Retrofitting residential septic infrastructure for decentralised biogas production: a scenario-based circular waste-to-energy assessment in Lagos

Purpose Rapid urbanisation in cities such as Lagos requires decentralised solutions that address energy insecurity, inadequate sanitation and environmental sustainability. This study evaluates the feasibility of retrofitting septic infrastructure within an ageing residential estate in Lagos to support decentralised sanitation-to-energy systems within a circular sanitation–energy framework. Design/methodology/approach A case study-based, engineering-calibrated methodology integrates infrastructure characterisation, wastewater estimation, organic loading analysis, infrastructure-constrained methane modelling and techno-economic evaluation. Real septic tank design data (˜20 m3) were used to estimate hydraulic retention time under operating conditions. Alternative retrofit configurations were assessed using multi-criteria decision analysis (MCDA), supported by sensitivity analysis and multi-layer validation incorporating parameter, structural, benchmarking and sensitivity-based checks. Findings The estate generates 43.2 m3/day of wastewater and 38.88 kg COD/day from 432 residents. The calibrated model estimates methane production of 10.89 m3/day, yielding 92 kWh/day and avoiding approximately 106 tonnes CO2-equivalent emissions annually. A hydraulic retention time of 0.46 days identifies reactor hydraulics as the principal technical constraint despite adequate organic loading. Sensitivity analysis identifies methane yield and capital cost as the principal determinants of economic performance. Although centralised digestion maximises energy output, MCDA results identify cluster-scale systems as the preferred retrofit option. Originality/value The study presents a unified retrofit assessment framework integrating infrastructure characterisation, infrastructure-constrained methane modelling, uncertainty assessment, techno-economic evaluation and multi-criteria decision support. The findings suggest that decentralised sanitation systems are feedstock-adequate but hydraulically constrained, providing a transferable framework for retrofit assessment in infrastructure-constrained urban environments.