Industrial-Scale Thermophilic Anaerobic Digestion of Untreated Cafeteria Waste: Techno-Economic Assessment of Renewable Energy and Digestate Valorization

Abstract

Thermophilic anaerobic digestion (TAD) of solid organic wastes for biogas production offers dual human-health and environmental benefits: source-separated waste management with reduced pathogen burden and renewable energy generation with reduced greenhouse-gas emissions. This study investigated a “Techno-Economic Assessment (TEA)” of an industrial-scale TAD process integrated with combined heat and power (CHP) generation and alternative digestate-management strategies. Two processing capacities, 7000 tons/year (Case I) and 2000 tons/year (Case II), were evaluated under four digestate-management scenarios using economic assumptions. For Case I, the total capital investment (TCI), including working capital equivalent to 5% of fixed capital investment, was estimated at $4.055 million, while the annual operation and maintenance (O&M) cost was estimated at $0.434 million. Scenario 1, involving solid–liquid separation with recovery of separated compost and liquid digestate, generated an annual revenue of $1.070 million, a 20-year net present value (NPV) of $0.869 million, a discounted payback period of 12.3 years, and an internal rate of return (IRR) of 13.2%. Scenario 2, based on direct utilization of whole digestate, produced the highest modeled economic return, with an NPV of $5.998 million and an IRR of 30.1%; however, its feasibility depends strongly on securing a reliable whole digestate offtake market. In contrast, Scenarios 3 and 4 resulted in negative NPVs under the base-case assumptions. A normalized one-at-a-time sensitivity analysis identified process-cycle duration and effective throughput, liquid digestate netback, tipping fee, CHP energy value, methane yield, capital cost, processing capacity, and discount rate as the principal economic drivers. Zero-revenue digestate cases produced negative NPVs for Scenarios 1 and 2, with estimated break-even plant-gate netbacks of $0.134/gal for liquid digestate and $0.121/gal for whole digestate. Overall, industrial TAD can be economically feasible, but commercial viability depends on feedstock supply, reactor productivity, internal energy demand, digestate offtake, heat utilization, and site-specific waste-management revenue.