GHG emissions and mitigation potential in decentralised composting supply chain: Insights from 517 residential sites across multiple cities.

Composting is recognised as one of the most reliable and environmentally-friendly options for organic waste management. Yet, its comprehensive greenhouse gas (GHG) mitigation potential remains poorly quantified due to methodological inconsistencies. This study analyses material flow from 517 decentralised composting sites in four major cities of India - Delhi NCR, Mumbai, Pune, and Bengaluru - processing a total of 2085 tonnes of organic waste during FY 2024-25. It quantifies GHG emissions and mitigation across various stages, comparing results against alternative accounting methodologies. The study focuses on small-scale, decentralised aerobic composting designed for on-site treatment of organic waste, commercially marketed under the name 'Aerobin'. The Aerobin systems in this study achieved net GHG emissions mitigation of 322 tonnes CO2-eq annually, equivalent to 0.15 tonnes CO2-eq per tonne of waste treated. Manufacturing of these composting units accounted for 81% of total GHG emissions, while operational emissions accounted for 16%. Methane emissions from landfilling (under the business-as-usual scenario), avoided due to diversion of waste for composting, accounted for approximately 95% of total GHG mitigation, with minor mitigation contributions from fertiliser substitution and avoided transport. Mitigation intensity varied across the four cities, influenced by methane generation in landfills rather than by operational scale. Sensitivity analysis revealed that net emissions are highly sensitive to landfill methane generation rates (±45% variation) and to the lifespan of composting infrastructure. Methodological comparison showed that IPCC guidelines and carbon crediting approaches yield 7-9% higher mitigation estimates than full life-cycle accounting, due to the exclusion of capital goods and product substitution. These findings demonstrate that decentralised composting delivers consistent climate benefits, but accurate GHG accounting should capture both upstream emissions and downstream substitution effects. The results support policy frameworks promoting decentralised organic waste management as a viable climate mitigation strategy in emerging economies, while highlighting the need for standardised monitoring protocols and boundary definition.