This study presents a comparative analysis of biogas yields from various organic waste sources in Nakisunga Parish, Mukono District, Uganda, a rural area with significant potential for biogas production. The primary objective was to identify the most efficient waste sources for biogas production, thereby offering insights into optimizing biogas systems in similar rural communities. The study utilized a combination of field surveys and laboratory analyses to evaluate the biogas production potential of farm waste, household waste, municipal waste, and food processing waste. Results indicated that farm waste, particularly swine manure, exhibited the highest biochemical methane potential (BMP), with a BMP value of 0.007 mL CH₄/g VS, making it the most promising feedstock for biogas production. Crop residues and cow dung also showed moderate potential, while household and food processing wastes had lower BMP values. The findings underscore the importance of selecting appropriate feedstocks to maximize biogas yields and highlight the viability of biogas production in rural Ugandan communities where agricultural activities dominate. This study provides localized data on biogas potential in rural Sub-Saharan Africa, addressing the gap in research regarding the comparative efficiency of various organic waste sources in these settings. The results have significant implications for sustainable energy policy and rural development, suggesting that targeted biogas initiatives could play a critical role in improving energy access and waste management in rural areas. Recommendations for future research include exploring co-digestion strategies to further enhance biogas production and examining the socio-economic impacts of biogas adoption in rural communities.
Research Article
Comparative Analysis of Biogas Yields from Different Organic Waste Sources in Rural Communities
https://doi.org/10.21203/rs.3.rs-5206514/v1
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Biogas
Organic waste
Biochemical methane potential (BMP)
Rural communities
Renewable energy.
Biogas production has emerged as a sustainable and environmentally friendly solution for addressing energy needs and managing organic waste, particularly in rural communities [1–3]. As global concerns about fossil fuel depletion and environmental degradation grow, the need for alternative energy sources becomes increasingly urgent [4–6]. Biogas, a renewable energy source produced through the anaerobic digestion of organic matter, offers significant potential for reducing reliance on traditional energy sources, while simultaneously addressing waste management challenges.
In rural communities, where agricultural activities generate substantial amounts of organic waste, biogas production presents a promising opportunity for converting waste into valuable energy. However, the efficiency and viability of biogas production depend heavily on the types of organic waste used as feedstock. Different organic materials, such as farm waste, household waste, municipal waste, and food processing waste, have varying biochemical properties that influence the quantity and quality of biogas produced [7–9].
This study investigated the biogas production potential of various organic waste sources in Nakisunga Parish, a rural area in Mukono District, Uganda. The district is characterized by agricultural activities, small-scale food processing, and a growing population, all of which contribute to the generation of significant organic waste. Despite the availability of these resources, there has been limited research on the specific biogas potential of these waste streams in this region.
The primary objective of this study was to compare the biogas yields from different organic waste sources, identifying the most efficient and effective feedstocks for biogas production. By doing so, the research provides valuable insights into the feasibility of implementing biogas systems in rural Ugandan communities, with potential applications for similar regions in Sub-Saharan Africa.
Through this comparative analysis, the study contributes to the broader understanding of sustainable energy solutions in rural contexts, offering practical recommendations for optimizing biogas production and supporting rural development. The findings of this research will inform policymakers, local authorities, and practitioners of the best practices for biogas production, ultimately contributing to energy security and environmental sustainability in rural communities.
Biogas production has long been recognized as a sustainable energy solution with the potential to address multiple challenges, including energy access, waste management, and environmental sustainability [2, 10, 11]. The anaerobic digestion process that produces biogas converts organic waste into methane-rich gas, which can be used for cooking, lighting, and generating electricity. Several studies have explored the efficiency of different organic waste feedstocks in producing biogas, though gaps remain in understanding the best feedstock combinations in specific rural settings like Nakisunga Parish, Uganda.
Biogas is primarily produced through anaerobic digestion, a biological process that breaks down organic material in the absence of oxygen [12–13]. Key stages include hydrolysis, acidogenesis, acetogenesis, and methanogenesis, with the final stage yielding methane (CH₄) and carbon dioxide (CO₂) as primary gases [14–15]. Biogas production efficiency is influenced by the type of organic waste used, the conditions of the anaerobic digester, and microbial activity within the system [16–17].
Different types of organic waste exhibit varying biochemical methane potential (BMP), which directly impacts the quantity of biogas produced. Farm-based organic waste such as animal manure, crop residues, and poultry droppings are common feedstocks in rural settings, as they are abundantly available [18–19]. Additionally, household organic waste, including food scraps and vegetable peelings, also contributes to biogas production but often has a lower methane yield due to higher water content and lower volatile solids [20–21]. Municipal organic waste and waste from food processing industries present another viable source for biogas production but are less studied in rural contexts.
Several studies have compared biogas yields from different types of organic waste. Hilgert et al. [22] found that livestock manure, especially from cattle and pigs, provides high methane yields due to its rich nutrient content, which fosters microbial activity during anaerobic digestion. Atelge et al. [23] similarly identified animal manure as a highly efficient substrate for biogas production, particularly in rural communities where agriculture is dominant. In contrast, food waste and crop residues tend to exhibit lower methane yields per kilogram of waste, although they are still valuable feedstocks when combined with higher-efficiency materials in co-digestion processes [24]. Co-digestion, which involves mixing different organic waste streams, has been shown to improve the stability and efficiency of anaerobic digesters by balancing nutrient content and enhancing microbial diversity [25]. Despite this, more research is needed to optimize the combinations of waste for biogas production in specific rural settings, as regional variations in waste composition can impact the digestion process.
In Sub-Saharan Africa, where energy poverty remains a pressing issue, biogas is seen as a promising solution for improving energy access, especially in rural areas. Studies have highlighted the potential of biogas to reduce dependence on traditional biomass (e.g., firewood and charcoal) and improve indoor air quality [26]. However, despite the theoretical potential, biogas technology adoption remains low in many parts of Africa due to challenges related to technical capacity, cost, and limited knowledge about the most suitable feedstocks [27–28]. In Uganda, efforts to promote biogas production have largely focused on farms, but there is a need for more localized studies that evaluate the specific types of organic waste available and their biogas yields.
Although numerous studies have evaluated the potential of organic waste for biogas production, there is a notable gap in localized research that examines specific rural communities in Sub-Saharan Africa. Much of the existing research has focused on large-scale or urban biogas systems, neglecting the unique conditions of rural settings where waste types and quantities may differ. Additionally, there is limited comparative analysis of the full range of organic waste streams such as farm waste, household waste, and food processing waste, within the same geographical area. Most studies emphasize animal manure or agricultural waste, while the role of municipal and food processing waste in biogas production remains underexplored, particularly in rural communities like Nakisunga Parish.
This study addresses these gaps by conducting a comprehensive comparative analysis of biogas yields from various organic waste sources in Nakisunga Parish, Mukono District, Uganda. By including a broad spectrum of waste streams such as farm waste, household waste, municipal waste, and food processing waste, this research aims to provide a more nuanced understanding of which feedstocks are most effective for biogas production in this rural setting. The study not only identifies the best individual feedstocks but also offers insights into the potential for co-digestion, which can improve biogas yields through the optimal combination of waste types.
Moreover, this research contributes to the body of knowledge on biogas potential in Sub-Saharan Africa, providing localized data that can inform policy decisions and practical biogas projects. By focusing on a rural Ugandan community, the study highlights the feasibility of biogas production in areas where organic waste is abundant but underutilized. This research will serve as a guide for local governments, farmers, and energy developers seeking to harness biogas as a sustainable energy solution, bridging the gap between theory and practical implementation in rural communities.
3.1 Study Area
The study was carried out in Nakisunga Parish, Nakasunga Sub County, Mukono district, which is a neighbourhood of Kampala, the capital city of Uganda. The district is located at coordinates 0.2835° N, 32.7633° E, with a temperature range varying from 17°C to 28°C [29].
3.2 Data collection and analysis
A structured questionnaire was distributed randomly to 20 farms, 30 households, and 10 food processing businesses in Nakisunga Parish. The aim was to establish an estimate of the quantities and types of organic feedstocks and whether they are sufficient for biogas production. After distributing the questionnaires, responses were collected during in-person visits. The collected responses were organized and stored securely for data entry. Each response from the questionnaire was manually entered into the tally Excel sheets. Tally marks were used to count the number of times a particular response was given. The collected data were analyzed by representing it on a frequency table and pie chart using Microsoft Excel 2020.
3.3 Sample collection and preparation
Five feedstock samples were gathered, each weighing 500 grams as weighed using an electronic beam balance. These were pig manure, kitchen scraps, cow dung, chicken droppings and crop residues. Each sample was put in a polythene bag with a clear label and properly sealed to prevent contamination. After four days of sun drying, the samples were crushed with a crusher and pestle to a powder, the samples were then sieved and dried again for a day. The samples were taken to the laboratory to determine their pH, volatile solids, and total solids.
3.4 PH determination
The materials used were pH meter, distilled water, Gloves, Mixing container, Stirring rod and sample containers. The collected sample of the substrate was dried and ground up into fine powder materials for pH testing. The ground powder is then soaked in distilled water for 1 day. Using a pH meter, the pH of every sample substrate was determined. To make a pH measurement, the electrode was immersed into the sample solution until a steady reading was achieved. This was done for every sample and the results were tabulated.
3.5 Determination of Total Solid (TS)
Materials used were crucible, laboratory oven, Desiccator, Electronic precision balance, Dish tongs, wash bottles and muffle furnace. The amount of total solids is the amount of solid present in the sample after the loss of water molecules present in it. These were the procedures followed;
i. A crucible weighing 0.236kg was properly washed and dried in the oven at a temperature of 105°C for one hour. The crucible was stored and cooled in a desiccator until needed.
ii. The crucible was re-weighed before use (0.236kg)
iii. The laboratory oven was switched on and allowed to reach a temperature of 105°C. This temperature was maintained throughout the experiment
iv. A 0.500kg of the collected sample substrate was added to the crucible and diligently placed in the laboratory oven at a temperature of 105°C. The substrate sample was dried to a constant mass for a period of 1 to 2 hours.
v. The crucible plus substrate residue were allowed to cool in a desiccator to balance temperature. Desiccators are designed to provide an environment of standard dryness. The desiccator was properly lubricated with grease and this was to prevent moisture from entering the desiccator as the test glassware cools.
vi. The crucible plus substrate (material) residue was weighed using an electronic precision balance. Equation 1 was used to calculate the percentage of total solids.
Where TS = Percentage total solid
m1 = Weight of dried crucible + dried residue
m2 = Weight of crucible
m3 = Weight of wet sample (substrate) + crucible [30].
3.6 Determination of Volatile Solid (VS)
The residue obtained as TS was ignited at 6000c for 30 minutes in a muffle furnace.
The crucible and the black mass of carbon were collected to cool partially in the air before it was transferred to the desiccator for complete cooling. After cooling, the sample was weighed. Percentage volatile solids were calculated as shown in equation 2.
Where m4 is weight of crucible plus weight of residue after ignition [30].
3.7 Biochemical methane potential
To calculate the methane potential of substrates, also known as the biochemical methane potential (BMP), an empirical model called the Buswell model equation was used [31].
It therefore calculated the methane potential based on the volatile solids content of the substrate already determined. The Buswell equation is as shown in equation 3
Where:
BMP is the biochemical methane potential (expressed in mL CH4/g VS),
VS is the volatile solids content of the substrate (expressed in g/L),
e is the base of the natural logarithm (approximately 2.71828).
This equation provides an estimate of the methane potential based on the volatile solids content of the substrate. It assumes that the methane yield is a function of the volatile solids content and that the methane production follows first-order kinetics.
Table 1 presents an estimate of organic wastes per source. Table 2 presents an estimate of total organic wastes per source.
Table 1: An estimate of available categories of organic wastes
|
Source |
Types of wastes |
Quantity(kg/day) |
|
Farm |
Swine manure Poultry droppings Cattle manure Crop residues |
20 30 50 100 |
|
Households |
Food scraps Vegetable peelings Fruit wastes |
20 15 10 |
|
Food processing units |
Fruit peels Vegetable peelings Spoiled food products |
15 10 35 |
|
Local council |
Sewage wastes Paper waste Brewery wastes |
15 15 30 |
Table 2: An estimate of total organic wastes per source
|
Source |
Quantity (kg/day) |
|
|
Farm |
200 |
|
|
food processing |
60 |
|
|
households |
45 |
|
|
Municipal wastes |
60 |
Figure 1 is the pie chart showing sources of wastes in percentage. The greatest contributors are the farms which produce 54.79% of the total wastes. Municipal and food processing wastes together account for 16% and 17% of the total waste. The least amount of waste produced accounting for 12.33% of the total is the households.
Table 3 presents the sample substrates with their BMP, which is further illustrated in Figure 2 using bar graph.
Table 3: Sample substrates with their BMP
|
Sample |
VS |
BMP |
|
Cow dung |
0.6476 |
0.005 |
|
Poultry drops |
0.4103 |
0.002 |
|
Swine manure |
0.8538 |
0.007 |
|
Food wastes |
0.2369 |
0.001 |
|
Crop residues |
0.6592 |
0.005 |
Swine manure has the highest BMP value of 0.007 mL CH4/g VS indicating that they have the greatest potential for methane production among the samples listed. Both cow dung and crop residues have a BMP value of 0.005mL CH4/g VS, indicating a moderate potential for methane production. Poultry droppings have a BMP value of 0.002, suggesting a lower potential for methane production compared to cow dung, crop residues, and swine manure. Food wastes have the lowest BMP value of 0.001, indicating the least potential for methane production among the samples.
The study successfully identified and evaluated the biogas production potential of various organic waste sources in Nakisunga Parish, Mukono District. The research findings highlight the significant contributions of farms, households, municipal, and food processing units to the overall organic waste output, with farms being the largest contributor. The analysis of the Biochemical Methane Potential (BMP) of different substrates further underlines the importance of selecting appropriate feedstock for maximizing biogas yield.
According to the data, farms account for 54.79% of the total garbage produced in Nakisunga Parish, making them the primary source of organic waste. This is in line with Atelge et al. [23] research which found that raising livestock is one of the main sources of organic waste that may be used to produce biogas. Similarly, Tjutju et al. [5] noted that because there is a lot of animal waste accessible, areas with extensive farming activities frequently have better potential for producing biogas. The high contribution from farms in Nakisunga Parish is indicative of the strong potential for sustainable biogas production, particularly from swine manure, cow dung, and crop residues.
Food scraps and vegetable peelings were among the household wastes, which made up 12.33% of the garbage generated. Even though this is a smaller percentage, feedstock from this source is nevertheless quite important. Research has demonstrated that, even in smaller amounts, domestic organic waste can serve as a useful feedstock for the production of biogas, especially when combined with other high BMP substrates [7–8].
Municipal and food processing wastes contributed 16% and 17%, respectively, to the total organic waste. These findings align with research by Ferdeș et al. [9] which demonstrated that municipal solid waste and food industry by-products are valuable resources for biogas production, especially when adequately sorted and pre-treated. The diversity in waste types from these sources suggests that a combination of substrates might be beneficial for optimizing biogas yields. In addition, the significant quantity of organic waste available indicates a strong potential for sustainable biogas production. Similar studies have emphasized the importance of identifying and quantifying feedstock for efficient biogas production [5, 6, 10]. Our findings align with these studies, confirming the feasibility of biogas production in Nakisunga parish.
The BMP analysis revealed that swine manure had the highest methane potential (0.007 mL CH₄/g VS), followed by cow dung and crop residues (0.005 mL CH₄/g VS each). This finding is consistent with previous studies, such as those by Somer et al. [11], which highlighted that swine manure typically exhibits higher BMP values compared to other agricultural wastes due to its balanced nutrient composition and higher biodegradability. The relatively high BMP of cow dung and crop residues also aligns with the results of Somer et al. [11], who found that these substrates are particularly effective when used in combination with other organic wastes.
Poultry droppings and food wastes, with BMP values of 0.002 mL CH₄/g VS and 0.001 mL CH₄/g VS, respectively, were found to have lower methane production potential. This is in line with the findings of (Tawfik et al.), who reported that poultry manure, despite being nutrient-rich, often produces lower methane yields due to its high nitrogen content, which can inhibit the anaerobic digestion process. Food wastes, while abundant, typically have lower BMP values, as noted by Yang et al. [20], who emphasized that the composition of food waste can vary greatly, affecting its biogas production efficiency.
One of the primary contributions of this study is its detailed examination of the specific waste streams in a rural Ugandan context, which has been underrepresented in biogas research. While previous studies have largely focused on general agricultural or urban waste in broader regions, this research provides granular data on the types and quantities of organic waste available in Nakisunga Parish. By quantifying the waste output from farms, households, municipal sources, and food processing units, this study offers a localized perspective on biogas feedstock availability, which is crucial for designing context-specific biogas systems.
Furthermore, this study's use of the Biochemical Methane Potential (BMP) model in a rural African setting is significant. While the BMP model has been widely used in other regions, its application in this context adds valuable data to the limited pool of research focused on biogas potential in Sub-Saharan Africa. The finding that swine manure has the highest BMP value among the substrates tested contributes new insights into the potential of livestock waste in rural Ugandan communities, where pig farming is prevalent.
The findings of this study have significant implications for biogas production in rural communities. The identification of farms as the largest contributors of organic waste suggests that agricultural areas could be key sites for biogas plants, potentially transforming waste management practices and enhancing energy security in these regions. The high BMP value of swine manure, in particular, indicates that targeted biogas production initiatives could be especially effective in communities where pig farming is common.
Moreover, the study highlights the importance of integrating multiple waste streams to optimize biogas yields. The inclusion of municipal and food processing wastes in the analysis suggests that a more holistic approach to waste management, which leverages a variety of organic materials, could enhance the efficiency and sustainability of biogas production systems. This has practical implications for policymakers and practitioners, as it suggests that biogas initiatives should not only focus on traditional agricultural wastes but also consider other readily available organic materials.
Beyond the local context, the broader implications of this research extend to the global challenge of sustainable waste management and renewable energy production. The study contributes to the growing body of evidence that biogas production can play a critical role in addressing energy needs in rural areas while simultaneously managing organic waste. This aligns with global sustainability goals, particularly those related to clean energy (SDG 7) and sustainable cities and communities (SDG 11). In addition, the research provides a model for other rural communities in developing countries, offering a framework for assessing local biogas potential that can be adapted and replicated elsewhere. By demonstrating the feasibility of biogas production in Nakisunga Parish, the study encourages the adoption of similar projects in other rural regions, potentially leading to broader environmental and economic benefits.
Sample Size and Geographic Scope
The study was limited to a specific rural community in Mukono District, with data collected from a relatively small number of farms, households, and food processing units. While the findings are significant for the area studied, they may not be fully representative of other regions with different socio-economic conditions, waste management practices, or agricultural profiles.
Temporal Variability
The study's data collection was conducted over a limited period, which may not capture the seasonal variations in organic waste generation. Agricultural waste, in particular, can fluctuate significantly depending on the time of year, affecting the availability and composition of feedstock for biogas production.
Simplified Biochemical Methane Potential (BMP) Model
The study used a standard BMP model to estimate the methane production potential of different substrates. While this model provides a useful estimate, it does not account for all the variables that can influence biogas production in real-world conditions, such as the presence of inhibitors, the efficiency of the digestion process, or the impact of co-digestion of multiple substrates.
Laboratory Conditions vs. Real-World Applications
The study's laboratory-based measurements of total solids, volatile solids, and BMP were conducted under controlled conditions that may differ from those in actual biogas digesters. Factors such as temperature fluctuations, microbial diversity, and digester design in real-world applications could lead to different outcomes in biogas yields.
Focus on Organic Waste Only
This study focused exclusively on organic waste streams, excluding other potential biogas feedstocks such as sewage sludge or industrial waste. While this approach was appropriate for the study's objectives, it may have limited the exploration of other viable substrates that could contribute to biogas production.
Limited Economic and Technical Analysis
Although the study identified potential feedstocks and estimated their methane production potential, it did not include a detailed economic analysis of biogas production. Factors such as the cost of setting up and maintaining biogas digesters, the availability of technology, and the financial viability of biogas production in rural communities were not explored in depth.
Community Acceptance and Social Factors
The study did not investigate the social factors or community acceptance related to the adoption of biogas technology. The success of biogas production initiatives often depends on local attitudes, awareness, and willingness to engage in new waste management practices, which were beyond the scope of this research.
This study has provided a detailed evaluation of the biogas production potential from various organic waste sources in Nakisunga Parish, Mukono District. The findings confirm that the area has a substantial quantity of organic waste, particularly from farms, which constitute the largest source of feedstock for biogas production. The study’s application of the Biochemical Methane Potential (BMP) model demonstrated that swine manure offers the highest potential for methane production, followed by cow dung and crop residues, highlighting these substrates as key candidates for optimizing biogas yields.
The research makes several important contributions to the field, particularly by offering localized data from a rural Ugandan context, where such studies are scarce. It underscores the viability of biogas production in rural communities, not only as a waste management solution but also as a means to enhance energy security. The study also illustrates the importance of considering a variety of waste streams, including those from food processing and municipal sources, to maximize biogas production potential.
However, the study also recognizes its limitations, including its geographic scope, the reliance on laboratory conditions, and the lack of a detailed economic analysis. These limitations suggest avenues for future research, particularly in expanding the study to other regions, conducting long-term assessments, and exploring the socio-economic aspects of biogas production.
In conclusion, this research provides a strong foundation for the development of biogas systems in Nakisunga Parish and similar rural communities. It demonstrates that with the right approach to waste management and the selection of optimal feedstocks, biogas production can be a viable and sustainable energy solution, contributing to both environmental sustainability and rural development.
Given the high Biochemical Methane Potential (BMP) value of swine manure, local authorities and development agencies should encourage pig farmers to utilize this waste as a primary feedstock for biogas production. Training programs on effective manure collection and biogas digester management could further enhance methane yields.
Data Availability
The datasets generated during and/or analysed during the current study are available from the corresponding author upon reasonable request.
Competing interests
The authors declare there is no conflict of interest.
Funding
None.
Authors' contributions
Conceptualization, M.W.; methodology, M.W; investigation, M.W.; supervision, A.U.; writing—original draft preparation, M.W., A.U.; writing—review and editing, M.W., A.U.; visualization, M.W., A.U.
Acknowledgements
None.
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No competing interests reported.