How does two-stage anaerobic digestion improve methane recovery after shutdowns in sugarcane vinasse treatment?

Short-term shutdowns and off-season interruptions can decouple treatment efficiency from bioenergy recovery during anaerobic digestion of sugarcane vinasse, yet reactor recovery remains poorly understood. This study compared a single-stage methanogenic reactor fed with raw vinasse and a second-stage methanogenic reactor fed with fermented vinasse under repeated shutdowns lasting 1-10 days at organic loading rates of 10 and 20 kg-COD m^-3 d^-1, followed by a 111-day off-season interruption. Soluble chemical oxygen demand (COD) removal returned to pre-shutdown levels within one day after short shutdowns and within five and four days after the off-season interruption in the single-stage and second-stage methanogenic reactors, respectively. In contrast, at 10 and 20 kg-COD m^-3 d^-1, post-recovery methane yields were 215 ± 38 and 206 ± 16 NmL-CH₄ g^-1-COD_removed in the single-stage reactor, respectively, compared with 266 ± 14 and 299 ± 32 NmL-CH₄ g^-1-COD_removed in the second-stage methanogenic reactor. After the 111-day off-season interruption, methane yield averaged 213 ± 15 NmL-CH₄ g^-1-COD_removed in the single-stage reactor and 307 ± 32 NmL-CH₄ g^-1-COD_removed in the second-stage methanogenic reactor during the three-day post-recovery window. Propionate and total volatile fatty acid accumulation were higher in the single-stage reactor. After the off-season, propionate reached 1.9 ± 0.6 g-COD L^-1, consistent with a less stable post-restart condition. These results show that assessing recovery from organic matter removal alone can underestimate methane production losses, particularly in single-stage methanogenic reactors, whereas two-stage anaerobic digestion better preserves bioenergy recovery.