Operational dual bicarbonate stabilization enables chemical-free high-rate vinasse second-stage methanogenesis with energy and cost benefits.

Second-stage anaerobic digestion is essential for maximizing energy recovery from sugarcane vinasse, yet stable high-rate methanogenesis typically demands substantial alkalinity inputs that raise operating costs and reduce net energy yield. This study demonstrates that these constraints can be alleviated by relocating buffering upstream through a dual stabilization strategy combining fermentation-derived bicarbonate and effluent recirculation. Two mesophilic structured-bed reactors were operated up to 25 g-CODt L-1 d-1 to isolate the effects of bicarbonate carryover (R1) and recirculation-supported alkalinity retention (R2). This carried-buffered methanogenesis approach supported acetate-based conversion pathways, maintained pH homeostasis, and enabled chemical-free operation under loading conditions where external NaHCO3 is typically required. A system-level comparison of three vinasse-to-biogas configurations showed that single-phase digestion suffers from sulfate reducing bacteria-methanogen competition and diluted biogas (64.3% CH4), while thermophilic dark fermentation improves sulfate reduction but lowers net energy due to substrate diversion and thermal demand. In contrast, the carried-buffered configuration achieved the highest methane fraction (84.5%) and the greatest global energy potential (140.8 GWh). A six-scenario OPEX analysis indicated that alkalinity economics are strongly influenced by where buffering is supplied, not only by total NaHCO3 dosing. By confining sulfidogenesis to fermentation and avoiding alkalinity supplementation in the methanogenic stage, the carried-buffered design could reduce annual chemical costs by approximately 1.1 million USD based on the preliminary OPEX assessment. These findings indicate that carried-buffered methanogenesis is a promising bench-scale strategy for enhancing methane productivity, although pilot-scale validation and broader techno-economic assessment are still required before industrial extrapolation.