Simultaneous biomethanisation of endogenous and imported CO2 in organically loaded anaerobic digesters

Abstract

In-situ biomethanisation reduces the CO2 in biogas to CH4 via direct H2 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 organicallyloaded anaerobic digester receiving supplementary biogas. Conventional stirred-tank digesters were first acclimated to H2 addition, increasing biogas methane content from 50% to 95% and VMP from 0.86 to 1.51 L L-1 day-1 at a moderate loading rate of 3 g organic chemical oxygen demand per L per day (g CODorg L-1 day-1). Externally-produced biogas was then added to demonstrate simultaneous biomethanisation of endogenous and imported CO2. This further increased VMP to 2.76 L L-1 day-1 without affecting organic substrate degradation. In-situ CO2 reduction can alter digester pH by reducing bicarbonate buffering: the combined process operated stably at around pH 8.0 with 3-5% CO2 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

A Rapid, Sensitive, Low-Cost Assay for Detecting Hydrogenotrophic Methanogens in Anaerobic Digesters Using Loop-Mediated Isothermal Amplification

“…Although our positive control indicated assay specificity, we were unable to detect Methanococcus in the available salt marsh samples, possibly due to low microbial abundance resulting in DNA concentrations below the sensitivity of our LAMP assay. Consistent with 16S rRNA amplicon sequencing [5], Methanococcus was not detected by LAMP in mesophilic lab scale digesters (Figure 5c, lanes 4, 5). Using DNA extracted from stool samples from human volunteers, we confirmed the presence of Methanobrevibacter by LAMP.…”

“…Additionally, our LAMP assay results are consistent with 16S rRNA amplicon profiles obtained for these samples in our previous work. We showed that CO 2 reductive biomethanisation systems were dominated by Methanoculleus genus hydrogenotrophs and that the abundance of these species increased with a rise in H 2 addition to these systems [5].…”

“…To identify hydrogenotrophic methanogen sequences, we sampled and sequenced a lab scale anaerobic digester receiving a supplementary feed of hydrogen to promote in situ CO 2 biomethanisation [5]. The resulting metagenomic dataset of 225,144,076 reads was assembled using the pipeline described above into 136,759 contigs from which we identified 230,339 putative open reading frames.…”

“…Hydrogen-scavenging methanogens can partner with syntrophic acetate-oxidizing bacteria (SAO) to support stable methane production in AD under certain conditions [4]. The presence and relative abundance of hydrogenotrophs is of particular importance for the reductive biomethanisation of CO 2 from both inorganic and organic sources, e.g., as a means of in situ biogas upgrading [5] or for ex situ carbon capture and utilization [6]. Recent studies reported microbial community shifts from Methanoculleus and Methanoregulaceae species towards hydrogenotrophic methanogenesis when ex situ configurations were employed under thermophilic and mesophilic conditions, respectively [7].…”

Temperature and Inoculum Origin Influence the Performance of Ex-Situ Biological Hydrogen Methanation

“…The so-called "catalytic" methanation, based on the Sabatier reaction, is carried out using chemical catalysts such as nickel at high temperatures (250-450 • C) [6,7]. The biological methanation (also called biomethanation) is based on microorganisms, the conversion being operated at moderate temperatures and pressures • C and atmospheric pressure up to 10 bars, respectively) [8][9][10]. In addition, when performed on biogas issued from anaerobic digestion (AD) plants, biological methanation can be considered as a substitute of a CO 2 purifier through biogas upgrading [7,11].…”