not only to quantify techno-economic BECCS potentials at the local and national scales, but also biomass feedstock availability to achieve net-zero emissions in hard-toabate sectors like chemical industry, agriculture, steel, cement, and aviation, which will need a combination among electrification, CCS, and biomass routes to achieve net-zero emissions [138].
The target to keep global warming within 1.5 • C or 2 • C by the end of the century is very ambitious and requires timely actions to reduce emissions and offset hard-to-abate emissions.Scenarios compiled by the Intergovernmental Panel on Climate Change suggest that limiting warming to 1.5 • C or 2 • C could require 5-10 Gt CO 2 per year of CDR via BECCS by 2100 [5].However, such estimates have been met with intense skepticism due to land use requirements [9].It is important that any attempt to develop and deploy biomass feedstock for BECCS considers possible loss of carbon in soils and forests as well as biodiversity and other eco-systems services.Land use change impacts of harvesting biomass for BECCS should be accounted for to avert overestimating the mitigation potential of BECCS and creating a lock-in on high and unsustainable consumption of biomass.Alternative waste biomass feedstocks, such as forestry residues and biomass grown on marginal land, can be used to produce additional BECCS [40,57,139,140].Additionally, alternative CDR techniques to BECCS, such as biochar, direct air capture and storage and enhance weathering, could be deployed to achieve climate goals [8,11,141].Importantly, waste biomass is a limited resource, starting the deployment of BECCS early allows additional CDR needed to balance our carbon budgets by the end of the century compared to a scenario where BECCS is implemented later [142,143].
Multiple benefits can be achieved by the BECCS supply chain envisioned in this study.Agricultural production accounts for 12% of global total GHG emissions excluding forestry and land use (7.1 Gt CO 2 equivalent per year) [49].We estimate that BECCS supply chains that use waste biomass and residues have the potential to reduce 21% of GHG emissions from agricultural production.In addition, the utilization of waste biomass and residues can avert numerous disposal problems, such as water pollution [144][145][146], air pollution, and GHG emission from crop residues, biomass burning, and manure management [147][148][149].For example, if waste biomass were not deployed for BECCS, wastewater would contribute to 0.65 Gt CO 2 equivalent, the burning and decomposition of agriculture residues and livestock manure would generate 1.6 Gt CO 2 equivalent [49], and the decomposition of organic matter in municipal solid waste results in CH 4 emission [150,151], which would contribute to 0.8 Gt CO 2 equivalent [152].Replacing fossil fuel with biomethane can avoid 1.1 Gt CO2 equivalent per year [133].As the increasing demand for food and other resources from a growing population results in an increasing amount of organic municipal solid waste, manure, crop residues, and wastewater [19,153], the BECCS supply chain envisioned in this study can achieve higher carbon capture potential in the future and play significant role as a negative emission technology.In addition, digestate produced from anaerobic digestion is a nutrient rich output that can recycle nutrients to croplands while offsetting carbon-intensive industrial fertilizers [138].

Biomethane potential
In recent years, there has been a passionate intellectual debate regarding the worldwide accessibility of sustainable biomass [14].There is a strong consensus that the potential for sustainable bioenergy on a global scale would be limited to approximately 100 EJ per year for energy purposes by 2050 to avoid significant pressure on the environment [1,154].However, there is still significant uncertainty surrounding the precise level of energy supply from biomass, pathways limiting warming to 1.5 • C by 2050 enable 67-310 EJ year −1 calculated by IPCC [154], IEA [155], and different integrated assessment models [156,157].In this study, we find that anaerobic digestion of waste biomass can generate 39 ± 6 EJ (10 781 ± 1721 TWh) of biomethane per year with minimum impacts on the environment, which corresponds for 39% of the estimated sustainable bioenergy potential.This number aligns with the 24 EJ bioenergy produced in 2018 [156] and approaches the IEA projection of 50 EJ from biomass by the year 2035 [14].
In terms of meeting final energy consumption for achieving net-zero emissions, the study suggests that biomethane production has the potential to fulfill 10% of the global energy demand.Among the top 20 countries with the highest biomethane production potential from anaerobic digestion, China can meet 8% of its final energy consumption with domestic biomethane production, the United States can meet 5%, and India can meet 16%.Among multiple sources of waste biomass, crop residues have 54% of the potential for biomethane production.
Upgraded biomethane possesses similar characteristics to natural gas, allowing it to seamlessly replace natural gas using existing infrastructure and technologies [158].Despite the fact that the current biomethane industry is generating a growing interests in the production of heat and electricity, mainly across Europe and North America, the scale of this industry remains small, accounting for about 0.1% of today's natural gas demand [19].Therefore, our results indicate a significant role for biomethane potential in the transformation of the global energy system.The production potentials of biomethane from waste biomass are globally distributed, providing local sources of power, heat, clean cooking fuels, and transportation fuels for households [19].Particularly in Europe, biomethane presents a significant opportunity for countries to reduce their dependence on imported natural gas.
Biomethane costs vary across regions, with estimates ranging from US$40-80/MWh (figure 6).In certain circumstances, biomethane costs can be competitive with current natural gas prices in North America, Eurasia, and Europe (figure 6).However, the widespread production of biomethane faces challenges due to the cost competitiveness of fossil natural gas.To overcome these challenges, governments must implement supportive policies and incentives to encourage biomethane initiatives, such as a tax and refund scheme, a flat-rate subsidy, etc [159].Such actions would drive the advancement of biomethane technology and facilitate the transition to a sustainable energy system.With appropriate incentives and subsidies, biomethane derived from waste biomass can become cost-competitive with fossil natural gas, enabling its widespread adoption as an environmentally friendly alternative.
Anaerobic digestion is a widely adopted technology globally for biogas production [19].Although using organic wastes for biogas production has the potential to reduce GHG emissions, it is crucial to address the issue of methane leakage.Methane can escape from biogas digesters, piping, and appliances, potentially increasing emissions instead.Therefore, it is important to consider the possibility of leaks in biomethane supply chains when planning.Current measurements indicate leakage rates of approximately 6% [111], which is much higher compared to current natural gas leaks and it is accounted in our assessment.To ensure that biogas plants have a positive environmental impact, it is essential to minimize methane fugitive emissions.

Conclusions
This study underscores the substantial potential of waste biomass for BECCS as a powerful tool in addressing climate change.Through a comprehensive analysis of BECCS supply chains worldwide, we have identified regions, such as the Midwestern United States, southern Brazil, western Europe, west Africa, India, Pakistan, and eastern China, where the anaerobic digestion of waste biomass can significantly contribute to CDR efforts.
These findings highlight the crucial role that waste biomass can play in mitigating climate change by reducing CO 2 emissions and promoting circular economy practices.Moreover, our spatial analysis provides valuable insights for decision makers, stakeholders, and practitioners involved in sustainable waste management and renewable energy strategies, guiding them to make informed investments and policy decisions that can maximize CDR outcomes.
Recognizing the CDR potential of waste biomass and taking proactive steps to facilitate its utilization can yield substantial environmental benefits, promote sustainable resource management, and make a significant contribution to the global fight against climate change.In essence, waste biomass is a valuable resource that, when harnessed effectively, can drive us closer to a low-carbon future.

Figure 1 .
Figure 1.Global geospatial distribution of (a) manure, (b) crop residues, (c) organic municipal solid waste.The figure shows at 10 km resolution the global distribution of waste biomass.

Figure 2 .
Figure 2. Global geospatial distribution of biogenic carbon dioxide removal potential from anaerobic digestion of waste biomass.The figure shows at 10 km resolution the distribution of biogenic CDR that can be produced from anaerobic digestion of livestock manure, crop residues, organic municipal solid waste, and wastewater sludge.

Figure 3 .
Figure 3. Biogenic CDR potential from anaerobic digestion of waste biomass.Gray shaded areas compare country-specific biogenic CDR potentials from anaerobic digestion with country-specific total GHG emissions, excluding land-use change and forestry in year 2019 [131].Uncertainty bars show the conservative and optimistic CDR scenarios considering the variability in biogas yields, CO2 content in biogas, and CO2 transport losses.

Figure 4 .
Figure 4. Biomethane potential from anaerobic digestion of waste biomass.Gray shaded areas in the background compare the share of energy produced from biomethane with country-specific final energy consumption in year 2019.Country-specific final energy consumption in year 2019 is defined as the energy that reaches the end users [132].The y axis on the left shows the bioenergy potential from biomethane and y axis on the right shows the quantity of biomethane produced from anaerobic digestion.Uncertainty bars show the conservative and optimistic biomethane production scenarios considering the variability in biomethane yields from waste biomass feedstocks, biogas upgrading, and methane leakages.

Figure 5 .
Figure5.The share of biomethane produced from anaerobic digestion compared with natural gas supply in each country.Total energy supply from natural gas data is taken from International Energy Agency for year 2019[132].Countries with no data on natural gas supply are depicted in grey.

Figure 6 .
Figure 6.Biomethane production cost by region with comparison to natural gas prices.Each bar represents the mean biomethane cost.The biomethane cost includes the costs of feedstocks, anaerobic technology, and biogas upgrading.The uncertainty bars show the range of minimum and maximum costs due to different feedstock types, biodigester sizes, and upgrading technologies (tableS4).The bar width represents the total bioenergy potential of the region.The average and 1 standard deviation of the United States (2014-2022) and European (2014-2020) natural gas prices are displayed as solid and dashed lines.The price of natural gas in Europe for the year 2022 increased sharply from previous years due to Russia's invasion of Ukraine, so it is displayed separately.In addition, natural gas prices in Europe for 2021-2022 are excluded from calculating the average European natural gas prices.The regions in the figure are defined by the International Energy Agency[19] (figureS5).