Specific inhibitors for identifying pathways for methane production from carbon monoxide by a nonadapted anaerobic mixed culture

Carbon monoxide inhibition on acidogenic glucose fermentation and aceticlastic methanogenesis

Syngas and CO-rich off-gases are key chemical platforms to produce biofuels and bioproducts. From the perspective of optimizing and up-scaling CO co-digestion with organic waste streams, this study aims at assessing and quantifying the inhibitory effects of CO on acidogenic glucose fermentation and aceticlastic methanogenesis. Mesophilic cultures were fed in two sets of batch assays, respectively, with glucose and acetate while being exposed to dissolved CO in equilibrium with partial pressures in the range of 0.25-1.00 atm. Cumulative methane production and microbial monitoring revealed that aceticlastic methanogenic archaea were significantly inhibited (2-20 % of the methane production of CO non-exposed cultures). The acidogenic glucose degrading community was also inhibited by CO, although, thanks to its functional redundancy, shifted its metabolism towards propionate production. Future work should assess the sensitivity of hereby estimated CO inhibition parameters, e.g., on the simulation output of a continuous syngas co-digestion process with organic substrates.

Biological treatment of biowaste as an innovative source of CO-The role of composting process

Carbon monoxide (CO) is an essential "building block" for producing everyday chemicals on industrial scale. Carbon monoxide can also be generated though a lesser-known and sometimes forgotten biorenewable pathways that could be explored to advance biobased production from large and more sustainable sources such as bio-waste treatment. Organic matter decomposition can generate carbon monoxide both under aerobic and anaerobic conditions. While anaerobic carbon monoxide generation is relatively well understood, the aerobic is not. Yet many industrial-scale bioprocesses involve both conditions. This review summarizes the necessary basic biochemistry knowledge needed for realization of initial steps towards biobased carbon monoxide production. We analyzed for the first time, the complex information about carbon monoxide production during aerobic, anaerobic bio-waste treatment and storage, carbon monoxide-metabolizing microorganisms, pathways, and enzymes with bibliometric analysis of trends. The future directions recognizing limitations of combined composting and carbon monoxide production have been discussed in greater detail.

Mitigating methane emission from oil sands tailings using enzymatic and lime treatments

Engineering strategies to reduce greenhouse gases (GHGs) emissions by inhibiting methanogenesis in oil sands tailings have rarely been examined. In this study, we explored the potential impact of chemical treatment (lime) and biological treatment using enzymes (lysozyme and protease) on inhibiting methane emissions from tailings. Overall, treatment with protease 3%, lysozyme 3%, and lime 5000 ppm reduced CH₄ production (by 52%, 28%, and 25%, respectively) and were weakly associated with the archaeal abundance. Enzymes treatment resulted in a higher reduction in CH₄ production compared with lime treatment. A 3% lysozyme treatment suppressed CH₄ production (the change in methane was 0.48 mmol) and reduced the degradation of hexane throughout the experiment. Similarly, 3% protease suppressed CH₄ production throughout the experiment (the change in methane was 0.78 mmol), which could be attributed to the pH reduction to pH 4.9 at week 23 resulting from the formation of volatile fatty acids. Another possible mechanism could be the formation of toxic compounds, such as high nitrogen content, after protease treatment that inhibited the microbial community. The toxicity effect to Vibrio fischeri was greater with lysozyme 3% and protease 3% treatment than with lime treatment (124 TU and 76 TU, respectively). Lime treatment resulted in the highest reduction in 16S rRNA gene copies from 5.7 × 10⁶ cells g^-1 (control) to 2.7 × 10⁵, 1.71 × 10⁵, and 1.4 × 10⁵ cells g^-1 for 1600, 3500, and 5000 ppm treatments, respectively. This study supports further work to examine and determine the optimum conditions (e.g., enzyme and lime dosages) for CH₄ inhibition.

A comprehensive review on current status and future perspectives of microbial volatile fatty acids production as platform chemicals

Volatile fatty acids (VFA), the secondary metabolite of microbial fermentation, are used in a wide range of industries for production of commercially valuable chemicals. In this review, the fermentative production of VFAs by both pure as well mixed microbial cultures is highlighted along with the strategies for enhancing the VFA production through innovations in existing approaches. Role of conventionally applied tools for the optimization of operational parameters such as pH, temperature, retention time, organic loading rate, and headspace pressure has been discussed. Furthermore, a comparative assessment of above strategies on VFA production has been done with alternate developments such as co-fermentation, substrate pre-treatment, and in situ removal from fermented broth. The review also highlights the applications of different bioreactor geometries in the optimum production of VFAs and how metagenomic tools could provide a detailed insight into the microbial communities and their functional attributes that could be subjected to metabolic engineering for the efficient production of VFAs.

Biomethanation of syngas at high CO concentration in a continuous mode

Syngas from pyrolysis/gasification process is a mixture of CO, CO₂ and H₂, which could be converted to CH₄, so called syngas biomethanation. Its development is obstructed due to the low productivity and CO inhibition. The aim of this study was to demonstrate the feasibility of using syngas as the only carbon source containing high CO concentration (40%) for biomethanation. Lab-scale thermophilic bioreactor inoculated with anaerobic sludge was operated continuously for over 900 h and the shift of microbial structure were investigated. Results showed that thermophilic condition was suitable for syngas biomethanation and the microbes could adapt to high CO concentration. Higher processing capacity of 12.6 m³/m³/d was found and volumetric methane yield of 2.97 m³/m³/d was observed. These findings could strengthen the theoretical basis of syngas biomethanation and support its industrialization in the future.

Carbon monoxide conversion and syngas biomethanation mediated by different microbial consortia

Syngas biomethanation is an attractive process for extending application of gasification products. In the present study, anaerobic sludges from three methanogenic reactors feeding cattle manure (CS), sewage sludge (SS) and gaseous H₂/CO₂ (GS) were used to investigate the effect of microbial consortia composition on syngas biomethanation. The results showed that CS presented the highest CO consumption rate due to its highest relative abundance of CO consuming bacteria. The CO was mainly converted to acetate, and syntrophic acetate oxidization (SAO) bacteria converted acetate to H₂/CO₂ for hydrogenotrophic methanogenesis in CS and SS. However, acetate was accumulated in GS for lacking acetoclastic methanogens and SAO bacteria, leading to lower biomethanation efficiency. Additionally, adding stoichiometric H₂ could convert CO and CO₂ to nearly pure methane, while, the CO consumption rate declined in H₂ added systems. The results present novel insights into microbial consortia on CO conversion and syngas biomethanation.

The Biotic and Abiotic Carbon Monoxide Formation During Aerobic Co-digestion of Dairy Cattle Manure With Green Waste and Sawdust

Carbon monoxide (CO), an air pollutant and a toxic gas to humans, can be generated during aerobic digestion of organic waste. CO is produced due to thermochemical processes, and also produced or consumed by cohorts of methanogenic, acetogenic, or sulfate-reducing bacteria. The exact mechanisms of biotic and abiotic formation of CO in aerobic digestion (particularly the effects of process temperature) are still not known. This study aimed to determine the temporal variation in CO concentrations during the aerobic digestion as a function of process temperature and activity of microorganisms. All experiments were conducted in controlled temperature reactors using homogeneous materials. The lab-scale tests with sterilized and non-sterilized mix of green waste, dairy cattle manure, sawdust (1:1:1 mass ratio) were carried out for 1 week at 10, 25, 30, 37, 40, 50, 60, 70°C to elucidate the biotic vs. abiotic effect. Gas concentrations of CO, O₂, and CO₂ inside the reactor were measured every 12 h. The CO concentrations observed for up to 30°C did not exceed 100 ppm v/v. For 50 and 60°C, significantly (p < 0.05) higher CO concentrations, reaching almost 600 ppm v/v, were observed. The regression analyses showed in both cases (sterile and non-sterile) a statistically significant effect (p < 0.05) of temperature on CO concentration, confirming that the increase in temperature causes an increase in CO concentration. The remaining factors (time, O₂, and CO₂ content) were not statistically significant (p > 0.05). A new polynomial model describing the effect of temperature, O₂, and CO₂ concentration on CO production during aerobic digestion of organic waste was formulated. It has been found that the proposed model for sterile variant had a better fit (R² = 0.86) compared with non-sterile (R² = 0.71). The model predicts CO emissions and could be considered for composting process optimization. The developed model could be further developed and useful for ambient air quality and occupational exposure to CO.

A novel concept for syngas biomethanation by two-stage process: Focusing on the selective conversion of syngas to acetate

Thermal gasification of nonrenewable and renewable sources produces syngas, containing CO, H₂, CO₂ and N₂. Anaerobic conversion of syngas to CH₄ is a promising way to replace natural gas. However, the high N₂ content (>50%) in syngas would result in the low CH₄ content in the biogas and CO in syngas also had serious inhibition on methanogens. The present study proposed a two-stage anaerobic process for syngas biomethanation, and syngas was first anaerobically converted to acetate by mixed culture, which could be further converted to methane easily without the negative effects of N₂ and CO. The results showed that mesophilic condition was more suitable for the conversion of syngas to acetate compared to thermophilic and ambient conditions at pH 5.5 considering the higher acetate yield and syngas conversion rate. Although CO was efficiently converted at thermophilic condition, it was mostly converted to H₂, which was then converted to acetate. CO was much easier to be converted compared to H₂. Further study showed that pH 6.5 and 7.5 were optimal for selective conversion of syngas to acetate. The other products including butyrate and ethanol were also detected in relatively higher amounts at pH 4.5 and 9.5. Although pH 5.5 and 8.5 had relatively lower syngas conversion rates compared to pH 6.5 and 7.5, they might inhibit methanogens naturally without adding methane inhibitors. Finally, batch experiments showed that the acetate concentration had obvious inhibition on syngas conversion when the acetate concentration was higher than 2 g/L.

A novel process for volatile fatty acids production from syngas by integrating with mesophilic alkaline fermentation of waste activated sludge

The present study proposed and demonstrated a novel process for the bioconversion of syngas (mainly CO and H₂) to valuable volatile fatty acids (VFA) by integrating with mesophilic alkaline fermentation of waste activated sludge (WAS). The results showed that although pH 9 was suitable for VFA production from WAS, 62.5% of the consumed CO was converted to methane due to the presence of hydrogenogenic pathway for CO conversion. The increase of pH from 9 to 9.5 inhibited the methane production from CO because of the possible presence of only acetogenic pathway for CO conversion. However, methane was still produced from H₂ contained in syngas through hydrogenotrophic methanogenesis, and around 32-34% of the consumed syngas was converted to methane. At both pH 9 and 9.5, methane was produced by hydrogenotrophic methanogens Methanobacteriales. Further increase of pH to 10 effectively inhibited methane production from syngas, and efficient VFA (mainly acetate with the concentration of around 135 mM) production by simultaneous conversion of syngas and WAS was achieved. High acetate concentrations (>150 mM) were shown to have serious negative effects on the conversion of syngas. The addition of syngas to the mesophilic alkaline fermentation of WAS at pH 10 not only resulted in the enrichment of some known bacteria related with syngas conversion, but also changed the microbial community compositions for the fermentation of WAS.

Biomethanation of blast furnace gas using anaerobic granular sludge via addition of hydrogen

The high concentrations of CO (toxic) and CO2 (greenhouse gases) in blast furnace gas (a by-product of steelworks) reflect its low calorific value. In this study, anaerobic granular sludge was used to convert carbon from blast furnace gas to methane via exogenous hydrogen addition. The inhibition of methane production by CO partial pressure (P CO) was found to start from 0.4 atm. The intermediate metabolites from CO to methane including acetate, propionate, and H2 accumulated at higher CO concentrations in the presence of 2-bromoethanesulfonic acid. After the introduction of H2 and blast furnace gas, although the hydrogen partial pressure (P H2 ) up to 1.54 atm resulted in the maximum CH4 yield, the whole system was not stable due to the accumulation of a large amount of volatile fatty acids. The optimum P H2 on CH4 production from the simulated blast furnace gas, 5.32 mmol g-1 VSS, was determined at 0.88 atm in this study.

Anaerobic granular sludge for simultaneous biomethanation of synthetic wastewater and CO with focus on the identification of CO-converting microorganisms

CO is a main component of syngas, which can be produced from the gasification of organic wastes and biomass. CO can be converted to methane by anaerobic digestion (AD), however, it is still challenging due to its toxicity to microorganisms and limited knowledge about CO converting microorganisms. In the present study, anaerobic granular sludge (AGS) was used for the simultaneous biomethanation of wastewater and CO. Batch experiments showed that AGS tolerated CO partial pressure as high as 0.5 atm without affecting its ability for synthetic wastewater degradation, which had higher tolerance of CO compared to suspended sludge (less than 0.25 atm) as previously reported. Continuous experiments in upflow anaerobic sludge blanket (UASB) reactors showed AGS could efficiently convert synthetic wastewater and CO into methane by applying gas-recirculation. The addition of CO to UASB reactor enhanced the hydrogenotrophic CO-oxidizing pathway, resulted in the increase of extracellular polymeric substances, changed the morphology of AGS and significantly altered the microbial community compositions of AGS. The microbial species relating with CO conversion and their functions were revealed by metagenomic analysis. It showed that 23 of the 70 reconstructed genome bins (GBs), most of which were not previously characterized at genomic level, were enriched and contained genes involved in CO conversion upon CO addition. CO-converting microorganisms might be taxonomically more diverse than previously known and have multi-functions in the AD process. The reductive tricarboxylic acid (TCA) cycle in combination with the oxidation of the CO was probably crucial for CO utilization by the majority of the GBs in the present study.

Biomethanation of Syngas Using Anaerobic Sludge: Shift in the Catabolic Routes with the CO Partial Pressure Increase

Syngas generated by thermal gasification of biomass or coal can be steam reformed and purified into methane, which could be used locally for energy needs, or re-injected in the natural gas grid. As an alternative to chemical catalysis, the main components of the syngas (CO, CO2, and H2) can be used as substrates by a wide range of microorganisms, to be converted into gas biofuels, including methane. This study evaluates the carboxydotrophic (CO-consuming) methanogenic potential present in an anaerobic sludge from an upflow anaerobic sludge bed (UASB) reactor treating waste water, and elucidates the CO conversion routes to methane at 35 ± 3°C. Kinetic activity tests under CO at partial pressures (pCO) varying from 0.1 to 1.5 atm (0.09-1.31 mmol/L in the liquid phase) showed a significant carboxydotrophic activity potential for growing conditions on CO alone. A maximum methanogenic activity of 1 mmol CH4 per g of volatile suspended solid and per day was achieved at 0.2 atm of CO (0.17 mmol/L), and then the rate decreased with the amount of CO supplied. The intermediary metabolites such as acetate, H2, and propionate started to accumulate at higher CO concentrations. Inhibition experiments with 2-bromoethanesulfonic acid (BES), fluoroacetate, and vancomycin showed that in a mixed culture CO was converted mainly to acetate by acetogenic bacteria, which was further transformed to methane by acetoclastic methanogens, while direct methanogenic CO conversion was negligible. Methanogenesis was totally blocked at high pCO in the bottles (≥1 atm). However it was possible to achieve higher methanogenic potential under a 100% CO atmosphere after acclimation of the sludge to CO. This adaptation to high CO concentrations led to a shift in the archaeal population, then dominated by hydrogen-utilizing methanogens, which were able to take over acetoclastic methanogens, while syntrophic acetate oxidizing (SAO) bacteria oxidized acetate into CO2 and H2. The disaggregation of the granular sludge showed a negative impact on their methanogenic activity, confirming that the acetoclastic methanogens were the most sensitive to CO, and a contrario, the advantage of using granular sludge for further development toward large-scale methane production from CO-rich syngas.

Anaerobic microbial community response to methanogenic inhibitors 2-bromoethanesulfonate and propynoic acid

Methanogenic inhibitors are often used to study methanogenesis in complex microbial communities or inhibit methanogens in the gastrointestinal tract of livestock. However, the resulting structural and functional changes in archaeal and bacterial communities are poorly understood. We characterized microbial community structure and activity in mesocosms seeded with cow dung and municipal wastewater treatment plant anaerobic digester sludge after exposure to two methanogenic inhibitors, 2‐bromoethanesulfonate (BES) and propynoic acid (PA). Methane production was reduced by 89% (0.5 mmol/L BES), 100% (10 mmol/LBES), 24% (0.1 mmol/LPA), and 95% (10 mmol/LPA). Using modified primers targeting the methyl‐coenzyme M reductase (mcrA) gene, changes in mcrA gene expression were found to correspond with changes in methane production and the relative activity of methanogens. Methanogenic activity was determined by the relative abundance of methanogen 16S rRNA cDNA as a percentage of the total community 16S rRNA cDNA. Overall, methanogenic activity was lower when mesocosms were exposed to higher concentrations of both inhibitors, and aceticlastic methanogens were inhibited to a greater extent than hydrogenotrophic methanogens. Syntrophic bacterial activity, measured by 16S rRNA cDNA, was also reduced following exposure to both inhibitors, but the overall structure of the active bacterial community was not significantly affected.