Anaerobic digestion of gliricidia sepium co-digested with pig manure using automated and portable digester – Nature

Report on Biogas Generation from Organic Waste Co-Digestion

Introduction: Addressing Sustainable Development Goals through Renewable Energy

The global pursuit of the Sustainable Development Goals (SDGs) necessitates innovative solutions for clean energy, waste management, and climate action. The over-reliance on fossil fuels has precipitated significant environmental degradation, directly challenging SDG 13 (Climate Action) and SDG 3 (Good Health and Well-being). In developing nations like Nigeria, energy poverty further impedes economic growth, highlighting a critical need to advance SDG 7 (Affordable and Clean Energy). This study investigates the potential of decentralized anaerobic digestion as a sustainable approach to simultaneously manage organic waste and produce renewable energy. By converting agricultural residues (Gliricidia sepium) and animal waste (pig manure) into biogas, this research provides a viable pathway towards achieving a circular economy, in line with SDG 12 (Responsible Consumption and Production) and fostering SDG 11 (Sustainable Cities and Communities). The primary objective was to evaluate the efficacy of co-digesting pig manure with Gliricidia sepium in both automated and locally fabricated digesters, assessing its potential for small-scale, household-level energy generation.

Methodology

Materials and Substrate Collection

• Substrates: Gliricidia sepium was sourced from the Omu-Aran community, and fresh pig manure was collected from Landmark University Farms in Kwara State, Nigeria.
• Storage: Samples were refrigerated at 4°C to prevent premature degradation before analysis and use.

Substrate Pre-treatment Process

To enhance the biodegradability of the lignocellulosic biomass and improve biogas yield, a thermo-alkaline pre-treatment was employed. This process is crucial for breaking down complex structures, making the organic matter more accessible for microbial action.

1. Mechanical Treatment: The Gliricidia sepium biomass was sun-dried, ground into fine particles, and sieved.
2. Thermal Treatment: The ground biomass underwent a 70-minute thermal treatment at 80°C in a water bath.
3. Alkaline Treatment: Following heating, the biomass was treated with 3g of sodium hydroxide (NaOH) per 100g of substrate at 55°C for 24 hours to further break down lignin and cellulose structures.

Experimental Design and Digester Setup

• Digester Types: The experiment utilized two types of digesters to compare efficiency and practicality: a controlled, automated batch digester and a 25-litre portable, fabricated digester designed for household-level application.
• Process Parameters: The anaerobic co-digestion was conducted for a hydraulic retention time (HRT) of 30 days.
• Mixing Ratio: A substrate-to-inoculum (pig dung) mixing ratio of 1:1 was used for all experiments.
• Monitoring: Daily measurements of biogas production, pH, and temperature were recorded. The final gas composition (CH₄, CO₂, H₂S) was analyzed using gas chromatography.

Key Findings and Discussion

Impact of Pre-treatment on Substrate Composition

The thermo-alkaline pre-treatment significantly altered the chemical structure of the Gliricidia sepium, making it more suitable for anaerobic digestion and contributing to higher energy output (SDG 7).

• Cellulose concentration was reduced from 9% to 3% post-treatment.
• Lignin content decreased from 6% to 4%, indicating successful delignification.
• An increase was observed in total solids, volatile solids, and key mineral elements (Iron, Zinc, Potassium, etc.), enhancing the nutrient profile for microbial activity.
• The final C/N ratio of the co-digestion mixture was approximately 12, falling within the optimal range for stable anaerobic digestion.

Biogas and Methane Yield Analysis

The study demonstrated successful biogas production, with notable differences between the two digester types. These results confirm the potential for decentralized systems to contribute to local energy needs, directly supporting SDG 7.

• Total Biogas Yield (30 days):
  • Automated Digester: 0.986772 m³
  • Fabricated Digester: 0.50845 m³
• Peak Methane (CH₄) Content:
  • Automated Digester: 58.85%
  • Fabricated Digester: 58.26%

The higher yield in the automated digester is attributed to its controlled environment. However, the significant methane content (over 58%) from the low-cost fabricated digester confirms its viability for practical, small-scale household applications in developing communities.

Process Stability Parameters

The pH levels remained within the optimal range of 6.0 to 8.86 throughout the 30-day digestion period. This stability indicates a healthy and efficient microbial process, crucial for sustained biogas production without process failure due to acidification.

Conclusion and SDG Implications

Summary of Findings

This research successfully demonstrated that the co-digestion of pre-treated Gliricidia sepium and pig manure is a highly effective method for biogas production. The thermo-alkaline pre-treatment was vital in breaking down the biomass, leading to a cumulative biogas yield of 0.986772 m³ and a methane content of 58.85% in the automated digester. Crucially, the portable fabricated digester also achieved a high methane content of 58.26%, proving its suitability for decentralized, community-level energy generation.

Contribution to Sustainable Development Goals

The outcomes of this study provide a tangible framework for addressing multiple SDGs through a single, integrated process:

• SDG 7 (Affordable and Clean Energy): The research validates a low-cost, decentralized method for producing clean cooking and lighting fuel from locally available waste, reducing energy poverty at the household level.
• SDG 11 and SDG 12 (Sustainable Communities and Responsible Production): It offers a circular economy solution by transforming agricultural and animal waste—often a source of pollution—into a valuable energy resource. This promotes sustainable waste management and resource efficiency.
• SDG 13 (Climate Action): By capturing methane from decomposing manure and providing a renewable alternative to fuelwood and fossil fuels, this process directly mitigates greenhouse gas emissions and reduces deforestation.
• SDG 2 (Zero Hunger) and SDG 3 (Good Health): The nutrient-rich digestate produced as a byproduct can be used as an organic fertilizer, enhancing soil fertility and food security. Furthermore, replacing solid fuels for cooking reduces indoor air pollution, a major health risk in developing regions.

Future research should focus on optimizing the process parameters for fabricated digesters to further enhance biogas yield and promote wider adoption in rural and peri-urban communities.

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 7: Affordable and Clean Energy

   The article’s primary focus is on promoting biogas as a renewable and sustainable energy source. It directly addresses the need for clean energy alternatives to fossil fuels, which are linked to environmental degradation and health challenges. The research explores decentralized anaerobic digestion systems to provide energy at the community and household levels, which is central to SDG 7.

2. SDG 11: Sustainable Cities and Communities

   The article connects to SDG 11 by proposing an innovative approach to managing organic waste. It notes that animal waste like pig manure “frequently emits an unpleasant odor and affects the surrounding community’s environment” and that “Waste accumulation poses a number of environmental challenges.” By converting this waste into energy, the research contributes to reducing the adverse environmental impact of communities, a key aspect of sustainable urban and rural living.

3. SDG 12: Responsible Consumption and Production

   This goal is addressed through the article’s emphasis on creating a circular economy model where waste is repurposed. The study uses organic waste (pig manure) and agricultural residue (Gliricidia sepium) as feedstocks for energy production. This method represents a sustainable management and efficient use of natural resources and a direct way to reduce waste generation, aligning with the principles of responsible production and consumption.

4. SDG 13: Climate Action

   The article explicitly links the dependence on fossil fuels to “a shift in the climate” and highlights the need to minimize greenhouse gas emissions. By developing a method to produce biogas (a renewable energy source), the research offers a direct strategy to mitigate climate change. Capturing methane from decomposing manure for energy use prevents its release into the atmosphere, where it acts as a potent greenhouse gas, thereby contributing to climate action.

2. What specific targets under those SDGs can be identified based on the article’s content?

• Under SDG 7 (Affordable and Clean Energy):

  • Target 7.1: By 2030, ensure universal access to affordable, reliable and modern energy services. The article highlights the energy crisis in Nigeria, stating that “fewer than 45% of the population has access to power” and the national infrastructure serves “only 40% of the country’s population.” The study’s focus on developing “portable fabricated anaerobic digester viability as a decentralized way of generating biogas for energy use in small household level” directly aims to improve energy access for underserved populations.
  • Target 7.2: By 2030, increase substantially the share of renewable energy in the global energy mix. The entire research is an effort to promote biogas as a renewable energy source to replace conventional fossil fuels. The introduction states, “The adoption of sustainable and renewable energy sources as alternatives to conventional energy sources has increased due to depletion and environmental harm.”

• Under SDG 11 (Sustainable Cities and Communities):

  • Target 11.6: By 2030, reduce the adverse per capita environmental impact of cities, including by paying special attention to air quality and municipal and other waste management. The article addresses this by proposing a method to manage organic waste like “Pig manure, cow dung, and chicken droppings,” which it notes can negatively affect the “surrounding community’s environment.” The anaerobic digestion process is a form of controlled waste management that mitigates these impacts.

• Under SDG 12 (Responsible Consumption and Production):

  • Target 12.2: By 2030, achieve the sustainable management and efficient use of natural resources. The study demonstrates how agricultural residues (Gliricidia sepium) and animal waste (pig manure) can be efficiently used as resources to produce energy, rather than being discarded.
  • Target 12.5: By 2030, substantially reduce waste generation through prevention, reduction, recycling and reuse. The co-digestion process described in the article is a form of waste recycling and reuse, converting organic waste streams into a valuable product (biogas), thereby reducing the overall volume of waste that needs to be managed or landfilled.

• Under SDG 13 (Climate Action):

  • Target 13.2: Integrate climate change measures into national policies, strategies and planning. The research provides a technological solution that supports climate change mitigation. The introduction states, “There is a growing focus on minimizing greenhouse gas emissions by promoting the production of renewable energy.” This research provides evidence and a practical method that could be integrated into energy and environmental strategies.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article mentions and implies several quantitative indicators that can be used to measure progress:

• For SDG 7 (Affordable and Clean Energy):

  • Indicator 7.1.1 (Proportion of population with access to electricity): The article explicitly mentions the low access to electricity in Nigeria (“serves only 40% of the country’s population”), providing a baseline against which the impact of decentralized energy solutions could be measured.
  • Indicator 7.2.1 (Renewable energy share in the total final energy consumption): The study provides direct measurements of renewable energy generation. The “cumulative biogas of 0.986772 m³ and 0.50845 m³” and the “methane content of 58.85% and 58.26%” are quantitative measures of renewable energy production that contribute to this indicator.

• For SDG 11 (Sustainable Cities and Communities):

  • Indicator 11.6.1 (Proportion of municipal solid waste collected and managed in controlled facilities): The article implies this indicator by focusing on managing organic waste (pig manure) through a controlled process (anaerobic digestion). The amount of waste processed in the digesters is a measure of waste being managed in a controlled facility rather than being left to decompose in the open.

• For SDG 12 (Responsible Consumption and Production):

  • Indicator 12.5.1 (National recycling rate, tons of material recycled): The methodology of using pig manure and Gliricidia sepium for biogas production is a direct example of material recycling. The quantities of substrates used (e.g., “mixing ratio of the substrate to an inoculum of 1:1”) and the resulting biogas yield are metrics for the recycling process.

• For SDG 13 (Climate Action):

  • Implied Indicator (Greenhouse Gas Emissions Reduction): The article’s goal is to replace fossil fuels and manage waste to reduce GHG emissions. The measurement of “methane content of gas (58.26%)” is a key performance indicator. This captured methane is used as fuel, preventing its release from waste decomposition and offsetting CO2 emissions from fossil fuels. The volume of methane captured and utilized can be converted into CO2-equivalent to measure climate impact.

4. Table of Findings

Source: nature.com