A review on the bioenergetics of anaerobic microbial metabolism close to the thermodynamic limits and its implications for digestion applications

Functional biochar and catalytic sustainable biosynthesis of PHA derived from waste activated sludge

Advancing circular wastewater management, this study developed an innovative solution for waste activated sludge (WAS) utilisation by constructing a system in whichWAS wasconverted to biochar and feedback catalysed biosynthesis of Polyhydroxyalkanoates (PHA). After PHA extraction, residual sludge was pyrolysed into biochar, which acted as a catalyst to further enhance PHA production by the self-fermentation conversion of WAS. Biochar significantly improved electron transfer during anaerobic digestion (by 61.36 %) and boosted the proportion of PHA-producing bacteria by 12.33%, resulting in an 81.04% increase in PHA yield, comprising 66.14% of the dry sludge weight. Multi-omics analysis showed that biochar amplified the expression of key PHA synthesis genes (phaA, 4.69-fold;phaB,10.04-fold; andphaC,12.02-fold). This study offers a sustainable approach for WAS management, thereby advancing resource efficiency and providing ecological benefits.

Enhanced anaerobic digestion model no.1 for high solids fermentation: Integrating homoacetogenesis and chain elongation

The production of volatile fatty acids (VFA) through high-solids anaerobic fermentation of organic waste offers a promising route for resource recovery. This study used a batch-mode anaerobic leach bed reactor (LBR) with leachate circulation to ferment the organic fraction of municipal solid waste, producing high concentrations of butyric acid, along with notable amounts of lactic and caproic acids. These results provide valuable insights and underscore the need for process optimization within conventional fermentation systems. To better model the LBR's complex dynamics, the Anaerobic Digestion Model No.1 (ADM1) was modified and extended to account for slow hydrolysis in dry fermentation conditions, thermodynamic constraints imposed by Gibbs Free Energy and hydrogen partial pressures, as well as lactic acid production and chain elongation pathways including homoacetogenesis and caproic acid formation. These enhancements provided deeper insights into high-solids anaerobic fermentation, advancing strategies for improved process control and system optimization.

Insight into physico-chemical properties and microbial community structure of biogas slurry from household biogas plants of sub-Himalaya for its implications in improved biogas production

Numerous metagenomics studies, conducted in both full-scale anaerobic digesters and household biogas plants, have shed light on the composition and activity of microbial flora essential for optimizing the performance of biogas reactors, underscoring the significance of microbial community composition in biogas plant efficiency. Although the efficiency of household biogas plants in the sub-Himalayan region has been reported, there is no literature evidence on the microbial community structure of such household biogas plants in the sub-Himalayan region. The current study evaluated the physico-chemical properties and bacterial community structure from the slurry samples of household biogas plants prevalent in the sub-Himalayan region. The slurry samples were observed to be rich in nutrients; however, their carbon and nitrogen contents were higher than the recommended standard values of liquid-fermented organic manure. The species richness and diversity indices (Chao1, Shannon, and Simpson) of household biogas plants were quite similar to the advanced biogas reactors operating at mesophilic conditions. 16S rRNA gene amplicon sequencing reveals microbial diversity, showing a higher abundance of Firmicutes (70.9%) and Euryarchaeota (9.52%) in advanced biogas reactors compared to household biogas plants. Microbial analysis shows a lack of beneficial microbes for anaerobic digestion, which might be the reason for inefficient biogas production in household biogas plants of the sub-Himalayan region. The lack of efficient bacterial biomass may also be attributed to the digester design, feedstock, and ambient temperatures. This study emphasized the establishment of efficient microbial consortia for enhanced degradation rates that may increase the methane yield in biogas plants.

The ambivalent role of graphene oxide in anaerobic digestion: A review

The capability of graphene oxide (GO) to enhance direct interspecies electron transfer (DIET) and improve anaerobic digestion (AD) performance is gaining attention in AD literature. The present review discusses the implications of GO and its ambivalent role in AD. Under anaerobic conditions, GO is rapidly converted to biologically reduced graphene oxide (bioRGO) through microbial respiration. GO addition could promote the release of extracellular polymeric substances and lead to toxic effects on anaerobic microorganisms. However, further research is needed to determine the GO toxic concentration thresholds. GO application can impact biogas production and organic micropollutants removal of anaerobic digesters. Nevertheless, most of the studies have been conducted at batch scale and further work in continuously operated anaerobic digesters is still needed. Finally, the review evaluates the economic potential of GO application in AD systems. Overall, this review lays the foundations to improve the applicability of GO in future full-scale digesters.

Effects of Decabromodiphenyl Ethane and Cadmium Coexposure on Their Bioaccumulation, Oxidative Stress, Root Metabolism, and Rhizosphere Soil Microorganisms in a Soil-Rice System

Decabromodiphenyl ethane (DBDPE) and cadmium (Cd) are typical pollutants in e-waste, seriously threatening crop growth. This study investigated the bioaccumulation and toxicity mechanisms of DBDPE and Cd in a soil-rice system. The results showed that 50 mg/kg DBDPE could reduce the level of accumulation of Cd in rice roots. DBDPE and Cd induced the antioxidant system (SOD, POD, and MDA) in rice seedlings. The combined exposure reduced the contents of carbohydrates, lipids, amino acids, and organic acids. Phenylalanine and phenylpropanoid metabolisms were identified as the key detoxification metabolic pathways under combined exposure. DBDPE and Cd disrupted the functional cycling of carbon and nitrogen in rhizosphere soil, while Gemmatimonadetes, Actinobacteria, and Bacteroidetes were the key bacterial groups responding to DBDPE and Cd stress. This work provides data for the toxicity risk evaluation of DBDPE and Cd combined exposure to food crops.

Sludge bound-EPS solubilization enhance CH4 bioconversion and membrane fouling mitigation in electrochemical anaerobic membrane bioreactor: Insights from continuous operation and interpretable machine learning algorithms

Bound extracellular polymeric substances (EPS) are complex, high-molecular-weight polymer mixtures that play a critical role in pore clogging, foulants adhesion, and fouling layer formation during membrane filtration, owing to their adhesive properties and gelation tendencies. In this study, a novel electrochemical anaerobic membrane bioreactor (EC-AnMBR) was constructed to investigate the effect of sludge bound-EPS solubilization on methane bioconversion and membrane fouling mitigation. During the 150-days' operation, the EC-AnMBR demonstrated remarkable performance, characterized by an exceptionally low fouling rate (transmembrane pressure (TMP) < 4.0 kPa) and high-quality effluent (COD removal > 98.2 %, protein removal > 97.7 %, and polysaccharide removal > 98.5 %). The highest methane productivity was up to 38.0 ± 3.1 mL/Lreactor/d at the applied voltage of 0.8 V with bound-EPS solubilization, 107.6 % higher than that of the control stage (18.3 ± 2.4 mL/Lreactor/d). Morphological and multiplex fluorescence labeling analyses revealed higher fluorescence intensities of proteins, polysaccharides, total cells and lipids on the surface of the fouling layer. In contrast, the interior exhibited increased compression density and reduced activity, likely attributable to compression effect. Under the synergistic influence of the electric field and bound-EPS solubilization, biomass characteristics exhibited a reduced propensity for membrane fouling. Furthermore, the bio-electrochemical regulation enhanced the electroactivity of microbial aggregates and enriched functional microorganisms, thereby promoting biofilm growth and direct interspecies electron transfer. Additionally, the potential hydrogenotrophic and methylotrophic methanogenesis pathways were enhanced at the cathode and anode surfaces, thereby increasing CH₄ productivity. The random forest-based machine learning model analyzed the nonlinear contributions of EPS characteristics on methane productivity and TMP values, achieving R² values of 0.879 and 0.848, respectively. Shapley additive explanations (SHAP) analysis indicated that S-EPSPS and S-EPSPN were the most critical factors affecting CH₄ productivity and membrane fouling, respectively. Partial dependence plot analysis further verified the marginal and interaction effects of different EPS layers on these outcomes. By combining continuous operation with interpretable machine learning algorithms, this study unveils the intricate impacts of EPS characteristics on methane productivity and membrane fouling behaviors, and provides new insights into sludge bound-EPS solubilization in EC-AnMBR.

Individual methanogenic granules are whole-ecosystem replicates with reproducible responses to environmental cues

BACKGROUND

In this study, individual methanogenic (anaerobic), granular biofilms were used as true community replicates to assess whole-microbial-community responses to environmental cues. The aggregates were sourced from a lab-scale, engineered, biological wastewater treatment system, were size-separated, and the largest granules were individually subjected to controlled environmental cues in micro-batch reactors (μBRs).

RESULTS

Individual granules were identical with respect to the structure of the active community based on cDNA analysis. Additionally, it was observed that the active microbial community of individual granules, at the depth of 16S rRNA gene sequencing, produced reproducible responses to environmental changes in pH, temperature, substrate, and trace-metal supplementation. We identified resilient and susceptible taxa associated with each environmental condition tested, as well as selected specialists, whose niche preferences span the entire trophic chain required for the complete anaerobic degradation of organic matter.

CONCLUSIONS

We found that single anaerobic granules can be considered highly-replicated whole-ecosystems with potential usefulness for the field of microbial ecology. Additionally, they act as the smallest whole-community unit within the meta-community of an engineered bioreactor. When subjected to various environmental cues, anaerobic granules responded reproducibly allowing for rare or unique opportunities for high-throughput studies testing whole-community responses to a wide range of environmental conditions.

Simultaneous Production of Biogas and Electricity from Anaerobic Digestion of Pine Needles: Sustainable Energy and Waste Management

Power scarcity and pollution can be overcome with the use of green energy forms like ethanol, biogas, electricity, hydrogen, etc., especially energy produced from renewable and industrial feedstocks. In hilly areas, pine needles are the most abundant biomass that has a low possibility of valorization due to high lignin content. On the other hand, anaerobic digestion (AD) of lignin and animal waste has low biogas yield due to poor conductivity. This study focuses on the simultaneous production of biogas and electricity through the co-digestion of cow dung and pine needles. The digester was initially established and stabilized in the lab to ensure a continuous supply of inoculum throughout the experiment. The optimization process involved the determination of an ideal cow dung-to-water ratio and selecting the appropriate conductive material that can enhance the energy generation from the feedstock. Afterward, both batch and continuous anaerobic digestion experiments were conducted. The results revealed that the addition of powdered graphite (5 mM), activated charcoal (15 mM), and biochar (25 mM) exhibited maximum voltage of 0.71 ± 0.013 V, 0.56 ± 0.013 V, and 0.49 ± 0.011 V on the 30th, 25th and 20th day of AD, respectively. The batch experiment showed that 5 mM graphite powder enhanced electron transfer in the AD process and generated a voltage of 0.77 ± 0.014 V on the 30th day, indicating an increase of ~1.5-fold as compared to the control (0.56 ± 0.019 V). The results from the continuous AD process showed that the digester with cow dung, pine needle, and a conductive material in combination exhibited the maximum voltage of 0.76 ± 0.012 V on the 21st day of AD, while the digester with cow dung only exhibited a maximum voltage of 0.62 ± 0.015 V on the 22nd day of AD, representing a 1.3-fold increase over the control. Furthermore, the current work used discarded plastic items and electrodes from spent batteries to emphasize waste management and aid in attaining sustainable energy and development goals.

Integration of Digestate-Derived Biochar into the Anaerobic Digestion Process through Circular Economic and Environmental Approaches-A Review

The growing energy consumption and the need for a circular economy have driven considerable interest in the anaerobic digestion (AD) of organic waste, offering potential solutions through biogas and digestate production. AD processes not only have the capability to reduce greenhouse gas emissions but also contribute to the production of renewable methane. This comprehensive review aims to consolidate prior research on AD involving different feedstocks. The principles of AD are explored and discussed, including both chemical and biological pathways and the microorganisms involved at each stage. Additionally, key variables influencing system performance, such as temperature, pH, and C/N ratio are also discussed. Various pretreatment strategies applied to enhance biogas generation from organic waste in AD are also reviewed. Furthermore, this review examines the conversion of generated digestate into biochar through pyrolysis and its utilization to improve AD performance. The addition of biochar has demonstrated its efficacy in enhancing metabolic processes, microorganisms (activity and community), and buffering capacity, facilitating Direct Interspecies Electron Transfer (DIET), and boosting CH4 production. Biochar also exhibits the ability to capture undesirable components, including CO2, H2S, NH3, and siloxanes. The integration of digestate-derived biochar into the circular economy framework emerges as a vital role in closing the material flow loop. Additionally, the review discusses the environmental benefits derived from coupling AD with pyrolysis processes, drawing on life cycle assessment investigations. Techno-economic assessment (TEA) studies of the integrated processes are also discussed, with an acknowledgment of the need for further TEA to validate the viability of integrating the biochar industry. Furthermore, this survey examines the techno-economic and environmental impacts of biochar production itself and its potential application in AD for biogas generation, aiming to establish a more cost-effective and sustainable integrated system.

Multivariate comparison of taxonomic, chemical and operational data from 80 different full-scale anaerobic digester-related systems

BACKGROUND

The holistic characterization of different microbiomes in anaerobic digestion (AD) systems can contribute to a better understanding of these systems and provide starting points for bioengineering. The present study investigates the microbiome of 80 European full-scale AD systems. Operational, chemical and taxonomic data were thoroughly collected, analysed and correlated to identify the main drivers of AD processes.

RESULTS

The present study describes chemical and operational parameters for a broad spectrum of different AD systems. With this data, Spearman correlation and differential abundance analyses were applied to narrow down the role of the individual microorganisms detected. The authors succeeded in further limiting the number of microorganisms in the core microbiome for a broad range of AD systems. Based on 16S rRNA gene amplicon sequencing, MBA03, Proteiniphilum, a member of the family Dethiobacteraceae, the genus Caldicoprobacter and the methanogen Methanosarcina were the most prevalent and abundant organisms identified in all digesters analysed. High ratios for Methanoculleus are often described for agricultural co-digesters. Therefore, it is remarkable that Methanosarcina was surprisingly high in several digesters reaching ratios up to 47.2%. The various statistical analyses revealed that the microorganisms grouped according to different patterns. A purely taxonomic correlation enabled a distinction between an acetoclastic cluster and a hydrogenotrophic one. However, in the multivariate analysis with chemical parameters, the main clusters corresponded to hydrolytic and acidogenic microorganisms, with SAOB bacteria being particularly important in the second group. Including operational parameters resulted in digester-type specific grouping of microbes. Those with separate acidification stood out among the many reactor types due to their unexpected behaviour. Despite maximizing the organic loading rate in the hydrolytic pretreatments, these stages turned into extremely robust methane production units.

CONCLUSIONS

From 80 different AD systems, one of the most holistic data sets is provided. A very distinct formation of microbial clusters was discovered, depending on whether taxonomic, chemical or operational parameters were combined. The microorganisms in the individual clusters were strongly dependent on the respective reference parameters.

Catabolism and interactions of syntrophic propionate- and acetate oxidizing microorganisms under mesophilic, high-ammonia conditions

Microbial inhibition by high ammonia concentrations is a recurring problem that significantly restricts methane formation from intermediate acids, i.e., propionate and acetate, during anaerobic digestion of protein-rich waste material. Studying the syntrophic communities that perform acid conversion is challenging, due to their relatively low abundance within the microbial communities typically found in biogas processes and disruption of their cooperative behavior in pure cultures. To overcome these limitations, this study examined growth parameters and microbial community dynamics of highly enriched mesophilic and ammonia-tolerant syntrophic propionate and acetate-oxidizing communities and analyzed their metabolic activity and cooperative behavior using metagenomic and metatranscriptomic approaches. Cultivation in batch set-up demonstrated biphasic utilization of propionate, wherein acetate accumulated and underwent oxidation before complete degradation of propionate. Three key species for syntrophic acid degradation were inferred from genomic sequence information and gene expression: a syntrophic propionate-oxidizing bacterium (SPOB) "Candidatus Syntrophopropionicum ammoniitolerans", a syntrophic acetate-oxidizing bacterium (SAOB) Syntrophaceticus schinkii and a novel hydrogenotrophic methanogen, for which we propose the provisional name "Candidatus Methanoculleus ammoniitolerans". The results revealed consistent transcriptional profiles of the SAOB and the methanogen both during propionate and acetate oxidation, regardless of the presence of an active propionate oxidizer. Gene expression indicated versatile capabilities of the two syntrophic bacteria, utilizing both molecular hydrogen and formate as an outlet for reducing equivalents formed during acid oxidation, while conserving energy through build-up of sodium/proton motive force. The methanogen used hydrogen and formate as electron sources. Furthermore, results of the present study provided a framework for future research into ammonia tolerance, mobility, aggregate formation and interspecies cooperation.

Unveiling the unique role of iron in the metabolism of methanogens: A review

Methanogens are the main participants in the carbon cycle, catalyzing five methanogenic pathways. Methanogens utilize different iron-containing functional enzymes in different methanogenic processes. Iron is a vital element in methanogens, which can serve as a carrier or reactant in electron transfer. Therefore, iron plays an important role in the growth and metabolism of methanogens. In this paper, we cast light on the types and functions of iron-containing functional enzymes involved in different methanogenic pathways, and the roles iron play in energy/substance metabolism of methanogenesis. Furthermore, this review provides certain guiding significance for lowering CH4 emissions, boosting the carbon sink capacity of ecosystems and promoting green and low-carbon development in the future.

Reverse electron transfer: Novel anaerobic methanogenesis pathway regulated through exogenous CO2 synergized with biochar

Acid accumulation and carbon emission are two major challenges in anaerobic digestion. Syntrophic consortia can employ reverse electron transfer (RET) to facilitate thermodynamically unfavorable redox reactions during acetogenesis. However, the potential mechanisms and regulatory methods of RET remain unclear. This study examines the regulatory mechanisms by which exogenous CO2 affects RET and demonstrates that biochar maximizes CO2 solubility at 25.8 mmol/L to enhance effects further. CO2 synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO2 decreases energy level to a maximum of -87 kJ/mol, strengthening the thermodynamic viability. The underlying mechanism can be attributed to RET promotion, as indicated by increased formate dehydrogenase and enrichment of H2/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % 13CH4 and methane contribution result show that CO2 accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO2 and biochar in AD surrounding RET.

Altering operational conditions during protein fermentation to volatile fatty acids modifies the associated bacterial community

In recent years, the production of volatile fatty acids (VFA) through mixed culture fermentation (MCF) has been gaining attention. Most authors have focused on the fermentation of carbohydrates, while other possible substrates, such as proteins, have not been considered. Moreover, there is little information about how operational parameters affect the microbial communities involved in these processes, even though they are strongly related to reactor performance and VFA selectivity. Hence, this study aims to evaluate how microbial composition changes according to three different parameters (pH, type of protein and micronutrient addition) during anaerobic fermentation of protein-rich side streams. For this, two continuous stirred tank reactors (CSTR) were fed with two different proteins (casein and gelatine) and operated at different conditions: three pH values (5.0, 7.0 and 9.0) with only macronutrients supplementation and two pH values (5.0 and 7.0) with micronutrients' supplementation as well. Firmicutes, Proteobacteria and Bacteroidetes were the dominant phyla in the two reactors at all operational conditions, but their relative abundance varied with the parameters studied. At pH 7.0 and 9.0, the microbial composition was mainly affected by protein type, while at acidic conditions the driving force was the pH. The influence of micronutrients was dependent on the pH and the protein type, with a special effect on Clostridiales and Bacteroidales populations. Overall, this study shows that the acidogenic microbial community is affected by the three parameters studied and the changes in the microbial community can partially explain the macroscopic results, especially the process selectivity.

Microbiome resistance mediates stimulation of reduced graphene oxide to simultaneous abatement of 2,2',4,4',5-pentabromodiphenyl ether and 3,4-dichloroaniline in paddy soils

Paddy soils near electrical and electronic waste recycling sites generally suffer from co-pollution of polybrominated diphenyl ethers and 3,4-dichloroaniline (3,4-DCA). This study tested the feasibility of reduced graphene oxide (rGO) to stimulate the simultaneous abatement of 2,2',4,4',5-pentabromodiphenyl ether (BDE99) and 3,4-DCA in percogenic paddy soil (PPS) and hydromorphic paddy soil (HPS). rGO improved the debromination extent of BDE99 and the transformation rate of 3,4-DCA in PPS, but did not affect their abatement in HPS. The inhibition of specific fermenters, acetogens, and methanogens after rGO addition contributed to BDE99 debromination by obligate organohalide-respiring bacteria (OHRB) in PPS, but relevant soil microbiomes (e.g., fermenters, acetogens, methanogens, and obligate OHRB) responded little to rGO in HPS. For 3,4-DCA, the enhanced activities of nitrogen-metabolic chloroaniline degraders by rGO increased its transformation rate in PPS, but was compensated by the decreased biotransformation from 3,4-DCA to 3,4-dichloroacetanilide after the addition of rGO to HPS. The discrepant stimulation of rGO between PPS and HPS was mediated by soil microbiome resistance. rGO has the application potential to stimulate the simultaneous abatement of polybrominated diphenyl ethers and chloroanilines in paddy soils with relatively low microbiome resistance.

Mechanistic insights into synergistic facilitation of copper/zinc ions and dewatered swine manure-derived biochar on anaerobic digestion of swine wastewater

Biochar-assisted anaerobic digestion (AD) has been proposed an advanced system for swine wastewater (SW) management. However, the effects of metallic nutrients in SW, such as copper/zinc ions (Cu2+/Zn2+), on the biochar-assisted AD of SW are not well understood. This study investigated the influences of individual Cu2+/Zn2+ or dewatered swine manure-derived biochar, as well as their combined additions, on the AD of SW. The results showed that exposure to 50 mg/L Cu2+/Zn2+ temporary inhibited methane production, but the addition of 20 g/L biochar alleviated this inhibition by shortening the methanogenic lag time and increasing methane yield. Following a period of acclimation, both Cu2+/Zn2+ and biochar promoted methane production, although metagenomic analysis revealed distinct mechanisms underlying their promotion. Cu2+/Zn2+ enhanced ATP processing, including electron exchange between NADH/NAD+ and succinate/fumarate transformation, by 26.0-35.8%. Additionally, the gene encoding Coenzyme M methylation was upregulated by 36.2% along with enrichments of Methanocullus and Methanosarcina, contributing to accelerated hydrolysis and methanogenesis rates by 54.7% and 44.8%, respectively. On the other hand, biochar mainly stimulated bacterial F-type ATPase activities by 28.4%, likely facilitating direct interspecies electron transfer between Geobacter and Methanosarcina for syntrophic methanogenesis. The combined addition of Cu2+/Zn2+ and biochar resulted in "win-win" benefits, significantly increasing the maximum methane production rate from 40.3 mL CH4/d to 53.7 mL CH4/d. Moreover, the introduction of biochar into AD of SW facilitated the transformation of more Cu2+/Zn2+ from a reducible Fe-Mn oxides form to a residual form, which potentially reduced the metallic toxicity of the digestate for soil amendment. The findings of this study provide novel insights into understanding the synergistic impacts of heavy metals and biochar in regulating SW during AD, as well as the management of associated digestate.

Effects of ozone on activated sludge: performance of anaerobic digestion and structure of the microbial community

The treatment and disposal of activated sludge are currently challenging tasks in the world. As a common biological engineering technology, biological fermentation exists with disadvantages such as low efficiency and complex process. Ozone pretreatments are commonly applied to improve this problem due to their high efficiency and low cost. In this study, the significant function of ozone in anaerobic fermentation gas production was verified with excess sludge. Compared with other untreated sludge, ozone pretreatment can effectively degrade activated sludge. After ozone treatment and mixing with primary sludge, the methane production of excess sludge increased by 49.30 and 50.78%, and the methanogenic activity increased by 69.99 and 73.83%, respectively. The results indicated that the mixing of primary sludge with excess sludge possessed synergistic effects, which contributed to the anaerobic fermentation of excess sludge. The results of microbial community structure exhibited that methanogenic processes mainly involve hydrogenogens, acidogens and methanogens. The relative abundance of both bacteria and microorganisms changed significantly in the early stage of hydraulic retention time, which coincided exactly with the gas production stage. This study provided a feasible pretreatment strategy to improve sludge biodegradability and revealed the role of microorganisms during anaerobic digestion.

Carbon, nitrogen, and phosphorus metabolic relationships and reaction mechanisms in SBBR processes in the plateau habitat

In this study, two laboratory-scale SBBR reactors were established in a plateau habitat. Using high flux sequencing, the SBBR process was compared by natural sediment and autotrophic sludge to characterize the functional modules and functional genes of carbon, nitrogen, and phosphorus metabolism under different working conditions and to analyze the reaction mechanism. The results showed that all the functional modules of carbon metabolism and nitrogen metabolism were found in the SBBR process, except for methane metabolism, which occurred at 25 °C in tank 2, the functional modules related to methane metabolism are enhanced at all working conditions. Except for methane metabolism, all functional genes in tank 2 are inhibited by different working conditions, whereas tank 1 shows a slight enhancement. The different working conditions in nitrogen metabolism demonstrate inhibition of functional modules and functional genes in both tanks. Oxidative phosphorylation was missing five functional modules, except for M00153, where only two genes, K00424 and K22501, are missing, all of the required genes are missing in the other four functional modules. Overall the different conditions demonstrated some inhibition in both reaction tanks of the SBBR process. It is preferable to use self-cultivated sludge for membrane acclimation when operating the SBBR process in a plateau habitat. The findings of this study can be used to further research microbial carbon, nitrogen, and phosphorus metabolism mechanisms in SBBR processes in plateau habitats.

Functional convergence in slow-growing microbial communities arises from thermodynamic constraints

The dynamics of microbial communities is complex, determined by competition for metabolic substrates and cross-feeding of byproducts. Species in the community grow by harvesting energy from chemical reactions that transform substrates to products. In many anoxic environments, these reactions are close to thermodynamic equilibrium and growth is slow. To understand the community structure in these energy-limited environments, we developed a microbial community consumer-resource model incorporating energetic and thermodynamic constraints on an interconnected metabolic network. The central element of the model is product inhibition, meaning that microbial growth may be limited not only by depletion of metabolic substrates but also by accumulation of products. We demonstrate that these additional constraints on microbial growth cause a convergence in the structure and function of the community metabolic network-independent of species composition and biochemical details-providing a possible explanation for convergence of community function despite taxonomic variation observed in many natural and industrial environments. Furthermore, we discovered that the structure of community metabolic network is governed by the thermodynamic principle of maximum free energy dissipation. Our results predict the decrease of functional convergence in faster growing communities, which we validate by analyzing experimental data from anaerobic digesters. Overall, the work demonstrates how universal thermodynamic principles may constrain community metabolism and explain observed functional convergence in microbial communities.

Impact of incineration slag co-disposed with municipal solid waste on methane production and methanogens ecology in landfills

Co-landfill of incineration slag and municipal solid waste (MSW) is a main method for disposal of slag, and it has the potential of promoting methane (CH₄) production and accelerating landfill stabilization. Four simulated MSW landfill columns loaded with different amount of slag (A, 0%; B, 5%; C, 10%; D, 20%) were established, and the CH₄ production characteristics and methanogenic mechanisms were investigated. The maximum CH₄ concentration in columns A, B, C and D was 10.8%, 23.3%, 36.3% and 34.3%, respectively. Leachate pH and refuse pH were positively correlated with CH₄ concentration. Methanosarcina was the dominant genus with abundance of 35.1%∼75.2% and it was positively correlated with CH₄ concentration. CO₂-reducing and acetoclastic methanogenesis were the main types of methanogenesis pathway, and the methanogenesis functional abundance increased with slag proportion during stable methanogenesis process. This research can help understanding the impact of slag on CH₄ production characteristics and microbiological mechanisms in landfills.

Impact of graphene oxide addition on pharmaceuticals removal in anaerobic membrane bioreactor

The addition of conductive materials to the anaerobic digestion bioreactor was suggested to enhance microbial activity. In the present work, an anaerobic membrane bioreactor treating municipal wastewater was operated for 385 days. The impact of different graphene oxide concentrations on the removal target pharmaceuticals and microbial community dynamics was investigated. The addition of graphene oxide did not impact the reactor stability, whereas the removals of antibiotics (e.g., trimethoprim and metronidazole) were enhanced. A shift in the microbial community was detected after the addition of 50-900 mg L^-1 of graphene oxide, with the proliferation hydrogenotrophic methanogens. The proliferation of syntrophic microorganisms may indicate interactions via direct interspecific electron transfer. The obtained results suggest that the addition of graphene oxide at low mg L^-1 concentrations to an anaerobic membrane bioreactor may be considered to improve the removal of antibiotics from municipal wastewater.

The modified properties of sludge-based biochar with ferric sulfate and its effectiveness in promoting carbon release from particulate organic matter in rural household wastewater

The scarcity of carbon sources presents a significant challenge for the bio-treatment of rural domestic wastewater (RDW). This paper presented an innovative approach to address this issue by investigating the supplementary carbon source through in-situ degradation of particulate organic matter (POM) facilitated by ferric sulfate modified sludge-based biochar (SBC). To prepare SBC, five different contents of ferric sulfate (0%, 10%, 20%, 25%, and 33.3%) were added to sewage sludge. The results revealed that the pore and surface of SBC were enhanced, providing active sites and functional groups to accelerate the biodegradation of protein and polysaccharide. During the 8-day hydrolysis period, the concentration of soluble chemical oxidation demand (SCOD) increased and peaked (1087-1156 mg L^-1) on the fourth day. The C/N ratio increased from 3.50 (control) to 5.39 (25% ferric sulfate). POM was degraded the five dominant phyla, which were Actinobacteriota, Firmicutes, Synergistota, Proteobacteria, and Bacteroidetes. Although the relative abundance of dominant phyla changed, the metabolic pathway remained unchanged. The leachate of SBC (<20% ferric sulfate) was beneficial for microbes, but an excessive amount of ferric sulfate (33.3% ferric sulfate) could have inhibition effects on bacteria. In conclusion, ferric sulfate modified SBC holds the potential for the carbon degradation of POM in RDW, and further improvements should be made in future studies.

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Inhibitory mechanisms on dry anaerobic digestion: Ammonia, hydrogen and propionic acid relationship

Inhibitory pathways in dry anaerobic digestion are still understudied and current knowledge on wet processes cannot be easily transferred. This study forced instability in pilot-scale digesters by operating at short retention times (40 and 33 days) in order to understand inhibition pathways over long term operation (145 days). The first sign of inhibition at elevated total ammonia concentrations (8 g/l) was a headspace hydrogen level over the thermodynamic limit for propionic degradation, causing propionic accumulation. The combined inhibitory effect of propionic and ammonia accumulation resulted in further increased hydrogen partial pressures and n-butyric accumulation. The relative abundance of Methanosarcina increased while that of Methanoculleus decreased as digestion deteriorated. It was hypothesized that high ammonia, total solids and organic loading rate inhibited syntrophic acetate oxidisers, increasing their doubling time and resulting in its wash out, which in turn inhibited hydrogenotrophic methanogenesis and shifted the predominant methanogenic pathway towards acetoclastic methanogenesis at free ammonia over 1.5 g/l. C/N increases to 25 and 29 reduced inhibitors accumulation but did not avoid inhibition or the washout of syntrophic acetate oxidising bacteria.

Biohythane Production in Hydrogen-Oriented Dark Fermentation of Aerobic Granular Sludge (AGS) Pretreated with Solidified Carbon Dioxide (SCO2)

Though deemed a prospective method, the bioconversion of organic waste to biohydrogen via dark fermentation (DF) has multiple drawbacks and limitations. Technological difficulties of hydrogen fermentation may, in part, be eliminated by making DF a viable method for biohythane production. Aerobic granular sludge (AGS) is a little-known organic waste spurring a growing interest in the municipal sector; its characteristics indicate the feasibility of its use as a substrate for biohydrogen production. The major goal of the present study was to determine the effect of AGS pretreatment with solidified carbon dioxide (SCO₂) on the yield of H₂ (biohythane) production during anaerobic digestion (AD). It was found that an increasing dose of SCO₂ caused an increase in concentrations of COD, N-NH₄⁺, and P-PO₄^3- in the supernatant at the SCO₂/AGS volume ratios from 0 to 0.3. The AGS pretreatment at SCO₂/AGS ratios within the range of 0.1-0.3 was shown to enable the production of biogas with over 8% H₂ (biohythane) content. The highest yield of biohythane production, reaching 481 ± 23 cm³/gVS, was obtained at the SCO₂/AGS ratio of 0.3. This variant produced 79.0 ± 6% CH₄ and 8.9 ± 2% H₂. The higher SCO₂ doses applied caused a significant decrease in the pH value of AGS, modifying the anaerobic bacterial community to the extent that diminished anaerobic digestion performance.

Alleviating acid inhibition via bentonite supplementation during acidulated swine manure anaerobic digestion: Performance enhancement and microbial mechanism analysis

Swine manure is usually transmitted by the "collection-storage-transport" mode of the biogas project. However, this particular application pattern results in high volatile fatty acids (VFAs) concentration due to the long transition time in the "collection-storage-transport" process. In this work, acidulated swine manure anaerobic digestion (AD) with bentonite supplementation was firstly investigated with an expectation of acid alleviation, performance enhancement and microbial mechanism. Results indicated that the methane production rate in the 20 g/L bentonite-added digester was 2.87 fold higher than that of the control digester. Chemical oxygen demand (COD) removal rate was elevated by 140.1% via bentonite supplementation. Besides, the rapid decrease of VFAs and ammonia indicated that bentonite supplementation could offer buffering capacity and alleviate acid inhibition. Microbial community analysis revealed that acetoclastic methanogenesis (Methanosaeta and Methanosarcina) was the predominant methanogenesis pathway in this AD system. Syntrophic acetate oxidation (SAO) bacteria were discovered in the bentonite-added digester, and they converted acetate into H₂/CO₂ to support hydrogenotrophic methanogenesis. This study could offer guidance for acidulated swine manure AD in the practical biogas project.

Anaerobic digestion of vinasse and water treatment plant sludge increases methane production and stability of UASB reactors

Studies are still needed to increase the stability and efficiency of methane production from vinasse. Therefore, operations strategies, such as the anaerobic digestion with one or more wastes and adding micronutrients, especially iron, become attractive. The performance of two treatment systems, each one composed of two UASB reactors in series, operated under mesophilic (R1_M and R2_M) and thermophilic (R1_T and R2_T) temperature conditions, was evaluated in the anaerobic digestion of vinasse (ADV). First, the reactors were operated with the effluent recirculation and increasing organic loading rate (OLR) up to 20 g COD_total L^-1d^-1 in the R1_M and R1_T. Then, the anaerobic digestion of vinasse and water treatment plant (WTP) sludge (ADVS) was performed in the proportions of 25:75 to 50:50 (% v/v) in both systems. In the ADV, applying the highest OLR, the mesophilic and thermophilic reactors instabilities happened. The ADVS of over 35% of WTP sludge promoted the recovery of the mesophilic and thermophilic UASB reactors with significantly reduced total volatile acids and increased alkalinity and biogas production. The higher average values of the volumetric methane production (VMP) occurred in the ADVS at 50% of WTP; in the R1_M and R1_T, they were 3.23 and 3.00 L CH₄ L^-1d^-1, respectively. In the ADV, the thermophilic system presented higher VMP concerning the mesophilic for OLR up to 15 g COD_total L^-1d^-1. For higher OLR, the mesophilic system showed better carrying capacity and stability. The ADVS with above 35% of WTP sludge promoted similar benefits in the two systems, with no significant differences in COD_total removal and VMP. Therefore, adding iron by WTP sludge in ADVS improves methane production and increases the stability of UASB reactors under mesophilic and thermophilic conditions.

Alkaline fermentation of refinery waste activated sludge mediated by refinery spent caustic for volatile fatty acids production

Volatile fatty acids (VFA), produced from waste activated sludge (WAS), provide unique opportunities for resource recovery in wastewater treatment plants. This study investigates the potential of refinery spent caustic (RSC) on VFA production during refinery WAS (RWAS) alkaline fermentation. The highest VFA yield was 196.3 mg/g-VS at a sludge retention time of 6 days. Amplicon sequencing revealed the enrichment of Soehngenia (20.21%), Bacilli (11.86%), and Brassicibacter (4.17%), which was associated with improved activities of protease (626%) and α-glucosidase (715%). Function prediction analysis confirmed that acetyl-CoA production and fatty acid biosynthesis were enhanced, while fatty acid degradation was inhibited. Accordingly, hydrolysis, acidogenesis, and acetogenesis were improved by 6.87%, 10.67%, and 28.50%, respectively; whereas methanogenesis was inhibited by 28.87%. The sulfate and free ammonia in RSC likely contributed to increased acetic acid production. This study showcases that RWAS alkaline fermentation mediated by RSC for VFA production is the practicable approach.

Chronic effects of benzalkonium chlorides on short chain fatty acids and methane production in semi-continuous anaerobic digestion of waste activated sludge

As an emerging pollutant, benzalkonium chlorides (BACs) potentially enriched in waste activated sludge (WAS). However, the microbial response mechanism under chronic effects of BACs on acidogenesis and methanogenesis in anaerobic digestion (AD) has not been clearly disclosed. This study investigated the AD (by-)products and microbial evolution under low to high BACs concentrations from bioreactor startup to steady running. It was found that BACs can lead to an increase of WAS hydrolysis and fermentation, but a disturbance to acidogenic bacteria also occurred at low BACs concentration. A noticeable inhibition to methanogenesis occurred when BAC concentration was up to 15 mg/g TSS. Metagenomic analysis revealed the key genes involved in acetic acid (HAc) biosynthesis (i.e. phosphate acetyltransferase, PTA), β-oxidation pathway (acetyl-CoA C-acetyltransferase) and propionic acid (HPr) conversion was slightly promoted compared with control. Furthermore, BACs inhibited the acetotrophic methanogenesis (i.e. acetyl-CoA synthetase), especially BAC concentration was up to 15 mg/g TSS, thereby enhanced short chain fatty acids (SCFAs) accumulation. Overall, chronic stimulation of functional microorganisms with increasing concentrations of BACs impact WAS fermentation.

Combined effects of liquid digestate recirculation and biochar on methane yield, enzyme activity, and microbial community during semi-continuous anaerobic digestion

The combined effects of liquid digestate recirculation (LDR) and biochar on methanogenesis and microbial communities were studied in semi-continuous anaerobic reactors fed with wheat straw and swine manure. The tolerated organic loading rate (OLR) was expanded from 5 g- volatile solids (VS)∙L-1∙d-1 in the control to higher than 6 g-VS∙L-1∙d-1 in the LDR. At the OLR of 5.0 g-VS∙L-1∙d-1, average special methane yield in LDR with biochar was 0.234 L∙g-VS-1, which was 5.4 % higher than that of the LDR alone. Moreover, enzyme activity and microbial community analysis indicated that LDR with biochar enhanced the processes of hydrolysis and methanogenesis, and balanced the pathway between hydrogenotrophic and acetoclastic methanogenesis. The co-application of LDR and biochar synergistically enhanced the degradation pathways of substrates and the loading shock resistance of anaerobic digestion system. This study could offer strategies for developing sustainable applications of full and continuous LDR in industrial biogas projects.

Effect of pH dynamic control on ethanol-lactic type fermentation (ELTF) performance of glucose

This study proposed a new ethanol-lactic type fermentation (ELTF) and explored the optimal control strategy. Using batch experiments, the effects of pH, temperature and organic loading (OL) on ELTF were investigated. The sum of ethanol and lactic acid yield was highest at whole-control pH value of 4.0, 35°C temperature and OL of 33 gCOD/L. To improve ELTF, the dynamic pH control in the long-term CSTR was adjusted at 4.0 (1-28 days), 5.0 (29-44 days) and 4.0 (46-62 days) successively. The high concentration of ethanol and lactic acid was 8190.5 mg/L at 16th day of pH 4.0. At pH of 5.0, the average acidogenesis rate and total concentration of fermentation products increased 111.0% and 128.0%, respectively. Organisms of Lactobacillus and Bifidobacterium were the predominant bacteria in reactor. It can achieve the directional regulation of ELTF and provides parameter support for the application of two-phase anaerobic digestion.

Role of interspecies electron transfer stimulation in enhancing anaerobic digestion under ammonia stress: Mechanisms, advances, and perspectives

Ammonia stress is a commonly encountered issue in anaerobic digestion (AD) process when treating proteinaceous substrates. The enhanced relationship between syntrophic bacteria and methanogens triggered by interspecies electron transfer (IET) stimulation is one of the potential mechanisms for an improved methane yield from the AD plant under ammonia-stressed condition. There is, however, lack of synthesized information on the mechanistic understanding of IET facilitation in the ammonia-stressed AD processes. This review critically discusses recovery of AD system from ammonia-stressed condition, focusing on H₂ transfer, redox compound-mediated IET, and conductive material-induced direct IET. The effects and the associated mechanisms of IET stimulation on mitigating ammonia stress and promoting methanogenesis were elucidated. Finally, prospects and challenges of IET stimulation were critically discussed. This review highlights, for the first time, the critical role of IET stimulation in enhancing AD process under ammonia-stressed condition.

Anaerobic digester microbiome dynamics in response to moderate and failure-inducing shock loads of fats, oils and greases

Accidental organic overloading (shock loading) is common during the anaerobic co-digestion of fats, oils and greases (FOG) and may lead to decreased performance or reactor failure due to the effects on the microbiome. Here, adapted and non-adapted lab-scale anaerobic digesters were exposed to FOG shocks of varying organic strengths. The microbiome was sequenced during the recovery periods employed between each shock event. Non-failure-inducing shocks resulted in enrichment of fermentative bacteria, and acetoclastic and methylotrophic methanogens. However, sub-dominant bacterial populations were largely responsible for increased biogas production observed after adaptation. Following failure events, early recovery communities were dominated by Pseudomonas and Methanosaeta while late recovery communities shifted toward sub-dominant bacterial taxa and Methanosarcina. Generally, the recovered microbiome structure diverged from that of both the initial and optimized microbiomes. Thus, while non-failure-inducing FOG shocks can be beneficial, the adaptations gained are lost after a failure event and adaptation must begin again.

Understanding the mechanisms behind enhanced anaerobic digestion of corn straw by humic acids

Humic acids (HAs) are abundant on earth, yet their effects on anaerobic digestion (AD) of cellulosic substrate are not fully uncovered. The effects of HAs on AD of corn straw and the mechanisms behind were analyzed in this study. Results showed that the effects of HAs on methane yield were closely related to the total solids (TS) content. At relative high TS content of 5.0%, HAs benefited AD process by increasing 13.8% of methane yield, accelerating methane production rate by 43% and shortening lag phase time by 37.5%. Microbial community analysis indicated that HAs increased the relative abundance of syntrophic bacteria (Syntrophomonadaceae and Synergistaceae), facilitating the degradation of volatile fatty acids. HAs might act as electron shuttles to directly transfer electrons to hydrogenotrophic methanogens for CO₂ reduction to CH₄. This study provides a simple and efficient strategy to facilitate the AD of cellulosic substrate by HAs addition.

Roadmap from Microbial Communities to Individuality Modeling for Anaerobic Digestion of Sewage Sludge

Biological models describing anaerobic digestion (AD) of sewage sludge have been widely applied to test various control and operation strategies. Anaerobic digestion model 1 (ADM1) provides a generic platform that includes the main processes of AD, excluding homoacetogenesis and the microbial structure. Homoacetogenic bacteria have been identified as important competitors for hydrogen consumption and acetate production. Although recent advances in meta-omics techniques have improved our characterization of AD microbial communities, conventional models treat functional groups as homogeneous and overlook the physiology and behavior of microbial individuality, limiting insights into mechanisms governing process performance. A novel microbial individuality model (MIM) that integrates kinetics, energetics, and agent-based modeling to describe a microbiome's behavior as heterogenic populations, including homoacetogenesis, was developed. The MIM was validated with two datasets from previous studies through daily biogas production, methane content, compound concentrations, and microbial relative abundance changes. The MIM identified the emergence of Methanosaeta at low concentrations of acetate. Moreover, this simulation supports experimental studies confirming that the overlooked homoacetogenesis is an important hydrogen sink in AD. Validated MIMs are expected to provide insights into syntrophic and competitive interactions among microbiomes in AD systems while testing different operational parameters in a virtual environment. The MIM offers a methodological framework to other biological treatment systems and their microbial community dynamics.

Hypersaline Wastewater Produced from Pickled Mustard Tuber (Chinese Zhacai): Current Treatment Status and Prospects

Pickled mustard tuber, a worldwide condiment, is increasing at a fast growth rate. Its production generates a considerable amount of hypersaline wastewater containing NaCl of 7 wt.%, COD of 30,000 mg L−1, NH3-N of 400 mg L−1, and TP of 300 mg L−1. Pickled mustard tuber wastewater (PMTW) has severe effects on crops, deterioration of water quality, soil infertility and ecological systems. Due to the technic difficulties and insufficient support from the local governments; however, PMTW has not yet been widely investigated and well summarized. Therefore, this manuscript reviewed the relatively latest advances in PMTW. Physicochemical and biological hybrid processes mainly treat PMTW and the corresponding cost is 6.00 US dollars per ton. In the context of double carbon capture capacity in China and the development of the pickled mustard industry, PMTW sauce and sustainable reuse such as nutrient recovery, acid and alkaline regeneration and renewable energy may be bright prospects.

Thermodynamics of volatile fatty acid degradation during anaerobic digestion under organic overload stress: The potential to better identify process stability

Anaerobic digestion (AD) operating under organic overload stress usually increases the potential for process instability, leading to significant economic and ecological consequences. Volatile fatty acids (VFAs) accumulation is regularly considered a major factor during AD and their degradation is subject to thermodynamic constraints. To date, no study has systematically investigated the mechanisms of VFA degradation on process stability from the perspective of thermodynamics. Hence, increased substrate-to-inoculum ratio was applied in this study to simulate organic overload stress using batch tests with Hybrid Pennisetum. As a result, VFAs accumulation increased, accompanied by decreased methane yield, slower methane production kinetics and even severe process instability. Metagenomic analysis demonstrated that the accumulated propionate and butyrate were degraded by methyl-malonyl-CoA and the β-oxidation pathway while syntrophic acetate oxidation was preferred during acetate degradation. The deviation of stability parameters to varying degrees from the recommended threshold values was observed. However, a subsequent thermodynamic analysis revealed that moderate organic overload stress merely retarded the syntrophic oxidation of propionate, butyrate, and acetate. As a result, the methanogenic activity decreased, and the lag phase of AD was extended, but no adverse thermodynamic effects actually occurred. Changes in the Gibbs free energy for syntrophic propionate and acetate oxidation have the potential to better identify process stability. This study provided novel insights into the underlying thermodynamic mechanisms of VFA degradation and may have important implications for improving the current diagnostic mode for AD process stability.

Syntrophic propionate-oxidizing bacteria in methanogenic systems

The mutual nutritional cooperation underpinning syntrophic propionate degradation provides a scant amount of energy for the microorganisms involved, so propionate degradation often acts as a bottleneck in methanogenic systems. Understanding the ecology, physiology and metabolic capacities of syntrophic propionate-oxidizing bacteria (SPOB) is of interest in both engineered and natural ecosystems, as it offers prospects to guide further development of technologies for biogas production and biomass-derived chemicals, and is important in forecasting contributions by biogenic methane emissions to climate change. SPOB are distributed across different phyla. They can exhibit broad metabolic capabilities in addition to syntrophy (e.g. fermentative, sulfidogenic and acetogenic metabolism) and demonstrate variations in interplay with cooperating partners, indicating nuances in their syntrophic lifestyle. In this review, we discuss distinctions in gene repertoire and organization for the methylmalonyl-CoA pathway, hydrogenases and formate dehydrogenases, and emerging facets of (formate/hydrogen/direct) electron transfer mechanisms. We also use information from cultivations, thermodynamic calculations and omic analyses as the basis for identifying environmental conditions governing propionate oxidation in various ecosystems. Overall, this review improves basic and applied understanding of SPOB and highlights knowledge gaps, hopefully encouraging future research and engineering on propionate metabolism in biotechnological processes.

New insights into the r/K selection theory achieved in methanogenic systems through continuous-flow and sequencing batch operational modes

Anaerobic digestion is achieved through cooperation among various types of microorganisms, and the regulation of microbial communities is key to achieving stable system operation. In this study, the r/K selection theory was adopted to examine the system performance and microbial characteristics in anaerobic reactors with different operating modes (continuous-flow reactors, CFRs; sequencing batch reactors, SBRs) and sludge retention times (25 and 10 days). Four lab-scale reactors (CFR_25d, CFR_10d, SBR_25d, and SBR_10d) were operated. In the cycle reaction, CFR_25d achieved the highest methane yield (678.0 mL/L) and methane production rate (140.8 mL/(L·h)); while those in CFR_10d were the lowest, which could have been due to an accumulation of volatile fatty acids. CFR could wash out r-strategists efficiently, such as Methanosarcina. CFR_25d and CFR_10d significantly enriched the K-strategist Geobacter, with the relative abundances of 34.0% and 72.6%, respectively. In addition, the hydrogenotrophic methanogens of Methanolinea and Methanospirillum (K-strategists) dominated in CFR_25d and CFR_10d. Methanobacterium adapted to the diverse operational conditions, but the slow grower Methanosaeta only accounted for 0.9% in CFR_10d. Failure to enrich propionate oxidizers resulted in a functional absence of propionate degradation in the CFRs.

Enhancement of methane production from waste activated sludge using hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) process - A review

Hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) was proved to increase methane productivity and methane yield of waste activated sludge (WAS) by establishing direct interspecies electron transfer method and enriching functional microorganisms. This review first summarized the pretreatment methods of WAS for MEC-AD and then reviewed the reactor configurations, operation parameters, and the economic benefit of MEC-AD. Furthermore, the enhancement mechanisms of MEC-AD were reviewed based on the analysis of thermodynamics and microbial community. It was found that the decrease of hydrogen partial pressure due to the hydrogenotrophic methanogens enriched in cathodic biofilm and direct interspecies electron transfer between exoelectrogens and anode were the core mechanisms for improving acidogenesis, acetogenesis, and methanogenesis. Finally, the potentially technological issues that need to be addressed to increase energy efficiency in large-scale MEC-AD processes were discussed.

Electron bifurcation reactions in dark fermentation: An overview for better understanding and improvement

Electron bifurcation (EB) is the most recently found mode of energy conservation, which involves both exergonic and endergonic electron transfer reactions to minimize energy loss. Several works have been devoted on EB reactions (EBRs) in anaerobic digestion but limited in dark fermentative hydrogen production (DF). Two main electron carriers in DF are ferredoxin (Fd) and reduced nicotinamide adenine dinucleotide (NADH), complicatedly involved in EB. Here, i) the importance of EB involvement in DF, ii) all EBRs possible to present in DF, as well as iii) the limitation of previous studies that tried incorporating any of EBRs in DF metabolic model, were highlighted. In addition, the concept of using metagenomic analysis for estimating the share of each EB reaction in the metabolic model, was proposed. This review is expected to initiate a new wave for studying EB, as a tool for explaining and predicting DF products.

Organic waste conversion through anaerobic digestion: A critical insight into the metabolic pathways and microbial interactions

Anaerobic digestion is a promising method for energy recovery through conversion of organic waste to biogas and other industrial valuables. However, to tap the full potential of anaerobic digestion, deciphering the microbial metabolic pathway activities and their underlying bioenergetics is required. In addition, the behavior of organisms in consortia along with the analytical abilities to kinetically measure their metabolic interactions will allow rational optimization of the process. This review aims to explore the metabolic bottlenecks of the microbial communities adopting latest advances of profiling and ¹³C tracer-based analysis using state of the art analytical platforms (GC, GC-MS, LC-MS, NMR). The review summarizes the phases of anaerobic digestion, the role of microbial communities, key process parameters of significance, syntrophic microbial interactions and the bottlenecks that are critical for optimal bioenergetics and enhanced production of valuables. Considerations into the designing of efficient synthetic microbial communities as well as the latest advances in capturing their metabolic cross talk will be highlighted. The review further explores how the presence of additives and inhibiting factors affect the metabolic pathways. The critical insight into the reaction mechanism covered in this review may be helpful to optimize and upgrade the anaerobic digestion system.

Anaerobic bacterial responses to carbonaceous materials and implications for contaminant transformation: Cellular, metabolic, and community level findings

Carbonaceous materials (CM) enhance the abundance and activity of bacteria capable of persistent organic (micro)pollutant (POP) degradation. This review synthesizes anaerobic bacterial responses to minimally modified CM in non-fuel cell bioremediation applications at three stages: attachment, metabolism, and biofilm genetic composition. Established relationships between biological behavior and CM surface properties are identified, but temporal relationships are not well understood, making it difficult to connect substratum properties and "pioneer" bacteria with mature microorganism-CM systems. Stark differences in laboratory methodology at each temporal stage results in observational, but not causative, linkages as system complexity increases. This review is the first to critically examine relationships between material and cellular properties with respect to time. The work highlights critical knowledge gaps that must be addressed to accurately predict microorganism-CM behavior and to tailor CM properties for optimized microbial activity, critical frontiers in establishing this approach as an effective bioremediation strategy.

Mechanisms Driving Microbial Community Composition in Anaerobic Co-Digestion of Waste-Activated Sewage Sludge

Anaerobic co-digestion (Co-AD) is used to increase the effectiveness of anaerobic digestion (AD) using local “wastes”, adding economic and environmental benefits. Since system stability is of existential importance for the operation of wastewater treatment plants, thorough testing of potential co-substrates and their effects on the respective community and system performance is crucial for understanding and utilizing Co-AD to its best capacity. Food waste (FW) and canola lecithin (CL) were tested in mesophilic, lab-scale, semi-continuous reactors over a duration of 120 days with stepwise increased substrate addition. Key performance indicators (biogas, total/volatile solids, fatty acids) were monitored and combined with 16S-rRNA amplicon sequencing to assess the impact of co-substrate addition on reactor performance and microbial community composition (MCC). Additionally, the latter was then compared with natural shifts occurring in the wastewater treatment plant (WWTP, source) at the same time. An almost linear increase in biogas production with both co-substrates at an approximate 1:1 ratio with the organic loading rate (OLR) was observed. The MCCs in both experiments were mostly stable, but also prone to drift over time. The FW experiment MCC more closely resembled the original WWTP community and the observed shifts indicated high levels of functional redundancy. Exclusive to the CL co-substrate, a clear selection for a few operational taxonomic units (OTUs) was observed. There was little evidence for a persistent invasion and establishment of microorganisms from typical primary substrates into the stable resident community of the reactors, which is in line with earlier findings that suggested that the inoculum and history mostly define the MCC. However, external factors may still tip the scales in favor of a few r-strategists (e.g., Prolixibacter) in an environment that otherwise favors K-strategists, which may in fact also be recruited from the primary substrate (Trichococcus). In our study, specialization and diversity loss were also observed in response to the addition of the highly specialized CL, which in turn, may have adverse effects on the system’s stability and reduced resilience and recovery.

Conversion of Carbon Monoxide to Chemicals Using Microbial Consortia

Syngas, a gaseous mixture of CO, H₂ and CO₂, can be produced by gasification of carbon-containing materials, including organic waste materials or lignocellulosic biomass. The conversion of bio-based syngas to chemicals is foreseen as an important process in circular bioeconomy. Carbon monoxide is also produced as a waste gas in many industrial sectors (e.g., chemical, energy, steel). Often, the purity level of bio-based syngas and waste gases is low and/or the ratios of syngas components are not adequate for chemical conversion (e.g., by Fischer-Tropsch). Microbes are robust catalysts to transform impure syngas into a broad spectrum of products. Fermentation of CO-rich waste gases to ethanol has reached commercial scale (by axenic cultures of Clostridium species), but production of other chemical building blocks is underexplored. Currently, genetic engineering of carboxydotrophic acetogens is applied to increase the portfolio of products from syngas/CO, but the limited energy metabolism of these microbes limits product yields and applications (for example, only products requiring low levels of ATP for synthesis can be produced). An alternative approach is to explore microbial consortia, including open mixed cultures and synthetic co-cultures, to create a metabolic network based on CO conversion that can yield products such as medium-chain carboxylic acids, higher alcohols and other added-value chemicals.

Effects of chlorine disinfectants on the microbial community structure and the performance of anaerobic digestion of swine manure

The residual chlorine disinfectants (CDs) in swine slurry could negatively impact the anaerobic digestion (AD). The objective of this study was to investigate the effects of CDs on mesophilic and thermophilic AD. The results indicated that CDs exerted inhibition effects on methanogenesis at the initial stage of mesophilic AD, leading to the extension of lag time from 0.62 days for control to 0.85, 1.9, 3.8, and 5.5 days with the increasing CDs concentrations of 50, 100, 200, and 400 mg/L, respectively. Under thermophilic condition, the inhibition effects reduced significantly at the initial stage but a decrease of CMP_u at later stage was observed. The microbial analysis revealed that CDs resulted in the enrichment of chlorine-resistant bacteria (Clostridum_sensu_stricto_1) and archaea (Methanosarcina). Addition of activated carbon (AC), zero-valent iron (ZVI) and biochar (BC) was evaluated for alleviating the inhibitions of CDs and proved to be feasible strategies to alleviate the inhibited AD.

Microbial communities and their enzymes facilitate degradation of recalcitrant polymers in anaerobic digestion

Microbial consortia efficiently degrade complex biopolymers found in the organic fraction of municipal solid waste (OFMSW). Through enzyme production and division of labor during anaerobic digestion, microbial communities break down recalcitrant polymers and make fermentation products, including methane. However, microbial communities remain underutilized for waste degradation as it remains difficult to characterize and predict microbial interactions during waste breakdown, especially as cultivation conditions change drastically throughout anaerobic digestion. This review discusses recent progress and opportunities in cultivating natural and engineered consortia for OFMSW hydrolysis, including how recalcitrant substrates are degraded by enzymes as well as the critical factors that govern microbial interactions and culture stability. Methods to measure substrate degradation are also reviewed, and we demonstrate the need for increased standardization to enable comparisons across different environments.

Delivery and effects of proton pump inhibitor on anaerobic digestion of food and kitchen waste under ammonia stress

Ammonia stress changes microbial metabolism of anaerobic digestion and decreases methane yield, where proton pump overactivated by free ammonia suggested to be the centre of the metabolism changes in anaerobic digestion under ammonia stress. The work demonstrated that proton pump inhibitor (PPI) could alleviate the overactivated proton pump and mitigate ammonia inhibition. Its impacts on iron transporter, substrates uptake, and energy conservation were investigated in anaerobic digestion treating food and kitchen waste. The PPI formed a stimuli-responsive drug delivery system driven by pH for the more inhibited microbe (p < 0.01), confirmed by FE-SEM/EDS and high throughput sequencing, implying the PPI was activated at inhibited microbe more than mixed liquor. Consistent microbial population increase observed in syntrophs and methanogens, who utilized the substrates for high yielding pathway and facilitated the energy sharing by direct interspecies electron transfer. These results demonstrated PPI could recovery methane production and could mitigate fatty-acid accumulation under high ammonia stress by delivery and activation in acetoclastic methanogen.