Process Optimization and Techno-Economic Evaluation of Single-Stage Biohythane Production from Green Waste: Low-Input and High-Conversion Operation

Single-stage dark fermentation offers a promising route to convert lignocellulosic green waste into biohythane, a mixture of biohydrogen (H2) and biomethane (CH4), but conditions maximizing hydrogen selectivity may increase inputs, with unclear economic consequences. This study examines the effect of food-to-microorganism (F/M) ratio, pH, agitation, and NaOH pretreatment on biohythane yield and evaluates the resulting process economics at campus, district, and city scales. The most H2-selective condition (F/M 0.5, pH 6.0, no agitation) produced 50.3 mL H2/g volatile solids (VS) and 245.9 mL CH4/g VS; alkaline operation with agitation and NaOH raised biodegradability to 81.1% of theoretical maximum but suppressed H2 to near zero. Techno-economic analysis showed the acidic, non-agitated condition achieved the highest net present value at district scale under two independent capital-cost methods (NPV = US$19.6 million and US$10.7 million; internal rate of return (IRR) = 55%): its CH4 revenue gain (+US$290,000/yr) was outweighed 3-fold by higher costs (+US$966,000/yr). This ranking held across scales and under H2-purification and tax-incentive scenarios. A low-input strategy therefore achieves superior returns by preserving CH4 revenue while avoiding agitation and pretreatment costs, with H2 better viewed as a co-benefit than the primary target, challenging the assumption that maximizing total biogas output is economically preferable within a scale-sensitive techno-economic framework.