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| Fig. 1: Simplified schematic of an anaerobic digester system converting livestock manure into biomethane. Methane is cleaned and used for heat, vehicle fuel, or electricity generation, while residual solids and liquids are recycled as compost and fertilizer. (Source: Wikimedia Commons) |
Estimating the maximum energy from global biomethane reserves requires quantifying manure conversion to methane (CH4) and CO2 via anaerobic digestion, accounting for methane yields from different sources, livestock statistics, the energy content of the methane bond (Joules), and the efficiency of methanogenic bacteria. [1,2] Fig. 1 shows a schematic for a typical anaerobic digester.
As shown in Table 1, the Food and Agriculture Organization (FAO) estimates that there were approximately 1.5 billion cattle, 1.4 billion pigs, and nearly 30 billion chickens worldwide in 2018. [3] According to the FAO, livestock manure contains 125 million tonnes of nitrogen, of which 88 million tonnes are left on pasture by grazing animals, 34 million tonnes are managed in manure systems, and 27 million tonnes are applied to solids for crop production. [4]
Representative amounts of manure and livestock are shown in Table 1. The statistics in this table allow the calculation of the total manure produced by each livestock category. These values are based on ASAE standards and allow for back-of-the-envelope estimates. Using livestock population data from the FAO and representative manure production rates from ASAE Standard D384s, global manure production can be estimated as
or 39 billion tonnes per year. This result is of the same order of magnitude as the nitrogen-based 2018 FAO estimate (= 25 billion tonnes/yr).
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| Table 1: Global livestock populations and representative manure production rates used to estimate annual manure generation. [3,5] |
The total energy that can be obtained from the anaerobic digestion of wastewater sludge from different livestock types can be calculated using anaerobic digestion parameters. Heating primary and secondary wastewater sludge usually yields about 20 MJ/kg on a dry, volatile-solids basis, although the higher heating value (HHV) varies among livestock. [6] Because of the presence of oxygen, ash, and other non-fuel components that dilute the energy content, dried, digestible manure solids have about 20 MJ of chemical energy per kilogram, which is significantly less than the HHV of pure methane (55.5 MJ/kg). [7,8] The HHV depends on the carbon, hydrogen, and oxygen content of the manure, and thus varies among dairy cows, market hogs, and other animals.
Anaerobic bacteria are typically needed to extract chemical energy from carbohydrate bonds in organic matter. About half of the carbon is oxidized by these bacteria to CO2, and the remaining carbon is reduced to CH4. They naturally form communities that facilitate effective digestion, and their metabolic pathways are optimized for energy conversion.
Manure is a heterogeneous mixture of processed food residues resulting from digestion. It contains volatile solids, which are digestible by bacteria and can be converted into methane and carbon dioxide, as well as non-volatile solids, which typically consist of dirt and other indigestible materials. The maximum methane yield (mCH4) varies significantly by manure type. The methane mass produced per animal is calculated as follows:
where η denotes the combined efficiency of volatile solids (VS) degradation and methane conversion. η represents the theoretical mass branching ratio between CH4 and CO2 during anaerobic digestion. Approximately 35% of the degraded volatile solids are converted to methane by mass. [2]
The volatile solid values in Table 2 are estimated based on several calculations. Møller et al. report 1,759 kg of volatile solids per dairy cow annually. Dividing by 365 yields 4.8 kg VS per day. We round this to 5.0 kg per day to account for other cattle. For pigs, chickens, sheep, and goats, the NRCS provides representative volatile-solids excretion rates that scale with animal size. [2]
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| Table 2: Calculation of daily and annual methane generation from livestock. [9-11] |
Using the FAO livestock populations from Table 1, we estimate the total annual methane production (MCH4) from complete digestion using MCH4 = mCH4 × N × 365:
| Cow: | 0.55 kg/d | × | 1.5 × 109 | × | 365 d/yr | = | 3.0 × 1011 kg/yr |
| Pig: | 0.24 kg/d | × | 1.4 × 109 | × | 365 d/yr | = | 1.2 × 1011 kg/yr |
| Chicken: | 0.0084 kg/d | × | 3.0 × 1010 | × | 365 d/yr | = | 9.2 × 1010 kg/yr |
| Sheep: | 0.063 kg/d | × | 1.4 × 109 | × | 365 d/yr | = | 3.2 × 1010 kg/yr |
| Goat: | 0.045 kg/d | × | 1.1 × 109 | × | 365 d/yr | = | 1.8 × 1010 kg/yr |
Methane has a higher heating value (HHV) of 55.5 MJ/kg, which is much higher than the dry manure volatile solids' value of about 20 MJ/kg. [8] Methane, which is significantly more energy-dense, is produced from some of the volatile solids in manure through anaerobic digestion. Consequently, the following formula is used to determine the total chemical energy of global methane derived from manure
Converting to exajoules (1 EJ = 1018 J), we obtain ECH4 ≈ 31 EJ per year.
Given that the world's total primary energy supply is roughly 548 EJ/yr, even perfect digestion and energy recovery of all livestock manure would contribute only about 5 - 6% of global energy demand. [11]
The total global energy available from manure is modest compared to national energy requirements, but locally, especially in rural or remote areas, biomethane provides accessible, impactful energy and effective waste management.
These calculations exclude minor livestock sources such as buffalo, camels, and horses. The analysis shows that while biomethane cannot shape global energy supply, it can offer meaningful local benefits.
© Arshia Sazi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[2] Y. Feng, and L. Rosa, "Global Biomethane and Carbon Dioxide Removal Potential Through Anaerobic Digestion of Waste Biomass," Environ. Res. Lett. 19, 024024 (2024).
[3] A. J. Garmyn, "More Than Meat: Contributions of Livestock Systems Beyond Meat Production," Anim. Front. 11, 3 (2021).
[4] Livestock and Environment Statistics: Manure and Greenhouse Gas Emissions," Food and Agriculture Organization of the United Nations, 2020.
[5] "ASAE Standard D384.1: Manure Production and Characteristics," American Society of Agricultural Engineers, February 2003.
[6] R. L. Skaggs et al., "Waste-to-Energy Biofuel Production Potential For Selected Feedstocks in the Conterminous United States," Renew. Sustain. Energy Rev. 82, 2640 (2018).
[7] A. D. Cullar, and M. E. Webber, "Cow Power: The Energy and Emissions Benefits of Converting Manure to Biogas," Environ. Res. Lett. 3, 034002 (2008).
[8] Y. Mezmur and W. Bogale, "Simulation and Experimental Analysis of Biogas Upgrading," AIMS Energy 7, 371 (2019).
[9] H. B. Møller, S. G. Sommer, and B. K. Ahring, "Methane Productivity of Manure, Straw and Solid Fractions of Manure," Biomass Bioenergy 26, 485 (2004).
[10] "Agricultural Waste Characteristics," in Agricultural Waste Management Field Handbook Part 651, U.S. Department of Agriculture, March 2008.
[11] G. A. Jones and K. J. Warner, "The 21st Century Population-Energy-Climate Nexus," Energy Policy 93, 206 (2016).
