Improving the energy balance of ethanol industry with methane production from vinasse and molasses in two-stage anaerobic reactors

Thermophilic anaerobic digestion is a viable option for processing sugarcane vinasse that recovers energy in the form of methane. However, restarting real-scale vinasse-treating reactors, at the resumption of the sugarcane harvest season, can lead to difficulties, making it desirable to keep them in continuous operation in the off-season. Thus, was evaluated the thermophilic anaerobic conversion of sugarcane vinasse in a two-stage upflow anaerobic sludge blanket (UASB) reactor (reactors R1 and R2), employing sugarcane molasses in the off-season for methane production, followed again by vinasse in the following harvest season. UASB reactor R2 yielded a higher methane percentage in biogas and higher volumetric methane production rates and methane yield than R1, accounting for a 58% increase in energy production in the two-stage system compared to the single-stage system using vinasse. It is observed that the molasses is an important source of carbon for the energy production using the thermophilic UASB reactors (18.01 MJ L−1 molasses), compared to alcoholic fermentation (8.30 MJ L−1 molasses). The methane produced in the reactors system (R1 + R2), after the off-season, can generate 5.1 kWh d−1 per m3 of vinasse, or 17% of the electricity needed to process a ton of sugarcane.