Anaerobic digestion of soybean molasses: comparison between single and two-stage systems, operational temperature and microbial communities.

Soybean molasses is an abundant, low-value by-product of soy protein concentrate production whose valorization through anaerobic digestion remains little explored, and no two-stage system has yet been reported for this substrate. This study evaluated methane production in a methanogenic anaerobic sequencing batch biofilm reactor fed with the effluent of a thermophilic acidogenic reactor treating soybean molasses, assessing the influence of applied organic loading rate (4-12 kg-COD/m3/d) and operational temperature (30 or 55 °C) on performance, stability, and microbial community structure. An energy analysis then compared four single- and two-stage configurations. The reactor was stable at every loading rate, consuming the volatile acids of the acidified influent almost completely while generating alkalinity internally. Mesophilic operation outperformed thermophilic operation, reaching a methane productivity of 152.8 mol-CH4/m3/d and a yield of 13.1 mol-CH4/kg-COD at the highest load, 69% and 62% above the thermophilic values. Community analysis showed that temperature governed structure: mesophilic reactors were diverse and acetoclastic, whereas thermophilic reactors were dominated by specialists relying on syntrophic acetate oxidation. Splitting the process into two stages did not increase the overall energy yield relative to single-stage methanogenesis, which was already efficient; its value lies instead in hydrogen co-production and process stability, with a thermophilic-acidogenic and mesophilic-methanogenic pairing representing the most rational and energetically competitive configuration for this by-product.