Introduction

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.

Methodology

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.

Results

Manure Storage Location

The results obtained from the monitoring system at the manure storage location show a directly proportional relationship with ambient temperature. It was noted that higher temperatures were associated with increased emissions, while lower temperatures resulted in reduced emission levels. Figure 1 clearly illustrates this behavior, where methane emissions at the manure storage site varied from a minimum of 2.07 to a maximum of 21.36 ppm of CH4, with peak concentrations identified in a temperature range between 10 and 15 °C.

Carbon dioxide CO₂ emissions at the manure storage site, like methane emissions, are the result of the decomposition process of organic matter present in the manure. Their generation is directly linked to the bacterial consortium present in the feces, which is sensitive to temperature variations. During the storage period, the most significant emissions were recorded in a temperature range between 5 and 24 °C, with concentrations ranging from 950 to 2167 ppm of CO₂, as illustrated in Figure 2.

Regarding hydrogen sulfide emissions, the research results revealed concentrations in the storage scenario ranging from 0.58 to 0.63 ppm, recorded in a temperature range between 15 and 22 °C, as shown in Figure 3. These findings indicate that during the storage period of equine manure, hydrogen sulfide emissions were produced at relatively low levels, showing some stability in concentrations across the evaluated thermal range.

Figure 1 Methane emissions CH at the manure storage site throughout the monitoring period. 

Figure 2 Carbon dioxide CO₂ emissions at the manure storage site throughout the monitoring period. 

Figure 3 Hydrogen sulfide H₂S emissions at the manure storage site throughout the monitoring period. 

Biodigester Outlet

Methane emissions at the biodigester outlet site showed significantly lower concentrations compared to those observed in manure storage. Values ranging from 0.54 to 2.57 ppm were recorded, as detailed in Figure 4. This indicates that the anaerobic digestion process applied in the biodigester has a positive impact on reducing methane emissions. This phenomenon is attributed to the effectiveness of the biodigester in treating manure, minimizing the release of greenhouse gases compared to conventional storage.

Figure 4 Methane emissions CH4 at the biodigester outlet throughout the monitoring period. 

Monitoring results of carbon dioxide CO₂ at the biodigester outlet revealed emissions in a range of 397 to 694 ppm, as illustrated in Figure 5. Although these figures are relatively low compared to CO₂ emissions at the manure storage site, the fact that elevated CO₂ emissions are still generated highlights the importance of evaluating and improving this tool to minimize environmental impact.

In the biodigester outlet scenario, hydrogen sulfide H₂S concentrations were recorded in a range of 0.20 to 0.33 ppm, as shown in Figure 6. These results contrast significantly with the concentrations observed at the manure storage site, where hydrogen sulfide emissions reached higher levels.

Figure 5 Carbon dioxide CO₂ emissions at the biodigester outlet throughout the monitoring period. 

Figure 6 Hydrogen sulfide H₂S emissions at the biodigester outlet throughout the monitoring period. 

Controlled Laboratory Conditions

In the third scenario, in the laboratory under controlled conditions, methane production increased progressively over time, facilitated by the device designed to create a hermetic environment. In this closed space, concentrations of CH₄, CO₂ and H₂S, and accumulated more markedly compared to the previous scenarios, as those were semi-open settings. Figure 7 shows that CH₄ production exhibited concentrations ranging from 170 to 2216 ppm. This increase is attributed to the constant temperature of 20 °C in the incubator, providing ideal conditions for the mesophilic and cryophilic bacterial consortium.

Figure 7 Methane emissions CH₄ under controlled laboratory conditions throughout the monitoring period. 

Regarding hydrogen sulfide H₂S emissions in the monitoring system under controlled conditions, as illustrated in Figure 8, concentrations ranging from 0.49 to 1.75 ppm of H₂S were recorded. Emissions show a similar behavior to methane, which is explained by the composition of biogas, where represents less than 1 %, along with CH₄ (50 - 70 %) and carbon dioxide (50 - 30 %) [⁶]. These results highlight the influence of controlled laboratory conditions on gas generation, providing insight into the behavior of such gas emissions. The theoretical estimation of CH₄ and CO₂ equivalent emissions for manure management in the Cavalry Canton, the method proposed by the IPCC Tier 1 was used for this calculation. Theoretical results indicate that the annual emission of CH₄ would be 2.69*10 Gg and for CO₂ the equivalent would be 0.08339 Gg. Comparing these values with those obtained from the monitoring system reveals notable differences. Theoretical data, based on the IPCC Tier 1 method and extrapolated for the entire manure management in the Cavalry Canton, suggest significant annual emissions of methane and carbon dioxide equivalent. Meanwhile, measurements from the monitoring system in specific scenarios (manure storage, biodigester outlet, and laboratory conditions) present lower concentrations of CH₄ and CO₂ equivalent. These values, although valid for specific areas and particular scenarios, are significantly lower than theoretical estimates.

This difference suggests that actual emissions, according to the specific measurements, are lower than theoretical projections. Various factors can explain this difference: such as the spatial variability of emissions in the facility, the efficiency of the biodigester in reducing emissions, and the specific temperature and bacterial consortium conditions in each scenario. However, although the concentrations measured at the storage site represent only a specific area (6 of 42 m² total), it suggests that total emissions in manure storage could be considerably higher, covering the entire storage space.

Table 1 Results of the physicochemical characterization of the influent and effluent. 

Proximate Analysis	Units	Influent	Effluent
DQO	gDQO/L	24.44±0.79	3.92±0.120
ST	g/kg	23.48±1.50	2.10±0.060
SV	g/kg	20.74±1.56	1.10±0.005

The significant reductions in COD (84 %) and VS (95 %) indicate that the organic matter in the manure is effectively degraded during the digestion process. These reductions in COD and VS are indicative of successful organic waste management, directly contributing to the reduction of GHG emissions, such as CH₄, CO₂ and H₂S associated with anaerobic decomposition.

Figure 8 Hydrogen sulfide H₂S emissions under controlled laboratory conditions throughout the monitoring period. 

Analysis of Results

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 Average emissions of CH₄, CO₂, H₂S over 45 days at the manure storage site. 

Figure 10 Average emissions of CH CO H₂S over 45 days at the biodigester outlet. 

Conclusions

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.