Insights of energy potential in thermophilic sugarcane vinasse and molasses treatment: does two-stage codigestion enhance operational performance?

The study evaluated the performance of thermophilic co-digestion in both single-stage methanogenic reactors (TMR) and two-stage systems, consisting of a thermophilic acidogenic reactor and a thermophilic sequential methanogenic reactor (TSMR). A 1:1 mixture of sugarcane vinasse and molasses was codigested in anaerobic fluidized bed reactors, with varying organic matter concentrations based on chemical oxygen demand (COD) ranging from 5 to 22.5 g COD L⁻¹. Both systems achieved high organic matter removal efficiency (51 to 86.5%) and similar methane (CH₄) yields (> 148 mL CH₄ g⁻¹COD_removed). However, at the highest substrate concentration (22.5 g COD L⁻¹), the TSMR outperformed the TMR in terms of energy generation potential (205.6 kJ d⁻¹ vs. 125 kJ d⁻¹). Phase separation in the two-stage system increased bioenergy generation by up to 43.5% at lower substrate concentrations (7.5 g COD L⁻¹), with hydrogen (H₂) generation playing a critical role in this enhancement. Additionally, the two-stage system produced value-added products, including ethanol (2.3 g L⁻¹), volatile organic acids (3.2 g lactate L⁻¹), and H₂ (0.6–2.7 L H₂ L⁻¹ d⁻¹). Microbial analysis revealed that Thermoanaerobacterium, Caldanaerobius, and Clostridium were dominant at 5 g COD L⁻¹, while Lactobacillus prevailed at concentrations of ≥ 15 g COD L⁻¹. The primary methane producers in the single-stage system were Methanosarcina, Methanoculleus, and Methanobacterium, whereas Methanothermobacter, Bathyarchaeia, and Methanosarcina dominated in the two-stage system.