Formation and characteristics of a ternary pH buffer system for in-situ biogas upgrading in two-phase anaerobic membrane bioreactor treating starch wastewater.

Graphical abstract Highlights • Higher average pH seized over 6% more CO 2 in stage-II. • Most CO 2 ·H 2 O converts to HCO 3 − and CO 3 2− when pH > 7.9. • Lack of CO 2 ·H 2 O limited the hydrogenotrophic methanogenesis. • Ammonia is not only an inhibitor but also a pH buffer capacity contributor. • Endogenous VFAs regulated pH cycle in stage-II. Abstract Biochemical biogas upgrading retaining more CO 2 from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO 2 ·H 2 O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO 2 in the biogas into CH 4 , via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml·h−1. Low pH (7.37–7.80) decrease CO 2 ·H 2 O as a substrate in stage-I, higher pH (7.40–8.41) enhances CO 2 to CO 2 ·H 2 O transfer by 6.29 ± 2.20% in stage-II. Because of that 95% CO 2 ·H 2 O converts to HCO 3 − and CO 3 2− when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH 4 was upgraded from 67.27 ± 5.21% to 73.56 ± 5.01%. [ABSTRACT FROM AUTHOR]