Pushing the organic loading rate in electrochemically assisted anaerobic digestion of blackwater at ambient temperature: Insights into microbial community dynamics

Highlights: • Microbial electrolysis cell assisted anaerobic digester operated with blackwater. • Stable long-term operation at ambient temperature was achieved. • Methane yield was enhanced by 46% at 3.0 g COD/L-d with applied potential. • Core microbiome and partnerships were identified on the electroactive biofilms. • Applied potential increased the archaeal abundance on the cathode. Decentralized blackwater treatment by anaerobic digestion is being considered as a sustainable sanitation concept. However, the low biodegradability and complex composition restrictedly limited the treatability of blackwater, resulting in requirements of low operational organic loading rates (OLRs). In this study, a microbial electrolysis cell assisted anaerobic digester (MEC-AD) treating vacuum toilet blackwater was successfully operated for 420 days at OLRs ranging from 0.77 to 3.03 g COD/L-d in 6 stages (including an open-circuit Stage 5) at ambient temperature. Based on the steady-state results from different stages, the highest methane yield (42.4% out of 45% biochemical methane potential value) was achieved in Stage 1 with an OLR of 0.77 g COD/L-d. At the same OLR of ~3.0 g COD/L-d, Stage 4 (32.4%) and Stage 6 (35.2%) showed significantly higher methane yield (p < 0.01) than open-circuit Stage 5 (24.1%). The lowest COD removal efficiency of 31.8% was observed in Stage 5 with short-chain volatile fatty acids (SCVFAs) accumulated to ~1000 mg/L, which was more than double the values of Stage 4 and 6. The microbial community analysis revealed that the applied potential did not significantly affect archaeal diversity but largely increased the archaeal abundance on the cathode, and led the bacterial community shift with the enrichment of specific electroactive bacteria. Microbial co-occurrence network analysis further confirmed the positive correlations between known electroactive bacteria and electrotrophic methanogens. Moreover, electric energy consumed by the MEC-AD system was fully recovered as biomethane.