In the 2022 climate change assessment report, the IPCC (Intergovernmental Panel on Climate Change) highlights the AFOLU sector (Agriculture, Forestry, and Other Land Use) as a significant source of greenhouse gas emissions, representing approximately 23 % of global anthropogenic emissions between 2010-2019 [2]. Globally, manure management generates around 230 million tons of CO₂ equivalent annually. Colombia, as the third-largest country in terms of greenhouse gas emissions from agriculture, contributes 65 Mt of CO₂ equivalent [3]. In the context of the Cavalry Canton in Bogotá, which houses around 100 equine specimens, the challenge is to manage these animals' feces. At present, a portion of the feces is utilized to feed a biodigester within the canton, while the remainder is collected by a waste management company. During storage and the anaerobic process in the biodigester, gases such as CH₄, CO₂, and H₂S are emitted.

Some previous studies have addressed greenhouse gas detection at the industrial level, such as in the dairy industry with implementations in Canada, the Netherlands, and Japan. However, there are no similar reports related to equine feces. Therefore, the concentration of CH₄, CO₂, and H₂S emitted by the spontaneous decomposition of these feces is not reliably reported. Given this, during this research, a monitoring system was implemented over a period of 45 days to measure the concentration of CH₄, CO₂, and H₂S in different scenarios: the manure storage site, the biodigester outlet, and under controlled laboratory conditions. The second stage focused on calculating CH₄ and CO₂ equivalent emission factors for the Cavalry Canton without implementing any treatment. Finally, in the third phase, the potential for transforming equine manure using a low-cost tubular digester and its contribution to reduce greenhouse gas emissions was evaluated.

During the initial phase of this research, a review of greenhouse gas emissions was conducted, identifying CH₄, Cd, and H₂S as the primary contributors. Based on this identification, sensors that integrated the system during the 45-day monitoring period in the three scenarios where the concentrations of these gases were measured were selected: the manure storage site, the biodigester outlet, and under controlled laboratory conditions. For each scenario, the system consisted of four sensors: TGS2611 and MQ-4 for CH4, SEN 0219 for Cd, and MQ-136 for H₂S with a measurement range of (200 - 10,000 ppm, 500 - 12,500 ppm, 0 - 5000 ppm, and 0 - 200 ppm) and detection levels of (10 ppm, 0.1 - 0.65 ppm, 40 ppm, 30 ppm). The second phase of this research involved calculating CH₄ and CO₂ emission factors following the guidelines established by the Intergovernmental Panel on Climate Change (IPCC) [4]. For estimating CH₄ emissions from manure, the IPCC establishes three levels, each depending on the waste production rate per animal, the number of animals, and how it is treated. Of the three levels, the one that best suited the context of the research was Level 1. This is a simplified method that only requires data on livestock population by species/ category and climate region or temperature, in combination with the IPCC's default emission factors to estimate emissions [4].

Equation 1 illustrates the calculation of CH₄ emissions from manure management, following the Level 1 methodology.

Where:

CH _4Manure = CH _₄ emissions from manure management for a defined population (GgCH_₄*yr^⁻¹).

EF _(T) = emission factor for the defined livestock population.

N(T) = number of heads of the species/category of livestock.

T = species/category.

Regarding CO₂ emissions estimation, the IPCC proposes an equation to And the CO₂ equivalent emissions of CH₄.

Where:

CO _{2 Manure} = CH _₄ emissions in CO₂eq from manure management (GgCO_₂eq*yr^⁻¹).

GWP = global warming potential over a 100-year time horizon.

Finally, in the third phase of this research, an analysis was conducted to evaluate the potential for transforming equine manure using a low-cost tubular digester, with the primary objective of analyzing its contribution to reducing greenhouse gas emissions. In this context, a detailed analysis of the organic matter input into the biodigester was performed, characterizing it through parameters such as total solids (TS), volatile solids (VS), and chemical oxygen demand (COD). This characterization is crucial as there is a direct correlation between the percentages of contaminant load removal and biogas production [5]. Moreover, the results obtained from the installed monitoring system were assessed to determine the effectiveness of the digester in reducing greenhouse gas emissions within the specific context of the canton.

A detailed analysis of emissions during the 45-day monitoring period reveals significant patterns, particularly when comparing emissions at the storage site and the biodigester outlet. By implementing two-time frames-day and night-we observed how light conditions affect gas emissions, confirming the direct influence of temperature on this process [6]. As shown in Figure 9, emissions at the storage site significantly increase during the day, with an 85 % increase in CH₄, a 61 % increase in H₂S, and a 99 % increase in CO₂. During the nighttime, average emissions were 4.83 ppm of CH₄, and 9.42 ppm during the day; H₂S, emissions were 0.58 ppm at night and 0.61 ppm during the day, while for CO₂, emissions were 1074 ppm at night and 1506 ppm during the day. These variations suggest a direct relationship between emissions, ambient temperature, and microbial activity. Elevated daytime temperatures accelerate microbial kinetics, increasing the rate at which organic substrates break down into simpler compounds, thus increasing GHG production. This finding supports the direct influence of temperature and sunlight on biological decomposition processes. In contrast, at the biodigester outlet (Figure 10), CH₄ emissions were 0.43 ppm during the night and 1.76 ppm during the day. For H₂S, concentrations were 0.23 ppm at night and 0.27 ppm during the day, while CO₂ emissions were 408 ppm at night and 523 ppm during the day. These data indicate that the anaerobic digestion process effectively mitigates CH₄, CO₂ and H₂S emissions by capturing the methane produced by the manure for use in biogas combustion. The conversion of methane into CO₂ and water reduces the greenhouse effect impact [6].

This analysis highlights the positive impact of anaerobic digestion, as it not only mitigates emissions but also captures and utilizes the generated methane as an energy source. The results show significant reductions, ranging from 8 to 87 % in CH₄, from 58 to 67 % in CO₂, and from 33 to 65 % in , when comparing emissions with untreated storage conditions.

Figure 9

Figure 10

Direct measurements taken at the manure storage site and the biodigester outlet revealed a significant reduction in emitted gas concentrations. Methane decreased by 8 to 87 %, carbon dioxide by 58 to 67 °%, and hydrogen sulfide by 33 to 65 %, compared to emissions recorded under untreated storage conditions. This reduction highlights the effectiveness of anaerobic digestion in mitigating GHGs.

Additionally, a directly proportional relationship between emissions and temperature was observed in both scenarios. Emission concentrations increased with higher temperatures and decreased with lower temperatures, aligning with the optimal conditions for microbial activity in the bacterial consortium. This behavior underscores the importance of maintaining ideal thermal conditions (35 - 37 °C) to enhance microbial activity and maximize the efficiency of the anaerobic digestion process.

The project, in essence, contributes to technological advancement by offering an alternative methodology for quantifying GHG emissions- specifically CH₄, CO₂ and H₂S-derived from manure management and biodigester outputs. This proposal stands out by providing a practical alternative to theoretical calculations in small-scale gas estimates, validating the effectiveness of anaerobic digestion as a sustainable alternative for mitigating and utilizing these gases.