Optimization of Anaerobic Digestion Systems for Biomethane Recovery from Septic Tank Sludge

ABSTRACT: This study presents a process design, simulation, and optimization framework for converting septic sludge into biomethane using Aspen Plus^®. The sludge was characterized, revealing carbon, hydrogen, and volatile matter contents of 33.80, 5.86, and 34.86 wt.%, respectively. The developed Aspen Plus^® model was validated against three literature datasets, achieving percentage errors below unity. Optimization using Response Surface Methodology-Central Composite Design (RSM-CCD) showed that the maximum biomethane yield was 58.227 vol% under optimal conditions: 25 °C hydrolysis temperature, 60 °C digester temperature, 35 days hydraulic retention time (HRT), and an organic loading rate (OLR) of kg·VS·m⁻³·day⁻¹, with a desirability score of 1.0. A techno-economic evaluation using the Aspen Process Economic Analyser (APEA) demonstrated the system’s economic feasibility, with a total capital investment of USD 3.19 million, an annual operating cost of USD 1.29 million, and a payback period of approximately 3.8 years. The optimized system achieved a net energy gain of 82.6%, IRR of 16.6%, and NPV of $4.64 M, confirming strong economic viability. Sensitivity analysis further revealed that CAPEX, OPEX, feedstock cost, and upgrading energy demand significantly influence system profitability, emphasizing the importance of process optimization and energy-efficient upgrading strategies. Environmental assessment showed that the optimized system improved methane recovery efficiency to 98.7% and achieved a CO₂ emission reduction potential of 0.49 kg CO₂-eq/kg CH₄, demonstrating strong greenhouse gas mitigation potential. Overall, the findings establish anaerobic digestion of septic sludge as a sustainable and cost-effective waste-to-energy pathway suitable for decentralized urban wastewater management, supporting circular economy and clean energy objectives in developing regions.