Assessing the potential benefits of methane oxidation technologies using a concentration-based framework

Lowering the atmospheric methane concentration is critical to reducing short-term global warming because of methane’s high radiative forcing and relatively short lifetime. Methane could be destroyed at its emissions sources or removed from the atmosphere by oxidizing it to carbon dioxide and water vapor, greatly lowering the warming effect. Here we provide, to the best of our knowledge, the first estimate of the amount of methane that is emitted at a given concentration. We use this to assess the potential benefits (global temperature, air quality, and economic) of various technologies that could oxidize methane above specific concentration thresholds. We estimate that global mean surface temperature could be reduced by 0.2 °C ± 0.1 °C by continuously oxidizing all anthropogenic methane emitted above 1000 parts per million (the lowest concentration addressable with current commercial technologies). Continuously oxidizing all methane currently emitted above ten parts per million could cause 0.4 °C ± 0.2 °C of cooling. For the economic benefit of removing atmospheric methane to outweigh the associated energy cost, we show that reactors that use heat to oxidize methane must operate at most 3 °C ± 2 °C above ambient temperature while those that use light must convert at least 9% ± 8% of photons into oxidized methane molecules. Our framework can be used by scientists, engineers, and policymakers to better understand the connections between methane sources, including their emission rates and concentrations, and the technologies that can oxidize those emissions.


Introduction
The global mean surface temperature, already 1.1 • C ± 0.1 • C above preindustrial levels, is increasing at ∼0.2 • C per decade [1].Carbon dioxide (CO 2 ) has understandably received the most attention of all anthropogenic greenhouse gases because it has contributed the most (0.8 • C) to the global temperature increase.Acting rapidly to lower methane (CH 4 ) concentrations is critical to reducing short-term warming because methane is much shorter-lived than carbon dioxide and causes far more radiative forcing per tonne.Methane's short lifetime of ∼12 years means that the ∼0.5 • C of warming above preindustrial temperature that it has caused could hypothetically be reversed over decadal timescales by lowering its atmospheric concentration, which is predicted to reach ∼2 parts per millions (ppm) by year 2030, to its preindustrial level of 0.7 ppm [1].
The most important step in lowering the atmospheric methane concentration is preventing emissions from reaching the atmosphere.Certain emissions avoidance measures pay for themselves, such as plugging natural gas leaks.(See table S1 for a summary of the terminology used throughout this paper and example mitigation measures.)However, ∼40% of anthropogenic emissions are estimated to be technologically infeasible to avoid [2,3].Furthermore, there are few technologies proposed to reduce natural emissions (primarily from wetlands and freshwaters) which are projected to increase as temperatures rise [4].Natural climate feedbacks and tipping points, such as abrupt permafrost thaw, may begin around 2 • C above preindustrial temperature, resulting in more emissions that cannot be addressed by avoidance alone [5].
To address these more diffuse sources and reduce atmospheric methane concentrations, some researchers have argued for removing atmospheric methane via oxidation, the process that converts methane into carbon dioxide and water.Various research groups have outlined the associated opportunities, challenges, and research directions to better understand the technical and economic feasibility of methane oxidation [6][7][8][9][10].Methane can also be oxidized at its emission sources where its concentration is higher than atmospheric levels.However, to the best of our knowledge, there are no quantitative assessments of the potential scale and technological feasibility of methane oxidation technologies based on the methane concentrations that they can address.
We develop a framework (figure 1) to connect methane oxidation technologies, the concentrations (ppm CH 4 ) and emission sources at which they could be applied, and the potential scale of their benefits.We first detail the relationships between a mass of oxidized methane and its global benefits of reduced temperature, improved air quality, and subsequent economic benefits.By synthesizing available emissions data, we then present what we believe to be the first estimate of the relationship between methane concentration and the mass of methane emitted globally at that concentration.We next examine the lowest concentrations of methane that proposed technologies have oxidized.We then show that technologies that use heat or light to oxidize methane have concentration thresholds imposed by their energy requirements.Finally, we discuss high-priority research directions based on our new framework.

Benefits of methane oxidation
The conditions under which methane is oxidized dictate how much it reduces global mean surface temperature.Over its lifetime, methane contributes to global warming by trapping heat before being naturally oxidized to carbon dioxide.In contrast to this 'direct effect' of trapping heat itself, methane's 'indirect effects' come from the global warming impacts of its oxidation products: mainly carbon dioxide and water vapor, but also ozone when methane is oxidized in the presence of nitrogen oxides.Avoiding methane emissions altogether results in a temperature benefit equal to methane's direct plus indirect effects.
Here, we consider an idealized total methane oxidation process (henceforth referred to as 'oxidation') where the products are only carbon dioxide and water vapor (equation ( 1)).
Ideal methane oxidation: When ideally oxidized, each tonne of methane produces 2.75 tonnes of carbon dioxide (along with water vapor that has an insignificant climate effect when emitted from the Earth's surface).Thus, ideally oxidizing 1 tonne CH 4 results in a temperature benefit equal to that of avoiding 1 tonne of methane, minus the warming effect of 2.75 tonnes of carbon dioxide.Avoidance and ideal oxidation have similar climate impacts over short timescales, but the production of long-lived carbon dioxide through oxidation means that the climate benefits of ideal oxidation are slightly lower over longer timescales (figure S1).
Because methane is a relatively short-lived greenhouse gas that naturally decays exponentially, there is a rough equivalence between a continuous oxidation rate and an asymptotic temperature impact [10].We calculate the asymptotic temperature reduction of a sustained methane oxidation rate by subtracting 2.75 times the absolute global temperature change potential of carbon dioxide from that of methane to get 0.0014 • C ± 0.0007 • C per teragram (Tg) of methane oxidized per year (figure S1) [11].The cooling effect per Tg CH 4 oxidized would be ∼50% higher in a future scenario (SSP3-7.0) in which methane emissions and concentration are higher, its sinks are diminished, and its lifetime is therefore longer [10].
A secondary benefit of ideal methane oxidation is improved air quality through lower surface ozone concentrations.Global surface ozone is lowered by 0.012 ± 0.002 ppb O 3 per Tg CH 4 oxidized yr −1 ; each 0.012 ppb O 3 causes ∼600 premature deaths globally every yr [10].We emphasize that these values, 0.012 ppb O 3 and 0.0014 • C per (Tg CH 4 oxidized yr −1 ), are the asymptotic benefits that would be achieved by a continuous oxidation rate (figure S1).
The impacts of carbon dioxide and methane can also be quantified economically with dollar valuations of a marginal unit of emission [12].Estimates of these 'social costs' are uncertain and scenariodependent, but they are currently the best estimates for how to financially value the impacts of these greenhouse gases.We take our social cost estimates from the U.S. Interagency Working Group on Social Cost of Greenhouse Gases for 2030, using a 3% annual discount rate [13].The social benefit of methane oxidation is the social cost of methane minus 2.75 times the social cost of carbon dioxide: $1800 ± $1400 per tonne of methane oxidized.

Concentrations of methane emissions
We now present, to the best of our knowledge, the first estimates of the mass of methane emitted globally at a given concentration.We synthesize data for the mass of methane emitted by source category and the typical concentration emitted by each source.Total emissions for each source category are taken from the Global Methane Budget 2020 bottom-up estimates for the year 2017 [14,15].
Because methane diffuses into the atmosphere, its concentration is lower further from emissions sources.Therefore, we consider the concentration at the locations closest to emissions sources (pipelines, dairy barns, landfill surfaces, etc) for which data are available.As an example, the concentration of methane in a natural gas pipeline is above 90%, whereas the concentration entering the atmosphere from an underground leak is typically lower due to dilution; we consider the former as the methane that could theoretically be avoided or oxidized.The concentration that can be addressed by a specific technology will depend on its implementation and the extent to which the methane diffuses into the local air before being oxidized; our estimates are therefore upper bounds for the concentration.
The concentrations at which methane is emitted vary widely (see table S1 for a categorization of emissions sources used throughout this paper).Point source methane concentrations are assumed to be normally distributed with means and standard deviations found from various sources in the literature (figure 2(a); details and references in table S2).For methane sources that emit across twodimensional areas, such as wetlands, freshwaters, and rice paddies, we use data from the closest measurement devices ('flux towers') to provide estimates of the concentration available for localized oxidation.The concentration available for oxidation would be higher at the surface of an area source, but much less data is available there.The concentrations (2-3 ppm CH 4 ) observed at measurement towers near these area sources is nearly as low as the background atmospheric concentration (figure 2(b)).Other area sources, including oceans and biomass burning, emit at even lower concentrations than wetlands.These sources are categorized together as 'Other'; details on their estimation based on the emissions rate data (in Tg CH 4 m −2 yr −1 ) from the Global Methane Budget [14] are given in the supplementary information (figure S3).
We combine the point source data (figure 2(a)) with the area source data (figure 2(b)) to estimate the total mass of methane emitted annually by concentration (figures 2(c) and (d)).Most methane emitted globally is at concentrations below 10 ppm (∼400 Tg yr −1 ), although substantial quantities are emitted between 100 and 1000 ppm (∼100 Tg yr −1 ) and above 100 000 ppm (∼100 Tg yr −1 ) (figure 2(c) and table 2).The mass of methane available for oxidation increases gradually as concentration decreases over orders of magnitude: for example, there is twice as much methane available to be oxidized above 10 ppm as there is above 1000 ppm (figure 2(d)).Only near 2 ppm CH 4 (the background atmospheric concentration) are there substantial increases in the total mass of methane that could be oxidized, from area sources and the ∼5300 Tg already in the atmosphere.

State-of-the-art methane oxidation technologies
We turn next to state-of-the-art methane oxidation technologies and the concentrations at which they can be feasibly deployed.We first outline the lowest concentrations of methane at which different oxidation technologies have been demonstrated.We focus on flow reactor technologies rather than surface or atmospheric oxidation enhancement technologies because the concentration of methane dictates the applicability of flow reactors (table S1).We categorize technologies based on their energy source: heat, light, or biological enzymes (table 1).Methane oxidation using electric potential may also be feasible, although current proposals primarily focus on converting high concentration methane streams into value-added end products [16].
A range of commercial technologies can oxidize methane down to 1000 ppm.Methane concentrations above ∼44 000 ppm sustain a flame in air.Subflarable concentrations as low as 2000 ppm are commercially oxidized with regenerative thermal oxidizers (RTOs), another heat-based technology currently used for coal mine ventilation air, among other applications [17].However, RTOs require air temperatures of ∼1000 • C. RTOs that incorporate noble metal catalysts, known as regenerative catalytic oxidizers (RCOs), operate down to 1000 ppm CH 4 [17].Zeolites, a type of porous, crystalline substrate that can be doped with metal catalysts, have been demonstrated for low-concentration, mediumtemperature methane oxidation: a copper zeolite has been shown to oxidize 2 ppm CH 4 if the air is heated to 330 • C, removing 95% of the methane with a residence time of ∼30 s (t R,95% = ∼30 s) [18].
Methanotrophs (bacteria that obtain energy by oxidizing methane) have been applied in biofilters [28] (flow reactors that facilitate bacterial growth) at landfills [29], wastewater treatment plants [30], and coal mines [31].The current lowest methane concentration oxidized by a bacterial biofilter is 1000 ppm (t R,55% = ∼360 s) [30], although modeling suggests that bacterial biofilters could operate down to 500 ppm CH 4 before the concentration is too low for bacteria survival [32].This

Concentration thresholds imposed by energetic constraints
We next examine the feasibility of proposed methane oxidation technologies based on their energy requirements.We consider a 2030 scenario in which best-case projections for energy cost and carbon intensity are met.
The first constraint that we consider is 'climate neutrality,' wherein the cooling benefit of oxidized methane equals the warming caused by the carbon intensity the energy used for oxidation.For climate neutrality, we convert methane to an equivalent mass of carbon dioxide using the global warming potential (GWP) that aligns with the Paris Agreement temperature goal of 1.5 • C. Because the endpoint year that aligns with this goal is 2045 and we consider the year 2030, we use the 15 year GWP value of 93 ± 30 [43].Subtracting 2.75 from this value gives the climate benefit of methane oxidation (in CO 2equivalents): 90 ± 30.The second constraint that we consider is 'cost neutrality,' wherein the economic benefit of the oxidized methane (1800 $/tonne CH 4 oxidized in 2030, see section 2 for details) equals the cost of the energy used.
We limit our focus to the energy consumption for the oxidation process itself, noting that our definitions of climate and cost neutrality are necessary but insufficient conditions.Other aspects of deploying flow reactors include capital investments, raw catalyst materials, and the energy required to move vast quantities of air and overcome any pressure drop in the system, all of which have associated greenhouse gas emissions and costs.However, we do not assume any particular system configuration and hence do not include these carbon intensities and costs in our calculations.For a specific system configuration where these are included, the climate and cost neutrality thresholds would be even more restrictive.

Heat-based concentration thresholds
We first apply these constraints to flow reactors that heat the entire air stream.We calculate the required thermal energy in kWh from the temperature difference between the inlet and outlet air streams of the reactor.If 2030 projections for industrial heat are met, geothermal will be the lowest cost and carbon intensity heat source with a levelized cost of heat of 0.015 ± 0.004 $ kWh −1 [44] and a carbon intensity of 0.012 ± 0.002 kg CO 2 e kWh −1 [45].For temperatures exceeding typical geothermal direct-use temperatures (⩽150 • C) [46], we take the levelized cost of heat to be 0.020 ± 0.006 $ kWh −1 [47] for solarbased electrical heating (the projected cheapest in 2030) and the carbon intensity to be 0.015 ± 0.01 kg CO 2 e kWh −1 [48] for wind-based electrical heating (the projected lowest carbon intensity in 2030).Furthermore, we assume that all heat released by the exothermic oxidation of methane is used and that a heat exchanger recovers 80% of exhaust heat.(See 'Heat-based concentration thresholds' in the Supplementary Information for a detailed description of our methods including a sensitivity analysis.) We establish relationships between the concentration of methane oxidized and the temperature at which a technology can operate while still meeting cost and climate neutrality constraints (figure 3(a)).Cost neutrality (a stricter constraint than climate neutrality) imposes a practical concentration threshold: the minimum concentration of methane that must be oxidized for a given temperature increase above ambient temperature is 0.8 ± 0.6 ppm CH 4 / • C for temperatures below 150 • C and 1.0 ± 0.7 ppm CH 4 / • C for temperatures above 150 • C (figure 3(a)).
Heat-based technologies with high required temperatures are unlikely to be feasibly applied to atmospheric methane concentrations.When 2 ppm CH 4 is oxidized, we estimate that the temperature increase can only be 3 • C ± 2 • C for cost neutrality (figure 3(a)).The copper zeolite described in section 4 operated at 330 • C (305 • C above ambient temperature), which has an associated concentration minimum of 300 ± 200 ppm CH 4 for cost neutrality (figure 3(a)).
In particular instances, temperature-based concentration thresholds may be less restrictive, especially for slightly elevated temperatures.Waste heat may have a low cost and associated carbon intensity and is readily abundant: approximately half of global energy use is lost as waste heat, primarily below 300 • C [53].Heat pumps are more efficient than electric heating and are well-suited to deliver low elevated temperatures in warm regions of the world, where they extract heat from the ambient environment.Passive solar heating can also deliver temperatures well above 50 • C.
The temperature-based concentration thresholds would be even more restrictive for heat sources with higher carbon intensities and costs or for heat exchangers with lower effectiveness (figure S5).If the cost per kWh decreased by two orders of magnitude holding all else constant, then climate neutrality would dictate the practical concentration threshold rather than cost neutrality.In any case, these results underscore the importance of carefully considering the heat source for any technology that requires elevated temperature to operate.

Light-based concentration thresholds
We next apply cost and climate neutrality constraints to light-based technologies, such as photocatalysis and photolysis.We calculate the electrical energy required from artificial light generation in kWh.We consider a best-case scenario in which the radiant efficiency of UV light generation is 70% [54], the We assume that all heat from the exothermic oxidation methane is used and that 80% of heat is recovered with a heat exchanger.We model the lowest cost and carbon intensive heat sources for 2030, using geothermal for ⩽150 • C applications and electric heating from solar and wind for >150 • C applications (this appears visually as steps at 150 • C).Alphanumeric labels compare state-of-the-art heat-based oxidation technologies in terms of their demonstrated temperature and lowest methane concentration oxidized.Thermocatalysts: Ba-Mn [49] (T1) and PdO-Pt/α-Al2O3 [50] (T2).Thermo-electrocatalysts: Co3O4/Ce0.75Zr0.25 [51] (TE1).Thermophotocatalysts: ZnO/La0.8Sr0.2CoO3[52] (TP1).Zeolites: Cu/mordenite [18] (Z1) and Pd/HZSM-5 [38] (Z2).(b): Artificial light-based methane oxidation.We model the projected lowest cost and carbon intensive electricity sources for 2030 using solar and wind energy, respectively.The alphanumeric label (P1) shows the photocatalyst Ag/ZnO [19] in terms of its wavelength of light and apparent quantum yield demonstrated.levelized cost of electricity is 0.020 ± 0.006 $/kWh for solar electricity [47] (the projected cheapest in 2030), and the carbon intensity is 0.015 ± 0.01 kg CO 2 e/kWh for wind electricity [48] (the projected lowest carbon intensity in 2030).(See 'Light-based concentration thresholds' in the supplementary information for a detailed description of our methods including a sensitivity analysis.) The key metric for these technologies is the apparent quantum yield (AQY), which we define as the percentage of incident photons that oxidize a methane molecule.(Note that AQY has been defined in different ways in the literature; if all eight electrons required to fully oxidize methane via photocatalysis come from the light source then the maximum AQY is 12.5% [19].)AQY depends on the wavelength of light used; light-based technologies using shorter wavelengths, which carry more energy, require higher AQYs to offset the cost and carbon intensity of the electricity consumed.AQY also depends on methane concentration: for dilute concentrations, the methane oxidation reaction can be limited by the number of methane molecules that adsorb onto a catalyst or collide with gas-based radicals.In these cases, lower methane concentration causes a lower reaction rate and lower AQY [55].
We establish relationships between AQY and the wavelength of light used for oxidation that must be met for cost and climate neutrality and show that cost neutrality imposes a stricter concentration threshold than climate neutrality (figure 3(b)).For the projected 2030 electricity cost and current state-of-the-art wavelength (365 nm) used for both photocatalysis and photolysis, we estimate that the minimum AQY is 9% ± 8% for cost neutrality.To our knowledge, the only peer-reviewed light-based technology for which AQY data is available, the Ag-ZnO photocatalyst, has an AQY of 1% at 10 000 ppm CH 4 (and a lower AQY at lower methane concentrations).Therefore, significant breakthroughs in catalytic efficiency are required to satisfy cost neutrality.There is currently no photocatalyst that we know of that satisfies cost neutrality at 10 000 ppm CH 4 ; doing so at 2 ppm CH 4 therefore appears infeasible without substantial improvements in AQY.
Figure 3(b) uses a best-case projection for 2030 electricity cost and carbon intensity; it would be even more restrictive for electricity sources with higher carbon intensities and costs or for lights with lower radiant efficiencies (figure S7).Our results are for artificial light with 365 nm wavelength; light-based concentration thresholds would be much less restrictive if visible wavelengths of light could be used, enabling the use of free sunlight instead of artificial light.However, making use of visible light for methane oxidation will require substantial advances in materials science [21,52].

Discussion
Sections 2-5 establish our methane concentrationbased framework for estimating the maximum potential benefits of a methane oxidation technology (figure 1).This framework can be applied to hypothetical scenarios in which technologies are deployed Table 2. Hypothetical scenarios demonstrating our concentration-based framework (figure 1).We consider scenarios in which all emissions above certain concentrations are oxidized with no negative climate effects considered.These scenarios require a continuous oxidation rate (shown in brackets) over a time period of at least a decade to reach the asymptotic benefits presented.For scale, there is currently ∼5300 Tg of CH4 in the atmosphere, ∼3200 Tg more than in preindustrial times [6].that can oxidize methane down to certain concentrations (table 2).
One key takeaway from table 2 is the importance of existing technologies (such as RTOs and RCOs) that can already oxidize methane at concentrations above 1000 ppm.Roughly 161 Tg CH 4 is emitted above 1000 ppm every year, equating to hundreds of billions of dollars of value if a market mechanism existed to incentivize methane oxidation.Significant portions of these emissions would be better addressed through simple avoidance measures like plugging natural gas leaks, but there are substantial opportunities for methane oxidation at difficult-to-avoid point sources such as coal mine ventilation air (figures 2(d) and S7).Efforts should go towards rapidly deploying methane oxidation technologies at the concentrations where they can already be applied.
We show that large quantities of methane are emitted annually at relatively low concentrations (<10 ppm CH 4 ) with no well-developed technologies available to address them (figure 2(c)).This technology gap is ripe for innovation, and its magnitude will only increase as natural emissions increase in a warmer future climate.However, our results show that reducing concentration thresholds by optimizing existing technologies alone is unlikely to address the bulk of natural area-source emissions which occur below 10 ppm CH 4 .Developing new technologies that can oxidize atmospheric methane should be a central goal to minimize further climate damage.
Significant technological breakthroughs are needed to allow any methane oxidation technology to address atmospheric methane (2 ppm CH 4 ).Biofilters have typically only been applied above 1000 ppm CH 4 , but the introduction of fungi or innovations in bioengineering may push that concentration threshold lower.To the best of our knowledge, no literature yet suggests that low-concentration electrocatalytic methane oxidation is feasible.Unless heat can be obtained at costs greatly under projected market prices, heat-based technologies cannot satisfy cost neutrality for low-concentration methane (figure 3(a)).For technologies that use light (photocatalysis and photolysis), substantial improvements in AQYs are required to reach the minimum value of 9% ± 8% for cost neutrality (figure 3(b)).
Developing technologies that concentrate dilute methane may help expand the potential applications of certain oxidation technologies; proposals to do so include porous polymer networks [56], pressure swing adsorption [57], and metal organic frameworks [58].However, concentrating dilute methane is difficult because of methane's nonpolar structure and low solubility [6].
Our work would be strengthened by improved measurement of methane emissions, both in situ and via satellites and drones.Having a wider dataset would give more precise estimates of the potential global scale of methane oxidation technologies.Additionally, better understanding the sources that are economically or technically feasible to mitigate (by emitted concentration) would allow researchers to clarify where oxidation could be deployed when avoidance is not feasible (figure S7 shows a first version of this.) Concentration thresholds are one of many important characteristics that dictate the potential feasibility of a methane oxidation technology.Governance and policy work are needed to determine market incentives and evaluate other climate and social impacts of these technologies before they can be deployed at scale.Nonetheless, we hope that our new concentration-based framework can be used by scientists, engineers, and policymakers to better understand the connections between methane sources, their emission rates and concentrations, and the technologies that can oxidize those emissions.

Figure 1 .
Figure 1.A concentration-based framework for assessing the potential scale of benefits of methane oxidation technologies.Sections 2-5 of the text explain these relationships in detail and derive the conversion factors shown.

Figure 2 .
Figure 2.Estimated methane emissions by concentration and quantity.(a): Normalized frequency distributions by source type for point sources (references in table S2), showing estimated concentrations (shaded areas) and means (filled circles).(b): Same as (a) but for area sources (details in figure S2).(c): The mass of methane theoretically available for oxidation at a given concentration.(d): Same data as (c) in cumulative form; each point on the curve indicates the mass of methane emitted (Tg yr −1 ) above a given concentration.

Figure 3 .
Figure 3. Cost and climate neutrality thresholds for methane oxidation, using the 15 year climate benefit of methane oxidation of 90 (based on the GWPs of methane and carbon dioxide) and a social benefit of methane oxidation of 1800 $/tonne CH4 oxidized (based on the social costs of methane and carbon dioxide).(a): Heat-based methane oxidation.We assume that all heat from the exothermic oxidation methane is used and that 80% of heat is recovered with a heat exchanger.We model the lowest cost and carbon intensive heat sources for 2030, using geothermal for ⩽150 • C applications and electric heating from solar and wind for >150 • C applications (this appears visually as steps at 150 • C).Alphanumeric labels compare state-of-the-art heat-based oxidation technologies in terms of their demonstrated temperature and lowest methane concentration oxidized.Thermocatalysts: Ba-Mn[49] (T1) and PdO-Pt/α-Al2O3[50] (T2).Thermo-electrocatalysts: Co3O4/Ce0.75Zr0.25[51](TE1).Thermophotocatalysts: ZnO/La0.8Sr0.2CoO3[52](TP1).Zeolites: Cu/mordenite[18] (Z1) and Pd/HZSM-5[38] (Z2).(b): Artificial light-based methane oxidation.We model the projected lowest cost and carbon intensive electricity sources for 2030 using solar and wind energy, respectively.The alphanumeric label (P1) shows the photocatalyst Ag/ZnO[19] in terms of its wavelength of light and apparent quantum yield demonstrated.

Table 1 .
Classification of proposed methane oxidation technologies according to energy source and process.
[33]centration threshold could theoretically be lowered by improved biofilter design or selecting for enzymatic methane affinity through genetic modification.One promising idea is the inclusion of fungi in biofilters; the fungi created a structural support for methanotrophs and allowed for oxidation down to 20 ppm CH 4 in one study (t R,24% = ∼25 s)[33].