From kitchen waste to green gold! Uncovering the sustainable transformation power of compost, biogas and organic fertilizers

Organic waste is no longer just garbage! This study provides an in-depth analysis of the feasibility, risks and international application cases of composting, biogas power generation and organic fertilizer manufacturing, and explores how to convert waste into energy and fertilizer to create a new future of circular economy and sustainable agriculture.

Global waste generation continues to rise, especially organic waste, which poses increasingly severe challenges to the environment and society. Traditional waste disposal methods, such as landfill, face multiple problems such as limited land resources, possible environmental pollution (leaseate and greenhouse gas emissions), and the inability to effectively utilize the potential resources in the waste. In this context, the concept of circular economy has received increasing attention. Its core concept is to minimize waste generation and regard waste as a resource that can be recycled and reused. The resource utilization of organic waste plays a key role in the circular economy, and composting, biogas power generation and organic fertilizer manufacturing are three important ways to convert organic waste into valuable products. This report aims to provide an in-depth analysis of the feasibility, potential risks and actual international application of these three resource utilization methods, with a view to providing relevant decision-makers with comprehensive information.

2.1. Feasibility analysis:

• 2.1.1. Technology maturity: Composting is a long-standing biological treatment technology that uses microorganisms widely found in nature to decompose organic matter into stable humus-like products.1. Its basic principles and operating procedures have been thoroughly studied and widely used, and it is a technically mature waste treatment method. An effective composting process relies on precise control of key environmental parameters, including temperature, pH, moisture content and carbon to nitrogen ratio (C/N)1。 * In-depth analysis: There are complex interactions between these parameters. For example, temperature directly affects the growth rate of microorganisms and enzyme activity, which in turn determines the rate of decomposition and the quality of the final compost. Proper high temperatures (usually between 55-65°C) are essential to kill most pathogens and weed seeds3. The carbon-nitrogen ratio affects the nutritional balance of microorganisms; too high a carbon content will slow down the decomposition rate, while too high a nitrogen content may cause the release of ammonia and produce odor.5。 * Idea guidance: If the compost temperature is too low (below 55°C) for a long time, pathogens may not be completely killed, posing potential risks to the environment and health6. On the other hand, if the temperature is too high (above 70°C), beneficial composting microorganisms may be inhibited or even killed7. An inappropriate carbon-to-nitrogen ratio (such as too high carbon or nitrogen) will not only affect decomposition efficiency, but may also cause bad odors and reduce the quality of the compost. Adequate ventilation, i.e. the supply of oxygen, is essential to maintain aerobic decomposition in a composting system. Aerobic decomposition is more efficient and produces less odor, which is more ideal than anaerobic decomposition that occurs under anoxic conditions.1。 * In-depth analysis: In order to ensure that there is enough oxygen inside the compost pile, different composting technologies use different methods. For example, windrow composting relies on passive aeration and regular turning of the pile, while forced-air composting and reactor composting use mechanical equipment to more precisely control the oxygen supply. The choice of technology depends on the scale of the process, the type of feedstock and the quality of compost required. The maturity and stability of the final compost product are important indicators to measure the feasibility of compost technology and product quality. Mature compost has decomposed enough that it will not negatively affect plant growth. Stable compost means that the organic matter has reached a level where it is no longer easy to decompose quickly, is easy to store and use, and will not produce adverse effects.1。 * In-depth analysis: Immature compost may contain substances harmful to plants, such as volatile organic acids and high concentrations of ammonia, which can inhibit seed germination and root development.4. Additionally, unstable compost may continue to decompose rapidly in the soil, depleting oxygen and even causing a lack of oxygen around plant roots. *Idea guidance: Methods of assessing compost maturity include measuring the carbon to nitrogen ratio (generally below 25:1 is considered mature), monitoring carbon dioxide release or oxygen consumption to assess microbial activity, and conducting germination tests to detect the presence of phytotoxic compounds4. The Solvita test provides a rapid on-site assessment of compost maturity by measuring CO2 and ammonia emissions.14。
• 2.1.2. Cost-effectiveness: Composting may be economically advantageous over traditional landfill, especially in areas where landfill charges continue to rise and transportation costs are high15. By converting organic waste into compost, the amount of waste that ends up in landfill can be significantly reduced, thus reducing disposal costs. *In-depth analysis: The economics of composting become more attractive as landfill capacity decreases and associated costs increase. Avoiding high landfill fees is one of the important economic motivations for composting. *Idea guidance: A comparative cost study conducted in Portage, Wisconsin, USA, shows that maintaining a composting facility may be more economical than directly transporting municipal solid waste to other landfills, after accounting for the potential market value of compost.16. In addition, selling the resulting compost as a soil conditioner to agriculture, horticulture and landscaping sectors can generate additional income, further increasing the economic viability of composting16。 * In-depth analysis: However, the market value of compost depends on its quality and consistency. Compost made from municipal solid waste may face market acceptance challenges due to the presence of contaminants such as heavy metals or plastic fragments, which may require additional investment for more refined processing to increase its market value16. Many cost-benefit analysis (CBA) studies evaluate the economic impact of establishing a composting facility, which typically consider capital investments in infrastructure and equipment, ongoing operating costs (including labor, energy, and maintenance expenses), and potential benefits through reduced landfill charges and the sale of compost18。 * In-depth analysis: A cost-benefit analysis of a university food waste composting program shows the program has the potential to generate profits by selling compost-grown vegetables to student restaurants and the local community, especially when educational and environmental benefits are also taken into account18. Similarly, a feasibility study on establishing a composting facility on Vashon Island, Washington, estimated annual savings on ferry and fuel costs if the island’s green waste was diverted to compost.15. By processing organic waste locally for composting, the need to transport waste long distances to landfill can be reduced, thereby saving on fuel and transportation costs, further improving the economics of composting15。
• 2.1.3. Environmental impact: One of the most significant environmental benefits of composting is the significant reduction of methane emissions from landfills. In the anaerobic environment of landfills, organic waste decomposes to produce methane, a more potent greenhouse gas than carbon dioxide3. Composting, as an aerobic process, can effectively avoid the large production of methane. *In-depth analysis: In the United States, food waste is the largest single substance in landfills and a significant source of methane emissions3. Diverting this food waste into compost provides an important opportunity to mitigate climate change. The end product of compost is a valuable soil amendment that, when applied to fields, gardens and landscapes, improves soil structure, improves water-holding capacity, increases nutrient availability, and promotes the growth of beneficial soil microorganisms, thereby promoting healthier plant growth and reducing the need for synthetic fertilizers and pesticides3。 * In-depth analysis: The addition of compost can also help prevent soil erosion and protect water quality by improving soil agglomeration and reducing runoff. Additionally, compost can even be used to improve poor soils and remediate contaminated sites3. The application of compost also sequesters carbon in the soil, helping to remove carbon dioxide from the atmosphere and slow climate change.3. Life Cycle Assessment (LCA) studies provide a comprehensive assessment of the overall environmental impact of composting, considering all stages from waste collection to compost application21。 * In-depth analysis: While composting is generally considered an environmentally friendly practice, life cycle assessment studies also point to some potential environmental burdens, such as the greenhouse gases (methane and nitrous oxide) produced during the composting process as well as the impact of the energy consumption of composting facilities and the transportation of waste and compost21. Different composting methods (e.g. trough composting vs. windrow composting) may also have different impacts on the overall environmental footprint21。

2.2. Potential risks:

• 2.2.1. Pathogen transmission: Organic waste, especially waste of animal origin (such as feces) or waste containing food waste, may carry a variety of pathogenic microorganisms, including bacteria (such as Salmonella, E. coli O157:H7, Listeria), viruses, protozoa and parasites1. If the composting process fails to achieve and maintain high enough temperatures (typically in the high temperature range of 55-65°C or 130-150°F) for a sufficient period of time, these pathogens may survive and pose a risk to human and animal health through direct contact, inhalation of aerosols, or contamination of crops grown in compost-amended soil. *In-depth analysis: Home or backyard composting systems often fail to consistently reach or maintain these high temperatures, increasing the likelihood of pathogen survival.6. Commercial composting facilities often use high-temperature composting to reduce this risk3。 * Idea guidance: Effective pathogen inactivation requires maintaining the entire compost pile within a specified high temperature range for a specific period of time (e.g., windrow composting at 55°C for 15 days with regular turning of the pile, or specific time-temperature requirements for other methods)4. The subsequent ripening phase lasts for weeks or months, helping to further kill any remaining pathogens6. Studies have shown that certain pathogens, such as Salmonella typhimurium, can survive in mature compost for considerable periods of time if initial compost conditions are not good26. There is also evidence that pathogens can be transferred to fresh produce from soil amended with compost that has not been properly treated26. To reduce the risk of pathogen transmission, guidelines for safe composting emphasize avoiding composting high-risk materials such as raw meat, poultry, pet waste and food waste from sick patients in home systems.6. The guidance also recommends managing compost piles to achieve and maintain high temperatures, practicing good personal hygiene after handling compost, and giving compost adequate time to mature and stabilize before applying it, especially to food crops.3. For manure compost for use in vegetable gardens, it is generally recommended to wait at least 120 days between application and harvest6。
• 2.2.2. Odor problem: The biological decomposition of organic matter during the composting process produces a variety of volatile organic compounds (VOCs), some of which may have unpleasant odors. These odors are often associated with anaerobic conditions inside the compost pile, which can be caused by insufficient ventilation, excessive moisture levels, or an imbalance in the carbon to nitrogen ratio of the feed mixture. Certain types of organic waste, such as animal manure, fish processing waste and food waste that is not properly mixed, are more likely to produce strong odors if not managed properly1。 * In-depth analysis: The specific type of odor produced can provide clues as to potential problems. For example, the smell of ammonia often indicates excess nitrogen or the presence of anaerobic zones within the compost pile9, whereas a rotten or sulfurous smell may be due to deep anaerobic decomposition due to lack of oxygen28. Effective odor control in composting operations relies on maintaining optimal process conditions that promote aerobic decomposition. Key strategies include ensuring adequate ventilation through regular pile turning or forced air systems, carefully balancing the proportions of carbon-rich (“brown”) and nitrogen-rich (“green”) materials in the feed mix, maintaining appropriate moisture levels (usually 45-60%), and mixing the materials thoroughly8. Avoiding materials that are not suitable for composting, such as large amounts of meat, dairy and oily waste, can also help reduce odor problems28。 * In-depth analysis: Commercial composting facilities often employ additional odor mitigation technologies, such as biofilters to scrub odorous air emissions, or closed composting systems, such as reactor composting, to control odors8. Maintaining the proper carbon to nitrogen ratio (25:1 to 40:1) in the feedstock mix is ​​also critical to minimizing odor development30。
• 2.2.3. Heavy metal pollution: Various organic waste streams used for composting may contain heavy metals, including lead, cadmium, arsenic, mercury, chromium, copper, nickel, and zinc. These contaminants can come from a variety of sources such as municipal solid waste (especially from batteries and electronics), industrial waste (e.g. metal plating sludge, coal ash) and agricultural inputs (e.g. animal feed containing metal additives, phosphate rock containing arsenic or cadmium)1. Unlike organic pollutants, heavy metals are elements and cannot be broken down during the composting process. They accumulate in compost and, if applied to soil, may be absorbed by plants and enter the food chain or contaminate soil and water resources. *In-depth analysis: Certain heavy metals, such as copper and zinc, are essential micronutrients for plants in small amounts but can be toxic in high concentrations32. Other heavy metals, such as lead and arsenic, are known human carcinogens and pose serious health risks even at low concentrations32。 * Idea guidance: The best way to minimize heavy metal contamination in compost is to control it at the source, carefully select the type of organic waste used as feedstock, and avoid using materials known to be contaminated1. Implementing an effective municipal waste separation and recycling program can help remove heavy metals before they enter the compost stream1. Using only mature and stable compost also helps immobilize heavy metals, reducing their bioavailability32. Many countries have established regulations or guidelines regarding heavy metal concentrations in commercially produced compost to protect the environment and human health.1。

2.3. Practical application cases of composting technology in the world:

• 2.3.1. Large-scale and commercial operation cases: San Francisco’s mandatory composting program, launched in 2009, has successfully diverted large amounts of organic waste (more than 600 tons per day) from landfills by providing residents with free compostable trash bags and convenient collection infrastructure37. The success of the scheme is underlined by high levels of resident satisfaction and increased participation in recycling. Multinational pharmaceutical and biopharmaceutical giant AstraZeneca has implemented an on-site food waste composting solution at its large manufacturing site in the UK. By investing in an A900 rocket composter, the company now processes 24 tonnes of food and green waste per year, creating a nutrient-rich resource for its campus. The program is in line with AstraZeneca’s ambitious sustainability goals and demonstrates what is possible for large businesses to adopt circular economy principles for waste management38. The Recycle Organics program documents several successful composting cases in Small Island Developing States (SIDS), including Pérez Zeledón, Costa Rica, which operates a large composting facility, and Desamparados, also in Costa Rica, which promotes home composting. These case studies illustrate the adaptability and scalability of composting solutions in diverse geographical and economic contexts39. Leading engineering firm Compost Systems has been involved in the design and construction of numerous large-scale municipal and commercial composting facilities across North America and New Zealand, processing a variety of feedstocks including biosolids and food waste using a variety of technologies, such as forced-air static piles and forced-air tumbling.40. As Zero Waste highlights, industrial composting facilities use a variety of methods, including windrow composting (suitable for agricultural and municipal waste), forced-air static composting (efficient and relatively low-cost), and reactor composting (the fastest and most tightly controlled method), to process large volumes of organic waste from a variety of sources41. The Schweizerhof agricultural-municipal composting facility in Switzerland is not only a site for the production of high-quality compost but also a center for compost scientific research and training for composters from around the world44。
• 2.3.2. Small-scale and community-based operation cases: Hostel International in New York City successfully implemented an on-site composting system using an EcoRich Elite II composter to process its organic waste, demonstrating the feasibility of decentralized composting solutions in space-constrained urban environments.45. In Alappuzha, India, a citywide program involving subsidized distribution of small biogas digesters and compost bins to city residents has resulted in a highly successful decentralized organic waste management system and led to the closure of the city’s main landfill46. Many community-scale composting schemes exist around the world, often initiated by local residents, and focus on diverting food and yard waste from landfills while creating valuable compost for community gardens and local use47。
• 2.3.3. Success factors and challenges: Analysis of various composting programs reveals several key factors that contribute to their success, including strong community engagement and collaboration, comprehensive education and training for participants, ongoing and regular maintenance of composting systems, and the establishment of clear and easy-to-use pathways to final compost utilization48. Common challenges faced by large-scale composting programs include maintaining consistent feedstock quality and minimizing contamination from non-composted materials, effectively managing odors to avoid neighbor complaints, controlling temperature and humidity inside large compost piles to optimize decomposition, preventing and controlling pathogen and pest problems, and ensuring compliance with increasingly stringent environmental regulations5. The economic feasibility of large-scale composting, particularly the costs associated with disposal, and the need to develop a stable compost market also pose an important barrier10. An important prerequisite for launching a successful composting program, regardless of size, is the conduct of a detailed feasibility study that not only assesses the technical and financial aspects of the composting unit but also thoroughly analyzes the existing and potential markets for the compost product54。
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3.1. Feasibility analysis:

• 3.1.1. Energy conversion efficiency: Biogas is a renewable energy source produced through the anaerobic digestion (AD) process of organic matter. Its main components are methane (CH4, 40-75%) and carbon dioxide (CO2, 25-60%), with trace amounts of other gases such as hydrogen sulfide (H2S) and ammonia (NH3)55. The energy content of biogas depends directly on the concentration of methane. The higher the methane content, the higher the calorific value.57. The efficiency of converting biogas into electricity depends on the technology used. Internal combustion engines are commonly used in combined heat and power (CHP) systems, and their power generation efficiencies range from 8% to 43%, with some high-efficiency models having power generation efficiencies as high as 43%, depending on the model and power output60. Gas turbines can provide higher power generation efficiencies in larger-scale applications, while fuel cells (such as solid oxide fuel cells, SOFCs) have demonstrated higher power generation efficiencies (around 53% in one case study)62. By simultaneously generating electricity and capturing and utilizing the waste heat generated in the process, combined heat and power (CHP) systems can achieve significantly higher overall energy conversion efficiencies (18% to 90%)60. This combined heat and power approach maximizes the use of energy produced by biogas, making it a more efficient use of resources. The efficiency of the anaerobic digestion process itself, which converts organic biomass into methane, is also a key factor. Research shows that some biogas plants have a biomass conversion efficiency of about 60%, which represents the ratio of the chemical energy in the produced methane to the primary energy in the input matrix55. Factors affecting the overall energy conversion efficiency of biogas power generation include the type and characteristics of the feedstock, the design and operating parameters of the anaerobic digester, the power generation technology used (engine, turbine, fuel cell) and the extent to which the heat generated is recovered and utilized60. Innovative technologies aimed at improving biomass conversion rates during anaerobic digestion can also play a role in improving overall efficiency63。
• 3.1.2. Source of raw materials: A wide variety of organic feedstocks can be used for anaerobic digestion, including livestock manure (cattle, pigs, poultry), agricultural residues (straw, stalks), organic fractions of municipal solid waste (food waste, green waste), wastewater sludge from municipal and industrial treatment plants, food processing wastes (from citrus, dairy, vegetable processing, breweries, sugar mills) and specially grown energy crops (sugar cane, sorghum, elephant grass)56. The availability of specific types of feedstock varies by geographic region and major industry or agricultural practice. For example, energy crops have played an important role in the development of the biogas industry in Germany, while the United States relies primarily on landfill gas. In China, domestic biogas digesters often utilize locally available agricultural and human and animal waste67. Co-digestion, mixing different types of organic waste in the same anaerobic digester, can often increase biogas production by optimizing the nutrient balance of the microbial community and improving the overall digestion process56. Proper characterization of potential feedstocks, including parameters such as moisture content, volatile solids concentration and chemical oxygen demand (COD), is crucial to accurately estimate biogas production potential and design efficient digestion systems68. In some regions, especially those where the biogas industry is rapidly expanding, competition for available biomass resources between biogas plants and other industries is increasing, which may lead to feedstock shortages and price fluctuations71。
• 3.1.3. Economic benefit analysis: Biogas power projects can generate revenue in a variety of ways, including selling the generated electricity to the grid or for on-site use, utilizing recovered heat in combined heat and power (CHP) applications, upgrading biogas to biomethane for use as renewable vehicle fuel or injection into natural gas pipelines, and selling nutrient-rich digestion residues as organic fertilizer or soil amendments65. Some biogas plants also charge treatment fees by receiving organic waste as feedstock from different sources such as food waste from commercial and industrial entities75. The economic viability of a biogas power project is typically assessed through a techno-economic feasibility analysis, which involves a detailed assessment of capital investment costs (anaerobic digesters, power generation equipment, biogas upgrade facilities), ongoing operating and maintenance expenses, feedstock costs, and projected energy and by-product sales revenue65. For biogas to become a competitive energy source, its production costs need to be comparable to or lower than traditional gaseous fuels such as natural gas77. Government policies and financial incentives, such as tax credits for renewable energy production, Renewable Energy Certificates (RECs), wholesale rates for biogas generation, and support for the Renewable Fuel Standard (RFS) and Low Carbon Fuel Standard (LCFS), play a crucial role in making biogas projects more economically attractive67. High initial capital investment costs are often a significant barrier to the development and deployment of commercial-scale biogas plants, especially in developing countries and smaller-scale projects79. Innovative financing mechanisms, supportive regulatory frameworks, and policies to reduce perceived investment risks are critical to overcoming this challenge and promoting wider adoption of biogas technologies.

3.2. Potential risks:

• 3.2.1. Methane leakage: Methane (CH4), the main component of biogas, is a powerful greenhouse gas with a global warming potential much higher than that of carbon dioxide (CO2).56. Accidental leaks of methane emissions throughout the biogas supply chain, from production to utilization, could seriously undermine the environmental benefits of biogas as a renewable energy source and exacerbate climate change81。 * In-depth analysis: Recent research suggests that actual methane leakage rates from biogas and biomethane supply chains may be much higher (two to four times) than previously estimated by organizations such as the International Energy Agency (IEA). These leaks are often concentrated in a relatively small number of facilities and equipment, called “super emitters,” although methane releases can occur at every stage of the process.82. Common sources of methane leaks from biogas plants include open storage tanks for digestion residues, incomplete combustion in gas engines used to generate electricity, pressure relief valves, biogas upgrade units, ventilation systems for biogas equipment in buildings, and leaks from pipes, storage tanks, and other biogas infrastructure components81. Minimizing methane losses is critical to ensuring biogas truly becomes a climate-friendly alternative to fossil fuels. Since methane has a high global warming potential, even a relatively small amount of methane leakage can have a significant negative impact on the overall carbon footprint of biogas production and utilization81。
• 3.2.2. Waste treatment challenges (digestion residues): Digestion residue is the material left after the anaerobic digestion process. It is rich in nitrogen, phosphorus, potassium and other valuable nutrients needed by plants, as well as organic matter, so it is suitable for use as organic fertilizer or soil amendment.56. However, if not managed and applied properly, high concentrations of nutrients, especially nitrogen and phosphorus, in digestion residues can lead to environmental problems such as water contamination, eutrophication, and harmful algal blooms84。 * In-depth analysis: The distribution of nitrogen and phosphorus in digestion residues can be uneven, with nitrogen generally being more concentrated in the liquid fraction and phosphorus being more present in the solid fraction. This uneven distribution complicates recycling of these nutrients back into agriculture in a balanced way and can lead to environmental problems84. Many regions have strict regulations and discharge limits on the amount of nitrogen and phosphorus that can be applied to farmland or discharged to water bodies. Biogas plant operators must comply with these regulations when managing and utilizing digestion residues84. Depending on the feedstock used in the anaerobic digestion process, the digestion residue may also contain residual organic matter, pathogens, heavy metals and other potentially harmful substances. Therefore, proper handling and disposal is required to ensure safe and sustainable use of digestion residues72。
• 3.2.3. Security issues: The main component of biogas is methane, which is highly flammable and explosive. Leakage in biogas production, storage, transportation or utilization systems can create serious fire and explosion hazards and therefore requires strict compliance with safety regulations and the use of appropriate safety equipment88. In addition to methane, biogas also contains other gaseous components that may be harmful to human health. Hydrogen sulfide (H2S) is a highly toxic gas that can cause serious injury or death even at relatively low concentrations. Ammonia (NH3) is another irritating and toxic gas in biogas57. Exposure to these gases requires appropriate ventilation, gas detection systems and personal protective equipment (PPE). In enclosed or poorly ventilated spaces, biogas (especially methane and carbon dioxide) can displace oxygen, causing suffocation. This is an important risk in areas where digestion residues are stored or processed and in confined spaces within biogas plants88. Routine maintenance and repair work on biogas plant equipment can expose workers to a variety of safety hazards, including exposure to hazardous gases, biohazards from handling organic materials and digestion residues, and physical risks associated with machinery and confined spaces88. In biogas plants that include digestion residue drying operations, accumulation of dust may create a risk of dust explosions, similar to other industries handling fine organic powders88。

3.3. Biogas risk prevention and response strategies:

• 3.3.1. Methane leakage prevention: Implement a comprehensive and ongoing monitoring program using advanced technologies such as infrared cameras, drone-borne sensors and fixed gas detectors to regularly scan for methane leaks in biogas facilities for early detection and timely repairs81. Ensure biogas plants are designed, built and operated to strict standards, adhering to industry best practices and engineering codes to minimize the potential for leaks in digesters, piping, storage tanks and upgraded equipment82. Adopt and implement biogas upgrading technology that not only purifies biogas to biomethane standards but also includes measures to prevent methane slippage or loss during the upgrading process57。
• 3.3.2. Digestion residue management strategy: Use solid-liquid separation technology, such as screw presses or centrifuges, to separate digestion residues into solid and liquid fractions for more targeted management and application of nutrients70. Utilize post-treatment processes, such as composting, to further stabilize digestion residues, reduce their volume and odor, and increase their value as organic fertilizers70. Thermal drying can also be used to reduce moisture content and improve handling and storage85. Implement nutrient recovery technologies such as membrane filtration (reverse osmosis, ultrafiltration), ammonia stripping and struvite precipitation to extract valuable nutrients (such as nitrogen and phosphorus) from digestion residues, produce concentrated fertilizer products and reduce the nutrient load in the remaining wastewater84. Develop and promote markets for products derived from digestion residues, including high-quality organic fertilizers, soil amendments and animal bedding, to create economic value and encourage the sustainable use of this by-product70。
• 3.3.3. Biogas plant safety measures: Develop and strictly enforce comprehensive safety procedures to address all potential hazards associated with biogas production, including the handling of flammable and toxic gases, biohazards from organic materials, risks of fire and explosion, and procedures for routine maintenance and emergency response88. Use corrosion-resistant materials in the construction of biogas plant components, especially those in contact with biogas and digestion residues, to prevent degradation and potential leaks due to corrosion by H2S and other compounds89. Install explosion protection devices in digesters and biogas storage tanks to reduce the risk of explosions89. Implement a continuous gas detection system to monitor concentrations of methane, H2S and other hazardous gases throughout the facility and trigger alarms when preset thresholds are exceeded to warn personnel of potential hazards89. Equip all biogas treatment systems with appropriate safety valves to protect against overpressure or vacuum conditions89. Install and maintain biogas torches to safely burn excess biogas during startup, shutdown or emergency situations89. Ensure adequate ventilation of all enclosed areas of the biogas plant, including digester buildings, upgrade facilities and digestion residue storage areas, to prevent the accumulation of harmful gases and maintain a safe working environment89. Implement strict confined space entry and hot work permitting systems to control the risks associated with these tasks88. Provide all plant operators and maintenance personnel with comprehensive and regular training on the characteristics and hazards of biogas, safe operating practices, emergency response procedures, and the proper use and maintenance of personal protective equipment (PPE) such as respirators, gas detectors, and protective clothing.89。

3.4. Practical application cases of biogas power generation in the world:

• • 3.4.1. Europe: Germany, Italy and France have mature agricultural biogas industries that mainly use energy crops, plant by-products and animal manure to generate electricity and heat through combined heat and power (CHP) systems67. Countries such as Sweden, Switzerland and Finland rely more heavily on municipal waste streams for biogas production, including food waste and wastewater sludge.69. Greece has landfill gas power plant that captures methane produced by decomposing waste to generate renewable electricity99. Romania is home to one of the largest biogas plants in the region, the Moara project, which uses renewable energy to produce biogas for electricity and heating99. Denmark and the Netherlands are leading the way in biogas upgrading technology, producing high-quality biomethane for injection into the gas grid and as a sustainable transport fuel98。
  • 3.4.2. Asia: There are a large number of domestic biogas digesters in rural areas of China, which mainly use agricultural residues, livestock manure and human and animal excrement to produce biogas for cooking and lighting.67. India is increasingly focusing on developing large-scale commercial biogas plants that utilize a variety of organic feedstocks, including industrial wastewater and agricultural waste, for power generation and other applications67. A case study in Vietnam demonstrates an innovative approach to using biogas generated from shrimp farm waste to power a solid oxide fuel cell (SOFC) system, providing an efficient and sustainable source of electricity for shrimp farming.62。
  • 3.4.3. North America: The United States has a well-developed landfill gas to electricity (LFGTE) industry, with many projects capturing methane emissions from landfills to generate electricity67. There is also a growing number of biogas projects producing renewable energy from agricultural waste, such as manure from dairy and pig farms, and food waste from commercial and industrial sources.100. Canada also has examples of successful integration of biogas production into growing biorefinery complexes100。
  • 3.4.4. Africa: Kenya has been actively promoting the adoption of household biogas digesters by farmers, using agricultural waste and animal manure to provide clean cooking fuel and improve sanitation conditions108. In Cameroon, a community-led biogas system using pig manure provides a village with sustainable energy for cooking, heating and electricity generation110. South Africa has several biogas projects utilizing a variety of feedstocks, including waste from the juice industry, dairy and pig farm operations, and organic waste from households and commercial entities109. In Morocco, some wastewater treatment plants are equipped with anaerobic digesters to capture the biogas produced by human and animal waste and use it to generate electricity109。

4.1. Feasibility analysis:

• 4.1.1. Production process: The manufacture of organic fertilizers typically involves a series of interrelated steps, starting with the pretreatment of the organic feedstock to reduce its size and homogenize it. This pretreatment may include size reduction by crushing, grinding or chopping, as well as adjusting moisture content and ensuring a suitable carbon to nitrogen ratio to optimize the subsequent decomposition process112. The heart of organic fertilizer production is usually the composting or fermentation stage, where microorganisms naturally break down organic matter into a more stable and nutrient-rich form. Use a variety of composting methods, including windrow composting (suitable for large quantities of a variety of feedstocks), forced-air static composting (providing better aeration and temperature control), reactor composting (providing controlled and often faster processing), and vermicomposting (using earthworms to break down, resulting in high-quality vermicomposting)41. The specific method chosen depends on the size of the operation, the type of feedstock available, and the desired qualities of the final product. After composting, the material may require further processing to enhance its properties and marketability. This may include grinding to create a finer texture, sifting to remove any remaining large particles or contaminants, and blending with other organic or mineral nutrient sources to achieve a specific nutrient formula112. Granulation, or granulation, is a common technique used to convert powdered organic fertilizers into granular form. Granules are easier to handle, store and administer, and often have improved sustained-release properties. The process involves the use of a specialized fertilizer granulator. The resulting granules can then be dried and cooled to reduce moisture content and ensure product stability before packaging and distribution112。
• 4.1.2. Obtaining raw materials: The wide range of raw materials available for organic fertilizer manufacturing covers a wide range of organic wastes and by-products. Common sources include livestock manure (cattle, poultry, pigs), plant-based materials (such as crop residues, leaves, and seaweed), food waste from households, restaurants, and the food processing industry, slaughterhouse waste (blood meal, bone meal, feather meal), and even certain natural minerals such as phosphate rock, limestone, and green sand113. Organic fertilizers can be roughly classified according to their main source: animal-based fertilizers are usually rich in nitrogen and other macroelements; plant-based fertilizers are valuable for improving soil structure and can also provide various nutrients; and mineral-based organic fertilizers provide specific mineral elements needed for plant growth.118. The feasibility of organic fertilizer manufacturing is also affected by the availability and cost of these raw materials. Establishing a reliable and consistent source of organic waste is critical to sustainable production operations. Depending on the type of feedstock, pre-treatment steps may be required to optimize the composting process and ensure the quality of the final product112。
• 4.1.3. Market demand and potential: The global organic fertilizer market is experiencing significant growth due to several factors, including growing consumer demand for organic food, increasing awareness about the environmental impact of synthetic chemical fertilizers, and the global shift towards more sustainable farming practices112. As consumers become increasingly health-conscious and concerned about the potential health risks posed by the use of synthetic chemicals in conventional farming, demand for organically grown products is rising, which in turn is driving the demand for organic fertilizers112. Governments in many regions are promoting organic farming through supportive policies, financial incentives, and research programs, further stimulating the organic fertilizer market112. North America and Europe are currently the largest markets for organic fertilizers, but Asia-Pacific, especially China and India, also show significant growth potential due to increased organic farming activities and government support124. In North America, animal-derived organic fertilizers (such as manure-based products) currently hold a significant market share due to their relative affordability and availability124。

4.2. Potential risks:

• 4.2.1. Unstable quality of raw materials: The quality of raw materials used in the production of organic fertilizers can be highly variable. For example, the nutrient content of livestock manure can vary depending on the animal species, its diet and storage conditions. Likewise, the composition of food waste can vary widely. This inconsistency makes it challenging to produce organic fertilizers with consistent and predictable nutrient content.132. In addition, raw materials such as feces and food waste may contain contaminants including pathogens, heavy metals, antibiotic resistance genes and microplastics22。
• 4.2.2. Contamination during production: The production process of organic fertilizers also involves certain risks of pollution. If the composting or fermentation phase is not managed properly and fails to achieve adequate temperatures and duration, the resulting fertilizer may still contain viable pathogens137. Heavy metal contamination can also occur if raw materials are contaminated or processing equipment introduces metals into the product136. In addition, the production process may also create environmental issues such as odor and noise emissions, and if wastewater is generated, it needs to be properly treated to prevent water pollution136. Dust generated during the handling and processing of dry fertilizer materials may also pose a respiratory hazard to workers140。
• 4.2.3. Product quality inspection: Maintaining consistent quality of the final organic fertilizer product is a key challenge. This includes ensuring that fertilizers have the required nutrient content, are free of harmful contaminants, and are stable and mature enough to be used safely and effectively. Immature or unstable organic fertilizers may have adverse effects on plant growth, such as nitrogen immobilization or phytotoxicity134. Insufficient pathogen inactivation during composting may also lead to product quality issues and potential health risks. In addition, the physical characteristics of a fertilizer, such as particle size and moisture content, affect its handling and application characteristics144。
•  

4.3. Quality management and control methods for organic fertilizer production:

• Implementing strict quality management measures is critical to mitigating risks and ensuring the production of safe and effective products. This includes careful selection and testing of ingredients to assess their nutrient content and identify potential contaminants145. Close monitoring and control of the composting or fermentation process is crucial, paying particular attention to maintaining optimal moisture levels (usually 50-60% early on, then 40-50%), reaching and maintaining thermophilic temperatures (55-60°C) that effectively kill pathogens, ensuring a balanced carbon to nitrogen ratio (ideally 20-30:1), providing sufficient air and oxygen for aerobic decomposition, and monitoring pH (usually targeting a slightly alkaline 7.5-8.5 range for optimal composting rate)147. The final organic fertilizer product is regularly tested to verify its nutrient content (macroelements such as N, P, K, and organic matter) and to check for the presence of pathogens (Salmonella, E. coli) and heavy metals (Arsenic, Cadmium, Lead, Mercury, Chromium)145. Ensure the composting process reaches and maintains high temperatures long enough to effectively kill pathogens, insect eggs and weed seeds144. Adhere to established quality standards and seek certification from accredited organic certification bodies to build consumer trust and ensure product integrity145. Assess compost maturity and stability using tests such as the Solvita test (which measures carbon dioxide and ammonia) and the Dewar self-heating flask test to ensure the product is ready for application and will not harm plants145. Establish an in-house analytical chemistry laboratory for ongoing monitoring and quality assurance, and regularly send samples to an independent accredited laboratory to verify results145. For biofertilizers containing specific microorganisms such as rhizobia and nitrogen-fixing bacteria, quality control includes checking viable cell count, culture purity, cell morphology and nitrogen-fixing ability150。

4.4. Practical application cases of organic fertilizer manufacturing in the world:

• In Dhaka, Bangladesh, research explores the feasibility of using fruit and vegetable waste to produce liquid organic fertilizer, highlighting its potential to meet the potassium needs of certain crops151. Research in the same area also investigated the feasibility of replacing chemical fertilizers with organic fertilizers in corn production, with promising results when organic and inorganic fertilizers were used in combination152. China has established commercial-scale animal manure (especially cow dung) organic fertilizer production, using specialized equipment for fermentation, crushing, mixing, granulation, drying, cooling and packaging113. A feasibility analysis in the United States has shown that it is economically feasible to establish an organic fertilizer plant, driven by the rising cost of chemical fertilizers and growing demand for organically grown products.112. A study compared traditional and microwave-assisted extraction technologies for the production of liquid organic waste fertilizer and found that traditional extraction under alkaline conditions is the most beneficial option for expanding production scale.153. Farmers around the world are increasingly adopting organic fertilizers. In California, improvements in grape quality and yields were observed after many vineyards switched to organic fertilizers. Similarly, in Brazil, soybean farmers report that organic farming helps reduce pests and improve profits154. Canadian company Acti-Sol converts layer hen manure into a range of manure-based fertilizers155. Talborne Organics in South Africa provides case studies on the successful application of its organic fertilizers to a variety of crops including pineapples, potatoes and cauliflower156. Research firm Landlab conducts case studies demonstrating the effectiveness of organic fertilizers in improving soil health, water retention and plant growth in corn and turf157. Italian research shows that “AnchoisFert” (anchovy residue) is a powerful fertilizer that is significantly better than commonly used chemical and organic fertilizers in promoting the growth of red onions.158. Ficosterra provides case studies highlighting the successful application of its ecological fertilizers in horticulture, resulting in increased crop yields and plant health159. An Indonesian study analyzes the behavior of organic farmers using organic fertilizers160。

• Composting, biogas power generation and organic fertilizer manufacturing are viable and increasingly important ways to recover sustainable resources from organic waste. Each technology offers a unique set of feasibility considerations, potential risks, and practical applications.
• Composting is a well-established and widely applicable method that is particularly effective in reducing waste from landfills and producing valuable soil amendments. However, careful management is required to mitigate risks associated with pathogens, odors, and heavy metals.
• Biogas power generation provides the dual benefits of renewable energy production and organic waste treatment. While energy conversion efficiency is improving, challenges related to methane leakage, digestion residue management and safety still need to be addressed through technological advances and strict operating procedures.
• Organic fertilizer manufacturing plays a vital role in supporting permaculture by converting organic waste into nutrient-rich products. Ensuring the quality and safety of organic fertilizers requires careful selection of raw materials and implementation of robust quality control measures throughout the production process.
• International case studies highlight the successful implementation of these technologies at different scales and contexts, demonstrating their adaptability and potential for widespread application. However, the success of each project depends on careful planning, appropriate technology selection, effective risk management and a thorough understanding of local conditions and market dynamics.
• To further promote sustainable resource recovery, policymakers should consider implementing supportive regulations and incentives, investing in research and development to improve technology efficiency and reduce risk, and promoting public awareness of and participation in waste separation and resource recovery initiatives. Integrated approaches that combine these technologies, such as using biogas digestion residues as feedstock for compost or organic fertilizer production, can further improve the sustainability and economic viability of organic waste management.
• Ultimately, the transition to a circular economy for organic waste requires governments, industry, researchers and communities to work together to adopt these resource recovery methods and work collaboratively towards a more sustainable future.

Valuable tables included in the report:

1. Table in Section 2.1.1 (Composting Technology Maturity): Comparison of different composting technologies (ribbons, forced-air static piles, reactors, vermicomposting) based on size (small/medium/large), feedstock suitability (yard waste, food waste, manure, mixed), ventilation method (passive/active), processing time (weeks/months), estimated capital cost (low/medium/high) and odor potential (low/medium/high).
2. Table in Section 3.1.1 (Biogas energy conversion efficiency): Range of energy conversion efficiencies (electricity, thermal, overall) for different biogas utilization pathways, including internal combustion engines (combined heat and power), gas turbines and fuel cells.
3. Tabled in Section 3.2.3 (Biogas Safety Issues): Health effects of hydrogen sulfide (H2S) at varying concentrations (parts per million, ppm) ranging from detectable odor to concentrations immediately dangerous to life or health (IDLH).
4. Table in Section 4.1.1 (Organic Fertilizer Production Process): Comparison of different organic fertilizer granulation methods (new/drum stirring type, disc type, flat die extrusion type, drum type) based on raw material moisture tolerance, production capacity (tons/hour), particle shape (spherical, cylindrical, irregular) and drying requirements.
5. Table in Section 4.3 (Quality Control of Organic Fertilizers): Key quality parameters and typical ranges of mature compost (e.g. carbon to nitrogen ratio, moisture content, pH, conductivity, germination index, heavy metal limits according to relevant standards).
6.  

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