Unleashing the full potential of vinasse fermentation in sugarcane biorefineries

High sulfate concentrations (>2.0 g L⁻¹) in sugarcane vinasse present challenges for single-phase anaerobic digestion (AD) systems due to microbial competition and sulfide toxicity. While two-phase AD systems have successfully reduced sulfate in thermophilic fermentative systems, similar success under mesophilic conditions remains undocumented. This study evaluated different strategies to establish and maintain stable long-term sulfidogenic activity in high-rate fermentative reactors under mesophilic conditions. Three reactors were tested, each inoculated differently: R1 with mesophilic naturally-fermented vinasse, R2 with thermophilic naturally-fermented vinasse, and R3 with granular sludge. All reactors were operated at 30 °C with a 12-h hydraulic retention time. The addition of 0.25 gNaHCO₃ g⁻¹CODt during inoculation effectively maintained pH levels higher than 6.5, stimulating sulfidogenic activity in all systems, regardless of sulfate loading rate variations (3.9–4.8 kgSO₄ m⁻³ d⁻¹). R3 demonstrated superior buffering capacity and robust sulfidogenesis, achieving sulfate removal efficiencies of 63 ± 14 % in R1, 72 ± 15 % in R2, and 83 ± 16 % in R3, primarily driven by Desulfovibrio. Hydrogenotrophic methanogenesis persisted in all reactors, driven by Methanofollis, Methanobacterium, and Methanosarcina in R1, Methanofollis in R2, and Methanoculleus in R3. Despite methanogenesis occurrence, R3 exhibited higher acetate accumulation (>3.5 gHAc L⁻¹), with great potential to boost acetoclastic methanogenesis in a two-stage AD scheme. The produced biogas was low in hydrogen (<1 %) but rich in sulfide (up to 9 %), necessitating further gas treatment. These findings reveal the high resilience of sulfate-reducing bacteria and methanogens to high organic loads, highlighting the complexity of AD of vinasse.