The Resilience of Sulfate-Reducing Bacteria and Methanogens Exposed to Harsh Fermentative Conditions

The presence of high sulfate concentrations (>2 g L−1) in sugarcane vinasse comprises one of the main bottlenecks in single-phase anaerobic digestion (AD) systems toward enhanced methane production. Recent studies on two-phase AD systems processing sugarcane vinasse have demonstrated the feasibility of reducing sulfate in thermophilic acidogenic stages, although no reports have yet demonstrated high sulfidogenic activity under mesophilic conditions. An Anaerobic Structured-Bed Reactor (AnSTBR) processing sugarcane vinasse at 30 ℃ and inoculated with granular sludge was innovatively operated for 89 days under six operating stages. Higher sulfidogenic activity (99%) was achieved in Stage I with pH levels kept above 6.5. The reduction of HRT from 12 h to 6 h in Stage II did not significantly affect sulfidogenesis (86%) nor reduced methane (CH4) content in biogas (12%). However, induced low pH levels on Stages III (4.5 ± 0.4) and IV (3.2 ± 0.2) indicated poor sulfate reduction (<30%). The sudden increase in pH levels (Stage V) led to increased methane content (66–99%), while H2 was kept below 1%. Sulfidogenesis was re-established (75%) as pH returned close to neutrality (Stage VI), and CH4 content finally achieved its minimum value (5%). These findings reveal the high resilience of sulfate-reducing bacteria and methanogenic archaea to high organic loads and pH fluctuations, characterizing the AD of vinasse as a highly challenging process.