Decentralized anaerobic digestion in emerging Indian cities: A systems perspective on urban waste management, bioenergy recovery, and circular economy transitions

Rapid urbanization in emerging Indian urban cities is increasing the pressure on municipal solid waste management. This review evaluates anaerobic digestion as an integrated option for urban organic waste management, renewable energy recovery, and circular economy development. A structured narrative review was used to assess feedstock characteristics, digester configurations, operating conditions, and biogas uses. Policy support and reforms are discussed, followed by techno-economic evaluation of decentralized anaerobic digestion, along with opportunities to diversify income through bio-CNG, electricity, and bio-fertilizer production. Life cycle studies indicate a potential to reduce greenhouse gas through methane capture, fossil fuel substitution, and comparison with conventional methods. The reviewed evidence indicates that decentralized anaerobic digestion is most suitable if the waste is source-separated, moisture-rich, and available within a short distance. These work best if the local demand for energy and digestate exists. The economic performance of this technology depends substantially on digester scale, capacity utilization, feedstock availability, capital and operating and maintenance costs, etc. Direct thermal use may require lower treatment and conversion. At the same time, electricity generation and biomethane upgrading provide higher-value energy products, but the latter involves greater capital cost, operational complexity, and energy demand. Environmental benefits are higher when methane leakage is controlled, energy recovered replaces conventional energy resources, and digestate is used safely as a nutrient source. From a circular economy perspective, decentralized anaerobic digestion can reduce the disposal of biodegradable waste, promote energy and resource recovery, and support localized waste-to-resource systems. However, poor waste segregation, inconsistent feeding, limited capacity utilization, poor maintenance, and inadequate digestate management remain major barriers. This review article concludes that decentralized anaerobic digestion can support sustainable urban waste management in India only if technology selection, feedstock management, energy utilization, financing, policy support, and community participation are integrated.