Starvation pretreatment enhances sulfidogenic operation of two-stage anaerobic digestion system for biogas production with low H₂S content

This study attempted to enhance sulfidogenic activity via sulfate-reducing bacteria (SRB) enrichment and minimize organic carbon loss by methanogen inhibition in the sulfidogenic stage of a two-stage anaerobic digestion system (TSADS). To enrich SRB in the sulfidogenic stage, batch tests were performed with various granular sludge pretreatments. Starvation was the most effective pretreatment, increasing SO₄²⁻ removal and minimizing chemical oxygen demand (COD) loss by inhibiting methanogen activity. Microbial community analysis showed that Desulfovibrio, Desulfotomaculum, and Syntrophobacter were the dominant SRB in the sulfidogenic stage (5.0%, 3.1%, and 2.4%, respectively). This enabled SO₄²⁻ reduction (86%) and volatile fatty acid production (55% of fed COD) at a hydraulic retention time (HRT) of 4 h. Conversely, biogas with a reduced H₂S content (110 ppm_v) was produced in the methanogenic stage (HRT = 6 h). A granular sludge comparison revealed differences in their ecology, structure, and extracellular polymeric substance characteristics. Economic feasibility analysis demonstrated that TSADS can lead to a cost reduction of $80–90/1,000 m³ CH₄ compared to single-stage anaerobic digestion.