Microbial population in the biomass adhering to supporting material in a packed-bed reactor degrading organic solid waste

Enhancing two-phase anaerobic digestion of mixture of primary and secondary sludge by adding granular activated carbon (GAC): Evaluating acidogenic and methanogenic efficiency

Although the granular activated carbon (GAC) has been proved to enhance conventional single-phase anaerobic digestion (AD), how it impacts on acidogenic and methanogenic fermentation is still unknown. In this study, GAC was introduced to elevate the efficiency of two-phase AD, with mixture of primary and secondary sludge as substrate. Five dosages: 0, 0.1, 0.3, 0.5 and 0.7 g GAC/g TSS (Total Suspended Solids) were investigated to determine influences of GAC. The variations of biogas (hydrogen and methane), volatile fatty acids (VFAs), organics degradation and transformation in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. Modified Gompertz model and first-order reaction equation was applied to analyze the kinetics of biogas yield and VFAs utilization, respectively. Sludge reduction, electrical conductance and pH were also quantified to evaluate the system performance. The results showed that GAC could improve two-phase AD performance by enhancing methane production and organics conversion.

Isolation and Use of Coprothermobacter spp. to Improve Anaerobic Thermophilic Digestion of Grass

The isolation of microorganisms was performed from digestate from the process of the anaerobic digestion (AD) of grass after hyperthermophilic pretreatment. The bacterium that was isolated and identified was Coprothermobacter sp. Using the isolated bacteria, an AD process on fresh grass (GB) and pretreated grass (PGB) was carried out with 10% of its addition. The highest methane yield of 219 NmlCH₄/gVS was recorded for PGB at 55 °C. In contrast, fresh grass subjected to thermophilic digestion produced only 63 NmlCH₄/gVS. Due to the addition of bacteria in the AD process, an increase in the efficiency of hydrogen and methane production was observed in both fresh grass and grass after pretreatment.

Enrichment of thermophilic methanogenic microflora from mesophilic waste activated sludge for anaerobic digestion of garbage slurry

This study investigated a startup strategy for thermophilic methanogenic enrichment. Conventional waste activated sludge (WAS) was used as the seed. The WAS seed was incubated at 55 °C in a continuous-flow stirred tank reactor, with garbage slurry fed continuously as a substrate. One of the two reactors (termed reactor-high, RH) was fed with a high concentration of substrate (30 g-COD/L), while the other (reactor-low, RL) received a lower concentration of feed (15 g-COD/L). The specific organic loading rate was 0.2 g-COD/L/day initially, which was gradually increased by shortening the hydraulic retention time. The final OLR was 3.2 g-COD/L/day, after more than 90% of the initial WAS got washed out from the reactor and thermophilic microorganisms became dominant in the reactors. Biogas production rate and methane conversion ratio depended on substrate concentration, although total chemical oxygen demand removal and methane content were almost the same in RH and RL. Biogas production rate in RH was 3.2 times higher than that in RL, while the conversion ratio of RH was 1.6 times higher than that of RL. Quantitative polymerase chain reaction analysis using specific primers for the mcrA gene and high-throughput sequencing analysis of 16S rRNA gene amplicons demonstrated post enrichment differences in the microbial community, relative to that in the WAS. There was no significant difference in the enriched microbial community composition between RH and RL. In conclusion, thermophilic methanogenic microflora can be enriched from mesophilic seeds. Methanothermobacter, Methanosarcina, and other thermophilic bacteria were enriched in the community over time, with these thermophiles collectively accounting for ∼80% of the stable thermophilic community.

Enhanced methane production from cellulose using a two-stage process involving a bioelectrochemical system and a fixed film reactor

BACKGROUND

It is desirable to improve the anaerobic digestion processes of recalcitrant materials, such as cellulose. Enhancement of methane (CH₄) production from organic molecules was previously accomplished through coupling a bioelectrochemical system (BES); however, scaling-up BES-based production is difficult. Here, we developed a two-stage process consisting of a BES using low-cost and low-reactive carbon sheets as the cathode and anode, and a fixed film reactor (FFR) containing conductive material, i.e., carbon fiber textiles (CFTs) (:BES → FFR). By controlling the cathodic current at 2.7 μA/cm² without abiotic H₂ production, the three-electrode BES system was operated to mimic a microbial electrolysis cell.

RESULTS

The thermophilic BES (inlet pH: 6.1) and FFR (inlet pH: 7.5) were operated using hydraulic retention times (HRTs) of 2.5 and 4.2 days, respectively, corresponding to a cellulose load of 3555.6 mg-carbon (C)/(L day). The BES → FFR process achieved a higher CH₄ yield (37.5%) with 52.8 vol% CH₄ in the product gas compared to the non-bioelectrochemical system (NBES) → FFR process, which showed a CH₄ yield of 22.1% with 46.8 vol% CH₄. The CH₄ production rate (67.5 mM/day) obtained with the BER → FFR process was much higher than that obtained using electrochemical methanogenesis (0.27 mM/day). Application of the electrochemical system or CFTs improved the yields of CH₄ with the NBES → FFR or BES → non-fixed film reactor process, respectively. Meta 16S rRNA sequencing revealed that putative cellulolytic bacteria (identified as Clostridium species) were present in the BES and NBES, and followed (BES→ and NBES→) FFR. Notably, H₂-consuming methanogens, Methanobacterium sp. and Methanosarcina sp., showed increased relative abundances in the suspended fraction and attached fraction of (BES→) FFR, respectively, compared to that of (NBES→) FFR, although these methanogens were observed at trace levels in the BES and NBES.

CONCLUSIONS

These results indicate that bioelectrochemical preprocessing at a low current effectively induces interspecies H₂ transfer in the FFR with conductive material. Sufficient electrochemical preprocessing was observed using a relatively short HRT. This type of two-stage process, BES → FFR, is useful for stabilization and improvement of the biogas (CH₄) production from cellulosic material, and our results imply that the two-stage system developed here may be useful with other recalcitrant materials.

Sparking Anaerobic Digestion: Promoting Direct Interspecies Electron Transfer to Enhance Methane Production

Anaerobic digestion was one of the first bioenergy strategies developed, yet the interactions of the microbial community that is responsible for the production of methane are still poorly understood. For example, it has only recently been recognized that the bacteria that oxidize organic waste components can forge electrical connections with methane-producing microbes through biologically produced, protein-based, conductive circuits. This direct interspecies electron transfer (DIET) is faster than interspecies electron exchange via diffusive electron carriers, such as H2. DIET is also more resilient to perturbations such as increases in organic load inputs or toxic compounds. However, with current digester practices DIET rarely predominates. Improvements in anaerobic digestion associated with the addition of electrically conductive materials have been attributed to increased DIET, but experimental verification has been lacking. This deficiency may soon be overcome with improved understanding of the diversity of microbes capable of DIET, which is leading to molecular tools for determining the extent of DIET. Here we review the microbiology of DIET, suggest molecular strategies for monitoring DIET in anaerobic digesters, and propose approaches for re-engineering digester design and practices to encourage DIET.

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.

Carbon cloth enhances treatment of high-strength brewery wastewater in anaerobic dynamic membrane bioreactors

Anaerobic dynamic membrane bioreactors (AnDMBRs) can improve the efficiency of organic matter removal during wastewater treatment at a low cost. However, application of AnDMBRs for treatment of high-strength wastewater is usually unsuccessful. This study investigated whether use of conductive carbon cloth as the supporting material in an AnDMBR permits higher organic loading rates for treatment of brewery wastewater than non-conductive polyester cloth. The AnDMBR with carbon cloth operated stably with a COD removal efficiency of 98% even when high concentrations of influent COD (10,000 mg/L) were provided, while the polyester cloth reactor deteriorated when reactors were fed only 5000 mg/L influent COD. Microorganisms capable of direct interspecies electron transfer (DIET), including Geobacter and Methanothrix species, dominated the surface of the carbon cloth. These results demonstrate that carbon cloth provides an excellent supporting material for AnDMBRs by stimulating growth of microorganisms that can directly transport electrons to and from conductive materials.

Towards engineering application: Potential mechanism for enhancing anaerobic digestion of complex organic waste with different types of conductive materials

Conductive materials have been widely investigated to accelerate and stabilize the conversion of organic wastes to methane. However, the potential mechanisms involved with different types of conductive materials are still unclear. In this study, magnetite (Fe₃O₄) and granular activated carbon (GAC), as the two typical conductive materials, were respectively supplemented to acidogenesis and methanogenesis of a two-phase anaerobic digestion (AD) system in an attempt to explore their different mechanisms. The results showed that, magnetite supplemented to the acidogenic phase could enhance the decomposition of complex organics into simples, but significantly raise the hydrogen partial pressure as well as enrich the hydrogen-utilizing methanogens, which were not expected for aceticlastic methanogenesis known as a mainstream of methanogenesis in most of traditional digesters. GAC supplemented to the methanogenic phase had less influences on syntrophic metabolism of alcohols and fatty acids when acidogenesis was ineffective or out of work. Microbial community analysis suggested that direct interspecies electron transfer (DIET) had been established on the GAC, though the insignificant improvement of performances. Once magnetite was supplemented to the acidogenesis to improve the acidification efficiency, the syntrophic conversion of alcohols and fatty acids to methane in the GAC-supplemented methanogenic phase was significantly improved. These results suggested that, DIET was unlikely to participate in the direct decomposition of complex organics, even in the presence of GAC, but it could work effectively once acidogenesis functioned well.

Non-autotrophic methanogens dominate in anaerobic digesters

Anaerobic digesters are man-made habitats for fermentative and methanogenic microbes, and are characterized by extremely high concentrations of organics. However, little is known about how microbes adapt to such habitats. In the present study, we report phylogenetic, metagenomic, and metatranscriptomic analyses of microbiomes in thermophilic packed-bed digesters fed acetate as the major substrate, and we have shown that acetoclastic and hydrogenotrophic methanogens that utilize acetate as a carbon source dominate there. Deep sequencing and precise binning of the metagenomes reconstructed complete genomes for two dominant methanogens affiliated with the genera Methanosarcina and Methanothermobacter, along with 37 draft genomes. The reconstructed Methanosarcina genome was almost identical to that of a thermophilic acetoclastic methanogen Methanosarcina thermophila TM-1, indicating its cosmopolitan distribution in thermophilic digesters. The reconstructed Methanothermobacter (designated as Met2) was closely related to Methanothermobacter tenebrarum, a non-autotrophic hydrogenotrophic methanogen that grows in the presence of acetate. Met2 lacks the Cdh complex required for CO2 fixation, suggesting that it requires organic molecules, such as acetate, as carbon sources. Although the metagenomic analysis also detected autotrophic methanogens, they were less than 1% in abundance of Met2. These results suggested that non-autotrophic methanogens preferentially grow in anaerobic digesters containing high concentrations of organics.

Microbial diversity in an anaerobic digester with biogeographical proximity to geothermally active region

Anaerobic digestion of agricultural biomass or wastes can offer renewable energy, to help meet the rise in energy demands. The performance of an anaerobic digester considerably depends upon the complex interactions between bacterial and archaeal microbiome, which is greatly influenced by environmental factors. In the present study, we evaluate a microbial community of digester located at two different geographical locations, to understand whether the biogeographical proximity of a digester to a geothermally active region has any influence on microbial composition. The comparative microbial community profiling, highlights coexistence of specific bacterial and archaeal representatives (especially, Prosthecochloris sp., Conexibacter sp., Crenarchaeota isolate (Caldivirga sp.), Metallosphaera sp., Pyrobaculum sp. and Acidianus sp.) in a digester with close proximity to geothermally active region (Site I) and their absence in a digester located far-off from geothermally active region (Site II). A Sörensen's index of similarity of 83.33% and 66.66% for bacterial and archaeal community was observed in both the reactors, respectively.

Enhancing syntrophic metabolism in up-flow anaerobic sludge blanket reactors with conductive carbon materials

Syntrophic metabolism of alcohols and fatty acids is a critical step in anaerobic digestion, which if enhanced can better stabilize the process and enable shorter retention times. Direct interspecies electron transfer (DIET) has recently been recognized as an alternative route to hydrogen interspecies transfer as a mechanism for interspecies syntrophic electron exchange. Therefore, the possibility of accelerating syntrophic metabolism of ethanol in up-flow anaerobic sludge blanket (UASB) reactors by incorporating conductive materials in reactor design was investigated. Graphite, biochar, and carbon cloth all immediately enhanced methane production and COD removal. As the hydraulic retention time was decreased the increased effectiveness of treatment in reactors with conductive materials increased versus the control reactor. When these conductive materials were removed from the reactors rates of syntrophic metabolism declined to rates comparable to the control reactor. These results suggest that incorporating conductive materials in the design of UASB reactors may enhance digester effectiveness.

Thermophilic anaerobic digestion of thermal pretreated sludge: role of microbial community structure and correlation with process performances

Thermal hydrolysis pretreatment coupled with Thermophilic Anaerobic Digestion (TAD) for Waste Activated Sludge (WAS) treatment is a promising combination to improve biodegradation kinetics during stabilization. However, to date there is a limited knowledge of the anaerobic biomass composition and its impact on TAD process performances. In this study, the structure and dynamics of the microbial communities selected in two semi-continuous anaerobic digesters, fed with untreated and thermal pretreated sludge, were investigated. The systems were operated for 250 days at different organic loading rate. 16S rRNA gene clonal analysis and Fluorescence In Situ Hybridization (FISH) analyses allowed us to identify the majority of bacterial and archaeal populations. Proteolytic Coprothermobacter spp. and hydrogenotrophic Methanothermobacter spp. living in strict syntrophic association were found to dominate in TAD process. The establishment of a syntrophic proteolytic pathway was favoured by the high temperature of the process and enhanced by the thermal pretreatment of the feeding sludge. Proteolytic activity, alone or with thermal pretreatment, occurred during TAD as proven by increasing concentration of soluble ammonia and soluble COD (sCOD) during the process. However, the availability of a readily biodegradable substrate due to pretreatment allowed to significant sCOD removals (more than 55%) corresponding to higher biogas production in the reactor fed with thermal pretreated sludge. Microbial population dynamics analysed by FISH showed that Coprothermobacter and Methanothermobacter immediately established a stable syntrophic association in the reactor fed with pretreated sludge in line with the overall improved TAD performances observed under these conditions.

Carbon cloth stimulates direct interspecies electron transfer in syntrophic co-cultures

This study investigated the possibility that the electrical conductivity of carbon cloth accelerates direct interspecies electron transfer (DIET) in co-cultures. Carbon cloth accelerated metabolism of DIET co-cultures (Geobacter metallireducens-Geobacter sulfurreducens and G.metallireducens-Methanosarcina barkeri) but did not promote metabolism of co-cultures performing interspecies H2 transfer (Desulfovibrio vulgaris-G.sulfurreducens). On the other hand, DIET co-cultures were not stimulated by poorly conductive cotton cloth. Mutant strains lacking electrically conductive pili, or pili-associated cytochromes participated in DIET only in the presence of carbon cloth. In co-cultures promoted by carbon cloth, cells were primarily associated with the cloth although the syntrophic partners were too far apart for cell-to-cell biological electrical connections to be feasible. Carbon cloth seemingly mediated interspecies electron transfer between the distant syntrophic partners. These results suggest that the ability of carbon cloth to accelerate DIET should be considered in anaerobic digester designs that incorporate carbon cloth.

Digestion performance and microbial community in full-scale methane fermentation of stillage from sweet potato-shochu production

Sweet potato shochu is a traditional Japanese spirit produced mainly in the South Kyushu area in Japan. The amount of stillage reaches approximately 8 x 10(5) tons per year. Wastewater mainly containing stillage from the production of sweet potato-shochu was treated thermophilically in a full-scale treatment plant using fixed-bed reactors (8 reactors x 283 m3). Following the addition of Ni2+ and Co2+, the reactors have been stably operated for six years at a high chemical oxygen demand (COD) loading rate of 14 kg/(m3 x day). Analysis of coenzyme content and microbial communities indicated that similar microbial communities were present in the liquid phase and on the fiber carriers installed in reactors. Bacteria in the phyla Firmicutes as well as Bacteroidetes were dominant bacteria, and Methanosarcina thermophila as well as Methanothermobacter crinale were dominant methanogens in the reactors. This study reveals that stillage from sweet potato-shochu production can be treated effectively in a full-scale fixed-bed reactor under thermophilic conditions with the help of Ni2+ and Co2+. The high diversity of bacterial community and the coexistence of both aceticlastic and hydrogenotrophic methanogens contributed to the excellent fermentation performance.

Substrate type drives variation in reactor microbiomes of anaerobic digesters

The goal of this study was to obtain causative information about beta-diversity (differentiation between microbiomes) by comparing sequencing information between studies rather than just knowledge about alpha-diversity (microbiome richness). Here, published sequencing data were merged representing 78 anaerobic digester samples originating from 28 different studies for an overall comparison of beta-diversity (measured using unweighted UniFrac). It was found that digester microbiomes based on bacterial sequences clustered by substrate type, independent of the study of origin, and that this clustering could be attributed to distinct bacterial lineages.

Microbial community structure of a pilot-scale thermophilic anaerobic digester treating poultry litter

The microbial community structure of a stable pilot-scale thermophilic continuous stirred tank reactor digester stabilized on poultry litter was investigated. This 40-m(3) digester produced biogas with 57% methane, and chemical oxygen demand removal of 54%. Bacterial and archaeal diversity were examined using both cloning and pyrosequencing that targeted 16S rRNA genes. The bacterial community was dominated by phylum Firmicutes, constituting 93% of the clones and 76% of the pyrotags. Of the Firmicutes, class Clostridia (52% pyrotags) was most abundant followed by class Bacilli (13% pyrotags). The bacterial libraries identified 94 operational taxonomic units (OTUs) and pyrosequencing identified 577 OTUs at the 97% minimum similarity level. Fifteen OTUs were dominant (≥2% abundance), and nine of these were novel unclassified Firmicutes. Several of the dominant OTUs could not be classified more specifically than Clostridiales, but were most similar to plant biomass degraders, including Clostridium thermocellum. Of the rare pyrotag OTUs (<0.5% abundance), 75% were Firmicutes. The dominant methanogen was Methanothermobacter which has hydrogenotrophic metabolism, and accounted for >99% of the archaeal clones. Based on the primary methanogen, as well as digester chemistry (high VA and ammonia levels), we propose that bacterial acetate oxidation is the primary pathway in this digester for the control of acetate levels.

Metaproteomics of cellulose methanisation under thermophilic conditions reveals a surprisingly high proteolytic activity

Cellulose is the most abundant biopolymer on Earth. Optimising energy recovery from this renewable but recalcitrant material is a key issue. The metaproteome expressed by thermophilic communities during cellulose anaerobic digestion was investigated in microcosms. By multiplying the analytical replicates (65 protein fractions analysed by MS/MS) and relying solely on public protein databases, more than 500 non-redundant protein functions were identified. The taxonomic community structure as inferred from the metaproteomic data set was in good overall agreement with 16S rRNA gene tag pyrosequencing and fluorescent in situ hybridisation analyses. Numerous functions related to cellulose and hemicellulose hydrolysis and fermentation catalysed by bacteria related to Caldicellulosiruptor spp. and Clostridium thermocellum were retrieved, indicating their key role in the cellulose-degradation process and also suggesting their complementary action. Despite the abundance of acetate as a major fermentation product, key methanogenesis enzymes from the acetoclastic pathway were not detected. In contrast, enzymes from the hydrogenotrophic pathway affiliated to Methanothermobacter were almost exclusively identified for methanogenesis, suggesting a syntrophic acetate oxidation process coupled to hydrogenotrophic methanogenesis. Isotopic analyses confirmed the high dominance of the hydrogenotrophic methanogenesis. Very surprising was the identification of an abundant proteolytic activity from Coprothermobacter proteolyticus strains, probably acting as scavenger and/or predator performing proteolysis and fermentation. Metaproteomics thus appeared as an efficient tool to unravel and characterise metabolic networks as well as ecological interactions during methanisation bioprocesses. More generally, metaproteomics provides direct functional insights at a limited cost, and its attractiveness should increase in the future as sequence databases are growing exponentially.

Candidatus Methanogranum caenicola: a novel methanogen from the anaerobic digested sludge, and proposal of Methanomassiliicoccaceae fam. nov. and Methanomassiliicoccales ord. nov., for a methanogenic lineage of the class Thermoplasmata

The class Thermoplasmata harbors huge uncultured archaeal lineages at the order level, so-called Groups E2 and E3. A novel archaeon Kjm51a affiliated with Group E2 was enriched from anaerobic sludge in the present study. Clone library analysis of the archaeal 16S rRNA and mcrA genes confirmed a unique archaeal population in the enrichment culture. The 16S rRNA gene-based phylogeny revealed that the enriched archaeon Kjm51a formed a distinct cluster within Group E2 in the class Thermoplasmata together with Methanomassiliicoccus luminyensis B10^T and environmental clone sequences derived from anaerobic digesters, bovine rumen, and landfill leachate. Archaeon Kjm51a showed 87.7% 16S rRNA gene sequence identity to the closest cultured species, M. luminyensis B10^T, indicating that archaeon Kjm51a might be phylogenetically novel at least at the genus level. In fluorescence in situ hybridization analysis, archaeon Kjm51a was observed as coccoid cells completely corresponding to the archaeal cells detected, although bacterial rod cells still coexisted. The growth of archaeon Kjm51a was dependent on the presence of methanol and yeast extract, and hydrogen and methane were produced in the enrichment culture. The addition of 2-bromo ethanesulfonate to the enrichment culture completely inhibited methane production and increased hydrogen concentration, which suggested that archaeon Kjm51a is a methanol-reducing hydrogenotrophic methanogen. Taken together, we propose the provisional taxonomic assignment, named Candidatus Methanogranum caenicola, for the enriched archaeon Kjm51a belonging to Group E2. We also propose to place the methanogenic lineage of the class Thermoplasmata in a novel order, Methanomassiliicoccales ord. nov.

Efficient production of methane from artificial garbage waste by a cylindrical bioelectrochemical reactor containing carbon fiber textiles

A cylindrical bioelectrochemical reactor (BER) containing carbon fiber textiles (CFT; BER + CFT) has characteristics of bioelectrochemical and packed-bed systems. In this study, utility of a cylindrical BER + CFT for degradation of a garbage slurry and recovery of biogas was investigated by applying 10% dog food slurry. The working electrode potential was electrochemically regulated at −0.8 V (vs. Ag/AgCl). Stable methane production of 9.37 L-CH₄ · L⁻¹ · day⁻¹ and dichromate chemical oxygen demand (CODcr) removal of 62.5% were observed, even at a high organic loading rate (OLR) of 89.3 g-CODcr · L⁻¹ · day⁻¹. Given energy as methane (372.6 kJ · L⁻¹ · day⁻¹) was much higher than input electric energy to the working electrode (0.6 kJ · L⁻¹ · day⁻¹) at this OLR. Methanogens were highly retained in CFT by direct attachment to the cathodic working electrodes (52.3%; ratio of methanogens to prokaryotes), compared with the suspended fraction (31.2%), probably contributing to the acceleration of organic material degradation and removal of organic acids. These results provide insight into the application of cylindrical BER + CFT in efficient methane production from garbage waste including a high percentage of solid fraction.

Operation of a cylindrical bioelectrochemical reactor containing carbon fiber fabric for efficient methane fermentation from thickened sewage sludge

A bioelectrochemical reactor (BER) containing carbon fiber fabric (CFF) (BER+CFF) enabled efficient methane fermentation from thickened sewage sludge. A cylindrical BER+CFF was proposed and scaled-up to a volume of 4.0-L. Thickened sewage sludge was treated using three types of methanogenic reactors. The working electrode potential in the BER+CFF was regulated at -0.8 V (vs. Ag/AgCl). BER+CFF showed gas production of 3.57 L L(-1) day(-1) at a hydraulic retention time (HRT) of 4.0 days; however, non-BER+CFF showed a lower gas production rate (0.83 L L(-1) day(-1)) at this HRT, suggesting positive effects of electrochemical regulation. A stirred tank reactor (without CFF) deteriorated at an HRT of 10 days, suggesting positive effects of CFF. 16S rRNA gene analysis showed that the BER+CFF included 3 kinds of hydrogenotrophic methanogens and 1 aceticlastic methanogen. These results demonstrate the effectiveness of the BER+CFF for scale-up and flexibility of this technology.

A pyrosequencing-based metagenomic study of methane-producing microbial community in solid-state biogas reactor

BACKGROUND

A solid-state anaerobic digestion method is used to produce biogas from various solid wastes in China but the efficiency of methane production requires constant improvement. The diversity and abundance of relevant microorganisms play important roles in methanogenesis of biomass. The next-generation high-throughput pyrosequencing platform (Roche/454 GS FLX Titanium) provides a powerful tool for the discovery of novel microbes within the biogas-generating microbial communities.

RESULTS

To improve the power of our metagenomic analysis, we first evaluated five different protocols for extracting total DNA from biogas-producing mesophilic solid-state fermentation materials and then chose two high-quality protocols for a full-scale analysis. The characterization of both sequencing reads and assembled contigs revealed that the most prevalent microbes of the fermentation materials are derived from Clostridiales (Firmicutes), which contribute to degrading both protein and cellulose. Other important bacterial species for decomposing fat and carbohydrate are Bacilli, Gammaproteobacteria, and Bacteroidetes (belonging to Firmicutes, Proteobacteria, and Bacteroidetes, respectively). The dominant bacterial species are from six genera: Clostridium, Aminobacterium, Psychrobacter, Anaerococcus, Syntrophomonas, and Bacteroides. Among them, abundant Psychrobacter species, which produce low temperature-adaptive lipases, and Anaerococcus species, which have weak fermentation capabilities, were identified for the first time in biogas fermentation. Archaea, represented by genera Methanosarcina, Methanosaeta and Methanoculleus of Euryarchaeota, constitute only a small fraction of the entire microbial community. The most abundant archaeal species include Methanosarcina barkeri fusaro, Methanoculleus marisnigri JR1, and Methanosaeta theromphila, and all are involved in both acetotrophic and hydrogenotrophic methanogenesis.

CONCLUSIONS

The identification of new bacterial genera and species involved in biogas production provides insights into novel designs of solid-state fermentation under mesophilic or low-temperature conditions.

Methanosarcina: the rediscovered methanogen for heavy duty biomethanation

Anaerobic digestion is an important technology in the framework of renewable energy production. The anaerobic digestion system is susceptible to perturbations due to the sensitivity of the methanogens towards environmental factors. Currently, technology is evolving from conventional waste treatment, i.e. the removal of pollutants, to very intensive biogas production from concentrated wastes, in the framework of bio-energy production. In the latter configuration Methanosarcina species appear to be of crucial importance. Methanosarcina sp. are, compared to other methanogens, quite robust towards different impairments. They are reported to be tolerant to total ammonium concentrations up to 7000 mg L(-1), salt concentrations up to 18,000 mg Na(+)L(-1), a pH shock of 0.8-1.0 units and acetate concentrations up to 15,000 mg CODL(-1). The possibilities of Methanosarcina sp. as key organisms in specific types of anaerobic digestion systems are demonstrated in this review.

Bioreactor performance and methanogenic population dynamics in a low-temperature (5-18 °C) anaerobic fixed-bed reactor

The effect of temperature on the functionality of microbial community structure in a low temperature, anaerobic fixed-bed reactor was studied by decreasing the operating temperature from 18 °C to 5 °C. The reactor was productive within 20 days and produced stable methane content in biogas (above 77%) throughout the trial period. At 17 °C and 15 °C, chemical oxygen demand (COD) removal efficiency and biogas production of reactor were significantly reduced. These might be temperature thresholds when fixed-bed reactors are operated under low temperatures. The methanogen community composition was analyzed using 16S rRNA gene clone library screening and quantitative PCR. At low ambient temperatures, Methanomicrobiales were dominant methanogens, and they preferentially adhered to the carbon fiber carrier. The results indicated that 16S rRNA levels of Methanomicrobiales and Methanosaetaceae in adhering sludge were higher than in deposited sludge, and they all contributed to the efficient performance of the fixed-bed reactor at low operating temperatures.

Changes in bacterial communities from anaerobic digesters during petroleum hydrocarbon degradation

Anaerobic biodegradation of petroleum hydrocarbons (PHC) to methane has been recognized to occur in oil reservoirs and contaminated surface sites alike. This process could be employed efficiently for the treatment of contaminated materials, including petrochemical wastes and PHC-contaminated soil, since no external electron acceptor is required. Moreover, the controlled production of methane in digestion plants, similarly to the anaerobic digestion (AD) of energy crops or organic residues, would enable for energy recovery from these wastes. At present, little is known about the bacterial communities involved in and responsible for hydrocarbon fermentation, the initial step in PHC conversion to methane. In the present study, the fate of two different methanogenic communities derived from the AD of wastewater (WWT) and of biowaste, mixed with PHC-contaminated soil (SWT), was monitored during incubation with PHC using denaturing gradient gel electrophoresis (DGGE) of 16S rDNA genes amplified with Bacteria-specific primers. During 11 months of incubation, slight but significant degradation of PHC occurred in both sludges and distinct bacterial communities were developing. In both sludges, Bacteroidetes were found. In addition, in WWT, the bacterial community was found to be dominated by Synergistetes and Proteobacteria, while Firmicutes and unidentified members were abundant in SWT. These results indicate that bacterial communities from anaerobic digesters can adapt to and degrade petroleum hydrocarbons. The decontamination of PHC-containing waste via fermentative treatment appears possible.

Kinetic modelling and microbial community assessment of anaerobic biphasic fixed film bioreactor treating distillery spent wash

Anaerobic digestion, microbial community structure and kinetics were studied in a biphasic continuously fed, upflow anaerobic fixed film reactor treating high strength distillery wastewater. Treatment efficiency of the bioreactor was investigated at different hydraulic retention times (HRT) and organic loading rates (OLR 5-20 kg COD m⁻³ d⁻¹). Applying the modified Stover-Kincannon model to the reactor, the maximum removal rate constant (U(max)) and saturation value constant (K(B)) were found to be 2 kg m⁻³ d⁻¹ and 1.69 kg m⁻³ d⁻¹ respectively. Bacterial community structures of acidogenic and methanogenic reactors were assessed using culture-independent analyses. Sequencing of 16S rRNA genes exhibited a total of 123 distinct operational taxonomic units (OTUs) comprising 49 from acidogenic reactor and 74 (28 of eubacteria and 46 of archaea) from methanogenic reactor. The findings reveal the role of Lactobacillus sp. (Firmicutes) as dominant acid producing organisms in acidogenic reactor and Methanoculleus sp. (Euryarchaeotes) as foremost methanogens in methanogenic reactor.

A bioelectrochemical reactor containing carbon fiber textiles enables efficient methane fermentation from garbage slurry

A packed-bed system includes supporting materials to retain microorganisms and a bioelectrochemical system influences the microbial metabolism. In our study, carbon fiber textiles (CFT) as a supporting material was attached onto a carbon working electrode in a bioelectrochemical reactor (BER) that degrades garbage slurry to methane, in order to investigate the effect of combining electrochemical regulation and packing CFT. The potential on the working electrode in the BER containing CFT was set to -1.0 V or -0.8 V (vs. Ag/AgCl). BERs containing CFT exhibited higher methane production, elimination of dichromate chemical oxygen demand, and the ratio of methanogens in the suspended fraction than reactors containing CFT without electrochemical regulation at an organic loading rate (OLR) of 27.8 gCODcr/L/day. In addition, BERs containing CFT exhibited higher reactor performances than BERs without CFT at this OLR. Our results revealed that the new design that combined electrochemical regulation and packing CFT was effective.

Description of Anaerobaculum hydrogeniformans sp. nov., an anaerobe that produces hydrogen from glucose, and emended description of the genus Anaerobaculum

A novel anaerobic, moderately thermophilic, NaCl-requiring fermentative bacterium, strain OS1T, was isolated from oil production water collected from Alaska, USA. Cells were Gram-negative, non-motile, non-spore-forming rods (1.7-2.7×0.4-0.5 µm). The G+C content of the genomic DNA of strain OS1T was 46.6 mol%. The optimum temperature, pH and NaCl concentration for growth of strain OS1T were 55 °C, pH 7 and 10 g l(-1), respectively. The bacterium fermented D-fructose, D-glucose, maltose, D-mannose, α-ketoglutarate, L-glutamate, malonate, pyruvate, L-tartrate, L-asparagine, Casamino acids, L-cysteine, L-histidine, L-leucine, L-phenylalanine, L-serine, L-threonine, L-valine, inositol, inulin, tryptone and yeast extract. When grown on D-glucose, 3.86 mol hydrogen and 1.4 mol acetate were produced per mol substrate. Thiosulfate, sulfur and L-cystine were reduced to sulfide, and crotonate was reduced to butyrate with glucose as the electron donor. 16S rRNA gene sequence analysis indicated that strain OS1T was related to Anaerobaculum thermoterrenum (99.7 % similarity to the type strain), a member of the phylum Synergistetes. DNA-DNA hybridization between strain OS1T and A. thermoterrenum DSM 13490T yielded 68 % relatedness. Unlike A. thermoterrenum, strain OS1T fermented malonate, maltose, tryptone, L-leucine and L-phenylalanine, but not citrate, fumarate, lactate, L-malate, glycerol, pectin or starch. The major cellular fatty acid of strain OS1T was iso-C15:0 (91 % of the total). Strain OS1T also contained iso-C13:0 3-OH (3 %), which was absent from A. thermoterrenum, and iso-C13:0 (2 %), which was absent from Anaerobaculum mobile. On the basis of these results, strain OS1T represents a novel species of the genus Anaerobaculum, for which the name Anaerobaculum hydrogeniformans sp. nov. is proposed. The type strain is OS1T (=DSM 22491T=ATCC BAA-1850T). An emended description of the genus Anaerobaculum is also given.

Dynamic transition of microbial communities in response to acidification in fixed-bed anaerobic baffled reactors (FABR) of two different flow directions

Two five-compartment fixed-bed anaerobic baffled reactors (FABRs) were operated under deteriorative and stable conditions. The FABRs were identical except for flow direction: one was horizontal (H-reactor) and the other was vertical (V-reactor). The microbial community dynamics in 1, 3 and 5 compartments were analyzed using denaturing gradient gel electrophoresis (DGGE), 16S rRNA gene clone library screening and quantitative PCR. After start-up, the Methanomicrobiales were typically dominant in adhering sludge of 5th compartments of two reactors. Because methanogenesis mainly occurred in the latter compartment, Methanomicrobiales were likely to play important roles in FABRs. FABRs recovered from performance deterioration very quickly. Meanwhile, methanogens and dominant methanogens greatly increased in every compartment of two reactors. Our results indicated that 16S rRNA levels of methanogens in the adhering sludge were higher than those in the deposited sludge and the adhering fraction of V-reactor held up more acid-resistant bacteria and methanogens than that of H-reactor.

Comparative and joint analysis of two metagenomic datasets from a biogas fermenter obtained by 454-pyrosequencing

Biogas production from renewable resources is attracting increased attention as an alternative energy source due to the limited availability of traditional fossil fuels. Many countries are promoting the use of alternative energy sources for sustainable energy production. In this study, a metagenome from a production-scale biogas fermenter was analysed employing Roche's GS FLX Titanium technology and compared to a previous dataset obtained from the same community DNA sample that was sequenced on the GS FLX platform. Taxonomic profiling based on 16S rRNA-specific sequences and an Environmental Gene Tag (EGT) analysis employing CARMA demonstrated that both approaches benefit from the longer read lengths obtained on the Titanium platform. Results confirmed Clostridia as the most prevalent taxonomic class, whereas species of the order Methanomicrobiales are dominant among methanogenic Archaea. However, the analyses also identified additional taxa that were missed by the previous study, including members of the genera Streptococcus, Acetivibrio, Garciella, Tissierella, and Gelria, which might also play a role in the fermentation process leading to the formation of methane. Taking advantage of the CARMA feature to correlate taxonomic information of sequences with their assigned functions, it appeared that Firmicutes, followed by Bacteroidetes and Proteobacteria, dominate within the functional context of polysaccharide degradation whereas Methanomicrobiales represent the most abundant taxonomic group responsible for methane production. Clostridia is the most important class involved in the reductive CoA pathway (Wood-Ljungdahl pathway) that is characteristic for acetogenesis. Based on binning of 16S rRNA-specific sequences allocated to the dominant genus Methanoculleus, it could be shown that this genus is represented by several different species. Phylogenetic analysis of these sequences placed them in close proximity to the hydrogenotrophic methanogen Methanoculleus bourgensis. While rarefaction analyses still indicate incomplete coverage, examination of the GS FLX Titanium dataset resulted in the identification of additional genera and functional elements, providing a far more complete coverage of the community involved in anaerobic fermentative pathways leading to methane formation.

Ammonium concentrations in produced waters from a mesothermic oil field subjected to nitrate injection decrease through formation of denitrifying biomass and anammox activity

Community analysis of a mesothermic oil field, subjected to continuous field-wide injection of nitrate to remove sulfide, with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes indicated the presence of heterotrophic and sulfide-oxidizing, nitrate-reducing bacteria (hNRB and soNRB). These reduce nitrate by dissimilatory nitrate reduction to ammonium (e.g., Sulfurospirillum and Denitrovibrio) or by denitrification (e.g., Sulfurimonas, Arcobacter, and Thauera). Monitoring of ammonium concentrations in producing wells (PWs) indicated that denitrification was the main pathway for nitrate reduction in the field: breakthrough of nitrate and nitrite in two PWs was not associated with an increase in the ammonium concentration, and no increase in the ammonium concentration was seen in any of 11 producing wells during periods of increased nitrate injection. Instead, ammonium concentrations in produced waters decreased on average from 0.3 to 0.2 mM during 2 years of nitrate injection. Physiological studies with produced water-derived hNRB microcosms indicated increased biomass formation associated with denitrification as a possible cause for decreasing ammonium concentrations. Use of anammox-specific primers and cloning of the resulting PCR product gave clones affiliated with the known anammox genera "Candidatus Brocadia" and "Candidatus Kuenenia," indicating that the anammox reaction may also contribute to declining ammonium concentrations. Overall, the results indicate the following: (i) that nitrate injected into an oil field to oxidize sulfide is primarily reduced by denitrifying bacteria, of which many genera have been identified by DGGE, and (ii) that perhaps counterintuitively, nitrate injection leads to decreasing ammonium concentrations in produced waters.

The effect of enzyme addition on anaerobic digestion of JoseTall Wheat Grass

The effects of the addition of enzyme products containing cellulase, hemicellulase, and beta-glucosidase to anaerobic digestion systems were studied using JoseTall Wheat Grass (wheat grass) as a model substrate. Anaerobic digestion tests were performed using batch reactors operated at 50 degrees C. The application of enzyme products in three digestion configurations were simulated and investigated: (1) enzyme addition to a single-stage digester, (2) pre-treatment of wheat grass with enzymes followed by a single-stage anaerobic digestion, and (3) enzyme addition to the first stage (hydrolysis and acidification) of a two-stage digestion system. The enzyme products showed positive effects on the solubilization of wheat grass when used alone to treat the wheat grass. However, no significant differences in biogas and methane yields, and volatile solids reduction resulted when the enzyme products were tested in the anaerobic digestion systems. This reveals that the microorganisms present in the inoculum were effective in carrying out the digestion of wheat grass. The types of microorganisms present in the inoculum were identified using 16S rRNA sequence analysis. A comparison of the sequences between the different inocula revealed that the prevalent operational taxonomic units were similar, but that the acidified inoculum contained a higher percentage of the species Thermotogae.

Phylogenetic characterization of a biogas plant microbial community integrating clone library 16S-rDNA sequences and metagenome sequence data obtained by 454-pyrosequencing

The phylogenetic structure of the microbial community residing in a fermentation sample from a production-scale biogas plant fed with maize silage, green rye and liquid manure was analysed by an integrated approach using clone library sequences and metagenome sequence data obtained by 454-pyrosequencing. Sequencing of 109 clones from a bacterial and an archaeal 16S-rDNA amplicon library revealed that the obtained nucleotide sequences are similar but not identical to 16S-rDNA database sequences derived from different anaerobic environments including digestors and bioreactors. Most of the bacterial 16S-rDNA sequences could be assigned to the phylum Firmicutes with the most abundant class Clostridia and to the class Bacteroidetes, whereas most archaeal 16S-rDNA sequences cluster close to the methanogen Methanoculleus bourgensis. Further sequences of the archaeal library most probably represent so far non-characterised species within the genus Methanoculleus. A similar result derived from phylogenetic analysis of mcrA clone sequences. The mcrA gene product encodes the alpha-subunit of methyl-coenzyme-M reductase involved in the final step of methanogenesis. BLASTn analysis applying stringent settings resulted in assignment of 16S-rDNA metagenome sequence reads to 62 16S-rDNA amplicon sequences thus enabling frequency of abundance estimations for 16S-rDNA clone library sequences. Ribosomal Database Project (RDP) Classifier processing of metagenome 16S-rDNA reads revealed abundance of the phyla Firmicutes, Bacteroidetes and Euryarchaeota and the orders Clostridiales, Bacteroidales and Methanomicrobiales. Moreover, a large fraction of 16S-rDNA metagenome reads could not be assigned to lower taxonomic ranks, demonstrating that numerous microorganisms in the analysed fermentation sample of the biogas plant are still unclassified or unknown.

H2 production potential in thermophilic mixed fermentation

The effect of pH on dark fermentative H(2) production at 55 degrees C was studied using three different inocula namely windrow yard waste compost (G), anaerobic organic waste compost (H) and activated sludge (A) and 2 g/L glucose as substrate. The sequential batch experiments were performed by controlling the pH at 5, 5.5 and 6. The highest molar H(2) yields were found to be 1.7 at pH 5 for G and H and 1.6 at pH 5.5 for A, while severe drops in the subsequent runs were associated with lactate fermentation. At pH 5.5 in G, despite the butyrate formation ceased, lower but more stable H(2) yields of about 1.1 were obtained through acetate-ethanol fermentation. Besides, the clostridial thermopiles prevalent at pH 5 were substituted by Thermoanaerobacterium-related species at pH 5.5 and 6. The findings suggested that for a stable H(2) yield through acetate-butyrate fermentation at 55 degrees C with compost inoculum G or H, the pH has to be controlled between pH 5 and 5.5. This is one of the few papers discussing the effects of pH on H(2) production through thermophilic mixed fermentation of glucose by using three different natural mixed culture inocula.