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

• Mesophilic fermentation produced acetate-rich, sulfate-free, buffered vinasse. • Methanogenesis reached 25 kg-COD m −3 d −1 with zero alkalinity addition. • Recirculation regenerated bicarbonate and stabilized high-load operation. • Methane productivity reached 5.5 L-CH 4 L −1 d −1 with > 80% methane. • Carried-buffered AD reduced NaHCO 3 -related OPEX in vinasse biorefineries 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-COD t 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 NaHCO 3 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% CH 4 ), 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 NaHCO 3 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.