Enhanced sludge digestion using anaerobic dynamic membrane bioreactor: Effects of hydraulic retention time

Performance of conventional anaerobic digestion is limited due to coupling of solid retention time (SRT) and hydraulic retention time (HRT). In this study, the sludge digestion efficiency was enhanced by decoupling SRT and HRT in a pilot-scale anaerobic dynamic membrane bioreactor, with a particular focus on the effects of HRT. With HRT decreased to 10 d, the total solids concentration reached 43.3 g/L in the membrane tank with the influent sludge concentration 23.4 +/- 0.7 g/L, while volatile solids (VS) reduction rate and specific biogas pro-duction were 41% and 0.59 L/g VS, respectively, achieving simultaneous thickening and digestion of sludge. Microbial community analysis showed that Cloroflexi and Spirochaetota, which had an increased abundance at low HRT, were beneficial to VS reduction and multiple methanogenesis pathways. Methanosaeta, being a strict acetoclastic methanogen dominant at HRT 30 d, decreased in the abundance from 78.4% to 50.7% with HRT changing from 30 d to 10 d. In contrast, the abundance of hydrogenotrophic and methylotrophic methanogens increased from 17.2% to 39.4% with the decrease of HRT, indicating the influence of HRT on methanogenic pathways. Furthermore, the predicted gene analysis confirmed the transformation from dominant acetoclastic methanogenesis to the coexistence of multiple methanogenesis pathways. Finally, the recovered energy from methane increased from 0.43 kWh/kg VS to 0.56 kWh/kg VS as HRT decreased to 10 d from 30 d, and the net energy consumption at HRT of 10 d was 0.47 kWh/kg VS, only 66% of that at HRT of 30 d.

Automated summary

The performance of anaerobic digestion can be improved by decoupling solid retention time and hydraulic retention time. This study focused on the effects of hydraulic retention time and found that reducing the time led to increased solids concentration, improved microbial communities, and enhanced energy recovery from methane.