초록
<P><B>Abstract</B></P> <P>In-situ biomethanisation reduces the CO<SUB>2</SUB> in biogas to CH<SUB>4</SUB> via direct H<SUB>2</SUB> injection into an anaerobic digester, but volumetric methane production (VMP) is limited by organic loading. Ex-situ biomethanisation, where gaseous substrates are fed to pure or mixed cultures of hydrogenotrophic methanogens, offers higher VMP but requires an additional reactor and supply of essential nutrients. This work combined the two approaches in a novel hybrid application achieving simultaneous in-situ and ex-situ biomethanisation within an organically-loaded anaerobic digester receiving supplementary biogas. Conventional stirred-tank digesters were first acclimated to H<SUB>2</SUB> addition, increasing biogas methane content from 50% to 95% and VMP from 0.86 to 1.51 L L<SUP>−1</SUP> day<SUP>−1</SUP> at a moderate loading rate of 3 g organic chemical oxygen demand per L per day (g COD<SUB>org</SUB> L<SUP>−1</SUP> day<SUP>−1</SUP>). Externally-produced biogas was then added to demonstrate simultaneous biomethanisation of endogenous and imported CO<SUB>2</SUB>. This further increased VMP to 2.76 L L<SUP>−1</SUP> day<SUP>−1</SUP> without affecting organic substrate degradation. In-situ CO<SUB>2</SUB> reduction can alter digester pH by reducing bicarbonate buffering: the combined process operated stably at around pH 8.0 with 3–5% CO<SUB>2</SUB> in the headspace. Microbial community analysis indicated the process was mediated by bacterial syntrophic acetate oxidation and highly enriched hydrogenotrophic methanogenic archaea (up to 97% of the archaeal population). This approach presents the opportunity to retrofit a single digester for H<SUB>2</SUB> injection to convert and upgrade biogas from several others, minimising capital and operating costs by utilising both existing infrastructure and waste-derived feedstock nutrients for simultaneous biogas upgrading and power-to-methane.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Novel simultaneous in-situ and ex-situ CO<SUB>2</SUB> biomethanisation demonstrated. </LI> <LI> Biogas methane content over 95% achieved from conventional anaerobic digesters. </LI> <LI> Volumetric methane production increased by more than 3-fold of its original value. </LI> <LI> Upgrading biogas from multiple digesters in a single retrofitted digester. </LI> <LI> CO<SUB>2</SUB> biomethanisation coupled AD bridges biomass and renewable energy industries. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>