Multi-cycle anaerobic digestion of hydrothermal liquefaction aqueous phase: Role of carbon and iron based conductive materials in inhibitory compounds degradation, microbial structure shaping, and interspecies electron transfer regulation

[Display omitted] •Addition of magnetite, biochar, and GAC improved anaerobic digestion of HTL-AP.•Recycle of sludge and conductive materials further boosted digestion efficiency.•Strengthened microbial enrichment and interactions pointed to DIET formation.•DIET potential microbes were enriched in first batch and stabilized in second batch.•Conductive materials’ addition helped remove toxic N-heterocyclics and aromatics. Anaerobic digestion is commonly performed to treat hydrothermal liquefaction aqueous phase (HTL-AP), but the conversion is limited by the inhibitory compounds. This study was conducted to investigate the effects of adding granular activated carbon, biochar, and magnetite on the digestion of HTL-AP. Moreover, the enriched aggregates and conductive materials were recovered after experiment, and then reused to study the effects of multi-cycle digestion on organics degradation, microbial activity, and methane production. Results showed that the conductive materials significantly contributed to the digestion performance and maintained high activity after recycling. The degradation of organic matters, especially the removal of toxic compounds, the enrichment of functional microbes, as well as the production of methane can be further enhanced with the reuse of conductive materials. Specifically, a 77% higher methane production rate was observed in magnetite assisted digester after recycling. More than 80% of the nitrogen organics were degraded with biochar, and all potential inhibitors were completely removed with GAC addition. The diversity and abundance of microbes were improved, and the dominant structure was established in the first batch, then the enrichment was further stabilized after the recycling. Meanwhile, the sludge conductivity was increased by 28.1 and 8.5 times in GAC and magnetite digesters in the first batch and was further enhanced by 8.0% and 16.4% after the recycling, indicating the improved expression of conductive pili and cytochromes of the microbes. These results point to the establishment of direct interspecies electron transfer. This study showed promising prospects for the utilization and recycling of conductive materials in the anaerobic digestion of HTL-AP and its long-term applications.