Methanosarcina: the rediscovered methanogen for heavy duty biomethanation

Characterisation of microbial communities for improved management of anaerobic digestion of food waste

Anaerobic digestion of food waste is an attractive and increasingly popular technology within waste management and energy recovery. A better understanding of the microbiology associated with anaerobic digestion of food waste will provide new insight into the operational conditions required for optimizing this process, as well as its potential for utilisation in co-digestion systems. Eighteen full-scale reactors processing varying proportions of food waste under diverse operational configurations were subjected to microbial community analysis by amplicon sequencing of the 16S rRNA and mcrA genes to capture the bacterial and methanogenic populations. Statistical correlations between microbial populations, plant design and operating conditions revealed that the microbial communities were shaped by operational parameters such as the primary substrate type and operational temperature, while the methanogenic communities showed a more reactor specific distribution. The distribution of microbes based on the waste processed in the surveyed digesters was explored, as well as the presence of specialist populations such as syntrophs and methanogens. Food waste digester communities were not associated with a strong microbial fingerprint compared to other waste types (wastewater and manure) but contained greater abundance and unique syntrophic acetate oxidising populations, suggesting that co-digestion with food waste may improve the functional diversity of anaerobic digesters.

Waste to Energy: A Focus on the Impact of Substrate Type in Biogas Production

Anaerobic digestion is an efficient technology for a sustainable conversion of various organic wastes such as animal manure, municipal solid waste, agricultural residues and industrial waste into biogas. This technology offers a unique set of benefits, some of which include a good waste management technique, enhancement in the ecology of rural areas, improvement in health through a decrease of pathogens and optimization of the energy consumption of communities. The biogas produced through anaerobic digestion varies in composition, but it consists mainly of carbon dioxide methane together with a low quantity of trace gases. The variation in biogas composition are dependent on some factors namely the substrate type being digested, pH, operating temperature, organic loading rate, hydraulic retention time and digester design. However, the type of substrate used is of greater interest due to the direct dependency of microorganism activities on the nutritional composition of the substrate. Therefore, the aim of this review study is to provide a detailed analysis of the various types of organic wastes that have been used as a substrate for the sustainable production of biogas. Biogas formation from various substrates reported in the literature were investigated, an analysis and characterization of these substrates provided the pro and cons associated with each substrate. The findings obtained showed that the methane yield for all animal manure varied from 157 to 500 mL/gVS with goat and pig manure superseding the other animal manure whereas lignocellulose biomass varied from 160 to 212 mL/gVS. In addition, organic municipal solid waste and industrial waste showed methane yield in the ranges of 143–516 mL/gVS and 25–429 mL/gVS respectively. These variations in methane yield are primarily attributed to the nutritional composition of the various substrates.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.

Gas ebullition from petroleum hydrocarbons in aquatic sediments: A review

Gas ebullition in sediment results from biogenic gas production by mixtures of bacteria and archaea. It often occurs in organic-rich sediments that have been impacted by petroleum hydrocarbon (PHC) and other anthropogenic pollution. Ebullition occurs under a relatively narrow set of biological, chemical, and sediment geomechanical conditions. This process occurs in three phases: I) biogenic production of primarily methane and dissolved phase transport of the gases in the pore water to a bubble nucleation site, II) bubble growth and sediment fracture, and III) bubble rise to the surface. The rate of biogenic gas production in phase I and the resistance of the sediment to gas fracture in phase II play the most significant roles in ebullition kinetics. What is less understood is the role that substrate structure plays in the rate of methanogenesis that drives gas ebullition. It is well established that methanogens have a very restricted set of compounds that can serve as substrates, so any complex organic molecule must first be broken down to fermentable compounds. Given that most ebullition-active sediments are completely anaerobic, the well-known difficulty in degrading PHCs under anaerobic conditions suggests potential limitations on PHC-derived gas ebullition. To date, there are no studies that conclusively demonstrate that weathered PHCs can alone drive gas ebullition. This review consists of an overview of the factors affecting gas ebullition and the biochemistry of anaerobic PHC biodegradation and methanogenesis in sediment systems. We next compile results from the scholarly literature on PHCs serving as a source of methanogenesis. We combine these results to assess the potential for PHC-driven gas ebullition using energetics, kinetics, and sediment geomechanics analyses. The results suggest that short chain <C₁₀ alkanes are the only PHC class that alone may have the potential to drive ebullition, and that PHC-derived methanogenesis likely plays a minor part in driving gas ebullition in contaminated sediments compared to natural organic matter.

Methanogenic pathway and microbial succession during start-up and stabilization of thermophilic food waste anaerobic digestion with biochar

One of the major obstacles for thermophilic anaerobic digestion is the process instability during start-up. This study proposed the use of a cost-effective additive, biochar, to accelerate and stabilize the start-up of thermophilic semi-continuous food waste anaerobic digestion. The results showed that the reactors with biochar addition resulted in up to 18% higher methane yield as compared to the control reactors (without biochar). The key microbial networks were elucidated through thermochemical and microbial analysis. Particularly, the addition of biochar promoted the growth of electroactive Clostridia and other electroactive bacteria, while the absence of biochar promoted the growth of homoacetogenic Clostridia and syntrophic acetate oxidizing bacteria. It was revealed that biochar promoted direct interspecies electron transfer between the microbes and was responsible for the faster degradation of volatile fatty acids. Furthermore, reactors with biochar also enhanced the thermodynamically favourable acetoclastic methanogenic pathway due to the higher abundance of Methanosarcina.

Regeneration of unconventional natural gas by methanogens co-existing with sulfate-reducing prokaryotes in deep shale wells in China

Biogenic methane in shallow shale reservoirs has been proven to contribute to economic recovery of unconventional natural gas. However, whether the microbes inhabiting the deeper shale reservoirs at an average depth of 4.1 km and even co-occurring with sulfate-reducing prokaryote (SRP) have the potential to produce biomethane is still unclear. Stable isotopic technique with culture-dependent and independent approaches were employed to investigate the microbial and functional diversity related to methanogenic pathways and explore the relationship between SRP and methanogens in the shales in the Sichuan Basin, China. Although stable isotopic ratios of the gas implied a thermogenic origin for methane, the decreased trend of stable carbon and hydrogen isotope value provided clues for increasing microbial activities along with sustained gas production in these wells. These deep shale-gas wells harbored high abundance of methanogens (17.2%) with ability of utilizing various substrates for methanogenesis, which co-existed with SRP (6.7%). All genes required for performing methylotrophic, hydrogenotrophic and acetoclastic methanogenesis were present. Methane production experiments of produced water, with and without additional available substrates for methanogens, further confirmed biomethane production via all three methanogenic pathways. Statistical analysis and incubation tests revealed the partnership between SRP and methanogens under in situ sulfate concentration (~ 9 mg/L). These results suggest that biomethane could be produced with more flexible stimulation strategies for unconventional natural gas recovery even at the higher depths and at the presence of SRP.

Diversity and Abundance of Microbial Communities in UASB Reactors during Methane Production from Hydrolyzed Wheat Straw and Lucerne

The use of straw for biofuel production is encouraged by the European Union. A previous study showed the feasibility of producing biomethane in upflow anaerobic sludge blanket (UASB) reactors using hydrolyzed, steam-pretreated wheat straw, before and after dark fermentation with Caldicellulosiruptor saccharolyticus, and lucerne. This study provides information on overall microbial community development in those UASB processes and changes related to acidification. The bacterial and archaeal community in granular samples was analyzed using high-throughput amplicon sequencing. Anaerobic digestion model no. 1 (ADM1) was used to predict the abundance of microbial functional groups. The sequencing results showed decreased richness and diversity in the microbial community, and decreased relative abundance of bacteria in relation to archaea, after process acidification. Canonical correspondence analysis showed significant negative correlations between the concentration of organic acids and three phyla, and positive correlations with seven phyla. Organic loading rate and total COD fed also showed significant correlations with microbial community structure, which changed over time. ADM1 predicted a decrease in acetate degraders after a decrease to pH ≤ 6.5. Acidification had a sustained effect on the microbial community and process performance.

Biomethanation processes: new insights on the effect of a high H2 partial pressure on microbial communities

BACKGROUND

Biomethanation is a promising solution to upgrade the CH₄ content in biogas. This process consists in the injection of H₂ into an anaerobic digester, using the capacity of indigenous hydrogenotrophic methanogens for converting the injected H₂ and the CO₂ generated from the anaerobic digestion process into CH₄. However, the injection of H₂ could cause process disturbances by impacting the microbial communities of the anaerobic digester. Better understanding on how the indigenous microbial community can adapt to high H₂ partial pressures is therefore required.

RESULTS

Seven microbial inocula issued from industrial bioprocesses treating different types of waste were exposed to a high H₂ partial pressure in semi-continuous reactors. After 12 days of operation, even though both CH₄ and volatile fatty acids (VFA) were produced as end products, one of them was the main product. Acetate was the most abundant VFA, representing up to 94% of the total VFA production. VFA accumulation strongly anti-correlated with CH₄ production according to the source of inoculum. Three clusters of inocula were distinguished: (1) inocula leading to CH₄ production, (2) inocula leading to the production of methane and VFA in a low proportion, and (3) inocula leading to the accumulation of mostly VFA, mainly acetate. Interestingly, VFA accumulation was highly correlated to a low proportion of archaea in the inocula, a higher amount of homoacetogens than hydrogenotrophic methanogens and, the absence or the very low abundance in members from the Methanosarcinales order. The best methanogenic performances were obtained when hydrogenotrophic methanogens and Methanosarcina sp. co-dominated all along the operation.

CONCLUSIONS

New insights on the microbial community response to high H₂ partial pressure are provided in this work. H₂ injection in semi-continuous reactors showed a significant impact on microbial communities and their associated metabolic patterns. Hydrogenotrophic methanogens, Methanobacterium sp. or Methanoculleus sp. were highly selected in the reactors, but the presence of co-dominant Methanosarcinales related species were required to produce higher amounts of CH₄ than VFA.

Novel Syntrophic Isovalerate-Degrading Bacteria and Their Energetic Cooperation with Methanogens in Methanogenic Chemostats

Isovalerate is an important intermediate in anaerobic degradation of proteins/amino acids. Little is known about how this compound is degraded due to challenges in cultivation and characterization of isovalerate-degrading bacteria, which are thought to symbiotically depend on methanogenic archaea. In this study, we successfully enriched novel syntrophic isovalerate degraders (uncultivated Clostridiales and Syntrophaceae members) through operation of mesophilic and thermophilic isovalerate-fed anaerobic reactors. Metagenomics- and metatranscriptomics-based metabolic reconstruction of novel putative syntrophic isovalerate metabolizers uncovered the catabolic pathway and byproducts (i.e., acetate, H₂, and formate) of isovalerate degradation, mechanisms for electron transduction from isovalerate degradation to H₂ and formate generation (via electron transfer flavoprotein; ETF), and biosynthetic metabolism. The identified organisms tended to prefer formate-based interspecies electron transfer with methanogenic partners. The byproduct acetate was further converted to CH₄ and CO₂ by either Methanothrix (mesophilic) and Methanosarcina (thermophilic), which employed different approaches for acetate degradation. This study presents insights into novel mesophilic and thermophilic isovalerate degraders and their interactions with methanogens.

Effect of carbon-rich-waste addition as co-substrate on the performance and stability of anaerobic digestion of abattoir wastewater without agitation

Multiple wastes’ co-digestion is one of the alternatives for improved anaerobic digestion (AD) process of industrial and municipal wastes. The present work investigated the influence of fruit–vegetable solid waste (FVW) addition as a co-substrate on the performance of AD of abattoir wastewater (AWW). The co-digestion was done at a lab-scale-based experiment under mesophilic condition using a two-phase anaerobic sequencing batch reactor without agitation. It was tested at different mixing ratios (100%AWW; 75%AWW:25%FVW; 50%AWW:50%FVW; 25%AWW:75%FVW; 100%FVW) with the intention of looking for the best mixing ratio with the best performance. It was fed on a semi-continuous basis and operated for 18 days (d) total retention time (HRT): 3 days for the acidogenesis reactor and 15 days for methanogenesis reactor. The addition of FVW enhanced biogas yield and VS removal by 70.26% and 57.11%, respectively, at optimum mixing ratio. Moreover, to some extent improvement of AD process stability verified by the decreased TVFA:TAlk ratio and free ammonia nitrogen was observed upon progressive addition of FVW. Finally, this co-digestion process should further be studied for its performance at different HRTs with agitation.

Microbiome Diversity and Community-Level Change Points within Manure-based small Biogas Plants

Efforts to integrate biogas plants into bioeconomy concepts will lead to an expansion of manure-based (small) biogas plants, while their operation is challenging due to critical characteristics of some types of livestock manure. For a better process understanding, in this study, three manure-based small biogas plants were investigated with emphasis on microbiome diversity. Due to varying digester types, feedstocks, and process conditions, 16S rRNA gene amplicon sequencing showed differences in the taxonomic composition. Dynamic variations of each investigated biogas plant microbiome over time were analyzed by terminal restriction fragment length polymorphism (TRFLP), whereby nonmetric multidimensional scaling (NMDS) revealed two well-running systems, one of them with a high share of chicken manure, and one unstable system. By using Threshold Indicator Taxa Analysis (TITAN), community-level change points at ammonium and ammonia concentrations of 2.25 g L^-1 and 193 mg L^-1 or volatile fatty acid concentrations of 0.75 g L^-1were reliably identified which are lower than the commonly reported thresholds for critical process stages based on chemical parameters. Although a change in the microbiome structure does not necessarily indicate an upcoming critical process stage, the recorded community-level change points might be a first indication to carefully observe the process.

Low-Temperature Pretreatment of Organic Feedstocks with Selected Mineral Wastes Sustains Anaerobic Digestion Stability through Trace Metal Release

A low-cost approach for enhancing mesophilic (37 °C) anaerobic digestion (AD) of organic waste using a low-temperature (37 °C) pretreatment with different mineral wastes (MW) was investigated. A higher and stable methane production rate, in comparison to MW-free controls, was achieved for 80 days at organic loading rates of 1-2 g VS/L·d, using a feed substrate pretreated with incinerator bottom ash (IBA). The boiler ash and cement-based waste pretreatments also produced high methane production rates but with some process instability. In contrast, an incinerator fly ash pretreatment showed a progressive decrease in methane production rates and poor process stability, leading to reactor failure after 40 days. To avoid process instability and/or reactor failure, two metrics had to be met: (a) a methanogenesis to fermentation ratio higher than 0.6 and (b) a cell-specific methanogenic activity to cell-specific fermentation activity ratio of >1000. The prevalence of Methanofastidiosum together with a mixed community of acetoclastic (Methanosaeta) and hydrogenotrophic (Methanobacterium) methanogens in the stable IBA treatment indicated the importance of Methanofastidiosum as a potential indicator of a healthy and stable reactor.

In-situ mineral CO2 sequestration in a methane producing microbial electrolysis cell treating sludge hydrolysate

Microbial electrolysis cell (MEC) has excellent CH₄ production performance, however, CO₂ still remains in the produced biogas at high content. For achieving in-situ CO₂ sequestration and thus upgrading biogas, mineral carbonation was integrated into a MEC treating sludge hydrolysate. With 19 g/L wollastonite addition, in-situ mineral CO₂ sequestration was achieved by formation of calcite precipitates. CH₄ content in the biogas was increased by 5.1 % and reached 95.9 %, with CH₄ production improved by 16.9 %. In addition, the removals of polysaccharide, protein, and chemical oxygen demand (COD) of the MEC were increased by 4.4 %, 6.7 %, and 8.4 %, respectively. The generated precipitates rarely accumulated on bio-cathode, and did not significantly affect the morphology of cathode biofilm. However, integrating mineral carbonation resulted in a higher relative abundance of Methanosarcina on anode and slightly decreased the ratio of Methanobacterium to Methanosaeta on cathode, which should be noticed. In conclusion, integrating mineral carbonation is an attractive way to improve the performance of MEC by achieving in-situ CO₂ sequestration, accompanied with CH₄ production enhancement.

Caproate production from xylose by mesophilic mixed culture fermentation

Caproate production by mixed culture fermentation (MCF) is economically attractive. Xylose is known as the second most abundant sugar in nature, however, producing caproate from xylose is never reported. In this study, caproate production from xylose by mesophilic MCF was firstly investigated. The results showed that as pH decreasing to 5.0, the caproate concentration was 2.06 g/L in a batch reactor and was between 0.45 and 1.07 g/L in a continuously stirred reactor. Microbial analysis illustrated that Caproiciproducens and Clostridium_sensu_stricto_12, as two main identified caproate producers, occupied over 50% and around 10% of mixed culture, respectively. Thus, caproate production from xylose was proposed via the fatty acid biosynthesis pathway, not the well-known reverse β-oxidation pathway. These unexpected differences from literatures gains more understanding about caproate production from organic substrates via MCF.

Stimulation of methane production from benzoate with addition of carbon materials

Huge amounts of wastewater that contain aromatic compounds such as benzene and phenols are discharged worldwide. Benzoate is a typical intermediate in the anaerobic transformation of those aromatic compounds. In this study, electrically conductive carbon-based materials of granulated activated carbon (GAC), multiwalled carbon nanotubes (MwCNTs), and graphite were evaluated for the ability to promote the benzoate degradation. The results showed that 82-93% of the electrons were recovered in CH₄ production from benzoate. The carbon materials stimulated benzoate degradation in the sequence of GAC (5 g/L) > MwCNTs (1 g/L) ~ Graphite (0.1 g/L) > Control. Acetate was the only detected intermediate in the process of benzoate degradation. Taxonomic analyses revealed that benzoate was degraded by Syntrophus to acetate and H₂, which were subsequently converted to methane by Methanosarcina (both acetoclastic methanogens and hydrogenotrophic methanogens) and Methanoculleus (hydrogenotrophic methanogens), and direct interspecies electron transfer (DIET) of Desulfovibrio and Methanosarcina. Thus, these results suggest a method to effectively enhance the removal of aromatic compounds and methane recovery.

High-efficiency methanogenesis via kitchen wastes served as ethanol source to establish direct interspecies electron transfer during anaerobic Co-digestion with waste activated sludge

Kitchen wastes (KW) have been widely investigated for bio-ethanol production, while no study utilizes KW as ethanol source to stimulate the methanogenic communities to perform direct interspecies electron transfer (DIET), since the excess acidity contained after the biological ethanol-type fermentation pretreatment (BEFP) can seriously inhibit the DIET-based syntrophic metabolism. In this study, a strategy that utilized waste activated sludge (WAS) as co-substrate to relieve the excess acidity after BEFP during anaerobic co-digestion (AcoD) was proposed. The results showed that, under the mixed ratio of 1:2 and 1:5 (KW:WAS, volume ratio), both methane production and organic compound removal evidently increased, compared with that treating the sole WAS. Conversely, under the other mixed ratios (sole KW, 5:1, 2:1 and 1:1), no methane but the evident hydrogen production was detected, and syntrophic metabolism of organic acids and alcohols was prevented. Three-dimensional excitation emission matrix (3D-EEM) analysis showed that the protein-like organic compounds contained in both KW and WAS were effectively degraded. Furthermore, the maximum methane production potential from WAS during AcoD (260.5 ± 4.1 and 264.3 ± 2.7 mL/g-COD) was higher than that treating sole WAS (250.8 ± 0.1 mL/g-COD). Microbial community analysis showed that, some genera capable of metabolizing the complex organic compounds with the reduction of the elemental sulfur or equipped with the electrically conductive pili were specially enriched during AcoD under the mixed ratio of 1:2 and 1:5. They might proceed DIET with methanogens, such as Methanosarcina and Methanospirillum species, to maintain the syntrophic metabolism effective and stable, since the abundance of both Methanosarcina and Methanospirillum species evidently increased.

Increasing sulfate levels show a differential impact on synthetic communities comprising different methanogens and a sulfate reducer

Methane-producing microbial communities are of ecological and biotechnological interest. Syntrophic interactions among sulfate reducers and aceto/hydrogenotrophic and obligate hydrogenotrophic methanogens form a key component of these communities, yet, the impact of these different syntrophic routes on methane production and their stability against sulfate availability are not well understood. Here, we construct model synthetic communities using a sulfate reducer and two types of methanogens representing different methanogenesis routes. We find that tri-cultures with both routes increase methane production by almost twofold compared to co-cultures and are stable in the absence of sulfate. With increasing sulfate, system stability and productivity decreases and does so faster in communities with aceto/hydrogenotrophic methanogens despite the continued presence of acetate. We show that this is due to a shift in the metabolism of these methanogens towards co-utilization of hydrogen with acetate. These findings indicate the important role of hydrogen dynamics in the stability and productivity of syntrophic communities.

Enhanced anaerobic digestion performance by two artificially constructed microbial consortia capable of woody biomass degradation and chlorophenols detoxification

Catalpa sawdust (CSW) is a promising biomass-based biofuel. However, the complex lignocellulosic structure limits its efficient utilization in biorefinery applications. It is even more so when chlorophenols (CPs), highly toxic organic substances widely used as wood preservatives, are present. Hence, it is crucial to develop effective and eco-friendly approaches to attain deconstruction of lignocellulose and chlorophenols simultaneously as well as to improve methane (CH4) production efficiently. This study might be the first to explore the performance of the novel constructed microbial consortia CS-5 and BC-4 on woody biomass degradation and CPs detoxification simultaneously with CH4 production. After the degradation of CSW and CPs for 15 days by C5-5 or BC-4, significant reduction in lignocellulosic components and CPs mixture was realized with a total weight loss of 69.2 and 56.3 % and CPs degradation of 89 and 95 %, respectively. The toxicity of individual or mixed CPs after 15 days of degradation was reduced by approximately 90 %. The synergistic action of CS-5 and BC-4 enhanced biogas and CH4 yields over 76 and 64 % respectively, higher than control. Furthermore, CH4 production increased by 113.7 % at the peak phase of AD process. Methanosataceae represented 45.1 % of the methanogenic Archaea in digester G-III.

Determination of Various Parameters during Thermal and Biological Pretreatment of Waste Materials

Pretreatment of waste materials could help in more efficient waste management. Various pretreatment methods exist, each one having its own advantages and disadvantages. Moreover, a certain pretreatment technique might be efficient and economical for one feedstock while not for another. Thus, it is important to analyze how parameters change during pretreatment. In this study, two different pretreatment techniques were applied: thermal at lower and higher temperatures (38.6 °C and 80 °C) and biological, using cattle rumen fluid at ruminal temperature (≈38.6 °C). Two different feedstock materials were chosen: sewage sludge and riverbank grass (Typha latifolia), and their combinations (in a ratio of 1:1) were also analyzed. Various parameters were analyzed in the liquid phase before and after pretreatment, and in the gas phase after pretreatment. In the liquid phase, some of the parameters that are relevant to water quality were measured, while in the gas phase composition of biogas was measured. The results showed that most of the parameters significantly changed during pretreatments and that lower temperature thermal and/or biological treatment of grass and sludge is suggested for further applications.

Specific surface area and electron donating capacity determine biochar's role in methane production during anaerobic digestion

The addition of biochar derived from different materials can have varying effects on anaerobic digestion (AD), depending on its physicochemical properties. Physicochemical properties of biochars, biomethanization performance and microbial communities were examined to evaluate the effectiveness of biochars made from different plant wastes on AD in this study. Results showed that all biochars significantly reduce the lag phases during AD, compared with a control treatment (CK). Woody biochars particularly performed much better than herbal ones. Correlation analysis revealed that specific surface area (SSA) and electron donating capacity (EDC) were the key properties of the plant-feedstock-derived biochar in AD enhancement. Microbial community structure analysis showed that higher SSA and EDC are conducive for the growth of bacteria decomposing glucose, further promoting daily methane production in the early AD stage. The results indicate that it is important to select biochar with higher SSA and EDC to enhance biomethanization in AD systems.

Sequential biodegradation of 1,2,4-trichlorobenzene at oxic-anoxic groundwater interfaces in model laboratory columns

Halogenated organic solvents such as chlorobenzenes (CBs) are frequent groundwater contaminants due to legacy spills. When contaminated anaerobic groundwater discharges into surface water through wetlands and other transition zones, aeration can occur from various physical and biological processes at shallow depths, resulting in oxic-anoxic interfaces (OAIs). This study investigated the potential for 1,2,4-trichlorobenzene (1,2,4-TCB) biodegradation at OAIs. A novel upflow column system was developed to create stable anaerobic and aerobic zones, simulating a natural groundwater OAI. Two columns containing (1) sand and (2) a mixture of wetland sediment and sand were operated continuously for 295 days with varied doses of 0.14-1.4 mM sodium lactate (NaLac) as a model electron donor. Both column matrices supported anaerobic reductive dechlorination and aerobic degradation of 1,2,4-TCB spatially separated between anaerobic and aerobic zones. Reductive dechlorination produced a mixture of di- and monochlorobenzene daughter products, with estimated zero-order dechlorination rates up to 31.3 μM/h. Aerobic CB degradation, limited by available dissolved oxygen, occurred for 1,2,4-TCB and all dechlorinated daughter products. Initial reductive dechlorination did not enhance the overall observed extent or rate of subsequent aerobic CB degradation. Increasing NaLac dose increased the extent of reductive dechlorination, but suppressed aerobic CB degradation at 1.4 mM NaLac due to increased oxygen demand. 16S-rRNA sequencing of biofilm microbial communities revealed strong stratification of functional anaerobic and aerobic organisms between redox zones including the sole putative reductive dechlorinator detected in the columns, Dehalobacter. The sediment mixture column supported enhanced reductive dechlorination compared to the sand column at all tested NaLac doses and growth of Dehalobacter populations up to 4.1 × 10⁸ copies/g (51% relative abundance), highlighting the potential benefit of sediments in reductive dechlorination processes. Results from these model systems suggest both substantial anaerobic and aerobic CB degradation can co-occur along the OAI at contaminated sites where bioavailable electron donors and oxygen are both present.

Sequential biodegradation of 1,2,4-trichlorobenzene at oxic-anoxic groundwater interfaces in model laboratory columns

Halogenated organic solvents such as chlorobenzenes (CBs) are frequent groundwater contaminants due to legacy spills. When contaminated anaerobic groundwater discharges into surface water through wetlands and other transition zones, aeration can occur from various physical and biological processes at shallow depths, resulting in oxic-anoxic interfaces (OAIs). This study investigated the potential for 1,2,4-trichlorobenzene (1,2,4-TCB) biodegradation at OAIs. A novel upflow column system was developed to create stable anaerobic and aerobic zones, simulating a natural groundwater OAI. Two columns containing (1) sand and (2) a mixture of wetland sediment and sand were operated continuously for 295 days with varied doses of 0.14-1.4 mM sodium lactate (NaLac) as a model electron donor. Both column matrices supported anaerobic reductive dechlorination and aerobic degradation of 1,2,4-TCB spatially separated between anaerobic and aerobic zones. Reductive dechlorination produced a mixture of di- and monochlorobenzene daughter products, with estimated zero-order dechlorination rates up to 31.3 μM/h. Aerobic CB degradation, limited by available dissolved oxygen, occurred for 1,2,4-TCB and all dechlorinated daughter products. Initial reductive dechlorination did not enhance the overall observed extent or rate of subsequent aerobic CB degradation. Increasing NaLac dose increased the extent of reductive dechlorination, but suppressed aerobic CB degradation at 1.4 mM NaLac due to increased oxygen demand. 16S-rRNA sequencing of biofilm microbial communities revealed strong stratification of functional anaerobic and aerobic organisms between redox zones including the sole putative reductive dechlorinator detected in the columns, Dehalobacter. The sediment mixture column supported enhanced reductive dechlorination compared to the sand column at all tested NaLac doses and growth of Dehalobacter populations up to 4.1 × 108 copies/g (51% relative abundance), highlighting the potential benefit of sediments in reductive dechlorination processes. Results from these model systems suggest both substantial anaerobic and aerobic CB degradation can co-occur along the OAI at contaminated sites where bioavailable electron donors and oxygen are both present.

Molecular and microbial insights towards understanding the effects of hydrochar on methane emission from paddy soil

Directly returning rice straw to the paddy soil would significantly stimulate methane emission, and hydrochar has potential to be used as soil conditioner. However, the effects of hydrochar on the methane emission from paddy soil and the related mechanisms are still unclear. In the present study, straw-based hydrochar obtained at 200 °C (HC200), 250 °C (HC250) and 300 °C (HC300) and hydrochar after removal of bio-oil at these temperatures (CHC200, CHC250, and CHC300) were prepared and added to the paddy soil. The application of HC200, HC250 and HC300 resulted in the enhanced methane production compared to the control, showing 4.3, 1.6 and 1.5-fold higher methane production, respectively. It was related to the large amount of dissolved organic matter (DOM) released from hydrochar. Excitation-emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC) showed that the hydrochar-derived DOM mainly included humic-like, phenolic and less aromatic structures, and with the increase of hydrothermal temperature, the content of humic-like substances and phenols increased, while biodegradable organics decreased. This was consistent with the maximum methane production by HC200. After incubation, there was no low-aromatic structures observed in the soil leachate, and the residual organics were mainly humus. The EEM-PARAFAC results were supported by compositional characterization of soil leachate by high-resolution mass spectrometry, and the refractory organics released from hydrochar was mainly lignins or (CRAM)-like structures in the range of H/C = 0.8-1.6 and O/C = 0.1-0.5. The organics dissolved from the washed hydrochar was significantly reduced, and some washed hydrochar (CHC250 and CHC300) even inhibited methane emission possibly due to their ability to adsorb organics. Microbial analysis further showed that the increased methane production resulted from hydrochar was associated with the enrichment of Janibacter, Anaeromyxobacter, Anaerolinea and Sporacetigenium. This present study provided a better understanding to the effect of hydrochar on methanogenesis in paddy soil.

Effect of nickel, cobalt, and iron on methanogenesis from methanol and cometabolic conversion of 1,2-dichloroethene by Methanosarcina barkeri

Methanogens are responsible for the last step in anaerobic digestion (AD), in which methane (a biofuel) is produced. Some methanogens can cometabolize chlorinated pollutants, contributing for their removal during AD. Methanogenic cofactors involved in cometabolic reductive dechlorination, such as F₄₃₀ and cobalamin, contain metal ions (nickel, cobalt, iron) in their structure. We hypothesized that the supplementation of trace metals could improve methane production and the cometabolic dechlorination of 1,2‐dichloroethene (DCE) by pure cultures of Methanosarcina barkeri. Nickel, cobalt, and iron were added to cultures of M. barkeri growing on methanol and methanol plus DCE. Metal amendment improved DCE dechlorination to vinyl chloride (VC): assays with 20 µM of Fe³⁺ showed the highest final concentration of VC (5× higher than in controls without Fe³⁺), but also in assays with 5.5 µM of Co²⁺ and 5 µM of Ni²⁺ VC formation was improved (3.5–4× higher than in controls without the respective metals). Dosing of metals could be useful to improve anaerobic removal of chlorinated compounds, and more importantly decrease the detrimental effect of DCE on methane production in anaerobic digesters.

Microbial Community Redundancy and Resilience Underpins High-Rate Anaerobic Treatment of Dairy-Processing Wastewater at Ambient Temperatures

High-rate anaerobic digestion (AD) is a reliable, efficient process to treat wastewaters and is often operated at temperatures exceeding 30°C, involving energy consumption of biogas in temperate regions, where wastewaters are often discharged at variable temperatures generally below 20°C. High-rate ambient temperature AD, without temperature control, is an economically attractive alternative that has been proven to be feasible at laboratory-scale. In this study, an ambient temperature pilot scale anaerobic reactor (2 m³) was employed to treat real dairy wastewater in situ at a milk processing plant, at organic loading rates of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m³/day and hydraulic retention times (HRT) ranging from 36 to 6 h. Consistent high levels of COD removal efficiencies, ranging from 50 to 70% for total COD removal and 70 to 84% for soluble COD removal, were achieved during the trial. Within the reactor biomass, stable active archaeal populations were observed, consisting mainly of Methanothrix (previously Methanosaeta) species, which represented up to 47% of the relative abundant active species in the reactor. The decrease in HRT, combined with increases in the loading rate had a clear effect on shaping the structure and composition of the bacterial fraction of the microbial community, however, without affecting reactor performance. On the other hand, perturbances in influent pH had a strong impact, especially when pH went higher than 8.5, inducing shifts in the microbial community composition and, in some cases, affecting negatively the performance of the reactor in terms of COD removal and biogas methane content. For example, the main pH shock led to a drop in the methane content to 15%, COD removals decreased to 0%, while the archaeal population decreased to ~11% both at DNA and cDNA levels. Functional redundancy in the microbial community underpinned stable reactor performance and rapid reactor recovery after perturbations.

Dynamic shifts within volatile fatty acid-degrading microbial communities indicate process imbalance in anaerobic digesters

Buildup of volatile fatty acids (VFAs) in anaerobic digesters (ADs) often results in acidification and process failure. Understanding the dynamics of microbial communities involved in VFA degradation under stable and overload conditions may help optimize anaerobic digestion processes. In this study, five triplicate mesophilic completely mixed AD sets were operated at different organic loading rates (OLRs; 1-6 g chemical oxygen demand [COD] L_R^-1day^-1), and changes in the composition and abundance of VFA-degrading microbial communities were monitored using amplicon sequencing and taxon-specific quantitative PCRs, respectively. AD sets operated at OLRs of 1-4 g COD L_R^-1day^-1 were functionally stable throughout the operational period (120 days) whereas process instability (characterized by VFA buildup, pH decline, and decreased methane production rate) occurred in digesters operated at ≥ 5 g COD L_R^-1day^-1. Though microbial taxa involved in propionate (Syntrophobacter and Pelotomaculum) and butyrate (Syntrophomonas) degradation were detected across all ADs, their abundance decreased with increasing OLR. The overload conditions also inhibited the proliferation of the acetoclastic methanogen, Methanosaeta, and caused a microbial community shift to acetate oxidizers (Tepidanaerobacter acetatoxydans) and hydrogenotrophic methanogens (Methanoculleus). This study's results highlight the importance of operating ADs with conditions that promote the maintenance of microbial communities involved in VFA degradation.

Acclimation of Acid-Tolerant Methanogenic Culture for Bioaugmentation: Strategy Comparison and Microbiome Succession

To enrich an acid-tolerant methanogenic culture used as bioaugmented seed under acidic conditions, we operated four semicontinuous digesters under various conditions of pH decline for producing methane at pH 5.0. 16S rRNA amplification was performed to unravel the association between declining pH and microbiome succession. The findings demonstrated that a gradual decrease of pH, at a step size of 0.5, and a prolonged run time at each pH could achieve a suitable microbial culture, in which acetoclastic Methanothrix and hydrogenotrophic Methanolinea represented the dominant methanogens. In contrast, a sharp decline in pH could result in heavy loss of the acetoclastic methanogen Methanothrix, leading to a cessation of methane production. Hydrogenotrophic methanogens exhibited high acid tolerance, and Methanospirillum could thrive despite a sudden low-pH shock. Although Methanolinea required a longer time to enrich, it played a substantial role in methane production under an acidic environment.

Why do DIETers like drinking: Metagenomic analysis for methane and energy metabolism during anaerobic digestion with ethanol

Stimulating Methanothrix-dominant communities with ethanol is recently considered as a promising strategy of improving the efficiency and stability of anaerobic digestion (AD), while the effects on methanogenic pathway and energy metabolism linked to the establishment of direct interspecies electron transfer (DIET) were not investigated yet. The results showed that, Methanothrix species were the dominant and metabolically active methanogens in the methanogenic sludge fed with the ethanol-type fermentation products, and the abundance of genes that encoded the key enzymes involved in the reduction of carbon dioxide was significantly higher than that fed with the other products, such as propionate and butyrate. Conversely, the abundance of genes that encoded the key enzymes involved in acetate decarboxylation among all the methanogenic sludge were nearly same. In the presence of ethanol, the abundance of gene for pilA significantly increased. The gene for pliA was primarily derived from Sphaerochaeta, Sedimentibacter and Pseudomonas species that were specially abundant and metabolically active. Further analysis showed that, the abundance of genes that encoded V/A-type ATPase in the methanogenic digesters fed with the ethanol-type fermentation products was 1.3-1.5 folds higher than that fed with the other products. As a result, the concentration of total ATP in the cells was increased by 1.8-2.3 folds. These results, and the fact that DIET is the only electron donor to support the reduction of carbon dioxide in Methanothrix species for the first time revealed the mechanisms involved in the establishment of DIET-based methanogenic metabolism with ethanol.

Detection of early imbalances in semi-continuous anaerobic co-digestion process based on instantaneous biogas production rate

The aim of this study was to investigate the use of biogas production rate kinetics for the monitoring of anaerobic co-digestion. Recent extensive studies of degradation pathways showed that acetoclastic methanogenesis is not always the main pathway. Hydrogenotrophic methanogenesis and syntrophic acetate oxidation can also dominate, mostly for operating conditions with high concentrations of ammonia or volatile fatty acids … These conditions are also known to cause instability in the digester's operation especially in co-digestion due to substrate variability. Therefore, co-digestion experiments were conducted with several co-substrates using a continuously stirred 35-L tank reactor. Degradation pathways and their potential shifts were identified by monitoring variations in biogas production rate kinetics using a principal component analysis model. The shifts in the degradation pathways were used to monitor the process. These shift points were found to provide early warnings of instabilities in the anaerobic co-digestion process.

Enrichment of specific microbial communities by optimum applied voltages for enhanced methane production by microbial electrosynthesis in anaerobic digestion

This study investigates the distribution of microbiome in microbial electrosynthesis systems at different applied voltages (0.5, 1.0, and 1.5 V) for methane production. Results revealed that more favorable conditions for methane production were observed with 1.0 V applied voltage. In Venn plots, the bioelectrodes at 1.0 V had higher numbers of unique operational taxonomic units compared to those at 0.5 and 1.5 V. Hierarchical cluster, non-metric multidimensional scaling, and principal component ordinate analyses revealed that the biocathode at 1.0 V clustered separately from the rest of the biofilms mainly because of the quantitative differences in the microbial distribution. Taxonomically, exoelectrogens (Geobacter spp.) dominated the bioanode at 1.0 V, while the syntrophic assemblages of hydrogen-producing bacteria (i.e., Bacteroidetes and Firmicutes) and hydrogen-consuming methanogens (i.e., Methanobacterium sp.) existed in the biocathode. These results suggest that the optimum applied voltage enriched specific microbial communities on the anode and cathode for enhanced methane production.

Effect of Hydraulic Retention Time on the Treatment of Real Cattle Slaughterhouse Wastewater and Biogas Production from HUASB Reactor

Anaerobic digestion technology provides an alternative route for sustainable management of organic waste. In this study, the performance of the hybrid upflow anaerobic sludge blanket (HUASB) reactor consisting of synthetic grass media as attached growth surface was investigated for the treatment of cattle slaughterhouse wastewater under mesophilic (35 ± 1 °C) condition. After acclimatization with synthetic wastewater, the reactor was loaded up to OLR 10 g L−1d−1, corresponding to 20 g COD/L at a varying hydraulic retention time (HRT) of 24, 30, 36, 42, and 48 h. The system attained a maximum COD removal efficiency of 97% total suspended solids (TSS), volatile suspended solids (VSS), fats, oil, and grease (FOG), color removal, and turbidity were found as 97%, 284 mg/L, 79%, 78%, and 91% respectively. The biogas production after 48 h was found as 38 L/d, with about 85% methane and specific methane production of 0.24 LCH4/gCODadded. The ratio of alkalinity was 0.22, while ammonia nitrogen concentration reached a maximum of 839 mg/L at a steady state. Scanning electron microscopic (SEM) analysis revealed a predominance of Methanosarcina bacteria with the coccoidal shape at the end of the performance study. Therefore, the results of the experiment showed that increasing HRT significantly affects the performance of the system.

Effect of a Profound Feedstock Change on the Structure and Performance of Biogas Microbiomes

In this study the response of biogas-producing microbiomes to a profound feedstock change was investigated. The microbiomes were adapted to the digestion of either 100% sugar beet, maize silage, or of the silages with elevated amounts of total ammonium nitrogen (TAN) by adding ammonium carbonate or animal manure. The feedstock exchange resulted in a short-range decrease or increase in the biogas yields according to the level of chemical feedstock complexity. Fifteen taxa were found in all reactors and can be considered as generalists. Thirteen taxa were detected in the reactors operated with low TAN and six in the reactors with high TAN concentration. Taxa assigned to the phylum Bacteroidetes and to the order Spirochaetales increased with the exchange to sugar beet silage, indicating an affinity to easily degradable compounds. The recorded TAN-sensitive taxa (phylum Cloacimonetes) showed no specific affinity to maize or sugar beet silage. The archaeal community remained unchanged. The reported findings showed a smooth adaptation of the microbial communities, without a profound negative impact on the overall biogas production indicating that the two feedstocks, sugar beet and maize silage, potentially do not contain chemical compounds that are difficult to handle during anaerobic digestion.

Microbial Community Analysis of Digested Liquids Exhibiting Different Methane Production Potential in Methane Fermentation of Swine Feces

Batch methane fermentation was conducted using seed sludge collected from six methane fermentation facilities. Swine feces were centrifuged and autoclaved, followed by its use as a substrate for methanogenesis. This "swine feces supernatant medium" facilitates the cultivation of the microbes of the seed sludge, sampling of the digested liquid using a syringe, and subculturing of the digested liquid in a subsequent medium using a syringe. Through 15 subcultures, digested liquids with high and low methane production potential were obtained, which were named "H-DS" and "L-DS," respectively. On the day 10 of cultivation, chemical oxygen demand (COD) of H-DS significantly decreased by 31% and that of L-DS did not differ significantly compared with that on the day 0 of cultivation. Acetic acid concentration of H-DS (1009 mg/L) was significantly lower than that of L-DS (2686 mg/L). These chemical characteristics indicate that organics decomposition in L-DS was not successful and suggest that H-DS has high relative abundance of bacteria decomposing organic matter and methanogen utilizing acetic acid compared with those in L-DS. Microbial community analysis revealed that Shannon index of H-DS was significantly higher than that of L-DS, and the relative abundance of acetogenic bacteria (e.g., Syntrophomonas) and acetic acid-utilizing methanogen (Methanosarcina) in H-DS was significantly higher than that in L-DS. Thus, the high methane production potential of H-DS might be attributable to the smooth flow from acetogenesis to methanogenesis step in the methane fermentation, compared with the case of L-DS.

Enhanced biogas production from municipal solid waste via co-digestion with sewage sludge and metabolic pathway analysis

The present study intends to evaluate the potential of co-digestion for utilizing Organic fraction of Municipal Solid Waste (OFMSW) and sewage sludge (SS) for enhanced biogas production. Metagenomic analysis was performed to identify the dominant bacteria, archaea and fungi, changes in their communities with time and their functional roles during the course of anaerobic digestion (AD). The cumulative biogas yield of 586.2 mL biogas/gVS with the highest methane concentration of 69.5% was observed under an optimum ratio of OFMSW:SS (40:60 w/w). Bacteria and fungi were found to be majorly involved in hydrolysis and initial stages of AD. Probably, the most common archaea Methanosarsina sp. primarily followed the acetoclastic pathway. The hydrogenotrophic pathway was less followed as indicated by the reduction in abundance of syntrophic acetate oxidizers. An adequate understanding of microbial communities is important to manipulate and inoculate the specific microbial consortia to maximize CH₄ production through AD.

Metabolic performance of anaerobic digestion of chicken manure under wet, high solid, and dry conditions

The anaerobic digestion (AD) of chicken manure as a solo substrate has been challenging due to the ammonium inhibition effects when adopting a high organic loading rate (OLR). In this study, through increasing both the total solid in the feeding materials from 5% to 20%, and the OLR from 1.7 to 7.1 g-volatile solids (VS)/(L·d), the AD of chicken manure under wet, high solid, and dry conditions, with a fixed hydraulic retention time of 20 days, was investigated. The results obtained indicated that the wet AD system could achieve a methane yield of 0.28 L/g-VS and a low volatile fatty acid level. However, the process deteriorated under dry conditions, and methane formed mainly through acetate oxidation and methanogenesis. Methanosarcina and Methanoplasma were found to be more tolerant But, whether the dry AD of chicken manure can survive an ammonia-stressed environment when the OLR is lowered, still needs investigation.

Microbial community acclimatization for enhancement in the methane productivity of anaerobic co-digestion of fats, oil, and grease

The methane productivity and long chain fatty acids (LCFAs) degradation capability of unacclimatized seed sludge (USS) and acclimatized seed sludge (ASS) at different substrate ratios of fats oil and grease (FOG) and mixed sewage sludge were investigated in this study. Biogas produced in ASS in initial phase of anaerobic digestion had higher methane content (65-76%) than that in USS (26-73%). The degradation of major LCFAs in the ASS was 22-80%, 33-191%, and 7-64% higher for the substrate ratios of 100:10, 100:20, and 100:30, respectively, as compared to the LCFAs' degradation in USS. Microbial acclimatization increased the population of Firmicutes (40%), Bacteroidetes (32%), Synergistetes (10%), and Euryarchaeota (8%) in ASS, which supported the faster rate of LCFAs degradation for its later conversion to methane. The significant abundance of Syntrophomonas and Methanosarcina genera in ASS supported faster generation rate of methane in an obligatory syntrophic relationship.

Co-digestion of microalgae with potato processing waste and glycerol: effect of glycerol addition on methane production and the microbial community

The production of methane-rich biogas from the anaerobic digestion (AD) of microalgae is limited by an unfavorable biomass carbon-to-nitrogen (C/N) ratio; however, this may be ameliorated using a co-digestion strategy with carbon-rich feedstocks. For reliable plant operation, and to improve the economics of the process, secure co-feedstock supply (ideally as a waste-stream) is important. To this end, this study investigated the feasibility of co-digesting microalgae (Chlorella vulgaris) with potato processing waste (potato discarded parts, PPW_dp; potato peel, PPW_p) and glycerol, while monitoring the response of the methanogenic community. In this semi-continuous study, glycerol (1 and 2% v/v) added to mixtures of C. vulgaris : PPW_dp enhanced the specific methane yields the most, by 53–128%, whilst co-digestion with mixtures of C. vulgaris : PPW_p enhanced the methane yields by 62–74%. The microbial communities diverged markedly over operational time, and to a lesser extent in response to glycerol addition. The acetoclast Methanosaeta was abundant in all treatments but was replaced by Methanosarcina in the potato peel with glycerol treatment due to volatile fatty acid (VFA) accumulation. Our findings demonstrate that the performance of microalgae co-digestion is substantially improved by the addition of glycerol as an additional co-feedstock. This should improve the economic case for anaerobically digesting microalgae as part of wastewater treatment processes and/or the terminal step of a microalgae biorefinery.

Glycerol as an additional co-substrate enhanced methane yields by up to 128% when co-digestion with microalgae and potato waste.

Performance of enhanced anaerobic digestion with different pyrolysis biochars and microbial communities

Anaerobic digestion (AD) is commonly used to treat biowastes, however, there are challenges in AD such as low methane yield, intermediate inhibition, and system instability. In this study, the effects of typical biochars on methane yield and microbial variation for AD with straw and cow manure were explored. The results indicated that cumulative methane yield with coconut shell biochar was higher than that without a biochar (319.44 vs. 282.77 mL/g VS). Interestingly, AD with biochars had a secondary methane yield peak, whereas control groups did not show this phenomenon. A suitable dosage (e.g., straw biochar of 2%) improved cumulative methane yield, but excessive addition (4%) could inhibit AD. AD system with biochar was more helpful for the growth of acetoclastic methanogens rather than hydrogenotrophic methanogens. The study demonstrated biochar can indeed enhance AD performance, and microbial community analyses could supply valuable information to elucidate the mechanism of enhancement.

Responses of Methanosarcina barkeri to acetate stress

BACKGROUND

Anaerobic digestion of easily degradable biowaste can lead to the accumulation of volatile fatty acids, which will cause environmental stress to the sensitive methanogens consequently. The metabolic characteristics of methanogens under acetate stress can affect the overall performance of mixed consortia. Nevertheless, there exist huge gaps in understanding the responses of the dominant methanogens to the stress, e.g., Methanosarcinaceae. Such methanogens are resistant to environmental deterioration and able to utilize multiple carbon sources. In this study, transcriptomic and proteomic analyses were conducted to explore the responses of Methanosarcina barkeri strain MS at different acetate concentrations of 10, 25, and 50 mM.

RESULTS

The trend of OD600 and the regulation of the specific genes in 50 mM acetate, indicated that high concentration of acetate promoted the acclimation of M. barkeri to acetate stress. Acetate stress hindered the regulation of quorum sensing and thereby eliminated the advantages of cell aggregation, which was beneficial to resist stress. Under acetate stress, M. barkeri allocated more resources to enhance the uptake of iron to maintain the integrities of electron-transport chains and other essential biological processes. Comparing with the initial stages of different acetate concentrations, most of the genes participating in acetoclastic methanogenesis did not show significantly different expressions except hdrB1C1, an electron-bifurcating heterodisulfide reductase participating in energy conversion and improving thermodynamic efficiency. Meanwhile, vnfDGHK and nifDHK participating in nitrogen fixation pathway were upregulated.

CONCLUSION

In this work, transcriptomic and proteomic analyses are combined to reveal the responses of M. barkeri to acetate stress in terms of central metabolic pathways, which provides basic clues for exploring the responses of other specific methanogens under high organics load. Moreover, the results can also be used to gain insights into the complex interactions and geochemical cycles among natural or engineered populations. Furthermore, these findings also provide the potential for designing effective and robust anaerobic digesters with high organic loads.

Controlling methane and hydrogen production from cheese whey in an EGSB reactor by changing the HRT

This study assessed the effects of hydraulic retention time (HRT; 8 h-0.25 h) on simultaneous hydrogen and methane production from cheese whey (5000 mg carbohydrates/L) in a mesophilic (30 °C) expanded granular sludge bed (EGSB) reactor. Methane production was observed at HRTs from 4 to 0.25 h. The maximum methane yield (9.8 ± 1.9 mL CH₄/g COD_ap, reported as milliliter CH₄ per gram of COD applied) and methane production rate (461 ± 75 mL CH₄/day L_reactor) occurred at HRTs of 4 h and 2 h, respectively. Hydrogen production increased as methane production decreased with decreasing HRT from 8 to 0.25 h. The maximum hydrogen yield of 3.2 ± 0.3 mL H₂/g COD_ap (reported as mL H₂ per gram of COD applied) and hydrogen production rate of 1951 ± 171 mL H₂/day L_reactor were observed at the HRT of 0.25 h. The decrease in HRT from 8 to 0.25 h caused larger changes in the bacterial populations than the archaea populations. With the decrease in HRT (6 h-0.25 h), the Shannon diversity index decreased (3.02-2.87) for bacteria and increased (1.49-1.83) for archaea. The bacterial dominance increased (0.059-0.066) as the archaea dominance decreased (0.292-0.201) with the HRT decrease from 6 to 0.25 h.

Microbial and Phenyl Acid Dynamics during the Start-up Phase of Anaerobic Straw Degradation in Meso- and Thermophilic Batch Reactors

Aromatic compounds like phenyl acids derived from lignocellulose degradation have been suspected to negatively influence biogas production processes. However, results on this topic are still inconclusive. To study phenyl acid formation in batch reactors during the start-up phase of anaerobic degradation, different amounts of straw from grain were mixed with mesophilic and thermophilic sludge, respectively. Molecular biological parameters were assessed using next-generation sequencing and qPCR analyses. Metagenomic predictions were done via the program, piphillin. Methane production, concentrations of phenylacetate, phenylpropionate, phenylbutyrate, and volatile fatty acids were monitored chromatographically. Methanosarcina spp. was the dominant methanogen when high straw loads were effectively degraded, and thus confirmed its robustness towards overload conditions. Several microorganisms correlated negatively with phenyl acids; however, a negative effect, specifically on methanogens, could not be proven. A cascade-like increase/decrease from phenylacetate to phenylpropionate, and then to phenylbutyrate could be observed when methanogenesis was highly active. Due to these results, phenylacetate was shown to be an early sign for overload conditions, whereas an increase in phenylbutyrate possibly indicated a switch from degradation of easily available to more complex substrates. These dynamics during the start-up phase might be relevant for biogas plant operators using complex organic wastes for energy exploitation.

No difference in inhibition among free acids of acetate, propionate and butyrate on hydrogenotrophic methanogen of Methanobacterium formicicum

Free volatile fatty acids such as free acetic acid (FAA) and free butyrate acid (FBA) are true inhibitors of hydrogenotrophic methanogens (HM) in mixed culture. However, their inhibitory effects on pure culture of HM remain unclear. In this study, a typical HM of Methanobacterium formicicum demonstrated no difference in toxicity conferred by FAA, free propionate acid (FPA), or FBA in regard to the specific methanogenic activity (SMA) based on the C_50% (0.19, 0.17, and 0.23 g/L, respectively) and recoverable concentration values (0.97, 0.69, and 0.61 g/L, respectively). These results were within the same order of magnitude. The concentrations of FAA, FBA, and FPA all correlated well with the SMA values according to the inhibition model. Additionally, changes in the activity of the electron transport system also agreed well with the trend in the SMA variation. Together, the results of this study provide a benchmark to control methanogenesis during industrial applications.

Molecular characterization of methanogenic microbial communities for degrading various types of polycyclic aromatic hydrocarbon

Knowledge on methanogenic microbial communities associated with the degradation of polycyclic aromatic hydrocarbons (PAHs) is crucial to developing strategies for PAHs bioremediation. In this study, the linkage between the type of PAHs and microbial community structure was fully investigated through 16S rRNA gene sequencing on four PAH-degrading cultures. Putative degradation products were also detected. Our results indicated that naphthalene (Nap)/2-methylnaphthalene (2-Nap), phenanthrene (Phe) and anthracene (Ant) sculpted different microbial communities. Among them, Nap and 2-Nap selected for similar degrading bacteria (i.e., Alicycliphilus and Thauera) and methanogens (Methanomethylovorans and Methanobacterium). Nap and 2-Nap were probably activated via carboxylation, producing 2-naphthoic acid. In contrast, Phe and Ant shaped different bacterial and archaeal communities, with Arcobacter and Acinetobacter being Phe-degraders and Thiobacillus Ant-degrader. Methanogenic archaea Methanobacterium and Methanomethylovorans predominated Phe-degrading and Ant-degrading culture, respectively. These findings can improve our understanding of natural PAHs attenuation and provide some guidance for PAHs bioremediation in methanogenic environment.

Improved biogas production of dry anaerobic digestion of swine manure

In this work, wrapped granular activated carbon (GAC) and acclimated sludge were employed to enhance the efficiency of the dry anaerobic digestion of swine manure in semi-continuous tests. The addition of wrapped GAC increased the volumetric biogas production rate by 10.6%, and the removal efficiencies of TS and VS were enhanced by 5.3% and 6.6%, respectively. The concentration of total volatile fatty acids (TVFA) was 30.3% lower in the GAC reactor, but the total ammonia nitrogen (TAN) content was 15.3% higher. Inoculating with acclimated sludge helped the system survive unfavorable conditions, where the TAN and TVFA contents were around 5,200 mg/L and 8,800 mg/L, respectively, but it failed to improve the biogas production efficiency_. The mechanisms that allowed GAC to increase the production of biogas were related to the improved hydrolysis process, enhanced microbial adhesion, the provision of electronic bridges, and enrichment of functional microorganism.

Intermittent micro-aeration: New strategy to control volatile fatty acid accumulation in high organic loading anaerobic digestion

This study developed an intermittent oxidation-reduction potential (ORP)-controlled micro-aeration system for high solids anaerobic digestion (AD) of lignocellulosic biomass without volatile fatty acids (VFA) accumulation at high organic loading rate (OLR). Traditional AD of Napier grass, a model lignocellulosic biomass, at an OLR of 5 g volatile solids (VS)/L/day resulted in an accumulation of total VFA concentration up to 9.2 g/L as acetic acid (HAc) equivalent, causing rapid drops in pH and methane yield, and driving the digester to the verge of failure. Once intermittent (every 24 h) ORP-controlled micro-aeration (at ORP of -470 mV) was initiated, the total VFA concentration rapidly decreased to 3.0 g HAc/L and the methane yield improved, resulting in stable digester performance without the need for alkalinity supplementation or OLR reduction. By combining reactor performance results, mass balance analyses, microbial community characterization data, and a bioenergetic evaluation, this study suggested that rapid VFA conversion and CH₄ production were carried out by facultative anaerobes and hydrogenotrophic methanogens under micro-aerobic conditions. This novel operating approach can be applied as an effective control strategy for high OLR AD processes especially in the event of VFA accumulation.