Selective syngas fermentation to acetate under acidic and psychrophilic conditions using mixed anaerobic culture.

Syngas fermentation to acetate offers a promising solution for its valorisation, particularly when syngas contains a high N2 concentration, which otherwise impedes the utilisation of syngas biomethanation gaseous product in cogeneration or upgrading units. In this study, continuous lab-scale syngas fermentation assessing the effects of acidic pH and psychrophilic conditions (28 °C and 20 °C) on bioconversion efficiency and anaerobic consortium diversity was studied. The results showed that as temperature and pH decrease, acetate yield increases. The highest H2 and CO consumption rates were observed at 20 °C and pH 4.5, reaching 48.4 mmol/(L·d) and 31.5 mmol/(L·d), respectively, and methanogenic activity was not completely suppressed. The microbial community composition indicated an enhanced abundance of acetate-producing bacteria and hydrogenotrophic methanogens at 28 °C. The PICRUSt2 prediction of metabolic potential indicated that temperature and pH changes appear to have a more pronounced impact on acetotrophic methanogenesis genes than carbon dioxide-based methanogenesis genes.

Biological conversion of carbon monoxide and hydrogen by anaerobic culture: Prospect of anaerobic digestion and thermochemical processes combination.

Waste biomass is considered a promising renewable energy feedstock that can be converted by anaerobic digestion. However, anaerobic digestion application can be challenging due to the structural complexity of several waste biomass kinds. Therefore, coupling anaerobic digestion with thermochemical processes can offset the limitations and convert the hardly biodegradable waste biomass, including digestate residue, into value-added products: syngas and pyrogas (gaseous mixtures consisting mainly of H2, CO, CO2), bio-oil, and biochar for further valorisation. In this review, the utilisation boundaries and benefits of the aforementioned products by anaerobic culture are discussed. First, thermochemical process parameters for an enhanced yield of desired products are summarised. Particularly, the microbiology of CO and H2 mixture biomethanation and fermentation in anaerobic digestion is presented. Finally, the state-of-the-art biological conversion of syngas and pyrogas to CH4 mediated by anaerobic culture is adequately described. Extensive research shows the successful selective biological conversion of CO and H2 to CH4, acetic acid, and alcohols. The main bottleneck is the gas-liquid mass transfer which can be enhanced appropriately by bioreactors' configurations. A few research groups focus on bio-oil and biochar addition into anaerobic digesters. However, according to the literature review, there has been no research for utilising all value-added products at once in anaerobic digestion published so far. Although synergic effects of such can be expected. In summary, the combination of anaerobic digestion and thermochemical processes is a promising alternative for wide-scale waste biomass utilisation in practice.

A novel two-stage process for biological conversion of syngas to biomethane.

This study shows biomethane production in a novel two-stage syngas biomethanation consisting of the thermophilic anaerobic digestion of sewage sludge combined with an injection of syngas as the first stage. Since the syngas does not contain enough reducing equivalents, the produced biogas was connected to the second stage consisting of the trickle-bed reactor with the external H2 addition (ex-situ) to increase the CH4 content further. The aim was to evaluate the influence of different syngas compositions on the biomethane production in both stages. The results showed that H2 concentration in syngas is the main factor affecting the CH4 content in biogas. Moreover, the ex-situ reactor with H2 addition served to convert the residual CO and CO2, achieving the maximum CH4 content of 94.7% in the produced biomethane. In summary, the two-stage process enables biomethane production without any inhibitory effects on anaerobic sludge digestion.

Bioconversion of carbon dioxide to methane using hydrogen and hydrogenotrophic methanogens.

Biogas produced from organic wastes contains energetically usable methane and unavoidable amount of carbon dioxide. The exploitation of whole biogas energy is locally limited and utilization of the natural gas transport system requires CO2 removal or its conversion to methane. The biological conversion of CO2 and hydrogen to methane is well known reaction without the demand of high pressure and temperature and is carried out by hydrogenotrophic methanogens. Reducing equivalents to the biotransformation of carbon dioxide from biogas or other resources to biomethane can be supplied by external hydrogen. Discontinuous electricity production from wind and solar energy combined with fluctuating utilization cause serious storage problems that can be solved by power-to-gas strategy representing the production of storable hydrogen via the electrolysis of water. The possibility of subsequent repowering of the energy of hydrogen to the easily utilizable and transportable form is a biological conversion with CO2 to biomethane. Biomethanization of CO2 can take place directly in anaerobic digesters fed with organic substrates or in separate bioreactors. The major bottleneck in the process is gas-liquid mass transfer of H2 and the method of the effective input of hydrogen into the system. There are many studies with different bioreactors arrangements and a way of enrichment of hydrogenotrophic methanogens, but the system still has to be optimized for a higher efficiency. The aim of the paper is to gather and critically assess the state of a research and experience from laboratory, pilot and operational applications of carbon dioxide bioconversion and highlight further perspective fields of research.

Sulfur-oxidizing bacteria in environmental technology.

Hydrogen sulfide is widely known as the most undesirable component of biogas that caused not only serious sensoric and toxic problems, but also corrosion of concrete and steel structures. Many agricultural and industrial waste used in biogas production, may contain a large amount of substances that serve as direct precursors to the formation of sulfide sulfur-sources of hydrogen sulfide in the biogas. Biological desulfurization methods are currently promoted to abiotic methods because they are less expensive and do not produce undesirable materials which must be disposed of. The final products of oxidation of sulfides are no longer hazardous. Biological removal of sulfide from a liquid or gaseous phase is based on the activity of sulfur-oxidizing bacteria. They need an oxidizing agent such as an acceptor of electrons released during the oxidation of sulfides-atmospheric oxygen or oxidized forms of nitrogen. Different genera of sulfur-oxidizing bacteria and their technological application are discussed.

Effect of S/N ratio on sulfide removal by autotrophic denitrification.

In this study a completely stirred tank reactor was used to study the effect of sulfide to nitrate (S/N) ratio on sulfide removal while nitrate was used as electron acceptor. Several S/N ratios were studied for this purpose ranging from 0.3 to 2.4 mol/mol. The complete sulfide removal was achieved when S/N ratio 0.85 mol/mol was used with the autotrophic denitrification efficiency up to 80 %. No nitrite accumulation was observed, and the main product of sulfide oxidation was sulfate. Dissimilatory nitrogen reduction to ammonia occurred and subsequently, elemental sulfur accumulated while S/N ratio was higher than 1.3 mol/mol. The specific autotrophic denitrification rates under S/N ratios 0.8 and 1.2 were 5 and 26 mg g(-1) h(-1) (N-NO3 (-), VSS), respectively. Thiobacillus denitrificans and Thiomicrospira denitrificans were detected in the reactor by fluorescent in situ hybridization, but their overall representation was not more than 5 % of the entire microbial populations.