SUSTAINABILITY ANALYSIS OF CIRCULAR ECONOMY BASED BIOGAS PLANTS FOR MANAGING ORGANIC WASTE

Abstract

The present study investigates the design, and sustainability analysis of circular economy (CE)-based biogas plants in India. Anaerobic digestion (AD) is a process that converts biowaste from agriculture, industry, and municipal sources into biogas. As most regions experience significant seasonal temperature variations, there is a need to maintain consistent temperatures within the digester to ensure optimal plant performance. A comparative techno-economic analysis of different thermal management approaches identified the combination of thermal insulation and a combined heat and power (CHP) unit for onsite electricity and heat demand, as the preferred approach. Based on the availability and potential, the organic wastes identified for rural areas are rice straw and cattle dung, and for the urban areas, are sewage sludge and the organic fraction of municipal solid waste. For a biogas plant capacity of 300 t/day based on feedstock supply, the rural and urban frameworks show positive net present value (NPV) of US $3.24 million and US $1.30 million, respectively. The higher NPV of the rural framework is because of higher total solids in rural feedstocks, resulting in higher amounts of compressed biomethane gas (CBG) and fertilizers. The selling price of CBG, CO2, and solid fertilizer, followed by feedstock buying price and discount rate, are the most sensitive parameters for the two CE-based frameworks. The life cycle assessment (LCA) results show that rural and urban frameworks' total climate change impact (kg CO2 eq./kg CBG) is 4.87 and 4.52, respectively. Higher compost quantity, feedstock transportation, methane leakage from membrane separation, and CHP emissions are responsible for higher emissions in rural areas. The emission distribution shows that leakages from the biogas plant contribute the highest to climate change, followed by composting and transportation. Despite numerous government initiatives and policies, the dissemination of commercial-scale biogas plants remains slow in the country. A social impact assessment is performed to identify strengths, weaknesses, opportunities, and threats (SWOT) of large-scale biogas plants. The common strengths of biogas plants are efficient waste management, renewable energy production, biofertilizer generation, and reduced pollution and health risks. It is found that social communities are least aware of biogas technology strengths. Local job creation, enhanced research and social collaboration, technological advancements, and loyalty programs for CBG and biofertilizer users present opportunities to boost the dissemination of biogas technology. The weaknesses and threats act as barriers to disseminating biogas plants in the market. Twenty-four such barriers obtained are categorized into four types and later prioritized using the arithmetic mean method. The economic barriers are the most significant, followed by political, technical, and social barriers. Among the sub-barriers, the most critical is the lack of a digestate market, followed by limited access to the gas grid and high capital and operating costs. In India, cattle dung has been the prime feedstock that has been utilized for biogas production. The country has about 300 million cattle and 6250 registered cattle shelters (gaushalas), with 15%–30% having more than 500 cattle. The dairy or cattle industry contributes around 5% to India’s GDP and provides employment to approximately 80 million farmers. Hence, illustrative examples of different sizes of cattle shelters are considered for a comparative techno-economic and GHG emission analysis of biogas plants for CHP and compressed biomethane gas (CBG) applications. For small shelters (500 cattle), the framework showed an NPV of US $0.089 million for CHP and US $0.07 million for CBG. For large shelters (5000 cattle), the framework showed an NPV of US $2.78 million for CHP and US $2.80 million for CBG. Availability of dung, discount rate, and the market price of solid fertilizers significantly influence the NPV of the biogas plant. The greenhouse gas (GHG) emission analysis shows that the CHP application has lower emissions than the CBG application, particularly for bigger plants. It is recommended that biogas plants be built inside or near cattle shelters with a minimum dung availability of 2.25 t/day for both applications. Based on the results obtained, recommendations are provided for biogas owners and policymakers to enhance the dissemination of biogas technology in India. The findings provide useful insights for implementing proposed CE-based frameworks in existing or upcoming biogas plants in urban and rural parts of India to make them sustainable.