Key syntrophic partnerships identified in a granular activated carbon amended UASB treating municipal sewage under low temperature conditions

Molecular and microbial insights towards anaerobic biodegradation of anionic polyacrylamide in oil sands tailings

Anionic polyacrylamide (A-PAM) is widely used as a flocculant in the management of oil sands tailings. Nevertheless, apprehensions arise regarding its potential biodegradation and environmental consequences within the context of oil sands tailings. Consequently, it is imperative to delve into the anaerobic biodegradation of A-PAM in oil sands tailings to gain a comprehensive understanding of its influence on tailings water quality. This work explored the dynamics of A-PAM biodegradation across concentrations: 50, 100, 250, 500, 1000, and 2000 mg/kg TS. The results showed a significant decrease in A-PAM concentration and molecular weight at lower concentrations (50 and 100 mg/kg TS) compared to higher ones, suggesting enhanced degradation efficiency. Likewise, the organic transformation and methane production exhibited dependency on A-PAM concentrations. The peak concentrations observed were 20.0 mg/L for volatile fatty acids (VFAs), 0.07 mg/L for acrylamide (AMD), and 8.9 mL for methane yield, with these maxima being recorded at 50 mg/kg TS. The biodegradation efficiency diminishes at higher concentrations of A-PAM, potentially due to the inhibitory effects of polyacrylic acid accumulation. A-PAM biodegradation under anaerobic condition did not contribute to acute toxicity or genotoxicity. SEM-EDS, FT-IR and XRD analyses further revealed that higher concentrations of A-PAM inhibited the biodegradation by altering floc structure and composition, thereby restricting the microbial activity. Major microorganisms, including Smithella, Candidatus_Cloacimonas, W5, XBB1006, and DMER64 were identified, highlighting A-PAM's dual role as a source of carbon and nitrogen under anaerobic conditions. The above findings from this research not only significantly advance understanding of A-PAM's environmental behavior but also contribute to the effective management practices in oil sands tailings.

Advances in microbial community, mechanisms and stimulation effects of direct interspecies electron transfer in anaerobic digestion

Anaerobic digestion (AD) has been proven to be an effective green technology for producing biomethane while reducing environmental pollution. The interspecies electron transfer (IET) processes in AD are critical for acetogenesis and methanogenesis, and these IET processes are carried out via mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET). The latter has recently become a topic of significant interest, considering its potential to allow diffusion-free electron transfer during the AD process steps. To date, different multi-heme c-type cytochromes, electrically conductive pili (e-pili), and other relevant accessories during DIET between microorganisms of different natures have been reported. Additionally, several studies have been carried out on metagenomics and metatranscriptomics for better detection of DIET, the role of DIET's stimulation in alleviating stressed conditions, such as high organic loading rates (OLR) and lower pH, and the stimulation mechanisms of DIET in mixed cultures and co-cultures by various conductive materials. Keeping in view this significant research progress, this study provides in-depth insights into the DIET-active microbial community, DIET mechanisms of different species, utilization of various approaches for stimulating DIET, characterization approaches for effectively detecting DIET, and potential future research directions. All these can help accelerate the field's research progress, enable a better understanding of DIET in complex microbial communities, and allow its utilization to alleviate various inhibitions in complex AD processes.

Enhancing methane production and organic loading capacity from high solid-content wastewater in modified granular activated carbon (GAC)-amended up-flow anaerobic sludge blanket (UASB)

Anaerobic digestion of high solid-content wastewater is hindered by high organic loading rates (OLRs). Granular activated carbon (GAC) was reported to promote direct interspecies electron transfer (DIET) and enhance reactor performance. In this study, three up-flow anaerobic sludge blanket (UASB) reactors were supplied with GAC in different locations: bottom (R1), top (R2), and bottom+top (R3). The performances of three reactors at different OLRs treating high solid-content wastewater were evaluated. At a low OLR, the highest methane yield (74 ± 4 %, g CH4-COD/g TCOD) was detected when GAC was supplied at top of the UASB (R2). When a high OLR was applied, the UASB supplemented with GAC at both bottom and top (R3) achieved the highest methane yield (66 ± 2 %, g CH4-COD/g TCOD), whereas the UASB supplemented with GAC at the top (R2) failed. Further studies on spatial distributions of sludge stability, specific methanogenic activities (SMAs), and microbial communities demonstrated the different impacts of GAC location on reactor performance and sludge characteristics under different OLRs. This study highlights the significance of considering organic loading capacity treating high solid-content wastewater when choosing GAC-based UASB systems.

Dominant factors analyses and challenges of anaerobic digestion under cold environments

With the development of fermentation technology and the improvement of efficiency, anaerobic digestion (AD) has been playing an increasingly primary role in waste treatment and resource recovery. Temperature is undoubtedly the most important factor because it shapes microbial habitats, changes the composition of the microbial community structure, and even affects the expression of related functional genes. More than half of the biosphere is in a long-term or seasonal low-temperature environment (<20 °C), which makes psychrophilic AD have broad application prospects. Therefore, this review discusses the influencing factors and enhancement strategies of psychrophilic AD, which may provide a corresponding reference for future research on low-temperature fermentation. First, the occurrence of AD has been discussed. Then, the adaptation of microorganisms to the low-temperature environment was analyzed. Moreover, the challenges of psychrophilic AD have been reviewed. Meanwhile, the strategies for improving psychrophilic AD are presented. Further, from technology to application, the current situation of psychrophilic AD in pilot-scale tests is described. Finally, the economic and environmental feasibility of psychrophilic AD has been highlighted. In summary, psychrophilic AD is technically feasible, while economic analysis shows that the output benefits cannot fully cover the input costs, and the large-scale practical application of psychrophilic AD is still in its infancy. More research should focus on how to improve fermentation efficiency and reduce the investment cost of psychrophilic AD.

Stimulating anaerobic digestion to degrade recalcitrant organic pollutants: Potential role of conductive materials-led direct interspecies electron transfer

This review aims to provide a comprehensive understanding of the potential of CMs-dominated DIET in the degradation of recalcitrant organic pollutants in AD. The review covers the mechanisms and efficiencies of recalcitrant organic pollutant degradation by CMs-dominated DIET, the comparison of degradation pathways between DIET and chemical treatment, recent insights on DIET-enhanced degradation, and the evaluation of the potential and future development of CMs-dominated DIET. The review emphasizes the importance of coupled syntrophic microorganisms, electron flux, and physicochemical properties of CMs in enhancing the degradation performance of AD. Additionally, it highlights the advantages of DIET-led syntrophic metabolism over traditional oxidation technologies in terms of environmental friendliness and efficiency. Finally, the review acknowledges the potential risks associated with introducing CMs into AD systems and provides guidance for waste treatment and energy recovery.

Model Organisms To Study Methanogenesis, a Uniquely Archaeal Metabolism

Methanogenic archaea are the only organisms that produce CH4 as part of their energy-generating metabolism. They are ubiquitous in oxidant-depleted, anoxic environments such as aquatic sediments, anaerobic digesters, inundated agricultural fields, the rumen of cattle, and the hindgut of termites, where they catalyze the terminal reactions in the degradation of organic matter. Methanogenesis is the only metabolism that is restricted to members of the domain Archaea. Here, we discuss the importance of model organisms in the history of methanogen research, including their role in the discovery of the archaea and in the biochemical and genetic characterization of methanogenesis. We also discuss outstanding questions in the field and newly emerging model systems that will expand our understanding of this uniquely archaeal metabolism.

Celebrating 50 years of microbial granulation technologies: From canonical wastewater management to bio-product recovery

Microbial granulation technologies (MGT) in wastewater management are widely practised for more than fifty years. MGT can be considered a fine example of human innovativeness-driven nature wherein the manmade forces applied during operational controls in the biological process of wastewater treatment drive the microbial communities to modify their biofilms into granules. Mankind, over the past half a century, has been refining the knowledge of triggering biofilm into granules with some definite success. This review captures the journey of MGT from inception to maturation providing meaningful insights into the process development of MGT-based wastewater management. The full-scale application of MGT-based wastewater management is discussed with an understanding of functional microbial interactions within the granule. The molecular mechanism of granulation through the secretion of extracellular polymeric substances (EPS) and signal molecules is also highlighted in detail. The recent research interest in the recovery of useful bioproducts from the granular EPS is also emphasized.

Boosting resilience of microbial electrolysis cell-assisted anaerobic digestion of blackwater with granular activated carbon amendment

Poor hydrolysis and methanogenesis efficiencies remain the main challenges for blackwater anaerobic digestion. This study investigated the performance of a granular activated carbon (GAC) amended microbial electrolysis cell-assisted anaerobic digester (MEC-AD) treating blackwater. Due to hydrolysis limitation, both MEC-AD and control reactors experienced performance declines as the organic loading rate increased from 3.0 to 4.5 g COD/L-d. Then, adding GAC without mixing formed GAC-sludge aggregates that improved methane yield to 38.3% and 32.3% in the MEC-AD and control reactor, respectively, and enhanced hydrolysis efficiency. The amended MEC-AD also successfully overcame the performance deterioration due to a temperature drop. Biomarker identification revealed the crucial roles of GAC biofilms and settled sludge in promoting methanogenesis and hydrolysis, respectively. This study demonstrated the GAC addition and the electrochemical environment could have a reciprocal influence, leading to more robust syntrophic microbial interactions, which could guide the future application of conductive materials in MEC-AD systems.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

Conductive materials as fantastic toolkits to stimulate direct interspecies electron transfer in anaerobic digestion: new insights into methanogenesis contribution, characterization technology, and downstream treatment

Direct interspecies electron transfer (DIET) stimulated by conductive materials (CMs) enables intercellular metabolic coupling that can address the unfavorable thermodynamical dilemma inherent in anaerobic digestion (AD). Although the DIET mechanism and stimulation have been extensively summarized, the methanogenesis contribution, characterization techniques, and downstream processes of CMs-led DIET in AD are surprisingly under-reviewed. Therefore, this review aimed to address these gaps. First, the contribution of CMs-led DIET to methanogenesis was re-evaluated by comparing the effect of various factors, including volatile fatty acids, free ammonia, and functional enzymes. It was revealed that AD systems are usually intricate and cannot allow the methanogenesis stimulation to be singularly attributed to the establishment of DIET. Additionally, considerable attention has been attached to the characterization of DIET occurrence, involving species identification, gene expression, electrical properties, cellular features, and syntrophic metabolism, suggesting the significance of accurate characterization methods for identifying the syntrophic metabolism interactions. Moreover, the type of CMs has a significant impact on AD downstream processes involving biogas purity, sludge dewaterability, and biosolids management. Finally, the central bottleneck consists in building a mathematical model of DIET to explain the mechanism of DIET in a deeper level from kinetics and thermodynamics.

Exploring key factors in anaerobic syntrophic interactions: Biomass activity, microbial community, and morphology

The present work evaluated the impacts of microbial communities, biomass activity and sludge morphology on anaerobic syntrophic reactions. Experiments were conducted using mature floc sludge and granular sludge under different food/microbes ratios, and with different sludge types (floc sludge, concentrated floc sludge and granular sludge) and sludge morphology (granules, vortexed granules, and granules with different particle sizes). The results show that the intact granules achieved the most effective syntrophic reaction among all sludge types. The granule structure facilitated the enrichment of syntrophic acetate oxidation bacteria (g_Syner-01 and g_Mesotoga) and methanogens, which corresponds to their superior specific methanogenic activity and high production of communication compounds. Despite the high diffusion and substrate uptake capacities, the disintegrated granules had low H₂ consumption rates, which led to poor syntrophic activities. The results underline the importance of sludge spatial structures in promoting excellent syntrophic activities and the development of diverse microbial communities.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Conductive biofilms in up-flow anaerobic sludge blanket enhanced biomethane recovery from municipal sewage under ambient temperatures

The feasibility of municipal sewage treatment in laboratory-scale up-flow anaerobic sludge blankets was investigated in this work. Unlike previous studies, granular activated carbon (conductive) or sponge (non-conductive) was introduced to hollow plastic balls as carriers and suspended in the middle and upper layers of the reactors. The two bioreactors were operated at four different hydraulic retention times (stepwise reduced from 24 h to 8 h) for 100 days at ∼18 °C. The conductive-amended treatment was more effective than the non-conductive treatment in enhancing reactor performance. Interestingly, in the reactor containing conductive carriers, microorganisms enriched in the conductive biofilm were also dominant in the suspended sludge. In the reactor containing sponge carriers, the dominant microorganisms differed between the non-conductive biofilm and the suspended sludge. This study underlines that the enrichment of functional microbial communities and the positive impacts of biofilm on suspended sludge are the keys to improving biomethane recovery.

A multifaceted screening of applied voltages for electro-assisted anaerobic digestion of blackwater: Significance of temperature, hydrolysis/acidogenesis, electrode corrosion, and energy efficiencies

A microbial electrolysis cell-assisted anaerobic digester (MEC-AD) was operated with vacuum toilet blackwater at different applied voltages (0-1.6 V) at room temperature (R₂₀). A parallel MEC-AD was operated at 35 °C (R₃₅) to provide a kinetics index at mesophilic temperature. Both reactors failed at 1.6 V due to the alkaline pH created by anodic corrosion. In R₂₀, the best performance was observed at 1.2 V, with methane yield, COD removal, hydrolysis and acidogenesis efficiency increased by 59.9%, 27.0%, 52.0%, and 44.9%, respectively, compared to those of 0 V. Enrichment of hydrolytic and syntrophic bacteria (e.g., Clostridium, Bacteroidales, Sedimentibacter, Syntrophomonas) and increased abundance of genes encoding complex organics (e.g., proteins, carbohydrates, lipids) metabolism in R₂₀ at 1.2 V corresponded to the enhanced hydrolysis/acidogenesis processes. R₂₀ at 1.2 V generated 1.16 times more net energy than R₃₅ at the optimum voltage for methane yield (0.8 V), indicating ambient temperature operation of MEC-AD systems would be a more sustainable strategy.

Enhanced anaerobic digestion of dairy wastewater in a granular activated carbon amended sequential batch reactor

This study investigated the potential of granular activated carbon (GAC) supplementation to enhance anaerobic degradation of dairy wastewater. Two sequential batch reactors (SBRs; 0.8 L working volume), one control and another amended with GAC, were operated at 37°C and 1.5-1.6 m/h upflow velocity for a total of 120 days (four cycles of 30 days each). The methane production at the end of each cycle run increased by about 68%, 503%, 110%, and 125% in the GAC-amended SBR, compared with the Control SBR. Lipid degradation was faster in the presence of GAC. Conversely, the organic compounds, especially lipids, accumulated in the absence of the conductive material. In addition, a reduction in lag phase duration by 46%-100% was observed at all four cycles in the GAC-amended SBR. The peak methane yield rate was at least 2 folds higher with GAC addition in all cycles. RNA-based bacterial analysis revealed enrichment of Synergistes (0.8% to 29.2%) and Geobacter (0.4% to 11.3%) in the GAC-amended SBR. Methanolinea (85.8%) was the dominant archaea in the biofilm grown on GAC, followed by Methanosaeta (11.3%), at RNA level. Overall, this study revealed that GAC supplementation in anaerobic digesters treating dairy wastewater can promote stable and efficient methane production, accelerate lipid degradation and might promote the activity of electroactive microorganisms.

Enhancing the resistance to H2S toxicity during anaerobic digestion of low-strength wastewater through granular activated carbon (GAC) addition

Low-strength wastewater was treated using two laboratory-scale up-flow anaerobic sludge blankets (UASB) for 130 days under sulfate-reducing conditions. Granular activated carbon (GAC) was added to one of the reactors. The GAC addition increased the total chemical oxygen demand removal by 21-28% and total methane production by 32-78%. The sludge from the GAC-amended UASB showed higher specific methanogenic activities (SMA) and higher activities in the presence of H₂S, indicating that the GAC addition enhanced the resistance of methanogens to H₂S toxicity. Further, the microbial communities showed that the GAC addition shifted microbial communities. A robust syntrophic partnership between bacteria (i.e., Bacteroidetes_vadinHA17 and Trichococcus) and methanogens was established in the GAC-amended UASB. Sulfate-reducing bacteria (SRB) were enriched in the GAC biofilm, indicating the coexistence of competition and cooperation between SRB and methanogens. These findings provide significant insights regarding microbial community dynamics, especially SRB and methanogens, in a GAC-amended anaerobic digestion process under sulfate-reducing conditions.

Metagenomic insights into direct interspecies electron transfer and quorum sensing in blackwater anaerobic digestion reactors supplemented with granular activated carbon

The addition of granular activated carbon (GAC) enhanced the performance of up-flow anaerobic sludge blanket (UASB) reactor treating blackwater at 35 °C. DNA were extracted from the sludge and biofilms attached to GAC and submitted for shotgun sequencing. In addition, the acyl-homoserine lactones (AHLs) were quantified. Diverse partners for direct interspecies electron transfer (DIET) were enriched in the sludge or biofilm (GAC-biofilm) of GAC amended UASB. Pedosphaera parvula, Syntrophus aciditrophicus and Syntrophorhabus aromaticivorans were dominant syntrophs. The analysis for type IV pilus assembly genes further suggested DIET may be functioned through GAC for GAC-biofilm, while through conductive pili for sludge aggregates. AHLs quantification and the analysis for quorum sensing (QS) related genes indicated higher QS activity at the population level was induced by GAC. Overall, the work illustrated the different DIET patterns, and suggested that QS played an important role in controlling the performance in GAC amended USAB.

Roles of granular activated carbon (GAC) and operational factors on active microbiome development in anaerobic reactors

Ambient temperature municipal sewage was treated using two laboratory-scale up-flow anaerobic sludge blanket reactors for 225 days. Granular activated carbon (GAC) was added to one reactor to facilitate the development of direct interspecies electron transfer (DIET). The GAC addition increased total chemical oxygen demand removal by 5% - 18%. In addition to assessing the relative abundance of active amplicon sequence variants (ASVs), the mass balance model, the Mantel test, and the generalized linear models were applied to evaluate the dynamics of the active ASVs and the key operational factors controlling the bioreactor microbial community. These results demonstrated that, in addition to the GAC addition, extrinsic engineering operational factors played important roles in controlling (active) microbial communities. This study underlines the importance of taking a wholistic approach to assess microbial population dynamics. Reactor design and performance prediction should consider key engineering parameters when using DIET-based AD reactors in the future.

Direct interspecies electron transfer mechanism in enhanced methanogenesis: A mini-review

The role of direct interspecies electron transfer (DIET) on enhancement of methanogenesis has been studied. This mini-review updated the current researches on the potential role of DIET on enhanced performance for anaerobic digestion of organic substrates with effective strategies implemented. Since most experimental observations correlated with the DIET mechanism are yet to be consolidated, this article categorized and discussed the current experimental observations supporting DIET mechanism for methanogenesis, mainly based on those with supplement of carbon materials, from which the prospects and challenges for further studies to confirm the role of DIET in anaerobic digestion processes were highlighted.

Electron transfer and mechanism of energy production among syntrophic bacteria during acidogenic fermentation: A review

Volatile fatty acids (VFAs) production plays an important role in the process of anaerobic digestion (AD), which is often the critical factor determining the metabolic pathways and energy recovery efficiency. Fermenting bacteria and acetogenic bacteria are in syntrophic relations during AD. Thus, clear elucidation of the interspecies electron transfer and energetic mechanisms among syntrophic bacteria is essential for optimization of acidogenic. This review aims to discuss the electron transfer and energetic mechanism in syntrophic processes between fermenting bacteria and acetogenic bacteria during VFAs production. Homoacetogenesis also plays a role in the syntrophic system by converting H₂ and CO₂ to acetate. Potential applications of these syntrophic activities in bioelectrochemical system and value-added product recovery from AD of organic wastes are also discussed. The study of acidogenic syntrophic relations is in its early stages, and additional investigation is required to better understand the mechanism of syntrophic relations.

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

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

Enhancing anaerobic syntrophic propionate degradation using modified polyvinyl alcohol gel beads

Modified polyvinyl alcohol (PVA) beads serve as effective anaerobic microbe immobilization carriers. PVA beads were mixed with different conductive materials, activated carbon, magnetite, and green tuff stone powder. In this study, modified PVA beads were used to investigate the effect of using, promote methane production, and enhance direct interspecies electron transfer (DIET) on the anaerobic syntrophic degradation of propionate, which is an essential intermediate process for generating methane in anaerobic digesters. The batch experiment showed that PVA mixed with activated carbon had the highest methane conversion rate of 72%, whereas the rates for control (sludge) was 61%. Moreover, the lag time during the second and third feedings was shorter by 5-fold than for the first feeding when modified PVA beads were added. The syntrophic propionate degrading microorganisms in the modified PVA beads was Syntrophobacter and Methanobacterium, either Methanoculleus or Methanosaeta. The modified PVA beads hold at least 10 times larger syntrophs than normal PVA. Therefore, composite PVA with conductive materials can promote methane production, accelerate propionate consumption, and enhance electron transfer in related microbial species.