Perspectives for anaerobic digestion

Mediation of Bacterial Interactions via a Novel Membrane-Based Segregator to Enhance Biological Nitrogen Removal

The regulation of microbial subpopulations in wastewater treatment plants (WWTPs) with desired functions can guarantee nutrient removal. In nature, "good fences make good neighbors," which can be applied to engineering microbial consortia. Herein, a membrane-based segregator (MBSR) was proposed, where porous membranes not only promote the diffusion of metabolic products but also isolate incompatible microbes. The MBSR was integrated with an anoxic/aerobic membrane bioreactor (i.e., an experimental MBR). The long-term operation showed that the experimental MBR exhibited higher nitrogen removal (10.45 ± 2.73 mg/L total nitrogen) than the control MBR (21.68 ± 4.23 mg/L) in the effluent. The MBSR resulted in much lower oxygen reduction potential in the anoxic tank of the experimental MBR (-82.00 mV) compared to that of the control MBR (83.25 mV). The lower oxygen reduction potential can inevitably aid in the occurrence of denitrification. The 16S rRNA sequencing showed that the MBSR significantly enriched acidogenic consortia, which yielded considerable volatile fatty acids by fermenting the added carbon sources and allowed efficient transfer of these small molecules to the denitrifying community. Moreover, the sludge communities of the experimental MBR harbored a higher abundance of denitrifying bacteria than those of the control MBR. Metagenomic analysis further corroborated these sequencing results. The spatially structured microbial communities in the experimental MBR system demonstrate the practicability of the MBSR, achieving nitrogen removal efficiency superior to that of mixed populations. Our study provides an engineering method for modulating the assembly and metabolic division of labor of subpopulations in WWTPs. IMPORTANCE This study provides an innovative and applicable method for regulating subpopulations (activated sludge and acidogenic consortia), which contributes to the precise control of the metabolic division of labor in biological wastewater treatment processes.

The role of microbial ecology in improving the performance of anaerobic digestion of sewage sludge

The use of next-generation diagnostic tools to optimise the anaerobic digestion of municipal sewage sludge has the potential to increase renewable natural gas recovery, improve the reuse of biosolid fertilisers and help operators expand circular economies globally. This review aims to provide perspectives on the role of microbial ecology in improving digester performance in wastewater treatment plants, highlighting that a systems biology approach is fundamental for monitoring mesophilic anaerobic sewage sludge in continuously stirred reactor tanks. We further highlight the potential applications arising from investigations into sludge ecology. The principal limitation for improvements in methane recoveries or in process stability of anaerobic digestion, especially after pre-treatment or during co-digestion, are ecological knowledge gaps related to the front-end metabolism (hydrolysis and fermentation). Operational problems such as stable biological foaming are a key problem, for which ecological markers are a suitable approach. However, no biomarkers exist yet to assist in monitoring and management of clade-specific foaming potentials along with other risks, such as pollutants and pathogens. Fundamental ecological principles apply to anaerobic digestion, which presents opportunities to predict and manipulate reactor functions. The path ahead for mapping ecological markers on process endpoints and risk factors of anaerobic digestion will involve numerical ecology, an expanding field that employs metrics derived from alpha, beta, phylogenetic, taxonomic, and functional diversity, as well as from phenotypes or life strategies derived from genetic potentials. In contrast to addressing operational issues (as noted above), which are effectively addressed by whole population or individual biomarkers, broad improvement and optimisation of function will require enhancement of hydrolysis and acidogenic processes. This will require a discovery-based approach, which will involve integrative research involving the proteome and metabolome. This will utilise, but overcome current limitations of DNA-centric approaches, and likely have broad application outside the specific field of anaerobic digestion.

Diversity of novel archaeal viruses infecting methanogens discovered through coupling of stable isotope probing and metagenomics

Diversity of viruses infecting non-extremophilic archaea has been grossly understudied. This is particularly the case for viruses infecting methanogenic archaea, key players in the global carbon biogeochemical cycle. Only a dozen of methanogenic archaeal viruses have been isolated so far. In the present study, we implemented an original coupling between stable isotope probing and complementary shotgun metagenomic analyses to identify viruses of methanogens involved in the bioconversion of formate, which was used as the sole carbon source in batch anaerobic digestion microcosms. Under our experimental conditions, the microcosms were dominated by methanogens belonging to the order Methanobacteriales (Methanobacterium and Methanobrevibacter genera). Metagenomic analyses yielded several previously uncharacterized viral genomes, including a complete genome of a head-tailed virus (class Caudoviricetes, proposed family Speroviridae, Methanobacterium host) and several near-complete genomes of spindle-shaped viruses. The two groups of viruses are predicted to infect methanogens of the Methanobacterium and Methanosarcina genera and represent two new virus families. The metagenomics results are in good agreement with the electron microscopy observations, which revealed the dominance of head-tailed virus-like particles and the presence of spindle-shaped particles. The present study significantly expands the knowledge on the viral diversity of viruses of methanogens.

Soil Microbial Diversity and Community Composition in Rice-Fish Co-Culture and Rice Monoculture Farming System

Simple Summary

The integration of fish in rice fields can influence the diversity and structural composition of soil microbial communities. Therefore, soil microorganisms between rice–fish co-culture (RF) and rice monoculture (MC) were compared. The key findings revealed that Actinobacteria, Chloroflexi, Proteobacteria, Acidobacteria, and Planctomycetes were the most dominant taxa across both paddy fields. The most abundant genus in MC belonged to Anaeromyxobacter, whereas that in RF was Bacillus. Nitrogen fixation, aromatic compound degradation, and hydrocarbon degradation were more abundant in RF. Phosphatase, β-glucosidase, cellulase, and urease enzymes were detected in both paddy fields. However, a 2-year conversion from organic rice to rice–fish co-culture may not be long enough to significantly alter alpha diversity indices.

Abstract

Soil microorganisms play an important role in determining nutrient cycling. The integration of fish into rice fields can influence the diversity and structural composition of soil microbial communities. However, regarding the rice–fish co-culture (RF) farming system in Thailand, the study of the diversity and composition of soil microbes is still limited. Here, we aim to compare the microbial diversity, community composition, and functional structure of the bacterial communities between RF and rice monoculture (MC) farming systems and identify the environmental factors shaping bacterial community composition. Bacterial taxonomy was observed using 16s rRNA gene amplicon sequencing, and the functional structures of the bacterial communities were predicted based on their taxonomy and sequences. The results showed that soil organic carbon, total nitrogen (TN), organic matter, available phosphorous, and clay content were significantly higher in RF than in MC. The most dominant taxa across both paddy rice fields belonged to Actinobacteria, Chloroflexi, Proteobacteria, Acidobacteria, and Planctomycetes. The taxa Nitrosporae, Rokubacteria, GAL15, and Elusimicrobia were significantly different between both rice fields. At the genus level, Bacillus, Anaeromyxobacter, and HSB OF53-F07 were the predominant genera in both rice fields. The most abundant genus in MC was Anaeromyxobacter, whereas RF belonged to Bacillus. The community composition in MC was positively correlated with magnesium and sand content, while in RF was positively correlated with pH, TN, and clay content. Nitrogen fixation, aromatic compound degradation, and hydrocarbon degradation were more abundant in RF, while cellulolysis, nitrification, ureolysis, and phototrophy functional groups were more abundant in MC. The enzymes involved in paddy soil ecosystems included phosphatase, β-glucosidase, cellulase, and urease. These results provide novel insights into integrated fish in the paddy field as an efficient agricultural development strategy for enhancing soil microorganisms that increase soil fertility.

A review of advances in valorization and post-treatment of anaerobic digestion liquid fraction effluent

Traditionally, digestate is considered a waste, which is used as fertiliser in the agriculture industry. Recent studies focus on increasing the profitability of digestate by extracting reusable nutrients to promote biogas plants cost-effectiveness, sustainable management and circular economy. This review focuses on the post-treatment and valorization of liquor which is produced by solid-liquid fractioning of digestate. Nutrient recovery and removal from liquor are possible through mechanical, physicochemical and biological procedures. The processes discussed involve complex procedures that differ in economic value, feasibility, legislative restrictions and performance. The parameters that should be considered to employ these techniques are influenced by liquor characteristics, topography, climate conditions and available resources. These are key parameters to keep in mind during designing and manufacturing a biogas plant. In the following chapters, a discussion on available liquor treatment methods takes place. The present study examines the critical aspects of the available liquor treatment methods.

Determining Optimal Temperature Combination for Effective Pretreatment and Anaerobic Digestion of Corn Stalk

Temperature is one of the important factors affecting both chemical pretreatment and anaerobic digestion (AD) process of corn stalk (CS). In this work, the combined ways between pretreatment temperature (40 °C and 60 °C) and AD temperature (35 °C and 55 °C) were selected to investigate the AD performance for sodium hydroxide (NaOH) pretreated CS. Three organic loading rates (OLRs) of 1.6, 1.8 and 2.0 g·L⁻¹·d⁻¹ were studied within 255 days using continuously stirred tank reactors (CSTR). The results revealed that biogas yields of CS after pretreated were higher than that of untreated groups by 36.79–55.93% and 11.49–32.35%, respectively. When the temperature of NaOH pretreatment changed from 40 °C to 60 °C, there was no significant difference in enhancing the methane yields during the three OLRs. The mesophilic AD (MAD) of CS pretreated with 2% NaOH under 40 °C and 60 °C conditions produced 275 and 280 mL·g_vs⁻¹ methane yield at OLR of 1.6 g·L⁻¹·d⁻¹. However, as the OLR increased, the methane yield of CS under thermophilic AD (TAD) condition was further higher than under MAD condition. Furthermore, from the perspectives of energy balance and economic analysis, AD of 40 °C-treated CS recovered more energy and TAD is less expensive. Therefore, temperature of 40 °C was considered as an appropriate for pretreatment whether in mesophilic or thermophilic AD system. On the other hand, TAD was chosen as the optimal AD temperatures for higher OLRs.

Effect of an Electromagnetic Field on Anaerobic Digestion: Comparing an Electromagnetic System (ES), a Microbial Electrolysis System (MEC), and a Control with No External Force

This study examined the application of an electromagnetic field to anaerobic digestion by using an electromagnetic system (ES), a microbial electrolysis cell (MEC), and a control with no external force. The experimental work was performed by carrying out biochemical methane potential (BMP) tests using 1 L biodigesters. The bioelectrochemical digesters were supplied with 0.4 V for 30 days at 40 °C. The electromagnetic field of the ES was generated by coiling copper wire to form a solenoid in the BMP system, whereas the MEC consisted of zinc and copper electrodes inside the BMP system. The best performing system was the MEC, with a yield of 292.6 mL CH4/g chemical oxygen demand removed (CODremoved), methane content of 86%, a maximum current density of 23.3 mA/m2, a coulombic efficiency of 110.4%, and an electrical conductivity of 180 µS/cm. Above 75% removal of total suspended solids (TSS), total organic carbon (TOC), phosphate, and ammonia nitrogen (NH3-N) was also recorded. However, a longer exposure (>8 days) to higher magnetic intensity (6.24 mT) on the ES reduced its overall performance. In terms of energy, the MEC produced the greatest annual energy profit (327.0 ZAR/kWh or 23.36 USD/kWh). The application of an electromagnetic field in anaerobic digestion, especially a MEC, has the potential to maximize the methane production and the degradability of the wastewater organic content.

Bacterial communities in paddy soil and ditch sediment under rice-crab co-culture system

As an important form of sustainable agriculture, rice-crab (Eriocheir sinensis) co-culture is rapid developing worldwide. However, the knowledge on the bacterial communities of the different components of the system is limited. In this study, we investigated the bacterial community structure in paddy soil and ditch sediment by using high-throughput sequencing technology. The results showed that compared with the ditch sediment, the content of NH₄⁺-N in paddy soil decreased by 62.31%, and the content of AP (available phosphorus) increased by 172.02% (P < 0.05). The most abundant phyla in paddy soil and ditch sediment were Proteobacteria, Bacteroidetes and Chloroflexi, whose relative abundance was above 65%. Among the dominant genera, the relative abundance of an uncultured bacterium genus of Saprospiraceae and an uncultured bacterium genus of Lentimicrobiaceae in paddy soil was significantly lower than ditch sediment (P < 0.05). Alpha diversity indicated that the bacterial diversity of paddy soil and ditch sediment was similar. The bacterial community structure was affected by the relative abundance of bacteria, not the species of bacteria. Redundancy analysis (RDA) showed that the bacterial communities in paddy soil and ditch sediment were correlated with physicochemical properties. Our findings showed that the bacterial community structure was distinct in paddy soil and ditch sediment under rice-crab co-culture probably due to their different management patterns. These results can provide theoretical support for improving rice-crab co-culture technology.

Enteric and Fecal Methane Emissions from Dairy Cows Fed Grass or Corn Silage Diets Supplemented with Rapeseed Oil

Simple Summary

In this study, we evaluated methane emissions from dairy cows fed grass or corn silage diets supplemented with rapeseed oil. Enteric methane emissions decreased on adding rapeseed oil to the diet, but methane emissions from feces of dairy cows fed diets supplemented with rapeseed oil did not differ. Thus, no trade-offs were observed between enteric and fecal methane emissions due to forage type or addition of rapeseed oil to diets fed to Swedish dairy cows.

Abstract

This study evaluated potential trade-offs between enteric methane (CH₄) emissions and CH₄ emissions from feces of dairy cows fed grass silage or partial replacement of grass silage with corn silage, both with and without supplementation of rapeseed oil. Measured data for eight dairy cows (two blocks) included in a production trial were analyzed. Dietary treatments were grass silage (GS), GS supplemented with rapeseed oil (GS-RSO), GS plus corn silage (GSCS), and GSCS supplemented with rapeseed oil (GSCS-RSO). Feces samples were collected after each period and incubated for nine weeks to estimate fecal CH₄ emissions. Including RSO (0.5 kg/d) in the diet decreased dry matter intake (DMI) by 1.75 kg/d. Enteric CH₄ emissions were reduced by inclusion of RSO in the diet (on average 473 vs. 607 L/d). In 9-week incubations, there was a trend for lower CH₄ emissions from feces of cows fed diets supplemented with RSO (on average 3.45 L/kg DM) than cows with diets not supplemented with RSO (3.84 L/kg DM). Total CH₄ emissions (enteric + feces, L/d) were significantly lower for the cows fed diets supplemented with RSO. Total fecal CH₄ emissions were similar between treatments, indicating no trade-offs between enteric and fecal CH₄ emissions.

Empirical Validation of a Biogas Plant Simulation Model and Analysis of Biogas Upgrading Potentials

Biogas plants may support the transformation towards renewable-based and integrated energy systems by providing dispatchable co-generation as well as opportunities for biogas upgrading or power-to-X conversion. In this paper, a simulation model that comprises the main dynamics of the internal processes of a biogas plant is developed. Based on first-order kinetics of the anaerobic digestion process, the biogas production of an input feeding schedule of raw material can be estimated. The output of the plant in terms of electrical and thermal energy is validated against empirical data from a 3-MW biogas plant on the Danish island of Bornholm. The results show that the model provides an accurate representation of the processes within a biogas plant. The paper further provides insights on the functioning of the biogas plant on Bornholm as well as discusses upgrading potentials of biogas to biomethane at the plant from an energy perspective.

A novel pretreatment biotechnology for increasing methane yield from lipid-rich wastewater based on combination of hydrolytic enzymes with Candida rugosa fungus

Lipid-rich wastewater from the local dairy industry (cheese whey) in the Galilee, Israel was hydrolyzed by using two different sources of lipase as hydrolytic enzymes: fungal (Candida rogusa lipase-AY) and animal porcine pancreatic lipase(PPL). Pretreatment efficiency was verified by comparative biodegradability tests of raw and treated wastewater samples. Simultaneous hydrolysis and anaerobic digestion in the same reactors were also tested. Enzymatic pretreatment of these samples at a concentration of 0.05 w v^-1 showed organic matter removal of 90% and methane formation increases of 140% for the fungal source enzyme (i.e., AY), while for the animal source enzyme (i.e., PPL) was 86 and 130%, respectively. Enzymatic pretreatment led to significant methane formation which was obtained only for moderate substrate concentration (initial chemical oxygen demand of 15 gL^-1); While in high concentrated lipid-rich wastewater led to methane yield inhibition. The main finding was that the combination of AY enzyme with Candida rugosa fungus (i.e., enzyme mixture) led to a high efficiency in methane production (+152%) and organic materials removal (more than 90%). In summary, the use of fungal hydrolytic lipase mixed with Candida rugosa fungus is a promising method for enhancing methane production during the biodegradation of fat and grease-rich wastewaters.

Enrichment and description of novel bacteria performing syntrophic propionate oxidation at high ammonia level

Inefficient syntrophic propionate degradation causes severe operating disturbances and reduces biogas productivity in many high-ammonia anaerobic digesters, but propionate-degrading microorganisms in these systems remain unknown. Here, we identified candidate ammonia-tolerant syntrophic propionate-oxidising bacteria using propionate enrichment at high ammonia levels (0.7-0.8 g NH₃ L^-1 ) in continuously-fed reactors. We reconstructed 30 high-quality metagenome-assembled genomes (MAGs) from the propionate-fed reactors, which revealed two novel species from the families Peptococcaceae and Desulfobulbaceae as syntrophic propionate-oxidising candidates. Both MAGs possess genomic potential for the propionate oxidation and electron transfer required for syntrophic energy conservation and, similar to ammonia-tolerant acetate degrading syntrophs, both MAGs contain genes predicted to link to ammonia and pH tolerance. Based on relative abundance, a Peptococcaceae sp. appeared to be the main propionate degrader and has been given the provisional name "Candidatus Syntrophopropionicum ammoniitolerans". This bacterium was also found in high-ammonia biogas digesters, using quantitative PCR. Acetate was degraded by syntrophic acetate-oxidising bacteria and the hydrogenotrophic methanogenic community consisted of Methanoculleus bourgensis and a yet to be characterised Methanoculleus sp. This work provides knowledge of cooperating syntrophic species in high-ammonia systems and reveals that ammonia-tolerant syntrophic propionate-degrading populations share common features, but diverge genomically and taxonomically from known species.

Effect of Barium Addition on Hydrolytic Enzymatic Activities in Food Waste Degradation under Anaerobic Conditions

Enzymatic hydrolysis of complex components of residual materials, such as food waste, is a rate-limiting step that conditionates the production rate of biofuels. Research into the anaerobic degradation of cellulose and starch, which are abundant components in organic waste, could contribute to optimize biofuels production processes. In this work, a lab-scale anaerobic semi-continuous hydrolytic reactor was operated for 171 days using food waste as feedstock; the effect of Ba2+ dosage over the activity of five hydrolytic enzymes was also evaluated. No significant effects were observed on the global performance of the hydrolytic process during the steady-state of the operation of the reactor, nevertheless, it was detected that Ba2+ promoted β-amylases activity by 76%, inhibited endoglucanases and α-amylases activity by 39 and 20%, respectively, and had no effect on β-glucosidases and glucoamylases activity. The mechanisms that rule the observed enzymatic activity changes remain unknown; however, the discussion in this paper provides hypothetical explanations for further research.

Two Phase Anaerobic Digestion System of Municipal Solid Waste by Utilizing Microaeration and Granular Activated Carbon

In an anaerobic digestion (AD) process, the hydrolysis phase is often limited when substrates with high concentrations of solids are used. We hypothesized that applying micro-aeration in the hydrolysis phase and the application of granular activated carbon (GAC) in the methanogenesis phase could make the AD process more efficient. A packed bed reactor (PBR) coupled with an up-flow anaerobic sludge blanket (UASB) was conducted, and its effects on methane generation were evaluated. The micro-aeration rate applied in PBR was 254 L-air/kg-Total solids (TS)-d was compared with a control reactor. Micro-aeration showed that it reduced the hydrolysis time and increased the organic matter solubilization as chemical oxygen demand (COD) increasing 200%, with a volatile fatty acids (VFAs) increment higher than 300%, compared to the control reactor (without aeration). Our findings revealed that the implementations of microaeration and GAC in the two-phase AD system could enhance methane production by reducing hydrolysis time, increasing solid waste solubilization.

Carbohydrate Hydrolytic Potential and Redundancy of an Anaerobic Digestion Microbiome Exposed to Acidosis, as Uncovered by Metagenomics

The enzymatic hydrolysis of lignocellulosic biomass is mainly driven by the action of carbohydrate-active enzymes. By characterizing the gene profiles at the different stages of the anaerobic digestion experiment, we showed that the microbiome retains its hydrolytic functional redundancy even during severe acidosis, despite significant changes in taxonomic composition. By analyzing reconstructed bacterial genomes, we demonstrate that Bacteroidetes hydrolytic gene diversity likely favors the abundance of this phylum in some anaerobic digestion systems. Further, we observe genetic redundancy within the Bacteroidetes group, which accounts for the preserved hydrolytic potential during acidosis. This work also uncovers new polysaccharide utilization loci involved in the deconstruction of various biomasses and proposes the model of acetylated glucomannan degradation by Bacteroidetes. Acetylated glucomannan-enriched biomass is a common substrate for many industries, including pulp and paper production. Using naturally evolved cocktails of enzymes for biomass pretreatment could be an interesting alternative to the commonly used chemical pretreatments.

Increased hydrolysis of easily digestible biomass may lead to acidosis of anaerobic reactors and decreased methane production. Previously, it was shown that the structure of microbial communities changed during acidosis; however, once the conditions are back to optimal, biogas (initially CO₂) production quickly restarts. This suggests the retention of the community functional redundancy during the process failure. In this study, with the use of metagenomics and downstream bioinformatics analyses, we characterize the carbohydrate hydrolytic potential of the microbial community, with a special focus on acidosis. To that purpose, carbohydrate-active enzymes were identified, and to further link the community hydrolytic potential with key microbes, bacterial genomes were reconstructed. In addition, we characterized biochemically the specificity and activity of selected enzymes, thus verifying the accuracy of the in silico predictions. The results confirm the retention of the community hydrolytic potential during acidosis and indicate Bacteroidetes to be largely involved in biomass degradation. Bacteroidetes showed higher diversity and genomic content of carbohydrate hydrolytic enzymes that might favor the dominance of this phylum over other bacteria in some anaerobic reactors. The combination of bioinformatic analyses and activity tests enabled us to propose a model of acetylated glucomannan degradation by Bacteroidetes.

IMPORTANCE The enzymatic hydrolysis of lignocellulosic biomass is mainly driven by the action of carbohydrate-active enzymes. By characterizing the gene profiles at the different stages of the anaerobic digestion experiment, we showed that the microbiome retains its hydrolytic functional redundancy even during severe acidosis, despite significant changes in taxonomic composition. By analyzing reconstructed bacterial genomes, we demonstrate that Bacteroidetes hydrolytic gene diversity likely favors the abundance of this phylum in some anaerobic digestion systems. Further, we observe genetic redundancy within the Bacteroidetes group, which accounts for the preserved hydrolytic potential during acidosis. This work also uncovers new polysaccharide utilization loci involved in the deconstruction of various biomasses and proposes the model of acetylated glucomannan degradation by Bacteroidetes. Acetylated glucomannan-enriched biomass is a common substrate for many industries, including pulp and paper production. Using naturally evolved cocktails of enzymes for biomass pretreatment could be an interesting alternative to the commonly used chemical pretreatments.

Slaughterhouse by-products treatment using anaerobic digestion

The objective of the present study is to evaluate the use of animal by-products (ABP) as substrates for anaerobic digestion, aiming at methane production. Specifically, four ABP of Category 2 and 3, namely (i) stomach and rumen, (ii) stomach contents, (iii) breasts and reproductive organs and (iv) bladders and intestines with their contents, were selected. The methane potential of each ABP was initially determined, while the feasibility of anaerobic co-digestion of ABP with two agroindustrial waste, i.e. orange peels and olive leaves was also studied. To this purpose, Biochemical Methane Potential (BMP), as well as semi-continuous assays were respectively conducted. In the latter, the effect of the variation in the organic loading rate (OLR) on methane production was investigated. Results obtained from BMP assays showed that the samples containing breasts and reproductive organs, bladders and intestine, and stomach and rumen, had higher methane potentials of 815, 787 and 759 mLCH_4,STP/gVS, respectively. Moreover, according to the results of the semi-continuous assays, maximum methane yields between 253 and 727mLCH₄/gVS_fed were obtained at an OLR of 0.8gVS/L/d. The only case in which methanogenesis inhibition phenomena, due to increased ammonia concentrations, were observed, was the assay being fed with a mixture of breasts and reproductive organs and orange peels, at the highest OLR. This inhibition phenomenon was attributed to an inappropriate C/N ratio.

Enhanced biogas yield by thermo-alkali solubilization followed by co-digestion of intestine waste from slaughterhouse with food waste

Intestine waste generated from slaughterhouse (IWS) is difficult to degrade in anaerobic process due to the presence of high protein and lipid contents. However, anaerobic co-digestion helps to increase the degradation of IWS by the addition of carbon-rich food waste (FW). To increase the biogas yield, thermo-alkali pretreatment may be more viable method for the anaerobic digestion of protein and lipid rich wastes. In the present study, Thermo-alkali pretreatment of intestine waste from slaughterhouse and food waste alone and mixing of IWS and FW with different ratios (1:1-1:3) on VS basis have been studied. To study the effect of Thermo-alkali pretreatment on solubilization of substrate, the substrate was mixed with alkali solutions (NaOH and KOH) at different concentrations of 1, 2, 3, 4 and 5% solutions. The results revealed that the maximum solubilization was observed to be 94.7% and 90.1% at KOH (1:3 and 5%) and NaOH (1:3 and 5%), respectively. Based on the study, enhancement in biogas yield by 16% (IWS), 11.5% (FW), 12.2% (1:1), 18.11% (1:2) and 22.5% (1:3) in KOH pretreated waste when compared with NaOH pretreated waste.

Unraveling the Microbial Interactions and Metabolic Potentials in Pre- and Post-treated Sludge from a Wastewater Treatment Plant Using Metagenomic Studies

Sewage waste represents an ecosystem of complex and interactive microbial consortia which proliferate with different kinetics according to their individual genetic as well as metabolic potential. We performed metagenomic shotgun sequencing on Ion-Torrent platform, to explore the microbial community structure, their biological interactions and associated functional capacity of pre-treated/raw sludge (RS) and post-treated/dried sludge (DS) of wastewater treatment plant. Bacterial phylotypes belonging to Epsilonproteobacteria (∼45.80%) dominated the RS with relatively few Archaea (∼1.94%) whereas DS has the dominance of beta- (30.23%) and delta- (13.38%) classes of Proteobacteria with relatively greater abundance of Archaea (∼7.18%). In particular, Epsilonproteobacteria appears as a primary energy source in RS and sulfur-reducing bacteria with methanogens seems to be in the potential syntrophic association in DS. These interactions could be ultimately responsible for carrying out amino-acid degradation, aromatic compound degradation and degradation of propionate and butyrate in DS. Our data also reveal the presence of key genes in the sludge microbial community responsible for degradation of polycyclic aromatic hydrocarbons. Potential pathogenic microbes and genes for the virulence factors were found to be relatively abundant in RS which clearly reflect the necessity of treatment of RS. After treatment, potential pathogens load was reduced, indicating the sludge hygienisation in DS. Additionally, the interactions found in this study would reveal the biological and environmental cooperation among microbial communities for domestic wastewater treatment.

Trace element and temperature effects on microbial communities and links to biogas digester performance at high ammonia levels

BACKGROUND

High levels of ammonia and the presence of sulphide have major impacts on microbial communities and are known to cause operating problems in anaerobic degradation of protein-rich material. Operating strategies that can improve process performance in such conditions have been reported. The microbiological impacts of these are not fully understood, but their determination could help identify important factors for balanced, efficient operation. This study investigated the correlations between microbial community structure, operating parameters and digester performance in high-ammonia conditions.

METHOD

Continuous anaerobic co-digestion of household waste and albumin was carried out in laboratory-scale digesters at high ammonia concentrations (0.5-0.9 g NH3/L). The digesters operated for 320 days at 37 or 42 °C, with or without addition of a trace element mixture including iron (TE). Abundance and composition of syntrophic acetate-oxidising bacteria (SAOB) and of methanogenic and acetogenic communities were investigated throughout the study using 16S rRNA and functional gene-based molecular methods.

RESULTS

Syntrophic acetate oxidation dominated methane formation in all digesters, where a substantial enhancement in digester performance and influence on microbial community by addition of TE was shown dependent on temperature. At 37 °C, TE addition supported dominance and strain richness of Methanoculleus bourgensis and altered the acetogenic community, whereas the same supplementation at 42 °C had a low impact on microbial community structure. Both with and without TE addition operation at 42 °C instead of 37 °C had low impact on digester performance, but considerably restricted acetogenic and methanogenic community structure, evenness and richness. The abundance of known SAOB was higher in digesters without TE addition and in digesters operating at 42 °C. No synergistic effect on digester performance or microbial community structure was observed on combining increased temperature with TE addition.

CONCLUSIONS

Our identification of prominent populations related to enhanced performance within methanogenic (high dominance and richness of M. bourgensis) and acetogenic communities are valuable for continued research and engineering to improve methane production in high-ammonia conditions. We also show that a temperature increase of only 5 °C within the mesophilic range results in an extreme dominance of one or a few species within these communities, independent of TE addition. Furthermore, functional stable operation was possible despite low microbial temporal dynamics, evenness and richness at the higher temperature.

Use of a marine microbial community as inoculum for biomethane production

Marine substrates are prominent candidates for the production of biofuels, especially for biogas, which is a well-established technology that accepts different types of substrates for its production. However, the use of marine substrates in bioreactors may cause inhibition of methanogenic bacteria due to the addition of seasalts. Here, we explore a simple and economically viable way to circumvent the problem of inoculum inhibition. Based on the current knowledge of the diversity of microorganisms in marine sediments, we tested the direct use of methanogenic bacteria from an anoxic marine environment as inoculum for biomethane production. Both marine and freshwater substrates were added to this inoculum. No pretreatment (that may have enhanced methane production, but would have made the process more expensive) was applied either to the inoculum or to the substrates. For comparison, the same substrates were added to a standard inoculum (cow manure). Both the marine inoculum and cow manure produced methane by anaerobic digestion of the substrates added. The highest methane production (0.299 m(3) kg VS(-1)) was obtained by adding marine microalgae biomass (Chlorella sp. and Synechococcus sp.) to the marine inoculum. No inhibitory effects were observed due to differences in salinity between the inocula and substrates. Our results indicate the potential of using both marine inoculum and substrates for methane production.

Microbial community dynamics in replicate anaerobic digesters exposed sequentially to increasing organic loading rate, acidosis, and process recovery

BACKGROUND

Volatile fatty acid intoxication (acidosis), a common process failure recorded in anaerobic reactors, leads to drastic losses in methane production. Unfortunately, little is known about the microbial mechanisms underlining acidosis and the potential to recover the process. In this study, triplicate mesophilic anaerobic reactors of 100 L were exposed to acidosis resulting from an excessive feeding with sugar beet pulp and were compared to a steady-state reactor.

RESULTS

Stable operational conditions at the beginning of the experiment initially led to similar microbial populations in the four reactors, as revealed by 16S rRNA gene T-RFLP and high-throughput amplicon sequencing. Bacteroidetes and Firmicutes were the two dominant phyla, and although they were represented by a high number of operational taxonomic units, only a few were dominant. Once the environment became deterministic (selective pressure from an increased substrate feeding), microbial populations started to diverge between the overfed reactors. Interestingly, most of bacteria and archaea showed redundant functional adaptation to the changing environmental conditions. However, the dominant Bacteroidales were resistant to high volatile fatty acids content and low pH. The severe acidosis did not eradicate archaea and a clear shift in archaeal populations from acetotrophic to hydrogenotrophic methanogenesis occurred in the overfed reactors. After 11 days of severe acidosis (pH 5.2 ± 0.4), the process was quickly recovered (restoration of the biogas production with methane content above 50 %) in the overfed reactors, by adjusting the pH to around 7 using NaOH and NaHCO3.

CONCLUSIONS

In this study we show that once the replicate reactors are confronted with sub-optimal conditions, their microbial populations start to evolve differentially. Furthermore the alterations of commonly used microbial parameters to monitor the process, such as richness, evenness and diversity indices were unsuccessful to predict the process failure. At the same time, we tentatively propose the replacement of the dominant Methanosaeta sp. in this case by Methanoculleus sp., to be a potential warning indicator of acidosis.

First genomic insights into members of a candidate bacterial phylum responsible for wastewater bulking

Filamentous cells belonging to the candidate bacterial phylum KSB3 were previously identified as the causative agent of fatal filament overgrowth (bulking) in a high-rate industrial anaerobic wastewater treatment bioreactor. Here, we obtained near complete genomes from two KSB3 populations in the bioreactor, including the dominant bulking filament, using differential coverage binning of metagenomic data. Fluorescence in situ hybridization with 16S rRNA-targeted probes specific for the two populations confirmed that both are filamentous organisms. Genome-based metabolic reconstruction and microscopic observation of the KSB3 filaments in the presence of sugar gradients indicate that both filament types are Gram-negative, strictly anaerobic fermenters capable of non-flagellar based gliding motility, and have a strikingly large number of sensory and response regulator genes. We propose that the KSB3 filaments are highly sensitive to their surroundings and that cellular processes, including those causing bulking, are controlled by external stimuli. The obtained genomes lay the foundation for a more detailed understanding of environmental cues used by KSB3 filaments, which may lead to more robust treatment options to prevent bulking.

Genetic resources for methane production from biomass described with the Gene Ontology

Methane (CH₄) is a valuable fuel, constituting 70–95% of natural gas, and a potent greenhouse gas. Release of CH₄ into the atmosphere contributes to climate change. Biological CH₄ production or methanogenesis is mostly performed by methanogens, a group of strictly anaerobic archaea. The direct substrates for methanogenesis are H₂ plus CO₂, acetate, formate, methylamines, methanol, methyl sulfides, and ethanol or a secondary alcohol plus CO₂. In numerous anaerobic niches in nature, methanogenesis facilitates mineralization of complex biopolymers such as carbohydrates, lipids and proteins generated by primary producers. Thus, methanogens are critical players in the global carbon cycle. The same process is used in anaerobic treatment of municipal, industrial and agricultural wastes, reducing the biological pollutants in the wastes and generating methane. It also holds potential for commercial production of natural gas from renewable resources. This process operates in digestive systems of many animals, including cattle, and humans. In contrast, in deep-sea hydrothermal vents methanogenesis is a primary production process, allowing chemosynthesis of biomaterials from H₂ plus CO₂. In this report we present Gene Ontology (GO) terms that can be used to describe processes, functions and cellular components involved in methanogenic biodegradation and biosynthesis of specialized coenzymes that methanogens use. Some of these GO terms were previously available and the rest were generated in our Microbial Energy Gene Ontology (MENGO) project. A recently discovered non-canonical CH₄ production process is also described. We have performed manual GO annotation of selected methanogenesis genes, based on experimental evidence, providing “gold standards” for machine annotation and automated discovery of methanogenesis genes or systems in diverse genomes. Most of the GO-related information presented in this report is available at the MENGO website (http://www.mengo.biochem.vt.edu/).

Microbial trophic interactions and mcrA gene expression in monitoring of anaerobic digesters

Anaerobic digestion (AD) is a biological process where different trophic groups of microorganisms break down biodegradable organic materials in the absence of oxygen. A wide range of AD technologies is being used to convert livestock manure, municipal and industrial wastewaters, and solid organic wastes into biogas. AD gains importance not only because of its relevance in waste treatment but also because of the recovery of carbon in the form of methane, which is a renewable energy and is used to generate electricity and heat. Despite the advances on the engineering and design of new bioreactors for AD, the microbiology component always poses challenges. Microbiology of AD processes is complicated as the efficiency of the process depends on the interactions of various trophic groups involved. Due to the complex interdependence of microbial activities for the functionality of the anaerobic bioreactors, the genetic expression of mcrA, which encodes a key enzyme in methane formation, is proposed as a parameter to monitor the process performance in real time. This review evaluates the current knowledge on microbial groups, their interactions, and their relationship to the performance of anaerobic biodigesters with a focus on using mcrA gene expression as a tool to monitor the process.

Effect of anaerobiosis on indigenous microorganisms in blackwater with fish offal as co-substrate

The aim of this study was to compare the effect of mesophilic anaerobic digestion with aerobic storage on the survival of selected indigenous microorganisms and microbial groups in blackwater, including the effect of addition of Greenlandic Halibut and shrimp offal. The methane yield of the different substrate mixtures was determined in batch experiments to study possible correlation between methanogenic activity in the anaerobic digesters and reduction of indigenous microorganisms in the blackwater. By the end of the experiments a recovery study was conducted to determine possible injury of the microorganisms. In both anaerobic and aerobic samples, survival of Escherichia coli was better in the presence of Greenlandic Halibut offal when compared to samples containing blackwater only and blackwater and shrimp offal, possibly due to more available carbon in the samples containing Greenlandic Halibut offal. Reduction of faecal streptococci was large under both anaerobic and aerobic conditions, and the results indicated a complete removal of faecal streptococci in the anaerobic samples containing blackwater and a mixture of blackwater and shrimp offal after 17 and 31 days, respectively. Amoxicillin resistant bacteria were reduced in the anaerobic samples in the beginning of the study but increased towards the end of it. The opposite pattern was observed in the aerobic samples, with a growth in the beginning followed by a reduction. During the anaerobic digestion tetracycline resistant bacteria showed the least reduction in the mixture of blackwater and shrimp offal, which had the lowest methane yield while the highest reduction was observed in the mixture of blackwater and Greenlandic Halibut, where the highest methane yield was measured Reduction of coliphages was larger under anaerobic conditions. Addition of fish offal had no effect on survival of coliphages. The results of the recovery study indicated that a fraction of the E. coli in the aerobic blackwater sample and of the faecal streptococci in both the anaerobic and aerobic samples containing blackwater and Greenlandic Halibut were injured only, and thus able to resuscitate during recovery. The use of anaerobic digestion in the Arctic is limited to substrate types like those tested in this study because of absence of agriculture. The results indicate that anaerobic digestion of wastewater could benefit from the addition of fish offal, with respect to both microbial reduction and energy production.

Diversity and variability of methanogens during the shift from mesophilic to thermohilic conditions while biogas production

Anaerobic digestion (AD) is the most popular path of organic waste disposal. It is often used in wastewater treatment plants for excessive sludge removal. Methanogenic fermentation had usually been performed under mesophilic conditions, but in the past few years the thermophilic processes have become more popular due to economics and sludge sanitation. Methanogens, the group of microorganisms responsible for methane production, are thought to be sensitive to temperature change and it has already been proven that the communities performing methanogenesis under mesophilic and thermophilic conditions differ. But in most cases the research performed on methanogen diversity and changeability was undertaken in two separate anaerobic chambers for meso- and thermophilic conditions. It is also known that there is a group of microorganisms performing AD which are insensitive to temperature. Also the linkage between digester performance and its microbial content and community changeability is still not fully understood. That is why in this experiment we analyzed the bacterial community performing methanogenesis in a pilot scale anaerobic chamber during the shift from mesophilic to thermophilic conditions to point at the group of temperature tolerant microorganisms and their performance. The research was performed with PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis). It occurred that the community biodiversity decreased together with a temperature increase. The changes were coherent for both the total bacteria community and methanogens. These bacterial shifts were also convergent with biogas production-it decreased in the beginning of the thermophilic phase with the bacterial biodiversity decrease and increased when the community seemed to be restored. DGGE results suggest that among a wide variety of microorganisms involved in AD there is a GC-rich group relatively insensitive towards temperature change, able to adapt quickly to shifts in temperature and perform AD effectively. The studies of this microbial group could be a step forward in developing more efficient anaerobic digestion technology.

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.

Improvement of biogas production by bioaugmentation

Biogas production technologies commonly involve the use of natural anaerobic consortia of microbes. The objective of this study was to elucidate the importance of hydrogen in this complex microbial food chain. Novel laboratory biogas reactor prototypes were designed and constructed. The fates of pure hydrogen-producing cultures of Caldicellulosiruptor saccharolyticus and Enterobacter cloacae were followed in time in thermophilic and mesophilic natural biogas-producing communities, respectively. Molecular biological techniques were applied to study the altered ecosystems. A systematic study in 5-litre CSTR digesters revealed that a key fermentation parameter in the maintenance of an altered population balance is the loading rate of total organic solids. Intensification of the biogas production was observed and the results corroborate that the enhanced biogas productivity is associated with the increased abundance of the hydrogen producers. Fermentation parameters did not indicate signs of failure in the biogas production process. Rational construction of more efficient and sustainable biogas-producing microbial consortia is proposed.

Proteomic profiling of the acid tolerance response (ATR) during the enhanced biomethanation process from Taihu Blue Algae with butyrate stress on anaerobic sludge

Enhanced biomethanation with acid stress on anaerobic sludge, dehydrogenase activity, protein expression, and the primary proteomic profiling of microbial communities during the enhanced anaerobic digestion process from Taihu Blue Algae were investigated. It was found that the accumulation of organic acids and the specific biogas accumulation rate were 1.8 and 1.3 times of the control, when 10 g/L and 7.5 g/L of butyrate were selected for acid stress, respectively. Meanwhile, dehydrogenase activity of the 7.5 g/L acid stress group exhibited an increase of 1.6 times of the control, and protein expression was also found to be enhanced sharply as revealed by 1D-PAGE. Finally, twenty of the matched protein spots through 2D-PAGE from both the control and the 7.5 g/L stress groups were identified by MALDI-TOF MS, and five of which were proved to be involved in bioenergy metabolism. Significantly, ATR related proteins might be induced as the pIs of which were acidic as 5.92, 5.51 and 5.54, respectively.

Co-digestion of source segregated domestic food waste to improve process stability

Cattle slurry and card packaging were used to improve the operational stability of food waste digestion, with the aim of reducing digestate total ammoniacal nitrogen concentrations compared to food waste only. Use of cattle slurry could have major environmental benefits through reducing greenhouse gas emissions associated with current management practices; whilst card packaging is closely linked to food waste and could be co-collected as a source segregated material. Both options increase the renewable energy potential whilst retaining organic matter and nutrients for soil replenishment. Co-digestion allowed higher organic loadings and gave a more stable process. A high ammonia inoculum acclimated more readily to cattle slurry than card packaging, probably through supplementation by trace elements and micro-organisms. Long-term operation at a 75-litre scale showed a characteristic pattern of volatile fatty acid accumulation in mono-digestion of food waste, and allowed performance parameters to be determined for the co-digestion substrates.

Experimental studies and mathematical modeling of an up-flow biofilm reactor treating mustard oil rich wastewater

Bioremediation of lipid-rich model wastewater was investigated in a packed bed biofilm reactor (anaerobic filter). A detailed study was conducted about the influence of fatty acid concentration on biomethanation of the high-fat liquid effluent of edible oil refineries. The biochemical methane potential (BMP) of the liquid waste was reported and maximum cumulative methane production at the exit of the reactor is estimated to be 785 ml CH(4) (STP)/(gVSS added). The effects of hydraulic retention time (HRT), organic loading rate (OLR) and bed porosity on the cold gas efficiency or energy efficiency of the bioconversion process were also investigated. Results revealed that the maximum cold gas efficiency of the process is 42% when the total organic load is 2.1 g COD/l at HRT of 3.33 days. Classical substrate uninhibited Monod model is used to generate the differential system equations which can predict the reactor behavior satisfactorily.

Polyphasic analyses of methanogenic archaeal communities in agricultural biogas plants

Knowledge of the microbial consortia participating in the generation of biogas, especially in methane formation, is still limited. To overcome this limitation, the methanogenic archaeal communities in six full-scale biogas plants supplied with different liquid manures and renewable raw materials as substrates were analyzed by a polyphasic approach. Fluorescence in situ hybridization (FISH) was carried out to quantify the methanogenic Archaea in the reactor samples. In addition, quantitative real-time PCR (Q-PCR) was used to support and complete the FISH analysis. Five of the six biogas reactors were dominated by hydrogenotrophic Methanomicrobiales. The average values were between 60 to 63% of archaeal cell counts (FISH) and 61 to 99% of archaeal 16S rRNA gene copies (Q-PCR). Within this order, Methanoculleus was found to be the predominant genus as determined by amplified rRNA gene restriction analysis. The aceticlastic family Methanosaetaceae was determined to be the dominant methanogenic group in only one biogas reactor, with average values for Q-PCR and FISH between 64% and 72%. Additionally, in three biogas reactors hitherto uncharacterized but potentially methanogenic species were detected. They showed closest accordance with nucleotide sequences of the hitherto unclassified CA-11 (85%) and ARC-I (98%) clusters. These results point to hydrogenotrophic methanogenesis as a predominant pathway for methane synthesis in five of the six analyzed biogas plants. In addition, a correlation between the absence of Methanosaetaceae in the biogas reactors and high concentrations of total ammonia (sum of NH(3) and NH(4)(+)) was observed.

Coproduction of bioethanol with other biofuels

Large scale transformation of biomass to more versatile energy carriers has most commonly been focused on one product such as ethanol or methane. Due to the nature of the biomass and thermodynamic and biological constraints, this approach is not optimal if the energy content of the biomass is supposed to be exploited maximally. In natural ecosystems, biomass is degraded to numerous intermediary compounds, and we suggest that this principle is utilized in biorefinery concepts, which could provide different fuels with different end use possibilities. In this chapter we describe one of the first pilot-scale biorefineries for multiple fuel production and also discuss perspectives for further enhancement of biofuel yields from biomass. The major fuels produced in this refinery are ethanol, hydrogen, and methane. We also discuss the applicability of our biorefinery concept as a bolt-on plant on conventional corn- or grain-based bioethanol plants, and suggest that petroleum-base refineries and biorefineries appropriately can be coupled during the transition period from a fossil fuel to a renewable fuel economy.