Two-Phase improves Bio-hydrogen and Bio-methane production of anaerobic membrane bioreactor from waste activated sludge with digestate recirculation

The conventional recirculated two-phase anaerobic digestion (R-TPAD) equipped with two continuous stirred tank reactors (CSTR) usually shows poor water quality, insufficient bioconversion degree and low methane productivity. In this study, a new R-TPAD with CSTR as acidogenic phase and anaerobic membrane reactor (AnMBR) as methanogenic phase was constructed to digest waste activated sludge (WAS) for renewable energy recovery (H₂ and CH₄). The effects of recirculation ratio (R) on start-up time, long-term performance, membrane fouling behaviors, microbial community evolution, and energy recovery efficiencies were investigated systematically. Result showed that the R-TPAD possessed high H₂ (6.1 mL/g-COD) and CH₄ (67.9 mL/g-COD) yields with the corresponding COD removal of up to 96.1% at R_0.2. During the whole process, the transmembrane pressure was lower than −3.3 kPa with negligible membrane fouling observed. The 16S rRNA gene analysis revealed that the synergistic role of membrane separation and recirculation system promoted the proliferation of fatty acids-producing and organics-degrading bacteria (i.e. Coprothermobacteraeota, Bacteroidetes, etc.) in acidogenic phase and CH₄-producing methanogens (i.e. Methanobacterium, Methanosaeta, etc.) in methanogenic phase, thus accelerating the biodegradation of WAS. This positive stimulation sustained the high diversity and the dynamic balance of key microorganisms in the R-TPAD process. Energy balance analysis further indicated that the R-TPAD with R_0.2 could increase net energy output by 77.9% compared to the control TPAD without recirculation. This work provides a fundamental data base for the scientific management of WAS and push forward the real-world application of R-TPAD technology in the real-world scenarios.